U.S. patent application number 11/524656 was filed with the patent office on 2007-01-18 for surface-normal optical path length for infrared photodetection.
This patent application is currently assigned to Sharp Laboratories of America, Inc.. Invention is credited to Sheng Teng Hsu, Jong Jan Lee, Jer-Shen Maa, Douglas J. Tweet.
Application Number | 20070012876 11/524656 |
Document ID | / |
Family ID | 34679288 |
Filed Date | 2007-01-18 |
United States Patent
Application |
20070012876 |
Kind Code |
A1 |
Lee; Jong Jan ; et
al. |
January 18, 2007 |
Surface-normal optical path length for infrared photodetection
Abstract
A SiGe surface-normal optical path photodetector structure and a
method for forming the SiGe optical path normal structure are
provided. The method comprises: forming a Si substrate with a
surface; forming a Si feature, normal with respect to the Si
substrate surface, such as a via, trench, or pillar; depositing
SiGe overlying the Si normal feature to a thickness in the range of
5 to 1000 nanometers (nm); and, forming a SiGe optical path normal
structure having an optical path length in the range of 0.1 to 10
microns. Typically, the SiGe has a Ge concentration in the range
from 5 to 100%. The Ge concentration may be graded to increase with
respect to the deposition thickness. For example, the SiGe may have
a 20% concentration of Ge at the Si substrate interface, a 30%
concentration of Ge at a SiGe film top surface, and a thickness of
400 nm.
Inventors: |
Lee; Jong Jan; (Camas,
WA) ; Maa; Jer-Shen; (Vancouver, WA) ; Tweet;
Douglas J.; (Camas, WA) ; Hsu; Sheng Teng;
(Camas, WA) |
Correspondence
Address: |
SHARP LABORATORIES OF AMERICA, INC.;C/O LAW OFFICE OF GERALD MALISZEWSKI
P.O. BOX 270829
SAN DIEGO
CA
92198-2829
US
|
Assignee: |
Sharp Laboratories of America,
Inc.
|
Family ID: |
34679288 |
Appl. No.: |
11/524656 |
Filed: |
September 21, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10746952 |
Dec 23, 2003 |
7129488 |
|
|
11524656 |
Sep 21, 2006 |
|
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Current U.S.
Class: |
250/338.4 ;
257/E27.133 |
Current CPC
Class: |
H01L 31/1812 20130101;
Y02E 10/547 20130101; H01L 31/109 20130101; H01L 21/0245 20130101;
H01L 31/028 20130101; H01L 31/108 20130101; H01L 31/105 20130101;
H01L 31/02327 20130101; H01L 27/14643 20130101; H01L 31/035281
20130101; H01L 27/14627 20130101; H01L 21/02381 20130101; H01L
21/0243 20130101; H01L 21/02639 20130101; H01L 21/02532
20130101 |
Class at
Publication: |
250/338.4 |
International
Class: |
G01J 5/20 20060101
G01J005/20; H01L 27/14 20060101 H01L027/14; H01L 31/00 20060101
H01L031/00 |
Claims
1. A method for forming a silicon-germanium (SiGe) optical path
length, normal to a silicon (Si) substrate surface, for infrared
(IR) photodetection, the method comprising: forming a Si substrate
with a surface; forming a Si feature, normal with respect to the Si
substrate surface; depositing SiGe overlying the Si normal feature;
and, forming a SiGe optical path overlying the normal feature
having an optical path length perpendicular to the substrate
surface.
2. The method of claim 1 wherein forming a Si feature, normal with
respect to the Si substrate surface includes forming a feature
selected from the group including a via, trench, and pillar.
3. The method of claim 1 wherein depositing SiGe overlying the Si
normal feature includes depositing SiGe to a thickness in the range
of 5 to 1000 nanometers (nm).
4. The method of claim 1 wherein forming a SiGe optical path
includes forming an optical path length in the range of 0.1 to 10
microns.
5. The method of claim 1 wherein depositing SiGe overlying the Si
normal feature includes depositing SiGe with a Ge concentration in
the range from 5 to 100%.
6. The method of claim 1 wherein depositing SiGe overlying the Si
normal feature includes depositing SiGe with a graded Ge
concentration that increases with respect to the deposition
thickness.
