Compact ellipsometer employing a static polarimeter
module with arrayed polarizer and wave-plate elements
Takashi Sato,1 Takeshi Araki,1 Yoshihiro Sasaki,2 Toshihide Tsuru,3 Toshiyasu Tadokoro,1and Shojiro Kawakami1,3
2

1Photonic Lattice, Inc., Aoba Incubation Square, Aoba, Aramaki, Aoba-ku, Sendai 980-0845, Japan
Graduate School of Engineering, Tohoku University, Aoba, Arakami, Aoba-ku, Sendai 980-8579, Japan
3SFAIS, Aoba Incubation Square, Aoba, Aramaki, Aoba-ku, Sendai 980-0845, Japan*Corresponding author: sato@photonic-lattice.com
Received 23 October 2006; revised 16 April 2007; accepted 22 April 2007;
posted 24 April 2007 (Doc. ID 75758); published 6 July 2007

A portable ellipsometer with a compact static polarimeter using an arrayed polarizer, an arrayed wave
plate, and a CCD image sensor is developed. A high level of repeatability at a measurement speed of 0.3 s
is demonstrated by measurement of SiO2 films ranging from 2 to 300 nm in thickness deposited on an Si
wafer. There is the potential to realize an ultracompact ellipsometer module by integrating the opticalsource and receiver, suitable for deployment in a variety of manufacturing equipment and measurement
instruments. © 2007 Optical Society of America
OCIS codes: 120.2130, 120.5410.

1. Introduction

The ellipsometer is a powerful tool for measuring thethickness and dielectric constants of thin films byanalyzing the state of polarization (SOP) of reflectedlight. Features of ellipsometry include high resolution for thin films and independent determination ofthickness and refractive index. Ellipsometry is currently used as a characterization technique for thinfilms in industries associated with semiconductors
and flat-panel displays. The majority of commerciallyavailable ellipsometers require mechanical rotationof a polarizer or a wave plate [1], or electric phasemodulation to measure the SOP of the reflected light.
This causes optical modules and equipment as awhole to be large. On the other hand, static ellipsometers, or snapshot ellipsometers, are reported [2– 6]that do not require either optical or electric modulation. Division-of-wavefront photopolarimeters [2]have a simple configuration and thus require no complicated alignment, unlike division-of-amplitude photopolarimeters [3– 6]. However, the polarizers and0003-6935/07/224963-05$15.00/0© 2007 Optical Society of America

wave plates used in Ref. 2 are discrete elements aslarge as several millimeters square.
We previously proposed and demonstrated a snapshot ellipsometer classified as a division-of-wavefront
polarimeter utilizing an arrayed micropolarizer and anarrayed micro wave plate with a CCD camera [7].
Since a relay lens was used in that experiment totransfer the image of the arrayed elements to a CCD
sensor, mechanical instability and interference causedby the lens remained as problems to solve. We report acompact static polarimeter module consisting of thearrayed optical elements attached directly to the CCD
image sensor. We also demonstrate a portable ellipsometer using the polarimeter with high repeatability.
2. Principle

Figure 1 shows a schematic illustration of the proposed polarimeter module for the ellipsometer [1].
The receiver module is composed of an array of quarter wave plates, an array of polarizers, and a CCD
image sensor that is used for analyzing the SOP. Thismodule has no moving parts, and simultaneously acquires information equivalent to that obtained byconventional rotating element instruments. The SOPof the detected light is determined from images ob1 August 2007 兾 Vol. 46, No. 22 兾 APPLIED OPTICS

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The direction of the polarization extinguished by
the polarizer and the fast axis of the wave plate bothcorrespond to the direction of the substrate grating.
Therefore, micropolarizers or wave plates with different optical axes can be monolithically integrated bypatterning the substrate grating and one deposition
process, as shown in Fig. 2. Compared with hybrid
integration of discrete elements, monolithic integration has the advantages of precision of the orientationof the optical axes, sharpness of boundaries, and thepossibility of a micron order precise unit.
3. Experiments

Fig. 1. Schematic illustration of the proposed polarimeter for theellipsometer. For simplicity, the optical axis is shown as a 22.5°
step over the range of 180°.

