IN-SITU DOPED POLY-SIGE LPCVD PROCESS USING BCL3
FOR POST-CMOS INTEGRATION OF MEMS DEVICES
Carrie W. Low1, Matthew L. Wasilik1, Hideki Takeuchi2, Tsu-Jae King2, Roger T. Howe1
Berkeley Sensor & Actuator Center1, Department of Electrical Engineering and
Computer Sciences2, University of California at Berkeley
497 Cory Hall, Berkeley, CA 94720-1774, USA
email: [email protected]
Abstract. The depositions of in-situ boron doped poly-SiGe films
with BCl3 as the dopant source in a conventional LPCVD reactor have
been studied for MEMS applications. BCl3 doped poly-SiGe films
deposited at 400°C - 450 °C with 65% - 75% Ge content have similar
electrical and mechanical properties as B2H6 doped films. To achieve
the same level of doping, the required BCl3 gas partial pressure is two
orders of magnitude lower than that of B2H6. Poly-SiGe films with
higher boron doping have larger average compressive stresses and BCl3
doped films are more compressive than B2H6 doped films for the same
doping level. Thicker poly-SiGe film is found to have larger grain size,
lower resistivity, less compressive stress, and smaller strain gradient.
The initial BCl3 doped process shows similar or even better process
uniformity and repeatability than a mature B2H6 doped process.
INTRODUCTION
Poly-SiGe is a promising material for the modular integration of MEMS and CMOS,
due to its low process thermal budget and its good electrical and mechanical properties
[1]. For MEMS applications, the poly-SiGe structural layer should have low resistivity,
small tensile stress, and low strain gradient. Furthermore, a uniform and repeatable
process is required for mass production of integrated systems.
Conventional LPCVD reactors can be used to deposit conformal poly-SiGe at
temperature range of 400 °C – 450 °C. P-type in-situ doping is preferred as it enhances
the deposition rate and enables low resistivity films. B2H6 has been previously used for
in-situ doping of poly-SiGe MEMS structural films. However, serious problems of the
B2H6 process make it difficult to control the electrical and mechanical properties of the
deposited material. Recently, a high-throughput LPVCD process was developed using
BCl3 as the dopant source for epitaxial SiGe growth [2, 3]. Also, BCl3 is being studied for
in-situ doping of poly-Si [4].
In this work, we investigate LPCVD processes using BCl3 as the dopant source for
deposition of poly-SiGe MEMS structural films. The deposition rate, crystallinity, dopant
incorporation, resistivity, residual stress, strain gradient, as well as effects of thickness on
electrical and mechanical properties, and process uniformity are presented in this paper.
EXPERIMENTAL DETAILS
P-type in-situ doped poly-SiGe films were deposited in a Tystar hot-wall horizontal
LPCVD reactor, approximately 50 inches in length and 9 inches in diameter (Fig. 1).
This furnace is capable of both B2H6 and BCl3 doping, and it operates in the temperature
range of 350 - 450 °C, and within a pressure range of 100 - 2000 mTorr. Pure SiH4,
Si2H6 and GeH4 were used as the gaseous Si and Ge precursors. 0.1% BCl3 diluted in He
was first used as the dopant gas, so that the BCl3 partial pressure would be comparable to
that in the previously reported epitaxial growth system [2, 3]. The resistivity of the films
with the 0.1% BCl3 concentration was found to be higher than desired, so the
concentration of BCl3 was changed to 1%. SiH4 and GeH4 were introduced at the gas
ring located at the load side of the tube. The BCl3/He mixture was introduced through
the gas ring for these experiments, even though introducing BCl3 via a multi-pore
injector located at the bottom of the wafer boats is another option. Twenty-five
4”-diameter and twenty-five 6”-diameter wafers placed in open boats at the center of the
reactor were used per load.
