U.S. patent application number 10/947557 was filed with the patent office on 2005-03-31 for microfabricated structures and processes for manufacturing same.
This patent application is currently assigned to The University of Cincinnati. Invention is credited to Ahn, Chong H., Appasamy, Sreeram C., Trichur, Krishnan.
Application Number | 20050067286 10/947557 |
Document ID | / |
Family ID | 34382319 |
Filed Date | 2005-03-31 |
United States Patent
Application |
20050067286 |
Kind Code |
A1 |
Ahn, Chong H. ; et
al. |
March 31, 2005 |
Microfabricated structures and processes for manufacturing same
Abstract
Various techniques for the fabrication of highly accurate master
molds with precisely defined microstructures for use in plastic
replication using injection molding, hot embossing, or casting
techniques are disclosed herein. Three different fabrication
processes used for master mold fabrication are disclosed wherein
one of the processes is a combination of the other two processes.
In an embodiment of the first process, a two-step electroplating
approach is used wherein one of the metals forms the
microstructures and the second metal is used as a sacrificial
support layer. Following electroplating, the exact height of the
microstructures is defined using a chemical mechanical polishing
process. In an embodiment of the second process, a modified
electroforming process is used for master mold fabrication. The
specific modifications include the use of Nickel-Iron (80:20) as a
structural component of the master mold, and the use of a higher
saccharin concentration in the electroplating bath to reduce
tensile stress during plating and electroforming on the top as well
as sides of the dummy substrate to prevent peel off of the
electroform. The electroforming process is also well suited towards
the fabrication of microstructures with non-rectangular cross
sectional profiles. Also disclosed is an embodiment of a simple
fabrication process using direct deposition of a curable liquid
molding material combined with the electroforming process. Finally,
an embodiment of a third fabrication process combines the
meritorious features of the first two approaches and is used to
fabricate a master mold using a combination of the two-step
electroplating plus chemical mechanical polishing approach and the
electroforming approach to fabricate highly accurate master molds
with precisely defined microstructures. The microstructures are an
integral part of the master mold and hence the master mold is more
robust and well suited for high volume production of plastic MEMS
devices through replication techniques such as injection
molding.
Inventors: |
Ahn, Chong H.; (Cincinnati,
OH) ; Trichur, Krishnan; (Cincinnati, OH) ;
Appasamy, Sreeram C.; (Cincinnati, OH) |
Correspondence
Address: |
HAHN LOESER & PARKS, LLP
One GOJO Plaza
Suite 300
AKRON
OH
44311-1076
US
|
Assignee: |
The University of
Cincinnati
|
Family ID: |
34382319 |
Appl. No.: |
10/947557 |
Filed: |
September 22, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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60506641 |
Sep 26, 2003 |
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60506226 |
Sep 26, 2003 |
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60506321 |
Sep 26, 2003 |
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60506424 |
Sep 26, 2003 |
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60506635 |
Sep 26, 2003 |
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Current U.S.
Class: |
205/70 |
Current CPC
Class: |
C25D 1/10 20130101; B81B
2203/0376 20130101; B81B 2203/0361 20130101; B81C 99/009 20130101;
B81B 2203/033 20130101 |
Class at
Publication: |
205/070 |
International
Class: |
C25D 001/10 |
Goverment Interests
[0002] The U.S. Government has a paid-up license in this invention
and the right in limited circumstances to require the patent owner
to license others on reasonable terms as provided for by the terms
of grant no. AF F30602-00-1-0569 awarded by the Defense Advanced
Research Projects Agency (DARPA).
Claims
What is claimed is:
1. A method to generate a master mold used for micro-fabricating
plastic substrates, said method comprising: depositing a
micro-structure pattern of photo-resist onto a substrate;
over-plating a first metal onto said pattern to generate a
plurality of metal micro-structures; removing said pattern of
photo-resist from said substrate; and over-plating a second metal
onto said substrate and said plurality of metal
micro-structures.
2. The method of claim 1 further comprising lapping said second
metal and said plurality of metal micro-structures to create a
planar surface.
3. The method of claim 2 further comprising removing said second
metal from said substrate and said plurality of metal
micro-structures, leaving said master mold comprising said
substrate and said plurality of metal micro-structures.
4. The method of claim 1 wherein said substrate comprises
nickel.
5. The method of claim 1 wherein said first metal comprises
nickel.
6. The method of claim 1 wherein said first metal comprises an
alloy of nickel and iron.
7. The method of claim 1 wherein said second metal comprises
copper.
8. The method of claim 1 wherein said depositing said
micro-structure pattern of said photo-resist comprises performing a
photolithography process.
9. The method of claim 1 wherein said over-plating said first metal
comprises performing an electroplating process such that said first
metal is electroplated past a height of said photo-resist.
10. The method of claim 1 wherein said removing said pattern of
said photo-resist comprises performing a stripping process using a
remover solution.
11. The method of claim 1 wherein said over-plating said second
metal comprises performing an electroplating process such that said
second metal is electroplated past a height of said plurality of
metal micro-structures.
12. The method of claim 2 wherein said lapping comprises performing
a chemical-mechanical polishing process.
13. The method of claim 3 wherein said removing said second metal
comprises performing an etching process.
14. A method to generate a master mold used for micro-fabricating
plastic substrates, said method comprising: generating a
micro-structure pattern on a substrate; heating said
micro-structure pattern to form a pattern of at least one
quasi-semi-spherical feature on said substrate; coating said
substrate and said pattern of at least one quasi-semi-spherical
feature with a seed layer; and depositing a metal layer onto said
seed layer.
15. The method of claim 14 further comprising polishing a back of
said metal layer to form a planar surface.
16. The method of claim 15 further comprising removing said
substrate from said metal layer, said seed layer, and said at least
one quasi-semi-spherical feature.
17. The method of claim 16 further comprising removing said seed
layer to leave only said polished metal layer as said master
mold.
18. The method of claim 14 wherein generating said micro-structure
pattern is accomplished using a photo-resist and photolithography
techniques.
19. The method of claim 14 wherein said micro-structure pattern
comprises photoresist.
20. The method of claim 14 wherein said substrate comprises one of
silicon, glass, ceramic, plastic, and metal.
21. The method of claim 14 wherein said metal layer comprises an
alloy of Nickel and Iron.
22. The method of claim 14 wherein said step of depositing a metal
layer comprises an electroplating step.
23. The method of claim 16 wherein said substrate is removed using
a chemical-mechanical polishing technique.
24. The method of claim 17 wherein said seed layer is removed using
an etching technique.
25. A method to generate a master mold used for micro-fabricating
plastic substrates, said method comprising: generating a
micro-structure pattern of at least one quasi-semi-spherical
feature on said substrate by dispensing droplets of a polymer
material onto said substrate; allowing said droplets of polymer
material to cure; coating said substrate and said pattern of at
least one quasi-semi-spherical feature with a seed layer; and
depositing a metal layer onto said seed layer.
26. The method of claim 25 further comprising polishing a back of
said metal layer to form a planar surface.
27. The method of claim 26 further comprising removing said
substrate from said metal layer, said seed layer, and said at least
one quasi-semi-spherical feature.
28. The method of claim 27 further comprising removing said seed
layer to leave only said polished metal layer as said master mold.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS/INCORPORATION BY
REFERENCE
[0001] This application claims priority to provisional U.S. Patent
Applications Ser. Nos. 60/506,641; 60/506,226; 60/506,321;
60/506,424; and 60/506,635 all filed on Sep. 26, 2003, and all of
which are incorporated herein by reference in their entirety.
