U.S. patent application number 14/456801 was filed with the patent office on 2014-11-27 for method for making a spacer in a photovoltaic substrate.
The applicant listed for this patent is The Charles Stark Draper Laboratory, Inc.. Invention is credited to Paul Greiff.
Application Number | 20140349436 14/456801 |
Document ID | / |
Family ID | 35655841 |
Filed Date | 2014-11-27 |
United States Patent
Application |
20140349436 |
Kind Code |
A1 |
Greiff; Paul |
November 27, 2014 |
METHOD FOR MAKING A SPACER IN A PHOTOVOLTAIC SUBSTRATE
Abstract
A micron gap thermo-photo-voltaic device including a
photovoltaic substrate, a heat source substrate, and a plurality of
spacers separating the photovoltaic substrate from the heat source
substrate by a submicron gap. Each spacer includes an elongated
thin-walled structure disposed in a well formed in the heat source
substrate and having a top surface less than a micron above the
heat source substrate. Also disclosed are methods of making the
spacers.
Inventors: |
Greiff; Paul; (Wayland,
MA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
The Charles Stark Draper Laboratory, Inc. |
Cambridge |
MA |
US |
|
|
Family ID: |
35655841 |
Appl. No.: |
14/456801 |
Filed: |
August 11, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13068207 |
May 5, 2011 |
8829335 |
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14456801 |
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10895762 |
Jul 21, 2004 |
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13068207 |
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Current U.S.
Class: |
438/54 |
Current CPC
Class: |
H02S 10/30 20141201;
H01L 31/024 20130101; Y02E 10/50 20130101 |
Class at
Publication: |
438/54 |
International
Class: |
H01L 31/024 20060101
H01L031/024; H01L 31/04 20060101 H01L031/04 |
Claims
1-9. (canceled)
10. A method of making a spacer separating a photovoltaic substrate
from a heat source by a submicron gap, the method comprising:
forming a well in the heat source substrate; growing or depositing
a thermally resistant material on the surface of the well; and
enlarging the well to form an elongated thin-walled structure
disposed in the well having a top surface less than a micron above
the heat source substrate.
11. The method of claim 10 in which the well is formed by
photolithography processes and plasma etching.
12. The method of claim 10 in which the well is greater than 15
microns deep and 15 microns wide.
13. The method of claim 10 in which the thermally resistant
material is silicon dioxide grown on the surface of the well.
14. The method of claim 13 in which the silicon dioxide is grown to
a thickness of greater than 0.5 microns.
15. The method of claim 10 in which enlarging the well includes
etching.
16. The method of claim 15 in which etching includes an anisotropic
etch.
17. The method of claim 10 in which a recess is formed in the heat
source substrate and the well is formed in the recess.
18. The method of claim 17 in which the recess is formed by
etching.
19. The method of claim 17 in which the thermally resistant
material is grown or deposited to a thickness greater than the
depth of the recess.
20. The method of claim 17 in which the thermally resistant
material is patterned in the vicinity of the well to form a flange
in the recess.
21. The method of claim 10 in which the thermally resistant
material is a dielectric deposited on the surfaces of the well.
22. The method of claim 21 in which the dielectric is zirconia.
23. The method of claim 21 further including the step of etching
the surface of the heat source substrate so that the thermally
resistant material extends above the surface of the heat source
substrate.
24-29. (canceled)
30. A method of making a spacer separating a first substrate from a
second substrate, the method comprising: forming a well in one said
substrate; growing or depositing a thermally resistant material on
the surface of the well; and enlarging the well to form an
elongated thin-walled structure disposed in the well having a top
surface above said substrate.
31. The method of claim 30 in which the first substrate is a
photovoltaic substrate.
32. The method of claim 30 in which the second substrate is a heat
source substrate.
33. The method of claim 32 in which the well is formed in the heat
source substrate.
34. The method of claim 17 in which the recess is formed by growth
and dissolution of thermally grown oxide.
35. A method of making a spacer separating a photovoltaic substrate
from atop surface of a heat source by a submicron gap, the method
comprising: forming a well in the heat source substrate that
extends from the top surface of the heat source substrate and
extends into the body of the heat source substrate; growing or
depositing a thermally resistant material on the surface of the
well; and enlarging the well to form an elongated thin-walled
structure disposed in the well having atop surface less than a
micron above the top surface of the heat source substrate.
