Al-2%Si Induced Crystallization of Amorphous Silicon
Frank W. DelRio,
a,z
Joanna Lai,
b
Nicola Ferralis,
a
Tsu-Jae King Liu,
b
and Roya Maboudian
a
a
Department of Chemical Engineering, and
b
Department of Electrical Engineering and Computer Sciences,
University of California, Berkeley, California 94720, USA
Al–2% Si induced crystallization of amorphous silicon a-Si is investigated. The 2% Si is found to enhance the crystallization
process, thereby reducing the initial crystallization temperature by 50°C. The enhancement is attributed to the presence of Si
precipitates in the Al–2% Si film, which act as nucleation sites for Si grain growth. As with the Al/a-Si system, adjacent Al–2%
Si and a-Si films undergo a layer exchange during isothermal annealing, resulting in a continuous polycrystalline silicon film with
good physical and electrical properties. The activation energy for the process is 0.97 ± 0.09 eV, indicating that the crystallization
is a diffusion-limited process.
© 2007 The Electrochemical Society. DOI: 10.1149/1.2776243 All rights reserved.
Manuscript submitted May 16, 2007; revised manuscript received June 20, 2007. Available electronically August 30, 2007.
Polycrystalline silicon polysilicon thin films are used to form
transistor gate electrodes in complementary metal-oxide-
semiconductor CMOS integrated circuits
1
and structural layers in
surface-micromachined microelectromechanical systems MEMS.
2
Typically, the polysilicon films are deposited directly by low-
pressure, chemical vapor deposition LPCVD at temperatures
above 600°C using silane as the precursor gas.
3
At these tempera-
tures, though, it is difficult to use low-cost substrates, such as glass
and plastic, and to build MEMS devices on completed CMOS elec-
tronics. As a result, a significant amount of attention has been di-
rected toward the formation of polysilicon by solid-phase crystalli-
zation SPC
4
and excimer-laser annealing ELA
5
of amorphous
silicon a-Si. However, SPC can take 20 h to reach completion
even at 600°C, while ELA is a complex and expensive process with
relatively poor uniformity over large area substrates.
Aluminum-induced crystallization AIC of a-Si is a low-
temperature alternative to these methods. The crystallization tem-
perature of a-Si is reduced significantly when it is in contact with
certain metals. Hiraki proposed that the mobile free electrons in the
metal layer electronically screen the Si covalent bonds.
6
As a result
of the screening, the Si covalent bonds become weaker, which low-
ers the activation energy for silicon dissolution and promotes the
transformation from the amorphous phase to the crystalline phase.
The formation of polysilicon by AIC of nonhydrogenated a-Si relies
heavily on the layer exchange of the adjacent Si and Al films. The
layer exchange occurs during isothermal annealing at temperatures
well below the eutectic temperature for the Si-Al binary system
T
eu
= 577°C. Nast and Wenham
7
and Nast and Hartmann
8
showed
that several factors affect the exchange of the Al and Si layers and,
consequently, have an impact on the characteristics of the polysili-
con film. These factors include annealing time and temperature,
layer ratio and deposition sequence, Al grain structure, and thickness
of the oxide at the Al/a-Si interface. In this paper, we demonstrate
that the presence of Si in the Al film also has a significant influence
on the AIC process. In particular, Al–2% Si is examined, which is
commonly used in integrated circuits to minimize junction spiking,
hillocks, and electromigration.
9
Experimental
To investigate Al–2% Si induced crystallization of a-Si, a series
of samples were prepared on 6 in., p–type silicon100 substrates. At
the start, the substrates were cleaned in piranha solution
H
2
SO
4
:H
2
O
2
at 120°C and hydrofluoric acid. LPCVD was used to
grow a 2 m thick SiO
2
film, which acts as a diffusion barrier
between the Si substrate and the Al-2%Si/a-Si stack. The wafers
were immediately transferred to a Novellus m2i sputtering system
base pressure 10
-8
Torr, which is a modular tool capable of depos-
iting multiple films without intermediate exposure to the ambient
environment. Al–2% Si was deposited at a temperature and pressure
of 25°C and 4 mTorr, respectively. The thickness of the Al–2% Si
film was 500 nm. The samples were subjected to an oxygen envi-
ronment at a pressure of 0.5 mTorr for 2 min. The interfacial oxide
acts as a membrane between the Al–2% Si and a-Si layers and most
likely controls the diffusion process via Al spikes
10
through the ox-
ide layer.
11
a-Si was deposited on top of the Al layer at a tempera-
ture and pressure of 25°C and 3.6 mTorr, respectively. The thick-
ness of the a-Si film was 750 nm. The a-Si layer must be at least as
thick as the Al layer to ensure a continuous polysilicon film.
7
Fol-
lowing the film depositions, the samples were annealed in vacuum
base pressure 10
-7
Torr at temperatures in the range of 250–375°C
for 10–180 min. It is important to note that a new sample was used
for each annealing experiment i.e., samples were not subjected to
multiple temperatures. Using the same processing parameters,
samples were also fabricated using pure Al as the metal layer for
comparison.
Results and Discussion
The chemical composition and bonding configuration of the
near-surface region were examined via ex situ X-ray photoelectron
spectroscopy XPS in an ultrahigh vacuum chamber base pressure
10
-9
Torr using an Al K excitation source Omicron DAR 400
and a hemispherical analyzer Omicron EA 125. The anode voltage
and emission current for the X-ray source were 15 kV and 20 mA,
respectively. Figures 1a and b show the Si
2p
and Al
2p
regions, re-
spectively, before and after annealing at various temperatures for
60 min. At temperatures below 300°C, the Si
2p
peak related to el-
emental silicon at 99.2 eV remains unchanged. At 300°C, how-
ever, we note a decrease in the Si
2p
peak intensity, accompanied by
the emergence of two peaks in the Al
2p
region. The first peak at
72.6 eV corresponds to elemental aluminum, while the second peak
at 74.4 eV can be attributed to aluminum oxide. Peak areas were
used to compute elemental ratios after correcting for the elemental
sensitivity factors. The Al/Si ratio increases monotonically from
0.59 to 1.64 as the temperature increases from 300°C to 375°C,
which is indicative of the layer exchange process.
12
In addition, at
300°C and above, a shift in the Si
2p
peak is observed from 99.2 eV
to 98.8 eV. A shift in the binding energy of core electrons is directly
related to a change in the Fermi level i.e., change in doping state.
13
The observed decrease in the Si
2p
binding energy is consistent with
p–type doping
14
of the Si layer, attributed to the presence of Al in
the crystallized film.
To monitor the crystallization process, X-ray diffraction XRD
was performed in an automated powder X-ray diffractometer Sie-
mens D5000 with Cu K radiation = 1.5418 Å and a graphite
monochromator. The accelerating voltage and beam current were
45 kV and 35 mA, respectively. Prior to heat treatment, the XRD
spectra were dominated by the Si400 peak from the Si substrate
z
E-mail: fwdelrio@berkeley.edu
Electrochemical and Solid-State Letters, 10 11 H337-H339 2007
1099-0062/2007/1011/H337/3/$20.00 © The Electrochemical Society
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