Photocopying – Projection printing and copying cameras – Focus or magnification control
Reexamination Certificate
2001-05-17
2004-07-20
Font, Frank G. (Department: 2877)
Photocopying
Projection printing and copying cameras
Focus or magnification control
C355S053000, C355S067000, C355S071000, C355S075000, C355S077000, C359S513000, C250S492200, C250S492220
Reexamination Certificate
active
06765647
ABSTRACT:
TECHNICAL FIELD
This invention relates to an exposure method and an exposure apparatus used to transfer a mask pattern onto a substrate in a lithography process to manufacture microdevices, such as semiconductor elements, image-capture elements (CCDs and similar), liquid crystal display elements, and thin film magnetic heads, and in particular a method and an apparatus suitable for use in an exposure apparatus comprising a mechanism to focus the image plane of a projection optical system on the substrate surface by an automatic focusing method. More specifically, this invention concerns an exposure apparatus comprising a mechanism to control the temperature of prescribed members.
BACKGROUND ART
With the advanced integration of semiconductor devices in recent years, steppers and other projection exposure apparatuses are required to project images of circuit patterns with fine line widths, with high resolution, onto a wafer (or glass plate or similar) covered with resist as the substrate in each shot area. To this end, the numerical aperture of the projection optical system must be increased, and the exposure wavelength must be shortened; however, this is accompanied by a tendency toward reduction of the focal depth of the projection optical system. Thus the need arises to increase the focusing accuracy of the automatic focusing mechanism comprised by a projection exposure apparatus, in order to perform exposure in which the wafer surface is accurately adjusted within the range of the reduced focal depth, with respect to the image plane of the projection optical system (the best focus position of the projected image of the mask or reticle pattern).
This automatic focusing mechanism comprises an auto-focus sensor (hereafter “AF sensor”) which detects the focus position on the wafer surface (the position in the optical axis direction of the projection optical system), and a stage system which controls the height of the wafer or reticle based on the measurement results of this AF sensor. As this AF sensor, conventionally an oblique-incidence AF sensor is used in which a slit image or similar is projected obliquely onto the surface for detection, without passing through the projection optical system, and light reflected from this surface for detection is detected, as disclosed for example in Japanese Patent Application Laid-open No.6-283403. While this oblique-incidence AF sensor has the advantage of enabling measurement of fluctuations in the focus position of the surface for detection even during exposure, measurement light does not pass through the projection optical system, so that if for example the position of the image plane of the projection optical system fluctuates due to the heat of irradiation of the illumination light (exposure light) used in exposure, it is difficult to directly measure the change in defocusing of the surface for detection.
In order to measure the defocusing amount of the surface for detection with respect to the actual image plane of the projection optical system, a TTR (through-the-reticle) AF sensor has been proposed which projects on the in wafer stage the image of a mark on the reticle through the projection optical system and measures the position of the image plane based on the contrast of this image, as disclosed for example in Japanese Patent Application Laid-open No. 9-283421. This TTR type AF sensor has the advantage of enabling direct measurement of the image plane of the projection optical system; but in order to perform this measurement, exposure of the wafer must be interrupted, and if the AF sensor is used too frequently, the throughput of the exposure process is lowered. Hence as one example of a conventional method, a TTR type AF sensor and an oblique-incidence AF sensor are combined; the oblique-incidence AF sensor is used for focusing during normal exposure, and the TTR type AF sensor is used to measure the position of the actual image plane for each lot, once every half-day, once a day, or with similar frequency; based on the measurement results, the values measured by the oblique-incidence AF sensor are calibrated.
In a conventional exposure method like that described above, by performing calibration of the oblique-incidence AP sensor at prescribed time intervals, for example, the focusing accuracy can be maintained within a prescribed tolerance without greatly lowering throughput. However, because an oblique-incidence AF sensor is positioned on one side of the projection optical system in the vicinity of the wafer, if the illumination heat of the exposure light causes the temperature of the wafer to gradually rise, the temperature of members comprised by the oblique-incidence AF sensor also rises, and gradual drifting of the measured focus position may occur. Further, slight irregular shifts in position of prescribed optical members comprised by the AF sensor may cause irregular drifting of measured values. Such drift amounts are minute; but if the exposure wavelength is made still shorter and the focal depth is further decreased in response to the still higher integration levels of future semiconductor devices, drift in the values measured by the oblique-incidence AF sensor may cause the wafer surface to deviate outside the range of the focal depth for the image plane.
In order to lessen the drift in values measured by such an AF sensor, the above-described TTR type AF sensor may be used to perform frequent calibration (for example, each time the wafer is changed); but if exposure is interrupted and the TTR type AF sensor is used with such high frequency, there is the problem that throughput is greatly reduced.
In order to transfer, at a prescribed magnification and with high fidelity, a reticle pattern for use as a mask onto a wafer (or glass plate or similar) covered with resist as a substrate, conventionally the principal parts of the exposure apparatus are housed on a box-shape chamber; dust removal from within this chamber is performed extensively, and air regulated at a nearly constant temperature is supplied.
In recent years, high-precision temperature control not only of the reticle and wafer, but of various optical systems in the exposure apparatus has been sought in order to accommodate the ever-finer details in semiconductor elements and similar. In order to improve the resolution of an exposure apparatus, the wavelength of the illumination light used in exposure (exposure light) has been shortened, from primarily the i-line of mercury lamps (wavelength 365 nm) to KrF excimer laser light (wavelength 248 nm), and then to ArF excimer laser light (wavelength 193 nm); hereafter the use of F
2
laser light (wavelength 157 nm) and similar is also being studied. When the exposure wavelength is thus shortened, the transmissivity in ordinary air of the illumination light for exposure (exposure light) declines, and so recent an exposure apparatus comprises a gas supply mechanism to supply a gas having comparatively good transmissivity for the exposure light in part of the optical path of the exposure light.
FIG. 24
shows an exposure apparatus comprising such a gas supply mechanism; in this
FIG. 24
, exposure light emitted from the exposure light source
250
incorporating an excimer laser light source passes through the light transmission unit
268
including a relay optical system and through the optical path correction unit
252
, and is incident on the exposure main unit. Exposure light incident on the exposure main unit passes through the illumination system
253
and illuminates the reticle
259
, and a projected image passes through the projection optical system
264
and is projected onto the wafer
262
. The wafer
262
is held on the wafer stage
261
, and the wafer stage
261
is mounted in freely movable fashion on the wafer base
260
; the projection optical system
264
is supported by the support member
263
of a column
257
embedded in the wafer base
260
, and the reticle
259
is mounted on a reticle stage, not shown, on a reticle base
258
fixed in place on the column
257
. A support member
255
suppor
Brown Khaled
Font Frank G.
Nikon Corporation
Oliff & Berridg,e PLC
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