U.S. patent application number 09/757306 was filed with the patent office on 2001-10-25 for electrostatic printing of a metallic toner applied to solid phase crystallization and silicidation.
This patent application is currently assigned to The Penn State Research Foundation. Invention is credited to Detig, Robert H., Fonash, Stephen J., Kalkan, Ali Kaan.
Application Number | 20010033985 09/757306 |
Document ID | / |
Family ID | 22223748 |
Filed Date | 2001-10-25 |
United States Patent
Application |
20010033985 |
Kind Code |
A1 |
Fonash, Stephen J. ; et
al. |
October 25, 2001 |
Electrostatic printing of a metallic toner applied to solid phase
crystallization and silicidation
Abstract
A metal-containing toner is electrostatically printed on a
semiconductor surface. Subsequently, this surface is annealed to
achieve certain material modifications selectively at the regions
where the toner is applied. If the toner contains a
crystallization-catalyst metal, such as, Pd, Ni, Pt, and Cr, and is
printed on an amorphous semiconductor film, annealing results in
conversion of the printed regions to polycrystalline. If the
metal-containing toner is printed on a silicon surface (i.e.,
amorphous/poly-Si layer or Si wafer) the printed regions are
selectively converted to a metal-silicide (with the sufficient
amount of metal applied on these regions) upon annealing.
Inventors: |
Fonash, Stephen J.; (State
College, PA) ; Kalkan, Ali Kaan; (State College,
PA) ; Detig, Robert H.; (New Providence, NJ) |
Correspondence
Address: |
Thomas J. Monahan, Esq.
Intellectual Property Office Action
The Pennsylvania State University
113 Technology Center
University Park
PA
16802-7000
US
|
Assignee: |
The Penn State Research
Foundation
|
Family ID: |
22223748 |
Appl. No.: |
09/757306 |
Filed: |
January 8, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60090663 |
Jun 25, 1998 |
|
|
|
Current U.S.
Class: |
430/117.2 ;
430/114; 430/117.5; 430/118.3; 430/198 |
Current CPC
Class: |
H01L 27/1277 20130101;
H01L 21/02672 20130101; H01L 21/02592 20130101; H01L 21/02532
20130101 |
Class at
Publication: |
430/119 ;
430/126; 430/198; 430/114 |
International
Class: |
G03G 013/10; G03G
013/16 |
Claims
What is claimed is:
1. A method for applying metallic toner onto an amorphous
semiconductor layer with the objective of selective area
crystallization, said method comprising the steps of: (a) exposing
a photo-sensitive surface to cause exposed areas of said surface to
crosslink and exhibit an increase in resistivity in comparison with
unexposed areas of said photo-sensitive surface; (b) charging said
photo-sensitive surface, said exposed areas of said photo-sensitive
surface retaining a charge longer than said unexposed areas; (c)
applying a toner to said photo-sensitive surface, said toner
attracted by retained charge on said exposed areas; (d) juxtaposing
said photo-sensitive surface toned in step (c) to a layer of
amorphous semiconductor and applying an electric field therebetween
to cause said toner that is adherent to said photo-sensitive
surface to migrate to said amorphous semiconductor layer; and (e)
annealing said toned amorphous semiconductor layer to enable
formation of polycrystalline semiconductor only in areas where said
toner is adherent.
2. The method as recited in claim 1, wherein said photo-sensitive
surface comprises a material selected from the group consisting of:
epoxy cationic, acrylic free radical, and photosensitive
polyimide.
3. The method as recited in claim 1, wherein said toner contains a
metal and said metal is in a form selected from the group
consisting of: metal complex, pure metal particle, coated metal,
and organometallic compound.
4. The method as recited in claim 3, wherein said toner is a
material selected from the group consisting of: a resin with metal
particles, an organosol with metal particles, a metallo-organic
decomposition compound, and a metallo-organic decomposition
compound with metal particles.
5. The method as recited in claim 4, wherein said metal or said
metal particle is selected from the group consisting of: palladium,
silver, tin, nickel, platinum, chromium and mixtures thereof.
6. The method as recited in claim 1, wherein step (d) further
comprises interposing a nonconductive fluid between said
photo-sensitive surface and said layer of amorphous semiconductor
prior to applying said electric field.
7. The method as recited in claim 1, wherein said amorphous
semiconductor layer comprises a material selected from a group
consisting of: silicon, germanium, silicon-germanium,
silicon-carbide, cadmium selenide, and indium antimonide.
8. The method as recited in claim 1, wherein said amorphous
semiconductor layer is disposed on a substrate.
9. The method as recited in claim 8, wherein said substrate
comprises at least one material selected from the group consisting
of silicon, metal, glass, and plastic.
10. A method for applying metallic toner onto an amorphous
semiconductor layer with the objective or selective area
crystallization, said method comprising the steps of: (a) charging
a photo-sensitive surface; (b) exposing said photo-sensitive
surface to an optical image or a digitally addressed beam to
produce a latent image of charges on the photo-sensitive surface;
(c) applying a toner to said photo-sensitive surface, said toner
attracted to the charged areas of said photo-sensitive surface; (d)
juxtaposing said photo-sensitive surface toned in step (c) to a
layer of amorphous semiconductor and applying an electric field
therebetween to cause said toner that is adherent to said
photo-sensitive surface to migrate to said amorphous semiconductor
layer; and (e) annealing said toned amorphous semiconductor layer
to enable formation of polycrystalline semiconductor only in areas
where said toner is adherent.