7. The method of claim 6 wherein the SiGe has a 20% concentration
of Ge at the Si substrate interface, a 30% concentration of Ge at a
SiGe film top surface, and a thickness of 400 nm.
8. The method of claim 1 further comprising: depositing a Si layer
overlying the SiGe; depositing SiGe overlying the Si layer; and,
wherein forming a SiGe optical path includes forming an optical
path with a plurality of SiGe layers.
9. The method of claim 1 wherein forming a Si feature, normal with
respect to the Si substrate surface includes forming a trench with
a pair of sidewalls; wherein depositing SiGe overlying the Si
normal feature includes depositing SiGe sidewalls overlying the
trench sidewalls; and, wherein forming a SiGe optical path normal
structure includes forming an optical path pair-structure.
10. The method of claim 1 wherein forming a Si feature, normal with
respect to the Si substrate surface includes forming a trench;
wherein depositing SiGe overlying the Si normal feature includes
filling the trench with SiGe; and, wherein forming a SiGe optical
path includes forming an optical path uni-structure.
11. The method of claim 1 wherein forming a Si feature, normal with
respect to the Si substrate surface includes forming a pillar with
two pairs of sidewalls; wherein depositing SiGe overlying the Si
normal feature includes depositing SiGe sidewalls overlying the two
pairs of pillar sidewalls; and, wherein forming a SiGe optical path
includes forming an optical path array-structure adjacent the
corresponding pillar sidewall pairs.
12. The method of claim 1 wherein forming a Si feature, normal with
respect to the Si substrate surface includes forming a via with two
pairs of sidewalls; wherein depositing SiGe overlying the Si normal
feature includes depositing SiGe sidewalls overlying the two pairs
of via sidewalls; and, wherein forming a SiGe optical path includes
forming an optical path array-structure adjacent the corresponding
via sidewall pairs.
13. The method of claim 1 wherein forming a Si feature, normal with
respect to the Si substrate surface includes forming a via; wherein
depositing SiGe overlying the Si normal feature includes filling
the via with SiGe; and, wherein forming a SiGe optical path
includes forming an optical path uni-structure.
14. The method of claim 1 further comprising: forming an interlayer
dielectric overlying the SiGe optical path; and, forming a
microlens overlying the interlayer dielectric in optical
communication with the SiGe optical path.
16. A method for forming an infrared (IR) photodetector with a
silicon-germanium (SiGe) optical path length, perpendicular to a
silicon (Si) substrate surface, the method comprising: forming a Si
substrate with a surface; forming an interconnect in electrical
communication with a CMOS active region selected from the group
including a source, drain, gate, and a diode region; forming a Si
feature, normal with respect to the Si substrate surface;
depositing SiGe overlying the Si normal feature; and, forming a
SiGe optical path with a path length perpendicular to the substrate
surface and in electrical communication with the active region,
through the interconnect.
16. The method of claim 15 further comprising: forming an
interlayer dielectric overlying the SiGe optical path; and, forming
a microlens overlying the interlayer dielectric in optical
communication with the SiGe optical path.
17-20. (canceled)
21. A silicon-germanium (SiGe) optical path with a path length
normal to a silicon (Si) substrate surface, for infrared (IR)
photodetection, the structure comprising: a Si substrate with a
surface; a Si feature, normal with respect to the Si substrate
surface; and, a SiGe optical path overlying the Si feature, having
an optical path length perpendicular to the substrate surface.
22. The structure of claim 21 wherein the Si feature is selected
from the group including a via, trench, and pillar.
23. The structure of claim 21 wherein the Si substrate surface is
formed in a first plane; and, wherein the SiGe optical path is
formed in a second plane, normal to the first plane, with a
thickness in the range of 5 to 1000 nanometers (nm).
24. The structure of claim 21 wherein the SiGe optical path has an
optical path length in the range of 0.1 to 10 microns, in the
second plane.
25. The structure of claim 21 wherein the SiGe optical path
includes a Ge concentration in the range from 5 to 100%.
26. The structure of claim 21 wherein the SiGe optical path
includes graded Ge concentration that increases with respect to the
deposition thickness.
