U.S. patent application number 12/801219 was filed with the patent office on 2011-01-06 for method and electrode for defining and replicating structures in conducting materials.
This patent application is currently assigned to Replisaurus Technologies AB. Invention is credited to Mikael Fredenberg, Patrik Moller, Peter Wiwen-Nilsson.
Application Number | 20110000784 12/801219 |
Document ID | / |
Family ID | 20284507 |
Filed Date | 2011-01-06 |
United States Patent
Application |
20110000784 |
Kind Code |
A1 |
Moller; Patrik ; et
al. |
January 6, 2011 |
Method and electrode for defining and replicating structures in
conducting materials
Abstract
The present invention concerns an electrochemical pattern
replication method, ECPR, and a construction of a conductive
electrode for production of applications involving micro and nano
structures. An etching or plating pattern, which is defined by a
conductive electrode, a master electrode, is replicated on an
electrically conductive material, a substrate. The master electrode
is put in close contact with the substrate and the etching/plating
pattern is directly transferred onto the substrate by using a
contact etching/plating process. The contact etching/plating
process is performed in local etching/plating cells, that are
formed in closed or open cavities between the master electrode and
the substrate.
Inventors: |
Moller; Patrik; (Sundbyberg,
SE) ; Fredenberg; Mikael; (Stockholm, SE) ;
Wiwen-Nilsson; Peter; (Stockholm, SE) |
Correspondence
Address: |
FINNEGAN, HENDERSON, FARABOW, GARRETT & DUNNER;LLP
901 NEW YORK AVENUE, NW
WASHINGTON
DC
20001-4413
US
|
Assignee: |
Replisaurus Technologies AB
Kista
SE
|
Family ID: |
20284507 |
Appl. No.: |
12/801219 |
Filed: |
May 27, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10734223 |
Dec 15, 2003 |
7790009 |
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12801219 |
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PCT/SE02/01179 |
Jun 17, 2002 |
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10734223 |
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Current U.S.
Class: |
204/252 ;
204/290.01; 204/290.05 |
Current CPC
Class: |
H05K 2203/0117 20130101;
H05K 3/00 20130101; C25D 5/02 20130101; C25D 7/12 20130101; C25F
3/14 20130101; H05K 3/205 20130101; H05K 3/07 20130101 |
Class at
Publication: |
204/252 ;
204/290.01; 204/290.05 |
International
Class: |
C25B 9/08 20060101
C25B009/08; C25B 11/02 20060101 C25B011/02 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 15, 2001 |
SE |
0102144-3 |
Claims
1-54. (canceled)
55. An electrode suitable for an etching or plating process,
comprising a counter electrode and a pattern defining structure of
an electro chemical etching or plating cell are integrated into a
master electrode, wherein the counter electrode is a conducting
electrode layer or a flexible conducting foil, and the pattern
defining structure is an insulating patter layer being applied on
said counter electrode.
56. The electrode according to claim 55, wherein the counter
electrode is inert.
57. The electrode according to claim 55, wherein a flexible
elastomer layer is applied on the insulting pattern layer.
58. The electrode according to claim 55, wherein the counter
electrode is applied on a mechanical support layer.
59. The electrode according to claim 58, wherein a conductive
elastomer layer is applied between the counter electrode and the
mechanical support layer.
60. The electrode according to claim 57, wherein an intermediate
metal layer is applied between the insulating pattern layer and the
flexible elastomer layer.
61. The electrode according to claim 55, wherein the flexible
conducting foil is made of titanium.
62. The electrode according to claim 61, wherein the master
electrode comprises two counter electrodes with a sacrificial
photo-resist layer applied in between and that contact parts of the
master electrode, structures of the insulating pattern layer, are
electrochemically anodised to form an isolating layer.
63. An apparatus according to claim 62, comprising a master
electrode and means for creating conformable contact between the
master electrode and a substrate.
64. The apparatus according to claim 63, wherein said means are one
or more elastomer layers in the master electrode construction.
65. The apparatus according to claim 63, wherein said means are
combined with a conformable membrane.
66. The apparatus according to claim 63, wherein there are
conducting means for electrical connection to the master electrode
on an outer side and electrical connection to the substrate on a
contact side.
67. The apparatus according to claim 63, wherein the master
electrode is fixed in the apparatus by an applied vacuum.
68. The apparatus according to claim 66, wherein said conducting
means for electrical connections is a conducting piece applied on
the outer side of the master electrode.
69. The apparatus according to claim 63, wherein the master
electrode is fixed in the apparatus by a pressure against a
conducting piece, said pressure exerted by the conformable membrane
and/or a piston.
70. The apparatus according to claim 69, wherein said pressure when
applied with the conformable membrane is combined with a reservoir
containing gas or liquid.
71. The apparatus according to claim 63, wherein gas bubbles are
eliminated from an electrolyte solution and/or the reservoir by the
use of an externally applied vacuum, ultrasound or a combination of
vacuum and ultrasound.
