U.S. patent application number 11/416985 was filed with the patent office on 2007-11-08 for fabrication of vertical sidewalls on (110) silicon substrates for use in si/sige photodetectors.
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 | 20070259467 11/416985 |
Document ID | / |
Family ID | 38661684 |
Filed Date | 2007-11-08 |
United States Patent
Application |
20070259467 |
Kind Code |
A1 |
Tweet; Douglas J. ; et
al. |
November 8, 2007 |
FABRICATION OF VERTICAL SIDEWALLS ON (110) SILICON SUBSTRATES FOR
USE IN SI/SIGE PHOTODETECTORS
Abstract
A method of fabricating vertical sidewalls on silicon (110)
substrates for use in Si/SiGe photodetectors includes preparing a
silicon (110) layer wherein the silicon (110) plane is parallel to
an underlying silicon wafer surface. Masking the silicon (110)
layer with mask sidewalls parallel to a silicon (111) layer plane
and etching the silicon (110) layer to remove an un-masked portion
thereof, leaving a patterned silicon (110) layer having vertical
silicon (111) sidewalls. Removing the mask; growing SiGe-containing
layers on the patterned silicon (110) layer; and fabricating a
photodetector.
Inventors: |
Tweet; Douglas J.; (Camas,
WA) ; Lee; Jong-Jan; (Camas, WA) ; Maa;
Jer-Shen; (Vancouver, WA) ; Hsu; Sheng Teng;
(Camas, WA) |
Correspondence
Address: |
David C. Ripma;Sharp Laboratories of America, Inc.
5750 NW Pacific Rim Boulevard
Camas
WA
97202
US
|
Assignee: |
Sharp Laboratories of America,
Inc.
|
Family ID: |
38661684 |
Appl. No.: |
11/416985 |
Filed: |
May 2, 2006 |
Current U.S.
Class: |
438/48 |
Current CPC
Class: |
H01L 31/1812 20130101;
Y02E 10/50 20130101 |
Class at
Publication: |
438/048 |
International
Class: |
H01L 21/00 20060101
H01L021/00 |
Claims
1. A method of fabricating vertical sidewalls on silicon (110)
substrates for use in Si/SiGe photodetectors, comprising: preparing
a silicon (110) layer wherein the silicon (110) plane is parallel
to an underlying silicon wafer surface; masking the silicon (110)
layer with mask sidewalls parallel to a silicon (111) layer plane;
etching the silicon (110) layer to remove an un-masked portion
thereof, leaving a patterned silicon (110) layer; removing the
mask; growing SiGe-containing layers on the patterned silicon (110)
layer; and fabricating a photodetector.
2. The method of claim 1 wherein said preparing a silicon (110)
layer includes preparing a bulk silicon (110) substrate.
3. The method of claim 1 wherein said preparing a silicon (110)
layer includes preparing a SOI wafer having a silicon (110) top
silicon layer thereon.
4. The method of claim 1 wherein said masking the silicon (110)
layer includes masking and patterning the silicon (110) layer with
a masking taken from the group of maskings consisting of
photoresist and hardmasks.
5. The method of claim 1 wherein said etching the silicon (110)
layer includes etching by a selective wet etch process.
6. The method of claim 1 wherein said etching the silicon (110)
layer includes etching by a RIE process and then further etching
the silicon (110) layer by a selective wet etch process which
etches silicon (110) planes faster than silicon (111) planes,
leaving a patterned silicon (110) surface having silicon (111)
sidewalls.
7. A method of fabricating vertical sidewalls on silicon (110)
substrates for use in Si/SiGe photodetectors, comprising: preparing
a silicon (110) layer wherein the silicon (110) plane is parallel
to an underlying silicon wafer surface, wherein said preparing a
silicon (110) layer includes preparing a bulk silicon (110)
substrate or preparing a SOI wafer having a silicon (110) top
silicon layer thereon; masking the silicon (110) layer with mask
sidewalls parallel to a silicon (111) layer plane; etching the
silicon (110) layer to remove an un-masked portion thereof, leaving
a patterned silicon (110) layer; removing the mask; growing
SiGe-containing layers on the patterned silicon (110) layer; and
fabricating a photodetector.