11. The method as recited in claim 10, wherein said photo-sensitive
surface comprises a material selected from the group consisting of:
organic photoreceptor surface, arsenic triselenide, selenium, and
silicon.
12. A method for the formation of silicide on a silicon surface
comprising the steps of: (a) exposing a photo-sensitive surface to
cause exposed areas of said surface to crosslink and exhibit an
increase in resistivity in comparison with unexposed areas of said
photo-sensitive surface; (b) charging said photo-sensitive surface,
said exposed areas of said photo-sensitive surface retaining a
charge longer than said unexposed areas; (c) applying a toner to
said photo-sensitive surface, said toner attracted by retained
charge on said exposed areas; (d) juxtaposing said photo-sensitive
layer surface toned in step (c) to a silicon surface and applying
an electric field therebetween to cause said toner that is adherent
to said photo-sensitive surface to migrate to said silicon surface;
and (e) annealing said toned silicon surface thereby enabling
formation of silicide only in areas where said toner is
adherent.
13. The method as recited in claim 12, wherein said photo-sensitive
surface comprises a material selected from the group consisting of:
epoxy cationic, acrylic free radical, and photosensitive
polyimide.
14. The method as recited in claim 12, wherein the silicon surface
is amorphous, polycrystalline, or single crystalline.
15. The method as recited in claim 12, wherein said toner contains
a metal and said metal is in a form selected from the group
consisting of metal complex, pure metal particle, coated metal, and
organometallic compound.
16. The method as recited in claim 15, wherein said toner is
selected from the group consisting of: a resin with metal
particles, an organosol with metal particles, a metallo-organic
decomposition compound, and a metallo-organic decomposition
compound with metal particles.
17. The method as recited in claim 16, wherein said metal or metal
particle is selected from the group consisting of: aluminum,
cobalt, chromium, hafnium, iron, magnesium, molybdenum, nickel,
niobium, palladium, platinum, tantalum, titanium, tungsten,
zirconium and mixtures thereof.
18. The method as recited in claim 12, wherein step (d) further
comprises interposing a nonconductive fluid between said
photo-sensitive surface and said silicon surface prior to applying
said electric field.
19. The method as recited in claim 12, wherein said silicon surface
is an amorphous or polycrystalline silicon thin film disposed on a
substrate.
20. The method as recited in claim 19, wherein said substrate
comprises at least one material selected from the group consisting
of: silicon, metal, glass, and plastic.
21. A method for the formation of silicide on a silicon surface
comprising the steps of: (a) charging a photosensitive surface; (b)
exposing said photosensitive surface to an optical image or a
digitally addressed beam to produce a latent image of charges on
the photosensitive surface; (c) applying a toner to said
photosensitive surface, said toner attracted to the charged areas
of said photosensitive surface; (d) juxtaposing said
photo-sensitive layer surface toned in step (c) to a silicon
surface and applying an electric field therebetween to cause said
toner that is adherent to said photo-sensitive surface to migrate
to said silicon surface; and (e) annealing said toned silicon
surface thereby enabling formation of silicide only in areas where
said toner is adherent.
22. The method as recited in claim 21, wherein said photo-sensitive
surface comprises a material selected from the group consisting of:
organic receptor surface, arsenic triselenide, selenium, and
silicon.
Description
[0001] This Application claims benefit from U.S. Ser. No.
09/340,009 filed Jun. 25, 1999 and U.S. Provisional Patent
Application Ser. No. 60/090,663, filed Jun. 25, 1998.
FIELD OF THE INVENTION
[0002] This invention relates to a method and apparatus for
crystallizing amorphous films into polycrystalline films and, more
particularly, to an electrostatic printing method and apparatus for
selective deposition of catalyst metals to achieving such selective
crystallization. This invention also relates to a method and
apparatus for forming metal silicide regions on amorphous/poly-Si
films or Si wafers and, more particularly, to an electrostatic
printing method and apparatus for selective deposition of metals of
the metal silicides to achieving such selective silicidation.
BACKGROUND OF THE INVENTION
[0003] Large area amorphous silicon layers are widely used to make
the transistors used for flat panel display devices. Indeed the
most widely used flat panel display, i.e., the active matrix liquid
crystal display (AMLCD), derives its name from an active matrix of
transistors that are arranged in both the X and Y directions. A
transistor made from amorphous silicon is placed at each picture
element (pixel) in each color for a color display (red, green, and
blue).
[0004] Transistors made from amorphous silicon exhibit low
performance characteristics (compared to those made from single
crystal silicon), with low carrier mobility being a determining
property. Researchers have recognized that converting amorphous
silicon to poly crystalline silicon (poly-Si) will enhance
performance considerably, even to a significant fraction of the
performance of single crystal silicon, the material from which all
integrated circuits are made.
[0005] Studies of poly-Si thin film transistors have concentrated
on methods for reducing their fabrication costs, either by reducing
the transistors' processing time or by lowering the processing
temperatures. The latter effect is important since it allows the
usage of less expensive substrates for the transistor arrays, e.g.,
glass, plastic, etc . . . . For instance, Czubatyj et al. in
"Low-Temperature Polycrystalline TFT on 7057 Glass", IEEE Electron
Device Letters, Vol. 10, pages 349-351, 1989, demonstrates that
polysilicon thin film transistors can be fabricated on 7059 glass
substrates using relatively low temperature furnace annealing for
crystallization. However, the crystallization process takes longer
than 75 hours and is therefore not practically applicable.
