U.S. patent application number 09/866371 was filed with the patent office on 2001-10-11 for method for selectively exposing a light pattern to a photosensitive work surface.
Invention is credited to Sanford, James E..
Application Number | 20010028993 09/866371 |
Document ID | / |
Family ID | 23425458 |
Filed Date | 2001-10-11 |
United States Patent
Application |
20010028993 |
Kind Code |
A1 |
Sanford, James E. |
October 11, 2001 |
Method for selectively exposing a light pattern to a photosensitive
work surface
Abstract
A maskless exposure system for selectively exposing a
photosensitive work surface, such as a photoresist layer, includes
a semiconductor substrate having an elongated aperture. A series of
shutters and associated guides are formed upon the substrate using
conventional wafer processing methods. The shutters move between a
first position covering the aperture and a second position exposing
the aperture. A corresponding series of computer-controlled
actuators, in the form of electromagnetic coils, cooperate with the
shutters for selectively sliding each shutter between its first and
second positions. A light beam is directed toward the aperture, and
the shutters create a patterned light beam exiting the aperture. A
computer-controlled stepper is synchronized with the shutter
actuators and adjusts the relationship between the patterned light
beam and the photosensitive work surface to direct the patterned
light beam at different portions of the work material.
Inventors: |
Sanford, James E.; (Tempe,
AZ) |
Correspondence
Address: |
Marvin A. Glazer, Esq.
CAHILL, SUTTON & THOMAS P.L.C.
155 Park One
2141 East Highland Avenue
Phoenix
AZ
85016
US
|
Family ID: |
23425458 |
Appl. No.: |
09/866371 |
Filed: |
May 25, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09866371 |
May 25, 2001 |
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09362276 |
Jul 27, 1999 |
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6248509 |
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Current U.S.
Class: |
430/313 ;
430/311; 430/317; 430/318; 430/322; 430/324; 430/397 |
Current CPC
Class: |
G02B 26/02 20130101;
G03F 7/70383 20130101; G03F 7/70291 20130101 |
Class at
Publication: |
430/313 ;
430/311; 430/317; 430/318; 430/322; 430/324; 430/397 |
International
Class: |
G03F 007/20 |
Claims
I claim:
1. A maskless exposure system for selectively exposing a work
material having a photosensitive work surface to light, said
maskless exposure system comprising in combination: a. a substrate
having an elongated aperture formed therein, said substrate
generally lying in a plane; b. a source of a light beam directed
generally toward the elongated aperture; c. a plurality of shutter
elements movably supported upon said substrate, each of said
shutter elements having a first position covering a portion of the
elongated aperture and preventing the passage of light through such
portion, and each of said shutter elements having a second position
exposing a portion of the elongated aperture for allowing the
passage of light through such portion, each of said plurality of
shutter elements extending generally parallel to the plane of said
substrate, said plurality of shutter elements creating a patterned
light beam exiting the elongated aperture of said substrate; d. a
plurality of actuators, each of said actuators cooperating with a
related one of said plurality of shutter elements for selectively
moving said related shutter element between its first position and
its second position; e. a work material support for supporting a
work material having a photosensitive work surface; and f. a
stepper for adjusting the relationship between the patterned light
beam exiting the elongated aperture of said substrate and the work
material support in order to direct the patterned light beam at
different portions of the photosensitive work surface of the work
material.
2. The maskless exposure system recited by claim 1 wherein said
substrate is formed of a predetermined semiconductor material, and
wherein said plurality of shutter elements are formed from a layer
of material that was once secured to said substrate.
3. The maskless exposure system recited by claim 2 wherein said
plurality of shutter elements are formed from of semiconductor
material.
4. The maskless exposure system recited by claim 2 wherein said
plurality of shutter elements are formed from a layer of metal
deposited upon said substrate.
5. The maskless exposure system recited by claim 1 wherein each of
said plurality of shutter elements slides in a linear motion
between its first and second positions.
6. The maskless exposure system recited by claim 5 including a
plurality of guides secured to said substrate, each of said guides
extending about one of said plurality of shutter elements for
guiding movement of said shutter element.
7. The maskless exposure system recited by claim 5 wherein said
plurality of shutter elements includes a first shutter element and
a second shutter element, the first shutter element and the second
shutter element each having a side edge, and wherein the side edge
of the first shutter element extends closely proximate the side
edge of the second shutter element when both the first and second
shutter elements are in the first position covering a portion of
the elongated aperture.
8. The maskless exposure system recited by claim 7 wherein said
further and second shutter elements lie on opposite sides of the
elongated aperture.
9. The maskless exposure system recited by claim 1 wherein each of
said plurality of shutter elements incorporates a small permanent
magnet, and wherein each of said plurality of actuators includes an
electromagnet in the form of a coiled electrical conductor for
repelling or attracting a related one of said plurality of shutter
elements.
10. The maskless exposure system recited by claim 9 wherein said
plurality of actuators includes first and second actuators, and
including magnetic shielding interposed between said first and
second actuators.
11. A method for selectively exposing a light pattern to a
photosensitive work surface of a work material, said method
comprising the steps of: a. providing a substrate having an
elongated aperture formed therein, the substrate generally lying in
a plane; b. providing a beam of light that can be directed
generally toward the elongated aperture; c. supporting a plurality
of shutter elements upon the substrate, each of the shutter
elements being movable between a first position covering a portion
of the elongated aperture and a second position exposing a portion
of the elongated aperture, each of the shutter elements extending
generally parallel to the plane of the substrate whether in the
first position or second position; d. moving each of the shutter
elements to either its first position or its second position to
selectively block or pass portions of the beam of light in order to
create a first patterned light beam exiting the elongated aperture
of the substrate; e. directing the beam of light toward the
elongated aperture for generating the first patterned light beam;
and f. directing the first patterned light beam upon a first
portion of the photosensitive work surface of the work
material.
