U.S. patent application number 12/504774 was filed with the patent office on 2009-12-17 for fabrication of freestanding micro hollow tubes by template-free localized electrochemical deposition.
Invention is credited to Yeu Kuang Hwu, Jung Ho Je, Seung Kwon Seol.
Application Number | 20090308754 12/504774 |
Document ID | / |
Family ID | 39636084 |
Filed Date | 2009-12-17 |
United States Patent
Application |
20090308754 |
Kind Code |
A1 |
Seol; Seung Kwon ; et
al. |
December 17, 2009 |
FABRICATION OF FREESTANDING MICRO HOLLOW TUBES BY TEMPLATE-FREE
LOCALIZED ELECTROCHEMICAL DEPOSITION
Abstract
The present invention provides a method of fabricating a micro
hollow tube, more specifically, a method of fabricating a micro
hollow tube by template-free localized electrochemical deposition,
in which the micro hollow tube is fabricated by the accurate
control of the distribution of the electric field strength during
deposition with precise interplay of the applied voltage and the
distance between the microelectrode and the grown structure.
Inventors: |
Seol; Seung Kwon;
(Pohang-si, KR) ; Je; Jung Ho; (Pohang-si, KR)
; Hwu; Yeu Kuang; (Nankang, TW) |
Correspondence
Address: |
CHRISTOPHER P. MAIORANA, P.C.
24840 HARPER SUITE 100
ST. CLAIR SHORES
MI
48080
US
|
Family ID: |
39636084 |
Appl. No.: |
12/504774 |
Filed: |
July 17, 2009 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
PCT/KR2007/000325 |
Jan 19, 2007 |
|
|
|
12504774 |
|
|
|
|
Current U.S.
Class: |
205/81 ;
205/118 |
Current CPC
Class: |
C25D 5/02 20130101; C25D
7/04 20130101; C25D 5/04 20130101 |
Class at
Publication: |
205/81 ;
205/118 |
International
Class: |
C25D 21/12 20060101
C25D021/12; C25D 5/02 20060101 C25D005/02 |
Claims
1. A method of fabricating a micro hollow tube by template-free
localized electrochemical deposition, the method comprising the
steps of: (a) placing a microelectrode (anode) very close to a
substrate (cathode) immersed in a plating bath; and (b) applying a
voltage greater than a critical voltage to the microelectrode and
the substrate via a electrochemical medium, and thereby to form a
micro hollow tube structure on the substrate, wherein the critical
voltage is defined as the applied voltage when a maximum electric
field position moves from the center of the end of the micro hollow
tube structure into the edge of the end of the micro hollow tube
structure just below the rim of the tip of the microelectrode.
2. The method of fabricating a micro hollow tube according to claim
1 further comprises (c) moving up the microelectrode from the
formed micro hollow tube structure, with a contact growth mode
being kept during deposition.
3. The method of fabricating a micro hollow tube according to claim
1, wherein a position of the microelectrode relative to the
substrate or the micro hollow tube structure is directly observed
by using an image collecting apparatus.
4. The method of fabricating a micro hollow tube according to claim
3, wherein the image collecting apparatus is a microradiographic
apparatus with coherent X-rays in real time.
5. The method of fabricating a micro hollow tube according to claim
4, wherein the microradiographic apparatus comprises a X-ray beam
source, a sample stage, and an image detecting means.
6. The method of fabricating a micro hollow tube according to claim
2, wherein the movement of the microelectrode is performed with
three stepping motors in sub-microns.
7. The method of fabricating a micro hollow tube according to claim
1, wherein the micro hollow tube comprises at least one selected
from the group consisting of a metal and a metal alloy.
Description
[0001] This is a continuation of International Application
PCT/KR2007/000325, with an international filing date of Jan. 19,
2007, currently pending, which is hereby incorporated by
reference.
TECHNICAL FIELD
[0002] The present invention relates to a method of fabricating a
micro hollow tube. More specifically, the invention relates to a
method of fabricating a micro hollow tube by template-free
localized electrochemical deposition, in which the micro hollow
tube is fabricated by the accurate control of the distribution of
the electric field strength during deposition with precise
interplay of the applied voltage and the distance between the
microelectrode and the grown structure.
