Radiant energy – Photocells; circuits and apparatus – With circuit for evaluating a web – strand – strip – or sheet
Reexamination Certificate
1997-08-01
2003-05-06
Le, Que T. (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
With circuit for evaluating a web, strand, strip, or sheet
C250S559300
Reexamination Certificate
active
06559465
ABSTRACT:
FIELD OF THE INVENTION AND RELATED ART
This invention relates to a surface position detecting method for detecting level (height) or tilt of a surface of an object such as a wafer, for example, having a region with a pattern structure. More particularly, the invention is concerned with a surface position detecting method usable in a slit scan type exposure apparatus, for example, for continuously detecting position or tilt of a surface of a wafer, for example, with respect to an optical axis direction of a projection optical system of the exposure apparatus.
The size of recent memory chips is increasing because of a difference between the trend of cell size or resolvable line width of an exposure apparatus and the enlargement trend of memory capacity. For a first generation of 256 M, for example, it is reported that the size is about 14×25 mm.
For this chip size and with an exposure region of a diameter 31 mm of a reduction projection exposure apparatus (stepper) which is currently used as an exposure apparatus for a critical layer, only one chip can be exposed per one exposure operation and the throughput is low. Thus, an exposure apparatus which enables a larger exposure area is required. For such a large field size exposure apparatus, there may be a semiconductor device exposure apparatus for a rough layer for which a high throughput is required or a reflection projection type exposure apparatus which is an exposure apparatus for a large picture field size liquid crystal display device such as a monitor. Such an apparatus is a slit scan exposure apparatus of a mask-to-wafer relative scan type wherein a mask is rectilinearly scanned with an arcuate slit-like illumination light and, by use of a concentric reflection mirror optical system, a wafer is exposed as a whole.
For focus adjustment of a mask image in such apparatus, level measurement and corrective drive for autofocusing or auto-leveling are performed continuously during the scan exposure operation so that the surface, to be exposed, of a photosensitive substrate (wafer or glass plate which is coated with a photoresist, for example) is brought into registration with a best imaging plane of a projection optical system.
A height and surface position detecting mechanism in such apparatus may use a method in which an oblique projection optical system for projecting light onto the surface of a wafer obliquely from above and reflection light from the photosensitive substrate is detected as a positional deviation upon a sensor, or a method in which a gap sensor such as an air microsensor or an electrostatic capacity sensor is used. From plural measured values during the scan, the amount of corrective drive for height and tilt as the measurement position passes the exposure slit region is calculated.
If only a projection system of a slit scan type exposure apparatus currently used is modified to provide a resolving power that meets 256 M or larger, the following problems arise.
As the numerical aperture (N.A.) of the reduction optical system is enlarged to meet miniaturization of a circuit pattern, the tolerable depth of focus during a circuit pattern transfer process is narrowed. Exposure apparatuses currently used for a rough layer process have a tolerable depth not less than 5 microns. Therefore, a measurement error included in measured values in continuous measurement during the scan exposure process or the effect of a surface step within a chip can be disregarded. However, for 245 M, the tolerable depth will be not greater than 1 micron. Thus, such measurement error or the effect of a surface step could not be disregarded. More specifically, when the focus (height and tilt) of a wafer surface is measured and focus correction is performed to keep the wafer surface within the tolerable depth, since the wafer surface contains surface irregularities, offset correction is necessary to bring the chip or shot as a whole in registration with the image plane. In such a case, unless the focus measurement point at each shot is the same as those during the offset measurement, accurate offset correction is not assured. It may be assured in a stepper in which measurement is performed as the wafer is stopped at each shot. However, in a scan system, it is not assured. When an accumulation type sensor is used and if the cycle of accumulation start is a free running type, there is an inaccuracy in position corresponding to the accumulation time, that is, a deviation between a focus measurement point and an offset measurement point. Thus, offset correction is inaccurate.
In a surface position detecting mechanism which uses an oblique light projection optical system, the reflectivity of a surface to be detected may change. A received light signal may be too strong and a light receiving system may be saturated. Alternatively, it may be too small and the signal-to-noise ratio (S/N ratio) may be degraded. It causes low surface position detection precision. Thus, during a scan exposure operation, it is necessary to adjust the gain (or light) of light receiving means or an emitted light quantity from light projecting means, in accordance with the reflectivity of the surface to be detected.
During continuous surface position measurement to a surface with different reflectivities during the scan, if the reflectivity of the surface is measured and the light is adjusted, the measurement time per one measurement operation may change and synchronism with respect to the stage position is broken. Thus, accurate offset correction is not attainable. Particularly, for an electric charge accumulation type sensor, an idle reading operation due to the light adjustment operation leads to a bottle neck of a response speed during the scan operation, and the throughput is lowered or there arise insufficiency of correction points. Thus, the surface position detection precision is degraded.
If a rectangular exposure area is sequentially transferred onto a wafer of a circular shape, as shown in
FIGS. 1-3
, at a peripheral portion of the wafer, a portion of the exposure area is out of the wafer region. In a wafer peripheral region, a multi-chip structure is adopted in which there are plural chips within the exposure area, this being to enable chip production even if a portion of the exposure area is missed (see FIG.
2
). Thus, for such an exposure area in which a portion thereof is missing (non-rectangular portion), an ordinary exposure procedure has to be performed. In a structure in which one chip is present in an exposure region, such as a case of a CPU, in the process of wafer surface clamping such as in ion injection step or an RIE step, a photoresist remaining in a peripheral portion of the wafer may be separated and it may move to the pattern portion of the chip region, causing a pattern defect or dimension defect. It leads to degradation of the chip yield. It is, therefore, necessary to perform exposure to a non-rectangular portion to thereby remove any excessive resist thereon. Conventionally, during wafer exposure, the same exposure procedure is made to a non-rectangular portion at the peripheral portion of the wafer, as done to the rectangular portions inside the wafer.
In a slit scan type exposure apparatus, for exposure of a peripheral portion, focus is measured in real time and the wafer surface is corrected toward the lens image plane. Since, however, the exposure region (shot) is not rectangular as the wafer central portion, the focus measurement may end in failure and the exposure operation may be interrupted. Even if the error is detected and processed by use of software and the exposure process is continued, there remain an increase of processing time and synchronism delay with respect to a correction system. Thus, correction precision is not good.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a surface position detecting method by which the position of a surface to be detected can be detected precisely without being affected by a surface irregularity of the surface to be detected. Particularly, it may be app
Uzawa Shigeyuki
Yamada Yuichi
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