[0003] This patent application is being filed concurrently with
U.S. Patent Applications having attorney docket numbers
200057.00008, 200057.00009, 200057.00010, and 200057.00011, which
are incorporated herein by reference in their entirety.
TECHNICAL FIELD
[0004] Embodiments of the present invention generally relate to the
fabrication of ultra-high precision master molds for high volume
production of microfabricated structures through plastic
replication processes like injection molding and hot embossing.
Herein is described an approach for fabricating microstructures
with extremely uniform features and high surface quality using a
two-step electroplating process followed by a planarization
approach where one of the metals is used as a sacrificial support
structure. This technique is particularly well suited for
fabricating rectangular cross-section microstructures as used in
microfluidic devices. Nickel Iron (80:20) alloy is introduced as a
low stress material for the fabrication of master molds using the
process of electroforming. The electroforming process can be used
for fabricating microstructures with rectangular cross-sections
suited towards microfluidic applications, as well as other shapes
more suited for optical applications. Also, a very simple technique
for manufacturing a high density microlens array for optical
applications is described herein.
BACKGROUND OF THE INVENTION
[0005] MEMS (Micro Electro Mechanical Systems) technology has
enabled fabrication of miniaturized devices with applications in a
variety of fields like aerospace, medicine and telecommunications.
Traditionally most MEMS based devices have been fabricated on
Silicon or Glass substrate using the same technology as developed
for the microelectronics industry. An example of this is disclosed
by Chow et al in U.S. Pat. No. 6,167,910 (incorporated herein in
its entirety by reference), wherein a three-dimensional
microfluidic device is fabricated by stacking glass/Silicon
substrates onto which microchannels for flow confinement have been
defined. However, specifically for microfluidic and more
specifically for BioMEMS related applications, Silicon and Glass
were observed to have many limitations in terms of the surface
exposed to the fluids and/or biological samples. One specific
problem is non-specific adsorption of proteins commonly encountered
in biological samples. Considerable care and conditioning are
required to use Silicon or glass substrates in such applications.
Furthermore, Silicon and Glass are processed using the so-called
"serial process" wherein, for some of the processing steps, each
substrate is processed individually (even if it forms part of a
batch). With the high intrinsic cost of these substrates, the
serial processing further adds to the cost of the device.
[0006] Hence, there has been considerable interest in using polymer
or plastic substrates for MEMS and BioMEMS applications. The
appropriate plastics offer numerous advantages such as low-cost,
high durability, high strength, and excellent biocompatibility
characteristics. Another area where plastic devices are obviously
suited is Optical MEMS wherein optical transmission characteristics
with respect to a range of wavelengths are critical.
Polymer/Plastic substrates can be broadly divided into two
categories: thermoplastic materials, which deform when heated, and
thermoset materials, which solidify after heating. Amongst the two,
thermoplastics have gained wider acceptance owing to the ability of
using mass-manufacturing techniques such as injection molding and
embossing. A notable exception to this is the use of
Poly-dimethylsiloxane (PDMS), which is a thermoset material for
rapid-prototyping of microfluidic devices as described in U.S. Pat.
No. 6,686,184 and WO02100542A1, incorporated herein in their
entirety by reference.
[0007] The most common approach to fabricate plastic microfluidic
devices is to create a "master mold" and a suitable replication
technique such as hot embossing or injection molding to transfer
the pattern to a thermoplastic substrate. In earlier approaches,
again Silicon was used as the material for the master mold and the
desired features were formed by either wet-chemical etching or deep
reactive ion etching (DRIE) as described in U.S. Pat. No. 6,136,243
and as discussed by G. Kovacs, Micromachined Transducers
Sourcebook, WCB-McGraw Hill, New York, 1998. However, Silicon
master-mold cannot be easily used with injection molding equipment
due to processing complexities and the relatively fragile nature of
the Silicon substrate. Silicon substrates are most commonly limited
to replication with hot-embossing techniques as described in
WO4022302A3 (incorporated herein in its entirety by reference). Hot
embossing is an inherently slow-process and cannot match the
short-cycle times of injection molding.
[0008] The most commonly accepted substrate material for injection
molding is some form of a metallic substrate. Metallic substrates
offer the desired strength and machining characteristics required
for injection molding equipment. A common technique for creating
master molds is the use of high-precision milling tools to define
precise features onto a metal substrate. However, the milling
process is limited in terms of achievable feature size and aspect
ratio (the ratio of height to width for a microstructure). UV-LIGA
is another one of the commonly used techniques to form micro
fabricated master molds. It involves creating a mold from a
photoresist, deposited onto a substrate, using photolithography
followed by electroplating within the photoresist mold. After
electroplating, the photoresist mold is dissolved using suitable
solvents and the electroplated metal pattern (usually of the same
material as the substrate) acts as a master mold for plastic
replication.
[0009] One of the difficulties in this method is the fabrication of
structures that do not have a rectangular cross-section. One
example of such a structure being the hemispherical shape required
to fabricated lenses or an array of microlenses for optical MEMS
applications. A possible solution to this problem is offered by the
use of the so-called "gray-scale" lithography techniques, wherein
different sections of the photoresist are exposed to continuously
varying UV energy thereby allowing for the formation of "sloped" or
"rounded" features as described in U.S. Pat. No. 6,410,213
(incorporated herein in its entirety by reference).
[0010] Another major drawback in UV-LIGA based master mold
fabrication is non-uniform electroplating thickness in photoresist
mold patterns having different dimensions (specifically different
widths). Due to the process described as "current-crowding", the
plating rate is faster in channels with smaller widths than in
channels with higher widths due to the concentrated current flux in
narrow channels. This may give rise to problems where the
microstructure needs to be very accurately defined e.g. in
microfluidic applications. Furthermore, another problem of the
current-crowding effect is the non-uniform cross-sectional profile
achieved after electroplating, wherein typically (along the
cross-section) the center of the microstructure is plated to a
lower height as compared to the edges of the microstructure. This
problem is discussed clearly in JP1165794A2 (incorporated herein in
its entirety by reference), wherein a sharpened anode was used to
plate within a narrow feature, and pulled out at approximately the
same rate as the plating occurred. Though, this approach allows for
uniform plating, it is fairly complex to set it up and may not be
suitable for plating large areas.
[0011] Another method for creating master molds is by using the
techniques of electroforming. In this approach, a photoresist mold
is created on a dummy substrate. Alternately, the mold pattern can
also be directly etched into the dummy substrate using wet chemical
etching, electro-discharge machining (EDM) or conventional
micro-milling. Following this, a metal seed layer is deposited on
the photoresist (or substrate) mold and an appropriate metal is
electroplated beyond the thickness of the mold. In most cases (for
microfabricated master molds), the electroformed metal is plated to
a thickness of <1 mm and then peeled off the dummy substrate.
The concept of electroforming is certainly not new as illustrated
by applications (JP54067561A2) dating back to 1979. The process of
electroforming is well known to those skilled in the art and
extensively described in WO03041934A1 and JP62111755A2,
incorporated herein in their entirety by reference, amongst
others.
[0012] One of the primary issues in electroforming is stress
control. When a metal is electroplated to large thicknesses, the
tensile and compressive stresses dominate the final electroform
shape. If the stress is not controlled properly, the electroformed
master mold is severely distorted making it useless for plastic
replication. This issue has limited the use of electroforming
techniques for injection molding applications.
