U.S. patent application number 11/240964 was filed with the patent office on 2007-04-05 for laser resist transfer for microfabrication of electronic devices.
This patent application is currently assigned to Eastman Kodak Company. Invention is credited to Yongtaek Hong, David B. Kay, Glenn T. Pearce, Scott E. Phillips, Timothy J. Tredwell, Lee W. Tutt.
Application Number | 20070077511 11/240964 |
Document ID | / |
Family ID | 37667173 |
Filed Date | 2007-04-05 |
United States Patent
Application |
20070077511 |
Kind Code |
A1 |
Tredwell; Timothy J. ; et
al. |
April 5, 2007 |
LASER RESIST TRANSFER FOR MICROFABRICATION OF ELECTRONIC
DEVICES
Abstract
A method for forming a resist pattern on a substrate (18) places
a donor element (12) having a layer of thermoresist material
proximate the substrate. A gap is maintained such that the surface
of the layer of thermoresist material is spaced apart from the
surface of the substrate by a number of spacing elements. Thermal
energy is directed toward the donor element (12) according to the
resist pattern, whereby a portion of thermoresist material is
transferred from the donor element (12) across the gap by ablative
transfer and is deposited onto the substrate (18) forming the
resist pattern.
Inventors: |
Tredwell; Timothy J.;
(Fairport, NY) ; Tutt; Lee W.; (Webster, NY)
; Kay; David B.; (Rochester, NY) ; Hong;
Yongtaek; (Webster, NY) ; Pearce; Glenn T.;
(Webster, NY) ; Phillips; Scott E.; (Rochester,
NY) |
Correspondence
Address: |
Mark G. Bocchetti;Patent Legal Staff
Eastman Kodak Company
343 State Street
Rochester
NY
14650-2201
US
|
Assignee: |
Eastman Kodak Company
|
Family ID: |
37667173 |
Appl. No.: |
11/240964 |
Filed: |
September 30, 2005 |
Current U.S.
Class: |
430/201 |
Current CPC
Class: |
B41M 2205/08 20130101;
H05K 2203/0528 20130101; B41M 5/38207 20130101; H05K 2203/107
20130101; H05K 3/0079 20130101; B41M 5/398 20130101 |
Class at
Publication: |
430/201 |
International
Class: |
G03C 8/00 20060101
G03C008/00 |
Claims
1. (canceled)
2. A method as in claim 14 wherein the spacing elements are
particles on the surface of the substrate.
3. A method as in claim 14 wherein the spacing elements are
particles on the surface of the layer of thermoresist material.
4. A method as in claim 14 wherein the spacing elements are
microbeads on the surface of the substrate.
5. A method as in claim 14 wherein the spacing elements are
microbeads on the surface of the layer of thermoresist
material.
6. A method as in claim 14 wherein the spacing elements are organic
particles on the surface of the layer of thermoresist material.
7. A method as in claim 14 wherein the spacing elements are
microbeads embedded on the surface of the substrate.
8. A method as in claim 14 wherein the spacing elements are
microbeads embedded on the surface of the layer of thermoresist
material.
9. A method as in claim 14 wherein the spacing elements are surface
features on the surface of the substrate.
10. A method as in claim 14 wherein the spacing elements are
surface features on the surface of the layer of thermoresist
material.
11. A method as in claim 14 wherein heat or pressure or both are
applied to enhance contact between the donor element and the
substrate.
12. (canceled)
13. A method as in claim 14 wherein the transfer thermoresist layer
is 0.25 to 10 microns thick.
14. A method for forming a resist pattern on a substrate
comprising: a) placing a donor element comprising a layer of
thermoresist material proximate the substrate; b) maintaining a gap
such that a surface of the layer of thermoresist material is spaced
apart from the surface of the substrate by a plurality of spacing
elements; c) directing thermal energy toward the donor element
according to said resist pattern, whereby a portion of thermoresist
material is transferred from the donor element across the gap by
ablative transfer and is deposited onto the substrate, forming the
resist pattern; wherein the donor element is comprised of a
support, a transfer assist layer, and a transfer thermoresist
layer; and wherein the transfer thermoresist layer comprises
polymeric resins, monomeric glasses, light to heat converting
substance, and beads.
15-19. (canceled)
20. A method as in claim 14 wherein the beads are 1 to 10 microns
diameter preferably 1 to 5 microns diameter.
21. A method as in claim 14 wherein the beads are carbon.
22. A method as in claim 14 wherein the transfer of said pattern of
thermoresist material is caused by optical radiation.
23. A method as in claim 14 wherein the transfer of said pattern of
thermoresist material is caused by thermal heating.
24. A method as in claim 14 wherein the transfer of said pattern of
thermoresist material is caused by pressure.
25. A method as in claim 14 wherein the step of directing a thermal
energy toward the donor element according to the resist pattern
comprises using a mask.
26. A method as in claim 14 wherein the step of directing a thermal
energy toward the donor element according to the resist pattern
comprises energizing a laser.
27. A method according to claim 26 wherein the laser emits
predominantly near IR wavelengths.
28. A method according to claim 14 wherein the spacing elements are
formed in the surface of the donor element.
29. A method as in claim 14 wherein said layer of material not
covered by said resist pattern is plasma etched.
30. A method as in claim 14 wherein vacuum is applied to enhance
contact between the donor element and said substrate.
31. (canceled)
32. (canceled)
33. A method of making microelectronic devices comprising: a)
placing a donor element comprising a layer of thermoresist material
proximate a substrate; b) maintaining a gap such that a surface of
the layer of thermoresist material is spaced apart from the surface
of the substrate by a plurality of spacing elements; c) directing a
thermal energy toward the donor element according to a resist
pattern, whereby a portion of thermoresist material is transferred
from the donor element across the gap by ablative transfer and is
deposited onto the substrate, forming the resist pattern; d)
removing the donor element; e) etching the substrate to remove
material not covered by the resist pattern; and f) removing the
resist pattern.
34. A method as in claim 33 wherein the pattern is optically
transferred from a photomask.
35. A method as in claim 33 wherein the pattern is directly written
from an optically modulated light beam.
