U.S. patent application number 12/469980 was filed with the patent office on 2009-09-10 for mobility enhancement in sige heterojunction bipolar transistors.
This patent application is currently assigned to INTERNATIONAL BUSINESS MACHINES CORPORATION. Invention is credited to Thomas N. Adam, Dureseti Chidambarrao.
Application Number | 20090224286 12/469980 |
Document ID | / |
Family ID | 37772509 |
Filed Date | 2009-09-10 |
United States Patent
Application |
20090224286 |
Kind Code |
A1 |
Adam; Thomas N. ; et
al. |
September 10, 2009 |
MOBILITY ENHANCEMENT IN SiGe HETEROJUNCTION BIPOLAR TRANSISTORS
Abstract
The present invention relates to a high performance
heterojunction bipolar transistor (HBT) having a base region with a
SiGe-containing layer therein. The SiGe-containing layer is not
more than about 100 nm thick and has a predetermined critical
germanium content. The SiGe-containing layer further has an average
germanium content of not less than about 80% of the predetermined
critical germanium content. The present invention also relates to a
method for enhancing carrier mobility in a HBT having a
SiGe-containing base layer, by uniformly increasing germanium
content in the base layer so that the average germanium content
therein is not less than 80% of a critical germanium content, which
is calculated based on the thickness of the base layer, provided
that the base layer is not more than 100 nm thick.
Inventors: |
Adam; Thomas N.;
(Poughkeepsie, NY) ; Chidambarrao; Dureseti;
(Weston, CT) |
Correspondence
Address: |
SCULLY, SCOTT, MURPHY & PRESSER, P.C.
400 GARDEN CITY PLAZA, Suite 300
GARDEN CITY
NY
11530
US
|
Assignee: |
INTERNATIONAL BUSINESS MACHINES
CORPORATION
Armonk
NY
|
Family ID: |
37772509 |
Appl. No.: |
12/469980 |
Filed: |
May 21, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11212187 |
Aug 26, 2005 |
7544577 |
|
|
12469980 |
|
|
|
|
Current U.S.
Class: |
257/191 ;
257/197; 257/E29.188 |
Current CPC
Class: |
H01L 29/165 20130101;
H01L 29/7378 20130101; H01L 29/161 20130101 |
Class at
Publication: |
257/191 ;
257/197; 257/E29.188 |
International
Class: |
H01L 29/737 20060101
H01L029/737 |
Claims
1. A heterojunction bipolar transistor comprising a collector
region, a base region, an extrinsic base region, and an emitter
region, wherein the base region comprises a SiGe-containing layer,
wherein the SiGe-containing layer has a thickness of not more than
about 100 nm and a predetermined critical germanium content
associated with said thickness, and wherein the SiGe-containing
layer has a germanium content profile with an average germanium
content of not less than about 80% of the predetermined critical
germanium content.
2. The heterojunction bipolar transistor of claim 1, wherein the
germanium content profile of the SiGe-containing layer is stepped
or graded, and wherein the average germanium content in the
SiGe-containing layer is determined by integrating germanium
content over the entire SiGe-containing layer, so as to determine
an integrated germanium content in the layer, and dividing the
integrated germanium content over the thickness of said layer.
3. The heterojunction bipolar transistor of claim 1, wherein the
average germanium content in the SiGe-containing layer is not less
than about 90% of the predetermined critical germanium content.
4. The heterojunction bipolar transistor of claim 1, wherein the
average germanium content in the SiGe-containing layer is not less
than about 95% of the predetermined critical germanium content.
5. The heterojunction bipolar transistor of claim 1, wherein the
average germanium content in the SiGe-containing layer is not less
than about 99% of the predetermined critical germanium content.
6. The heterojunction bipolar transistor of claim 1, wherein the
average germanium content in the SiGe-containing layer is
substantially equal to the predetermined critical germanium
content.
7. The heterojunction bipolar transistor of claim 1, wherein the
predetermined critical germanium content of the SiGe-containing
layer is not less than about 10 atomic %.
