U.S. patent application number 11/362632 was filed with the patent office on 2006-06-29 for layered microelectronic contact and method for fabricating same.
This patent application is currently assigned to FormFactor, Inc.. Invention is credited to Igor Y. Khandros, Charles A. Miller, Stuart W. Wenzel.
Application Number | 20060138677 11/362632 |
Document ID | / |
Family ID | 33130885 |
Filed Date | 2006-06-29 |
United States Patent
Application |
20060138677 |
Kind Code |
A1 |
Khandros; Igor Y. ; et
al. |
June 29, 2006 |
Layered microelectronic contact and method for fabricating same
Abstract
A microelectronic spring contact for making electrical contact
between a device and a mating substrate and method of making the
same are disclosed. The spring contact has a compliant pad adhered
to a substrate of the device and spaced apart from a terminal of
the device. The compliant pad has a base adhered to the substrate,
and side surfaces extending away from the substrate and tapering to
a smaller end area distal from the substrate. A trace extends from
the terminal of the device over the compliant pad to its end area.
At least a portion of the compliant pad end area is covered by the
trace, and a portion of the trace that is over the compliant pad is
supported by the compliant pad. A horizontal microelectronic spring
contact and method of making the same are also disclosed. The
horizontal spring contact has a rigid trace attached at a first end
to a terminal of a substrate. The trace is free from attachment at
its second end, and extends from the terminal in a direction
substantially parallel to a surface of the substrate to the second
end. At least a distal portion of the trace extending to the second
end is spaced apart from the surface of the substrate. The
spaced-apart distal portion is flexible in a plane parallel to the
substrate.
Inventors: |
Khandros; Igor Y.; (Orinda,
CA) ; Miller; Charles A.; (Fremont, CA) ;
Wenzel; Stuart W.; (San Francisco, CA) |
Correspondence
Address: |
N. KENNETH BURRASTON;KIRTON & MCCONKIE
P.O. BOX 45120
SALT LAKE CITY
UT
84145-0120
US
|
Assignee: |
FormFactor, Inc.
|
Family ID: |
33130885 |
Appl. No.: |
11/362632 |
Filed: |
February 27, 2006 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
10410948 |
Apr 10, 2003 |
7005751 |
|
|
11362632 |
Feb 27, 2006 |
|
|
|
Current U.S.
Class: |
257/780 ;
257/E23.021; 257/E23.078 |
Current CPC
Class: |
H01L 2924/00014
20130101; H01L 24/72 20130101; H01L 2224/05001 20130101; H01L
2224/13 20130101; H01L 2924/01015 20130101; H01L 2924/01027
20130101; H01L 2924/01033 20130101; H01L 2924/01075 20130101; H05K
2201/09909 20130101; H05K 2201/0133 20130101; H01L 2224/05027
20130101; H01L 24/81 20130101; H01L 2224/81901 20130101; H01L 24/05
20130101; H01L 2924/01039 20130101; H01L 2924/10253 20130101; H01L
2924/01013 20130101; H01L 2924/3025 20130101; H01L 2924/014
20130101; H01L 2924/01058 20130101; H01L 24/13 20130101; H01L
2924/14 20130101; Y10T 29/49147 20150115; H01L 2924/3011 20130101;
H01L 2924/01022 20130101; H01L 2924/19041 20130101; H01L 2924/01046
20130101; H01L 2924/01047 20130101; H01L 2924/01079 20130101; H01L
2224/05568 20130101; H01L 2924/01082 20130101; H01L 2224/05548
20130101; H05K 2201/0367 20130101; H01L 2924/01078 20130101; H01R
43/007 20130101; H05K 3/4007 20130101; H01L 2224/13099 20130101;
H05K 3/326 20130101; H01L 2924/351 20130101; H01R 12/57 20130101;
H01L 2924/01006 20130101; H01L 2224/05573 20130101; H01L 2224/73251
20130101; H01L 2924/01074 20130101; H01L 2924/10253 20130101; H01L
2924/00 20130101; H01L 2924/351 20130101; H01L 2924/00 20130101;
H01L 2224/73251 20130101; H01L 2224/16 20130101; H01L 2224/72
20130101; H01L 2224/13 20130101; H01L 2924/00 20130101; H01L
2224/81901 20130101; H01L 2224/72 20130101; H01L 2924/00014
20130101; H01L 2224/05599 20130101; H01L 2924/00014 20130101; H01L
2224/05099 20130101 |
Class at
Publication: |
257/780 |
International
Class: |
H01L 23/48 20060101
H01L023/48 |
Claims
1-23. (canceled)
24. A resilient microelectronic contact, comprising an at least
partially self-supporting trace attached at a first end to a
terminal of a substrate and free from attachment at a second end
thereof, extending from the terminal to the second end, having at
least a distal portion extending to the second end spaced apart
from the surface of the substrate and free to flex in a plane
parallel to the surface of the substrate.
25. The microelectronic contact of claim 24, wherein the distal
portion has at least one bend for resiliency of the trace in a
plane parallel to the substrate.
26. The microelectronic contact of claim 24, wherein the distal
portion of the trace is patterned to follow a path having a shape
selected from zigzag, crenulated, hair-pin shaped, and
serpentine.
27. The microelectronic contact of claim 24, further comprising a
contact tip connected to the second end of the rigid trace.
28. The microelectronic contact of claim 27, wherein the contact
tip is flat and pad-shaped.
