Photocopying – Projection printing and copying cameras – Step and repeat
Reexamination Certificate
2001-04-09
2003-12-16
Mathews, Alan A. (Department: 1756)
Photocopying
Projection printing and copying cameras
Step and repeat
C355S075000, C355S077000, C430S005000
Reexamination Certificate
active
06665049
ABSTRACT:
TECHNICAL FIELD
This invention relates to a photomask on which a master pattern of patterns has been formed which are to be transferred (projected) onto substrates in lithographic processes for producing devices such as semiconductor integrated circuits, imaging devices (such as CCD), liquid-crystal display devices or thin-film magnetic heads.
BACKGROUND ART
In the fabrication of devices such as semiconductor integrated circuits, a transfer method is used in which, using a photomask having a master pattern of a circuit pattern to be formed, having been magnified, e.g., about four to five times, the pattern of this photomask is reduction-projected on an exposure-target substrate such as a wafer or a glass plate via a reduction projection optical system. What is used when the pattern of such a photomask is transferred is the projection exposure apparatus. Photomasks used in reduction projection type exposure apparatus of a step-and-repeat system or a step-and-scan system are also called reticles.
In the transfer of the pattern of such a photomask, where any foreign matter such as dust with a size larger than tolerance limits stands adherent to the pattern surface, an image of the foreign matter may also inevitably be transferred to the substrate such as a wafer, and there is a possibility that integrated circuits finally manufactured may function erroneously. Accordingly, a thin film called a pellicle which is comprised of an organic material of about 1 to 10 &mgr;m thick and transmits exposure light is conventionally spreadingly provided at a position about 5 mm apart from the photomask pattern surface so that the pellicle can prevent the foreign matter from adhering to the pattern surface. Also, the pellicle is spread over a metallic support frame called a pellicle frame bonded to the photomask pattern surface, and the space between the photomask pattern surface and the pellicle is substantially isolated from the open air and is so constructed that any foreign matter contained in the open air does not adhere to the photomask pattern surface.
As stated above, the pellicle comprised of a thin film of an organic material is spread over the pattern surface of any conventional photomask and this pellicle prevents dust from adhering. Also, as exposure light of projection exposure apparatus making use of photomasks, i-rays (wavelength: 365 nm) of mercury lamps have chiefly be used in conventional apparatus, where even conventional pellicles have a sufficient durability against exposure light having a wavelength of such a level.
On the other hand, exposure wavelengths of projection exposure apparatus are recently being replaced with much shorter wavelengths in order to make adaptation to semiconductor integrated circuits having been made finer, and recently KrF excimer laser light (wavelength: 248) is becoming prevailing. Then, at present, shorter-wavelength ArF excimer laser light (wavelength: 193 nm) is being on the stage of practical use. A projection exposure apparatus making use of much shorter-wavelength F
2
laser light (wavelength: 157 nm) is also on research.
As exposure wavelengths are replaced with such shorter wavelengths, it has become difficult for conventional organic type pellicles to ensure a sufficient durability. More specifically, since exposure light having a shorter wavelength has a higher energy per one photon, chemical bonds of organic molecules constituting a pellicle may be broken by exposure light. Hence, it is becoming difficult to provide a pellicle comprised of an organic material sufficiently durable to short-wavelength exposure light.
However, if the pellicle is not used, the foreign matter can not be prevented from adhering to the photomask pattern surface, bringing about a difficulty that the manufacture of semiconductor integrated circuits results in a low yield.
Accordingly, it is desirable that any substance other than the pellicle comprised of an organic material be installed in the vicinity of the position where the pellicle has conventionally been provided. Conventional pellicles are workable in very thin gage and moreover soft, and hence any optical influence is almost negligible as long as deflection is removed by applying a tension. However, almost no substance is known which has the same nature as conventional pellicles in respect of exposure light of 200 nm shorter and yet can ensure transmittance.
In the case when the exposure light of 200 nm or shorter is used, it is also known that impurities such as moisture and organic matter present there may contaminate light transmission surface or cause a decrease in transmittance through space. Accordingly, the greatest possible care must also be taken for contamination of any light transmission surface of an article corresponding to the pellicle provided at the position set on the same optical path of the pellicle.
In addition, as disclosed in WO99/49366, it is proposed to use a transmitting plate in place of conventional pellicles as a countermeasure for the trend toward shorter wavelength of exposure light. However, some problems may arise when such a transmitting plate is used. First, the transmitting plate has a considerable thickness compared with the pellicle, and hence it is necessary to measure a figure tolerance (surface precision) on the pellicle side and the projection lens side. However, in the measurement of the figure tolerance (surface precision), where the both surfaces are flat and substantially in parallel, fringes of equal-thickness interference occur when light is made incident on the surfaces. An influence of such fringes of equal-thickness interference provides a problem that it is difficult to analyze interference fringes for measuring the figure tolerance (surface precision).
There is also a problem that aberrations may occur since the transmitting plate having a thickness is inserted in the optical path to any reduction projection optical system in which various aberrations have been well corrected in the state where any transmitting plate is not present. To cope with this problem, as disclosed in WO99/49366, a method is proposed in which the space between lenses of a reduction projection optical system is adjusted according to the thickness of the transmitting plate. This method may be easy and effective because any lens component may be moved in a relatively small extent when, e.g., the error (aberration) is regulated in the state the transmitting plate has a thickness set previously. However, where a plurality of reticles having transmitting plates in various thickness are used, the lens component must be moved in so large an extent that there is a possibility of exceeding the stroke of adjustment which is the range within which the lens is movable. Also, even if it is moved within the adjustable range, residual aberrations may occur.
Moreover, in the case when the transmitting plate is inserted to the optical system, it is preferable for the transmitting plate not to lower the transmittance as far as possible, i.e., to be low reflective. In addition, it is preferable to cause flare or the like as less as possible.
DISCLOSURE OF THE INVENTION
The present invention was made taking account of the problems discussed above. Accordingly, an object of the present invention is to provide a photomask which has a sufficient durability to short-wavelength exposure beams, too, and also can prevent any foreign matter from adhering to patterns for transfer.
Another object of the present invention is to provide a photomask which is a photomask having a transmitting plate and in which the figure tolerance (surface precision) of the transmitting plate can be measured with ease and also the transmitting plate is low reflective, and to provide a photomask, and a projection exposure apparatus, which can make aberrations less occur even when the transmitting plate has a variety in thickness.
To achieve the above objects, first invention provides a photomask on which a transfer pattern to be transferred to an exposure-target substrate is formed and through which a stated exposure beam app
Mathews Alan A.
Miles & Stockbridge P.C.
Nikon Corporation
LandOfFree
Photomask, method for manufacturing the same, projection... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Photomask, method for manufacturing the same, projection..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Photomask, method for manufacturing the same, projection... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3126482