Title: Scanning charged particle microscope
Abstract: The present invention sets out to provide a scanning charged particle microscope equipped with a rapid control function capable of extrapolating an in-focus point from image information for a single frame and an automatic focusing system capable of reliably and precisely carrying out a focusing operation for a horizontal pattern image. The automatic focusing system provided in the scanning charged particle microscope of the present invention is provided with means for changing a focal point each raster scan line, and control means for comparing image information each scanning line and extrapolating focusing positions. The scanning line can then be made to be an inclined scanning line that is a combination of a horizontal component and a vertical component with respect to a chip array on a semiconductor wafer. Further, a method is adopted comprising a first step of reliably taking in a coarse in-focus point and a second step of detecting the in-focus point with a high degree of precision.
Patent Number: 6,852,973 Issued on 02/08/2005 to Suzuki, et al.
| Inventors:
|
Suzuki; Hidekazu (Chiba, JP);
Uemoto; Atsushi (Chiba, JP)
|
| Assignee:
|
SII NanoTechnology Inc. (Chiba, JP)
|
| Appl. No.:
|
410224 |
| Filed:
|
April 9, 2003 |
Foreign Application Priority Data
| Apr 10, 2002[JP] | 2002-108312 |
| Current U.S. Class: |
250/306; 250/307; 250/310; 250/396R |
| Intern'l Class: |
G01N 013/16 |
| Field of Search: |
250/310,307,397,311,306,309,396 ML,396 R,491.1
|
References Cited [Referenced By]
U.S. Patent Documents
| 4600832 | Jul., 1986 | Grund | 250/201.
|
| 4675528 | Jun., 1987 | Langner et al. | 250/396.
|
| 4827125 | May., 1989 | Goldstein | 250/234.
|
| 5445011 | Aug., 1995 | Ghislain et al. | 73/105.
|
| 5557156 | Sep., 1996 | Elings | 310/316.
|
| 5627373 | May., 1997 | Keese | 250/310.
|
| 6621082 | Sep., 2003 | Morita et al. | 250/310.
|
Primary Examiner: Wells; Nikita
Assistant Examiner: Smith, II; Johnnie L
Attorney, Agent or Firm: Adams & Wilks
Claims
What is claimed is:
1. A scanning charged particle microscope comprising: a charged particle
beam source for producing a charged particle beam; a charged particle beam
optical system for focusing the charged particle beam; a beam deflecting
apparatus for causing the focused charged particle beam to perform raster
scanning across a sample having a pattern thereon such that each raster
scan line is inclined relative to a direction of a boundary of the
pattern; and a control unit for changing a focal point of the focused beam
for each raster scan line, comparing an image definition between
successive raster scan lines, and determining an in-focus point based on
the comparison.
2. A scanning charged particle microscope according to claim 1; wherein the
pattern comprises a two-dimensional matrix extending in first and second
directions, and each raster scan line is inclined relative to the first
and second directions.
3. A scanning charged particle microscope according to claim 1; wherein the
sample comprises a semiconductor wafer, the pattern comprises a plurality
of semiconductor chips formed on the wafer and arranged in the
two-dimensional matrix extending in first and second directions, and each
raster scan line is inclined relative to the first and second directions.
4. A scanning charged particle microscope according to claim 1; wherein the
control unit discriminates between different scanning sections of the
sample based on the number of stepwise changes of an image signal produced
by each raster scan line.
5. A scanning charged particle microscope according to claim 1; wherein the
control unit first changes the focal point of the focused beam by a
relatively larger amount to determine a coarse in-focus point, and then
changes the focal point of the focused beam by a relatively smaller amount
in the vicinity of the first in-focus point to determine a fine in-focus
point.
6. A method for performing automatic focusing in a scanning charged
particle microscope which irradiates and scans a charged particle beam
over a sample having a pattern thereon and obtains a sample image,
comprising the steps of:
scanning the charged particle beam across the pattern in such a manner that
each scan line is inclined relative to a boundary of the pattern;
changing a focal point of the charged particle beam for each scan line;
comparing image definition between images obtained for each scan line; and
obtaining a position where the clearest image definition is obtained as an
in-focus point.