36. The method of claim 34 wherein the heat source substrate is
made of silicon, the spacer is made of silicon dioxide; and the
well is greater than 15 microns deep and 15 microns wide.
Description
RELATED APPLICATIONS
[0001] This application is a continuation of prior U.S. patent
application Ser. No. 10/895,762 filed Jul. 21, 2004 which is hereby
incorporated by reference herein, and to which this application
claims the benefit of and priority to under 35 U.S.C.
.sctn..sctn.119, 120, 363, 365, and 37 C.F.R. .sctn.1.55 and
.sctn.1.78.
FIELD OF THE INVENTION
[0002] This invention relates to a micron gap thermo-photo-voltaic
device.
BACKGROUND OF THE INVENTION
[0003] In a thermo-photo-voltaic device, a thermal heat source is
mated with a III-V photovoltaic cell. Thermal photons are radiated
from the heat source and those with energy greater than the bandgap
of the photovoltaic cell will generate electricity in the same
manner as the more familiar solar cell. A micron-gap thermo
photovoltaic device operates in similar fashion except the heat
source and the photovoltaic device are separated by a gap of less
than one micron. See the paper entitled "Micron-gap ThermoPhoto
Voltaic (MTPV)", DiMatteo et al., proceedings of the Fifth TPV
Conference, 2002 incorporated herein by this reference.
[0004] For experimental purposes, the heat source is a heater chip
which is heated with an on-board electrical resistance heater but
in application the heat could be derived from solar energy,
combustion of fuel, or the like. In one application, the system
consists of a photovoltaic cell, a heat source substrate and the
heat source itself. The function of the heat source substrate is to
foim a high temperature radiating surface at a distance less than
one micron from the photovoltaic surface. The thermal energy is
applied to the back of the heat source substrate by convection or
radiation. Various researchers have been looking at conventional
TPV technology for about 50 years. The applicants' approach is
unique is that the heat source is spaced from the photovoltaic cell
by a gap of less than a micron, This micron gap spacing gets around
Plank's law and allows the system to function as though the black
body emissivity were greater than one. A factor of ten has been
predicted and observed for 0.1 micron gaps. That is, the
photocurrent obtained with a 0.1 micron spacing is about ten times
that obtained when the heater chip is moved about two microns or
more away from the photovoltaic cell. This is a very large effect
and has the potential to revolutionize the field of
thermo-photo-voltaics. For a given temperature of operation, one
can decrease the size of the overall system and still achieve the
same power or one could operate at a lower temperature easing
materials problems and helping to make the use of
thermo-photo-voltaics more practical.
[0005] To avoid thermal shorting, the system operates in a vacuum
and spacers of silicon dioxide are employed to set the gap between
the heater source and the photovoltaic cell in a manner which
minimizes the heat transfer through the spacers. Phonons or
non-radiated energy carriers are a source of inefficiency as they
transfer energy from the source but do not have the individual
potential energy to excite electrons across the bandgap.
[0006] As described in the above referenced paper, the previous
method of forming the spacers between the heat source and the
photovoltaic cell was to grow a thick oxide on the heater chip and
pattern the spacers to be about six microns in diameter. One
disadvantage of this former method is that the spacers permit too
large a heat loss from the heater. Despite the fact that the
diameter of the spacers is small and the thermal resistance of
silicon dioxide is greater than that of silicon, about 30% to 50%
of the parasitic heat loss from the heater is due to these spacers.
Typically, spacers are used in the four corners of the heater and
there are at least three spacers per location. A high heat loss
means that the efficiency of conversion of heat to electricity is
low and also the cooling requirements on the photovoltaic cell are
increased.
[0007] Another disadvantage is that the spacers can cause damage to
the photovoltaic cell surface. Since the etching of the silicon
dioxide is isotropic, the spacers are etched inwards at the
surface. The base of the spacers is the mask dimension, about six
microns. However, the tops of the spacers are narrowed to about two
microns. The small size of the tips of the spacers causes the
spacers to dig into certain photovoltaic cells because the material
is relatively soft. Also, pressure applied to the
heater/photovoltaic cell assembly can break the spacers causing
debris and limiting the effectiveness of the spacers.
