Photocopying – Projection printing and copying cameras – Step and repeat
Reexamination Certificate
2000-09-29
2003-11-25
Mathews, Alan A. (Department: 2851)
Photocopying
Projection printing and copying cameras
Step and repeat
C356S401000
Reexamination Certificate
active
06654097
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an exposure apparatus,-and more particularly, to a projection exposure apparatus used in manufacture of semiconductor devices (integrated circuit (IC), large-scale integration (LSI) circuits, or the like), image pick-up devices (CCD or the like), liquid crystal display devices, thin layer magnetic heads, or the like.
2. Discussion of the Related Art
In recent years, following advances in higher integration of semiconductor devices, finer mask patterning is increasingly becoming a necessity in a projection exposure apparatus. To cope with such high resolution, focusing accuracy in the image of the mask pattern formed via the projection optical system needs to be improved. In other words, it is necessary to position the exposure surface of the photosensitive circuit board within the depth of focus for the image formation surface of the projection optical system. Variety of methods have been proposed to meet this requirement. For example, by installing a sensor that measures the vertical position of a substrate stage (photosensitive substance) relative to the projection optical system, the exposure surface of the photosensitive substrate is matched with the focused image plane using the origin of the signal from the detection sensor as a reference. In this case, the focal point of the mask pattern image using a fiducial plate surface placed on the substrate stage is measured, and the origin of the signal from the sensor is set to this focusing point.
In the method described above, the origin in the sensor may deviate from the actual focal point of the projection optical system due to fluctuation in environment of the exposure unit, type of mask used, or fluctuation in the imaging characteristics of the projection optical system over time. Therefore, it is necessary to reset the origin of the sensor to calibrate the sensor every once in a while at regular intervals. An example of such calibration of the sensor is disclosed in Japanese Laid-Open Publication No. 05-160003. In this reference, light emitted from a mark on a fiducial plate on the substrate stage passes through a projection optical system, and is reflected at the mask surface. The reflected light returns to the fiducial plate, and is measured when it is received at the light-emitting portion. Then, the focusing condition of the fiducial plate substrate is derived based on the received light quantity.
In the method described above, the illumination system for alignment focal position measurement is different from that for exposure, i.e, focus measurement is carried out under different conditions from that for exposure, which may lead to measurement errors. Also, there is a limit in reducing manufacturing tolerance to the width of the mark on the fiducial plate. Therefore, it is almost impossible to perform the focus measurement using the minimum line width for the mask. (This is because the projection optical system for exposure is typically a reduction type. Thus, the manufacturing tolerance of the mask pattern is much larger than that of the fiducial mask pattern.) Therefore, when an L/S (line and space mark) formed on the fiducial plate is used as the mark, focusing errors may occur due to the difference between the minimum line width of an L/S mark on the mask and the minimum line width of the L/S mark on the fiducial board.
Referring to
FIG. 25
, the operation of a conventional exposure apparatus is explained.
FIG. 25
is a schematic diagram of the conventional projection exposure apparatus.
When shutter
904
is opened by a shutter drive unit
902
, light emitted from an illumination light source
900
progresses in the direction A in the figure and impinges on a mirror
906
. The light deflected from the mirror
906
enters an illumination optical system
908
and is tuned to yield a uniform illumination field suitable for exposure. Then, the tuned light illuminates a reticle
910
. The light, which passes through reticle
910
, enters a projection lens
912
and is incident on a predetermined area of a wafer
916
on a wafer stage
914
. This way, the image of the pattern formed on reticle
910
is projected and transferred onto the wafer
916
.
In general, the above-mentioned projection exposure of the reticle pattern onto the wafer is repeatedly performed for multiple patterns. After processes of etching and film deposition, etc., ICs, LSIs, etc. are formed on the wafer. During such processes, the pattern that was projected onto the wafer in the previous process needs to be superimposed (aligned) with a reticle pattern for the next layer.
To perform such alignment, it is necessary to obtain a relationship between a coordinate system fixed on the reticle and a coordinates system fixed on the wafer. In this example above, the correspondence is obtained using a fiducial plate
918
placed on the wafer stage
914
. A wafer alignment mark
920
formed on the fiducial plate
918
and a reticle mark
922
formed on the reticle
910
are observed at the same time to determine the positional relationship.
In more detail, when the shutter drive unit
902
closes the shutter
904
, light reflected from a metal surface of the shutter
904
progresses in the direction B in the figure and enters a split mirror
926
of an alignment optical system
924
. Then, the light reflected by the split mirror
926
illuminates the reticle mark
922
, passes through the projection lens
912
, and illuminates the fiducial mark
920
as shown in the figure. This light, carrying information of the alignment marks, is reflected by the fiducial plate
918
and goes back along its incoming path, and enters the split mirror
926
. again. Then, the light that passes through the split mirror
926
reaches a two-dimensional image sensor
928
to image the marks
920
and
922
on the image sensor
928
. The image of each alignment mark captured by the image sensor
928
is input into an image processor
930
, and are analyzed to derive the relative positional difference between the reticle
910
and the wafer stage
914
. Such positional difference need be taken into account when moving the wafer stage
914
to align the existing pattern on the wafer
916
with the pattern of the reticle
910
being projected is conducted.
As mentioned above, in the exposure apparatus, the illumination condition of the exposure optical system and that of the alignment optical system are not necessarily consistent with each other. To exposure finer patterns (finer line width), which is becoming popular in recent years, various improvement have been introduced in an exposure apparatus: reducing the wavelength &lgr; of illumination light, increasing the numerical aperture NA of the projection lens, using a modified illumination technique, etc. With respect to the illumination wavelength &lgr;, the illumination condition of the exposure optical system and that of the alignment optical system can be made equivalent by relaying light from a single light source by a separate optical system. However, to achieve other methods such as increasing numerical aperture, it is necessary to build a large, complicated alignment illumination system, which is, however, not practical. Therefore, it is difficult to obtain the same illumination conditions between exposure and alignment.
To overcome such difficulties, a method of receiving the actual exposure light in a slit sensor located on the wafer stage has been developed. Although it is possible to match the illumination conditions between exposure and alignment with this technique, this method has a disadvantage in that it is not applicable to a high speed alignment operation using an RA (reticle alignment) image processing system, which is disclosed in Japanese Laid-Open publication No. 05-21314. Furthermore, when signals are obtained by integrating the amount of light passing through a light using slit measurement, this method is not suitable for measuring the width of the L/S (line and space) mark near maximum resolution.
Some of the prob
Mathews Alan A.
Morgan & Lewis & Bockius, LLP
Nikon Corporation
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