U.S. patent application number 15/498994 was filed with the patent office on 2017-11-30 for surface mount solar cell with integrated coverglass.
The applicant listed for this patent is Solar Junction Corporation. Invention is credited to Sathya Chary, Ewelina Lucow, Sabeur Siala, Ferran Suarez, Ali Torabi, Lan Zhang.
Application Number | 20170345955 15/498994 |
Document ID | / |
Family ID | 59009383 |
Filed Date | 2017-11-30 |
United States Patent
Application |
20170345955 |
Kind Code |
A1 |
Chary; Sathya ; et
al. |
November 30, 2017 |
SURFACE MOUNT SOLAR CELL WITH INTEGRATED COVERGLASS
Abstract
Photovoltaic cells, methods for fabricating surface mount
multijunction photovoltaic cells, methods for assembling solar
panels, and solar panels comprising photovoltaic cells are
disclosed. The surface mount multijunction photovoltaic cells
include through-wafer-vias for interconnecting the front surface
epitaxial layer to a contact pad on the back surface. The
through-wafer-vias are formed using a wet etch process that removes
semiconductor materials non-selectively without major differences
in etch rates between heteroepitaxial III-V semiconductor
layers.
Inventors: |
Chary; Sathya; (San
Francisco, CA) ; Lucow; Ewelina; (Los Gatos, CA)
; Siala; Sabeur; (Sunnyvale, CA) ; Suarez;
Ferran; (San Jose, CA) ; Torabi; Ali; (Los
Gatos, CA) ; Zhang; Lan; (Palo Alto, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Solar Junction Corporation |
San Jose |
CA |
US |
|
|
Family ID: |
59009383 |
Appl. No.: |
15/498994 |
Filed: |
April 27, 2017 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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15288559 |
Oct 7, 2016 |
9680035 |
|
|
15498994 |
|
|
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|
62342660 |
May 27, 2016 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01L 31/047 20141201;
H01L 31/03048 20130101; Y02E 10/544 20130101; H01L 31/02327
20130101; H01L 31/02245 20130101; H01L 31/0516 20130101; Y02E 10/50
20130101; H01L 31/0725 20130101; H01L 31/044 20141201; H01L 31/188
20130101; H01L 31/048 20130101; H01L 31/02168 20130101; H01L 31/186
20130101; H01L 31/02008 20130101; H01L 31/022441 20130101 |
International
Class: |
H01L 31/0224 20060101
H01L031/0224; H01L 31/05 20140101 H01L031/05; H01L 31/0304 20060101
H01L031/0304; H01L 31/0232 20140101 H01L031/0232; H01L 31/047
20140101 H01L031/047; H01L 31/18 20060101 H01L031/18; H01L 31/044
20140101 H01L031/044 |
Claims
1. A surface mount multijunction photovoltaic cell, comprising: a
substrate having a front substrate surface and a back substrate
surface; a heteroepitaxial layer overlying the front substrate
surface; a first portion of an antireflection coating overlying a
first portion of the heteroepitaxial layer; a patterned cap region
overlying a second portion of the heteroepitaxial layer; a
through-wafer-via formed through the substrate and through the
heteroepitaxial layer and electrically interconnecting the front
side metal and the front surface solder pad; a second portion of
the antireflection coating overlying a third portion of the
heteroepitaxial layer and disposed on sidewalls of the
through-wafer-via; a passivation layer underlying a portion of the
back substrate; a front surface solder pad underlying a portion of
the passivation layer; a front side metal overlying the patterned
cap region, and the second portion of the antireflection coating,
wherein the front side metal and the front surface solder pad are
electrically interconnected; a backside metal electrically
interconnected to the back substrate surface; an optical adhesive
overlying the first portion of the antireflection coating, the
patterned cap region, and the front side metal; and a coverglass
overlying the optical adhesive.
2. The surface mount multijunction photovoltaic cell of claim 1,
wherein the substrate is less than 150 .mu.m thick.
3. The surface mount multijunction photovoltaic cell of claim 1,
wherein the substrate is less than 100 .mu.m thick.
4. The surface mount multijunction photovoltaic cell of claim 1,
wherein the surface mount multijunction photovoltaic cell is
characterized by a unit mass per area of less than 0.09
g/cm.sup.2.
5. The surface mount multijunction photovoltaic cell of claim 1,
wherein the heteroepitaxial layer comprises at least two
junctions.
6. The surface mount multijunction photovoltaic cell of claim 1,
wherein the sidewalls have smooth surfaces and the back substrate
surface is free of pitting.
7. The surface mount multijunction photovoltaic cell of claim 1,
further comprising a metal filling the through-wafer-via.
8. The surface mount multijunction photovoltaic cell of claim 1,
wherein the heteroepitaxial layer comprises at least one dilute
nitride junction.
9. The surface mount multijunction photovoltaic cell of claim 1,
wherein the at least one dilute nitride junction comprises
GaInNAsSb, GaInNAsBi, GaInNAsSbBi, GaInNAs, GaNAsSb, GaNAs,
GaNAsBi, or GaNAsSbBi.
10. The surface mount multijunction photovoltaic cell of claim 9,
wherein, the at least one dilute nitride junction comprises
Ga.sub.1-xIn.sub.xN.sub.yAs.sub.1-y-zSb.sub.z; and the content
values for x, y, and z are within a range from
0.03.ltoreq.x.ltoreq.0.22, 0.007.ltoreq.y.ltoreq.0.055 and
0.001.ltoreq.z.ltoreq.0.05.
11. The surface mount multijunction photovoltaic cell of claim 9,
wherein the at least one dilute nitride junction comprises
Ga.sub.1-xIn.sub.xN.sub.yAs.sub.1-y-zSb.sub.z having an antimony
content within a range from 0.004.ltoreq.z.ltoreq.0.02,
0.005.ltoreq.z.ltoreq.0.018, 0.006.ltoreq.z.ltoreq.0.016,
0.008.ltoreq.z.ltoreq.0.015, 0.01.ltoreq.z.ltoreq.0.018,
0.01.ltoreq.z.ltoreq.0.014, 0.004.ltoreq.z.ltoreq.0.014, or
0.004.ltoreq.z.ltoreq.0.01.
12. The surface mount multijunction photovoltaic cell of claim 9,
wherein the at least one dilute nitride junction comprises
Ga.sub.1-xIn.sub.xN.sub.yAs.sub.1-y-zSb.sub.z having a nitrogen
content within a range from 0.02.ltoreq.y.ltoreq.0.10,
0.021.ltoreq.y.ltoreq.0.09, 0.024.ltoreq.y.ltoreq.0.085,
0.026.ltoreq.y.ltoreq.0.08, 0.03.ltoreq.y.ltoreq.0.06, or
0.04.ltoreq.y.ltoreq.0.06.
13. The surface mount multijunction photovoltaic cell of claim 9,
wherein the at least one dilute nitride junction comprises
Ga.sub.1-xIn.sub.xN.sub.yAs.sub.1-y-zSb.sub.z having: an antimony
content within a range from 0.004.ltoreq.z.ltoreq.0.02,
0.005.ltoreq.z.ltoreq.0.018, 0.006.ltoreq.z.ltoreq.0.016,
0.008.ltoreq.z.ltoreq.0.015, 0.01.ltoreq.z.ltoreq.0.018,
0.01.ltoreq.z.ltoreq.0.014, 0.004.ltoreq.z.ltoreq.0.014, or
0.004.ltoreq.z.ltoreq.0.01; and a nitrogen content within a range
from 0.02.ltoreq.y.ltoreq.0.10, 0.021.ltoreq.y.ltoreq.0.09,
0.024.ltoreq.y.ltoreq.0.085, 0.026 .ltoreq.y.ltoreq.0.08,
0.03.ltoreq.y.ltoreq.0.06, or 0.04.ltoreq.y.ltoreq.0.06.
14. The surface mount multijunction photovoltaic cell of claim 1,
wherein the substrate comprises Ge or GaAs.
15. A photovoltaic module comprising a plurality of the surface
mount multijunction photovoltaic cells of claim 1.
16. The photovoltaic module of claim 15, comprising: an
interconnection substrate; and wherein the plurality of surface
mount multijunction photovoltaic cells are mounted to the
interconnection substrate.
17. The photovoltaic module of claim 16, wherein the
interconnection substrate comprises: interconnects between each of
the plurality of surface mount multijunction photovoltaic cells;
and a plurality of bypass diodes, wherein each of the plurality of
bypass diodes is interconnected to one or more of the plurality of
surface mount multijunction photovoltaic cells, wherein each of the
plurality of bypass diodes is mounted to the interconnection
substrate.
18. The photovoltaic module of claim 15, wherein, the photovoltaic
module comprises a front surface area; and the plurality of surface
mount multijunction photovoltaic cells cover at least 70% of the
front surface area.
19. A power system comprising at least one photovoltaic module of
claim 15.
20. A method of fabricating a photovoltaic module, comprising
interconnecting at least one surface mount multijunction
photovoltaic cell of claim 1 to an interconnection substrate.
21. The method of claim 20, wherein interconnecting comprises
surface mounting.
Description
[0001] This application is a Divisional of U.S. application Ser.
No. 15/288,559, filed on Oct. 7, 2016, which claims the benefit
under 35 U.S.C. .sctn.119(e) of U.S. Provisional Application No.
62/342,660 filed on May 27, 2016, each of which is incorporated by
reference in its entirety.
FIELD
[0002] This disclosure relates to photovoltaic cells, methods for
fabricating photovoltaic cells, methods for assembling solar
panels, and solar panels comprising photovoltaic cells.
Particularly, the disclosure relates to surface mount multijunction
photovoltaic cells. The surface mount multijunction photovoltaic
cells include through-wafer-vias for interconnecting the front
surface epitaxial layer to a contact pad on the back surface. The
through-wafer-vias are formed using a wet etch process that removes
semiconductor materials non-selectively without major differences
in etch rates between heteroepitaxial III-V semiconductor
layers.
BACKGROUND
[0003] Conventional multi junction solar cells have been widely
used for terrestrial and space applications because of their high
efficiency. Multijunction solar cells (100), as shown in FIG. 1,
include multiple diodes in series connection, known in the art as
junctions or subcells (106, 107, and 108), realized by growing thin
regions of epitaxy in stacks on semiconductor substrates. Each
subcell in a stack possesses a unique bandgap and is optimized for
absorbing a different portion of the solar spectrum, thereby
improving efficiency of solar energy conversion. These subcells are
chosen from a variety of semiconductor materials with different
optical, electrical, and physical properties in order to absorb
different portions of the solar spectrum. The materials are
arranged such that the bandgap of the subcells becomes
progressively smaller from the top subcell (106) to the bottom
subcell (108). Thus, high-energy photons are absorbed in the top
subcell and less energetic photons pass through to the lower
subcells where they are absorbed. In every subcell, electron-hole
pairs are generated and current is collected at ohmic contacts in
the solar cell. Semiconductor materials used to form the subcells
include, for example, germanium and alloys of one or more elements
from group III and group V on the periodic table. Examples of these
alloys include, for example, indium gallium phosphide, indium
phosphide, gallium arsenide, aluminum gallium arsenide, indium
gallium arsenide, and dilute nitride compounds. For ternary and
quaternary compound semiconductors, a wide range of alloy ratios
can be used.
[0004] Using conventional photovoltaic cells, solar arrays used to
power space satellites are typically assembled manually which
results in high cost and introduces the risk of reliability issues.
Nearly all currently available space photovoltaic cells employ
welded interconnect tabs for interconnecting adjacent cells, and a
welded or monolithically integrated bypass diode on each individual
photovoltaic cell. Photovoltaic cells assembled with bypass diodes,
interconnects, and coverglass are referred to in the aerospace
industry as "Coverglass Interconnected Cells" or "CICs". These CICs
are typically assembled using manual process steps. The mechanical
design of commercially available CICs has not changed substantially
in the past two decades.
[0005] To reduce the number of overall steps associated with the
expensive, manual process steps used in both CIC and solar array
assembly, the industry has been moving to increasingly larger CICs
using both 4-inch and 6-inch Ge substrates.
[0006] Normally, a photovoltaic cell contributes around 20% to the
total cost of a photovoltaic power module. Higher photovoltaic cell
efficiency means more cost effective modules. Fewer photovoltaic
devices are then needed to generate the same amount of output
power, and the generation of higher power with fewer devices leads
to reduced system costs, such as costs for structural hardware,
assembly processes, wiring for electrical connections, etc. In
addition, by using high efficiency photovoltaic cells to generate
the same power, less surface area, fewer support structures, and
lower labor costs are required for assembly installation.
[0007] Photovoltaic modules are a significant component in
spacecraft power systems. Lighter weight and smaller photovoltaic
modules are always preferred because the lifting cost to launch
satellites into orbit is very expensive. Efficient surface area
utilization of photovoltaic cells is especially important for space
power applications to reduce the mass and fuel penalty associated
with large photovoltaic arrays. Higher specific power (watts
generated over photovoltaic array mass), which reflects the power
one solar array can generate for a given launch mass, can be
achieved with more efficient photovoltaic cells because the size
and weight of the photovoltaic array will be less for the same
power output. Additionally, higher specific power can be achieved
using smaller cells more densely arranged over a photovoltaic array
of a given size and shape.
[0008] Interconnection of multijunction photovoltaic cells is
typically accomplished by welding interconnect ribbons to front
side and back side contacts on the p- and n-sides of the device.
Interconnecting multijunction photovoltaic cells using these
methods can be costly. To minimize interconnection costs it can be
desirable to use larger area photovoltaic cells to reduce the
number of interconnects that need to be formed for a given panel
area. This can lead to a reduction in surface area utilization.
Interconnect welding is usually the most delicate operation in CIC
assembly.
[0009] It is desirable to develop alternative device structures and
methods for interconnecting multijunction photovoltaic cells to
solar cell subsystems.
SUMMARY
[0010] According to aspects of the invention, a surface mount
multijunction photovoltaic cell comprises: a substrate having a
front substrate surface and a back substrate surface; a
heteroepitaxial layer overlying the front substrate surface; a
front surface contact overlying a portion of and electrically
connected to the heteroepitaxial layer; an optical adhesive
overlying the front surface contact and the heteroepitaxial layer;
a coverglass overlying the optical adhesive; a back surface solder
pad underlying a portion of and electrically connected to the back
substrate surface; a front surface solder pad underlying and
insulated from the back substrate surface; and a through-wafer-via
interconnecting the front surface solder pad and the front surface
contact.
[0011] According to aspects of the invention, a photovoltaic module
comprises a plurality of the surface mount multijunction
photovoltaic cells according to the present invention.
[0012] According to aspects of the invention, a power system
comprises a photovoltaic module according to the present
invention.
[0013] According to aspects of the invention, a method of
fabricating a multijunction photovoltaic cell comprises: providing
a semiconductor wafer, wherein the semiconductor wafer comprises: a
substrate having a front substrate surface and a back substrate
surface; a heteroepitaxial layer overlying the front substrate
surface; a front surface contact overlying and electrically
connected to a portion of the heteroepitaxial layer; an optical
adhesive overlying the front surface contact and the
heteroepitaxial layer; and a coverglass overlying the optical
adhesive layer; and thinning the substrate.
[0014] According to aspects of the invention, a surface mount
multijunction photovoltaic cell comprises: a substrate having a
front substrate surface and a back substrate surface; a
heteroepitaxial layer overlying the front substrate surface; a
front surface contact overlying a portion of and electrically
connected to the heteroepitaxial layer; an optical adhesive
overlying the front surface contact and the heteroepitaxial layer;
a coverglass overlying the optical adhesive; a passivation layer
underlying a portion of the back substrate surface; a back metal
pad underlying a portion of the passivation layer; a
through-wafer-via electrically interconnecting the front metal
contact and the back metal pad; and a backside metal electrically
connected to the back substrate surface.