tained from the CCD sensor, and the film thickness
and optical constants are immediately obtained.
This type of ellipsometer is characterized by compactness and a high level of repeatability, which arerealized because there are no moving parts. Therealso is the potential to realize an ultracompact ellipsometer module integrating the optical source andreceiver, suitable for deployment in a variety of manufacturing and measuring instruments.
The key elements are the arrayed polarizers andwave plates. As shown in Fig. 2, they are composed ofphotonic crystals; that is two-dimensional periodic
structures of submicron periodicity. What we believe
to be a unique structure of the photonic crystal elements is formed by depositing dielectric thin films on
a grating substrate. Under a deposition process basedon bias sputtering, known as autocloning [8], layerswith a triangular corrugation are formed on the rectangular grating and are replicated layer by layer. Byvarying the stacking period L1, the grating pitch L2,and the number of layers, we can design polarizers
and wave plates for an arbitrary operating wavelength [9 –10].

Fig. 2. Arrayed polarizers and wave plates, which are composedof a two-dimensional periodic structure of submicron period. Arrows indicate the optical axes.
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Polarizers and wave plates were designed and fabricated arrayed as follows. The width of each array is50 ␮m and the optical axis varies by an 11.25° stepover the range of 180° (11.25° ⫽ 180°兾16). The initial
gratings are formed on a silica substrate by electron
beam lithography and reactive ion etching. The
pitches of the grating for polarizers and wave plates
are 300 and 200 nm, respectively. We selected Ta2O5and SiO2 as materials for the multilayer structure
because they are transparent at the operating wavelength of 655 nm and have a high index contrast
共2.2兾1.45兲. The triangular corrugation is formed independently of the orientation. At the wavelength of
655 nm, the transmittance and extinction ratio of thepolarizer are better than 95% and 35 dB, respectively, and the transmittance and retardation of thewave plate are better than 98% and 90 ⫹\⫺ 1° respectively.
The polarizer and wave-plate array are fixed to one
another and diced to a chip of 2.8 mm ⫻ 3.8 mm. The
optical axes of each array are varied in mutuallyorthogonal directions as illustrated. This chip is attached directly to a CCD image sensor with 1 ⫻ 106
pixels, as shown in Fig. 3. Since approximately 100
pixels of 4 ␮m square are included in a cell of a given
polarizer orientation and wave-plate orientation, thesignal-to-noise ratio is sufficient. A compact polarimeter module of 24 mm ⫻ 35 mm ⫻ 78 mm was produced including the driver and analog兾digital (A兾D)
converter electronics.
Figures 4(a) and 4(b) show images acquired fromthe CCD sensor on detecting light beams with differ-Fig. 3. (a) A compact polarimeter module, (b) and an arrayedpolarizer and arrayed wave plate attached to a CCD image sensor.

Fig. 4. Images outputted from the CCD sensor on detecting
light beams of differing SOPs, (a) ␧ ⫽ ⫺0.018, ␥ ⫽ 23.8°, and (b)␧ ⫽ ⫺0.353, ␥ ⫽ ⫺68.8°.

ent SOPs. The axes of the polarizers vary along thehorizontal direction, and those of the wave plates
along the vertical direction. The second, third, andfourth terms of Eq. (1) are derived from twodimensional Fourier transforms of the image, andthus the principal axis of the polarization ellipsoid ␥and the ellipticity ␧ are obtained:ⱍu共␾, ␪兲ⱍ2 ⫽1 1 1 ⫺ ␧2
⫹ ⫻1 ⫹ cos ␣兲 ⫻ cos共2␾ ⫺ 2␥兲
2 4 1 ⫹ ␧2 共⫹⫹

1 1 ⫺ ␧2⫻
1 ⫺ cos ␣兲 ⫻ cos共4␪ ⫺ 2␾ ⫺ 2␥兲
4 1 ⫹ ␧2 共
␧
1 ⫹ ␧2sin ␣ cos共2␪ ⫺ 2␾ ⫹ ␲兾2兲.

