Phase-shift-moiré focus monitor

Details Phase-shift-moiré focus monitor Phase-shift-moiré focus monitor

2001-08-31

2003-03-18

Font, Frank G. (Department: 2877)

Optics: measuring and testing

Focal position of light source

C356S124000, C356S399000, C356S400000

Reexamination Certificate

active

06535280

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to optical apparatus in semiconductor technology, and more particularly, to a test monitor for use in focusing an image on a semiconductor wafer.
2. Discussion of the Related Art
Typically, an optical system
30
(
FIG. 1
) used for patterning photoresist
32
on a semiconductor wafer
34
comprises a light source
36
, a mask or reticle
38
having opaque lines
40
and transparent portions
42
, and a lens
44
, the light from the light source
36
passing through the transparent portions
42
of the mask/reticle
38
through the lens
44
and to the photoresist
32
, with light being blocked from reaching the lens
44
(and photoresist
32
) by the opaque lines
40
of the
38
mask/reticle.
As is well known, there is a need to position the wafer
34
at a proper distance with respect to the lens
44
so that images that fall on the photoresist
32
of the wafer
34
will be in the plane of best focus.
Typically, prior to actual fabrication of semiconductor devices, a test focus monitor in the form of for example a reticle is used as part of the overall system to achieve proper focus of the image on the wafer. An example of such a monitor is shown and described in the paper entitled “New Phase Shift Gratings For Measuring Aberrations”, by Hiroshi Nomura, published by SPIE, dated Feb. 27, 2001, which is herein incorporated by reference.
FIGS. 2-4
herein show a monitor
50
configured as shown in
FIGS. 3 and 5
of that paper. The monitor
50
is made up of a quartz base
52
which is transparent to light, and which has a plurality of parallel, opaque, spaced apart lines
54
on the base
52
, the lines
54
having a first set of adjacent ends
55
, and a second, opposite set of adjacent ends
56
. The area between each adjacent pair of lines
54
is transparent to light and is made up of regions
58
which pass light therethrough without changing the phase thereof, and regions
60
which pass light therethrough which shift the phase of such light by 90° (the phase shifting caused by recesses
62
in the base
52
—
FIGS. 3 and 4
and the above cited paper). Each of the lines
54
has a region
58
and a region
60
which are aligned along and on one side thereof, and a region
58
and a region
60
which are aligned along and on the opposite side thereof. Each of the lines
54
has a region
58
on one side thereof opposite a region
60
on the other side hereof, these regions
58
,
60
running from end
55
of that line to adjacent to the middle thereof, and furthermore, each of the lines
54
has a region
60
on the one side thereof opposite a region
58
on the other side thereof, these regions
60
,
58
running from end
56
to adjacent the middle thereof.
FIGS. 3 and 4
are views similar to that shown in
FIG. 1
, but incorporating the monitor
50
of
FIG. 2
as a part of the system
30
.
FIG. 3
includes a sectional view of the monitor
50
taken along the line
3
—
3
of
FIG. 2
, showing a cross-section of the upper area
50
A of the monitor
50
, wile
FIG. 4
includes a sectional view of the monitor
50
taken along the line
4
—
4
of
FIG. 2
, showing a cross-section of the lower area
50
B of the monitor
50
. As will be seen, with reference to the upper area
50
A of the monitor
50
(FIG.
3
), moving the wafer
34
and lens
44
relatively together and apart causes the shadows
64
A,
64
B,
64
C formed on the photoresist
32
of the wafer
34
(formed by the opaque lines
54
) to shift (downward as the wafer
34
and lens
44
are moved relatively further apart). Meanwhile, with reference to the lower area
50
B of the monitor
50
(FIG.
4
), moving the wafer
34
and lens
44
relatively together and apart causes the shadows
64
D,
64
E,
54
F formed on the photoresist
32
of the wafer
34
to shift (upward as the wafer
34
and lens
44
are moved relatively further apart). The dotted lines