27. The structure of claim 26 wherein the SiGe optical path has a
20% concentration of Ge at the Si substrate interface, a 30%
concentration of Ge at a SiGe film top surface, and a thickness of
400 nm.
28. The structure of claim 21 further comprising: at least one Si
layer overlying SiGe; and, wherein the SiGe optical path includes a
plurality of SiGe layers overlying Si.
29. The structure of claim 21 wherein the Si feature is a trench
with a pair of sidewalls; and, wherein SiGe optical path is an
optical path pair-structure adjacent the trench sidewalls.
30. The structure of claim 21 wherein the Si feature is a trench;
and, wherein the SiGe optical path is an optical path uni-structure
filling the trench.
31. The structure of claim 21, wherein the Si feature is a pillar
with two pairs of sidewalls; and, wherein the SiGe optical path is
an optical path array-structure adjacent the corresponding pillar
sidewall pairs.
32. The structure of claim 21 wherein the Si normal feature is a
via with two pairs of sidewalls; and, wherein the SiGe optical path
is an optical path array-structure adjacent the corresponding via
sidewall pairs.
33. The structure of claim 21 wherein the Si normal feature is a
via; and, wherein the SiGe optical path is a optical path
uni-structure filing the via.
34. An infrared (IR) photodetector comprising: a CMOS active region
formed in a silicon (Si) substrate with a surface, the active
region selected from the group including a transistor source,
drain, gate, and a diode region; an interconnect in electrical
communication with the active region; a Si feature, normal with
respect to the Si substrate surface, and in electrical
communication with the interconnect; and, a SiGe optical path
overlying the Si feature having an optical path length
perpendicular to the substrate surface.
35. The photodetector of claim 34 further comprising: an interlayer
dielectric overlying the surface-normal SiGe optical path; and, a
microlens overlying the interlayer dielectric in optical
communication with the SiGe optical path.
Description
RELATED APPLICATIONS
[0001] This application is a Continuation of a pending patent
application entitled, SURFACE-NORMAL OPTICAL PATH STRUCTURE FOR
INFRARED PHOTODETECTION, invented by Lee et al., Ser. No.
10/746,952, filed Dec. 23, 2003, which is incorporated herein by
reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] This invention generally relates to integrated circuit (IC)
fabrication processes and, more particularly, to a surface-normal
infrared optical path structure and corresponding fabrication
method.
[0004] 2. Description of the Related Art
[0005] There are many applications for photodetection in the near
infrared region (the wavelength between 0.7 micron to 2 microns),
such as in fiber-optical communication, security, and thermal
imaging. Although III-V compound semiconductors provide superior
optical performance over their silicon (Si)-based counterparts, the
use of Si is desirable, as the compatibility of Si-based materials
with conventional Si-IC technology promises the possibility of
cheap, small, and highly integrated optical systems.
[0006] Silicon photodiodes are widely used as photodetectors in the
visible light wavelengths due to their low dark current and the
above-mentioned compatibility with Si IC technologies. Further,
silicon-germanium (Si.sub.1-xGe.sub.x) permits the photodetection
of light in the 0.8 to 1.6 micron wavelength region.
[0007] However, the SiGe alloy has larger lattice constant than the
Si lattice, so film thickness is a critical variable in the
epitaxial growth of SiGe on Si substrates. While a thick SiGe is
desirable for light absorption, too thick of a SiGe film causes a
defect generation that is responsible for dark currents. This
critical SiGe thickness is dependent upon the Ge concentration and
device process temperature. Higher Ge concentrations and higher
device process temperatures result in the formation of thinner SiGe
film thicknesses. In common practice, the SiGe critical thickness
is in the range of a few hundred angstroms, to maximum of a few
thousand angstroms. Once the SiGe thickness is grown beyond its
critical thickness, lattice defects in SiGe are inevitable. As
mentioned above, an IR photo detector built from a SiGe film with
lattice defects generates large dark currents and noise.
[0008] Quantum efficiency is a measure of the number of
electron-hole pairs generated per incident photon, and it is a
parameter for photodetector sensitivity. Quantum efficiency is
defined as: .eta.=(I.sub.p/q)/(P.sub.opt/h.nu.)
