U.S. patent application number 11/180348 was filed with the patent office on 2005-11-17 for alignment correction prior to image sampling in inspection systems.
This patent application is currently assigned to KLA INSTRUMENTS CORPORATION. Invention is credited to Babian, Fred E., Chadwick, Curt H., Douglas, Kent E., Kroeze, Roger, Szabo, Nicholas, Young, Scott A..
Application Number | 20050254698 11/180348 |
Document ID | / |
Family ID | 24145667 |
Filed Date | 2005-11-17 |
United States Patent
Application |
20050254698 |
Kind Code |
A1 |
Young, Scott A. ; et
al. |
November 17, 2005 |
Alignment correction prior to image sampling in inspection
systems
Abstract
A method and apparatus, and variations of each, for inspecting a
wafer defining at least one die thereon is disclosed. The present
invention first obtains the electronic image equivalent of two die,
and then determines the x and y offset between those electronic
images. Prior to inspection for defects, those two electronic
images are aligned by adjusting the x and y positions of one
electronic image of one die with respect to the electronic image of
the other die. Once that is accomplished, the those electronic
images are compared to detect any defects that may exist on one of
the die.
Inventors: |
Young, Scott A.; (Soquel,
CA) ; Kroeze, Roger; (Tracy, CA) ; Chadwick,
Curt H.; (Los Gatos, CA) ; Szabo, Nicholas;
(Cupertino, CA) ; Douglas, Kent E.; (San Martin,
CA) ; Babian, Fred E.; (Boulder Creek, CA) |
Correspondence
Address: |
ALLSTON L. JONES
PETERS, VERNY, JONES & SCHMITT, L.L.P.
Suite 230
425 Sherman Avenue
Palo Alto
CA
94306-1850
US
|
Assignee: |
KLA INSTRUMENTS CORPORATION
|
Family ID: |
24145667 |
Appl. No.: |
11/180348 |
Filed: |
July 13, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11180348 |
Jul 13, 2005 |
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10314546 |
Dec 9, 2002 |
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10314546 |
Dec 9, 2002 |
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10066161 |
Jan 31, 2002 |
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10066161 |
Jan 31, 2002 |
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09702943 |
Oct 30, 2000 |
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09702943 |
Oct 30, 2000 |
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08884466 |
Jun 27, 1997 |
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6141038 |
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08884466 |
Jun 27, 1997 |
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08538137 |
Oct 2, 1995 |
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Current U.S.
Class: |
382/145 ;
348/87 |
Current CPC
Class: |
G06T 2207/30148
20130101; G01N 21/9501 20130101; G01N 21/95607 20130101; G06T 7/001
20130101 |
Class at
Publication: |
382/145 ;
348/087 |
International
Class: |
G06K 009/00; H04N
007/18 |
Claims
What is claimed is:
1. A method for inspecting a wafer defining at least one die
thereon, said method comprising the steps of: a. obtaining the
electronic image equivalent of two die; b. determining the x and y
offset between the electronic images of said two die of step a.; c.
aligning said electronic images of said two die by adjusting the x
and y positions of one electronic image of one die with respect to
said electronic image of said other die; d. comparing said
electronic images from step C.; e. identifying image differences
between the two die compared in step d.
2. An apparatus to inspect a wafer defining at least one die
thereon comprising: an x-y stage to transport said die; a scanner
to obtain an electronic image equivalent of said at least one die
as said x-y stage transports said die; a first comparator coupled
to said scanner to determine the x and y offset between the
electronic images of two die; an alignment computer to reposition
said scanner to adjust the x and y positions of one electronic
image of one die with respect to said electronic image of said
other die; a second comparator coupled to said scanner to compare
said electronic images of said first and second die following the
operation of said alignment computer; and a defect detector coupled
to said second comparator to identify defect differences between
said electronic images compared by said second comparator.
Description
FIELD OF THE INVENTION
[0001] The present invention is related to sub-pixel image
alignment in wafer inspection machines, particularly to the
alignment of images both prior to and subsequent to scanning. Two
alternate methods are taught, one for laser scanning and the other
for scanning with a linear array.
BACKGROUND OF THE INVENTION
[0002] It is well known in the wafer inspection art that when two
similar images are to be compared, sub-pixel alignment is often
necessary to obtain the degree of accuracy that is desired.
Traditionally that alignment was accomplished by digitally
interpolating the image after scanning.