[0015] According to aspects of the invention, a photovoltaic module
comprises a plurality of the surface mount multijunction
photovoltaic cells according to the present invention.
[0016] According to aspects of the invention, a method of
fabricating a photovoltaic module comprises interconnecting at
least one of the surface mount multijunction photovoltaic cells
according to the present invention to an interconnection
substrate.
[0017] According to aspects of the invention, a method of
fabricating a multijunction photovoltaic cell comprises: providing
a semiconductor wafer, wherein the semiconductor wafer comprises: a
substrate having a front substrate surface and a back substrate
surface; a heteroepitaxial layer overlying the front substrate
surface; and a patterned cap region overlying a first portion of
the heteroepitaxial layer; etching a through-wafer-via extending
from the heteroepitaxial layer to within the substrate; depositing
an antireflection coating on a second portion of the
heteroepitaxial layer and on a sidewall and a bottom of the
through-wafer-via; etching the antireflection coating on the bottom
of the through-wafer-via to expose the substrate; depositing a
front surface contact overlying at least a portion of the patterned
cap region, the antireflection coating within the patterned cap
region, the sidewalls of the through-wafer-via, and the bottom of
the through-wafer-via; applying an optical adhesive overlying the
front surface contact, the patterned cap region, and the
antireflection coating; applying a coverglass overlying the optical
adhesive; and thinning the substrate.
BRIEF DESCRIPTION OF DRAWINGS
[0018] The drawings described herein are for illustration purposes
only. The drawings are not intended to limit the scope of the
present disclosure.
[0019] FIG. 1 shows a cross-section of a multijunction photovoltaic
cell.
[0020] FIG. 2A shows a cross-section of a multijunction
photovoltaic cell with TWVs fabricated by dry etching.
[0021] FIG. 2B shows a bottom view of the multijunction
photovoltaic cell shown in FIG. 2A.
[0022] FIG. 3A shows a cross-section of a multijunction
photovoltaic cell with TWVs fabricated by dry etching.
[0023] FIG. 3B shows a bottom view of the multijunction
photovoltaic cell shown in FIG. 3A.
[0024] FIG. 4A shows a schematic cross-section of a multijunction
solar cell semiconductor wafer representative of a scanning
electron microscopy image illustrating wafer damage caused using
dry etch methods.
[0025] FIG. 4B shows a schematic cross-section of a semiconductor
wafer representative of a scanning electron microscopy image
fabricated using methods provided by the present disclosure.
[0026] FIGS. 5-14 illustrate a process flow for fabricating a
multijunction photovoltaic cell having TWVs according to certain
embodiments provided by the present disclosure.
[0027] FIG. 15 shows a cross-section of a multijunction
photovoltaic cell with a TWV fabricated using the method
illustrated in FIGS. 5-14.
[0028] FIG. 16A shows a top view of the multijunction photovoltaic
cell shown in FIG. 15.
[0029] FIG. 16B shows a bottom view of the multijunction
photovoltaic cell shown in FIG. 15.
[0030] FIGS. 5 and 17-26 illustrate a process flow for fabricating
a multijunction photovoltaic cell with TWVs according to certain
embodiments provided by the present disclosure.
[0031] FIG. 27 shows a cross-section of a multijunction
photovoltaic cell with a TWV fabricated using the method
illustrated in FIGS. 5 and 17-26.
[0032] FIG. 28 shows a cross-sectional view of a multijunction
photovoltaic cell with a TWV fabricated using a dry etch.
[0033] FIGS. 29A-29C show cross-sectional views of TWVs fabricated
using a dry etch.
[0034] FIGS. 30A-30C show cross-sectional views of a multijunction
photovoltaic cell with a TWV fabricated using a wet etch method
provided by the present disclosure.
[0035] FIG. 31 shows a cross-section view of an example of a
surface mount multijunction photovoltaic cell according to the
invention.
[0036] FIG. 32 shows a bottom view of an example of a surface mount
multijunction photovoltaic cell according to the invention.
[0037] FIG. 33 shows a cross-section view of a coverglass
interconnect cell (CIC).
[0038] FIG. 34 shows a cross-section view of an example of a
surface mount coverglass cell (SMCC) according to the
invention.
[0039] FIGS. 35A-35C show solar cell panel utilization using full
wafers (35A), half wafers (35B), and SMCCs provided by the present
disclosure (35C).
[0040] FIGS. 36A-36B show front and back views of a SMCC device
according to the present disclosure with an example of a gridline,
busbar, and TWV configuration.
[0041] In the following detailed description, reference is made to
the accompanying drawings that illustrate specific embodiments.
[0042] Reference is now made in detail to certain embodiments of
the present disclosure. While certain embodiments of the present
disclosure are described, it will be understood that it is not
intended to limit the embodiments of the present disclosure to the
disclosed embodiments. To the contrary, reference to embodiments of
the present disclosure is intended to cover alternatives,
modifications, and equivalents as may be included within the spirit
and scope of the embodiments of the present disclosure as defined
by the appended claims.
DETAILED DESCRIPTION
[0043] The devices and methods of the present invention address and
in certain aspects eliminate a number of complicated, manual
processes in the assembly of CICs. Also, from a cost perspective,
the devices and methods of the present invention facilitate high
speed, low cost, automated assembly of solar arrays for use in
satellites and other solar energy systems. This is achieved using
photovoltaic cells with through-wafer-vias, all-backside surface
mount contacts with coverglass integrated at the wafer-level. These
devices are referred to as surface mount coverglass cells (SMCCs).
With all of the electrical contacts on the backside of the
photovoltaic cell, individual photovoltaic cells can be assembled
onto printed wiring boards (PWBs), printed circuit boards (PCBs) or
other interconnection substrate to provide a solar array using
standard electronics industry pick-and-place assembly equipment and
practices. SMCC multijunction photovoltaic cells (SMCC) can be
surface mounted to a variety of substrates using well-known, low
cost, high throughput, surface mount methods used throughout the
semiconductor industry. With the resulting decrease in
interconnection costs and assembly costs, smaller SMCC die can be
economically employed to provide improved solar array area
utilization. Surface mount interconnects eliminate the need to weld
interconnect tabs or a bypass diode to the frontside metal. This
results in the elimination of costly manufacturing processes and
thereby reduces the overall cost of fabricating and assembling a
solar array. The use of well-developed automated assembly methods
eliminates workmanship issues resulting in higher reproducibility
and reliability. Because automated assembly methods can be used,
SMCCs can have smaller dimensions, which results in higher wafer
and solar array area utilization.
[0044] Because a surface mount cell with integrated coverglass
(SMCC) eliminates the need for post-cell-processing application of
coverglass to individual photovoltaic cells the size of the
photovoltaic cell can be reduced without the corresponding increase
in assembly processes and costs associated with CIC production,
thereby increasing the number of photovoltaic cells that can be
included in a given area, and increasing the active area on a
particular solar array. This provides a significant increase in
power per area (power density) compared to a traditional solar
array employing traditional CICs. Because SMCCs eliminate the need
for welded interconnections, the distance between adjacent
photovoltaic cells is reduced, and photovoltaic cells can be
densely packed. Additionally, the small photovoltaic cell size also
results in higher cell efficiency due to a reduction in grid line
resistance loss. Furthermore, the overall wafer yield is increased
because small photovoltaic cells that do not meet electrical
performance specifications can be yielded out of the diced wafer,
compared to a defect on a large area cell that may result in the
need to discard the full wafer.
[0045] Applying coverglass at the wafer-level facilitates the
ability to thin the substrate. The coverglass can serve as a
carrier during subsequent process of the photovoltaic cell. The
process for forming through-wafer-vias (TWVs) can be combined with
a step in which the substrate is thinned Thinning the substrate can
significantly reduce the mass of the photovoltaic cell, which can
be important, for example, in space applications.
[0046] Radiation resistant coverglass is bonded to the epitaxial
layers and substrate at the wafer-level and is used as a rigid
carrier for subsequent process steps including substrate thinning.
Bonding the coverglass at the wafer level eliminates the need to
apply coverglass to each individual photovoltaic cell during CIC
manufacturing. Use of the coverglass as a wafer carrier also
facilitates the use of low cost etch and via deposition processes
to provide all-backside electrical contacts for surface mounting.
The surface mount photovoltaic cells can be assembled onto a
circuit board using high speed pick-and-place equipment and
methods.
[0047] FIG. 31 shows a cross-sectional schematic view of an example
of a SMCC device provided by the present disclosure. The SMCC shown
in FIG. 31 includes semiconductor 1, which includes heteroepitaxial
layers overlying a substrate, and a coverglass 3, bonded to the
front surface 9 of the semiconductor 1 using an optical adhesive 2.
Front side contact 4 on front surface 9 of semiconductor 1 is
interconnected to back surface contact pad 6 by through-wafer-via
(TWV) 5. Back surface contact pad 6 and TWV 5 are electrically
insulated from semiconductor 1. Back surface 11 of semiconductor 1
is interconnected to back surface contact 7. Surface mount solder
pad 8 is disposed on and electrically insulated from back surface
11 and is electrically interconnected (not shown) to back surface
contact 7.
[0048] When referring to the various surfaces of a multijunction
solar cell, the front surface or top surface refers to the surface
designed to face incident solar radiation, and the back surface or
bottom surface refers to the side of the solar cell facing away
from the incident radiation. The front surface is also referred to
as the frontside surface, and the back surface is also referred to
as the backside surface.
[0049] FIG. 32 shows a back surface view of the SMCC device shown
in FIG. 31. Back surface contact pads 6 are interconnected to the
front side contact (not shown) of the semiconductor. Back surface
contact pads 6 are interconnected to front side surface mount
solder pad 12 for interconnecting the SMCC device to a printed
circuit board or other interconnection substrate. Back surface
contact 7 is disposed on the back surface 10 of the SMCC device and
is interconnected to back surface mount solder pad 8 for
interconnecting the SMCC device to a printed circuit board.
[0050] The coverglass 3 (FIG. 31) can be any suitable optically
transparent dielectric material appropriate for use in solar cells.
The coverglass can be a sheet of material. In certain embodiments,
the coverglass is radiation resistant. The coverglass can be any
suitable thickness for protecting the solar cell from the
environment and radiation. For example, the coverglass can be from
20 .mu.m to 600 .mu.m thick, from 40 .mu.m to 500 .mu.m thick, from
50 .mu.m to 400 .mu.m thick, or from 75 .mu.m to 300 .mu.tm
thick.
[0051] The optical adhesive 2 (FIG. 31) can be any suitable optical
adhesive capable of bonding the coverglass to underlying layers
including a heteroepitaxial layer and/or metal contact layers. An
example of a suitable optical adhesive is Dow Corning .RTM.93-500
space grade encapsulant. The optical adhesive layer can be, for
example, from 2 .mu.m to 200 .mu.m thick, from 5 .mu.m to 150 .mu.m
thick, or from 10 .mu.m to 100 .mu.m thick.
[0052] Front side contact 4 (FIG. 31) can comprise one or more
layers and can be, for example, less than 0.2 .mu.m thick, less
than 10 .mu.m thick, less than 20 .mu.m thick, or less than 40
.mu.m thick. Thicker front contact layers can comprise multiple
layers such as, for example, layers of Au, Ag Ti, Ni, Cr, or
combinations of any of the foregoing. Each layer can be, for
example, from 1 .mu.m to 10 .mu.m thick, or from 0.1 .mu.m to 1
.mu.m thick.
[0053] Semiconductor layer 1 (FIG. 31) can comprise a
heteroepitaxial layer on a substrate. Semiconductor layer 1
comprises the active multijunction photovoltaic cell. The
multijunction photovoltaic cells can comprise one or more subcells.
Examples of multijunction photovoltaic cells are disclosed in U.S.
application Ser. No. 62/350,430 filed on Jun. 15, 2016, U.S.
application Ser. No. 14/887,021 filed on Oct. 19, 2015, U.S.
Application Publication No. 2013/0118566, and U.S. Application
Publication No. 2013/0130431, each of which is incorporated by
reference in its entirety. The heteroepitaxial layer can include
multiple layers of semiconductor material used to fabricate a
multijunction photovoltaic cell such as shown in FIG. 1. In certain
multijunction photovoltaic cells, at least one of the junctions
comprises a dilute nitride material such as GaInNAsSb, GaInNAsBi,
or GaInNAsSbBi. In certain embodiments, each of the subcells is
lattice matched to each of the other subcells forming the
multijunction photovoltaic cell and to the substrate. A substrate
can be active and comprise one of the active junctions of the
photovoltaic cell, or the substrate can be inactive. An example of
an active substrate is Ge. A Ge substrate can be less than 200 um
thick, less than 175 um thick, less than 150 um thick, or less than
100 um thick. A Ge substrate can be, for example, from 20 .mu.m to
175 .mu.m thick, from 50 .mu.m to 175 .mu.m thick, or from 50 .mu.m
to 80 .mu.m thick. An example of an inactive substrate is GaAs,
which can be, for example, from 10 .mu.m to 400 .mu.m, from 40
.mu.m to 90 .mu.m, from 50 .mu.m to 80 .mu.m, or from 50 .mu.m to
70 .mu.m thick.
[0054] Backside contact 6 (FIG. 31 and FIG. 32) can include one or
more layers of electrically conductive metals such as Au, Ag, Ti,
Ni, Cr, or a combination of any of the foregoing. The contact layer
6 can be, for example, less than 0.2 .mu.m thick, less than 0.5
.mu.m thick, or less than 1 .mu.m thick, and each of the
electrically conductive layers can be, for example, from 0.1 .mu.m
to 1 .mu.m thick, from 1 .mu.m to 20 .mu.m thick, or from 1 .mu.m
to 10 .mu.m thick.
[0055] FIG. 33 shows a cross-section view of an example of a
conventional coverglass interconnect cell (CIC). The CIC shown in
FIG. 33 includes a coverglass 1 bonded to heteroepitaxial layer 5
with optical adhesive 2. However, heteroepitaxial layers 5 overly
substrate 6. Front side contact 3 interconnects to the front
surface of heteroepitaxial layer 5, and back side contact 7
interconnects to the back surface of substrate 6. Interconnection
tabs 8 and 9 are welded to front side contact 3 and back side
contact 7, respectively.
[0056] FIG. 34 shows a cross-section view of an example of a
surface mount coverglass cell (SMCC) provided by the present
disclosure. The SMCC device includes coverglass 1 bonded to the
front surface of heteroepitaxial layer 5 with optical adhesive 2.
Heteroepitaxial layers 5 overly substrate 6. Substrate 6 can be a
thinned substrate. Front side contact 9 is interconnected to the
front surface of heteroepitaxial layers 5. Front side contact 9 is
interconnected to contact 8 on the back side surface of the device
by TWV 4. Back surface contact 7 underlies and is interconnected to
substrate 6. It can be appreciated that many details of a SMCC are
not shown in FIG. 34.
[0057] As illustrated by comparing the CIC device shown in FIG. 33
with the SMCC device shown in FIG. 34, the substrate in the SMCC
can be much thinner than the substrate in the CIC. Also, because
the SMCC solder pads in FIG. 34 are amendable to surface mount
assembly, the welded interconnect tabs shown in FIG. 33 are not
necessary.
[0058] FIGS. 35A-35C show top views of three different solar cell
panels. The mount area is 100 cm.sup.2. FIG. 35A shows the area
covered by three 59.42-cm.sup.2 photovoltaic cells with a surface
utilization of 59.42%. FIG. 35B shows the area covered by six
29.95-cm.sup.2 photovoltaic cells with a surface utilization of
59.49%. FIG. 35C shows the area covered by 243, 1-cm.sup.2 SMCC
photovoltaic cells corresponding to a surface utilization of
81%.