[0009] where I.sub.p is the current generated by the absorption of
incident optical power P.sub.opt at the light frequency .nu..
[0010] FIG. 1 is a graph showing the relationship between quantum
efficiency and the percentage of Ge in a SiGe film. One of the key
factors in determining quantum efficiency is the absorption
coefficient, .alpha.. Silicon has a cutoff wavelength of about 1.1
microns and is transparent in the wavelength region between 1.3 to
1.6 microns. The SiGe absorption edge shifts to the red with an
increasing Ge mole fraction and is shown in FIG. 1. The absorption
coefficient of any SiGe alloy is relatively small and the limited
thickness dictated by the critical thickness further limits the
ability of SiGe films to absorb photons.
[0011] As noted above, the major goals of SiGe-based photodetection
are high quantum efficiency and the integration of these SiGe
photodetectors with the existing Si electronics. One way to
increase the optical path, and improve the quantum efficiency, is
to form the optical path in the same plane as the SiGe film, along
the substrate surface in which the SiGe is deposited. Thus, light
propagates parallel to the heterojunction (SiGe/Si) interface.
However, this optical path design necessarily limits the design of
IR detectors.
[0012] It would be advantageous if an efficient SiGe IR
photodetector could be fabricated having an optical path that need
not be formed in parallel with a Si substrate surface.
SUMMARY OF THE INVENTION
[0013] The present invention SiGe optical path structure absorbs IR
wavelength light that is normal to a silicon substrate surface and
parallel to the SiGe/Si heterojunction interface, increasing the
length of the optical path. Therefore, a two-dimensional IR image
detection can be realized with a thin SiGe thickness. Because of
the relatively poor quantum efficiencies associated with SiGe, the
IR absorption length of SiGe must be long, and conventionally a
thick SiGe layer is needed to absorb high amounts of IR energy.
However, it is very difficult to grow defect-free thick SiGe film
on Si substrate because of the lattice mismatch between these two
materials. The present invention eliminates the need for a thick
SiGe film. SiGe film is grown on the sidewall of a Si substrate
trench or pillar, forming a relatively long optical path for light
normal to the substrate surface. The present invention's use of
relatively thin SiGe films permits a SiGe IR photodetector to be
easily integrated with Si CMOS devices, with minimal lattice
mismatch.
[0014] Accordingly, a method is provided for forming a SiGe optical
path structure, normal to a Si substrate surface, for the purpose
of IR photodetection. The method comprises: forming a Si substrate
with a surface; forming a Si feature, normal with respect to the Si
substrate surface, such as a via, trench, or pillar; depositing
SiGe overlying the Si normal feature to a thickness in the range of
5 to 1000 nanometers (nm); and, forming a SiGe optical path normal
structure having an optical path length in the range of 0.1 to 10
microns.
[0015] In some aspects of the method, depositing SiGe overlying the
Si normal feature includes depositing SiGe with a Ge concentration
in the range from 5 to 100%. In other aspects, the SiGe is
deposited with a graded Ge concentration that increases with
respect to the deposition thickness. For example, the SiGe may have
a 20% concentration of Ge at the Si substrate interface, a 30%
concentration of Ge at a SiGe film top surface, and a thickness of
400 nm.
[0016] Additional details of the above-described method and a SiGe
optical path structure, normal to a Si substrate surface, are
provided below.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] FIG. 1 is a graph showing the relationship between quantum
efficiency and the percentage of Ge in a SiGe film.
[0018] FIG. 2 is a cross-sectional view of the present invention
SiGe optical path structure, normal to a Si substrate surface, for
IR photodetection.
[0019] FIG. 3 is a cross-sectional view of an alternate aspect of
the SiGe optical path structure of FIG. 2.
[0020] FIG. 4 is a plan view of the present invention optical path
structure.
[0021] FIG. 5 is a cross-sectional view of the present invention IR
photodetector.
[0022] FIG. 6 is a cross-sectional view of FIG. 2, featuring an
alternate aspect of the invention.
[0023] FIG. 7 is a cross-sectional view of a preliminary step in
the formation of a PIN diode SiGe IR photodetector using a trench
surface-normal feature.