[0003] The most frequently used method for automatic inspection of
photomasks or patterned semiconductor wafers utilizes comparison to
detect defects. Typically, two supposedly identical patterns are
compared by scanning and digitizing the images. The digitized
images are then compared in high speed digital logic, or an image
is compared with data stored in the CADS (Computer Aided Design
System) database with data representing the desired pattern.
[0004] In the comparison process to detect differences between the
two patterns some form of image subtraction is most frequently
employed. However, image subtraction is contingent on sampling the
two images (or the image and image data from the database) at
nearly identical points for both images.
[0005] Early mask inspection systems, such as taught by Levy, et
al., in U.S. Pat. No. 4,247,203, were able to guarantee only a
.+-.21/2 pixel registration accuracy between the two images.
Because of the limited registration accuracy, Levy required that
the defect detection algorithm use feature extraction, followed by
the matching of these features, rather than image subtraction. Some
time later Levy, U.S. Pat. No. 4,579,455, taught area subtraction,
but because of the limited registration accuracy computed the
intensity difference at several possible registrations. If, for any
of these registrations the absolute value of the intensities was
less than a predetermined threshold, no defect was recorded at that
particular pixel. Subsequently, Specht, et al., in U.S. Pat. No.
4,805,123, taught a method of achieving image subtraction by first
reducing the registration error between the two images to less than
a pixel. However, the Specht method had the shortcoming that in
re-registering (also known as resampling) the two images with
respect to each other, interpolation of the scanned image was used,
which in turn introduced errors in determining the intensities of
the resulting pixels. These errors limited sensitivity (the
smallest detectable defect).
[0006] As will be shown subsequently, the maximum intensity error
determines the maximum detectable defect-to-pixel ratio. Since
inspection speed, at a given sensitivity, defines the productivity
of an inspection system, for a fixed sampling rate, it is desirable
to maximize the pixel size. Therefore, to achieve the maximum
throughput, one must minimize the registration error. The present
invention teaches methods for minimizing the registration error for
the two most common scanning methods: scanning with a laser and
scanning with a linear array.
SUMMARY OF THE INVENTION
[0007] The present invention is a method and apparatus, and
variations of each, for inspecting a wafer defining at least one
die thereon. The present invention first obtains the electronic
image equivalent of two die, and then determines the x and y offset
between those electronic images. Prior to inspection for defects,
those two electronic images are aligned by adjusting the x and y
positions of one electronic image of one die with respect to the
electronic image of the other die. Once that is accomplished, the
those electronic images are compared to detect any defects that may
exist on one of the die.
BRIEF DESCRIPTION OF THE FIGURES
[0008] FIG. 1 illustrates the pixelization of a surface by an
inspection system and the mis-alignment between two images.
[0009] FIG. 2 is a block diagram of a diode array scanning system
embodiment of the present invention.
[0010] FIG. 2a is the transparent reticle version of the system of
FIG. 2.
[0011] FIG. 3a illustrates the scanning of multiple patterns from
die-to-die inspection.
[0012] FIG. 3b illustrates the scanning of a single pattern for
die-to-database inspection.
[0013] FIG. 4 is a block diagram of a laser scanning system
embodiment of the present invention.
[0014] FIG. 4a is the transparent reticle version of the system of
FIG. 4.
[0015] FIG. 5 is a sketch of a signal that is representative of the
signal applied to the acousto-optic deflector/driver of FIG. 4 to
correct for coarse x-direction mis-alignment of the wafer of the
stage.
DETAILED DESCRIPTION OF THE PRESENT INVENTION
[0016] The key to the present invention is the use of the same
sampling points for both images, or the image of the die being
viewed and the die equivalent in the data base, to be compared as
will be seen from the following discussion.
[0017] FIGS. 3a and 3b illustrate the typical serpentine scanning
technique for multiple patterns and for a single pattern,
respectively. In FIG. 3a wafer 14 is scanned in a serpentine path
31, sweeping out several dies 33, 35 and 37 in die-to-die
inspection, and in FIG. 3b only a single die is scanned in
serpentine path 31' when die-to-database inspection is employed.
Each sweep of the path is designated a swath. A typical swath may
have a height of 500 to 2,000 pixels and may have a length of
500,000 pixels.
