Laser pattern generator

Details Laser pattern generator Laser pattern generator

1999-03-19

2004-05-04

Nguyen, Lamson (Department: 2861)

Incremental printing of symbolic information

Light or beam marking apparatus or processes

Scan of light

C347S256000, C347S239000, C347S241000, C250S234000, C359S204200, C359S197100

Reexamination Certificate

active

06731320

ABSTRACT:

BACKGROUND
1. Field of the Invention
This invention relates to printing systems and methods and particularly systems and methods using multiple scan beams that have wide lateral separations.
2. Description of Related Art
Printing systems including scanners are suitable for a variety of applications including printing text on paper, patterning photoresist during integrated circuit manufacture, and creating masks or reticles for projection-type photolithography systems. For integrated circuit applications, the printing systems typically require submicron precision.
FIG. 1A
illustrates the basic architecture of a precision printing systems
100
that employs scanning. System
100
includes: a light source
110
such as a laser; an acousto-optic modulator
120
that controls intensity of one or more input beams
135
; pre-scan optics
130
that control the position, shape, and collimation of input beams
135
; a scanning element
140
such as a polygon mirror that sweeps scan beams
145
along a scan direction; and post-scan optics
150
that focus scan beams
145
on an image plane
160
. Scanning of scan beams
145
forms scan lines that expose a pattern in an image area of plane
160
. Acousto-optic modulator
120
modulates the intensity of input beams
135
to select the pattern that scan beams
145
expose.
A conventional acousto-optic modulator includes a block of material such as fused silica through which input beams propagate. To turn on, turn off, or change the intensity of an input beam, a transducer generates an acoustic wave that crosses the path of the input beam in the block. The acoustic wave locally changes the optical properties of the block and deflects part of the input beam. Typically, a beam stop later in the optical train blocks the undeflected portion of the beam.
A concern for a precision scanner having a conventional acousto-optic modulator is the orientation of the scanning direction relative to propagation of the acoustic waves that modulate the input beams. If the propagation direction and the scanning direction are not collinear, the turning on and turning off of beams can reduce sharpness of edges or create undesired skew or directional bias in a pattern being illuminated.
FIG. 1B
illustrates an illuminated region
170
of a scan line formed when an acoustic wave deflects an input beam in a direction
178
that (after convolution through the system optics
130
and
150
) is perpendicular to a scan direction
172
. Deflection direction
178
typically corresponds to the direction of propagation of the acoustic wave in the acousto-optic modulator. As acousto-optic modulator
120
turns on input beam
135
, a cross-section
174
of the beam expands in direction
178
. Accordingly, the initially illuminated part of region
170
is narrow and toward one edge until the input beam has a fully illuminated cross-section such as cross-section
175
. Similarly, when acousto-optic modulator
120
turns off input beam
135
, one edge of the input beam darkens first, and a shrinking cross-section
176
of the beam causes illuminated region
170
to recede toward the opposite edge. This reduces sharpness at the edges of illuminated regions formed by multiple scan lines, skews rectangular illuminated areas, and causes pattern lines at 45° to the scan direction to differ in thickness from pattern lines at 135° to the scan direction. However, to provide independent control of the beam intensities and a narrow scan brush, acoustic waves in an acousto-optic modulator generally propagate at an angle relative to the scan direction.
As shown in
FIG. 1C
, a separation
133
between beams
132
,
134
,
136
, and
138
inside acousto-optic modulator
120
must be sufficient for acoustic waves
122
,
124
,
126
, and
128
to independently modulate respective beams
132
,
134
,
136
, and
138
. Typically, separation
133
must be more than a beam diameter. To avoid the separation causing gaps between scan lines, a scanning direction
172
is selected so that beams
132
,
134
,
136
, and
138
overlap when viewed along the scan direction
172
. An advantage of overlapping beams is the narrow width
180
of the scan brush. Narrow brushes reduce scan line bow which is common for conventional f-&thgr; scan lenses. (Scan line bow is the curvature of scan lines that are off the optical axis of a scan lens.) Also, scanning overlapping beams along scan direction
172
forms a band of scan lines without intervening gaps, which simplifies indexing of scan lines to cover the image area. As indicated above, disadvantages of the configuration of
FIG. 1C
are reduced sharpness at the edges in the image, skew of rectangular areas, and 45°/135° line thickness bias.
As shown in
FIG. 1D
, the scan direction
172
can alternatively be the same as or opposite to the direction of propagation of acoustic waves
122
,
124
,
126
, and
128
in acousto-optic modulator
120
. With this configuration, the separation
133
required for independent modulation of beams controls the separation between the scan lines. This creates a scan brush that is wider than the brush of
FIG. 1C
, and the wider scan brush increases scan line bow from a conventional f-&thgr; scan lens, making the accuracy required for integrated circuit applications difficult to achieve. Other types of scan lenses can reduce scan line bow but generally cause scan beams to move with non-uniform velocity and therefore can distort the image.
Systems and methods are sought that use simultaneous scan beams for faster scanning but avoid scan line bow and image distortion and also avoid the skew, blurred edges, and directional bias associated with acousto-optic modulators having acoustic waves propagating at an angle to the scan direction.
SUMMARY
In accordance with the invention, a multi-beam scanner has a wide scan brush, a modulator that controls intensity of pixels in scan beams, an optical system that minimizes scan line bow at the expense of non-uniform scanning beam velocity, and a timing generator that generates a pixel clock signal having a variable period that compensates for the non-uniformity of pixel velocity. The wide separation of scan beams permits the modulator to turn beams on or off with a direction of brightening or darkening in the cross-section of the beams being opposite to the scanning direction. This allows the brightening direction to be opposite the scan direction to improve edge sharpness, avoid skew in rectangular regions, and avoid a directional bias in line thickness.
A novel arrangement of the beams in the brush permits a uniform indexing step size to uniformly expose an image region. In particular, a brush with b beams spaced a distance n apart uniformly covers the image region after repeated scanning and indexing by a distance m if the number of beams b and the distances n and m are such that the ratio of m to n is equal to the ratio of b to an integer q that has no common factors with b. In one embodiment, a diastemal brush has a top half including b beams uniformly spaced distance n apart and a bottom half including b beams uniformly spaced distance n apart. The distance between the top and bottom halves is 1.5*n. With this diastemal brush and a uniform indexing distance m, the top half forms uniformly spaced scan lines, and the bottom half forms scan lines midway between adjacent scan lines that the top half forms. Other embodiments of the scan bush include three or more sections of equal spaced beams separated by two or more diastema.
In one embodiment, the timing generator includes: a source of pixel period values and a counter. The counter loads a first part of a pixel period value selected for a pixel, counts for a period of time indicated by the first part, and asserts a signal marking an end of the period. An additional delay calculator circuit can delay the signal from the counter for a time shorter than the period of a clock signal to the counter. A second part of the pixel period value controls the delay. The combination of the times for the count and the delay forms the complete p

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