Technique for writing with a raster scanned beam

Details Technique for writing with a raster scanned beam Technique for writing with a raster scanned beam

2002-04-01

2004-12-07

Lee, John R. (Department: 2881)

Radiant energy

Irradiation of objects or material

Irradiation of semiconductor devices

C250S492220, C250S492230, C250S311000, C250S307000

Reexamination Certificate

active

06828570

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates generally to pattern generation systems. More specifically, the present invention relates to a column in a lithographic pattern generation system that employs a raster scanned beam writing technique.
FIG. 1
shows an exemplary prior art column
10
employed in a photolithographic pattern generation system that includes a high brightness electron source
12
such as a ZrO Schottky emission cathode with extraction energy of 10 kV. Source
12
produces an electron beam
14
that is directed along a path
16
. Disposed in path
16
are a focusing lens
18
, a first stop
20
, a second stop
22
and an objective lens
24
. First stop
20
includes a square aperture
20
a
that lies in path
16
, and second stop
22
includes a rectangular aperture
22
a
that lies in path
16
. Disposed between first stop
20
and second stop
22
is a first deflector
26
. A second deflector
28
is disposed between second stop
22
and objective lens
24
.
Lens
18
may be a series of magnetic lenses or electrostatic lenses and is used to focus electrons in beam
14
to pass through square aperture
20
a
. First deflector
26
deflects beam
14
through an angle &thgr;
d
with respect to second aperture
22
a
, systematically allowing a portion of beam
14
to propagate through objective lens
24
, discussed more fully below.
Objective lens
24
defines an object plane
30
located between first deflector
26
and second deflector
28
, proximate to second stop
22
. Although object plane
30
is shown positioned between first deflector
26
and second stop
22
, object plane
30
may be positioned between second deflector
28
and second stop
22
. Objective lens
24
images object plane
30
onto an image plane
32
. Beam
14
impinges upon image plane
32
as a shadow, as opposed to a focused image, of the overlay of square aperture
20
a
and rectangular aperture
22
a
. With this configuration, the area of the shadow impinging upon image plane
32
is determined by the focus of objective lens
24
instead of image magnification. The area of the shadow may be much smaller than the physical size of either first aperture
20
a
or second aperture
22
a
, and its size may be adjusted by varying the distance between the cathode crossover
16
a
and object plane
30
. Positioned in object plane
30
is a substrate
36
, upon which a pattern is written.
When writing a pattern, it is desireable to provide the highest quality pattern in a minimum amount of time, which is expressed in terms of the pattern coverage rate (R). R specifies the pattern area exposed per second of writing time. R is normally expressed having the dimensions of square centimeters per second (cm
2
/sec). Thus, it is desireable to employ a pattern writing technique having a high R.
One such pattern writing technique is described by Rishton et al. in
Raster shaped beam pattern generation
J. Vac. Sci. Tech. B17:6, p. 2927 (1999) and employs a graybeam data format to specify a fraction of patterned area within each pixel on a regular grid. The pixel grid is further partitioned into a flash grid, where flash sites include four graybeam data pixels in a 2×2 array. The beam is scanned periodically over the substrate. An exposure amplitude retrograde scan is added to the uniform saw tooth raster scan, so that the beam appears to dwell on each area of exposure for approximately 10 nsec. At each exposure, the pattern is composed using a shaped beam, allowing edges to be positioned on an address grid that is much finer than the pixel grid. The size and shape of the beam is derived from a 4×4 pixel array of graybeam data surrounding the exposure area. The exposure time is varied between about 30%-80% of the exposure cycle time to correct for proximity scattering and other dose error effects.
Pattern exposure is controlled as a function of the flashing and blanking of beam
14
. The flash is a portion of the pattern written in image plane
32
during one cycle of an exposure sequence by the presence of beam
14
in image plane
32
. The blank is the absence of beam
14
in image plane
32
.
Referring to both
FIGS. 1 and 2
, a flash occurs when first deflector
26
deflects beam
14
so that a shadow of square aperture
20
a
superimposes a portion of rectangular aperture
22
a
, referred to as a flash position
34
. A blank occurs when second deflector
28
deflects beam
14
so that no portion of the shadow of square aperture
20
a
superimposes rectangular aperture
22
a
, referred to as a blank position
36
. First deflector
26
systematically flashes and blanks beam
14
in accordance with the pattern to be written.
Referring to
FIGS. 1 and 3
, a prior art blanking technique is shown. At the commencement of the flash cycle, the shadow of square aperture
20
a
impinges upon the surface of stop
22
, referred to as blank position
40
. During the flash cycle, beam
14
is deflected so that the shadow of square aperture
20
a
moves along a first direction to a flash position
42
, in which a portion
44
thereof superimposes rectangular aperture
22
a
. At the end of the flash cycle, the shadow of the square aperture returns to blank position
40
. To that end, beam
14
is deflected so that the shadow of square aperture
20
a
moves along a second direction, opposite to the first direction.
A drawback with this blanking technique is that it results in an undesirable “shutter” effect, due to the limited bandwidth and settling time of the deflection drive electronics. Specifically, beam
14
impinges upon regions of substrate
36
that should not be exposed when proceeding to the final position. In addition, the presence of a single blanking position, such as blank position
40
, results in regions of substrate
36
being exposed longer to beam
14
than other regions. The net result is a non-uniform dose distribution that causes errors in both the location and size of pattern features.
Referring to
FIGS. 1 and 4
, shown is another prior art blanking technique for an alternate embodiment of stop
122
. Stop
122
includes four apertures
122
a
,
122
b
,
122
c
and
122
d
. Beam
14
is deflected so that the shadow of square aperture
20
a
moves back and forth in opposite directions when traveling between a blank position
140
and a flash position
142
. The choice of aperture
122
a
,
122
b
,
122
c
and
122
d
selected for a flash position depends upon the shape of the region on the substrate to be exposed. This depends upon the relationship between pattern features to be written and the flash grid. As discussed above with respect to
FIG. 2
, this blanking technique also results in non-uniform dose distribution.
What is needed, therefore, is a blanking technique that provides improved dose uniformity.
SUMMARY OF THE INVENTION
Provided is technique for generating patterns with a photolithographic system that employs a multiple blank position flash cycle. In accordance with one embodiment of the present invention, a beam, creates a shadow of a first aperture that impinges upon a region of a stop, referred to as a first blank position. The beam is deflected so that the shadow of the first aperture moves along a first direction A to a flash position, in which a portion thereof superimposes a second aperture that is located in the stop. To complete the flash cycle, the beam is deflected so that the shadow of the aperture impinges upon a second region of the stop, referred to as a second blank position. As a result, during the flash cycle, the beam is deflected in one direction to impinge upon two different blank positions. During a subsequent flash cycle, the beam moves the shadow of the first aperture along a second direction, which is opposite to first direction. In this manner, the shadow of the aperture moves from blank position and impinges upon the aperture of the second stop. Thereafter, the beam is deflected to move the shadow of the first aperture of the first stop, along the second direction, from impinging upon the second aperture located in the sto

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