[0024] FIG. 8 is a cross-sectional view of the photodetector of
FIG. 7 following a photoresist process to form trenches in the Si
substrate (N-well).
[0025] FIG. 9 is a cross-sectional view of the photodetector of
FIG. 8 following the epitaxial growth of SiGe on photodiode
area.
[0026] FIG. 10 is a cross-sectional view of the photodetector of
FIG. 9 following a photoresist and etching of the P+ Si and SiGe
layers.
[0027] FIG. 11 is a cross-sectional view of the photodetector of
FIG. 10 following an ILD deposition and the formation of interlevel
contact to the CMOS transistors and the present invention IR
photodiode.
[0028] FIG. 12 is a cross-sectional view of SiGe optical path
structure of FIG. 6 with the addition of a microlens.
[0029] FIG. 13 is a cross-sectional view of the photodetector of
FIG. 11 with the addition of a microlens.
[0030] FIG. 14 is a cross-sectional view of a Schottky diode IR
photodetector using a surface-normal SiGe optical path.
[0031] FIG. 15 is a cross-sectional view of an npn bipolar IR
detector using a surface-normal SiGe optical path
[0032] FIG. 17 is a flowchart illustrating the present invention
method for forming a SiGe optical path structure, normal to a Si
substrate surface, for IR photodetection.
[0033] FIG. 18 is a flowchart illustrating the present invention
method for forming an IR photodetector with a SiGe optical path
structure, normal to a Si substrate surface.
[0034] FIG. 16 is a flowchart illustrating the present invention
method for photodetecting IR energy using a SiGe surface-normal
optical path structure.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0035] FIG. 2 is a cross-sectional view of the present invention
SiGe optical path structure, normal to a Si substrate surface, for
IR photodetection. The structure 200 comprises a Si substrate 202
with a surface 204. A Si feature 206 is normal with respect to the
Si substrate surface 204. As shown, the feature 206 can be a via
206a, a trench 206b, or a pillar 206c. A surface-normal SiGe
optical path 208, shown with double cross-hatched lines, overlies
the Si feature 206.
[0036] The Si substrate surface (interface) 204 is formed in a
first plane 210 parallel to the substrate surface 204. SiGe is
epitaxially grown on the Si surface 204 and Si feature 206. The
surface-normal SiGe optical path 208 is formed in a second plane
212, normal to the first plane 210. That is, the optical path 208
is normal to the substrate surface 204. Alternately stated, the
feature 206 has an element or structure in a vertical plane that is
perpendicular to the horizontal surface 204. Note, that the feature
206 may also include an element or structure, a trench bottom or
pillar top for example, that is in a plane parallel to the first
plane 210.
[0037] The optical path 208 has a thickness 214 in the range of 5
to 1000 nanometers (nm). The surface-normal SiGe optical path 208
has an optical path length 216 in the range of 0.1 to 10 microns,
in the second plane 212.
[0038] In some aspects, the surface-normal SiGe optical path 208
includes a Ge concentration in the range from 5 to 100%. In other
aspects, the surface-normal SiGe optical path 208 includes graded
Ge concentration that increases with respect to the deposition
thickness. For example, the surface-normal SiGe optical path 208
may have a 20% concentration of Ge at the Si substrate interface
204, a 30% concentration of Ge at a SiGe film top surface 220, and
a thickness 214 of 400 nm.
[0039] FIG. 3 is a cross-sectional view of an alternate aspect of
the SiGe optical path structure of FIG. 2. As in FIG. 2, a Si
substrate 202 has a surface 204. The structure 200 further
comprises at least one Si layer 300 overlying SiGe 302, so that the
surface-normal SiGe optical path 208 includes a plurality of SiGe
layers overlying Si. In this example, a via 206a is shown, with a
SiGe first layer 302 and a second SiGe layer 304. Although an
optical path 208 is shown with two SiGe layers (302/304) and a
single interposing Si layer 300, the present invention is not
limited to any particular number of SiGe/Si interfaces or layers.
Further, the final SiGe layer (304 in this example) may fill the
via 206a. Neither is the multilayer optical path structure limited
to just a via surface-normal feature.
