Radiation imagery chemistry: process – composition – or product th – Imaging affecting physical property of radiation sensitive... – Electron beam imaging
Reexamination Certificate
1999-06-07
2001-08-21
Young, Christopher G. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Imaging affecting physical property of radiation sensitive...
Electron beam imaging
C430S030000, C430S942000
Reexamination Certificate
active
06277542
ABSTRACT:
FIELD OF THE INVENTION
The present invention pertains to microlithography reticles and methods for their use in transferring a pattern defined on the reticle to a suitable sensitized substrate using a charged particle beam (e.g., electron beam or ion beam). Such reticles and methods have especial utility in the manufacture of semiconductor devices and displays. More specifically, the invention is directed to reducing errors in focal-point positions, distortions, and astigmatic blurring of transferred patterns caused by the Coulomb effect and related effects.
BACKGROUND OF THE INVENTION
Conventional electron-beam microlithography apparatus offer prospects of high-accuracy and high-resolution exposures but suffer from low throughput. Various technologies have been investigated in efforts to correct this fault. For example, certain pattern-portion batch exposure methods such as “cell projection,” “character projection,” and “block exposure” methods have received considerable attention. In a pattern-portion batch exposure method, small portions (e.g., 5 &mgr;m square units of a pattern defining a memory portion) of an overall pattern (e.g., of an entire integrated circuit) that are repeated many times in the overall pattern are defined by respective regions on the reticle. The region on the reticle corresponding to the small portion is typically used repeatedly many times during the transfer of a die pattern to the substrate (e.g., semiconductor wafer) to form the overall pattern on the substrate. Portions of the overall pattern that are not repeated are typically transferred using a different method such drawing using a variable shaped beam. Unfortunately, such methods have very low throughput.
An electron-beam “reduction” (i.e., demagnifying) projection-transfer apparatus has been proposed that purportedly achieves higher throughput than pattern-portion batch exposure methods. In this type of projection-transfer apparatus, the reticle defines the entire die pattern (i.e., the entire pattern destined to be exposed onto a separate “chip” on the substrate). The pattern on the reticle is typically divided (“segmented”) into multiple exposure units that are exposed sequentially by the electron beam onto the substrate. As the electron beam passes through an exposure unit, an image of the respective exposure unit is formed on a corresponding region of the substrate using a projection lens. The image is demagnified as projected onto the substrate, by which is meant that the image is smaller than the corresponding exposure unit as defined on the reticle.
In an attempt to improve the throughput of divided projection-transfer methods and apparatus, simultaneous irradiation of the entire reticle (i.e., “batch” exposure of the entire reticle defining an entire die pattern or even multiple die patterns) has been proposed. Unfortunately, such a technique exhibits poor transfer accuracy and poor edge resolution. It is also very difficult to produce a reticle that defines an entire die pattern (or multiple die patterns) to be transferred in one “shot” to the substrate.
Hence, divided projection exposure remains the favored technique for achieving projection exposure using a charged particle beam. According to one approach in divided projection exposure, the optical field of the projection-optical system is increased to allow projection of larger portions of the pattern during each shot. In any event, in divided projection-exposure methods, aberrations can arise during exposure of each exposure unit. Certain conventional divided projection-exposure apparatus achieve real-time correction of aberrations such as distortion or variations in the focal points of the images of the exposure units as formed on the substrate. Such corrections tend to improve the resolution and accuracy of pattern transfer over the entire die region compared to batch-transfer methods.
In exposure apparatus that employ a charged particle beam, exposed patterns can exhibit blurring (e.g., astigmatic blurring) and distortion. In a conventional variable-spot method or cell-projection method, each exposure unit is typically less than about 5 &mgr;m square. In the conventional divided transfer methods and apparatus summarized above, the exposure units are typically larger, approximately 100 &mgr;m square or larger (to increase throughput). With such large exposure units (each defining a respective portion of the overall pattern), if the features of the respective pattern portion are not evenly distributed, then the Coulomb effect can have a variable effect on image quality depending upon the distribution of pattern features in the exposure unit.
An example of an exposure unit having a non-uniform distribution of pattern features is shown in FIG.
5
. In
FIG. 5
, the exposure unit
81
comprises multiple pattern features
87
,
89
. The features
87
are smaller than and spaced farther apart than the features
89
. Also, the features
87
are congregated in a region
83
and the features
89
are congregated in a region
85
. Hence, the feature density in the region
83
is lower than the feature density in the region
85
. Each of the features
87
and
89
is defined on the reticle as an aperture (if the reticle is a stencil reticle) or a local region highly transmissive to charged particles (if the reticle is a membrane reticle). Hence, charged particles passing through any of the features
87
,
89
apply a corresponding local dosage of charged particles on the substrate. (Such features are termed “positive” features.) The complementary portions of the exposure unit
81
tend to block transmission of charged particles and are termed “negative” features.
In
FIG. 5
, the higher-density region
85
within the exposure unit
81
has a feature density of 50% and the lower-density region
83
has a feature density of 10%. The local beam current of the beam passing through the higher-density region
85
will be higher than the local beam current of the beam passing through the lower-density region
85
. As a result, the Coulomb effect will be more pronounced in the higher-density region
85
. The differential impact of the Coulomb effect causes the point of best focus of the beam passing through the higher-density region
85
to be axially displaced relative to the point of best focus of the beam passing through the lower-density region
83
.
Conventionally, transfer of the exposure unit
81
is performed at a “compromise” focal point for the regions
83
and
85
. The compromise focal point, however, is not optimal for either of the regions
83
,
85
. This results in a corresponding decrease in overall resolution of the transferred image of the exposure unit
81
than if each region
83
,
85
were exposed separately. The distortion in an image of an exposure unit
81
as projected is also different than any distortion in an image of an exposure unit with a more uniform feature density.
SUMMARY OF THE INVENTION
In view of the shortcomings of conventional methods and apparatus as summarized above, an object of the invention is to provide, inter alia, charged-particle-beam (CPB) projection-exposure methods that reduce variations in focal-point positions, distortions, or astigmatic blurring of the transferred pattern caused by variations in the Coulomb effect originating from variations in the beam-current distribution over the pattern as transferred to the substrate.
To such end, and according to a first aspect of the invention, methods are provided for projection-exposing a pattern onto a sensitive substrate using a charged particle beam and a projection-optical system. According to a representative embodiment of such a method, the pattern is defined on a reticle. The reticle defines multiple features of the pattern. The reticle also defines multiple “micro features” (as defined herein) each sized below a resolution limit of the projection-optical system. A region of the reticle is illuminated using the charged particle beam such that a portion of the charged particle beam passing through the illuminated region of the reticle bec
Kawata Shintaro
Okino Teruaki
Klarquist Sparkman Campbell & Leigh & Whinston, LLP
Nikon Corporation
Young Christopher G.
LandOfFree
Charged-particle-beam projection-exposure methods exhibiting... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Charged-particle-beam projection-exposure methods exhibiting..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Charged-particle-beam projection-exposure methods exhibiting... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2471186