29. The microelectronic contact of claim 27, further comprising a
dollop of bonding material on the contact tip.
30. The microelectronic contact of claim 29, wherein the bonding
material is a solder paste.
31. The microelectronic contact of claim 27, further comprising a
compliant pad disposed on the substrate under the contact tip.
32. The microelectronic contact of claim 31, wherein the compliant
pad has a base adhered to the substrate, and side surfaces
extending away from the substrate tapering to a end area distal
from the substrate, wherein the end area is substantially smaller
than the base.
33. The microelectronic contact of claim 31, wherein the compliant
pad is at least partially supporting the contact tip.
34. A method for making a resilient microelectronic contact,
comprising: depositing a first layer of a sacrificial material on a
semiconductor device; patterning the first layer to expose a
terminal of the device; depositing a conductive seed layer over the
first layer and terminal; depositing a second layer of a
sacrificial material directly over the seed layer; patterning the
second layer to expose the seed layer along a path running from the
terminal to a position distal from the terminal; plating a metallic
material along the path of the exposed seed layer; and removing the
first layer, the second layer, and an unplated portion of the seed
layer, thereby exposing a resilient microelectronic contact
attached at a first end to a terminal of a substrate and free from
attachment at a second end thereof.
35. The method of claim 34, wherein the patterning step further
comprises exposing the path having at least one bend.
36. The method of claim 34, wherein the patterning step further
comprises exposing the path having a shape selected from zigzag,
crenulated, hairpin-shaped and serpentine.
37. The method of claim 34, further comprising placing a dollop of
bonding material on the distal portion of the microelectronic
contact.
38. The method of claim 34, further comprising attaching a
compliant pad to the substrate prior to the first depositing
step.
39. The method of claim 38, wherein the attaching step further
comprises attaching the compliant pad having a base adhered to the
semiconductor device, side surfaces extending away from the
semiconductor device and tapering to a end area distal from the
semiconductor device, wherein the end area is substantially smaller
than the base.
40. The method of claim 39, wherein the patterning step further
comprises exposing the path leading to a tip portion of the
compliant pad.
41. A semiconductor device configured for flip-chip mounting to a
substrate, comprising: a semiconductor device having a plurality of
terminals on a surface thereof; a plurality of resilient
microelectronic contacts, each comprising a rigid trace attached at
a first end to each terminal of the device and free from attachment
at a second end thereof, extending from each terminal in a
direction substantially parallel to the surface of the device to
the second end, and having at least a distal portion extending to
the second end spaced apart from the surface and compliant in a
plane parallel to the surface of the semiconductor device.
42. The semiconductor device of claim 41, wherein the plurality of
terminals are spaced apart from one another for a first pitch
distance within a first portion of the surface, and wherein the
surface has a second portion that is essentially free of terminals,
the second portion being larger than the first portion.
43. The semiconductor device of claim 42, wherein the second ends
of the plurality of contacts are disposed over the second portion
of the surface and spaced apart from one another for a second pitch
distance, the second pitch distance being greater than the first
pitch distance.
44. The semiconductor device of claim 41, wherein the distal
portion of each microelectronic contact has at least one bend for
resiliency of the microelectronic contact in a direction parallel
to the substrate.
45. The semiconductor device of claim 41, wherein the distal
portion of each microelectronic contact has a shape selected from
zigzag, crenulated, hairpin-shaped and serpentine.
46. The semiconductor device of claim 41, wherein the surface of
the semiconductor device is essentially free of elastomer
material.
47. The semiconductor device of claim 41, further comprising a
dollop of a bonding material disposed on a distal tip of the distal
portion of each microelectronic contact.
48. The semiconductor device of claim 41, further comprising a
compliant pad disposed between a distal tip of the distal portion
of each microelectronic contact and the substrate.
49. The semiconductor device of claim 48, wherein the compliant pad
has a base adhered to the semiconductor device and side surfaces
extending away from the semiconductor device and tapering to a end
area distal from the semiconductor device, and wherein the end area
is substantially smaller than the base.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to microelectronic contacts
for use with semiconductor devices and the like.
[0003] 2. Description of Related Art
[0004] The demand for ever-smaller and more sophisticated
electronic components has driven a need for smaller and more
complex integrated circuits (ICs). The ever-smaller ICs and high
lead counts, in turn, require more sophisticated electrical
connection schemes, both in packaging for permanent or
semi-permanent attachment, and for readily demountable applications
such as testing and burn-in.
[0005] For example, many modern IC packages have smaller
footprints, higher lead counts and better electrical and thermal
performance than IC packages commonly used only a few years ago.
One such compact IC package is the ball grid array (BGA) package. A
BGA package is typically a rectangular package with terminals,
normally in the form of an array of solder balls, protruding from
the bottom of the package. These terminals are designed to be
mounted onto a plurality of bonding pads located on the surface of
a printed circuit board (PCB) or other suitable substrate. The
solder balls of the array are caused to reflow and bond with
bonding pads (terminals) on a mating component, such as by passing
the component with the mounted BGA package through an ultrasound
chamber or like thermal energy source, and then removing the energy
source to cool and harden the solder and form a relatively
permanent bond. Once melted and re-hardened, the solder ball
connections cannot readily be re-used, if at all. Hence, separate,
readily demountable contact elements are required to contact the
terminal pads of the IC or the solder balls of the BGA package
during testing and burn-in.
