U.S. patent application number 09/779960 was filed with the patent office on 2001-08-09 for confocal microscope with a motorized scanning table.
Invention is credited to Czarnetzki, Norbert, Derndinger, Eberhard, Ott, Peter, Scherubl, Thomas.
Application Number | 20010012069 09/779960 |
Document ID | / |
Family ID | 7825633 |
Filed Date | 2001-08-09 |
United States Patent
Application |
20010012069 |
Kind Code |
A1 |
Derndinger, Eberhard ; et
al. |
August 9, 2001 |
Confocal microscope with a motorized scanning table
Abstract
A confocal microscope has a motorized scanning table for moving
the sample perpendicularly to the optical axis of the microscope.
The object is illuminated simultaneously at many places by means of
a light source array. The light reflected or scattered at the
object is detected by means of a diaphragm array, which is
conjugate to the object and to the light source array. A sensor
array is provided as a detector and makes a displacement of charges
possible between individual positions in the scanning direction.
The sensor is a so-called TDI sensor. The displacement of the
charges is synchronized with the motion of the object corresponding
to the motion of the image points in the plane of the sensor array.
The image data can thereby be recorded during the motion of the
object, so that even large object fields can be sensed in a short
time with high lateral resolution. The motion of the object takes
place along linear paths (if necessary linear paths combined in a
meander form) and the motion along the linear paths takes place
uniformly. The microscope is particularly suitable for inspection
in the semiconductor industry (wafer inspection, LCD
inspection).
Inventors: |
Derndinger, Eberhard;
(Aalen, DE) ; Czarnetzki, Norbert; (Jena, DE)
; Ott, Peter; (Aalen, DE) ; Scherubl, Thomas;
(Berlin, DE) |
Correspondence
Address: |
M. Robert Kestenbaum
11011 Bermuda Dunes NE
Albuquerque
NM
87111
US
|
Family ID: |
7825633 |
Appl. No.: |
09/779960 |
Filed: |
February 9, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
09779960 |
Feb 9, 2001 |
|
|
|
08923470 |
Sep 4, 1997 |
|
|
|
Current U.S.
Class: |
348/295 ;
348/335; 348/79 |
Current CPC
Class: |
G02B 21/0036 20130101;
G02B 21/0056 20130101; G02B 21/0072 20130101; G02B 21/008 20130101;
G02B 21/0068 20130101; G02B 21/004 20130101 |
Class at
Publication: |
348/295 ;
348/335; 348/79 |
International
Class: |
H04N 003/14; H04N
009/47 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 7, 1997 |
DE |
197 14 221.4 |
Claims
We claim:
1. A confocal microscope having an optical axis and an objective
(7) with a focal plane, comprising: a motorized scanning table (9)
for moving an object (8) at right angles to said optical axis of
said microscope, a diaphragm array (4, 4a, 4b) in a plane that is
conjugate to said focal plane of said microscope objective (7), a
sensor array (11) following said diaphragm array (4, 4a, 4b) in an
observation direction, with a plurality of photosensitive elements,
change storage elements associated with said photosensitive
elements, and a device for displacing charges stored in said charge
storage elements from one charge storage element to another charge
storage element, and a synchronizing unit (13, 24) for effecting
displacement of said charges corresponding to motion of an image
point of an object point in a plane of said sensor array (11).
2. The confocal microscope according to claim 1, wherein said
scanning table (9) is arranged to move said object (8) along linear
paths.
3. The confocal microscope according to claim 1, wherein said
diaphragm array (4, 4a, 4b) is fixed relative to an observation
beam path during motion of said object.
4. The confocal microscope according to claim 3, wherein said
diaphragm array (4, 4a, 4b) has a plurality of transparent regions
(4.sub.1-4.sub.20) that are arranged such that image paths of said
plurality of transparent regions in said focal plane-of said
objective (7) fill a portion of said focal plane of said objective
(7) without gaps.
5. The confocal microscope according to claim 1, wherein a light
source array (4, 4a) is arranged for producing a plurality of
mutually spaced-apart light sources in a plane conjugate to said
focal plane of said objective (7), and positions of said plurality
of light sources are conjugate to positions of transparent regions
(4.sub.1-4.sub.20) of said diaphragm array (4, 4a).
6. The confocal microscope according to claim 1, wherein said
sensor array (11) has a plurality of mutually parallel linear
sensor columns and said charges are displaced in the direction of
said sensor columns.
7. The confocal microscope according to claim 6, wherein said
diaphragm array has a plurality of transparent regions
(4.sub.1-4.sub.20), and each column of said sensor array has at
least one of said transparent regions imaged on it.