[0040] FIG. 4 is a plan view of the present invention optical path
structure. Viewing both FIGS. 2 and 4, when the Si feature is a
trench 206b, there are a pair of sidewalls 400a and 400b. In one
aspect, the surface-normal SiGe optical path 208 is an optical path
pair-structure adjacent the trench sidewalls 400a and 400b (see
FIG. 4). Of course, the trench 206b typically has ends (not shown),
which would constitute a second pair of sidewalls. Then, the
optical path 208 might additionally include optical paths adjacent
these trench-end sidewalls. In a different aspect, the
surface-normal SiGe optical path 208 is a uni-structure that fills
the trench 206b (see FIG. 2).
[0041] Returning to FIG. 4, when the Si feature is a pillar 206c,
the pillar 206c has two pairs of sidewalls, a first pair 402a and
402b and a second pair 404a and 404b. The surface-normal SiGe
optical path 208 is an optical path array-structure adjacent the
corresponding pillar sidewall pairs 402a/402b and 404a/404b. In
other aspects (not shown) the optical path structure is composed of
SiGe layers adjacent a subset of the pillar sidewalls. Likewise, an
optical path may be formed adjacent a pillar with rounded
sidewalls.
[0042] In other aspects, the Si normal feature is via 206a with two
pairs of sidewalls, a first pair of sidewalls 410a and 410b, and a
second set of sidewalls 412a and 412b. Then, the surface-normal
SiGe optical path 208 is an optical path array-structure adjacent
the corresponding trench sidewall pairs 410a/410b and 412a/412b.
Alternately, the surface-normal SiGe optical path is a
uni-structure that fills the via (see FIG. 2). Likewise, an optical
path may be formed adjacent a via with rounded sidewalls.
[0043] FIG. 5 is a cross-sectional view of the present invention IR
photodetector. The photodetector 500 comprises an interconnect 502
in electrical communication with a CMOS active region (not shown)
formed in a Si substrate 504. The active region can be transistor
source, drain, gate, or a diode region. The Si substrate 504 has a
surface (interface) 505. A Si feature 506 is shown normal with
respect to the Si substrate surface 505 in electrical communication
with the interconnect 502. In this example, the feature 506 is a
via. However, in other aspects the feature can be a trench or a
pillar. A surface-normal SiGe optical path 508 overlies the Si
feature 506.
[0044] In some aspects, an interlayer dielectric 510, such as SiO2,
overlies the surface-normal SiGe optical path 508, and a microlens
512 overlies the interlayer dielectric 510 in optical communication
with the surface-normal SiGe optical path 508. Additional details
of the SiGe optical path are presented above in the explanations of
FIGS. 2-4. Examples of CMOS active regions follow.
Functional Description
[0045] FIG. 6 is a cross-sectional view of FIG. 2, featuring an
alternate aspect of the invention. The present invention optical
structure is created normal to a Si substrate surface. This can be
accomplished using standard Si IC trench, pillar, or hole (via)
processes. A SiGe (Ge concentration 5% to 100%) is epitaxially
deposited on the Si. Two simple structures, a Si trench and Si
pillar are shown. The SiGe is epitaxially deposited on Si to a
thickness that is less than the critical thickness, so that no
defects are generated. As an alternative to SiGe deposition with
fixed concentration of Ge, a graded SiGe layer can be deposited.
Another alternative is to form a quantum well SiGe structure that
includes multiple Si and SiGe layers (SiGe/Si/SiGe/Si . . . ). SiGe
can be used to either fill the trench or line the trench sidewalls
FIG. 7 is a cross-sectional view of a preliminary step in the
formation of a PIN diode SiGe IR photodetector using a trench
surface-normal feature. This invention can be incorporated with
various device types to fabricate high efficient IR photodetectors.
These devices include, but are not limited to, PN diodes, PIN type
diodes, heterojunction phototransistors, quantum well photodiodes,
and Schottky diodes. Standard CMOS devices can be integrated with
the IR detectors on a single Si wafer. As with any conventional
CMOS procedure, an interlevel dielectric (ILD) deposition is
performed. An N-well can be used as the n-layer of the PIN diode,
or additional processes can be performed to form an n-layer.
