U.S. patent application number 10/337678 was filed with the patent office on 2003-05-29 for metal plated spring structure.
This patent application is currently assigned to Xerox Corporation. Invention is credited to Fork, David Kirtland.
Application Number | 20030100145 10/337678 |
Document ID | / |
Family ID | 25340651 |
Filed Date | 2003-05-29 |
United States Patent
Application |
20030100145 |
Kind Code |
A1 |
Fork, David Kirtland |
May 29, 2003 |
Metal plated spring structure
Abstract
Efficient methods are disclosed for fabricating metal plated
spring structures in which the metal is plated onto the spring
structure after release. A conductive release layer is deposited on
a substrate and a spring metal layer is then formed thereon. A
first mask is then used to form a spring metal finger, but etching
is stopped before the release layer is entirely removed. A second
mask is then deposited that defines a release window used to remove
a portion of the release layer and release a free end of the spring
metal finger. The second mask is also used to plate at least some
portions of the free end of the finger and selected structures
exposed through the second mask. Remaining portions of the release
layer are utilized as electrodes during electroplating. The
resulting spring structure includes plated metal on both upper and
lower surfaces of the finger.
Inventors: |
Fork, David Kirtland; (Los
Altos, CA) |
Correspondence
Address: |
BEVER, HOFFMAN & HARMS, LLP
2099 GATEWAY PLACE
SUITE 320
SAN JOSE
CA
95110
US
|
Assignee: |
Xerox Corporation
|
Family ID: |
25340651 |
Appl. No.: |
10/337678 |
Filed: |
January 7, 2003 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
10337678 |
Jan 7, 2003 |
|
|
|
09863237 |
May 21, 2001 |
|
|
|
6528350 |
|
|
|
|
Current U.S.
Class: |
438/117 ;
257/735; 257/E23.068 |
Current CPC
Class: |
H01F 17/0006 20130101;
H01L 21/4853 20130101; H01L 23/49811 20130101; H01L 2924/0002
20130101; H01L 2924/0002 20130101; H01F 17/02 20130101; H05K 3/4092
20130101; H01L 2924/00 20130101 |
Class at
Publication: |
438/117 ;
257/735 |
International
Class: |
H01L 023/48; H01L
021/44 |
Claims
1. A spring structure comprising: a substrate; a release material
portion located over the substrate; a spring metal finger having an
anchor portion attached to the release material portion such that
the release material portion is located between the anchor portion
and the substrate, the spring metal finger also having a free
portion extending over the substrate, the free portion having
opposing first and second surfaces; and a plated metal layer formed
on both the first and second surfaces of the free portion of the
spring metal finger.
2. The spring structure according to claim 1, wherein the spring
metal finger comprises at least one of Molybdenum (Mo) and Chromium
(Cr), and the plated metal layer comprises Nickel (Ni).
3. The spring structure according to claim 1, wherein the release
material portion is electrically conductive.
4. The spring structure according to claim 3, wherein the release
material portion comprises at least one metal selected from the
group consisting of Ti, Cu, Al, Ni, Zr, and Co.
5. The spring structure according to claim 3, wherein the release
material portion comprises heavily doped silicon.
6. The spring structure according to claim 3, further comprising a
conductor formed on the substrate, wherein the spring metal finger
is electrically connected to the conductor via the release material
portion.
7. The spring structure according to claim 1, wherein the plated
metal layer includes a second portion formed on the anchor portion
of the spring metal finger.
Description
RELATED APPLICATIONS
[0001] The present application is a divisional of commonly owned
co-pending U.S. patent application Ser. No. 09/863,237, "METHOD FOR
FABRICATING A METAL PLATED SPRING STRUCTURE", filed May 21, 2001 by
David K. Fork.
FIELD OF THE INVENTION
[0002] This invention generally relates to stress-engineered metal
films, and more particularly to photo lithographically patterned
spring structures formed from stress-engineered metal films.