8. The confocal microscope according to claim 7, wherein said
transparent regions (4.sub.1-4.sub.20) of said diaphragm array (4,
4a, 4b) form a two-dimensional rhombic grid arrangement.
9. The confocal microscope according to claim 7, wherein said
transparent regions (4.sub.1-4.sub.20) of said diaphragm array (4,
4a, 4b) form a two-dimensional rectangular grid arrangement.
10. The confocal microscope according to claim 9, wherein said
diaphragm array (4, 4b) is imaged on said sensor array (11).
11. The confocal microscope according to claim 6, wherein said
sensor array (37) comprises a plurality of mutually independent
partial sensor arrays (38, 39, 40) arranged one behind the other in
a columnar direction, mutually offset in a row direction by a
distance (.DELTA.) equal to d/n, where d is the spacing of
individual sensors in said row direction and n is the number of
said partial sensor arrays.
12. The confocal microscope according to claim 11, wherein said
diaphragm array (41) is anamorphotically imaged on said sensor
array (37).
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] This invention relates to a confocal microscope and, more
particularly, to a confocal microscope with a motorized scanning
table for moving a sample perpendicularly to the optical axis of
the microscope.
[0003] 2. Discussion of Prior Art
[0004] A confocal microscope with a motorized scanning table to
move a sample perpendicularly to the optical axis of the microscope
is known from U.S. Pat. No. 5,239,178. Furthermore, the microscope
has a light source array in a plane conjugate to the focal plane of
an objective, and a detector array with numerous light-sensitive
elements, also in a plane conjugate to the focal plane of the
microscope objective. The movement of the specimen perpendicularly
to the optical axis of the microscope takes place primarily in the
microscopic region in order to increase the resolution, otherwise
defined by the raster spacing of the light source array,
perpendicular to the optical axis.
[0005] With this confocal microscope, sensing large object fields
that are substantially greater than the visual field imaged by the
objective is only possible to a limited extent. A series of
individual images of the object must be recorded. Between each
individual image, the object must be displaced over a path length
corresponding to the image field diameter.
[0006] A Nomarski microscope (not confocal) is designed for taking
and storing corresponding series of images, and is described, for
example, in European Patent EP 0 444 450-A1. Since this Nomarski
microscope is not confocal, it has only a small resolution in the
direction of the optical axis. Furthermore, this microscope is much
too slow when the image data in a large number of image fields must
be sensed. The sensing of large object fields in the shortest
possible time, with high resolution, is imperative in inspection
equipment used in production processes, for example, in the
semiconductor industry or in LCD production.
[0007] A microscope used for wafer inspection, also not confocal,
is described in U.S. Pat. No. 5,264,912. In it, filtering takes
place in the Fourier plane of the objective. The transmission
characteristic of the spatial filter in the Fourier plane
corresponds to the inverse diffraction figure of the integrated
circuit (IC) that is being produced. Consequently, the filter
transmits light only when the diffraction image of the momentarily
imaged IC deviates from the diffraction image of the reference IC,
and it can be concluded that the structure of the observed IC
deviates from the reference structure. In this microscope, a CCD
array or, alternatively, a high speed multiple output time delay
integration (TDI) sensor is provided as the light detector.
However, the reason for using a TDI sensor is not stated.
Furthermore, because of the non-confocal arrangement, this
microscope also has only a small resolution in the direction of the
optical axis.
[0008] Furthermore, U.S. Pat. No. 5,365,084 includes an arrangement
for inspecting a running length of fabric during its manufacture,
in which a TDI sensor is used, synchronized with the motion of the
length of fabric. However, such a video inspection device cannot be
considered for inspecting semiconductors in a production process,
because of its low resolution both in the direction of the optical
axis and perpendicular to the optical axis.
SUMMARY OF THE INVENTION
[0009] The object of the present invention is to provide an
arrangement that can be used for the optical inspection of
semiconductors in the production process. With this arrangement, a
further object is to achieve a sufficient resolution both in the
direction of, and also perpendicular to, the optical axis. At the
same time, an object is to sense large image fields in the shortest
possible time. These objects are achieved by a confocal microscope
including:
[0010] A motorized scanning table for moving an object at right
angles to the optical axis of the microscope;
[0011] A diaphragm array in a plane that is conjugate to the focal
plane of the microscope objective;
[0012] A sensor array following the diaphragm array in an
observation direction with a plurality of photosensitive elements,
charge storage elements associated with the photosensitive
elements, and a device for displacing charges stored in the charge
storage elements from one storage element to another storage
element; and
[0013] A synchronizing unit for effecting displacement of the
charges corresponding to motion of an image point of an object
point in a plane of the sensor array.
