Hydrogen peroxide etching and stability of P-type poly-SiGe films ...

HYDROGEN PEROXIDE ETCHING AND STABILITY OF P-TYPE POLY-SIGE FILMS
'B.L. Bircumshaw, 'ML. Wasilik, 'E.B. Kim, 'ER. Su, 2H. Takeuchi, 'C.W. Low, 'G. Liu,
'.'A.P. Pismo, 2T.-.I King, and ','R.T. Howe
I
Departments of Mechanical Engineering and 'Electrical Engineering and Computer Sciences
Berkeley Sensor & Actuator Center, University of California, Berkeley, CA 94720-1774, USA
ABSTRACT
In this paper, a model is developed for the etching of as-
deposited, in-situ, boron-doped, LPCVD poly-SiGe films
in hydrogen peroxide. The model is corroborated by
etching results for poly-SiGe alloys with Ge content
between SS and 70%. The results indicate that Ge
content in the 55 to 65% range is desirable for
maintaining high selectivity with respect to poly-Ge
sacrificial layers in a peroxide etch while attaining a
polycrystalline film structure (at ahont 42S'C, 50% Ge
content poly-SiGe films are amorphous). The drift in
residual stress of poly-SiGe films at room temperature in
dry and wet ambients is also reported. Finally, the
etching of in-sifu, boron-doped, LPCVD poly-Ge films in
hydrogen peroxide is studied.
1. INTRODUCTION
Polycrystalline silicon-germanium (poly-SiGe) is a
promising material for the integration of surface
micromachined MEMS (microelectromechanical systems)
with electronics. The process flow for poly-SiGe MEMS
fabrication is similar to that for conventional poly-Si MEMS
fabrication. In this case, however, poly-SiGe replaces poly-
Si as the structural material, while poly-Ge can serve as the
sacrificial material instead of silicon dioxide. This enables
the use of hydrogen peroxide (H202) as the release etchant
instead of hydrofluoric acid (HF). Peroxide selectively
etches poly-Ge over poly-SiGe. The selectivity, though, is
dependent on doping conditions and the Ge content of the
SiCe alloy.
Conventional LPCVD (low pressure chemical vapor
deposition) techniques can be used to deposit conformal
poly-SiGe and poly-Ge films at temperatures below 425'C
[1,2]. Consequently, poly-SiGe MEMS can be micro-
machined on top of modem foundry CMOS circuitry.
Furthermore, research by Sedky et al. has shown that Al-
metallized CMOS can be annealed at temperatures up to
525T for 90 minutes without significant harm to the
underlying electronics [3].
Micromachined resonators can exhibit very high
mechanical quality factors. Indeed, unannealed poly-SiGe
MEMS resonators deposited at 425'C have been reported to
have Qs in excess of 30,000 [4]. Due to their high Q's,
MEMS resonators arc promising as replacements for discrete
filters and oscillators in wireless communications systems
[5,6]. Integrating RF MEMS directly with CMOS promises
to drop parasitic capacitances and inductances, as well as
0-7803-8265-X/04/$l7.00 02004 IEEE. 514
reduce fabricationiintegration costs and the form factor of
telecommunications devices.
After reviewing the problem with etch selectivity that
motivated this research, we discuss how Ge content was
determined in our processes. A model for the peroxide
etching of in-situ, boron-doped (p-type), LPCVD poly-SiGe
is then presented, and the etch rate of in-situ, boron-doped,
LPCVD poly-Ge is tabulated. Finally, the stability of in-sifu,
boron-doped poly-SiGe films at room temperature is studied
by measuring their average stress in dry and wet ambients.
2. SIDEWALL SPACER RF RESONATOR PROCESS
A five-mask, sidewall-spacer process was employed in an
attempt to fabricate poly-SiGe RF electrostatic resonators
with ultra-narrow gaps (Fig. 1). To remain compatible with
post-CMOS fabrication, all process temperatures were kept
at or helow 425T, with the exception of two LTO (low
temperature oxide) depositions at 450'C. Gaps between 80
and 100 nm were achieved on 2.4 pm poly-SiCe films
(Fig. 2a). After a one-hour release in 90T hydrogen
peroxide, however, the gaps widened precipitously (Fig. 2h).
EEzEz3
*540.Y.J.T0 SD~rn_, ..d,Idll
P*~GIIIP
, *,. , ., ., (.ld"".M ly.ee
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\I" (. cr sa- s"b-.c.
