Circular scanning patterns

Details Circular scanning patterns Circular scanning patterns

2002-06-03

2003-08-05

Phan, James (Department: 2872)

Optical: systems and elements

Deflection using a moving element

Using a periodically moving element

C359S201100, C359S216100, C359S900000, C356S237100

Reexamination Certificate

active

06603589

ABSTRACT:

FIELD AND BACKGROUND OF THE INVENTION
The present invention relates to a method of inspecting surfaces and, in particular, it concerns a method for inspecting surfaces having a periodic pattern such as dies and cells that are produced on the surface of silicon wafers used in the Integrated Circuits (IC) industry.
Many known inspection methods for defect detection are based on a comparison principal. According to this principal the gray levels of each pixel in the digital image acquired from the inspection region are subtracted from the gray levels of their corresponding pixels in a digital reference-image related to the inspection region. A defect is indicated if the subtraction results for any pixel of the digital image are greater than a predetermined threshold value.
When the acquired image of the inspection region contains a large amount of pixels due to one or a combination of the size of the inspection region or high resolution of the image, the reference-image has to be stored in a large volume memory which is very expensive.
For some applications such as high-resolution inspection of silicon wafers, the cost of large volume memory makes the use of the above methods impractical. An alternative inspection method is also based on a comparison technique and is useful for surfaces having a periodic pattern. In this method, the scan is performed along stripes or bands known as swaths. The swaths are aligned with the periodic structure of the periodic pattern. Since the pattern contains many periodic fragments, such as cells or dies and the swaths are relatively narrow, the information in the swath relating to each periodic fragment is dramatically reduced. As the inspection area has a periodic pattern, the surface can be inspected by making a comparison between swaths that are in related fragments of the periodic pattern. Accordingly, this comparison method requires a relatively small memory-volume and it also eliminates the need for a reference-image.
A difference detected by a comparison between two fragments indicates that one of the fragments is defective, but the defective fragment is not identifiable. Three-fragment comparison is needed to identify the fragment and the defect location within the identified fragment and not just to detect the existence of a defect without the ability to indicate the exact location of the defect. Three-fragment comparison is performed by comparing the fragment under inspection with two adjacent fragments. Statistically, it is assumed that there is a very low probability that a defect will repeat itself at the same position in two other fragments. Therefore, a defect is defined as a deviation that appears twice in the two comparisons and the fragment that contains the defect is the one that differs from the other two fragments. The three-fragment comparison method is also known as Cell-to-Cell or Die-to-Die comparison. The three-fragment comparison method is only effective when the scan direction of the swaths is aligned with a periodic structure that typically has Cartesian symmetry.
In the IC industry there is continuing demand for miniaturization of the wafer patterns. This is leading to a reduction in the dimensions of electrical components produced on silicon wafers. Therefore, there is a need for improving the detection capability of the inspection machines by improving resolution, signal to noise ratio and contrast. Moreover, the quantity of inspection data is increasing with the increase in resolution and therefore there is a need for inspection machines with a higher throughput.
Recently a novel scanning system was described in a U.S. Pat. No. 6,310,710 to Shahar et al., entitled “High-Resolution Reading and Writing Using Rotating Beams and Lenses Rotating at Equal or Double Speed”. The aforementioned system provides better resolution and higher throughput as compared to other scanning systems. Accordingly, the aforementioned system is very attractive to the IC industry for fulfilling the current and future demands in the field of silicon wafers inspection. The scanning system described in the aforementioned system is a circular scanner including at least one scanning beam that is operated with or without confocal mode. The circular scanner produces a scan along a circular path and therefore does not have the symmetry of a Cartesian coordinate system. Therefore, performing the Cell-to-Cell or Die-to-Die comparison method with the rotating microscope leads to a mismatch between the circular paths of the scanner and the orthogonal symmetry of the silicon wafers. Therefore, large quantities of inspection data need to be stored relating to the area of several dies. Therefore a very large memory-volume is required, which makes the use of the circular scanner impractical, in spite of all its advantages.
Reference is now made to FIG.
1
and FIG.
2
.
FIG. 1
is a side view of a scanning arrangement
5
configured to perform circular scanning paths that is constructed and operable in accordance with the prior art.
FIG. 2
is a plan view of scanning arrangement
5
. Scanning arrangement
5
includes a circular scanner
6
, a stage
19
and a drive mechanism
21
(not shown). Circular scanner
6
includes a spindle
8
, a polygon
10
, a disk
12
and at least one scanning head
14
. Circular scanner
6
has an axis of rotation
15
about which the rotating elements of circular scanner
6
rotate. Spindle
8
rotates polygon
10
at a rate W about axis of rotation
15
. Spindle
8
rotates disk
12
at a rate 2 W about axis of rotation
15
. Disk
12
carries scanning head
14
. Therefore, scanning head
14
performs a circular scanning motion about axis of rotation
15
due to the rotation of disk
12
over an inspection area
18
of a sample. The sample containing inspection area
18
is mounted on stage
19
. Drive mechanism
21
is configured to provide relative linear movement between stage
19
and axis of rotation
15
in a direction perpendicular to axis of rotation
15
in order to enable circular scanner
6
perform an area scan. Circular scanner
6
also includes a light source
20
, an optical apparatus
24
, an auto focus system
26
and a light detector
30
. The optical path of a light beam
16
originates from light source
20
. Light beam
16
is transmitted from light source
20
through optical apparatus
24
and optional auto focus system
26
to polygon
10
. Light beam
16
is reflected by the surfaces of polygon
10
along a path
32
to scanning head
14
(FIG.
2
). It is shown in the prior art that path
32
is equivalent to a path
34
and therefore circular scanner
6
preserves the length of the optical path of light beam
16
at all times (FIG.
2
). Light beam
16
is projected to a point
28
which is on inspection area
18
by scanning head
14
. Light beam
16
is reflected from inspection area
18
via scanning head
14
, polygon
10
, auto focus system
26
and optical apparatus
24
to light detector
30
. Light beam
16
is a single beam or a collection of multiple beams. The scanning path produced by light beam
16
is referred to as a scanning swath. If light beam
16
includes a collection of multiple beams, then the scanning path produced by each multiple beam is referred to as a curved scanning path. Therefore, if light beam
16
includes a collection of multiple beams, there will be a plurality of curved scanning paths per scanning swath.
Reference is now made to
FIG. 3
, which is a schematic plan view of a scanning pattern
35
produced by scanning arrangement
5
in accordance with the prior art. This particular example describes a system having four scanning beams included within light beam
16
. Circular scanner
6
scans inspection area
18
by moving a light spot
38
, a light spot
40
, a light spot
42
and a light spot
44
along a curved scanning path
46
, a curved scanning path
48
, a curved scanning path
50
and a curved scanning path
52
respectively. Inspection area
18
has Cartesian symmetry is composed of a periodic pattern which is schematically shown as a plurality of

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