[0018] FIG. 1 illustrates two identical forms 20 and 30
superimposed on a grid that represents the boundaries of pixels 10
as defined by the inspection system of the present invention. The
nominal sampling point of each pixel is the center of that pixel
however in reality the scanner measures the total light energy that
falls on an area of approximately the size of a pixel 10. The
idealized intensity value of each pixel is the normalized intensity
value expressed as a percentage of the maximum. FIG. 1 shows two
identical geometric forms 20 and 30, each consisting of a rectangle
of opaque material (e.g., chromium) on a transparent medium, such
as quartz. In this configuration, pixels 40A and 40B have different
measured values since the sampling points (the centers of the
pixels) are not equidistant from the corresponding one of two forms
20 and 30, respectively. Consequently, pixels 40A and 40B, as shown
in FIG. 1 have measurable values of 76% and 92%, respectively.
[0019] Clearly, if pixel-to-pixel comparison is used for defect
detection, the sampling points must nearly coincide with respect to
the forms. It can readily be seen that the registration error (the
relative displacement of the sampling points between the two forms
20 and 30) determines the maximum possible intensity difference
between any two pixels to be compared. Assuming that .DELTA.I is
the maximum possible intensity difference attributable to the
registration error, then the defect detectors intensity threshold
must be at least .DELTA.I. For binary images, i.e. where at every
sampling point the transmittance is either 0 or 100%, the minimum
detectable defect size (in terms of area) is merely D.sub.x times
D.sub.y, where D.sub.x and D.sub.y are the maximum x and y
directional registration errors (see FIG. 1 for the D.sub.x and
D.sub.y between forms 20 and 30 for example).
[0020] In the prior art, as stated above in the Background of the
Invention section, registering the two images was accomplished by
first scanning both images. Next, integer pixel misalignment was
corrected as taught by Levy, by shifting the image in the digital
memory the appropriate number of locations. Fractional pixel
registration was achieved by resampling one of the images as taught
by Specht.
[0021] In the present invention, for both scanning techniques, a
coarse correction is made prior to sampling, the image is scanned
and then stored in memory. For diode array scanning (FIG. 2) coarse
correction in the X-direction is implemented by a mechanical
movement of a mirror, while for laser scanning (FIG. 4)
X-directional coarse correction uses timing control of the
sampling. In the Y-direction, both scanning techniques use timing
control of the sampling.
[0022] The purpose of the present invention is to minimize the
intensity error caused by the registration error of sampling points
with respect to the two forms to be compared whether die-to-die or
die-to-data base.
[0023] The present invention is an improvement over the Specht
method in that a coarse correction of the misregistration error is
achieved in both X and Y prior to the scanning of the pattern, or
patterns. The residual error after coarse correction and subsequent
to scanning is then further reduced by interpolation of the
intensities. Since the residual alignment error after coarse
correction is now small, the error contributed by interpolation is
significantly smaller than when the Specht alignment and inspection
method is used. Hence, with the present invention, the two images
used in image subtraction are much better aligned with respect to
each other and consequently the minimum detectable defect, as a
percentage of the pixel size, is significantly smaller than as in
the prior art. Consequently, a larger pixel size can be used for a
given minimum detectable defect. A larger pixel size, for a given
minimum detectable defect and for a constant pixel rate translates
into a higher throughput than in the prior art. Higher throughput
produces more defect data which in turn results in more reliable
diagnosis of the problems and better yield management.
[0024] One significant concept of the present invention is that one
may employ a pixel that is significantly larger than the minimum
detectable defect or even the minimum feature size (geometric
figure on the mask or wafer), provided the two images are
registered accurately with respect to each other.
[0025] The present invention relates to two different scanning
embodiments and how improved registration may be achieved using the
present invention. These scanning embodiments are: Scanning with a
Diode (or TDI) Array, and Scanning with a Laser Beam. These two
embodiments are discussed separately below. Additionally, it should
be kept in mind that both embodiments lend themselves to scanning
with both transmitted and reflected light, either separately or
together in the same system.