36. A method as in claim 33 wherein directing a radiant energy
comprises the step of energizing a laser.
37. A method as in claim 33 comprising the additional step of:
hardening the pattern of resist material prior to etching.
38. A method as in claim 26 wherein the hardening is by optical
radiation.
39. A method as in claim 26 wherein the hardening is by formal
heating.
40. A method as in claim 26 wherein the hardening is by chemical
modification.
41. A method of making microelectronic devices comprising: a)
placing a donor element comprising a layer of thermoresist material
proximate a substrate; b) maintaining a gap such that a surface of
the layer of thermoresist material is spaced apart from the surface
of the substrate by a plurality of spacing elements; c) directing a
thermal energy toward the donor element according to a resist
pattern, whereby a portion of thermoresist material is transferred
from the donor element across the gap by ablative transfer and is
deposited onto the substrate, forming the resist pattern; d)
directing light energy toward the donor element for at least one
additional time, according to the resist pattern; e) removing the
donor element; f) etching the substrate to remove material not
covered by the resist pattern; and g) removing the resist
pattern.
42. A method of making microelectronic devices comprising: a)
placing a donor element comprising a layer of thermoresist material
proximate a substrate; b) maintaining a gap such that a surface of
the layer of thermoresist material is spaced apart from the surface
of the substrate by a plurality of spacing elements; c) directing a
thermal energy toward the donor element according to a resist
pattern, whereby a portion of thermoresist material is transferred
from the donor element across the gap by ablative transfer and is
deposited onto the substrate, forming the resist pattern; d)
directing light energy toward the donor element for at least one
additional time, according to the resist pattern; e) removing the
donor element; f) depositing an overlying layer of material; and g)
removing the resist pattern and material overlying the resist
pattern.
43. A method of making microelectronic devices comprising: a)
placing a first donor element comprising a layer of a first
thermoresist material proximate a substrate; b) maintaining a gap
such that a surface of the layer of the first thermoresist material
is spaced apart from the surface of the substrate by a plurality of
spacing elements; c) directing a thermal energy toward the donor
element according to a resist pattern, whereby a portion of the
first thermoresist material is transferred from the first donor
element across the gap by ablative transfer and is deposited onto
the substrate, forming the resist pattern; and d) replacing the
first donor element with a second donor element comprising a layer
of a second thermoresist material and repeating steps b) and c) to
transfer the second thermoresist material from the second donor
element onto the substrate, forming a resist pattern thereby.
44. The method of claim 43 further comprising: e) depositing an
overlying layer of material onto the resist pattern on the
substrate; and f) removing a portion of the layer of material
overlying the resist pattern.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] Reference is made to commonly-assigned copending U.S. patent
application Ser. No. ______ (Attorney Docket No. 89243/RLO), filed
herewith, entitled PATTERNING OLED DEVICE ELECTRODES AND OPTICAL
MATERIAL by Newman et al.; and U.S. patent application Ser. No.
10/944,586, filed Sep. 17, 2004, entitled METHOD OF FORMING A
STRUCTURED SURFACE USING ABLATABLE RADIATION SENSITIVE MATERIAL, by
Ali et al., the disclosures of which are incorporated herein.
FIELD OF THE INVENTION
[0002] The present invention relates, in general, to fabrication of
microelectronic devices and in particular to fabrication of
microelectronic devices using a dry process with a resist formed by
laser transfer.
BACKGROUND OF THE INVENTION
[0003] Lithographic patterning techniques have been employed in
conventional fabrication of microelectronic devices, including thin
film transistor (TFT) arrays for flat panel application.
Conventional photoresist lithographic techniques applied to
microfabrication have proved capable of defining structures and
forming regions of material on a substrate to within dimensions of
about 100 nm.
[0004] Based on a printing model, the lithographic process forms a
pattern of areas that are either receptive or repellent
(non-receptive) to a coating (such as ink) or to some other
treatment. Conventional photolithography requires a small number of
basic steps, with variations according to the materials used and
other factors. A typical sequence is as follows: [0005] (i) wet
coating of a positive-working or negative-working photoresist (such
as by spin-coating); [0006] (ii) prebake of the photoresist; [0007]
(iii) forming the pattern by exposure to some form of
electromagnetic radiation through an overlay mask using an optical
mask aligner; [0008] (iv) curing of the masked pattern, such as by
postbake; and [0009] (v) removal of the uncured portion, using a
liquid etchant.
[0010] Following subsequent coating or treatment of the surface,
the protective photoresist pattern can then itself be removed.
[0011] Steps (i)-(v) may be performed in air, such as in a clean
room environment, and are typically performed using separate pieces
of equipment. Alternately, one or more steps, such as coating
deposition, may be performed in vacuum. Because of the very
different nature of processes carried out in each of these steps,
it would be difficult and costly to combine steps (i)-(v) in any
type of automated, continuous fabrication system or apparatus.
[0012] Considerable effort has been expended to improve upon
conventional methods as listed in steps (i)-(v) above in order to
achieve better dimensional resolution, lower cost, and eliminate
the use of chemicals such as etchants. One improvement of
particular benefit has been the refinement of plasma etching
techniques that eliminate the need for liquid etchants. With
reference to step (v) listed above, the use of plasma etching is an
enabler for performing microlithographic fabrication in a dry
environment.
[0013] As is well known to those skilled in the microlithographic
art, conventional photoresist materials follow "reciprocity law,"
responding to the total exposure received, the integral of
illumination over time. Among disadvantages of conventional
photoresist use is the requirement for careful control of ambient
illumination until curing is complete. Conventional photoresists
are typically exposed with light in the UV portion of the spectrum,
where photon energy is particularly high. Examples of photoresists
used microfabrication of semiconductor components are given in U.S.
Pat. No. 6,787,283 (Aoai et al.)
[0014] Due to this response characteristic and other shortcomings
of photoresists, another type of resist material, namely
thermoresist material, has proved to be more suitable for
microlithographic fabrication. Thermoresist responds to heat energy
rather than to accumulated exposure over time. Most thermoresists
respond to radiation in the IR and near-IR range and may be
casually termed "IR resists." However, there can also be
alternative thermoresists in the UV region, as described, for
example, in U.S. Pat. No. 6,136,509 (Gelbart).
