U.S. patent application number 10/978323 was filed with the patent office on 2005-05-26 for method for making sol gel spacers for flat panel displays.
This patent application is currently assigned to Micron Technology, Inc.. Invention is credited to Hofmann, James J., Kraus, Brenda D., Vaartstra, Brian A., Westmoreland, Donald L..
Application Number | 20050112298 10/978323 |
Document ID | / |
Family ID | 33300165 |
Filed Date | 2005-05-26 |
United States Patent
Application |
20050112298 |
Kind Code |
A1 |
Hofmann, James J. ; et
al. |
May 26, 2005 |
Method for making sol gel spacers for flat panel displays
Abstract
The present invention describes thick film photolithographic
molds, methods of making thick film photolithographic molds, and
methods of using thick film photolithographic molds to form spacers
on a substrate. The thick film photolithographic molds preferably
comprise an epoxy bisphenol A novolac resin. The present invention
also describes sol gel spacers comprising sodium silicates and
potassium silicates. The thick film photolithographic molds and sol
gel spacers of the present invention can be used in flat panel
displays, such as field emission displays and plasma displays.
Inventors: |
Hofmann, James J.; (Boise,
ID) ; Vaartstra, Brian A.; (Nampa, ID) ;
Kraus, Brenda D.; (Boise, ID) ; Westmoreland, Donald
L.; (Boise, ID) |
Correspondence
Address: |
WILMER CUTLER PICKERING HALE AND DORR LLP
60 STATE STREET
BOSTON
MA
02109
US
|
Assignee: |
Micron Technology, Inc.
Boise
ID
|
Family ID: |
33300165 |
Appl. No.: |
10/978323 |
Filed: |
November 1, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10978323 |
Nov 1, 2004 |
|
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09560260 |
Apr 26, 2000 |
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6812990 |
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Current U.S.
Class: |
428/1.5 ;
313/238; 349/155 |
Current CPC
Class: |
Y10T 428/1059 20150115;
G02F 1/1339 20130101; B29C 33/3857 20130101; G03F 7/0017 20130101;
H01J 2211/36 20130101; G02F 1/13394 20130101; C09K 2323/05
20200801 |
Class at
Publication: |
428/001.5 ;
349/155; 313/238 |
International
Class: |
G02F 001/1339 |
Goverment Interests
[0001] The present invention was made with Government support under
Contract No. DABT63-97-C-0001 awarded by the Advanced Research
Projects Agency (ARPA). The Government may have certain rights in
this invention.
Claims
What is claimed is:
1. A mold for forming spacers on a flat panel display comprising a
photolithographic material.
2. The mold of claim 1, wherein the photolithographic material is
an epoxy bisphenol A novolac resin.
3. The mold of claim 2, wherein the epoxy bisphenol A novolac resin
is SU-8.
4. The mold of claim 2, further comprising a crosslinking
agent.
5. The mold of claim 4, wherein the crosslinking agent is a
polyol.
6. A mold for forming spacers on a flat panel display comprising a
photolithographic material, wherein the photolithographic material
comprises anepoxy bisphenol A novolac resin and a crosslinking
agent.
7. The mold of claim 6, wherein the epoxy bisphenol A novolac resin
is SU-8.
8. The mold of claim 6, wherein the crosslinking agent is a
polyol.
9. The mold of claim 8, wherein the polyol is at least one of triol
and a diol.
10. The mold of claim 9, wherein the polyol is a diol.
11. The mold of claim 10, wherein the diol is at least one of a
propanediol, a hexanediol and an ethylene glycol.
12. The mold of claim 10, wherein the ratio of terminal groups in
the epoxy bisphenol A novolac resin to terminal groups in the diol
is about 1.5 to about 1.
13. A mold for forming spacers on a flat panel display, wherein the
mold is prepared by the process comprising: preparing a composition
comprising a photolithographic material; applying the composition
on a support; applying a photomask over the composition; curing
exposed portions of the composition; and removing un-cured portions
of the composition to reveal the mold.
14. The mold of claim 13, wherein the photolithographic material is
an epoxy bisphenol A novolac resin.
15. The mold of claim 14, wherein the epoxy bisphenol A novolac
resin is SU-8.
16. The mold of claim 13, wherein the composition further comprises
a crosslinking agent.
17. The mold of claim 16, wherein the crosslinking agent is a
diol.
18. The mold of claim 17, wherein the diol is propanediol,
hexanediol or ethylene glycol.
19. The mold of claim 13, wherein the composition further comprises
a solvent and a photoinitiator.
20. The mold of claim 19, wherein the photolithographic material is
present in an amount of 200-600 parts by weight, wherein the
solvent is present in an amount of 200-600 parts by weight, and
wherein the photoinitiator is present in an amount of about
1-100parts by weight.
21. The mold of claim 13, further comprising priming the support
with a priming agent prior to applying the composition on the
support.
22. The mold of claim 13, wherein the mold is a negative mold.
23. The mold of claim 13, wherein the mold is a positive mold.
24. A method for adjusting the adhesion between a support and a
photolithographic material comprising: providing a support;
applying a priming agent to the support; preparing a composition
comprising a photolithographic material; and applying the
composition on the support.
25. The method of claim 24, wherein the priming agent comprises at
least one polyol.
26. The method of claim 25, wherein at least one polyol is
triethanol amine or ethylene glycol.
27. The method of claim 24, wherein the priming agent is a silane
bonding agent.
28. The method of claim 27, wherein the silane bonding agent
comprises a silane, a cyclohexane and an epoxy group.
29. The method of claim 27, wherein the silane bonding agent is
2(3,4-epoxycyclohexyl)ethyltrimethoxysilane.
30. The method of claim 24, wherein the priming agent is at least
one of phosphoric acid, hydrochloric acid, NH.sub.4OH, KOH, HF, and
NH.sub.4OH:H.sub.2O.sub.2.
31. The method of claim 24, wherein the photolithographic material
comprises an epoxy bisphenol A novolac resin.
32. The method of claim 31, wherein the epoxy bisphenol A novolac
resin is SU-8.
33. The method of claim 31, wherein the photolithographic material
further comprises a crosslinking agent.
34. A method for forming a negative mold for forming spacers on a
flat panel display comprising: providing a positive mold comprising
at least one post and a photolithographic material; covering the
positive mold with a composition; allowing the composition to
harden; and removing the positive mold to reveal the negative mold
comprising the hardened composition.
35. The method of claim 34, wherein the photolithographic material
comprises an epoxy bisphenol A novolac resin.
36. The method of claim 35, wherein the epoxy bisphenol A novolac
resin is SU-8.
37. The method of claim 35, wherein the photolithographic material
further comprises a crosslinking agent.
38. The method of claim 34, wherein the composition comprises
latex.
39. A mold for forming spacers on a flat panel display prepared by
the process comprising: adding a dry photolithographic material to
a mixture comprising a solvent and a photoinitiator; applying the
mixture to a support; heating the mixture until the mixture forms a
flowable composition on the support; applying a photomask over the
composition; curing exposed portions of the composition; and
removing un-cured portions of the composition to reveal the
mold.
40. The mold of claim 39, wherein the photolithographic material
comprises an epoxy bisphenol A novolac resin.
41. The mold of claim 40, wherein the epoxy bisphenol A novolac
resin is SU-8.
42. The mold of claim 40, wherein the photolithographic material
further comprises a crosslinking agent.
43. The mold of claim 39, wherein the photolithographic material is
present in an amount of 200-600 parts by weight, wherein the
solvent is present in an amount of 200-600 parts by weight, and
wherein the photoinitiator is present in an amount of about
1-100parts by weight.
44. The mold of claim 39, further comprising priming the support
with a priming agent prior to applying the mixture on the
support.
45. The mold of claim 44, wherein the priming agent comprises at
least one polyol.
46. The mold of claim 44, wherein the priming agent comprises at
least one of triethanol amine, ethylene glycol,
2(3,4-epoxycyclohexyl)ethyltrimetho- xysilane, phosphoric acid,
hydrochloric acid, NH.sub.4OH, KOH, HF, and
NH.sub.4OH:H.sub.2O.sub.2.
47. A method for forming a mold comprising: providing a support;
placing alignment marks on the support; applying a thick film
photolithographic material on the support, wherein the thick film
photolithographic material is positioned within the alignment marks
on the support; and curing the thick film photolithographic
material to form the mold.
48. The method of claim 47, wherein the photolithographic material
comprises an epoxy bisphenol A novolac resin.
49. The method of claim 48, wherein the epoxy bisphenol A novolac
resin is SU-8.
50. The method of claim 48, wherein the photolithographic material
further comprises a crosslinking agent.
51. The method of claim 47, further comprising priming the support
with a priming agent prior to applying the thick film
photolithographic material on the support.
52. A method for forming a mold comprising: providing a support;
placing alignment marks on the support; applying a transparent
photolithographic material over the support; placing a photomask on
the photolithographic material, wherein the photomask is positioned
within the alignment marks on the support; curing the
photolithographic material; and removing the un-cured
photolithographic material to reveal the mold.
53. The method of claim 52, wherein the photolithographic material
comprises an epoxy bisphenol A novolac resin.
54. The method of claim 53, wherein the epoxy bisphenol A novolac
resin is SU-8.
55. The method of claim 53, wherein the photolithographic material
further comprises a crosslinking agent.
56. The method of claim 52, further comprising priming the support
with a priming agent prior to applying the transparent
photolithographic material on the support.
57. A method of forming at least one spacer on a substrate
comprising: applying a negative mold on a substrate, wherein the
negative mold comprises a photolithographic material, and at least
one hole; applying a spacer material to the negative mold, wherein
the spacer material fills the at least one hole; hardening and
bonding the spacer material to the substrate; and removing the
negative mold from the substrate, wherein the hardened spacer
material remains bonded to the substrate.
58. The method of claim 57, wherein the method is performed in a
vacuum.
59. The method of claim 57, wherein the photolithographic material
comprises an epoxy bisphenol A novolac resin.
60. The method of claim 59, wherein the epoxy bisphenol novolac
resin is SU-8.
61. The method of claim 57, wherein the spacer material comprises a
sodium silicate and a potassium silicate.
62. The method of claim 57, wherein the substrate is the face plate
or the base plate of a flat panel display.
63. A method of forming a spacer on a substrate comprising:
preparing a mold comprising at least one hole; filling the at least
one hole in the mold with a spacer material; contacting the mold
with a substrate; allowing the spacer material to harden and bond
to the substrate; and removing the mold from the substrate, wherein
the hardened spacer material remains bonded to the substrate.
64. The method of claim 63, wherein the spacer material comprises a
sodium silicate and a potassium silicate.
65. The method of claim 63, wherein the substrate is the face plate
or the base plate of a flat panel display.
66. A spacer for a flat panel display comprising sodium silicate
and potassium silicate.
