Radiant energy – Means to align or position an object relative to a source or...
Reexamination Certificate
1998-11-12
2001-03-20
Anderson, Bruce C. (Department: 2881)
Radiant energy
Means to align or position an object relative to a source or...
C250S492200, C250S292000, C250S397000, C250S398000
Reexamination Certificate
active
06204509
ABSTRACT:
FIELD OF THE INVENTION
The invention pertains to charged-particle-beam (CPB) projection-microlithography masks (reticles), apparatus, and methods for transferring a pattern, as defined by the mask, onto a sensitized substrate. The reticles, apparatus, and methods of the present invention are particularly suitable for correcting projected pattern-image errors resulting from reticle distortion, and for transferring the mask pattern onto the sensitized substrate while correcting such errors.
BACKGROUND OF THE INVENTION
In charged-particle-beam (CPB) projection microlithography as used in the fabrication of integrated circuits, a circuit pattern defined by a reticle or mask (these terms are used interchangeably herein) is irradiated with a charged particle beam to transfer the pattern defined by the reticle to a sensitized substrate (e.g., a semiconductor wafer). In recent years, CPB projection-microlithography apparatus (“pattern-transfer apparatus”) have been developed that exhibit improved resolution of the transferred pattern and improved product throughput (i.e., the number of semiconductor wafers that can be exposed with a pattern per unit time). With certain conventional CPB pattern-transfer apparatus, one or more entire die patterns defined on a reticle are transferred onto the wafer in a single exposure. A “die” is a pattern coextensive with the bounds of an integrated circuit or other device to be transferred onto the wafer (usually multiple dies are exposed at respective locations on the wafer).
It is difficult to produce a reticle for a CPB pattern-transfer apparatus that transfers an entire die in a single exposure while also providing the high resolution and circuit densities demanded in recent years. In addition, conventional CPB pattern-transfer apparatus that transfer an entire die per exposure cannot satisfactorily control aberrations arising in the CPB optical system through which the charged particle beam passes, especially over a large optical field covering one or more dies. To solve this problem, CPB pattern-transfer apparatus have been proposed in which a pattern to be transferred is divided into multiple field segments (termed “mask subfields”) that are individually and separately exposed. Such a pattern is typically transferred using a “step-and-repeat” transfer scheme in which the individual mask subfields are sequentially transferred to corresponding “transfer subfields” on a wafer or other sensitized substrate. The transfer subfields are produced on the wafer surface in locations relative to each other such that the transfer subfields are “stitched” together in the correct order and alignment to reproduce the entire die pattern on the wafer surface.
Various mechanical and environmental conditions can cause distortions in the mask (or reticle) from which the patterns are transferred. Such distortions include, but are not limited to, distortions that occur as a mask pattern is formed on the reticle, mechanical distortions of the reticle that occur when the reticle is mounted to a reticle stage or other reticle holder, and thermal distortions arising from changes in reticle temperature due to exposure of the reticle to a charged-particle beam. Whenever a mask pattern is transferred to a substrate using a distorted reticle, the projected mask pattern on the substrate is typically distorted. Such distortion decreases resolution of the transferred subfield images and the positional accuracy with which mask-subfield images are formed on the substrate. This results in poor accuracy with which the transfer subfields are “stitched” together.
Accordingly, there is a need to provide CPB projection-microlithography reticles, apparatus, and methods for correcting optical errors in pattern images projected from a distorted reticle before exposure of the substrate to the pattern defined by the reticle.
SUMMARY OF THE INVENTION
In light of the foregoing deficiencies in the prior art, the present invention provides charged-particle-beam (CPB) projection-microlithography reticles, apparatus, and methods (also termed “pattern-transfer reticles,” “pattern-transfer apparatus,” and “pattern-transfer methods”, respectively) for transferring a pattern image, as defined by the reticle, onto a sensitized substrate using a charged particle beam. The pattern-transfer reticles, apparatus, and methods of the present invention provide correction of pattern-image errors resulting from a distorted reticle and provide transfer of the corrected mask-pattern images onto a substrate with high accuracy.
According to a first aspect of the invention, reticles are provided for exposing a pattern onto a sensitized substrate using a charged particle beam. According to one embodiment, such a reticle comprises a thin film defining a mask pattern, a support grid, and an alignment mark (also termed a “distortion-measurement mark”). The support grid comprises intersecting struts and has a thin-film-facing portion and an opposing surface. The thin-film-facing portion supports the thin film. The alignment mark can be formed on or in the support grid or generally on or in the thin film. The alignment mark is used for measuring reticle distortion such as before exposing a subfield of the reticle to the charged particle beam.
The membrane portions of the mask subfields are preferably very thin in the thickness dimension to suppress unwanted absorption or scattering of a charged particle beam in the reticle.
Typically, the pattern defined by the thin film is segmented into multiple mask subfields, in which instance the mask subfields are separated from one another by respective boundary regions defined on the thin film such that respective struts of the support grid surround a respective membrane of each respective mask subfield. A respective alignment mark can be situated on or in the support grid adjacent at least one corner of at least one mask subfield bounded by respective struts. Each such alignment mark is preferably a stencil mark.
The thin film and the support grid are preferably made from one body etched from a single silicon substrate. The struts of the support grid are preferably in registration with the boundary regions.
The mask subfields can be separated from one another by respective boundary regions defined on the thin film such that respective struts of the support grid surround a membrane of each respective mask subfield. In such an instance, a respective alignment mark is situated on or in the support grid adjacent at least one side of at least one mask subfield bounded by respective struts.
Further alternatively, one or more alignment marks can be situated in one or more mask subfields.
Another embodiment of a reticle according to the invention comprises a thin film defining a mask pattern divided into multiple mask subfields each comprising a membrane defining a respective portion of the mask pattern through which a charged particle beam may pass. A respective boundary region is situated between adjacent reticle membranes. A support grid, having a thin-film-facing portion and an opposing surface, of intersecting struts extends along respective boundary regions so as to support the thin film with the membranes extending between respective adjacent struts. At least one alignment mark can be situated on or in the thin film, or situated on or in the support grid for measuring and correcting distortion of the reticle, such as before exposing a mask subfield to the charged particle beam. Such a reticle preferably has multiple alignment marks situated at prescribed locations on or in the support grid or on or in the thin film. The alignment marks can face downstream during use of the reticle for exposing the sensitized substrate to the mask pattern. A respective alignment mark can be situated on or in the boundary region at each location where struts intersect one another. Alternatively, a respective alignment mark can be situated on the thin film at each location corresponding to a midpoint of a side of a respective mask subfield. Further alternatively, certain mask subfields can include o
Hirayanagi Noriyuki
Yahiro Takehisa
Anderson Bruce C.
Klarquist Sparkman Campbell & Leigh & Whinston, LLP
Nikon Corporation
Wells Nikita
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