U.S. patent application number 10/950644 was filed with the patent office on 2005-03-03 for maskless lithography systems and methods utilizing spatial light modulator arrays.
This patent application is currently assigned to ASML Holding N.V.. Invention is credited to Bleeker, Arno, Cebuhar, Wenceslao A., Hintersteiner, Jason D., McCullough, Andrew W., Wasserman, Solomon.
Application Number | 20050046819 10/950644 |
Document ID | / |
Family ID | 33131643 |
Filed Date | 2005-03-03 |
United States Patent
Application |
20050046819 |
Kind Code |
A1 |
Bleeker, Arno ; et
al. |
March 3, 2005 |
Maskless lithography systems and methods utilizing spatial light
modulator arrays
Abstract
A maskless lithography system that writes patterns on an object.
The system can include an illumination system, the object, spatial
light modulators (SLMs), and a controller. The SLMs can pattern
light from the illumination system before the object receives the
light. The SLMs can include a leading set and a trailing set of the
SLMs. The SLMs in the leading and trailing sets change based on a
scanning direction of the object. The controller can transmit
control signals to the SLMs based on at least one of light pulse
period information, physical layout information about the SLMs, and
scanning speed of the object. The system can also correct for dose
non-uniformity using various methods.
Inventors: |
Bleeker, Arno; (Westerhoven,
NL) ; Cebuhar, Wenceslao A.; (Norwalk, CT) ;
Hintersteiner, Jason D.; (Bethel, CT) ; McCullough,
Andrew W.; (Newtown, CT) ; Wasserman, Solomon;
(Long Beach, NY) |
Correspondence
Address: |
STERNE, KESSLER, GOLDSTEIN & FOX PLLC
1100 NEW YORK AVENUE, N.W.
WASHINGTON
DC
20005
US
|
Assignee: |
ASML Holding N.V.
|
Family ID: |
33131643 |
Appl. No.: |
10/950644 |
Filed: |
September 28, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10950644 |
Sep 28, 2004 |
|
|
|
10449908 |
May 30, 2003 |
|
|
|
Current U.S.
Class: |
355/67 ;
355/68 |
Current CPC
Class: |
G03F 7/70466 20130101;
G03F 7/70283 20130101; G03F 7/70358 20130101; G03F 7/70291
20130101; G03F 7/70558 20130101 |
Class at
Publication: |
355/067 ;
355/068 |
International
Class: |
G03B 027/54 |
Claims
What is claimed is:
1. A maskless lithography system, comprising: an illumination
system; an object; spatial light modulators (SLMs) that pattern
light from the illumination system before the light is received by
the object, the SLMs including a leading set and a trailing set of
the SLMs, the SLMs in the leading and trailing sets changing based
on a scanning direction of the object; and a controller that
transmits control signals to the SLMs based on at least one of
light pulse period information, physical layout information about
the SLMs, and scanning speed of the object.
2. The system of claim 1, wherein the object is a semiconductor
wafer.
3. The system of claim 1, wherein the object is a glass
substrate.
4. The system of claim 1, wherein the glass substrate is part of a
liquid crystal display.
5. The system of claim 1, further comprising: a beam splitter that
directs light from the light source to the SLMs and from the SLMs
to the object, wherein the SLMs are reflective SLMs.
6. The system of claim 1, wherein the SLMs are transmissive
SLMs.
7. The system of claim 1, wherein the SLMs are liquid crystal
displays.
8. The system of claim 1, further comprising: a beam splitter that
directs light from the light source to the SLMs and from the SLMs
to the object, wherein the SLMs are digital micromirror devices
(DMDs).
9. The system of claim 1, wherein the control signal to the SLMs
allows for more than one at least partial exposure of a same area
of the object during a single scan.
10. The system of claim 1, wherein the SLMs are positioned a
predetermined distance apart such that more than one SLM exposes an
exposure area on the object for pulses occurring during continuous
movement of the object based on the control signal.
11. The system of claim 1, wherein the SLMs are configured in a
two-dimensional array.
12. The system of claim 1, wherein a first edge of a first one of
the SLMs lies in a same plane as a second edge of an adjacent,
second one of the SLMs.
13. The system of claim 1, wherein the SLMs are staggered with
respect to adjacent SLMs.
14. The system of claim 1, wherein a set of the SLMs that writes to
a same exposure area on the object includes four columns of the
SLMs.
15. The system of claim 14, wherein the columns are positioned
one-half of an active area apart.
16. The system of claim 14, wherein each of the SLMs has an
inactive area and wherein a top of the inactive area of one of the
SLMs is aligned with a bottom of the inactive area of an adjacent
one of the SLMS.
17. The system of claim 14, wherein each of the columns includes
two of the SLMs.
18. The system of claim 14, wherein each of the columns includes
four of the SLMs.
19. The system of claim 14, wherein the exposure area on the object
moves a distance approximately equal to two active areas of the
SLMs during each pulse of the illumination system.
20. The system of claim 1, wherein a set of the SLMs that writes to
a same exposure area on the object includes six columns of the
SLMs.