[0013] On the other hand, electroforming techniques offer the
advantage of non-rectangular cross-sections that are suitable for
plastic replication techniques. Specifically, if a master mold
feature is wider at the top than the bottom, it is not possible to
injection mold (or emboss) a copy of it since the mold pattern will
be "stuck" into the plastic substrate. However, when the master
mold pattern is narrower at the top, it is easier to separate the
master mold and the replicated plastic part. Such profiles are very
difficult to create (except by using complex gray-scale lithography
techniques as explained earlier) using UV-LIGA techniques. An
innovative approach is described in JP2297409A2 (incorporated
herein in its entirety by reference), wherein the photoresist mold
is heated beyond the heat resistance temperature of the photoresist
thereby causing reflow of the photoresist, leading to tapered
structures, which are narrower at the top. Electroforming is done
over the reflowed photoresist mold to create a master mold
especially designed for easy separation of the master mold from
replicated plastic.
[0014] For Optical MEMS applications, an array of recessed
semi-spherical cavities within the master mold can be used to
replicate Piano-Convex microlens arrays onto the replicated
plastic. Several innovative approaches have been used to create
this pattern in a master mold designed for electroforming. For
example, in WO03041934A1 and JP1212789A2 (incorporated herein in
their entirety by reference), the dummy substrate is directly
machined to form suitable shapes and then the master mold is
electroformed over it. U.S. Pat. No. 5,705,256 (incorporated herein
in its entirety by reference), discloses a technique wherein, the
recessed cavities are defined by isotropic, wet chemical etching-
and subsequently used for electroforming of the master mold and yet
another approach is described in U.S. Pat. No. 6,436,265
(incorporated herein in its entirety by reference). JP2001290006A2
(incorporated herein in its entirety by reference), discloses a
method wherein the microlenses are formed directly onto a flat
plastic substrate by depositing a controlled amount of another
plastic material and heating the deposited plastic to its reflow
temperature thereby forming the microlens array.
[0015] Finally, an issue of concern in master molds created using
either UV-LIGA or electroforming techniques, is the surface
roughness of the microstructures. For most BioMEMS applications,
increased surface roughness leads to poor performance; specifically
in the case of Capillary Electrophoresis (CE) chips, wherein poor
surface quality can render the chip unusable for separation
applications. Obviously, for Optical MEMS applications, even slight
surface imperfections can lead to deviation in the optical path
characteristic ergo, poor device performance.
[0016] For injection molding, the typical setup includes an
arrangement to inject molten plastic material into a mold block.
The mold block contains the features to be replicated. In most
cases, the master mold is fabricated either as an integral part of
the mold block or as a component, which can be assembled into the
mold block. Obviously, when microfabrication techniques, such as
UV-LIGA, are used the master mold is always an independent piece
that is assembled into the mold block as explained in R. Trichur et
al. in the Proceedings of the 6th International Conference on Micro
Total Analysis Systems (micro-TAS 2002), Nara, Japan, Nov. 3-7,
2002, pp. 560-562, and A. Puntambekar et al. in Proceedings of the
6th International Conference on Micro Total Analysis Systems
(micro-TAS 2002), Nara, Japan, Nov. 3-7, 2002, pp. 422-424 and
J.-W. Choi et al. in Proceedings of the 5th International
Conference on Micro Total Analysis Systems (micro-TAS 2001),
Monterey, Calif., Oct. 21-25, 2001, pp. 411-412.
SUMMARY OF THE INVENTION
[0017] Certain embodiments of the present invention seek to address
the shortcoming listed above to develop a master mold for plastic
replication with precisely defined microstructures and ultra-low
surface roughness. Also, modifications to the electroforming
process are disclosed to make it more amenable towards fabrication
of master molds for injection molding. Finally, a simple yet
elegant approach is presented to fabricate a master mold for
microlens arrays on plastic substrates.
[0018] Disclosed herein is an embodiment of a two-step
electroplating technique, wherein the height of the microstructures
on the master mold is controlled precisely using a polishing step
after electroplating of the microstructures. The structural
integrity of the microstructures is preserved by the use of a
second sacrificial support metal which is also electroplated,
following the first electroplating step. The second sacrificial
metal is selectively etched out after the polishing step. The
polishing step ensures that all the microstructures are of uniform
height (across the entire substrate) and furthermore each
microstructure is exhibits uniform height along its cross-sectional
profile. The polishing step also ensures that the surface roughness
of the microstructures is minimized to yield high-quality
replicated features.
[0019] Also disclosed herein are embodiments for making
modifications to the electroforming technique to make it suitable
for master mold fabrication for injection molding applications.
More particularly, certain embodiments of the present invention use
Nickel-Iron (80:20) alloy as a low stress material extremely
suitable for electroforming where the stress can be controlled
easily just by adjusting the current density, temperature, pH, and
composition of the electroplating bath to obtain electroforms that
are suited as master molds for plastic replication. Previously,
Nickel-Iron (80:20) electroplating has been used to develop soft
magnetic material due to this material's excellent magnetic
properties. This is the first application where the alloy is used
as a structural component of the master mold. Other modifications
to the electroforming process will be apparent in the section
entitled "Detailed Description of the Invention".
[0020] Certain embodiments of the present invention provide an
elegant solution to creating a negative image of an array of
Piano-Convex microlenses on a master mold and subsequently
replicating the Plano-Convex lens array structure on the plastic
substrate.
[0021] Embodiments of the present invention overcome the
deficiencies and inadequacies in the prior art as described in the
previous section and as generally known in the industry.
[0022] Certain embodiments of the present invention are concerned
with developing a master mold for plastic replication, wherein the
master mold is a discrete component, which can be easily assembled
into the injection mold block for plastic replication.
[0023] Certain embodiments of the present invention are concerned
with developing a master mold using a modified UV-LIGA fabrication
process, specifically the two-step electroplating process with one
of the electroplating used for sacrificial metal deposition, to
create master molds with microstructures of uniform height across
the entire master mold.
[0024] Other embodiments of the present invention are concerned
with developing a master mold using a modified UV-LIGA fabrication
process, specifically the two-step electroplating process with one
of the electroplating used for sacrificial metal deposition, to
create master molds wherein, the microstructures have uniform
height across their cross-section.
[0025] Certain embodiments of the present invention are concerned
with developing a master mold using a modified UV-LIGA fabrication
process, specifically the two-step electroplating process with one
of the electroplating used for sacrificial metal deposition, to
create master molds with surface roughness less than 50 nanometers
(nm).
[0026] Certain embodiments of the present invention are concerned
with developing a master mold suitable for replication of
microfluidic structures onto a plastic substrate.
[0027] Alternate materials have been investigated for the
electroforming process thereby allowing for fabrication of very
thick (more than 0.5 mm) electroforms.
[0028] Certain embodiments of the present invention are concerned
with developing a fabrication process, using the alternate
materials, for fabricating planar electroforms with minimal bending
and/or curvature along the diameter of the master mold.
[0029] Certain embodiments of the present invention are concerned
with developing a master mold suitable for replication of a
microlens array onto a plastic substrate.
[0030] Certain embodiments of the present invention are concerned
with developing a simplified process for the fabrication of the
microlens array on a plastic substrate by replication
techniques.
[0031] Other embodiments, features and advantages of the present
invention will become apparent from the detailed description of the
invention when considered in conjunction with the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0032] FIGS. 1a-1b are schematic sketches illustrating the
replaceable mold disk technology and also shows actual photographs
of the mold block and master mold with microfabricated features, in
accordance with various embodiments of the present invention.