8. The method of claim 7 wherein said masking the silicon (110)
layer includes masking and patterning the silicon (110) layer with
a masking taken from the group of maskings consisting of
photoresist and hardmasks.
9. The method of claim 7 wherein said etching the silicon (110)
layer includes etching by a selective wet etch process.
10. The method of claim 7 wherein said etching the silicon (110)
layer includes etching by a RIE process and then further etching
the silicon (110) layer by a selective wet etch process which
etches silicon (110) planes faster than silicon (111) planes,
leaving a patterned silicon (110) surface having vertical silicon
(111) sidewalls.
11. A method of fabricating vertical sidewalls on silicon (110)
substrates for use in Si/SiGe photodetectors, comprising: preparing
a silicon (110) layer wherein the silicon (110) plane is parallel
to an underlying silicon wafer surface; masking the silicon (110)
layer with mask sidewalls parallel to a silicon (111) layer plane;
etching the silicon (110) layer to remove an un-masked portion
thereof, leaving a patterned silicon (110) layer, wherein said
etching the silicon (110) layer includes etching by a RIE process
and then further etching the silicon (110) layer by a selective wet
etch process which etches silicon (110) planes faster than silicon
(111) planes, leaving a patterned silicon (110) surface having
vertical silicon (111) sidewalls.; removing the mask; growing
SiGe-containing layers on the patterned silicon (110) layer; and
fabricating a photodetector.
12. The method of claim 11 wherein said preparing a silicon (110)
layer includes preparing a bulk silicon (110) substrate.
13. The method of claim 11 wherein said preparing a silicon (110)
layer includes preparing a SOI wafer having a silicon (110) top
silicon layer thereon.
14. The method of claim 11 wherein said masking the silicon (110)
layer includes masking and patterning the silicon (110) layer with
a masking taken from the group of maskings consisting of
photoresist and hardmasks.
15. The method of claim 11 wherein said etching the silicon (110)
layer includes etching by a selective wet etch process.
Description
FIELD OF THE INVENTION
[0001] This invention relates to photodetectors, and particularly
to near-infrared photodetectors integrated on silicon
substrates.
BACKGROUND OF THE INVENTION
[0002] Photo detecting in the near infrared regime, i.e., having a
wavelength of between 0.7 .mu.m to 2 .mu.m, has many applications,
such as in fiber-optic communication, security applications,
machine vision and night vision imaging. Although III-V compound
semiconductors provide superior optical performance over their
silicon-based counterparts, the compatibility of silicon based
materials with current silicon-IC technology provides the
possibility of making cheap, small and highly integrated optical
systems. The following references provide additional background for
the invention: Maiti et al., Strained Silicon Heterostructures:
Materials and Devices, Chapter 10: Si/SiGe Optoelectronics, The
Institution of Electrical Engineer, 2001; Murtaza et al., Room
Temperature Electroabsorption in Ge.sub.xSi.sub.1-xPIN Photodiode,
IEEE Trans. on Electron Devices, 2297-2300, Vol. 41, No. 12, 1994;
Tashiro et al., A Selective Epitaxial SiGe/Si Planar Photodetector
for Si-Based OEICs, IEEE Trans. on Electron Devices, 545-550, Vol.
44, No. 4, 1997; Vonsovici et al., Room Temperature Photocurrent
Spectroscopy of SiGe/Si p-i-n Photodiodes Grown by Selective
Epitaxy, IEEE Trans. on Electron Devices, 538-542, Vol. 45, No. 2,
1998; and Jones et al., Fabrication and Modeling of Gigahertz
Photodetectors in Heteroepitaxial Ge-on-Si using Graded Buffer
Layer Deposited by Low Energy Plasma Enhanced CVD, IEDM, 2002.
[0003] Silicon photodiodes are widely used as photodetectors for
visible light due to their low dark current and compatibility with
silicon IC technologies. The use of Si.sub.1-xGe.sub.x (SiGe)
alloys in silicon processing permits photo detection operating in
the 0.8 .mu.m to 1.6 .mu.m wavelength regime.