[0059] Using the smaller, surface mountable SMCC devices (FIG.
35C), a solar panel can be tiled to fill the panel with little area
between adjoining cells. The wafer is also more efficiently
utilized. Using CICs (FIGS. 35A and 35B), there is significant
wasted space between adjacent cells in part due to the welded
interconnection tabs. Because of the high cost associated with
welded interconnects it is desirable to reduce the number of
interconnections in a solar cell array. This can be accomplished by
using larger photovoltaic cells. Although the use of larger
photovoltaic cells reduces the number of interconnects between
photovoltaic cells, the panel area utilization is reduced. FIG. 35C
shows the high panel utilization provided by assembling SMCCs onto
a panel with little separation between adjacent photovoltaic cells.
FIG. 35B shows panel tiling using half-wafer sections with area
between adjacent photovoltaic cells required for welded tab
interconnects. It can be appreciated that using, for example,
quarter-wafer photovoltaic cells will increase the number of welded
tab interconnects and also reduce the panel utilization.
[0060] Using through-wafer-vias, the coverglass can be applied to
the front surface of the photovoltaic cells at the wafer-level. The
coverglass can be used as a carrier to thin the semiconductor
substrate. For example, the epitaxial layers of a multijunction
solar cell can be grown on a thick substrate such as a 140 .mu.m
thick Ge substrate as is usually the case for conventional three
junction space cells. The thickness of the substrate can be
reduced, for example, from 140 .mu.m to 50 .mu.m for Ge, and down
to as thin as 10 .mu.m for GaAs substrates. As an example, a SMCC
with a solar cell on a GaAs substrate thinned-down to 50 .mu.m,
results in a 43% reduction in the mass of the photovoltaic cell,
relative to a conventional cell on a 140 .mu.m-thick Ge substrate.
For a satellite with a 550 W BOL power requirement, replacing
conventional CICs with SMCC devices provided by the present
disclosure can reduce the mass by over 0.75 kg.
[0061] The fabrication of SMCC multijunction photovoltaic cells
includes forming high quality through-wafer-vias (TWVs) across the
complex heteroepitaxial structure.
[0062] Conventional multi junction solar cells have been widely
used for terrestrial and space applications because of their high
efficiency. Multijunction solar cells (100), as shown in FIG. 1,
include multiple diodes in series connection, known in the art as
junctions or subcells (106, 107, and 108), realized by growing thin
regions of epitaxy in stacks on semiconductor substrates. Each
subcell in a stack possesses a unique bandgap and is optimized for
absorbing a different portion of the solar spectrum, thereby
improving efficiency of solar energy conversion. These subcells are
chosen from a variety of semiconductor materials with different
optical and electrical properties in order to absorb different
portions of the solar spectrum. The materials are arranged such
that the bandgap of the subcells becomes progressively narrower
from the top subcell (106) to the bottom subcell (108). Thus,
high-energy photons are absorbed in the top subcell and less
energetic photons pass through to the lower subcells where they are
absorbed. In every subcell, electron-hole pairs are generated and
current is collected at ohmic contacts in the solar cell.
Semiconductor materials used to form the subcells include, for
example, germanium and alloys of one or more elements from group
III and group V on the periodic table. Examples of these alloys
include, for example, indium gallium phosphide, indium phosphide,
gallium arsenide, aluminum gallium arsenide, indium gallium
arsenide, and dilute nitride compounds. For ternary, quaternary,
and quinary compound semiconductors, a wide range of alloy ratios
can be used.
[0063] Solar cells are manufactured on a wafer scale using
conventional semiconductor processing methods known to
practitioners skilled in the art. Danzilio (CS MANTECH Conference,
May, 14-17, 2007, Austin, Tex., pp. 11-14) summarizes the
processing steps for making a typical multijunction solar cell.
[0064] A through-wafer via (TWV) is an electrical interconnect
between the top (front) and bottom (back) surfaces of a
semiconductor chip. TWVs are routinely used for a variety of
applications in the field of semiconductor devices including
photovoltaic cells. FIG. 2A and FIG. 3A show examples of TWVs (200
and 300) for photovoltaic cells with front and back electrical
contacts. TWVs are electrically isolated from the photovoltaic cell
substrate (202 and 302) and all the epitaxial regions (203 and
303), and are electrically connected to the patterned cap regions
(204 and 304). The patterned cap regions are patterned such that
they surround the TWV structures on the top surface of the
photovoltaic cell. Front side metal pads (201 and 301) lay over
patterned cap regions (204 and 304). TWVs also comprise back side
metal (205 and 305), via metal (206 and 306), passivation layer
(207 and 307), via contact metal region (208 and 308) and gap (209
and 309) between passivation layer 207 and back side metal 205. In
some examples of TWVs, a recess structure 310 is present in the TWV
design. Methods to fabricate TWVs are known to practitioners
skilled in the art of semiconductor fabrication. For example, Chen
et al. (Journal of Vacuum Science and Technology B, 27(5), p. 2166,
2009) disclose a semiconductor device with TWVs for a high mobility
electron transport device application.
[0065] TWVs are also used to provide back-contact packaging in
photovoltaic cells. Back-contact cells have both positive and
negative external contact pads disposed on the back surface, which
allows for optimized module efficiency by increasing the packing
density of solar cells. Shading losses and resistive losses are
also significantly reduced. Van Kerschaver et al. (Progress in
Photovoltaics: Research and Applications 2006; 14:107-123)
summarizes several approaches for back-contact solar cells.
[0066] Dry etching is a routine method used in semiconductor
fabrication but has found limited use in solar cell manufacturing.
Dry etching involves the removal of semiconductor material by
exposing the material to a plasma of reactive gases in a vacuum
chamber. Dry etching of heteroepitaxial layers in multijunction
photovoltaic cells involves added complexity because each class of
semiconductor material requires a unique etch condition. This
complexity causes a slower net etch rate and a bottleneck in
manufacturing. As etching proceeds across multiple layers of
heterogeneous semiconductor materials, re-deposition of etched-off
material causes rough sidewalls and is unavoidable. A mask is used
to protect wafer areas where etching is not required. A
photosensitive polymer is typically used as the mask, but a
photosensitive polymer mask cannot withstand the long etch times
and high heat required for dry etching. The photosensitive polymer
mask is often destroyed, leading to pitting and. significantly to
the generation of rough surfaces, which complicates subsequent
sidewall passivation processing and decreases reliability of
manufactured devices. FIG. 4A depicts a schematic of a wafer
cross-section imaged by scanning electron microscopy, damaged with
pitting and rough sidewall surfaces (408 and 409). The device shown
in FIG. 4A includes coverglass 407, front side metal pad 406, ARC
405, heteroepitaxial layer 403, substrate 402, patterned cap region
404, and back side 401 of substrate 402. The sidewall 409 of
heteroepitaxial layer 403 is characterized by a rough surface
including pitting and/or undercutting resulting from the dry etch.
Pitting 408 is also shown on the back side surface 401 of substrate
402, which can also be caused by the dry etch. Alternative masking
methods such as dielectric hard masks can be used in place of a
photosensitive polymer, but these masks require elaborate
downstream steps for removal from the wafer. Dry etching also
involves expensive equipment setup and maintenance. In summary, dry
etching presents the following complications:
[0067] (1) electroplating or electrografting to protect wafer areas
where etching is not desired, requiring expensive and specific
equipment;
[0068] (2) low processing throughput and longer processing time
because dry etching can be carried out on only a few wafers at a
time;
[0069] (3) difficulty in controlling etch rate as well as etch
stop, leading to insufficient etching or over-etching;
[0070] (4) uneven etching of heteroepitaxial III-V semiconductor
layers results in pitting and rough sidewall surfaces, which
complicate subsequent sidewall passivation;
[0071] (5) higher possibility of device failure due to insufficient
sidewall passivation;
[0072] (6) more chemical, water, and energy consumption during
fabrication; and
[0073] (7) higher cost from equipment procurement and
maintenance.
[0074] Wet etching, another method for removing semiconductor
material by using chemicals in liquid phase, is not without
shortcomings. Typically, wet etchants used for etching one class of
semiconductor materials is selective and will not etch certain
other classes of semiconductor materials. A comprehensive list of
wet etchants, etch rates and selectivity relationships was
published by Clawson, Materials Science and Engineering, 31 (2001)
1-438. The selectivity of a wet etchant may also depend on alloy
concentration of the compounds. Consequently, etching
heteroepitaxial layers can require application of multiple wet etch
chemistries. Using multiple applications of different selective wet
etchants typically results in jagged, non-smooth, and/or irregular
TWV sidewalls (as shown in FIG. 4A). This is observed in
photovoltaic cell fabrication where different etch chemistries are
used for each class of semiconductor material in the
heteroepitaxial layers, resulting in distinctively different etch
profiles and rough sidewall surfaces throughout the wafer. Zaknoune
et al., J. Vac. Sci. Technol. B 16, 223 (1998) reported a wet
etching method that is nonselective for III-V phosphides and
arsenides as an alternative to using multiple wet etchants.
Although the method is nonselective, the etching of gallium
arsenide results in very rough morphology and involves an etch rate
10 times faster than the etch rate of aluminum gallium indium
phosphide. Zaknoune et al. describe a system with one layer of
epitaxy, such as that found in heterojunction bipolar transistors
(HBT), quantum well lasers (QWL) and high electron mobility
transistors (HEMT). The Zaknoune et al. method does not address any
sidewall problem related to heteroepitaxial layers that is
characteristic of multijunction solar cells.
[0075] Typically, rough/jagged TWV sidewalls complicate subsequent
sidewall passivation, leading to an increase in device failures and
lower fabrication yield. In addition, the use of multiple etchants
has other disadvantages compared to single-etch chemistries,
including, for example:
[0076] (1) increased difficulty in controlling the etch rate and
undesirable lateral undercutting of layers;
[0077] (2) uneven etching of different semiconductor layers and
increased difficulty in subsequent sidewall passivation
processing;
[0078] (3) higher possibility of device failure due to insufficient
sidewall passivation;
[0079] (4) longer processing time due to complications and
unpredictability inherent in the method;
[0080] (5) more chemical, water, and energy consumption during
fabrication; and
[0081] (6) more chemical waste generation.
[0082] The abovementioned conventional processes have hindered
cost-effective fabrication of multijunction photovoltaic cells.
There were attempts to explore non-selective etchants and certain
examples are briefly described. Zaknoune et al. (J. Vac. Sci.
Technol. B 16, 223, 1998) report an etching procedure that is
nonselective for gallium arsenide and aluminum gallium indium
phosphide, where the aluminum gallium indium phosphide quaternary
compound has 35% aluminum phosphide, 15% gallium phosphide, and 50%
indium phosphide. The etching procedure described by Zaknoune et
al. uses a diluted solution of hydrochloric acid, iodic acid, and
water to etch 300 nm of the quaternary compound grown on a gallium
arsenide substrate using a photosensitive polymer mask. The main
application areas described in the paper by Zaknoune et al. are
heterojunction bipolar transistors (MIT), various quantum well
lasers QWL), and high electron mobility transistors (HEMT) for
which large conduction and valance band discontinuities are
required. These devices are majority carrier devices in which the
large bandgap materials are typically used as barrier materials for
majority carriers. Zaknoune et al. describe a system with one layer
of epitaxy and do not recognize any sidewall problem related to
multilayer epitaxy that is characteristic of photovoltaic
cells.
[0083] The device requirements for multijunction solar cells are
significantly different than for HBTs, QWLs, and HEMTs, largely
because multijunction photovoltaic cells are minority carrier
devices. Consequently the procedure described by Zahnoune et al.
has no direct application to etching multijunction solar cell
structures, which include a wide variety of semiconductor materials
with a wide range of bandgaps (for example, from 0.67 eV to 2.25
eV).
[0084] The present disclosure describes a TWV fabrication method
that overcomes complications with existing methods. The various
advantages include the following:
[0085] (1) when anti-reflective coating (ARC) is deposited, as part
of routine solar cell fabrication, a pattern is added where the TWV
is to be constructed, i.e. the ARC is used as a dielectric etch
stop between the semiconductor and the metal pads on top of the
wafer. This additional function of the ARC simplifies TWV
fabrication by eliminating the application of an extra etch
stop;
[0086] (2) standard manufacturing processing steps are employed,
including photolithography, wet etching and thin film
evaporation;
[0087] (3) significant cost reduction due to the use of inexpensive
equipment, chemicals and methods;
[0088] (4) processing throughput is higher because multiple wafers
can be etched at the same time and fewer etching process steps are
required;
[0089] (5) areas of wafer that need to be protected from etching
can be protected by a photosensitive polymer, employing a lower
cost material and simpler method than electroplating photoresist or
electrografting;
[0090] (6) smooth, 100% passivated TWV walls, which improves
manufacturing yield by lowering the risk of device failure; and
[0091] (7) a thinner substrate results from these processing steps,
making the photovoltaic cells lighter and appropriate for space
applications, simplifies fabrication of the TWV, and improves
thermal properties.
[0092] U.S. Application Publication No. 2015/0349181 to Fidaner et
al. discloses a method of etching mesa sidewalls in multijunction
photovoltaic cells using a single-step wet etch process, where the
etchant comprises a mixture of hydrochloric acid and iodic acid,
which is incorporated by reference in its entirety. Fidaner
demonstrates that the iodic etchant can be used to etch
heteroepitaxial layers such as characteristic of multijunction
photovoltaic cells having smooth sidewalls.
[0093] A wet etchant used to etch the TWVs can comprise iodic acid,
hydrochloric acid, and water prepared in the molar ratios of
1:62:760, respectively. The molar ratios of iodic acid and
hydrochloric acid can be within, for example, a variance of .+-.5%,
such that the molar ratios in the mixture are within the ranges
(0.95-1.05):(59-65):760, for iodic acid, hydrochloric acid, and
water, respectively. The molar ratios of iodic acid and
hydrochloric acid can be within, for example, a variance of
.+-.10%, such that the molar ratios in the mixture are within the
ranges (0.90-1.10):(56-68):760, for iodic acid, hydrochloric acid,
and water, respectively. The molar ratios of iodic acid and
hydrochloric acid can be within, for example, a variance of
.+-.15%, such that the molar ratios in the mixture are within the
ranges (0.85-1.15):(53-71):760, for iodic acid, hydrochloric acid,
and water, respectively.
[0094] In terms of vol %, the iodic acid, hydrochloric acid and
water can be combined in a 1:2:3 ratio by volume, wherein the
aqueous solution of hydrochloric acid can be 38% .+-.3% by weight
and the aqueous solution of iodic acid can be 6.6% .+-.1% by
weight. The aqueous solution of hydrochloric acid can be 38% .+-.6%
by weight and the aqueous solution of iodic acid can be 6.6% .+-.5%
by weight. It is within the contemplation of the invention to use
another solute or liquid mixtures besides water in the wet etch
process, although water is the most readily available. Similarly,
other acids of different molar concentration can be substituted for
hydrochloric acid to yield the same result.
[0095] The wet etch results cross-sectional shape of the side wall
profile characterized by a substantially macroscopically smooth
curved profile, that is, having a substantially macroscopically
smooth surface without significant undercutting of a junction
region compared to other junction regions.