21. The system of claim 20, wherein a top of an inactive area of
one of the SLMs is aligned with a bottom of an adjacent inactive
area of another one of the SLMs.
22. The system of claim 20, wherein each of the columns includes
four of the SLMs.
23. The system of claim 20, wherein the columns are positioned
one-half of an active area apart.
24. The system of claim 20, wherein the columns are positioned one
active area apart.
25. The system of claim 1, wherein a set of the SLMs that writes to
a same exposure area on the object includes two columns of the
SLMs.
26. The system of claim 25, wherein a top of an inactive area of
one of the SLMs is aligned with a bottom of an adjacent inactive
area of another of the SLMs.
27. The system of claim 25, wherein each of the columns includes
four of the SLMs.
28. The system of claim 25, wherein the columns are positioned
one-half of an active area apart.
29. The system of claim 25, wherein the exposure area on the object
moves a distance approximately equal to one active area of the SLMs
during each pulse of the illumination system.
30. The system of claim 1, wherein the SLMs are coupled to a
support device in a two dimensional array.
31. The system of claim 30, wherein the support device comprises
cooling channels running therethrough.
32. The system of claim 30, wherein the support device comprises
control circuitry.
33. The system of claim 1, wherein each one of the SLMs comprise: a
active area section; and a package section.
34. The system of claim 33, wherein the package section comprises:
control circuitry to control devices in the active area.
35. The system of claim 34, wherein the control circuitry receives
the control signal from the controller.
36. The system of claim 1, wherein a set of the SLMs that writes to
a same exposure area on the object includes six columns of the
SLMs.
37. The system of claim 1, wherein a set of the SLMs that writes to
a same exposure area on the object includes eight columns of the
SLMs.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional application of U.S.
application Ser. No. 10/449,908, filed May 30, 2003, which is
incorporated by reference herein in its entirety.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates generally to lithography. More
particularly, the present invention relates to maskless
lithography.
[0004] 2. Related Art
[0005] Lithography is a process used to create features on the
surface of substrates. Such substrates can include those used in
the manufacture of flat panel displays (e.g., liquid crystal
displays), circuit boards, various integrated circuits, and the
like. A frequently used substrate for such applications is a
semiconductor wafer or glass substrate. While this description is
written in terms of a semiconductor wafer for illustrative
purposes, one skilled in the art would recognize that this
description also applies to other types of substrates known to
those skilled in the art.
[0006] During lithography, a wafer, which is disposed on a wafer
stage, is exposed to an image projected onto the surface of the
wafer by exposure optics located within a lithography apparatus.
While exposure optics are used in the case of photolithography, a
different type of exposure apparatus can be used depending on the
particular application. For example, x-ray, ion, electron, or
photon lithography each can require a different exposure apparatus,
as is known to those skilled in the art. The particular example of
photolithography is discussed here for illustrative purposes
only.
[0007] The projected image produces changes in the characteristics
of a layer, for example photoresist, deposited on the surface of
the wafer. These changes correspond to the features projected onto
the wafer during exposure. Subsequent to exposure, the layer can be
etched to produce a patterned layer. The pattern corresponds to
those features projected onto the wafer during exposure. This
patterned layer is then used to remove or further process exposed
portions of underlying structural layers within the wafer, such as
conductive, semiconductive, or insulative layers. This process is
then repeated, together with other steps, until the desired
features have been formed on the surface, or in various layers, of
the wafer.
[0008] Step-and-scan technology works in conjunction with a
projection optics system that has a narrow imaging slot. Rather
than expose the entire wafer at one time, individual fields are
scanned onto the wafer one at a time. This is accomplished by
moving the wafer and reticle simultaneously such that the imaging
slot is moved across the field during the scan. The wafer stage
must then be asynchronously stepped between field exposures to
allow multiple copies of the reticle pattern to be exposed over the
wafer surface. In this manner, the quality of the image projected
onto the wafer is maximized.
[0009] Conventional lithographic systems and methods form images on
a semiconductor wafer. The system typically has a lithographic
chamber that is designed to contain an apparatus that performs the
process of image formation on the semiconductor wafer. The chamber
can be designed to have different gas mixtures and grades of vacuum
depending on the wavelength of light being used. A reticle is
positioned inside the chamber. A beam of light is passed from an
illumination source (located outside the system) through an optical
system, an image outline on the reticle, and a second optical
system before interacting with a semiconductor wafer.
[0010] A plurality of reticles are required to fabricate a device
on the substrate. These reticles are becoming increasingly costly
and time consuming to manufacture due to the feature sizes and the
exacting tolerances required for small feature sizes. Also, a
reticle can only be used for a certain period of time before being
worn out. Further costs are routinely incurred if a reticle is not
within a certain tolerance or when the reticle is damaged. Thus,
the manufacture of wafers using reticles is becoming increasingly,
and possibly prohibitively expensive.
[0011] In order to overcome these drawbacks, maskless (e.g., direct
write, digital, etc.) lithography systems have been developed. The
maskless system replaces a reticle with a spatial light modulator
(SLM) (e.g., a digital micromirror device (DMD), a liquid crystal
display (LCD), or the like). The SLM includes an array of active
areas (e.g., mirrors or transmissive areas) that are either ON or
OFF to form a desired pattern. A predetermined and previously
stored algorithm based on a desired exposure pattern is used to
turn ON and OFF the active areas.
