Pattern defect inspecting apparatus

A high-speed pattern defect inspecting apparatus with a high sensitivity and less erroneous detection. In the pattern defect inspecting apparatus, a wafer is scanned by an electron beam, secondary electron signal generated by the scanning is stored in an image memory, and the stored image is used to cause a display unit to be subjected to a brightness modulation. A reference pattern image previously stored in the image memory is compared with a detected wafer pattern image to find a difference between the both images, and the difference is detected as a defect in a wafer pattern. The wafer scanning of the electron beam is carried out only for an arbitrary specified part thereon.

BACKGROUND OF THE INVENTION 
The present invention relates to pattern defect inspecting apparatus and 
more particularly, to a pattern defect inspecting apparatus which can 
suitably inspect a defect in a pattern in fabrication of semiconductor 
devices, image pick-up elements, display elements, etc. 
Major examples of pattern defect inspecting apparatuses associated with the 
present invention include a scanning electron microscope (SEM), a laser 
scanning microscope, and a scanning inter-atomic force microscope. 
Explanation will be made in connection with an example of semiconductor 
fabrication as a typical application field. The SEM has been widely used 
in inspection of pattern defect. Thus explanation will be made in 
connection with the use of the SEM. 
FIG. 1 is an arrangement of an SEM for explaining its basic principle, in 
which an electron beam is used as a probe to perform a raster scan on the 
entire surface of a field of view. 
An electron beam 2 emitted from an electron gun 1 is accelerated, and 
converged through a condenser lens 3 and an objective lens 4, and then 
focused on a surface of a wafer 5 as an sample. Concurrently, the electron 
beam 2 is bent in its locus by a deflector 6 so that the beam 
two-dimensionally or one-dimensionally scans the entire surface of the 
wafer. Meanwhile, a part of the wafer, when subjected to an irradiation of 
the electron beam 2, emits secondary electrons. The secondary electrons 
are detected by a secondary electron detector 8 to be converted to an 
electric signal, the signal is converted by an A/D converter 9 to a 
digital signal and stored in an image memory 10. The stored signal is 
processed by an image processor 11 to be used for brightness modulation or 
Y modulation of a display unit 12. The display unit 12 is scanned 
similarly to the scanning of the electron beam 2 on the wafer, so that a 
sample image is formed on the display unit 12. When two-dimensional 
scanning and brightness modulation are carried out, an image appears on 
the display unit; whereas, when the Y modulation is carried out, a line 
profile is depicted on the display. 
Here is an example of procedure of inspecting a pattern defect with use of 
the SEM. 
A sheet of wafer 5 to be measured is extracted from a wafer cassette 13, 
and then subjected to a pre-aligning operation. The pre-alignment is to 
align the wafer direction with respect to an orientation flat or notch 
formed in the wafer as a reference. More in detail, after being subjected 
to the pre-alignment, the wafer 5 is fed into a sample chamber 14 kept in 
a vacuum and then placed on an X-Y stage 15 in the chamber. The wafer 5 
placed on the X-Y stage 15 is aligned with use of an optical microscope 16 
mounted in an upper part of the sample chamber 14. The alignment in this 
example is to correct a relationship between a positional coordinate 
system of the X-Y stage 15 and a pattern positional coordinate system of 
the wafer, for which end an alignment pattern formed on the wafer is used. 
More specifically, an image generated by the optical microscope 16 is 
converted by a CCD element or the like to an electric signal, converted by 
an A/D converter 17 to a digital signal, and then stored in the image 
memory 10. The stored signal is coupled to the display unit 12 via the 
image processor 11 so that the image of the optical microscope appears on 
the display unit 12. The image of the optical microscope magnified to a 
size about several hundred times the size of the alignment pattern is 
compared with a reference image of an alignment pattern previously 
registered, and the stage positional coordinate system is corrected so 
that its field of view exactly overlaps with the field of view of the 
reference image. After the alignment, a raster scan is carried out on the 
entire surface of a required inspection zone on the wafer using 
combination of the electron beam scan and stage movement to thereby form 
an SEM image. The formed SEM image is compared with the reference SEM 
image so that a difference between the images is detected as a pattern 
defect. Generally used as the reference SEM image is an SEM image of the 
same part in a chip or cell already inspected. 
In this connection, control over the storing and reading operation of the 
image signal, the processing of the image signal, pattern matching, etc. 
is carried out under control of a computer/controller 18. 
There is a reciprocal relationship between detection sensitivity for 
pattern defects and an inspection rate therefor. Both the detection 
sensitivity and inspection rate depend on pixel size (the number of pixels 
in the view field). When the pixel size is made small (when the number of 
pixels in the view field is increased), the pattern defect detection 
sensitivity can be made high but the inspection rate is decreased. That 
is, when a pattern defect is to be detected with a high sensitivity, its 
required inspection time is increased. 