[0014] The arrangement according to the invention is a confocal
microscope with a motorized scanning table to move the specimen
perpendicularly of the optical axis of the microscope. It has a
diaphragm array with numerous light transmitting regions, so-called
pinholes, in a plane that is conjugate to the focal plane of the
microscope objective. The diaphragm array is followed by a sensor
array that has numerous photosensitive elements. Each
photosensitive element is associated with a charge storage element.
Furthermore, the sensor array has a device for displacing the
charges stored in the charge storage elements from one storage
element to another storage element, as in the case in the so-called
TDI sensors. Furthermore, a synchronizing unit is provided that
effects displacing charges corresponding to the movement of the
image point of a specimen point in the plane of the sensor
array.
[0015] In the confocal microscopic arrangement, high resolution
both in the direction of the optical axis and perpendicular to the
optical axis, which is usual for confocal microscopes, is attained.
The resolution that can be attained by using a strong magnifying
objective, for example, one having a magnification of 20-120 times,
is sufficient for semiconductor inspection. By using a diaphragm
array, and the numerous parallel confocal beam paths associated
with the diaphragm array, a number of object positions is sensed
that correspond to the number of pinholes in the diaphragm array.
By synchronizing the displacement of the charges in the sensor
array corresponding to the motion of the image point of an object
point, the measurement takes place while the sample is in motion.
Preferably, the motion of the sample takes place along linear paths
that extend over the complete length of the sample in the direction
of motion. For sensing large, two-dimensional surfaces,
corresponding linear paths can be combined in a meander form. Short
acceleration or deceleration segments, during which no signal
recording takes place, occur respectively at the start and at the
end of each linear path. Between these acceleration and
deceleration segments, the motion of the sample is uniform, so that
the movement of charge between the storage elements of the sensor
array and the motion of the image point on the sensor array are
mutually synchronized.
[0016] In order to produce the parallel confocal beam paths, a
light source array that has numerous mutually spaced-apart light
sources is arranged in a plane conjugate to the focal plane of the
objective. The positions of the individual light sources are
conjugate to the positions of the transparent regions of the
diaphragm array. Corresponding light source arrays can be formed in
different ways. The simplest variant results when the diaphragm
array is arranged in a common portion of the illumination and
observation beam paths, and the diaphragm array is illuminated from
the back. However, this simple arrangement has a disadvantage, in
that a substantial portion of the illuminating light is reflected
at the back side of the diaphragm array and thus produces a strong
signal background on the sensor array. Such a strong signal
background can be prevented by providing two separate diaphragm
arrays, one in the illuminating beam path and the other in the
observation beam path or measuring beam path. The diaphragm array
in the illumination beam path is then again illuminated from the
back. For a more effective use of light, the diaphragm array in the
illumination beam path can be preceded by a lens array, as
described in U.S. Pat. No. 5,239,178. In an alternative to using
diaphragm arrays illuminated at the back, the light source array
can also be formed by light-conducting fibers with their end
surfaces arranged in an array. Likewise, as an alternative to a
lens array, a correspondingly constructed diffractive element may
be used.
[0017] As the sample is scanned, the diaphragm array, the light
source array, and the sensor array are at rest. All three
components are mutually stationary.
[0018] Preferably, the sensor array is a two-dimensional array of
photosensitive elements and charge storage elements associated with
the photosensitive elements, with numerous columns arranged
parallel to each other. The direction of the columns is then
defined by the direction in which the charges are displaced between
the charge storage elements. On the one hand, the light source
array and diaphragm array, and on the other hand, the sensor array,
are arranged relative to each other so that at least one
transparent region of the diaphragm array is imaged on each of the
mutually parallel columns of the sensor array.
[0019] TDI sensors may be used as the corresponding sensor array.
To the extent that such TDI sensors have light-insensitive regions
between the photosensitive surfaces, these can be arranged, and the
imaging between the diaphragm array and the sensor can be chosen so
that the transparent regions of the diaphragm array are exclusively
imaged on the photosensitive regions.
[0020] The transparent regions of the diaphragm array are formed,
corresponding to the direction of motion of the scanning table and
to the imaging ratio between the object plane and the diaphragm
array, so that the paths of the images of all the transparent
regions, closely fill, preferably without a gap, a portion of the
focal plane, while maintaining the confocal filtering. With linear,
one-dimensional scanning of the object, the image data for a strip
whose width corresponds to the width of the image section sensed
perpendicularly to the direction of motion is sensed completely
confocally, without micro-movements perpendicular to the direction
of motion required. For this purpose, the transparent regions of
the diaphragm array may be arranged in the form of a
two-dimensional rhombic grid. The midpoint of each transparent
region then corresponds to the position of the theoretical grid
point. However, it is particularly advantageous to arrange the
transparent regions of the diaphragm array in the form of a
rectangular grid, the grid axes of which are rotated relative to
the linear direction of motion. Such a rectangular geometry confers
advantages when the light source array is formed in the form of a
fiber illumination, a lens array, or as a diffractive element
producing a corresponding illumination.
