Optics: measuring and testing – By light interference – Having light beams of different frequencies
Reexamination Certificate
2002-10-30
2004-10-19
Turner, Samuel A. (Department: 2877)
Optics: measuring and testing
By light interference
Having light beams of different frequencies
C356S493000, C356S500000
Reexamination Certificate
active
06806960
ABSTRACT:
BACKGROUND
Interferometers commonly use polarization encoding to distinguish reference beams from measurement beams. In a plane-mirror interferometer
100
illustrated in
FIG. 1
, for example, an input beam IN contains two linearly polarized components having orthogonal linear polarizations. A polarizing beam splitter
110
in interferometer
100
separates the two components creating a reference beam and a measurement beam.
In
FIG. 1
, polarizing beam splitter
110
reflects the component corresponding to the reference beam. The reference beam thus travels down a path R
1
through a quarter-wave plate
120
to a reference mirror
130
. Reference mirror
130
has a fixed mounting relative to polarizing beam splitter
110
and is aligned perpendicular to path R
1
so that the reference beam reflects from a reference mirror
130
and travels back through quarter-wave plate
120
along path R
1
. Passing twice through quarter-wave plate
120
effectively rotates the polarization of the reference beam by 90° so that the reference beam returning on path R
1
passes through polarizing beam splitter
110
and enters a cube corner reflector
140
along a path R
2
.
Cube corner reflector
140
reflects the reference beam onto an offset path R
3
, and the reference beam traverses polarizing beam splitter
110
directly to a collinear path R
4
. The reference beam then continues along a path R
4
through quarter-wave plate
120
before again reflecting from reference mirror
130
and returning through quarter-wave plate
120
along path R
4
. The second pair of trips through quarter-wave plate
120
changes the polarization of the reference beam, so that polarizing beam splitter
110
reflects the reference beam from path R
4
onto an output path ROUT.
Polarizing beam splitter
110
of
FIG. 1
transmits the input polarization component corresponding to the measurement beam so that the measurement beam travels along a path M
1
through a quarter-wave plate
150
to a measurement mirror
160
. Measurement mirror
160
is on an object such as a translation stage in processing equipment for integrated circuit fabrication. Measurement mirror
160
is ideally perpendicular to path M
1
, but generally, measurement mirror
160
may have an angular orientation that is subject to variations as the object moves.
FIG. 1
shows a configuration where measurement mirror
160
has a non-zero yaw angle relative to path M
1
. As a result, the measurement beam reflected from measurement mirror
160
returns along a path M
1
′ that forms a non-zero angle (i.e., twice the yaw angle) with path M
1
.
The measurement beam, which passes twice through quarter-wave plate
150
, has its linear polarization rotated by 90°, so that polarizing beam splitter
110
reflects the measurement beam from path M
1
′ to a path M
2
into cube corner
140
. From cube corner
140
, the measurement beam travels path M
3
, reflects in polarizing beam splitter
110
to a path M
4
through quarter-wave plate
150
to measurement reflector
160
. The measurement beam then returns from measurement reflector along a path M
4
′ through quarter-wave plate
150
. Path M
4
′ forms an angle with path M
4
according to the orientation of measurement mirror
160
and is parallel to path M
1
. Polarizing beam splitter
110
transmits the measurement beam from path M
4
′ to an output path MOUT.
Interferometer electronics (not shown) can analyze phase information from a combination of the reference and measurement beams to measure movement of measurement mirror
160
. In particular, a merged beam resulting from merging the reference and measurement beams can be made to interfere to produce a measurement signal. When measurement mirror
160
is moving along the direction of the measurement beam, each reflection of the measurement beam from measurement mirror
160
causes a Doppler shift in the frequency of the measurement beam and a corresponding change in the beat frequency of the merged beam. In a DC interferometer where the measurement and reference beams initially have the same frequency, the beat frequency of the merged beam corresponds to the Doppler shift. In an AC interferometer where the measurement and reference beams initially have slightly different frequencies, the change in the beat frequency indicates the Doppler shift.
AC interferometers typically use an input beam having orthogonal, linear polarization components with slightly different frequencies. Imperfect polarization separation of the frequency components of the input beam can cause cyclic errors in the Doppler shift measurement. If the reference beam, for example, contains some light at the frequency intended for the measurement beam, the reference beam by itself gives rise to an error signal having the beat frequency depending on the frequencies of the input components. If the error signal becomes too large when compared to the measurement signal, accurate measurements become difficult. Accordingly, maximizing the measurement signal is important for accurate measurements.
Maximizing the measurement signal for AC or DC interferometers requires efficient combination of the measurement and reference beams, and combination of the reference and measurement beams is most efficient when the output paths ROUT and MOUT for the reference and measurement beams are collinear. Achieving collinear output beams from interferometer
100
depends on proper alignment of reference mirror
130
and measurement mirror
160
.
In the properly aligned configuration, measurement mirror
160
is perpendicular to path M
1
, and reflected paths M
1
′ and M
4
′ are collinear with incident paths M
1
and M
4
. As a result, measurement paths M
2
, M
3
, and MOUT respectively coincide with reference paths R
2
, R
3
, and ROUT when measurement mirror
160
is ideally aligned. If measurement mirror
160
is out of alignment, paths M
1
and M
1
′ form an angle that depends on the misalignment of measurement mirror
160
, and the reference and measurement paths are skewed relative to each other. The angular difference or separation between the measurement and reference paths continues until the second reflection from measurement mirror
160
. After the second reflections, measurement path M
4
′ and output path MOUT become parallel to the output path ROUT for the reference beam. However, the angular variation of measurement mirror
160
still displaces the measurement beam output path MOUT relative to the reference beam output path ROUT. This phenomenon is commonly referred to as beam walk-off.
When the beam walk-off is negligible compared to the diameters for the reference and measurement beams, the merged beam provides a strong measurement signal. However, a misalignment of measurement mirror
160
by about 0.001 radians or more in concert with a large distance (on the order of 0.5 meters or more) between beam splitter
110
and mirror
160
in some precision interferometers causes a walk-off that is a significant fraction of the beam diameters. (The walk-off in a plane-mirror interferometer is generally about 4L&agr;, where L is the distance between the interferometer and measurement mirror
160
and &agr; is the angular misalignment in radians of measurement mirror
160
.) The resulting decrease in the overlapped area of the measurement and reference beams causes a significant drop off in the measurement signal, making the cyclic error signal more significant and making accurate measurements difficult.
Another problem arising from beam walk-off is the dynamic range of measurement signal during operation of interferometer
100
. In particular, the light intensity in the overlapped beam can vary from a best case having a maximum overlap to a worst-case have a relatively small overlap. The intensity of the measurement signal thus depends on the alignment of measurement mirror
160
, and the alignment changes during operation of interferometer
100
, particularly when the object being measured moves. The input beam must have sufficient power to provide
Bagwell Kerry D.
Bockman John J.
Felix Greg C.
Ray Alan B.
Agilent Technologies , Inc
Turner Samuel A.
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