12. The method recited by claim 11 including the additional steps
of: g. moving each of the shutter elements to either its first
position or its second position to selectively block or pass
portions of the beam of light in order to create a second patterned
light beam exiting the elongated aperture of the substrate; h.
directing the beam of light toward the elongated aperture for
generating the second patterned light beam; and i. directing the
second patterned light beam upon a second portion of the
photosensitive work surface of the work material.
13. The method recited by claim 12 wherein the steps recited in
claim 12 are repeated for third and subsequent portions of the
photosensitive work surface of the work material until
substantially all portions of the photosensitive work surface have
been exposed to a respective patterned light beam.
14. The method recited by claim 11 wherein the substrate is a wafer
made of a semiconductor material, and wherein the shutter elements
are formed by the steps of: a. forming a first sacrificial layer of
a material upon a surface of the substrate; b. depositing a first
deposited layer of material upon the first sacrificial layer; c.
patterning the first deposited layer of material to form the
shutter elements; and d. etching the first sacrificial layer
underlying the patterned first deposited-layer of material for
allowing the shutter elements to move relative to the
substrate.
15. The method recited by claim 14 wherein the first deposited
layer of material is semiconductor material.
16. The method recited by claim 14 wherein the first deposited
layer of material is a metal.
17. The method recited by claim 14 wherein the first sacrificial
layer is composed of a material selected from the group consisting
of oxides and nitrides of the semiconductor material
18. The method recited by claim 14 wherein the first sacrificial
layer is a polymer material.
19. The method recited by claim 14 further including the steps of:
e. forming a second sacrificial layer of a material upon the
surface of the substrate overlying the first deposited layer; f.
depositing a second deposited layer of material upon the second
sacrificial layer; g. patterning the second deposited layer to form
guide elements that bridge across a shutter element; and h. etching
the second sacrificial layer underlying the patterned second
deposited second layer for allowing the shutter elements to slide
within the guide elements.
20. The method recited by claim 11 wherein the step of moving each
of the plurality of shutter elements between its first and second
positions includes the step of sliding each such shutter element
along a linear path.
21. The method recited by claim 20 including the step of
positioning each shutter element within at least one surrounding
guide for guiding sliding movements of each such shutter
element.
22. The method recited by claim 20 including the step of
positioning successive shutter elements on alternating sides of the
elongated aperture.
23. The method recited by claim 20 wherein the step of moving each
of the shutter elements includes the steps of: a. providing a small
permanent magnet on each shutter element; b. forming an
electromagnet proximate to each shutter element, each such
electromagnet having an electrical coil through which electrical
current can flow; and c. controlling the direction of current flow
through each such electrical coil in order to repel or attract each
of the shutter elements.
24. A method of forming an electrical coil upon a semiconductor
substrate, the electrical coil extending generally along a
longitudinal axis said method comprising the steps of: a.
depositing a first layer of metal upon the semiconductor substrate;
b. patterning the deposited first layer of metal to form a
plurality of lower rungs of the electrical coil, the plurality of
lower rungs extending generally perpendicular to the longitudinal
axis of the electrical coil, each of the plurality of lower rungs
having opposing first and second ends; c. forming a first
insulative layer of material over the substrate and above the
patterned deposited first layer of metal; d. forming a layer of
magnetic material extending the longitudinal axis of the electrical
coil and extending above the first insulative layer and extending
across the plurality of lower rungs of the electrical coil; e.
forming a second insulative layer of material over the substrate
and extending above both the first insulative layer and the layer
of magnetic material; f. etching holes through the first and second
insulative layers above the opposing first and second ends of the
plurality of lower rungs to expose such opposing first and second
ends; g. depositing a second metal layer upon the substrate over
the second insulative layer and within the etched holes exposing
the opposing first and second ends of the plurality of lower rungs;
and h. patterning the second metal layer to connect the second end
of each lower rung to the first end of a next succeeding lower rung
to form the electrical coil.
25. The method recited by claim 24 including the steps of: i.
forming first and second electrical coils using the steps recited
in claim 24; j. depositing magnetic shielding material between the
first and second electrical coils, the magnetic shielding material
being spaced apart from the first and second electrical coils; and
k. forming a third insulative layer of material over the substrate
and extending above both the first and second electrical coils to
protect the first and second electrical coils from contacting the
magnetic shielding material.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates generally to the fields of
image transfer and the photolithographic transfer of images to a
semiconductor wafer, printed circuit board or other substrate, and
more particularly, to an apparatus and method for selectively
exposing a photosensitive layer of material to a patterned source
of light without the need for a mask.
[0003] 2. Description of the Relevant Art
[0004] Image transfer of complex patterns onto substrates such as
semiconductor wafers, printed circuit boards, flat panel displays,
and the like commonly employ the use of a photolithographic
apparatus containing a light source, a system of lenses and/or
mirrors and a photomask, mask or reticle. In common
step-and-repeat, or step-and-scan systems, light moves from a light
source, through a lens/mirror assembly, and through a patterned
mask onto the substrate which is covered with a photosensitive
polymer resist. The mask is a two dimensional stencil of the
pattern to be transferred. Often, thirty or more different
patterns, or layers, are transferred to a given substrate during
the manufacturing process, requiring a different mask for each such
layer. The mask manufacturing process has proven to be a costly and
time-consuming process. The mask itself is easily damaged during
everyday handling; accordingly, it is common to produce duplicate
masks in order to replace masks that become damaged. Elimination of
the various masks would save time and money in the manufacturing
process.
[0005] The advantages inherent in eliminating the need for such
exposure masks has long been recognized, and those skilled in the
art have explored mask free lithography in depth. Current
direct-write methods are known, such as laser, electron or ion beam
lithography, wherein a fine beam of light is selectively steered to
shine on each point of the photosensitive film that needs to be
exposed. For example, in U.S. Pat. No. 5,451,489 to Leedy, an
electron beam is used to selectively expose a photoresist layer on
a semiconductor wafer without the use of a mask. Similarly, in U.S.
