Photocopying – Projection printing and copying cameras – Step and repeat
Reexamination Certificate
2000-03-07
2004-10-26
Nguyen, Henry Hung (Department: 2851)
Photocopying
Projection printing and copying cameras
Step and repeat
C355S077000, C318S575000
Reexamination Certificate
active
06809798
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a method of controlling a stage in a stage apparatus that requires a high-speed, high-precision positioning capability. More particularly, the invention relates to the field of substrate scanning and exposure, and especially to a method of controlling the stage of a scanning exposure apparatus in which the pattern of a semiconductor or liquid crystal display elements is transferred to a substrate by photolithography. Further, the present invention falls under the category of exposure methods and apparatus as well as device manufacturing methods that utilize such a stage control method.
BACKGROUND OF THE INVENTION
A process for forming fine patterns on a device substrate in the manufacture of semiconductors and liquid crystal display devices generally utilizes a photolithography technique. An original pattern to undergo transfer is formed as a raised or recessed pattern on the light shielding surface of a glass substrate referred to as a reticle or mask. The pattern on this master plate exposed by illuminating light is projected for exposure onto a semiconductor wafer (referred to simply as a wafer below) or glass substrate for a liquid crystal, which wafer or substrate has been coated with a photosensitive photoresist, via projection optics, whereby the latent image of the pattern is transferred to the resist. The substrate to be exposed is itself worked by carrying out etching in which the ratio of the selectivity of the worked surface to that of the resist image formed by developing is high.
In photolithographic processes especially for manufacturing semiconductor devices, use of light in an exposure apparatus referred to as a stepper has been in the forefront until recently. This exposure method employs a step-and-repeat scheme in which exposure areas (shot areas) obtained by subdividing a wafer are successively moved into an exposure field of the projection aligner optics and exposed to the pattern on the reticle after the wafer is positioned and stopped.
There is an increasing demand for semiconductor devices of higher speed, greater capacity and lower cost and, in terms of the production line, there is a need also of an exposure apparatus that makes it possible to attain much finer working in small chip areas. Accordingly, there has been proposed a step-and-scan scheme that makes it possible to realize, with a high degree of productivity, an improvement in the resolving power of the projection optics and better pattern uniformity in the exposure area.
This scheme scans the reticle and wafer synchronously with respect to the projection optics and performs exposure at the same time that the shot area is scanned relative to the exposure field. The reticle and wafer are adjusted to have a demagnification ratio that the projection optical system provides both longitudinally and transversely, whereby the reticle pattern is projected onto the wafer at the demagnification ratio. It is required that the scanning speed of the exposure area be controlled so as to be constant in order to control the amount of exposure to a prescribed amount in conformity with the properties and film thickness of the resist and in dependence upon the reticle pattern. To this end, the reticle and wafer must each attain a prescribed scanning speed before the reticle and the area to be exposed on the wafer enter the slit-shaped exposure area of the exposure apparatus.
FIGS. 10A and 10B
illustrate the control sequence of a stage device holding a reticle or wafer.
FIG. 10A
illustrates the velocity profile (target velocity vs. time) of the stage and
FIG. 10B
illustrates the acceleration profile (target acceleration vs. time) of the stage.
In
FIGS. 10A and 10B
, an elapsed time from a start of scanning is plotted along the horizontal axis. The end time indicates the time relating to one shot of exposure (shot processing time). The shot processing time is an important indicator that decides the processing time of one wafer.
As shown in
FIG. 10A
, shot processing time is divided into five intervals. Stage operation will now be described in relation to these intervals.
First, the stage is accelerated along the scanning direction from the at-rest position until it attains a target scanning velocity. This is the acceleration time and corresponds to interval I. After acceleration ends, velocity is controlled in such a manner that a variation in sychronization will fall within a tolerance. This is the settling time and corresponds to interval II. The distance traveled by the stage during the total of the acceleration time and settling time is referred to as the approach distance. The stage is then caused to travel at a constant velocity to perform exposure. This is the exposure time and corresponds to interval III. Next, the stage is allowed to travel for a time approximately equal to the settling time while the scanning velocity thereof is held constant. This is the post-settling time and corresponds to interval IV. Deceleration of the stage then starts and the stage comes to rest with respect to the scanning direction. This is the deceleration time and corresponds to interval IV. The distance traveled by the stage during the total of the post-settling time and deceleration time is referred to as the overrun distance. The approach distance and overrun distance are approximately equal.
In most cases of stage operation described above, the scanning direction is reversed after the stage is decelerated and stopped, and then the operation of intervals I to V is repeated. In rare cases, scanning is repeated in the same direction.
In the case of a wafer stage, movement of the exposure field is started toward the scanning starting position of the next exposure shot after the exposure in interval III ends. The move operation is referred to as a stepping operation. The start of stepping movement of the wafer stage in a direction perpendicular to the scanning direction is carried out immediately after exposure ends. In the scanning direction, however, there is no particular stepping movement or stepping movement is started at the end of operation in interval V. The reason for this is as follows: In many cases the next exposure shot is immediately adjacent in the direction perpendicular to the scanning direction and, as a result, the position at which the stage in interval V ends is the starting position of the next scan, thus making it unnecessary to take the trouble to perform the stepping operation. In other words, the operations in intervals IV and V themselves may be said to be transposable in terms of operation in the scanning direction until the next exposure shot.
The following problems arise in the prior art:
(1) Exposure energy Ed per wafer unit area is proportional to the illuminance of the slit light and inversely proportional to scanning velocity. In the example of the prior art described above, scanning velocity is held substantially constant during post-settling time. Consequently, if the photoresist has little sensitivity and the scanning velocity is low, overrun time lengthens and throughput declines. This tendency becomes more pronounced if settling time is prolonged in order to improve synchronization precision at the time of exposure.
(2) In the prior art, the operations of intervals I to V are repeated for all shot areas. Accordingly, if, when a row of shots is changed, the scanning direction of the final shot of the initial row is the forward direction and the direction of step and scan for the first shot of the next row is the reverse direction, then, after exposure of the final shot of the row ends, it is required that the wafer stage be reversed after traveling a prescribed overrun distance in the Y direction and that the stage travel a further distance equal to the sum of the overrun distance and the stepping distance. Accordingly, the amount of unnecessary travel equivalent to double the overrun distance inevitably causes a decline in throughput. In addition, after exposure of the final shot of the wafer, the overrun operation for exposure of
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