Multi-beam scanner including a dove prism array

Details Multi-beam scanner including a dove prism array Multi-beam scanner including a dove prism array

1999-03-19

2001-08-07

Schwartz, Jordan M. (Department: 2873)

Radiant energy

Photocells; circuits and apparatus

Optical or pre-photocell system

C359S205100, C359S640000, C347S241000, C347S256000

Reexamination Certificate

active

06271514

ABSTRACT:

BACKGROUND
1. Field of the Invention
This invention relates to printing systems using multiple scan beams and particularly to optical systems associated with acousto-optic modulators in such systems.
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 reticules 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
; prescan 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 in image plane
160
.
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. A beam stop later in the optical train blocks the undeflected part 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, turning beams on or off 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
(after convolution through the system optics
130
and
150
) that is perpendicular to a scan direction
172
. Deflection direction
178
typically corresponds to the direction of propagation of the acoustic wave. 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.
Acoustic waves in an acousto-optic modulator propagating opposite the scan direction (after convolution through scanner optics) eliminates skew and 45°/135° bias and sharpens edges of illuminated regions. However, in scanning systems using multiple beams, projections of the scan beams along the scan direction typically overlap. For example, as shown in
Fig. 1C
, beams
132
,
134
,
136
, and
138
overlap when viewed along scan direction
172
. This creates a brush that illuminates a strip in the image plane without gaps between adjacent beams. With this configuration, an acoustic wave propagating along or opposite scan direction
172
would affect multiple beams. Generally, the separation
133
between beams inside acousto-optic modulator
120
must be more than a beam diameter to permit acoustic waves
122
,
124
,
126
, and
128
to independently modulate respective beams
132
,
134
,
136
, and
138
. Accordingly, to provide independent control of the intensities of beams
132
,
134
,
136
, and
138
, acoustic waves
122
,
124
,
126
, and
128
in acousto-optic modulator
120
must propagate at an angle relative to scan direction
172
.
Systems and methods are sought that use simultaneous scan beams for faster scanning but 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 includes an array of optical elements such as dove prisms. Each optical element effectively rotates the direction in which illumination progress across the cross-section of an associated beam when an acousto-optic modulator turns on the beam. The amount of rotation is selected so that at the image plane where scan lines form, the beams cross-sections expand or brighten along a direction opposite the scan direction.
One embodiment of the invention is a scanning system that includes: a source of multiple beams; a modulator positioned to separately control intensities of the beams, an array of optical elements associated with the beams; a scanning element that sweep the beams along a scanning direction; and post-scan optics that direct the beams to form scan lines in an image plane. When the modulator turns on one of the beams, illuminated areas in a cross-section of the beam progress in a brightening direction. Each optical element acts on the associated beam to change brightening direction so that in the image plane the brightening direction is along or opposite the scanning direction. In one specific embodiment, the array of optical elements is an array of dove prisms.
To avoid varying in the relative positions of beams, dove prisms in an array have uniform geometry and are uniformly positioned relative to central axes of the beams. A process for making dove prisms with uniform geometry attaches multiple rods or pieces of fiber optic material to an optical flat with sides of the rods in contact to keep the rods parallel to each other. The combined assembly including the rods and the flat are then ground or polished to form planar surfaces at opposite ends of the rods. Forming these planar surface forms front and back facets of the dove prisms. The lengths of the dove prisms (i.e., the distances between front and back facets) are uniform since all rods are polished at once. A top surface of the assembly is ground or polished to form side facets where total internal reflections occur in the dove prisms. Once polishing of the assembly forms the dove prisms, the dove prisms are removed from the optical flat for mounting on a substrate to form the array. To prepare the substrate, known integrated circuit processing techniques such as photolithography and etching form parallel grooves with precise spacing, shape, and size in the substrate. The dove prisms are placed in the grooves with the side facets of the dove prisms oriented as required to rotate each beam by the desired amount.


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patent: 4796038 (1989-01-01), Allen et al.
patent: 4956650 (1990-09-01), Allen et al.
patent: 5386221 (1995-01-01), Allen et al.
patent: 5631762 (1997-05-01), Kataoka
patent: 5923359 (1999-07-01), Montgomery
patent: 92307098 (1993-10-01), None

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