Id $" Lar 7e"ps.(lu. Odd. @To)
3Ln#*C?.N LilicOS.b,ed- 1
Figure I: Cross-seclions of the low-temperarure,
sidewall-spacer process employed to fabricate highfrequency
resonators. Poly-SiGe is used as the structural
material, whilepoly-Ge is used as the sacrrficial material.

By visual examination of Fig. 2, and assuming a linear
etch rate, we found that in-situ, boron-doped poly-Sio.31Geo.c9
etches at a rate of approximately 0.24 pulu (40 kmin).
This etch rate is at least 40 times higher than that reported for
undoped poly-Sio.,GeO7 films [2,7]. As will he discussed in
Section 4, the etch rate of poly-SiGe is, in fact, non-linear
From the SEMs, in-situ, boron-doped poly-Ge was found to
etch at a rate of at least 0.5 Wmin; at least 60% faster than
undoped poly-Ge [SI.
It is clear from these experimental results that boron
doping greatly enhances the etch rate of both poly-Ge and
poly-SiGe films in peroxide. This enhancement is greater for
SiGe compared to Ge, reducing the selectivity of peroxide as
a release for boron-doped SiGe structures with Ge as the
sacrificial. Moreover, the Ge content of the deposited films
was several atomic percent higher than expected. These
findings indicated the need for us to more fully characterize
our poly-SiGe and poly-Ge films.
3. CHARACTERIZING GE CONTENT OF THE
LPCVD POLY-SIGE
Precise control of Ge content is critical for poly-Sil.,Ge,
MEMS technology. Peroxide etch rate, internal stress,
resistivity, strain gradient, and deposition rate are all
dependent to some degree on Ge content.
Unless othenvise stated, all poly-SiGe films described in
this paper were deposited at 425T and 400mTorr in a
conventional, horizontal LPCVD furnace. The total gas flow
rate was fixed at 220 sccm, and the flow rates of SiH4 and
GeH4 were varied. For in-silu, boron-doped processes, the
B2H6 (10% in H2) flow rate was set at 60 sccm, while for
undoped processes, the N2 flow rate was set at 60 sccm.
Figure 2: Cross-sectional SEMs of completed slruclures
fabricated with the sidewall-spacer process @er release in
90%peroxide for: I30 seconds (a) andone hour (b).
515
To study the dependence of Ge content on process
conditions, multiple poly-SiGe films of varying Ge content
were deposited, one on top of another. After depositions,
SlMS (secondaly ion mass spectrometly) was used to
quantify the atomic composition of these multilayer stacks.
For each set of process conditions, wafer position
dependence was also examined.
It was found that the Ge content of the films varied nearly
linearly with the ratio of GeH4 to (GeH4 + SiH,) flow rates
over the 62 to 75% Ge content region (Fig. 3). The
difference in Ge content between wafer positions is attributed
to the loading effect inside the furnace.
It is important to note that the Ge contents of the films
used in this paper were determined by curve-fitting the
results in Fig. 3, not direct SlMS analysis of each film. The
direct accuracy of SIMS analysis for Ge content is about
i 2% (Ge content). The SIMS data indicated that Ge and/or
Si interdiffusion took place at the interface hetween different
Ge content poly-SiGe alloys at 425OC. This phenomenon has
been reported upon in the literature [9]. Despite the
interdiffusion, the multilayer stacks exhibited a stepped
profile.
To ensure that the multilayer Ge contents were correct,
SlMS analysis was used to measure the Ge content in single
layer poly-SiGe films deposited under the same conditions as
specific layers of the multilayer stacks. The single film and
multilayer Ge contents were compared and found to he in
agreement to within i 1.8% (typically, the agreement was
better than i 0.8%). The discrepancy between the two
measurements is attributed to fluctuations in LPCVD
conditions (flow rates, doping concentrations, temperature,
etc.) and Ge interdiffusion in the multilayer films.
4. PEROXIDE ETCHING OF POLY-SIGE
Qualitative Observations of Peroxide Etching
Peroxide etching of high Si content (i.e., < 60% Ge content)
poly-SiGe alloys results in little alteration of the original film
for etches under one hour long. Peroxide etching of high Ge
content poly-SiGe alloys results in films that steadily darken
and become more “glassy” (exhibiting reflective properties
and coloration common to thin oxides) the longer the films
are etched (Fig. 4). An HF dip makes the films duller,
indicating that the glassy substance has been removed.