8. The heterojunction bipolar transistor of claim 1, wherein the
SiGe-containing layer having a thickness of not more than about 50
nm, and wherein the predetermined critical germanium content of the
SiGe-containing layer is not less than about 17 atomic %.
9. The heterojunction bipolar transistor of claim 1, wherein the
base region comprises two epitaxial semiconductor layers, and
wherein the SiGe-containing layer is sandwiched between said two
epitaxial semiconductor layers.
10. The heterojunction bipolar transistor of claim 9, wherein the
two epitaxial semiconductor layers both consists essentially of
silicon.
11. A heterojunction bipolar transistor comprising a
SiGe-containing base layer having a thickness of not more than
about 50 nm and a germanium content profile with an average
germanium content ranging from about 16.5 atomic % to about 17.5
atomic %.
Description
RELATED APPLICATION
[0001] This application is a divisional of U.S. patent application
Ser. No. 11/212,187, filed Aug. 26, 2005.
FIELD OF THE INVENTION
[0002] The present invention relates to a SiGe-based heterojunction
bipolar transistor, and more particularly, to a SiGe-based
heterojunction bipolar transistor that has improved performance due
to mobility enhancement. The present invention is also related to a
method of fabricating such a SiGe-based heterojunction bipolar
transistor.
BACKGROUND OF THE INVENTION
[0003] In the state-of-the-art SiGe heterojunction bipolar
transistor (HBT) devices, the base material is epitaxdially
deposited by means of chemical vapor deposition (CVD) or molecular
beam epitaxy (MBE) as front-end-of-line (FEOL) films relatively
early in the manufacturing process. This offers the possibility of
tailoring specific base profiles in both alloy and dopant and
allows pseudomorphic growth of alloys of silicon with germanium and
carbon, which can be used to improve performance of the HBT
devices.
[0004] Specifically, incorporation of substitutional germanium into
the crystal lattice of the silicon creates a compressive strain in
the material, because the Ge atom requires a larger atomic
separation. It also reduces the bandgap of the material. In some
SiGe-based heterojunction bipolar transistor (HBT) devices, the Ge
content increases abruptly to a constant value across the entire
base region (single rectangular profile) or parts thereof (stepped
profile). In a "graded" SiGe HBT device, the Ge content in the base
region is not a constant, but instead increases from a low Ge
concentration near the emitter-base junction to a high Ge
concentration deeper into the base region, thus creating a drift
field with decreasing bandgap in the direction of the electron
flow. The electrons injected from the emitter of the HBT device
face a reduced injection barrier due to the low Ge concentration at
the emitter-base junction, and then experience an accelerating
field across the base region due to the increasing Ge content
deeper into the base region. The low Ge content at the emitter-base
junction increases the electron injection into the base, thus
increasing the current gain. The Ge grading in the base region has
the effect of speeding the transport of electrons across the
device, resulting in reduced transit time through the base, which
is of particular importance in scaling the device to a higher-speed
performance. Such a desired Ge grading can be readily created by
time-dependent programming of the Ge precursor flows during the
SiGe film deposition.
[0005] However, when the strain in the pseudomorphically grown SiGe
film reaches a critical level, either due to increase of the SiGe
film thickness or increase of the Ge content, it can no longer be
contained by elastic energy stored in the distorted SiGe crystal
structure. Instead, a portion of the strain will be relaxed through
generation of misfit dislocations in the heteroepitaxial interface.
Therefore, for a SiGe film of a specific Ge content, there exists a
"critical thickness," defined as the maximum thickness for the
pseudomorphic growth of the SiGe film, below which the strain
caused by lattice mismatch between Si and Ge is contained by
elastic energy stored in crystal lattice distortion, and above
which a portion of the strain is relaxed through generation of
misfit dislocations in the heteroepitaxial interface. Similarly,
for a SiGe film of a specific thickness, there exists a "critical
Ge content," which is defined as the maximum germanium content that
can be incorporated into the pseudomorphic SiGe film, below which
the strain caused by lattice mismatch between Si and Ge is
contained by elastic energy stored in crystal lattice distortion,
and above which a portion of the strain is relaxed through
generation of misfit dislocations in the heteroepitaxial
interface.