[0026] Diode (or TDI) Array Scanning
[0027] FIG. 2 is a block diagram of a diode (or TDI) array scanning
system using reflected light. A wafer, or reticle, 14 is mounted on
X/Y stage 50, with X-Y scales 51 mounted thereon to determine stage
position, and an illuminator (not shown) illuminates the area of
wafer 14 under objective lens 52. The light reflected from wafer 14
travels through objective lens 52, is reflected by tilted mirror 54
to lens 57 through which a portion of the wafer image is projected
onto linear diode array 59. Mirror 54 shifts the image of wafer 14
onto diode array 59 by pivoting about an axis perpendicular to the
plane of the paper under the control of piezo-electric actuator 56
with the shift occurring in the y-direction. Each time stage 14
travels the distance of a pixel, array 59 serially reads out a
(y-directional) column of intensities which are digitized by A/D
converter 58. This information flows from converter 58 into each of
pixel memory 60, first-in-first-out (FIFO) memory 64 and alignment
computer 62. Pixel memory 60 is a two-dimensional memory of the
width of a swath and a length somewhat greater than the widest
(x-directional dimension) die to be inspected. Pixel memory 60 is
essentially also a FIFO memory, i.e. its input accepts a column of
pixels at a time and outputs them at the other end. Pixel memory 60
has output registers which are capable of shifting one pixel, on a
command from alignment computer 62, the data in either the x or y
direction, prior to producing an output, similar to the method
taught by U.S. Pat. No. 4,247,203 by Levy et al. The purpose of
pixel memory 60 is to store pixel data from one die while the next
die is being scanned so that the two dies can be compared.
[0028] This operation is illustrated by the following example.
Referring to FIGS. 2 and 3a as die 33 is scanned on the first pass
across wafer 14, the information flows into pixel memory 60. Then,
as the scanner starts to scan die 35, the information from die 33
is read from pixel memory 60 correctly aligned to the closest
integer pixel to the image of die 35. Alignment computer 62
performs running alignment computation to determine the
misalignment between the two data streams corresponding to the
first swath across die 33 and the present time swath across die 35.
The alignment error of these two data streams is computed as
described by Specht. Integer alignment errors are corrected by the
output registers of pixel memory 60, while the fractional error is
corrected by alignment corrector 66 by using resampling as
discussed below.
[0029] Overall, the two data streams, one from FIFO memory 64 and
the other from alignment corrector 66, arrive at defect detector 74
aligned with a precision of such as {fraction (1/256)} of a pixel
is achievable.
[0030] In addition to the alignment correction commands fed to
alignment corrector 66 and pixel memory 60, alignment computer 62
produces three other signals. Two of these, one to stage drive 70
and a second to tilt mirror actuator 56, are intended to provide
low frequency alignment correction signals. The signal to tilt
mirror actuator 56 provides y-directional control, while the signal
to stage drive 70 exercises control in the x-direction. The purpose
of these is to make sure that the misalignment between die does not
exceed the dynamic range that the correction system can rectify.
Alignment computer 62 also produces a strobe signal to initiate the
readout of a column of pixels from linear diode sensor 59. Since
stage 50 travels approximately at a constant speed, slightly
varying the time between strobe pulses allows fine alignment in the
x-direction. The strobe is generated in alignment computer 62 by a
phase-locked loop which derives its input from the x-directional
alignment error and from a linear scale mounted on stage 50 that
measures the position of stage 50 by alignment computer 62. U.S.
Pat. No. 4,926,489 by Danielson, et al., describes a similar
implementation using a phase-locked loop.
[0031] FIFO memory 64 is a short memory of the same width as the
swath height. Its purpose is to delay the flow of pixel information
into defect detector 74 sufficiently to make sure that alignment
computer 62 has enough image data to correct the alignment error,
prior to the two image data streams reach defect detector 74.
[0032] In defect detector 74 the corresponding intensity values of
the two images are compared and if the absolute value of the
difference exceeds a predetermined threshold, an error flag is
raised. The error data is then sent to general purpose computer 72
(e.g. a Sun workstation), where adjacent defect locations are
combined to permit a determination of the size and shape of the
defects. This information is then used by yield management
programs.
[0033] The basic philosophy behind this embodiment of the present
invention is that tilting mirror 54 and proper strobing of linear
diode sensor 59 provide first order alignment corrections which
reduce the needed dynamic range for the fine correction. Since the
amount of error contributed by the resampling is a function of the
dynamic range of the correction needed, the error intensity into
defect detector 74 is smaller than would be achievable without
correcting the alignment prior to sampling the image.
[0034] In the case where the comparison is die-to-data base, data
is obtained from a die 14 on stage 50 with switch 61 in the
position shown, then switch 61 is switched to the other position
and data from data base generator 63 is connected to supply the
second data set. The overall operation is therefore the same as
described above.
[0035] The subject invention may also be used to inspect
transparent substrates, such as a reticle. FIG. 2a illustrates the
system in that case. Substrate 14', a reticle, is illuminated from
below and the only difference between this implementation and the
one that uses transmitted light, is the location of the source of
the illumination.