[0006] The advantages of readily demountable contact elements for
use in compact packaging and connection schemes have previously
been recognized. Readily demountable, flexible and resilient
microelectronic spring contacts for mounting directly to substrates
such as ICs are described in U.S. Pat. No. 5,917,707 to Khandros et
al. Among other things, the '707 patent discloses microelectronic
spring contacts that are made using a wire bonding process that
involves bonding a very fine wire to a substrate, and subsequent
electro-plating of the wire to form a resilient element. These
microelectronic contacts have provided substantial advantages in
applications such as back-end wafer processing, and particularly
for use as contact structures for probe cards, where they have
replaced fine tungsten wires. These same or similar contact
elements may also be used to make electrical connections between
semiconductor devices in general, for making both temporary
(readily demountable) and more permanent electrical connections in
almost every type of electronic device.
[0007] Presently, however, the cost of fabricating fine-pitch
spring contacts has limited their range of applicability to less
cost-sensitive applications. Much of the fabrication cost is
associated with manufacturing equipment and process time. Contacts
as described in the aforementioned patents are fabricated in a
serial process (i.e., one at a time) that cannot be readily
converted into a parallel, many-at-a-time process. Thus, new types
of contact structures, referred to herein as lithographic type
microelectronic spring contacts, have been developed, using
lithographic manufacturing processes that are well suited for
producing multiple spring structures in parallel, thereby greatly
reducing the cost associated with each contact.
[0008] Exemplary lithographic type spring contacts, and processes
for making them, are described in the commonly owned, co-pending
U.S. patent applications Ser. No. 09/032,473 filed Feb. 26, 1998,
by Pedersen and Khandros, entitled LITHOGRAPHICALLY DEFINED
MICROELECTRONIC CONTACT STRUCTURES," and Ser. No. 60/073,679, filed
Feb. 4, 1998, by Pedersen and Khandros, entitled "MICROELECTRONIC
CONTACT STRUCTURES." These applications disclose methods for
fabricating the spring structures using a series of lithographic
steps, thereby building up the height of the spring contact with
several layers of plated metal that may be patterned using various
lithographic techniques. Microelectronic spring contacts are
preferably provided with ample height to compensate for any
unevenness in the mounting substrate and to provide space for
mounting components, such as capacitors, under the spring
contact.
[0009] Methods of achieving adequate height in a single
lithographic step, i.e., a single resilient layer, and exemplary
structures made thereby, are disclosed in the commonly owned,
co-pending U.S. patent applications Ser. No. 09/364,788, filed Jul.
30, 1999 by Eldridge and Mathieu, entitled "INTERCONNECT ASSEMBLIES
AND METHODS," and Ser. No. 09/710,539, filed Nov. 9, 2000, by
Eldridge and Wenzel, entitled "LITHOGRAPHIC SCALE MICROELECTRONIC
SPRING STRUCTURES WITH IMPROVED CONTOURS." The foregoing
applications disclose spring elements made from a single layer of
metal. The metal layer is plated over a patterned three-dimensional
layer of sacrificial material, which has been shaped using a
micromachining or molding process. The sacrificial layer is then
removed, leaving a free-standing spring contact having the
contoured shape of the removed layer.
[0010] A need therefore exists for an improved microelectronic
spring contact, and method of making it, that achieves or improves
upon the performance of multi-layer and single-layer spring
contacts at a substantially lower cost. The spring contact should
be useful in very dense fine-pitch arrays for directly connecting
to IC's and like devices, and be capable of making both relatively
demountable and relatively permanent (e.g., soldered)
connections.
[0011] Moreover, it is desirable that the microelectronic spring
contact be useful in compact packaging schemes, where low cost,
demountability, and resiliency are important. Exemplary
applications may include portable electronic components (cellular
phones, palm computers, pagers, disk drives, etc.), that require
packages smaller than BGA packages. For such applications, solder
bumps are sometimes deposited directly onto the surface of an IC
itself and used for attachment to the printed circuit board (PCB).
This approach is commonly referred to as direct chip attach or
flip-chip. The flip-chip approach is subject to various
disadvantages.
[0012] One key disadvantage is the requirement for a polymer
underfill beneath a die. The underfill is required to reduce
thermal stresses caused by the relatively low thermal expansion of
the silicon die relative to the typically much higher expansion of
resin-based PCB's. The presence of the underfill often makes it
infeasible to rework the component. Consequently, if the IC or its
connection to the PCB is defective, the entire PCB usually must be
discarded.
[0013] Another type of BGA package, the chip-scale ball grid array
or a chip scale package (CSP), has been developed to overcome this
disadvantage of flip-chips. In a chip scale package, solder ball
terminals are typically disposed underneath a semiconductor die in
order to reduce package size, and additional packaging elements are
present to eliminate the need for underfill. For example, in some
CSP's, a soft compliant elastomer layer (or elastomer pad) is
disposed between the die and the solder ball terminals. The solder
ball terminals may be mounted onto a thin 2-layer flex circuit, or
mounted to terminals on the complaint member. The IC is typically
connected to terminals on the flex circuit or elastic member using
a wire or tab lead, and the entire assembly (except the ball grid
array) is encapsulated in a suitable resin.
[0014] The elastomeric member is typically a polymer, such as
silicone, about 125 .mu.m to 175 .mu.m (5-7 mils) thick. The
elastomer pad or layer essentially performs the function of and
replaces the underfill used in flip-chips, that is, minimizes
thermal mismatch stress between the die and the PCB. In other CSP
designs, the IC is adhered directly to the surface of a two-layer
flex circuit, and connected to terminals on the chip side of the
flex circuit using wire leads. Solder balls are mounted on an
opposite surface of the flex circuit. This design lacks an
elastomer layer for decoupling the die from the PCB and, therefore,
may not eliminate the need for underfill.
