U.S. patent application number 12/423197 was filed with the patent office on 2009-08-13 for pattern measuring method and pattern measuring device.
Invention is credited to Hitoshi KOMURO, Ryoichi MATSUOKA, Hidetoshi MOROKUMA, Akiyuki SUGIYAMA, Takumichi SUTANI.
Application Number | 20090200465 12/423197 |
Document ID | / |
Family ID | 36931972 |
Filed Date | 2009-08-13 |
United States Patent
Application |
20090200465 |
Kind Code |
A1 |
SUTANI; Takumichi ; et
al. |
August 13, 2009 |
PATTERN MEASURING METHOD AND PATTERN MEASURING DEVICE
Abstract
A pattern measuring method and device are provided which set a
reference position for a measuring point to be measured by a
scanning electron microscope and the like, based on position
information of a reference pattern on an image acquired from the
scanning electron microscope and based on a positional relation,
detected by using design data, between the measuring point and the
reference pattern formed at a position isolated from the measuring
point.
Inventors: |
SUTANI; Takumichi;
(Hitachinaka, JP) ; MATSUOKA; Ryoichi;
(Hitachinaka, JP) ; MOROKUMA; Hidetoshi;
(Hitachinaka, JP) ; KOMURO; Hitoshi; (Hitachinaka,
JP) ; SUGIYAMA; Akiyuki; (Hitachinaka, JP) |
Correspondence
Address: |
ANTONELLI, TERRY, STOUT & KRAUS, LLP
1300 NORTH SEVENTEENTH STREET, SUITE 1800
ARLINGTON
VA
22209-3873
US
|
Family ID: |
36931972 |
Appl. No.: |
12/423197 |
Filed: |
April 14, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11359374 |
Feb 23, 2006 |
7518110 |
|
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12423197 |
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Current U.S.
Class: |
250/311 |
Current CPC
Class: |
H01J 37/265 20130101;
H01J 2237/2817 20130101; H01J 37/28 20130101 |
Class at
Publication: |
250/311 |
International
Class: |
G21K 7/00 20060101
G21K007/00 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 25, 2005 |
JP |
2005-049923 |
Claims
1. A pattern measuring method for acquiring an image of a pattern
on a specimen and measuring the pattern on the image, the pattern
measuring method comprising the steps of: acquiring an image of a
reference pattern arranged at a position isolated from a measuring
point on the specimen; and setting a reference position for the
measurement based on a position of the reference pattern on the
image and based on design data for a portion including the
reference pattern and the measuring point.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This application is a Continuation of U.S. application Ser.
No. 11/359,374, filed Feb. 23, 2006, which claims priority from
Japanese Patent Application No. 2005-049923, filed Feb. 25, 2005,
the contents of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0002] The present invention relates to a pattern measuring method,
a pattern measuring device and a program, and more particularly to
a method, device and program for measuring a pattern in an acquired
image.
[0003] It has been known to measure a pattern on a semiconductor
integrated circuit by using CAD (Computer Aided Design) data.
Design data such as CAD data represents intended, ideal geometries
of semiconductor devices, so comparison between the CAD data and an
actually formed pattern can evaluate a semiconductor manufacturing
process. In U.S. Pat. No. 6,868,175B1 and U.S. 2002/0015518A1 is
disclosed a technology which detects an amount of deformation of a
pattern with respect to design data by detecting an edge of a
pattern to be inspected and an edge of a reference pattern and
comparing these detected edges.
[0004] As described above, the actually formed pattern exhibits a
shape different from that of the design data because of
manufacturing process effects. Many different shapes of patterns
are formed on a semiconductor wafer. There is no definite criterion
for position alignment between the design data and the actual
pattern and thus it is not possible to measure, according to some
reference, a degree to which the pattern being inspected is
deviated from an ideal pattern represented by the design data or
how much the pattern is deformed.
SUMMARY OF THE INVENTION
[0005] To solve the problem described above, a reference position
for a measuring point to be measured by a SEM (scanning electron
microscope) and the like is set based on position information of a
reference pattern on an image acquired from the SEM and based on a
positional relation, detected by using design data, between the
measuring point and the reference pattern formed at a position
isolated from the measuring point.