[0046] FIG. 8 is a cross-sectional view of the photodetector of
FIG. 7 following a photoresist process to form trenches in the Si
substrate (N-well). The trenches have a depth in the range of 0.1
to 10 microns.
[0047] FIG. 9 is a cross-sectional view of the photodetector of
FIG. 8 following the epitaxial growth of SiGe on photodiode area.
SiGe is non-doped to form a SiGe intrinsic layer. Note, a SiGe
deposition on top of ILD becomes polycrystalline. The SiGe
thickness is 0.05 to 0.5 microns. Next, Si is epitaxially grown and
doped to be P+. The P+ doping can be an result of in-situ doping
during Si growth, or ion implantation after Si growth. The P+
thickness is in the range of 0.05 to 1 micron.
[0048] FIG. 10 is a cross-sectional view of the photodetector of
FIG. 9 following a photoresist and etching of the P+ Si and SiGe
layers.
[0049] FIG. 11 is a cross-sectional view of the photodetector of
FIG. 10 following an ILD deposition and the formation of interlevel
contact to the CMOS transistors and the present invention IR
photodiode.
[0050] FIG. 12 is a cross-sectional view of SiGe optical path
structure of FIG. 6 with the addition of a microlens.
[0051] FIG. 13 is a cross-sectional view of the photodetector of
FIG. 11 with the addition of a microlens. IR detectors with SiGe
vertical sidewalls have improved quantum efficiency, but the
effective area for the IR detection (the optical path length)
depends on the surface-normal feature (trench/via/pillar) layout.
Another way to improve the area efficiency is to add a microlens,
to focus the incident IR into the trench, as shown in FIGS. 12 and
13. The IR reflection at the Si/SiGe interface can also improve the
light absorption. Note the reflection of light at the Si/SiGe
interfaces. Also note that in FIG. 13 that each trench has its own
microlens to focus the light and maximize the IR absorption in
SiGe.
[0052] FIG. 14 is a cross-sectional view of a Schottky diode IR
photodetector using a surface-normal SiGe optical path. The diode
can be formed in either a P-well or an N-well. The metal deposition
can be a material such as Pt, Ir, or Pt/Ir.
[0053] FIG. 15 is a cross-sectional view of an npn bipolar IR
detector using a surface-normal SiGe optical path. As shown, the
transistor is formed in an N-well. The SiGe is p-type doped with
boron either by in-situ doping or by ion implantation after
deposition. The overlying Si layer is in-situ P-doped or As-doped
n-type Si. Alternately, intrinsic Si can be implanted with dopants
of As or P to form n-type Si layer.
[0054] FIG. 17 is a flowchart illustrating the present invention
method for forming a SiGe optical path structure, normal to a Si
substrate surface, for IR photodetection. Although the method (and
the method describing FIGS. 18 and 16, below) is depicted as a
sequence of numbered steps for clarity, no order should be inferred
from the numbering unless explicitly stated. It should be
understood that some of these steps may be skipped, performed in
parallel, or performed without the requirement of maintaining a
strict order of sequence. The method starts at Step 1700.
[0055] Step 1702 forms a Si substrate with a surface. Step 1704
forms a Si feature, such as a via, trench, or pillar, normal with
respect to the Si substrate surface. Note, the invention is not
necessarily limited to just these three example features. Step 1706
deposits SiGe overlying the Si normal feature (and Si substrate
surface), to a thickness in the range of 5 to 1000 nanometers (nm).
Step 1708 forms a SiGe optical path normal structure having an
optical path length in the range of 0.1 to 10 microns. As used
herein, a normal structure is intended to describe a Si substrate
surface-normal structure. Alternately expressed, the surface-normal
features have a length of 0.1 to 10 microns.
[0056] Typically, Step 1706 deposits SiGe with a Ge concentration
in the range from 5 to 100%. In some aspects, SiGe is deposited
with a graded Ge concentration that increases with respect to the
deposition thickness. For example, the SiGe may have a 20%
concentration of Ge at the Si substrate interface, a 30%
concentration of Ge at a SiGe film top surface, and a thickness of
400 nm.