Description
TECHNICAL FIELD
[0001] The present invention relates to a new etching or plating
method for simplifying production of applications involving micro
and nano structures by using a special electrode, according to the
appended claims.
[0002] The present invention is closely related to electrochemical
etching, plating, photolithography and pattern replication and is
within the micro- and nanotechnic area.
[0003] The method is particularly useful for fabrication of PWB
(printed wiring boards), PCB (printed circuit boards), MEMS (micro
electro mechanical systems), sensors, flat panel display, magnetic
and optical storage devices. Integrated circuits, different types
of structures in conductive polymers, structures in semiconductors,
structures in metals, and others are possible to produce using this
method. Even 3D-structures in silicon, by using formation of porous
silicon, are possible.
BACKGROUND OF THE INVENTION
[0004] The ever-increasing demand for smaller, faster and less
expensive microelectronic and micro-electromechanical systems
requires corresponding development of efficient and suitable
manufacturing techniques.
[0005] Either additive or subtractive techniques are used in the
fabrication of micro- and/or nano-structures on a surface. One
general subtractive technique is etching and one general additive
technique is plating.
[0006] The etching methods are usually divided into two subgroups,
dry- and wet etching. In general, dry etching is used for submicron
structures and/or where straight sidewalls are important. Wet
etching is used for large structures where some undercutting is
acceptable or sometimes desirable. The wet etching techniques can
be divided into chemical- and electrochemical etching.
[0007] The advantage of dry etching compared to wet etching is that
anisotropic etched profiles can be generated in both crystalline
and polycrystalline/amorphous material. Some of the disadvantages
of dry etching are high equipment costs, lack of selectivity,
problems with re-deposition on the sample, environmentally
hazardous chemicals, surface damages on the etched sample and
safety and disposal problems.
[0008] The advantage of wet etching is that it is a simple and
inexpensive process. One of the disadvantages is that it does not
involve any directional driving force and therefore the etching
rate is the same in all directions, which results in an isotropic
etchprofile. Some other disadvantages are that wet etching baths
generally contain aggressive and toxic chemicals, which results in
safety and disposal problems. In many wet etching processes waste
treatment and disposal costs often surpass actual etching costs,
and the same drawback applies for dry etching.
[0009] Detailed descriptions regarding the above mentioned etching
processes are considered known by a man skilled in the art and will
not be presented in this paper. Because of the close relationship
between the etching method according to the present invention and
the electrochemical etching some details regarding the later will
be presented as follows.
[0010] Electrochemical etching is a simple and inexpensive etching
method, which makes it possible to achieve high etch rates and
accurate process control. In electrochemical etching an external
electrical potential is applied between an etched sample and a
counter electrode, all immersed in a liquid etchant. An
electrochemical cell with the working electrode, the sample, as
anode and the counter electrode as cathode are formed, as shown in
FIG. 1. An external potential is applied to drive the oxidation
process at the working electrode. The corresponding reduction at
the cathode is usually hydrogen gas formation. As electrolyte, and
etchant, neutral salt solutions or very diluted mixtures of
conventional etchant can be used. The applied potential and the
electric field from it give a directional etching in the vertical
direction.
[0011] One problem the designers of electrochemical etching cells
are facing is that, to reduce the resistive losses from charge
transfer in the electrolyte, one wants a small electrode distance.
A small distance, which makes just a tiny unevenness in the
electrode, give rise to a relatively big .DELTA.d that, gives a
non-uniform current density distribution. The result is that some
parts of the sample are over-etched while some parts are not etched
to the desired depth. No mechanical support is possible to keep the
electrode in position over the whole surface, since no contact
between sample and counter electrode is allowed.
[0012] Another problem in electrochemical etching is non-uniform
current density distribution arising from accumulated currents from
non-etched areas, due to the fact that all parts of the counter
electrode are in contact with the electrolyte, and not only the
desired areas above the etched parts.
[0013] The second option, additive techniques, for pattern transfer
is to add material in the structure formed on top of the substrate
by the pattern-defining step. Electro-chemical deposition, for
which the persons skilled in the art also use the term
"electroplating", physical vapour deposition and chemical vapour
deposition are examples of additive processes. It is known in the
field that, by using electroplating, well defined patterns,
vertical sidewalls and high aspect ratio structures can be
fabricated. However, common industrial problems are associated with
the known electroplating process, namely non uniform current
density distribution resulting in a deposition rate depending on
the pattern surrounding each structure that is plated. Furthermore,
such differences in current density also result in different
material composition when plating alloys, as well as differences in
height of electroplated structures on a substrate. Up to now, these
undesired uneven distributions typically have to be rectified using
planarization methods in a subsequent process step.