7. A method for performing automatic focusing according to claim 6; wherein
the step of scanning the charged particle beam across the pattern
comprises scanning the charged particle beam such that successive scan
lines are at a broad spacing to obtain a coarse in-focus point, and then
scanning the charged particle beam such that successive scan lines are at
a narrow spacing narrower than the broad spacing in the vicinity of the
range including the coarse in-focus point to obtain a fine in-focus point.
8. A method for performing automatic focusing according to claim 6; wherein
the step of comparing image definition between images obtained for each
scan line comprises the step of comparing differences between image
signals for each scan line at the boundary of the pattern; and the step of
obtaining a position where the clearest image definition is obtained
comprises the step of obtaining a position at which the differences are at
a maximum value.
9. A method for performing automatic focusing according to claim 6; wherein
the step of obtaining a position where the clearest image definition is
obtained as an in-focus point comprises the step of obtaining an in-focus
point at a plurality of areas of the sample; and further comprising the
steps of storing the in-focus points; and observing the sample to detect
defects using the obtained in-focus points.
10. A method for performing automatic focusing according to claim 6;
wherein the pattern comprises a two-dimensional matrix extending in first
and second directions, and each scan line is inclined relative to the
first and second directions.
11. A method for performing automatic focusing according to claim 6;
wherein the sample comprises a semiconductor wafer, the pattern comprises
a plurality of semiconductor chips formed on the wafer and arranged in the
two-dimensional matrix extending in first and second directions, and each
scan line is inclined relative to the first and second directions.
12. A scanning charged particle microscope for irradiating and scanning
with a charged particle beam semiconductor chips arranged in a
two-dimensional matrix extending in first and second directions on a
semiconductor wafer to obtain a sample image, comprising:
scanning means for performing raster scanning of the semiconductor chips
with the charged particle beam in such a manner that each scan line is
inclined relative to the first and second directions of the semiconductor
chips on the semiconductor wafer;
focal point changing means for changing a focal point for each scan line;
and
control means for comparing an image definition between successive images
obtained for each scan line and determining an in-focus point therefrom.
13. A scanning charged particle microscope according to claim 12; wherein
the scanning means comprises a beam deflecting apparatus which deflects
the charged particle beam in an inclined direction relative to the first
and second directions.
14. A scanning charged particle microscope according to claim 12; wherein
the control means discriminates between different scanning sections of the
semiconductor chips arranged in the two-dimensional matrix based on a
number of stepwise changes of the image signal obtained for each scanning
line.
15. A scanning charged particle microscope according to claim 12; wherein
the focal point changing means changes the focal point by a relatively
larger amount to determine a coarse in-focus point, and then by a
relatively smaller amount in the vicinity of the range including the first
in-focus point to determine a fine in-focus point.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method and system for reliably,
accurately and rapidly executing an automatic focusing function for a
scanning charged particle microscope typified by an electron microscope,
etc.
2. Description of Related Art
Tests are implemented using defect checking devices to detect defects such
as the adhesion of foreign matter that are the main cause of defective
device operation as a means of managing yield in semiconductor device
manufacturing processes. The defect checking devices detect defects and
store the number and position of these defects in a defect file for
subsequent processes. Semiconductor device wafers are typically
substantially circular plates as shown in FIG. 4. A lattice of a large
number of the same chips 2 are transferred onto a single wafer 1. The
checking device then scans the surface of the wafer manufactured in this
manner using an optical probe so as to detect surface defects. When a
defect is detected, a chip number (for example, a way of showing which row
of which column) specifying at which chip the defect exists and internal
chip coordinate information specifying the position within the chip are
stored in memory as a data file. Monitoring and analysis of the defects is
carried out by various microscopes and analysis apparatus based on this
storage information and one of these is high resolution defect monitoring
using an electron microscope for defect monitoring. During this time,
positioning of the noted defects in the field of vision of the microscope
is carried out based on storage information of the defect checking device.
However, focal adjustment of the optical system of the microscope in order
to observe the defects using an electron microscope are carried out by the
electron microscope itself which is typically provided with an Auto Focus
function.
Conventionally, a so-called "frame focusing control method" is widely
adopted for these automatic focusing mechanisms. This frame focusing
control method is a method whereby frame images are sequentially read in
while moving the focal point and is based on the theory that a difference
signal with neighboring pixels occurring at an outline portion is bigger
for clearer images. A differential value is obtained for the image signal
and the focal point is moved in a direction giving a larger value. An
image expressing the maximum value is then traced or extrapolated and the
in-focus point is obtained. However, a time on the order of three to ten
seconds, depending on the application, is required in order to carry out
the operation of taking in a large number of frame images while moving the
focal point.
