Effects of hydrogen surface pretreatment of silicon dioxide on the nucleation and surface roughness of polycrystalline silicon films prepared by rapid thermal chemical vapor deposition

Article abstract of DOI:10.1063/1.118148

It is well known that Si surface treatment is crucial for low-temperature Si epitaxy. Although considerable work exists which is aimed at elucidating the effects of Si surface pretreatments on Si epitaxy, little is known about the effects of SiO₂ surface pretreatments for polycrystalline silicon (poly-Si) growth. We report on a study of SiO₂ surface pretreatment effects on poly-Si nucleation and film surface roughness using a low energy hydrogen ion beam (200 eV) and H₂ gas annealing (850°C) in a rapid thermal chemical vapor deposition system. In situ real-time ellipsometry was used to monitor the surfaces during pretreatment and observe the nucleation. The microstructure and surface roughness of the deposited poly-Si films are determined by analysis of in situ spectroscopic ellipsometry (SE) and atomic force microscopy (AFM) measurements. Hydrogen ion beam pretreatment was found to produce higher nuclei density and a smoother poly-Si surface than nonpretreated substrates, and the opposite was found for hydrogen gas annealing giving lower nuclei density and rougher poly-Si.

Full text of DOI:10.1063/1.118148

Effects of hydrogen surface pretreatment of silicon dioxide on the nucleation and
surface roughness of polycrystalline silicon films prepared by rapid thermal chemical
vapor deposition
Y. Z. Hu, C. Y. Zhao, C. Basa, W. X. Gao, and E. A. Irene
Citation: Applied Physics Letters 69, 485 (1996); doi: 10.1063/1.118148
View online: http://dx.doi.org/10.1063/1.118148
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Effects of hydrogen surface pretreatment of silicon dioxide
on the nucleation and surface roughness of polycrystalline silicon films
prepared by rapid thermal chemical vapor deposition
Y. Z. Hu, C. Y. Zhao, C. Basa, W. X. Gao, and E. A. Irenea)
Department of Chemistry, University of North Carolina, Chapel Hill, North Carolina 27599
͑Received 2 April 1996; accepted for publication 17 May 1996͒
It is well known that Si surface treatment is crucial for low-temperature Si epitaxy. Although
considerable work exists which is aimed at elucidating the effects of Si surface pretreatments on Si
epitaxy, little is known about the effects of SiO2 surface pretreatments for polycrystalline silicon
͑poly-Si͒ growth. We report on a study of SiO2 surface pretreatment effects on poly-Si nucleation
and film surface roughness using a low energy hydrogen ion beam ͑200 eV͒ and H2 gas annealing
͑850 °C͒ in a rapid thermal chemical vapor deposition system. In situ real-time ellipsometry was
used to monitor the surfaces during pretreatment and observe the nucleation. The microstructure and
surface roughness of the deposited poly-Si films are determined by analysis of in situ spectroscopic
ellipsometry ͑SE͒ and atomic force microscopy ͑AFM͒ measurements. Hydrogen ion beam
pretreatment was found to produce higher nuclei density and a smoother poly-Si surface than
nonpretreated substrates, and the opposite was found for hydrogen gas annealing giving lower nuclei
density and rougher poly-Si. © 1996 American Institute of Physics. ͓S0003-6951͑96͒03330-X͔
It is well known that Si surface treatment is crucial for
low-temperature Si epitaxy ͑see for example, Refs. 1–3͒.
However there are few reports on SiO2 surface pretreatment
for polycrystalline silicon ͑poly-Si͒ depositions. The use of
poly-Si films in applications involving integrated circuit and
thin-film transistor ͑TFT͒ technology for active matrix liquid
crystal displays has received much attention.4 For this appli-
cation, glass substrates must be used with limit the tempera-
ture at which the silicon layers are formed to around 620 °C,
the softening point of glass.5 Smooth poly-Si films with large
grain size are desirable, in order to achieve a high mobility