[0033] FIGS. 2a-2g are schematic sketches explaining the UV-LIGA
process normally used for master mold fabrication.
[0034] FIGS. 3a-3j are schematic sketches explaining the sequence
of steps used for fabricating the master mold using the two-step
electroplating technique, in accordance with an embodiment of the
present invention.
[0035] FIGS. 4a-4i are schematic sketches explaining the sequence
of steps used for fabricating the master mold using the
electroforming approach, in accordance with an embodiment of the
present invention.
[0036] FIGS. 5a-5f are schematic sketches explaining the
fabrication process for the manufacture of the master mold and
subsequently the plastic replica with an array of microlenses, in
accordance with an embodiment of the present invention.
[0037] FIGS. 6a-6k are schematic sketches showing the sequence of
steps in the modified electroforming process for the manufacture of
a highly accurate and precise master mold, in accordance with an
embodiment of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0038] Broadly stated, certain embodiments of the present invention
provide two technologies intended for developing master molds for
plastic replication using injection molding techniques. The first
part herein discloses an embodiment of a two-step
electroplating/polishing approach wherein, one of the electroplated
metals is used as a sacrificial support layer during polishing. The
second part herein describes embodiments that provide modifications
to the electroforming process, specifically: (a) the use of
Nickel-Iron (80:20) as a structural component; (b) modifications to
the electroplating bath for generating low-stress electroforms; and
(c) modifications to the sequence of steps followed during
electroforming for fabricating a uniform electroform. Also
disclosed in the second part herein is the use of thermal treatment
to the photoresist mold to generate an array of microlens pattern
on the master mold. The second part herein also discloses a
simplified fabrication process for manufacturing an array of
microlenses on the master mold using electroforming techniques.
Finally, a combined fabrication process that uses the
electroforming approach after the two-step electroplating and
planarization (or polishing) technique is disclosed for the
fabrication of robust master molds suitable for high volume
productions.
[0039] Definitions
[0040] The process of "Microfabrication" as described herein
relates to the process used for manufacture of micrometer sized
features on a variety of substrates using standard microfabrication
techniques as understood widely by those skilled in this art. The
process of microfabrication typically involves a combination of
processes such as photolithography, wet etching, dry etching,
electroplating, laser ablation, chemical deposition, plasma
deposition, surface modification, injection molding, hot embossing,
thermoplastic fusion bonding, low temperature bonding using
adhesives and other processes commonly used for manufacture of MEMS
(microelectromechanical systems) or semiconductor devices.
"Microfabricated" or "microfabricated devices" as referred to
herein refers to the patterns or devices manufactured using the
microfabrication technology.
[0041] The term "BioMEMS" as used herein describes devices
fabricated using MEMS techniques specifically applied towards
biochemical applications. Such applications may include detection,
sample preparation, purification, isolation etc. and are generally
well know to those skilled in the art. One such technique that is
commonly used in BioMEMS applications is that of "Capillary
Electrophoresis" (CE). CE refers to the process wherein an
electrical field is applied across a liquid column leading to the
separation of its constituents based on their mass/charge ratio.
The term "CE Chips" refers to microfluidic BioMEMS devices
specifically used for CE applications.
[0042] The term "Optical MEMS" as used herein describes devices
fabricated using MEMS techniques specifically applied towards
optical applications. The term "MOEMS", which is an abbreviation
for "Micro-Opto-Electro-Mechan- ical-Systems", is also used
interchangeably with "Optical MEMS" herein.
[0043] The term "chip", "microchip", or "microfluidic chip" as used
herein means a microfluidic device generally containing a multitude
of microchannels and chambers that may or may not be interconnected
with each another. Typically, such biochips include a multitude of
active or passive components such as microchannels, microvalves,
micropumps, biosensors, ports, flow conduits, filters, fluidic
interconnections, electrical interconnects, microelectrodes, and
related control systems. More specifically the term "biochip" is
used to define a chip that is used for detection of biochemically
relevant parameters from a liquid or gaseous sample. The
microfluidic system of the biochip regulates the motion of the
liquids or gases on the biochip and generally provides flow control
with the aim of interaction with the analytical components, such as
biosensors, for analysis of the required parameter.
[0044] The term "microstructure" as used herein, describes a
structure created using well-known microfabrication processes
wherein at least one of the dimensions of the microstructure ranges
from 1 .mu.m to 1000 .mu.m. In the case of microfluidic devices,
the microstructures may be referred to as "microchannels" or simply
"channels" whereas for Optical MEMS devices, the microstructures
may be referred to as "microlens" or "microlens array". It is to be
understood that "microstructures" is a generic term whereas more
specific terms are used in contexts where the microstructures are
used for a specific application. Microstructures are generally
characterized by their "aspect ratio", which used herein describes
the ratio of the height to width of the microstructure. The term
"surface roughness" refers to the root mean square (rms) value of
surface irregularity for the surface of the microstructure which
extends out of the plane of the substrate and is parallel to the
plane of the substrate. The term "cross-section" as used herein
follows the commonly accepted meaning, specifically the area
created by a plane cutting through the microstructure.
[0045] The term "microchannel" as used herein refers to a groove or
plurality of grooves created on a suitable substrate with at least
one of the dimensions of the groove being in the micrometer range.
Microchannels can have widths, lengths, and/or depths ranging from
1 .mu.m to 1000 .mu.m. It should be noted that the terms "channel"
and "microchannel" are used interchangeably in this description.
Microchannels can be used as stand-alone units or in conjunction
with other microchannels to form a network of channels with a
plurality of flow paths and intersections.
[0046] The term "microfluidic" generally refers to the use of
microchannels for transport of liquids or gases. The microfluidic
system consists of a multitude of microchannels forming a network
and associated flow control components such as pumps, valves and
filters. Microfluidic systems are ideally suited for controlling
minute volumes of liquids or gases. Typically, microfluidic systems
can be designed to handle fluid volumes ranging from picoliter to
milliliter ranges.
[0047] The term "microlens" as used herein refers to a physical
configuration on the substrate that can be used for focusing or
diverging an incident beam of light, and where at least one
dimension of the microlens ranges from 1 .mu.m to 1000 .mu.m. The
term "microlens array" is used herein to describe a plurality of
microlenses wherein at least 2 microlenses are fabricated in close
proximity of each other. The microlens array may be symmetric (i.e.
a 2.times.2 or a 4.times.4 etc. arrangement of microlenses) or
asymmetric (i.e. a 2.times.1 or a 5.times.2 etc. arrangement of
microlenses). The term "Plano-Convex" describes a lens structure
wherein one side of the lens is flat and the other side has a
convex structure.
[0048] The term "substrate" as used herein refers to the structural
component used for fabrication of the micrometer sized features
using microfabrication techniques. A wide variety of substrate
materials are commonly used for microfabrication including, but not
limited to; silicon, glass, polymers, plastics, ceramics to name a
few. The substrate material may be transparent or opaque,
dimensionally rigid, semi-rigid or flexible, as per the application
they are used for. Generally, microfluidic devices consist of at
least two substrate layers where one of the faces of one substrate
layer contains the microchannels and one face of the second
substrate is used to seal the microchannels. The terms "substrate"
and "layer" are used interchangeably in this description. Also, the
terms "plastics" and "polymers" are used interchangeably. It is to
be understood that the terms "plastics" or "polymers" encompass
thermoplastics (material which deform when pressurized at elevated
temperatures), thermosets (materials which "set" or attain final
shape at elevated temperatures) as well as two-part polymers (that
are mixed for curing). The choice of the plastic substrate is
dictated by the application area of the MEMS device (e.g. for
BioMEMS application; biocompatibility is an important criteria;
whereas optical transmission properties may be of greater
importance for MOEMS applications) and not limited to a particular
material or even a set of materials.
