Radiant energy – Inspection of solids or liquids by charged particles – Electron microscope type
Reexamination Certificate
2000-04-11
2003-05-27
Anderson, Bruce (Department: 2881)
Radiant energy
Inspection of solids or liquids by charged particles
Electron microscope type
C250S3960ML, C250S3960ML, C250S492200, C250S398000
Reexamination Certificate
active
06570155
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
The present invention relates to the lithographic writing of patterns on substrates by means of directed energy beams, typically electron beams. More particularly, the present invention relates to increasing the throughput of beam lithography by utilizing bi-directional scanning of the beam over the substrate including correction of the beam scanning characteristics in both scan directions to achieve high accuracy in the patterns being written.
2. Description of Related Art
The fabrication of integrated circuits (“ICs”) requires ever more accurate methods for creating patterns on a wafer substrate. Two basic processes are commonly used. In one process, the pattern may be created on a resist-coated wafer by exposing the wafer to a beam of energy directed onto the substrate through a mask containing the desired pattern. “Positive resists” require transparent regions of the mask for those areas in which resist removal is desired. “Negative resists” require opaque regions of the mask for those areas in which resist removal is desired. Both positive and negative resists are commercially useful. Exposure through a mask is typically performed with electromagnetic radiation although exposure by means of electron, ion or particle beams impinging on the mask are not excluded. “Photolithography” denotes the exposure of a resist-coated substrate through a patterned mask, typically by means of electromagnetic radiation.
The second method of writing patterns on a resist makes use of a beam of energy directed only to those regions of the resist-coated surface requiring exposure without screening by an intervening mask. A suitable beam steering mechanism is typically employed along with suitable on-off controls to insure that only the regions of the surface requiring exposure are contacted by the incident beam. The beam may be electrons, ions, neutral particles or collimated laser light or other electromagnetic radiation. However, to be definite in our discussion, we will emphasize the example of a beam of electrons impacting the resist-coated substrate (e-beam lithography), not excluding thereby other forms lithography by means of directed energy beams.
Direct beam writing of patterns onto a resist-coated surface is the method presently preferred for creating the masks used in photolithography, but the technique offers other advantages as well. Among these other advantages of direct beam writing are the avoidance of complications of alignment and registration of the mask with the substrate and the possibility of creating more precise patterns with the use of accurately focused beams. One disadvantage of direct beam patterning in comparison with photolithography is the relatively smaller throughput possible with direct beam writing. Increasing the throughput of direct beam writing is one objective of the present invention.
Considering e-beam lithography by way of example and not limitation, the presently employed writing techniques may be classified into one of two general categories, vector scan or raster scan. Vector scan typically directs the beam while off to a region of the substrate requiring exposure, then by turning the beam on exposes a contiguous region of the substrate to the energy of the beam before moving to another region for exposure. Simply stated, vector scanning “paints” or “tiles” a region of the substrate with beam energy before moving on to expose another region. Most conveniently, beam direction, scanning trajectories, pixel spot size and/or intensity are under computer control, defining the pattern to be written.
Raster scanning directs the beam to all regions of the substrate no matter what pattern requires exposure and adjusts the beam intensity at each point scanned to effect the correct pattern of exposure. The simplest beam control during raster scanning entails having the beam on or off as each pixel is scanned. However, adjustment of beam intensity to numerous levels between full-on and full-off (gray scales) is also feasible in some raster scanning procedures. For example, see the work of Abboud et. al. U.S. Pat. No. 5,393,987.
One common type of raster scanning is unidirectional, denoting the scanning geometry of writing an entire line from beginning to end followed by beam-off “flyback” to begin writing the next line adjacent to the first pixel of the just-completed line, as depicted schematically in
FIG. 1-I
. Bi-directional scanning denotes the writing of a line in one direction (say bottom to top) then writing the immediately adjacent line in the opposite direction (top to bottom) in which the first pixel of the second line is adjacent to the last pixel of the first line, as depicted schematically in FIG.
1
-II. The non-productive “flyback” period is typically much shorter when bi-directional scanning is used than when writing by means of the uni-directional scanning of
Figure 1-I
.
Usage in the art may use “raster scan” as a generic term to distinguish “vector scan” including within the concept of raster scanning both uni-directional and bi-directional scanning. Unfortunately, “raster scan” may also be used to indicate just uni-directional scanning as in
FIG. 1-I
in contrast to “serpentine scanning” of FIG.
1
-II. Hereinafter, we will use “raster scan” to distinguish vector scan to indicate that every pixel in the scan pattern has the beam directed in its direction with the beam off for pixels not written, including therein both unidirectional and bi-directional scanning of FIG.
1
. To be definite in expression,
FIG. 1-I
we denote as “unidirectional” while FIG.
1
-II we denote as either “bi-directional” or “serpentine.”
“Writing a line” is used herein to distinguish from “flyback,” even though few or none of the pixels in a particular line “written” actually receive any beam energy. That is, “writing a line” denotes scanning the beam over a line of pixels on the substrate under a condition that, depending on the particular pattern, may or may not receive beam energy striking pixels in the line. In contrast, “flyback” denotes the act of repositioning the beam following completion of the writing of a particular line in preparation for writing the next line of the pattern. During flyback, the beam is off. That is, the beam is typically directed to a blanking plate, beam dump, or in some other fashion caused not to impact onto the substrate although an actual energy beam may be generated in the writing system. Genuine termination of the beam at its source is also possible for the “beam-of” condition. Thus, the beam is typically fully off during flyback but, during writing, impacts each pixel in the line or pattern being written with the appropriate amount of energy (which may be zero).
Writing precise patterns on the substrate requires (among other things) precise control of the beam path during writing and precise positioning of the beam during flyback to insure correct placement of lines on the substrate. Beam deflection signals typically include both electronic and electron optic effects to direct the beam in the desired writing-pattern. However, such beam deflection signals are typically not linear to the required degree of accuracy, leading to imprecise patterns. One way to achieve high accuracy in e-beam writing and positioning is to measure the beam distortion in a calibration step and apply beam correction signals (“dynamic corrections”) during actual writing. Prior work of the present inventor related to uni-directional scanning, U.S. Pat. No. 5,345,085 (“'085”) describes methods and systems for determining the dynamic corrections during a calibration step, storing correction signals in a table look-up arrangement then applying the proper correction signals to the main beam deflection signal as the pattern is being written. Increased pattern accuracy results.
However, since the flyback time for unidirectional scanning is typically larger than the flyback time for bi-directional scanning, throughput of e-beam patterning systems may be increased if bi-directional scanning is employed. The w
Abboud Frank
Prior Richard
Anderson Bruce
Applied Materials Inc.
Hashmi Zia R.
Kuo Jung-hua
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