"Focal point adjustment methods occurring in charged particle beam device"
was therefore proposed in Japanese Patent Laid-open Publication No. Hei.
7-16132 as a means for overcoming the fact that this control operation is
too time-consuming. This reference discloses a focal point adjustment
method for a charged particle beam device comprising a focusing lens for
focusing a charged particle beam onto a sample, scanning means for
scanning an irradiation position of the charged particle beam on the
sample, a detector for detecting a signal obtained by irradiation of the
sample with a charged particle beam, and means for sequentially changing
the focusing of the charged particle beam on the sample. Here, the
focusing of the charged particle beam is sequentially changed in
synchronization with a vertical scanning signal and detection signals
occurring for each focused state of the charged particle beam are
accumulated with regards to signals detected by the detector. Each
accumulated signal is then stored and an optimum focal point position is
obtained from the stored series of accumulated values. The focusing lens
is then set to the optimum focusing position. In this method, the focal
point position is changed every time the vertical position of the scanning
line changes rather than being changed once for every frame and image
definition is compared for each scanning line. Control can therefore be
implemented more rapidly compared with the related art where images are
compared for every frame. However, in this method, there is a problem in
that a pattern extending in a vertical direction of the sample image as
shown in FIG. 1A is necessary in order to implement automatic focal point
adjustment. If this is not present, it is not possible to perform a
comparison of every scanning line. Namely, image definition for each
scanning line can be discerned using the differences in image signals
occurring at points passing through boundary regions. Therefore, when the
image is a linear pattern going along the direction of the scanning lines
as shown in FIG. 1B, the scanning lines do not pass through the boundary
region and the focusing operation therefore does not operate with this
method. In other words, the image information in the scanning direction in
this case is uniform and difference signals for neighboring pixel
information are therefore all zero. Semiconductor patterns differ from
typical images taken of scenery or people in that vertical direction
boundaries and horizontal direction boundaries are common, which means
that such problems cannot be neglected in these kinds of situations.
SUMMARY OF THE INVENTION
The present invention provides a rapid control function capable of
extrapolating an in-focus point from image information for a single frame
and an automatic focusing system capable of reliably and precisely
carrying out a focusing operation for a horizontal pattern image.
The automatic focusing system of the present invention is provided with
means for changing a focal point for each raster scan line, and control
means for comparing image information for each scanning line and
extrapolating in-focus positions, with the scanning line being made to be
an inclined scanning line that is a combination of a horizontal component
and a vertical component with respect to a chip array on a semiconductor
wafer. As means to implement this, there is a method carried out using
beam deflection or a method of setting the stage in such a manner that the
direction of the sample and the horizontal scanning direction intersect
each other.
In order that peak values, i.e. in-focus points can be obtained even when
the pattern shapes through which each of the scanning lines pass differ,
control where image definition is compared each raster line and in-focus
points are extrapolated is such that scanning sections are discriminated
from changes in step shapes for between each scanning line and peak values
from small consecutive changes between each scanning line are
extrapolated.
Further, in order to implement automatic focal point control, a method is
adopted comprising a first step of taking coarse in-focus points using
coarsely taken large differences between focal points for between scanning
lines, and a second step of detecting in-focus points with a high degree
of precision using small differences in focal points between scanning
lines based on the coarse in-focus point information.
There is therefore adopted a method for implementing automatic focusing
control comprising the steps of: recording pre-selected in-focus points
taking into consideration the pattern arrangement of semiconductor chips
arrayed on the wafer and being capable of covering the entire region,
accessing the in-focus points covering positions of noted defects obtained
by a defect checking device and executing a focusing operation, and
consecutively monitoring defects in positions covered by the in-focus
points in the state of a focal position of a lens obtained at this time.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1A-1E are views illustrating the theory of the present invention
using the relationship between the pattern on the sample surface and the
beam scanning lines.
FIG. 2 is a view illustrating the automatic focusing operation of the
present invention using a two step method.