[0049] The term "dummy substrate" as used herein refers to a
substrate that is used in the fabrication process and is
selectively destroyed as a part of the fabrication process such
that it does not constitute a part of the final product.
[0050] The term "UV-LIGA" describes a photolithography process
modeled on the "LIGA" fabrication approach. LIGA refers to the
microfabrication process for creating microstructures with high
aspect ratio using synchrotron radiation and thick photoresists
(ranging in film thickness from 1 .mu.m to 5 mm). The LIGA process
is used to form a template that can be used directly or further
processed using techniques such as electroplating to create the
microfluidic template. UV-LIGA uses modified photoresists that can
be spin coated in thicknesses of 1 .mu.m to 1 mm and are sensitive
to UV radiation. UV radiation sources are commonly used in
microfabrication facilities and hence UV-LIGA offers a lower cost
alternative to LIGA for fabrication of high aspect ratio
microstructures.
[0051] The term "current crowding" as used herein refers to the
phenomenon wherein the current flux is concentrated in certain
areas of the substrate during the electroplating process. Current
crowding in turn results in non-uniform electroplating rates.
[0052] The term "reflow" as used herein, refers to the process
where a thermoplastic material (such as photoresist) is heated
beyond a critical temperature at which stage it starts changing to
a liquid phase. Surface tension forces will then alter the shape of
the molten thermoplastic material to minimize the free surface
energy.
[0053] The term "sacrificial" as used herein refers to a component
used during the fabrication process which is only present in
intermediate steps of the fabrication process and is completely
destroyed in the process such that it does not form a part of the
fabricated device.
[0054] The term "chemical mechanical polishing (CMP)" as used
herein describes a process wherein a reasonably flat substrate is
polished on one or both sides to achieve parallelism between two
opposite surfaces as well as desired surface uniformity on one or
both surfaces. This process is also referred to as "planarization"
and the two terms are used interchangeably in this description.
[0055] The term "electroforming" as used herein carries the same
meaning as the conventionally accepted meaning to those skilled in
this art. Specifically, it is "An electrochemical process of master
mold fabrication using an electrolyte, an anode to supply the
metal, and a control of the electrical current and of the
deposition of metal onto a suitable mold, which is fabricated on a
dummy substrate."
[0056] The term "master mold" as used herein refers to a
replication template, typically manufactured on a metallic or
Silicon substrate. Specifically herein, "master mold" refers to a
metallic mold wherein the microstructures are either an integral
part of the master mold or are deposited on one of the surfaces of
the master mold. The features of the master mold are fabricated
using the UV-LIGA and other microfabrication processes. The
microstructures created on the master mold may be of the same
material as the master mold substrate e.g. Nickel microstructures
on a Nickel substrate or may be a dissimilar material e.g.
photoresist on a Silicon surface. The master mold is typically used
for creating patterns on a polymer substrate using techniques such
as hot embossing, injection molding, and casting.
[0057] The term "bonding" as used herein refers to the process of
joining at least two substrates, at least one of which has
microfabricated structures, e.g. a microchannel, on its surface to
form a robust bond between the two substrates such that any liquid
introduced in the microchannel is confined within the channel
structure. A variety of techniques can be used to bond the two
substrate including thermoplastic fusion bonding, liquid adhesive
assisted bonding, use of interfacial tape layers, etc.
Specifically, in this description, the terms "bonding" and
"thermoplastic fusion bonding" are used interchangeably.
Thermoplastic fusion bonding involves heating the two substrates to
be joined to their glass transition temperature and applying
pressure on the two substrates to force them into intimate contact
and cause bond formation. Another bonding process, namely the use
of UV-adhesive assisted low temperature bonding, is also described
herein and is specifically and completely referred to in all
occurrences.
[0058] The intent of defining the terms stated above, is to clarify
their use in this description and does not explicitly or implicitly
limit the application of embodiments of the present invention by
modifications or variations in perception of said definitions.
[0059] Two-step electroplating and planarization technique for
master mold fabrication using UV-LIGA process:
[0060] FIG. 1a shows the basic concept of a replaceable master mold
disk within the mold block. As shown in FIG. 1a, the injection mold
block consists of two halves A 102 and B 103. Mold Block A 102,
houses the master mold whereas Mold block B 103, has two cavities
opposite to the master molds and the depth of these cavities
defines the thickness of the injection molded plastic part. The
replaceable master mold 100 contains microfabricated features 101
and is mounted on the A block 102. The replaceable mold disks are
mounted on mounting cylinders 105 as shown in FIG. 1b and the
mounting cylinder plus mold disk assembly is positioned within the
mold block half A 102 (note that the schematic sketch shows only
one mold disk whereas the actual mold block can contain two master
molds at the same time). After inserting the mounting cylinders and
master mold, the two mold blocks are assembled together and mounted
in an injection molding machine (not shown). The injection molding
machine contains a hopper to store plastic pellets from which they
are fed into an extrusion screw, which is maintained at an elevated
temperature. The plastic pellets are melted in the screw and forced
out as molten plastic through a nozzle which connects to the inlet
in mold block B 103. The molten plastic is then injected through an
inlet 106 in the B block 103 shown in FIG. 1b. The molten plastic
fills up the cavity in mold block B 103. One end of this cavity is
exposed to the master mold and the molten plastic will replicate
the shape of the master mold. Upon cooling, the plastic part will
solidify and is subsequently ejected using ejector pins 104. It is
clear from this description that the microstructures in the plastic
part will be a negative image of those on the mold master and using
appropriate injection molding conditions very high fidelity
reproductions can be achieved. Hence, it is critical that the
master mold microstructures are very precisely and accurately
defined.
[0061] FIGS. 2a-2g show the conventional UV-LIGA process that is
commonly used to make the master mold. As shown in FIG. 2a,
initially photoresist 210 is spin-coated onto a (typically)
metallic substrate 200. Most commonly a Nickel substrate is used
because of its excellent mechanical properties as well as the ease
of subsequent electroplating of Nickel microstructures. The
negative photoresist 210, is exposed to a UV-radiation 213 through
opening 215 in a photomask 214. Next, the exposed photoresist is
developed in a suitable developer to retain photoresist in the
exposed areas 216, and washed out to expose the underlying metal
layer in the unexposed areas 218, as shown in FIG. 2c and FIG. 2d.
Note that the location of the resist areas and cavities can be
reversed by using a positive photoresist, while using the same
photomask. Following development, Nickel 219 (or the same metal as
the underlying substrate) is electroplated within the cavities of
the photoresist mold. After this step, the photoresist mold is
dissolved using a suitable solvent to leave behind free standing
microstructures 201 as shown in FIG. 2f.
[0062] Although this process or minor variations thereof is very
commonly used, it suffers from a couple of serious drawbacks. As
shown in the magnified view 212 of FIG. 2b, the spin-coated
photoresist does not coat the entire substrate uniformly.
