Internal stress in Cat-CVD microcrystalline Si:H thin films
Laxmi Sahu a, Nitin Kale b, Nilesh Kulkarni c, R. Pinto b, R.O. Dusane a,*, B. Schröder d
a
d
Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology, Bombay, Mumbai-400076, India
b
Department of Electrical Engineering, Indian Institute of Technology, Bombay, Mumbai-400076, India
c
Tata Institute of Fundamental Research, Colaba, Mumbai-400005, India
Department of Physics/Center for Optical Technologies and Laser Controlled processes, University of Kaiserslautern, Kaiserslautern, Germany
Abstract
Stress in the Cat-CVD Ac-Si:H films is of concern for the performance of the flexible solar cells and MEMS devices. We report the results
of our initial studies on stress determination of the HWCVD deposited Ac-Si:H films and its variation with thermal treatment. From the
analysis of the stress values of the intrinsic Ac-Si:H films it is seen (at least in the preliminary results obtained) that films deposited around
250 -C show a lower stress which could be due to the better network and optimum hydrogen content in the films. Secondly the doped films
show an order of magnitude larger internal stress compared to the intrinsic films while the grain size is comparable. However upon annealing
the stress minimizes and we get films with very low stress.
Keywords: Residual stress; Grain size; X-ray diffraction
1. Introduction
Low pressure chemical vapor deposited (LPCVD) polycrystalline silicon thin films have wide applicability in
electronic, opto-electronic and, now, in surface micromachined
(MEMS) devices [1–5]. Another important application of
these thin films is that they can act as piezoresistive sensors if
coupled with the mechanical devices due to their high gauge
factor. However there are two issues with the films deposited
by LPCVD. One is that the temperatures used for deposition
are above 600 -C, which would never allow plastic substrates,
and the second is that these films have a large amount of
residual stress. Hence there have been a few attempts to
employ the low temperature CVD processes particularly the
recently introduced hot-wire CVD (HWCVD) technique to
develop low temperature polycrystalline silicon thin films to be
used for piezoresistive sensor applications. One such attempt
has been made by Conde et al. [6].
We are in a process to develop flexible solar cells and
MEMS devices based on Ac-Si:H. In the MEMS cantilever
based devices these films will be used as both the structural
layer as well as the piezoresistive sensing element. Thus the
structural layer would largely be of intrinsic in nature while
the piezo would be a p-type film. The choice of the p-type
films is driven by the fact that the piezoresistive coefficient
given as DR/R is larger in p-type films compared to the n-type
films. However we find that there is a lot of stress in the
multilayer structures which lead to the bending or even
rolling off of the cantilevers as shown in Fig. 1a and b. In this
paper we make an attempt to evaluate and analyze the residual
stress in the Ac-Si:H films deposited by the HWCVD
technique [6 – 8] since these would be an important component of the MEMS devices as well as flexible solar cells. This
could help us to optimize the process conditions or post
deposition treatment to achieve stress-free intrinsic as well as
doped films which would give optimum device performance.
2. Experimental
Ac-Si:H films were deposited on p-type Si wafer (100)
with resistivity of 8 – 12 V cm having a thermally grown
oxide layer of thickness approximately 15 nm and also on
118
in which d o is the lattice spacing for standard Si powder
sample and d n is experimentally observed value for our
samples from XRD, E is Young’s modulus, which is taken
as 170 GPa and m is the Poisson’s ratio taken as 0.17.
2. From the sin2W method where the stress r in the
direction / is given as
r/ ¼ ð E=ð1 þ mÞÞsin2 Wðdi dn Þ=dn
where d i is the d-spacing at any inclination W and d n is
d-spacing at W = 0. In the sin2W method we do not need
the data of any standard Si. From the X-ray peak width
we have also determined the grain size in these films by
applying the Scherer’s formula after doing the corrections of instrumental contribution to peak width.
3. Results and discussion
plain Si substrates (with native oxide). The process
conditions employed during the deposition were
Filament temperature = 1900 -C,
Gas pressure = 70 mbar,
SiH4 to H2 ratio=1:25,
Substrate temperature = 150 -C to 350 -C,
Filament geometry—a multiple W shape filament with
an area of 1 cm by 2 cm.
100
400
90
350
80
300
70
60
250
50
200
40
150
30
100
20
Residual stress (MPa)
Stress measurements were carried out on different sets of
samples. In one set the intrinsic Ac-Si:H films with a
thickness around 4000 Å were deposited at different
substrate temperatures from 150 to 350 -C. The second
set of samples contained films of different thicknesses
deposited at a substrate temperature of 250 -C under similar
other process conditions and in the third set a doped film
was annealed at 400 -C for different times. X-ray measurements were done with the help of a Siemens D-500 X-ray
Diffractometer and a Philips ExPert machine. From the Xray data the strain ( can be calculated by two methods
Grain Size (nm)
Fig. 1. A cantilever which has layers with lower stress only bend (a) but
those with high stress completely roll over (b).