67. A method for preparing a spacer for a flat panel display
comprising: preparing a composition comprising formamide, potassium
silicate and sodium silicate; allowing the composition to form a
gel; and firing the gel to produce the spacer.
68. A flat panel display comprising: a face plate; a base plate;
and at least one spacer between the face plate and the base plate,
wherein the spacer comprises a sodium silicate and a potassium
silicate.
69. A method of forming spacers on a display comprising: providing
a resilient negative mold, the negative mold comprising a first
layer and a second layer, the first layer being characterized by a
first durometer, the second layer being characterized by a second
durometer, the second durometer being higher than the first
durometer, the first layer defining a plurality of apertures and a
plurality of vacuum pump out channels; placing the negative mold in
contact with a substrate; using the vacuum pump out channels to
remove gas from between the substrate and the negative mold;
filling the apertures with a spacer material; allowing some of the
spacer material to cure and to become attached to the substrate;
separating the substrate from the negative mold.
70. A method according to claim 69, wherein the negative mold
comprises an alignment sheet, the alignment sheet defining at least
two alignment apertures.
71. A method according to claim 70, wherein the alignment sheet is
metallic.
Description
FIELD OF THE INVENTION
[0002] The present invention describes thick film photolithographic
molds, methods of making thick film photolithographic molds, and
methods of using thick film photolithographic molds to form spacers
on a substrate. The thick film photolithographic molds preferably
comprise an epoxy bisphenol A novolac resin. The present invention
also describes sol gel spacers comprising sodium silicates and
potassium silicates. The thick film photolithographic molds and sol
gel spacers of the present invention can be used in flat panel
displays, such as field emission displays and plasma displays.
BACKGROUND OF THE INVENTION
[0003] FIG. 1 shows a cross sectional view of a portion of a prior
art FED 100. FED 100 includes a cathode, or baseplate 102 and an
anode, or faceplate 104. Baseplate 102 includes a substrate 106, a
plurality of conical field emitters 108, an insulating layer 110,
and a conductive grid layer 112. Insulating layer 110 is disposed
over substrate 106, and grid layer 112 is disposed over insulating
layer 110. Insulating layer 110 defines a plurality of void regions
114, and each emitter 108 is disposed over substrate 106 in one of
the void regions 114. Grid layer 112 defines a plurality of
apertures 116. Each aperture 116 corresponds to, and overlies, one
of the void regions 114. The apertures 116 are positioned so that
(1) the grid layer 112 does not obstruct a path 117 between the
upper tips of the emitters 108 and the faceplate 104 and (2) a
portion of the grid layer 112 is proximal to the tip of each
emitter 108. Baseplate 102 also includes a plurality of conductive
row and column lines 118 disposed between emitters 108 and
substrate 106.
[0004] Faceplate 104 includes a glass plate 120, a transparent
conductor 122, and a phosphor layer 124. Transparent conductor 122
is disposed on one major surface of glass plate 120, and phosphor
layer 124 is disposed on transparent conductor 122. The faceplate
104 and baseplate 102 are spaced apart from one another and are
disposed so that the phosphor layer 124 is proximal to the grid
layer 112.
[0005] FED 100 also includes a plurality of spacers 130 disposed
between the faceplate 104 and baseplate 102. The spacers 130
maintain the orientation between baseplate 102 and faceplate 104 so
that the baseplate and faceplate are substantially parallel to one
another. Outer walls (not shown) seal the outer periphery of FED
100 and the space between baseplate 102 and faceplate 104 is
substantially evacuated. Since the space between faceplate 104 and
baseplate 102 is substantially evacuated, atmospheric pressure
tends to press baseplate 102 and faceplate 104 together. However,
spacers 130 resist this pressure and maintain the spacing between
baseplate 102 and faceplate 104.
[0006] FED 100 also includes a power supply 140 for (1) charging
the transparent conductor 122 to a highly positive voltage; (2)
charging the grid layer 112 to a positive voltage; and (3)
selectively charging selective ones of the row and column lines 118
to a negative voltage.
[0007] In operation, voltages applied to the row and column lines
118, the grid layer 112, and the transparent conductor 122 cause
emitters 108 to emit electrons 150 that travel along path 117
towards, and impact on, phosphor layer 124. Incident electrons 150
on phosphor layer 124 cause phosphor layer 124 to emit photons and
thereby generate a visible display on faceplate 104.
[0008] The visible display of FED 100 is normally arranged as a
matrix of pixels. Each pixel in the display is typically associated
with a group of emitters 108, with all the emitters 108 in a group
being dedicated to controlling the brightness of their associated
pixel. For example, FIG. 1 shows a single pixel 160, with pixel 160
being associated with emitters 108a, 108b, 108c, and 108d. Pixel
160 could be a single pixel of a black and white display or a
single red, green, or blue dot associated with a single pixel of a
color display. Charging line 118a to a negative voltage
simultaneously activates emitters 108-d causing emitters 108-d to
emit electrons that travel towards and impact on phosphor layer 124
in the region of pixel 160. Normally, the row and column lines are
arranged so that the emitters associated with one pixel can be
controlled independently of all other emitters in the display and
so that all emitters associated with a single pixel are controlled
in unison.
[0009] The top of each spacer 130 contacts a portion of faceplate
104. Electrons emitted by emitters 108 can not impact phosphor
layer 124 in the regions where the spacers 130 touch the phosphor
layer 124. So, each spacer 130 creates a black, or dark, region of
the display that can not be illuminated. The presence of dark
regions created by spacers 130 does not significantly degrade the
quality of the display of FED 100 as long as the area of the dark
regions is small compared with the area of the illuminated pixels.
Four or more spacers 130 are normally disposed around the periphery
of each pixel. It is therefore important for the cross section of
the spacers 130 to be relatively small compared with the area of
each pixel. Ideally, the spacers 130 are characterized by a
relatively high aspect ratio (i.e., the spacer's height is larger
than its width). Such a high aspect ratio (1) does not create dark
regions large enough to degrade the display quality and (2)
provides sufficient spacing between the baseplate 102 and faceplate
104 to permit electrons traveling from emitters 108 towards
faceplate 102 to acquire sufficient energy to cause phosphor layer
124 to emit photons. The spacers 130 must also provide sufficient
structural strength to resist the atmospheric pressure and thereby
maintain the desired spacing between baseplate 102 and faceplate
104. It is also desirable for all spacers 130 to have exactly the
same height so they can provide uniform spacing between the
baseplate 102 and the faceplate 104. It is also important for the
spacers to be properly aligned with respect to the array of pixels
so the dark regions created where the spacers 130 contact the
faceplate do not interfere with the display (e.g., it is desirable
for the bottom of the spacers 130 to contact the grid layer 112 at
selected locations that are between the apertures 116 and are
equidistant from all the adjacent emitters). Since the spacers 130
are disposed within a vacuum, it is also important for the spacers
to be formed from a vacuum compatible material (e.g., a material
that does not outgas significantly).
[0010] Prior art spacers 130 are typically constructed using glass
posts. After the posts are prepared they are bonded to the grid
layer 112. Following this bonding, the faceplate 104 is fitted onto
the posts. Functioning FEDs may be constructed using these
techniques, however, these techniques have several disadvantages.
For example, when the spacers are fabricated from glass posts, it
is difficult to insure that every spacer has precisely the same
height. Variation in spacer height degrades the parallel alignment
between the faceplate and baseplate and thereby degrades the
quality of the FED. Another problem with prior art spacer
manufacturing techniques is that they do not permit precise
alignment of the spacers with respect to the pixel array. As stated
above, any deviation from the desired alignment can cause the dark
regions created where the spacers contact the faceplate 104 to
degrade the quality of the display. Ideally, the bottom of each
spacer 130 contacts the grid layer 112 at a point that is
equidistant from all the adjacent emitters, however, prior art
spacer manufacturing techniques make it difficult to achieve this
ideal. It would therefore be desirable to develop a new technique
for fabricating spacers 130 for use in FEDs that provides improved
control in manufacturability.
[0011] The use of porous, low-density xerogel materials for forming
spacers in field emission flat panel displays and vacuum
microelectronics is described in U.S. Pat. No. 5,658,832. In the
methods described therein, a mold is placed on a substrate, such as
the baseplate or faceplate in a field emission flat panel display.
A mold release agent, such as glycerol, silicone or wax, is applied
to the mold, and then a liquid xerogel precursor is placed in the
mold. After the xerogel hardens, the mold is removed, such that a
xerogel spacer is formed on the base plate or face plate. There are
several problems associated with this process. First, the positive
mold was created by using a saw to remove material from a four inch
by four inch square substrate and thereby form an array of posts.
Molds larger than four inches by four inches are generated by
aligning an array of four inch by four inch tiles. These methods
prevent the posts from being accurately positioned. Second, the
posts formed by the techniques disclosed therein tend to be
contaminated by air bubbles. That is, air bubbles tend to form near
the base of the posts thereby weakening the posts as well as
weakening the attachment between the posts and the substrate.
[0012] There is a need in the art for new and improved materials
for spacers for flat panel displays, and for new and improved
methods for forming the spacers on the face plates and base plates
in flat panel displays. The present invention is directed to these,
as well as other, important ends.
SUMMARY OF THE INVENTION
[0013] The present invention comprises methods of making and using
molds for forming spacers for flat panel displays. Preferably,
photolithography is used to make the molds. Molds are fabricated by
depositing a thick film of photoresist, or photolithographic
material, onto a support and by using photolithography to pattern
the thick film photoresist into a shape for a desired mold.
Positive molds define an array of posts, each of the posts
corresponding to one spacer of the display that will ultimately be
formed by the molding process. Negative molds define a plurality of
apertures, each aperture corresponding to one spacer of the display
that will ultimately be formed by the molding process. The
photoresist can be patterned to form either positive molds (with
arrays of posts) or negative molds (defining arrays of
apertures).
[0014] One preferred thick film photoresist for use in fabricating
the molds comprises an epoxy bisphenol A novolac resin, such as
EPON.RTM. Resin SU-8. Methods are disclosed below for depositing a
thick film of an epoxy bisphenol A novolac resin onto a support
(e.g., a glass panel), for patterning the epoxy bisphenol A novolac
resin layer into a desired shape for a mold, for improving the
ability of the epoxy bisphenol A novolac resin to adhere to the
support, for making the cured epoxy bisphenol A novolac resin less
brittle or more resilient, and for planarizing a thick layer of
uncured epoxy bisphenol A novolac resin.
[0015] The fabricated molds may be used to form spacers on a
display. The mold is filled with a spacer material, brought into
contact with a substrate (e.g., a faceplate of an FED), and the
spacer material is allowed to cure into hardened spacers. After
curing, the mold and substrate are separated leaving cured spacers
attached to the substrate. Thereafter, the substrate and spacers
may be dried and fired to further harden or cure the spacers.
