Optics: measuring and testing – Dimension – Width or diameter
Reexamination Certificate
2002-02-26
2004-01-20
Pham, Hoa Q. (Department: 2877)
Optics: measuring and testing
Dimension
Width or diameter
C348S079000
Reexamination Certificate
active
06680781
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a critical dimension measurement method and apparatus utilizing an optical microscope and a two-dimensional image sensor such as a CCD camera. In particular, the present invention relates to a very narrow line size measurement method and apparatus suitable for, although not restricted to, measuring a critical dimension, such as a magnetic head track width and a line width of a photomask for semiconductor fabrication, in a contactless manner.
JP-A-59-176605, for example, describes a basic configuration of a size measurement apparatus. A configuration similar thereto is shown in FIG.
11
.
In
FIG. 11
, an image of an object
307
projected by an optical microscope
305
is picked up by a CCD camera
308
. A size measurement processor
309
electrically measures sizes of desired sections of the image obtained by the CCD camera
308
. The image and values of sizes of the object
307
are displayed on a video monitor
310
. Numeral
306
denotes a light source.
FIG. 6
shows the display screen of the video monitor
310
and a luminance distribution of an image of an object (here a magnetic head surface)
307
picked up by the CCD camera
308
of
FIG. 11
on an object scanning line
6
(Li) of the video monitor
310
. In this case, as is clear from the luminance distribution on the measurement object scanning line
6
(Li), the luminance becomes largest at the pole
5
portion of the magnetic head.
This critical dimension measurement apparatus obtains the size of a line width of an measurement object (in this case, the size of the track width which is the width of the pole
5
) such that a video signal corresponding to the scanning line
6
(Li) is stored in a frame memory (not illustrated) provided in the size measurement processor
309
pixel by pixel to determine respective N-divided pixel positions and obtain the line width of the measurement object based on the N-divided pixel positions and their respective luminance values. More specifically, a maximum luminance level
61
and a minimum luminance level
62
in the luminance distribution are assumed to be 100% and 0%, respectively. A positional difference Nab between an a-th pixel and a b-th pixel which each correspond to a luminance level
63
of 50% is obtained. The positional difference Nab is multiplied by a coefficient k determined according to a magnification factor of the optical microscope
305
and a light receiving size of the CCD camera
308
to obtain a value of size M of the object
307
in accordance with equation (1) below. This method is referred to as edge detection method for the sake of convenience.
M=k×Nab
(1)
On the other hand, a minimum size limit in size measurement becomes the resolution of the optical microscope. The resolution &agr; is represented by equation (2).
&agr;=&lgr;/(2×
NA
) (2)
where &lgr; is the wavelength, and NA is the numerical aperture of the object lens.
However, the above-described conventional method has a problem that the measurement becomes impossible when the actual size of the object is smaller than the resolution &agr; of the optical microscope. A microscopic size measurement method for solving this problem is disclosed in JP-B-6-103168 (Japanese Patent No. 1967489).
SUMMARY OF THE INVENTION
Hereafter, the problem of the above-described conventional technique will be described by referring to FIG.
7
.
FIG. 7
is a diagram showing relations between the resolution &agr; of an optical microscope and the luminance distribution. In
FIG. 7
, numeral
34
denotes a luminance distribution obtained by picking up the image of a white line having a width 2&agr;, numeral
37
a minimum luminance level, numeral
38
a maximum luminance level (VH), and numeral
41
a threshold level, that is, an intermediate level (50% level) between the maximum luminance level
38
and the minimum luminance level
37
. Furthermore, numeral
35
denotes a luminance distribution of a white line having an width &agr;, numeral
39
a maximum luminance level, and numeral
42
a threshold level (50% level). Furthermore, numeral
36
denotes a luminance distribution of a white line having a width less than &agr;, numeral
40
a maximum luminance level, and numeral
43
a threshold level (50%). As shown in
FIG. 7
, the luminance distribution
34
of the white line with the width 2&agr;, the luminance distribution
35
of the white line with the width &agr;, and the luminance distribution of the white line
36
with the width less than &agr; have sizes
53
,
54
and
55
according to the edge detection method, respectively. Although the relation
53
>
54
holds true, it becomes impossible to determine which one of
54
and
55
is greater or less than the other, which means that, from the luminance distribution of the white line having the width less than &agr;, it is impossible to measure the size of the white line having the line width less than &agr;.
Furthermore, in
FIG. 7
, numeral
44
denotes a luminance distribution of a black line having a width 2&agr;, numeral
47
a minimum luminance level, and numeral
50
a threshold level (50% level). Furthermore, numeral
45
denotes a luminance distribution of a black line having a width a, numeral
48
a minimum luminance level, and numeral
51
a threshold level (50% level). Furthermore, numeral
46
denotes a luminance distribution of a black line having a width less than &agr;, numeral
49
a minimum luminance level, and numeral
52
a threshold level (50% level). As shown in
FIG. 7
, the luminance distribution
44
of the black line with the width 2&agr;, the luminance distribution
45
of the black line with the width &agr;, and the luminance distribution
46
of the black line with the width less than a have sizes
56
,
57
and
58
according to the edge detection method, respectively. Although the relation
56
>
57
holds true, it becomes impossible to determine which is greater,
57
or
58
, and the size of the black line having the width less than a cannot be measured.
It is now assumed that the numerical aperture of the object lens having a magnification factor of 100 is NA=0.95.
When a visible-ray optical microscope is used and the light source wavelength is &mgr;=0.55 &mgr;m, the resolution is, &agr;=0.29 &mgr;m;
when an ultraviolet-ray optical microscope is used and the light source wavelength is &lgr;=0.365 &mgr;m, the resolution is, &agr;=0.19 &mgr;m; and
when a deep ultraviolet-ray optical microscope is used and the light source wavelength is &lgr;=0.248 &mgr;m, the resolution is, &agr;=0.13 &mgr;m. It is impossible to measure objects having the sizes less than these sizes.
In order to address this problem, a method described in JP-B-6-103168 (Japanese Patent No. 1967489) has already been proposed.
FIG. 8
is a diagram for explaining the critical dimension measurement according to this conventional technique. An image of the object
307
shown in
FIG. 11
is picked up by the CCD camera
308
. A video signal corresponding to one scanning line for size measurement is taken in the size measurement processor
309
. Its luminance level is digitized by an A-D converter (not illustrated). The digitized luminance level is stored in a frame memory (not illustrated) formed of a series of storage elements on a pixel by pixel basis. A luminance level characteristic in each pixel position at this time is shown in FIG.
8
. It is now assumed that pixel addresses on the frame memory are 0 to N and a luminance level of an address i is Vi. It is also assumed that a maximum value
61
of a stored luminance level
60
is its 100% level and a minimum value
62
of the stored luminance level
60
is its 0% level. A value of a threshold level T
L
63
is set to be, for example, a 50% level. Addresses of pixels a and b having the same luminance level as the threshold level T
L
63
are obtained. And luminance levels at all addresses between the pixels a and b (i.e., luminances above the threshold l
Kosuge Shogo
Shimizu Takahiro
Hitachi Kokusai Electric Inc.
Pham Hoa Q.
LandOfFree
Critical dimension measurement method and apparatus capable... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Critical dimension measurement method and apparatus capable..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Critical dimension measurement method and apparatus capable... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3218445