[0006] Poly-Si films can be deposited, deposited and
recrystallized, or deposited in the amorphous (.alpha.-Si) form and
then crystallized into poly-Si films. There are three principal
crystallization processes: furnace annealing, rapid thermal process
(RTP) and laser annealing. The first two are solid phase
crystallization techniques, while the third is a liquid phase
crystallization technique. Although reported laser annealing
techniques have the potential for effecting low temperature
crystallization, laser crystallization suffers from the need to
raster the laser beam; raising throughput issues. Laser annealing
also exhibits other difficulties, e.g. reproducibility, uniformity
and peel-off. The most commonly used methods for producing large
grain poly-Si films are furnace annealing of .alpha.-Si films at
temperatures of at least 600.degree. C., with very long processing
times (16-30 hours or longer for .alpha.-Si films) or the RTP
approach (e.g., 700.degree. C./5 mins).
[0007] In "Low Thermal Budget Poly-Silicon Thin Film Transistors on
Glass", Japanese Journal of Applied Physics, Vol. 30, pages
L269-L271, 1991, it was demonstrated that thin film transistors can
be fabricated on poly-Si films made by the crystallization of
pre-cursor .alpha.-Si films. Those polycrystalline films were
obtained by a rapid thermal annealing of the precursor films for
five minutes at 700.degree. C. on Corning 7059 glass
substrates.
[0008] In U.S. Pat. No. 5,147,826 to Liu et al., it was shown that
a thermal anneal procedure at 700.degree. C. (for converting
.alpha.-Si to poly-Si) can be reduced to a range of from
550.degree. C. to 650.degree. C. This improvement is accomplished
by depositing a thin discontinuous film of a nucleating site
forming material over an already deposited layer of .alpha.-Si. The
.alpha.-Si film is then rapidly thermally annealed, with the
nucleating site forming material enabling crystallization of the
underlying .alpha.-Si at temperatures lower than theretofore
reported.
[0009] Liu et al. also report in the '826 patent that .alpha.-Si
can be selectively crystallized by depositing the nucleating site
performing material in a pattern thereon and subsequently
subjecting the patternized surface to an anneal procedure. Because
the nucleating site forming material is a metal, the treated
surface of the subsequently crystallized silicon is not optimal for
structures. As a result, additional processing steps are required
to allow untreated surfaces to become boundaries for devices to be
grown.
[0010] In U.S. Pat. No. 5,275,826 of Fonash et al., a fabrication
process for polycrystalline silicon thin film transistors is
described that commences with a deposition of an ultra-thin
nucleating-site forming layer onto the surface of an insulating
substrate (e.g., 7059 glass, plastic). Next, an .alpha.-Si film is
deposited thereover and the combined films are annealed at
temperatures that do not exceed 600.degree. C. By patterning the
deposition of the nucleating site forming material on the glass
substrate, the subsequently deposited .alpha.-Si film can be
selectively crystallized only in areas in contact with the
nucleating-site forming material.
[0011] Ohtani et al. in U.S. Pat. Nos. 5,585,291, 5,612,250,
5,643,826, 5,543,352, and 5,654,203 describe a solution method for
applying a catalyst metal to enhance subsequent .alpha.-Si
crystallization.
[0012] The aforesaid thus clearly indicates that catalysts can be
used to reduce the time-temperature thermal budget needed for
crystallization of semiconductor materials. For example, catalytic
agents like palladium or nickel can be deposited by various
techniques like vacuum evaporation or from solution and such
catalytic agents can substantially impact the thermal budget. The
crystallization time may be shortened to as low as 4 minutes at
550.degree. C. by such metal treatments.
[0013] Each of the above-cited references has employed some form of
photolithographic masking to achieve selective deposition of the
catalytic metal on selected parts of a substrate. Such procedures
require a number of steps and add to the cost of the ultimate
product made thereby.
[0014] Accordingly, it is an object of this invention to provide an
improved method and apparatus for applying a crystallization
catalyst onto an amorphous semiconductor film.
[0015] Besides selective area crystallization of an amorphous film,
another microelectronic fabrication process of interest involving
the selective area application of a metal on an amorphous or
polycrystalline Si film or a Si wafer is selective area
silicidation. A wide range of noble and refractory metals form
compounds with Si called silicides. As in the case of metal-induced
crystallization, silicidation requires annealing of the related
metal layer in contact with Si. Minimum annealing temperature
depends on the silicide to be formed and varies from 400 to
1000.degree. C. Silicides exhibit conductivities close to metals
(0.1-0.01 S/cm) and in certain applications are preferable to
metals where a better chemical stability or lattice match is
desired. Applications of silicides include; electrical
interconnects, Schottky contacts to form Schottky barrier diodes,
gate electrodes in transistors, and source and drain contacts in
transistors. As different from metal-induced crystallization,
silicidation requires a thicker layer of metal be deposited. This
is because, in crystallization, the catalyst layer (which may be
pure metal, or a metal containing material) deposited acts as the
catalyst or seed layer and does not need to be thicker than a few
tens of .ANG.. On the other hand, in applications of silicides as
stated above, the silicide layer is required to be at least
hundreds of .ANG.. Hence, a metal layer of similar thickness
(hundreds of .ANG.) is needed which is to be consumed during
silicidation process. As in the case of selective area
crystallization, fabrication of silicide structures or patterns on
a Si surface requires the metal layer to be patterned.