Feedback Control
Pressure Gauge
Heater
50
1
……
Baffle
Thermocouple
MFC
Injector
Heater
B2H6 / H2
BCl3 / He
N2
MFC
MFC
MFC
MFC
MFC
SiH4
MFC
Si2H
MFC
GeH
MFC
Fig. 1 LPCVD reactor schematic
Door
Pump
Gas Ring
Heat
……
Gate
Valve
The process conditions of various depositions are summarized in Table I. Poly-SiGe
films were deposited on top of Si wafers coated with ~2 µm thick low temperature
(450 °C) LPCVD oxide. A ~5 nm thick undoped amorphous Si (a-Si) seeding layer was
deposited first using 100 sccm of Si2H6 to promote adhesion of SiGe to SiO2. The a-Si
layer was deposited at 300 mTorr for 15 minutes at various temperatures. For the
poly-SiGe deposition, the process pressure was held constant at 400 mTorr. Temperature,
BCl3 partial pressure, and deposition time were varied. The SiH4 to GeH4 gas flow ratio
was held constant in order to target a 70% Ge content. The flow rates of the two gases
however were reduced in some cases to increase the partial pressure of BCl3. The high Ge
content was chosen to ensure crystallinity for consistent resistivity measurement with
boron doping. It should be noted that the selectivity of pure Ge to Si30Ge70 for H2O2
etching is degraded to 10:1 [5]. High peroxide etching selectivity is desired for integrated
MEMS applications because the use of pure Ge as the conformal sacrificial layer
eliminates the need to passivate the underlying CMOS.
Experimental data was collected with five 4”-diameter wafers that were placed in
slots 3, 8, 13, 18, and 23 of the wafer boats, counting from the gas inlet side. A four-point
probe instrument was used to measure the sheet resistance. The films were patterned and
etched for the thickness measurement using a stylus-based profiler. Wafer curvature was
measured before and after SiGe deposition (backside SiGe film removed) to determine
the average residual stress of the film. A cantilever beam array was patterned and
released for strain gradient measurement. The strain gradient was calculated as the
reciprocal of the radius of curvature of the cantilevers with various lengths. Resistivity,
thickness, and strain gradient were measured at various points on each wafer, and average
numbers are reported in this paper. Ge content, B and Cl concentrations were determined
by secondary ion mass spectrometer with wafers at the gas inlet side of the load. The
crystallinity of selected films was determined by transmission electron microscopy.
RESULTS AND DISCUSSIONS
Overall results of average deposition rate, resistivity, residual stress, Ge content, and
B doping level, along with uniformity of the BCl3 and B2H6 doped poly-SiGe processes
are summarized in Table I. Both B2H6 and BCl3 doped SiGe films have similar Cl
concentration, all below 2×1016 cm-3, which indicates Cl incorporation is not a problem
for the BCl3 doping process.
Table I Process Summary
Deposition Recipe
Run
#
a-Si
Results
SiGe layer
Entire Load
Avg. DR
(nm/min)
DR
XW
unif.
DR
XL
unif.
Gas Inlet Side
Avg. Res.
(mΩ-cm)
Res.
XW
unif.
Res.
XL
unif.
Avg.
stress
(MPa)
Stress
stdev
Ge
cont.
B.
conc.
(cm-3)
Res.
(mΩ-cm)
Temp
(°C)
Temp
(°C)
SiH4
(sccm)
GeH4
(sccm)
BCl3
/He
(sccm)
T22
450
350
0
100
16.5
0.1%
60
7
2%
3%
70
7%
12%
-240
26
100%
8.0e18
65
T2
400
400
100
60
9.2
0.1%
60
4.4
3%
5%
673
32%
68%
-268
21
74%
3.4e18
180
T4
400
400
100
60
16.5
0.1%
60
4.5
2%
2%
175
21%
60%
-301
13
74%
1.0e19
58
T5
425
425
100
60
1.8
0.1%
60
8.6
3%
8%
893
4%
10%
-74
16
70%
1.4e17
790
T1
425
425
100
60
9.2
0.1%
60
8.9
2%
6%
412
7%
25%
-61
16
70%
5.6e17
264
T6
425
425
100
60
16.5
0.1%
60
8.8
2%
6%
199
10%
41%
-100
17
70%
5.8e18
93.6
BCl3
conc.
