Rotating-compensator multichannel ellipsometry: Applications for real time
Stokes vector spectroscopy of thin film growth
Joungchel Lee, P. I. Rovira, Ilsin An,
a)
and R. W. Collins
b)
Materials Research Laboratory and The Department of Physics, The Pennsylvania State University,
University Park, Pennsylvania 16802
͑Received 3 November 1997; accepted for publication 13 January 1998͒
A multichannel spectroscopic ellipsometer based on the rotating-compensator principle was
developed and applied to measure the time evolution of spectra ͑1.5–4.0 eV͒ in the normalized
Stokes vector of the light beam reflected from the surface of a growing film. With this instrument,
a time resolution of 32 ms for full spectra is possible. Several advantages of the
rotating-compensator multichannel ellipsometer design over the simpler rotating-polarizer design
are demonstrated here. These include the ability to: ͑i͒ determine the sign of the p-s wave
phase-shift difference ⌬, ͑ii͒ obtain accurate ⌬ values for low ellipticity polarization states, and ͑iii͒
deduce spectra in the degree of polarization of the light beam reflected from the sample. We have
demonstrated the use of the latter spectra to characterize instrument errors such as stray light inside
the spectrograph attached to the multichannel detector. The degree of polarization of the reflected
beam has also been applied to characterize the time evolution of light scattering during the
nucleation of thin film diamond by plasma-enhanced chemical vapor deposition, as well as the time
evolution of thickness nonuniformities over the probed area of the growing diamond film. In this
article, a detailed description of calibration and data reduction for the new instrument is provided.
Future applications of this instrument are expected for real time characterization of film growth and
etching on patterned surfaces for microelectronics and on thick transparent substrates for large area
displays and photovoltaics. © 1998 American Institute of Physics. ͓S0034-6748͑98͒03004-4͔
I. INTRODUCTION
The first rapid-scanning spectroscopic ellipsometer with
sufficient speed and sensitivity to perform real time measure-
ments of film growth on the monolayer scale was described
by Muller and Farmer in 1984.
1
This instrument operated on
the self-compensating principle with Faraday cell modula-
tors. The combination of a Xe source and rotating interfer-
ence filter served as a rapid-scanning monochromator, and a
photomultiplier tube served as the detector. Successive
‘‘nulling angles’’ were obtained with an acquisition time of
t
a
ϳ1 ms, and related to the wavelength scan through the
output of an encoder on the rotating filter. With this instru-
ment, spectra consisting of ϳ400 pairs of ellipsometry
angles ͑
, ⌬͒ between 400 and 700 nm could be measured
with a repetition time of t
r
ϳ3 s. The orders of magnitude
difference between t
a
and t
r
in this case is a characteristic of
serial-scanning spectroscopic ellipsometers. The serial scan-
ning approach leads to a poorer time resolution ͑Ͼ1 s, in this
case͒ and signal-to-noise ratio in comparison to an equiva-
lent parallel detection approach. The advantages of the self-
compensating instrument, however, are the same as those for
a manual null ellipsometer: uniform sensitivity for all reflect-
ing surfaces and the capability of measuring ⌬ over its full
range (Ϫ180°Ͻ⌬р180°) without ambiguity.
Performance specifications of an alternative instrument
design for high-speed spectroscopic ellipsometry ͑SE͒ were
reported in 1990 and 1991.
2–5
This instrument operated on
the rotating-polarizer principle, and its design was based on
the observation that the readout time t
s
of multichannel de-
tectors (t
s
ϳ2–10 ms) is shorter than the optical period T
P
͑one-half the mechanical period͒ accessible to a rotating po-
larizer ͑T
P
ϳ15–50 ms for rotation frequencies of 10–35
Hz͒.
6,7
In this instrument, white light from a Xe source is
linearly polarized by the rotating polarizer and reflected from
the surface. The detection system, mounted after a fixed ana-
lyzer, consists of a spectrograph and a 1024-pixel linear sili-
con photodiode array, which is grouped by 8 to yield 128
spectral points. Such multichannel detectors are normally op-
erated in an integrating mode.
6
As a result, the minimum
acquisition and repetition times t
a
and t
r
for ellipsometric
spectra are both essentially equal to T
P
. This implies that all
pixel groups are measuring the photon flux virtually at all
times, thus yielding fully parallel operation. Photons are lost,
however, only during the brief interval when the integrated
irradiance at each pixel group is read out. For the instrument
of Ref. 2, this interval is ϳ35
s over a 10 ms integration
time; thus, only 0.3% of the photons are lost.
3,4
A third type of instrument for high-speed SE based on
the phase-modulation principle was first described in 1993.
8,9
In this instrument, white light from a Xe source is first po-
larized and then phase modulated by a piezobirefringent el-
ement. As in the rotating-polarizer instrument, the detection
system consists of a spectrograph and photodiode array ͑40
pixels in Refs. 8 and 9͒. Because the array scanning time is at
a͒
Permanent address: Department of Physics, Han Yang University, Ansan,
Seoul, Korea.
b͒
Electronic mail: rwc6@psu.edu
REVIEW OF SCIENTIFIC INSTRUMENTS VOLUME 69, NUMBER 4 APRIL 1998
18000034-6748/98/69(4)/1800/11/$15.00 © 1998 American Institute of Physics