[0006] Dislocation defects originated from strain relaxation are
electrically active and can cause increased carrier scattering,
carrier trapping, and carrier recombination. Therefore, in the
past, the Ge content and total thickness of a SiGe base layer were
carefully designed not to exceed the critical values, in order to
avoid formation of dislocation defects in the device structure.
[0007] Recent aggressive scaling of the SiGe HBT devices in both
the vertical and lateral directions has led to significant
reductions in device dimensions, including significant reduction in
the base layer thickness. Further, recent high-frequency
measurements indicate that carriers traveling through ultra-thin
base layers of high-performance HBTs (e.g., having a thickness of
not more than about 100 nm) have already reached a saturation
velocity at the today's aggressive Ge grading. In other words,
increased Ge grading in the ultra-thin base layers does not yield
further improvements in carrier velocity.
[0008] As a result, state-of-the-art SiGe-based HBT devices (see
Khater et al., "SiGe HBT Technology with fMax/fT=350/300 GHz and
Gate Delay Below 3.3 ps," IEEE Electron Devices Meeting, IEDM
Technical Digest, 13-15 Dec. 2004, pp. 247-250) have base layers
with Ge content and thickness that are well below the critical
values.
SUMMARY OF THE INVENTION
[0009] The present invention seeks to further improve performance
of currently available SiGe-based HBT devices by increasing biaxial
strain in the base region of the HBT devices, which in turn
increases carrier mobility in the base region.
[0010] The present invention discovers that although a further
increase in the Ge content of the ultra-thin base layers of the
currently available SiGe-based HBT devices does not further
increase carrier velocity, it can cause increase in biaxial strains
near the base region, i.e., increased compressive strain along the
direction parallel to the substrate surface (i.e., the lateral
direction) and increased tensile strain along the direction
perpendicular to the substrate surface (i.e., the vertical
direction), which functions to enhance mobility of holes laterally
flowing through the base region and electrons vertically traversing
the base region.
[0011] Since the carrier base-transit time depends not only on
carrier velocity, but also on carrier mobility, the carrier
base-transit time of the currently available SiGe-based HBT devices
can be further reduced by increasing the Ge content of the
ultra-thin base layers of such HBT devices to near-critical
value.
[0012] Further, the base resistance of the SiGe-based HBT devices
also depends on the carrier mobility, so an increase of the base
layer Ge content to near-critical value can also be used to reduce
the base resistance.
[0013] In one aspect, the present invention relates to a HBT device
containing a collector region, a base region, an extrinsic base
region, and an emitter region. The base region of the HBT device
comprises an ultra-thin SiGe-containing layer, i.e., having a
thickness of not more than about 100 nm. A critical germanium
content can be predetermined for such an ultra-thin SiGe-containing
layer, based on its thickness, and the SiGe-containing layer is
arranged and constructed so that it has a germanium content profile
with an average germanium content of not less than about 80% of the
predetermined critical germanium content.
[0014] Preferably, the average germanium content in the ultra-thin
SiGe-containing layer is not less than 90%, more preferably not
less than 95%, and still more preferably not less than 99%, of the
predetermined critical germanium content. Most preferably, the
average germanium content in the ultra-thin SiGe-containing layer
is substantially equal to (i.e., with .+-.0.1% difference) the
predetermined critical germanium content.
[0015] Critical germanium content for the ultra-thin
SiGe-containing layer can be readily calculated by various
conventionally known methods, as described hereinafter in greater
detail, and the present invention selects the average calculated
critical germanium content for controlling the actual germanium
content in the SiGe-containing layer, so as to minimize the risk of
dislocation generation. For example, for a SiGe-containing layer of
about 50 nm thick, the calculated critical germanium content is
between about 16 atomic % to about 18 atomic %, while the average
value of 17 atomic % is selected as the predetermined critical
germanium content in the present invention. For another example,
the calculated critical germanium content of a 100 nm thick
SiGe-containing layer is between about 9 atomic % to about 11
atomic %, and the average value of 10 atomic % is selected as the
predetermined critical germanium content for practice of the
present invention.