BACKGROUND OF THE INVENTION
[0003] Photo lithographically patterned spring structures have been
developed, for example, to produce low cost probe cards, and to
provide electrical connections between integrated circuits. A
typical spring structure includes a spring metal finger having an
anchor portion secured to a substrate, and a free portion initially
formed on a pad of release material. The spring metal finger is
formed from a stress-engineered metal film (i.e., a metal film
fabricated such that its lower portions have a higher internal
tensile stress than its upper portions), such that the spring metal
finger bends away from the substrate when the release material is
etched away. The internal stress gradient is produced in the spring
metal by layering different metals having the desired stress
characteristics, or using a single metal by altering the
fabrication parameters. Such spring metal structures may be used in
probe cards, for electrically bonding integrated circuits, circuit
boards, and electrode arrays, and for producing other devices such
as inductors, variable capacitors, and actuated mirrors. Examples
of such spring structures are disclosed in U.S. Pat. No. 3,842,189
(Southgate) and U.S. Pat. No. 5,613,861 (Smith).
[0004] The present inventors believe that many, if not most,
potential commercial applications of spring structures will require
metal plating on the free (released) portion of the spring metal
finger. In some of these applications, the present inventors
believe the spring metal structures will also require metal plating
on the anchored portion of the spring metal finger. Accordingly,
what is needed is a cost effective method for fabricating spring
structures from stress-engineered metal film that include plated
metal on the spring metal fingers.
SUMMARY OF THE INVENTION
[0005] The present invention is directed to efficient methods for
fabricating spring structures in which a plated metal layer is
formed on spring metal fingers after release from an underlying
substrate. By plating the spring metal finger after release (i.e.
after the finger is allowed to bend upward from the substrate due
to internal stress), plated metal is formed on both the upper and
lower surfaces of the spring metal finger simultaneously, thereby
producing a low-cost spring structure exhibiting superior
stiffness, thickness and electrical conductivity when compared to
non-plated spring structures, or to spring structures plated before
release.
[0006] In accordance with the disclosed fabrication method, a
conductive release layer is deposited on a substrate, and then a
stress-engineered (spring) metal film is formed on the release
material layer. A first mask is then used to etch an elongated
spring metal island from the metal film, but etching is stopped
before the release layer is entirely removed to prevent
undercutting that can cause premature release of the spring metal
island. A release (second) mask is then deposited that defines a
release window exposing a portion of the spring metal island and
the release material layer surrounding this exposed portion.
Subsequent removal of the release material layer exposed by the
release mask causes the exposed portion of the spring metal island
to bend away from the substrate due to its internal stress, thereby
becoming the free portion of a spring metal finger, which also
includes an anchored portion covered by the release mask. The
release mask is then used to perform metal plating during which a
plated metal layer is formed on the free portion of the spring
metal finger, along with other selected structures exposed through
the release mask.
[0007] In one embodiment, the plated metal is formed using an
inexpensive electroplating procedure in which a conductive release
layer is utilized as a cathode, thereby providing a thick spring
structure that is significantly less expensive than spring
structures having comparable thicknesses entirely formed by
sputtering.
[0008] In another embodiment, the release mask, which is also used
during the plating process, is provided with a channel extending
over the anchored (i.e., non-released) portion of the spring metal
finger, thereby facilitating the formation of plated metal on the
anchor portion to improve conductivity.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] These and other features, aspects and advantages of the
present invention will become better understood with regard to the
following description, appended claims, and accompanying drawings,
where:
[0010] FIG. 1 is a plan view showing a spring structure according
to a first embodiment of the present invention;
[0011] FIG. 2 is a cross-sectional side view of the spring
structure taken along section line 2-2 of FIG. 1;
[0012] FIG. 3 is a cut-away perspective view of the spring
structure shown in FIG. 1;
[0013] FIGS. 4(A) through 4(J) are cross-sectional side views
showing fabrication steps associated with the production of the
spring structure shown in FIG. 1;
[0014] FIGS. 5(A) and 5(B) are plan views showing the spring
structure of FIG. 1 during selected fabrication steps;
[0015] FIG. 6 is a cut-away perspective view showing a spring
structure according to a second embodiment of the present
invention; and
[0016] FIG. 7 is a plan view showing a release mask utilized to
fabrication the spring structure shown in FIG. 6.
