Radiation imagery chemistry: process – composition – or product th – Radiation modifying product or process of making – Radiation mask
Reexamination Certificate
2000-03-01
2002-03-05
Young, Christopher G. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Radiation modifying product or process of making
Radiation mask
C430S030000, C430S296000, C430S942000, C250S492220, C250S492300
Reexamination Certificate
active
06352799
ABSTRACT:
FIELD OF THE INVENTION
This invention pertains to microlithography (pattern projection and transfer from a mask or reticle to a substrate) as used in the manufacture of semiconductor integrated circuits and displays. More specifically, the invention pertains to microlithography using a charged particle beam (e.g., electron beam or ion beam) to transfer a circuit pattern or the like to a substrate (e.g., semiconductor wafer) at a resolution (minimum linewidth) of 0.1 &mgr;m or less on the substrate.
BACKGROUND OF THE INVENTION
As feature sizes in circuit patterns for integrated circuits, displays, and the like progressively have been miniaturized, the resolution limits of optical microlithography have become increasingly apparent. This has resulted in intensive efforts to develop practical microlithography apparatus and methods exploiting an exposure technology offering prospects of substantially greater resolution than obtainable using optical microlithography. Optical microlithography utilizes a beam of light (typically ultraviolet light) as a pattern-transfer energy beam. One candidate alternative technology to optical microlithography involves the use of a charged particle beam (e.g., electron beam or ion beam) rather than a light beam as an energy beam.
Whereas charged-particle-beam (CPB) microlithography (e.g., electron-beam microlithography) offers prospects of high resolution, many technical problems must be solved in order to develop practical CPB microlithography apparatus and methods. One technical problem pertains to beam drift, i.e., changes in actual beam position relative to desired beam position. As can be surmised, in order to achieve a pattern-feature resolution on the order of 0.1 &mgr;m or less, the position of a charged particle beam as used for pattern transfer must be controlled extremely accurately and precisely. If beam drift is excessive, then the “CPB optical system” (i.e., assembly of “lenses”, deflectors, and the like for shaping and guiding the beam from a source to the substrate) conventionally must be disassembled, cleaned, and reassembled.
Most instances of beam drift arise from the accumulation of contaminants in the CPB optical system. Deposits of contaminants in a CPB optical system tend to accumulate static charges that can have a significant electrostatic effect on the beam. I.e., propagation of the beam past contaminant deposits presenting an unwanted electrostatic charge to the beam can cause the beam to be deflected or distorted in undesirable ways. Some causes of beam drift can be attributed to parameters that can be controlled in the optical system such as variations in lens-induction current, deflection current, voltage, temperature, and the like. Nevertheless, beam drift (especially beam drift caused by factors that cannot be controlled directly) remains an important problem requiring effective solution.
Whereas the beam current in certain types of CPB microlithography apparatus (specifically, conventional electron-beam “variable-shaped beam apparatus”) is usually small (approximately 1 &mgr;A or less), a beam current of, e.g., 20 times greater (i.e., 20 to 25 &mgr;A) is used in other types of apparatus such as “divided-pattern” CPB microlithography. Exposure of a resist on the surface of a wafer with these higher beam currents typically generates large amounts of volatile by-products of the resist. The volatile by-products tend to deposit in various locations inside the CPB optical system, and the rate of deposition tends to increase with increases in beam current. To achieve and maintain maximal resolution of pattern transfer, beam stability (freedom from significant drift) must be maintained at a high level. However, to maintain such stability at high beam currents, affected components of the CPB optical system must be disassembled and cleaned progressively more frequently. In addition, especially at higher beam currents, resulting variations in temperature of the components in the CPB optical system can cause significant beam drift, even in instances in which the CPB optical system is not “dirty.”
SUMMARY OF THE INVENTION
This invention addresses these problems and its purpose is to provide, inter alia, methods for realizing high-precision pattern transfer, even when there is a certain amount of beam drift.
A charged-particle-beam CPB) microlithography (“projection-transfer” or “projection-exposure”) system according to the invention employs a reticle in which the pattern field is divided or “segmented” into multiple portions defining respective portions of the pattern. More specifically, the pattern field is divided into multiple “stripes” that are typically rectangular in shape. Each stripe has a width (shorter dimension) that is within the deflectable field of the CPB optical system. Each stripe is divided further into multiple parallel “deflection fields” each having a length extending the width of the stripe. The overall pattern field is transferred stripe-by-stripe and each stripe is transferred deflection-strip-by-deflection-strip. To transfer a stripe, the charged particle beam is deflected, in a scanning manner, across the width of the stripe to scanningly expose each deflection field. As each deflection field is exposed, the reticle and substrate are mechanically displaced as required (in the length dimension of the stripe) to bring the next deflection field into position for exposure.
The reticle includes beam-drift test patterns in certain deflection fields (a beam-drift test pattern desirably is located in a terminus of the respective deflection field) that is scanned by the charged particle beam (functioning as a “detection beam”). Corresponding beam-test marks are disposed on or at a substrate in locations where the respective test-pattern-containing deflection fields will be exposed by the charged particle beam. The beam-test marks on the substrate are irradiated scanningly by the detection beam (passing through corresponding beam-test patterns on the reticle) prior to pattern transfer. The positions of the test patterns are detected (desirably iteratively) by scanning the corresponding beam-test marks on the substrate multiple times with the detection beam passing through the corresponding beam-drift test patterns on the reticle, thereby providing positional data for detecting beam drift. From such positional data, the magnitude and directions of corrective deflections to the beam are calculated. The beam position during subsequent pattern transfer is corrected according to the results of these calculations to correct the beam drift and achieve a more accurate pattern transfer.
Whenever the charged particle beam is being used constantly under identical conditions, the magnitude of beam drift over time tends to be minimal. On the other hand, comparatively large beam drifts tend to occur whenever beam parameters are changed. For example, substantial beam drift can occur immediately after resuming use of a beam that has been “blanked” for a long period of time. Substantial beam drift also can occur whenever the beam current is suddenly and substantially changed from a high beam current to a low beam current, for example, or immediately after the beam is subjected to a large deflection. Even though such drifts are regarded as irregular, a degree of repeatability can be discerned in them under similar beam parameters.
In CPB microlithography apparatus that perform pattern transfer using reticles having identical specifications, essentially the same exposure operations normally are repeated at locations on the reticle at which the beam is either blanked or deflected, but not at locations on the reticle at which the beam current normally changes with a change in the pattern. Similar magnitudes and directions of the beam drift tend to be evident at the respective repeated locations. By correcting these repeatable components of beam drift, it is possible to perform high-accuracy pattern transfer even when there is a small residual amount of beam drift.
According to the invention, changing ratios of beam drift or simple d
Klarquist & Sparkman, LLP
Nikon Corporation
Young Christopher G.
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