Cross-sectional SEMs of a poly-SiGe alloy with
E 74%
E 72% GPE Inlet Position
70%
’ 62%
6 66%
0 64%
60%
0.35 0.40
Flow Ratio[ GeH,I(GeH,+SiH,)l
Figure3: Ge content vs. CeH4 to (GeH4 + SiHJ flow
ratio for in-situ, boron-dopedprocesses.

approximately 66% Ge content that has been etched under a
variety of conditions is presented in Fig. Sa-d. A post-
peroxide etch HF dip removes the silicon dioxide created
during the H202 etch (compare Figs. 5c and 5d). An HF dip
before the H202 etch will enhance peroxide etching (compare
Figs. 5h and 5d).
Fig. 5d appears to indicate that the H202 is attacking
along grain boundaries (spaced several 100 nm apart at this
deposition temperature). The exact mechanism for enhanced
etching along grain boundaries and along interfacial surfaces
is unknown. From SIMS analysis, however, it is known that
- 66% Ge min
mi" min min
0 30 60
-65%"
- 62% Ge
min
0
mm
30
min
60
-63%'' mill
min
min min
Figure 4: Qualitative assessment of the characteristics oj
90°C peroxide etching offilms of varying Ge content. The
left-handfigure shows the effect ofvaiying Ge content for
different etch times. The right-hand figure provides a
derailed view of haw a single film darkens as the etch
proceeds. The schematics below the figures indicate the
approximate Ge content ofthefilms and the amount oftime
they were etched in H20,. No pre- or post-etch HF dip was
performed on these samples. The poly-SiGe alloys were
depositedat 450°C and 600 mTorr.
10secHF(10:1),3OminH,O, 10secHF(10:1),30minH20,,
lOsecHF(10:l)
Figure 5: Cross-sectional SEMs ofa poly-SiGe alloy with
- 66% Ge content before and after c1 30 minute immersion
in 90°C H20, with and without a preceding HF dip. The
poly-SiGefilm was deposited at 450°C and 600 mTorr.
516
the films are degenerately boron-doped (- 3~10'~ to 5~10~'
boron atomsicm', with this variation exhibited across the
load). Thus, the grain boundary enhanced etching is likely
' due to boron segregation.
It should he noted that a separate injector is used for
introducing B2H6 into our LPCVD furnace. The high degree
of cross-load variation in the boron-doping appears to he due
to non-uniformities in the B2H6 flow profile resulting from
the condition of the current injector.
Proposed Poly-SiCe Etch Mechanism
Previous work had assumed that the etch rate of poly-SiGe in
peroxide is linear with respect to time [7,8]. Heck observed
SiOz and GeOz growth on the surface of poly-SiGe during
peroxide etching [SI. He proposed that once the
concentration of Si atoms in the surface oxide reached a
critical value (- 50%), the etch would be arrested. For Si
contents below this critical value, he predicted a linear etch
rate with time.
In this paper, we propose an etch mechanism analogous
to the model developed by Deal and Grove in 1965 to
explain the thermal oxidation of silicon [lo,] I]. Chemically,
the model can be described as follows:
2H202 + SiGe + GeO + Si0 + 2Hz0
H2O2 + Si0 -* Si02 + HzO
H2OZ + GeO a GeO, f HZO
HzO + Ge02(s) + H20 + Ge02(aq)
As a strong oxidizer, hydrogen peroxide is presumed to
oxidize Si and Ge atoms exposed on the surface of the poly-
SiGe film. The Si is converted into Si02 and the Ge into
Ge02. While SiOz is stable in H20, Ge02 is water-soluble.
Because peroxide is typically available as a saturated
solution of 30% H202 and 70% H20, the germanium dioxide
is dissolved, leaving behind a porous silicon dioxide film.
Overall, for films with high Ge content, material volume is
removed as the Ge02 is dissolved, resulting in the etching of
poly-SiGe micromachined structures.
At first, the etching is reaction-rate limited. However, as
the etch continues, the porous Si02 layer thickens, restricting
the flux of reactants to, and the flux of products from, the
SiGeSi02 interface. Hence, the etch rate is diffusion-
limited for long etch times.
