U.S. patent application number 11/015364 was filed with the patent office on 2005-07-21 for liga developer apparatus system.
Invention is credited to Bankert, Michelle A., Boehme, Dale R., Christenson, Todd R..
Application Number | 20050155707 11/015364 |
Document ID | / |
Family ID | 26873786 |
Filed Date | 2005-07-21 |
United States Patent
Application |
20050155707 |
Kind Code |
A1 |
Boehme, Dale R. ; et
al. |
July 21, 2005 |
LIGA developer apparatus system
Abstract
A system to fabricate precise, high aspect ratio polymeric molds
by photolithograpic processe is described. The molds for producing
micro-scale parts from engineering materials by the LIGA process.
The invention is a developer system for developing a PMMA
photoresist having exposed patterns comprising features having both
very small sizes, and very high aspect ratios between part minimum
feature size and part overall dimension. The developer system of
the present invention comprises a developer tank, an intermediate
rinse tank and a final rinse tank, each tank having a source of
high frequency sonic agitation, temperature control, and continuous
filtration. It has been found that by moving a patterned, LIGA
wafer, through a specific sequence of developer/rinse solutions,
wherein the solutions are agitated with a source of high frequency
sonic vibration, wherein the solution temperature of each tank is
adjusted and closely controlled, and wherein the solutions are
continuously recirculated and filtered, it is possible to maintain
the kinetic dissolution of the exposed PMMA polymer as the rate
limiting step.
Inventors: |
Boehme, Dale R.;
(Pleasanton, CA) ; Bankert, Michelle A.; (San
Francisco, CA) ; Christenson, Todd R.; (Albuquerque,
NM) |
Correspondence
Address: |
KURT C. OLSEN
SANDIA LABS - MAIL STOP 9031
P O BOX 969
LIVERMORE
CA
94551-0969
US
|
Family ID: |
26873786 |
Appl. No.: |
11/015364 |
Filed: |
December 15, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11015364 |
Dec 15, 2004 |
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09493926 |
Jan 28, 2000 |
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6517665 |
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60177929 |
Jan 25, 2000 |
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Current U.S.
Class: |
156/345.18 ;
156/345.11 |
Current CPC
Class: |
G03F 7/3014 20130101;
G03F 7/40 20130101; B29C 33/40 20130101; B29L 2031/756
20130101 |
Class at
Publication: |
156/345.18 ;
156/345.11 |
International
Class: |
C23F 001/00 |
Goverment Interests
[0001] The United States Government has rights in this invention
pursuant to Contract No. DE-AC04-94AL85000 between the United
States Department of Energy and the Sandia Corporation for the
operation of the Sandia National Laboratories.
Claims
1. (canceled)
2. (canceled)
3. (canceled)
4. (canceled)
5. (canceled)
6. (canceled)
7. (canceled)
8. (canceled)
9. (canceled)
10. (canceled)
11. (canceled)
12. (canceled)
13. (canceled)
14. (canceled)
15. (canceled)
16. (canceled)
17. A polymeric mold for producing a micro-scale part comprising: a
mold substrate, said substrate having an intermediate adhesion
layer; one or more polymer layers on said intermediate layer, said
one or more layers having a thickness of about 50 microns greater
than a thickness of a part to be produced; and features within said
one or more layers having dimensions of up to three orders of
magnitude smaller than said part thickness or said part diameter or
length.
18. The mold of claim 17, wherein the photoresist layer further
comprises PMMA.
19. The mold of claim 17, wherein the mold substrate is selected
from the group of materials consisting of silicon, metals, or
ceramics, and combinations thereof.
20. (canceled)
Description
BACKGROUND OF THE INVENTION
[0002] The present invention relates to the production of
microstructures and/or microparts, and particularly to a system for
developing a polymeric mold used for the production of microparts.
More particularly, the present invention relates to an apparatus
for producing precise, high aspect ratio polymer molds which may be
adapted for fabricating micro-scale, metal, polymer, or ceramic
parts using the so-called LIGA process.
[0003] LIGA, is an acronym derived from the German words for
Lithography, Electroforming, and Molding. The LIGA process is being
evaluated worldwide as a method to produce microstructures and/or
microparts from engineering materials.
[0004] The LIGA process was pioneered in the early 1980s as a
method to produce precise, high aspect ratio microstructures from
engineering materials, such as various metals, polymers, and
ceramics. See E. W. Becker, et al., Microelectronic Engr. 4 (1986)
35; and W. Ehrfeld, et al., KFK-Nachrichten 10(4) (1987) 167.