[0096] The wet etchant can comprise a volumetric ratio of
hydrochloric acid from 10%-50% and the volumetric ratio of iodic
acid in the mixture can be 10%-50%, where the aqueous solution of
hydrochloric acid is 38% .+-.3% by weight and the aqueous solution
of iodic acid can be 6.6% .+-.1.0% by weight, or 38% .+-.5% by
weight and the aqueous solution of iodic acid is 6.6% .+-.5.0% by
weight. It is to be understood the same molar ratios of the
constituent chemicals can be provided using different volumetric
ratios with different molarities in the aqueous solutions used.
During processing, the temperature of the wet etchant can be
maintained between 10.degree. C. and 140.degree. C., such as, for
example, from 20.degree. C. to 100.degree. C., from 20.degree. C.
to 60.degree. C., or from 30.degree. C. to 50.degree. C.
[0097] A wet etchant can comprise volumetric ratio of hydrochloric
acid from 30% to 35% and a volumetric ratio of iodic acid from 14%
to 19%, using the molarities in the aqueous solutions of the
constituent chemicals, and the temperature of the mixture can be
maintained between 30.degree. C. and 45.degree. C. A wet etchant
can comprise volumetric ratio of hydrochloric acid from 27% to 38%
and a volumetric ratio of iodic acid from 11% to 22%, using the
molarities in the aqueous solutions of the constituent chemicals,
and the temperature of the mixture can be maintained between
30.degree. C. and 45.degree. C.
[0098] Single step wet etch processes are described to produce
semiconductor devices that have back contacts, i.e. electrical
contacts on the back side of the device, that can be employed to
provide a SMCC. Specifically, TWVs for back-contact multijunction
solar cells are fabricated with this wet etch method. TWVs are
fabricated that are electrically isolated from the photovoltaic
cell substrate and all epitaxial regions, except for the patterned
cap regions. The method of wet etch chemistry employed removing
semiconductor materials non-selectively without major differences
in etch rates between different heteroepitaxial layers. This is
useful for multijunction photovoltaic cells, which comprise
multiple heterogeneous semiconductor layers epitaxially grown on
the semiconductor substrate. Multijunction solar cells thus formed
lack pitting on the wafer surfaces and on the TWV sidewalls, and
have smooth sidewall surfaces within the TWVs. This process employs
standard wafer batch processing, significantly reduces fabrication
complexity and cost, increases processing throughput, and improves
device performance and reliability by ensuring complete passivation
of TWV walls.
[0099] The process steps described herein can be modified or
adapted provided that the removal of semiconductor material in
exposed areas is achieved using a single-step wet etch process. It
is to be understood that additional process steps can be inserted
in all semiconductor processes that require TWV fabrication.
[0100] In certain aspects of the invention, TWVs can be etched from
the back side of a semiconductor wafer. The semiconductor wafer has
front side metal pads, patterned cap regions, metal regions that
lay over each patterned cap region, and an ARC that result from
front side wafer processing. The front side of the semiconductor
wafer can also be bonded to coverglass with an optically clear
adhesive. The semiconductor can be thinned from its back side. TWV
holes can be etched from the back side of the semiconductor wafer
so that the TWVs extend from the back side surface of the
semiconductor wafer to the ARC overlying the top of the
heteroepitaxial layer. Wafer areas, where etching is not desired,
can be protected by resist patterns. Then, multiple layers of
semiconductor material can be wet etched where TWVs are desired;
etching can be carried out in a single step with wet chemistry that
may comprise the use of an iodic acid-hydrochloric acid mixture.
The ARC can serve as a dielectric etch stop and can protect the
front side metal pad from being etched. The ARC can then be removed
to expose the bottom side of the front metal pads. A passivation
layer can subsequently be deposited over the smooth TWV sidewalls.
This can be followed by the deposition of a metal isolation resist
pattern, protecting semiconductor wafer areas where metal is not
required. Then, metal can be deposited on the bottom of the TWV and
on the sidewalls of the TWV and on the back side of the wafer.
Finally, the metal isolation resist pattern and sacrificial metal
can be removed.
[0101] In another aspect of the invention, TWVs can be etched from
the front side of a semiconductor wafer. The semiconductor wafer
has a cap layer overlying the heteroepitaxial layer. TWV holes can
be etched from the front side of the semiconductor wafer into the
substrate layer using a single-step wet chemistry that may include
the use of an iodic acid-hydrochloric acid mixture. Wafer areas
where etching is not desired can be protected by resist patterns.
Then, patterned cap regions can be formed from the cap layer. ARC,
which functions as a passivation layer, can be applied on the front
side of the semiconductor wafer on regions surrounding the
patterned cap regions as well as on the smooth surfaces of the TWV
holes. The ARC that lines the bottom surface of TWV holes can be
removed to expose the substrate. Then, metal can be deposited on
the TWVs and on the front side of the semiconductor wafer, except
on semiconductor wafer areas where metal is not desired and the
semiconductor wafer can be protected by another resist pattern.
This resist pattern can be removed and gold or other electrically
conductive metal or alloy can be deposited to fill the TWVs. Gold
can be deposited by electroplating. The semiconductor wafer can be
mounted on coverglass with optically clear adhesive. Then, from the
back side, the semiconductor wafer can be thinned and a passivation
layer can be patterned onto this back side surface with a hard
baking step. This can be followed by metal deposition, guided by a
metal isolation resist pattern, on the back side of the
semiconductor wafer. Finally, the metal isolation resist pattern
and sacrificial metal can be removed.
[0102] Semiconductor devices formed using the single-step wet etch
processes described lack pitting on the wafer surfaces as well as
on the TWV sidewalls. Pitting morphology is typical if dry etching
is employed to fabricate TWVs. The TWV sidewalls fabricated by this
single-step wet etch method also have substantially smooth sidewall
surfaces. Semiconductor devices formed by this method include
back-contact-only multijunction photovoltaic cells.
[0103] SEM (scanning electron microscopy) images showing
cross-sections of TWVs fabricated using dry etch methods or
fabricated using wet etch methods provided by the present
disclosure are presented in FIGS. 28-30.
[0104] FIG. 28 shows a cross-section of a multijunction solar cell
structure with a TWV fabricated using a dry etch process, including
back side via metal 2801, passivation layer 2802, GaAs substrate
2803, bottom subcell 2804, middle subcell 2805, top subcell and
contact layers 2806, adhesive 2807, and coverglass 2808. The
surface of the GaAs substrate is characterized by pitting due to
compromise of the etch mask. The side wall of the via is also rough
and pitted. The rough surface results in the passivation layer that
is applied to the side wall is not completely conformal. The
purpose of the passivation layer is to electrically isolate the TWV
metal from the semiconductor layers such as the substrate and the
heteroepitaxial layers. A high quality passivation layer will be
conformal to the underlying layer such as the substrate and the
side wall of the TWV and will be free of pinholes. In FIG. 28 there
is poor passivation over the sharp edges of the side walls and the
pits in the substrate can reach the heteroepitaxial layers.
[0105] FIGS. 29A-29C also show cross-sections of TWVs fabricated
using dry etch methods. FIG. 29A shows electroplated back side
metal 2901, passivation layer 2902, GaAs substrate 2903, bottom
subcell 2904, middle subcell 2905, top subcell and contact layers
2906, adhesive 2907, coverglass 2908, and top side metal pad 2909.
There is no passivation on the rough side wall surfaces. FIG. 29B
shows that dry etching can produce smooth side walls in a GaAs
substrate; however, as shown in FIG. 29C, a dry etch of both GaAs
and an overlying heteroepitaxial layer produces rough side wall
surfaces that are difficult to passivate. FIG. 29C shows both a
cross-sectional view and a top view of a TWV structure having both
GaAs and heteroepitaxial layers.
[0106] For dry etch TWV structures, because the post-etch substrate
and via wall topography is rough and/or pitted, the passivation
layer coating quality is poor, especially around the via edges
where the passivation thickness is less than 1 .mu.m and there are
a large number of pinholes in the passivation layer. These pinholes
serve as a source for electrical shorting. Dry etching also
generates etch mask residue such as burned resist that cannot be
removed from the wafer without employing harsh cleaning and
processing methods that can compromise the via structure. The
burned resist results from the prolonged dry etch of the III-V
heteroepitaxial stack and tends to accumulate around the TWV
openings and also contributes to the formation of pinholes in the
passivation coating.
[0107] FIGS. 30A-30C show cross-sectional views of TWVs prepared
using wet etch methods provided by the present disclosure. FIG. 30A
shows deposited back side metal 3001, passivation layer 3002, GaAs
substrate 3003, bottom subcell 3004, middle subcell 3005, top
subcell and contact layers 3006, optically clear adhesive 3007,
coverglass 3008, and ARC etch stop 3009. As shown in FIG. 30A, the
top surface of the substrate and the side wall of the TWV are
smooth and free of pitting and undercutting. The passivation layer
conformably coats the surfaces that were etched using the iodic
acid wet etch method provided by the present disclosure. The wet
etched surfaces can comprise traces of iodine. FIG. 30B shows a
cross-section with some lateral undercutting of the heteroepitaxial
layer but with sufficiently smooth surfaces that the passivation
layer conformally coats the side wall of the TWV. FIG. 30C shows
another view of a TWV structure fabricated using the iodic acid wet
etch method provided by the present disclosure. FIG. 30C also shows
the bottom of the via metal in the TWV structure. The passivation
thickness is 3 .mu.m at the edges of the TWV. As shown in these
figures, because the substrate and TWV surfaces are smooth and free
of post-etch contamination, the passivation coating quality is high
and is 100% conformal.
[0108] As shown in FIG. 1, multijunction photovoltaic cells 100 can
include a substrate 5, back metal contact 52, top metal contact 2
including cap regions 3 and heteroepitaxial layers 45 forming the
subcells. An ARC 1 overlies metal contact 2, cap regions 3, and the
front surface of the uppermost subcell 106. The multijunction
photovoltaic cell in FIG. 1 includes three subcells 106, 107, and
108. Each subcell can comprise a front surface field 4 and emitter
102 forming element 132, depletion region 103, base 104, back
surface field 105, and tunnel junction 167. An ARC can cover the
top surface of the multijunction solar cell. Tunnel junction 178
interconnects second subcell 107 and third subcell 108.
Heteroepitaxial layers 45 overly substrate 5 and a metal contact 52
is disposed on the back side of substrate 5.
[0109] At least one of the subcells can comprise a dilute nitride
subcell. Examples of dilute nitride subcells include GaInNAsSb,
GaInNAsBi, GaInNAsSbBi, GaInNAs, GaNAsSb, GaNAsBi, and
GaNAsSbBi.
[0110] The process flow described herein is merely an example.
Other process flows with different steps can be used to achieve
TWVs on semiconductor material such as multijunction photovoltaic
cells.
[0111] FIGS. 5-15 illustrate an aspect of the invention associated
with etching TWVs from the front side of a semiconductor wafer in
the fabrication of a back-contact solar cell. FIGS. 5-8 show steps
involved in front side processing. FIGS. 8-15 show steps associated
with back side processing including the wet etch steps provided by
the present disclosure. The process steps and final product
described can be modified by one skilled in the art to accommodate
a wide variety of semiconductor devices; the steps and final
product are not limited to solar cells and are applicable to other
semiconductor devices and in particular to minority carrier
devices. The semiconductor wafer cross-sections shown in FIGS. 5-15
can be summarized as follows: FIG. 5 shows a heteroepitaxial layer
on an unmodified substrate; FIG. 6 shows a wafer after contact cap
layer patterning; FIG. 7 shows a wafer following application of an
ARC; FIG. 8 shows a wafer following application of a front side
metal pad; FIG. 9 shows a wafer after wafer bonding, back-grinding
and wet etch back-thinning; FIG. 10 shows a wafer after via hole
lithography and wet etch; FIG. 11 shows a wafer after via etch stop
(ARC/dielectric) removal; FIG. 12 shows a wafer after passivation
layer patterning and hard bake; FIG. 13 shows a wafer after back
side and via-metal isolation lithography; FIG. 14 shows a wafer
after back side and TWV-metal deposition; and FIG. 15 shows a
completed device after metal lift off (TWV metal and back side
metal separation) .
[0112] A semiconductor wafer can first undergo front side
processing (FIGS. 5-8). As shown in FIG. 5, a semiconductor wafer
can comprise a substrate layer 505 and the back side 506 of the
wafer, and a heteroepitaxial layer 504 overlying the substrate
layer 505. Materials used to form the substrate include, for
example, germanium, gallium arsenide, alloys of germanium, and
alloys of gallium arsenide. The substrate can be, for example, from
100 .mu.m to 1000 .mu.m thick, or from 100 .mu.m to 700 .mu.m
thick. For example, a GaAs substrate can be from 100 .mu.m to 700
.mu.m thick, and a Ge substrate can be from 100 .mu.m to 200 .mu.m
thick. Materials used to form the heteroepitaxial layer include,
for example, alloys of one or more elements from group III and
group V on the periodic table, such as indium gallium phosphide,
indium phosphide, gallium arsenide, aluminum gallium arsenide,
indium gallium arsenide, and dilute nitride compounds. FIGS. 5-6
show cap region 502 and patterned cap regions 602A, which are
formed on the front side of the semiconductor wafer, adjacent to
the heteroepitaxial layer (504 and 604). The patterned cap regions
602A may be patterned in a disk shape, but can also be patterned in
a variety of geometric configurations, as well as shaped to
function as gridlines, busbars, pads or any type of conductive
component of an electrical device. FIG. 6 shows substrate 605, back
side 606 of substrate 605, heteroepitaxial layer 604, and patterned
cap regions 602A following post-cap etch.
[0113] In some embodiments, an ARC (703 in FIG. 7) may be applied
over the heteroepitaxial layer 704. FIG. 7 shows substrate 705,
back side 706 of substrate 705, heteroepitaxial layer 704, ARC 703,
and patterned cap regions 702A following post-cap etch. Some
embodiments may also employ the application of front side metal
pads (801 in FIG. 8) and narrow metal gridlines (not shown). At the
end of front side processing, a semiconductor wafer with an
unmodified back side 806 of substrate 805 can be obtained, as shown
in FIG. 8. FIG. 8 shows substrate 805, back side 806 of substrate
805, heteroepitaxial layer 804 overlying substrate 805, ARC 803
disposed between patterned cap regions 802A, and front side metal
pad 801 electrically connected to patterned cap regions 802A.
[0114] In FIG. 9, the semiconductor wafer shown in FIG. 8 can be
permanently bonded to a coverglass 908 with an optically clear
adhesive 907. In some embodiments, the coverglass 908 may be space
grade coverglass such as radiation resistant coverglass, which may
be made of borosilicate glass. The back side of the substrate (806
in FIG. 8) can be thinned (909 in FIG. 9) by wet etching,
back-grinding, or other methods. In some embodiments, the thinned
substrate 905 can be between 20 .mu.m and 200 .mu.m thick
post-thinning, such as from 20 .mu.m to 100 .mu.m, from 20 .mu.m to
80 .mu.m, or from 20 .mu.m to 50 .mu.m. Thinned devices are
desirable in some applications, for example, in space solar cells
to reduce the weight of the photovoltaic cell. It is also possible
that a substrate can be bonded to a carrier using, for example, a
sacrificial or lift-off layer. The carrier is then removed before
forming TWVs. A thinned substrate also facilitates the formation of
high quality TWVs. FIG. 9 shows thinned substrate 905, back side
909 of thinned substrate 905, heteroepitaxial layer 904, ARC 903,
patterned cap regions (post-cap etch) 902A, front side metal pad
901, optically clear adhesive 907, and coverglass 908.
[0115] In FIG. 10, the back side of the substrate 1009 is patterned
with a photosensitive polymer or any suitable type of suitable
masking material in a desired TWV pattern (not shown), aligning TWV
holes with front side metal pads 1001 and patterned cap regions
1002A that end up forming a perimeter around the ARC-adjacent
region of the TWV holes 1010. Etching TWV holes 1010 starts from
the back side 1009 of the substrate 1005 and stops at the ARC 1003.