Pat. No. 5,109,149 to Leung, a laser beam is used in conjunction
with a polygonal mirror, a beam expander, and a lens to selectively
direct light onto the surface of a wafer mounted on an X-Y axis
motorized table. However, these direct-write techniques have been
proven to be too slow for economic commercial use.
[0006] In U.S. Pat. No. 5,691,541 to Ceglio, et al., a lithography
system is described wherein a programmable array of "light
switches", in the form of an array of digital micro-mirror devices,
is provided to control the passage of light from a source to a
photosensitive layer to be exposed. Each micro-mirror is either
deflected through an angle to form a dark portion of the pattern,
or undeflected to form a bright portion of the pattern. However,
the device described by Ceglio, et al. is dependent upon the proper
alignment of many small reflecting mirrors in order to reflect the
desired image to the substrate. Reflective aberrations and mirror
mis-alignment cause inaccuracies in the image that is projected
onto the substrate.
[0007] In U.S. Pat. No. 5,781,331 to Carr, et al., an optical
micro-shutter array is described that can be produced using known
semiconductor fabrication processes. The disclosed optical shutter
includes an aperture plate positioned in a light path, the aperture
plate having an array of apertures formed therein. A series of
microcantilevers are used to selectively cover the array of
apertures, each microcantilever being associated with one of the
apertures. Carr, et al. describe such microcantilevers as
preferably being formed of two layers of material having different
thermal coefficients of expansion, and preferably being
thermally-actuated, although Carr, et al. also state that
piezoelectric and electrostatic-originating forces may also be
employed. When a microcantilever is heated by passing an electrical
current through an associated resistor, the microcantilever curls
away from the associated aperture, thereby allowing light to pass
through such aperture. One disadvantage of using such
microcantilevers is that their up and down curling/flexing motion
tends to cause unwanted interference between two adjacent
microcantilevers; if a first microcantilever is trying to curl up
away from its aperture, and a second adjacent microcantilever is
trying to move down toward its adjacent aperture, then the two
microcantilevers may contact each other.
[0008] U.S. Pat. No. 5,808,384 to Tabat describes a micromechanical
actuator that can be formed on substrates using lithographic
processing techniques. Tabat describes such devices as being useful
for, among other things, forming optical switches. A plunger having
two magnetic heads is supported within a gap of a magnetic core to
which an electrical coil is coupled. A pair of springs bias the
plunger to a central position. The application of electrical
current to the electrical coil moves the plunger back and forth in
a linear movement depending upon the direction of current flow.
However, the necessity of having the actuators pass through the
core of the electrical coil places restrictions on how close two or
more of such actuators can be positioned relative to each other.
Moreover, the need to form spring-like biasing members within the
substrate further complicates the fabrication of such devices.
[0009] Accordingly, it is an object of the present invention to
provide a maskless photoresist exposure system which eliminates the
need for masks in order to selectively expose photosensitive layers
applied to semiconductor wafers, printed circuit boards, or other
substrates.
[0010] Another object of the present invention is to provide such a
maskless photoresist exposure system capable of using a
conventional photolithographic light source and avoiding the need
for lasers, electron beams, or ion beams.
[0011] Still another object of the present invention is to provide
such a maskless photoresist exposure system which operates quickly
enough to prove economically feasible for commercial use.
[0012] A further object of the present invention is to provide such
a maskless photoresist exposure system which avoids the need for
precise alignment of small mirrors in order to produce a patterned
light image.
[0013] A still further object of the present invention is to
provide such a maskless photoresist exposure system which includes
a series of optical shutters that can be disposed closely proximate
one another to form adjacent pixels of light, yet wherein movement
of one such shutter does not interfere with movement of shutters
adjacent thereto.
[0014] A yet further object of the present invention is to provide
such a maskless photoresist exposure system wherein the
aforementioned series of optical shutters can themselves be formed
using known photolithographic semiconductor processing
techniques.
[0015] Yet another object of the present invention is to provide
such a maskless photoresist exposure system wherein the
aforementioned series of optical shutters need not themselves pass
through the core of an electrical coil.
[0016] Still another object of the present invention is to provide
a method of performing mask free photolithography.
[0017] An additional object of the present invention is to provide
a novel method of forming an electrical coil, suitable for use in
forming an electromechanical actuator, on a semiconductor substrate
using known semiconductor wafer processing techniques.
[0018] These and other objects of the present invention will become
more apparent to those skilled in the art as the description of the
present invention proceeds.
SUMMARY OF THE INVENTION
[0019] Briefly described, and in accordance with the preferred
embodiment thereof, the present invention relates to a maskless
exposure system for selectively exposing a photosensitive work
surface of a work material to light. Such photosensitive work
surface could be a photosensitive film applied over the work
material, such as a photoresist layer; alternatively, the work
material may itself be photosensitive, such as photo-imagable
glass, in which case the photosensitive work surface is merely an
outer surface of the work material. The maskless exposure system of
the present invention includes a generally planar substrate having
an elongated aperture formed therein. A light beam from a source of
light having a desired wavelength is directed generally toward the
elongated aperture of the aforementioned substrate. The
aforementioned source of light may include a projection system of
lenses and/or mirrors to focus the beam of light toward the
elongated aperture of the substrate. A plurality of shutter
elements are formed upon the substrate generally parallel thereto
and movably supported thereon. Each of the shutter elements is
adapted to move between a first position covering a portion of the
elongated aperture, and thereby preventing the passage of light
through such portion, and a second position exposing a portion of
the elongated aperture for allowing the passage of light
therethrough. In order to control movement of the various shutter
elements, a series of actuators are also included; each actuator
cooperates with one of the shutter elements for selectively moving
it between its first and second positions. These actuators are
preferably computer-controlled so that desired movements of the
shutter elements can be programmed. The combination of the covered
and exposed portions of the elongated aperture creates a patterned
light beam exiting the elongated aperture of said substrate.
