Radiant energy – Irradiation of objects or material – Irradiation of semiconductor devices
Reexamination Certificate
2000-07-25
2002-08-13
Nguyen, Kiet T. (Department: 2881)
Radiant energy
Irradiation of objects or material
Irradiation of semiconductor devices
C250S492230
Reexamination Certificate
active
06433348
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to lithography and more specifically to scanning beam lithography using beams that have a shaped cross section.
DESCRIPTION OF RELATED ART
The field of lithography is well known, especially for use in the semiconductor industry. Typical is semiconductor fabrication by electron beam lithography or laser beam lithography where the electron beam or laser beam is scanned across a sensitive layer. This process is used to fabricate masks or to direct write semiconductor wafers. Lithography systems generate or expose patterns on a substrate which is typically the semiconductor wafer or mask blank by controlling the flow of energy (the beam) from a source to the substrate coated with a resist 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 the 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. In some embodiments, the amount of dose, or energy, at each site is varied as required. Hence, exposure data can be organized in a time order related to the regular scanned 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 (Gaussian) beam, exposing sites one at a time, a periodic scan moving sequentially to each site of the grid, and a rasterized representation of data corresponding to the required dose for each site or “pixel” of the grid. By Gaussian is meant a beam that is most intense at the center and whose intensity falls off (to good approximation) in accordance with a Gaussian curve towards its perimeter.
Also known in the lithography field is vector scan writing wherein 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 the stage and beam movement in what is referred to as a semi-random scan. Pattern data must be provided that includes bout 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 different size and/or shape, in terms of its cross section, for each flash. The pattern is then composed from these variable shapes. For an example of variable shaped beam see Rishton et al., “Raster Shaped Beam Electron Beam Exposure Strategy Using a Two-Dimensional Multi Pixel Flash Field”, U.S. patent application Ser. No. 09/226,361, filed Jan. 6, 1999, and commonly owed with the present disclosure, the disclosure of which is incorporated herein by reference in its entirety. The shaped beam in that disclosure is accomplished by associated circuitry and software including shape codes which specify rectangular shaped exposed areas with four different rotations. Various shape codes specify exposed areas that can be either square or rectangular shaped with four different rotations. Other codes represent L-shaped exposed areas with four different rotations. In other embodiments, shape coudes can represent other shapes of the beam. By shape is meant the cross section of the beam where it is incident on the substrate being exposed.
Such a system includes typically a shaper/blanker driver that includes a translator, output device, timer, and can include a retrograde scan device. The shaper/blanker driver request and receives flash data, i.e., shape data and corresponding dose values from respective flash converter and dose value circuitry. A translator receives the flash data and converts the shape data in corresponding dose values into respective voltage values and an exposure time. The translator provides exposure time to a timer and provides both values to an output device.
Additionally, in one embodiment for each input shape data there is an entry in a shape lookup table in a memory and which outputs four voltage values, to various multiplexers. The voltage values specify a two-dimensional electric field deflection by an upper deflector (this is in the context of an electron beam) that effectively control a shaping of the electron beam cross section by controlling a location that the electron beam intersects a lower aperture. Two voltage values specify a two-dimensional electric field deflection by a lower deflector that effectively offsets any deflection by the upper deflector and positions the shaped electron beam on an intended portion of the target substrate. The location at which the electron beam intersects the lower aperture is adjustable by a large number of incremental distance units in either the horizontal or vertically directions within the plane of the lower aperture. This fine incremental positioning allows for offsetting fine errors due, for example, to variations in an opening defined by the lower aperture over time (aperture erosion).
Furthermore, in one embodiment a retrograde scan device adjusts the voltages provided to the lower deflector to offset the movement of the position of the beam on the substrate during the raster scan. The retrograde scan prevents the electron beam column from spreading a flash field beyond its intended area. Further details are in the above-referenced disclosure. Of course, that disclosure is not limiting as to shaped beams. Shaping of laser beams cannot be done using deflectors but instead is accomplished using various aperture arrangements and optics components. As examples, laser beams can be shaped by mechanically changing the shape of an aperture, or by deflecting the laser beam across an aperture (in analogy to the procedure described for electron beams).
Also well known in the electron beam lithography area is raster gray beam writing used in lithography tools known commercially as MEBES®((manufacturing electron beam exposure system) available from Etec Systems, Inc. See also U.S. Pat. No. 3,900,737 to Collier et al. Examples are the Etec MEBES® 4500, 5000 and 5500 electron beam lithography systems. For an example of a similar type raster scan electron beam system see Abboud et al. U.S. Pat. No. 5,393,987 issued Feb. 28, 1995, assigned to Etec Systems, Inc. and incorporated herein by reference in its entirety.
Raster gray beam writing refers to use of beams which have graduations of dose intensity on a pixel-by-pixel basis. In addition, gray beam writing can also be achieved by scanning the substrate several times. The flash data can vary from one scan to the next, in order to create the desired gradient on the substrate. This provides benefits in terms of faster write time and improved position-related accuracy of the features being exposed by the beam.
An undesirable effect called edge blur is caused by such gray scale writing techniques. This refers to blurring of the edges of features as exposed. This effect has been minimized in the past by using high contrast resist as the sensitive (resist) layer and a dry etch process for subsequent processing steps. However, there still remain the problems of corner rounding, line end shortening and line edge roughening. Corner rounding refers to features, which typically are intended to be square or rectangular, having their
Abboud Fayez E.
Chabala Jan M.
Kuo Jung-hua
Nguyen Kiet T.
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