[0006] In this construction, since the position of the reference
pattern is detected from the image acquired from the SEM and, with
this position as a reference, the reference position for
measurement is set using the design data, it is possible to
evaluate to what extent an actual pattern is deviated or deformed
from an ideal pattern location or pattern shape, by using the
design data and the position information based on a SEM image.
[0007] Other objects, features and advantages of the invention will
become apparent from the following description of the embodiments
of the invention taken in conjunction with the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] FIG. 1 illustrates an outline of a SEM (scanning electron
microscope).
[0009] FIG. 2 shows circuit design data and a pattern superimposed
together.
[0010] FIG. 3 is a flow chart from a generation of a dimension
measuring recipe to an evaluation based mainly on the circuit
design data.
[0011] FIG. 4 shows an example of an image acquisition point.
[0012] FIG. 5 shows an example of a box-in-box pattern.
[0013] FIG. 6 shows where on a chip of a wafer the design data
corresponds to.
[0014] FIG. 7 shows an example pattern used to determine a distance
between the design data and a position on the wafer.
[0015] FIG. 8 is a diagram showing a positional relation between a
first addressing point and an image acquisition point.
[0016] FIG. 9 is a diagram showing how a second addressing point is
searched.
[0017] FIG. 10 is a diagram showing a positional relation between
the searched second addressing point and the image acquisition
point.
[0018] FIG. 11 shows a detailed example on the circuit design data
of FIG. 10.
[0019] FIG. 12 shows an example in which positions of a SEM image
and the circuit design data are aligned at the second addressing
point.
[0020] FIG. 13 shows an example image acquisition point cut off
FIG. 12.
[0021] FIG. 14 shows an example in which a measuring point is
determined from an image obtained from FIG. 13.
[0022] FIG. 15 shows an example in which a distance is measured
between an imaginary line segment not present on a wafer and a
pattern.
[0023] FIG. 16A and FIG. 16B show a location of measurement
performed in FIG. 15 and an intensity of a secondary electron
signal at that location, respectively.
[0024] FIG. 17 is a flow chart showing a detailed sequence of
information setting in a SEM recipe.
[0025] FIG. 18 is a conceptual diagram of a generated recipe.
DESCRIPTION OF THE EMBODIMENTS
[0026] An outline of a SEM (scanning electron microscope) will be
explained in the following. An electrooptical system of FIG. 1
focuses a charged particle beam (electron beam) 2 emitted from a
charged particle source (electron gun) 1--that releases electrons,
or charged particles--by a lens 3 onto a specimen 4 and scans the
specimen in a desired sequence. Secondary particles (e.g.,
secondary electrons) 5 produced at the surface of the specimen 4 as
a result of application of the electron beam are detected by a
secondary particle detection system 6, from which they are supplied
as image data to a control system 7 (control processor) with an
image calculation control function. The specimen 4 can be moved in
all three-dimensional directions by an X-Y-Z stage 8. The control
system 7 performs control on the charged particle source (electron
gun) 1, lens 3, secondary particle detection system 6, X-Y-Z stage
8 and also on a display 9.
[0027] In this example, the electron beam 2 is scanned over the
specimen 4 two-dimensionally (in X-Y directions) by a scanning coil
not shown. A signal detected by a secondary electron detector in
the secondary particle detection system 6 is amplified by a signal
amplifier in the control system 7 and then transferred to an image
memory and shown as a specimen image on the display 9. The
secondary signal detector may be one that detects secondary
electrons or reflected electrons or one that detects light or X
rays.
[0028] An address signal corresponding to a memory position on the
image memory is generated in the control system 7 or in a separate
computer, converted into an analog signal and supplied to the
scanning coil. When, for example, the image memory is 512.times.512
pixels, an address signal in the X direction is a repetitive
digital signal ranging from 0 to 512 and an address signal in the Y
direction is a repetitive digital signal ranging from 0 to 512 that
is incremented by 1 when the X-direction address signal reaches a
value of 512. These address signals are converted into analog
signals.
[0029] Since the address of the image memory matches an address of
a deflection signal for scanning the electron beam, the image
memory is recorded with a two-dimensional image of a beam
deflection area in which the scanning coil deflects the electron
beam. The signal in the image memory can be chronologically read
out in sequence by a read address generation circuit synchronized
with a read clock. The signal read out based on the address is
converted into an analog signal that becomes a luminance modulation
signal of the display 9.