In some embodiments, the etchant mixture used can comprise a
volumetric ratio of 10% to 50% hydrochloric acid with a volumetric
ratio of 10% to 50% iodic acid in deionized water. The etchant
mixture can have a temperature that ranges from 10.degree. C. to
140.degree. C. As shown in FIG. 10, etching proceeds from the back
side 1009 through thinned substrate 1005 and through
heteroepitaxial layer 1004. Etching stops at the ARC 1003, which
serves as a selective dielectric etch stop layer 1011. Then, the
patterned photosensitive polymer/masking material (not shown) and
the ARC 1003 that is exposed in the TWV hole 1010 are removed. FIG.
10 also shows heteroepitaxial layer 1004, optically clear adhesive
1007, and coverglass 1008.
[0116] Suitable wet etchant mixtures comprising hydrochloric acid
and iodic acid are disclosed, for example, in U.S. Application
Publication No. 2013/0312817, which is incorporated by reference in
its entirety. Smooth sidewalls etched with the etchant mixture can
comprise traces of iodine. The heteroepitaxial sidewalls can be
characterized by a macroscopically smooth surface without
significant undercutting and that continuously widens from the
substrate to the ARC. In some embodiments, the etchant mixture used
can comprise a volumetric ratio of 30% to 35% hydrochloric acid
with a volumetric ratio of 14% to 19% iodic acid in deionized
water. The etchant mixture can have a temperature within the range
from 30.degree. C. to 45.degree. C.
[0117] FIG. 11 shows the result of the steps described with
reference to FIG. 10. FIG. 11 shows the exposed bottom 1112 of the
front side metal pad 1101 after the ARC is removed from TWV hole
1110. The sidewalls 1010 of the TWV holes (1010 and 1110) are
smooth, as shown in FIG. 4B; there is an absence of pitting (411)
and rough sidewall surfaces that results using prior art methods
(FIG. 4A). There is also an absence of pitting on the back side
(410) of the wet etched back-thinned substrate (1009 and 1109) as
shown in FIG. 4B. The semiconductor wafer is sufficiently protected
by a photosensitive polymer/masking material (not shown) from
etching that deviates from a desired etching pattern. The device
shown in FIG. 4B includes coverglass 407, front side metal pad 406,
patterned cap regions 404, ARC 405, heteroepitaxial layer 403,
substrate 402, and back side surface 401 of substrate 402. The TWV
sidewalls of heteroepitaxial layer 411 are smooth, without pitting
and with reduced undercutting. Also, no pitting 410 is present on
the back side surface 401 of substrate 402. FIG. 11 shows front
side metal pad 1101, patterned cap region (post-cap etch) 1102A,
ARC (dielectric) 1103, heteroepitaxial layer 1104, substrate 1105,
optically clear adhesive 1107, coverglass 1108, backside of the wet
etched back-thinned substrate 1109, TWV hole 1110, and exposed
bottom of the front side metal pad 1112 after TWV etch stop
removal.
[0118] The ARC at the top of the TWV 1110 serves as an etch stop
for the wet etch. After the wet etch and via formation the ARC at
the top of the TWV can subsequently be removed, for example by dry
etching or by wet etching using, for example, hydrofluoric acid, to
expose front side metal pad 1112.
[0119] Residual ARC 1109 can remain between the patterned cap
region 1102A and the TWV 1110. In certain embodiments, cap regions
may not be present and the metal pad may overly only the ARC layer.
After wet etch and TWV formation, a portion or the entire ARC layer
previously underlying the metal pad may be removed to expose the
lower surface of the metal pad. If a portion of the ARC layer is
removed there will be an ARC layer between a portion of the metal
pad and the heteroepitaxial layer.
[0120] The profiles shown FIG. 4A and FIG. 4B are for illustration
purposes and other etch profiles may be characterized by other
roughened and/or pitted surfaces. It is to be understood that the
examples of semiconductor morphology illustrated in the present
disclosure are not limited to the substrate, heteroepitaxial and
processing layers. It is known to one skilled in the art that other
embodiments may be present in semiconductor structures and
devices.
[0121] In FIG. 12, a passivation layer 1213 is applied over the
back side 1209 of the wet etched back-thinned substrate 1205
according to a desired pattern to passivate the substrate 1205 from
metal contact. The passivation layer 1213 also lines the walls of
the TWV holes 1210. The passivation layer 1213 can be applied using
standard deposition techniques, including for example,
photosensitive polymer application, plasma-enhanced chemical vapor
deposition, atomic layer deposition, and electrografting. In some
embodiments, hard baking can be used in this step. The bottom 1212
of the front side metal pad 1201 remains exposed after TWV etch
stop removal and deposition of the passivation layer 1213. FIG. 12
shows front side metal pad 1201, patterned cap regions (post-cap
etch) 1202A, ARC 1203, heteroepitaxial layer 1204, thinned
substrate 1205, optically clear adhesive 1207, coverglass 1208,
back side 1209 of the wet etched back-thinned substrate 1205, TWV
hole 1210, exposed bottom 1212 of the front side metal pad 1201
after TWV etch stop removal, and passivation layer 1213.
[0122] In FIG. 13, TWV metal isolation resist pattern 1314 can be
formed with a photosensitive polymer. This patterning is carried
out, for example, by photolithography techniques which may or may
not require hard baking, depending on the specific embodiment. The
bottom 1312 of the front side metal pad 1301 remains exposed 1312.
FIG. 13 shows front side metal pad 1301, patterned cap regions
(post-cap etch) 1302A, ARC 1303, heteroepitaxial layer 1304,
thinned substrate 1305, optically clear adhesive 1307, coverglass
1308, back side of the wet etched back-thinned substrate 11309, TWV
hole 1310, exposed bottom 1312 of the front side metal pad 1301
after TWV etch stop removal, passivation layer 1313, and back side
and TWV metal isolation resist pattern 1314.
[0123] In FIG. 14, TWV metal 1415 is applied such that the TWV
metal 1415 lines the previously exposed bottom of the front side
metal pad 1401 and lines the sidewalls 1416 of TWV holes 1410,
forming an electrical interconnection to the TWV front side metal
pad 1407. The TWV metal 1415 also lines a portion of the back side
of the substrate (1417 and 1419), bounded by the resist 1414 from
the previous step (FIG. 13). In some embodiments, these TWV and
back side substrate metals (1415, 1416, 1417 and 1419) can be
applied in a single deposition step. Sacrificial metal 1418 and
metal isolation resist pattern 1414 are then lifted off to isolate
positive and negative electrical contacts (front side and back side
electrical contacts), leading to the product shown in FIG. 15. FIG.
14 shows front side metal pad 1401, patterned cap regions (post-cap
etch) 1402A, ARC 1403, heteroepitaxial layer 1404, substrate 1405,
optically clear adhesive 1407, and coverglass 1408, on the top side
of the wet etched back-thinned substrate 1405, TWV hole 1410,
passivation layer 1413, back side TWV metal isolation resist
pattern 1414, TWV metal 1415 deposited on the bottom of the TWV
interconnecting directly to the top side metal pad 1401, TWV metal
1416 deposited along the sidewalls of the TWV isolated from the
heteroepitaxial stack and from the substrate by the passivation
layer 1413, TWV metal 1417 deposited over a portion of passivation
layer 1413, back side metal 1419 deposited on the back side of
substrate 1405, and sacrificial metal 1418 on top of the isolation
resist 1414.
[0124] The example of a completed TWV structure shown in FIG. 15
includes front side metal pad 1501, patterned cap regions (post-cap
etch) 1502A, ARC 1503, residual ARC 1503A, heteroepitaxial layer
1504, substrate 1505, optically clear adhesive 1507, coverglass
1508, TWV hole 1510, ARC layer 1503, TWV metal 1515 deposited on
the bottom of the TWV (electrically connecting directly to the top
side metal pad 1501), TWV metal 1516 deposited along the sidewalls
of the TWV and electrically isolated from the heteroepitaxial stack
and from the substrate by the passivation layer 1513, TWV metal
1517 deposited on the back side of the device, and back side metal
1519 electrically connected to substrate 1505.
[0125] A TWV can be, for example, from 20 .mu.m to 50 .mu.m deep,
or from 10 .mu.m to 200 .mu.m deep. A TWV can have a width, for
example, from about 10 .mu.m to 500 .mu.m, from 10 .mu.m to 400
.mu.m, from 100 .mu.m to 400 .mu.m, or from 100 .mu.m to 250 .mu.m.
A TWV can be characterized, for example, by an aspect ratio from
0.5 to 1.5 from 0.8 to 1.2, or from 0.9 to 1.1.
[0126] Referring to FIG. 15, depending on the width of the top of
the TWV structure, there can be a residual ARC layer 1503A or
section between a portion of the front side metal 1501 and the
heteroepitaxial layer 1504. The residual ARC layer can be between
the patterned cap region 1502A and the passivation layer 1513 on
the sidewalls of the TWV. If the width of the top of the TWV
structure is large, then there may not be a residual ARC layer in
the top of the TWV within the patterned cap region.
[0127] FIG. 16A and FIG. 16B each show a cross-section of a
completed device viewed from the top of the semiconductor wafer and
from the bottom of the semiconductor wafer, respectively. This
device was manufactured using the processes illustrated in FIGS.
5-15. FIG. 16A and FIG. 16B represent an example of a particular
embodiment and do not limit the present disclosure. Modifications
in the processes and the resulting devices by one skilled in the
art may result in final products with variations. Possible
variations include device structure, shape, materials and
dimensions. For example, although the patterned cap regions 1602A
and front side metal pad 1601 are shown to be annular, they are not
limited to this shape and represent only an embodiment of the
present disclosure. Other shapes that may be used include, for
example, squares and rectangles. In the example of a device that is
manufactured by processes shown in FIGS. 5-15, a front side metal
pad lies directly over the TWV hole. In another example, where the
processes are as described in FIGS. 17-27, a gold plug can be
present in a device that is manufactured by processes shown in
FIGS. 5 and 17-27, while a front side metal pad is absent (not
shown). From the top side (FIG. 16A), the following components of
the device are visible: front side metal pad 1601, patterned cap
regions 1602, and ARC 1603. The TWV is directly beneath front side
metal pad 1601, and includes ARC 1603, passivation layer 1613, and
TWV metal 1615 that connects directly to the back side of top side
metal pad 1601. From the bottom side (FIG. 16B), the following
components of the device are visible: back side metal 1619,
passivation layer 1613, TWV metal 1615 on back side of and
electrically connected to the top side metal pad 1501, TWV metal
1616 along the sidewalls of the TWV isolated from the
heteroepitaxial layer and from the substrate by the passivation
layer, and TWV metal 1617 deposited on the back side of the device.
These are examples of a particular embodiment and do not limit the
scope of the disclosure. Modifications in the method and the device
disclosed may result in final products with variations. The final
product fabricated by methods in the disclosure will have smooth
sidewalls 411 instead of lateral undercutting and pitting of the
semiconductor wafer as shown, for example, in (FIGS. 4A and 4B).
This is an advantageous improvement over prior art, resulting in
improved fabrication reliability and yield of devices that comprise
a heteroepitaxial layer. Bonding the coverglass to the front
surface of the device before fabrication of the TWV provides a
carrier for subsequent processing. Most importantly the thick
substrate used during epitaxial growth can be thinned using one or
more methods to provide a thin substrate. The thinned substrate
facilitates the formation of high quality TWVs using wet etching,
and can significantly reduce the overall weight of the
multijunction photovoltaic cell.
[0128] FIGS. 5 and 17-27 show an aspect of the invention that
comprises etching TWVs from the front side of a semiconductor wafer
in the fabrication of a back-contact surface-mountable photovoltaic
cell. FIGS. 5 and 17-23 show steps associated with front side
processing, including the wet etch steps highlighted in the
disclosure. FIGS. 24-27 show steps involved in back side
processing. The process steps and final product described can be
modified by one skilled in the art to accommodate a wide variety of
semiconductor devices; the steps and final product are not limited
to solar cells. The process steps illustrated in FIGS. 5 and 17-27
can be summarized as follows: FIG. 5 shows a heteroepitaxial layer
on an unmodified substrate; FIG. 17 shows a wafer after via hole
lithography and wet etch; FIG. 18 shows a wafer after contact cap
layer patterning; FIG. 19 shows a wafer after ARC and passivation
layer application, FIG. 20 shows a wafer after passivation layer
removal from the bottom of TWV holes; FIG. 21 shows a wafer after
front side metal seed layer lithography and evaporation; FIG. 22
shows a wafer after gold plug lithography and electroplating; FIG.
23 shows a wafer after mounting on coverglass; FIG. 24 shows a
wafer after back-grinding and wet etch back-thinning; FIG. 25 shows
a wafer after back side passivation layer patterning and hard bake;
and FIG. 26 shows a wafer after back side and via-metal isolation
lithography; and FIG. 27 shows a completed device after metal lift
off (TWV metal and back side metal separation).
[0129] A semiconductor wafer (FIG. 5) can be provided comprising a
heteroepitaxial layer 504 overlying the front side of the substrate
505, and a cap layer 502 overlying the front side of the
heteroepitaxial layer 504. Cap layer 502 is electrically connected
to the topmost subcell. The substrate includes back side 506.
Materials used to form the substrate include for example,
germanium, gallium arsenide, germanium alloys, and gallium arsenide
alloys. Materials used to form the heteroepitaxial layer include,
for example, alloys of one or more elements from group III and
group V on the periodic table, such as indium gallium phosphide,
indium phosphide, gallium arsenide, aluminum gallium arsenide,
indium gallium arsenide, and dilute nitride compounds. The
semiconductor wafer can undergo front side processing (FIGS. 5 and
17-22). TWV holes (1707 in FIG. 17) can be formed by wet etching as
determined by a photosensitive polymer pattern or any type of
suitable masking pattern (not shown). Etching of TWV holes 1707
starts from the front side of the cap layer 1702 extends through
heteroepitaxial layer 1704, and stops in the substrate 1705 at any
desired wafer depth before the wafer is etched completely through
to the back side 1706. In some embodiments, the etchant mixture
used is a volumetric ratio of 10% to 50% hydrochloric acid with a
volumetric ratio of 10% to 50% iodic acid and deionized water. The
mixture can have a temperature that ranges from 10.degree. C. to
140.degree. C. The patterned photosensitive polymer/masking
material (not shown) can be removed.
[0130] After wet etching TWV holes (1807 in FIG. 18), patterned cap
regions 1802A are formed, determined by a photosensitive polymer
pattern or any type of suitable masking pattern (not shown). The
patterned cap regions 1802A may be patterned in a disk shape, but
can also be patterned in a variety of geometric configurations, as
well as shaped to function as gridlines, busbars, pads and any type
of conductive component of an electrical device. FIG. 18 shows
patterned cap regions (post-cap etch) 1802A, heteroepitaxial layer
1804, substrate 1805, back side 1806 of the substrate, and TWV hole
1807.
[0131] In FIG. 19, ARC (1903, 1908) functions as a passivation
layer after it is applied over the heteroepitaxial layer 1904,
surrounding patterned cap regions 1902A and over the TWV sidewalls
and bottom of TWV hole 1907. Photosensitive polymers can also be
used as a passivation layer instead of an ARC. The passivation
layer can be applied using standard deposition techniques,
including, for example, photosensitive polymer application,
plasma-enhanced chemical vapor deposition, atomic layer deposition,
and electrografting. In some embodiments, hard baking is used in
this step. FIG. 19 also shows substrate 1905 and back side 1906 of
substrate 1905.