Specifically, because of difference in centripetal force
experienced during spin-coating and due to surface tension forces
at the edge of the wafer, the photoresist tends to form a so-called
"edge-bead" along the circumference of the substrate. This problem
can be partly circumvented by using alternate coating techniques
such as spray-coating or dip-coating but these techniques are not
widely accepted in the art.
[0063] Another and more serious problem is due to the phenomenon of
current crowding. In the electroplating setup, the anode is
(typically) approximately equal in area and dimensions to the
substrate (which acts as the cathode in the electrochemical
reactions involved in electroplating). The anode and the substrate
are maintained at a specific distance from each another and a
constant (or pulsed) current is applied across this arrangement.
Because of the difference in widths, the current flux tends to be
more concentrated at the narrower features leading to a faster
electroplating rate and higher microstructures 223 when compared to
microstructures with larger widths 224. Furthermore, even along the
width of the microstructures, due to current flux concentration at
the edges, the edges of the microstructures 225 tend to be taller
than the middle sections 226. These problems are well known in the
art and are tolerated as there has been no solution proposed which
can effectively address these problems. For example, if a pulsed
current is used wherein an asymmetrical square wave current with
longer duration in the positive cycle is used, it has been
demonstrated that this problem is minimized yet it is not fully
addressed and variations, though of a lesser magnitude, are still
observed.
[0064] The non-uniform microstructure dimensions can be a
significant problem in microfluidics applications. In this case,
the microchannels need to be of precise dimensions to accurately
govern the flow characteristics, and even small variations in the
dimensions can lead to substantial changes in flow characteristics.
For example, it has been observed in our experiments that a 20
.mu.m wide channel is electroplated almost twice as fast as a 500
.mu.m wide channel leading to significant variations in the volumes
that can be accommodated in these channels. Furthermore, this
discrepancy is not even predictable and can be severely affected by
the overall layout, density and physical proximity of microchannels
of different widths.
[0065] FIGS. 3a-3j show the sequence of events for the two-step
electroplating and planarization method, in accordance with an
embodiment of the present invention, which addresses most of the
problems listed above. As is readily obvious, steps shown in FIGS.
3a, 3b and 3c are similar to those described above. However, as
shown in FIG. 3d, the microstructures 319 are electroplated beyond
the thickness of the photoresist so that each microstructure
extends beyond 320 the photoresist mold. Following this step, the
photoresist is dissolved in a suitable solvent as shown in FIG. 3e.
Also, some sections of the substrate are blocked using an
electrical non-conductive layer 321 to prevent electroplating in
these areas. Then a second sacrificial metal layer 330 is
electroplated on the substrate such that this metal layer extends
well beyond the thickness of the electroplated microstructures and
effectively covers all the microstructures. In an embodiment of the
present invention, the materials involved are a Nickel substrate,
electroplated Nickel microstructures, and a sacrificial Copper
layer. These are by no means a unique combination and indeed any
combinations of metallic substrates and sacrificial layer may be
used with equal success. The only criterion for choosing the metal
pair is that the sacrificial metal layer can be etched without
affecting the substrate and electroplated microstructures.
Secondary factors such as cost prohibit the use of obviously
precious metals such as Gold though technically that may also be
used since it can be selectively etched against (in this case) the
Nickel patterns.
[0066] Following electroplating of the sacrificial metal layer, the
substrate wafer is flipped over and polished using CMP techniques.
Initially the sacrificial metal layer is polished out using a
coarse grit, until the first microstructures (which would be the
narrowest and hence the tallest) start showing through. Following
this the CMP is done using a fine grit to ensure slow removal and
low surface roughness on the top surface of the microstructures. An
intermediate step in the polishing process is when all the
microstructures can be seen without any sacrificial metal on top.
At this stage all the microstructures are of exactly the same
height and furthermore, have uniform height across the width of the
pattern. The height 331 of the polished microstructures and
sacrificial metal layer is checked periodically as shown in FIG. 3g
and lapping is continued until the desired height is obtained. As
is readily apparent from the drawing, the polishing step could also
have been started after step 4 (FIG. 3d) in the sequence. However,
the relatively soft photoresist material cannot provide enough
structural support to high aspect ratio microstructures during the
polishing process. A very high shear force is exerted on the
microstructures during polishing and if they are not properly
supported in the sides, they can easily peel off the substrate. The
sacrificial metal layer is used precisely for this reason to
provide the strong support required.
[0067] Following the polishing step, the sacrificial metal layer is
selectively etched using an etchant that does not affect the
substrate or electroplated metal. At this stage the master mold
350, contains microstructures 301 which are of extremely uniform
height across the entire mold surface. Furthermore, as shown in the
magnified view 322, of FIG. 3i the patterns are not only of exactly
the same height but also are perfectly uniform across the entire
width of the patterns. FIG. 3j shows the replicated plastic part
399, using the microfabricated master mold 350.
[0068] The surface roughness on the top of the microstructures is
directly governed by the grit size of the polishing compound use
during the final steps of the polishing process. By choosing an
appropriate grit size (which are available down to nm size), a very
uniform surface with ultra-low rms roughness value can be achieved.
In the preferred embodiment, the final grit size used for polishing
is a 150 nm grit which yields rms surface roughness of <50 nm
across all the microstructures. This can obviously be changed
easily by using different grit sizes.
[0069] The two-step electroplating described above is well suited
for certain MEMS applications such as microfluidics. Without intent
of limiting the scope of embodiments of the present invention, it
is anticipated that this technique could primarily benefit the
development of high accuracy master molds for microfluidic plastic
devices. However, this technique is not well suited to the
fabrication of microstructures with non-rectangular cross sectional
areas. The electroforming process described in the next two
sub-sections is more appropriate for creating features with
different cross-sectional profiles. Another issue that needs to be
addressed is the longevity and durability of the master mold.
Since, in the two-step electroplating process (or in the commonly
accepted UV-LIGA process) the microstructures are deposited onto a
substrate, there are inherent problems with the adhesion of the
microstructures to the base substrate layer. In accordance with an
embodiment of the present invention described above, Nickel
microstructures are deposited onto a Nickel substrate, or more
generically stating, the microstructures are commonly the same
material as the substrate to ensure that there is good adhesion and
matching of the compressive or tensile stress between the two.
Nevertheless, the microstructures are still "foreign" to the
substrate and after repeated injection molding cycles, the
microstructures eventually start peeling off because of the high
shear rate exerted on them during the injection molding process. In
the electroforming process, the microstructures form an integral
part of the substrate since the base substrate layer is "grown" on
top of the microstructures hence, in this case adhesion failure is
not an issue of concern. Hence, for applications where it is
anticipated that a very large volume of plastic parts will be
replicated (e.g. in high volume manufacturing scenarios) the
electroforming process may be more suitable than the UV-LIGA or
two-step electroplating process. The process for developing master
molds with the same high accuracy is described later in this
description.
[0070] Master Mold Fabrication Using a Modified Electroforming
Process:
[0071] As explained previously, the electroforming process is
particularly well suited for fabricating features with
non-rectangular cross-sectional profiles. One such process is
illustrated schematically in FIGS. 4a-4i. Again, as in previous
cases, a layer of photoresist 410 is deposited onto a dummy
substrate 400. In accordance with an embodiment of the present
invention, the dummy substrate is a 2 mm thick Silicon wafer. Of
course, as is obvious to those skilled in the art, a number of
different dummy substrate material may be used such as, but not
limited to, Glass, Ceramics, rigid plastics, or even highly
polished metallic substrates. After depositing the photoresist, it
is exposed to UV-radiation 413 through openings 415 in a photomask
414 as shown in FIG. 4b. Following this, the exposed photoresist is
developed with a suitable developer solution and the unexposed
regions (in the case of a negative resist) are washed away.