[0030] The control system 7 is provided with an input device not
shown, by which an image retrieve condition (scan speed and total
number of imaged pages), a viewing field correction method and an
image output and storage can be specified.
[0031] The device of this example has a function to form a line
profile based on detected secondary electrons or reflected
electrons. The line profile is formed according to an amount of
electrons detected when scanning a primary electron beam
one-dimensionally or two-dimensionally, or according to brightness
information of the specimen image. The line profile thus obtained
is used, for example, to measure dimensions of a pattern formed on
a semiconductor wafer.
[0032] While in FIG. 1 the control system 7 has been described to
be integral with the SEM or configured in an equivalent state, it
is not limited to these conditions. For example, processing
explained in the following may be performed by a control processor
provided separately from the SEM. In that case, it is necessary to
provide a transmission medium by which to transfer a detection
signal detected by the secondary signal detector to the control
processor or transfer a signal from the control processor to the
lens or deflector of the SEM, and also an input/output terminal to
input or output signals transferred via the transmission
medium.
[0033] The device of this example also has a function which stores,
as a recipe in advance, conditions (such as measuring locations and
optical conditions of the SEM) for observing a plurality of points
on a semiconductor wafer and performs measurements and observations
according to a content of the recipe.
[0034] It is also possible to register a program for the processing
explained in the following with a storage medium and execute the
program by the control processor that supplies necessary signals to
the SEM. That is, an example explained below also covers a program
or program product that can be employed in a charged particle beam
device such as a SEM with an image processor.
[0035] Further, the control system 7 includes a design data
management unit 10 which stores design data of a pattern formed on
the semiconductor wafer and converts it into data required for the
control of the SEM. The design data management unit 10 has a
function to generate a recipe for controlling the SEM based on the
design data of the semiconductor pattern entered from an input
device not shown. The design data management unit 10 also has a
function to rewrite the recipe according to a signal transferred
from the control system 7. Although, in this example, the design
data management unit 10 is described to be separate from the
control system 7, it may be otherwise. For example, the control
system 7 and the design data management unit 10 may be integrated
with each other.
[0036] This example takes a wafer in the semiconductor product
manufacturing process as the specimen 4. A resist pattern formed on
the wafer in the lithography process was used. For comparison with
the resist pattern, semiconductor circuit design data (CAD data)
that constitutes a base for the pattern was used. The semiconductor
circuit design data used here represents an ideal geometry for the
final semiconductor circuit pattern. Although in the following
description, the subject to be inspected is taken to be a
semiconductor wafer, other subjects may be used as long as there is
a correspondence between the design data and a subject to be
evaluated. The circuit design data may be of any kind if software
that displays the circuit design data can display its format and
handle it as graphical data.
Embodiment
[0037] Conventionally, in a SEM an observer manually specifies
measuring points. Thus, the locations of the measuring points need
to be found on the wafer but it is very difficult to determine the
specified locations on the wafer. After the measuring points are
specified, the observer must set conditions necessary to prepare a
recipe for the SEM, such as addressing points and autofocus points,
for each measuring point. Therefore, the precision of this manual
setting necessarily depends on the experience of the observer.
[0038] Further, since there is a limiting condition that the
preparation of the dimension measuring recipe requires a SEM and a
wafer, an efficient work has not been possible. If, for example, a
pattern is chosen as an addressing point, whether that choice is
appropriate or not cannot be known until it is actually matched to
the pattern on the wafer. By repeating such a trial and error the
dimension measuring recipe for the SEM is made. A prolonged time
taken by the recipe preparation means a reduction in the
efficiency. Another factor that should be pointed out for the bad
efficiency of the conventional method is that the SEM must be used
even during the generation of the dimension measuring recipe,
during which time other measurements cannot be made.
[0039] As the design rule of semiconductor devices is getting
smaller in recent years, the use of an exposure wavelength shorter
than a pattern critical dimension (CD) necessarily increases an
optical proximity effect and an optical proximity correction
technology has a growing significance. The optical proximity effect
is a phenomenon in which even patterns with the same reticle CD may
have different pattern CDs depending on the environment in which
the patterns are placed.