BACKGROUND ART
[0003] The fabrication of micro-devices is a fundamental issue in
modern technology. Diverse techniques have been developed to
fabricate microstructures consisting of semiconductors, metals and
polymers. Especially, freestanding three-dimensional (3D) hollow
tubes are particularly promising for broad applications in diverse
areas such as optics, electronics, medical technology and
microelectromechanics.
[0004] Such structures are typically fabricated by conventional
lithographic process, LIGA process (Marc J. Madau, Fundamentals of
Microfabrication: The Science of miniaturization (CRC press,
1997)), track-etch method(Martin C R, Van Dyke L S, Cai Z, Liang W,
J. Am. Chem. Soc. 112, 8976 (1990)) and laser-assisted chemical
vapor deposition (LCVD) (Lehmann O, Stuke M, Science 270, 1644
(1995)).
DISCLOSURE
Technical Problem
[0005] So far, the most useful technique for producing 3D
structures was the LIGA process using synchrotron x-rays--that
combines lithography with electrochemical metal deposition.
Although quite successful, this process is affected by some
significant problems: it implies multiple fabrication steps, long
fabrication times, high cost due to the use of sophisticated masks
or moulds; furthermore, the electroplating solution cannot easily
fill high aspect ratio trenches encountered during the process. In
general, LIGA finds it difficult to produce complex 3D
structures.
Technical Solution
[0006] Therefore, the present invention has been made in view of
the above problems, and it is an object of the present invention to
provide a low cost, fast and simple method for fabricating a
micro-tube with the high aspect-ratio and uniform property.
[0007] To accomplish the above object, according to one aspect of
the present invention, there is provided a novel method of
fabricating a micro hollow tube by template-free localized
electrochemical deposition, the method comprising the steps of: (a)
placing a microelectrode (anode) very close to a substrate
(cathode) immersed in a plating bath; and (b) applying a voltage
greater than a critical voltage to the microelectrode and the
substrate via a electrochemical medium, and thereby to form a micro
hollow tube structure on the substrate, wherein the critical
voltage is defined as the applied voltage when a maximum electric
field position moves from the center of the end of the micro hollow
tube structure into the edge of the end of the micro hollow tube
structure just below the rim of the tip of the microelectrode.
[0008] Preferably, the method of fabricating a micro hollow tube
further comprises (c) moving up the microelectrode from the formed
micro hollow tube structure, with a contact growth mode being kept
during deposition.
[0009] Preferably, a position of the microelectrode relative to the
substrate or the micro hollow tube structure is directly observed
by using an image collecting apparatus.
[0010] Preferably, the image collecting apparatus is a
microradiographic apparatus with coherent X-rays in real time.
[0011] Preferably, the microradiographic apparatus comprises a
X-ray beam source, a sample stage, and an image detecting
means.
[0012] Preferably, the movement of the microelectrode is performed
with three stepping motors in sub-microns.
[0013] Preferably, the micro hollow tube comprises at least one
selected from the group consisting of a metal and a metal
alloy.
Advantageous Effects
[0014] In conclusion, we demonstrated that a careful manipulation
of the electric field strength distribution and in general of the
growth parameters enables LECD to fabricate well-defined metallic
micro hollow tubes. These results were made possible by a careful
control of the interplay between migration and diffusion, in turn
determined by the field strength.
DESCRIPTION OF DRAWINGS
[0015] The above and other objects, features and advantages of the
present invention will be apparent from the following detailed
description of the preferred embodiments of the invention in
conjunction with the accompanying drawings, in which:
[0016] FIG. 1 shows FE-SEM images of a 3D copper wire fabricated
with an applied voltage of 4.5 V and with an electroplating
solution of CuSO.sub.4.H.sub.2O (250 g/L), H.sub.2SO.sub.4 (75
g/L). In (a) we see an overall picture of the wire revealing two
different growth regimes, illustrated in detail in (b) and (c).