[0005] In the general LIGA process x-ray radiation from a
synchrotron source passes through, and is patterned by, a specially
designed mask to produce deep exposures in a x-ray resist,
typically polymethylmethacrylate (PMMA), with precise lateral
dimensions. The PMMA, after exposure, is placed in a chemical
developer to remove the exposed material and produce thereby a
polymeric mold. This mold is most commonly used as an
electroplating template to produce metal microparts or a metal
master mold. If a metal master mold is made, it can be used to
produce cost-effective replicates in other materials, primarily
polymers. Finally, the process can be used also to directly produce
PMMA microstructures and/or microparts.
[0006] One of the appeals of LIGA as a fabrication methodology is
the ability to produce precise, micro-scale parts with high aspect
ratios made from traditional metallic materials. Applications such
as motors, spinnerets, and switches have been explored using metal
microparts fabrication from LIGA. Over the past few years there has
been a growing interest in plastic parts for applications such as
spectrometers, microanalytical instrumentation, and medical
applications. Also emerging is an interest in ceramic materials in
LIGA fabricated structures. Ceramic materials allow improved
magnetic properties, piezoelectric properties, and application at
higher temperatures.
[0007] In order to produce metal, plastic, or ceramic LIGA parts,
it is necessary to have the appropriate equipment, systems, and
processes in order to conduct synchrotron exposures and subsequent
development of the exposed PMMA to produce the required polymer
mold. The present invention involves an apparatus for practicing
the photoresist development step of the LIGA process. In
particular, the present invention is drawn to a photoresist
developer system comprising a group of developer tanks; appropriate
developer solutions, high frequency solution agitation, continuous
solution circulation and filtration, and close temperature control
of the tank contents.
SUMMARY OF THE INVENTION
[0008] The present invention involves a developer station which
enables a user to readily produce polymeric molds to be used in the
production of precision micro-scale mechanical parts by the LIGA
process.
[0009] It is, therefore, an object of the invention to provide an
apparatus for preparing polymeric molds for subsequent production
of micro-scale parts from engineering materials.
[0010] A further object of the invention is to provide a developer
station to enable the preparation of polymeric molds to produce
precise, high aspect ratio micro-scale parts from engineering
materials and also molds having widely differing aspect ratios.
[0011] Still another object of the present invention is the
production of molds exhibiting feature sizes which may be several
orders of magnitude smaller than an overall mold dimension.
[0012] More specifically the present invention involves making a
polymeric mold by carefully developing an exposed image in a
photoresist substrate using the developer station described
below.
[0013] Other objects and advantages of the present invention will
become apparent from the following description and accompanying the
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] The accompanying drawings, which are incorporated into and
form a part of the disclosure, illustrate an embodiment of the
invention and, together with the description, serve to explain the
principles of the invention.
[0015] FIGS. 1-5 illustrate the processing operations of the prior
art to produce micro-scale molds or parts, as shown in FIG. 5.
[0016] FIG. 6 schematically illustrates a three-tank system for
development after exposure.
[0017] FIG. 7 schematically illustrates an assembly including the
tank system of FIG. 6 and the controls therefor.
[0018] FIG. 8 schematically illustrates an enlarged view of one of
the developer tanks having a filter and cooling coil with
temperature control.
[0019] FIG. 9 illustrates an embodiment of a vertical wafer or
substrate retaining assembly for use in the development tanks and
subsequent plating.
[0020] FIG. 10 illustrates an embodiment of a horizontal substrate
or wafer retaining assembly for use in the development tanks and
subsequent plating.
DETAILED DESCRIPTION OF THE INVENTION
[0021] The present invention is drawn to a photoresist developer
station optimized to produce high aspect ratio polymeric parts or
polymeric molds from which micro-scale parts from engineering
materials can be produced. X-ray radiation from a synchrotron is
passed through a specially designed mask to allow deep exposures in
a x-ray resist, such as polymethylmethacrylate (PMMA) with precise
lateral dimensions. The PMMA, after exposure to the synchrotron
radiation, is placed in a chemical developer, and a polymeric mold
is obtained by dissolution of the exposed portion of the PMMA
resist layer. In this invention the chemical developer station
includes the use of a series of developer tanks and solutions,
including at least a chemical developer tank, an intermediate rinse
tank, and a water rinse tank. Each tank can include a means to
maintain temperature control in the tank solution. Each tank can
also include a closed recirculation means and continuous
filtration. Finally, a means to effectively agitate the solution
with a source of high frequency, or "megasonic" vibration (as used
hereinafter the term "megasonic" is intended to refer to sonic
frequencies above about 500 kHz).