[0015] The use of thermoresist offers advantages for providing a
dry process alternative to conventional coating (step (i) given
above). Moreover, where a thermoresist pattern can be applied
directly to a substrate using highly focused radiant energy, the
need for masks is eliminated or minimized and both pre-bake and
curing processes (steps (ii) and (iv) above) may no longer
apply.
[0016] Recognizing these advantages, a number of patent disclosures
have proposed thermoresist application using laser energy,
including the following: [0017] U.S. Patent No. 5,858,607 (Burberry
et al.) discloses a method for directly transferring patterning
material from a donor sheet to a lithographic printing plate. A
hydrophilic lithographic printing support such as aluminum or
coated polyester is overlaid with a coated donor sheet. The donor
sheet contains a transfer layer containing a material that absorbs
laser radiation and a polymeric binder having reoccurring units of
the following units of the following formula: ##STR1## [0018]
Wherein R1 represents cyano, isocyanate, azide, sulfonyl, nitro,
phosphoric, phosphonyl, hetroaryl, or R2 is Hydrogen, alkyl or from
the same list as R1 and a receiver element consisting of a support
having a hydrophilic surface such that upon imagewise heating the
binder is transferred to the hydrophilic receiver surface. The
assemblage is image wise exposed with a high intensity laser beam
that transfers binder to receiver. The transfer requires relatively
low exposure, with no chemical or solution processing of the plate
and no post-bake or other post processing.
[0019] U.S. Pat. No. 6,855,384 (Nirmal et al.) discloses a process
for patterning a light emitting polymer, forming the emissive layer
of an organic electroluminescent device. The Nirmal et al. '384
process provides a transfer donor sheet, bringing the donor sheet
into close proximity with a receptor substrate, and selectively
thermally transferring the transfer layer from the donor to the
receptor. The donor sheet includes a substrate and a transfer layer
that includes a blend of a light emitting polymer and an additive.
The additive can be selected to promote high fidelity thermal
transfer of the transfer layer. U.S. Patent Application Publication
No. 2005/0074705 (Toyoda) discloses adhesive transfer of a resist
material from a donor sheet to a substrate. In the Toyoda '74705
method, a layer of resist material is fused or melted onto the
substrate by means of an irradiating beam of energy. The unfused
donor material is then lifted off, leaving the resist pattern
adhered to the substrate surface.
[0020] Advantageously, the methods of the Burberry et al. '607,
Nirmal et al. '384, and Toyoda '74705 disclosures provide a dry
process, eliminating any requirement to coat the substrate
initially with uncured resist material, eliminating or reducing
masking requirements, and allowing the use of plasma etching
techniques. However, in spite of these advantages, some performance
drawbacks remain.
[0021] Adhesive transfer, used in the methods of each of these
disclosures, has inherent limitations for maintaining precise
tolerances. To illustrate this, FIGS. 1 and 2 represent the
adhesive transfer process in simplified schematic form, showing
side views (not to scale). Referring to FIG. 1, in an adhesive
transfer patterning apparatus 10, a donor sheet 70 having a
transfer layer 68, such as a layer of thermoresist material, on a
support 72 is placed directly against a substrate 18. A laser beam
26, or some other form of highly focused radiant energy, is applied
to donor sheet 70. In the immediate area where laser beam 26 is
incident, a photothermal conversion takes place, melting a
corresponding portion of transfer layer 68 and thus adhering this
portion to the surface of substrate 18. Adhesive transfer is
sometimes suitably termed "melt" transfer.
[0022] FIG. 2 shows a familiar problem with adhesive transfer. This
problem occurs as donor sheet 70 is pulled away from substrate 18.
As shown more clearly in the magnified area labeled Q, a feature 74
is formed wherever some portion of transfer layer 68 is adhered to
the surface of substrate 18. Here, feature 74 is a portion of the
thermoresist pattern, with transfer layer 68 being a thermoresist
material. Ideally, at the edge of feature 74 or other structure of
resist material, a clean separation occurs between that portion of
the resist donor that is intended to stick to substrate 18 and the
other portion that is lifted off and "torn away" from adhered
feature 74. One of two problems is possible, however, as
illustrated in FIG. 2. First, as shown by an excess portion 76 in
FIG. 2, a portion of the un-adhered resist donor may not separate
cleanly when overlaying donor sheet 70 is removed. Excess portion
76 may even be inadvertently adhered to the substrate 18 surface,
such as through overheating, or may be torn from the balance of
transfer layer 68. The existence of excess portion 76 may lead to
jagged edges of circuit traces, for example.
[0023] A second possible problem relates to a portion of the
adhered resist transfer layer 68 that is not perfectly affixed to
substrate 18, due to some slight surface imperfection for example,
or due to excessive thickness or strength of the surrounding
un-adhered resist donor on resist transfer layer 68. In FIG. 2, a
torn portion 78 is lifted off along with donor sheet 70. Where torn
portion 78 is small, there may be no perceptible effects. However,
in some cases, this effect could cause jagged edges of surface
features where excessive resist material has been removed.
[0024] Various measures can be taken to lessen the likelihood of
torn portions and to improve overall adhesion bonding. For example,
to counter such effects and obtain clean separation from the donor
sheet, the Toyoda '74705 disclosure even suggests the addition of a
mold-releasing lubricant or other agent in the donor sheet
structure. But because it is not possible to obtain perfect
separation between adhered and un-adhered portions of a pattern,
adhesive transfer, as proposed in the Burberry et al. '607, Nirmal
et al. '384, and Toyoda '74705 disclosures, suffers from inherent
problems in maintaining precision edge definition. This, in turn,
limits the dimensional resolution that can be obtained for a resist
pattern formed using adhesion bonding methods.
[0025] A further, significant disadvantage of adhesive transfer
relates to overall energy level requirements. When a donor sheet is
in flush contact with a receiver substrate, the laser spot
necessarily loses some amount of heat through thermal diffusion.
This effect, in turn, requires the use of higher exposures to
effect any physical change needed for melting and adhesion. Thermal
diffusion can be particularly troublesome when the receiving
substrate is a metal surface with a high thermal conductivity.