[0021] Preferably, a particularly advantageous sensor array has
several mutually independent two-dimensional partial sensor arrays
that are arranged one behind the other in the column or stage
direction, and that are respectively offset, perpendicularly to the
column direction or stage direction, by a distance .DELTA.=d/n from
each other, where d is the spacing of the individual sensors
perpendicularly to the column direction and n is the number of
two-dimensional partial arrays. Such an offset arrangement of
several two-dimensional sensor arrays has an image field that is
larger by the number of two-dimensional arrays in anamorphotic
imaging of the diaphragm array on the sensor array, in contrast to
an arrangement of a single sensor array with the same number of
photosensitive elements, so that a correspondingly large
signal/noise ratio results.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] Details of the invention are described in further detail
herein below taken together with the accompanying drawings, in
which:
[0023] FIG. 1a comprises a schematic of the principles of a first
embodiment of the invention, with a single pinhole array arranged
in the common portion of the illuminating and observation beam
paths;
[0024] FIG. 1b shows a second embodiment of the invention with
separate light source array and diaphragm array;
[0025] FIG. 1c is a schematic explaining the principle of the
synchronization between object motion and charge displacement in
the sensor array;
[0026] FIG. 2a is a block circuit diagram for the synchronization
between the object motion and the charge displacement in the sensor
array;
[0027] FIG. 2b is a detailed representation of the functioning
sequence in the microcontroller of FIG. 2a;
[0028] FIGS. 3a-3c show sections of a diaphragm array forming a
rhombic grid and the associated image points in the object plane
and in the plane of the sensor array;
[0029] FIGS. 4a-4c show sections of a diaphragm array forming a
rectangular grid and the associated image points in the object
plane and the plane of the sensor array;
[0030] FIG. 5a shows a schematic representation of a sensor array
consisting of several two-dimensional partial sensor arrays that
are arranged mutually offset;
[0031] FIG. 5b is a schematic of the principle of a pinhole array
suitable for the sensor array of FIG. 5a.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0032] In the confocal microscope according to the invention shown
in FIG. 1a, a single diaphragm array (4) with numerous transparent
regions or holes is arranged in the common portion of the
illumination and observation beam paths. This arrangement forms, at
one and the same time, the diaphragm array for the detection beam
path and the light source array for the illumination of the object
(8). For this purpose, the diaphragm array (4) is uniformly
illuminated from the back side by a light source (1) that is
followed by a condenser (2). Each transparent region, or each
pinhole, of the diaphragm array (4) thus forms a secondary light
source.
[0033] A tube lens (5) with a microscope objective (7) is arranged
after the diaphragm array (4) in order to image the diaphragm array
(4) on the object (8) positioned on the motorized scanning table
(9). The microscope objective (7) is shown, greatly simplified, as
a single lens in FIG. 1a. The microscope objective (7) is corrected
to an infinite focal intercept, and thus to an infinite image
distance. This is indicated in FIG. 1a by the telecentering
diaphragm (6).
