Photocopying – Projection printing and copying cameras – Step and repeat
Reexamination Certificate
1997-05-08
2001-03-20
Mathews, Alan A. (Department: 2851)
Photocopying
Projection printing and copying cameras
Step and repeat
C356S401000, C430S005000, C430S022000
Reexamination Certificate
active
06204912
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates generally to exposure apparatus and methods and, more particularly, to an exposure method, an exposure apparatus, and a mask that are suitable for, for example, manufacturing an active matrix liquid crystal display (liquid crystal panel) having a switching device.
In recent years, a high-resolution color liquid crystal display (LCD) with a wide screen has been used as a display for personal computers or television sets. The screen size (screen diagonal) of a current LCD is typically 10-12 inches; however, a wide screen LCD with a 16-inch screen, 20-inch screen, or still wider screen, is being developed. In response to the increased screen size, the resolution is also improved, and an LCD having a VGA (640X480) pixel matrix, XGA (1024X768) pixel matrix, or SXGA (1280X1024) pixel matrix is being manufactured. An active matrix LCD is superior in the response characteristic of image display, wide view angle characteristics, and multi-tone characteristics. In many applications, a thin film transistor LCD (TFT/LCD), which uses a thin film transistor as a switching device in each pixel, has been used.
FIG. 16
illustrates an example of the pixel structure of a TFT/LCD in a conventional display device as an enlarged plan view, which shows a portion of the array substrate (on which TFTs are formed). A plurality of gate lines
132
are formed on the glass substrates
142
along the horizontal direction of a display screen, and a plurality of data lines
130
are formed in the vertical direction. Areas defined by the gate lines
132
and the data lines
130
are display pixel areas, in which a transparent pixel electrode
134
made of ITO (indium tin oxide) is formed. A gate electrode
136
, which is derived from the gate line
132
, is formed in a corner of each display area. A channel layer
144
made of, for example, amorphous silicon (&agr;-Si) is formed on the gate electrode
136
with a gate insulation film (not shown) therebetween. A drain electrode
140
, which is derived from the data line
130
, and a source electrode
138
, which is electrically connected to the transparent electrode
134
, are formed on the channel layer
144
simultaneously. A TFT is composed of a gate electrode
136
, a gate insulation film, a channel layer
144
, and source and drain electrodes
136
,
140
.
The circuit pattern in each layer, which constitutes a TFT/LCD, is formed through a photolithographic process, in which a projection exposure apparatus is used to expose the circuit pattern formed on a photomask or reticle (collectively, referred to as a reticle) onto a resist layer (photosensitizer) formed on the glass substrate
142
. The resist layer is developed and used as a mask on which the circuit pattern has been transferred. Using the photoresist mask, a semiconductor layer made of, for example, &agr;-Si is etched to form the channel layer
144
. The gate line
132
, gate electrode
136
, data line
130
and source/drain electrodes
138
,
140
are formed by etching a metal interconnection layer.
There are two types of exposure apparatus, namely, a step-and-repeat-type and a scanning-type. In a scanning-type exposure apparatus, the reticle and the glass substrate are moved in synchronization with each other.
In a step-and-repeat-type apparatus, the photosensitive substrate (glass substrate) mounted on the movable stage is driven in a step-and-repeating manner to successively expose a portion of the reticle pattern onto a predetermined area on the photosensitive substrate in a section-by-section manner. In this type of exposure apparatus, a plurality of reticles are held in a reticle changer. The reticle changer and the stage are driven so as to successively expose a portion of the multiple reticle patterns onto one of the divided pattern areas on the photosensitive substrate, thereby forming a first layer. Other patterns on different reticles held in the reticle changer are subsequently exposed to form a second layer over the first layer.
FIG. 17
illustrates the first and second layers LY
1
and LY
2
exposed onto the photosensitive substrate P, which is mounted on the movable stage of an exposure apparatus. Unit patterns LY
1
A and LY
1
B of the first layer LY
1
are exposed successively onto the photosensitive substrate P, and LY
1
A and LY
1
B are combined through a stitching portion JN. Similarly, unit patterns LY
2
A and LY
2
B of the second layer LY
2
are successively exposed over the first layer, and LY
2
A and LY
2
B are combined through the stitching portion JN.
The movable stage (not shown in
FIG. 17
) that supports the photosensitive substrate P is moved within the X-Y plane in a controllable manner, and the position of the photosensitive substrate P mounted on the movable stage is controlled within the X-Y coordinate system. If, for example, the first unit pattern LY
1
A of the first layer LY
1
is exposed onto the substrate with an offset of −&Dgr;x from the target exposure position, and the first unit pattern LY
2
A of the second layer LY
2
is exposed with an offset of +&Dgr;x from the target exposure position, then the offset of the first pattern LY
2
A of the second layer LY
2
becomes +2&Dgr;x relative to the first unit pattern LY
1
A of the first layer LY
1
, which corresponds to the distance between the exposure positions of LY
1
A and LY
2
A.
If the second unit pattern LY
1
B of the first layer LY
1
is exposed with an offset of +&Dgr;x from the target exposure position, and the second unit pattern LY
2
B of the second layer LY
2
is exposed with an offset of −&Dgr;x from the target exposure position, then the offset of the second unit pattern LY
2
B of the second layer LY
2
becomes −2&Dgr;x relative to the second unit pattern LY
1
B of the first layer LY
1
, which is the distance between the exposure positions of LY
1
B and LY
2
B. Accordingly, the total offset of the second layer LY
2
relative to the first layer LY
1
becomes +4&Dgr;x with respect to the stitching JN, as shown in FIG.
18
.
If such an offset occurs during the exposure process, in a thin film transistor of the liquid crystal panel, the drain electrode DR and the source electrode SO formed in the second layer LY
2
are offset by +4&Dgr;x relative to the gate electrode GA formed in the first layer LY
1
, as shown in FIG.
19
.
The hatched areas PIL
1
and PIL
2
of the drain electrodes DR, which overlap the gate electrodes GA, define the capacitor capacitance generated between the gate electrode GA and the drain electrode DR. Change in the capacitance results in variation in the holding voltage of the thin film transistor. If the overlapping areas PIL
1
and PIL
2
differ in the left and right sides of the liquid crystal panel with the stitching portion JN as a boundary, the light-permeability of the liquid crystal panel varies from area to area. Consequently, the contrast differs between the left and right halves of the liquid crystal panel, separated at the stitching portion JN.
As the glass substrate
142
is enlarged along with the increased size of TFT/LCDs, a scanning-type projection exposure apparatus with a plurality of projection lens systems has been preferably used to increase the projection exposure area of the apparatus. In such a scanning-type projection exposure apparatus, the circuit pattern on a reticle is divided into multiple trapezoid areas when exposed onto a glass substrate. The reticle and the glass substrate are synchronously scanned with respect to the projection lens systems. In this manner, the entire area of the reticle circuit pattern is transferred to the glass substrate.
FIG.
20
(
a
) shows a portion of the projection area formed on the glass substrate
142
by a scanning-type projection exposure apparatus. The trapezoid projection areas
150
,
152
formed through individual projection lens systems overlap each other in the Y direction by a predetermined amount. This arrangement enables the circuit pattern to be illuminated uniformly. In the figure, the glass substrate
142
Fujimori Nobutaka
Nara Kei
Seki Masami
Toguchi Manabu
Tsuchiya Makoto
Mathews Alan A.
Nikon Corporation
Nixon & Vanderhye P.C.
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