[0016] Preferred materials for use as the spacer material are also
disclosed below. Preferably, the spacer material is a liquid or gel
that can be used to fill the apertures in a mold and that then
cures into a hardened spacer. Preferred spacer materials comprise
sodium silicate and potassium silicate. Other preferred materials
comprise compositions comprising formamide, potassium silicate and
sodium silicate.
[0017] Primary positive molds (fabricated using photolithography)
may be used to form secondary negative molds. Such a secondary
negative mold is preferably made by depositing a resilient
material, such as latex, silicone, or plastic, over the primary
positive mold so that the material conforms to the positive mold,
allowing the material to cure, and then separating the cured
resilient material from the positive mold. The resulting secondary
negative mold can then be filled with a spacer material and placed
in contact with a substrate so as to form spacers on the
substrate.
[0018] Alignment of the mold and the substrate can be achieved by
including an alignment template in the mold. One preferred method
of achieving alignment is to provide some alignment marks near the
border of a primary positive mold. A thin sheet of perforated metal
(e.g., spring steel) that is flexible and resistant to permanent
bending is then fabricated that replicates the alignment marks of
the positive mold. The metal sheet is then positioned so that the
alignment marks of the metal sheet are aligned with the alignment
marks of the positive mold. Resilient material used for making the
negative mold is then poured over the positive mold so that the
resilient material encases the perforated portion of the metal
sheet and so that the portion of the metal sheet including the
alignment marks protrudes through the exterior of the resilient
material. After curing, the resilient material is permanently
bonded to the metal sheet. Thereafter, when the negative mold is
used to apply spacers onto a substrate, the alignment marks of the
metal sheet may be aligned with alignment marks on the
substrate.
[0019] The molds and spacer materials constructed according to the
invention are suitable for forming spacers on large flat panel
displays. For example, spacers may be formed on displays with
rectangular active areas measuring at least eight inches by ten
inches.
[0020] Still other objects and advantages of the present invention
will become readily apparent to those skilled in the art from the
following detailed description wherein several embodiments are
shown and described, simply by way of illustration of the best mode
of the invention. As will be realized, the invention is capable of
other and different embodiments, and its several details are
capable of modifications in various respects, all without departing
from the invention. Accordingly, the drawings and description are
to be regarded as illustrative in nature, and not in a restrictive
or limiting sense, with the scope of the application being
indicated in the claims.
BRIEF DESCRIPTION OF THE FIGURES
[0021] For a fuller understanding of the nature and objects of the
present invention, reference should be made to the following
detailed description taken in connection with the accompanying
drawings in which the same reference numerals are used to indicate
the same or similar parts wherein:
[0022] FIG. 1 is a cross section of a portion of a prior art field
emission display (FED).
[0023] FIG. 2 shows the chemical structure of EPON.RTM. Resin
SU-8.
[0024] FIGS. 3A and 3 B show side and top views, respectively, of a
primary positive mold constructed according to the invention.
[0025] FIG. 3C shows the mold of FIG. 3A being used according to
the invention to form a secondary negative mold.
[0026] FIG. 3D shows the finished negative mold, the fabrication of
which was illustrated in FIG. 3C.
[0027] FIG. 3E shows a view of the negative mold taken in the
direction of line 3E-3E as shown in FIG. 3D.
[0028] FIG. 3F shows the negative mold of FIG. 3D being used to
apply spacers to a substrate.
[0029] FIG. 3G shows the substrate of FIG. 3F after the spacers
have cured and the negative mold has been separated from the
substrate.
[0030] FIGS. 3H and 3I show side and top views, respectively, of a
primary negative mold constructed according to the invention.
[0031] FIG. 3J shows the mold of FIGS. 3H and 3I being used to
apply spacers to a substrate.
[0032] FIG. 3K shows the substrate of FIG. 3J after the spacers
have cured and the negative mold has been separated from the
substrate.
[0033] FIG. 4 shows a thick film of photoresist that has been
applied according to the invention to a support.
[0034] FIG. 5 shows the general reaction scheme for bonding a
silicate to a glass surface.
[0035] FIG. 6A shows a negative mold disposed adjacent to a
substrate to which spacers will be attached in which the apertures
of the mold have been filled to overflowing with spacer material
according to the invention.
[0036] FIG. 6B shows a magnified view of one of the spacers that
has been attached to the substrate.
[0037] FIG. 6C shows a magnified view of one of the apertures in
the mold of FIG. 6A while the spacer material is curing and
expelling liquid.
[0038] FIG. 7A shows one method according to the invention of
filling apertures in a mold so that the spacer material does not
overflow the apertures.
[0039] FIG. 7B shows another mold constructed according to the
invention that includes a lip around the end of each aperture.
[0040] FIG. 8 shows a dual durometer secondary negative mold
constructed according to the invention.
[0041] FIG. 9A shows a top view of a primary positive mold
constructed according to the invention that includes row and column
lines.
[0042] FIG. 9B shows a view of the mold along line 9B-9B as shown
in FIG. 9A.
[0043] FIG. 9C shows the mold of FIG. 9A being used to construct a
negative mold.
[0044] FIG. 9D shows the mold of FIG. 9A being used to construct
another negative mold.
[0045] FIG. 9E shows a bottom view of the negative mold of FIG. 9D
after the negative mold has been separated from the positive
mold.
[0046] FIG. 9F shows a view of the mold along line 9F-9F of FIG.
9E.
[0047] FIG. 9G shows the mold of FIGS. 9E and 9F being used to
apply spacers to a substrate.
[0048] FIG. 9H shows the top view of a mold positioned over a
substrate.
[0049] FIGS. 10A and 10B show side and top views, respectively, of
a primary positive mold constructed according to the invention that
includes alignment marks.
[0050] FIG. 10C shows an alignment sheet constructed according to
the invention.
[0051] FIG. 10D shows a negative mold being formed according to the
invention using the alignment sheet of FIG. 10C and the positive
mold of FIGS. 10A and 10B.
[0052] FIG. 11 shows a preferred secondary negative mold
constructed according to the invention.
[0053] FIG. 12 is a crack test graph showing that surface
treatments of a glass support with a priming agent (i.e.,
2(3,4-epoxycyclohexyl)ethyltrie- mthoxysilane) significantly
increased the adhesive strength between the glass support and the
SU-8 resist when compared to the adhesive strength between an
unprimed glass support and the SU-8 resist.
[0054] FIGS. 13A-B show that mixing propanediol (FIG. 13A) and
hexanediol (FIG. 13B) with an SU-8 resist produce an SU-8 resist
with superior properties.
DETAILED DESCRIPTION OF THE INVENTION
[0055] In the present invention, molds are used to form spacers in
flat panel displays. Thick film photolithographic materials are
used to make the molds. A thick film photolithographic material, or
photoresist, is a material that can be used to form features that
are at least about 400 to about 500 .mu.m thick using standard
photolithographic techniques. Preferably, the thick film
photolithographic material is an epoxy bisphenol A novolac resin.
Epoxy bisphenol A novolac resins include, for example, EPON.RTM.
Resin SU-2.5 (Shell Chemical), EPON.RTM. Resin SU-3 (Shell
Chemical), and EPON.RTM. Resin SU-8 (Shell Chemical), and the like.
Preferably, the epoxy bisphenol A novolac resin is EPON.RTM. Resin
SU-8 (hereafter "SU-8"). SU-8 has the structure shown in FIG. 2.
Polymerization of the monomer occurs at the terminal oxygen rings.
SU-8 was originally developed for Micro Electro-Mechanical Systems
(MEMS), and can form structures up to 1200 .mu.m thick using
contact printing (Lorenz et al, "SU-8: A Low-Cost Negative Resist
For MEMS," J. Micromech. Microeng., 7:121-124 (1997)).
[0056] After the molds have been formed, the molds are filled with
a spacer material and then the spacer material is allowed to cure
onto a portion of an FED so as to form spacers for the FED.
Preferred compositions of the spacer material will be discussed
below. Generally, the spacer material is preferably a liquid or gel
(1) that may be used to fill apertures in the molds and (2) that
hardens or cures to a relatively rigid material.
[0057] Prior to discussing certain aspects of the present invention
in detail, the use of molds for forming spacers according to the
invention will be briefly reviewed. FIGS. 3A through 3G illustrate
how a primary positive mold may be used to form a secondary
negative mold and how that negative mold may be used to form
spacers on an FED. FIGS. 3A and 3B show side and top views,
respectively, of a primary positive mold 302. Mold 302 includes a
support 310 and a plurality of posts 320. As will be discussed in
greater detail below, the posts are preferably formed by using
conventional photolithography steps to pattern a layer of thick
film photoresist material. As will also be discussed below, each of
the posts 320 of positive mold 302 will be used to form a single
spacer in an FED. Although mold 302 is illustrated as only
including twelve posts 320, it will be appreciated that positive
molds constructed according to the invention typically include
thousands of posts, one post for every spacer on an FED. It will
further be appreciated that, for convenience of illustration, FIG.
3A as well as the other figures are not drawn to scale. For
example, each of the posts 320 is typically about 350-700 microns
high and about 25 microns wide. The support 302 is normally several
centimeters thick so that the mold can be easily handled. The
support of a mold constructed according to the invention is
typically rectangular and may be about ten inches by about twelve
inches. The active area of the mold (i.e., the portion including
the posts 320) may be about eight inches by ten inches for use with
displays having rectangular active areas that are eight by ten
inches.
[0058] As shown in FIGS. 3C, 3D, and 3E, primary positive mold 302
may be used to form a secondary negative mold 304. Negative mold
304 is normally formed by depositing a material that is relatively
flexible or resilient when cured, such as latex, silicone, or
low-melting-point plastic, over primary mold 302 so that the
material conforms to the positive mold as shown in FIG. 3C. One
preferred material for forming negative mold 304 is a natural latex
rubber mold-making material manufactured by B.F. Goodrich and sold
under the name ARTCRAFT.RTM.. After curing, the negative mold 304
is separated from positive mold 302. FIG. 3D shows a side view of
negative mold 304 after it has been separated from positive mold
302, and FIG. 3E shows a view of mold 304 taken in the direction
indicated by line 3E-3E as shown in FIG. 3D. As shown in FIGS. 3D
and 3E, negative mold 304 defines a plurality of apertures 322
corresponding to the posts 320 of positive mold 302. Regarding the
scale of FIG. 3D, it will again be appreciated that the apertures
322 in mold 304 are typically about 350-700 microns high, and that
the entire mold 304 is normally several centimeters thick so that
the mold may be easily handled (i.e., the portion of the mold
extending above the tops of apertures 322 is much larger than is
indicated in the figure). It is normally advantageous to spray or
apply a release agent (e.g., Krylon) to the primary positive mold
302 before depositing the material used to form mold 304 over mold
302. The release agent facilitates separation of the positive mold
302 and the cured negative mold 304.