[0012] Conventional SLM-based writing systems (e.g., Micronic's
Sigma 7000 series tools) use one SLM as the pattern generator. To
achieve linewidth and line placement specifications, gray scaling
is used. For analog SLMs, gray scaling is achieved by controlling
mirror tilt angle (e.g., Micronic SLM) or polarization angle (e.g.,
LCD). For digital SLMs (e.g., TI DMD), gray scaling is achieved by
numerous passes or pulses, where for each pass or pulse the pixel
can be switched either ON or OFF depending on the level of gray
desired. Because of the total area on the substrate to be printed,
the spacing between active areas, the timing of light pulses, and
the movement of the substrate, several passes of the substrate are
required to expose all desired areas. This results in low
throughput (number of pixels packed into an individual optical
field/number of repeat passes required over the substrate) and
increased time to fabricate devices. Furthermore, using only one
SLM requires more pulses of light or more exposure time to increase
gray scale. This can lead to unacceptably low levels of
throughput.
[0013] Therefore, what is needed is a maskless lithography system
and method that can expose all desired areas on a substrate for
each pattern during only one pass of a substrate.
SUMMARY OF THE INVENTION
[0014] The present invention provides a maskless lithography
system. The system can include an illumination system, an object,
spatial light modulators (SLMs), and a controller. The SLMs can
pattern light from the illumination system before the object
receives the light. The SLMs can include a leading set and a
trailing set of the SLMs. The SLMs in the leading and trailing sets
change based on a scanning direction of the object. The controller
can generate control signals to the SLMs based on at least one of
light pulse period information, physical layout information about
the SLMs, and scanning speed of the object.
[0015] Other embodiments of the present invention provide a method
for controlling dose in maskless lithography. The method includes
measuring a dose delivered in each pulse in a series of pulses from
SLMs, calculating a dose error based on the measuring steps,
calculating a correctional blanket dose based on the dose error,
and applying the correctional blanket dose using a final set of
SLMs.
[0016] Still other embodiments of the present invention include a
method for controlling dose in maskless lithography. The method
includes measuring an intensity of a dose from a leading set of
SLMs, subtracting the measured intensity from a predetermined value
to generate an error signal, delaying the error signal, adding the
delayed signal another predetermined value to generate a control
signal, and using the control signal to control dose from a
trailing set of SLMs.
[0017] Further embodiments, features, and advantages of the present
inventions, as well as the structure and operation of the various
embodiments of the present invention, are described in detail below
with reference to the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES
[0018] The accompanying drawings, which are incorporated herein and
form a part of the specification, illustrate the present invention
and, together with the description, further serve to explain the
principles of the invention and to enable a person skilled in the
pertinent art to make and use the invention.
[0019] FIG. 1 shows a maskless lithography system having reflective
spatial light modulators according to embodiments of the present
invention
[0020] FIG. 2 shows a maskless lithography system having
transmission spatial light modulators according to embodiments of
the present invention.
[0021] FIG. 3 shows a spatial light modulator according to an
embodiment of the present invention.
[0022] FIG. 4 shows more details of the spatial light modulator in
FIG. 3.
[0023] FIGS. 5, 6, 7, 8, 9, and 10 show two-dimensional arrays of
spatial light modulators according to various embodiments of the
present invention.
[0024] FIG. 11 shows an exposure diagram for sequential pulses of
light from an illumination source according to various embodiments
of the present invention.
[0025] FIG. 12 is a system 1200 that can control dose and/or
uniformity for a multiple SLM pattern generation array, according
to an embodiment of the present invention.
[0026] FIG. 13 is a flow chart depicting a method according to
embodiments of the present invention.
[0027] FIGS. 14 and 15 show two-dimensional arrays of spatial light
modulators according to various embodiments of the present
invention.
[0028] The present invention will now be described with reference
to the accompanying drawings. In the drawings, like reference
numbers may indicate identical or functionally similar elements.
Additionally, the left-most digit(s) of a reference number may
identify the drawing in which the reference number first
appears.
DETAILED DESCRIPTION OF THE INVENTION
[0029] Overview
[0030] While specific configurations and arrangements are
discussed, it should be understood that this is done for
illustrative purposes only. A person skilled in the pertinent art
will recognize that other configurations and arrangements can be
used without departing from the spirit and scope of the present
invention. It will be apparent to a person skilled in the pertinent
art that this invention can also be employed in a variety of other
applications.
[0031] An embodiment of the present invention utilizes an array of
SLMs in a maskless lithography system in order to allow for
multiple exposures to the same area on an object surface during
each scanning pass. Using the array of SLMs can increase throughput
and lower costs compared to conventional maskless systems using
only one SLM.
[0032] By integrating multiple SLMs into one mechanical assembly, a
field replaceable unit can be made. This unit could integrate
mechanical and thermal stability, cooling channels, purge gas
channels, and electrical connections. Drive electronics, including
wiring, memory, and processors, could also be integrated into
assembly 500, either on a backside or in the empty space on a front
side of assembly 500.