[0026] Yet other disadvantages of adhesive transfer relate to the
need for intimate, planar surface contact between donor and
receiver substrate. Accuracy and high resolution require that the
donor be in contact with the receiver during adhesive transfer. The
presence of any type of surface features on the receiver surface
tends to separate the donor from the receiver surface, resulting in
less-than-ideal bonding conditions for precision transfer using
adhesive transfer techniques. Similarly, dust or dirt particles,
inevitable even in controlled "clean room" environments, may settle
between the surfaces of the donor and receiver substrate. Imperfect
adhesion bonding caused by dust or other particulate can have a
pronounced effect, resulting in a drop-out near the point of
contact.
[0027] Thus, there is a need for an apparatus and method for
thermoresist patterning on a substrate using dry media that allows
improved edge definition between adhered resist and surrounding
areas, that does not direct excessive heat levels onto the donor or
receiver substrate, that works well for transfer onto a featured
surface, and that is more robust with respect to dust and dirt than
is conventional adhesive transfer.
SUMMARY OF THE INVENTION
[0028] In response to the need for improved thermoresist
patterning, the present invention provides a method for forming a
resist pattern on a substrate comprising: [0029] a) placing a donor
element comprising a layer of thermoresist material proximate the
substrate; [0030] b) maintaining a gap such that the surface of the
layer of thermoresist material is spaced apart from the surface of
the substrate by a plurality of spacing elements; and [0031] c)
directing a thermal energy toward the donor element according to
the resist pattern, whereby a portion of thermoresist material is
transferred from the donor element across the gap by ablative
transfer and is deposited onto the substrate, forming the resist
pattern.
[0032] The invention and its objects and advantages will become
more apparent in the detailed description of the preferred
embodiment presented below.
BRIEF DESCRIPTION OF THE DRAWINGS
[0033] While the specification concludes with claims particularly
pointing out and distinctly claiming the subject matter of the
present invention, it is believed that the invention will be better
understood from the following description when taken in conjunction
with the accompanying drawings, wherein:
[0034] FIG. 1 is a block diagram showing a patterning apparatus
using conventional adhesive transfer;
[0035] FIG. 2 is a block diagram showing a significant problem with
adhesive transfer upon donor removal;
[0036] FIG. 3 is a block diagram showing a patterning apparatus for
applying a thermoresist pattern according to the present
invention;
[0037] FIG. 4 is a cross-sectional side view, not to scale, showing
components at the donor/substrate interface;
[0038] FIG. 5 is a cross-sectional side view, in an optional
embodiment, showing components at the donor/substrate interface;
and
[0039] FIG. 6 is a block diagram showing process steps for forming
a surface element using the methods and apparatus of the present
invention.
DETAILED DESCRIPTION OF THE INVENTION
[0040] The present invention will be directed in particular to
elements forming part of, or in cooperation more directly with the
apparatus in accordance with the present invention. It is to be
understood that elements not specifically shown or described may
take various forms well known to those skilled in the art.
[0041] Unlike adhesive transfer methods described earlier in the
background section, the patterning process of the present invention
utilizes ablative transfer of a resist material from a donor
element onto the layer to be patterned using thermal or optical
energy. In this process, a donor element, such as a donor sheet or
other article coated with a resist material (organic or inorganic)
is brought into very close proximity to a receiver, such as a TFT
substrate, to be patterned with a layer of a material (such as
aluminum). A scanned laser beam (or laser head) transfers the
resist material from the donor element to the receiver in a
pattern, wherever the donor was exposed. The resist pattern can be
written onto the substrate through a mask. Alternately, a digital
writing or direct-write process, such as that used for silicon
wafers (that is, maskless lithography) can be employed. The resist
pattern formed in this manner need not be further treated or cured
in most applications.
[0042] After removal of the donor element, the substrate continues
into etching, either wet etch or plasma etch. As in conventional
etching, the deposited layer is removed wherever the resist was not
protecting that layer. The resist could then be stripped, such as
by oxygen plasma, and the substrate would continue to further
fabrication steps, such as additional pattern deposition.
[0043] The background section given above described problems with
adhesive transfer of thermoresist material from a donor medium. To
overcome problems inherent to adhesive transfer, the present
invention employs ablative material transfer, across a gap, from
the donor medium to a receiver substrate. The term "ablative
transfer" is broadly understood to be a heat-induced transfer from
the donor medium, wherein at least a portion of a component of the
donor medium is converted to a gaseous state. The material that is
converted to gaseous state may be the resist material itself or may
be some other component material of the donor that thus serves as a
propellant for ablative transfer. In either case, expansion to
gaseous form creates a propellant force that acts as the transfer
mechanism in ablative transfer. The broad classification of
ablative transfer can include sublimation transfer in which some or
all of the resist donor material that is heated is converted from a
solid to a vapor. Ablative transfer also includes fragmentation
transfer or particulate transfer, in which at least some portion of
the donor material may not actually be converted to gaseous state,
but is effectively fragmented and propelled by the conversion to
vapor form of some heated component of the donor. In ablative
transfer, the donor resist material is propelled across a gap
between the surface of the donor and receiver substrate. The
vaporization and gaseous flow mechanisms of ablative transfer
differentiate this transfer method from conventional adhesive
transfer, which relies on intimate contact (that is, having no gap)
between donor and receiver, and which uses some type of melting
that transfers the resist material between donor and receiver.
[0044] Initially developed in order to support colorant transfer
from a donor to a receiver medium, the ablative transfer technique
takes advantage of the gap between the donor and receiver surfaces
to prevent some of the problems inherent to adhesive transfer, as
was described with reference to FIG. 1. In the transfer process,
thermoresist material to be deposited, or some other material in
the donor that serves as a propellant, is heated to a state of
sublimation or ablation, causing partial or full vaporization of at
least some component of the donor, rather than melting of
thermoresist material. Under suitable heating from a laser or other
source, the resulting vapor and/or ablated solids travel across the
gap, from the donor to the receiver surface, in a partially or
fully vaporized form. At the receiver (substrate 18), deposition of
the resulting vapor and ablated solids builds up the surface
features 74 (as were shown in FIG. 2) that form the thermoresist
pattern.