Time
(min)
T11
425
425
100
60
16.5
0.1%
60
8.9
2%
6%
221
12%
41%
-94
26
70%
5.2e18
100
T14
425
425
100
60
16.5
0.1%
120
8.7
1%
6%
131
13%
46%
-70
18
70%
6.2e18
54
T12
425
425
100
60
16.5
0.1%
180
8.8
2%
7%
110
13%
48%
-34
9
70%
6.0e18
46
T17
425
425
80
48
16.5
0.1%
90
7.6
2%
8%
180
12%
46%
-70
11
70%
6.9e18
77
T18
425
425
50
30
16.5
0.1%
120
5.7
2%
9%
142
13%
50%
-139
19
68%
9.3e18
58
T9
425
425
20
12
16.5
0.1%
180
3.1
NA
NA
45
NA
NA
-99
8
64%
2.2e19
32
T3
450
450
100
60
9.2
0.1%
60
13.4
2%
8%
655
5%
20%
-15
9
67%
3.3e17
472
T19
425
425
100
60
3.6
1%
60
8.5
3%
8%
69
17%
64%
-130
18
73%
2.8e19
21
T20
425
425
100
60
9.2
1%
60
8.4
3%
9%
9.3
15%
60%
NA
NA
73%
1.5e20
3.6
T23
425
425
100
60
9.2
1%
60
8.4
3%
10%
9.2
16%
63%
-157
13
NA
NA
3.5
T21
425
425
100
60
16.5
1%
60
8.5
3%
12%
2.7
9%
40%
-163
14
73%
3.1e20
1.6
R3*
400
400
100
60
60 B2H6
10%
60
7.9
2%
3%
0.87
2%
3%
-240
28
NA
NA
0.90
R1*
425
425
100
60
60 B2H6
10%
60
9.2
2%
9%
7.2
7%
86%
-108
34
72%
4.0e19
16
R2*
450
450
100
60
60 B2H6
10%
60
13.1
2%
13%
65.7
6%
66%
-36
16
NA
NA
119
*B2H6 was introduced from the pump side through a multi-pore injector. Caged boats were used. Thirty 4”-diameter and fifteen 6”-diameter wafers were used per load.
A. Deposition Rate
Comparison of Runs T5, T1, T6, T19, T20, and T21 with BCl3 partial pressure as the
only difference, we can see deposition rate is independent of BCl3 flow. Also, depositions
with identical parameters but various deposition times (Runs T11, T14, and T12) show
the same deposition rate, which suggests that there is no incubation period at the
beginning of the deposition.
An Arrhenius plot of deposition rate is shown in Fig. 2 for 100 sccm SiH4, 60 sccm
GeH4 depositions at 400 mTorr for both BCl3 and B2H6 processes. The deposition
temperatures of these runs are rather low and the processes are believed to be operating in
reaction-limited regime. The activation energies of the BCl3 and the B2H6 doped
processes are extracted to be 0.94 eV and 0.42 eV, respectively. They are of the same
order of magnitude as the undoped poly-SiGe process reported before [6]. For the
425 °C and 450 °C depositions, the deposition rates are similar for both dopant gases.