[0016] The ultra-thin SiGe-containing layer of the present
invention may have a flat Ge content profile (i.e., a substantially
uniform Ge content is provided across the entire SiGe-containing
layer), a multi-step Ge content profile (i.e., multiple plateaus of
uniform Ge content are provided across the entire SiGe-containing
layer), or a graded Ge content profile (i.e., the Ge content
changes in the SiGe-containing layer). The term "Ge content
profile" or "germanium content profile" as used herein refers to a
plot of germanium contents in a structure as a function of
thickness or depth in the structure. Preferably, the ultra-thin
SiGe-containing layer has a graded Ge content profile, which may
have any suitable shape, either regular or irregular. For example,
such an ultra-thin SiGe-containing layer may have a triangular Ge
content profile, or a trapezoidal Ge content profile.
[0017] For a simple (i.e., stepped) or complicated (graded)
SiGe-containing layer, the "average Ge content" is determined by
first integrating the Ge content over the entire SiGe-containing
layer, i.e., so as to determine the total or integrated Ge content
in the layer, and then dividing the integrated Ge content over the
thickness of the layer. A SiGe-based HBT is found to be stable in
further high-temperature processing steps, which are required to
finish the HBT device, as long as the average Ge content in the
base layer of such a SiGe-based HBT device remains below or equal
to a critical Ge content corresponding to the thickness of the base
layer. The critical Ge content can be readily determined, for
example, from the Matthew/Blakeslee line (MBL) that is to be
described in greater detail hereinafter. Moreover, certain
deposition techniques, such as ultra-high vacuum chemical vapor
deposition (UHVCVD) and high-temperature bake-conditioned remote
plasma-enhanced chemical vapor deposition (RPCVD), allow the
SiGe-containing base layer to be deposited with an average Ge
content that is very close (more than 95%) to the critical Ge
content.
[0018] In another aspect, the present invention relates to a
heterojunction bipolar transistor that comprises a SiGe-containing
base layer having a thickness of not more than about 50 nm and a
germanium content profile with an average germanium content ranging
from about 16.5 atomic % to about 17.5 atomic %.
[0019] In a further aspect, the present invention relates to a
method for enhancing carrier mobility in a heterojunction bipolar
transistor that has an ultra-thin SiGe-containing base layer,
without changing quasi-static drift field of the base layer. The
quasi-static drift field of a SiGe-containing layer depends on the
Ge grading rate or the shape of the Ge content profile, but not the
absolute Ge content, in the SiGe-containing layer.
[0020] Therefore, a uniform increase in the Ge content across the
ultra-thin SiGe-containing base layer can be used to reach
near-critical Ge content in the base layer, thereby maximizing the
biaxial strain and the carrier mobility in the base layer, but it
does not change the Ge grading rate or the shape of the Ge content
profile and thus maintains the same quasi-static drift field in the
base layer.
[0021] In one embodiment, the method of the present invention
comprises: [0022] measuring the thickness of the SiGe-containing
base layer; [0023] calculating a critical germanium content based
on the thickness of the SiGe-containing base layer; [0024]
measuring germanium content in the SiGe-containing base layer to
determine the germanium content profile of said SiGe-containing
base layer; and [0025] changing the germanium content profile of
the SiGe-containing base layer, by uniformly increasing the
germanium content in the SiGe-containing base layer by a sufficient
amount so that the changed germanium content profile has an average
germanium content of not less than about 80% of the calculated
critical germanium content.