[0059] Negative mold 304 may then be used for forming spacers on an
FED as illustrated in FIGS. 3F and 3G. Initially, the apertures 322
of negative mold 304 are filled with a spacer material and then the
mold 304 is placed in contact with a substrate 306 (e.g., faceplate
104 or baseplate 102 of the type shown in FIG. 1) as shown in FIG.
3F. After the spacer material in apertures 322 has cured, the mold
304 and substrate 306 may be separated thereby leaving a plurality
of cured spacers 130 attached to substrate 306 as shown in FIG. 3G.
Each of the spacers 130 corresponds to one of the posts 320 of
positive mold 302. After separation, the substrate 306 and spacers
130 would normally be air dried and then fired to further cure and
harden the spacers 130.
[0060] It will be appreciated that the negative mold 304 is
reusable and may accordingly be used to form spacers on many
substrates. Similarly, the positive mold 302 is also reusable and
may be used to form many negative molds.
[0061] As discussed above, the posts 320 of positive mold 302 are
preferably made by patterning a layer of thick film photoresist
material. Instead of patterning thick film photoresist material to
form posts, it is also possible to pattern a layer of thick film
photoresist material to form apertures and thereby use photoresist
material to form a negative mold. FIG. 3H and 3I show side and top
views, respectively, of such a negative mold 308. Negative mold 308
includes a support 310 (typically several centimeters thick) and a
layer of thick film photoresist 309 that has been patterned so that
layer 309 defines a plurality of apertures 326. Layer 309 is
typically about 400 to 800 microns thick and the apertures 326 are
typically about 350-700 microns high. The apertures 326 of negative
mold 308 may be filled with a spacer material and then the mold 308
may be placed in contact with a substrate 306 (or another portion
of an FED) as shown in FIG. 3J. After the spacer material has
cured, the negative mold 308 and the substrate 306 may be separated
thereby leaving a plurality of cured spacers 130 attached to the
substrate as shown in FIG. 3K. Each of the spacers 130 corresponds
to one of the apertures 326 of negative mold 308. It will be
appreciated that negative mold 308 is also reusable and may be used
to apply spacers to numerous FEDs.
[0062] Although negative molds (such as mold 308) made using
photolithography are useful, it is currently preferred to use
secondary negative molds (such as mold 304) that are made by
depositing a material over a positive mold (such as mold 302) that
conforms to the positive mold and then cures to a resilient
material. This is because negative molds made using
photolithography (such as mold 308) tend to be rigid or brittle and
it is more convenient to use a resilient mold such as mold 304.
[0063] Now that a brief background on the use of molds for forming
spacers in FEDs has been provided, the spacer material that is used
to fill the apertures in the molds and that cures or hardens into
the spacers for an FED will be discussed. While any spacer material
known in the art can be used in the embodiments of the present
invention described herein, the spacer material is preferably the
novel sol gel of the present invention. A sol gel is a
high-density, non-porous structure usually composed of silicates.
"High-density" preferably means that the sol gel is about 80 to
about 90% SiO.sub.2. The common starting materials for forming the
sol gels of the present invention are silicates in an aqueous or
alcohol solution. The basic starting material is preferably an
alkali silicate solution. Alkali silicate solutions include, for
example, Kasil 2130 (PQ Corp.) or N-silicate (PQ Corp.).
[0064] Silicate solutions are clear and slightly viscous with a pH
typically over 11. Formamide (HCONH.sub.2), acting as a weak acid
and an initiator, converts the alkali hydroxide into a formate
while lowering the overall pH of the solution. At a certain
critical pH, the silica present as hydrated micelles several
nanometers in diameter begin to contact each other and react.
Hydroxyl groups on the surface of the micelles are eliminated via
hydrolysis, leaving a network of three-dimensionally connected
chains of silica. Using formamide as an initiator is described by
Ulrich, Chemtech, pages 242-249 (April 1988). Besides formamide,
other initiators known in the art, such as propylene carbonate can
be used.
[0065] The three-dimensional silica network along with the water in
the original solution comprise a gel. Depending on the degree of
connectivity, the gel can be soft with long strands of polymer or
hard with short frequently branched strands of polymer. The more
frequently connected the polymer strands the more water is
eliminated from the bulk of the gel.
[0066] When a large amount of formamide is used (e.g., more than
one part formamide to one part silica), gelation begins immediately
upon mixing, and within minutes a soft gel is formed. A short time
later the gel shrinks, turns cloudy and eliminates water from the
bulk. Within one half hour after mixing, the gel is a hard white
solid. Most of the original liquid in the silicate solution is
eliminated from the interior volume of the gel.
[0067] When a slightly smaller amount of formamide is used (e.g.,
less than about one part formamide to one part silica), it leads to
a soft gel that eliminates a minor amount of water and dries
without cracking. In formamide:silica ratios of less than 1:1 to
1:10, a gel is formed which is soft, clear and only slightly moist
to touch. These gels may be air dried, then heat treated to over
500.degree. C. without problems. Weight loss experiments indicate
nearly all detectable water is driven off, the unreacted formamide
is decomposed as NH.sub.4 and CO.sub.2, and the alkali formate
byproducts are converted to oxalates.
[0068] 1 part formamide in 20 or more parts Kasil lead to gels
which require several days to cure at room temperature. Clear soft
and dry to touch, these gels collapse when dried and release all
detectable water when baked from 480-500.degree. C.
[0069] The best formulation for preparing Kasil-based sol gels is
1/4 to 1 part formamide to 1 part silica, preferably about 1/4 part
formamide to 1 part silica. The gelation time is several hours at
room temperature. During gelation, the silica network pulls
together, and shrinks away from the confines of the mold,
eliminating water. As a supernatant liquid, this water lubricates
the void between the gel surface and the mold allowing easy
separation of the two parts.
[0070] When propylene carbonate is used as the initiator, it works
about ten times faster than formamide. The gels of the present
invention can be cast in about six minutes at room temperature when
using about 12 grams of silicate to about 1 to 11/2 grams of
propylene carbonate.
[0071] In any of the embodiments of the present invention, drying
of the spacers is preferably accomplished in two stages, an air dry
which removes a large portion of water, and a firing that may
ultimately reach 500.degree. C. A preferred drying rate is
described in Example 3.
[0072] Drying the gel in air leads to a collapse of the hollow
pores which were filled with and supported by water. This volume
loss can be related to the degree of connectivity. Simple drying at
ambient temperatures will not in general make a solid compact
body.
[0073] The sol gel is normally fired to create a dense structure.
Firing at a temperature of 150-200.degree. C. is required to
decompose any unreacted formamide and loss experiments carried out
on the reactants indicate an oxalate reaction occurs.
[0074] While ceramic bodies composed of discrete micron sized
particles require firing at temperatures where the viscosity is
about 8 to 8.8 to achieve a dense non-porous structure, sol gels
begin to compact and densify at a viscosity of about 11. The
densification of gels is describe in Ultrastructure Processing of
Ceramics, Glasses, and Composites, Hench and Ulrich, Eds.,
Wiley-Interscience Publication, pages 76-87, the disclosure of
which is incorporated by reference herein in its entirety.
[0075] A formulation of a sol gel containing 6 parts N-Silicate
(sodium silicate) to 10 parts Kasil (potassium silicate) to 0.5
parts Formamide began to densify at 520.degree. C. Larger
proportions of sodium silicate will lower the densification
temperature.
[0076] Densification temperatures of compositions containing
varying amounts of Na.sub.2O, K.sub.2O and SiO.sub.2 are shown in
Table 1.
[0077] The densified material formed two phases, one phase was a
glassy material containing mostly silica, potassium and sodium, the
other phase was in the form of long needle-like crystals containing
sodium and oxygen, as measured by LDS. In cross sections of samples
run at 480.degree. C. and 500 .degree. C., the crystals were
present in the pores of the dried gel. In the 520.degree. C. run,
the crystals were incorporated into the silica rich matrix. At
540.degree. C., the sol gel become glassy, as evidenced by brittle
fracture lines when the bulk gel was split for a cross sectional
view.
[0078] Differential scanning calorimetry revealed two peaks in
sodium-potassium silicate glasses, the first peak occurred for all
compositions at a potassium:sodium silica ratio of about 10:6 to
about 10:10 at 450.degree. C. A second peak roughly 20.degree. C.
higher shifted lower in temperature as the sodium content was
raised. The first peak is narrow and fairly deep indicating some
sort of crystallization. It is near the predicated
sintering/compaction temperature. Water in the glass precursor is
responsible for dropping the sinter temperature to the 450.degree.
C. region, normally for a dry glass of this composition the sinter
temperature is 100.degree. C. higher. The reaction should be
irreversible, leading to a glass that sinters and compacts at one
temperature then becomes stable afterwards to a higher temperature.
This property is engineerable based on the water content of the
sol-gel and the degree of ripening.
[0079] The spacers can have their surface resistivity adjusted by
including a semi-conductive material in or on the spacer. Such
semi-conductive materials include, for example, tin oxide, lead
oxide, zinc oxide and/or titanium oxide.
[0080] A major difficulty in filling the apertures of the molds of
the present invention with the sol gels of the present invention is
the CO.sub.2 reaction that occurs on exposure of the silicates in
the sol gels to air. CO.sub.2 diffuses into the surface where it
forms a weak acid, in turn reacting with the alkaline solution.
This initiates a hydrolysis reaction that forms a skin over the top
of the solution with a film of silica. The silica prevents the
liquid from wetting the surface. One way to prevent the skin
formation is to perform the mold filling operations described
herein in an inert gas, such as argon.
[0081] Preferably, each spacer in the present invention can hold
about 0.05 pounds or more of pressure, more preferably about 0.07
pounds or more of pressure. Accordingly, about 422 spacers per
square inch are preferably used in a flat panel display. Since
there are generally about 10,000 spacer bonding sites per square
inch in a flat panel display, the yield (i.e., the number of
spacers that bond to the substrate) does not have to be
particularly high in order to achieve good results.
[0082] The spacers and molds described herein can be used in flat
panel displays in field emission displays or plasma displays. In
field emission display applications, the spacers are generally
about 350-700 .mu.m tall. In plasma display applications, the
spacers are generally about 80-100 .mu.m tall.
[0083] Now that the spacer material has been discussed, the use of
thick film photoresist (e.g., an epoxy bisphenol A novolac resin)
for forming the molds will be discussed. An initial step in the
formation of such a mold (e.g., primary positive mold 302 as shown
in FIG. 3A or negative mold 308 as shown in FIG. 3H) is to form a
thick layer (e.g., 130-775 .mu.m) of photoresist material over a
support. FIG. 4 illustrates a thick layer of photoresist material
404 (e.g., about 700 microns) that has been deposited over a
support 402 (support 402 is typically several centimeters thick).
