Radiant energy – Photocells; circuits and apparatus – Photocell controls its own optical systems
Reexamination Certificate
1999-12-22
2002-12-17
Allen, Stephone (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Photocell controls its own optical systems
C356S400000, C355S053000
Reexamination Certificate
active
06495847
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a stage apparatus which is used in a semiconductor exposure apparatus, inspection apparatus, or the like, and aligns an exposure master disk, an object to be exposed, or an object to be inspected to a predetermined position, and its control method.
In general, as exposure apparatuses used in the manufacture of semiconductor elements, an apparatus called a stepper, and an apparatus called a scanner are known. The stepper projects a pattern image formed on a reticle on a semiconductor wafer on a stage apparatus in a reduced scale via a projection lens while stepping the wafer beneath the projection lens, thus sequentially forming the pattern image by exposure on a plurality of portions on the single wafer. The scanner moves a semiconductor wafer on a wafer stage and a reticle on a reticle stage relative to a projection lens (scan movement), and projects a reticle pattern on the wafer by irradiating it with slit-patterned exposure light during the scan movement. The stepper and scanner are considered as mainstreams of the exposure apparatus in terms of their resolution and superposing precision.
FIG. 21
is a schematic top view of a wafer stage used in such an exposure apparatus.
A wafer
102
to be exposed is mounted on a wafer stage
101
via a wafer chuck (not shown). With reference to an exposure optical system, the position of an exposure optical axis
103
is fixed in FIG.
21
. Hence, the wafer stage
101
must move in the X- and Y-directions with respect to the exposure optical axis
103
to expose the entire surface of the wafer. The wafer
102
must also move in the Z- and tilt directions to adjust the imaging focal point, but a detailed description thereof will be omitted herein. The position measurement of the wafer stage
101
in the X- and Y-directions uses a high-resolution laser interferometer to realize high-precision alignment. In order to use the laser interferometer, a reflection mirror
107
for reflecting a laser beam must be provided on the wafer stage
101
. However, since this reflection mirror
107
must reflect the laser beam to cover the entire moving range of the wafer stage
101
, it requires a length equal to or larger than the moving distance of the wafer stage
101
. That is, if Ly represents the stage moving distance in the Y-direction, the length Lx of a reflection mirror for X-measurement requires Ly or more (=Ly+&agr;).
The moving range of the wafer stage
101
need only be slightly larger than the wafer diameter if exposure alone is done. However, in practice, the wafer stage
101
moves not only in an exposure operation but also in other operations.
In order to expose the wafer
102
, alignment must be done with high precision with respect to the imaging point. Various alignment methods are available, and a method that irradiates an alignment mark, which has been exposed and transferred onto a wafer in advance, with alignment light to obtain any alignment error amount from light reflected by the mark is prevalently used. In this alignment method, the alignment optical axis center does not often match the exposure optical axis center.
FIG. 22
shows a general wafer stage when an alignment optical axis center
104
does not match the exposure optical axis center
103
. Referring to
FIG. 22
, the alignment optical axis center
104
strikes a wafer at a position a given distance L
2
from the exposure optical axis center
103
. The wafer stage
101
must move the wafer
102
in the X- and Y-directions to expose the entire surface of the wafer, and must also move it in the X- and Y-directions to irradiate the entire surface of the wafer with the alignment light
104
.
For this purpose, the movable range of the wafer stage
101
must be broadened by the displacement between the alignment optical axis
104
and exposure optical axis
103
. Consequently, the length of the reflection mirror
107
must be increased by the broadened size of the moving range of the wafer stage.
Referring to
FIG. 22
, a length Lx
2
of the reflection mirror in the X-direction must be increased by a displacement L
2
between the alignment optical axis
104
and exposure optical axis
103
.
More specifically, in
FIG. 22
, assume that an X-interferometer optical axis
105
passes through the exposure optical axis center
103
, and a Y-interferometer optical axis
106
passes through the exposure optical axis center
103
and alignment optical axis center
104
. Let Ly be the stage moving distance in the Y-direction (the distance between the position (solid line) where the wafer stage
101
has moved a maximum distance in a +Y-direction, and the position (broken line) where the stage
101
has moved a maximum distance in a −Y-direction), and L
2
be the distance between the exposure optical axis center
103
and alignment optical axis
104
. Then, the minimum required length Ly
2
of the reflection mirror
107
is given by Ly
2
=Ly+L
2
.
Especially, in recent years, the wafer diameter becomes as large as 300 mm to improve productivity. In order to expose the entire surface of the wafer, the wafer stage must have a moving range at least equal to or larger than the wafer diameter. As the wafer alignment position may be different from the exposure position, and wafers must be exchanged, the moving range must be further broadened. Inevitably, the length of the reflection mirror increases.
However, a reflection mirror
107
having such a large length is not preferable since (1) it is hard to prepare a long reflection mirror having a high-precision mirror surface, (2) high cost is required to prepare the mirror surface of such a long reflection mirror, (3) the weight of the reflection mirror itself increases and results in an increase in total weight of the stage, (4) heat produced by a stage driving device increases due to an increase in stage weight, and (5) the characteristics of a control system deteriorate due to a decrease in natural frequency of a mechanical system of the stage.
In order to solve this problem, an arrangement described in Japanese Patent Laid-Open No. 7-253304 has been proposed. This apparatus comprises a laser interferometer distance measuring device, movable mirror, X-Y moving stage, and arithmetic device. The movable mirror has a length shorter than the stage moving distance in the Y-direction, and a plurality of X-interferometers are provided. The spacing between neighboring X-interferometers is shorter than the length of the movable mirror, and the movable mirror is irradiated with measurement light of one of the X-interferometers independently of the current position of the stage and is sometimes irradiated with measurement light from two X-interferometers. Which of the X-interferometers is ready to measure is determined by the arithmetic device based on the value of a Y-interferometer, and a measurement result in the X-direction is obtained. Upon moving the stage in the Y-direction, a new X-interferometer which becomes ready to measure undergoes recovery operation at a predetermined position using the value of the interferometer that has been used in measurement so far. By passing the values in turn, movement over a broad range is measured using the short movable mirror.
According to the arrangement described in Japanese Patent Laid-Open No. 7-253304, size and weight reductions of a position measurement mechanism (reflection mirror) to be mounted on a stage can be achieved. However, since the plurality of interferometers are selectively used, the process throughput lowers, and the measurement precision is not sufficiently high. According to the findings of the present inventors, such shortcomings are caused for the following reasons.
The recovery operation of each interferometer requires a certain time, and cannot fall within one sample time of a stage control system. Hence, even by the recovery operations of the interferometers like in the conventional system, measurement values cannot be continuously passed to a control system.
The X-Y stage must be a
Asano Toshiya
Inoue Mitsuru
Sakamoto Eiji
Allen Stephone
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
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