[0014] When the purpose of etching is to provide a structure in the
etching material by etching away selected parts thereof, the
etching material which is not to be etched away is usually coated
with an etching preventing layer, a so called mask or resist. The
primary technique to define patterns to be etched is
photolithography and a common etching preventing layer is a
photo-resist. The photo-resist is exposed by electromagnetic
radiation and developed to transfer the pattern where etching is
wanted. Every sample that is etched has to be coated with resist,
pre-baked, exposed, developed and hard-baked before the etching
process can start.
[0015] Most of today's micro-devices are built up by a large number
of functional layers and each layer has to be patterned and aligned
in a photolithography process followed by a pattern transfer
process. FIG. 6 shows a conventional etching process with the
lithography process. The complicated nature of the pattern defining
lithography process and the large number of lithography steps
needed to fabricate a micro-device makes it to a major time and
cost carrier in the total manufacturing chain.
[0016] From the European patent publication EP 1060299 it is known
to use a method of making, by etching, depressions in selected
portions of an etching surface by using an electrode with
electrically conductive electrode portions in selected portions of
an electrode surface, where the electrode portions is forming an
electrode pattern which corresponds to the etching pattern. The
method is different compared to the present invention by using
electromagnetic radiation to dissolve a passivating layer, which is
formed on the etching material. During etching the electrode is
placed at a distance from the electrically conductive etching
material, which also differs from the present invention. The
electrodes according to EP 1060299 have to be transparent to
electromagnetic radiation and they do not compensate for unevenness
in the micro/nano areas.
[0017] WO 9845504 discloses a method for electroplating using an
electroplating article, an anode and a substrate. The
electroplating article is put in contact with the substrate. In one
embodiment, the external anode is placed separated from the
substrate and the electroplating article, all immersed in an
electrolyte. According to the disclosure, a potential is applied
over the external anode and the substrate, resulting in material
transferred from the anode, through the porous carrier of the
electroplating article and plated on the substrate in a pattern
defined by the insulating mask of the electroplating article. The
electrolyte volume between the electroplating article and the anode
can be agitated to improve mass transfer of electroactive ions.
However, the disclosed method struggles with the same problems and
drawbacks as associated with conventional electroplating, namely
non-uniform plating rates as a result of non-uniform current
density distribution due to the anode having areas with a surface
size differing from the surface size of corresponding cathode areas
on the patterned substrate. Thus, differences in reaction rates in
different cavities result in plated microstructures with different
heights depending on the pattern surrounding each structure. The
problem is usually solved by a subsequent planarization process
step like lapping or CMP (Chemical Mechanical Polishing). When
plating alloys, the method described in WO9845504 suffers from the
same problems as conventional plating processes, namely differences
in material composition because of non uniform current density
distribution.
[0018] Furthermore, the mentioned embodiment disclosed in WO
9845504 requires an electroplating article fabricated with a porous
material that is permeable for ions in the electrolyte, which gives
rise to limitations in how small dimensions that can be defined,
depending on the pore size of the material.
[0019] In a second embodiment disclosed in WO 9845504 it is
mentioned an electroplating article that consists of a patterned
mask placed onto an anode. The anode can be soluble or insoluble
and can include an erodable layer. In the method using a soluble
anode, the material is transferred from the anode material in the
electroplating article, thus the electroplating article is eroded
during use, but can be periodically redressed and reused. However,
the problem of non uniform current density distribution also
applies to this method, as the patterned mask still is placed as a
separate layer onto the anode layer, i.e. the current density
distribution is only at the beginning of a plating process uniform,
whereas the contact surface of the electrolyte with the anode
material increases differently in each local plating cell,
depending on its size, as anode material is consumed. Moreover, the
maximum aspect ratio, i.e. height/width ratio, of structures that
can be plated is limited by the fact that the erosion of the anode
material in the electroplating article undercuts the insulating
pattern mask. Undercutting the mask layer during use is also
associated with reliability problems, since the patterned mask
layer will be completely undercut and disintegrated from the
electroplating article if the electroplating process is not
terminated in time. The problems described are inherently
associated with the method because the soluble anodic material is
transferred directly from the electroplating article itself, even
in the case where the electroplating article consists of different
layers of soluble and insoluble material.
SUMMARY OF THE INVENTION
[0020] One object of the present invention is to simplify
production of applications involving micro and nano structures
where an etching or plating pattern, which is defined by a
conductive electrode, a master electrode, is replicated on an
electrically conductive material, a substrate. Also, the master
electrode should be possible to reuse many times to fabricate
replicas according to the method. More specifically, an object of
the invention is to avoid unnecessary process steps, such as the
above mentioned planarization process steps, during said production
of said structures, and to enable an accurately controlled
electrochemical etching or plating process without limitations in
maximum aspect ratio of deposited structures, variations in
material composition of deposits and reliability problems in large
scale production.