Conventionally, this procedure is also performed by
photolithography and requires a number of steps increasing the
processing cost.
[0016] Accordingly, it is also an object of this invention to
provide an improved method and apparatus for applying a metal onto
a Si surface (Si wafer, amorphous/poly-Si film) with the purpose of
obtaining silicide regions.
[0017] The process of the invention is simple, low cost and is much
like a copy machine and enables the printing of a toner for the
purpose of selective area crystallization or silicidation,
preferably on a silicon layer that resides on a low cost substrate.
Glass, plastics and metal foils covered by an insulating layer can
be used. The patterning and image registration can be performed to
high accuracy using the process of the present invention.
[0018] In the case of crystallization or silicidation of thin films
according to the present invention, the process sequence may be
modified by applying the catalyst-containing or metal containing
toner to the substrate prior to semiconductor film deposition and
annealing of the semiconductor film. The semiconductor film can be
a material other than Si, e.g., carbon, germanium and alloys
thereof.
SUMMARY OF THE INVENTION
[0019] The present invention is directed to a method for applying
metallic toner onto an amorphous semiconductor layer with the
objective of selective area crystallization, the method comprising
the steps of: (a) exposing a photo-sensitive surface to cause
exposed areas of the surface to crosslink and exhibit an increase
in resistivity in comparison with unexposed areas of the
photo-sensitive surface; (b) charging the photo-sensitive surface,
the exposed areas of the photo-sensitive surface retaining a charge
longer than the unexposed areas; (c) applying a toner to the
photo-sensitive surface, the toner attracted by retained charge on
the exposed areas; (d) juxtaposing the photo-sensitive surface
toned in step (c) to a layer of amorphous semiconductor and
applying an electric field therebetween to cause the toner that is
adherent to the photo-sensitive surface to migrate to the amorphous
semiconductor layer; and (e) annealing the toned amorphous
semiconductor layer to enable formation of polycrystalline
semiconductor only in areas where the toner is adherent. In another
embodiment of the invention, step (d) further comprises interposing
a nonconductive fluid between the photo-sensitive surface and the
silicon surface prior to applying the electric field.
[0020] In one embodiment of the invention, the photo-sensitive
surface comprises a material such as epoxy cationic, acrylic free
radical, and photosensitive polyimide.
[0021] In another embodiment of the invention, the toner contains a
metal (a) chemically, wherein the toner is a compound or solution
of the metal; (b) physically, wherein the metal is contained in the
toner as metal particles; or (c) both. Preferably, the metal
contained in the toner is in a form selected from the group
consisting of metal complex, pure metal particle, coated metal, and
organometallic compound. More preferably, the toner is a material
selected from the group consisting of: a resin with metal
particles, an organosol with metal particles, a metallo-organic
decomposition compound, and a metallo-organic decomposition
compound with metal particles. The metal or the metal particle is
comprised of a metal selected from the group consisting of:
palladium, silver, tin, nickel, platinum, chromium and mixtures
thereof.
[0022] In a further embodiment of the invention, the amorphous
semiconductor layer comprises a material such as silicon,
germanium, silicon-germanium, silicon-carbide, cadmium selenide,
and indium antimonide. Preferably, the amorphous semiconductor
layer is disposed on a substrate, which comprises at least one
material, such as silicon, metal, glass, or plastic.
[0023] The present invention is also directed to a method for
applying metallic toner onto an amorphous semiconductor layer with
the objective or selective area crystallization, the method
comprising the steps of: (a) charging a photo-sensitive surface;
(b) exposing the photo-sensitive surface to an optical image or a
digitally addressed beam to produce a latent image of charges on
the photo-sensitive surface; (c) applying a toner to the
photo-sensitive surface, the toner attracted to the charged areas
of the photo-sensitive surface; (d) juxtaposing the photo-sensitive
surface toned in step (c) to a layer of amorphous semiconductor and
applying an electric field therebetween to cause the toner that is
adherent to the photo-sensitive surface to migrate to the amorphous
semiconductor layer; and (e) annealing the toned amorphous
semiconductor layer to enable formation of polycrystalline
semiconductor only in areas where the toner is adherent. In an
embodiment of the invention, the photo-sensitive surface comprises
a material such as organic photoreceptor surface, arsenic
triselenide, selenium, and silicon. In another embodiment of the
invention, the exposing of the photo-sensitive surface in step (b)
is to an optical image or digitally addressed beam, such as, a LED
or laser.
[0024] The present invention is further directed to a method for
the formation of silicide on a silicon surface comprising the steps
of: (a) exposing a photo-sensitive surface to cause exposed areas
of the surface to crosslink and exhibit an increase in resistivity
in comparison with unexposed areas of the photo-sensitive surface;
(b) charging the photo-sensitive surface, the exposed areas of the
photo-sensitive surface retaining a charge longer than the
unexposed areas; (c) applying a toner to the photo-sensitive
surface, the toner attracted by retained charge on the exposed
areas; (d) juxtaposing the photo-sensitive layer surface toned in
step (c) to a silicon surface and applying an electric field
therebetween to cause the toner that is adherent to the
photo-sensitive surface to migrate to the silicon surface; and (e)
annealing the toned silicon surface thereby enabling formation of
silicide only in areas where the toner is adherent. In accordance
with still another embodiment of the invention, step (d) further
comprises interposing a nonconductive fluid between the
photo-sensitive surface and the silicon surface prior to applying
the electric field.