[0034] The diaphragm array (4), and thus also the light source
array imaged by the diaphragm array (4), is arranged, by means of
the tube lens (5) and the telecentric imaging, to be confocal with
the focal plane of the objective (7). A pattern of illumination
corresponding to the image of the diaphragm array (4) arises in the
focal plane of the objective (7). The object (8) is illuminated at
the points that are conjugate to the transparent regions of the
diaphragm array (4). The light scattered or reflected by the object
(8) is imaged backward again, by the objective (7) with the
subsequent tube lens (5), onto the diaphragm array (4). In this
backward imaging, the diaphragm array (4) effects confocal
filtering, resulting in only such light being transmitted through
the transparent regions of the diaphragm array (4) as was scattered
or reflected in regions of the object (8) that are confocal to the
transparent regions of the diaphragm array (4). In contrast to
this, the light that is scattered or reflected on the object (8)
above or below the focal plane of the objective (7) is trapped by
the non-transparent regions of the diaphragm array (4). This
confocal microscope results in high resolution in the direction of
the optical axis (z-direction), denoted by a dot-dash line. For
separating the illumination and observation beam paths, a
beam-splitter mirror (3) is arranged between the diaphragm array
(4) and the condenser (2), and a portion of the light scattered or
reflected at the object (8) and transmitted through the diaphragm
array (4) is reflected out towards the sensor array (11). A further
imaging optics (10) that images the diaphragm array (4) on the
sensor array (11) is provided in the reflected beam path; that is,
between the beam-splitter mirror (3) and the sensor array (11). The
sensor array (11) is a so-called TDI sensor (Time Delay and
Integration), such as is offered, for example, by DALSA Inc.,
Ontario, Canada, under the reference IT-E1 or IT-F2. Such a TDI
sensor has 2048 columns each with 96 TDI stages or rows. A
photosensitive region and a charge storage element is associated
with each TDI stage in each column, so that the number of pixels
(photosensitive regions) and charge storage elements amounts to
96.times.2048. The diaphragm array (4) has at least a number of
transparent regions corresponding to the number of columns of the
TDI sensor, so that at least one transparent region of the
diaphragm array (4) is imaged on each column of the TDI sensor. The
detailed imagewise arrangement of the pixels of the TDI sensor and
of the transparent regions is described in more detail herein below
with reference to FIGS. 3a-3c and 4a-4c.
[0035] The scanning table (9) can be moved by motor drive in two
directions perpendicular to the optical axis, and senses large
object regions. Its motion is sensed by means of two position
measuring systems (12). The summed charges in the charge storage
elements of the TDI sensor (11) are displaced in the stage
direction by means of a synchronization unit (13), corresponding to
the motion of the scanning table (9). For this purpose, the motion
of the scanning table takes place along (possibly several) linear
paths of movement, so that on the TDI sensor (11) the image point
belonging to an object point is displaced along the columns. This
state of affairs will be explained with reference to the simplified
representation of FIG. 1c. Suppose that, at a first instant, an
object point (8a) is imaged at an image point (11a) on the TDI
sensor (11). Due to the motion of the scanning table (9), a motion
of the object (8) results in the direction of the arrow (P1) and at
a somewhat later instant the object point (8a) has traveled to
position (8b). Simultaneously with the motion of the object (8),
the charges stored in the charge storage elements of the TDI sensor
(11) are displaced in the direction of the arrow (P2) from the
stage (11a) to the stage (11b). Measurement can proceed during the
motion of the object (8) due to this synchronization between the
motion of the object (8) and the motion of the charges. The motion
of the object (8) therefore does not take place in start-stop
operation but uniformly during the measurement. Substantially
shorter measurement times are attained at the same signal/noise
ratio compared to arrangements in which the object motion takes
place in start/stop operation and a measurement takes place when
the object is stationary.
[0036] The complete scanning of the object field at right angles to
the direction of motion of the scanning table (9) takes place
through an arrangement of the transparent regions that is offset at
right angles to the direction of motion. In combination with the
synchronization of the charge displacement in the sensor array
corresponding to the motion of the image point of an object point,
the whole object field, which corresponds to the row width of the
sensor array, is sensed. Due to the offset arrangement of the
diaphragms in the diaphragm array, the paths of the image points of
the diaphragms lie close together, without gaps, in the focal plane
of the objective (7). Complete sensing of the image field is
possible without any micro-displacements at right angles to the
direction of motion. This reduces the costs of data storage (data
sorting) and reduces the tolerance requirements on the motion of
the scanning table.
[0037] In the embodiment according to FIG. 1b, components
corresponding to the individual components of the embodiment
according to FIG. 1a are referenced with the same symbols as in
FIG. 1a. The difference between the embodiment according to FIG. 1a
and in FIG. 1b is that the diaphragm array (4b) is arranged
following the beam splitter (31) in the observation beam path or
the detection beam path. The illumination beam path has its own
diaphragm array (4a), which forms the light source array. The two
diaphragm arrays (4a) and (4b) are arranged conjugate to each other
and conjugate to the focal plane of the objective (7). The
transparent regions of the two diaphragm arrays (4a) and (4b) are
also mutually conjugate. The use of separate diaphragm arrays (4a,
4b) in the illumination and observation beam paths avoids producing
a large signal background on the TDI sensor (11) due to the
relatively large proportion of light reflected at the diaphragm
array (4a) of the illumination beam path.