[0045] There are a number of propellant materials that can be used
for ablative transfer. Propellant materials would be disposed
within the donor (for example, in radiation-absorbing layer 36,
FIG. 4) such that heat absorbed by the propellant material from the
laser or light source generates gas or vapor that provides the
force needed to direct the resist material across the gap and onto
the receiving substrate. Materials suitable for this function
include polycyanoacrylates (PCyA) and copolymers of Maleic
anhydride (Gantrez Resins). Other types of polymers may also be
suitable.
[0046] Turning now to FIG. 3, there is shown a patterning apparatus
100 for transferring material from a donor transfer element such as
a donor sheet, which will hereinafter be referred to as a donor
element 12, onto substrate 18 in accordance with the ablative
transfer of the present invention. Laser 14 of the patterning
apparatus 100 can be a diode laser or any other high power laser
that produces laser beam 26. More than one laser 14 or laser beam
26 can be used simultaneously in this invention. In order to scan
laser beam 26 to provide relative movement between laser beam 26
and donor element 12, a galvanometer 22 that includes a moveable
mirror scans the beam through an f-theta lens 24 to form a scan
line in direction X. Those skilled in the art will understand that
scanning the laser beam can also be accomplished by other kinds of
moveable reflective surfaces, such as rotating polygons with mirror
faces, or by other devices such as rotating diffraction gratings.
Alternately, the needed relative movement could be provided by
moving substrate 18 relative to laser beam 26.
[0047] In the embodiment shown in FIG. 3, donor element 12 and
substrate 18 are transported in a direction Y, which is orthogonal
to the scan line, by a translation stage 32 allowing the full area
to be scanned. The intensity of beam 26 at any point in the scan is
controlled by the laser power control line 30 using instructions
from a control logic processor 28. Alternatively, the intensity of
laser beam 26 can be controlled by a separate modulator such as an
acousto-optic modulator (not shown), as is well known by those
skilled in the art of laser optics. In an alternative embodiment,
substrate 18 can remain stationary and the laser writing apparatus
made to move or laser beam 26 redirected optically. The important
feature is that there is relative movement between laser beam 26
and substrate 18 in order to allow full area scanning.
[0048] As shown in FIG. 3, donor element 12 is positioned in
transfer relationship to display substrate 18. Structure,
materials, and fabrication of donor element 12 and substrate 18 are
discussed in more detail below. Donor element 12 and substrate 18
may be held in position by clamping, application of pressure,
adhesives, or the like, with optional heating of either or both
donor element 12 and substrate 18. One example of a fixture for
this positioning is disclosed in commonly-assigned U.S. Pat. No.
6,695,029 (Phillips et al.), the disclosure of which is hereby
incorporated by reference. It is preferred that transfer take place
under an inert atmosphere, such as nitrogen or argon, or under
vacuum. As shown in FIGS. 3 and 4, there is a gap G maintained
between donor element 12 and the portions of substrate 18 to which
material transfer is desired. Spacers 88, such as embedded beads or
other spacing elements, are used to maintain gap G.
[0049] Referring to both FIGS. 3 and 4, F-theta lens 24 focuses
laser beam 26 onto a radiation-absorbing layer 36 of donor element
12 while galvanometer 22 scans the beam. Laser beam 26 must have
sufficient power to heat radiation-absorbing layer 36 to a
temperature high enough to cause ablative transfer so that the
material in a transfer thermoresist layer 38, also termed a resist
layer, is propelled toward display substrate 18, thereby forming a
transferred resist portion 44 on substrate 18. FIG. 4 represents
feature 74 that is to be formed in dotted outline.
[0050] In one embodiment, ablative transfer occurs by partial or
full vaporization of the material in thermoresist layer 38 with
subsequent deposition onto the display substrate 18. The spot size
geometry of f-theta lens 24 dictates the area of thermoresist layer
38 that will be transferred. The arrangement is such that, when
laser beam 26 has sufficient power for a given rate of scan, the
spot size causes material from the irradiated portion of
thermoresist layer 38 to be selectively transferred, across gap G,
from donor element 12 to designated areas similar to "pixels" on
substrate 18. In FIG. 3, laser beam 26 is represented as two spaced
arrows. For convenience of illustration, it should be understood
that laser beam 26 has actually been moved between two different
positions where it is turned on for transferring portions of resist
layer 38.
[0051] In a preferred embodiment (FIG. 3), laser beam 26 is
continuously scanned by galvanometer 22 across donor element 12
while the laser power is modulated by instructions from control
logic processor 28. The modulation of laser power incident on donor
element 12 causes transfer of the resist material in thermoresist
layer 38 in selectable amounts in selected regions of the scan to
substrate 18. In a preferred embodiment, most or all of the
material in resist layer 38 is transferred to substrate 18.
[0052] Laser 14 must be matched to suit the composition of the
thermoresist material in resist layer 38 and to suit other
materials used for ablative transfer in radiation-absorbing layer
36. For most types of thermoresists that can be used, laser 14 can
be an infrared solid-state laser, a neodynium YAG laser or any
other laser providing a sufficient power to effect transfer of the
resist layer. The power necessary depends on matching the
absorption of radiation-absorbing layer 36 with the wavelength of
laser 14. The spot shape may be oval to allow small lines to be
written while using low cost multimode lasers, as taught in
commonly-assigned U.S. Pat. No. 6,252,621 (Kessler et al.), the
disclosure of which is hereby incorporated by reference.
[0053] In the preferred embodiment, donor element 12 used in the
transfer process comprises a donor support 34 that is transmissive
to the laser light, radiation-absorbing layer 36 for transfer
assist that converts the laser light into heat, and resist layer 38
to be transferred. In separate embodiments, resist layer 38 can
also act as a radiation-absorbing layer in addition to layer 36, or
alternatively, layer 36 can be eliminated. As yet another
alternative, support layer 34 can also serve as the
radiation-absorbing layer, and separate layer 36 can be eliminated.
Examples of donor support and radiation-absorbing materials that
can be used in this invention can be found in U.S. Pat. No.