[0025] In one embodiment of the above invention, the
photo-sensitive surface comprises a material such as epoxy
cationic, acrylic free radical, and photosensitive polyimide. The
silicon surface for the above method can be amorphous,
polycrystalline, or single crystalline. In an embodiment of the
invention, the photo-sensitive surface comprises a material such as
organic photoreceptor surface, arsenic triselenide, selenium, and
silicon.
[0026] In an embodiment of the invention, the toner contains a
metal as detailed above. Preferably, the metal or metal particle is
a metal of the desired metal-silicide including: aluminum, cobalt,
chromium, hafnium, iron, magnesium, molybdenum, nickel, niobium,
palladium, platinum, tantalum, titanium, tungsten, zirconium and
mixtures thereof.
[0027] In another embodiment of the invention, the silicon surface
is an amorphous or polycrystalline silicon thin film disposed on a
substrate. Preferably, the substrate comprises at least one
material, such as, silicon, metal, glass, or plastic.
[0028] The present invention is still further directed to a method
for the formation of silicide on a silicon surface comprising the
steps of: (a) charging a photosensitive surface; (b) exposing the
photosensitive surface to an optical image or a digitally addressed
beam to produce a latent image of charges on the photosensitive
surface; (c) applying a toner to the photosensitive surface, the
toner attracted to the charged areas of the photosensitive surface;
(d) juxtaposing the photo-sensitive layer surface toned in step (c)
to a silicon surface and applying an electric field therebetween to
cause the toner that is adherent to the photo-sensitive surface to
migrate to the silicon surface; and (e) annealing the toned silicon
surface thereby enabling formation of silicide only in areas where
the toner is adherent. In an embodiment of the invention, the
photo-sensitive surface comprises a material selected from the
group consisting of: organic receptor surface, arsenic triselenide,
selenium, and silicon. In one embodiment of the invention, the
exposing of the photo-sensitive surface in step (b) is to an
optical image or digitally addressed beam, such as, a LED or
laser.
[0029] This invention utilizes electrophotography to apply a
crystallization catalyst to an amorphous semiconductor layer. The
catalyst is subsequently employed to convert areas of the amorphous
semiconductor layer to discrete, defined polycrystalline
regions.
[0030] This invention also utilizes electrophotography to apply a
metal or metal containing material to a Si surface. The metal or
metal containing material is subsequently employed to convert areas
of the Si surface to discrete, defined metal-silicide regions.
BRIEF DESCRIPTION OF THE DRAWINGS
[0031] FIG. 1 illustrates a first step in the process of the
invention wherein a photosensitive plate material is selectively
cross-linked by application of actinic energy.
[0032] FIG. 2 illustrates a second step in the process of the
invention wherein a photosensitive plate material is
electrostatically charged.
[0033] FIG. 3 illustrates a third step in the process of the
invention wherein catalyst-containing or metal containing toner is
applied to the charged photosensitive plate material.
[0034] FIG. 4 illustrates a fourth step in the process of the
invention wherein the catalyst-containing toner is transferred from
the charged photosensitive plate material to an amorphous
semiconductor layer (or to a Si surface) by action of an applied
electric field.
[0035] FIGS. 5a, 5b illustrate an alternate fourth step in the
process of the invention wherein the metal-containing toner is
transferred from the charged photosensitive plate material to a
substrate, followed by deposition thereon of a Si or other
semiconductor layer and then followed by an anneal operation to
achieve selective crystallization or silicidation of the Si layers
that are in contact with the toner.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0036] In application to selective area crystallization a catalytic
liquid toner is electrostatically printed on an amorphous silicon
layer (or a substrate that is to support such a layer) in an
image-wise fashion. After the liquid toner is dried, the amorphous
silicon layer is heated, preferably using rapid thermal annealing,
to approximately 550.degree. C. for about 2 minutes to complete the
polycrystalline conversion process. The toner used during the
printing action is a dispersion of resin particles which contains a
modest amount of metallic catalyst, such as palladium, silver, tin,
nickel, platinum or chromium.
[0037] In application to selective area silicidation, a metal
containing toner is electrostatically printed on a Si surface (Si
wafer, amorphous/poly-Si layer). Upon annealing, the Si regions in
contact with the metal toner are converted to metal-silicide. The
conversion temperature depends on the type of silicide. For
example, Pd.sub.2Si forms at .about.400.degree. C., while WSi.sub.2
forms at .about.1000.degree. C.
[0038] The printing step initially forms latent images on a
photo-sensitive receptor plate or drum. Images can be created
either electrophotographically, as in a Xerox-graphic copier, or
digitally as in a laser printer. The latent images are developed by
application of a liquid toner. The toner is then transferred to the
Si surface. After the toner is dried, the patterning process is
complete. The printed Si surface is now heat processed to complete
the crystallization or silicidation process.
[0039] The remaining unprinted Si regions are unconverted to
poly-Si or metal silicide (in crystallization and silicidation,
respectively) and need not be removed, a significant process saving
step unless required by demands such as stress control or light
transmission.
