Optics: measuring and testing – Inspection of flaws or impurities
Reexamination Certificate
2002-10-07
2004-12-14
Pham, Hoa Q. (Department: 2877)
Optics: measuring and testing
Inspection of flaws or impurities
C356S399000, C359S216100, C250S235000, C347S250000
Reexamination Certificate
active
06831736
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to a method and apparatus for correcting inaccuracies in an optical scanner, and more particularly, to a method and apparatus for compensating for static and dynamic inaccuracies in an optical scanner.
2. Description of the Related Art
In the fabrication of modem semiconductor integrated circuits, inspection systems are used to inspect semiconductor wafers, photomasks, reticles, and other surfaces. A typical surface inspection system detects light that is scattered, reflected, or otherwise modified from a small area of an inspected surface. In order to detect errors or anomalies, data from the inspection system, in the form of a quantity of light detected, for example, may be compared with some kind of reference data. The reference data may come from a database, from another portion of the surface being inspected, or from other suitable sources.
Erroneous detection of errors or anomalies can occur when the data for a sampled surface area is not matched correctly with reference data. One source of such errors can occur when the optical system used in scanning the inspected surface is misaligned.
FIG. 1
shows a general example of an inspection system, in which an article
10
being inspected is placed on an inspection stage
20
. A light source
60
, which may be a laser or any suitable light source, provides its output to an acousto-optic deflector (AOD)
70
, which in turn causes a light beam
55
to be output. Control of AOD
70
, which for example may be accomplished, in whole or in part, by RF generator
80
under entire or partial control of a stage control
25
, causes the light beam
55
to scan across the surface of the article
10
. Other types of systems for causing the light beam
55
to scan as indicated may be used.
In this type of inspection system, the light beam
55
generally moves in the direction of a first axis, while the stage
20
moves the article
10
along a second, generally perpendicular axis. Relative movement between the light beam
55
and the article
10
in these respective axes may be accomplished in any known or desired manner. For example, the light beam
55
may move, and the article
10
(or stage
20
) may remain stationary; the light beam
55
may remain stationary, and the article
10
(or stage
20
) may move; or both the light beam and the article (stage) may move. For purposes of the following discussion, it will be assumed that the light beam scans the surface of the article
10
along the X axis, while the inspection stage moves along the Y axis.
In
FIG. 1
, a stage control
25
controls movement of the stage
20
in the Y axis. AOD
70
causes the light beam
55
, generated by light source
60
and under control of an RF generator
80
, to scan in the direction of the X axis. The RF generator
80
communicates with stage control
25
to control the scanning of light beam
55
in accordance with the movement of the stage
20
.
FIG. 2
depicts generally an ideal relative movement between the stage
20
and the light beam
55
. That is, ideally, in the configuration shown, for a given article
10
being inspected, the stage
20
always has the article
10
positioned precisely on it, and always begins to move at the same point along the Y axis, while the light beam
55
always begins a scan line at the same point along the X axis. With this kind of perfect alignment, the light information produced by the scan at a given position (coordinate pair) on the article will always match the data at the corresponding coordinate pair in the reference data.
Obviously, misalignment along either the X axis or the Y axis can cause misregistration, which could be sufficient to cause scan data to be matched incorrectly with reference data. As a result, an error could be misidentified in at least one of two ways. Either the system may identify a non-existent error; or the system may fail to identify an error. Accordingly, one problem that needs to be addressed is the non-ideal nature of the (x, y) grid over which the scanning occurs.
Imperfections in the (x, y) grid can result from various problems. For example, in certain kinds of inspection stage movement systems, incremental stage movements can be sufficiently imprecise to cause misalignment, with resulting inaccuracy in the position of the stage
20
relative to the light beam
55
. For example, there may be an instruction to the stage
20
to move to coordinate (1000, 1000). The units of the coordinate system may be a function &mgr; of the resolution of the inspection system. Merely by way of example, &mgr; may be 40 nm in one type of inspection system, so that coordinate 1000 along the Y axis is actually at a location that is 40,000 nm away from the origin along that axis. As a result, if there is an error, an instruction to move to (1000,1000) actually may result in movement to coordinate (1000, 1000+&Dgr;y), where &Dgr;y is the amount of error. In the example just given, &Dgr;y may have a relation to &mgr;. To correct for this kind of error, it would be desirable to be able to provide an amount of compensation for &Dgr;y, particularly as a function of &mgr;.
This type of error, which is referred to as a “static” error, may not occur in certain types of stages, such as an interferometer controlled stage. Such an error may be an inherent function of the resolution of the stage control
25
.
A second type of error, resulting from scan lines starting at different positions along the X axis, is referred to as “dynamic” error, because the amount of error can vary from scan to scan. One version of this dynamic error phenomenon is known as “jitter”. To compensate for jitter, it is necessary to monitor the starting position of the light beam along the Y axis, and provide appropriate compensation at the start of each scan line.
FIG. 3
shows an example of different degrees of jitter. To compensate for jitter, the starting position for each scan line should be normalized to a position that is an amount &Dgr;x
0
away from the Y-axis. For successive scans at times t
1
, t
2
, and t
3
, the jitter adjustment would be dx
1
, dx
2
, and dx
3
, respectively.
In order to correct for the kinds of static and dynamic error just described, it would be desirable to provide an approach which facilitates starting both the initial movement of the stage, and each scan line, at the same location every time.
One way of compensating for the foregoing types of misalignments and positioning errors is known as registration. In one such registration method, a scanned image is compared to a reference image. During this comparison, the scanned and reference images are aligned, to determine the existence of a positioning error. If differences between the scanned and reference images are detected during the comparison, then compensating factors may be determined.
In the kind of registration system just described, the reference image typically is obtained from a predetermined location on a scanning surface. Other systems may generate a reference image by utilizing a previously scanned image, or even by averaging a number of previously scanned images.
One drawback of the registration method is that registration accuracy relies heavily on the quality of the scanned images. As a result, if either the scanned image or the reference image is inaccurate, the errors detected as a result of the comparison will be incorrect. It can be appreciated readily that the problem will be even worse if both the scanned image and the reference image are inaccurate.
It would be desirable to provide a system that compensates for misalignment without the need for image comparison techniques such as the just-described registration method. It also would be desirable to provide accurate scanner misalignment correction without having to rely on the accuracy of sample or reference images.
SUMMARY OF THE INVENTION
In accordance with this and other aspects of the present invention, a method is provided that compensates for one or more sources of a scanner&apos
Elichai Rami
Margulis Pavel
Naftali Ron
Schwartz Gilad
Slobodnik Igor
Applied Materials Israel, Ltd.
Pham Hoa Q.
Sughrue & Mion, PLLC
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