[0040] The environment referred to here is, for example, a pattern
pitch. Under the condition that a pattern CD on a mask and a
pattern CD on a wafer are equal in a dense environment, an isolated
pattern in a coarse environment become narrower in CD. This
phenomenon is called the optical proximity effect and a technology
to compensate for this effect is the optical proximity correction.
Increasing the reticle CD of the isolated pattern can adjust the
pattern CD on a wafer to be equal to that of the dense pattern.
[0041] Such a simple optical proximity correction can be evaluated
by measuring the pattern CD. In recent years, an evaluation method
has been proposed which measures a pattern shape in order to make
an optical proximity correction with higher precision. One such
example involves measuring a pattern position with respect to a
reference.
[0042] FIG. 2 represents a case where design data and a pattern on
the wafer can be superimposed precisely. In this case, not only can
the two dimensions be measured but their positional relation can
also be measured. Measuring the positions can provide more
information than can the pattern CD. So, the position information
can be expected to contribute to more precise optical proximity
correction. Since there is no such reference on the wafer, the
pattern positions have been difficult to measure. But this
measurement can be made by overlapping the circuit design data over
the wafer pattern in an image acquired by the SEM.
[0043] This example is briefly explained as follows by using a flow
chart of FIG. 3. First, using the circuit design data, evaluation
points to be inspected are determined. For each of the evaluation
points, a recipe for making measurements by SEM is generated.
According to the recipe thus prepared, an image of the evaluation
points on the wafer to be inspected is acquired by using the SEM.
The acquired image is compared to the circuit design data for
evaluation. While in this example, the comparison is made with the
circuit design data, other reference patterns may be used as long
as they can be compared. For example, comparison may be made
between a simulated image obtained from the design data and an
image acquired by the SEM.
[0044] Individual steps in the flow chart of FIG. 3 are explained
below one by one. In this example, design data used for evaluation
is a simulated circuit design pattern with a minimum line width of
90 nm which is generated in GDSII format. In a step of determining
evaluation points in this simulated circuit design pattern, a
lithography simulator is used to inspect a wafer following the
lithography process.
[0045] When inspecting a wafer following other processes,
inspection points may be determined by using an associated
simulator. For example, in a pattern following an etching process
an etching simulator may be used to determine inspection points. In
this example, a lithography simulator of Sigma-C make, Solid-C, was
used. This simulator can directly handle GDSII format data of the
circuit design pattern and, by using process conditions of the
lithography process from the circuit design pattern, it is possible
to specify in what pattern the circuit design pattern will be
obtained following the lithography process.
[0046] A condition used for simulation is: a projection optical
system reduction ratio of 1/4, an exposure wavelength of 193 nm, a
Numerical Aperture (NA) of 0.73, a coherence factor .sigma. of
0.75, a ring shield factor .epsilon. of 0.67, and a set exposure
energy of 28 mJ/cm.sup.2. This condition is the same as used for
wafers manufactured later. A design rule checker was used to
select, from a pattern geometry obtained by the simulation, those
locations that are considered likely to cause defects. This is a
tool that automatically detects locations in the pattern formed by
the simulation where the dimensions will become short. In this
example, a design rule checker incorporated in Calibre of Mentor
Graphics was used. The design rule checker detects as hazardous
points those locations where lines are formed to less than 80 nm in
width or to more than 100 nm. In this example, only the line width
are checked for uncertain locations, it is also possible to extract
uncertain locations in other respects. The detected uncertain
points were output as coordinates on the circuit design
pattern.
[0047] That is, for a design rule of 90 nm, lower and upper
threshold are set at 80 nm and 100 nm, respectively, and those
locations where the line width is equal to or in excess of these
thresholds are extracted as hazardous points and their coordinates
with respect to the design data are output. In this example, the
extracted information is shown on a display device or display 9 in
the design data management unit 10. This arrangement facilitates a
decision making as to where on a semiconductor wafer formed with a
plurality of different patterns the measuring points (evaluation
points) should be set. It is also possible to provide an input
device such as a pointing device in the design data management unit
10 or control system 7 and to select, from among the uncertain
points, desired locations as the measuring points. By making
arrangements so that the above selection causes the recipe to be
automatically rewritten, the operator can specify, from among the
uncertain points, the locations to be measured based on his or her
experience, realizing both the proper selection of uncertain points
and the more efficient evaluation in terms of time. An arrangement
may also be made which involves automatically recording the
detected uncertain points in the recipe as locations to be measured
and later deleting unnecessary measuring locations from them.