B2H6 process has a much higher deposition rate at 400 °C. The resistivity of the 400 °C
B2H6 recipe is very low, and boron incorporation for this film is estimated to be on the
order of 1×1021 cm-3. B2H6 is known to enhance deposition for poly-Si, but the
temperature effect and the doping effect cannot be separated in Fig. 2.
o
Temperature ( C)
Deposition Rate (nm/min)
20
450
425
400
400 mTorr
100 sccm SiH4
60 sccm GeH4
10
Ea= 0.42 eV
Ea= 0.94 eV
4
9.2 sccm 0.1% BCl3 -- T2, T1, T3
60 sccm 10% B2H6 -- R3, R1, R2
1.38
1.40
1.42
1.44
1.46
1.48
1.50
-1
1000/T (K )
Fig. 2 Deposition rate vs. Temperature
B. Crystallinity
Cross-sectional TEMs for films of different thicknesses are shown in Fig. 3. These
two films have exactly the same recipe except the deposition times are 1 hour and 3 hours
for the thin film and the thick film, respectively. Both films have columnar grain structure
with finer grains at the bottom. The two TEMs are shown on the same scale. As the film
gets thicker, the grains grow significantly larger.
Fig. 3 Cross-sectional TEM: (a) Run T6 - 0.5 um film; (b) Run T12 - 1.5 um film
C. Dopant incorporation
Fig. 4 plots boron concentration vs. partial pressure for all 425 °C poly-SiGe
depositions. As a comparison, data for epi-SiGe deposited at 470 °C [2, 3] is also shown.
The difference in dopant incorporation of the two systems comes from several sources:
the deposition temperature of the poly-SiGe film is much lower, the oxygen
contamination level of the poly-SiGe system is an order of magnitude higher, the
poly-SiGe film is deposited on an oxide surface, whereas epi-SiGe film is deposited on
an ultra-clean Si surface, the germanium content of the poly-SiGe film is much higher,
and the deposition rate of the poly-SiGe film is more than 10× faster than that of the
epi-SiGe. All of the above differences result in more than two orders of magnitude offset
in boron incorporation between the films.
A data point from the B2H6 process is also shown in Fig. 4. A high B2H6 partial
pressure is required to achieve similar doping levels in the film while other deposition
conditions are identical. The consumption of B2H6 is mainly due to the decomposition
inside injector rather than the disassociation on the wafer surface. Comparing the three
B2H6 depositions (R3, R1 and R2) at various temperatures provides further evidence of
the temperature instability of B2H6. Resistivity is found to be an order of magnitude
higher for an increment in temperature of 25 °C with the same B2H6 flow rate.
-3
Boron Concentration (cm )
Poly-Si28Ge72
10
20
10
19
o
425 C
400 mTorr
Epi-Si73Ge27
Poly-Si30Ge70
o
470 C
228 mTorr
10
10
o
425 C
400 mTorr
18
epi-SiGe BCl3 doping [2, 3]
poly-SiGe BCl3 doping
poly-SiGe B2H6 doping
17
10
-4
10
-3
10
-2
10
-1
10
0
10
1
Dopant Gas Partial Pressure (mTorr)
Fig. 4 Boron concentration vs. Dopant gas partial pressure
Resistivity vs. boron concentration for the BCl3 doped process is plotted in Fig. 5 for
~70% germanium content films deposited at 425 °C with similar thicknesses. As
expected, resistivity decreases linearly with boron doping. The resistivity of poly-SiGe is
more than 10× higher than that of single crystalline films due to carrier trapping at the
grain boundaries.
Resistivity (mΩ-cm)
1000
poly-SiGe
o
425 C
400 mTorr
~ 70% Ge content
~ 0.6 µm thick
100
10
poly-SiGe BCl3 doping
poly-SiGe B2H6 doping
single crystal Si [7]
single crystal Ge [7]
1
10
17
10
18
10
19
10
20
-3
Boron Concentration (cm )
Fig. 5 Resistivity vs. Boron concentration
A comparison of BCl3 runs (T1, T2, and T3) with same deposition conditions except
temperatures shows higher temperature gives lower doping level and higher resistivity.
This phenomenon could be explained by a lower boron surface reaction while the
deposition rate goes up with temperature.