DETAILED DESCRIPTION OF THE DRAWINGS
[0017] FIGS. 1, 2 and 3 show a spring structure 100 according to a
first embodiment of the present invention. FIG. 1 is a plan view of
spring structure 100, FIG. 2 is a cross-sectional side view taken
along section line 2-2 of FIG. 1, and FIG. 3 is a perspective view
with a cut-away section indicated by section line 3-3 in FIG.
1.
[0018] Spring structure 100 generally includes a substrate 101, a
release material portion 110, and a spring metal finger 120.
Substrate 101 (e.g., glass) includes an optional conductor 105 that
can take several forms. For example, if substrate 101 includes an
integrated circuit, conductor 105 may be a portion of the
metallization that is exposed through an opening in a passivation
layer, otherwise referred to as a via. In this case, conductor 105
may be electrically connected to electrical elements of the
integrated circuit. Alternatively, if substrate 101 is printed
circuit board, printed wiring board, silicon device, or interposer,
then conductor 105 may be an exposed portion of conducting material
that is electrically connected to redistribution traces through
substrate vias, solder bumps, solder balls, mounted electrical
components, integrated passive components, or interconnect pads.
Release material portion 110 is formed on an upper of substrate 101
such that it contacts conductor 105 (if present). Spring metal
finger 120 includes an anchor portion 122 and a free (i.e.,
cantilevered) portion 125. Anchor portion 122 is attached to
release material portion 110 (i.e., such that release material
portion 110 is located between anchor portion 122 and substrate
101). Free portion 125 extends from anchor portion 122 over
substrate 101, and includes an upper (first) surface 126 and an
opposing lower (second) surface 127 that define a thickness T1.
[0019] Similar to prior art spring structures, spring metal finger
120 is etched from a stress-engineered metal film that is formed by
DC magnetron sputtering one or more metals using gas (e.g., Argon)
pressure variations in the sputter environment during film growth
in accordance with known techniques. By carefully selecting the
metals and/or processing parameters, sputtered metal films can be
used to form tightly curved spring metal fingers, or very stiff
spring metal fingers, but not both simultaneously because
increasing the film thickness (which is necessary to increase
stiffness) also increases the radius of the resulting spring metal
finger. Further, the internal stress of the stress-engineered metal
film cannot be increased arbitrarily because of material
limitations.
[0020] In accordance with an aspect of the present invention, a
plated metal layer 130 (e.g., Nickel (Ni)) is formed on free
portion 125 of spring metal finger 120 after free portion 125 is
released (i.e., after release material located under free portion
125 is removed, thereby allowing internal stress to bend free
portion 125 away from substrate 101). Because plated metal layer
130 is formed after free portion 125 is released, plated metal
layer 130 is deposited on both upper surface 126 and lower surface
127 of free portion 125, thereby providing structural and
electrical characteristics that are superior to spring structures
that are formed without plated metal, or having plated metal formed
only on one side. Several of the benefits provided by plated metal
layer 130 are described in the following paragraphs.