4,772,582 (DeBoer). Donor support 34 must be capable of maintaining
the structural integrity during the light-to-heat-induced transfer.
Support materials meeting these requirements include, for example,
glass, metal foils, plastic (polymeric) foils, and fiber-reinforced
plastic foils. Plastic foils are highly advantaged for most
applications. While selection of suitable support materials can be
based on their suitability to handling the incident laser
radiation, using known engineering approaches, it will be
appreciated that other aspects of a selected support material may
merit special consideration when configured as a donor support for
the present invention. For example, some support materials may
require a multistep cleaning and surface preparation process prior
to precoating with transferable resist material.
[0054] The material used in radiation-absorbing layer 36 may be a
dye such as the dyes specified in commonly assigned U.S. Pat. No.
4,973,572 (DeBoer) and U.S. Pat. No. 5,578,416 (Tutt), or a pigment
such as carbon black. Radiation-absorbing layer 36 may be a metal
such as chromium, nickel or titanium, or a layered stack of
materials that absorb radiation by virtue of their antireflection
properties. The main criteria is that radiation-absorbing layer 36
absorb laser light, at the given wavelength, at a high enough
intensity for transfer of material from resist layer 38. The
efficiency of this transfer is well known to depend on the laser
fluence, spot size, beam overlap and other factors. Generally, the
optical density of radiation-absorbing layer 36 should be at least
0.1 (20% of the light is absorbed).
[0055] Radiation-absorbing layer 36, in addition to having a
light-absorbing material, may preferably contain polymeric binders
or amorphous organic solids. These binders and solids are
preferably thermally labile or gas-producing substances such as
polycyanoacrylate, nitrocellulose, copolymers of maleic anhydride,
and materials disclosed in U.S. Pat. No. 6,190,827 (Weidner) and
U.S. Pat. No. 6,165,671 (Weidner et al.) and references cited
therein, as components of a propellant layer in laser donor
elements. Radiation-absorbing layer 36 may also contain other
polymeric and organic solids necessary to ensure the physical
integrity of the layer.
[0056] Resist layer 38 may comprise a polymeric binder, an
amorphous organic solid, and a particle or spherical bead.
Alternatively, resist layer 38 may also contain a mixture of these
materials and radiation-absorbing materials and thermally labile
substances as described with reference to radiation-absorbing layer
36.
[0057] The polymeric binders of layer 38 may include polymers known
to provide resist properties in lithographic processes, such as
Shipley G-Line or Microposit.TM. photoresists from Shipley Company,
Marlborough, Mass., and including materials such as
polymethacrylates and acrylates, polystyrene and copolymers with
butadiene, acrylates and methacrylates, acrylonitrile, polyesters,
polyamides, polyimides, polyvinylchlorides and copolymers with
vinyl acetate, polyvinyl esters, polyurethanes, polyvinyl acetals
and butyrals, epoxides, polyimides, Novolac resins, polyvinyl
phenol resins, and the like.
[0058] The amorphous organic solids of resist layer 38 may be
monomeric resins as described in previously cited U.S. Pat. No.
6,165,671, such as hydrogenated and partially hydrogenated rosin
esters and similar rosin esters. Commercially available materials
include the glycerol ester of hydrogenated wood rosin, such as
Staybellite Ester 10 (Hercules), the glycerol ester of hydrogenated
rosin, such as Foral 85 (Hercules) and the pentaerythritol ester of
modified rosin, such as Pentalyn 344 (Hercules). The amorphous
organic solids of layer 38 may also include monomeric glassy solids
as described in commonly-assigned U.S. Pat. No. 5,891,602
(Neumann), as binder elements in a dye donor laser transfer sheet.
The amorphous organic solids of layer 38 may also be oligomeric
resins with a molecular weight of less than about 4000 such as the
polyester tone 260.
Spacing Elements
[0059] The particles, beads, or other spacing elements, such as
spacers 88 in FIG. 4, used between donor element 12 and substrate
18 may be any of a number of types of regularly or irregularly
shaped or spherical particle, either organic or inorganic in
composition. Spacing elements can be of a size between about 0.1
and 100 microns, and are preferably of somewhat uniform size in the
range of about 1 to 20 microns in diameter.
[0060] The use of spacers for maintaining gap G between donor
element 12 and substrate 18 is described in more detail in commonly
assigned U.S. Patent Application Publication No. 2003/0162108
(Burberry et al.); the disclosure of which is incorporated herein
by reference. Examples of spacing beads are disclosed in U.S. Pat.
No. 4,876,235 (DeBoer).
[0061] Spacers 88 or other spacing elements may be embedded in
resist layer 38 or may be formed within radiation-absorbing layer
36 or donor support 34, or may be formed within any combination of
these three layers. Spacing elements could alternately be formed on
the surface of substrate 18 or could be introduced onto either
surface prior to resist pattern transfer processing. As yet another
alternative, the roughness of either donor element 12 or substrate
18 could be used to provide gap G.
[0062] Any of the layers of donor element 12 may include, in
addition to the materials already disclosed, surface active agents
necessary as coating aids and used for the modification of surface
properties, hardeners, adhesion promoting agents, and the like,
necessary for the physical integrity and manipulation of the
manufactured devices. Dyes and pigments may also be added to any of
the layers of donor element 12 to provide process
visualization.
[0063] The donor element 12 substrate can be any self supporting
material including sheet material, for example, metal or polymer
film. The preferred embodiment of this invention the receiver
support is Polyester (such as Estar.TM.).
[0064] Examples of material that could be patterned are all
etchable metals or conductors, semiconductors, dielectrics and
polymers. Examples that follow show materials that can be patterned
using thermoresist and laser etching. Some metals or conductors
commonly used with these resists are aluminum, titanium, silver,
gold, chromium, tin, ITO, platinum, etc. Semiconductors commonly
used include poly silicon, amorphous silicon, doped silicon,
gallium arsenide (GaAs), gallium nitride (GaN), Indium antimonide
(InSb), germanium, and others. Some commonly used dielectrics
include single or multiple layers of combinations of silicon
nitride, silicon oxide, silicon oxynitride, metal oxide including
aluminum oxide, titanium oxide, and organic materials including
benzocyclobutene (BCB), spin-on glass, acrylic, Teflon, polyimide,
and others. Commonly used polymers include cellulose triacetate,
poly thiophene, etc.
