Optics: measuring and testing – Inspection of flaws or impurities – Surface condition
Reexamination Certificate
1997-12-15
2001-05-22
Font, Frank G. (Department: 2877)
Optics: measuring and testing
Inspection of flaws or impurities
Surface condition
C356S343000
Reexamination Certificate
active
06236454
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to laser scanning in general and to laser scanning within inspection systems in particular.
BACKGROUND OF THE INVENTION
Laser scanning involves moving a laser beam along a surface to be scanned and can be used for both writing and reading purposes. For example, laser scanning is used for writing in printing systems, where the scanned beam activates spots on a printing medium, and in cutting systems where the scanned beam cuts material. For reading, laser scanning is used in inspection systems and in copiers which use the scanned beam to illuminate consecutive spots of a surface to be viewed or a page to be copied.
FIG. 1
, to which reference is now made, schematically illustrates a laser scanning system for printing and a surface
10
of a medium to be activated. The system includes a laser
12
, a pre-scan optical system
16
, a scan unit
14
, and a post-scan optical system
20
. The scan unit
14
can be an acousto-optic deflector, a polygon deflector a hologon deflector or an oscillating mirror.
The laser
12
produces a beam
22
, the pre-scan optical system
16
provides the scanned beam with the desired optical properties and the scan unit
14
deflects the beam
22
to provide the scanning motion, as indicated by arrow
24
. The post-scan optical system
20
focuses the scanned beam on the medium
10
, thereby to produce the printing spot, and converts the angular scan of arrow
24
to a linear scan, as indicated by arrow
26
.
Due to the action of the scan unit
14
, the focused beam scans a portion of the medium
10
, as indicated by arrow
26
, in one direction, known as the “fast scan direction”. The medium
10
typically is moved, as indicated by arrow
26
, in a second direction, orthogonal to the fast scan direction. This is generally known as the slow scan direction. The fast and slow scan directions together provide two-dimensional scanning. Alternatively, the scan unit
14
can produce two-dimensional scanning if it includes means for deflecting the beam along a second direction.
The scanning rate (defined as pixels/sec or spots/sec) of any laser scanning system is a function of the velocity of the spot and the size of the spot, both of which are functions of the limitations of the scan unit. The scanning rate is thus limited by the fundamental parameters and quality of the scan unit. It will be appreciated that, for a given pixel or spot size, the scanning rate determines the throughput (e.g. number of pages printed or number of wafers inspected within a given period of time).
It is known to increase the throughput of a laser scanning system for printing by increasing the number of beams being scanned at one time. One such system, with 32 beams, is the ALTA-3500, commercially available from Etec Systems Inc. of California, USA.
FIG. 2
, to which reference is now briefly made, schematically shows the system, but with only three beams
30
. The multiple beams can be aligned along the fast scan direction, as shown, or along the slow scan direction. A beam generating unit
32
, such as multiple lasers or a single laser with multiple beam splitters, produces the multiple beams
30
. The multiple beams
30
pass through a system similar to that shown hereinabove for
FIG. 1
but whose elements are designed for multiple beams. Thus, the scan unit and pre- and post-optical systems carry similar reference numerals as those of the scanning system of
FIG. 1
but are additionally marked with an apostrophe (').
The multiple processed beams, labeled
34
, are scanned along the surface of the medium
10
, thereby generating multiple parallel scan lines at one time. This typically increases the throughput of the scanning system by the number N of beams used, where an N of two to many hundreds are known.
Laser scanning systems for inspection systems utilize the scanned light for illumination of an article to be inspected by one or more detectors. Such a system is shown schematically in
FIG. 3
, to which reference is now made. Like the previous scanning systems, it also includes laser
12
, scan unit
14
, pre-scan optical system
16
and post-scan optical system
20
. However, the inspection system also includes multiple light detectors
40
for detecting the shape of features on a surface
42
, such as the surface of a semiconductor wafer, from different viewing perspectives. The movement of the surface
42
is indicated by arrow
44
.
The scanning elements illuminate the surface
42
from above and the surface
42
scatters the light in many directions, as a function of the optical characteristics of the features thereon. The inspection system of
FIG. 3
is a “dark field” inspection system since its detectors
40
collect the light scattered from the surface
42
at an oblique angle &bgr; which is outside of the convergence angle of the post-scan optical system
20
.
The oblique angle &bgr; varies depending on the type of surface to be inspected and the type of features to be inspected. The light detectors
40
are typically non-imaging detectors, such as a photomultiplier tubes, which measure the changing intensity, over time, of the light impinging upon them. As is known to those skilled in the art, in order to differentiate the light from different pixels on the surface
42
, the signal from the photomultiplier tube must be sampled at a rate corresponding to the spot size and to the velocity of the spot on the surface
42
. This may be called “temporal resolution”.
As in the other scanning systems, the scanning rate of the inspection system of
FIG. 3
is a function of the fundamental parameters and the quality of the scan unit
14
. Of course, as in other scanning systems, it is desirable to increase the scanning rate of the inspection system. However, an inspection system does not easily lend itself to operating with multiple beams. One reason is that non-imaging detectors do not discern the position from which the light was scattered. Adding other beams would, therefore, cause cross-talk on the detectors caused by the signals from the other spots. Imaging detectors cannot easily be incorporated into a dark field imaging system since, due to oblique incidence angle &bgr;, the collection optics cannot resolve sufficiently small pixels such as is possible with detectors placed at a non-oblique angle.
SUMMARY OF THE INVENTION
An object of the present invention is to provide multiple scanning beams within an inspection system.
There is therefore provided, in accordance with a preferred embodiment of the present invention, an inspection system using at least dark field imaging which includes a multiple beam laser scanning unit and at least one multiple beam dark field imaging unit. The laser scanning unit generates multiple beams which illuminate multiple spots on a surface to be scanned. Each imaging unit collects light from one viewing perspective and separately detects light scattered from the multiple spots.
Moreover, in accordance with a preferred embodiment of the present invention, each imaging unit includes a plurality of photodetectors, at least one per spot, spaced apart from each other and collection optics directing light scattered from each spot to an assigned one of the photodetectors.
In one embodiment, the collection optics and photodetectors are arranged according to the principles of Scheimpflug imaging. For example, the collection optics are mounted so that the longitudinal axis of its thin lens equivalent is at a first non-parallel angle to the surface, the multiple photodetectors are mounted along an image plane of the collection optics and the image plane is at a second non-parallel angle to the longitudinal axis.
Additionally, in accordance with a preferred embodiment of the present invention, the collection optics may include a spatial filter which limits the range of angles of scattered light which are received by the multiple photodetectors. The limiting unit can be an aperture stop. The collection optics can also include wavelength filters and/or polarization filters.
Further, in accor
Applied Materials Inc.
Font Frank G.
Smith Zandra V
Sughrue Mion Zinn Macpeak & Seas, PLLC
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