[0033] Maskless Lithography Systems
[0034] FIG. 1 shows a maskless lithography system 100 according to
an embodiment of the present invention. System 100 includes an
illumination system 102 that transmits light to a reflective
spatial light modulator 104 (e.g., a digital micromirror device
(DMD), a reflective liquid crystal display (LCD), or the like) via
a beam splitter 106 and SLM optics 108. SLM 104 is used to pattern
the light in place of a reticle in traditional lithography systems.
Patterned light reflected from SLM 104 is passed through beam
splitter 106 and projection optics 110 and written on an object 112
(e.g., a substrate, a semiconductor wafer, a glass substrate for a
flat panel display, or the like).
[0035] It is to be appreciated that illumination optics can be
housed within illumination system 102, as is known in the relevant
art. It is also to be appreciated that SLM optics 108 and
projection optics 110 can include any combination of optical
elements required to direct light onto desired areas of SLM 104
and/or object 112, as is known in the relevant art.
[0036] In alternative embodiments, either one or both of
illumination system 102 and SLM 104 can be coupled to or have
integral controllers 114 and 116, respectively. Controller 114 can
be used to adjust illumination source 102 based on feedback from
system 100 or to perform calibration. Controller 116 can also be
used for adjustment and/or calibration. Alternatively, controller
116 can be used for turning ON and OFF active devices (e.g.,
pixels, mirrors, locations, etc.) 302 (see FIG. 3) on SLM 104, as
was described above, to generate a pattern used to expose object
112. Controller 116 can either have integral storage or be coupled
to a storage element (not shown) with predetermined information
and/or algorithms used to generate the pattern or patterns.
[0037] FIG. 2 shows a maskless lithography system 200 according to
a further embodiment of the present invention. System 200 includes
an illumination source 202 that transmits light through a SLM 204
(e.g., a transmissive LCD, or the like) to pattern the light. The
patterned light is transmitted through projection optics 210 to
write the pattern on a surface of an object 212. In this
embodiment, SLM 204 is a transmissive SLM, such as a liquid crystal
display, or the like. Similar to above, either one or both of
illumination source 202 and SLM 204 can be coupled to or integral
with controllers 214 and 216, respectively. Controllers 214 and 216
can perform similar functions as controller 114 and 116 described
above, and as known in the art.
[0038] Example SLMs that can be used in systems 100 or 200 are
manufactured by Micronic Laser Systems AB of Sweden and Fraunhofer
Institute for Circuits and Systems of Germany.
[0039] Merely for convenience, reference will be made only to
system 100 below. However, all concepts discussed below can also
apply to system 200, as would be known to someone skilled in the
relevant arts.
[0040] FIG. 3 shows details of an active area 300 of SLM 104.
Active area 300 includes an array of active devices 302
(represented by dotted patterns in the figure). Active devices 302
can be mirrors on a DMD or locations on a LCD. It is to be
appreciated that active devices 302 can also be referred to as
pixels, as is known in the relevant art. By adjusting the physical
characteristics of active devices 302, they can be seen as being
either ON or OFF. Digital or analog input signals based on a
desired pattern are used to turn ON and OFF various active devices
302. In some embodiments, an actual pattern being written to object
112 can be detected and a determination can be made whether the
pattern is outside an acceptable tolerance. If so, controller 116
can be used to generate analog or digital control signals in real
time to fine-tune (e.g., calibrate, adjust, etc.) the pattern being
generated by SLM 104.
[0041] FIG. 4 shows further details of SLM 104. SLM 104 can include
an inactive packaging 400 surrounding active area 300. Also, in
alternative embodiments, a main controller 402 can be coupled to
each SLM controller 116 to monitor and control an array of SLMs
(see discussion below). As discussed below, adjacent SLMs may be
offset or staggered with respect to each other in other
embodiments.
[0042] Spatial Light Modulator Array Configurations
[0043] FIG. 5 shows an assembly 500 including a support device 502
that receives an array of SLMs 104. In various embodiments, as
described in more detail below, the array of SLMs 104 can have
varying numbers of columns, rows, SLMs per column, SLMs per row,
etc., based on a number of desired exposures per pulse, or other
criteria of a user. The SLMs 104 can be coupled to a support device
502. Support device 502 can have thermal control areas 504 (e.g.,
water or air channels, etc.), areas for control logic and related
circuitry (e.g., see FIG. 4 showing elements 116 and element 402,
which can be ASICs, A/D converters, D/A converters, fiber optics
for streaming data, etc.), and windows 506 (formed within the
dashed shapes) that receive SLMs 104, as is known in the relevant
art. Support device 502, SLMs 104, and all peripheral cooling or
control devices are referred to as an assembly. Assembly 500 can
allow for a desired step size to produce the desired stitching
(e.g., connecting of adjacent elements of features on object 112)
and overlap for leading and trailing SLMs 104. By way of example,
support device 502 can be 250 mm.times.250 mm (12 in.times.12 in)
or 300 mm.times.300 mm (10 in.times.10 in). Support device 502 can
be used for thermal management based on being manufactured from a
temperature stable material.