[0021] Generally, this object is met by a special contact
electrochemical etching/plating method that is called the
electrochemical pattern replication method. To simplify the
description of the electrochemical pattern replication method
according to the present invention it is stated as the "ECPR"
method further in this description. This method is based on a
structured electrode device, an electrochemical etching/plating
method, and an apparatus to perform the process in, according to
different aspects of the invention as defined by the appended
independent patent claims.
[0022] The master electrode and the substrate are put in close
contact, where local etching/plating cells are formed in the open
or closed cavities between the master electrode and the substrate.
A setup with an internal counter electrode surface inside each
local electrochemical etch or plating cell, each defined by the
walls of an insulating pattern layer, enables a uniform current
density distribution independent of the pattern. To enable the
internal counter electrode principle of ECPR in closed cavity
electrochemical micro- and nano cells, predeposition of soluble
anode material inside the cavities in the master electrode is being
done prior to ECPR plating, and during ECPR etching electroplating
of excess ions in the electrolyte created from substrate etching is
being done. This results in uniform current density distribution of
ECPR, independent of any pattern applied, solves the
above-mentioned drawbacks associated with the prior art, namely
different deposition speed depending on the pattern that is plated.
Moreover ECPR eliminates the need for a subsequent planarization,
since deposited structures already have the same height when being
plated with the ECPR method. ECPR also solves the problems with
limitations in maximum aspect ratio of structures deposited in each
plating cycle and reliability problems associated with prior art.
Furthermore, when plating alloys, ECPR also solves the
above-mentioned problem of different material composition of
different structures depending on the pattern surrounding each
structure. Thus the object of the invention is met. Another
advantage of the ECPR method, when used for etching, is that it
enables a high and well controlled anisotropic etch profile, etch
rate and surface finishing and uniformity, a possibility of
accurate process control, minimised undercut, environmentally
friendly process (since electrolytic or very diluted etchants is
used) and low costs.
[0023] Another object is to design the master electrode, which is
used in the ECPR method.
[0024] This object is met by integrating a counter electrode and
pattern defining structures of an electrochemical etching/plating
cell into one device, the master electrode. This master electrode
will operate both as counter electrode and pattern master in the
local etching/plating cell used in the ECPR method. The substrate,
the sample on which the pattern is to be etched or plated on,
operates as a working electrode in the etching/plating cell used in
the ECPR method.
[0025] By using this master electrode combined with the ECPR
method, several replicas can be produced in conducting materials by
electrochemical material removal or addition inside each local
electrochemical micro- or nano-cell defined by the master
electrode.
[0026] Further objects and advantages of the present invention will
be obvious to a person skilled in the art from reading the detailed
description below of preferred embodiments.
BRIEF DESCRIPTION OF THE DRAWINGS
[0027] The invention will be described more closely below by way of
examples and with reference to the enclosed drawings. In the
drawings:
[0028] FIG. 1 is a sectional view of an etching cell used for
conventional electrochemical etching.
[0029] FIGS. 2a to 2f are sectional views, which illustrate one of
the fabrication processes of a master electrode, according to the
present invention, based on open local electrochemical cells.
[0030] FIG. 3 is a sectional view of an etching/plating cell,
according to the present invention.
[0031] FIG. 4a is a sectional view of an etching cell, where the
master electrode and the substrate are compressed and closed local
etching cells are formed, according to the present invention.
[0032] FIG. 4b is a sectional view of an etch cell, where the
pattern has been etched on the substrate, according to the present
invention.
[0033] FIG. 5a is a sectional view of a plating cell, where the
master electrode and substrate are compressed and closed local
plating cells are formed, according to the present invention.
[0034] FIG. 5b is a sectional view of a plating cell, where the
pattern is replicated on the substrate, according to the present
invention.
[0035] FIG. 6 is a flowsheet of a microfabrication process, with a
photolithography process.
[0036] FIG. 7 is a flowsheet of the ECPR process, according to the
present invention.
[0037] FIG. 8 is a sectional view of a principal apparatus used for
single sided etching/plating with the ECPR method, according to the
present invention.
[0038] FIG. 9a is a side view of an example of another apparatus
used for etching/plating with the ECPR method, according to the
present invention.
[0039] FIG. 9b is an end view of the same apparatus that is shown
in FIG. 9a.
[0040] FIGS. 10a to 10h are sectional views of different exemplary
combinations of designs and materials of a master electrode,
according to the present invention.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0041] A master electrode 8 of the present invention operates both
as a counter electrode 1 and a pattern defining master, and a
substrate 9 operates as a working electrode 2 in an etching/plating
cell, which is shown in FIG. 3, used in the ECPR process, according
to the present invention.
[0042] Further on in the description an exemplary etching or
plating process is mentioned, but it should be noted that it is
obvious to a man skill in the art that it also concerns and applies
correspondingly to the respective plating or etching process.