Resistivity vs. film thickness is plotted in Fig. 6(a) for runs having the same
deposition conditions but different deposition times. Wafer positions are also labeled on
the graph. For wafers from the same run, higher resistivity and lower deposition rate are
observed at the gas outlet due to the gas depletion effect. Comparing wafers at the same
position from different runs, thicker films are found to have lower resistivity. This might
be a result of furnace annealing and/or crystal growth during deposition. A two-hour
425 °C anneal in N2 ambient was done for several 1-hour deposition films to discriminate
the two effects. There is no change in resistivity after the annealing. As shown in Fig. 3
earlier, thicker films have larger grains, which is consistent with this lower resistivity
observation since carrier trapping is more significant with higher grain boundary density
[8].
350
Stress (MPa)
Res. (mΩ-cm)
o
300 Gas Outlet
250
23
200
13
150
8
18
100
50
0
-20
-40
-60
-80
-100
-120
-140
3
23
18
13
8
Gas Inlet
23
18
13
3
T6 - 1 hr depo.
T14 - 2 hr depo.
T12 - 3 hr depo. 13 8 3
3
13
18
23
425 C, 400 mTorr
100 sccm SiH4
60 sccm GeH4
16.5 sccm 0.1% BCl3
8
8
(a)
3
13
18
23
18
23
8
3
Gas Inlet
Gas Outlet
(b)
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4
Thickness (µm)
Fig. 6 (a) Resistivity and (b) Stress vs. Film thickness
D. Stress
Fig. 6(b) shows the average residual stress vs. film thickness. Thicker film has less
compressive stress. Furnace annealing at 425 °C for 2 hours does not cause significant
change in residual stress. This reduction in residual stress magnitude with thickness
allows the growth of thick films without peeling off. However, the origins of the residual
stress and the reasons for its decrease with film thickness are not completely understood.
Comparing films with similar thickness and boron doping as the only process variable,
we can see that the average residual stress is more compressive as boron doping increases,
as shown in Fig. 7. This possibly indicates the boron atoms are not residing in
substitutional lattice sites, but interstitial lattice sites or grain boundaries. A data point
from the B2H6 process shows that the average stress for B2H6 doped film is less
compressive for the same level of doping. Further investigation on crystallinity might be
able to explain this phenomenon. However, 0.6 µm films are too thin for most MEMS
applications, and the 1.7 µm thick BCl3 doped film with 6.0×1018 cm-3 doping yields
reasonably low residual stress at -20.9 MPa.
0
BCl3 doping
B2H6 doping
Average Stress (MPa)
-20
-40
-60
-80
-100
-120
o
425 C, 400 mTorr
100 sccm SiH4
60 sccm GeH4
~ 0.6 µm
-140
-160
17
10
18
10
19
10
10
20
21
10
-3
B Doping (cm )
Fig. 7 Average stress vs. doping
E. Strain gradient
The cantilever arrays from the thick films are released for strain gradient
measurement. Other films with thickness less than 1 µm are too thin for reliable strain
gradient measurements. As shown in Fig. 8, the error increases as the film gets thinner.
Strain gradient is smaller for thicker film, and the reason is not well-understood. On the
other hand, the trend for strain gradient vs. resistivity is not very clear, which indicates
doping is a second order effect on strain gradient.
-1
Strain Gradient (µm )
1.1x10
-3
1.0x10
-3
9.0x10
-4
8.0x10
-4
7.0x10
-4
6.0x10
-4
5.0x10
-4
4.0x10
-4
3.0x10
-4
2.0x10
-4
1.0x10
-4
T14 - 2 hour Depo
T12 - 3 hour Depo
o
425 C, 400 mTorr
100 sccm SiH4
60 sccm GeH4
16.5 sccm 0.1% BCl3
0.0
0.9
1.0
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
Thickness (µm)
Fig. 8 Strain gradient vs. thickness
F. Process uniformity
Process uniformity summarized in Table I is being calculated as the normalized
standard deviation across the wafer and across the load. A few B2H6 doped depositions
done in the same LPCVD reactor are listed at the bottom of Table I for comparison. The
dopant gas B2H6 is introduced from the pump side via a multi-pore injector located
underneath the wafer boats to minimize the depletion effect. Caged boats were used in
the B2H6 doped process. The pump-side injector and caged boats for the B2H6 doped
process are not shown in Fig. 1. As a comparison, the BCl3 process provides similar
uniformity within a wafer and better cross load uniformity in most cases with open boats
and no gas injector.