[0026] In a still further aspect, the present invention provides a
method for fabricating a high performance SiGe-based HBT device,
by: [0027] determining a projected thickness and a projected
germanium profile for a SiGe-containing base layer of the
SiGe-based HBT device, wherein said projected thickness is not more
than about 100 nm; [0028] calculating a critical germanium content
based on the projected thickness and an average germanium content
based on the projected germanium profile and the critical germanium
content, wherein said average germanium content is not less than
80% of the critical germanium content; [0029] forming a collector
for the HBT device in a semiconductor substrate; [0030] depositing
over the collector a SiGe-containing base layer, which has the
projected thickness, the projected germanium profile, and the
calculated average germanium content; and [0031] forming an
extrinsic base and an emitter for the HBT device.
[0032] The projected thickness and the projected germanium profile
can be readily determined by theoretical band-structure
calculations and historical base profile scaling, which are known
in the art and therefore are not described in detail herein.
Preferably, the projected germanium profile provides for germanium
grading over the base layer, which establishes a quasi-static drift
field for accelerating carriers across the base layer.
[0033] Other aspects, features and advantages of the invention will
be more fully apparent from the ensuing disclosure and appended
claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0034] FIG. 1 shows a cross-sectional view of an exemplary prior
art SiGe-based HBT device.
[0035] FIG. 2 shows two prior art base Ge content profiles for
SiGe-based HBT devices.
[0036] FIG. 3 shows an improved base Ge content profile for a high
performance SiGe HBT device, in comparison with a prior art base Ge
content profile, according to one embodiment of the present
invention.
[0037] FIG. 4 shows another improved base Ge content profile for a
high performance SiGe HBT device, in comparison with a prior art
base Ge content profile, according to one embodiment of the present
invention.
[0038] FIG. 5 shows yet another improved base Ge content profile
for a high performance SiGe HBT device, in comparison with a prior
art base Ge content profile, according to one embodiment of the
present invention.
[0039] FIG. 6 shows a Matthews-Blakeslee curve that can be used for
determining the critical Ge content for a SiGe-containing layer
based on its thickness.
DETAILED DESCRIPTIONS OF THE INVENTION, AND PREFERRED EMBODIMENTS
THEREOF
[0040] A typical SiGe-based HBT having a deep trench isolation and
a T-shaped emitter is shown in FIG. 1 (hereinafter FIG. 1).
Specifically, FIG. 1 includes a semiconductor structure 10 that
includes at least a collector 12 that is positioned between two
shallow trench isolations regions 14R and 14L. The shallow trench
isolation region on the left hand side of the drawing, represented
by 14L, has a deep trench 16 extending from a bottom walled surface
of the shallow trench. The semiconductor structure shown in FIG. 1
also includes a first epitaxial silicon layer 18, a SiGe base 20,
and a second epitaxial silicon layer 22, which are located atop the
trench isolation regions and the collector 12. The structure shown
in FIG. 1 also includes an extrinsic base 24 and an oxide layer 26
which are patterned to expose a surface of the second epitaxial
silicon layer 22 that is located above the SiGe base 20. Nitride
spacers 28 are located on sidewalls of the oxide layer 26 and the
extrinsic base 24. A T-shaped emitter 30 is present in the
structure as shown in FIG. 1.
[0041] The HBT shown in FIG. 1A is fabricated utilizing
conventional bipolar processing techniques that are well known in
the art. For example, a heterojunction Si-containing base,
particularly comprised of SiGe, is epitaxially grown on a collector
pedestal surrounded by isolation oxide.
[0042] During epitaxial growth, sophisticated boron, germanium, and
carbon content profiles (with either exponential or polynomial
ramps) can readily be created by time dependent programming of
precursor flows. Graded germanium content profiles are desirable
for creating built-in drift fields that accelerate carriers across
the otherwise neutral base region of the transistor, thus
drastically reducing the transit time.