[0043] Master Electrode
[0044] The purpose of the master electrode 8 is to provide a well
defined predeposited anode material electrical connection to all
local plating cells 14 formed when compressing the master electrode
8 and the substrate 9 and, at the same time, to provide electrical
insulation to the areas where electrochemical action is undesired,
i.e. at the contact areas between an insulating pattern layer 3 and
the substrate 9. To enable a well-defined pattern transfer, even
for relatively rough substrate surfaces, a conformable behaviour is
needed, both globally over the entire substrate surface and locally
at each insulating structure of the pattern layer in contact with
the substrate surface. This is satisfied by a flexible behaviour of
the entire master electrode globally on the macro scale and a
compressible elastomer layer 20, 21 within the master electrode
construction on the local micro scale.
[0045] The insulating pattern layer 3 is fabricated by using an
electrically insulating material that is chemically inert in the
electrolytes that are used, enables high aspect ratio structures
and is easily patterned using i.e. UV, X-ray, electron beam, laser
or etching/plating combined with an insulating process. Examples of
insulating materials, which may be used are polyimide, SU-8, SC
100, MRL 6000, ED-resist and Teflon materials. In another
embodiment the insulating portions are made by anodising a
conducting material, e.g. a metal.
[0046] The counter electrode 1 comprises a conducting electrode
layer 1'. Alternatively, the conducting electrode layer may also
comprise a flexible conducting foil 1'', a solid metal sheet or a
thin conducting layer on a mechanical support layer 23. When the
conducting electrode layers 1', 1'' are deposited on a mechanical
support layer 23 or an elastomer layer 21 with a very high surface
uniformity, the two features planarity and high surface uniformity
are combined. Crucial material characteristics for the conducting
electrode layer 1', 1'' are high conductivity, chemically inertness
in the electrolytes used, good seed layers for electrochemical
material deposition and suitable methods for depositing or in other
ways incorporating the layer into the integrated master electrode
construction. A non-limiting list of examples of conducting
electrode layer 1', 1'' materials used comprises stainless steel,
platinum, palladium, titanium, gold, graphite, chromium, aluminium
and nickel.
[0047] According to an embodiment, the master electrode is
manufactured by using a conventional microfabrication method, which
is illustrated in FIG. 6. The different embodiments of master
electrodes used for ECPR processing are described in FIGS. 10a-10h.
All different electrode layer 1', 1'' embodiments may be combined
with all different combinations of insulating pattern layer 3,
flexible elastomer layer 20, 21, mechanical support layer 23 and
intermediate metal layers 22. All these configurations may be used
for both an open cavity concept and a closed cavity concept. These
concepts will be explained further on in the present document.
[0048] Master electrodes for the open cavity configuration may be
fabricated using the method described below.
[0049] The master electrode used for open cavity configuration is
fabricated in two major steps. In the first step the counter
electrode layer 1 is shaped and prepared to meet the different
requirements stipulated as crucial for successful ECPR processing.
After meeting these requirements an insulating pattern layer 3 is
deposited and patterned on the counter electrode layer 1.
[0050] In the preferred embodiment titanium has been chosen as a
master electrode material since it is inert in the electrolytes
being used. Furthermore, anodising can form a dense insulating
outer layer of TiO.sub.2 at the contact areas. It is possible to
use other materials as well, which has been mentioned above.
[0051] Since the master electrode 8 is in contact with the working
electrode 2, some parts of the master electrode have to be made of
an insulating material, an insulating pattern layer 3 on the
contact side, the master side 11. The insulating pattern layer 3
prevents the areas where etching is undesired from etchant
contact.
[0052] All the manufacturing steps of the master electrode 8 may be
carried out with conventional microfabrication processes, known
from the prior art, wherein characteristic steps are shown in FIG.
6.
[0053] Accordingly, the master electrode 8 will be fabricated out
of two titanium foil layers 16, as stated before, which is shown in
FIGS. 2a to 2e, with a sacrificial photo-resist layer 17 in
between, to form gas/electrolyte transport channels. An example of
how the fabrication of this master electrode may be performed is as
follows: [0054] 1. The starting material, the sample in FIG. 6, is
a 4 .mu.m Ti-foil layer 16. A 1 .mu.m sacrificial photo-resist
layer 17 is electrochemical deposited, as shown in FIG. 2a. To form
fluidic channels, the resist forms square with 4 .mu.m width,
separated with 1 .mu.m resist lines, as shown in FIG. 2b. A second
Ti-foil layer 16, 3 .mu.m, is deposited on top of the sacrificial
resist layer 17, as shown in FIG. 2c. [0055] 2. Both long sides of
the "sandwich", which are shown in FIG. 2c, are coated with
ED-resist 18, as shown in FIG. 2d. The master side 11 is patterned
with desired master pattern and the outer side 10 is patterned with
1 .mu.m holes, according to the pattern definition process shown in
FIG. 6. [0056] 3. Double-sided electrochemical etching is
performed, according to the pattern transferring process shown in
FIG. 6. The outer side 10 is etched to the sacrificial resist layer
and the master side 11 is etched to a depth of 3 .mu.m, saving 1
.mu.m for gas traps. A new layer of ED resist is deposited. The
contact areas are exposed and developed. The contact areas are
anodised and isolating TiO.sub.2 is formed, as shown in FIG. 2e.