[0038] In addition, in the embodiment according to FIG. 1b, the
beam splitter (3') is constructed as a polarizing beam splitter,
and the illumination of the diaphragm array (4a) in the
illumination beam path also takes place with polarized light,
denoted by a polarizer (2a) preceding the diaphragm array (4a). In
addition, a quarter wavelength plate (14) is provided on the object
side of the beam splitter (3') and, in a known manner, effects a
rotation of 90.degree. in the polarization of the light that is
transmitted twice through the quarter wavelength plate (14). Using
polarized light, a polarizing beam splitter (3') and a quarter
wavelength plate (14) results in a better use, by a factor of four,
of the light present behind the condenser (2), compared to the
embodiment according to FIG. 1a. However, a corresponding
arrangement of polarizing beam splitter, polarizing filter and
quarter wavelength plate is also possible in the embodiment with
only one diaphragm array according to FIG. 1a.
[0039] A first embodiment of a diaphragm array (4, 4a, 4b) is shown
in FIG. 3b. The diaphragm array (4) contains a number of
transparent regions, of which only 20 (4.sub.1-4.sub.20), are shown
in FIG. 3b for reasons of clarity. The diameter of each transparent
region (4.sub.1-4.sub.20) corresponds to about half the diameter of
the Airy disk, and with an objective of numerical aperture NA=0.95
and for a wavelength lambda=365 nm amounts to about 0.25 .mu.m
multiplied by the imaging scale between the object (8) and the
diaphragm array (4, 4a, 4b). In order to obtain the best possible
confocal filtering, the spacing of closest neighboring transparent
regions is at least 4 times the diameter of the transparent
regions. The transparent regions (4.sub.1-4.sub.20) form a
two-dimensional rhombic grid. The angle between the two grid axes
is chosen so that, taking into account the imaging ratio between
the diaphragm array (4, 4b) and the TDI sensor (11), the center of
respective closest neighboring transparent regions is imaged on
neighboring columns of the TDI sensor (11). This imagewise
arrangement is shown in FIG. 3c. Each square in FIG. 3c represents
a photosensitive region. The 96 stages are represented in the
vertical direction, and a section of the 2,048 columns in the
horizontal direction, the columns being denoted by (P1, P2, P10,
P11). As can be gathered from the view of FIGS. 3b and 3c, the
transparent region (41) is imaged on the column (P1); the
transparent region (42) on the column (P2); and so on, on different
columns of the TDI sensor (11). At the same time, the transparent
regions (4.sub.1-4.sub.10) are imaged on different stages. The
stage position, again corresponding to the stage position of the
region (4.sub.1) is the stage position on which the transparent
region (4.sub.11) is imaged.
[0040] FIG. 3a shows the image of the diaphragm array (4) and the
TDI sensor (11) in the focal plane of the objective (7), and hence
in a sectional plane of the object (8). The images of the
transparent regions of the diaphragm array (4) are denoted using
the same symbols as in FIG. 3b. Each square that has been drawn
represents the image of the associated photosensitive region of the
TDI sensor 11. The linear direction of motion of the scanning table
(9) on the long meander paths is denoted by the arrow (S).
[0041] The same situation as in FIGS. 3a-3c is shown in principle
in FIGS. 4a-4c for an alternative diaphragm array (4') (see FIG.
4b). In this alternative embodiment for the diaphragm array (4'),
the transparent regions correspond in their diameter and their
distance to the neighboring transparent region to those of FIG. 3b.
These transparent regions are arranged so that a rectangular
two-dimensional grid of transparent regions results. The grid axes
of the rectangular grid are rotated relative to the scanning
direction (arrow S) so that here (as in previously described the
embodiments according to FIGS. 3a-3c) a respective transparent
region (4.sub.1'-4.sub.6') is imaged on a respective column of the
TDI sensor (11). In FIG. 4a the image of the diaphragm array (4')
and of the TDI sensor (11) are again shown in the focal plane of
the objective (7).
[0042] The rectangular grid arrangement of the transparent regions
confers constructional advantages when the light source array (4a)
is not constituted solely by a diaphragm array that is
homogeneously illuminated from the back, but by a diaphragm array
with a preceding lens array, a diffractive element, or a preceding
fiber array for better illumination of the transparent regions of
the diaphragm array (4a). With resulting secondary light sources
that are sufficiently formed as points, an illuminating diaphragm
array (4a) may even be dispensed with.
[0043] The electronics required for controlling the object motion
and the simultaneous synchronization of the charge displacement is
now described, with reference to the block circuit diagrams in
FIGS. 2a and 2b.