[0044] Support device 502 can be utilized as a mechanical backbone
to ensure spacing control of SLMs 104 and for embedding the
circuitry and the thermal controls areas 504. Any electronics can
be mounted on either or both of a backside and front side of
support device 502. For example, when using analog based SLMs or
electronics, wires can be coupled from control or coupling systems
504 to active areas 300. Based on being mounted on support device
502, these wires can be relatively shorter, which reduces
attenuation of analog signals compared to a case where the
circuitry is remote from the support device 502. Also, having short
links between the circuitry and active areas 300 can increase
communication speed, and thus increase pattern readjustment speed
in real time.
[0045] In some embodiments, when SLM 104 or electrical devices in
the circuitry wear out, assembly 500 can easily be replaced.
Although it would appear replacing assembly 500 is more costly than
just a chip on assembly 500, it is in fact easier and quicker to
replace the entire assembly 500, which can save production costs.
Also, assembly 500 can be refurbished, allowing for a reduction in
replacement parts if end users are willing to use refurbished
assemblies 500. Once assembly 500 is replaced, only verification of
the an overall alignment is needed before resuming fabrication. In
some examples, kinematic mounting techniques can be used to allow
for repeatable mechanical alignments of assembly 500 during field
replacements. This may eliminate a need for any optical adjustment
of assembly 500.
[0046] FIGS. 6, 7, 8, 9, and 10 show how one exposure area of
object 112 is patterned by a section of an SLM array. Thus, the
figures show how the section of the SLM array will look from the
perspective of the one exposure area of object 112.
[0047] FIGS. 6 and 7 show alternative embodiments for how sections
650 and 750 of an array of SLMs 104 will fall within exposure areas
660 and 760, respectively. Sections 650 and 750 both include four
columns having two equivalent SLMs 104 each. Thus, sections 650 and
750 include eight equivalent SLMs 104. SLMs 104 in one column can
be staggered with respect to SLMs 104 in adjacent columns. Each
column is spaced a width of one-half an active area 300 apart.
[0048] In one example, active area 300 can be 4.8 mm.times.30 mm
and each active device 302 can be about 6 .mu.m.times.6 .mu.m. This
can produce about 150.times. magnification. In this example, if an
entire SLM 104 is about 4 megapixels (e.g., 4096 active devices
302.times.1024 active devices 302), each section 650 or 750 can be
about 797 .mu.m.times.240 .mu.m and each exposure area 660 or 760
can be about 120 mm.times.36 mm.
[0049] In this example, there is about a 4.8 nm step size between
light pulses at a SLM plane and about a 34 .mu.m step between
exposure periods at an object plane. Object 112 can be moving at
approximately 128 mm/sec in a direction of arrow A. A data refresh
rate and/or pulse rate of illumination source can be around 4 kHz.
With these parameters, an expected throughput of up to about 5
wafers per hour (wph) can be possible. Thus, if an object's speed
was about one active area width traveled per light pulse, each
exposure area 660 and 760 would receive two pulses of light during
each scan period of object 112.
[0050] FIG. 8 shows another embodiment of an array of SLMs 104
having a section 850 writing to exposure an area 860. Section 850
includes eight columns having four SLMs 104 each. Thus, section 850
includes 32 SLMs 104. SLMs 104 in one column can be staggered with
respect to SLMs 104 in adjacent columns. Each column is spaced a
width of one-half an active area 300 apart.
[0051] In one example, active area 300 can be about 8.192
mm.times.32.768 mm and each active device 302 can be about 6
.mu.m.times.6 .mu.m. This can produce about 400.times.
magnification. In this example, if an entire SLM 104 is about 1
megapixel (e.g., 2048 active devices 302.times.512 active devices
302). each section 850 can be about 567.5 .mu.m.times.344 .mu.m and
each exposure area 860 can be about 227 mm.times.137.2 mm.
[0052] In this example, there is about a 16.4 mm step size between
light pulses at a SLM plane and about a 43.52 .mu.m step between
exposure periods at an object plane. Object 112 can be moving at
approximately 40.96 mm/sec in a direction of arrow B. A data
refresh rate and/or pulse rate of illumination source can be around
1 kHz. With these parameters, an expected throughput of up to about
1.2 wph can be possible. Thus, if an object's speed was about two
active area widths traveled per light pulse, each exposure area 860
would receive two pulses of light during each scan period of object
112. In an alternative example, if an object's speed was about one
active area width traveled per light pulse, each exposure area 860
would receive four pulses of light during each scan period of
object 112.
[0053] FIG. 9 shows another embodiment of an array of SLMs 104
having a section 950 writing to an exposure area 960. Section 950
includes six columns alternating between having three or four SLMs
104. Thus, section 950 includes 14 SLMs 104. SLMs 104 in one column
can be staggered with respect to SLMs 104 in adjacent columns. Each
column is spaced one width of one active area apart.
[0054] In one example, active area 300 can be about 8.192
mm.times.32.768 mm and each active device 302 can be about 6
.mu.m.times.6 .mu.m. This can produce about 400.times.
magnification. In this example, if an entire SLM 104 is about 1
megapixel (e.g., 2048 active devices 302.times.512 active devices),
each section 950 can be about 567.5 .mu.m.times.344 .mu.m and each
exposure area 960 can be about 227 mm.times.137.2 mm.