[0057] 4. The photo-resist is stripped thoroughly in alkaline
solution to dissolve outer layers and sacrificial layer, as shown
in FIG. 2f.
[0058] All fabrication steps for the outer side 10 of the master
electrode 8 are standardised and do not depend on what kind of
master structure that is used. Universal standard masks may be
used. Only the masks for the master-side 11 have to be selected for
every specific master structure. The master electrode is ready to
be mounted in an etching cell.
[0059] The fabrication of a closed cavity master electrode may be
performed in the same way as the above described fabrication
process of the open cavity master electrode except for the
sacrificial resist layer. Several combinations of material are
shown in FIGS. 10a to 10h.
[0060] A very important part of the ECPR process is to use a
suitable insulating layer. One of many benefits of the process is
that it would no longer be needed to apply a resist on each sample
but instead the resist would be out on a reusable master. For this
to be a benefit it of course requires that the resist withstand
several process, cycles. Besides that, the resist also governs how
small structures that can be made, what volume electrolyte to
sample depth ratio one can have and also, how easy it is to keep
all structures in contact with the sample. Electro-deposited
photo-resist, ED resist, which is often used for lithography
processes, is suitable for these etching processes as it can be
deposited with very precise thickness control.
[0061] The embodiments of the master electrode according to the
present invention are in no way limited to the exemplary
constructions and design shown neither in FIGS. 2a-2i, or 10a-10h,
nor to the materials listed as suitable in the description
above.
[0062] Substrate
[0063] Any electrically conductive material durable to
electrochemical stresses, e.g. copper, may be used as substrate
material.
[0064] Electrolyte
[0065] The electrolyte composition is crucial in controlling an
electrochemical process and its different features. Conductivity,
ion mobility, ionic atmosphere, relaxation, migration, diffusion
and transport numbers are important concepts.
[0066] When an electrolytic etchant is used there is no or less
chemical etching and the negative influences will be negligible on
the replicated structures. The existence of chemical etching
depends on if there is a chemical oxidation agent present in the
electrolyte solution.
[0067] One important issue that the electrolyte has to take care of
is to optimise a mass-transport of electro-active ions in local
electrochemical cells, which has to occur to achieve an optimised
ECPR-process. The optimisation of the electrolyte to cause an
optimised mass-transport is described below, after the description
of the ECPR-process.
[0068] Reducing components, e.g. metal ions, could be added to the
electrolyte solution if one wants to prevent deposition of
substrate material and to cause an etch process to stop in a
natural way. When reducing components are added the reduction
process will take place in the electrolyte and-there will be a
natural ending of the etching process when there is a balance
between the reduction components and the deposited components.
[0069] ECPR Process
[0070] The substrate 9 and the master electrode 8 are put together
in close contact and form an etching cell, as shown in FIG. 4a.
[0071] They will be mounted in an apparatus where the ECPR process
will take place. This apparatus will be described in more detail
below. One of its main issues is to keep the electrodes in exact
place once they are put in contact and to supply them with a
conformable contact.
[0072] The insulating pattern layer 3 defines the distance between
the counter electrode 1 part of the master electrode and the
substrate 9. Thanks to the fact that the distance is short and
precise all over the surface it solves the problems with
non-uniform current density distribution and non-etched areas. It
also minimises the resistive losses from charge transfer in the
electrolyte.
[0073] The structure is replicated on the substrate 9 because the
field and motion of the ions in the etching/plating solution is
controlled in vertical direction by the master electrode 8.
[0074] Since the master electrode 8 and the substrate 9 are in
close contact, closed or open cavities, local etching cells 12, are
provided between the electrode surfaces. If the cavities are open
or closed depends on how master electrode 8, that is used, is
constructed, with or without a sacrificial resist layer 17. The
cavities are considered to be closed further in the document.
These, very small and well-controlled, spaces between the
electrodes provide an effective etching with high precision. Every
local etching cell 12 has a surface on the master electrode 8,
which corresponds to a surface on the substrate 9 which is to be
etched away and thereby avoiding the problems with fluctuating
current density distribution in the vicinity of large insulating
areas with adjacent small structures.
[0075] According to the invention, an ECPR method for etching
selected parts of a surface defined by the master electrode, which
was described above, has thus been provided.