There is also a correlation between the pattern defect detection 
sensitivity and error detection frequency. The defect detection 
sensitivity is proportional to the resolution of the SEM. When the SEM 
resolution is increased in order to increase the defect detection 
sensitivity, a fine structure other than the pattern edge becomes also 
clear. The clear fine structure is frequently erroneously judged as a 
pattern edge, which leads to noise caused by the pattern defect (error 
detection factor). In other words, when the pattern defect is detected 
with a high sensitivity, the error detection rate is increased. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide a pattern 
defect inspecting apparatus which can suitably perform pattern defect 
inspection with a high sensitivity at a high rate while preventing 
increase of an error detection rate. 
In accordance with the present invention, there is provided a pattern 
defect inspecting apparatus which scans a surface of a sample by means of 
a probe to inspect a defect in a pattern on the basis of information on 
the pattern imaged on the sample surface, and wherein the scanning of the 
probe on the sample is carried out for an arbitrarily specified area of 
the sample surface. 
The scanning of the present invention is based on a so-called vector scan. 
For this reason, even when pixel size is made small, a scanning area of a 
fine structure is narrow, so that the number of pixels is not so large. 
That is, even when the pixel size is made small to detect a pattern defect 
with a high sensitivity, only a short inspection time is required. 
Further, since information on areas of the fine structure other than the 
specified area cannot be obtained, it can be avoided that the 
non-specified areas of the fine structure result in noise caused by 
pattern edge detection. That is, even when the resolution is made and a 
high pattern defect is detected with a high sensitivity, the error 
detection rate will not increase. 
In accordance with the present invention, in this way, pinhole defects or 
island-like defects in the areas other than the specified area will not be 
detected. In addition, such defects will lead to a fatal defect in the 
element with a small probability and thus missing inspection will not 
involve a big disadvantage.

DETAILED DESCRIPTION 
Hardware of a pattern defect inspecting apparatus of the present invention 
is the same as that of FIG. 1, and thus explanation thereof is omitted to 
avoid repetition. 
Different methods of performing vector scan on a specified pattern area are 
shown, in model form, by chain-dotted lines, e.g., in FIGS. 2A, 2B and 2C. 
More in detail: 
(1) As shown in FIG. 2A, an electron beam is scanned on an area which is 
defined between an outside chain-dotted line and an inside chain-dotted 
line in FIG. 2A, that is, on an outer periphery of a pattern to be 
inspected, including a pattern edge. This method is valid when a line 
pattern is inspected or when such a convex defect as a residual resist or 
etched remainders is inspected. 
(2) As shown in FIG. 2B, an electron beam is scanned on an area surrounded 
by a chain-dotted line in FIG. 2B, that is, on an inside area of a pattern 
to be inspected, including a pattern edge. This method is valid when a 
hole pattern is inspected or when such a concave defect as a defect caused 
by improperly narrow resist or missing of etching is inspected. 
(3) As shown in FIG. 2C, an electron beam is scanned on a pattern edge area 
defined between a chain-dotted line located inside of a pattern to be 
inspected and a chain-dotted line located outside thereof. This method can 
also be applied to both of the above convex and concave defect 
inspections. When the methods of FIGS. 2A, 2B and 2C can be more 
effectively used in their combination for a part of the inspection pattern 
seemingly regarded as weak in the pattern formation. Any combination of 
the methods of FIGS. 2A, 2B and 2C may be employed in a single inspection 
work. 
Ones of methods for judging a pattern defect are shown, in model form, 
e.g., in FIGS. 3A, 3B, 3C, 4A, 4B, 4C, 5A, 5B and 5C. More specifically: 
(1) An SEM image (see FIG. 3A) is compared with a reference image (see FIG. 
3B) previously registered, e.g., by the aforementioned pattern matching 
technique to detect a difference between the both images as a pattern 
defect (see FIG. 3C). The comparison with the reference image may be 
achieved by pattern shape collation or pattern edge matching. 
(2) When a pattern image within a SEM image (see FIG. 4A) is overlapped 
with a specification range (an area surrounded by dotted lines in FIG. 4B) 
defined by predetermined upper and lower limit lines to find a part not 
overlapped and when a non-overlapped part is found, the part is detected 
as a defect (see FIG. 4C). 
(3) A sample line profile (see FIG. 5A) is compared with a reference line 
profile (defined by upper and lower dotted lines in FIG. 5B) previously 
registered, and a difference between the both profiles is detected as a 
pattern defect. 
(4) A part of the sample line profile (see FIG. 5A) located outside of a 
predetermined specification range of the reference line profile (see FIG. 
5B) is detected as a defect (see FIG. 5C). 
When the above methods (3) and (4) are used as combined with the method of 
FIG. 2C for scanning the pattern edge with a single stroke of electron 
beam, high speed inspection can be effectively achieved. In either case, a 
plurality of reference images or specification ranges as upper and lower 
specification limits may be set. 