[0065] The receiver substrate can be any self-supporting material
including, for example, metal, glass, silicon, and polymer film. In
one embodiment of this invention, substrate 18 is glass.
[0066] The transfer atmosphere can be vacuum, atmosphere, or ultra
low humidity environments. The lighting can be complete darkness,
yellow light (no UV), full spectrum light or standard room lighting
with incandescent or fluorescent lights.
[0067] The etch process used can be done in vacuum, using plasma
with a reactive ion etch, with a plasma torch, or with wet etches.
Examples of etch gases used for plasma etching include Cl.sub.2,
BCl.sub.3, SF.sub.6, O.sub.2, H.sub.2, CH.sub.4, chloroform and
N.sub.2. Exemplary wet etches include HCl, nitric acid, acidic
acid, phosphoric acid, ferric chloride, and ferric nitrate.
EXAMPLE 1
[0068] A donor element comprising a 102 microns polyethylene
teraphthalate support containing a subbing layer of 0.43 g/m2
gelatin hardened with dihydroxydioxane was subsequently coated with
a transfer assist layer containing (0.332 g/m2) polycyanoacrylate,
(0.054 g/m2) IR dye 1, with the following structure: ##STR2## from
a mixture of cyclopentanone, N-methylpyrollidone, and methanol with
0.01 wt % surfactant Silwet L7001 (GE Silicones). This assembly was
then overcoated with the resist transfer layer comprising (1.08
g/m2) Foral 85 (hydrogenated rosin ester, Hercules Corp.) binder,
(1.08 g/m2) Organic Pigment T-11 from DayGlo Corporation (solid dye
solution in a Thermoset Resin Bead of 4-5 micron diameter), (0.118
g/m2) polyvinyl butyral, (0.027 g/m2) Rhodamine 6G dye, and DC1248
(Dow Corning), a silicone surfactant at 0.02 wt % of the solution,
from a mixture of methyl ethyl ketone and ethyl alcohol.
[0069] Donor element 12 was placed on substrate 18, which was
composed of a glass plate coated with 500 angstroms of aluminum,
with the resist coated side adjacent to the aluminum on the
substrate surface. Laser transfer of thermoresist material was
effected from donor element 12 to substrate 18 by irradiation of
donor element 12 through substrate 18 with an infrared laser beam.
The beam size was approximately 16 micrometers by 80 micrometers to
the 1/e.sup.2 point scanning along the wide beam direction. The
dwell time was 27 microseconds with a suitable power density.
Transfer was effected in regions, which were desired to be covered
by the resist material, and in which the aluminum was to remain
after etching.
[0070] Donor element 12 was removed after resist transfer, and the
sample placed in a Lam model 4600 chlorine based plasma etch tool,
product of Lam Research Corporation, Fremont, Calif. The pattern
was etched into the metal layer. The sample was then placed in an
oxygen plasma asher and the remaining thermoresist material
removed. This process transferred the laser resist pattern into the
aluminum. Removal of the resist resulted in an aluminum pattern
with high fidelity with a completely dry patterning process. Some
pinholes were observed where the beads shadowed the transfer
process.
EXAMPLE 2
[0071] For this two-pass example, the steps of Example 1 were
completed in a first pass. Then, as the second pass, a second
resist material was applied, this time where no spacer particles or
beads were used between donor element 12 and substrate 18. This is
permissible since existing structures formed in the first pass
provide the needed gap G (FIG. 4). That is, for the second pass,
existing surface features 74 that were formed during the first pass
effectively act as spacers, eliminating the need for spacers 88 as
were shown in FIG. 4. Donor element 12 for the second pass used a 4
mil PET support, coated with a first subbing layer of tetrabutoxy
titanate (Tyzor TBT, Dupont) coated from a mixture of n-butanol and
n-propyl acetate, and a second transfer layer of 0.16 g/m2
cellulose acetate propionate, 0.054 g/m.sup.2 IR-2, 0.077 g/m2 Cyan
Dye 1, with structure: ##STR3## 0.075 Magenta Dye 1, with
structure: ##STR4## 0.069 g/m2 Yellow Dye 1, with structure:
##STR5## coated from a mixture of MIBK (40%), n-propyl acetate
(20%), proylene glycol monomethyl ether (20%) and methanol
(10%).
[0072] This second resist transfer was applied directly over the
first resist pattern such that the second transfer exactly matched
the first pattern. The registration was accomplished by replacing
the first donor element 12 with the second donor element 2, while
not removing receiver substrate 18 from the laser stage platform,
then performing the laser-activated transfer using the same pattern
as was used for the first pattern. After the second transfer, the
display substrate was subjected to the same chlorine etch and O2
plasma cleaning as in Example 1. This secondary transfer mitigated
bead shadowing, resulting in improved conductor line quality.
EXAMPLE 3
[0073] This example followed the same process steps as those
described for Examples 1 or 2, with minor exceptions, as follows:
[0074] (i) the coated material on the glass substrate is poly
silicon; and [0075] (ii) the etch gas atmosphere is sulfur
hexafluoride.
EXAMPLE 4
[0076] For this next example, an overcoat was applied to donor
element 12 and radiation-absorbing layer 36 of Example 1. The
overcoat was a thermoresist transfer layer comprising 1.51 g/m2
Staybilite 10 (a Rosin Ester), 0.116 g/m2 polyvinylbutyral,
0.016g/m2 IR Dye 2, 0.054 g/m2 Cyan Dye 1, 0.0038 g/m2
divinylbenzene beads (4 microns diameter) and DC 1248 surfactant
from Dow Corning at 0.036 wt % of the coating solution, with
methylethyl ketone solvent. This layer was transferred in a pattern
to substrate 18 as in Example 1, and a second identical pattern was
transferred, without separating elements, as in Example 2. The
chlorine etch followed by the O2 plasma cleaning was performed as
in Examples 1 and 2 to provide the aluminum conductor pattern.