[0048] Next, a recipe is generated for the SEM to inspect the
output coordinates. To measure the evaluation points by the SEM, it
is necessary to change, according to the environment in which the
evaluation points are placed, the set conditions including wafer
alignment points for correcting not only the coordinates of
evaluation points but also stage coordinates and chip coordinates,
addressing points leading to the wafer alignment points used to
make position correction step by step, and locations where
autofocus and autostigma are performed.
[0049] In this example, areas for the addressing points and
autofocus and autostigma points are automatically determined from
the design data.
[0050] Further, since the design data can be handled as an
addressing template, the generation of the dimension measuring
recipe has become possible without a wafer on which the dimension
measurement is to be performed. The settings of other than the
above coordinates (magnification factor on the addressing points
and measuring points, acceleration voltage, specimen current,
electrooptical system condition and contrast condition) were
specified as the measurement conditions by the observer. This is
because these settings cannot be determined from the circuit design
data but are the information that should be determined by the
observer.
[0051] The recipe for the SEM involves first performing the wafer
alignment to correct the coordinates between chips formed on the
wafers. This allows coordinates on the chips to be used in the SEM.
Next, images of individual hazardous points are acquired. In this
invention the addressing is performed in steps. The first
addressing corrects errors in the stage precision of the SEM and
the next addressing corrects the position of the patterns to be
measured. Since the guaranteed range of a stage stopping accuracy
in the SEM used in this invention is 4 .mu.m, the magnification at
the first addressing point was fixed at 20K, which sets one side of
the viewing field (square) to 6.75 .mu.m. It is of course possible
to perform the addressing with a smaller magnification, i.e., with
a viewing field measuring more than 4 .mu.m on one side. In this
example, the magnification was set to the one at the first
addressing point.
[0052] As described above, the first addressing point depends on
the stage stopping accuracy of the SEM used. The second addressing
point is selected within the same viewing field as the final
measuring point. An example case will be explained as follows in
which an image is stored using 2048.times.2048 pixels. If
individual pixel areas in the SEM image are considered equal, the
2048.times.2048 pixels produce a SEM image in the viewing field
four times the side, and 16 times the area, of the viewing field
with 512.times.512 pixels. The 2048.times.2048-pixel area includes
not only the measuring point but also the second addressing point.
The final image to be acquired is a 512.times.512-pixel image with
one side measuring 900 nm. Other magnification factors may be used
as long as they produce a desired resolving power.
[0053] In this example, it is already known in advance that this
magnification factor produces a desired resolving power, so this
magnification factor was used. Under this condition, the second
addressing point is a square area which includes a 900-nm square
and which measures 4.times.900 nm, or 3600 nm, on each side.
Because the number of pixels in each side of the acquired image is
quadrupled, the side of the image area is also quadrupled. If an
image of this square area is acquired and if the pattern acquired
by SEM can be matched to the circuit design data at appropriate
locations/areas in this square area, then the finally obtained
image lies at a position matching the design data.
[0054] FIG. 4 schematically shows hazardous locations output from
the lithography simulator and the design rule checker, i.e., a
900-nm square area with a point from which an image should be
acquired located at the center, and a 3600-nm square area including
the 900-nm square area. At this stage, however, the second
addressing point is not yet determined, so the process for
specifying the second addressing point will be described later.
[0055] For the wafer alignment point that is set at the initial
stage, an alignment precision correction mark located on the chip
is used. In manufacturing a semiconductor device, a device layer is
laid over an immediately preceding layer to form an intended
device. The alignment precision correction mark is used to check if
the overlapping precision is within a range of specification and,
in the processes following the overlapping step, the overlapping
precision is verified by a dedicated device. Representative of this
mark is a pattern called a box in box, such as shown in FIG. 5. Two
squares are each on separate layers and a deviation between the
center coordinates of the two boxes is detected as an alignment
accuracy.