Layers 402 and 404 are each typically rectangular and measure
several inches in length and width (e.g., eight inches by ten
inches). The thick layer of photoresist material 404 can be formed
in several ways, including, for example, by spin coating, dip
coating and meniscus coating. Methods for spin coating, dip coating
and meniscus coating are known in the art. Preferably, the
photoresist material 404 is an epoxy bisphenol A novolac resin,
more preferably SU-2.5, SU-3 or SU-8, most preferably SU-8. The
support 402 can be any support known in the art, including, for
example, a glass panel. In a preferred embodiment, the support is
covered with a first thin layer (e.g., 30-75 .mu.m) of photoresist
material, flood exposed and post-exposure baked to create a flat,
well-bonded base layer on the support. Thereafter, a second thick
layer (e.g., 100-700 .mu.m) of photoresist material is applied,
exposed using a photomask, and developed to form the desired
pattern.
[0084] A novel method for forming layer 404 is by powder coating.
In one example of powder coating, dry SU-8 powder is applied to the
support 402 and baked at temperatures required for reflow.
Specifically, solid SU-8 is mixed with a solvent, preferably a
general organic solvent, such as ethyl lactate, and a
photoinitiator. Any photoinitiator known in the art can be used,
such as, for example, Cyracure 6974 photoinitiator, Cyracure 6990
photoinitiator, and the like. The components are thoroughly mixed
until the dry SU-8 powder dissolves. Thereafter, the solvent is
removed by heating, and the SU-8 powder is finely ground.
Conventional powder coating techniques are then used to apply a
uniform thickness of dry powder to the support. When heated to
about 120-130.degree. C., the SU-8 powder melts, reflows, and
planarizes during reflow due to its high surface tension to form,
for example, a layer such as layer 404 shown in FIG. 4.
[0085] Meniscus coating can also be used to form a layer 404 of
thick film photoresist material that is about 400-450 .mu.m thick.
Preferably, the thick film photoresist material is an epoxy
bisphenol A novolac resin, more preferably SU-8. This is done by
using a meniscus coater to form a series of about 80-200 .mu.m
thick coatings, one on top of another. Preferably, after each
meniscus coating is deposited, the support and coatings are baked
(e.g., at about 95.degree. C. for about 20 minutes, then at about
110.degree. C. for about 25 minutes) before the next meniscus
coating is deposited. This method removes all of the solvents from
the thick film photoresist material. The thickness increment is
limited by the solids content of the thick film photoresist
material, which is in turn limited by the specific pumping
equipment of the meniscus coater.
[0086] Exemplary methods for forming layer 404 are described in
more detail herein. A practical composition comprising SU-8 was
described in several papers (e.g., Lorenz et al, "EPON SU-8: A Low
Cost Negative Resist For MEMS," Sensors & Activators, A64:33-39
(1998)), the disclosure of which is incorporated by reference
herein in its entirety), although a more preferred composition is
described herein. Preferably, the composition comprises about
200-600 parts by weight solvent (e.g., ethyl lactate thinner),
about 200-600 parts by weight SU-8, and about 1-100 parts by weight
photoinitiator (e.g., Cyracure 6974 or 6990 photoinitiator). More
preferably, the composition comprises about 300-500 parts by weight
solvent (e.g., ethyl lactate thinner), about 300-500 parts by
weight SU-8, and about 10-70 parts by weight photoinitiator (e.g.,
Cyracure 6974 or 6990 photoinitiator). Most preferably, the
composition comprises about 400 parts by weight solvent (e.g.,
ethyl lactate thinner), about 400 parts by weight SU-8, and about
40 parts by weight photoinitiator (e.g., Cyracure 6974 or 6990
photoinitiator).
[0087] After the solvent and photoinitiator are mixed together,
SU-8 is added. After all the SU-8 dissolves, ultrasonic agitation
can be used to de-gas the material. A meniscus coater is then
preferably used to successively deposit several layers of the
material over a substrate 402 to form a thick layer 404 of
photolithographic material. The support speed in the meniscus
coater can be about 0.1-1.0 cm/second, preferably about 0.55
cm/second. Any soft-bake temperatures and time that slowly drive
off the solvent can be used, such as, for example, about 95.degree.
C. for 30 minutes followed by 130.degree. C. for 30 minutes.
[0088] After layer 404 has been formed (e.g., by meniscus coating,
powder coating, or spin coating), the upper or "contact" surface
406 of layer 404 is sometimes "wavy" or non-planar. As shown in
FIG. 4, contact surface 406 is the surface of layer 404 that is
distal from the support 402. With reference to FIGS. 3H and 3J it
can be appreciated that it is advantageous for the contact surface
406 (as shown in FIG. 4) to be planar, or as planar as possible. If
the surface of the photoresist layer that contacts the substrate
306 during the molding process is not planar, the spacer material
in apertures 326 may not bond well to the substrate. Developed SU-8
is brittle. However, unexposed SU-8 will cold flow under pressure,
when layer 404 is formed from SU-8. So, planarization of the
contact surface 406 of the undeveloped layer 406 may be achieved by
using a press (i.e., by using a platen of the press to apply
pressure to the contact surface 406). Prior to applying the platen
to contact surface 406, it may be advantageous to apply a release
agent to the contact surface 406 or the platen to prevent the
platen from bonding to the contact surface 406. Preferably, the
release agent is Krylon industrial mold release (Borden Co.). The
release agent can be sprayed on from an-aerosol can. Applying the
release agent to the platen and allowing the organic solvents to
escape is the preferred application method. The support is placed
in the press and force is applied to the panel to cold flow the
SU-8. Several microns of waviness may be removed from contact
surface 406 in this manner. These methods produce a SU-8 layer over
a support, having a relatively planar contact surface 406.
[0089] Since developed SU-8 is brittle, when negative mold 308 is
used to apply spacers to a substrate (as discussed above in
connection with FIGS. 3H, 3I, 3J , and 3K), it is advantageous for
the contact surface 406 (as shown in FIG. 4) of the mold to be as
planar as possible. However, when a negative mold made from latex,
or another resilient material, (e.g., such as negative mold 304) is
used to apply spacers to the substrate, it is less important for
the contact surface to be planar. This is because pressure can be
applied to hold the mold and substrate tightly together while the
spacer material is curing. Such pressure can temporarily "smooth
out" or planarize the contact surface insuring that the spacers
contact and bond to the substrate.
[0090] One problem with thick film photoresists is that they are
sometimes characterized by poor adhesion to a support. It can
therefore be advantageous to prime the support 402 prior to forming
the thick film photoresist layer 404 to improve adhesion between
the support 402 and the thick film photoresist layer 404. One
reason for desiring good adhesion between the photoresist layer and
the support can be illustrated with reference to FIGS. 3A through
3D. As shown in FIG. 3A, the positive mold 302 comprises a
plurality of photoresist posts 320 disposed over, and attached to,
support 310. The negative secondary mold 304 is formed by
depositing a compound over positive mold 302, allowing the compound
to cure, and then separating the positive mold 302 from the
negative mold 304. If the posts 320 do not strongly adhere to the
support 310, the posts 320 may break off of the support 310 when
the cured negative mold 304 is separated from the positive mold 302
thereby damaging the positive mold. Therefore, to preserve the
integrity of the positive mold 302, and to allow the positive mold
302 to be used to fabricate a large number of secondary negative
molds, it is desirable to provide good adhesion between the
photoresist posts 320 and the support 302.
[0091] Conventional primers that have been used to improve adhesion
of photoresist materials include silanes, such as HMDS, which have
silanol groups at one end and organic methyl groups at the other.
These priming materials generally work for novolac resins, but are
generally ineffective for SU-8 because SU-8 is an epoxy, and the
methyl groups do not participate in bonding. As described below in
Example 1, making the support 402 as polar as possible and
terminating it in hydroxyl groups or amine groups provides the best
possible surface for wetting and adhesion to the photolithographic
material. For example, triethanol amine and/or ethylene glycol can
be applied to the upper surface of support 402 as a priming agent
before the photolithographic material is deposited onto the upper
surface of the support 402. Adhesion of the layer 404 to the
support 402 can be greatly improved by, for example, mixing
triethanol amine as a 1% solution in IPA, coating the substrate
with the solution for three minutes, and then rinsing off the
solution.
[0092] Alternatively, a silane bonding agent can be used to improve
adhesion between the support 402 and the thick film photoresist
layer 404. A silane bonding agent preferably comprises silane,
cyclohexane and an epoxy group, such as
2(3,4-epoxycyclohexyl)ethyltrimethoxysilane (manufactured by
Gelest). This material, used as a primer from a deionized
water/alcohol bath at a pH of 4.5-5.5, forms a monomolecular film
that hydrolyzes to the support 402 via the silane groups and
permanently bonds to the layer 404 with a ring opening
polymerization reaction. The general reaction scheme for bonding a
silane to a glass surface is shown in FIG. 5.
[0093] After a thick film layer of photoresist has been formed over
a support, conventional photolithographic processing (e.g.,
exposure followed by developing or etching) can be used according
to the invention to pattern the layer 404 of photoresist material
and thereby convert layer 404 into a mold (e.g., such as positive
mold 302 as shown in FIG. 3A or as negative mold 308 as shown in
FIG. 3H). Exposure of the photoresist material can be by broadband
ultraviolet light (preferably about 365-400 nm) or by contact
printing. Exposure can be performed until a latent image is formed.
A post-exposure bake can be done at 95.degree. C. for 8-30 minutes,
depending on the thickness of the SU-8, followed by 25 minutes at
115.degree. C. Use of the two temperatures is desirable for
reducing distortion that occurs during post exposure bake. The
95.degree. C. firing softens and melts the unexposed SU-8. The
115.degree. C. firing develops full adhesion between the SU-8 and
support.
[0094] PGMEA (propylene glycol monomethyl ether acetate) can be
used to develop the exposed photoresist material. For example, two
baths of PGMEA can be used, where the first bath is used to remove
all of the undeveloped SU-8, and the second bath is used as a rinse
to remove the SU-8-loaded developer from the first bath. A final
rinse in IPA can then be performed. Going from PGMEA developer to
IPA without the intermediate rinse in clean PGMEA results in
scumming, as the dissolved resist coagulates in the incompatible
IPA solvent.
[0095] A post develop firing may also be performed to increase
adhesion and dry out any remaining developer or solvent (e.g.,
about 85-95.degree. C. for about 10-20 minutes).
[0096] It has been discovered that exposed and developed SU-8 is a
hard and brittle plastic that cracks under stress and is not
particularly compliant. The brittle nature of the cured SU-8 can
impose restrictions on the flatness of an SU-8 mold's contact
surface. For example, if SU-8 is used as the photolithographic
layer 309 in negative mold 308 (as shown in FIG. 3H), the upper
surface of the layer 309 must be flat (or planar). If this surface
is not flat, then the brittle nature of the cured SU-8 will prevent
portions of the negative mold 308 from contacting the substrate 306
when the mold and substrate are placed in contact as shown in FIG.