[0008] Another disadvantage relates to the eventual use of micron
gap thermo-photovoltaic devices for generating power which requires
large area devices. One method of building a large area working
system of heaters on the photovoltaic surface is to braze
individual heater chips down on the photovoltaic cell creating a
"tiled" surface. See U.S. Pat. No. 6,232,546 incorporated herein by
this reference. A single large heater chip cannot be used because
the heater is operated at about 1000.degree. C. and the
photovoltaic cell must be kept at room temperature to function
effectively as a collector of photons and a generator of electrons.
The difference in thermal expansion between the heater and the
photovoltaic cell as the heater chip is heated from room
temperature to 1000.degree. C. can break the spacers or distort the
geometry during the temperature excursion if there is a rigid
attachment. This necessitates the use of tiling and the issue now
becomes how to attach the heater tiles to the photovoltaic cell
without creating a thermal short. With the tips of the spacers at 2
to 6 microns in diameter, there may not be enough room for the
deposition of sufficient braze material to make this a practical
method of attaching the heater chips to the photovoltaic cell.
SUMMARY OF THE INVENTION
[0009] It is therefore an object of this invention to provide a
micron gap thermo-photo-voltaic device.
[0010] It is a further object of this invention to provide a method
of making spacers for a micron gap thermo-photo-voltaic device.
[0011] It is a further object of this invention to provide spacers
with increased thermal resistance.
[0012] It is a further object of this invention to provide a micron
gap thermo-photo-voltaic device with increased thermal
efficiency.
[0013] It is a further object of this invention to provide for
precise and uniform setting of the gap dimension between the
photovoltaic substrate and the heat source substrate.
[0014] It is a further object of this invention to provide spacers
which are rugged, durable, and strong in compression.
[0015] It is a further object of this invention to provide spacers
having a geometry which does not damage the photovoltaic
substrate.
[0016] The invention results from the realization that conventional
photolithography processes can be employed in a novel way to
provide an elongated thin walled spacer disposed in a well formed
in the heat source substrate. The top surface of the thin walled
structure is less than a micron above the heat source substrate
defining a submicron gap between the heat source and the
photovoltaic. The well surrounding the thin walled structure
thermally isolates the thin walled structure from the heat source.
No longer is the length of the spacer dictated by the thickness of
the deposited or grown spacer material.
[0017] The subject invention, however, in other embodiments, need
not achieve all these objectives and the claims hereof should not
be limited to structures or methods capable of achieving these
objectives.
[0018] This invention features a micron gap thermo-photo-voltaic
device comprising a photovoltaic substrate, a heat source
substrate, and a plurality of spacers separating the photovoltaic
substrate from the heat source substrate by a submicron gap. Each
spacer includes an elongated thin-walled structure disposed in a
well formed in the heat source substrate and having a top surface
less than a micron above the heat source substrate.
[0019] Each spacer top surface optionally includes a flange
extending radially outward from the thin-walled structure. In one
embodiment, the heat source substrate is made of silicon, and the
spacer is made of silicon dioxide or a dielectric material such as
zirconia. In one example, the thin-walled structure is in the shape
of a cylinder, the well is cup shaped and is greater than 15
microns deep and 15 microns wide.
[0020] A method of making a spacer separating a photovoltaic
substrate from a heat source by a submicron gap, in accordance with
this invention, includes forming a well in the heat source
substrate, growing or depositing a thermally resistant material on
the surface of the well, and enlarging the well to form an
elongated thin-walled structure disposed in the well having a top
surface less than a micron above the heat source substrate.
[0021] The well may be formed by photolithography processes and
plasma etching to have a depth greater than 15 microns and a width
greater than 15 microns. In one example, thermally resistant
material is silicon dioxide grown on the surface of the well to a
thickness of greater than 0.5 microns. The step of enlarging the
well may include etching and, in one example, an anisotropic etch
is used. In the preferred embodiment, a recess is formed in the
heat source substrate by etching and the deep well is formed in
recess. The thermally resistant material is grown or deposited to a
thickness greater than the depth of the recess and patterned in the
vicinity of the well to form a flange in the recess.