[0015] Current chip-scale package designs have a number of
shortcomings. The elastomeric materials tend to absorb moisture,
and if excessive moisture is absorbed, rapid outgassing of this
moisture at reflow temperatures may cause the formation of voids in
the elastomer layer, or bursting of the package. For example,
moisture may be released from polymer materials in the elastomer
and become trapped within the die attachment adhesive. Voids may
then be formed when this trapped moisture expands during board
assembly heating operations, typically causing cracking and package
failure. Formation of such voids may be particularly problematic
during reflow attachment to a PCB.
[0016] Another difficulty with chip-scale package designs is the
process for integrating the elastomer member, which is typically
done by picking and placing elastomer pads onto individual sites,
or by screen printing and subsequently curing a fluid polymer. In
either case, it may be difficult to meet the tight tolerances and
package flatness required for a CSP application. For example, in a
typical CSP design, the package flatness (planarity) should be less
than about 25 .mu.m (1 mil) to ensure that all solder balls
establish contact with PCB upon reflow. This level of flatness may
be difficult to achieve using prior art processes for depositing
the elastomeric materials.
[0017] Therefore, it is further desirable to provide an improved
microelectronic contact element for applications such as CSPs and
flip-chips.
SUMMARY OF THE INVENTION
[0018] The structure of the spring contacts according to the
present invention may be understood by considering an exemplary
method by which they may be fabricated. In an initial step of the
method, a precisely shaped pit, such as a pyramidal pit, is formed
in a sacrificial substrate using any suitable technique, for
example, etching or embossing. Typically, a large array of
identical pits will be formed at the same time in the sacrificial
substrate, arranged in a pattern corresponding to the desired
position of the contact tips to be formed on the electronic device.
The surface of the pits may then be coated, if necessary, with a
thin layer of a suitable release material, such as
polytetrafluoroethylene (PTFE). The pits may then be filled with a
suitable fluid elastomer, or similar compliant material. The
elastomer or compliant material is preferably free of any filler
materials, such as conductive fillers. The sacrificial substrate
may then be mated to the device substrate on which the spring
contacts are to be formed, the elastomer cured (solidified) in
place, thereby adhering the elastomer to the device, and the
sacrificial substrate removed. In the alternative, the elastomer or
compliant material may be cured before the sacrificial substrate is
mated to the device substrate, and the compliant members adhered to
the device process by some other method, such as application of
heat or by a suitable adhesive. As yet another alternative, dots of
a polymer material may be applied to the device substrate by, for
example, screen printing, and the pit features then pressed against
the dots to mold the dots.
[0019] As a consequence of the foregoing steps, the device
substrate should be populated with at least one compliant pad or
protrusion, and typically, a plurality of compliant pads,
positioned away from the working terminals of the device substrate.
For most applications, the pads are preferably of similar or nearly
identical height and shape, having a relatively wide base and a
pointed top. Of course, the pads may be different sizes and/or
shapes depending on the requirements of the intended application.
Suitable shapes may include pyramids, truncated pyramids, stepped
pyramids, prisms, cones, quadrangular solids, and similar shapes.
The pads may be essentially solid and homogenous, or may include
voids, bubbles, layers, and the like. It is not necessary that
conductive contact be established between the compliant members and
the device substrate. To the contrary, the compliant members are
preferably positioned so as avoid contact with terminals on the
device substrate. Also, the compliant pads will generally be
distributed in a pitch-spreading pattern relative to the terminals
on the device substrate.
[0020] In an embodiment of the invention, the compliant pads are
primarily elastic, meaning that they are configured to spring back
to their original positions after an applied load is removed. In
alternative embodiments, the compliant pads may be primarily
inelastic, meaning that they will not spring back to their original
positions after the applied load is removed; or the compliant pads
may be configured to exhibit some combination of elastic and
inelastic behavior. One of ordinary skill may select different
materials and pad geometries to obtain the desired response
characteristics under anticipated load conditions.
[0021] In an embodiment of the invention, the device substrate,
including the protrusions, may be coated with a thin metallic seed
layer, such as a titanium-tungsten layer, applied by any suitable
process such as sputtering. One or more uniform conformal layers of
a sacrificial material, such as an electrophoretic resist material,
is then applied over the device substrate. The sacrificial layer is
then patterned as desired to expose the seed layer in a pattern of
traces extending from the terminals of the device substrate to
respective tops of the compliant pads. The trace pattern may be
made wider over the compliant pads for greater stiffness and
strength of the resulting contact structures.
[0022] A metallic resilient and/or conductive layer is then plated
to the desired depth over the partially exposed seed layer. Nickel
or nickel alloy material is generally preferred, plated to a depth
sufficient to be suitably strong and resilient. In an embodiment,
the nickel material is plated to sufficient depth so the resulting
trace is stiffer than the compliant pads. Optionally, the resilient
layer is coated with a protective and conductive layer, such as a
thin layer of gold, after the plating step. After the desired
metallic layers are applied, the layer of sacrificial material and
the excess seed layer are removed using processes that leave the
compliant protrusions and metal traces on the device substrate.