Specifically, (c) is the dense growth obtained for L.apprxeq.40
.mu.m and (b) the porous one produced when L is reduced to a few
micrometers. The top view in the inset of FIG. 1b reveals a
hollow-shaped feature with diameter not far from the 50 .mu.m value
of the microelectrode (dashed circle);
[0017] FIG. 2 is the distribution map of the electric field
strength for different values of the applied voltage V and of the
distance L. (a) V=4.5V and L=40 .mu.m; (b) V=4.5V and L=5 .mu.m;
(c) V=10.0V and L=5 .mu.m;
[0018] FIG. 3 is LECD growth of well-defined hollow tube. Top:
FE-SEM images and (inset) tomographic slices of the grown
structures; Bottom: real-time images of the LECD process by
coherent x-ray microradiography. The images show: (a) a dense wire
obtained at 4.5V in the no-contact growth mode; (b) a porous wire
obtained at 4.5V in the contact growth mode; (c) a well-defined
hollow tube obtained at 10.0V, again in the contact growth mode by
the method of the present invention. The contrast difference in (c)
between the inner (black arrow) and outer (white arrow) regions in
the radiographic image reveals the formation of a hollow tube.
MODE FOR INVENTION
[0019] The preferred embodiments of the invention will be hereafter
described in detail, with reference to the accompanying
drawings.
[0020] We developed a novel approach based on localized
electrochemical deposition (LECD) with significant advantages with
respect to LIGA for the fabrication of metallic micro hollow tubes.
Specifically, it is a simple, inexpensive, and damage free
method.
[0021] The LECD approach is based on electrochemical deposition:
the microelectrode (anode) is placed very close to the conducting
substrate (cathode) immersed in the plating bath. As the voltage is
applied and the microelectrode is moved up, a metallic
microstructure is fabricated that protrudes towards the
microelectrode. The process is thus particularly suitable for
producing high-aspect-ratio metallic structures with a variety of
features. This simple approach can be applied to different
materials such as metals, metal alloys, conducting polymers and
semiconductors to fabricate objects in the micrometer,
sub-micrometer, and nanometer scale.
[0022] We conducted experiments at room temperature using 1.05M
CuSO.sub.4.H.sub.2O, 0.8M H.sub.2SO.sub.4. The microelectrode with
50 .mu.m in diameter was prepared by sealing Pt wire (99.95%, Alfa
Aesar) in a glass tube and then by polishing the surface. Platinum
coated silicon wafers were used as cathodes. The microelectrode
position was accurately controlled by three stepping motors. The
experiments were performed at the "7B2 X-ray Microscopy" beamline
of the Pohang Light Source (PLS), Korea. Additional tests such as
field emission scanning electron microscopy (FE-SEM, JEOL JSM6330F)
were also used to study the microscopic characteristics of the
grown structures. The microradiographic monitoring of the LECD
process was implemented in situ in a specially designed miniature
electrochemical cell machined from a Teflon block and sealed by
Kapton films that were x-ray transparent and stable for most
chemical reactions. The distance between the two cell windows was
optimized to .apprxeq.5 mm to avoid unnecessary x-ray absorption by
the plating electrolyte. For the micro-tomography, the grown
structure was mounted on a translation/rotation stage with precise
positioning (250 nm/0.002 .mu.m) and one thousand projection
radiographs were taken while rotating the sample between 0.degree.
and 180.degree. The slice images of the grown structure were then
reconstructed by using a self-developed reconstruction
algorithm.
[0023] FIG. 1 shows FE-SEM (field-emission scanning electron
microscope) images of a 3D copper wire fabricated by the LECD
process with an applied voltage of 4.5 V. FIG. 1(a) shows that the
wire so produced reflects two growth regimes: the upper part [shown
in detail in FIG. 1(b)] corresponds to a regime yielding a porous
microstructure, whereas the other regime results in a dense uniform
microstructure [FIG. 1(c)]. The dense uniform growth was obtained
with a relatively large distance between the microelectrode and the
growing structure, L=40 .mu.m (no-contact growth mode). The dense
uniform growth abruptly changed to a porous growth when L was
reduced to a few micrometers, (contact growth mode).