[0021] First, forming plated metal layer 130 after release allows
spring structure 100 to be relatively thick (and, therefore,
stiff), thereby increasing the spring force constant of spring
structure 100 at a lower cost than un-plated spring structures, or
spring structures having plated metal formed only on one side. As
indicated in FIG. 2, the plating process increases a total
thickness T2 of free portion 125 over thickness T1 by twice the
thickness of plated metal layer 130. It is well established that
increasing thickness by metal plating is significantly less
expensive than by sputtering. Therefore, spring structure 100 is
significantly less expensive to produce than an un-plated spring
structure having the same thickness. Further, plated metal layer
130 forms on both upper surface 126 and lower surface 127
simultaneously, thereby reducing the required plating period (and,
hence, manufacturing costs) when compared to pre-release plating
methods. Moreover, as described below, plated metal layer 130 is
formed at very low cost because the basic two-mask process utilized
for making un-plated spring structures is not violated (i.e., no
additional masks are used to perform the plating process).
[0022] Second, forming plated metal layer 130 after release allows
spring structure 100 to be both tightly curved and relatively thick
(and, therefore, stiff) at a lower cost than un-plated spring
structures, or spring structures having plated metal formed only on
one side. As indicated in FIG. 2 and discussed above, the curvature
R of free portion 125 is partially determined by the thickness T1
of the stress-engineered metal film from which it is etched. In
order to generate a tightly curved spring structure, a relatively
thin metal film is required. According to the present invention,
spring structure 100 can be both tightly curved and relatively
thick by forming spring metal finger 120 from a thin
stress-engineered metal film, and then forming a relatively thick
of plated metal layer 130.
[0023] Third, forming plated metal 130 on both upper surface 126
and lower surface 127 increases the conductivity of spring metal
finger 120, when compared to spring structures without plated metal
or having plated metal formed only on one side. Due to the
fabrication processes typically used to form the stress-engineered
metal film (e.g., sputtering), these metal films are inherently
poor electrical conductors. Therefore, in applications requiring
high conductivity, plated metal layer 130 may be added to increase
the total electrical conductivity of spring structure 100.
[0024] Plated metal layer 130 provides several other potentially
important benefits. For example, plated metal layer 130 may be used
to electroform the closure of mechanically contacted elements
(e.g., an out-of-plane inductor formed using a series of spring
metal fingers bent such that the free end of each spring metal
finger contacts the anchor portion of an adjacent spring metal
finger). Plated metal layer 130 may also be used to passivate
spring metal finger 120, which is important because most springy
metals, such as stress-engineered metal film 210, form surface
oxides. Plated metal layer 130 may also be added to increase wear
resistance and lubricity. Plated metal layer 130 may also be added
to resist delamination of free portion 125 of spring metal finger
120 by balancing the peeling tendency of the stress gradient in the
stress-engineered metal film. Plated metal layer 130 can also
provide a compression stop to limit spring compression. Moreover,
plated metal layer 130 may be added to strengthen spring structure
100 by adding ductility. Finally, plated metal layer 130 may be
added to blunt the radii of process features and defects that can
arise on spring metal finger 120. The above-mentioned benefits are
not intended to be exhaustive.
[0025] Note that optional conductor 105 is included to provide
electrical coupling of spring structure 100 to an external
electrical system (not shown). Note also that the electrical
coupling between spring metal finger 120 and conductor 105
necessitates using an electrically conductive release material to
form release material portion 110. However, electrical coupling can
also be provided directly to spring metal finger 120 by other
structures (e.g., wire bonding), thereby allowing the use of
non-conducting release materials. Further, the cost-to-thickness
(stiffness) characteristics discussed above may also be
beneficially exploited in applications in which spring metal finger
120 is not used to conduct electric signals.
[0026] FIGS. 4(A) through 4(I) and FIGS. 5(A) and 5(B) illustrate a
method for fabricating spring structure 100 (described above).
[0027] Referring to FIG. 4(A), the fabrication method begins with
the formation of a conductive release material layer 210 over a
glass (silicon) substrate 101. When electroplating is utilized (see
step described below), release material layer 210 is formed from an
electrically conductive material, and a portion 210A of release
material layer 210 contacts conductor 105 that is exposed on the
upper surface of substrate 101. In one embodiment, release material
layer 210 is Titanium (Ti) that is sputter deposited onto substrate
210 to a thickness of approximately 0.2 microns or greater.