3J. That is, waviness in the contact surface of the SU-8 mold, can
cause areas of the mold to drop out of contact with the substrate,
such that the spacers will not adhere to the substrate, which
leaves bare patches on the substrate. Generally applying pressure
up to 14 psi will not make the SU-8 mold conform flat against the
substrate.
[0097] Accordingly, it is desired to modify the properties of cured
SU-8 so that the mold can better contact the substrate. As shown in
Example 2, it has been unexpectedly discovered that crosslinking
agents, such as polyols, can be used to successfully modify the
properties of cured SU-8. For example, polyols added to uncured,
undeveloped SU-8 increase flexibility, photospeed, and adhesive
strength. The polyol is preferably a triol or a diol, more
preferably a diol. Preferred diols include propanediol, hexanediol,
ethylene glycol and the like. The base ratio of SU-8 terminal
groups to diol terminal groups is preferably 1.5 to 1. The quantity
of the diol can be increased or decreased, depending on the desired
properties of the resin.
[0098] Without intending to be bound by any theory of the
invention, the property of the diols that make them an effective
additive is the increase in crosslinking speed that occurs with
their use. For best adhesion, the post-exposure bake (PEB)
temperature of the SU-8 resist was about 115.degree. C. to
125.degree. C. This exceeds the melting point of the unexposed SU-8
resin, which is 83.degree. C. This creates a problem in properly
baking microstructures that do not bend or otherwise deform under
the stress of the unexposed resist expanding and melting. The diols
rapidly crosslink the exposed resist during the initial part of the
bake, making it strong enough to prevent warping under stress. The
increase in viscosity during PEB also affects the resolution of the
resist. A fast rise in viscosity prevents diffusion of the acid
groups from the exposed to the unexposed regions, which can lead to
blurring. This is seen in developing small holes and bars. With
base resin, the diffusion of the acid generator into the unexposed
regions creates a viscous layer around the features which does not
develop away. This leads to stringers, at best, and filled holes
and gaps, at worst. The polyol, such as propanediol and hexanediol,
appears to have rapidly allowed the SU-8 to polymerize. This would
have the effect of blocking diffusion quickly so that blurring does
not occur.
[0099] It has also been discovered that developed SU-8 can be
further treated so that it reacts better with the spacer material
that is used to fill apertures in the molds (e.g., apertures 326 in
cured photoresist layer 309 of negative mold 308 as shown in FIG.
3H). After development, the SU-8 layer is hydrophobic, which means
that the liquid spacer material added thereto may bead up rather
than conform to the apertures in the layer. Thus, it has been
discovered that an oxygen glow discharge can be used to treat the
surface of the -hydrophobic SU-8. After such treatment, the surface
of the SU-8 mold will be hydrophilic, as proven by water drop
contact angle measurements. Alternatively, UV based ozone
treatments can be used to treat the surface of the mold to make it
hydrophilic. Chemical treatments, such as surfactants, may also be
used to change the degree of surface polarity. Making the mold
hydrophilic allows for better conformation of the liquid spacer
material therein.
[0100] Preferred spacer materials for use with the invention and
preferred methods of treating photoresist according to the
invention have been discussed above. Additional factors concerning
how to form spacers with molds according to the invention will now
be discussed.
[0101] FIG. 6A shows a small portion of a negative mold 610
constructed according to the invention. It will be appreciated that
negative mold 610 could be for example of the type shown at 304 in
FIG. 3D or of the type shown at 308 in FIG. 3H. The apertures 620
of negative mold 610 have been filled with a spacer material of the
type discussed above (e.g., one of the above discussed sol gels).
As shown, the apertures 620 have been filled to overflowing so that
a small amount of spacer material 622 extends over each aperture
620. FIG. 6A also shows a substrate 306 (e.g., a faceplate of an
FED) positioned adjacent the mold. 610. Spacers are formed on the
substrate by bringing the substrate 306 and mold 610 into contact
and allowing the spacer material to cure. The substrate 306 and
mold 610 are preferably left in contact until gelation or curing of
the spacer material occurs. The mold 610 and substrate 306 may then
be peeled apart, at which time the hardened spacers will be bonded
to the substrate 306.
[0102] FIG. 6B shows a magnified view of a portion of one of the
spacers 130 that are attached to substrate 306 after the mold 610
and substrate 306 are separated. As shown, the base of spacer 130
includes a portion 624 that is "flared out" where the spacer 130
contacts the substrate 306. It is advantageous for the spacers 130
to include such flared out bases since they increase the strength
of the bond between the spacers and the substrate. It has been
found that filling the apertures 620 of the mold so that the spacer
material overflows as shown at 622 in FIG. 6A advantageously
increases the likelihood that spacers will be formed with flared
out bases 624. One disadvantage of filling the apertures so the
spacer material overflows as shown at 622 is that it tends to form
unwanted residues of hardened spacer material on the substrate.
[0103] FIG. 6C shows a magnified view of one of the apertures 620
of the mold 610. When sol gels of the type discussed above are used
to fill the apertures 620 of the mold, the spacer material cures
into a hardened or gelled portion 130 and an expelled liquid 630.
Also, as the spacer material cures, the hardened portion 130 tends
to shrink and to bond or attach itself to substrate 306. As the
hardened portion 130 shrinks, the expelled liquid 630 tends to fill
the remainder of the aperture 620 so that the liquid separates the
hardened portion 130 from the surface of the mold 610. When the
substrate 612 is removed from the mold 610, the expelled liquid 630
acts as a lubricator and facilitates separation of the substrate
306 and the mold 610. The liquid 630 tends to prevent the cured
spacer material 130 from breaking off of substrate 306 or remaining
attached to mold 620.
[0104] FIG. 7A illustrates an alternative method according to the
invention for forming spacers. FIG. 7A shows a small portion of a
negative mold 710 constructed according to the invention. Mold 710
defines apertures 720 that have been filled with a spacer material
722. Rather than filling the apertures to overflowing (as shown in
FIG. 6A, the apertures have been filled so that a meniscus 724 of
the spacer material 722 is recessed from the end of the aperture
720. Application of pressure to the mold 710 causes the meniscus
724 of the liquid spacer material to contact the surface of the
substrate to which the spacers are to be attached. Preferably,
pressure is applied until all the spacers have hardened and
attached to the substrate. This method of forming spacers is
particularly preferred when the mold is flexible, such as when the
mold is made of latex or silicone. One advantage of filling
apertures of the mold with spacer material so that the meniscus of
spacer material is recessed (as shown in FIG. 7A) is that it tends
to eliminate the unwanted residue that is sometimes formed when the
apertures are filled to overflowing (as shown in FIG. 6A).
[0105] FIG. 7B shows an embodiment of a small portion of another
negative mold 740 constructed according to the invention in which
the end of each aperture 742 is surrounded by a lip or protrusion
744. Such a lip or protrusion 744 around the apertures 742 allows
additional liquid spacer material to accumulate at the intersection
of the substrate and the mold, permitting each spacer to be formed
with a uniformly rounded or flared base 624, as described herein
and shown for example in FIG. 6B.
[0106] Molds such as mold 740 can be formed by applying and imaging
successive layers of photoresist (e.g., SU-8). For example, a
positive mold for forming negative mold 740 may be constructed by
applying, imaging, developing, and curing a first layer of
photoresist material to define a plurality of shallow trenches.
These trenches in the positive mold will later correspond to the
lips 744 in the negative mold. Then a second layer of photoresist
may be applied over the first layer and then imaged, developed, and
cured to define a plurality of posts, each of the posts being
surrounded by the previously formed trenches. The posts correspond
to the apertures 742 in mold 740. Such a positive mold may then be
used to form negative molds such as mold 740.
[0107] As an example of the above methods used in development work,
liquid spacer material (e.g., silicate solution) was filled into
all the apertures of a negative mold constructed according to the
invention such that an excess of the spacer material protruded from
the mold apertures to a depth of {fraction (1/16)} to {fraction
(3/32)} of an inch. The wet mold was placed in a vacuum chamber and
a moderate vacuum was pulled on it, whereby the air bubbles were
removed from the mold volume, and, at the same time, the solution
was degassed. The vacuum prevents the above-discussed "skinning
over" effect. After the solution was fully degassed and the holes
filled, the spacer material-filled mold was removed from the vacuum
chamber and excess spacer material was squeezed off. An excess of
spacer material (e.g., silicate solution) is preferably used to
prevent the skin that forms on the spacer material (e.g., silicate)
during vacuum exposure from covering the mold openings and trapping
the air bubbles. Degassing the spacer material solution has the
unexpected effect of delaying the gelation time. This is mainly due
to the loss of CO.sub.2 incorporated in the solution during
mixing.
[0108] In a second implementation, the mold was placed in vacuum
chamber in a vertical position (i.e., in a position that is rotated
clockwise ninety degrees from the orientation shown in FIG. 6A
about an axis perpendicular to the plane of the page in FIG. 6A). A
high vacuum was drawn on the chamber. A spacer material (e.g.,
silicate solution) was introduced into the chamber, and the spacer
material was drawn into the mold holes by surface tension/capillary
action. Due to its viscosity, the spacer material may not be drawn
into small features. After allowing the spacer material (e.g.,
silicate solution) to wet and flow as well as possible, the spacer
material feed and vacuum valves were closed off and an inert gas,
such as argon, was introduced to pressurize the chamber. This over
pressure can force the spacer material (e.g., silicate solution)
into small pores. After a given time the spacer material feed valve
was opened and the chamber drained under pressure. The excess
spacer material was removed from the mold surface. The mold was
then placed in contact with a substrate and the molded spacer
material then cured onto the substrate.
[0109] Referring again to FIGS. 3A through 3G, it has been found
that primary positive molds such as mold 302 may be used to produce
about ten to fifteen secondary negative molds such as mold 304
before the primary mold 302 becomes unsuitable for further use. A
few of the posts 320 of the primary positive mold 302 tend to break
off of the support 310 each time a secondary negative mold 304 is
separated from the primary mold 302. Since, as discussed above, the
necessary yield for producing spacers for an FED is relatively low,
a primary mold 302 can be used to produce secondary negative molds
304 even if several of the posts 320 have broken off. However,
after about ten to fifteen castings (i.e., after use of the same
primary positive mold to produce about ten to fifteen secondary
negative molds), enough posts 320 have typically broken off to make
the primary mold 302 unsuitable for producing further secondary
molds.
[0110] One preferred method for using secondary molds, such as mold
304, for producing spacers is as follows. First, the secondary mold
is preferably washed to remove any release agents that were used to
separate the primary positive and secondary negative molds. The
secondary mold is then placed in a vacuum chamber and the apertures
of the mold are filled with a spacer material while the mold is
under a vacuum. After filling, the surface of the mold is cleaned
with a scalpel to remove any excess spacer material from the mold.
