Photocopying – Projection printing and copying cameras – Step and repeat
Reexamination Certificate
1997-01-09
2002-01-01
Adams, Russell (Department: 2851)
Photocopying
Projection printing and copying cameras
Step and repeat
C355S055000, C430S030000
Reexamination Certificate
active
06335784
ABSTRACT:
FIELD OF THE INVENTION AND RELATED ART
This invention relates to an exposure apparatus for use in the manufacture of semiconductor devices and, more particularly, to a projection exposure apparatus and method for transferring, by projection, a photomask pattern onto a wafer. Also, the invention relates to a semiconductor device manufacturing method. More specifically, the present invention is concerned with a scan type exposure apparatus and method, or to a semiconductor device manufacturing method using the same, wherein, for projection exposure of a wafer to a photomask pattern, the mask and a wafer are scanningly moved in synchronism relative to a projection optical system (projection exposure system). As an example, the invention is suitably applicable to a scan type projection exposure apparatus which is usable in a lithographic process, among device manufacturing processes for the manufacture of semiconductor devices such as ICs or LSIs, image pickup devices such as CCDs, display devices such as liquid crystal panels, or magnetic heads, for example, to perform wafer alignment when a pattern of a reticle (first object) is to be projected by a projection optical system onto the surface of a wafer (second object).
The density of integration of a semiconductor device such as an IC or an LSI has increased considerably. As regards projection exposure apparatuses, filling the major role in the fine processing technology for semiconductor wafers, there are various exposure apparatuses developed, such as a unit-magnification projection exposure apparatus (mirror projection aligner) wherein an exposure process is performed while scanningly moving a mask and a photosensitive substrate relative to a unit-magnification mirror optical system having an arcuate exposure region, or a reduction projection exposure apparatus (stepper) wherein an image of a mask pattern is formed through a refraction optical system on a photosensitive substrate and the photosensitive substrate is exposed in a step-and-repeat process.
The size of the chip pattern of one semiconductor device is increasing. Thus, for a projection exposure apparatus, it is required to enlarge the exposure area by which a large area pattern of a mask can be printed on a photosensitive substrate.
In order to meet these requirements, various proposals have been made in relation to a step-and-scan type scanning projection exposure apparatus (exposure apparatus) which provides a high resolution and an enlarged picture field size. In such a scan type exposure apparatus, a pattern on the surface of a reticle is illuminated with a slit-like light beam and, while the pattern as illuminated with the slit-like light beam is projected by a projection system (projection optical system), it is transferred onto a wafer through a scanning operation.
Many proposals have been made on such a scan type projection exposure apparatus, and there is an example in which a unit-magnification scan type exposure apparatus with a conventional reflection projection optical system is modified and refraction elements are incorporated into the projection optical system, such that reflection elements and refraction elements are used in combination. Another example is a scan type exposure apparatus wherein a reduction projection optical system comprising refraction elements only is used and wherein both of a mask stage and a stage (wafer stage) for a photosensitive substrate are scanningly moved in synchronism with each other at a speed ratio corresponding to the reduction magnification.
FIG. 23
is a schematic view of a main portion of a scanning exposure apparatus. In the drawing, a mask (reticle)
1
on which an original is formed is supported by a mask stage
3
. Wafer (photosensitive substrate)
13
is supported by a wafer stage
5
. The mask
1
and the wafer
13
are disposed in an optically conjugate relationship with each other, with respect to a projection optical system
2
. Slit-like exposure light
12
coming from an illumination system (not shown) and being elongated in the Y direction in the drawing, illuminates the mask
1
by which it is imaged upon the wafer
13
with a size corresponding to the projection magnification of the projection optical system
2
. A scan exposure process is performed by moving both of the mask stage
3
and the wafer stage
5
relative to the slit-like exposure light
12
, in other words, relative to the projection optical system
2
, at a speed ratio corresponding to the optical magnification, to scan the mask
1
and the wafer
13
. By this, the whole device pattern of the mask
1
is transferred onto a transfer region
10
on the wafer
13
.
Practically, the scan exposure process is performed to two types of wafers as roughly classified:
(a1) Wafer to which no mask pattern has been transferred (hereinafter “first wafer”); and
(a2) Wafer on which a mask pattern or patterns are already formed (hereinafter “second wafer”).
When a mask pattern is to be superposedly printed on a second wafer, patterns of a mask
1
and of a wafer
13
may be detected by using mark detecting means
15
and through an alignment optical system
4
. The result of the detection may be processed by an operation processing circuit
16
. On the basis of this and of the positions of the mask stage and wafer stage as monitored by laser interferometers
7
and
8
, mask and wafer alignment may be done. Thereafter, the mask stage
3
and the wafer stage
4
may be moved in synchronism with each other under control by drive control means
17
, to perform the scan exposure.
SUMMARY OF THE INVENTION
If a reduction projection optical system is used, it is necessary that a mask
1
to be used is larger than a pattern region, to be defined on a wafer
13
, by an amount corresponding to the projection magnification. In order to support such a large mask
1
, the mask stage
3
has to be large. Taking into account a drive system for such a mask, an increase in weight is large. This may result in a problem that sliding motion of the mask stage
3
during an actual exposure process causes a shift of the gravity center of the mask stage
3
, which in turn may cause an error in attitude of the projection optical system
2
, or a considerable shift of it.
Further, driving such a heavy mask stage
3
may cause a problem of vibration of the projection optical system
2
, due to a reaction in accelerating the mask stage
3
to a certain speed, or pitching of the mask stage
3
itself. Also, such vibration may cause a change in attitude of the projection optical system
2
, which in turn may cause a problem that an image is printed with deviation from a desired exposure region. An image shift on an exposure plane may also result from pitching of the mask stage. Such a deviation attributable to vibration, if it occurs in the scan direction, leads to deterioration of superposition precision. Vibration in a direction perpendicular to the scan direction causes a focus error (defocus and tilt) on the wafer surface. It results in degradation of exposure precision during first wafer processing and second wafer processing.
In order to avoid the effect of vibration of the projection optical system
2
, attributable to driving the mask stage
3
, it may be necessary that, from a start of acceleration, the mask stage
3
is moved to a position where the effect of vibration can be disregarded, and that the exposure process is initiated from that position. If driving the mask stage causes a large vibration of the projection optical system, a substantial time has to be set from a start of scan motion to a start of exposure. Thus, throughput will be lowered.
Also, the scan distance of the mask stage to the above-described position becomes longer. Since in a scan type exposure apparatus, the scan direction of a mask stage is usually reversed between an odd-number shot and an even-number shot, on a wafer, for enhancement of throughput, the prolongation of scan distance appears both on the opposite sides of the scan direction. This leads to bulkiness of the whole exposure apparatus
Adams Russell
Kim Peter B.
LandOfFree
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