[0022] Referring now to the drawings, FIGS. 1-5 illustrate the
prior art method of the LIGA process. FIG. 1 illustrates a LIGA
wafer generally indicated at 10 and broadly composed of a substrate
11, a metal layer (electrode) 12, a layer 13 of x-ray photoresist
such as polymethylmethacrylate (PMMA), and a mask 14 having
patterned shapes 15 therein. By way of example, the substrate 11 is
composed of silicon with a thickness of 600 .mu.m and a diameter
larger than the 3- or 4-inch diameter mask 14. The metal layer 12
may be composed of three layers, about a 700 .ANG. titanium layer,
a 4000 .ANG. nickel or copper layer, and 700 .ANG. layer of
titanium. The PMMA layer 13 may vary in thickness of 100 .mu.m to 3
mm, but is typically used with a thickness of about 50 .mu.m
greater than the final thickness of the microstructures being
fabricated. Thus, the substrate 11, metal layer 12, and PMMA layer
13 may be obtained as an off-the-shelf component. The mask 14, for
example, is formed from a 100 .mu.m thick, 3 or 4-inch diameter
metallized silicon substrate, and patterned using one or two layers
of photoresist, as described above, and gold is electroplated in
the photoresist pattern using a gold sulfide bath. The gold
thickness ranges from about 8.mu. to 30 .mu.m depending on the
desired lateral feature sizes and tolerance control. Thus, the mask
14 is generally described as a x-ray mask composed of a gold
absorber patterned on a 100 .mu.m metallized silicon substrate.
Note, as shown as in FIG. 1, that the substrate 11 and metal layer
12, are larger than the PMMA layer 13 and the mask 14.
[0023] As shown schematically in FIG. 2, the LIGA wafer 10 is
exposed to x-rays as indicated by arrow 16 from a synchrotron, such
as by scanning of the mask 14 and PMMA resist layer 13 through a
stationary x-ray beam to provide an evenly distributed exposure
over the LIGA wafer 10.
[0024] FIG. 3 schematically illustrates the prior art chemical
development process. As shown in FIG. 3, the LIGA wafer 10 after
x-ray exposure, as shown in FIG. 2, is placed in a chemical
developer tank 17 for development of patterned openings as
indicated at 18, in the PMMA layer 13 under the patterned areas 15
of the mask 14.
[0025] As shown in FIG. 4, the developed LIGA wafer 10 of FIG. 3,
is placed in a plating tank 20 containing a plating solution 21
whereby the openings 18 in PMMA layer 13 are filled by an
electroplating process to produce patterned microstructures 22. In
the electroplating process, the metal layer 12 functions as an
electrode and is connected to a power source 23 by a lead 24, with
the tank 20 functioning as the electrode and connected to power
source 23 by a lead 25. FIG. 5 illustrates the electroplated LIGA
wafer 10 with patterned microstructures 22 formed on metal layer
12.
[0026] The final step in producing metal microparts is a lapping
and polishing process used to bring the parts to their to the final
thickness.
[0027] Following lapping and polishing, the microstructures 22 may
be removed from the wafer 10 to produce microparts or, if
microstructures 22 are to serve as molds, they are retained on the
wafer.
THE PREFERRED EMBODIMENT OF THE PRESENT INVENTION
[0028] The invention is a development system for developing a PMMA
photoresist having exposed patterns comprising both very small
feature sizes, and very high aspect ratios between part minimum
feature size and part overall extent. The development system of the
present invention is described in greater detail hereinafter with
respect to FIG. 6. The apparatus comprises a developer tank, an
intermediate rinse tank and a final rinse tank, each tank having a
source of high frequency sonic agitation, temperature control, and
continuous filtration. The inventors have found that by moving the
patterned, LIGA wafer 10, of FIG. 3, through a specific sequence of
developer/rinse solutions, wherein the solutions are agitated with
a source of high frequency sonic vibration, wherein the solution
temperature of each tank is adjusted and closely controlled, and
wherein the solutions are continuously recirculated and filtered,
it is possible to maintain the kinetic dissolution of the exposed
PMMA polymer as the rate limiting step. This condition is important
because, when the kinetic dissolution is the rate limiting step
instead of mass-transport of the developer solution, features of
widely varying dimensions can be "developed," i.e., dissolved, at
essentially the same rate. This means that development of small
features do not dictate the conclusion of the development step.