[0076] FIGS. 3, 4a and 4b show the different steps in the ECPR
etching process, according to the present invention. The steps are
as follows: [0077] 1. The master electrode 8 and the substrate 9,
are immersed into an electrolyte solution 6, which will be
described later, as shown in FIG. 3. [0078] 2. They are compressed
and an etching cell with local etching cells 12, filled with
electrolyte solution 6, is formed. This is shown in FIG. 4a. It is
also possible to apply the electrolyte solution as a very thin
layer of liquid on one of the surfaces before the electrodes are
compressed, e.g. by dipping the surfaces into the electrolyte
solution before the compress procedure, or to supply the
electrolyte solution to the etching cell, after compressing the
electrodes, through the layer on the outer side 10 in the master
electrode 8. [0079] 3. An external pulsed voltage with or without
additional ultrasound is applied over the etching cell, where the
substrate 9 becomes the anode and the master electrode 8 becomes
the cathode. [0080] 4. FIG. 4b shows how the pattern 3, which is
defined by the master electrode 8, is replicated on the substrate
9. The material that has been etched away has been deposited on the
master electrode 8, a deposit material 13, all inside each local
electrochemical cell. [0081] 5. Since some of the substrate
material that is etched from the anode is deposited in the
structure on the master electrode 8 it will eventually be filled
with substrate material, deposit material 13, and therefore it is
essential to have an easy way to clean the master electrode. After
a number of etching cycles, a cleaning process is normally
performed. The deposit material 13 is etched away from the master
electrode 8.
[0082] FIGS. 5a and 5b shows the different steps in the ECPR
plating process, according to the present invention. The plating
process is almost the same as the etching process except the
following steps: [0083] 1. Before the electrodes 8, 9 are
compressed and immersed into electrolyte solution, plating material
15 has been deposited on the master electrode 8 in the cavities,
which are defined by the insulating pattern layer 3. When a certain
height of the plating structure has been reached will the space,
formed by the local plating cells 14 between the master electrode 8
and the substrate 9, be filled with electrolyte solution 6, as
shown in FIG. 5a. [0084] 2. The pattern, which is defined by the
master electrode 8, replicates on the substrate 9 when the external
pulsed voltage is applied over the plating cell 14, where the
master electrode 8 becomes the anode and the substrate 9 becomes
the cathode. Consequently, the plating material 15, which was
deposited on the master electrode 8, has been plated on the
substrate 9, as shown in FIG. 5b. Since all plating material, which
can be plated on the substrate, has, from the beginning, been
deposited on the master structure, the amount of plating material,
which is plated on the substrate, is controlled with high
precision.
[0085] Major advantages using the ECPR process are uniform current
density distribution in each local electrochemical cell and
globally over the entire substrate independent of cell size, shape
and neighbouring cells according to the pattern. As mentioned in
the above summary of the invention, this solves the problems of
non-uniform height of plated structures, the problem with
non-uniform material composition when plating alloys, and
eliminates the need for a subsequent planarization process. It also
enables deposition of structures with high aspect ratio, i.e.
height/width ratio, and a highly reliable process for large scale
production.
[0086] An optimised mass-transport of electro-active ions in these
cells has to occur to achieve an optimised ECPR-process. The
mass-transfer, with transport of material from one location in
solution to another location, arises from differences in electrical
or chemical potential at two locations, or from movement of a
volume element of solution. There are three modes of mass-transfer,
migration, diffusion and convection. For thin-layer electrochemical
cells, as is in this case, there is a much larger A/V ratio than
for regular macroscopic cells. The high A/V ratio implies large
frictional forces per unit volume, making all electrolyte volumes
to stagnant layers. This means no forced convective mass transfer
occurs, except when using ultrasound, leaving only the diffusion
and migration mechanisms to exert the material transport. This
concerns the closed cavity master electrode. In the open cavity
master electrode there is a micro-convection because of the
sacrificial resist layer, where the channels in the layer allow a
micro-convection mechanism.
[0087] Following actions is made to optimise the
mass-transport:
[0088] 1. Electrolyte Solution
[0089] The parameters that were adjusted in the solution were the
pH-value and the electro-active species/supporting electrolyte
ratio.
[0090] In one embodiment acid copper electrolyte was used as
electrolyte solution. The pH-value was changed by adding either
H.sub.2SO.sub.4 or diluted NaOH. Several experiments were made to
establish which pH-value was the best. It was settled, in this
embodiment, that a pH-value of 2 to 5 was satisfying.
[0091] No or less supporting electrolyte in combination with a
higher concentration of electro-active species, compared to
standard electrolytes, also improves the mass-transport. A
concentration of electro-active species of 10 to 1200 mM is
preferred.
[0092] The ECPR process involves both electrochemical etching and
electro-deposition at the same time. Electro-deposition is the
reversed electrochemical etching process, where ions from the
electrolyte is reduced and deposited on the cathode. The same
conditions apply and the same parameters control the two processes.
With conventional electroplating processes there is a tendency to
obtain a higher deposition rate at the top of a cavity, than at the
bottom, when the high aspect ration structures are to be filled.