[0036] When the reticles, rather than wafers, are inspected,
ordinarily the inspection is a comparison with the data base. The
data base generator, at its output, produces a data stream that
simulates the desired optical image. Switch 61 allows either the
datastream from A/D converter 58 or from database generator 63 to
flow into pixel memory 60.
[0037] Laser Scanning
[0038] The same general approach taught above with respect to FIG.
2 may also be used with laser scanning. The laser scanner here can
be adapted from the implementation of the KLA 301 Reticle and Mask
Inspection Unit, made by the assignee. FIG. 4 illustrates such a
laser scanner embodiment of the present invention. Laser 80 directs
coherent light to acousto-optic deflector/driver 82 which deflects
the light in the y-direction, as described by Evelet in U.S. Pat.
No. 3,851,951 (High Resolution Laser Beam Recorder with
Self-focusing Acousto-optic Scanner). The y-deflected light beam
from acousto-optic deflector/driver 82 is then applied to
beamsplitter 84 through which the laser beam passes and proceeds to
lens 86 which focuses the laser beam on wafer 14 on X/Y stage 50.
Some of the light incident on wafer 14 is then reflected back into
lens 86 and proceeds to beamsplitter 84, where portions of the
reflected light are reflected to condenser lens 88 where it is
refracted and collected on the surface of single diode sensor 90.
The resultant electrical signal from diode 90 is then applied to
A/D converter 100. The remaining components of the laser
implementation, with the exception of alignment computer 62',
function as for the diode array implementation of FIG. 2.
Consequently, pixel memory 60, alignment corrector 66, FIFO 64,
defect detector 74, general purpose computer 72, stage drive 70 and
X/Y stage 50 function as described above for the diode array
implementation shown in FIG. 2 with stage 14 executing the same
serpentine scanning travel as described previously with respect to
FIG. 3.
[0039] In addition to the functions outlined above, A/D converter
100 and alignment computer 62' perform additional functions that
are necessary to control the operation of acousto-optic
deflector/driver 82. Acousto-optic deflector/driver 82 is driven by
a saw tooth signal (see FIG. 5) generated by alignment computer
62'. That saw tooth signal includes two components, a ramp 92 and
variable time delay 96 between consecutive ramps. X-directional
coarse correction is implemented by varying time-delay 96 between
successive ramps 92, since the stage travels at a constant speed.
The timing of the start of ramp 92 is controlled by a phased-locked
loop oscillator of alignment computer 62' that derives its control
signal from the x-directional alignment error determined by
alignment computer 62'. Alignment computer 62' also generates
strobe pulses to control when A/D converter 100 samples the video
signal from diode sensor 90. Since the laser beam sweeps across
wafer 14 at a constant speed, the y-coordinates of the samples are
determined by the timing of the strobe pulses. These strobe pulses
are also driven by the phase-locked loop oscillator of alignment
computer 62' which is controlled by the y-directional alignment
error. The fine corrections in both X and Y are executed in
alignment corrector 66, as discussed for the diode array embodiment
of FIG. 2.
[0040] Also, for the die-to-data base situation, the use of switch
61 and data base generator 63 is as discussed above for FIG. 2.
[0041] For the laser scanner implementation using transmitted light
as in FIG. 4a, reticle 14' is placed on stage 50 and the
implementation is virtually identical to the one shown in FIG. 4
except that diode detector 90 is now under stage 50 to collect, via
condenser lens 88', the light transmitted through reticle 14'. In
most instances, the inspection will be against the CADS database
for which DataBase Generator 63 provides a simulated image.
[0042] While the forgoing techniques are most beneficial in defect
detection where image subtraction is used, all known techniques,
such as those using feature extraction and comparison,
specifically, operate more efficiently when registration errors are
minimized. Of course, these methods may also be used when a single
image is derived physically and is compared with computer generated
data. Furthermore, these alignment techniques are useful in all
image processing applications that depend on alignment.
[0043] While the present invention has been described in several
embodiments and with exemplary routines and apparatus, it is
contemplated that persons skilled in the art, upon reading the
preceding descriptions and studying the drawings, will realize
various alternative approaches to the implementation of the present
invention. It is therefore intended that the following appended
claims be interpreted as including all such alterations and
modifications that fall within the true spirit and scope to the
present invention and the appended claims.
* * * * *