[0020] The material bearing the photosensitive work surface is
supported by a holder. A stepper device is provided for adjusting
the relationship between the patterned light beam exiting the
elongated aperture of the substrate and the work material holder in
order to direct the patterned light beam at different portions of
the work material. This stepper is again preferably controlled by a
computer for programming movements of the patterned light beam
relative to the work material, and to synchronize the operation of
the shutter elements with the operation of the stepper. In one
embodiment of the present invention, the stepper is a scanning
mirror which is disposed at various angles to reflect the patterned
light beam onto different portions of the work material; in an
alternate embodiment, the patterned light beam is aimed directly
onto the work material, and the stepper is an indexing system for
physically shifting the work material holder until all portions of
the work material have been exposed to the patterned beam.
[0021] Preferably, the aforementioned substrate in which the
aperture is formed is constructed from a semiconductor material
such as silicon, and the series of shutter elements are also formed
of semiconductor material, using conventional semiconductor wafer
processing techniques. In this regard, the semiconductor material
forming such shutter elements can be semiconductor material that
was initially deposited upon the substrate using known chemical
vapor deposition techniques.
[0022] Ideally, each of the shutter elements slides in a linear
motion between its first position covering a portion of the
aperture, and its second position exposing such portion of the
aperture. To facilitate such sliding motion, guides can be formed
upon the substrate. Such guides can be disposed below and/or extend
around each shutter element for guiding the movement thereof.
[0023] In those instances wherein two adjacent shutter elements
both assume their first positions for blocking the passage of light
through the aperture, it is desired that no light gap exist between
the two adjacent shutter elements. Accordingly, abutting side edges
of two adjacent shutter elements preferably extend closely
proximate one another to avoid creation of any light gaps
therebetween. In order to help position the abutting side edges of
adjacent shutter elements as close together as possible, Applicant
has found it to be helpful to position successive shutter elements
on opposite sides of the elongated aperture.
[0024] In the preferred embodiment of the present invention, the
shutter elements are actuated electromagnetically. In this regard,
each shutter element includes a small permanent magnet affixed to
an end of the shutter element, and each actuator includes an
electromagnet in the form of a coiled electrical conductor for
repelling or attracting the permanent magnet formed upon the
related shutter element. The electrical coil extends along a
longitudinal axis that is generally parallel to, and coaxial with,
the axis of linear movement of the shutter element. To prevent one
such coil from influencing a neighboring shutter element, magnetic
shielding is preferably interposed between adjacent actuators.
[0025] The present invention also relates to Applicant's method for
selectively exposing a light pattern to a photosensitive work
surface of a work material. In practicing such method, a generally
planar substrate is provided having an elongated aperture formed
therein. A beam of light is directed generally toward the elongated
aperture of the substrate. A series of shutter elements are
supported upon the substrate, each of the shutter elements being
movable between a first position covering a portion of the
elongated aperture and a second position exposing a portion of the
elongated aperture. These shutter elements extend generally
parallel to the plane of the substrate whether in the
aforementioned first position or second position. Each of the
shutter elements is moved to either its first position or its
second position to selectively block or pass portions of the beam
of light in order to create a patterned light beam that exits
through the elongated aperture of the substrate; preferably, each
of the shutter elements moves between its first and second
positions by a sliding motion along a linear path. The resulting
patterned light beam is then directed toward a first portion of the
photosensitive work surface of the work material. After selectively
exposing the first portion of the photosensitive work surface, the
step of moving the shutter elements is repeated to create a new
patterned light beam that is directed at a second portion of the
photosensitive work surface of the work material. This process can
be repeated until substantially all portions of the photosensitive
work surface have selectively been exposed to an appropriately
patterned beam of light.
[0026] Preferably, the method described above includes the step of
forming the substrate from a wafer made of a semiconductor
material. The step of forming the series of shutter elements
preferably includes the step of forming a first sacrificial layer
of a material upon a surface of the substrate; this first
sacrificial layer is an oxide or nitride of the semiconductor
material that forms the substrate. Next, a first layer of
semiconductor material is deposited upon the first sacrificial
layer, and this first layer is then patterned to form the shutter
elements. The first sacrificial layer underlying the patterned
deposited first layer of semiconductor material is then etched to
allow the shutter elements to move relative to the substrate.
[0027] If desired, the method of the present invention may also
include the step of forming a second sacrificial layer of material
upon the surface of the substrate overlying the deposited first
layer of semiconductor material; this second sacrificial layer may
also be an oxide or nitride of the semiconductor material. A second
layer of semiconductor material is then deposited upon the second
sacrificial layer, and is patterned to form guide elements that
bridge across the shutter elements. The second sacrificial layer
that underlies the patterned deposited second layer of
semiconductor material is then etched away for allowing the shutter
elements to slide within the guide elements. Each shutter element
is preferably positioned within at least one surrounding guide for
guiding sliding movements of each such shutter element. Ideally,
successive shutter elements are positioned on alternating sides of
the elongated aperture.
[0028] In order to move the shutter elements, the method of the
present invention preferably includes the steps of providing a
small permanent magnet on each shutter element, forming an
electromagnet, including an electrical coil through which
electrical current can flow, proximate each shutter element, and
controlling the direction of current flow through each such
electrical coil in order to repel or attract each of the shutter
elements.
[0029] The electrical coils described in the preceding paragraph
can advantageously be formed by the method of depositing a first
layer of metal upon the semiconductor substrate, and patterning the
deposited first layer of metal to form a number of lower rungs of
the electrical coil. A first insulative layer of material is then
formed over the substrate and above the patterned deposited first
layer of metal. A layer of magnetic material is then formed above
the first insulative layer and extending across the plurality of
lower rungs of the electrical coil. A second insulative layer of
material is formed over the substrate above both the first
insulative layer and the layer of magnetic material. Holes are then
etched through the first and second insulative layers above the
opposing first and second ends of the lower rungs to expose the
ends of such lower rungs. A second metal layer is deposited upon
the substrate over the second insulative layer and within the
etched holes that expose the ends of the lower rungs. The second
metal layer is patterned to connect the second end of each lower
rung to the first end of a next succeeding lower rung to form the
electrical coil. A third insulative layer of material is then
formed over the substrate, extending above the electrical coils to
insulate the second metal layer portion of the electrical coils.