The mold is then placed in contact with a substrate (e.g., a
faceplate of an FED) with the apertures facing down and the
substrate below the mold as depicted in FIG. 3F. A rubber membrane
is then placed over the mold and a vacuum is drawn in the volume
beneath the membrane to remove any air that may be trapped at the
interface of the mold and the substrate. The rubber membrane is
then drawn down tightly to apply uniform pressure against the mold
and thereby squeeze the mold and substrate together. The volume
above the -rubber membrane is then pressurized in excess of
atmospheric pressure to clamp the mold and the substrate together
and thereby insure that the spacer material in the mold apertures
is in contact with the substrate. After the spacer material has
cured, the secondary mold and the substrate are separated leaving
an array of spacers on the substrate.
[0111] Another variation of this technique is to use a relatively
thin ledge (e.g., several millimeters thick) to separate the
substrate and a negative mold that has been filled with spacer
material. The ledge preferably extends around the outer periphery
of the negative mold. When pressure is applied to the top of the
mold, the pressure initially pushes the center of the mold into
contact with the substrate, but the ledge prevents the outer
periphery of the mold from contacting the substrate. As the
pressure increases, more and more of the mold is pushed into
contact with the substrate and the area of contact gradually
extends from the center of the mold out towards the outer
periphery. The ledge is preferably placed far enough away from the
active area of the mold (i.e., the portion of the mold that
includes apertures that have been filed with spacer material) so
that the entire active area may be brought into contact with the
substrate. The mold is then held in place until the spacer material
cures. This technique minimizes the amount of air that is sometimes
trapped at the interface of the mold and the substrate and thereby
facilitates placing the spacer material into good contact with the
substrate.
[0112] One problem that has been encountered in use of negative
molds such as mold 304 is that cured spacers may break off of the
substrate when the mold and substrate are separated. If the mold is
hard or brittle, then the mold fails to deform around the spacers
as the mold is pulled away from the substrate and tends to break
the spacers off. Conversely, if the mold is soft enough to deform
around the spacers then the mold tends to stretch while it is being
separated from the substrate and this stretching also tends to
break off the spacers. One solution to this problem is to construct
a negative mold by bonding two layers of rubber together, each
layer being characterized by a different durometer. FIG. 8 shows an
example of such a dual durometer mold 802. Mold 802 includes a
rubber layer 810 that is characterized by a relatively low
durometer (e.g., about 8-10 on the Shore A scale of the American
Society for Testing Materials) and a rubber layer 812 that is
characterized by a higher durometer (e.g., greater than sixty on
the Shore A scale). Layers 810 and 812 are bonded together. As
shown, the apertures 822 of the mold are fully defined by the low
durometer portion 810. The low durometer portion 810 is high enough
to accommodate the apertures 822 (e.g., the low durometer portion
may be about 800 microns thick) and the high durometer portion 812
is typically several centimeters thick so that the mold 802 can be
easily handled. When the mold 802 is separated from a substrate,
the low durometer portion 810 is able to deform around the spacers
(rather than breaking them) and the high durometer portion 812
prevents the low durometer portion 810 from stretching enough to
break off the spacers.
[0113] FIG. 9A shows a top view of a small portion of another
primary positive mold 902 constructed according to the invention.
FIG. 9B shows a view of a small portion of mold 902 taken in the
direction of line 9B-9B as shown in FIG. 9A. Similarly to mold 302
(FIG. 3A), mold 902 includes a support 902 (that is typically
several centimeters thick) and a plurality of posts 920 (that are
about 350 to 700 microns high). However, mold 902 additionally
includes a network of row and column lines 922. Each of the posts
920 is intersected by at least one of the-row and column lines 922.
The row and column lines are substantially shorter than the posts
920. In one embodiment the posts are 350 microns high and the row
and column lines are twenty to twenty-five microns high. As will be
discussed below, the row and column lines 922 are used to form a
network of "vacuum pump out channels" in secondary negative molds
formed on positive mold 902.
[0114] Molds such as mold 902 can be formed by applying and imaging
successive layers of photoresist (e.g., an epoxy bisphenol A
novolac resin, such as SU-8) to the support 902. In one preferred
method, a layer of SU-8 that is fifty microns thick is deposited
over the support 902 and developed to establish a base. Then a
second layer of SU-8 is deposited over the first layer and the
second layer is then imaged to form the row and column lines. Then
a third, thick layer of SU-8 is deposited over the second layer and
the third layer is then imaged to form the posts. In general,
multiple layers of photoresist can be successively applied and
imaged to generate complex shapes such as that of mold 902.
[0115] FIG. 9C shows a small portion of a secondary negative mold
950 being formed on positive mold 902. As shown, mold 950 is formed
by pouring silicone or latex over the positive mold 902 so that the
posts 920 project through the silicone layer 950. FIG. 9D shows a
small portion of another secondary negative mold 960 being formed
on positive mold 902. As shown, mold 960 is similar to mold 950.
However, mold 960 is made of a thicker layer of material so that
the posts 920 of positive mold 902 do not project through the mold
960. The apertures formed in mold 950 by posts 920 are
"through-holes" since they pass entirely through the mold, whereas
the apertures formed in mold 960 by posts 920 are "blind holes"
(such as the apertures of mold 304 shown in FIG. 3D). After the
material used to form the negative mold has cured, the mold (either
mold 950 or 960) is separated from the positive mold 902.
[0116] FIG. 9E shows a bottom view of a small portion of the
completed mold 960. FIG. 9F shows a side view a small portion of
mold 960 taken in the direction indicated by line 9F-9F as shown in
FIG. 9E. As shown, mold 960 defines a plurality of apertures (or
"blind holes") 970 that correspond to the posts 920 of positive
mold 902. Also, mold 960 defines a plurality of vacuum pump out
channels 972 that correspond to the row and column lines 922 of
positive mold 902. It will be appreciated that mold 950 (FIG. 9C)
also defines a plurality of vacuum pump out channels.
[0117] One problem with negative molds such as mold 304 (FIG. 3D)
is that air (or gas or gaseous material) tends to get trapped
between the mold and the substrate and this trapped air tends to
prevent the spacer material from becoming reliably attached to the
substrate. Even the method discussed above of using a ledge so that
contact between the mold and substrate initiates at the center of
the mold and gradually extends out to the periphery does not fully
eliminate this trapped air. The vacuum pump out channels provide a
mechanism for removing air at the interface of the mold and
substrate.
[0118] FIG. 9G shows a side view of a small portion of mold 960 in
contact with a substrate 306 (e.g., a faceplate of an FED). FIG. 9H
shows a top view of the entire mold 960 positioned over the entire
substrate 306. As shown in FIG. 9H, the substrate 306 is preferably
centered under the mold 960 and the edges of the mold 960 normally
extend beyond the edges of the substrate. The active area (i.e.,
the area within which all the emitters, posts, or pixels are
disposed) of the substrate 306 is indicated in FIG. 9H by dashed
rectangle 982. A clamp ring 980 preferably pushes the mold 960 down
onto the substrate 306, and the clamp ring 980 is preferably
disposed so that it surrounds the active area 982. The six dashed
lines 984 (three horizontal lines and three vertical lines) in FIG.
9H illustrate the vacuum pump out channels 972 that extend
throughout the entire mold. The mold 960 also defines an injection
port 986. As shown in FIG. 9H, the injection port 986 is preferably
positioned within the clamp ring 980 and outside the active area
982. The injection port 986 is an aperture extending vertically
through the entire mold 960.
[0119] In operation, after the clamp ring has been applied to hold
the mold onto the substrate, the vacuum pump out channels 972 can
be used to remove any air that has been trapped between the mold
and the substrate. For example, the entire assembly including
substrate 306, mold 960, and clamping ring 980 may be placed in a
vacuum chamber. Evacuating the chamber evacuates all the gas from
the environment surrounding the assembly, and the vacuum in the
surrounding environment causes any gas trapped between the mold and
substrate to be removed via the vacuum pump out channels.
Thereafter, the spacer material can be injected into the top of
injection port 986. The injected spacer material travels down
through the mold towards the vacuum pump out channels 972 (i.e.,
towards the interface of the substrate and the mold) and suction
from the vacuum pump out channels causes the spacer material to
spread across the entire mold and to fill all the apertures of the
mold. After all apertures have been filled, the spacer material is
then allowed to cure. After the spacer material has cured, the mold
960 and substrate 970 are separated leaving the cured spacers
attached to substrate 306. Air can be injected into the vacuum pump
out channels to facilitate separation of the mold and substrate.
During the molding process, the substrate and mold are normally
supported by some kind of surface or table. It may also be useful
to use a flange to push the outer perimeter of the mold down onto
the table during the molding process.
[0120] Mold 950 may be used in a similar fashion as mold 960.
However, rather than injecting the spacer material into the mold,
the spacer material is filled into the mold apertures through the
top of the mold (i.e., at the top of the through hole) and a doctor
blade can be run over the top of the mold to remove excess spacer
material. Although mold 950 is useful, mold 960 is currently
preferred because separation of the mold and substrate tends to
break off more spacers when mold 950 is used.
[0121] When using a mold to form spacers on a display, it is very
desirable to be able to accurately align the mold with the
substrate so that the spacers are located at desirable positions
within the display. One method according to the invention for
improving the alignment of the mold with respect to the substrate
of the display shall now be described. Initially a primary positive
mold (e.g., such as mold 302 of FIG. 3A) is prepared so that
alignment marks are included on the mold. FIGS. 10A and 10B show
side and top views, respectively, of an example of such a mold 1002
that includes a support 1010, a plurality of posts 1020 and a three
alignment marks 1022. Mold 1002 is shown including three T-shaped
alignment marks, but it will be appreciated that any configuration
of alignment marks may be used. Mold 1002 is preferably formed
using multiple layers of photoresist as has been discussed above.
That is, one layer of photoresist is applied to the support and
imaged to form the alignment marks. Then, another layer of
photoresist is applied and imaged to form the posts 1020.
[0122] The mold 1002 is then used to prepare a secondary negative
mold. The first step in preparing the secondary negative mold is to
prepare a sheet of perforated metal (e.g, spring steel that is
0.001 inches thick) that has alignment apertures that match the
marks of positive mold 1002. FIG. 10C shows a top view of such a
sheet of spring steel 1030. Sheet 1030 includes apertures 1032 that
exactly match the alignment marks 1022 of mold 1002. Sheet 1030
also includes a plurality of perforations 1034.