FIG. 3 is a view illustrating the automatic focusing operation of the
present invention using an AF point selection registration method.
FIG. 4 is a view schematically showing chips formed in an array on a
semiconductor wafer.
FIG. 5 is a scanning charged particle microscope having an automatic
focusing system.
DETAILED DESCRIPTION OF THE INVENTION
The present invention provides a technology for performing more detailed
checks using an electron microscope for defects such as those caused by
foreign bodies becoming attached to some of the same chips arrayed in a
lattice on a semiconductor wafer that are detected using a defect checking
device, and as such it is important to acquire clearly defined images of
defects constituting the subjects of such checks. The situation in the
related art where related autofocusing methods were used to perform frame
focusing control on image information for a plurality of frames was
excessively time-consuming whatever the approach adopted. A method where
the focal point is changed for every horizontal scanning line which
switches vertical deflection so that image definition is compared for
every scanning line to exert focusing control is therefore preferred.
However, there is still occasionally the inconvenience that this does not
operate very well with images for regions where the pattern only exists in
the horizontal direction as described previously. The cause of this is the
fact that a pattern extending in the vertical direction does not exist, so
that the horizontal scanning lines do not pass through the boundary of the
pattern and results are obtained where the image information is seen to be
uniform. The present applicant therefore gave consideration to
implementing scanning in an inclined direction, i.e. providing the
microscope beam deflection mechanism with a scanning rotation function in
order to ensure that the scanning lines always cross this horizontal
pattern even when the image pattern is an interconnect portion extending
along a boundary line of an element in a horizontal direction or extending
in a horizontal direction.
If the direction of the scanning lines is at an incline, then boundary
points are always passed through even if the pattern of the observed
region is a vertical pattern or a horizontal pattern. Since patterns for
chips arrayed on a semiconductor wafer in a vertical direction or
horizontal direction are common, this autofocus function is effective,
with the exception of occasional regions where there is no pattern.
FIG. 5 shows a scanning charged particle microscope having an automatic
focusing system of the present invention. Charged particle beam 65
generated by the charged particle source 41 is condensed by condenser lens
52. Deflector 54 deflects the charged particle beam condensed by the
condenser lens 52 so as to make the charged particle beam scan the sample
55. The objective lens 53 focuses the charged particle beam 65 on the
sample 55. The secondary electron detector 58 detects secondary electrons
generated from the sample 55 owing to the irradiation of the charged
particle beam 65. The image processor 64 processes signals from the
secondary electron detector 58 and a sample image is displayed on the
display unit 63.
Signals from the computer 59 make the DAC 67 change currents flowing in
coils of an X-deflector and a Y-deflector comprising the beam deflector
54. Changing the currents in the coils of the X-deflector and Y-deflector
enables a change in scanning direction of the charged particle beam in
desired directions on the sample 55. Namely, for example, the charged
particle beam is able to scan a semiconductor chip diagonally. It is
possible to make the charged particle beam always scan across a stripe
pattern whether the stripe pattern on the semiconductor is vertical or
horizontal. In this condition, a focal distance of the objective lens 53
is changed for every scanning line, which is done via the lens adjustment
unit 66 based on signals from "means for changing the focal distance 61"
that is provided in the computer 59. The image obtained at every scanning
line is taken into "means for comparing image definitions and finding an
in-focus point 62" and image definitions are compared here. To compare the
image definitions, the difference of image signals between a background
and the stripe pattern obtained when the charged particle beam passes
across the boundary of the stripe pattern and the background. The position
where the maximum difference is obtained is judged as the position where
the image is the clearest. And the position is judged as an in-focus
point.
A description is now given of the theory of the present invention with
reference to FIG. 1. The case of a pattern that is a vertical stripe at
the sample surface is shown in FIG. 1A with each horizontal scan therefore
passing through this vertical stripe without fail so that the image
information changes substantially twice at the points where both
boundaries are passed through. An image is acquired while sequentially
changing the focal point of the electron beam every scanning line. Images
where the focus differs respectively each scanning line are therefore
composed. Of these images, images occurring at scanning lines at positions
where the focus was correct are most clearly defined. The difference
signals for between neighboring pixels at the boundary positions of these
scanning lines therefore give the largest output. This focal point between
scanning lines giving this maximum value is then the in-focus point.