Preferably, magnetic shielding material is deposited between, but
spaced apart from, adjacent electrical coils to isolate the
electromagnetic field in one coil from adjacent coils.
BRIEF DESCRIPTION OF THE DRAWINGS
[0030] FIG. 1 is a schematic view of an optical projection system
using the apparatus and method of the present invention.
[0031] FIG. 2 is a top view of a slit aperture formed in a
substrate, and including a series of sliding shutter elements used
to selectively pattern the light beam passing through the slit
aperture.
[0032] FIG. 3A is a cross-sectional drawing of an aperture formed
in a semiconductor wafer.
[0033] FIG. 3B is a cross-sectional drawing similar to that shown
in FIG. 3A but wherein the dimensions of the aperture are
determined by an additional layer of material disposed upon the
semiconductor wafer.
[0034] FIG. 4 is a perspective view of a series of shutter elements
positioned on both sides of a slit aperture formed in the
substrate.
[0035] FIG. 5 is a perspective view of a shutter element and
showing guides for guiding movement of the shutter element, as well
as sacrificial layers used in manufacturing such shutter
elements.
[0036] FIG. 6A is a perspective view of a shutter element being
repelled by an associated electromagnet to cover a portion of the
aperture.
[0037] FIG. 6B is a perspective view of the shutter element shown
in FIG. 6A, but being attracted by its associated electromagnet to
expose a portion of the aperture.
[0038] FIG. 7 is a cross-sectional drawing of an electrical coil
assembly including stationary magnetic material disposed within
such coil, insulating material, and magnetic shielding material
separating adjacent electrical coils.
[0039] FIG. 8 is a top view of the photosensitive work surface to
be exposed, broken into portions, and showing the configuration of
shutter elements used to form the pattern to expose one of such
portions.
[0040] FIG. 9A is a schematic drawing showing the use of a planar
scanning mirror for stepping the patterned light beam onto a
corresponding portion of the photosensitive work surface to be
exposed.
[0041] FIG. 9B is a schematic drawing similar to FIG. 9A but
showing the use of a concave scanning mirror for stepping the
patterned light beam onto a corresponding portion of the
photosensitive work surface to be exposed.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0042] As indicated in FIG. 1, an optical projection system using
the present invention includes a mirror 21 and a light source 22.
Light source 22 provides light of a desired wavelength suitable for
exposing a photosensitive coating or other work surface of a work
material. The light beam formed by light source 22 and mirror 21 is
directed through a condenser lens 23 which focuses the light
through a narrow slit aperture assembly 24 formed upon a substrate
29. In this sense, light source 22, mirror 21, and condenser 23
collectively form a source of a light beam that is directed toward
slit aperture assembly 24. In a manner to be described in greater
detail below, slit aperture assembly 24 has the ability to
selectively block portions, or pixels, of the light beam
transmitted by condenser lens 23. The patterned light beam passing
through slit aperture assembly 24 is intercepted by a reduction
lens 25 used to demagnify the projected image to a desired pixel
size. A scanning mirror 26 then scans the projected image across a
portion of a substrate 27 to be exposed, for example, a
semiconductor wafer bearing a photoresist coating, or a
photosensitive substrate. Substrate 27 thereby represents some form
of work material that has a photosensitive work surface. Substrate
27 is supported by a precisely controlled indexed stage (not shown)
which can be moved in incremental steps along an x-axis and y-axis
to allow the desired portions of substrate 27 to be exposed by
correspondingly patterned beams of light. The patterned light beam
produced by slit aperture assembly 24 can be stepped across the
photosensitive work surface of substrate 27, either by holding
substrate 27 fixed while moving scanning mirror 26, by holding
scanning mirror 26 fixed while moving the indexed stage that
supports the work material, or by combined motion of both scanning
mirror 26 and the indexed stage. In either of these cases, the
relationship between the patterned light beam exiting the aperture
assembly 24 and the work material support is adjusted in order to
direct the patterned light beam at different portions of
the-photosensitive work surface of the work material.
[0043] Light source 22 may produce light of any desired wavelength
ranging from visible light to wavelengths down to 5 nm, or soft
X-rays. The lens system, including condenser lens 23 and
demagnifying lens 25, must be adjusted to support the desired
wavelength of light. Transmissive optics of current step-and-repeat
optical projection systems can support wavelengths down to 193 nm.
Future transmissive lens materials such as CaF are being developed
for even shorter wavelengths. Reflective optics would need to be
used for very short wavelengths such as extreme ultraviolet (EUV)
wavelengths of 5 to 20 nm. As in prior art step-and-repeat optical
projections systems, color filters (not shown) may be used to allow
only desired wavelengths of light to pass through the optics.
[0044] The preferred embodiment of slit aperture assembly 24 is
shown in FIG. 2. Substrate 29 is preferably in the form of a
generally planar semiconductor wafer made from silicon or a similar
material. Substrate 29 has a long, narrow opening, or aperture 30,
etched or otherwise formed through it. A series of micro
electromechanical system, or MEMS, shutters, including those
designated by reference numerals 33, 33A, and 33B, are placed
alternately on either side of slit aperture 30, and are movably
supported upon substrate 29 in such a way as to selectively block
certain portions of aperture 30. Each of the shutters, including
shutters 33, 33A, and 33B, extends generally parallel to the plane
of substrate 29, and each is free to move back and forth, with a
generally linear, sliding motion, between first and second end
positions, to alternately cover or expose a selected portion of
elongated aperture 30. For example, as shown in FIG. 2, shutter
elements 33 and 33B are each extended to their first positions for
covering a portion of elongated aperture 30 to preventing the
passage of light therethrough, while shutter element 33A is
retracted to its second position for exposing a portion of
elongated aperture 30 for allowing the passage of light through
such portion. In this manner, the series of shutter elements,
including 33, 33A, and 33B, can be used to create a patterned light
beam that exits through elongated aperture 30 of substrate 29.