[0056] Since this pattern is put at a predetermined position on
each device layer, its coordinates can be specified from the design
data. As a template image for alignment, design data was used.
Other patterns than the box-in-box pattern may be used to perform
the wafer alignment.
[0057] Next, the image acquired by the SEM and the design data must
be matched in position. Generally, there is no particular rules
regarding a coordinate origin for the design data. So, unless a
correlation value between them is given, there is no association
between the coordinates of the design data and the SEM image. In
FIG. 6 a hatched area is assumed to represent the design data and
an outer frame a chip of a wafer. It is very difficult to handle
the circuit design data for the same area as the wafer chip area
because of the data size. In this example, the circuit design data
was prepared for only a limited area in which a point to be
measured exists. This arrangement made the data easy to handle.
[0058] In order to match the positions of the design data and the
SEM image, coordinates of a particular position are determined in
each coordinate system and their coordinates are made equal. In
this example, an H-shaped character pattern of an appropriate size
is arranged on the design data as shown in FIG. 7, and the
coordinates in the two systems of the same location are used. The
SEM pattern, unlike the design data, is curved at the corners, so
two line segments are extended and an intersecting point is taken
to be the corner.
[0059] Under the above conditions, detailed information on the SEM
recipe is determined. FIG. 17 is a flow chart showing details of an
information setting sequence. First, a final measuring point and
its magnification factor are specified (S0001). Although this
example first specifies the measuring point and the magnification
factor, others may be specified.
[0060] Next, to determine a point for correcting the position of
the SEM image, i.e., an addressing point, a candidate area is
determined. More specifically, the candidate area needs only to be
within a range in which the viewing field can be changed by moving
the electron beam path. If the candidate area is an area extending
15 .mu.m up/down and left/right from the evaluation point as the
center, the addressing point is determined within this range. In
other words, the addressing point needs to be set in an area 30
.mu.m on each side.
[0061] Even if the SEM stage stopping accuracy is lowest, when the
stage fails to stop within the viewing field, the SEM image cannot
be matched and its position not corrected. So, the magnification
factor and the viewing field at the first addressing point depend
on the stage stopping accuracy of the SEM. The size of the
addressing template is determined within a 30 .mu.m square area. In
this example, the size of the template image of the addressing
point is determined by limiting the viewing field to a square area
6750 nm in one side within the 30 .mu.m square area and a location
suited for addressing is searched as shown in FIG. 8 (S0002)
(S0003).
[0062] In this example, since the side of the viewing field was
6750 nm that gave a wider area than the stage stopping accuracy. A
wider viewing area will pose no problem at all in terms of system
because the template image falls in this viewing field if there is
an error in the movement of the SEM stage. In this example, a
square area 6750 nm in one side was automatically determined as a
template in the square viewing field 30 .mu.m in one side by
applying a normalization correlation method.
[0063] While in this example an area suited for addressing was
automatically determined by using a method described in "Iwanami
Course: Multimedia Information 5, Information Processing on Image
and Space, p. 56". The addressing area was determined by excluding
an area of the final image to be acquired.
[0064] In this example, a cross pattern as shown in the figure was
determined as the first addressing point. Next, the autofocus point
and the autostigma point were automatically determined from the
design data in a way similar to that of the first addressing point.
The determination of the autofocus point and the auto stigmapoint
differs from that of the first addressing point in that their
appropriate areas are determined in the same viewing field as the
final image acquisition magnification factor (in this invention, a
900-nm square area).
[0065] Next, a decision is made as to whether the area in which the
first addressing point was set is allowed to overlap the measuring
area (S0004). If there is a pattern in the addressing area, the
autofocus can be executed in that area. The operator makes a
decision as to whether the addressing point should be searched in
an area including the measuring area (S0005) or in an area not
including the measuring area (S0006). In the last step, the
operator determines the position/area of the first addressing point
(S0007).
[0066] Next, a location for the second addressing point is
determined. In a 3600-nm square area encompassing the final,
required image area of 900-nm square, the addressing point needs to
be determined by the method described above in an area excluding
the image acquisition point. It is noted, however, that at the
first addressing point one pixel of the SEM image is about 13-nm
square area and that an addressing error necessarily occurs within
the range of this pixel size.
