Optics: measuring and testing – By alignment in lateral direction
Reexamination Certificate
2000-10-27
2002-06-04
Font, Frank G. (Department: 2877)
Optics: measuring and testing
By alignment in lateral direction
Reexamination Certificate
active
06400456
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a plane positioning apparatus and, more particularly, to one suitable for application to an autofocusing mechanism or an autoleveling mechanism in a so-called step-and-scanning type exposure apparatus which sequentially exposes a pattern on a reticle onto each shot area on a photosensitive substrate by synchronously scanning the reticle and the substrate in respect to a slit radiating region that is, for example, a rectangular or an arc in shape.
Conventionally, in manufacturing a semiconductor device, a liquid crystal display element, or a thin film magnetic disk using a photolithography technique, a projection exposure apparatus has been used for exposing a pattern on a photomask or reticle (hereinafter generally called a “reticle”) onto a wafer (or glass plate, etc.) on which photoresist or the like is applied, through an optical projection system. Generally, because high resolution is required for the projection exposure apparatus, and the numerical aperture of the installed optical projection system is high, the depth of focus (focus margin) of the projected image is reduced in inverse proportion to the square of the numerical aperture. Then, to align each shot region of a wafer within the range of the depth of focus in respect to the imaging plane of the optical projection system, the projection exposure apparatus has been provided with an autofocus mechanism which aligns the focus position of the wafer, at a predetermined reference point in an exposure field, with the imaging plane by the optical projection system, and an autoleveling mechanism which sets the inclination of the exposure surface of the wafer in the exposure field parallel to the imaging plane.
A conventional autofocus mechanism, on the one hand, comprises a focus position detection sensor for detecting the amount of defocus in the focus position (the position of the optical projection system in the direction of the optical axis) of a predetermined measurement point in each shot region of the wafer from the imaging plane (hereinafter called the AF sensor), and a servo system for controlling the height of the Z stage to make its amount of defocus fall within an allowable range. In the AF sensor, a grazing-incidence detector reimages the slit pattern image, which is projected on a predetermined measurement point in the exposure field at an angle, on the reception section, and detects the focus position at the measurement point by utilizing the fact that the deviation of the focus position on the wafer surface causes the position of the reimaged slit pattern image to change.
On the other hand, the autoleveling mechanism comprises a leveling sensor which detects focus positions at three or more measurement points on each shot region on the wafer, and a servo system which makes the mount of shift of the inclination of the average plane determined by the focus positions at these three or more measurement points within an allowable range.
In this regards, in a conventional batch projection exposure apparatus, such as the stepper which is generally used, because the wafer whose focus position is subject to detection is stationary during exposure, it is possible to manage the decrease in the depth of focus by improving the resolution and accuracy of the AF sensor and the leveling sensor which detect the mount of defocus, and by tuning the accuracy of the mechanism for the Z stage in the servo system, even if the numerical aperture of the optical projection system is further increased.
Recently, an one chip pattern for a semiconductor and the like has come to have a larger and larger size so that the projection exposure is required to have a larger size for projecting a pattern with a larger area on the reticle onto the wafer.
It has also become necessary to improve the resolution of the optical projection system as the pattern for a semiconductor and the like becomes finer. There is a disadvantage in that it is difficult to enlarge the exposure field of the optical projection system in its design or manufacture to improve the resolution of the optical projection system. In particular, when a catadioptric system is used as the optical projection system, an exposure field without aberration is given by a circular arc in shape.
To manage such an enlargement of the transfer pattern and the constraints of the exposure field of the optical projection system, a so-called step-and-scanning type projection exposure apparatus has been developed which synchronously scans a reticle or wafer to, for example, a rectangular, circular arc, or a hexagonal radiating region (hereinafter called a slit radiating region) to sequentially project a pattern on the reticle, which has a larger area than the slit radiating region, onto each shot region on the wafer.
Such a projection exposure apparatus also requires an autofocus mechanism and an autoleveling mechanism, both of which align the exposure surface of the wafer under exposure with the imaging plane. However, in the step-and-scanning type system, because the wafer whose focus position is to be detected moves during exposure, the output signal of the AF sensor or leveling sensor indicating the focus position of the measurement point varies as a function of the position in the scanning direction. Thus, if signal processing and control similar to that of the batch projection exposure apparatus is conducted, it results in poor follow-up capability in respect to the movement of the focus position of the wafer. Therefore, there is the disadvantage such that the exposure surface of the wafer is difficult to align with the imaging plane within the depth of focus. This disadvantage is described in detail in the following:
In the step-and-scanning type projection exposure apparatus, as described-earlier, the detection signal of a focus position is observed as a function of the position in the scanning direction in a time series. Therefore, when this signal is merely caused as a deviation signal to operate a closed-loop servo, control is performed in such a manner that the Z stage dynamically follows up the time-series signal if the system has a sufficiently fast response. If the width of the slit exposure region (exposed field) in the scanning direction is sufficiently small in respect to the scanning speed, this causes no particular disadvantage. However, the width of the slit exposure region in the scanning direction usually has a non-negligible value in respect to the scanning speed.
Therefore, even if the center of an exposure region is moved while caused to completely follow the detection signal of a focus position, the movement of the Z stage when a point on the wafer passes through the slit exposure region adversely affects the imaging characteristics at that point as vibration. Moreover, because the slit exposure region has a finite width in the scanning direction, follow-up essentially cannot be performed for unevenness finer than the limit, which is the unevenness of one cycle in the width on the wafer. The best control in this case is to perform no control. If the center of the exposure region is caused to follow the fine unevenness as in the above, any control may deteriorate an image by deterioration of the focusing accuracy.
On the one hand, the wider the width of the slit exposure region along the scanning direction, the higher the step-and-scanning type system throughput. On the other hand, if this width is large, light may leak from an area outside the blocking area at the end of the reticle to cause unnecessary exposure. Thus, as disclosed in Japanese Patent Public Disclosure No. HEI 4-196513, a method has been developed wherein the exposure region is made narrower in the scanning direction near the starting and terminating ends of the reticle, while it is made wider in the scanning direction between the starting and terminating ends of the reticle. Such a method wherein the width of the exposure region in the scanning direction is changed during exposure is not attained in a conventional stepper or the like, so a prefe
Font Frank G.
Lee Andrew H.
Oliff & Berridg,e PLC
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