X-ray or gamma ray systems or devices – Specific application – Lithography
Reexamination Certificate
1999-07-20
2001-05-08
Porta, David P. (Department: 2882)
X-ray or gamma ray systems or devices
Specific application
Lithography
C378S035000
Reexamination Certificate
active
06229871
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a reticle chuck used in projection lithography that preferably employs soft x-rays and in particular to a reticle chuck having a surface that is contoured to compensate for distortions in the projected image to the reticle.
BACKGROUND OF THE INVENTION
Projection lithography is a powerful and essential tool for microelectronics processing and has supplanted proximity printing. “Long” or “soft” x-rays (a. k. a. Extreme UV) (wavelength range of &lgr;=100 to 200 Å) are now at the forefront of research in efforts to achieve the smaller desired feature sizes. With projection photolithography, a reticle (or mask) is imaged through a reduction-projection lens onto a wafer. Reticles for EUV projection lithography typically comprise a silicon or glass substrate coated with an EUV reflective multilayer material and an optical pattern fabricated from an EUV absorbing material that is formed on the reflective material. As is apparent, projection lithography systems are non-telecentric in that incident radiation from the condenser is not normal to the reflecting reticle surface.
In operation, EUV radiation from the condenser is projected toward the surface of the reticle and radiation is reflected from those areas of the reticle reflective surface which are exposed, i.e., not covered by the EUV absorbing material. The reflected radiation effectively transcribes the pattern from the reticle to the wafer positioned downstream from the reticle. Conventional EUV lithographic projection system designs employ reflecting reticles that have flat surfaces to avoid contributing to distortions of the projected image. A scanning exposure device uses simultaneous motion of the reticle and wafer, with each substrate being mounted on a chuck that is attached to an X-Y stage platen, to continuously project a portion of the reticle onto the wafer through a projection optics. Scanning, as opposed to exposure of the entire reticle at once, allows for the projection of reticle patterns that exceed in size that of the image field of the projection system. Laser interferometry is typically used to determine the actual stage platen position.
Among the problems encountered in EUV projection lithography are point-to-point position variations caused by non-flatness in the reflective mask. One result is that the radiation reflected to the wafer is shifted relative to the ideal position. The art is in search of techniques to reduce such distortions.
SUMMARY OF THE INVENTION
The present invention is based in part on the recognition that the surface of a reflecting reticle can be employed to correct some distortions of the projection lithography system. Further, it is recognized that given the thinness and pliability of the reticle, the reticle's surface contour will conform to that of the chuck holding the reticle.
Accordingly, in one embodiment, the invention is directed to a chuck for holding a reflective reticle, which reflects EUV radiation that is projected from an illumination device, wherein the chuck includes an insulator block that is adapted to support the reflective reticle and the insulator block has a non-planer surface contoured to cause distortion correction of the EUV radiation. Preferably, the EUV radiation is projected from a first direction that is parallel to the scanning direction and the non-planer surface is contoured to displace EUV radiation in the first direction when reflected from the reflective reticle.
In another embodiment, the invention is directed to a chuck reticle assembly that includes:
a chuck that comprises an insulator block; and
a reflective reticle, that is positioned on a surface of the insulator block, wherein the reflective reticle has a non-planer surface that reflects EUV radiation that is projected from an illumination device and that is contoured to cause distortion correction of the EUV radiation.
In a further embodiment, the invention is directed to a photolithography system that includes:
chuck reticle assembly that comprises:
(i) a chuck that comprises an insulator block; and
(ii)a reflective reticle, that is positioned on a surface of the insulator block, wherein the reflective reticle has a non-planer surface that reflects EUV radiation that is projected from an illumination device and that is contoured to cause distortion correction of the EUV radiation;
a wafer positioned downstream from the reflective reticle;
an illumination device positioned upstream of the reticle chuck that projects a beam of radiation onto the mask pattern of the reflective reticle from a first direction; and
a projection optics device that collects radiation reflected from the reflective reticle and directing the radiation toward the wafer, wherein the reflective reticle cancels distortions present in the projection optics device by causing the beam of radiation to be displaced in the first direction when reflected from the reflective reticle.
In yet another embodiment, the invention is directed to a photolithography process implemented in a system that includes a reticle chuck that has a surface that supports a reflective reticle, a wafer that is positioned downstream from the reflective reticle, an illumination device that projects radiation toward the reflective reticle, and a projection optics device that collects radiation reflected from the reflective reticle and directs the radiation toward the wafer, wherein the process includes the steps of:
measuring distortions in an image that is projected toward the wafer; and
modifying the reflective reticle so that its reflective surface onto which the projected radiation is directed is non-planer and has a compensating contour effective to correct the distortions.
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Burns Doane Swecker & Mathis L.L.P.
EUV LLC
Porta David P.
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