[0067] To prevent a possible positional deviation at the second
addressing point due to this error, a location for the second
addressing point is searched in a 3550-nm square area, more than 15
nm inside the 3600-nm square area. Since the final acquired image
needs only to be included in the 3550-nm square area, the second
addressing point is searched in a square area measuring 3550
nm.times.2-900 nm=6200 nm in one side with the image acquisition
point at the center as shown in FIG. 9. In this example, the
magnification factor (viewing field) at the second addressing point
was set equal to that of the measuring area (S0008). In generating
a dimension measuring recipe, before determining the position and
area of the second addressing point (S0013), the search area for
the second addressing point is set (S0009) and the operator decides
whether the second addressing area is allowed to overlap the
measuring area (S0010), as in the case of the first addressing
point. The operator decides whether the addressing point should be
searched in an area including the measuring area (S0011) or in an
area not including the measuring area (S0012), and then determines
the position/area of the second addressing point.
[0068] If, as a result of the search, the second addressing point
is determined as shown in FIG. 10, a middle point on a line segment
connecting the centers of the image acquisition point and the
second addressing point is taken as the center of the final,
acquired image (S0014). Since the final image acquisition area is a
3600-nm square area, this area includes the second addressing point
and is an area of the final image acquired by the SEM. Further,
addressing is performed in this area to extract only a 900-nm
square area and overlap the design data and the SEM image, thereby
producing a completely aligned, overlapped image.
[0069] A recipe for acquiring the SEM image is generated in the
manner described above for each hazardous point output from the
process simulator. The result of automatically determining the
second addressing point in the 6200-nm square area with the image
acquisition point at the center is shown in FIG. 11. That is, this
area is the most characteristic shape and therefore suited for the
addressing point. From the second addressing point and the image
acquisition point, the final, acquired image area was
determined.
[0070] A pattern is formed on a wafer, for example, in a process
described below. First, a coat type reflection prevention film is
spin-coated on the wafer to a thickness of about 60 nm. This is
further spin-coated with a chemically amplified, positive resist
film about 200 nm thick. This wafer is masked with a photomask
formed from the design data used in this example and exposed under
the same condition as used in the process simulator: a projection
optical system reduction ratio of 1/4, an exposure wavelength of
193 nm, an NA of 0.73, a coherence factor .sigma. of 0.75, a ring
shield factor .epsilon. of 0.67, and set exposure energy of 28
mJ/cm.sup.2. After exposure, the wafer is subjected to a post
exposure baking (PEB) at 100.degree. C. for about 90 seconds and
then immersed in an alkaline developing liquid of 0.21 N for about
60 seconds for development to form a transferred pattern on the
wafer.
[0071] An image of hazardous locations on the wafer is acquired by
the SEM. The recipe used for the acquisition is generated from the
design data by the method described above. Since the recipe is
based on the design data, if the design data is not matched to the
position of the image acquired by the SEM, the addressing point
does not function and thus the final, required image cannot be
acquired. In this example, a method described in JP-A-2002-328015
was used as the matching method.
[0072] This method makes it possible to use the design data as the
template for the SEM, thus acquiring a SEM image of hazardous
points. The result of matching the acquired SEM image to the design
data in the 3600-nm square area including the second addressing
point and the hazardous point is schematically shown in FIG. 12.
This shows a SEM image in an image area including the second
addressing point and the design data corresponding in position to
the SEM image and overlapped on it. They are matched in the viewing
field.
[0073] In this example, the second addressing point isolated from
the hazardous points is taken as a reference pattern, and a
reference position for measurement (e.g., line portion of a line
pattern at left in FIG. 12) is set based on the information of
reference pattern position on the image and the design data for a
portion including the reference pattern and the hazardous points
(points to be measured). As shown in FIG. 12, the second addressing
point and the position on the SEM image corresponding to the second
addressing point are matched, overlapping the SEM image and the
design data. This overlap allows the reference position for
measurement to be set by using the actual position information
obtained from the actual SEM image and an ideal position relation
between the reference pattern obtained from the design data and the
measured location.
[0074] Particularly in this example, since the measured location is
a line pattern extending vertically, there is a problem that the
reference position for measurement in the vertical direction in the
figure is difficult to set precisely. However, by taking a pattern
extending in both vertical and horizontal directions in the figure
as the second addressing point, a position alignment precision in
both vertical and horizontal directions in the figure can be
secured, allowing the measurement reference position in the
vertical direction of the figure to be located with high precision.