(1)Here, ␾ and ␪ are the orientations of the polarizer
and quarter wave plate, and ␣ is the retardation of
the quarter wave plate. Before taking the Fouriertransform, the intensity distribution of the incident
light beam is compensated so that it will be flat byusing a fitted Gaussian function, to reduce errors in
the Fourier coefficients.
To confirm the operation of the polarimeter, measurement is performed by rotating a commercial polarizer or quarter wave plate placed between the lightsource and the polarimeter. Figure 5(a) indicates therelation between the measured ␥ and the azimuthal
angle of a polarizer, while Fig. 5(b) indicates the relation between the measured ␧ and the azimuthal
angle of a quarter wave plate. Both relations agree
well with the theoretical curves, verifying sufficientaccuracy for use of the module as a polarimeter.
Figure 6 shows an ellipsometer utilizing the above
polarimeter. The angle of incidence is configurable bya pair of wedges of the desired angle. The footprint isA4 in size, and the weight is approximately 4 kg,
which is one-tenth that of currently available ellipsometers. Several silicon dioxide films on Si wafer
were measured at an angle of incidence of 60°. Lightis launched into a polarization maintaining fiber froma laser diode light source, and collimated. Figure 7
shows simulated trajectories in the ␧–␥ plane as afunction of the thickness of SiO2 films deposited on Si

Fig. 5. (a) Relation between measured ␥, the principal axis of thepolarization ellipsoid, and the azimuthal angle of a polarizer. (b)Relation between the measured ellipticity ␧ and the azimuthal
angle of a quarter wave plate.

wafer. The thickness and兾or optical constants can bederived from the measured ␧ and ␥ in the same waythat conventional ellipsometers use ⌿ and ⌬.
Figure 8 shows the correlation between the thickness measured by the ellipsometer reported here andthe thickness as measured by a spectroscopic ellip1 August 2007 兾 Vol. 46, No. 22 兾 APPLIED OPTICS

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Fig. 6. Ellipsometer utilizing an arrayed polarizer and an arrayed wave plate. The footprint is reduced to A4, and the weight is
as low as 4 kg.

someter (J. A. Woollam, M–2000). Over the range
from 2 to 300 nm, the results agree well, indicating
accurate operation of this ellipsometer. Figure 9shows repeatability for a sample with a thickness of
126 nm and an n of 1.46. One hundred measured
values obtained at an interval of 0.3 s are plotted. The
speed of measurement mostly depends on the CCDsensor and electronic interface. It will be improved toless than 0.1 s by using the appropriate sensor andinterface. The standard deviation of the thickness

Fig. 7. Simulated trajectories in the ␧–␥ plane as a function of thethickness of SiO2 film deposited on Si wafer. The angles of incidence for A and B are 60° and 75°, and the incident polarization for
A and B are 45° and 10°, respectively. (The wavelength is 655 nm
and the refractive index of SiO2 at 655 nm is 1.456.)
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APPLIED OPTICS 兾 Vol. 46, No. 22 兾 1 August 2007Fig. 8. Correlation between the thickness measured by the ellipsometer reported here and the exact thickness as measured by aspectroscopic ellipsometer.

and refractive index are as low as 0.04 nm and3 ⫻ 10⫺4, respectively.
Next, the incident conditions were changed toattain the high resolution required for thin films.
Here, the incident angle is set to 75°, which is close
to Brewster’s angle. The incident polarization is set
at 10° from the plane of incidence to compensate forthe difference between the reflection coefficients ofFig. 9. Repeatability for a sample having a thickness of 126.6 nm
and n of 1.463. One hundred measured values obtained at aninterval of 0.3 s are plotted. The standard deviation of the thickness and n are as low as 0.04 nm and 3 ⫻ 10⫺4, respectively.

data are the average values of 100 points. As shownin Fig. 10(b), the standard deviation is less than
0.03 nm. Thus a high resolution of 0.1 nm at an interval of 0.3 s is realized. These results demonstratehigh resolution, good repeatability, and a measurement speed high enough for the majority of applications.
4. Conclusions

A compact ellipsometer utilizing an arrayed polarizer, an arrayed wave plate, and a CCD image sensor
has been developed. A high level of repeatability andmeasurement speed high enough for most applications have been confirmed by measurement of SiO2
films ranging from 2 to 300 nm in thickness depositedon an Si wafer. The next stage of research will focus
on improvements to the module for use in manufacturing processes.
The authors thank M. Yamamoto of IMRAM,
Tohoku University, for his valuable advice andcomments.
References

Fig. 10. (a) Correlation between thicknesses over the range from
1.6 to 4 nm measured by the ellipsometer reported here and exact
thicknesses as measured by a spectroscopic ellipsometer. (b) One
hundred measured values obtained at an interval of 0.3 s are plotted.
The standard deviation of the thickness is as low as 0.03 nm.

the p- and s- waves. Figure 10(a) shows a good correlation between the thicknesses measured by theellipsometer of this report and the thicknesses measured by a spectroscopic ellipsometer. The plotted

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