[0022] In another example, the thermally resistant material is a
dielectric deposited on the surfaces of the well. In one
embodiment, the dielectric is zirconia. The surface of the heat
source substrate is etched so that the thermally resistant material
extends above the surface of the heat source substrate.
[0023] In accordance with another example, the subject invention
includes a plurality of spacers separating a first substrate from a
second substrate by a gap. Each spacer includes an elongated
thin-walled structure disposed in a well formed in one of the first
and second substrates and having a top surface above the substrate.
In one example, the first substrate is a photovoltaic substrate,
the second substrate is a heat source substrate, and the well is
formed in the heat source substrate.
[0024] One method of making a spacer separating a first substrate
from a second substrate in accordance with the subject invention
features forming a well in one substrate, growing or depositing a
thermally resistant material on the surface of the well, and
enlarging the well to form an elongated thin-walled structure
disposed in the well having a top surface above the substrate. In
one example, the first substrate is a photovoltaic substrate, the
second substrate is a heat source substrate, and the well is formed
in the heat source substrate.
BRIEF DESCRIPTION OF THE DRAWINGS
[0025] Other objects, features and advantages will occur to those
skilled in the art from the following description of a preferred
embodiment and the accompanying drawings, in which:
[0026] FIG. 1 is a schematic three-dimensional exploded view of a
micron gap thermo-photo-voltaic device in accordance with the
subject invention;
[0027] FIGS. 2A-2F are schematic cross-sectional views depicting
the primary steps associated with one method of making a spacer
separating a photovoltaic substrate from a heat source substrate in
accordance with the subject invention;
[0028] FIGS. 3A-3E are schematic cross-sectional views depicting
the primary steps associated with another method of making a spacer
separating a photovoltaic substrate from a heat source substrate in
accordance with the subject invention; and
[0029] FIGS. 4A-4C are schematic cross-sectional views showing the
primary steps associated with still another method of making a
spacer separating a photovoltaic substrate from a heat source
substrate in accordance with the subject invention.
DISCLOSURE OF THE PREFERRED EMBODIMENT
[0030] Aside from the preferred embodiment or embodiments disclosed
below, this invention is capable of other embodiments and of being
practiced or being carried out in various ways. Thus, it is to be
understood that the invention is not limited in its application to
the details of construction and the arrangements of components set
forth in the following description or illustrated in the drawings.
If only one embodiment is described herein, the claims hereof are
not to be limited to that embodiment. Moreover, the claims hereof
are not to be read restrictively unless there is clear and
convincing evidence manifesting a certain exclusion, restriction,
or disclaimer.
[0031] Micron gap thermo-photo-voltaic device 10, FIG. 1, in
accordance with one embodiment of the subject invention, includes
photovoltaic substrate 12, heat source substrate 14, and spacers 16
which separate substrate 12 from substrate 14 by a submicron gap.
Each spacer is an elongated thin-walled structure (e.g., a hollow
cylinder) disposed in well 20 formed in substrate 14. The top
surface of each thin-walled spacer, typically in a form of flange
18 extending radially outward from the thin-walled structure, is
preferably less than a micron above the substrate to define a gap
of less than a micron between substrate 14 and substrate 12.
[0032] In one embodiment, heater source substrate 14 is made of
silicon and spacers 16 are made of silicon dioxide. In another
embodiment, however, spacers 16 are made of another thermally
resistant material, e.g., a dielectric material such as zirconia.
Wells 20 are typically cup-shaped and greater than 15 microns deep
and 15 microns wide. In one example, each well 20 is 20 microns
deep and over 20 microns wide. The wall thickness of the
thin-walled spacer is typically on the order of about 1.0 microns.
The gap height defined by the distance between the top of flange 18
and the top of substrate 14, in one example, was 0.2 microns. The
thermal resistance of each spacer 16 is proportional to the length
of the cylinder divided by its cross-sectional area. In this
example, the cross-sectional area of the spacer wall is five times
that of a single prior art six-micron diameter spacer and the
thermal resistance is four times greater. When a single spacer 16,
in accordance with this invention, is compared to the use of three
prior art spacers per location, the thermal resistance of spacers
16 is twelve times greater.