[0004] SiGe alloys have larger lattice constants than pure silicon,
thus, the epitaxial growth of SiGe on silicon has a critical
thickness, above which the film begins to relax by the nucleation
of dislocations. The critical thickness of SiGe depends on the
germanium concentration and device process temperature. Houghton,
Strain relaxation kinetics in Si.sub.1-xGe.sub.x/Si
heterostructures, J. Appl. Phys. Vol. 780, No. 4, 1991.
[0005] A high germanium concentration and high device process
temperature result in a smaller critical thickness. In common
practice, the SiGe critical thickness is in the range of few
hundred angstroms to a maximum of a couple thousand angstroms. Once
the SiGe thickness is grown above its critical thickness, lattice
defects in SiGe are inevitable. An IR photo detector built on SiGe
containing lattice defects will have a high dark current and
produce electronic noise.
[0006] Quantum efficiency is the number of electron-hole pairs
generated per incident photon and is a parameter for photo detector
sensitivity. The quantum efficiency is defined as:
.eta.=(I.sub.p/q)/(P.sub.opt/h.nu.) (1) where Ip is the
photo-generated current by the absorption of incident optical power
P.sub.opt at the light frequency .nu., where q is the electron
charge, and h is Planck's constant.
[0007] One of the key factors that determines the quantum
efficiency is the absorption coefficient, .alpha.. Silicon has a
cutoff wavelength of about 1.1 .mu.m and is transparent to
wavelengths beyond .about.1.2 .mu.m. The SiGe absorption edge
shifts to the red with increasing germanium mole fraction and is
shown in FIG. 1. The absorption coefficient of SiGe alloy is small
and the critical thickness limits the absorbing layer thickness.
The major aim of SiGe based photo detectors is to achieve high
quantum efficiency and integration with existing silicon
electronics.
[0008] One way to increase the optical path and improve the quantum
efficiency is to illuminate the edge of the photo detector with
light so that the light propagates parallel to the heterojunction
(SiGe/Si) interfaces. However, this does not allow the device to be
used in image detection. Growing strained, defect-free SiGe films
or SiGe/Si multilayer structures on the sidewalls of etched silicon
structures has been disclosed by Lee et al., Surface-Normal Optical
Path Structure for Infrared Photodetection, U.S. Patent Publication
No. 2005/0136637-A1, published Jun. 23, 2005; and Tweet et al.,
Vertical Optical Path Structure for Infrared Photodetection, U.S.
Patent Publication No. 2005/0153474-A1, published Jul. 14, 2005. In
devices incorporating the technology described in the two preceding
references, illumination of the device is normal to the silicon
substrate, however, light travels parallel to the heterojunction
interface to increase the optical path length. Therefore,
two-dimensional IR image detection may be achieved within thin SiGe
or SiGe/Si film thicknesses.
[0009] Fabrication of high quality, defect-free strained SiGe films
requires SiGe growth on a defect-free silicon surface. However,
referring to FIG. 2, reactive ion-etching (RIE) of silicon usually
results in a sloped sidewall, shown generally at 10, and in poor
crystal quality near the sidewall surface, as shown generally at
12, where a damaged area of silicon is formed near an RIE-etched
sidewall. Also, the surface is often rather rough. Ideally, these
defects may be cured by use of a selective etch, which etches the
desired sidewall crystal plane more slowly than it does other
planes. For various kinds of silicon device fabrication, silicon
substrates having a (001) plane parallel to the wafer surface are
most commonly used. Etching of a trench in this substrate renders
the sidewalls parallel to the (110) planes or (100) planes,
depending on the azimuthal rotation of the wafer. Making these
sidewalls more vertical requires a selective etch, which etches the
(110) or (100) planes, respectively, more slowly than other planes.
However, such an etch process is not known in the prior art.
Instead, there are well-known selective etches which etch the (111)
plane much more slowly than other planes.
[0010] One way around this problem is to change the substrate to
one with the silicon (110) plane parallel to the wafer surface, Liu
et al., Multi-Fin Double-Gate MOSFET Fabricated by using
(110)-Oriented SOI wafers and Orientation-Dependent Etching,
Electrochemical Society Proceedings, vol. 2003-06, 566 (2003).