Observations
[0077] In various embodiments transfer thermoresist layer 38 is
comprised of one or more polymeric resins, 10 grams per meter
square, preferably 1 to 2 grams per meter square of molecular
weight 1000 to 2 million preferably 50,000 to 200,000. The transfer
thermoresist layer can be composed of monomeric glasses with a
typical molecular weight of 250 to 1000 and typical glass
transition temperatures ranging from 25 to 175 degrees centigrade,
preferably 40 to 100 degrees centigrade. As spacers 88 (FIG. 4),
transfer thermoresist layer can include beads, typically 1 to 10
microns diameter, preferably 1 to 5 microns diameter. Beads of
various materials, such as carbon, could be used, as described
previously.
[0078] As Example 2 and subsequent examples show, there can be
significant value in performing multiple-pass deposition using the
method of the present invention, each pass with a fresh donor
element 12. The pattern traced by laser beam 26 is the same for the
first pass, the second pass, and any subsequent passes. The benefit
of multiple pass deposition relates to shadowing effects caused by
spacers 88. That is, a tiny "pinhole" can be caused where a
particular spacer 88 obstructs a portion of feature 74 so that
transferred resist layer 44 (FIG. 4) cannot be properly formed in a
single pass. Where spacers 88 are used for the second pass, it is
unlikely that the same area of substrate 18 will be blocked by a
spacer 88 on two successive passes.
[0079] An alternative to spacers 88 for second and subsequent
passes is noted in Example 2, above. That is, once some portion of
features 74 are deposited, the surface of substrate 18 may be able
to maintain sufficient gap G distance without requiring spacers 88.
In yet another embodiment, spacing elements can be provided by
surface roughness itself, at either the surface of substrate 18 or
the surface of donor element 12. In one embodiment, the surface of
resist layer 38 is featured with peaks 84 so that donor element 12
effectively supports itself over substrate 18, maintaining gap G as
shown in FIG. 5.
[0080] By maintaining gap G between donor element 12 and receiver
substrate 18, the ablative transfer method of the present invention
offers advantages over other transfer methods such as the adhesion
transfer method described in the background section given earlier.
One significant advantage relates to thermal isolation. Heat that
is generated in order to effect transfer is effectively buffered or
insulated from reaching the surface of receiver substrate 18 by
air, or other gas, within gap G. This is particularly beneficial
where substrate 18 has high thermal conductivity, such as would be
true of a metallic material, for example. A highly thermal
conductive surface could draw excessive amounts of heat from the
transfer area, even inhibiting transfer in some cases. This
inherent thermal isolation also makes ablative transfer more
promising for use with organic materials or flexible substrates
that may be sensitive to heat. Yet another advantage of ablative
transfer relates to the tendency of the donor material to stick to
the wrong surface, as was described with reference to adhesion
transfer in FIG. 2. In ablative transfer, a gap distance is
maintained between the donor and receiving surface, minimizing any
bonding between deposited and non-deposited donor material, thus
reducing or eliminating the likelihood that, when the donor sheet
is pulled away, donor material is unintentionally removed from the
receiving surface or that excessive donor material is removed from
the donor element and clings to the receiving surface.
Fabrication System
[0081] Referring to FIG. 6, there is shown a block diagram of the
steps used in a fabrication system for applying a pattern of a
material 92 onto substrate 18 using the method of the present
invention. The process executed by such a fabrication system is a
dry fabrication process, eliminating drying stages, liquid
etchants, and liquid coating processes. In an entry step 110,
substrate 18 has a support 94 coated with material 92. In a
thermoresist deposition step 120, donor element 12 is positioned in
proximity to substrate 18, separated by gap G. Laser 14 writes
feature 74, a trace line in the example shown, transferring
thermoresist material from donor element 12 onto substrate 18. In a
donor removal step 130, donor element 12 is removed. Thermoresist
deposition step 120 and donor removal step 130 may be repeated for
multiple passes, as described earlier. A material etching step 140
etches material 92 from the surface of support 94, except where
protected by thermoresist, that is, beneath feature 74. In a resist
etching step 150, feature 74 is itself etched from the surface,
leaving a trace 96, or other patterned element, that is made of the
original material 92.
[0082] The sequence of FIG. 6 is greatly simplified, showing only
the basic process for forming a single element onto substrate 18.
As would be clear to one skilled in the microfabrication arts, the
processes shown in FIG. 6 would be repeated a number of times in
order to form circuit components onto substrate 18.
[0083] The invention has been described in detail with particular
reference to certain preferred embodiments thereof, but it will be
understood that variations and modifications can be effected within
the scope of the invention. For example, while most thermoresists
are activated by IR or near-IR radiation, this method could also be
used with a thermoresist activated by sufficiently intense visible
or UV light. Laser scanning mechanisms could include flat platen as
well as drum embodiments, such as a vacuum drum, for example.
Auxiliary cleaning steps might be needed following thermoset
deposition (with respect to FIG. 6, following thermoresist
deposition step 120), including the use of etchant. The method of
the present invention can be used with either plasma or liquid
etchants, as is most suitable for the device being fabricated.
Material 92 (FIG. 6) could be a metal, organic or inorganic
semiconductor, or an organic or inorganic dielectric material, for
example.
[0084] Thus, what is provided is an apparatus and method for
fabrication of microelectronic devices using a dry process with a
resist formed by ablative laser transfer.
Parts List
[0085] 10 patterning apparatus [0086] 12 donor element [0087] 14
laser [0088] 18 substrate [0089] 22 galvanometer [0090] 24 lens
[0091] 26 laser beam [0092] 28 control logic processor [0093] 30
control line [0094] 32 translation stage [0095] 34 donor support
[0096] 36 radiation-absorbing layer [0097] 38 resist layer [0098]
44 transferred resist portion [0099] 68 transfer layer [0100] 70
donor sheet [0101] 72 support [0102] 74 feature [0103] 76 excess
portion [0104] 78 torn portion [0105] 88 spacer [0106] 92 material
[0107] 94 support [0108] 96 trace [0109] 100 patterning apparatus
[0110] 110 entry step [0111] 120 thermoresist deposition step
[0112] 130 donor removal step [0113] 140 material etching step
[0114] 150 resist etching step
* * * * *