66
in
FIGS. 3 and 4
indicate the traverse of the shadows
64
A,
64
B,
64
C,
64
D,
64
E,
64
F as the wafer
34
is so moved relatively toward and away from the lens
44
.
These paths are plotted in
FIG. 5
, and the intersections thereof indicate the best focus of the image on the wafer
34
.
FIG. 6
includes
FIGS. 6A-6F
which are views taken along the lines
6
A—
6
A,
6
B—
6
B,
6
C—
6
C,
6
D—
6
D,
6
E—
6
E, and
6
F—
6
F of
FIGS. 3 and 4
. With the wafer
34
and lens
44
closest together as shown in
FIGS. 3 and 4
,
FIGS. 6A and 6D
show the simultaneous positions of the shadows
64
A-
64
F on the photoresist
32
determined by the respective areas
50
A,
50
B of the monitor
50
With the wafer
34
and lens
44
so positioned relative to each other, the photoresist
32
is exposed to light from the light source
36
and is then developed to determine photoresist lines, which corresponds to the positions of the shadows
64
A-
64
F. As will be seen, the lines of
FIGS. 6A and 6D
are misaligned As the wafer
34
and lens
36
are moved relatively further apart to an intermediate position as shown in
FIGS. 3
in
4
,
FIGS. 6B and 6E
show the simultaneous positions of the shadows
64
A-
64
F on the photoresist
32
determined by the respective areas
50
A,
50
B of the monitor
50
. Again, the photoresist
32
is exposed to light from the light source
36
and is then developed to determine photoresist lines that correspond to the positions of the shadows
64
A-
64
F. As will be seen, the lines of
FIGS. 6B and 6E
are substantially in alignment. Then, as the wafer
34
and lens
44
are moved relatively further apart, i.e., to their most far apart positions as shown in
FIGS. 3 and 4
,
FIGS. 6C and 6F
show the simultaneous is positions of the shadows
64
A-
64
F on the photoresist
32
determined by their respective areas
50
A,
50
B of the monitor
50
. Again, with the wafer
34
and lens
44
so positioned relative to each other, the photoresist
32
is exposed to a light from the light source
36
and is then developed to determine photoresist lines, which correspond to the positions of the shadows
64
A-
64
F. As will be seen, the lines of
FIGS. 6E and 6F
are misaligned.
It will be seen that changing the distance between the lens
44
and wafer
34
causes the shadows
64
A-
64
C to move further in and out of alignment with the shadows
64
D-
64
F. The process of moving the lens
44
and wafer
34
relatively closer together and further apart, along with the corresponding exposure and development of the photoresist
32
accompanying each adjustment is repeated until the lines formed in the photoresist
32
are substantially straight. This is illustrated in
FIG. 6
of the above cited paper.
While such an approach is useful, only a relatively coarse reading of focus is achievable. For example, with reference to
FIG. 6
of the above cited paper, only a small shift in the pattern from top to bottom is shown over a range of 400 nm of relative movement between the wafer
34
and lens
44
. With device dimensions continually being reduced, there is a need to achieve a proper reading of focus within a much smaller range of lens-wafer relative movement, for example, 200 nm or less.
Therefore, what is needed is an apparatus which is capable of providing proper focus of an image on a wafer through a very small range of relative movement between a lens and a wafer.
SUMMARY OF THE INVENTION
In the present invention, an optical tool is provided, made up of a tool body having a first plurality of parallel, substantially opaque, spaced apart lines thereon, and a second plurality of parallel, substantially opaque, spaced apart lines thereon with a relatively small angle between the first and second pluralities of lines. As an image of the lines of the first plurality thereof is provided on a semiconductor body, such line images move relative to the semiconductor body as the semiconductor body is moved relatively toward and away from the optical tool. Furthermore, as an image of the lines of the second plurality thereof is provided on the semiconductor body, s

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