Then, the sidewall plane may be silicon (111) or any other related,
equivalent orthogonal plane. Liu et al. used a 2.38%
tetramethylammonium hydroxide (TMAH) solution to etch vertical
sidewalls and form a silicon fin structure with rectangular
cross-section on silicon-on-insulator wafers. These were then used
to fabricate silicon-FINFET (FIN Field-Effect Transistor) devices.
Liu et al. report that TMAH etches (110) planes 23-25 times faster
than silicon (111) planes. In addition, the use of a selective wet
etch instead of RIE results in undamaged crystalline silicon at the
sidewall surfaces.
SUMMARY OF THE INVENTION
[0011] A method of fabricating vertical sidewalls on silicon (110)
substrates for use in Si/SiGe photodetectors includes preparing a
silicon (110) layer wherein the silicon (110) plane is parallel to
an underlying silicon wafer surface. Masking the silicon (110)
layer with mask sidewalls parallel to a silicon (111) layer plane
and etching the silicon (110) layer to remove an un-masked portion
thereof, leaving a patterned silicon (110) layer. Finally, the mask
is removed, and SiGe-containing layers are grown on the patterned
silicon (110) layer. The photodetector is then completed.
[0012] It is an object of the invention to improve the quantum
efficiency of photodetectors, e.g., near-IR photodetectors,
integrated on silicon substrates.
[0013] Another object of the invention is to provide a method for
using silicon (110) substrates and selective chemical etching to
fabricate vertical sidewalls on which to grow strained SiGe and/or
SiGe/Si multilayer structures for use in photodetectors, such as
near-IR photodetectors.
[0014] This summary and objectives of the invention are provided to
enable quick comprehension of the nature of the invention. A more
thorough understanding of the invention may be obtained by
reference to the following detailed description of the preferred
embodiment of the invention in connection with the drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] FIG. 1 represents normalized photocurrent spectra for
various SiGe sample.
[0016] FIG. 2 is a schematic profile of RIE-etched silicon
structures.
[0017] FIG. 3 is a block diagram of the method of the
invention.
[0018] FIG. 4 depicts use of a bulk silicon (110) substrate.
[0019] FIG. 5 depicts use of a SOI (110) substrate.
[0020] FIG. 6 depicts bulk silicon (110) substrate following direct
use of selective wet etch.
[0021] FIG. 7 depicts SOI (110) substrate following selective wet
etch.
[0022] FIG. 8 depicts bulk silicon (110) substrate following
anisotropic RIE etch.
[0023] FIG. 9 depicts SOI (110) substrate following anisotropic RIE
etch.
[0024] FIG. 10 depicts bulk silicon (110) substrate following
anisotropic RIE etch and selective wet etch.
[0025] FIG. 11 depicts SOI (110) substrate following anisotropic
RIE etch and selective wet etch.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0026] The objective of this invention is to improve the quantum
efficiency of photodetectors, e.g., near-IR photodetectors,
integrated on silicon substrates. This is done by providing silicon
structures with nearly vertical sidewalls and low crystalline
defects to enable the growth of low-defect strained, epitaxial SiGe
and/or SiGe/Si multilayers on the sidewalls. By so doing, the
optical path length for absorption of light is be determined by the
height of the sidewalls, and not by the SiGe film thickness. In
this way the quantum efficiency of the photodetector is
improved.
[0027] As previously noted, the use of reactive ion-etching (RIE)
to etch silicon structures results in rough, damaged sidewalls with
unacceptably large slopes. The readily available selective etch
processes do not improve this situation on silicon (001)
substrates. The present invention uses silicon (110) substrates and
selective chemical etching to fabricate vertical sidewalls on which
to grow strained SiGe and/or SiGe/Si multilayer structures for use
in photodetectors, such as near-IR photodetectors.
