Radiant energy – Photocells; circuits and apparatus – Photocell controls its own optical systems
Reexamination Certificate
2000-03-23
2003-03-25
Pyo, Kevin (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Photocell controls its own optical systems
C356S401000
Reexamination Certificate
active
06538260
ABSTRACT:
FIELD OF THE INVENTION AND RELATED ART
This invention relates to a position measuring method and a semiconductor exposure apparatus using the same. More particularly, the invention is concerned with a position measuring method and a semiconductor exposure apparatus using the same, which is particularly suitably applicable, for example, in a semiconductor exposure apparatus for projecting and printing an electronic circuit pattern onto a semiconductor substrate, to perform position measurement for an alignment mark of a wafer, to be used for precise alignment, or to perform relative alignment between a wafer and a mask, between a mask and a certain reference position in an apparatus, and between components in an apparatus, for example.
Precise position measurement for an article is used in various fields, such as work machines or robots, for example, and further improvements in its precision have been desired. As regards recent semiconductor devices, the degree of integration of each device is increasing more and more, as can be represented by DRAM, and the linewidth of a pattern to be formed on the semiconductor device is decreasing more and more. In these situations, in a process for measuring the relative position of a reticle and a wafer and for aligning them with each other, which process is an essential technique in a semiconductor exposure apparatus, a further improvement of precision is a critical matter.
A conventional semiconductor exposure apparatus as well as examples of conventional wafer alignment methods will be described below.
FIG. 2
is a schematic view of a semiconductor exposure apparatus. Denoted in the drawing at R is a reticle (original), and denoted at W is a wafer (substrate). Denoted at
1
is a projection optical system. Denoted at
2
is an alignment illumination means, and denoted at
3
is a beam splitter. Denoted at
4
and
5
are imaging optical systems. Denoted at
6
is an image pickup device, and denoted at
7
is an A/D (analog-to-digital) converter. Denoted at
8
is an integration device. Denoted at
10
is a stage driving means, and denoted at
11
is an X-Y stage which is movable two-dimensionally.
While in
FIG. 2
only one alignment optical system G for X-direction measurement is illustrated, there is an additional alignment optical system (not shown) for performing Y-direction measurement, like the X-direction measurement. In the semiconductor exposure apparatus shown in
FIG. 2
, the relative position of the reticle R and the wafer W is detected, and then they are brought into alignment with each other. Thereafter, exposure light is projected from an exposure illumination light source (not shown), by which an electronic circuit pattern formed on the reticle R is projected and transferred to the wafer W, placed on the X-Y stage
11
, through the projection optical system
1
.
The mask-to-wafer alignment in the apparatus of
FIG. 2
will be described below.
The alignment illumination device
2
emits non-exposure light (to which a resist is not sensitive). The light emitted from the illumination device
2
goes through the beam splitter
3
, the reticle R and the projection optical system
1
, and it illuminates an alignment mark formed on the wafer W.
FIGS. 3A and 3B
are schematic views for explaining the alignment mark, and it comprises plural rectangular patterns of the same shape. The light reflected by the alignment mark goes again through the projection optical system
1
and the reticle R, and it is reflected by the beam splitter
3
. Then, after passing through the imaging optical system
5
, it produces an image W
M
of the alignment mark upon the image pickup surface of the image pickup device
6
. The image pickup device
6
then functions to photoelectrically convert the thus formed image W
M
of the mark, and a signal converted is applied to the A/D converter
7
where it is transformed into a two-dimensional digital signal array. The integration device
8
serves to set a processing window W
P
(
FIG. 3B
) to the wafer mark image as digitalized by the A/D converter
7
. Further, the integration device
8
operates to perform integration processing in the window W
P
along the Y direction, to transform the two-dimensional imagewise signal into a one-dimensional mark waveform S(x) such as shown in FIG.
3
A. The position measuring device
9
in
FIG. 2
serve to measure the position of the alignment mark, on the basis of the one-dimensional waveform S(x) as outputted from the integration device
8
.
The procedure described above is repeated, by which positional information is produced in relation to plural measurement points. On the basis of this positional information as well as information related to the relative position of the reticle R and the image pickup device
6
, having been detected beforehand, the stage driving means
10
moves the X-Y stage
11
to accomplish alignment between the mask and the wafer.
Next, the method of measuring the alignment mark position in the position measuring device
9
will be explained.
FIG. 4
illustrates an example of a conventional alignment mark position measuring method. In regard to
FIG. 4
, for simplicity of explanation, a case where an alignment mark is provided by a single rectangular pattern will be described. If an alignment mark is provided by plural rectangular patterns, similar operations may be repeated.
In
FIG. 4
, a registration calculation step S
102
is a process for calculating the centricity (degree of registration) of the mark, and repeated calculations are made with respect to a certain mark position measurement range having been preset. For example, as shown in
FIG. 5
, by repeating the registration calculation step S
102
to a mark waveform S(x)
1
, the registration degree r(x)
1
can be determined.
Now, two examples for conventional registration degree calculating processes will be explained.
In a first example, the degree of registration between a mark waveform (detected waveform) and a preset template waveform (reference waveform) is calculated while shifting the template position. The template position where the registration degree becomes highest is taken as the mask position. Hereinafter, this method will be called a “template matching method”. The registration degree can be calculated, on the basis of the difference between the mark waveform and the template waveform. The registration degree r(x) at a position x upon the mark waveform can be determined in accordance with equation (1) or equation (2), below.
r
(
x
)
=
1
∑
k
=
-
w
/
2
w
/
2
|
S
(
x
+
k
)
-
T
(
k
)
|
(
1
)
r
(
x
)
=
1
∑
k
=
-
w
/
2
w
/
2
{
S
(
x
+
k
)
-
T
(
k
)
}
2
(
2
)
in equation (1) and (2), S(x) is the mark waveform, T(x) is the template waveform, and w is the waveform width for calculating the registration degree, and it corresponds to the width of the template.
A second example is that a mark waveform is laterally and symmetrically folded at a certain position and the registration degree between the left-hand side and right-hand side mark waveforms is calculated while shifting the folding position. The folding position with which the registration degree becomes highest is taken as the mark central position. Hereinafter, this method will be called a “folding method”. The registration degree can be calculated on the basis of a difference between the left-hand and right-hand mark waveforms. The registration degree r(x) at a position x upon the mark waveform can be determined in accordance with equation (3) or equation (4) below.
r
(
x
)
=
1
∑
k
=
0
w
/
2
|
S
(
x
+
k
)
-
S
(
x
-
k
)
|
(
3
)
r
(
x
)
=
1
∑
k
=
0
w
/
2
{
S
(
x
+
k
)
-
S
(
x
-
k
)
}
2
(
4
)
A highest registration calculation step S
103
in
FIG. 4
is a process for determining the highest registration degree position (template position or folding position) as can be calculated by the registration calculation at step S
102
, such that the thus determined position is taken as the mark center position. The position where the
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
Pyo Kevin
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