[0055] In this example, there is about a 8.2 mm step size between
light pulses at a SLM plane and about a 21.76 .mu.m step between
exposure periods at an object plane. Object 112 can be moving at
approximately 1 Khz or 20.48 mm/sec in a direction of arrow C. With
these parameters, an expected throughput of up to about 0.6 wph can
be possible. Thus, if an object's speed was about one active area
width traveled per light pulse, each exposure area 960 can receive
two pulses of light during each can period of object 112.
[0056] FIG. 10 shows another embodiment of an array of SLMs 104
having a section 1050 writing to an exposure area 1060. Section
1050 can include two columns having four SLMs 104 each. Thus,
section 1050 includes 8 SLMs 104. SLMs 104 in one column can be
staggered with respect to SLMs 104 in adjacent columns. Each column
is spaced one-half an active area width apart.
[0057] In one example, active area 300 can be about 4.5 mm.times.36
mm and each active device 302 can be about 6 .mu.m.times.6 .mu.m.
This can produce about 150.times. magnification. In this example,
if an entire SLM 104 is about 4 megapixels (e.g., 6000 active
devices 302.times.750 active devices 302), each section 1050 can be
about 1593 .mu.m.times.96 .mu.m and each exposure area 1060 can be
about 239 mm.times.14 mm.
[0058] In this example, there is about a 4.5 mm step size between
light pulses at a SLM plane and about a 31.5 .mu.m step between
exposure periods at an object plane. Object 112 can be moving at
approximately 64 mm/sec in a direction of arrow D. A data refresh
rate and/or pulse rate of illumination source can be around 4 kHz.
With these parameters, an expected throughput of up to about 5.1
wph can be possible. Thus, if an object's speed was about one-half
active area width traveled per light pulse, each exposure area 1060
could receive two pulses or light during each scan period of an
object 112.
[0059] Exposure Diagrams for Arrays of Spatial Light Modulators
[0060] FIG. 11 is one example of an exposure diagram for three
sections 1150 of an array having four SLMs 104 per section as they
write to a same row of exposure areas 1160 on object 112 during
five pulses of light. Sections 1150-1 and 1150-3 can be part of a
first (e.g., leading) set of SLMs and Section 1150-2 can be part of
a second (e.g., trailing) set of SLMs. This exposure diagram is
shown from the perspective of object 112 as it is moving in the
direction of the arrow with an equivalent step of two widths of
active areas 300 per light pulse. During Pulse 1, the array has not
overlapped object 112. During Pulse 2, a pattern generated by the
array for SLMs 104 in a first section 1150-1 is written to a first
exposure area 1160-1. During Pulse 3, either the same or a
different pattern is written to exposure area 1160-1 by section
1150-2 and either the same or different pattern is written to
exposure area 1160-2 by section 1150-1. Thus, the trailing set in
section 1150-2 writes over a same exposure area 1160-1 later in
time as the leading set in section 1150-1. This general exposure
process is repeated for Pulses 4 and 5, as is shown.
[0061] It is to be appreciated this is a very simple example of the
exposure process that can occur using an array of SLMs 104 in a
maskless lithography system. It is being used to demonstrate how
using an array of SLMs 104 allows for multiple exposures in each
exposure area 1160 during each scan period, which increases
throughput compared to a conventional system using one SLM.
[0062] Operation
[0063] In this example, light is scanned across object 112, while
each SLM 104 receives updated pattern data. This results in
multiple pulses reflecting from multiple SLMs 104 as scanning
occurs. In each direction, a first set (e.g., a leading set) of
SLMs 104 directs a first pulse and second set (e.g., a trailing
set) of SLMs 104 comes up behind the first set and directs the
second pulse (e.g., trailing SLMs). Hence, at any instance in time
a single pulse is directed by varying pattern profiles on SLMs 104
to write varying patterns to object 112.
[0064] For example, during the duration between pulses, object 112
is stepped either all or a portion of a width of active area 300.
Then, 3-4 pulses later, a trailing SLM 104 can overlap something
printed 3-4 pulses ago by a leading SLM 104. System 100 can
continuously or periodically update the pattern, accordingly. This
allows for printing during multiple passes with SLMs 104, while
keeping object 112 continuously moving and only doing one pass over
object 112 to achieve higher throughput compared to conventional
systems using only one SLM.
[0065] In essence, system 100 allows for exposing multiple patterns
during one pass by using multiple SLMs 104. There could be full
overlap, half overlap, etc. of patterns generated by leading and
trailing SLMs 104 in order to allow for stitching or other
effects.
[0066] Some features of the various embodiments of the present
invention described above may be that it allows for: process
flexibility in terms of number of pulses to deliver each dose,
while maintaining a continuously moving wafer, easy algorithm
development for pattern rasterization by pre-defining the geometric
relationship between leading and trailing SLMs, dead pixels on one
SLM to be compensated for by corresponding pixels on other SLMs,
and a mechanism by which an array of multiple SLMs can be
field-replaceable on a single mechanical unit with only minor
electrical, mechanical, pneumatic, and cooling connections and a
quick optical adjustment.