The wafer flat region has lower resistivity for the BCl3 process, which is due to the
fact that the 4” wafers are placed below the center axis in the 9”-diamter reactor and there
is more open space for gas flow on the top of the wafer flat. As a comparison, 6” wafers,
which are nearly centered in the reactor, have better cross-wafer uniformity in resistivity.
Deposition rate and resistivity of selected 425 °C runs are plotted vs. wafer position
in Fig. 9 to demonstrate the cross load uniformity of the BCl3 process. Wafers at the gas
inlet side have higher deposition rate and lower resistivity, which suggests a gas depletion
effect. Increasing the gas flow rate along with using an injector to introduce BCl3 could
result in better cross load uniformity. A 425 °C B2H6 deposition is also shown in Fig. 9 as
a reference. Cross load variation in deposition rate and resistivity is similar to the BCl3
process, despite the fact that B2H6 is introduced via a multi-pore injector. Cross load
resistivity of the B2H6 shows the opposite trend compared to the BCl3 process, because
B2H6 was introduced from the pump side.
Res. (mΩ-cm)
100
10
1
100 sccm SiH4, 60 sccm GeH4
o
(a)
425 C, 400 mTorr
Dep. Rate (nm/nim)
0.1
10
9
T19 - 1.8 sccm 1% BCl3
T20 - 9.2 sccm 1% BCl3
T21 -16.5 sccm 1% BCl3
R1 - 60 sccm 10% B2H6
8
(b)
7
25
20
15
10
5
Gas Outlet
Wafer Position
0
Gas Inlet
Fig. 9 Deposition rate and resistivity across load
Runs T6 and T11, as well as Runs T20 and T23 have exactly the same deposition
parameters, but they were done separated by a few other depositions. The results show
that the BCl3 process is repeatable. Moreover, results of other similar depositions are
reasonably consistent, which indicates that the process is operating in a relatively stable
manner. In comparison, the decomposition of B2H6 causes periodic clogging of the
injector, which results in large run-to-run variation unless the injector is changed
approximately every 30 hours of deposition. Decomposition of BCl3 is rather slow
compared to B2H6, and injector clogging is not anticipated to be a problem. On the other
hand, introducing B2H6 through the gas ring to avoid injector clogging is not an option
since large across load variation in film thickness and resistivity [9] resulted from its
tendency to decompose at low temperature.
CONCLUSIONS
We have investigated the depositions and properties of LPCVD poly-Si30Ge70 using
BCl3 as a dopant source. The desired doping level for poly-Si30Ge70 is achieved with 1%
BCl3 concentration. The better uniformity and higher doping efficiency for the BCl3
process is clearly advantageous over the B2H6 process for poly-SiGe films, but
degradations in deposition rate and increases in residual stress are the tradeoffs. Tradeoffs
between residual stress and resistivity are also involved in optimizing boron
concentration for the BCl3 doped process. Both residual stress and resistivity can be
improved by increasing the film thickness. Thicker films also have advantages in strain
gradient and grain crystallinity, and thicker films are desired for better electro-mechanical
performance in MEMS applications. Further modifications of the LPCVD reactor will be
investigated to improve the BCl3 process.
ACKNOWLEDGEMENTS
The authors would like to thank Dr. Yasuo Kunii and his group for providing helpful
information about the epitaxial LPCVD system, UC Berkeley Microfabrication
Laboratory staff for the modification and maintenance of the equipment, and Mr. Kieran
Nunan and Dr. Tim Brosnihan of Analog Devices Inc. for their technical supports of this
research. Finally, funding for this project is provided by the DARPA NMASP program.
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