Titanium provides desirable characteristics as a conductive release
material layer due to its plasticity (i.e., its resistance to
cracking) and its strong adhesion. Other release materials having
the beneficial plastic characteristics of titanium may also be
used. In other embodiments, release material layer 210 includes
another metal, such as Copper (Cu), Aluminum (Al), Nickel (Ni),
Zirconium (Zr), or Cobalt (Co). Release material layer 210 may also
be formed using heavily doped silicon (Si). Further, two or more
release material layers can be sequentially deposited to form a
multi-layer structure. In yet another possible embodiment, any of
the above-mentioned release materials can be sandwiched between two
non-release material layers (i.e., materials that are not removed
during the spring metal release process, described below). Note
that when an electroless plating process is utilized, the release
material layer 210 can be a non-conducting material, such as
Silicon Nitride (SiN).
[0028] FIG. 4(B) shows a stress-engineered metal film 220 formed on
release material layer 210 using known processing techniques such
that it includes internal stress variations in the growth
direction. For example, in one embodiment, stress-engineered metal
film 220 is formed such that its lowermost portions (i.e., adjacent
to release material layer 210) has a higher internal tensile stress
than its upper portions, thereby causing stress-engineered metal
film 220 to have internal stress variations that cause a spring
metal finger to bend upward away from substrate 201 (discussed
below). Methods for generating such internal stress variations in
stress-engineered metal film 220 are taught, for example, in U.S.
Pat. No. 3,842,189 (depositing two metals having different internal
stresses) and U.S. Pat. No. 5,613,861 (e.g., single metal sputtered
while varying process parameters), both of which being incorporated
herein by reference. In one embodiment, which utilizes a 0.2 micron
Ti release material layer, stress-engineered metal film 220
includes Molybdenum and Chromium (MoCr) sputter deposited to a
thickness of 1 micron. In other embodiments, a Mo spring metal
layer can be formed on SiN release material layers.
[0029] Note that when conductive release material is used,
stress-engineered metal film 220 is separated from contact pad 105
by portion 210A of release material layer 210. Accordingly, a
separate masking step utilized in conventional fabrication methods
to form an opening in the release material is not required, thereby
reducing fabrication costs. Instead, as discussed below, the
present embodiment utilizes the conductivity of release material
layer 210 to provide electrical connection between contact pad 105
and stress-engineered metal film 220.
[0030] Note also that an optional passivation metal layer (not
shown) may be deposited on the upper surface of stress-engineered
metal film 220 at this stage of the fabrication process. Such a
passivation metal layer (e.g., Au, Pt, Pd, or Rh) is provided as a
seed material for the subsequent plating process if
stress-engineered metal film 220 does not serve as a good base
metal. The passivation metal layer may also be provided to improve
contact resistance in the completed spring structure.
[0031] Referring to FIGS. 4(C) and 5(A), elongated spring metal
(first) masks 230 (e.g., photoresist) are then patterned over a
selected portion of stress-engineered metal film 220. Note that
each spring metal mask 230 extends over an associated contact pad
105 (if present), as shown in FIG. 5(A).
[0032] Next, as indicated in FIG. 4(D), exposed portions of
stress-engineered metal film 220 surrounding the spring metal mask
230 are etched using one or more etchants 240 to form a spring
metal island 220-1. Note that this etching process is performed
such that limited etching is performed in portions 210B of release
layer 210 that surround spring metal island 220-1 such that at
least a partial thickness of release layer portion 210B remains on
substrate 101 after this etching step. In one embodiment, the
etching step may be performed using, for example, a wet etching
process to remove exposed portions of stress-engineered metal film
220. This embodiment was successfully performed using cerric
ammonium nitrate solution to remove a MoCr spring metal layer. In
another embodiment, anisotropic dry etching is used to etch both
stress-engineered metal film 220 and the upper surface of release
layer portion 210B. This embodiment may be performed, for example,
with Mo spring metal, and Si or Ti release layers. Mo, Si and Ti
all etch in reactive fluorine plasmas. An advantage of dry etching
the spring metal film is that it facilitates finer features and
sharper tipped spring metal fingers. Materials that do not etch in
reactive plasmas may still be etched anisotropically by physical
ion etching methods, such as Argon ion milling. In yet another
possible embodiment, the etching step can be performed using the
electrochemical etching process described in IBM J. Res. Dev. Vol.