[0132] In FIG. 20, the ARC or passivation layer is removed from the
front side of the bottom of the TWV hole 2007 to expose the front
side 2009 of the wafer substrate that lies at the bottom of the TWV
hole 2007. FIG. 20 shows patterned cap regions (post-cap etch)
2002A, ARC 2003, heteroepitaxial layer 2004, substrate 2005, back
side 2006 of the substrate, TWV hole 2007, ARC 2008, and exposed
bottom 2009 of the TWV after removal of the passivation layer.
[0133] In FIG. 21, metal is deposited from the front side of the
semiconductor wafer, such that a metal seed layer lines the TWV
holes 2107 and overlies the TWV sidewalls 2111, the bottom of the
TWV 2112 and the front side of the patterned cap regions 2102A and
certain desired areas of the ARC 2103. Metal
deposition/metallization is determined by metal isolation resist
pattern (not shown) that can be formed with a photosensitive
polymer. This patterning can be carried out, for example, using
standard photolithography techniques which may or may not require
hard baking, depending on the specific embodiment. In some
embodiments, this metallization step uses an evaporation method.
The deposited metal can function as a front side metal pad as well
as a conducting metal seed layer for electroplating the TWV
sidewalls and the TWV bottom. FIG. 21 shows patterned cap regions
(post-cap etch) 2102A, ARC 2103, heteroepitaxial layer 2104,
substrate 2105, back side 2106 of the substrate 2105, TWV hole
2107, ARC 2108 within the via, front side metal 2110, metal layer
2109 deposited along the sidewalls 2111 of the TWV isolated from
the heteroepitaxial stack and the substrate by the passivation
layer (ARC 2108), and metal layer 2112 deposited on the bottom of
the TWV 2107.
[0134] In FIG. 22, gold or other electrically conductive metal or
alloy can be applied within the TWV by lithography and
electroplating to form a gold plug 2213 in the TWV, directly
contacting the metal layer deposited on the TWV bottom and
sidewalls (2211, 2212). The gold plug mechanically reinforces the
TWV structure, allowing conduction of higher current density with
low resistive losses. FIG. 22 includes patterned cap regions
(post-cap etch) 2202A, ARC 2203, heteroepitaxial layer 2204,
substrate 2205, backside 2206 of the substrate, ARC 2208 within and
around the TWV, front side metal 2210, metal layer 2211 deposited
along the sidewalls of the TWV isolated from the heteroepitaxial
stack and the substrate by the passivation layer (ARC 2208), metal
layer 2212 deposited on the bottom of the TWV, and electroplated
gold plug 2213.
[0135] In FIG. 23, the front side of the semiconductor wafer can be
permanently bonded to coverglass 2315 with an optically clear
adhesive 2314. In some embodiments, the coverglass may be space
grade coverglass, which may be made of borosilicate glass. The
coverglass can serve as a carrier for further processing from the
back side of the semiconductor wafer. FIG. 23 includes patterned
cap regions (post-cap etch) 2302A, ARC 2303, heteroepitaxial layer
2304, substrate 2305, back side 2306 of the substrate, ARC 2308
within the TWV, front side metal 2310, metal layer 2311 deposited
along the sidewalls of the TWV isolated from the heteroepitaxial
stack and the substrate by the passivation layer (ARC 2308), metal
layer 2312 deposited on the bottom of the TWV, electroplated gold
plug 2313, optically clear adhesive 2314, and coverglass 2315.
[0136] In FIG. 24, the back side of the substrate 2416 can be
thinned by wet etching, back-grinding, or other methods. In some
embodiments, the substrate can be, for example, between 20 .mu.m
and 200 .mu.m thick, less than 20 .mu.m thick, or from 40 .mu.m to
80 .mu.m thick, post-thinning. Thinned devices are desirable in
some applications, including, for example, space solar cells. FIG.
24 includes patterned cap regions (post-cap etch) 2402A, ARC 2403,
heteroepitaxial layer 2404, substrate 2405, ARC 2408 within TWV,
front side metal 2410, metal layer 2411 deposited along the
sidewalls of the TWV isolated from the heteroepitaxial stack and
the substrate by the passivation layer (ARC 2408), metal layer 2412
deposited on the bottom of the TWV, electroplated gold plug 2413,
optically clear adhesive 2414, coverglass 2415, and back side of
the wet etched back-thinned substrate 2416.
[0137] In FIG. 25, a passivation layer 2517 can be applied on the
back side 2516 of the substrate 2505 according to a desired pattern
to passivate the substrate from metal contact. The passivation
layer 2508 also lines the walls of the TWV holes. The passivation
layer can be applied using standard deposition techniques,
including, for example, photosensitive polymer application,
plasma-enhanced chemical vapor deposition, atomic layer deposition,
and electrografting. In some embodiments, hard baking is used in
this step. The bottom of the front side metal pad remains exposed
2512. FIG. 25 includes patterned cap regions (post-cap etch) 2502A,
ARC 2503, heteroepitaxial layer 2504, substrate 2505, ARC 2508
lining the TWV, front side metal 2510, metal layer 2511 deposited
along the sidewalls of the TWV isolated from the heteroepitaxial
stack and the substrate by the passivation layer, metal layer 2512
deposited on the bottom of the TWV, electroplated gold plug 2513,
optically clear adhesive 2514, coverglass 2515, back side of the
wet etched back-thinned substrate 2516, and passivation layer 2517
on backside surface 2516.
[0138] In FIG. 26, back side and TWV metal isolation resist pattern
2618 can be applied to determine the subsequent deposition of back
side metal (2719 in FIG. 27) and TWV metal (2720 in FIG. 27)
deposition. In some embodiments, these back side and TWV metals can
be applied in a single deposition step, or can be applied in more
than one deposition steps. FIG. 26 includes patterned cap regions
(post-cap etch) 2602A, ARC 2603, heteroepitaxial layer 2604,
substrate 2605, ARC 2608 lining the sidewalls of the TWV, front
side metal 2610, metal layer 2611 deposited along the sidewalls of
the TWV isolated from the heteroepitaxial stack and the substrate
by the passivation layer, metal layer 2612 deposited on the bottom
of the TWV, electroplated gold plug 2613, optically clear adhesive
2614, coverglass 2615, back side of the wet etched back-thinned
substrate 2616, passivation layer 2617, and back side and TWV metal
isolation resist pattern 2618.
[0139] FIG. 27 shows a completed device after the back side
sacrificial metal and metal isolation resist pattern are lifted off
to isolate positive and negative electrical contacts. FIG. 27 shows
patterned cap regions (post-cap etch) 2702A, ARC 2703,
heteroepitaxial layer 2704, substrate 2705, ARC 2708 lining the
sidewalls of the TWV, front side metal 2710, metal layer 2711
deposited along the sidewalls of the TWV isolated from the
heteroepitaxial stack and the substrate by the passivation layer,
metal layer 2712 deposited on the bottom of the TWV, electroplated
gold plug 2713, optically clear adhesive 2714, coverglass 2715,
passivation layer 2717, back side metal 2719 electrically connected
to the back side of substrate 2705, and TWV metal 2720 deposited on
the back side of the semiconductor wafer and electrically connected
to the TWV, the front side metal 2710, the cap regions 2702A and
the front side of heteroepitaxial layer 2704. TWV metal 2720 is
electrically insulated from substrate 2705 by passivation layer
2717.
[0140] Methods of forming a semiconductor device can comprise the
steps of: providing a semiconductor wafer, wherein the
semiconductor wafer comprises: a substrate region comprising a
front side and a back side; a heteroepitaxial layer overlying the
front side of the substrate region, wherein, the heteroepitaxial
layer comprises a first subcell and at least one additional subcell
overlying the first subcell; and at least one of the first subcell
or the at least one additional subcell comprises an alloy
comprising one or more elements from group III of the periodic
table, N, As, and an element selected from Sb, Bi and a combination
thereof; a plurality of patterned cap regions overlying the
heteroepitaxial layer; an anti-reflective coating overlying the
heteroepitaxial layer; and a corresponding metal region overlying
each of the plurality of patterned cap regions; bonding a
coverglass to the front side of the semiconductor wafer with an
optically clear adhesive; removing a desired amount from the
semiconductor wafer by a thinning of the substrate region from the
back side of the semiconductor wafer; patterning the back side of
the semiconductor wafer with a back etch through-wafer via pattern;
etching from the back side of the semiconductor wafer a plurality
of through-wafer vias using a single wet etchant mixture, wherein
each of the plurality of through-wafer vias extends from the back
side of the semiconductor wafer to the anti-reflective coating
overlying the heteroepitaxial layer; removing the anti-reflective
coating to expose a bottom side of the corresponding metal region
with a subsequent wet etching method, wherein the subsequent wet
etching method is specific for the removal of the anti-reflective
coating; depositing a passivation layer on the through-wafer via
walls with standard deposition techniques; depositing a resist
pattern on the back side of the semiconductor wafer for back side
metal isolation, wherein the resist pattern underlies the
passivation layer; depositing a metal on the back side of the
semiconductor wafer and on the through-wafer via; and removing the
resist pattern and a sacrificial metal.
[0141] In certain embodiments, methods of forming a semiconductor
device comprise the steps of: providing a semiconductor wafer,
wherein the semiconductor wafer comprises: a substrate region
comprising a front side and a back side; a heteroepitaxial layer
overlying the front side of the substrate region, wherein, the
heteroepitaxial layer comprises a first subcell and an at least one
additional subcell overlying the first subcell; at least one
subcell comprises an alloy comprising one or more elements from
group III of the periodic table, N, As, and an element selected
from Sb, Bi and a combination thereof; and a cap layer overlying
the heteroepitaxial layer; patterning the front side of the
semiconductor wafer with a front etch through-wafer via pattern;
etching from the front side of the semiconductor wafer a plurality
of through-wafer vias using a single wet etchant mixture, wherein,
each of the plurality of through-wafer vias extends from the front
side surface of the semiconductor wafer into the substrate;
patterning the plurality of patterned cap regions on the
heteroepitaxial layer on the front side of the semiconductor wafer;
depositing an anti-reflective coating overlying the heteroepitaxial
layer and the through-wafer via sidewalls; removing, from the front
side, the anti-reflective coating from the bottom of the
through-wafer via holes; depositing a front side resist pattern
from the front side of the semiconductor wafer, wherein the front
side resist pattern guides metal layer lithography; and depositing
a metal on the front side of the semiconductor wafer, on the
through-wafer via sidewalls and on the through-wafer via
bottom.
[0142] Semiconductor devices can comprise a heteroepitaxial layer,
further comprising an alloy comprising one or more elements from
group III of the periodic table, N, As, and an element selected
from Sb, Bi and a combination thereof; and a plurality of
through-wafer vias characterized by the absence of pitting on
smooth sidewall surfaces formed by a method provided by the present
disclosure.
[0143] Through-wafer via structures can comprise a substrate
comprising a back side and a front side; a heteroepitaxial layer
overlying the front side of the substrate; an antireflection
coating overlying a first portion of the heteroepitaxial layer; a
patterned cap region overlying a second portion of the
heteroepitaxial layer; a front side metal pad overlying and
electrically connected to the patterned cap region, wherein the
front side metal pad comprises a bottom surface; and a
through-wafer via extending from the back side of the substrate to
the front side metal pad, wherein the through-wafer via comprises
sidewalls; a passivation layer overlying a portion of the back side
of the substrate and the sidewalls of the through-wafer via; and a
metal layer overlying the passivation layer and the bottom surface
of the front side metal pad within the through-wafer via.
[0144] Through-wafer via structures can comprise a substrate
comprising a back side and a front side; a heteroepitaxial layer
overlying the front side of the substrate; an anti-reflection
coating overlying a first portion of the heteroepitaxial layer; a
patterned cap region overlying a second portion of the
heteroepitaxial layer; a front side metal overlying a portion of
the anti-reflection coating and the patterned cap region; a
through-wafer via extending from the back side of the substrate
through a portion of the anti-reflection coating; a passivation
layer overlying side walls of the through-wafer via; a metal seed
layer overlying the passivation layer and plugging the bottom of
the through-wafer via; and a metal overlying the metal seed layer
and filling the through-wafer via.
[0145] SMCCs provided by the present disclosure facilitate
low-cost, low-complexity, high-speed fabrication of solar arrays
with low mass and high reliability. This is accomplished by
eliminating welding processes and bulky interconnects, reducing the
thickness and cost of the backside metal, reducing the overall mass
of the photovoltaic device by using a thin substrate, integrating
the coverglass during wafer processing, increasing solar array area
utilization with the interconnections and bypass diodes integrated
with interconnection substrates such as PWBs/PCBs, and increasing
wafer utilization with small cells.
[0146] SMCC photovoltaic cells can be used with well-known, highly
automated surface mount equipment, SMCC cells can be mounted
directly to a PWB, PCB, or other interconnection substrate, which
includes the interconnects between subcells. By eliminating welding
interconnection strings between subcells, it becomes cost-effective
to use smaller photovoltaic cells. Smaller SMCC photovoltaic cells
facilitate more efficient and economical use of solar array surface
area. More effective utilization of solar array area results in
higher power, lower weight, and lower cost per solar cell array
area.
[0147] SMCC devices provided by the present disclosure can have a
front surface area of 5 cm.sup.2 or less, 4 cm.sup.2 or less, 3
cm.sup.2 or less, 2 cm.sup.2 or less, or 1 cm.sup.2 or less. For
example, a SMCC device provided by the present disclosure can have
a front surface area from 0.5 cm.sup.2 to 5 cm.sup.2, from 0.5
cm.sup.2 to 4 cm.sup.2, from 0.5 cm.sup.2 to 3 cm.sup.2, from 0.5
cm.sup.2 to 2 cm.sup.2, or from 0.5 cm.sup.2 to 1 cm.sup.2. SMCC
photovoltaic cells can also have other dimensions.
[0148] SMCC devices provided by the present disclosure, such as
three junction SMCC devices, can have a unit mass per area less
than 0.10 g/cm.sup.2, a unit mass per area, for example, less than
0.09 g/cm.sup.2, less than 0.08 g/cm.sup.2, less than 0.07
g/cm.sup.2, or less than 0.06 g/cm.sup.2. SMCC devices provided by
the present disclosure, such as three junction SMCC devices, can
have a unit mass per area, for example, from 0.05 g/cm.sup.2 to
0.10 g/cm.sup.2, from 0.06 g/cm.sup.2 to 0.09 g/cm.sup.2, or from
0.06 g/cm.sup.2 to 0.08 g/cm.sup.2.
[0149] For example, solder balls or solder paste can be applied to
the contact pads. The SMCC devices with applied solder are then
assembled onto corresponding reciprocal contact pads on a printed
circuit board and the solder reflowed to interconnect the SMCC to
the printed circuit board.
[0150] FIG. 36A shows a top view of a front side of an example of a
SMCC device including gridlines 3601 interconnected to busbar 3602,
which is interconnected to back side surface mount pads (not shown)
with TWVs 3603.
[0151] FIG. 36B shows a view of the back side of an example of a
SMCC device including surface mount solder pads 3607 interconnected
to back side contact 3606, surface mount solder pads 3605
interconnected to a front side contact and busbar 3602 (in FIG.
36A) by TWVs 3603. The front side surface mount pads 3605 are
electrically insulated from the back surface of the SMCC device and
from the back surface contact 3606 by insulator 3604.
[0152] Individual SMCC die can be assembled onto a substrate, which
can be a printed circuit board or other suitable support structures
such as semiconductor wafers
[0153] The front surface solder pads and the back surface solder
pads can have any suitable shape, dimensions, and layout suitable
for surface mount assembly. An example of a surface mount
configuration is shown in FIGS. 31 and 32.