This might result in voids, affecting the mechanical and electrical
properties of the microstructure in a negative Way. The geometry of
the local electrochemical cell and the use of additives are
solutions to enable "bottom-up-filling" without any voids.
Additives are added to give the electrolyte a sufficiently
controlled electro-deposition. Additives are often used in plating
processes to make the plating even. It contains several active
components but predominantly it prevents the forming of pillars by
being attracted to and covered high current density areas as soon
as the pillars start forming. This turned out to be a key to the
problem and as soon as it was used a clean and solid substrate
material was formed on the cathode. Several commercial systems have
been tested with satisfactory results. Coveted additives are
wetting agents, which lowers the surface tension, accelerators,
which are molecules that locally increases current density where
they absorb, suppressors, which are polymers which tend to form
current-suppressing film on the entire substrate surface (could
sometimes use chloride as co-suppressor) and levelers, which are
current suppressing molecules with mass transfer dependent
distribution.
[0093] To avoid a far too high concentration of electro-active
species at the anode, which give a local saturated compound and
deposition of solid salt, the counter-ions are exchanged to ones,
which provide a higher solubility product. Further, a sequestering
agent could be added, e.g. EDTA, to dissolve more metal ions
without causing any further precipitation.
[0094] 2. Voltage
[0095] Pulsed-voltage was chosen because it enhances mass transfer
and disturbs the formation of blocking layers at the
electrode-solution interface. Tests were made to determine what
kind of frequencies, duty cycles and potentials to use. Both
periodic pulse reverse voltage (PPR) and complex waveforms have
been used with success. Frequencies of 2 to 20 kHz have been tested
with satisfactory results but also higher frequencies are possible.
In the described embodiment the frequency of 5 kHz is preferred.
The potential is from 0 to 10 V.
[0096] 3. Ultrasound
[0097] Ultrasound may be used together with pulsed voltage to
enhance the mass-transport by micro-convection.
[0098] A machine solution to exert the actions described in this
document is a crucial part of the invention. The purpose of the
machine is to compress the two electrode surfaces, the master
electrode and the substrate, to create the micro/nano cavities
where the local electrochemical cells are formed. To enable
conformable surfaces in both micro/nano- and macro scale, flexible
layers in the machine, for macro scale conformable behaviour and
plane parallelism, are combined with flexible layers within the
master electrode, which enables micro- and nano scale conformable
behaviour. In this way both bent and dented substrates with a
rather high surface roughness can be used for ECPR processing.
[0099] Before compressing the electrodes 8, 9 to create the local
etching cells 12, all gas has to be evacuated from the solution and
from the solid/liquid interface between the master electrode
8/electrolyte 6 and electrolyte 6/the substrate 9. In one
embodiment this is done using a vacuum system and in another using
ultrasound. The two gas bubble elimination methods can also be
combined. Evacuated gas and electrolyte has been taken care of by a
buffer volume connected between a reaction chamber and a vacuum
system.
[0100] To enable the ECPR process, both master electrode 8 and
substrate 9 have to be electrically contacted in the same machine
solution. This has been done with outer side 10 contacting on the
master electrode 8 and front side, the contacting side, contacting
on the substrate 9. The invention is however in no way depending on
this configuration.
[0101] There are two main machine embodiments to perform the
desired actions for ECPR processing.
[0102] The first embodiment, which is shown in FIG. 8, is based on
a membrane solution where a pressurised membrane 24 is expanded
against the master electrode 8 or the substrate 9. The medium 19
inside the pressure volume can be both gas and liquid. Gas bubbles
are eliminated by a combination of ultrasound and vacuum, or just
using ultrasound. In this embodiment electrical contact to the
master electrode 8 is provided from the outer side 10, i.e. from
the membrane 24 and contact to the substrate 9 from the front side.
Plane parallelism is ensured by the nature of the expanding
membrane, applying an even pressure in a conformable way. Both
flexible and rigid master electrodes and substrates can be used in
this embodiment.
[0103] The second embodiment is based on a cylinder, which is shown
in FIG. 9, containing a movable piston, not shown in the figure.
The entire system is confined. Pressure is applied to compress the
two electrodes 8,9 pneumatically using a combination of vacuum and
overpressure or hydraulically using a hydraulic piston or
mechanically using a screw. Gas bubbles are eliminated by a
combination of ultrasound and vacuum. In this embodiment electrical
contact 26 to the master electrode is provided from the outer side
10 and contact to the substrate 25 from the front side using
conducting movable rods. Plane parallelism is ensured by two
flexible elastomer layers between the sample and the piston, one
being more compressible than the other is. These elastomer layers
can also be placed behind the master electrode 8, i.e. between
master electrode and cylinder wall. Both flexible and rigid master
electrodes and substrates can be used in this embodiment.
[0104] The invention is in no way limited to the embodiments
illustrated and described above, and several modifications are
feasible within the scope of protection as defined in the appended
claims.
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