͑10–100 cm2/V s͒ TFT.6 It has been found that the grain size
of recrystallized films formed from Si2H6 chemical vapor
deposition ͑CVD͒ is larger than that formed from SiH4.7
Good quality poly-Si films have been obtained by deposition
in the amorphous phase and subsequent crystallization via
low-temperature annealing.8–11
ion beam to pretreat SiO2 covered Si substrates before the
CVD from Si2H6. In situ ellipsometry was used for real time
observation of the surface change during pretreatment and
nucleation. In situ spectroscopic ellipsometry ͑SE͒ and
atomic force microscopy ͑AFM͒ were used to characterize
the film roughness and microstructure. For comparison with
the ion beam pretreatments, results of H2 gas pretreatment at
2 Torr and 850 °C are also presented.
Surface pretreatments and poly-Si film depositions were
performed in situ in a custom built rapid thermal processing
system consisting of a water cooled, turbopumped vacuum
chamber ͑base pressure of 10Ϫ8 Torr͒ with a rotating ana-
lyzer in situ ellipsometer that was described in greater detail
elsewhere.12,13 The samples are heated from the back side,
and the ellipsometric measurements are made on the sample
front side. The SE measurements were made with photon
energies between 2.0 and 5.0 eV and at room temperature
after cooling the sample. The real time single wavelength
In this letter, we report on the surface roughness and
microstructure of poly-Si films using a low energy hydrogen
FIG. 1. AFM results of the poly-Si film surface deposited on in situ pretreated SiO2 substrates using Si2H6 ͑5% in He͒ at 0.02 Torr and 650 °C. rms: root mean
square, Zmax : maximum height, Dg : average grain size. ͑a͒ Nonpretreated sample, rmsϭ22 nm, Zmaxϭ140 nm, Dgϭ270 nm. ͑b͒ H 2 gas bake pretreated
sample, rmsϭ33 nm, Zmaxϭ190 nm, Dgϭ440 nm. ͑c͒ H ion beam pretreated sample, rmsϭ6 nm, Zmaxϭ58 nm, Dgϭ180 n
a͒
Electronic mail: gene-irene@unc.edum.
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Appl. Phys. Lett. 69 (4), 22 July 1996
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© 1996 American Institute of Physics
485
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larly sensitive to the semiconductor surface condition, and
that a maximum value for the peaks indicates the best sur-
face. We found featureless ⑀2 spectra from the poly-Si film
deposited on the substrate pretreated by H2 bake which is
indicative of a rough surface while the film deposited on
Hϩ ion beam pretreated substrate shows the Si interband
peaks indicating a higher quality and smoother film. The SE
data were analyzed using the well-established linear regres-
sion technique employing the Bruggeman effective medium
approximation.16 Previously, we have found that the best-fit
optical model for poly-Si film deposited on SiO2 is a two
layer model.17 The top layer is a roughness layer and the
bottom layer is the poly-Si film. The best-fit models reveal
that the films are mixture of amorphous and crystalline
phases, and that the films deposited from H2 gas baked
samples have more voids, less c-Si component and a much
thicker roughness layer ͑47Ϯ5 nm͒ than that formed from
ion beam pretreated samples ͑3.7Ϯ0.3 nm͒. A series of pre-
treatment experiments with different ion energies and ion
doses have also been done and show that reducing the ion
doses from 9.8ϫ1016 to 2.4ϫ1016 ions/cm2 increases poly-Si
roughness from 4 to 8 nm, and increasing ion energies from
200 to 1000 eV the roughness only changes by a fraction of
a nm. In order to confirm the SE results, the film surfaces
were examined using AFM. Figure 1 displays AFM images
with the root mean square ͑rms͒ and maximum roughness
values of the surface roughness. The trends of the values are
similar to the results from SE models.
In order to understand the effect of the surface pretreat-
ments on the deposited poly-Si film structures, we monitored
both the surface changes with a real-time SWE during pre-
treatment, and then with SE after pretreatments. Real-time
SWE at 340 nm did not yield observable changes in ␺ and ⌬