Residual stresses in thin films are of the following types:
(i) Epitactic stresses which arise due to misfit of inter atomic
distances. (ii) Thermal stresses arising from difference in
coefficient of thermal expansion and (iii) intrinsic stresses
due to physical, chemical and structural transformation of
film, film densification, vacancy, interstitial or doping
particle diffusion, chemical reactions and phase transformations like oxidation. Such stresses could affect the
thin film device in various ways. The entire substrate can
bend or even roll if it is too thin as shown in the Fig. 1a and
b for a MEMS cantilever. It can also sometimes lead to
peeling of the film.
In Fig. 2 we show the variation of the grain size and the
associated residual stress calculated by the h – 2h method of
the HWCVD intrinsic Ac-Si:H films as a function of
substrate temperature. The first observation from this data
is that the residual stress is in the range of a few hundreds of
MPa and shows a dip at around 250 -C substrate temperature. On the other hand the grain size variation is less
pronounced with a small dip around 250 -C. It is interesting
50
10
150
200
250
300
350
o
1. The h –2h where the stress r y in the y-direction is given as,
ry ¼ ð E=mÞðdn do Þ=do :
TS( C)
Fig. 2. Variation of the grain size (h) and the residual stress (‚) in the
HWCVD deposited Ac-Si:H films as a function of substrate temperature.
119
4000
50
3500
Grain size A°
40
3000
2500
30
2000
1500
20
1000
Residual stress (MPa)
to note that the substrate temperature (Ts) has a significant
effect on the internal stress in the films. Ts of 250 -C yields
films with minimum stress. It is known that both the
hydrogen coverage and the Ts play an important role during
the growth kinetics of these films. We think that below 250
-C hydrogen coverage is important while above this the Ts
becomes important. Over a small regime around Ts = 250 -C
the conditions are such that the grain growth is restricted
due to a lower surface mobility of the ad-atoms and yields a
smaller grain size and subsequently lower internal stress.
In Fig. 3 we show the variation of the grain size and the
residual stress in Ac-Si:H films with increasing film thickness. In this figure we have also plotted the residual stress
determined by the sin2W method from our recently acquired
Philips X-ray diffractometer. It is interesting to note that the
value of the residual stress determined by the sin2W method
is about 6 times lower than that determined by the h – 2h
method. The h2h method forces us to use the standard d o
value in calculating stress. We have observed a large
difference between the d-spacing measured at W = 0 - and
the standard d o of powder b111 Si. This large difference
gives a large stress value (in few GPa), whereas the sin2W
method considers only the measured d-spacing obtained at
different W inclinations to calculate stress. The difference
between these d-spacings is small and therefore the
calculated stress is small (at 200 MPa). This value agrees
with earlier reported values (Peiro et al., ‘‘Stress measurements in poly crystalline Silicon films grown by HWCVD’’,
Materials Letters, 30 (1997) 239– 243).
However from both the methods we see that the stress is
tensile in nature and increases with film thickness. Interestingly the grain size also increases indicating grain growth.
Such an increase in the residual stress with thickness or the
grain size has significant implications in a tandem cell with
thick Ac-Si:H intrinsic layer. Therefore it would be an
important effort to minimize this stress for thick films.
500
10
0
-1
0
1
2
3
4
5
Annealing time (Hr)
Fig. 4. Variation of the grain size (h) and the residual stress (‚) as a
function of annealing time at 400 -C.
In Fig. 4 we show the variation of the grain size and the
residual stress in the p-type Ac-Si:H film (2000A-) for the as
deposited and annealed at 400 -C for different times. It is
quite obvious that since the annealing temperature is not
very high one cannot expect grain growth, which is also
seen in the figure. However the residual stress decreases
very sharply and we get almost stress-free films after just 1 h
of annealing which does not change for further annealing
steps. Stress reduction with annealing in materials is a
commonly observed phenomenon. However in the present
case the reduction is very sharp and we think that this is may
be due to relaxations in the network structure which may be
associated with redistribution of hydrogen in the network.
However we have not been able to identify any such
changes in the hydrogen related signatures in the infrared
data.
4. Conclusion
sin2ψ method
d0 method
24
22
2000
20
1500
18
16
1000
14
Grain Size (nm)
Residual Stress (MPa)
2500
500
12
0
From the initial studies on the determination of the
residual stress in HWCVD deposited Ac-Si:H films we see
that the substrate temperature significantly affects the growth
process which results in a variation of both grain size and the
residual stress in the films. Though the minimum in both the
grain size and the residual stress observed around 250 -C is
not very easily explainable we think that understanding the
role of hydrogen is key to the optimization of the these two
parameters of the Ac-Si:H films which are important from the
point of technological applications. We believe in the stress
data obtained by sin2W measurements reflects a correct
picture for the following reasons:
10
0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2
Film Thickness (µm)
Fig. 3. Variation of the residual stress and grain size (‚) for films with
different thicknesses. The stress is determined by two methods: (a) by the
h – 2h method (?) and (b) by the sin2W method (h).
1. This method does not force us to use standard d-spacing
data, use of which gave us erroneously large values of
stress.
2. The stress values calculated by sin2W agrees with
reported data.
120
Further, annealing of the doped films yields almost stressfree material which is another useful result from the sensor
application point of view since this material has very good
piezoresistive properties which allow it to be integrated with
the MEMS devices. The observation that the stress increases
with increasing thickness is very important from the
application of Ac-Si:H films for tandem cells.
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