42, No. 5, page 655 (Sep. 5, 1998), which is incorporated herein by
reference. Many additional process variations and material
substitutions are therefore possible and the examples given are not
intended to be limiting.
[0033] FIG. 4(E) shows spring metal island 220-1 and release
material 210 after spring metal mask 230 (FIG. 4(D)) is removed.
Note again that electrical connection between contact pad 105 and
spring metal island 220-1 is provided through portion 210A of
release material layer 210.
[0034] Referring to FIG. 4(F), release (second) mask 250 (e.g.,
photoresist) is then formed on a first portion 220-1A of spring
metal island 220-1. Release mask 250 defines a release window RW,
which exposes a second portion 220-1B of spring metal island 220-1
and surrounding portions 210B of release material layer 210.
[0035] Referring to FIG. 4(G), a release etchant 260 (e.g., a
buffered oxide etch) is then use to selectively remove a portion of
the release material layer from beneath the exposed portion of the
spring metal island to form spring metal finger 120 (discussed
above with reference to FIGS. 1-3). Specifically, removal of the
exposed release material causes free portion 125 to bend away from
substrate 101 due to the internal stress variations established
during the formation of the spring metal film (discussed above).
Note that anchor portion 122 remains secured to substrate 101 by
release material portion 110, which are protected by release mask
250. Note also that when release material portion 110 is formed
from a conductive release material, the resulting spring structure
is electrically coupled to contact pad 105.
[0036] Note that in region OH (FIG. 4(G)) the undercut edge of
release mask 250 overhangs remaining portions 210C of the release
material layer. During subsequent metal plating (discussed below),
metal can become plated in overhang region OH under the overhanging
mask structure. This is a potential problem as it could lead to
shorted structures caused by bridging strips of plated metal that
become separated from the edges of the release window when the
release window is subsequently stripped.
[0037] FIGS. 4(H) and 5(B) depict optional steps for avoiding the
bridging strips of plated metal that can become plated along the
edge of release mask 250. First, as indicated in FIG. 4(H), a
reflow process that may be performed in which the temperature of
release mask 250 is raised above its glass transition temperature
or melting point in order to collapse the edge 251 of release
window 250 to close off the overhanging resist. Closing off this
overhanging resist will prevent the plating solution from forming
potentially bridging strips. A modest amount of over etch, if
needed, during the release process will produce the overhanging
resist such that release mask 250 will flow to close off the edge
of release layer portion 210C. The inventors have observed that
during reflow, capillary forces in the liquefied release window
material cause it wet to and stick to the substrate, thereby
closing off the gap produced by the undercut. Referring to the
upper portion of FIG. 5(B), eliminating the overhang facilitates
relatively closely spaced spring structures because it allows more
than one spring metal island (i.e., 220-1 and 220-2) to be exposed
through a single release window RW. However, as indicated in the
lower portion of FIG. 5(B), a second approach avoids the reflow
step entirely by forming a separate release window RW2 for each
spring metal island 220-3, thereby preventing bridging strips from
contacting more than one spring structure. Note that separate
release window RW2 requires a relatively wide spacing between
spring metal islands, thereby resulting in relatively widely spaced
spring structures.
[0038] In accordance with another aspect of the present invention,
metal plating is applied to the released spring metal finger using
the release mask and remaining portions of the release metal layer
(i.e., those portions that are not etched away during the release
operation, discussed above).