[0067] The geometrical layout of assembly 500 (e.g., the spaces
between SLMs 104) can be a function of: active area 300 on each SLM
104, the area taken up by packaging 400 for each SLM 104, the
number of pulses desired to deliver a particular dose to a
particular exposure area, the maximum object stage speeds
achievable, and the maximum lens diameter in projection optics
110.
[0068] In one example, an amount of exposures for each exposure
area can be increased by a factor of two (i.e. 2, 4, 8, 16, etc.)
using the same SLM array layouts by just halving the object stage
scan speed. Scan speed should remain constant, and is defined by
the geometrical relationship between SLMs 104. The amount of
overlap between leading and trailing SLMs 104 depends on the
overall stitching strategy employed. Different examples of this
include full overlap, half overlap, or shifted overlap (e.g., full
or half overlap where the pixels on trailing SLMs 104 are offset by
a fraction of a pixel in X and Y as compared to leading SLMs 104).
The spacing between leading and trailing SLMs can be on the order
of the smallest possible multiple of
(active_area_width)/(#_of_exposures)- , plus the stitching overlap,
compatible with the physical packaging of the SLM.
[0069] Dose and Uniformity Control System and Method Using
Monitoring
[0070] FIG. 12 is a system 1200 that can control dose and/or
uniformity for a multiple SLM pattern generation array, according
to an embodiment of the present invention. The control of SLMs 104
can be based upon measurements 1202 of an intensity using
controller 1204 (e.g., a dose/uniformity manipulator for leading
pattern generation SLMs (high transmission measured)). Controller
1204 measures leading SLMs 104 at the point in time that the
leading SLMs 104 are exposed. This measurement is subtracted from a
predetermined value 1206 (e.g., setpoint/dose uniformity value in
leading SLMs) using subtractor 1208 to generate an error signal
1210 (e.g., dose/uniformity error in leading SLMs 104. Error signal
1210 can be delayed using delay device 1212 that receives a delay
signal 1214. Delay signal 1214 can be based on the number of pulses
between leading and trailing pulses of the SLM array. The delayed
signal 1212 is added to a predetermined value 1216 (e.g., a
setpoint dose/uniformity value in the trailing SLMS) using adder
1218 to generate a control signal 1220. Controller 1222 receives
control signal 1220, which can be a dose/uniformity manipulator for
the trailing pattern generation SLMs 104. Controller 1222 may be
low transmission controllable.
[0071] If the controlled SLM 104 has sufficient zones, it can also
be used to vary intensity along the height of the exposure for the
trailing SLM 104 to compensate for non-uniformities in the beam
during the leading pulse. To accommodate stitching, which may cause
two "first pulses"to be overlapped with one "second pulse," the
trailing portion can be further subdivided into bands that are
commanded with the appropriate correction. The shot energy in the
trailing SLMs 104 can be selected so as to accomplish
stitching.
[0072] In order to successfully compensate for dose variations
during the leading pulse without worrying about induced errors from
trailing pulses, the energy in the leading pulse can be
significantly higher than the trailing pulses. As an example for a
two-pulse system, a ratio of 90% dose for leading SLMs 104, 10%
dose for trailing SLMs 104 could be envisioned, meaning that the
error in dose on the trailing SLMs 104 would be 9.times. lower than
the error in dose on the leading SLM 104. Continuing the example,
if the dose on a given set of leading SLMs 104 was measured at 85%
instead of the 90% nominal, the attenuation of the trailing SLMs
104 during the appropriate pulse could be set to allow 15% dose
transmission, instead of the nominal 10% dose.
[0073] The SLM 104 can be constructed to cover both sides of the
beam. This would allow for reversal in exposure scan direction
(which reverses the leading and trailing SLMs 104) as well as to
provide the capability for correcting offsets in transmission and
uniformity for the leading SLMs 104.
[0074] It is to be appreciated that this concept is readily
extendable to a single SLM system or any functional multi-SLM
array, and can be used in any lithographic printing strategy with
two or more pulses per point on the wafer being applied to deliver
dose. One advantage for this embodiments is that it can improve
dose control in a direct-write lithographic system use of
conventional lithographic lasers, which have relatively poor
pulse-to-pulse energy intensity variability and uniformity
performance.
[0075] Dose Control System and Method Using a Correctional Blanket
Dose
[0076] In maskless lithography only a very limited number of laser
pulses are used to expose the resist. This is to maintain a
reasonable throughput in a maskless lithography tool. For example,
a number of laser flashes exposing the resist can be limited to 2
to 4 at each site on the wafer. The dose repeatability of the
commonly used excimer laser is typically in the 1 to 3% 1.sigma.,
while the required exposure dose needs to be within 0.5% 3.sigma..
Without monitoring this would result in an unacceptable dose
variations.
[0077] Embodiments of the present invention can use 3 or 4 laser
flashes (exposures) in which the last pulse only contains a small
(e.g., 5%) fraction of the total dose needed to expose the resist.