[0045] Still referring to FIG. 2, the shutter elements disposed
above aperture 30, including shutter element 33, are held to the
surface of substrate 29 by a silicon bridge structure 34. Likewise,
the shutter elements disposed below aperture 30, including shutter
elements 33A and 331B, are held to the surface of substrate 29 by
silicon bridge structure 34A. A magnetic material 35 with a
permanent polarity (i.e., a permanent magnet) is deposited on one
end of shutter element 33 and is influenced by an adjacent magnetic
material 36 which extends within a small electrical coil structure
37. Magnetic material 36 and electrical coil 37 constitute an
electromagnet for selectively repelling or attracting permanent
magnet 35, and hence, shutter element 33. Similarly, magnetic
material 35A with a permanent polarity is deposited on one end of
shutter element 33A and is influenced by an adjacent magnetic
material 36A which extends within electrical coil structure
37A.
[0046] The direction of the current running through electrical coil
37 determines the polarity of magnetic material 36, and therefore
determines whether magnetic material 36 attracts or repels magnetic
material 35. Conducting current through electrical coil 37 in a
first direction causes the polarity of magnetic material 36 to be
the same as that of magnetic material 35, thereby repelling
magnetic material 35, and moving shutter element 33 to the position
shown in FIG. 2 for blocking the related portion of aperture 30. In
contrast, conducting current through electrical coil 37 in the
opposite, second direction causes the polarity of magnetic material
36 to be the opposite to that of magnetic material 35, thereby
attracting magnetic material 35, and moving shutter element 33 to
its alternate position for exposing the related portion of aperture
30.
[0047] Similarly, the direction of the current running through
electrical coil 37A determines the polarity of magnetic material
36A, and thereby determines whether magnetic material 36A attracts
or repels magnetic material 35A. Conducting current through
electrical coil 37A in the aforementioned second direction causes
the polarity of magnetic material 36A to be the opposite that of
magnetic material 35A, thereby attracting magnetic material 35A,
and moving shutter element 33A to the position shown in FIG. 2 for
exposing the related portion of aperture 30. In contrast,
conducting current through electrical coil 37A in the
aforementioned first direction causes the polarity of magnetic
material 36A to be the same as that of magnetic material 35A,
thereby repelling magnetic material 35A, and moving shutter element
33A to its alternate position for blocking the related portion of
aperture 30.
[0048] Attraction of all shutter elements leaves aperture 30
completely open, allowing a continuous slit of light to be
projected to the substrate to be exposed. Conversely, repulsion of
all shutter elements causes aperture 30 to be completely blocked,
allowing no light to pass. It will be appreciated that the
electrical coils, including 37 and 37A, serve as actuators, each
cooperating with a related one of the shutter elements for
selectively moving its related shutter element between its first
position and its second position. Magnetic shielding 38 should be
used to isolate one coil structure 37 from the next, thereby
preventing electromagnetic coupling between two adjacent coils.
[0049] The width of aperture 30 is generally equal to one pixel
multiplied by the demagnification of the optics. If the optics
demagnifies the image 10 times (i.e. 10.times.), and if the desired
pixel size is 0.5 microns, then the width of aperture 30 would be
5.0 microns. The width of the slit forming aperture 30 is
determined by the field size desired multiplied by the
demagnification factor. The field size should be a multiple of the
pixel size. If a field size of 1,000 microns is desired with a
demagnification of 10.times., then a 10,000 microns-wide slit
aperture is required.
[0050] Referring to FIG. 3A, slit aperture 30 may be made by
etching though substrate 29, which may again be formed from a
simple silicon wafer. Alternatively, as shown in FIG. 3B, an
additional layer 41, which is placed on top of silicon substrate
42, may be used to define the size of aperture 30, thereby allowing
the aperture 39 etched within silicon substrate 42 to be somewhat
oversized. The composition and thickness of this additional layer
41 may be chosen to allow approximately 10% transmission and a 180
degree phase change of the light. This will provide a phase shift
effect as described by Watanabe. Therefore, the composition and
thickness of layer 41 is necessarily dependent upon the wavelength
of light used.
[0051] In order to ensure smooth linear motion of the shutter
elements, aperture assembly 24 preferably includes a series of
guides secured to substrate 29 for guiding the movement of the
shutter elements. Referring to FIG. 4, shutter element 44 slides
back and forth along a pair of guides 43 that extend below, and
along opposite edges of, shutter 44, and which allow only back and
forth movement of shutter 44. Guides 43 may be formed of silicon
that is deposited upon substrate 29. Formation of shutter element
44 on top of guides 43 provides shutter 44 with a topography that
enhances the strength of shutter 44.
[0052] Likewise, shutter 46 on the opposite side of elongated
aperture 30 is formed upon a single, central underlying guide 47.
Guide 47 again limits travel of shutter 46 to back and forth
movement. Guide 47 can again be formed from silicon deposited upon
substrate 29. It will be noted in FIG. 4 that shutter element 44 is
generally V-shaped in cross-section, while shutter element 46
generally forms an inverted V-shape in cross-section. This allows
shutters 44 and 46 to move into their extended (blocking) positions
across aperture 30 without making contact with, or otherwise
interfering with, the adjacent shutters coming from the opposite
side of aperture 30. Shutter elements 44 and 46 must be
sufficiently long to extend fully across aperture 30 in order to
selectively block light from passing through their respective
portions of aperture 30. Preferably, guides 43 and 46 are of
approximately the same length as shutter elements 44 and 46,
respectively.