[0033] One exemplary method of making spacer 16, FIG. 1 is depicted
in FIGS. 2A-2F. In FIG. 2A, photoresist 30 has been applied to
substrate 14 and patterned using photolithography and silicon
plasma etch processing to form shallow recess 32 in the area where
a spacer is to be located. The depth of recess 32 should be precise
because it is one of the two factors that will determine the height
of the resulting spacer over the surrounding silicon of substrate
14 which in turn becomes the gap between the heater substrate and
the photovoltaic substrate. Recess 32 maybe formed by plasma
etching or by KOH etching with an oxide mask protecting the
silicon. In FIGS. 2B-2C, photoresist 34 and photo-patterning and
plasma etching is used to create well 36 nominally 20 microns wide
and 20 microns deep in substrate 14. A wide latitude is possible
for these dimensions depending on a trade off of thermal resistance
and spacer ruggedness. The dimensions of well 36 do not affect the
gap dimension discussed above. In FIG. 2D, silicon dioxide oxide
layer 38 is grown to a thickness of greater than 0.5 microns,
typically about 1.0 microns but in any case greater than the depth
of recess 32. The thickness of the oxide layer 38 is the second
factor which determines the gap as discussed below. Typically, the
thickness of the oxide layer 38 is very uniform over all the
contours in the surface of substrate 14. In FIG. 2E, oxide 38 has
been patterned in the vicinity of well 36. Preferably, a thick
resist (not shown) is used because it is easier to fill well 36
with resist rather than try to get the resist to cover the steep
walls of the well. The size of the resulting flange 18 has no
effect on either the gap or the thermal performance of the spacer.
A large flange is more convenient when using a deposited braze in
order to secure the heater substrate to the photovoltaic substrate
and a smaller flange is typically more rugged. One requirement for
accurate gap dimension, however, is that the flange terminate
within recess 32. FIG. 2F shows the completed spacer 16. A metal
film, not shown, is deposited on substrate 14 and patterned using a
resist. A selective etchant such as EDP or TMAH is used to etch
away the silicon from the immediate vicinity of spacer 18 to form
well 20 without etching the silicon dioxide material of the spacer
and then the metal film is removed. The size of the area of silicon
removed is not a significant factor. Anisotropic etchants can be
used for this etch although any silicon etch that does not attack
silicon dioxide rapidly may also be used. The height of gap g, FIG.
2F is determined by the thickness of the oxidation 38, FIG. 2D less
the depth of recess 32, FIG. 2A. For the nominal dimension used in
this example, the gap will be 0.2 microns above the silicon outside
the area of the recess. Note that spacer 18 is anchored in
substrate 14 as shown at 21. This method is advantageous when the
deposited dielectric is not readily etched with etchants compatible
with photolithography.
[0034] In another example, as shown in FIGS. 3A-3E, well 36 is
etched in silicon substrate 14 by plasma etching to about 40
microns deep and 20 microns in diameter. In FIG. 3B, a thick layer
of a dielectric such as zirconia is deposited on a surface of
substrate 14 and down into well 36. Dielectric layer 40 maybe tens
of microns thick and a nominal thickness example is 4 microns. In
FIG. 3C, substrate 14 is planarized by polishing to form a flat
surface as shown. In FIG. 3D, the silicon surface of substrate 14
is recessed by a selective etchant that does not affect dielectric
40. For the example shown, the etch-back is 0.2 microns--a depth
which sets the gap between the heat source substrate and the
photovoltaic substrate to give the height of the top surface 15 of
spacer 16a above the top surface of substrate 14. The completed
structure is shown in FIG. 3E after the silicon around spacer 16a
has been etched to thermally isolate spacer 16a from substrate 14
thereby forming well 20.
[0035] If the deposited dielectric is etch compatible with
photolithography the following process may be used and a flange may
also be created for a dielectric deposited in this way as shown in
FIGS. 4A-4C. In FIG. 4A a deep well and recess 32a have been formed
by similar techniques as in the previous processes. Dielectric 40
is now applied by deposition instead of thermal growth. This allows
a wider choice of materials and thicknesses of dielectric. In FIG.
