Photocopying – Projection printing and copying cameras – Step and repeat
Reexamination Certificate
2001-04-27
2004-03-23
Adams, Russell (Department: 2851)
Photocopying
Projection printing and copying cameras
Step and repeat
C355S055000, C355S077000, C430S005000, C235S462050
Reexamination Certificate
active
06710847
ABSTRACT:
TECHNICAL FIELD
The present invention relates to an exposure method used when producing a photomask used for producing a semiconductor integrated circuit, a liquid crystal display device, a thin film magnetic head, or another microdevice by photolithography and an exposure apparatus for working the exposure method.
BACKGROUND ART
In photolithography process of a semiconductor device, a pattern of a photomask is transferred on to a wafer or glass plate coated with a photoresist (hereinafter also called a “photosensitive substrate”). As this type of projection exposure apparatus, wide use has been used in the past of a step-and-repeat type exposure apparatus (stepper). This step-and-repeat type exposure apparatus exposes and transfers a pattern of a photomask by reduction projection on individual shot areas of the wafer. Therefore, when exposure of one shot area ends, the wafer is moved for exposure of the next shot area. This process is successively repeated.
Further, to increase the range of exposure of the mask pattern, a step-and-scan type exposure apparatus (scanning stepper) has been developed which synchronously moves the mask and wafer in scanning motion with the projection optical system in a state of restricting the exposure light from the illumination system into a slit shape (for example, rectangular shape) and projecting reduction image of a part of a mask pattern using the slit light. This step-and-scan type exposure apparatus (scanning stepper) has the advantages of an aligner transfer method of transferring the pattern of the entire surface of the mask to the entire surface of the wafer at an equal magnification by a single scan exposure and the advantages of the stepper transfer method. Note that a photomask used in a step-and-repeat type or step-and-scan type reduction projection type exposure apparatus is also called a “reticle”.
The photomask used in such an exposure apparatus has conventionally been produced by drawing a master pattern on a photomask substrate using an electron beam lithography system or a laser beam lithography, system. That is, a mask material is formed on the substrate, a resist is coated on it, then the master pattern is drawn using an electron beam lithography system or laser beam lithography system. Next, the resist is developed and etched etc. to form the master pattern by the mask material. In this case, if the magnification rate of the reduction projection type exposure apparatus using this photomask is 1/&bgr;, the master pattern drawn on the photomask may be the pattern of the device enlarged &bgr;-fold, therefore the drawing error due to the lithography system is reduced to about 1/&bgr; on the device. Therefore, it becomes possible to form the pattern of the device by a resolving power of about 1/&bgr; of the resolving power of the lithography system.
As explained above, in the past, the master pattern-of the photomask has been drawn by an electron beam lithography system or laser beam lithography system. These lithography systems draw master patterns directly based on the drawing data from a control computer. Recent LSIs and other devices, however, have become larger in size and improved in fineness and integration, so the master pattern of the photomask required for exposure also becomes larger in area and finer. Further, as the photomask, use is also made of a reticle for double exposure provided with a correction pattern for preventing transfer of unnecessary patterns, a so-called phase shift reticle provided with a phase shifter between adjoining patterns, etc. With these special photomasks, however, the amount of the drawing data tends to become greater than that of other photomasks. Due to this, the amount of drawing data required in an exposure apparatus for producing a photomask becomes massive.
Therefore, the drawing time required for drawing a master pattern of a photomask by such a lithography system has recently grown from 10 hours to around 24 hours. This increase in the drawing time is becoming a factor behind the rising cost of manufacture of a photomask.
In this regard, in an electron beam lithography system, it is necessary to correct the proximity effect caused by the back scattering distinctive to an electron beam. Further, it is necessary to correct the uneven electric field around the substrate due to the charging of the surface of the substrate. Therefore, to draw a master pattern as designed, it is necessary to measure the error of the drawing position etc. in advance under various conditions and make complicated corrections at a high accuracy and stability at the time of drawing. Making such complicated corrections during an extremely long drawing time such as the above with a high accuracy and stability on a continuous basis, however, is difficult. The disadvantage arises of drift of the drawing position during the drawing. Further, it is possible to suspend the drawing for calibration, but this has the disadvantage of the overall drawing time becoming even longer.
Further, the resolving power and other characteristics of the resist for electron beam use have not been improved that much. No rapid improvement in these characteristics is expected in the future as well. Therefore, if the pattern rule of semiconductor devices becomes finer in the future, the drawing time for the master pattern of a photomask is liable to become too long and the resolving power of the electron beam resist is liable to approach its limit making the required drawing accuracy impossible to obtain. Further, the amount of the drawing data in the control computer is also becoming massive to-the extent of difficulty for use in a single drawing operation.
A laser beam lithography system draws a master pattern using an ultraviolet band laser beam. There are the advantages that it is possible to use a resist giving a higher resolving power compared with an electron beam lithography system and that there is no proximity effect due to back scattering. The resolving power of a laser beam lithography system is inferior to that of an electron beam lithography system, however. Further, in a laser beam lithography system, since a master pattern is drawn directly in this system, the amount of drawing data becomes massive and data processing becomes difficult. Further, the drawing time becomes extremely long. Therefore, the required drawing accuracy is liable to not be able to be obtained due to drift of the drawing position etc.
To solve the above problem, the present assignee previously proposed an apparatus which enlarges the pattern for transfer, divides the pattern into a plurality of patterns of master masks, and successively projects and exposes images of the plurality of patterns of the mask patterns reduced by the projection optical system on the surface of the mask substrate (blank) while stitching them (hereinafter sometimes called a “reticle exposure apparatus” or a “mask exposure apparatus”).
When producing a photomask used for the production of a microdevice (working mask) using a reticle exposure apparatus, a thin film of a mask material is formed on a mask substrate as the photomask substrate and a resist or other photosensitive material is coated on it. Next, reduced images of the plurality of patterns of master masks are transferred to the photosensitive material for example by an optical type reduction projection exposure apparatus by the step-and-repeat system or the step-and-scan system. By etching using the pattern of the remaining photosensitive material as a mask, a desired pattern for transfer (master pattern) is formed.
At this time, if the magnification rate of a for example optical type exposure apparatus for producing a photomask is made 1/&agr; (where a is an integer or fraction etc. larger than 1), the transfer pattern, that is, the master pattern, is enlarged &agr;-fold. This enlarged master pattern is divided into for example a x &agr; number of patterns of master masks. If the magnification rate is 1/5 (&agr;=5), 5×5=25 master masks are provided. As a result, since the patterns formed on t
Kim Peter B
Nikon Corporation
Oliff & Berridg,e PLC
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