[0040] FIG. 1 shows the first step in the electrostatic printing
process of the invention, i.e., the making of the printing plate. A
photosensitive surface such as a photopolymer material 10,
preferably in a dry film form, is laminated to a grounded substrate
12. Photopolymer 10 exhibits the characteristics of photoresists
that are used for photolithography applications (e.g., etch
resistance). A preferred photopolymer is Dynachem 5038, available
from the Dynachem Corporation, Tustin Calif. Another photopolymer
that is acceptable is Riston 4615, a product of the Dupont
Corporation, Wilmington, Del.
[0041] Photopolymer 10 should have the characteristic of
crosslinking in areas exposed to actinic energy. As shown in FIG.
1, photopolymer 10 is exposed through a photo tool to actinic
radiation in the 300 to 400 nm range or the near ultra violet
region of the spectrum. Exposure levels are typically from 50 to
500 millijoules per cm.sup.2. Such exposure causes areas 14 to
crosslink and at this stage, the plate making step is complete. To
achieve selective image-wise charging, a modulated laser beam may
be swept across the surface of photosensitive material 10 in the
manner of a laser printer. A similar result can be achieved through
use of a line of modulated laser diodes that are moved over the
surface of photosensitive material 10. Further, it is to be
understood that while the foregoing description will consider use
of a flat plate photopolymer, that the invention can be carried out
using a flexible photopolymer that is imaged by either a swept
modulated laser beam or a line of modulated laser diodes.
[0042] Photosensitive material 10 is now sensitized by charging it,
for example with a corona unit, as shown in FIG. 2. A positive
charge is shown as being applied but the photosensitive material 10
can accept either positive or negative charge.
[0043] Where photosensitive material 10 is exposed, the resulting
crosslinking raises the electrical resistivity of the material by 4
to 6 orders of magnitude. This enables photosensitive material 10
to retain its charge in the crosslinked areas after the charging
step, while unexposed regions quickly discharge.
[0044] In FIG. 3, the previously charged areas of photosensitive
material 10 are "toned" with liquid toner particles as indicated by
the negatively charged spheres 16. Each sphere 16 comprises a metal
catalyst particle encompassed by a polymeric shell. Details of the
method of manufacture of toner particles 16 are given below.
[0045] Next, as shown in FIG. 4, the plate including photosensitive
material 10 is placed close to a Si surface 20. Here, in
particular, the Si surface is that of a Si film 20 coated on a
glass substrate 22. A conductive layer 24 is disposed on the
opposite face of glass plate 22 and is connected to a voltage
supply 26. The region between photosensitive material 10 and
amorphous silicon layer 20 is filled with a nonconductive fluid,
e.g., Isopar G, a product of the Exxon Corporation. The mechanical
gap between amorphous silicon layer 20 and photoconductor 10 is
preferably of the order of 50 to 150 microns. Thereafter, toner
particles 16 are transferred across the fluid filled mechanical gap
to amorphous silicon layer 20 by means of an electric field that is
created when a transfer voltage is applied to conductor 24 by
voltage supply 26. The transfer voltage is typically in the range
of 500 to 2000 volts, with a polarity opposite to that of the toner
particles. Accordingly, the toner particles are attracted to Si
surface 20 by the electric field and remain restricted to areas in
alignment with those on photoconductor 10.
[0046] In the case of crystallization, the toner "imaged" amorphous
silicon layer 20 is now removed and dried before being furnace
treated or subjected to a rapid thermal anneal process to produce
Poly-Si where the toner was imaged. The selective crystallization
of amorphous silicon layer 10 occurs as described by Liu et al. in
U.S. Pat. No. 5,147,826 or Fonash et al. in U.S. Pat. No.
5,275,851, both described above and incorporated by reference
herein. In the case of silicidation, 20 represents the Si surface
(Si wafer, amorphous/poly-Si film) on which a silicide pattern is
to be defined. In this case, the toner transfer prior to annealing
takes place the same way as described above for the crystallization
case.
[0047] FIG. 5a illustrates an alternate fourth step in the process
of the invention wherein the catalyst-containing toner is
transferred from charged photosensitive plate material 10 to
substrate 22, followed by deposition thereon of amorphous
semiconductor layer 20 (FIG. 5b). Then an anneal operation is
performed to achieve selective crystallization of the amorphous
semiconductor layer portions that are in contact with the
toner.
EXAMPLE
[0048] Samples of amorphous silicon layers were prepared by
RF-PECVD from hydrogen diluted silane at 250.degree. C. on Corning
7059 glass. These amorphous Si layers were then imaged in the
following manner:
[0049] 1.) An electrostatic printing plate ESP-4 from the Electrox
Corporation; Newark, N.J. was charged to approximately -1000 v by
means of a corona charge.
[0050] 2.) The plate was developed with palladium toner (Electrox
EPT1-b) by ordinary means.
[0051] 3.) Using 125 micron thick polyester film spacer strips, the
Si coated glass was spaced away from the ESP-4 plate by a
mechanical gap of 125 micron filled with Isopar G (Exxon).
[0052] 4.) With a voltage of -1500 v applied to the amorphous
silicon, the palladium toner particles transferred across the gap
in an orderly, image wise fashion to the amorphous silicon.
[0053] 5.) The toned silicon coated Corning 7059 glass was lifted
off the ESP-4 plate and spacers and the excess Isopar G liquid was
dried.
[0054] 6.) The amorphous Si layer was subjected to a rapid thermal
anneal (RTA) process at 550-600.degree. C. for 5 to 10 minutes at
Penn State University. Poly silicon features were demonstrated in
the areas covered with the palladium toner.