[0043] Although germanium content profiles and germanium grading in
conventional SiGe-based HBT devices were used to be limited by the
critical thickness and critical germanium content of the SiGe base
layer, recent aggressive scaling of the SiGe HBT devices has led to
significant reductions in device dimensions, including significant
reduction in the base layer thickness. Further, since recent
studies indicate that carriers traveling through ultra-thin base
layers have already reached a saturation velocity at the today's
moderate Ge grading and that increased Ge grading in the ultra-thin
base layers does not yield further improvements in carrier
velocity, currently available SiGe-based HBT devices have base Ge
content well below the critical value.
[0044] FIG. 2 shows two exemplary graded Ge profiles in currently
available SiGe-based HBT devices. The average Ge contents for these
two graded Ge profiles (x.sub.A1 and x.sub.A2, respectively) are
well below the critical Ge content (x.sub.c) of the ultra-thin SiGe
base layers of such devices.
[0045] The present invention discovers that although further
increase in the Ge content of the ultra-thin base layers of the
currently available SiGe-based HBT devices does not further
increase carrier velocity, it can cause increase in biaxial strains
near the base region, thereby enhancing carrier mobility in the
base region and reducing the carrier base-transit time as well as
the base resistance in the SiGe-based HBT devices.
[0046] Therefore, the present invention utilizes near-critical
average Ge content in the ultra-thin base region of a SiGe HBT, so
as to increase carrier mobility and further reduce base resistance
and carrier transit time through the neutral base region. The
method described by the present invention can be used to modify and
improve the performance of an existing SiGe HBT device, or to
fabricate a high performance SiGe HBT device ab initio.
[0047] In order to maintain the same drift fields created by the
graded Ge content profiles in the ultra-thin base region of the
existing SiGe HBT, the present invention proposes modification of
the existing SiGe HBT, by uniformly increasing the germanium
content in the SiGe base layer of the existing HBT device by a
sufficient amount so that the average germanium content of the SiGe
base layer is close to, or at least near, 80% of the critical
germanium content.
[0048] FIG. 3 shows a graded Ge content profile 14, which is
created by uniformly increasing Ge content in the graded Ge content
profile 12 of an existing SiGe HBT device having an ultra-thin base
region, according to one embodiment of the present invention. The
increased Ge content is referred to as .DELTA.x, and the average Ge
content (x.sub.A) in the new graded Ge content profile 14 is
significantly closer to the critical germanium content (x.sub.c)
than the average Ge content (not shown) in the prior art Ge content
profile 12.
[0049] Similarly, FIG. 4 shows a graded Ge content profile 24,
which is created by uniformly increasing Ge content (by .DELTA.x)
in the prior art graded Ge content profile 22 of an existing SiGe
HBT device having an ultra-thin base region, according to one
embodiment of the present invention. The average Ge content
(x.sub.A) in the new graded Ge content profile 14 is significantly
closer to the critical germanium content (x.sub.c) than the average
Ge content (not shown) in the prior art Ge content profile 12.
[0050] FIG. 5 shows another graded Ge content profile 34, which is
created by uniformly increasing Ge content by .DELTA.x in both the
ultra-thin SiGe base region of an existing SiGe HBT and the two
epitaxial silicon layers flanking the ultra-thin SiGe base (i.e.,
layers 18 and 22 of FIG. 1), according to another embodiment of the
present invention. The Ge content increases in the two epitaxial
silicon layers are indicated by ramps 36a and 36b in FIG. 5, and
the average Ge content (x.sub.A) in the new graded Ge content
profile 34 is significantly closer to the critical germanium
content (x.sub.c) than the average Ge content (not shown) in the
prior art Ge content profile 32.
[0051] Therefore, the increase in Ge content can be either limited
to only the ultra-thin SiGe base region, so that the epitaxial
silicon layers flanking such an ultra-thin SiGe base consist
essentially of silicon, with little or no Ge therein, or it can be
extended to also the flanking epitaxial silicon layers, forming an
extended SiGe epitaxial base region.
[0052] The present invention provides a method to enhance the
carrier mobility in a SiGe-based HBT device while reducing the base
resistance of the transistor. In accordance with the present
invention, carrier mobility enhancement is achieved by changing the
Ge profile in the ultra-thin base region of the HBT device, without
negatively impacting the drift fields that are typically associated
with bipolar transistors.