Assuming first order kinetics and steady state diffusion,
the etch mechanism described by Eqn. 1 can be modeled by a
simple quadratic equation:
where I is time and X = X(t) is the thickness of removed
material (drop in step height), while A, B, and rare constants.
A thin layer of Si02 and Ge02 is present on poly-SiGe films
exposed to oxygen. The time it would take peroxide to
create and etch this oxide is represented by r.
For short etches, where (t + r)

.
In this regime, the etch rate is proportional to the square root
of time, and B is the parabolic etch rate constant.
Etch Rate Measurement Method
Single layer, in-situ, boron-doped poly-SiCe thin films of
varying Ge content were deposited on single crystal silicon
(SCS) wafers, as well as SCS wafers covered by a 1 to 3 pm
LTO film. The SiGe films were then patterned
lithographically using reactive ion etching (RIE) to create
test structures with twenty 15 pm wide steps, each spaced
15 pm apart. The initial step height was recorded using a
stylus-hued profilometer with sub-Angstrom resolution and
8 A or 0.1% step height repeatability. Due to the high index
of refraction of high Ge content films and the high IR (infra-
red) absorption of boron, the optical techniques available to
us could not he employed for step height measurements.
Each step height was an average of the twenty step
heights of the test structure. Standard deviations over the
twenty measurements ranged from I to 10 nm, and were
typically on the order of 2 to 3 nm. Each wafer was
measured in two to ten different locations.
After the initial step height measurements, the patterned
wafers were dipped in a bath of heated peroxide. The
temperature of the bath was fixed at 90°C using a hotplate,
temperature sensor @laced roughly two inches above the hot
plate), and a feedback controller. Additionally, a
thermometer was used to verify the temperature of the bath.
Due to the wafer carrier, in nearly all cases, the poly-SiGe
etch structures were exposed to H20, at a temperature
slightly above 90'C (between 90 and 96'C, accounting for
fluctuations in temperature).
After a specific timed etch, the wafers were removed and
washed thoroughly with deionized water. The average step
heights of the test structures were recorded, and then the
same wafers were placed.hack in the H202 bath for a
specified time. This process was repeated over a total etch
period of between 30 minutes and one hour.
It is known that peroxide decomposes over time [IZ]. It
is therefore critical that we rnle out the possibility of our
measurements being affected by this decomposition. The
oxidation of Ge and dissolution of Ge02 does not vary with
peroxide concentrations above 5% [13]. Moreover, it is
-
E
I 300 , , I
5 250
.c
._ al
f 200
P
150
._ c
100 1slTesl Best-Fit
._
L
a 0
50 2nd Test Best-Fit
0 500 1000
Time (sec)
1500 2000
Figure 6: Etched material vs. time for a SiGe alloy oj
70.3% Ge content.
517
known that a 30% peroxide bath heated at 90'C for 20 hours
will maintain an H202 concentration above 5% [SI. All of
our measurements were taken within 20 hours of heating a
fresh peroxide bath. However, to ensure that the peroxide
did not significantly decompose, we topped off the bath with
fresh H202 periodically. Significant etch rate fluctuations
were not observed after refreshing the bath, indicating that
the bath remained effective over the period of the tests.
Poly-SEe Etch Results
Fig. 6 is an example of one set of experimental results, along
with best-fit quadratic models (Eqn. 2). In this case, a wafer
was broken in half and tested on two different days. As can
he seen, OUT model fits the ohserved etch phenomena very
well. However, the model does deviate noticeably in the
short term. This is due to the fact that variations in testing
conditions (temperature, precise etch time, placement of the
profilometer stylus, etc.) are integrated over the etch time.
For short etches, minor fluctuations have a strong effect on
the measurements. For long etches, variations are averaged
together and tend to cancel out.
The parabolic and linear etch rate constants were
extracted using a least squares quadratic hest fit. In all cases,
the time constant r was set to zero, which is reasonable since
the initial oxide on the steps was on the order of 5 to 30 A
thick. The averaged constants are presented in Figs. 7 and 8.
1000
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100
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0.1
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2 0.07
e
h
0,001
i --
o Undoped
55% 60% 65% 70% 759
Ge Content
Figure 7:
The undnped data was extracted from 121
U 0.2
& 0.1
Parabolic etch rate constant, B vs. Ge content.
3
-0.1
55% 60% 65%
Ge Content
70% 75:
Figure 8: Linear etch rate constunt, BIA vs. Ge content.