This method of setting the measurement reference proves very
effective, for example, in evaluating by how much the front end of
the line pattern has shrunk with respect to the design data.
[0075] An example dimension measuring method using the measurement
reference position set by the above method will be explained.
[0076] FIG. 13 shows only an image acquisition area extracted from
the acquired SEM image and design data overlapped on the viewing
field image. Although in this viewing field, the design data and
SEM image seem deviated in position, their positional relation is
correct because the viewing field is cut off a larger area that has
been matched with the design data.
[0077] A SEM image obtained from this figure is evaluated with
respect to the design data. To make an evaluation in terms of a
line width detected by the design rule checker, the line width of
the SEM image in this viewing field is first measured.
[0078] In the case of FIG. 13, it can be evaluated how much the
actually formed pattern is deviated from the design data as a
reference.
[0079] In the case of FIG. 14, a difference between the SEM image
and the design data is taken to be an evaluation value. Comparison
with the design data allows for an evaluation in other respects
than the line width. A distance between a line segment of the
design data and an edge segment of the SEM image can be taken as an
evaluation item. In the case of FIG. 14, it is possible to evaluate
how much the pattern has shrunk in the vertical direction or a
degree of lateral deviation with respect to the design data as a
reference. In the case of a contact hole pattern, though not shown,
a new contact hole evaluation method can be proposed which makes
comparison between the reference positions and the upper and bottom
parts of the contact hole. In this case, the reference positions
for both the upper and bottom parts of the contact hole may be made
settable.
[0080] Further, data thus obtained may be displayed as a wafer map.
An example method for displaying the data as a wafer map is
described in JP-A-2001-110862 (U.S. Pat. No. 6,765,204).
[0081] These distances are the values that can only be obtained if
the SEM image and the associated design data are overlapped
correctly. Additional overlap of the design data corresponding to
the next process of the device pattern allows for an evaluation in
other respects.
[0082] Square design data shown dashed in FIG. 15 represents an
ideal position of a pattern formed in the next process of the
semiconductor device fabrication. Since there is no overlay error
for each piece of design data, a degree of superimposition with the
data shown in a dashed line may be used as an evaluation reference.
In this example, a distance was measured between the dashed line
design data and the pattern end. Comparison between the SEM image
pattern acquired in this manner and the design data allowed a new
measurement value to be calculated. Parameters required for
measurement were not determined from the circuit design data but
determined optimally from a SEM image after it was acquired. In
this example, a linear approximation method was used for the
measurement based on the SEM image. A threshold in this method was
set at 50% of a maximum intensity of secondary electron signal.
[0083] FIG. 16A shows a measuring point in a SEM image. A secondary
electron signal was detected over an area from the top to the
bottom of the figure. FIG. 16B shows an intensity of a secondary
electron signal. The position of the right side of the figure is
determined by the linear approximation method with its threshold
set at 50%. The position of the left side of the figure was already
determined by the matching between the circuit design data and the
SEM image, so the dimension could be measured from both of their
values. Measurements were able to be made at other locations by the
similar method.
[0084] FIG. 18 is a conceptual diagram showing a generated SEM
measurement recipe. Such a conceptual diagram is shown on a display
during the recipe generation process so that a person making the
recipe (operator) can decide whether the image acquisition area is
appropriate or not. In this example, areas to be set in the recipe
generation process are shown on the circuit design data to make it
possible to ascertain whether there is an overlapping in the region
where the electron beam is scanned. Further, displaying these areas
in different colors for discrimination and indicating their roles
allows them to be easily identified.
[0085] For example, AP1/AF indicates that the area of interest is a
first addressing area in which the autofocus is to be executed. If
the electron beam scanning is performed for addressing and
autofocus before the measurement is made, the measured value may
differ from that when such an electron beam scanning is not
performed. Therefore, the above indication is effective.
[0086] It should be further understood by those skilled in the art
that although the foregoing description has been made on
embodiments of the invention, the invention is not limited thereto
and various changes and modifications may be made without departing
from the spirit of the invention and the scope of the appended
claims.
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