Photocopying – Projection printing and copying cameras – Step and repeat
Reexamination Certificate
2000-05-10
2002-03-19
Adams, Russell (Department: 2851)
Photocopying
Projection printing and copying cameras
Step and repeat
C355S055000, C355S057000, C355S060000, C355S067000, C355S077000, C355S030000, C359S512000, C250S216000, C250S492200, C250S548000
Reexamination Certificate
active
06359678
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Cross-reference
This application is a Continuation of International Application PCT/JP98/05118 which was filed on Nov. 13, 1998 claiming the conventional priority of Japanese patent application Nos. 9-330861 and 9-330862 filed on Nov. 14, 1997, respectively.
2. Field of the Invention
The present invention relates to an exposure apparatus and an exposure method based on the use of a reflecting type mask. In particular, the present invention relates to an exposure apparatus and a method for producing the same, as well as an exposure method to be used, for example, when a circuit device such as a semiconductor element and a liquid crystal display element is produced in accordance with a lithography step.
3. Description of the Related Art
At present, a circuit device (for example, D-RAM of 64 M (mega) bits) having a minimum line width of about 0.3 to 0.35 &mgr;m is mass-produced in the production site for the semiconductor device by using a reduction projection exposure apparatus, i.e., a so-called stepper based on the use of an illumination light beam of an i-ray of a mercury lamp having a wavelength of 365 nm. Simultaneously, an exposure apparatus has been introduced in order to produce a circuit device of the next generation having a degree of integration of about 256 M bits or 1 G (giga) bits D-RAM class with a minimum line width of not more than 0.25 &mgr;m.
A scanning type exposure apparatus based on the step-and-scan system is developed as an exposure apparatus for producing the next generation circuit device, in which the illumination light beam is an ultraviolet pulse laser beam having a wavelength of 248 nm radiated from a KrF excimer laser light source or an ultraviolet pulse laser beam having a wavelength of 193 nm radiated from an ArF excimer laser light source, and a mask or a reticle (hereinafter generally referred to as “reticle”) with a depicted circuit pattern and a wafer as a photosensitive substrate are subjected to relative one-dimensional scanning with respect to a projection field of a reduction projection optical system to thereby repeat the stepping operation between shots and the scanning exposure operation for transferring the entire circuit pattern on the reticle into one shot area on the wafer.
It is certain that the degree of integration of the semiconductor device may be further increased to be high in future, and those having 1 G bits may be replaced with those having 4 G bits. In such a situation, the device rule is 0.1 &mgr;m, i.e., about 100 nm L/S. There are numerous technical tasks if such a situation is dealt with the exposure apparatus which uses the illumination light beam of the ultraviolet pulse laser beam having the wavelength of 193 nm as described above. The resolution of the exposure apparatus to represent the device rule (practical minimum line width) is generally represented by the following expression (1) with the exposure wavelength &lgr; and the numerical aperture N.A. of the projection optical system.
(Resolution)=k·&lgr;/N.A. (1)
In the expression, k represents a positive constant called “k factor” of not more than 1, and it differs depending on, for example, the characteristic of the resist to be used.
As clarified from the foregoing expression (1), it is extremely effective to decrease the wavelength &lgr; in order to enhance the resolution. Therefore, recently, the development is started for an EUV exposure apparatus which uses, as an exposure light beam, a light beam in the soft X-ray region having a wavelength of 5 to 15 nm (in this specification, the light beam is referred to as “EUV (Extreme Ultra Violet) light beam” as well). Such an EUV exposure apparatus attracts the attention as a hopeful candidate for the next generation exposure apparatus having a minimum line width of 100 nm.
The EUV exposure apparatus generally uses a reflecting type reticle. An illumination light beam is radiated obliquely onto the reflecting type reticle. A reflected light beam from the reticle surface is projected onto a wafer via a projection optical system. Thus, a pattern in an illumination area on the reticle is transferred onto the wafer. In the case of the EUV exposure apparatus, the scanning exposure method is adopted as follows, in order to transfer the pattern by utilizing only a portion of the projection optical system in which the image formation performance is excellent. That is, a ring-shaped illumination area is set on the reticle, and the entire surface of the pattern on the reticle is successively transferred onto the wafer via the projection optical system by relatively scanning the reticle and the wafer with respect to the projection optical system.
The reason why the reflecting type reticle is used is as follows. That is, there is no substance for producing a reticle which efficiently transmit the light without any absorption at the wavelength (5 to 15 nm) of light used for the EUV exposure apparatus. Further, it is also difficult to prepare a beam splitter. Therefore, it is necessary that the illumination light beam is radiated obliquely with respect to the reticle.
For this reason, the side of the reticle is non-telecentric. The displacement of the reticle in the direction along the optical axis appears as the change in magnification in the longitudinal direction and as the change in position in the transverse direction of the ring-shaped exposure area on the wafer (area on the wafer corresponding to the ring-shaped illumination area on the reticle).
Explanation will be made referring to specified numerical values. It is assumed that an EUV light beam having a wavelength of 13 nm is used as an exposure light beam to design a projection optical system having a resolution of 100 nm L/S.
The foregoing expression (1) can be converted into the following expression (2).
N.A.=k·&lgr;/(resolution) (2)
It is now assumed that k=0.8 is given. According to the expression (2), it is comprehensive that N.A. necessary to obtain the resolution of 100 nm L/S is N.A.=0.104≈0.1. Of course, this N.A. represents a value on the wafer side, which is different from N.A. on the reticle side.
It is now assumed that the projection magnification of the projection optical system is 4:1 which is generally used for the conventional far ultraviolet ray exposure apparatus (DUV exposure apparatus) which uses the exposure light beam of i-ray, g-ray, KrF excimer laser beam, or ArF excimer laser beam. If N.A. is 0.1 on the wafer side, N.A. on the reticle side is ¼ of this value, i.e., 0.025. This fact means the fact that the illumination light beam radiated onto the reticle has a broadening of an angle of about ±25 mrad with respect to the main ray of light. Therefore, in order not to superimpose the incoming light beam an the reflected light beam with each other, it is necessary that the angle of incidence is not less than 25 mrad at the minimum.
For example, with reference to
FIG. 16
, it is assumed that the angle of incidence &thgr; (=outgoing angle &thgr;) is 50 mrad. The lateral discrepancy &egr; of the circuit pattern depicted on the reticle R with respect to the displacement AZ in the Z direction of the pattern plane of the reticle R (hereinafter appropriately referred to as “displacement of the reticle in the Z direction” as well) is represented by the following expression (3).
&egr;=&Dgr;Z·tan &thgr; (3)
According to the expression (3), the following fact is comprehensible. That is, for example, if the reticle R is displaced by 1 &mgr;m in the vertical direction (Z direction) in
FIG. 16
, the lateral discrepancy of the image on the reticle pattern plane is about 50 nm. On the wafer, the image shift occurs in an amount of ¼ of this value, i.e., 2.5 nm. It is also approved that the overlay error (superimposing error), which is allowable in the semiconductor process with the device rule of 100 nm L/S, is not more than 30 nm. The occurrence of the overlay error of 12.5 nm, which is caused by only the displacement of the reti
Adams Russell
Brown Khaled
Nikon Corporation
Oliff & Berridg,e PLC
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