However, in the case of the horizontal stripe pattern at the sample
surface shown in FIG. 1B, there is no intersection between each horizontal
scanning line and this horizontal stripe. Rather than just regions where
there is no pattern, even in pattern regions the scanning lines may scan
along the pattern and therefore do not pass through a boundary. There is
therefore no large change in the image information. As a result it is not
possible to implement this kind of autofocus method where comparisons are
made while shifting the focal point each scanning line for this kind of
region.
Therefore, in the present invention, the main scanning of the beam is given
an angle of inclination with respect to the direction of the chip array,
as shown in FIG. 1C and FIG. 1D. While scanning in this manner,
sub-scanning is also carried out in a direction orthogonal to the main
scanning direction and the focal point is changed every time the
sub-scanning position is changed. As a result of scanning in this manner,
at least some of the scanning lines will always pass through the pattern
boundary even in the case of the vertical stripe pattern region shown in
FIG. 1C or in the case of the horizontal stripe pattern region shown in
FIG. 1D. By slightly changing the focal point of the electron beam each
time the scanning line positions are changed for the scanning lines, it is
possible to obtain an in-focus point or a position close to the focal
point with any kind of scanning lines. Namely, when scanning commences and
sequential sub-scanning proceeds, the difference signal for neighboring
pixels becomes larger and the image gradually becomes more clearly
defined. When the sub-scanning then progresses further, next, the
difference signal for neighboring pixels becomes smaller and the image
therefore gradually becomes less well defined. This peak position is then
the in-focus point. When the difference signals for neighboring pixels
becomes small as the sequential sub-scanning progresses, movement is in a
direction away from the in-focus point. It is therefore necessary to
change the direction of movement and when it is taken that the difference
signal has been large all along up to the end of the sub-scanning, it is
taken that the in-focus point does not exist within this width of changing
the focal point. Scanning is then executed against from this focal point
position so that a peak position is detected. This operation is
essentially the same as the frame focusing control of the related art but
differs in that rather than changing the position of the focal point every
frame, the position of the focal point is changed for every scanning line.
Further, the direction of the scanning lines is not horizontal but rather
is inclined. By adopting this approach it is possible to ensure the
rapidness of the focusing operation and to ensure a steady operation that
is not influenced by the pattern shape.
The pattern conditions are all the same as for that of the related art
where definition is compared every frame but in the autofocus operation of
the present invention, the pattern captured by each scanning line is not
necessarily the same. That shown in FIG. 1A where a uniform vertical
stripe pattern is scanned in the horizontal direction is given as an
exceptional example where the pattern conditions are the same for all of
the scanning lines. However, in the case of this invention, there may be a
vertical pattern or there may be a horizontal pattern and the pattern
conditions for each scanning line are not all the same. Looking at an
example of the case where the inclined scanning method of the present
invention is implemented on the horizontal stripe pattern shown in FIG.
1D, as shown in FIG. 1E, in a first scanning section a the scanning lines
do not pass through the pattern. In a following scanning section b, a
boundary from a plain-colored region to a pattern region is entered one
time. In a middle scanning section c, a boundary from a plain-colored
region to a pattern region is entered one time, and a boundary from a
pattern region exiting to a plain colored region is passed through one
time. Next, in a scanning section d, a boundary exiting from the pattern
region to the plain-colored region is passed through one time. Finally, in
a scanning section e, the scanning lines do not pass through the pattern.
It can therefore be determined that comparisons of the scanning line
images are meaningless in at least the scanning sections a and e where
autofocus control does not function. At the scanning sections b and d, the
directions of the changes in signal level have inverse relationships but
if absolute value comparisons are made then the conditions become the same
and comparison is possible. The conditions are the same within the
scanning section c but the orientation of the change in signal level at
the boundary upon entering the pattern region from the plain colored
region and the orientation of the change in signal level at the boundary
upon exiting from the pattern region to the plain colored region are
opposite to each other and it is therefore preferable to add their
absolute values. The number of times the boundaries are passed through at
the scanning section c and the scanning sections b and d is different and
a simple comparison is therefore meaningless. A corresponding relationship
is therefore composed where the signals for the scanning sections b and d
are doubled and compared.
Changes in conditions of patterns which the scanning lines pass through in
this manner corresponds to sub-scanning and are detected as step-shapes.