for both the 5 min H2 gas and Hϩ ion beam pretreatments. In
situ SE results before and after the pretreatments show only
small changes. AFM revealed no changes after the substrate
pretreatments. We have found from our previous thin-film
nucleation studies11,12,17,18 that nucleation is very sensitive to
the substrate surface conditions. Therefore, a series of nucle-
ation experiments after in situ surface pretreatments were
performed. The ␺-time curves in Fig. 2 show that the time
prior to observable nucleation, the incubation time (tinc), for
FIG. 2. Initial evolution of ␺ as a function of time from real time single
wavelength ellipsometry ͑␭ϭ340 nm͒ measurements at 650 °C and 0.02
Torr of Si2H6 ͑5% in He͒ for different surface pretreated samples.
ellipsometry ͑SWE͒ was carried out at 340 nm ͑3.65 eV͒,
since this wavelength is relatively insensitive to temperature
for Si.14
The samples were silicon wafers, with about 50 nm thick
thermally grown SiO2. The sample pretreatments used in this
study consisted of a H2 bake ͑2 Torr and 850 °C͒ and Hϩ ion
beam bombardment with low ion energies ͑100–1000 eV͒ at
650 °C for 5 min. Hydrogen ion beam pretreatment was per-
formed using a 3 cm Kaufman ion beam source operated at
1–10
mA,
with
calibrated
doses
of
about
1016–1017 ions/cm2 for 5 min. The ion beam was directed
normal to the substrate. Substrates were heated using quartz-
halogen lamps. A shutter located in front of the substrate is
used during Hϩ ion beam stabilization and during rapid ther-
mal CVD ͑RTCVD͒. The poly-Si films were deposited on
the pretreated samples using Si2H6 ͑5%, diluted with He͒ at a
pressure of 0.02 Torr and 650 °C in the RTCVD system.
Immediately after pretreatment the substrates were subjected
to Si deposition by RTCVD using Si2H6 under the same
conditions ͑temperature of 650 °C and pressure of 0.02 Torr
for 5 min͒. The SE spectra were obtained at room tempera-
ture after cooling the samples, because of the unavailability
of reliable optical data for all the components present at the
higher temperatures. Aspnes15 has shown that the interband
peaks in the ⑀2 spectrum at about 3.4 and 4.3 eV are particu-
FIG. 3. AFM images of initial nucleation stage of RTCVD Si nuclei on different pretreated SiO2 surfaces. In order to make a good comparison the CVD were
stopped at the certain time corresponding to ␺ϭ65° for all samples from Fig. 2. t: deposition time, n: nuclei density, r: mean nuclei radius, h: average nuclei
height. ͑a͒ Nonpretreated sample, tϭ2 min, nϭ4.2ϫ109 cm Ϫ2, rϭ67 nm, hϭ20 nm. ͑b͒ H2 gas pretreated sample, tϭ2.5 min, nϭ1.5ϫ109 cmϪ2
rϭ91 nm, hϭ33 nm. ͑c͒ H ion beam pretreated sample, tϭ1 min, nϭ7.0ϫ1010 cm Ϫ2, rϭ12 nm, hϭ9.3 nm.
,
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486 Appl. Phys. Lett., Vol. 69, No. 4, 22 July 1996 Hu et al.
131.104.248.64 On: Wed, 10 Dec 2014 15:53:14

TABLE I. A comparison of effect of different SiO2 surface in situ pretreat-
ments on the nucleation and poly-Si film surface roughness using RTCVD
from Si2H6.
nuclei layer. The difference of pretreatment mechanism be-
tween H2 gas annealing and Hϩ is not completely under-
stood. However, we believe from the SE and AFM results
͑i.e., the inability to observe significant changes͒ that both
pretreatments before deposition only strongly influence the
top surfaces.
The authors would like to thank Dr. J. Boland for helpful
discussions. This work was supported in part by the National
Science Foundation, NSF, Engineering Research Center at
North Carolina State University, and Northern Telecom
Electronics, NTE, Ltd.
H2 gas annealing H ϩ ion beam
Parameters of
Non-
film and nucleation
pretreatment
pretreatment
pretreatment
Incubation time ͑min͒
Nucleation point
1.2
A
1.7
B
0.7
C
Nuclei density ͑cmϪ2
Nuclei radius ͑nm͒
Nuclei height ͑nm͒
Film roughness ͑nm͒
SE ͑model͒
͒
1.5ϫ109
91
7.0ϫ1010
12
4.2ϫ109
67
20
33
9.3
22
22
47
33
4
6
AFM ͑rms͒
Grain size ͑nm͒
270
440
180
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