Metal-containing Toner Composition
[0055] An organosol toner was selected for use with the present
invention. A preferred organosol is similar to organosol
compositions reported in U.S. Pat. No. 3,900,412 (G. Kosel). This
patent discloses a class of liquid toners that make use of
self-stable organosols as polymeric binders to promote self-fixing
of a developed latent image. Self-stable organosols are colloidal
(0.1-1 micron diameter) particles of polymeric binder which are
typically synthesized by nonaqueous dispersion polymerization in a
low dielectric hydrocarbon solvent. The organosol particles are
sterically-stabilized with respect to aggregation by the use of a
physically-adsorbed or chemically-grafted soluble polymer. Details
of the mechanism of such steric stabilization are provided by
Napper in "Polymeric Stabilization of Colloidal Dispersions",
(Academic Press: New York, 1983). Procedures for effecting the
synthesis of self-stable organosols, generally involving nonaqueous
dispersion polymerization, are known to those skilled in the art
and are described in detail in "Dispersion Polymerization in
Organic Media", K. E. J. Barrett ed., (John Wiley: New York,
1975).
[0056] In simplified terms, nonaqueous dispersion polymerization is
a free radical polymerization carried out when one or more
ethylenically-unsaturated (typically acrylic) monomers, soluble in
a hydrocarbon medium, are polymerized in the presence of a
preformed amphipathic polymer. The preformed amphipathic polymer,
commonly referred to as the stabilizer, has two distinct functional
blocks, one essentially insoluble in the hydrocarbon medium, the
other freely soluble. When the polymerization proceeds to a
fractional conversion of monomer corresponding to a critical
molecular weight, the solubility limit is exceeded and the polymer
precipitates from solution, forming a core particle. The
amphipathic polymer then either adsorbs onto or covalently bonds to
the core, which continues to grow as a discrete particle. The
particles continue to grow until monomer is depleted. The adsorbed
amphipathic polymer "shell" acts to sterically-stabilize the
growing core particles with respect to aggregation. The resulting
core/shell polymer particles comprise a self-stable, nonaqueous
colloidal dispersion (organosol) comprised of distinct spherical
particles in the size (diameter) range 0.1-1 microns.
[0057] The composition of the insoluble organosol core is
preferentially manipulated such that the organosol exhibits an
effective glass transition temperature (T.sub.g) of less than the
development temperature (typically 23.degree. C.), thus causing a
toner composition containing the organosol as a major component to
undergo rapid film formation (rapid self fixing) in printing or
imaging processes that are carried out at temperatures greater than
the core T.sub.g. Rapid self fixing is a liquid toner performance
requirement to avoid printing defects (such as smearing or loss of
image resolution) in high speed printing. The use of low T.sub.g
resins to promote rapid self fixing of printed or toned images is
known in the art, as exemplified by "Film Formation" (Z. W. Wicks,
Federation of Societies for Coatings Technologies, 1986, p. 8).
[0058] The resulting organosols can be subsequently converted to a
liquid toner by incorporation of the metal catalyst and charge
director, followed by high shear homogenization, ball-milling,
attritor milling, high energy bead (sand) milling or other means
known in the art for effecting particle size reduction in a
dispersion. The input of mechanical energy to the dispersion during
milling acts to break down aggregated particles into primary
particles (0.05-1.0 micron diameter) and to "shred" the organosol
into fragments which adhere to the newly-created metal catalyst
surface, thereby acting to sterically-stabilize the metal particles
with respect to aggregation. The charge director may be physically
or chemically adsorbed onto the metal surface, the organosol or
both. The result is a sterically-stabilized, charged, nonaqueous
metal catalyst dispersion in the size range 0.1-2.0 microns, with
typical toner particle diameters between 0.1-0.5 microns.
[0059] In summarizing the properties of organosol formulations, it
is convenient to denote the composition of each particular
organosol in terms of the ratio of the total weight of monomers
comprising the organosol core relative to the total weight of graft
stabilizer comprising the organosol shell. This ratio is referred
to as the core/shell ratio of the organosol. In addition, it will
be useful to summarize the compositional details of each particular
organosol by ratioing the weight percentages of monomers used to
create the shell and the core. For example, the preferred organosol
can be designated LMA/HEMA-TMI//MMA/EA(97/3-4.7//25/75%w/w), and
comprises a shell composed of a graft stabilizer precursor which is
a copolymer consisting of 97 weight percent lauryl methacrylate
(LMA) and 3 weight percent hydroxyethylmethacrylate (HEMA), to
which is covalently bonded a grafting site consisting of 4.7 weight
percent TMI (dimethyl-m-isopropanol benzylisocyanate, from CYTEC
Industries) based upon the total weight of the graft stabilizer
precursor. This graft stabilizer is subsequently covalently bonded
to an organosol core which is comprised of 25 weight percent methyl
methacrylate (MMA) and 75 weight percent ethyl acrylate (EA). The
weight ratio of core to shell in the preferred organosol is
adjusted to 4.
[0060] The preferred organosol makes use of an LMA/HEMA graft
stabilizer precursor which is similar to the LMA/GMA (glycidyl
methacrylate--precursor described in Example IV of U.S. Pat. No.
3,900,412; however, the grafting site was changed to permit
grafting via formation of a polyurethane linkage between a hydroxyl
group and an isocyanate, as opposed to grafting via formation of an
epoxide linkage between glycidyl methacrylate and methacrylic acid.