In the judging operations of FIG. 3A, 3B, 3C, 4A, 4B, 4C, 5A, 5B and 5C, 
information about defect size and defect mode such as defect size, convex 
or concave defect can be obtained. It is also possible to add operational 
results classified according to the defect size and defect mode to the 
contents of the defect judgement. Even in the absence of a defect, the 
sample image may be different in its brightness or contrast between its 
chips or cells. 
When image parameters such as brightness, saturation and contrast of the 
sample and reference images are arranged to be modified respectively 
independently, or when profile parameters such as amplitude and contrast 
of the sample and reference line profile are arranged to be modified 
respectively independently; relative accuracy can be improved. 
Shown in FIG. 6 is an example of an operational procedure in the present 
invention. 
The wafer 5 is extracted from the wafer cassette 13 and then subjected to a 
prealigning operation (step S1). After the prealignment, a wafer number 
formed on the wafer 5 is read out by a wafer number reader (not shown) 
(step S2). The wafer number is uniquely given to the wafer. A recipe of 
the wafer previously registered in the computer/controller 18 is read out 
based on the read-out wafer number as a key (step S2). Prescribed in the 
recipe are the inspection procedure and conditions of the wafer. The 
subsequent operations are carried out automatically or semiautomatically 
according to the recipe. After the recipe is read out, the wafer 5 is fed 
into the sample chamber 14 kept in a vacuum and then placed on the X-Y 
stage 15 (step S3). The wafer 5 placed on the stage is aligned with use of 
the optical microscope 16 mounted in the upper part of the sample chamber 
14, and an alignment pattern formed on the wafer 5 (step S4). An image of 
the alignment pattern formed in the optical microscope is compared with a 
reference image of the alignment pattern previously stored in the image 
memory 10 as associated with the recipe, and a stage positional coordinate 
system is corrected so that its field of view is overlapped with a field 
of view of the reference image. After the alignment, a pattern formed on 
the wafer 5 to be inspected is accurately positioned with use of a 
positioning pattern formed on the wafer 5 (step S5). The positioning 
pattern is moved through a movement of the stage so as to be subjected to 
an electron beam, whereby its image is formed. The image of the 
positioning pattern, like the aligning operation, is compared with a 
reference image of the positioning pattern previously registered in the 
image memory 10 as associated with the recipe, and then the scanning area 
of the electron beam is finely adjusted so that the both images just 
overlaps with each other. The positioned wafer 5 is inspected for a 
pre-specified inspection area as to whether or not there is a defect in 
the pattern (step S6). For the pattern defect inspection, the 
aforementioned vector scan method and pattern defect detection means are 
used. 
Such inspection data as a coordinate position of a pattern defect part and 
an image thereof are registered in the computer/controller 18. Further, 
when a reference image therefor is also stored in the image file, 
reviewing operation after the inspection can be facilitated. 
In this manner, inspection of a sheet of the wafer is completed. When there 
are a plurality of wafers to be inspected still remaining in the wafer 
cassette, the next wafer is extracted from the wafer cassette and then 
repetitively subjected to the aforementioned inspecting operations 
according to the procedure of FIG. 6. 
The reference image may be previously registered prior to the inspecting 
operations or may be newly registered or re-registered in the inspecting 
operations. For example, the image of the same pattern part in the 
just-previously inspected chip or cell can be repetitively re-registered 
as a reference image in the inspecting operations. As the reference image, 
pattern design information can be used in place of the sample image. 
In the case where such a charged particle beam as an electron beam or an 
ion beam is used, with respect to such an sample that it takes a 
considerable time before its charge-up (charge accumulation on an 
insulator surface caused by the irradiation of the charged particle beam) 
is saturated, the charged particle beam may be irradiated for a 
predetermined period of time and then the sample image may be captured. 
Although an X-Y stage has been used in the above embodiment, the X-Y stage 
may be replaced by an X-Y-T stage (T meaning tilt) to perform the pattern 
defect inspection under such a condition that a sample is tilted. 
Only the pattern defect inspection has been explained above. However, when 
an abnormality is recognized in the inspection result, such analysis 
function as is done in a characteristic X-ray analyzer or an Auger 
electron analyzer may be attached as necessary so that analyzed data of 
the defect part can be also acquired. 
Though an electron beam has been employed for image formation in the above 
embodiment, the electron beam may be replaced by an ion beam, an optical 
beam or a mechanical probe. 
The above explanation has been made in connection with the case of one 
probe and one pixel, but the image formation may be carried out based on 
multi-probes and multi-pixels. 
The above explanation has been made in connection with the case where a 
semiconductor wafer is observed in the foregoing embodiment, but the 
semiconductor wafer may be replaced by an image pick-up element, display 
element wafer or a sample of any shape other than the wafer. 
It will be appreciated from the above explanation that compatibility can be 
realized between the pattern defect inspection with a high sensitivity and 
the high speed inspection with less-erroneous information. 
In accordance with the foregoing embodiment of the present invention, there 
is provided a pattern defect inspecting apparatus which can suitably 
perform pattern defect inspection with a high sensitivity at a high speed 
while avoiding an increase of error detection rate.