[0023] The resulting structure is then ready to use without further
processing, and comprises a metal trace integral with a spring
contact running from each desired terminal of the device substrate
to the top of a respective one of the compliant pads. Preferably, a
pointed top of each compliant pad has imparted a relatively sharp
pointed tip to each spring contact by the highly conformal plating
process. Each contact extends both laterally and vertically from
the base of each compliant pad to the top of each pad, providing a
cantilevered structure that imparts a beneficial wiping action to
the motion of the contact tip when the spring contact is deflected.
The spring contacts are advantageously supported by the compliant
pad during use.
[0024] The support of the compliant material may enable use of a
thinner plated layer for the spring contacts than would otherwise
be required to provide adequate contact forces. The thinner plated
layer, in turn, may save substantial processing time during the
plating step. Also, the foregoing method avoids any need for
contouring or molding of a sacrificial layer, any need for separate
forming steps for providing a sharp contact tip, and any need for a
separate step to provide redistribution traces.
[0025] In an alternative embodiment, the plating step and the
related steps of applying the seed layer and applying and
patterning the resist layer are omitted. Instead, the desired
traces and contact elements are patterned directly onto the device
substrate and elastomer protrusions by a method such as sputtering
or vapor deposition.
[0026] In another alternative embodiment, the traces are configured
for a flip-chip application that requires no elastomer pad or
underfill. The traces are shaped to be resilient in a direction
parallel to the device substrate. For convenience, such traces are
referred to herein as "horizontal springs," and it should be
apparent that "horizontal" is not limiting except in the sense of
describing resiliency in the direction parallel to the device
substrate. The horizontal resiliency compensates for thermal
mismatch between the device substrate and the PCB or other member
to which it is mounted, and thereby eliminates the requirement for
underfill and for elastomer members. Optionally, the traces may
also be made resilient in a direction perpendicular to the device
substrate, like the spring contacts described in the references
cited above.
[0027] Preferably, the horizontal spring contacts are formed on a
sacrificial layer on the device substrate. Each horizontal spring
contact runs between a terminal of the device and a bonding pad,
such as a pad for bonding to a corresponding pad of a PCB using a
solder ball or adhesive connection. Horizontal flexibility may be
provided by patterning the trace in any suitable fashion, such as
in a zigzag, pleated, crenulated, or serpentine pattern. The
sacrificial layer is then removed, leaving each horizontal spring
contact suspended above the device substrate, except where it is
attached to its respective terminal. Each trace is thus made
flexible in the direction parallel to the device substrate. When
the free end of each trace is bonded to a mating substrate, stress
arising from thermal mismatch between the device and the mating
substrate is relieved by deflection of the horizontal spring
contacts. Optionally, a compliant pad may be located under a
contact tip of the horizontal spring contact, for additional
vertical support.
[0028] A more complete understanding of the layered microelectronic
contact and the horizontal spring contact will be afforded to those
skilled in the art, as well as a realization of additional
advantages and objects thereof, by a consideration of the following
detailed description of the preferred embodiment. Reference will be
made to the appended sheets of drawings which will first be
described briefly.
BRIEF DESCRIPTION OF THE DRAWINGS
[0029] FIG. 1 is an enlarged perspective view of an exemplary
microelectronic spring contact according to the invention with a
pyramidal compliant pad.
[0030] FIG. 2 is an enlarged plan view of an array of
microelectronic spring contacts of the type shown in FIG. 1,
showing a portion of a pitch-spreading array.
[0031] FIG. 3 is an enlarged perspective view of exemplary
microelectronic spring contacts using a shared prism-shaped
compliant pad.
[0032] FIG. 4 is an enlarged perspective view of an exemplary
microelectronic spring contact using a hemispherical compliant
pad.
[0033] FIG. 5 is an enlarged perspective view of an exemplary
microelectronic spring contact using a conical compliant pad.
[0034] FIG. 6 is an enlarged side view of an exemplary
microelectronic spring contact using a compliant pad in the shape
of a stepped pyramid.
[0035] FIG. 7 is an enlarged side view of an exemplary
microelectronic spring contact using a compliant pad in the shape
of a truncated pyramid.
[0036] FIG. 8 is an enlarged side view of an exemplary
microelectronic spring contact with a pyramidal compliant pad,
showing deflection characteristics of a spring contact having a
metallic trace that is relatively stiff compared to the compliant
pad.
[0037] FIG. 9 is an enlarged side view of an exemplary
microelectronic spring contact with a pyramidal compliant pad,
showing deflection characteristics of a spring contact having a
metallic trace that is relatively flexible compared to the
compliant pad.
[0038] FIG. 10 is a flow diagram of showing exemplary steps of a
method for forming a microelectronic spring contact according to
the invention.
[0039] FIG. 11 is a flow diagram showing exemplary steps of a
method for depositing a conductive trace between a terminal and a
compliant pad.
[0040] FIG. 12 is an enlarged plan view of an exemplary
microelectronic spring contact having a relatively thin and
flexible metal trace deposited over a pyramidal compliant pad.
[0041] FIG. 13 is an enlarged perspective view of the spring
contact shown in FIG. 12.
[0042] FIG. 14 is an enlarged perspective view of a spring contact
with offset openings in a relatively thin and flexible metal trace,
for enhanced lateral flexibility.
[0043] FIG. 15A is a plan view of an exemplary flip-chip
semiconductor device having an array of microelectronic spring
contacts according to the invention.