[0039] Metal plating can be performed through the release mask
using either electroless plating techniques or electroplating
techniques. However, electroplating is preferred due to simplicity,
cost, and material quality. The spring metal finger component may
be thought of as a scaffold or skeleton upon which additional
material is added by plating. The high-cost component (sputtered
metal) is minimized and augmented by the low-cost batch material
(plated metal). Accordingly, metal plating a relatively thin spring
metal finger provides a substantially less expensive method of
achieving a thick, stiff spring structure than sputtering alone.
The release window is used as the plating mask to plate metal onto
the exposed metal including the release springs, and depending on
design, other exposed metal.
[0040] As indicated in FIG. 4(I), in one embodiment release
material portion 110 (which is located under anchored portion
220-1A) and remaining portions 210C (which are located under
release material mask after the release operation) can be utilized
to facilitate electroplating by providing a suitable common
electrical path for the electroplating cathode. In one embodiment,
electroplating is performed, for example, using a metal source 270
(e.g., Ni, Au, Cu, Pd, Sn solder, Rh and/or alloys thereof) and
known parameters. More than one of these metals may be plated in
succession (e.g., Ni for stiffness followed by Au for passivation).
The electrical (cathode) connection can be made directly to these
remaining release layer portions, through conductor 105, or through
spring metal 220 or its optimal passivation metal (not shown). On a
typical wafer containing many devices to be plated, current can be
supplied to all of the devices through a small number of contacts
located at the periphery of the wafer. After the release operation,
these release material portions are still connected without
isolated islands, although they do have many openings beneath free
portions 125. These release layer portions therefore provide a
suitable conducting contact for the electroplating cathode.
[0041] FIG. 4(J) shows spring structure 100 after release mask 250
and remaining portions 210C of the release layer (see FIG. 4(I))
are removed using known techniques.
[0042] FIG. 6 shows a spring structure 300 according to a second
embodiment of the present invention. Similar to spring structure
100 (discussed above), spring structure 300 includes a release
layer portion 110 formed to contact a conductor 105, a spring metal
finger 120 formed on release layer portion 110, and a plated metal
layer 330 formed on spring metal finger 120. However, spring
structure 300 differs from spring structure 100 in that plated
metal layer 330 is formed on both free portion 125 and anchored
portion 122 of spring metal finger 120 (referring to FIG. 3, plated
metal layer 130 only covers free portion 125). Specifically, plated
metal layer 330 is formed on both sides of free portion 125, as
described above, and is also formed on an upper surface of anchored
portion 122. As mentioned above, it is well established that
resilient springy metals such as MoCr exhibit relatively high
resistance in comparison to many forms of plated metal, such as Ni,
Au and Cu. Accordingly, by extending plated metal layer 330 over
anchored portion 122, currents passing between free portion 125 and
conductor 105 are subjected to less resistance than in spring
structure 100 due to the presence of plated metal layer 330 on
anchor portion 122.
[0043] FIG. 7 is a plan view showing a release mask 450 utilized in
the fabrication of spring structure 300 (FIG. 6). Release mask 450
is similar to release mask 250 (shown in FIG. 5(B)), except that
the release window defined by release mask 450 exposes part of the
anchored portion of each spring metal island 230(1) through 230(3).
For example, referring to spring metal island 230(1), release
window 450 includes a channel 455 that extends over anchored
portion 222. Note that channel 455 overlaps the outer edge 229 of
anchor portion 222 by an overlap width OL of 1 to 10 microns to
prevent unintended release of anchor portion 222. Referring briefly
to FIG. 6, is overlap produces a step structure shoulder 325
extending along the edge of anchor portion 222.
[0044] Although the present invention has been described with
respect to certain specific embodiments, it will be clear to those
skilled in the art that the inventive features of the present
invention are applicable to other embodiments as well, all of which
are intended to fall within the scope of the present invention.
* * * * *