4B the flange has been etched inside the recess 32a. In FIG. 4C,
the same deep etch as discussed above with referenced to FIGS. 2F
and 3E is performed to form well 20.
[0036] In any embodiment, the result is a micron gap
thermo-photo-voltaic device and in particular novel methods of
making spacers for micron gap thermo-photo-voltaic devices. The
spacers have an increased thermal resistance and the resulting
micron gap thermo-photo-voltaic device has increased thermal
efficiency. The methods of this invention provide for precise and
uniform setting of the gap dimension between the photovoltaic
substrate and the heat source substrate. The spacers of this
invention are rugged, durable, and strong in compression and the
spacers have a geometry which does not damage the photovoltaic
substrate.
[0037] By employing conventional photolithography processes in a
novel way, an elongated thin walled spacer is formed and disposed
in a well in the heat source substrate. The top surface of the thin
walled structure is preferably less than a micron above the heat
source substrate defining a submicron gap between the heat source
and the photovoltaic. The well surrounding the thin walled
structure thermally isolates the thin walled structure from the
heat source. No longer is the length of the spacer dictated by the
thickness of the deposited or grown spacer material.
[0038] The spacer of this invention is preferably a hollow cylinder
in shape and 10 to 30 or more times longer in the direction of the
heat flow than the present spacers. However, since the deep well is
formed by photo lithography and plasma etching, a variety of shapes
other than a cylinder may be employed if deemed advantageous. The
shape of the long portion of the spacer will not affect the gap
obtained. The resultant structure is both larger in area and
results in an order of magnitude improvement in thermal resistance.
In addition, the spacer of his invention may have a flange on the
edge of the cylinder that can be made arbitrarily large without
affecting the thermal resistance to provide a significant area for
braze attachment. Despite the large increase in size, the process
for forming the spacer allows for very precise and uniform setting
of the gap dimension.
[0039] The new spacer formation processes result in spacers having
a much greater thermal resistance. A factor of 12 is obtained with
the geometry shown. Higher factors may be readily obtained. The
spacers are larger in area so that spacers will not dig into softer
photovoltaic surfaces and offset the gap. A large flange may be
used that facilitates the placement of braze material and provides
a practical method of attaching heater tiles to a large area
photovoltaic without creating a low resistance thermal path. The
larger cross sectional area results in greater mechanical strength
in compression.
[0040] The resulting process creates a tubular dielectric spacer
which is stronger and has higher thermal resistance than a spacer
than can be made by just patterning dielectrics that can only be
grown or deposited to a relatively short height. A precise gap to
0.1 micron or less is made by controlling a shallow recess and the
oxide thickness, in spite of using a spacer 20 or more microns
long. The flange on the spacer tube allows brazing or other
purposes without interfering with the thermal resistance function
of the spacer. The result is a practical high efficiency conversion
of heat to electricity having many commercial as well as military
uses.
[0041] Although specific features of the invention are shown in
some drawings and not in others, this is for convenience only as
each feature may be combined with any or all of the other features
in accordance with the invention. The words "including",
"comprising", "having", and "with" as used herein are to be
interpreted broadly and comprehensively and are not limited to any
physical interconnection. Moreover, any embodiments disclosed in
the subject application are not to be taken as the only possible
embodiments. Other embodiments will occur to those skilled in the
art and are within the following claims. For example, spacers
suitable for closely separating substrates other than photovoltaic
substrates and heat source substrates are within the scope of the
following claims.
[0042] In addition, any amendment presented during the prosecution
of the patent application for this patent is not a disclaimer of
any claim element presented in the application as filed: those
skilled in the art cannot reasonably be expected to draft a claim
that would literally encompass all possible equivalents, many
equivalents will be unforeseeable at the time of the amendment and
are beyond a fair interpretation of what is to be surrendered (if
anything), the rationale underlying the amendment may bear no more
than a tangential relation to many equivalents, and/or there are
many other reasons the applicant can not be expected to describe
certain insubstantial substitutes for any claim element
amended.
* * * * *