Although an example system and method are found in WO 99/45435,
embodiments of the present invention can have several advantages
over this system, such as substantially no throughput loss and very
limited increase in the cost of goods manufactured.
[0078] Embodiments of the present invention divide the dose over
the laser flashes, such that the last flash only delivers a small
fraction, say 5%, of the total dose. Measuring the first two or
three doses then defines the dose in the last pulse.
[0079] In one example, the last exposure can have the full
patterning information. In this case, the data path needs to be
fully loaded to generate that information. Moreover, if the
exposures are delivered sequentially, as has been done in
conventional systems, the last exposure decreases the throughput of
the tool considerably. In another example, the last exposure can
have substantially no patterning information, as is described
below.
[0080] Accordingly, embodiments on the present invention provide a
final exposure to correct for dose errors in the previous
exposures. The final exposure will be delivered as a blanket
exposure. This means that the final exposure does not contain any
pattern information. The final exposure thus does not need an
extensive (and thus expensive) data path.
[0081] FIG. 13 is a flow chart depicting a method 1300 according to
embodiments of the present invention. In step 1302, a first set of
SLMs measures a dose delivered in the first pulse. In step 1304, a
second set of SLMs measures a dose delivered in a second pulse. In
step 1306, a dose error is calculated. In step 1308, a correctional
blanket dose is calculated. In step 1310, the correctional blanket
dose is applied through a final set of SLMs.
[0082] FIG. 14 shows a layout of SLMs having 21 SLMs. The SLMs
generate three shots 1402, 1404, and 1406 at three different
exposure times. In this configuration, a lens (not shown) in a
projection optical system can have a diameter of about 271 mm.
[0083] FIG. 15 shows a layout of SLMs having 24 SLMs. The SLMs
generate three shots 1502, 1504, and 1506 at three different
exposure times. In this configuration, a lens (not shown) in a
projection optical system can have a diameter of about 302 mm.
[0084] In the proposed layout for FIGS. 14 and 15, all three
exposures (1402-1406 or 1502-1506) are done within a single
exposure field. At each position on the wafer the dose is delivered
sequentially. For example, in FIG. 15 if pulse N delivers the first
dose then pulse N+3, N+4, N+5, or N+6 delivers the second dose,
depending on the precise layout of the SLM array. This means that a
single SLM-sized field will experience four potentially different
doses. The SLM in the 3rd shot (column) 1406/1506 should then be
delivering four different correctional doses. Exposures 1402/1502
and 1404/1504 both hold complete data information. This means that
SLM columns 1402/1502 and 1404/1504 are connected to an extensive
data path.
[0085] The data path is one of the most expensive components of the
maskless lithography tool. In conventional systems, an addition of
the final shot would increase the costs even more because it would
add about 50% to the data path. To avoid these extra costs,
embodiments of the present invention apply a blanket exposure with
the final SLM column 1406/1506. This means that the final exposure
will contain no pattern data. The purpose is to add an additional
background in a controlled manner. The only "pattern" on the SLMs
is there because it will need to correct for potentially four
different doses within the SLM field. This, however is a very
simple pattern that needs only a very limited amount of
electronics.
[0086] Consider an aerial image f(x) and a resist threshold th. The
boundary x.sub.th between exposed and non-exposed resist is then
given by:
f(x.sub.th)=th. (1)
[0087] Now assume that the delivered dose deviates from the ideal
dose by a factor b, i.e.:
delivered_dose(x)=bf(x). (2)
[0088] Clearly (1) does not hold anymore. To restore the condition
laid down in (1), add (1-b) th to (2) and obtain:
dose(x)=bf(x)+(1-b)th=th+b(f(x)-th). (3)
[0089] Now dose(x.sub.th)=th produces
th+b(f(x.sub.th)-th)=th, (4)
[0090] which implies (1). Therefore the correctional background
dose is given by:
D=(1-b)th, (5)
[0091] which is independent of the actual pattern. This holds for
every value of b. In our case however, b will be close to but less
then one. As an example the dose in the first two exposures can be
96% and in the final nominally 4%. Then b will be 0.96. The dose
correction method as proposed above does have a small negative
effect on the exposure latitude. The exposure latitude is given by
the slope of the aerial image at the resist threshold: 1 S = y x x
= x th ( 6 )
[0092] Again assume the dose deviates from the ideal dose by a
factor b then S will be given by: 2 S = b y x x = x th ( 7 )
[0093] So the exposure latitude will be degraded by a factor 1-b.
In the example given above the degradation will be 4% (e.g. from
10% to 9.6%). Error budgeting indicates that this decrease of
exposure latitude can be absorbed. However, in preferred
embodiments the correctional dose is maintained as small as
possible.
[0094] Conclusion
[0095] While various embodiments of the present invention have been
described above, it should be understood that they have been
presented by way of example only, and not limitation. It will be
apparent to persons skilled in the relevant art that various
changes in form and detail can be made therein without departing
from the spirit and scope of the invention. Thus, the breadth and
scope of the present invention should not be limited by any of the
above-described exemplary embodiments, but should be defined only
in accordance with the following claims and their equivalents.
* * * * *