[0044] Essentially, the object table or stage (9) consists of table
elements that are displaceable in two mutually perpendicular
directions, the motorized drives (20, 21), the position measuring
systems (22, 23), and a microcontroller (24). The object table
itself (9) is displaceably received, for a focusing in the
direction of the optical axis, on a stand (not shown). The two
motorized drives (20, 21), for producing motion in two orthogonal
directions are preferably constituted as linear drives. The
position measuring systems (22, 23) that sense the motion or
deflection of the table (9) independently of each other in the two
mutually perpendicular directions, are constructed as length
measuring interferometers. When the table moves in the direction of
the measuring beam path of the associated interferometer, these
interferometers provide an intensity of irradiation on a radiation
sensor that has a sinusoidal dependence on the path traveled. The
period of the sinisoidal signal which is proportional to the
wavelength of the measuring light used is then directly associated
with the distance traveled. At the beginning of a measurement, a
null position is traveled to, since the measuring signal has
ambiguities for long traveled paths, and an absolute calibration is
required. At each later instant, the present position is then given
in relation to this null position by the number of times the
interferometer signal passed through zero, together with the phase
difference of the detected sine wave signal in the calibration
position and the present position.
[0045] The microcontroller (24) controls the drives (20, 21) of the
object table (9) corresponding to the present position values that
are supplied by the measuring systems (22, 23), and to the
reference position values that are determined by a host computer
(not shown) via a bus line (29). FIG. 2b shows (on a larger scale),
the controller circuit required for this purpose within the
microcontroller 24. The data supplied via the control bus, for
example, a CAN bus, is converted in an arithmetic logic unit (ALU)
(33) into the present reference positions. In a further ALU (32)
that follows, the values determined in the ALU (33) are
respectively subtracted from the values supplied from the two
measuring systems (22, 23), so that the difference represents the
amount of deviation between the actual position and the reference
position. This difference is integrated over time in an integrator
(34) and then multiplied in a unit (35) by a factor that gives the
amplification of the open control circuit. This factor is as a rule
negative, in order to effect a phase displacement of 180.degree..
This amplified and time integrated difference signal then
represents the drive signal for the drives (20, 21).
[0046] The values of the present reference positions in the two
mutually perpendicular directions are simultaneously passed on by
the ALU (32) via data leads (30, 31) to a further microcontroller
(28), a drive (27) for the reading out, or the cycle timing, of the
TDI sensor (11), and an image processing electronics (25). The
drive (27) (driven by the microcontroller (24)), effects a
displacement of the charges stored in the TDI sensor corresponding
to the travel of each image point on the TDI sensor (11). The
charge data read out from the TDI sensor (11) are digitized by an
A/D converter (26) and are then also passed on to the image
processing electronics (25). In this manner, the image processing
electronics (25) obtains the information for which table position
the radiation intensities recorded with the TDI sensor are to be
entered into the image to be produced. Here, the electronics takes
into consideration the delays which are caused by the systematic
properties of the TDI sensor. Should the table be located at a
position outside the region to be sensed by the recording, the
values given by the TDI sensor remain unconsidered.
[0047] The image processing electronics first carries out a
restoration of the recording. Constant and linear errors (that can
arise, for example, due to changes of the radiation intensity, or
due to deviations of the dimensions of the transparent regions
within the diaphragm array, or deviations of the table speed from
the reference speed, or different sensitivity characteristics of
the pixels of the TDI sensor) are thereby compensated. After such
constant or linear errors are compensated, the structures of the
object (for example, of the illuminated wafer) can be suppressed
somewhat by suitable filtering, in order to better establish the
existence of errors between the dies.
[0048] In order to carry out a so-called die-to-die comparison, the
portions of the recording that are to be compared with each other
are brought to cover one another, with pixel accuracy, taking into
account errors in the table system. The portions of the recording
to be compared are then subtracted one from another, the die-to-die
comparison is carried out, and defects such as contaminating
particles are detected by exclusive threshold formation.
[0049] With reference to FIGS. 2a and 2b, in the control circuit
described above, the nominal desired speed and the course of the
table are predetermined by the host computer. With the aid of the
clock (36) built into the microcontroller (24), the microcontroller
calculates from the speed standards the reference position of the
table and the cycle time according to which the table is regulated,
and the cycle times are set for the drive (27) for the TDI sensor
and for the image processing electronics. As an alternative to
this, the cycle times for reading out the TDI scanner and the image
processing electronics are set directly from the host computer. In
this case, the reference position is not passed on via the data
leads (30, 31), but the momentary actual positions are passed on to
the image processing electronics (25).
[0050] Preferably, the image recording of a large object field
takes place by an object table motion of meander form, in which the
long motion is oriented so that the image points travel in the
direction of the 96 stages of the TDI sensor. The motion then takes
place at a constant speed over the image region to be recorded.