[0072] At this stage, a free standing photoresist pattern is formed
on the dummy substrate as shown in FIG. 4c. After this, the
substrate plus photoresist patterns are heated to the so-called
glass transition temperature of the photoresist. Most photoresist
consist of a polymeric or epoxy backbone with light sensitive
chemicals added thereafter. The photosensitive material confers the
patterning ability. However, most of the polymeric or epoxy bases
used for photoresist are thermoplastic in nature wherein
application of heat (to cured or hardened photoresist) causes it to
gradually transition from a solid to a gel to a liquid. Surface
tension forces come into effect during the transition and modify
the shape of the photoresist patterns. The surface tension forces
always try to minimize the surface area in order to minimize the
free surface energy. It is well known that a sphere has the lowest
surface area for a given volume and the surface tension forces try
to shape the molten resist into a spherical shape. However, surface
tension forces are also active at the interface between the molten
resist and the substrate material which prevent formation of the
spherical shape. For any liquid-solid pair there exists a contact
angle depending on the various surface tension forces. As shown in
FIG. 4d, upon heating, the rectangular cross-section is initially
rounded at the edges then the vertical sidewalls become more sloped
and eventually in the ideal case, the molten photoresist will
achieve a shape resembling a section of a sphere. The final shape
of the molten photoresist depends upon the temperature, the
duration of exposure to the elevated temperature, the volume of the
photoresist, the initial shape of the photoresist, the composition
of the photoresist itself, the contact area per unit volume between
the photoresist and the substrate and the contact angle between the
molten resist and the substrate. Varying any one of these
conditions can lead to a different shape. The concept of melting
photoresist to achieve non rectangular cross-sectional areas has
never been applied to generating a quasi semi-spherical shape
before. In accordance with an embodiment of the present invention,
the photoresist is heated to approximately 10.degree. C. higher
than the glass transition temperature and the temperature is
maintained for an extended period of time (ranging from a few
minutes to more than 2 hours) to allow the photoresist to reflow
completely. Thus, in this case the primary governing factors
determining the final shape are very stable and repeatable for a
given photoresist/substrate combination. Of course, any of the
intermediate shapes shown in FIG. 4d can also be achieved.
[0073] As shown in FIG. 4e, depending on the initial shape (narrow
or wide) the photoresist will reflow to quasi semi-spherical shapes
with different radii of curvature 417, 427 and 437. As is readily
apparent, this is of great significance for MOEMS applications,
wherein each different radius of curvature can generate a lens with
a different focal point.
[0074] Following the photoresist reflow step, the dummy substrate
is completely cooled down (to approximately room temperature or
.about.25.degree. C.) and the top and sides are coated with a
metallic seed layer 433. One of the novel ideas exercised in this
invention is that the SIDES as well as the "top" surface of the
substrate is coated with the seed layer whereas the back side does
not have a seed layer coating. The reasons for this will be
apparent from further disclosures later in this section. The dummy
substrate is then immersed in an electroplating bath for deposition
of the actual substrate material. As explained previously, normally
Nickel is used as a substrate material owing to its suitable
mechanical properties. For this particular application, Nickel is
not a suitable substrate material since electroplated Nickel films
develop very high stress and are likely to peel off the dummy
substrate.
[0075] In accordance with an embodiment of the present invention,
we disclose the use of Nickel-Iron (80:20) as a substrate material.
Formerly, Nickel-Iron (80:20) has been extensively used as a
material with excellent magnetic properties. However, in accordance
with an embodiment of the present invention, we have used
Nickel-Iron (80:20) for the first time as a structural component of
the master mold. Electroplated Nickel-Iron (80:20) has considerably
lower stress than Nickel thereby allowing for much thicker
electroplated films. Indeed the examples listed in the Background
section herein all report electroformed film with thicknesses
ranging from 200 .mu.m to 500 .mu.M. The only cases which report
higher thicknesses use electroforming on large scale substrates and
are not related to microfabrication processes. Furthermore, we have
also modified the electroplating bath composition used for
Nickel-Iron (80:20) electroplating. Normally, a Nickel-Iron (80:20)
electroplating bath contains 3 gm/l of saccharin to reduce the
tensile stress on the electroformed film. In accordance with an
embodiment of the present invention, we have used 5 gm/l of
saccharin which is an optimum concentration to minimize the tensile
stress for creating thick electroforms. The composition of the
Nickel-Iron (80:20) electroplating bath (total volume 8 liters) is
shown in Table 1.
1 TABLE 1 Compound concentration (g/l) Nickel sulphate 200 Iron
sulphate 8 Nickel chloride 5 Boric Acid 25 Saccharin 5
[0076] In order to further reduce the probability of the
electroform peeling off the dummy substrate, electroplating is also
done of the sides of the substrate as shown in FIG. 4f. From our
experiments we have concluded that without electroplating on the
sides of the substrate it is almost impossible to create a
electroform of substantial thickness since even with the reduced
stress of Nickel-Iron (80:20) and the extra Saccharin, the
electroform will still peel away if it is not clamped at the sides.
The growth of the electroform on the sides of the dummy wafer
anchors the electroform firmly to the dummy substrate and allows
for higher thicknesses.
[0077] It is well known in the art that a number of parameters such
as pH, temperature and current density affect the electroplated
Nickel Iron alloy. Lower current densities lead to decreased
current efficiency that leads to secondary cathode reactions like
evolution of hydrogen that significantly increases stress, whereas
higher current densities lead to very fast electroplating, which is
usually under massive tensile stress. Our experiments have
demonstrated that an optimum current density of 10 mA/cm.sup.2
leads to increased current efficiency and low stress deposition.
During electroplating the pH was maintained between 2.9-3.1. The pH
increases with plating duration and is controlled via the addition
of dilute sulfuric acid. The plating rate was .about.9-10
micrometer/hour. The electroplating was carried out at room
temperature to avoid thermally induced stress. The electroplating
was carried out for a period of around 160 hours to obtain a plated
thickness of .about.1.6 mm. It is impossible to avoid the roughness
432 at the back end of the electroform 430.
[0078] Following this step, the back of the electroform is polished
to make it planar as shown in FIG. 4g. By visual inspection, no
curvature or bending due to stress was observed during actual
fabrication processes. The thick silicon dummy substrate 400 is
then removed from the electroplated Nickel Iron layer by
chemical-mechanical polishing from the dummy substrate side (note
that the dummy substrate can also be removed by using a selective
etchant that does not affect the electroform). Then the seed layer
is etched out using suitable selective metal etchants. Finally, the
electroform is machined to trim the edges and create a finished
circular substrate 450 with concave depression as shown in FIG. 4h.
FIG. 4i shows the replicated plastic part clearly illustrating that
each concave depression in the master-mold results in a
Plano-Convex lens shape of the plastic substrate. Since this
process is based on microlithography techniques it is possible to
create and array of microlenses with equal ease.
[0079] If the intent of the fabrication process is specifically the
development of microlens arrays, a highly simplified yet much more
versatile approach can also be used as illustrated in FIGS. 5a-5f.