Various correspondence is then possible because the change can be
distinguished by signal changes which occur gradually by changing the
focal point. For example, in the case of scanning sections "a" and "e",
the difference signals for within the scanning lines are extremely small
and can be discerned as being no pattern. Meanwhile, movement of the focal
point is stopped for the next scanning line and it is rational to execute
scanning for the same focal point position. This is because the focusing
operation does not function at the scanning sections "a" and "e" for
whatever reason. When the scanning section b is entered, a change in the
step shape exceeding a threshold value can be seen in the difference
signal for the scanning line image. At this time it is detected that the
pattern region has been entered from the plain-colored region and
comparisons of signals are executed every scanning line while changing the
focal point position by a prescribed amount while moving to the next
sub-scanning position. The difference signals for between scanning lines
at this section are more like small consecutive changes rather than being
step-shaped and the focusing operation is then to obtain the peak values
for these changes. When scanning section c is entered, the signal again
changes to a step shape so that it is detected that a scanning region
having a boundary entering from the plain colored region to the patterned
region or a scanning region having a boundary exiting from the pattern
region to the plain colored region has been entered. At this time, if the
signal for the scanning section and the definition of the image are
compared, it is possible to double the signal for the scanning section b
and make the comparison. Changes in the step shape are typically such that
the boundary has increased or decreased by one and consecutive comparison
is then possible if correction is performed according to this number.
Scanning section d can then be handled in the same way as scanning section
b and scanning section e can be handled in the same way as for scanning
section a. However, in the present invention, the important thing is to
find peak positions for image difference signals between scanning lines.
If the peak value is therefore obtained by excluding the scanning sections
a and e where there is no pattern and performing comparisons between the
sections b, c and d, this position is then the in-focus point. In the
scanning method of the present invention, discontinuous points that change
into the step-shape occur but adding the correction described previously
so as to obtain intercorrelation is not always necessary. The step shape
signal changes can be used as a section separating signal and can be made
to correspond to operations every section.
In the above description, a description is given assuming inclined scanning
composed of horizontal scanning and vertical scanning is implemented using
deflection means during beam scanning in order to ensure that the scanning
lines reliably pass through a horizontal pattern (scan rotation). However,
the main objective is that the beam scanning lines pass through the
pattern whether it is a horizontal pattern or a vertical pattern. The same
results can therefore also be obtained even when executing a focusing
operation where the sample stage is rotated so that the horizontal
scanning direction and the horizontal pattern of the sample cross each
other.
Next, a description is given of a method for operating the autofocus of the
present invention. In the present invention, the main scanning is executed
while changing the focal point for every sub-scanning position. Namely, as
shown in FIG. 2, the position of the focal point of the electron beam 3
can be changed from a deep position to a shallow position, with these
positions sandwiching the surface position 21 of a semiconductor chip 2
constituting a sample and it is detected at which position the clearest
image is obtained. At this time, when the width of the focal positions
which are changed every scanning line becomes narrower, the precision of
the position specifying the in-focus point (resolution) becomes high,
while on the other hand there is the danger that the surface position 21
will not be captured within the width of change. Therefore, in the present
invention, as a first step, the width of the focal point positions which
are changed for every scanning line is made broad so that the focal point
position is changed over a broad focal point region L1 so that the surface
position 21 may be reliably caught within this width and the focusing
operation is then executed. However, as the width of focal point positions
for between scanning lines is broad in this case, rough position
information can be obtained in a short period of time but reliable
focusing position information cannot be obtained. In a second step, a
narrow region L2 reliably including the surface position 21 within it's
width is set based on this coarse position information. The width of focal
point positions changing every scanning line is then made narrow and the
focusing operation is executed once again. It is therefore possible to
detect the in-focus point with a high degree of precision because the
width of the in-focus point positions for each scanning line during this
time is set to be narrow. The focusing operation in the second step access
of the present invention is therefore capable of extrapolating the focal
point reliably over a broad range to give a precise result over a short
time.