[0028] Referring now to FIG. 3, the method of the invention is
depicted generally at 14, includes preparation of a substrate,
usually in the form of a silicon wafer, 16. The starting substrate
has a silicon (110) plane parallel to the wafer surface. The
substrate may be a bulk silicon (110) wafer 18, or it may be a
silicon-on-insulator (SOI) wafer in which the top silicon has the
(110) plane parallel to the wafer surface, referred to herein as a
SOI(110) wafer 20. In the latter case, the top silicon needs to be
sufficiently thick, e.g., 0.1 micron to 1.0 micron, or more, so as
to result in a longer optical path length of the deposited SiGe
film than can be achieved by the usual deposition on a planar
substrate, i.e., the critical thickness. Note that it is only
necessary for the top silicon to have the (110) plane parallel to
the wafer surface. If a wafer-bonding technique, such as
SmartCut.RTM. or Eltran.RTM., is used, the silicon substrate
beneath the buried-oxide (BOX) may have the usual (100)
orientation. This procedure is likely less expensive than using a
substrate of silicon (110).
[0029] In preparation for forming the vertical silicon surfaces, a
photoresist, or hardmask, such as CVD deposited SiO.sub.2, pattern
is fabricated, by any state-of-the art method, 22. It is important
that the sidewalls of the photoresist or hardmask are parallel to
the silicon (111) or equivalent plane, as shown in FIG. 4, wherein
a silicon (110) substrate 24 has a mask 26 thereon. The mask
sidewalls are parallel to the silicon (111) plane. FIG. 5 depicts a
SOI (110) wafer 28, having a silicon substrate 30, a BOX layer 32
thereon, and a top silicon (110) layer 34. Mask elements 26 are
formed on top silicon layer 24, again so that the mask sidewalls
are parallel to the silicon (111). Vertical silicon surfaces with
silicon (111) sidewalls can then be fabricated by one of two
methods:
Wet Etch Only
[0030] A selective chemistry, such as a 2.38% tetramethylammonium
hydroxide (TMAH) solution, is used to etch the (110) planes while
barely etching the (111) planes, 36, to remove a portion of the
un-masked silicon (110) layer. The mask is then removed. This
solution etches silicon (110) about 23-25 times faster than it does
silicon (111), which results in a sidewall slope of .about.4% from
vertical, or a 86.degree. sidewall, as shown in FIGS. 6 and 7. The
height of the sidewall is determined by the etch time for the bulk
silicon (110) wafer, FIG. 6. In the case of SOI (110), FIG. 7, the
etch stops at the BOX, so the sidewall height is determined by the
thickness of the top silicon. The advantage of this method is that
there is little or no damage to the crystal lattice along the
sidewall surface. The disadvantage is that the sidewall angle is
directly limited by etch selectivity, i.e., the higher the
selectivity, the more vertical the sidewall.
Reactive Ion Etch (RIE) Followed by Selective Wet Etch
[0031] RIE may be used to anisotropically etch 38 most of the
un-masked silicon (110) layer, FIGS. 8 and 9. Then, with the
photoresist or hardmask pattern still intact, a short selective wet
etch may be used to etch (110) while barely etching (111), as shown
in FIGS. 10 and 11. The mask is then removed. The advantage of this
method is that a more vertical sidewall results, i.e., closer to
90.degree.. The disadvantage is that crystal damage near the
sidewall needs to be removed. This may be done by several
techniques, such as growing a thin thermal oxide and removing it
with an HF etch, and/or annealing the etched silicon substrate to
recrystallize the damaged region.
[0032] In either case, the height of the resulting sidewall may be
adjusted to meet the needs of the application. In general,
sidewalls with heights of 0.1 micron to 1.0 micron, or more, can be
fabricated. Strained SiGe films or strained SiGe/Si multilayer
structures having a germanium content from 5% to 100% can then be
epitaxially grown, 40, on these patterned, remaining silicon (110)
structures and used in photodetector fabrication, 42, as described
in by Lee et al., supra, for non-selective epitaxy, and by Tweet et
al., supra, for selective epitaxy, i.e., epitaxial growth on
silicon but not on oxide or nitride.
[0033] Thus, a method for fabrication of vertical sidewalls on
(110) silicon substrates for use in Si/SiGe photodetectors has been
disclosed. It will be appreciated that further variations and
modifications thereof may be made within the scope of the invention
as defined in the appended claims.
* * * * *