In this case, the mold material 507, which can be any polymer
composition that is easily cured at room temperature, or at
elevated temperatures or by exposure to (say) UV light etc., is
directly deposited onto the dummy substrate in the form of droplets
508. Conventional micro-dispensers 509 capable of accurate
dispensing in the microliter-nanoliter range are well known in the
art and are in fact commercially available. Furthermore, such
dispensing systems also incorporate a high precision X-Y stage such
that the liquid can be dispensed at precise locations on the
substrate over the entire area of the substrate. In addition to
simplicity, another significant advantage of this technique is the
wide choice of materials that are available for this application.
Since the mold material in this case does not have to be a
photosensitive material such as photoresist, a huge variety of
materials can be used for this application. The only constraints in
the choice of the mold material are that (a) it should exhibit good
adhesion to the dummy substrate and (b) it can selectively be
etched against the electroform material. Furthermore, in order to
control the spread of the dispensed liquid onto the substrate, the
surface energy (and hence that contact angle) of the substrate can
be modified by using a wide variety of techniques well known in the
art.
[0080] Following deposition of the droplets onto the dummy
substrate, the surface tension forces will govern the final shape
of the droplet as explained previously. All the factors listed
previously also apply here with the exception of the high
temperature and the duration of exposure to elevated temperatures
since these are not required in this case. The quasi semi-spherical
droplets 507 will then be cured to a solid in their final positions
as shown in FIG. 5b. Since the lens array mold is created by direct
deposition of the mold material, it is also conceivable that a
variety of materials be used for creating lenses with slightly
different shapes. For example, if a low viscosity and another high
viscosity mold material are used on the same substrate, the low
viscosity material would generate lens shapes with very long focal
distance (due to larger radius of curvature) whereas, the high
viscosity material would create lens shapes with shorter focal
lengths (due to smaller radius of curvature). Using this approach,
it is possible to create lenses with different focal lengths as
part of the same array or such that each lens array has identical
lenses but that each array varies from another one on the same
substrate.
[0081] After this step, the dummy substrate and the mold pattern
are electroplated to create the electroform 550 and, subsequently,
the replicated plastic device 599 with the array of Plano-Convex
lenses, as shown in FIGS. 5e-5f, in a process similar to the one
explained previously.
[0082] In yet another embodiment, the two processes described above
can be combined to fabricate a master mold with the high accuracy
and precision of the two-step electroplating process as well as the
robust characteristics of the electroforming process. This process
is illustrated schematically in FIGS. 6a-6k.
[0083] The initial sequence of events is exactly the same as the
two-step electroplating process wherein photoresist 610 is coated
onto a dummy substrate 600, then exposed to UV radiation 613
through and appropriate photomask 614 to create a photoresist mold
pattern 616 and 618. In accordance with an embodiment of the
present invention, Nickel is then electroplated 619 beyond the
thickness of the photoresist mold 620. Then, the photoresist mold
is removed and copper 630 is electroplated to cover the Nickel
microstructures. Then the plated patterns are planarized and
polished to the desired height 631.
[0084] Following this step, the NICKEL is now selectively etched
out (instead of etching out the copper as is done in the previous
approach) to leave a free standing copper microstructure pattern
611 as shown in FIG. 6h. Then a metal seed layer is deposited in
the microstructures, the exposed areas on the top of the dummy
substrates as well as the sides of the dummy substrate. Following
this, the assembly is immersed in a Nickel-Iron (80:20) plating
bath and an electroplating process similar to the one described in
the electroforming approach is used to create a Nickel-Iron (80:20)
electroform. Then, the dummy substrate is removed by CMP and the
copper microstructures are selectively etched out. Then, the
electroform is machined to the desired dimensions to achieve the
final master mold 650 shown in FIG. 6j and subsequently used for
plastic replication to create patterns as shown in FIG. 6k.
[0085] This approach combines the advantages of the two techniques
disclosed previously. Since, the two-step electroplating process
and planarization approach is used to define the copper patterns on
the dummy substrate, all the microstructures are of uniform height,
exhibit uniform height across their widths, and have very low
surface roughness. Since the master mold is actually an
electroform, the microstructures on the master mold are now an
integral part of the mold and hence are not subject to peeling and
consequently can be used for many more replication cycles.
[0086] It will be readily obvious to those skilled in the art that
the choice of dummy substrate, electroplated microstructure
material, sacrificial support layer material and electroforming
material is not unique and can be easily extended to include other
materials. Furthermore, the application areas listed for the above
disclosures are by no means complete and the techniques disclosed
in accordance with various embodiments of the present invention can
be readily applied to a broader spectrum of applications where high
accuracy and precision are important aspects for the master mold
design. Such modifications do not depart from the essential novelty
of the present invention and are hereby incorporated within the
scope of the present invention.
[0087] The aforementioned fabrication processes offer numerous
advantages as well as fabrication options for MEMS devices, a few
of which are enumerated hereafter.
[0088] An advantage of certain embodiments of the present invention
is the ability to fabricate extremely uniform microstructures (in
terms of height) across the entire area of a large master mold.
[0089] Another advantage of certain embodiments of the present
invention is the ability to fabricate a master mold containing
microstructures with extremely uniform height across the entire
width of the microstructure, irrespective of the width itself.
[0090] Yet another advantage of certain embodiments of the present
invention is the ability to fabricate a master mold containing
microstructures with ultra-low surface roughness on the top surface
of the microstructures.
[0091] Yet another advantage of certain embodiments of the present
invention is the ability to fabricate a master mold with high
aspect ratio microstructures that incorporate the advantage listed
above.
[0092] Yet another advantage of certain embodiments of the present
invention is the ability to fabricate a thick (greater than 1 mm
thickness) master mold using modifications to the electroforming
techniques and specifically tailoring it towards MEMS based master
mold development.
[0093] Yet another advantage of certain embodiments of the present
invention is the ability to create electroforms with low tensile
stress.
[0094] Yet another benefit of certain embodiments of the present
invention is the ability to use existing electroplating setups and
other standard microfabrication techniques without the need to
develop specialized equipment.
[0095] Yet another advantage of certain embodiments of the present
invention is the ability to create robust master molds wherein the
microstructures are an integral part of the master mold thereby
allowing use of the master mold for high volume production runs of
plastic replication.
[0096] Yet another advantage of certain embodiments of the present
invention is the ability to manufacture precise microlens arrays
with a wide range of focal lengths using a highly simple
fabrication process.
[0097] Yet another advantage of certain embodiments of the present
invention is the quick turn-around time offered by the use of the
replaceable mold disk technology with the high accuracy, robust
master molds thereby avoiding the necessity of having to change the
entire mold block for a new design,
[0098] Yet another advantage of certain embodiments of the present
invention is the reduction in manufacturing costs achieved by
eliminating the need to replace the entire mold block for each new
design.
[0099] Yet another advantage of certain embodiments of the present
invention is the ability to develop a wide variety of cross
sectional profiles including square/rectangular, rounded
trapezoidal, quasi-semi circular, and semi-circular.
[0100] Yet another advantage of certain embodiments of the present
invention is the wide choice of material that can be employed for
use as substrate, dummy substrate, sacrificial support layer and
mold material.
[0101] While the invention has been described with reference to
certain embodiments, it will be understood by those skilled in the
art that various changes may be made and equivalents may be
substituted without departing from the scope of the invention. In
addition, many modifications may be made to adapt a particular
situation or material to the teachings of the invention without
departing from its scope. Therefore, it is intended that the
invention not be limited to the particular embodiment disclosed,
but that the invention will include all embodiments falling within
the scope of the appended claims.
* * * * *