[0044] FIG. 15B is an enlarged plan view of the flip-chip device
shown in FIG. 15A.
[0045] FIG. 16 is an enlarged side view of an exemplary flip-chip
device with readily demountable microelectronic spring contacts
according to the invention.
[0046] FIG. 17 is an enlarged side view of an exemplary flip-chip
device with solderable microelectronic spring contacts according to
the invention.
[0047] FIG. 18 is an enlarged perspective view of a horizontal
spring contact according to the invention.
[0048] FIG. 19 is an enlarged plan view of a serpentine horizontal
spring contact according to the invention.
[0049] FIG. 20 is an enlarged plan view of a horizontal spring
contact having a hairpin-shaped beam portion.
[0050] FIG. 21 is a flow diagram showing exemplary steps of a
method for making horizontal spring contacts according to the
invention.
[0051] FIG. 22 is an enlarged plan view of an exemplary flip-chip
device with an array of horizontal spring contacts.
[0052] FIG. 23 is an enlarged side view of the flip-chip device
shown in FIG. 22 in contact with terminals of a substrate.
[0053] FIG. 24 is an enlarged perspective view of a horizontal
spring contact in combination with a pyramidal compliant pad.
[0054] FIG. 25 is an enlarged side view of a horizontal spring
contact in combination with a compliant pad in the shape of a
truncated pyramid.
[0055] FIG. 26 is an enlarged perspective view of a horizontal
spring contact in combination with a compliant pad in the shape of
a stepped pyramid.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0056] The present invention provides microelectronic spring
contacts that overcome limitations of prior art spring contacts. In
the detailed description that follows, like element numerals are
used to describe like elements appearing in one or more of the
figures.
[0057] The present invention achieves the benefits of multi-layer
and single-layer lithographic spring contacts as disclosed in the
patent applications referenced herein, at a potentially lower cost,
and provides additional advantages for certain packaging and
connecting applications. The spring contacts of the present
invention are believed especially suitable for compact packaging
applications, such as flip-chip packages and CSP's, where they may
replace or augment the use of ball grid arrays as connection
elements.
[0058] With proper selection of materials, the spring contacts may
also be used for testing and burn-in applications. It is therefore
within the scope and intent of the invention that spring contacts
according to the invention be fabricated directly on the devices of
an unsingulated wafer for initial testing and/or burn-in; remain on
the devices after testing for burn-in testing before or after
packaging, if desired; and then be used as the primary connection
element (i.e., with or without solder or conductive adhesive) for
final assembly to an electronic component. In the alternative, the
spring contacts of the present invention may be used for any
selected one or combination of the foregoing applications, used as
secondary connection elements (e.g., IC to flex circuit) within a
package incorporating other connection elements such as a BGA, used
as the contact elements or interposer elements of a test probe,
used within a connector such as a Land Grid Array (LGA) socket, or
for any other suitable connection application.
[0059] An exemplary layered microelectronic spring contact 100 is
shown in FIG. 1. Spring contact 100 comprises two primary layers of
material: a first non-conductive elastomer layer in the form of
pyramidal compliant pad I 10, and a second conductive and resilient
layer in the form of metallic trace 102. Spring contact 100 is
described as layered because at least a part of a conductive layer
(trace 102) overlies a non-conductive layer (pad 110) and the two
layers together define the contact 100.
[0060] Compliant pad 10 may be any suitable shape within the
parameters described herein. In an embodiment of the invention, it
is a precisely formed shape, such as a molded shape in alternative
embodiments, pad 110 may be a less well-defined shape, such as a
relatively amorphous dollop. The morphology of the pad may be
imparted to a relatively rigid metallic tip and beam that are
deposited over the pad surface. To ensure a high degree of
uniformity across densely populated spring contact arrays, each pad
may be formed using a parallel process that minimizes variability
between pads. Parallel formation, such as molding en masse,
provides the further benefit of requiring less time than individual
dollop formation.
[0061] Specifically, pad 110 has a pyramid shape, although other
suitable shapes may be used such as, for example, the pad shapes
described herein. In more general terms, the pad 110 may be
described as a tapered mass having a relatively large and flat base
area 112 where the pad is adhered to a substrate 116, and free side
surfaces 109 that extend away from the substrate and taper to a
relatively small end area distal from the substrate. The end area
is hidden from view in FIG. 1 by the overlaying metallic tip 104.
This tapered shape maximizes the area for adhesion to the substrate
116 while efficiently supporting a defined tip structure. In this
embodiment, the pyramidal shape reduce the potential for outgassing
from the elastomeric material, to ventilate contact 100 from any
outgassing that may occur, and to provide increased lateral
flexibility for thermal stress relief across contact arrays.
[0062] A pyramidal compliant pad may be particularly suitable
because pyramid shapes with the desired tapered characteristics may
readily be formed with great precision and at extremely small
scales by exploiting the properties of commonly available
crystalline silicon materials. It is well known that a pyramidal
pit, with side surfaces defined by the orientation of crystal
planes in the silicon material, may readily be produced by exposing
a silicon substrate covered with a suitably patterned layer of
photo-resist to a suitable etchant, such as KOH. An array of
substantially identical pyramidal pits may thus be produced in a
silicon substrate, and the substrate with pits may be used as a
mold for forming an array of identical pyramidal compliant pads.
Related shapes such as prisms, truncated pyramids or prisms, and
stepped pyramids or prisms may be similarly formed using suitable
etching and masking process, as should be apparent to one of
ordinary skill in the art.
