Optics: measuring and testing – By polarized light examination – Of surface reflection
Reexamination Certificate
2002-01-09
2004-06-01
Font, Frank G. (Department: 2877)
Optics: measuring and testing
By polarized light examination
Of surface reflection
C356S364000
Reexamination Certificate
active
06744510
ABSTRACT:
FIELD OF THE INVENTION
The present invention is related to an ellipsometer and alignment method for the incident angle of the ellipsometer, and particularly to an ellipsometer which is capable of performing three steps and one corrective sub step and precisely and repeatedly measuring surface characteristic of a specimen by scanning the surface by varying the incident angles of the light to the specimen and detecting the reflecting light from the specimen, and precision auto alignment method for incident angle of the ellipsometer.
BACKGROUND OF THE INVENTION
It is well known that the ellipsometric method is one of the most accurate optical methods to study reflecting surfaces through the measurement of the optical constants of a material or thin-layer parameters.
However, it makes use of optical and mechanical components that are always prone to induce more or less important errors. In addition, the alignment relative to the incident and reflected light beams, always delicate, must not be altered by the rotation of some of the optical components. If this alignment is not accurate, a systematic error could be included in the ellipsometric measurements. Therefore, the incident angle of the light toward the specimen must be correctly aligned. Here, the process to correct the altered incident angle is called as “incident angle alignment”, which comes from misalignments of components including the specimen.
Recently, some investigators have shown that many cases of angle-of-incidence dependence of optical constants of specula surfaces can be attributed to azimuthal misalignments.
FIG. 1
shows a schematic block diagram of a conventional ellipsometer. As shown in the drawing, the conventional ellipsometer comprises a polarizing unit
1
for inputting a light from a light source and polarizing the light, a specimen stage
4
for supporting a specimen thereon and letting the polarized light from the polarizing unit
1
be incident on the specimen, a detecting unit
3
for detecting the reflecting light from the specimen and analyzing the detected light, and a focusing microscopy
4
for adjusting the incident angle of the polarized light from the polarizing unit
1
to the specimen.
FIG. 2
shows a detailed view of FIG.
1
. As shown in the Figure, the polarizing unit
1
includes a polarizer
1
b
polarizing the light from the light source and a modulator
1
a
modulating the polarized light from the polarizer
1
b
and outputting the modulated light to the specimen
4
a.
Also, the detecting unit
3
includes an analyzer
3
a
inputting the reflection light from the specimen
4
a
and analyzing its polarizing state, and a detector
3
b
changing the light from the analyzing light into an electrical signal. Here, the dashed line stands for an optical path when the components are in perfect alignment and the continuous lines denotes an optical path when they are in misalignment.
However, the incident angle is altered by the misalignments of the components as well as the specimen stage. Namely, according to the state of the specimen stage
4
and the position of the specimen
4
a
on the specimen stage
4
, there are h translation error and &agr; tilt angle error and &bgr; tilt angle error. The h translation error occurs when components such as the polarizing prism and the light source, are set and the &agr; tilt angle error and &bgr; tilt angle error rise, ie, when a new specimen is put on the specimen stage. The &agr; and &bgr; tilt angle errors and the h translation error of the specimen stage may arise because the dimension of the present specimen may differ from the previous.
When a specimen, for example SiO
2
(of 100 nm) Si, is on the specimen stage
4
and the incident angle of the light onto the specimen
4
a
is 70, the &agr; and &bgr; tilt angle errors and translation error occurs in the process for the measurements of the specimen and their quantities are as below.
When the analyzer prism is misaligned by 3 tilt angle error and rotates for measurement, the spot also rotates and is partially blocked by the detector's entrance aperture. The trajectory of the spot when the analyzer prism rotates is shown in
FIG. 3
, which was embodied in a Cartesian coordinate system from the electrical signals converted from the detecting light. The light is totally received or partially received from the detector's entrance aperture. Here, the dashed circle, G1, is a trajectory of the detector's entrance aperture, G2 is the trajectory of the spots and G3 is a trajectory of moving the center of the focus according to the rotation of the detector.
Thus, the signal of detector changes according to the fraction of light that is arrived at detector through the entrance aperture, as shown in FIG.
4
.
FIG. 4
shows graphs where the detector converts the reflected signal into an electrical signal when the analyzer prism is misaligned. Here, G4 is a trajectory drawn by the detected signal at the detector when the components are misaligned, G5 is a trajectory drawn by the detected signal when the components 25 are perfectly aligned, and G6 is a trajectory drawn by a fraction of light which has arrived at the detector through the entrance aperture. Here, a value of 1 for the fraction of light at the axis of the normalized signal magnitude indicates that the spot has totally arrived at detector through the entrance aperture, provided that the detector is not saturated. Thus, the signal of the detector is the product of the intensity and the fraction of light.
Meanwhile, the orientation angle error of analyzer is, 3 the film thickness is calculated as 95.31 m nm, that is a 4.79 nm thickness error. In this case, the alignment precision depends on the precision of manufacture and assemblage.
When exchanging a specimen, the tilt angle errors and translation error of the specimen stage may arise. Because the dimension of the currently putting specimen may differ from that of the previous one. The alpha tilt angle error does not alter the incident angle but both beta tilt angle error and translation error alter the incident angle. The beta tilt angle error and translation error are related by the geometry of system.
FIG. 5
shows a measurement error of the film thickness with tilt angle error of specimen stage, which is the same as the beta tilt angle error, and
FIG. 6
shows a measurement error of the film thickness with translation error of specimen stage, which is geometrically related with translation error.
The measurement error was proportional to both &bgr; tilt angle error and h translation error. 1 Å measurement error of film thickness is due to each 0.022° tilt angle error and 80 &mgr;m translation error of the specimen stage. From these results and rule of thumb, the resolutions of the specimen stage should be higher than 0.0022° and 8 &mgr;m each for tilt and translation motion, for the purpose of assuring 1 Å precision of measurement.
However, the conventional ellipsometers must use auxiliary equipment such as a focusing microscopy and 3-axis specimen stage for alignment of incident angle, which makes the system expensive. Also, it is difficult to compensate for the misalignment of incident angle, thereby it may not accurately analyze the surface characteristic of a specimen and not provide the complete information obtained therefrom either.
Also, another conventional ellipsometer using step motors may easily align the spot at the center of the detector's aperture though, but it still has a translation error of specimen stage cause incident angle error.
SUMMARY OF THE INVENTION
The main object of the present invention is to provide an ellipsometer which is capable of supplying a various incident angles of light onto a specimen and easily aligning the incident angles of light using kinematic coupling.
Another object of the present invention is to provide an ellipsometer which is capable of supplying a various incident angles of light onto a specimen and easily aligning the incident angles of light using a 3-axis specimen stage and a detector outputting a signal.
Gweon Dae Gab
Jeong Jae Wha
Park Sung Lim
Duane Morris LLP
Font Frank G.
Korea Advanced Institute of Science and Technology
Punnoose Roy M.
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