[0029] As shown schematically in FIG. 6, the development apparatus,
generally indicated at 30, comprises three tanks 31, 32, and 33
located in an atmosphere controlled fume hood 34. The tanks
comprise a developer tank 31, an intermediate rinse tank 32, and a
water rinse tank 33, within each of which is contained a solution
35, 36, and 37, respectively, and the solutions are continuously
filtered with filters 38, 39, and 40, which may be 0.2 micron
polytetrafluoroethylene filters, with the solutions being drawn
through the filters by peristalic pumps 40, 41, and 42 via inlet
lines 43, 44, and 45 and outlet lines 46, 47, and 48. Pumps 40, 41,
and 42 are connected to a pneumatic pump air supply 49 via lines
50. Due to the corrosive nature of solution 36, tank 31 and all of
the fluid inlet and outlet lines and fittings must be constructed
of a chemically inert material, such as glass, ceramic, or
polytetrafluoroethylene.
[0030] Tank 31 is provided with an immersion coil 51 connected to a
temperature control 52 via line 53. Similarly, tanks 32 and 33 are
provided with coils 54 and 55 connected to a temperature control 56
via lines 57. Coils 51, 54, and 55 are designed to operate by
circulating warm water therein by temperature controls 52 and 56.
The solution in each tank, therefore, is maintained, for example,
at 25.degree. C..+-.3.degree. C. via the immersion coils 51, 54,
and 55 which are constructed of gold plated copper tubing to
minimize corrosion and tank solution contamination.
[0031] Each of tanks 31-33 is provided with a high frequency, sonic
agitation unit indicated at 58 and hereinafter referred to as a
megasonic unit. While the beneficial effect of this agitation is
important in each of tanks 31-33, it is most important, and indeed
essential, in the developer tank 31 where the agitation is used to
loosen and remove the dissolving PMMA and to assist with the
transport of fresh developer solution to the PMMA interface. Each
of the megasonic units 58 is connected to a single megasonic
control 59 or a separate control unit (not shown) via line 60. The
megasonic transducer element is placed in the bottom of each tank
but is not required to by fixed in any particular location.
[0032] By way of example, the developer solution 35 in developer
tank 31 may comprise a solution of di-(ethylene glycol) butyl
ether, morpholine, ethanolamine; and water. The present invention
utilizes a known solution composition consisting of 60w/o
di-(ethylene glycol) butyl ether, 20w/o morpholine, 5w/o
ethanolamine, and 15w/o water. Solution 36 in intermediate rinse
tank 32 may, for example, comprise a solution of di-(ethylene
glycol) butyl ether and water; and is in the present invention
consists essentially of 80w/o solution of di-(ethylene glycol)
butyl ether in water. Solution 37 in rinse tank 33 is essentially
pure water.
[0033] Tank solutions 35-37 may, for example, be circulated by
pumps 40-42 through the filter 38-40. Circulation rates may be from
nearly stagnant to several liter-per-minute in order to accommodate
the wide range of development rates necessitated by the different
designs and features used in LIGA. It is to be understood that the
composition of solution in developer tank 31, will be dependent,
particularly on the composition of the components of the wafer
10.
[0034] It is important to note that while the function of the
developer solution is obvious, the development process is not.
Rather, developing the PMMA is multi-step process which includes,
among other steps, a gellation step which can, and often does,
leave partially developed, or "gelled" PMMA on the surfaces of the
microstructures 22 of FIG. 5 especially in regions of widely
varying feature size. The inventors have found that a critical
aspect of the instant invention is the use of the intermediate
rinse solution 84. In particular, it was been found that by placing
a wafers taken from the developer tank 31 directly into water rinse
tank 33 without the intermediate rinse in tank 32, any partially
developed PMMA photoresist immediately recrystallizes or "freezes"
in place and ruins that mold. This step is crucial to the
preparation of high quality microstructures 22 since any
recrystallized PMMA cannot be further developed.
[0035] By using the intermediate rinse step in a solution
containing di-(ethylene glycol) butyl ether and water after initial
development, the tendency for recrystallization of gelled PMMA is
virtually eliminated by continuing the development process.
Together with the megasonic agitation the intermediate rinse
removes any latent developed PMMA.
[0036] Similarly, the application of megasonic agitation to a
photoresist development process is also not obvious. This equipment
was originally designed to aid in cleaning flat silicon wafers
prior to further microelectronics processing and while use of
megasonic agitation has been suggested, in U.S. Pat. Ser. No.
4,213,807 (col. 3, line 19), to shorten a silicon oxide etching
process step the teaching of this patent further associates
megasonic agitation with a "cleaning" step (col. 3, line 28) to
mechanically remove adhering particles. Application of megasonic
agitation as a means for helping move product/reactants to and from
a reacting (developing) photolithographic interface, therefore, is
believed not to have been reported its application as an aid in
improving the ability to prepare and produce micro-parts exhibiting
a wide variance in dimension (high aspect ratio) is, therefore,
believed to be unique.