[0057] Other aspects of the method include additional steps. Step
1707a deposits a Si layer overlying the SiGe. Step 1707b deposits
SiGe overlying the Si layer. Then, forming a SiGe normal optical
path structure in Step 1708 includes forming a normal optical path
structure with a plurality of SiGe layers. Note, Steps 1707a and
1707b may be iterated a number of times to build up a plurality of
SiGe/Si layers.
[0058] For example, if Step 1704 forms a trench with a pair of
sidewalls, Step 1706 may deposit SiGe sidewalls overlying the
trench sidewalls. Then, Step 1708 forms a SiGe optical path
pair-structure. Alternately, Step 1706 fills the trench with SiGe
and Step 1708 forms a SiGe optical path uni-structure.
[0059] In another example, Step 1704 forms a pillar with two pairs
of sidewalls and Step 1706 deposits SiGe sidewalls overlying the
two pairs of pillar sidewalls. Then, Step 1708 forms an optical
path array-structure adjacent the corresponding pillar sidewall
pairs. Alternately, the SiGe optical path structure can be formed
on a subset of the four pillar sidewalls.
[0060] In another example, Step 1704 forms a via with two pairs of
sidewalls and Step 1706 deposits SiGe sidewalls overlying the two
pairs of via sidewalls. Then, Step 1708 forms an optical path
array-structure adjacent the corresponding via sidewall pairs. As
above, the optical path structure can be formed on a subset of the
via sidewalls. Alternately, Step 1706 fills the via with SiGe and
Step 1708 forms an optical path uni-structure.
[0061] Other aspects of the method include further steps. Step 1710
forms an interlayer dielectric overlying the SiGe optical path
normal structure. Step 1712 forms a microlens overlying the
interlayer dielectric in optical communication with the SiGe
optical path normal structure.
[0062] FIG. 18 is a flowchart illustrating the present invention
method for forming an IR photodetector with a SiGe optical path
structure, normal to a Si substrate surface. The method starts at
Step 1800. Step 1802 forms a Si substrate with a surface. Step 1804
forms an interconnect in electrical communication with a CMOS
active region such as a source, drain, gate, or a diode region.
Examples of such active regions are presented in FIGS. 5 through
15. Step 1806 forms a Si feature, normal with respect to the Si
substrate surface. Step 1808 deposits SiGe overlying the Si normal
feature. Step 1810 forms a SiGe optical path normal structure in
electrical communication with the CMOS active region, through the
interconnect. Step 1812 forms an interlayer dielectric overlying
the SiGe optical path normal structure. Step 1814 forms a microlens
overlying the interlayer dielectric in optical communication with
the SiGe optical path normal structure. Details of the SiGe optical
path structure are presented in the explanation of FIG. 17,
above.
[0063] FIG. 16 is a flowchart illustrating the present invention
method for photodetecting IR energy using a SiGe surface-normal
optical path structure. The method starts at Step 1900. Step 1902
accepts IR photons having a trajectory normal to a Si substrate
surface. Step 1904 absorbs the IR photons through a SiGe
surface-normal optical path structure. Step 1906 generates a
current in response to absorbing the IR photons. Step 1908 conducts
the current into a CMOS active region.
[0064] In some aspects, accepting IR photons having a trajectory
normal to a Si substrate surface in Step 1902 includes accepting IR
photons having a wavelength in the range of 0.8 to 1.6 microns.
[0065] In other aspects, absorbing the IR photons through a SiGe
surface-normal optical path structure in Step 1904 includes
absorbing 1.1 micron wavelength IR photons with an efficiency of
approximately 7%, responsive to an optical path structure length of
10 microns. In a different aspect Step 1904 absorbs 1.1 micron
wavelength IR photons with an efficiency in the range of 0.07 to 7%
efficiency, responsive to an optical path structure length in the
range of 0.1 to 10 microns.
[0066] A surface-normal SiGe optical path structure and
corresponding fabrication process have been presented. Simple
surface-normal features such as vias, trenches, and pillars have
been used to illustrate the invention. However, the invention may
also be applied to more complicated features. Likewise, although
SiGe films have been described, the invention is not necessarily
limited to a particular light-absorbing film or a particular
wavelength of light. Other variations and embodiments of the
invention will occur to those skilled in the art.
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