[0154] The SMCC devices can be mounted to an interconnection
substrate such as PWB or PCB using any suitable surface mount
assembly method and using any suitable surface mount assembly
materials.
[0155] The interconnection substrate such as PWB or PCB can be made
of any suitable material, which can depend on the application. For
example, for space applications, the printed circuit board will be
qualified for space applications. A PWB or PCB can comprising
solder pads for surface mounting the SMCCs and interconnects for
connecting each of the SMCC devices. Bypass diodes can be mounted
on the printed circuit board such as on the side of the printed
circuit board opposite the side on which the SMCC devices are
mounted. A bypass diode may be interconnected to one or more SMCC
devices.
[0156] The front surface of the epitaxial layer can comprise front
contacts in the form of thin lines forming a grid. The grid can be
interconnected to a busbar. TWVs interconnect the busbar to front
contact pads located on the back side of the SMCC.
[0157] After SMCC structures are fabricated at the wafer-level,
each of the SMCC devices can be tested, and then singulated to
provide individual SMCC devices. The individual SMCC devices can be
surface mounted to a carrier such as a printed circuit board.
[0158] Another advantage of the present invention is eliminating
the need to incorporate a bypass diode within each device as is
required for CIC devices. Bypass diodes are used to protect a solar
array from failure of individual solar cells forming the array. In
prior art photovoltaic cells a silicon bypass diode is either
attached to the solar cell or the bypass diode is monolithically
integrated into the photovoltaic cell.
[0159] In configurations in which a bare-chip bypass diode is
welded onto the photovoltaic cell as part of CIC assembly, a
custom-made bare-die bypass diode chip is required. This increases
the cost relative to the use of generic or off-the-shelf bypass
diodes that can be used for other electronics applications. SMCC
allows the use of generic, packaged bypass diodes.
[0160] To highlight the advantages of eliminating the need to
attach a bypass diode as done for traditional CIC sub-assembly it
can be useful to consider the individual steps involved with
assembling bypass diodes to a CIC. Attaching a discrete bare die
bypass diode to a CIC involves the delicate operation of welding
one end of a metallic tab to the bare silicon bypass chip, then
welding the other end of the metallic tab to a thick metallization
on the semiconductor material forming the multi junction
photovoltaic cell. Welding on semiconductor material, especially
III-V material requires thick metallization that can include
several micrometers of silver, which not only adds to the cost of
the metallization, but also reduces manufacturing yield by
introducing additional warp, bow, and stress on solar cell wafers.
The welding operation itself is delicate, requiring stringent
process control, and can be a significantly reduce manufacturing
yield.
[0161] For the case of monolithically integrated bypass diodes, in
traditional CICs, bypass diode integration consumes real-estate on
solar cell epitaxial wafers, which is by far the most expensive
component of the CIC, and also complicates epitaxial growth by
adding additional process steps and conditions. With SMCC, bypass
diodes do not bring those penalties.
[0162] In the SMCCs disclosed herein a low cost bare or packaged
bypass diode can be assembled to a printed circuit board or printed
wiring board using mature, automated, ultra-high volume
pick-and-place equipment and methods used in the electronics
industry. SMCC also allows for alternative bypass diode
interconnection configurations. In a traditional design, a bypass
diode is interconnected to or integrated within each CIC. SMCC
provides the ability to interconnect the solar cell array through
the PCB/PWB such that more than one SMCC device can share the same
bypass diode. Fewer bypass diodes can be used and the number of
bypass diodes can be optimized. With fewer protective devices,
traditional bypass diodes can be replaced with more sophisticated
protective devices without prohibitively increasing the overall
cost.
[0163] Compared to non-surface mount photovoltaic cells, SMCC
photovoltaic cells provided by the present disclosure have the
following advantages:
[0164] (1) rather than dispensing adhesive and applying coverglass
to each cell, the adhesive and coverglass can be applied and bonded
at the wafer-level prior to backside processing;
[0165] (2) rather than using welded interconnects to the frontside
and backside of the solar cells, the photovoltaic cell is
interconnected to surface mount pads;
[0166] (3) rather than using discrete or monolithically integrated
bypass diodes, low cost bypass diodes can be integrated into the
PWB/PCB, thereby increasing the active surface area on the solar
array panel and reducing the cost and complexity of the assembly
process;
[0167] (4) the need to weld together strings of individual
photovoltaic cells is eliminated;
[0168] (5) with welded stings of photovoltaic cells, the string is
manually assembled onto a substrate using space grade adhesive;
however, SMCCs can be assembled directly onto a PCB using high
speed automated pick-and-place assembly methods; and
[0169] (6) whereas space solar arrays typically require a manual
wiring process on the backside of an array substrate to connect
strings of CICs, the use of SMCCs eliminates this process by
utilizing electronics industry standard production of PWBs/PCBs
with integrated electrical connections.
[0170] SMCC multijunction photovoltaic cells provided by the
present disclosure can be integrated to photovoltaic modules,
photovoltaic sub-systems, and photovoltaic power systems for space
or terrestrial applications. A photovoltaic modulate can comprise a
plurality of SMCC multijunction photovoltaic cells mounted on a
substrate panel.
[0171] Photovoltaic modules of the present invention can comprise a
front surface area and a plurality of surface mount multijunction
photovoltaic cells of the present disclosure overlying the front
surface area, wherein the photovoltaic modules cover, for example,
at least 70% of the front surface area, at least 80% of the front
surface area, or at least 90% of the front surface area.
Photovoltaic modules of the present invention can comprise a
plurality of SMCC devices of the present disclosure mounted to a
front surface of the module, wherein the SMCC devices cover, for
example, from 60% to 90% of the front surface area of the module,
from 65% to 85%, or from 70% to 80% of the front surface area of
the photovoltaic module.
[0172] Surface mount multijunction photovoltaic cells of the
present invention can comprise a substrate having a front substrate
surface and a back substrate surface; a heteroepitaxial layer
overlying the front substrate surface; a front surface contact
overlying a portion of and electrically connected to the
heteroepitaxial layer; an optical adhesive overlying the front
surface contact and the heteroepitaxial layer; a coverglass
overlying the optical adhesive; a back surface solder pad
underlying a portion of and electrically connected to the back
substrate surface; a front surface solder pad underlying and
insulated from the back substrate surface; and a through-wafer-via
interconnecting the front surface solder pad and the front surface
contact.
[0173] Surface mount multijunction photovoltaic cells of the
present invention can have a substrate that is less than 150 .mu.m
thick.
[0174] Surface mount multijunction photovoltaic cells of the
present invention can have a substrate comprising Ge that is from
20 .mu.m to 175 .mu.m thick.
[0175] Surface mount multijunction photovoltaic cells of the
present invention can have a substrate comprising GaAs that is less
than 100 .mu.m thick.
[0176] Surface mount multijunction photovoltaic cells of the
present invention can be characterized by an area less than 4
cm.sup.2.
[0177] Surface mount multijunction photovoltaic cells of the
present invention can be characterized by a unit mass per area of
less than 0.09 g/cm.sup.2.
[0178] Surface mount multijunction photovoltaic cells of the
present invention can have an heteroepitaxial layer comprises at
least two junctions.
[0179] Surface mount multijunction photovoltaic cells of the
present invention can have an heteroepitaxial layer comprising
Ga.sub.1-xIn.sub.xN.sub.yAs.sub.1-y-zSb.sub.z; and the content
values for x, y, and z are within composition ranges as follows:
0.03.ltoreq.x.ltoreq.0.22, 0.007.ltoreq.y.ltoreq.0.055 and
0.001.ltoreq.z.ltoreq.0.05.
[0180] Ga.sub.1-xIn.sub.xN.sub.yAs.sub.1-y-zSb.sub.z subcells can
comprise a low antimony content such as, for example,
0.004.ltoreq.z.ltoreq.0.02, 0.005.ltoreq.z.ltoreq.0.018,
0.006.ltoreq.z.ltoreq.0.016, 0.008.ltoreq.z.ltoreq.0.015,
0.01.ltoreq.z.ltoreq.0.018, 0.01.ltoreq.z.ltoreq.0.014,
0.004.ltoreq.z.ltoreq.0.014, or 0.004.ltoreq.z.ltoreq.0.01.
[0181] Ga.sub.1-xIn.sub.xN.sub.yAs.sub.1-y-zSb.sub.z subcells can
comprise a high nitrogen content such as, for example, 0.02
.ltoreq.y.ltoreq.0.10, 0.021.ltoreq.y.ltoreq.0.09,
0.024.ltoreq.y.ltoreq.0.085, 0.026.ltoreq.y.ltoreq.0.08,
0.03.ltoreq.y.ltoreq.0.06, or 0.04.ltoreq.y.ltoreq.0.06.
[0182] Ga.sub.1-xIn.sub.xN.sub.yAs.sub.1-y-zSb.sub.z subcells can
comprise a low antimony content such as, for example,
0.004.ltoreq.z.ltoreq.0.02, 0.005.ltoreq.z.ltoreq.0.018,
0.006.ltoreq.z.ltoreq.0.016, 0.008.ltoreq.z.ltoreq.0.015,
0.01.ltoreq.z.ltoreq.0.018, 0.01<z.ltoreq.0.014,
0.004.ltoreq.z.ltoreq.0.014, or 0.004.ltoreq.z.ltoreq.0.01; and a
high nitrogen content such as, for example,
0.02.ltoreq.y.ltoreq.0.10, 0.021.ltoreq.y.ltoreq.0.09,
0.024.ltoreq.y.ltoreq.0.085, 0.026.ltoreq.y.ltoreq.0.08,
0.03.ltoreq.y.ltoreq.0.06, or 0.04.ltoreq.y.ltoreq.0.06.
[0183] Ga.sub.1-xIn.sub.xN.sub.yAs.sub.1-y-zSb.sub.z subcells can
comprise x, y, and z values of 0.025.ltoreq.x.ltoreq.0.19,
0.008.ltoreq.y.ltoreq.0.09, and 0.001.ltoreq.z.ltoreq.0.02;
0.025.ltoreq.x.ltoreq.0.19, 0.008.ltoreq.y.ltoreq.0.09, and
0.001.ltoreq.z.ltoreq.0.018; 0.025.ltoreq.x.ltoreq.0.19,
0.008.ltoreq.y.ltoreq.0.09, and 0.001.ltoreq.z.ltoreq.0.016;
0.025.ltoreq.x.ltoreq.0.19, 0.02.ltoreq.y.ltoreq.0.09, and
0.001.ltoreq.z.ltoreq.0.02; 0.025.ltoreq.x.ltoreq.0.19,
0.02.ltoreq.y.ltoreq.0.09, and 0.001.ltoreq.z.ltoreq.0.018;
0.025.ltoreq.x.ltoreq.0.19, 0.02.ltoreq.y.ltoreq.0.09, and
0.001.ltoreq.z.ltoreq.0.016; 0.025.ltoreq.x.ltoreq.0.19,
0.008.ltoreq.y.ltoreq.0.09, and 0.001.ltoreq.z.ltoreq.0.016;
0.025.ltoreq.x.ltoreq.0.19, 0.022.ltoreq.y.ltoreq.0.09, and
0.001.ltoreq.z.ltoreq.0.02; 0.025.ltoreq.x.ltoreq.0.19,
0.022.ltoreq.y.ltoreq.0.09, and 0.001.ltoreq.z.ltoreq.0.018; or
0.025.ltoreq.x.ltoreq.0.19, 0.022.ltoreq.y.ltoreq.0.09, and
0.001.ltoreq.z.ltoreq.0.016.
[0184] Surface mount multijunction photovoltaic cells of the
present invention can further comprise an antireflection coating
overlying a first portion of the heteroepitaxial layer; a patterned
cap region overlying a second portion of the heteroepitaxial layer;
a front side metal pad overlying and electrically connected to the
patterned cap region, wherein the front side metal pad comprises a
bottom surface; and a through-wafer-via extending from the back
substrate surface to the front side metal pad, wherein the
through-wafer-via comprises sidewalls; a passivation layer
overlying a portion of the back substrate surface and the sidewalls
of the through-wafer-via; and a metal layer overlying the
passivation layer and the bottom surface of the front side metal
pad within the through-wafer-via.
[0185] Surface mount multijunction photovoltaic cells of the
present invention can comprise an heteroepitaxial layer comprising
one or more elements from group III of the periodic table, N, As,
and an element selected from Sb, Bi, and a combination thereof.
[0186] Surface mount multijunction photovoltaic cells of the
present invention can comprise an heteroepitaxial layer comprises
one or more subcells of a multijunction solar cell, wherein at
least one of the subcells comprises one or more elements from group
III of the periodic table, N, As, and an element selected from Sb,
Bi, and a combination thereof
[0187] Surface mount multijunction photovoltaic cells of the
present invention can have a through-wafer-via that is
characterized by smooth sidewall surfaces and the back substrate
surface is free of pitting.
[0188] Surface mount multijunction photovoltaic cells of the
present invention can further comprise an anti-reflection coating
overlying a first portion of the heteroepitaxial layer; a patterned
cap region overlying a second portion of the heteroepitaxial layer;
a front side metal overlying a portion of the anti-reflection
coating and the patterned cap region; a through-wafer via extending
from the back substrate surface through a portion of the
anti-reflection coating; a passivation layer overlying side walls
of the through-wafer-via; a metal layer overlying the passivation
layer and plugging the bottom of the through-wafer via; and a metal
overlying the metal layer and filling the through-wafer-via.
[0189] Photovoltaic modules of the present invention can comprise a
plurality of the surface mount multijunction photovoltaic cells of
the present invention.
[0190] Photovoltaic modules of the present invention can comprise
an interconnection substrate comprising a front interconnection
substrate surface and a back interconnection substrate surface; and
a plurality of surface mount multijunction photovoltaic cells of
claim 1 mounted to the interconnection substrate.
[0191] Photovoltaic modules provided by the present invention can
comprise an interconnection substrate comprising interconnects
between each of the plurality of surface mount multijunction
photovoltaic cells of the present invention; and a plurality of
bypass diodes, wherein each of the plurality of bypass diodes is
interconnected to one or more of the plurality of surface mount
multijunction photovoltaic cells of the present invention.
[0192] Photovoltaic modules of the present invention can comprise a
plurality of bypass diodes mounted to the back interconnection
substrate surface.
[0193] Photovoltaic modules of the present invention can comprise a
front surface area and a plurality of surface mount multijunction
photovoltaic cells of the present disclosure overlying the front
surface area, wherein the photovoltaic modules cover, for example,
at least 70% of the front surface area.
[0194] Power systems of the present invention can comprise a
photovoltaic module of the present disclosure and/or at least one
surface mount multijunction photovoltaic cell of the present
disclosure.
[0195] Methods of fabricating a photovoltaic module of the
invention can comprise interconnecting at least one of the surface
mount multijunction photovoltaic cells of the present disclosure to
an interconnection substrate.
[0196] Methods of fabricating a photovoltaic module of the
invention can comprise interconnecting a surface mount photovoltaic
cell of the present disclosure to an interconnection substrate by
surface mounting.
[0197] Methods of fabricating a multijunction photovoltaic cell of
the present invention can comprise providing a semiconductor wafer,
wherein the semiconductor wafer comprises a substrate having a
front substrate surface and a back substrate surface; a
heteroepitaxial layer overlying the front substrate surface; a
front surface contact overlying and electrically connected to a
portion of the heteroepitaxial layer; an optical adhesive overlying
the front surface contact and the heteroepitaxial layer; and a
coverglass overlying the optical adhesive layer; and thinning the
substrate.