[0053] More particularly, the present invention provides a method
in which the Ge content profile in the ultra-thin base region of a
SiGe HBT device is changed to provide the simultaneous application
of lateral compressive and vertical tensile strain. This change in
Ge content profile as described by the present invention does not
negatively affect, or significantly alter, the quasi-static drift
field created by the amount of Ge grading in the ultra-thin base
region. By adding a uniform amount of additional Ge to the base Ge
content profile and increasing the average Ge content in the
ultra-thin base layer to near-critical value, the internal biaxial
layer strain can be greatly enhanced up to the apparent and
metastable critical point of relaxation.
[0054] The critical Ge content for a SiGe base layer of a specific
thickness can be readily determined by various methods, as
described by J. C. Bean et al., "Ge.sub.xSi.sub.1-x/Si
Strained-Layer Superlattice Grown by Molecular Beam Epitaxy," J.
VAC. SCI. TECHNOL., Vol. A2, No. 2, pp. 436-440 (1984); J. H. van
der Merwe, "Crystal Interfaces. Part I. Semi-Infinite Crystals," J.
APPL. PHYS., Vol. 34, No. 1, pp. 117-122 (1963); J. M. Matthews and
A. E. Blakeslee, "Defects in Epitaxial Multilayers I. Misfit
Dislocations in Layers," J. CRYSTAL GROWTH, Vol. 27, pp. 118-125
(1974); S. S. Iyer et al., "Heterojunction Bipolar Transistors
Using Si--Ge Alloys," IEEE TRANSACTIONS ON ELECTRON DEVICES, Vol.
36, No. 10 (October 1989); R. H. M. van der Leur et al., "Critical
Thickness for Pseudomorphic Growth of Si/Ge Alloys and
Superlattice," J. APPL. PHYS., Vol. 64, No. 5, pp. 3043-3050 (15
Sep. 1988); and D. C. Houghton et al., "Equilibrium Critical
Thickness for Si.sub.1-xGe.sub.x Strained Layers on (100) Si,"
APPL. PHYS. LETT., Vol. 56, No. 5, pp. 460-462 (29 Jan. 1990).
[0055] FIG. 6 shows a Matthews-Blakeslee curve that correlates the
critical thickness of a SiGe-containing film with the Ge content
therein, which can be readily used to determine the critical Ge
content giving a specific thickness of the SiGe film.
[0056] The critical Ge contents calculated by using different
methods may differ slightly from one another, due to the different
models used and different parameters considered. The present
invention selects the average calculated critical germanium content
for controlling the actual germanium content in the SiGe-containing
layer. For example, for a SiGe-containing layer of about 50 nm
thick, the calculated critical germanium content is between about
16 atomic % to about 18 atomic %, while the value of 17 atomic % is
selected as the predetermined critical germanium content in the
present invention. For another example, the calculated critical
germanium content of a 100 nm thick SiGe-containing layer is
between about 9 atomic % to about 11 atomic %, and the value of 10
atomic % is selected as the predetermined critical germanium
content for practice of the present invention.
[0057] Preferably, the ultra-thin SiGe base layer with the
near-critical Ge content is pseudomorphically grown by chemical
vapor deposition (CVD), with well-established process control and
proven repeatability and suitable for batch processing and
large-scale manufacturing. In addition, CVD process requires no
plasma treatment, and the substitutional Ge atoms are electrically
inactive, save for minute changes in band structure and ensuring
ultra-low contamination levels in the base layers.
[0058] While the present invention has been particularly shown and
described with respect to preferred embodiments thereof, it will be
understood by those skilled in the art that the foregoing and other
changes in forms and details may be made without departing from the
spirit and scope of the present invention. It is therefore intended
that the present invention not be limited to the exact forms and
details described and illustrated, but fall within the scope of the
appended claims.
* * * * *