A BEST MODE FOR PRACTICING THE PRESENT INVENTION
[0037] FIG. 7 illustrates a second embodiment of a three tank
developer assembly, with each of the three tanks including
filtration means, temperature control means, and megasonic
agitation units. The developer assembly generally indicated at 70
is designed as a stand-alone developer work station and includes
stand 71 mounted to floor 72. Positioned on stand 71 is a box 73
having a hood 74 which includes a window 75, the hood 74 being
shown in raised position. Box 73 may be constructed of stainless
steel, polytetrafluoroethylene, DELRIN.TM., polyethylene or any
other corrosion resistant material. Window 75 is usually
constructed with a high strength tempered glass or, if required, a
blast-resistant transparent material such as a polycarbonate. The
box 73 may be atmospherically controlled, and includes a horizontal
support member 76 and two end vertical support members 77 to which
support member 76 is connected at each end, and another vertical
support member 79 for horizontal support member 76. Tanks 80, 81,
and 82 are constructed of a corrosion resistant material such as
glass, polytetrafluoroethylene, DELRIN.TM., or polyethylene and are
retained in, or mounted to, horizontal support member 76, with tank
80 containing a developer solution 83, tank 81 containing an
intermediate rinse solution 84, and tank 82, containing a rinse
solution 85 of water. Each tank 80, 81, and 82 is provided with a
filter 86 and a circulation pump 87, as described with respect to
FIG. 6. Each tank is also provided with a temperature control coil
and at least tanks 80 and 81 are provided with a megasonic
agitation unit, again as described above with respect to FIG.
6.
[0038] Positioned adjacent to stand 71 is a control rack 88 having
mounted therein a temperature control 89 and a megasonic control
90. A LIGA wafer or substrate support assembly 91 is mounted in lid
or top section 92 of box 73 and includes a handle or rod 93 which
retains one or more LIGA wafers, as shown in FIG. 8. The wafer
support assembly 91 is moved from tank 80 to tank 81 to tank 82
during the development process, thereby avoiding unwanted delays
and maintaining control over the development process.
[0039] The vertical wafer or substrate support assembly 91 is
illustrated in detail in FIGS. 8 and 9. Illustrated components, and
the developer tank 80 are similar to those of FIG. 7 and are given
corresponding reference numerals. As shown in FIG. 8, the developer
tank 80 is provided with a temperature control coil 94 and
controller 89; and provided with megasonic agitation units 96 and
controller 90. The vertical LIGA wafer or substrate support
assembly, as shown in FIGS. 8 and 9, includes a holder 97 having
lower side slits 98 into which is retained a LIGA wafer 10' having
a metallized substrate 11' a layer of PMMA 13' with patterned
shapes (parts) 15' formed in the PMMA layer 13 by x-ray exposure as
described above. After a predetermined time, the wafer or substrate
support assembly 91 is withdrawn from tank 80 and placed in tank
81, and then into tank 82 for completion of the chemical
development process. The use of the support assembly allows quickly
moving wafer 10' from tank to tank without allowing the surface and
features of the patterned shapes (parts) 15' formed in the PMMA
layer to dry.
[0040] FIG. 10 illustrates a horizontal LIGA wafer support assembly
91' having a handle or rod 93' connected to a holder 97' which
includes an opening 99 and groove 100 into which a LIGA wafer is
slid and retained during the development process. Furthermore,
wafer 10' is inserted into support assembly 91' such that the
reacting surface of the PMMA is facing out into the tank solution
and is oriented, therefore, in an inverted position. The inventors
have discovered that the horizontal support assembly is required in
many cases to maintain adequate mass transport at the reacting
(developing) interface of the PMMA, for successful development of
certain high aspect ratio microstructure devices. Presumably, this
behavior is due to the intrinsic flow characteristics of the
dissolving PMMA as it migrates out of the micro-channels created in
the developing mold at sites of very small feature, such at the
tips of gear teeth, and the like. It is therefore a critical aspect
of this invention to orient the developing wafer 10' in the
developer solution such that it is held in an inverted
position.
[0041] It has thus been shown-that the present invention provides
the required system for producing microstructures and/or microparts
utilizing the LIGA process. The method utilizes a multiple tank
development process, whereby precise, high aspect ratio
microstructures are produced in polymeric materials such as PMMA.
The parts thus produced may be utilized as molds for polymeric and
ceramic parts or may be finished as microparts for complex
assemblies.
[0042] While a particular embodiment, process sequence, materials,
development tank arrangement, and wafer holders have been described
and or illustrated, such are not intended to be limiting.
Modifications and changes may become apparent to those skilled in
the art, and it is intended that the scope of the invention be
limited only by scope of the appended claims.
* * * * *