[0123] FIG. 10D illustrates the production of a secondary mold 1050
that includes sheet 1030. First, the sheet 1030 is held in place so
that the alignment apertures 1032 of sheet 1030 are aligned with
the alignment marks 1022 of positive mold 1002. Then, silicone or
another resilient material 1040 is poured over mold 1002 so that
the material 1040 surrounds the perforated portion of sheet 1030,
conforms to the posts 1020 of mold 1002, and does not extend over
the alignment apertures of sheet 1030. After the material 1040
cures, the sheet 1030 is permanently embedded into (and becomes a
"spine" of) the material 1040. The finished mold 1050 includes the
cured material 1040 and the sheet 1030.
[0124] After the mold 1050 is separated from positive mold 1002,
the sheet 1030 comprises an alignment template for the mold 1050.
The sheet 1030 is flexible and permits the mold to flex or bend,
but the sheet 1030 will not stretch or otherwise permanently
deform. Mold 1050 may then be used with substrates that include
alignment marks matching the alignment apertures of the sheet 1030.
The alignment apertures of the mold 1050 are aligned with the
alignment marks of the substrate and this alignment is maintained
as the substrate and mold are brought into contact. In this way,
the accuracy of the photolithography process (used to create the
positive mold 1002) is transferred to the substrate and the spacers
are placed at desired locations on the substrate.
[0125] FIG. 11 shows an example of a preferred secondary negative
mold 1102 constructed according to the invention. Mold 1102
includes a plurality of apertures 1120 corresponding to posts of a
positive mold (and to spacers of a display). Mold 1102 also
includes a plurality of vacuum pump out channels 1122 (similar to
the channels 972 of mold 960 shown in FIG. 9F). Mold 1102 also
includes a metal alignment sheet 1130 (similar to sheet 1030 of
mold 1002 shown in FIGS. 10C and 10D). Mold 1102 also includes a
low durometer portion 1140 bonded to a high durometer portion 1142.
Low durometer portion 1140 defines all of the apertures 1120. The
alignment sheet is shown as being embedded in the low durometer
portion 1140 however, the alignment sheet could alternatively be
embedded into the high durometer portion.
[0126] In use, the alignment apertures of the alignment sheet 1130
of mold 1102 are first aligned with the alignment marks of a
substrate. Then the mold 1102 and substrate are brought into
contact while maintaining alignment of the alignment marks of the
substrate and the alignment apertures of the sheet 1130. Then a
clamping ring (similar to the ring 980 shown in FIG. 9G) is applied
to the outer periphery of the mold 1102 to hold the outer periphery
securely against the substrate. After the clamping ring has been so
applied, the vacuum pump out channels 1122 are used to evacuate
trapped gas (e.g., air) from between the interface of the substrate
and the mold 1102. Then the spacer material is injected into the
apertures of the mold 1102 using standard injection molding
techniques. About five cubic centimeters of spacer material are
typically injected into the mold to provide enough material for all
the spacers in a display having an active area of 80 square inches.
Then the spacer material is allowed to cure. After this curing, the
mold 1102 and substrate are separated. Air may be pumped into the
vacuum pump out channels to facilitate this separation. Thereafter,
the hardened spacer material that has attached to the substrate may
be air dried and then fired to fully cure the spacers.
EXAMPLES
[0127] The following examples are for purposes of illustration
only, and are not intended to limit the scope of the invention or
the claims.
Example 1
Glass-Resist Adhesion
[0128] A mold fabricated from SU-8 can be formed as described
herein. Briefly, a support, such as soda lime glass, is cleaned and
coated with a thin film of SU-8 resist (e.g., about 50 .mu.m),
which is then flood exposed to completely cure the SU-8 resist. A
thicker film of the SU-8 resist is then applied (e.g., 200-600
.mu.m), exposed with the desired photopattern, baked, and developed
in a PGMEA solvent. Negative resist allows the process of coating,
exposing and developing to be repeated, such that a complex 3D
shape can be formed.
[0129] As described herein, adhesion of the SU-8 resist to a glass
support can be poor, such that delamination occurs when the SU-8
resist exceeds a thickness of 200 .mu.m. A search for possible
priming methods was carried out in three broad categories: (1)
cleaning processes that leave the surface hydrophilic; (2) cleaning
processes that leave the surface hydrophobic; and (3) depositing
several monolayers of molecules to promote adhesion. Table 2 shows
the wetting behavior of the support for different surface
treatments. Treatments that left the surface hydrophilic
promoted-wetting of the SU-8 resist to the substrate. On the other
hand, adhesion promoters such as HMDS, surfasil, FRSiCl.sub.3 and
octadecyltrichlorosilane failed to promote adhesion between the
glass support and the SU-8 resist.
[0130] Taking the surface conditions into account along with the
solubility of the monomer in polar organic solvents, it was decided
that a better adhesion promoter would be terminated in hydroxyl
groups. Accordingly, triethanol amine was dissolved in methanol,
and glass supports were soaked for 3-5 minutes therein, then air
dried. A two level Toguchi test was performed with factor one being
the presence of the priming agent and factor two being the
temperature of the post-exposure bake. Temperature was the primary
factor that effected adhesion with 56% probability, and the primer
was the secondary factor that effected adhesion with 24%
probability comparing 95.degree. C. PEB with 115.degree. C.
PEB.
[0131] Surface treatments with triethanol amine significantly
increased the adhesive strength. This was demonstrated by a major
reduction in undercutting of small features during development.
Environmental testing of the resist in the presence of water,
however, showed the same failure rates as unprimed material.
[0132] Surface treatments with
2(3,4-epoxycyclohexyl)ethyltrimethoxysilane significantly increased
the adhesive strength between the glass support and the SU-8
resist, as shown in the crack test graph in FIG. 12. A sharp
indenter was pressed against the SU-8 resist until it broke through
to the glass support. The area delaminated by this process was
measured using a microscope. The relative areas lifted are
inversely proportional to the bond strength. As shown in FIG. 12,
the indenter produced smaller cracks in the
2(3,4-epoxycyclohexyl)ethyltrimethoxysilane-primed areas than in
the unprimed areas. Specifically, the change in area from unprimed
to 2(3,4-epoxycyclohexyl)ethyltrimethoxysilane-primed was 36 times.
In some of the 2(3,4-epoxycyclohexyl)ethyltrimethoxysilane-primed
cases, no lifting was detected except for the immediate damage
caused by the indenter.
Example 2
Modification of SU-8
[0133] Propanediol and hexanediol were added to an SU-8 resist to
determine the effects on the properties of the SU-8. As shown in
FIGS. 13A-B, the photospeed of the propanediol/SU-8 mix and the
hexanediol/SU-8 mix was faster than the photospeed of the SU-8
alone, and the adhesive strength of the propanediol/SU-8 mix with
the support and the hexanediol/SU-8 mix with the support was better
than the adhesive strength of the SU-8 alone.
Example 3
Sol Gel Post-Mold Treatment
[0134] After the sol gel spacers are formed, they must be washed
and dried, as described herein.
[0135] Washing the wet gels in deionized water can lead to swelling
and cracking if care is not taken to balance the osmotic pressure
between the solvent and the gel network. Sol gels will match the
dilution of the ionic species it is immersed in by swelling or
shrinking. Since the sol gels of the present invention contain a
large amount of sodium and potassium, they will swell and rupture
when placed in deionized water. When the sol gels of the present
invention are washed in a solution of deionized water saturated
with NaCl, they tend to shrink and expel water because the ion
concentration gradient is higher outside the gel than inside. Thus,
it has been found that a 1-10% solution of HCI in deionized water
can effectively wash the sol gels of the present invention without
swelling.
[0136] After washing, the gels must be dried. An increasing
temperature cycle produces dense sol gel spacers that are
well-bonded to the substrate.
[0137] First, a temperature of 110.degree. C. is required to remove
bulk water from the gel. This may be performed in air or a vacuum.
Initially, a 40-70.degree. C. pre-bake is performed to remove the
bulk water by evaporation. This was shown to close off the pores
and seal the interior of the gel. When heated to about 100.degree.
C., the gel would swell from steam build-up, creating bubbles which
led to rupture. Going directly from the acid wash to 110.degree. C.
has been shown to be an effective means of drying. The time
required for this step is about 24 hours.
[0138] The next ramp temperature is 120.degree. C. There is a
transition between 110.degree. C. and 120.degree. C. in the
behavior of the drying process. Below 120.degree. C., the bulk
water is evaporated from the gel. At 120.degree. C. and above,
water adsorbed on the surface of the silica begins to evaporate.
The time required for this step is about 4 hours.
[0139] At 150.degree. C., the hydroxyl groups on the surface of the
silica begin to condense in the reaction:
--Si--OH+HO--Si"Si--O--Si--+H.sub.2O. This process is responsible
for turning the gel into an insoluble porous glass.
[0140] The process continues at 170-180.degree. C., where the
silanol groups quickly condense. At this stage, the mechanical
properties of the glass improve as the bonds between the particles
of silica in the network get stronger.
[0141] Organic burnout is accomplished at 270.degree. C., followed
by a ramp to 430-460.degree. C. where the porous gel
consolidates.
[0142] Each of the patents and publications cited herein is
incorporated by reference herein in its entirety.
[0143] Although the invention has been set forth in detail and with
reference to examples, one skilled in the art will appreciate that
changes and modifications can be made to the invention without
departing from the spirit and scope of the invention.
1TABLE 1 Na.sub.2O--K.sub.2O--SiO.sub.2 Systems mol % by batch t,
.degree. C. at log .eta. (P) Na.sub.2O K.sub.20 8 9 10 11 12 13 3 2
-- 808 758 712 667 628 2 3 -- 796 746 702 658 621.5 1 4 -- 797 744
694 651 612 0 5 838 777 722 675.5 632 592 13 0 -- 712 675 641 607
578 11 2 -- 662 624 588 557 524 9 4 692 645 601 562 526 493 7 6 688
637 591 547 511 476 5 8 693 643 597 557 517 482 3 10 683 633 589
550 512 479 0 13 702 655 612 578 547 517 20 0 652 607.5 571 532 502
473 18 2 616 571 532 502 472 444 16 4 600 561 527 493 466 437 14 6
608 557 517 485 456 428.5 12 8 609 563 524 489 458 429 10 10 605
563 526 493 465.5 437 5 15 632 582 543 512 479 451 0 20 662 621 583
550 518 491
[0144] The data shown in Table 1 was taken from page 30 of the
Handbook of Glass Data, Part C, Ternary Silicate Glasses, by
Mazurin, Streltsina and Shvaiko-Shvaikovskaya.
2 TABLE 2 Surface Treatment Results Surfasil poor wetting hot
phosphoric acid wet surface HMDS poor wetting FRSiCl.sub.3 would
not wet Con HCl, IPA very good wetting control good wetting
NH.sub.4OH very good wetting KOH, 100 C good wetting HF 100:1 good
wetting octodecyltrichlorosilane patchy wetting control wet
NH.sub.4OH:H.sub.2O.sub.2 good wetting
* * * * *