Radiant energy – Irradiation of objects or material – Irradiation of semiconductor devices
Reexamination Certificate
1999-01-06
2001-07-17
Arroyo, Teresa M. (Department: 2881)
Radiant energy
Irradiation of objects or material
Irradiation of semiconductor devices
C250S492220, C250S492200
Reexamination Certificate
active
06262429
ABSTRACT:
BACKGROUND
1. Field of the Invention
The present invention relates to lithography and to electron (or other energy) beam columns and more specifically to a structure and method for generating variable shaped beams.
2. Description of the Related Art
It is well known in the field of lithography (pattern generation) that it is desirable to increase the throughput of pattern generation systems. Two main applications for such pattern generation systems are making masks for use in semiconductor fabrication by electron beam lithography and electron beam direct writing of patterns onto wafers to form semiconductor devices.
Lithography systems generate or expose patterns by controlling the flow of energy (the beam) from a source to a substrate coated with a layer sensitive to that form of energy. Pattern exposure is controlled and broken into discrete units commonly referred to as flashes, wherein a flash is that portion of the pattern exposed during one cycle of an exposure sequence. Flashes are produced by allowing energy from the source, for example light, electron or other particle beams, to reach the coated substrate within selected pattern areas. The details of flash composition, dose and exposure sequence used to produce a pattern, and hence the control of the lithographic system, make up what is known as a writing strategy.
A traditional raster scan writing strategy employs a uniform periodic raster scan, somewhat similar to television raster scanning. A mechanical stage moves a substrate, for example placed on a table, uniformly in a direction orthogonal to the direction of the uniform scan of an energy beam. In this manner a pattern is composed on a regular grid with a regular scan trajectory resulting from the orthogonal movement of the stage and beam. When the beam is positioned over a grid site requiring exposure, the beam is unblanked and the underlying site exposed. Only the amount of dose, or energy, at each site is varied as required. Hence, exposure data can be organized in a time sequence corresponding to the regular scan trajectory, and only the dose for each site need be specified. The distinguishing characteristics of a traditional raster scan writing strategy are a small round beam exposing one site at a time, a periodic scan moving sequentially to each site of a grid and a rasterized representation of data corresponding to the required dose for each site or “pixel” of the grid.
On the other hand, in a typical vector scan writing strategy, the beam is positioned only over those sites that require exposure and then unblanked to expose the site. Positioning is accomplished by a combination of stage and beam movement in what is often referred to as a semi-random scan. Thus, data must be provided that includes both the dose and position of each flash or site exposed. Frequently vector scan strategies use a variable shaped beam, that is a beam capable of having a different size and/or shape for each flash. The pattern is then composed from these variable shapes. A shaped beam is capable of exposing multiple pixel sites simultaneously instead of one pixel site at a time as in a raster scan writing strategy. Where a variable shaped beam is used, the data must additionally include the location, size and shape for each flash. Thus the distinguishing characteristics of traditional vector scan writing strategies are a variable shaped and sized beam exposing multiple pixel sites in a single flash, a semi-random scan encompassing only those portions of a pattern to be exposed, and a vectorized representation of data including the location, size, shape and dose of each flash.
Both vector and raster scan writing strategies have advantages and disadvantages. Vector scan strategies can offer fine pattern definition. However, vector scan flash rates are typically slower than raster scan strategies due to settling time required between the relatively large beam deflections of the semi-random scan trajectory. For patterns with exposed portions that are finely detailed, vector scan strategies are relatively slower due to delays in settling of the electron beam shaping components which are capable of shaping the beam over a wide range of dimensions. Also, current density (current per unit area) is generally lower in vector scan strategies due to the need for the electron source to be capable of covering larger areas simultaneously, again leading to lower throughput. A drawback of raster scan writing processes is a relatively coarse pattern definition.
Thus it is desirable to develop an improved writing strategy that combines the advantages of a vector scan strategy, namely, fine pattern definition, with those of a raster scan strategy, namely, increased speed, to increase the throughput of pattern generation systems.
SUMMARY
An embodiment of the present invention provides an apparatus to write flash fields on a substrate in a raster scan, where the flash fields define a pattern, where the apparatus includes: a rasterizer which rasterizes a surface of the substrate into pixels and outputs gray level values, where the gray level values specify a proportion of a pixel that overlaps with the pattern; a buffer coupled to receive and store the gray level values from the rasterizer; a flash converter coupled to receive the gray level values from the buffer, where the flash converter outputs shape data that define a flash field; a dose value circuitry coupled to the rasterizer, where the dose value circuitry computes dose values associated with the shape data; a converter coupled to receive the shape data from the flash converter and associated dose values from the dose value circuitry, where the converter outputs signals that specify a shape of the flash field, duration of the flash field, and a position of the flash field on the substrate; and a charged particle beam column coupled to receive the signals from the converter, and which generates the flash field as specified by the signals from the converter.
An embodiment of the present invention provides a method of calculating and generating flash fields to be written onto a substrate, the method including the acts of: representing the substrate as a grid of pixels; representing each pixel as a gray level value that specifies a proportion of each pixel that is to be exposed by a charged particle beam; determining an exposure time associated with a quadrant of the pixels; representing the quadrant as a shape code; and generating a flash field, where the shape code and the exposure time specify a shape and position on the substrate of the flash field.
An embodiment of the present invention provides an electron (or other charged particle) beam column for writing variable shaped beams onto a surface, and includes: a source of a charged particle beam; a transfer lens positioned downstream of the source; a first aperture element coaxial with the beam and positioned downstream of the source and that defines an opening; a first deflector coaxial with the beam and positioned downstream of the first aperture element, and which generates an electric field; a second aperture element coaxial with the beam and positioned downstream of the first deflector and defining at least one opening, where the electric field directs the beam onto the at least one opening thereby to variably shape the beam; a second deflector coaxial with the beam and positioned downstream of the second aperture element, and which generates a second electric field; magnetic coil deflectors positioned downstream of the second deflector thereby to raster scan the beam; and an objective lens, where the objective lens focuses the variably shaped beam onto the surface and controls a final size of the variably shaped beam on the surface. In one embodiment, the at least one opening includes four openings, each of the four openings being L shaped and the four openings being arranged as corners of a square shape. In one embodiment, the at least one opening is a cross shaped opening. In one embodiment, the beam is directed in a raster scan.
An embodiment of the present invention provides a me
Rishton Stephen A.
Sagle Allan L.
Varner Jeffery K.
Veneklasen Lee H.
Wang Weidong
Arroyo Teresa M.
Etec Systems, Inc.
Leitich Greg
Wells Nikita
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