Television – Special applications – Flaw detector
Reexamination Certificate
1998-07-10
2001-11-20
Lee, Richard (Department: 2613)
Television
Special applications
Flaw detector
C348S135000
Reexamination Certificate
active
06320609
ABSTRACT:
BACKGROUND
1. Field of the Invention
This invention relates to measurement and inspection systems that use polar coordinate stages to position samples.
2. Description of Related Art
Many measurement and inspection systems mount samples such as semiconductor wafers on X,Y stages. An X,Y stage can move a sample in two independent orthogonal directions X and Y to select an area of the sample for viewing, imaging, or measurement. For example, an X,Y stage can move a wafer to select and position an area of the wafer in the field of view of an imaging system. The travel distances of the X,Y stage in the X and Y directions determine the size of the largest sample that can be inspected from edge to edge, and large samples require large travel distances. Accordingly, inspection systems have become larger to accommodate larger samples, for example, larger diameter semiconductor wafers.
The space required to accommodate the range of motion of an X,Y stage has a width that is equal to or greater than the width of the sample plus the travel distance in the X direction and a length that is equal to or greater than the length of the sample plus the travel distance in the Y direction.
FIG. 1
illustrates a system
100
that uses an X,Y stage to position a circular sample
110
. System
100
includes an imaging and/or measurement system (not shown) that can be, for example, a video camera, a microscope, an interferometer, a reflectometer, an ellipsometer, an FTIR spectrometer, or any type of spectrophotometer. Such systems typically have a field of view
130
that is much smaller than sample
110
. To view the left edge of sample
110
, the X,Y stage moves sample
110
to a position
112
where the left edge of sample
110
is in field of view
130
. Position
112
is offset to the right from the central position of sample
110
by the radius r of sample
110
. A position
116
for viewing the right edge of sample
110
is offset a distance r to the left along the X axis from the central position. Accordingly, the X,Y stage must have a travel distance of 2r along the X axis for edge-to-edge inspection of sample
110
. Similarly, the X,Y stage must have a travel distance of 2r along the Y axis between positions
114
and
118
, and a minimum area
120
required for an X,Y stage capable of positioning sample
110
for edge-to-edge viewing is about 16*r
2
.
Many applications require the sample to be accurately positioned and oriented or at least require accurate information regarding the position and orientation of the sample relative to the X,Y stage. This requirement is common in automated semiconductor manufacturing where the samples are generally round semiconductor wafers. A wafer's position can be accurately determined by rotating the wafer about a rotation axis and monitoring the variation in the perimeter location of the wafer as a function of the rotation. An analysis of the measured perimeter variations can accurately determine the offset from the rotation axis to the center of the wafer. Additionally, the process can identify the orientation of the wafer because most semiconductor wafers have an orientation indicator such as a notch or a flat on its perimeter. An edge detector detects when the flat or notch in the wafer's perimeter rotates past. Examples of such position detector systems, which are often referred to as prealigners, are described in U.S. Pat. No. 4,457,664 of Judell et al., U.S. Pat. No. 5,308,222 of Bacchi et al., U.S. Pat. No. 5,511,934 of Bacchi et al., and U.S. Pat. No. 5,513,948 of Bacchi et al. Prealignment for an X,Y stage requires addition of structure such as a separate prealignment station, from which the wafer is transferred to the X,Y stage after prealignment, or a rotatable sub-stage on the X,Y stage for rotating the wafer.
FIG. 2
illustrates a system
200
using a polar coordinate stage
220
to position sample
110
. Polar coordinate stage
220
has a rotatable platform mounted on a linear drive mechanism. The linear drive mechanism moves the platform and a sample along a coordinate axis R, and the platform rotates the sample about the rotation axis of the platform. Polar coordinate stage
220
requires significantly less area when positioning sample
110
for edge-to-edge inspection. In particular, a travel distance r (the radius of the sample) along axis R out to a position
212
is sufficient to center in field of view
130
any radial coordinate &rgr; in the range from 0 to r. Rotation of sample
110
then selects an angular coordinate &thgr; so that any point on sample
110
can be positioned in field of view
130
. Since polar coordinate stage
220
only requires one-dimensional linear motion and half the travel distance of an X,Y stage, the polar coordinate stage takes much less area than an X,Y stage requires. In particular, a polar coordinate stage needs an area of about 6*r
2
, which is less than 40% of the area that an X,Y stage requires.
A disadvantage of a polar stage is the portion of sample
100
in field of view
130
generally appears to rotate when the stage rotates sample
100
to move from one inspection location to another. Thus, different areas appear to have different orientations when an operator or machine vision software views the sample through an imaging system. Additionally, the speed of movement generally varies from one location to another for any constant stage rotation speed. In some measurement systems, an operator observes an image of a portion of the sample being measured or inspected and controls movement of the sample to select which areas are measured or inspected. With a polar stage, image rotation and variable image motion can easily confuse or disorient the operator when the operator is continuously viewing or inspecting sample
110
and moving the sample from one position to another. Accordingly, systems and methods are sought that provide the area savings of a polar coordinate stage but avoid the confusion of image rotation and variable speeds of motion.
SUMMARY
In accordance with an aspect of the invention, a system including a polar coordinate stage and an imaging system rotates an image to continuously compensate for image rotation that results when the polar coordinate stage moves from one part of a sample to another. In one embodiment, a control system accepts from a control such as a joystick, a mouse, or an external computer, control commands which define the desired direction and speed of an image shift. The control system determines the required motion of the polar coordinate stage and the required image correction to achieve the desired image shift. The control system generates the signal required to conform to the control commands and applies the required signals to the &rgr; and &thgr; drives in the polar coordinate stage. Image correction is performed by mechanically varying the imaging system or by processing image signals to rotate the image being viewed.
In one embodiment of the invention, the imaging system includes active opto-mechanical image correction. For example, when the imaging system includes an optical microscope, an optical element such as a dove prism rotates an image by an amount that depends on the variable property of the optical element. When the imaging system includes a scanning beam microscope, such as a scanning electron beam microscope, the active image rotation unit rotates the scan direction to rotate the image. The control system calculates and applies the required signals to adjust the active image correction device and achieve the necessary image correction. For example, the control system can rotate a dove prism or a beam deflector at the appropriate rate and direction to maintain the image orientation while the stage moves. Alternatively, the imaging system provides a first image signal representing an image that rotates as the stage rotates the sample, and the control system electronically processes the first image signal to generate a second image signal that maintains the desired orientation while the stage rotates.
In accordance with another embodimen
Buchanan Robert
Spady Blaine R.
Yarussi Richard A.
Halbert Michael J.
Lee Richard
Nanometrics Incorporated
Ogonowsky Brian D.
Skjerven Morrill & MacPherson LLP
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