It cannot be said that the focusing operation for an electron microscope is
such that once a focusing position is captured, then this is effective
over the whole of the surface of the sample on the stage. Depending on
whether the electron microscope is a high-resolution electron microscope,
the focal point may be undermined by microscopic undulations and curves in
the sample surface or by the operation of the stage drive mechanism. The
effectiveness of the obtained in-focus point position information is
roughly for around 5 mm. In the case as shown in FIG. 4 where
semiconductor chips are arrayed on a wafer, one chip may be 10 mm square
or 20 mm square, for example, so that the in-focus point information for
one point may not be effective over the whole region. Therefore, in the
present invention, a number of locations covering the whole region of the
chips are set and a location in the vicinity in which the pattern best
suited to a focusing operation exists (hereinafter referred to as an AF
point) is selected. When defects within the chip are then monitored, a
focusing operation is first carried out at the AF point covering this
defect position and the focus value for the lens that is obtained is used
during defect observation. Namely, as there are regions within the chip
where there is no pattern as well as locations where an AF point cannot be
applied, an appropriate location is selected based on the chip pattern and
recorded. Examples of AF points (P.sub.0, P.sub.1, P.sub.2, P.sub.3 and P4
in the drawings) selected for a chip 2 are shown in FIG. 3. AF points are
selected taking into consideration positions that cover the entire region
of the chip 2 and the shape of the pattern. The pattern structures for the
chips arrayed on the wafer 1 are all the same and the selected points AF
may therefore be locations corresponding to each chip respectively. When
noted defects are observed based on defect position information provided
by a defect checking device, AF points of the chips in the vicinity are
first accessed and the focusing operation is executed at these locations.
The noted defects are then accessed with the lens at the focusing
positioned obtained at this time and monitoring is performed. In this way,
AF points applicable to the focusing operation are recorded in advance.
This means not only that a reliable focusing operation can be executed,
but also that a region where the in-focus point information obtained at
the AF points is valid is known. When a plurality of defects then exists
within this region, the focusing operation is not repeated each time and
defects can be observed consecutively. This is therefore extremely
beneficial in that the operation time can be made short.
In the above description, an electron microscope is taken as the object but
the present invention can also be applied as is to an ion microscope that
is the equivalent to an electron microscope for scanning with a particle
beam and detecting secondary charged particles.
The automatic focusing system for a scanning charged particle microscope of
the present invention is provided with means for changing a focal point
each raster scan line, and control means for comparing image definition
each raster line and extrapolating an in-focus point. The focusing
operation is therefore dramatically faster compared to the control of
related art where the focal point position is changed every frame, image
definition is compared every frame, and a in-focus point is then
extrapolated. Further, the scanning line is an inclined scanning line that
is a combination of a horizontal component and a vertical component with
respect to a chip array direction on a semiconductor wafer. A reliable
focusing operation can therefore be achieved regardless of the fact that
the image region is a horizontal pattern or a vertical pattern.
In the automatic focusing system of the present invention, as a specific
method of control for comparing image definition each raster line and
extrapolating an in-focus point, scanning sections are distinguished from
changes in step shapes for between each scanning line and peak values are
extrapolated from small consecutive changes between each scanning line.
Therefore, even when the shapes of the patterns through which each
scanning lines passes is different, a peak value, i.e. a in-focus point,
can be obtained in a reliable and straightforward manner.
Moreover, in the present invention, in a method for implementing automatic
focusing control in a scanning charged particle microscope being equipped
with means for changing a focal point each raster scan line, and control
means for comparing image definition each raster line and extrapolating a
in-focus point, there is provided a first step of taking coarse in-focus
points using taken large differences between focal points for between
scanning lines, and a second step of detecting in-focus points with a high
degree of precision using small differences focal points between scanning
lines based on the coarse in-focus point information. It is therefore
possible to obtain an in-focus point both rapidly and reliably.
Further, a method for implementing automatic focusing control in the
present invention comprises the steps of: recording pre-selected focal
points taking into consideration the pattern arrangement of semiconductor
chips arrayed on the wafer and being capable of covering the entire
region, accessing the in-focus points covering positions of noted defects
obtained by a defect checking device and executing a focusing operation,
and consecutively monitoring defects in positions covered by the in-focus
points in the state of a focal position of a lens obtained at this time.
This means not only that a reliable focusing operation can be executed,
but also that a region where the in-focus point information obtained at
the AF points is valid is known. When a plurality of defects then exists
within this region, the focusing operation is not repeated each time and
defects can be observed consecutively. This has the benefit that the
operation time can be made shorter.
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