[0053] Shutter elements 33, 33A, and 33B (see FIG. 2) and 44 and 46
(see FIG. 4) can be formed of semiconductor material, such as
silicon that is deposited onto silicon substrate 29. More details
regarding the process used to deposit and pattern such shutter
elements are provided below. At this point in the description, it
might be mentioned that such shutter elements can be formed from a
layer of material, such as a deposited layer of silicon, that was
initially secured to the underlying substrate, and which is
subsequently patterned and etched to form such shutter elements.
Other opaque materials such as tantalum or aluminum can be used to
enhance the silicon shutters; alternatively, such metals can be
used instead of silicon to form such shutter elements. Once again,
such shutter elements can be formed from a layer of metal that was
initially deposited upon the substrate, and which is subsequently
patterned and etched to form the shutter elements.
[0054] The width of each shutter element is equal to the desired
pixel size times the demagnification of the optics. In other words,
referring to FIG. 1, if the length of the image projected onto
substrate 27 by reduction lens 25 is, for example, one-tenth of the
length of aperture assembly 24, then the width of each shutter
element is ten times the desired pixel width of the demagnified
beam that strikes substrate 27. The MEMS shutter elements may be
made to essentially block the transmission of all light, or as
described above, to allow approximately 10% transmission and a
180-degree phase change of the light for the described phase shift
effect.
[0055] Referring now to FIG. 5, the shutter elements can be built
up from substrate 29 using sacrificial oxide or nitride layers
which are removed later in the manufacturing process to allow each
shutter element to slide freely. First, the silicon guides 43, 47
are formed upon substrate 29, as by depositing or growing a layer
of silicon upon substrate 29, and then patterning such layer to
etch away all portions of such layer except those forming guides
43, 47. The upper surface of substrate 29 is then covered with a
thin sacrificial layer of silicon nitride 51, which bridges over
the patterned guides 43, 47, as shown in FIG. 5. Next, a second
layer of silicon, designated in FIG. 5 by reference numeral 46, is
deposited over the nitride layer 51 to ultimately form the silicon
shutter elements. This second layer of silicon is then patterned
and etched to form individual shutter elements, such as shutter
element 46.
[0056] Still referring to FIG. 5, the substrate is then covered by
a second thin sacrificial layer 53 of silicon nitride. A third
layer of silicon is then deposited over second sacrificial nitride
layer 53; this third layer of silicon is then patterned and etched
to form the silicon retaining bridge 34 that extends above each of
the shutters elements. Finally, the sacrificial silicon nitride
layers 51 and 53 are selectively etched away to free shutter
element 46 from substrate 29, guide 47 and bridge 34, thereby
allowing free sliding movement of shutter 46 relative thereto.
[0057] As mentioned above, a magnetic material such as NiFe is
deposited on top of one end of each shutter element with a
permanent polarity in the same axis as the shutter movement, thus
forming a permanent magnet on the end of the shutter element.
Referring to FIGS. 6A & 6B, the polarity of the permanent
magnet 68 is influenced by an adjacent non-movable magnet 60, which
is surrounded by an electromagnetic coil 61 that extends along a
longitudinal axis that is essentially colinear with non-movable
magnet 60 and colinear with the sliding motion of shutter element
46. The coil configuration is such that as electrical current is
moved through the coil in a first direction (see FIG. 6A), the
non-movable magnet 68 is polarized to repel the adjacent shutter
46, thereby extending it over a portion of aperture 30, and
therefore blocking the passage of light therethrough. When the
current is reversed in coil 61 (see FIG. 6B), the polarity of
non-movable magnet 60 reverses, attracting the shutter 46 and
pulling it away from the aperture 30, thereby allowing the light to
pass through the selected portion of aperture 30. Magnetic
shielding 67, such as NiFe or other soft magnetic material, is
interposed between adjacent coils to isolate each of the electrical
coil assemblies so that the operation of one assembly does not
interfere with the operation of the adjacent assemblies.
[0058] FIG. 7 illustrates the manner by which electrical coil 61 of
FIGS. 6A/6B is produced. Initially, a first layer 70 of aluminum or
other metal is deposited upon substrate 29; this first layer 70 of
metal is then patterned and etched to form the lower rungs of
electrical coil 61, as well as the lower rungs of the other
electrical coils to be formed on substrate 29. As shown in FIGS.
6A/6B and FIG. 7, each of the lower rungs 70 of electrical coil 61
extend generally perpendicular to the longitudinal axis of
electrical coil 61. In addition, each such lower rung has a first
end 71 and an opposing second end 71A. After patterning the lower
rungs 70, substrate 29 is then covered with an insulating material
72, such as silicon dioxide, which extends over substrate 29 and
over the patterned lower rungs 70. Magnetic material 73 is then
deposited above first insulative layer 72 and above, and
perpendicular to lower rungs 70, along the longitudinal axis of
electrical coil 61; this magnetic material may again consist of
NiFe, extends and ultimately forms the non-movable magnet 60.
Another encapsulating insulating layer of material 74 is deposited
over substrate 29, covering first insulative layer 72 and magnetic
material 73. Holes are then etched through the first insulative
layer 72 and second insulative layer 74 above the first and second
opposing ends 71 and 71A, respectively, of each lower rung to
expose such ends. A further metal layer 75 is then deposited over
substrate 29, covering second insulative layer 74, but extending
within the aforementioned etched holes. This second metal layer 75
is then patterned to form the diagonal connecting arms 75 shown in
FIGS. 6A/6B and FIG. 7, in order to connect the first end of one
lower rung to the second end of the next succeeding lower rung,
thereby forming electrical coil 61. Finally, a third insulative
layer of material 76 is deposited over the substrate and entire
coil assembly to insulate the diagonal connecting arms 75 of the
electrical coils, such as coil 61.