For poly-SiGe films, the correlation factor (R2) is typically
greater than 80% above 64% Ge content, the correlation
factor is 99% or better. The correlation factor does drop off
as the Ge content falls. This is due to the fact that very little
material is etched (- I nm), resulting in significant errors in
measurement stemming from inconsistent placement of the
profilometer stylus. Overall, the most reliable data is the
parabolic etch rate constants for the high Ge content films.
It is important to realize that the data in Figs. 6-8 simply
records the drop in step height. It does not necessarily relate
to the lateral etch rate. Moreover, the conductive part of the
films is actually etched more severely than at first glance:
the step heights recorded here are for poly-SiGe covered wiih
porous SO2.
5. ETCH RATE OF POLY-GE
The etch mechanism for Poly-Ge is analogous to that of
poly-SiGe [SI:
HIOl + Ge + GeO + H1O
HZ02 + GeO + GeOz + HZO
(5)
HIO + Ge02(s) - H,O + Ge02(aq)
In the case of poly-Ge, however, porous silicon dioxide is not
formed. Hence, the etch rate is, to first order, reaction rate
limited and, therefore, linear with time.
The lateral etch rate of in-situ, boron-doped (p-type),
LPCVD poly-Ge was found by depositing a layer of poly-Ge
on SCS and LTO wafers. The poly-Ge film was then
covered with a 1 pm LTO film. Next, the LTO film was
patterned into a series of ever-widening oxide bridges, each
bridge 1 pm wider than the last. Dice were then etched for
various periods of time, and the etch length determined by
the widest oxide bridge undercut. The results were then
plotted and fit via a least squares method to determine the
etch rate. The results are tabulated in Table 1
One of the wafers tested was accidentally dipped for three
seconds in piranha (a 120T solution of roughly 56:l
H2S04:H101). The poly-Ge appeared to he slightly etched.
Despite this, we continued processing the wafer without
incident. When the etch rate of the piranha-dipped poly-Ge
was measured, however, we found it to he enhanced by about
80% over those wafers not dipped in piranha.
6. STABILITY OF POLY-SIGE
The stability of poly-SiGe films when stored at room
temperature was also studied. Roughly 1 pm of boron-doped
poly-SiGe was deposited on oxidized Si wafers at 450°C and
600 mTorr (approximately 65 to 66% Ge content). As a
control, roughly 1 pm of in-situ, phosphorous-doped (ntype),
LPCVD poly-Si was deposited on oxidized Si wafers
at 605°C.
Some of the wafers were then annealed for one hour in a
nitrogen ambient at 825°C (1050T for the poly-Si control
wafers); the other wafers remained unannealed. The
backside film of all wafers was then stripped. The wafers
were then placed in a desiccator or exposed to laboratory
ambient conditions for different lengths of time. During the
study, some wafers were oxidized or coated with a SAM
(self-assemhled monolayer). Using wafer curnature, average
518
Table I: Etch rate of doped and undoped poly-Ge, and
the corelaiion coeficient of the least squares bestrfit line.
ne undoped data is from 181.
Undoped Poly-Ge I 0.31 pdmin I NIA
Doped Poly-Ge I OS2 pdmin I 91.6%
Doped Poly-Ge After
3 Sec Dip in Piranha
0,94 pdmin 84.0%
film stress was measured regularly over a 7%-month period
(Fig. 9).
Unannealed SiGe wafers in ambient conditions exhibited
an increasingly tensile average film stress, while those in the
desiccator maintained a constant stress. We observed no
stress drift in the annealed poly-SiGe wafers, even in ambient
conditions. The unannealed and annealed poly-Si control
wafers exhibited no stress drift, regardless of ambient
conditions (wet or dry).
Low temperature (360°C) wet oxidation does not appear
to affect the stress drift in poly-SiGe. Stresses in those
wafers oxidized jumped after the oxidation, but then
continued their original trend, either drifting upward (i.e.,
unannealed wafers in the ambient) or maintaining a constant
stress (i.e., unannealed wafers in the desiccator or annealed
wafers in the ambient or desiccator). Hexamethyldisilazane
(HMDS) and the water-based SAM coating Siliclad (from
Gelest Inc., [14]) do not appear to halt the stress drift of the
unannealed SiGe films in ambient conditions.