After the object has been scanned in one direction, a displacement
of the table takes place in the direction perpendicular to this, so
that now when scanning the nearest long meander path, the
neighboring object regions are imaged on the TDI sensor. Scanning
out then takes place in the opposite direction, wherein at the same
time the direction of the charge transport between the storage
elements of the TDI sensor is reversed. Here it is of course
required that the TDI sensor have bidirectional scanning
properties, so that the charges are displaceable in the two
opposite directions. The sensor can be, for this purpose, an
IT-F2-Type of DALSA, Inc.
[0051] The frequency that is predetermined by the host computer or
by the clock (36) of the microcontroller (24) is determined so that
the object table is moved at the maximum speed possible for a
readout of the TDI rows with the maximum frequency, while taking
into account the imaging scale and the image drift.
[0052] A change of the objective (7) is required to change the
imaging scale. Preferably, this takes place by means of a coded
revolving nosepiece, where the scale data of the objectives
belonging to the positions of the revolving nosepiece are stored in
a memory. A matching of the mutually synchronized speed between the
reading out of the TDI sensor and the object table can then also
occur when the revolving nosepiece position is changed.
[0053] As a rule, a change in the imaging scale is associated with
a change in the diaphragm array, since the diameter of the
transparent regions remains matched to the size of the Airy disk,
which depends on the numerical aperture of the objective.
[0054] A particularly advantageous arrangement of a TDI sensor in
combination with the present invention is shown in FIG. 5a. The TDI
sensor (37) consists of several partial sensors (38, 39, 40), that
are arranged one after the other in the stage direction, and that
are mutually offset in the pixel direction (the horizontal, in FIG.
5a) by the distance .DELTA.=d/n, where d is the pixel spacing and n
is the number of partial sensors. Together with an anamorphotic
imaging of the diaphragm array (41) (FIG. 5b) on the composite TDI
sensor, an improvement in the signal/noise ratio corresponding to
the number of partial sensors (38, 39, 40) arranged one behind the
other results, compared to a TDI sensor having an identical total
surface area. In the embodiment shown in FIG. 5a, a total of 9
partial sensors 20. (38, 39, 40), again with 96 stages
respectively, are arranged one behind the other. The stage
direction here again corresponds to the motion of the object point
when the object is scanned. The imaging of the diaphragm array (41)
then takes place with a 9 times greater imaging scale in the
scanning direction than in the direction at right angles to it. By
this anamorphotic imaging, the transparent regions lying in the
first two rows (Z.sub.1, Z.sub.2) of the diaphragm array (41) are
then imaged on the first partial sensor (38); the two succeeding
rows (Z.sub.3, Z.sub.4) are imaged on the second partial sensor
(39); and so on. This anamorphotic imaging is shown in FIG. 5a by
the oval images of the circular transparent regions of the
diaphragm array (41). First, the offset arrangement of several
partial sensors makes it possible to image the transparent regions
that are imaged on each partial sensor as right-angled, partial
grids directed parallel to the rows and columns of the partial
sensors. Second, at the same time, the partial grids are mutually
offset in correspondence with the mutual offset of the partial
sensors, so that the whole image field is sensed without gaps when
the image data of the partial TDIs are correspondingly sorted to
obtain the correct sequence. Several transparent regions can
thereby be imaged on one column of each partial sensor at different
stage positions, resulting in the improved signal/noise ratio. In
the illustrations of FIGS. 5a and 5b, two transparent regions are
imaged on each pixel position at correspondingly offset stage
positions of the same partial sensor (38). However, the use of only
two transparent regions per pixel position serves only for
illustration. In order to optimally use the surface of the sensor
(37) at a predetermined ratio of diameter of the transparent
regions to the spacing of the transparent regions, the number of
the transparent regions can be chosen corresponding to the number
of partial sensors (38, 39, 40), so that with 9 partial sensors, an
amount of light per pixel result that is greater by a factor of 9
than in the embodiments according to FIGS. 3a-3c and 4a-4c, so that
with the same signal/noise ratio, the scanning of the object can
take place at 9 times the speed.
[0055] Due to the anamorphotic imaging, all columns of all the
partial sensors contribute to image production. A sensor array of
several partial sensors may also be put to use in combination with
a normal, non-anamorphotic imaging of the diaphragm array on the
sensor array. In this case, only a portion of the columns of the
partial sensors contributes to the formation of the image.
[0056] Instead of TDIs as the partial sensors, an arrangement of a
corresponding number of row sensors in a mutually offset
arrangement is conceivable. Such an arrangement can be compared, in
terms of light sensitivity, with the embodiments according to FIGS.
3a-3c. Of course, in comparison, the sensor surface used is clearly
reduced.
* * * * *