[0059] As mentioned above, it is desirable to isolate adjacent
electrical coils from each other by disposing magnetic shield
material between such coils. As shown in FIG. 7, magnetic shielding
material 67 is deposited along both sides of electrical coil 61,
but spaced apart therefrom, to isolate each such electrical coil 61
from its adjacent neighboring coils.
[0060] Referring to FIGS. 1 and 8, the image resulting from the
light passing through the slit aperture assembly 24 is projected to
scanning mirror 26 which reflects the image to the substrate 27 to
be exposed. As scanning mirror 26 is rotated, the shutter elements
described above, including shutter elements 44 and 46, move in such
a way over aperture 30 as to create the two dimensional image 81
shown in FIG. 8. Within FIG. 8, the image shown in the row
designated 82 corresponds to the pattern of shutter elements
covering or exposing aperture 30.
[0061] Referring to FIG. 9A, flat scanning mirror 90 must rotate
about the reflective plane 93 to limit scan distortions. On the
other hand, the one-dimensional concave scanning mirror 91 shown in
FIG. 9b offers several benefits over the flat scanning mirror 90
shown in FIG. 9A. First, by further focusing the incoming patterned
beam of light 92 in one dimension, the intensity is increased
allowing for higher doses of exposure and/or throughput. Second,
because the projected image is less than one pixel wide, the
shutter movement can be more precisely controlled reducing exposure
gradients at the edge of features.
[0062] As mentioned above with respect to FIG. 1, an alternative to
the use of a scanning mirror is to project the image directly to
the substrate 27, and to move the stage which supports substrate 27
by incremental, indexed movements, to effectively scan the entire
photosenstive surface of substrate 27 through the stationary image.
Each time the substrate supporting stage is advanced, the shutter
elements are moved to form a new patterned beam of light. The
combination of stage indexing and shutter movements creates a
two-dimensional image upon substrate 27, similar to that shown in
FIG. 8. The precisely controlled substrate support stage must be
synchronized with the shutter movement.
[0063] Several exposure techniques can be use to enhance the image
quality. First, the scan rate can be adjusted to vary the exposure
dose in a desired area on the substrate. Second, a retrace scan can
be used to re-expose all or part of a given exposure. This will
allow portions of a given pattern to be exposed at a different dose
or doses than the bulk of the pattern. Third, optical proximity
corrections as described in Glendinning can be used by simply
correcting the pattern data file in a computer.
[0064] Apart from the new and improved maskless exposure apparatus
described above, the present invention also encompasses a new
method for exposing a photosensitive surface to a desired light
pattern. In practicing such method, a substrate is provided like
described substrate 29, including an elongated aperture 24/30. A
beam of light, such as that produced by light source 22 and mirror
21 is directed generally toward the elongated aperture 24/30. A
series of shutter elements 33, 33A, 33B are supported upon the
substrate 29, each of the shutter elements being movable between a
first position covering a portion of the elongated aperture, as
indicated for example by FIG. 6A, and a second position exposing a
portion of the elongated aperture, as indicated by FIG. 6B. Such
shutter elements extend generally parallel to the plane of the
substrate, whether in the first position or second position, and
movement of such shutter elements is a generally linear sliding
motion.
[0065] To control the pattern of light emitted by the aperture
24/30, the method includes the step of moving each of the shutter
elements to either its first position (FIG. 6A) or its second
position (FIG. 6B) to selectively block or pass portions of the
beam of light, thereby creating a first patterned light beam
exiting the elongated aperture 24/30 of the substrate 29. This
first patterned light beam is directed upon a first portion of the
photosensitive work surface of a work material, such as a
photoresist coating applied to substrate 27. The steps of moving
each of the shutter elements to either its first position or its
second position is repeated in a subsequent exposure cycle to
create a second patterned light beam exiting the elongated aperture
24/30. The second patterned light beam is then directed upon a
second portion of the photosensitive work surface. These steps are
repeated for third and subsequent portions of the photosensitive
work surface, as indicated in FIG. 8, until substantially all
portions of the photosensitive work surface have been exposed to a
respective patterned light beam.
[0066] Those skilled in the art will now appreciate that the
present invention provides an apparatus and method for eliminating
the need for conventional photolithography masks. The single narrow
slit aperture, combined with the series of micro electromechanical
system, or MEMS, shutter elements creates desired patterns of
light. The shutter elements are positioned in such a way as to
allow portions of the slit aperture to be selectively covered,
therefore blocking the light transmission. The resulting image
projected onto the substrate is a row of pixels with only the
desired pixels illuminated. The projected image is then advanced to
the next row on the substrate, and the shutter elements are
reconfigured to produce the desired light pattern. This process is
repeated until all of the rows of the target substrate have been
selectively exposed. The described shutter elements slide back and
forth without contact while still blocking the path of the
light.
[0067] Those skilled in the art will also appreciate that the
maskless exposure system and related method described above can be
used with any appropriate wavelength of light, including DUV and
EUV, because the desired light rays do not travel through any solid
medium, as is true for chromeon-quartz masks. Further, the
above-described apparatus and method can be used at any of the
common magnifications, including 1.times., 5.times. or 10.times..
In addition, because the shutter elements and related
electro-magnet actuators are so small, the aperture assembly
reduces to a very dense structure. In addition, the use of the
electromagnetic coils to actuate the shutter elements magnetically
allows the size of the shutter elements and the size of the
electrical coil to be more independent of each other; the coil can
be as long as necessary to move the shutter element without
affecting the size of the shutter element itself All movement is
positively induced by magnetic forces, and no spring mechanisms or
other attached devices are required to return the shutter elements
to their initial position.
[0068] While the present invention has been described with respect
to preferred embodiments thereof, such description is for
illustrative purposes only, and is not to be construed as limiting
the scope of the invention. Various modifications and changes may
be made to the described embodiments by those skilled in the art
without departing from the true spirit and scope of the invention
as defined by the appended claims.
* * * * *