The grafting site was changed in order to take advantage of raw
materials already available. In addition, the polymerization of the
preferred organosol was carried out in ISOPAR L (the carrier liquid
selected for use in fabricating toners) using
azobisisobutyronitrile (AZDN from Elf-Atochem) as the free radical
initiator. The AZDN initiator was selected to provide a higher
effective initiator concentration and lower initiator half-life
relative to benzoyl peroxide, thereby limiting the molecular weight
of the graft stabilizer to values below 500,000 Daltons.
[0061] The actual process for making the toner is as follows.
Aldrich Chemical Company sells a number of palladium powders, one
of which (Product #32666-6) is certified 99.9 percent by weight
sub-micron with a number mean diameter of 0.33 micron. A 5 gram
sample of this material was acquired and prepared 120 g of the
following electroless plating toner was formulated.
1 Preferred Organosol: 17 g #32666-6 Colloidal Pd: 2 g Zirconium
HEX-CEM (12%) 1 g ISOPAR L: 100 g
[0062] This toner was milled for 1.5 hours@2000 RPM using 1-2 mm
stainless steel shot. The mean particle size was 0.333 microns. It
appears that milling was effective at reducing the palladium powder
to primary particles.
[0063] An alternate group of materials to serve as metal-containing
toners are metallo-organic decomposition (MOD) compounds. MOD
compounds are pure synthetic metallo-organic compounds, which
decompose cleanly at low temperature to precipitate the metal as
the metallic element. The organic moiety is bonded to the metal
through a heteroatom providing a weak link that provides for easy
decomposition at low temperature. Hence, once heated to 125.degree.
C. or higher, (preferably 125.degree. C. to 250.degree. C.) the
organic constituents evolve out as CO.sub.2 and H.sub.2O or other
hydrocarbon fragments leaving a well-bonded metallic trace on the
surface, where the MOD compound was applied. Therefore, use of a
MOD compound toner in the invention described here excludes the
need for metallic particles. In other words, the function of the
MOD compound is two-fold; (1) electrographic (charging), similar to
organosol, and (2) to contain the metal; the catalyst for
crystallization of an amorphous semiconductor layer (e.g., Pd, Ni,
Cr, Pt) or the essential ingredient for converting a Si surface to
a certain metal-silicide. The MOD compound functions
electrographically by serving as the charge control agent bound to
the particle which forms acid/base couples with the charge director
dissolved in the diluent liquid. Use of a MOD toner is advantageous
for the following reasons. First, the metal is contained chemically
in the toner solution, and therefore the distribution of metal is
very uniform (in the molecular scale). On the other hand, when
metal is included in toner in terms of particles, the resolution of
the metal print will be limited by the particle size as well as how
good the metal particles are distributed. Second, since MOD reduces
to a pure metal upon low temperature annealing, there won't be an
organic residue blocking the diffusion of metal to semiconductor
surface during crystallization or silicidation. For these reasons,
the reproducibility is improved significantly. Examples of several
MOD compounds are; silver neodecanoate, gold amine
2-ethylhexanoate, platinum amine 2-ethyl hexanoate which are listed
in the U.S. Pat. No. 6,036,889, and can be used to print the noble
metals Ag, Au, and Pt, respectively. MOD compounds containing
catalyst metals for crystallization (also applicable to
silicidation) are palladium(II) acetate (Pd(Oac).sub.2),
palladium(II) formate, palladium(II) propionate, palladium(II)
fumarate, palladium(II) stearate, palladium(II) benzoate,
diacetatobis (triphenylphosphine) palladium(II), (U.S. Pat. No.
5,332,646), palladium neodecanoate (U.S. Pat. No. 4,262,040), and
nickel formate Ni(HCO.sub.2).sub.2 (U.S. Pat. No. 3,897,285). Also,
several MOD compounds, which contain catalyst metals for
crystallization are produced by Engelhard Co., NJ, and are sold
under the catalog names 52D (Cr), A6051 (Cr), 58A (Ni), A2985 (Pd),
A1121 (Pt), A6054A (Pt), M603B (Pt).
[0064] The toner may also be composed of metal particles coated
with a MOD compound coating. In this case, the MOD coating need not
even contain the same metal as the particle to be coated. For
instance, 50 nm Pd particles could have a thin layer of silver
neodeconoate as a charge control agent. Once printed and dried, the
Pd particle would have trace levels of silver on it. Then, in
crystallization, the primary catalytic function is performed by the
palladium particle, with the silver, inert to further
processing.
[0065] It should be understood that the foregoing description is
only illustrative of the invention. Various alternatives and
modifications can be devised by those skilled in the art without
departing from the invention. In particular, annealing can be
carried out by variety of means, e.g., furnace annealing, rapid
thermal annealing, inductive heating, microwave heating, etc.
Furthermore, the processed material can be a material other than
Si, e.g., carbon, germanium and alloys thereof. In case of a thin
film material, it may reside on various substrates, e.g., silicon,
metal, glass or plastics. The toner can generally contain the
catalyst (for crystallization, e.g., Pd, Ni, Pt, Cr) or metal (for
silicidation, i.e., metal of desired silicide) either chemically
(the toner is a compound or solution of the metal) or physically
(the metal is contained in the toner in terms of particles) or
both. Accordingly, the present invention is intended to embrace all
such alternatives, modifications and variances which fall within
the scope of the appended claims.
* * * * *