[0063] Compliant pad 110 may be made of any suitable material. For
example, suitable elastomer materials may include silicone rubber,
natural rubber, rubberized plastics, and a wide variety of other
organic polymer materials. One of ordinary skill in the art may
select a suitable material by considering the intended operating
environment (such as temperature or chemical environment) and
desired structural characteristics of the spring contact. For
example, a suitably soft and resilient material may be selected
once the contact geometry, desired range of compressibility, and
maximum contact force are defined. Preferably, the pad material is
a homogenous plastic material free of any particulate filler
material, and is inherently non-conductive. Homogenous plastic
material may be more readily formed into a precise pad shape at
small scales, such as for compliant pads that are less than about 5
mils (about 130 .mu.m) wide.
[0064] The compliant pad 110 is adhered to substrate 116 at a
location spaced apart from terminal 114 for which an electrical
connection is desired. A conductive trace 102 is then deposited
from the terminal 1 14 to the end area of the compliant pad, by a
process such as electroplating. Trace 102 may be comprised of any
suitable metal or metal alloy, and may include one or more layers.
For example, trace 102 may be comprised of a relatively thick layer
of nickel or nickel alloy for strength and rigidity, covered with a
relatively thin layer of gold for conductivity. Trace 102 is
preferably an integral piece of metal having a contact tip portion
104 deposited over the end area of pad 110, a pad-supported beam
portion 106 running from the base 112 of pad 110 to the contact tip
104, and a substrate-supported redistribution trace portion 108
connecting the beam portion 106 to the terminal 114. Contact tip
104 may be relatively pointed (as shown) for penetrating oxide and
contamination layers of a mating terminal. In the alternative, the
contact tip 104 may be relatively flat for supporting features such
as solder balls. Beam portion 106 may be tapered from a greater
width at base 112 to a narrower neck at tip 104, as shown. This
tapered design has the advantage of more uniformly distributing
stresses along the beam length. In the alternative, beam 106 may be
of constant width, be provided with a reverse taper (wider at the
top), or have any other suitable shape. Substrate 116 may be any
suitable electronic device, including but not limited to a
semiconductor die or wafer, a connector or socket for a die or
wafer, and a printed circuit board.
[0065] Spring contacts 100 may readily be used in a pitch-spreading
array 118, as shown in FIG. 2. Terminals 114 on substrate 116 are
disposed at a first pitch P1, and contact tips 104 are disposed at
a coarser pitch P2, wherein P2 is greater than PI. FIG. 2 also
shows various ways for positioning the redistribution portion 108
of trace 102. As shown at the bottom right of FIG. 2, the
redistribution trace 108 for a more distant contact 100' may be
routed completely around the compliant pad 110 of a closer contact.
In the alternative, as shown at the bottom left of FIG. 2, trace
108 for a more distant contact 100'' may be deposited directly over
the compliant pad 110 of a less distant contact, adjacent to its
base 112. Positioning traces over free areas of the compliant pads
may be advantageous in very dense arrays for which space for
positioning the redistribution traces is limited. Such positioning
may also relieve stress in the materials from which the spring
contact is formed.
[0066] FIGS. 3-7 show various alternative embodiments of the
invention. FIG. 3 shows a prism-shaped compliant pad 124 supporting
a plurality of spring contacts 122. The end area of pad 112 is
partially exposed. Other features of the contacts 122 are similar
to those described for spring contact 100. FIG. 4 shows a spring
contact 130 with a hemispherical pad 132. Contact tip 104 is
relatively flat. FIG. 5 shows a spring contact 134 with a conical
compliant pad 136. FIG. 6 is a side view of a spring contact 140
having a compliant pad 142 in the shape of a stepped pyramid.
Compared to a regular pad, the stepped pyramid pad 142 provides a
lower aspect ratio, that is, a lower height for a base of given
size. The lower aspect ratio may be advantageous for providing a
firmer contact for applications in which a higher contact force is
desired. FIG. 7 shows a side view of a spring contact 150 having a
compliant pad 152 in the shape of a truncated pyramid. The
truncated pyramid shape also provides a lower aspect ratio pad, and
may be suitable for applications in which a flat contact tip 104 is
desired. Spring contacts may be provided in various other shapes
and configurations different from those depicted herein, without
departing from the scope of the invention.
[0067] The relative structural properties of the compliant pad and
the overlying conductive trace may be varied. In an embodiment of
the invention, the compliant pad is relatively soft and flexible
compared to the conductive trace. FIG. 8 shows a deflection mode of
a spring contact 100 having a relatively flexible pad 110 and a
relatively stiff beam 106. In this embodiment, the characteristics
of the spring contact 100 are dominated by the properties of the
beam 106, which will deflect under the influence of a contact force
in a mode similar to how it would deflect were it not supported by
the compliant pad. The contact tip 104 will accordingly move a
lateral distance "dx" corresponding to a vertical displacement
"dz," thereby providing a beneficial wiping action to the contact
tip.
[0068] In an alternative embodiment, the conductive trace can be
made relatively flexible compared to the compliant pad. FIG. 9
shows a deflection mode of a spring contact 160 having a
pad-supported beam 166 that is relatively flexible compared to
compliant pad 162. To achieve greater flexibility, contact tip 164,
beam 166 and redistribution trace 168 may be deposited as a
relatively thin layer, which advantageously may be accomplished
more quickly than depositing a relatively thick beam like beam 106.