[0024] In order to understand this change in the growth
characteristics, we must consider the mass transport mechanisms of
metal ions. Diffusion of metal ions from the bulk solution to the
cathode dominates conventional electrochemical deposition; in LECD,
however, we must take into account the migration of metal ions that
is driven by strong localized electric fields. The distance L
determines the interplay between diffusion and migration.
Specifically, diffusion prevails at large L-values whereas
migration increasingly dominates as L decreases. When L reaches the
critical value at which migration replaces diffusion as the
dominating factor, the deposition rate rapidly increases because of
the strong electric fields, changing the grown structure from dense
to porous as seen in FIG. 1. The top view shown in the inset of
FIG. 1(b) reveals a hollow feature within the porous wire; the
diameter of this feature is not far from that of the microelectrode
(dashed line).
[0025] One of the factors that affect the growth characteristics is
the electric field strength distribution near the grown feature. We
modeled this distribution and the results are illustrated in FIG.
2. For a low applied voltage of 4.5V and a large L-distance of 40
.mu.m, the electric field strength exhibits a maximum value at the
center of the grown feature [FIG. 2(a)]. We expect, therefore, the
formation of a wire with a cone on top. As L decreases to 5
.mu.m--a value much lower than the critical level [FIG. 2(b)]--the
maximum field position moves to the edge of the grown structure
just below the rim of the microelectrode. Thus, the formation of
the porous region with the hollow feature of FIG. 1(b) can be
explained by the electric field edge enhancement at the
microelectrode rim that induces a high migration rate below the
rim.
[0026] As the applied voltage increases from 4.5 to 10V for L=5
.mu.m, the electric field strength sharply increases at the
microelectrode rim, enhancing the field strength difference with
respect to the microelectrode core--see FIGS. 2(b) and 2(c).
Consequently, the electrochemical deposition is also enhanced below
the rim. The consistent results of the field simulation and of the
actual growth thus suggest that it is possible to change the copper
grown structure from a dense wire to a well-defined hollow shape
simply by controlling the electric field distribution near the
microelectrode.
[0027] These findings lead us to suitable strategy to fabricate
well-defined hollow tubes. The strategy is based on the control of
the electric field strength near the grown structure as suggested
by FIG. 2: as the applied voltage is increased in the contact
growth mode (the migration dominant regime), the enhancement of the
growth below the microelectrode rim eventually produces a tube
rather than a wire.
[0028] FIG. 3 is the results of this approach for the production of
copper grown structures on Pt substrates. The top of FIG. 3 shows
FE-SEM images while the bottom demonstrates microradiographic
images obtained by real-time coherent x-ray imaging. A dense wire
is produced at 4.5V by the no-contact growth mode [FIG. 3(a)]. The
tomographic slice reconstruction in the inset of FIG. 3(a) shows
that the wire is not only dense but also uniform. The cone shape on
top of the wire is the result of the field-induced local migration
discussed above--see FIG. 2(a).
[0029] On the other hand a porous structure is obtained at 4.5V in
the contact growth mode as illustrated in FIG. 3(b) but a dense rim
feature is present around the porous structure (white arrow), as
confirmed by the x-ray tomographic slice in the inset of FIG. 3(b,
top). As the applied voltage is increased to 10V, the grown
structure changes to a well-defined hollow tube with a very uniform
wall thickness (.apprxeq.5 .mu.m)--as shown by the FE-SEM image of
FIG. 3(c, top) and by the corresponding tomographic slice in the
inset. This is the limit result of the migration enhancement near
the rim produced by a highly confined, strong electric field. The
coherent x-ray micro images of FIG. 3(c, bottom) illustrate this
process in real time.
INDUSTRIAL APPLICABILITY
[0030] The practical cases discussed here are only a few examples
of the broad variety of metal structures that our novel LECD
approach can produce by appropriate tuning of the growth
parameters.
[0031] While the present invention has been described with
reference to the particular illustrative embodiments, it is not to
be restricted by the embodiments but only by the appended claims.
It is to be appreciated that those skilled in the art can change or
modify the embodiments without departing from the scope and spirit
of the present invention.
* * * * *