SlMS analysis of the atomic composition of an annealed
and unannealed, 63% Ge content SiGe alloy exposed to
0 50 100 150 200
Time (days)
Figure 9: At Day 0, SiGeJ, 4, 5, and 6 were ununnealed.
while SiGe9 was annealed at 825°C in N2 for one hour. All
wafers were left out in ambient conditions on Day 0. The
events labeled in thefigure above are as follows:
I.
2.
3.
4.
5.
6.
7.
8.
Day 4: placed SiGe3 in desiccator
Day 47: took SiGe3 out of desiccator
Day 55: placed SiGe6 in desiccator
Day 70: placed SiGe4 and 5 in desiccator
Dq 147: took SiCe6 out of desiccator, oxidized
SiGe-3. 6, and 9 in steam at 360°C for 1 hr
Day 165: took SiGe5 out of desiccator
Day 174: coated SiGe3 with HMDS
Day 21 7: coated SiGe5 and 6 with Siliclad

ambient conditions is presented in Fig. IO. Two days after
deposition of the film, the wafer was broken in half; one half
of the wafer was annealed for five hours in N2 at 450"C,
while the other half remained unannealed. The SlMS
analysis was performed 29 days after the film deposition. In
the interim, between deposition and SIMS analysis, the wafer
halves were stored in laboratory ambient conditions.
The B, Ge, and Si profiles (not shown) were not
noticeably affected by the anneal. The atomic concentrations
of H and 0, on the other hand, increased more substantially
at the surface of the unannealed film compared to the
annealed film. Integrating the curves, it was found that the
unannealed sample contained 1.9 and 1.4 times more H and
0, respectively, than the annealed sample. Accounting for
residual 0 and H from the deposition process, we found that
the unannealed film absorbed about 1.8 times more 0 than H.
The higher 0 incorporation into the unannealed film suggests
that the stress drift of SiGe films is due to a water-mediated
introduction of oxygen and hydrogen into the SiGe film, not
simply water absorption. Exposing the front side of the
unannealed SiGe wafers in the stress drift experiment to HF
results in virtually no change in the average stress. This
indicates that oxidation is not the mechanism causing the
stress drift (Fig. 9) and higher 0 incorporation (Fig. IO).
7. CONCLUSIONS
In order to use poly-Ge sacrificial layers, release steps for
SiGe films with t 65% Ge content must he time-limited due
to the high etch rate of SiGe in peroxide. To maintain their
stress stability, unannealed poly-SiGe MEMS should be
encapsulated in a relatively dry ambient.
For RF MEMS resonators, the restrictions are more
severe. Most RF electrostatic resonators require gaps of
- 100 nm or smaller [IS]. Unfortunately, a IS-minute
release etch will more than double the gap width of a poly-
Sio.,lGeo.6g resonator, increasing the motional resistance of
the device more than 16-fold. However, this problem can he
resolved without significant alteration of the process flow by
increasing the Si content of the poly-SiGe films, thereby
lE121
._
1 2 1E+20 ~
I 5= 9
' 1E+19
9
-HAnnealed -0Annealed
-H Unanneeled -0 Unannealed
1E+18
0 01 02 03 04 05 06 0
DeDth luml
Figure IO: SlMS analysis of the atomic composition of an
unannealed and annealed @ve hours in N2 at 450°C). 63%
Ge content SiGe alloy 29 days afrer deposition (27 days ajier
the anneal) The Si, Ge, and B profiles (not shown) were no/
noticeably affected by the anneal. The poly-SiCe fihn wus
deposited at 425°C and 600 mTorr.
lowering the etch rate of the films in peroxide and increasing
the etch selectivity between poly-Ge and poly-SiGe.
One should note, however, that, as the Ge content
approaches 50%, the SiGe films become amorphous at a
deposition temperature of 425°C [Z]. Therefore, the optimal
Ge content which achieves both sufficient etch selectivity
and polycrystalline film structure is on the order of 55 to
65%. Finally, because the gas flow rates have been shown to
strongly affect Ge content in the poly-Si,.,Ge, LPCVD
process, precise control of gas flow rates is critical.
8. ACKNOWLEDGEMENTS
The authors would like to thank Marie Eyoum for
contributions to Fig. 10, Dimitry Kouzminov for the SlMS
measurements, and Marilyn Kushner for making mask sets.
Finally, we wish to thank the DARF'A NMASP program for
their financial support of this research.
519
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