[0198] Methods of the present invention can comprise forming a
through-wafer-via interconnecting the front surface contact to a
front contact pad underlying the back substrate surface.
[0199] Methods of the present invention can comprise forming a back
surface contact interconnected to the back substrate surface.
[0200] Methods of the present invention can comprise methods of
thinning the substrate by wet etching, back-grinding, lift-off, or
any combination of any of the foregoing.
[0201] Methods of forming a semiconductor device of the present
invention can comprise the steps of: providing a semiconductor
wafer, wherein the semiconductor wafer comprises: a substrate
region comprising a front side and a back side; a heteroepitaxial
layer overlying the front side of the substrate region, wherein,
the heteroepitaxial layer comprises a first subcell and at least
one additional subcell overlying the first subcell; and at least
one of the first subcell or the at least one additional subcell
comprises an alloy comprising one or more elements from group III
of the periodic table, N, As, and an element selected from Sb, Bi
and a combination thereof; a plurality of patterned cap regions
overlying the heteroepitaxial layer; an anti-reflective coating
overlying the heteroepitaxial layer; and a corresponding metal
region overlying each of the plurality of patterned cap regions;
bonding a cover glass to the front side of the semiconductor wafer
with an optically clear adhesive; removing a desired amount from
the semiconductor wafer by a thinning of the substrate region from
the back side of the semiconductor wafer; patterning the back side
of the semiconductor wafer with a back etch through-wafer via
pattern; etching from the back side of the semiconductor wafer a
plurality of through-wafer vias using a single wet etchant mixture,
wherein each of the plurality of through-wafer vias extends from
the back side of the semiconductor wafer to the anti-reflective
coating overlying the heteroepitaxial layer; removing the
anti-reflective coating to expose a bottom side of the
corresponding metal region with a subsequent wet etching method,
wherein the subsequent wet etching method is specific for the
removal of the anti-reflective coating; depositing a passivation
layer on the through-wafer via walls with standard deposition
techniques; depositing a resist pattern on the back side of the
semiconductor wafer for back side metal isolation, wherein the
resist pattern underlies the passivation layer; depositing a metal
on the back side of the semiconductor wafer and on the
through-wafer via; and removing the resist pattern and a
sacrificial metal.
[0202] Methods of forming a semiconductor device of the present
invention can comprise the steps of: providing a semiconductor
wafer, wherein the semiconductor wafer comprises: a substrate
region comprising a front side and a back side; a heteroepitaxial
layer overlying the front side of the substrate region, wherein,
the heteroepitaxial layer comprises a first subcell and an at least
one additional subcell overlying the first subcell; at least one
subcell comprises an alloy comprising one or more elements from
group III of the periodic table, N, As, and an element selected
from Sb, Bi and a combination thereof; and a cap layer overlying
the heteroepitaxial layer; patterning the front side of the
semiconductor wafer with a front etch through-wafer via pattern;
etching from the front side of the semiconductor wafer a plurality
of through-wafer vias using a single wet etchant mixture, wherein,
each of the plurality of through-wafer vias extends from the front
side surface of the semiconductor wafer into the substrate;
patterning the plurality of patterned cap regions on the
heteroepitaxial layer on the front side of the semiconductor wafer;
depositing an anti-reflective coating overlying the heteroepitaxial
layer and the through-wafer via sidewalls; removing, from the front
side, the anti-reflective coating from the bottom of the
through-wafer via holes; depositing a front side resist pattern
from the front side of the semiconductor wafer, wherein the front
side resist pattern guides metal layer lithography; and depositing
a metal on the front side of the semiconductor wafer, on the
through-wafer via sidewalls and on the through-wafer via
bottom.
[0203] Methods of the present invention can comprise an
anti-reflective coating that serves as a passivation layer.
[0204] Methods of the present invention can have a passivation
layer that comprises photosensitive polymers.
[0205] Methods of the present invention can have a wet etchant
mixture comprising a volumetric ratio of hydrochloric acid of 10%
to 50%; volumetric ratio of iodic acid of 10% to 50%; and deionized
water, wherein the single wet etchant mixture has a temperature of
10.degree. C. to 140.degree. C.
[0206] Methods of the present invention can have a back etch
through-wafer via pattern and a front etch through-wafer via
pattern formed using a photoresist, using a hard mask, or using
both a photoresist and a hard mask.
[0207] Methods of the present invention can comprise a
semiconductor device comprising a photovoltaic cell such as a
multijunction photovoltaic cell.
[0208] Methods of the present invention can have a semiconductor
device comprising a solar cell or a back-contact solar cell.
[0209] Methods of the present invention can comprise filling each
of the plurality of through-wafer vias with gold.
[0210] Methods of the present invention can comprise bonding a
cover glass to the front side surface of the semiconductor wafer
with an optically clear adhesive; removing a desired amount of the
semiconductor wafer by a thinning of the substrate region from the
back side of the semiconductor wafer; depositing a passivation
layer with standard deposition techniques or lithography on the
back side of the semiconductor wafer, wherein the passivation layer
is guided by a passivation layer pattern; depositing a back side
metal isolation resist pattern on the back side of the
semiconductor, wherein the back side metal isolation resist pattern
underlies the passivation layer; depositing a metal on the back
side of the semiconductor wafer; and removing the back side metal
isolation resist pattern and a sacrificial metal.
[0211] Methods of the present invention can comprise the thinning
of the substrate region from the back side of the semiconductor
wafer by wet etching, back-grinding, substrate lift-off, or a
combination of any of the foregoing.
[0212] According to an aspect of the invention, a surface mount
multijunction photovoltaic cell comprises: a substrate having a
front substrate surface and a back substrate surface; a
heteroepitaxial layer overlying the front substrate surface; a
front surface contact overlying a portion of and electrically
connected to the heteroepitaxial layer; an optical adhesive
overlying the front surface contact and the heteroepitaxial layer;
a coverglass overlying the optical adhesive; a back surface solder
pad underlying a portion of and electrically connected to the back
substrate surface; a front surface solder pad underlying and
insulated from the back substrate surface; and a through-wafer-via
interconnecting the front surface solder pad and the front surface
contact.
[0213] According to any of the preceding aspects, a substrate is
less than 150 .mu.m thick.
[0214] According to any of the preceding aspects, a substrate
comprises Ge and is from 20 .mu.m to 175 .mu.m thick.
[0215] According to any of the preceding aspects, a substrate
comprises GaAs and is less than 100 .mu.m thick.
[0216] According to any of the preceding aspects, a surface mount
multijunction photovoltaic cell is characterized by an area less
than 4 cm.sup.2.
[0217] According to any of the preceding aspects, a surface mount
multijunction photovoltaic cell is characterized by a unit mass per
area of less than 0.09 g/cm.sup.2.
[0218] According to any of the preceding aspects, a heteroepitaxial
layer comprises at least two junctions.
[0219] According to any of the preceding aspects, the
heteroepitaxial layer comprises
Ga.sub.1-xIn.sub.xN.sub.yAs.sub.1-y-zSb.sub.z; and the content
values for x, y, and z are within composition ranges as follows:
0.03.ltoreq.x.ltoreq.0.22, 0.007.ltoreq.y.ltoreq.0.055 and
0.001.ltoreq.z.ltoreq.0.02.
[0220] According to any of the preceding aspects, a surface mount
multijunction photovoltaic cell further comprises: an
antireflection coating overlying a first portion of the
heteroepitaxial layer; a patterned cap region overlying a second
portion of the heteroepitaxial layer; a front side metal pad
overlying and electrically connected to the patterned cap region,
wherein the front side metal pad comprises a bottom surface; and a
through-wafer-via extending from the back substrate surface to the
front side metal pad, wherein the through-wafer-via comprises
sidewalls; a passivation layer overlying a portion of the back
substrate surface and the sidewalls of the through-wafer-via; and a
metal layer underlying the passivation layer and the bottom surface
of the front side metal pad within the through-wafer-via.
[0221] According to any of the preceding aspects, a heteroepitaxial
layer comprises one or more elements from group III of the periodic
table, N, As, and an element selected from Sb, Bi, and a
combination thereof
[0222] According to any of the preceding aspects, a heteroepitaxial
layer comprises one or more subcells of a multijunction solar cell,
wherein at least one of the subcells comprises one or more elements
from group III of the periodic table, N, As, and an element
selected from Sb, Bi, and a combination thereof.
[0223] According to any of the preceding aspects, a
through-wafer-via is characterized by smooth sidewall surfaces and
the back substrate surface is free of pitting.
[0224] According to any of the preceding aspects, a surface mount
multijunction photovoltaic cell further comprises: an
anti-reflection coating underlying a first portion of the
heteroepitaxial layer; a patterned cap region underlying a second
portion of the heteroepitaxial layer; a front side metal overlying
a portion of the anti-reflection coating and the patterned cap
region; a through-wafer via extending from the back substrate
surface through a portion of the anti-reflection coating; a
passivation layer overlying side walls of the through-wafer-via; a
metal layer overlying the passivation layer and plugging the bottom
of the through-wafer via; and a metal overlying the metal layer and
filling the through-wafer-via.
[0225] According to any of the preceding aspects, a surface mount
multijunction photovoltaic cell further comprises an ARC layer
between a portion of the front side metal and the heteroepitaxial
layer.
[0226] According to any of the preceding aspects, a surface mount
multijunction photovoltaic cell further comprises an ARC layer
between a portion of the through wafer via sidewall and the
patterned cap region.
[0227] According to an aspect of the invention, a photovoltaic
module comprises a plurality of the surface mount multijunction
photovoltaic cells according to the present invention.
[0228] According to any of the preceding aspects, a photovoltaic
module comprises: an interconnection substrate comprising a front
interconnection substrate surface and a back interconnection
substrate surface; and a plurality of surface mount multijunction
photovoltaic cells according to the present invention mounted to
the interconnection substrate.
[0229] According to any of the preceding aspects, an
interconnection substrate comprises: interconnects between each of
the plurality of surface mount multijunction photovoltaic cells
according to the present invention; and a plurality of bypass
diodes, wherein each of the plurality of bypass diodes is
interconnected to one or more of the plurality of surface mount
multijunction photovoltaic cells according to the present
invention.
[0230] According to any of the preceding aspects, each of the
plurality of bypass diodes is mounted to the interconnection
substrate.
[0231] According to any of the preceding aspects, the module
comprises a front surface area; and the plurality of surface mount
multijunction photovoltaic cells cover at least 70% of the front
surface area.
[0232] According to an aspect of the invention, a power system
comprises a photovoltaic module according to the present
invention.
[0233] According to the present invention, a method of fabricating
a photovoltaic module comprises interconnecting at least one of the
surface mount multijunction photovoltaic cells according to the
present invention to an interconnection substrate.
[0234] According to any of the preceding aspects, interconnecting
comprises surface mounting.
[0235] According to aspects of the invention, a method of
fabricating a multijunction photovoltaic cell, comprises: providing
a semiconductor wafer, wherein the semiconductor wafer comprises: a
substrate having a front substrate surface and a back substrate
surface; a heteroepitaxial layer overlying the front substrate
surface; a front surface contact overlying and electrically
connected to a portion of the heteroepitaxial layer; an optical
adhesive overlying the front surface contact and the
heteroepitaxial layer; and a coverglass overlying the optical
adhesive layer; and thinning the substrate.
[0236] According to any of the preceding aspects, a method further
comprising, forming a through-wafer-via interconnecting the front
surface contact to a front contact pad underlying the back
substrate surface.
[0237] According to any of the preceding aspects, a method further
comprising forming a back surface contact interconnected to the
back substrate surface.
[0238] According to any of the preceding aspects, thinning the
substrate comprises wet etching, back-grinding, lift-off, or any
combination of any of the foregoing.
[0239] According to aspects of the invention, a surface mount
multijunction photovoltaic cell comprises: a substrate having a
front substrate surface and a back substrate surface; a
heteroepitaxial layer overlying the front substrate surface; a
front surface contact overlying a portion of and electrically
connected to the heteroepitaxial layer; an optical adhesive
overlying the front surface contact and the heteroepitaxial layer;
a coverglass overlying the optical adhesive; a passivation layer
underlying a portion of the back substrate surface; a back metal
pad underlying a portion of the passivation layer; a
through-wafer-via electrically interconnecting the front metal
contact and the back metal pad; and a backside metal electrically
connected to the back substrate surface.
[0240] According to any of the preceding aspects, a surface mount
multijunction photovoltaic cell comprises a patterned cap region
overlying a portion of the heteroepitaxial layer; and an
antireflection coating overlying a portion of the heteroepitaxial
layer; wherein the front surface contact overlies the patterned cap
region and is electrically connected to the patterned cap
region.
[0241] According to any of the preceding aspects, the
antireflection coating overlies the heteroepitaxial within the
patterned cap region; and the antireflection coating overlies the
sidewalls of the through-wafer-via.
[0242] According to any of the preceding aspects, a surface mount
multijunction photovoltaic cell comprises a metal plug at least
partially filling the through-wafer-via.
[0243] According to an aspect of the invention, a photovoltaic
module comprises a plurality of the surface mount multijunction
photovoltaic cells according to the present invention.
[0244] According to any of the preceding aspects, a photovoltaic
module comprises: an interconnection substrate comprising a front
interconnection substrate surface and a back interconnection
substrate surface; and a plurality of surface mount multijunction
photovoltaic cells according to the present invention mounted to
the interconnection substrate.
[0245] According to any of the preceding aspects, an
interconnection substrate comprises: interconnects between each of
the plurality of surface mount multijunction photovoltaic cells
according to the present invention; and a plurality of bypass
diodes, wherein each of the plurality of bypass diodes is
interconnected to one or more of the plurality of surface mount
multijunction photovoltaic cells according to the present
invention.
[0246] According to any of the preceding aspects, each of the
plurality of bypass diodes is mounted to the interconnection
substrate.
[0247] According to any of the preceding aspects, the module
comprises a front surface area; and the plurality of surface mount
multijunction photovoltaic cells cover at least 70% of the front
surface area.
[0248] According to an aspect of the invention, a power system
comprises the photovoltaic module according to the present
invention.
[0249] According to any of the preceding aspects, a method of
fabricating a photovoltaic module comprises interconnecting at
least one of the surface mount multijunction photovoltaic cells
according to the present invention to an interconnection
substrate.
[0250] According to any of the preceding aspects, interconnecting
comprises surface mounting.
[0251] According to an aspect of the invention, a method of
fabricating a multijunction photovoltaic cell comprises: providing
a semiconductor wafer, wherein the semiconductor wafer comprises: a
substrate having a front substrate surface and a back substrate
surface; a heteroepitaxial layer overlying the front substrate
surface; and a patterned cap region overlying a first portion of
the heteroepitaxial layer; etching a through-wafer-via extending
from the heteroepitaxial layer to within the substrate; depositing
an antireflection coating on a second portion of the
heteroepitaxial layer and on a sidewall and a bottom of the
through-wafer-via; etching the antireflection coating on the bottom
of the through-wafer-via to expose the substrate; depositing a
front surface contact overlying at least a portion of the patterned
cap region, the antireflection coating within the patterned cap
region, the sidewalls of the through-wafer-via, and the bottom of
the through-wafer-via; applying an optical adhesive overlying the
front surface contact, the patterned cap region, and the
antireflection coating; applying a coverglass overlying the optical
adhesive; and thinning the substrate.
[0252] According to any of the preceding aspects, a method further
comprises forming a back surface contact interconnected to the back
substrate surface.
[0253] There are alternative ways of implementing the embodiments
disclosed herein. Accordingly, the present embodiments are to be
considered as illustrative and not restrictive. Furthermore, the
claims are not to be limited to the details given herein, and are
entitled their full scope and equivalents thereof.
* * * * *