Being symmetrically supported, pad 162 will deflect a vertical
distance "dz" without appreciable lateral deflection. Beam 166 and
contact tip 164 bend to follow the contour of pad 162.
[0069] It should be appreciated that FIGS. 8 and 9 show deflection
modes that are at opposite ends of two extremes. It may be
desirable to configure a contact that operates in a mode that is
intermediate between the modes shown in FIGS. 8 and 9. In an
intermediate mode, the spring contact will exhibit characteristics
of both deflection modes. For example, the contact tip will undergo
some lateral deflection or wipe, while at the same time being
substantially supported by the compliant pad. Thus, in an
intermediate mode the advantages of both deflection modes--i.e.,
wiping action, and a thin, rapidly formed trace--may both be
realized to a degree. One skilled in the art may construct a spring
contact that operates in any desired deflection mode. For a given
geometry and selection of materials, the beam thickness may be
varied until the desired deflection mode is achieved. Computer
modeling may be useful in the design phase to predict the
deflection characteristics of a particular spring contact
design.
[0070] FIG. 10 shows exemplary steps of a method 200 for forming a
microelectronic spring contact according to the invention. In
initial step 202, a compliant pad is formed on a sacrificial
substrate. To form an array of compliant pads, precision pits in a
sacrificial substrate, such as a silicon substrate, in a pattern
corresponding to the desired arrangement of contact tips in the
spring contact array that is to be formed. The precision pits are
formed in a shape corresponding to the desired shape of compliant
pad, for example, a pyramidal pit is used to form a pyramidal pad,
and so forth. Any suitable method may be used for forming the
precision pits; in particular, various lithographic/etching
techniques may be employed to form pits of various shapes. After
the pits have been created, the sacrificial substrate is preferably
coated with a thin layer of a suitable release agent, such as a
PTFE material or other fluoropolymer. An alternative method of
forming a compliant pad is by deposition of a dollop of uncured or
softened elastomer material directly on a substrate, and then
curing or hardening the elastomer in place.
[0071] After the sacrificial substrate has been prepared, the pits
may be filled with the selected elastomeric material, preferably in
a liquid state. The substrate on which the contacts are to be
formed (the "device substrate") may then be mounted to the
sacrificial substrate, and the elastomeric material cured or
hardened with the device substrate in place, thereby adhering the
compliant pads to the substrate. The substrate and its attached
pads may then be removed from the sacrificial substrate,
transferring the pads to the device substrate as indicated at step
204. The sacrificial substrate may be re-used as desired.
[0072] In the alternative, after the pits in the sacrificial
substrate are filled with the liquid elastomer, the elastomer
material may be cured or hardened with the sacrificial substrate
left free and open. The sacrificial substrate may then be coated
with a suitable adhesive material, thereby coating the exposed
bases of the compliant pads. Preferably, the adhesive material is
patternable, so that it may be removed from the sacrificial
substrate except in regions over the elastomer material. In
addition, the adhesive material is preferably pressure-sensitive,
so that it will adhere on contact with a mating substrate. The
compliant pads may then be transferred to the device substrate as
desired.
[0073] With the compliant pads in place on the device substrate, at
step 206, a conductive trace is deposited between a terminal of the
device substrate and the top of a corresponding pad. FIG. 11 shows
exemplary steps of a method 210 for depositing a conductive trace
on a device substrate and compliant pad. At step 212, a seed layer
is deposited over the entire surface of the device substrate and
its attached compliant pads. One suitable seed layer is a sputtered
titanium-tungsten layer; a suitable seed layer may be selected by
one skilled in the art.
[0074] At step 214, a sacrificial layer is deposited over the seed
layer. The sacrificial layer is a patternable material, such as a
photoresist material, and is preferably applied as a highly
conformal layer over the device substrate and its protruding
elastomeric pads. Various methods may be used to deposit a
conformal layer of resist material. One suitable coating method for
thicknesses up to about 35 .mu.m is electrodeposition
(electrophoretic resist). Other methods may include spray coating,
spin coating, or meniscus coating, in which a laminar flow of
coating material is passed over the device substrate. A greater
depth may be built up by successively coating and curing layers of
material. The minimum depth of the sacrificial layer is preferably
equal or greater than the desired thickness of the metallic trace
to be deposited.
[0075] At step 216, the sacrificial layer is patterned to expose
the seed layer in the areas where the conductive traces are to be
deposited. Generally, patterning may be accomplished using any
suitable photo-patterning technique as known in the art. At step
218, the conductive trace material is deposited to the desired
depth over the exposed areas of the seed layer, such as by
electroplating. Successive layers of different materials, such as a
relatively thick layer of nickel or nickel alloy, followed by a
relatively thin layer of gold or other suitable contact metal such
as palladium, platinum, silver, or alloys thereof, may be applied
as desired. At step 220, the sacrificial layer is removed, such as
by dissolving in a suitable solvent. The device is thereby provided
with an array of spring contacts according to the invention.
[0076] For spring contacts in which the metal trace is to be
relatively thin and flexible, the metal trace need not be deposited
by electroplating, and may preferably be deposited by a method such
as sputtering or vapor deposition. In such case, the entire surface
of the device substrate and compliant pad may be coated with a thin
layer or layers of metal to the desired depth, as if with a seed
layer. Then, a photoresist layer may be applied and patterned to
protect those areas of the device substrate where a metallic trace
layer is desired, and the remaining unprotected areas of the metal
layer removed in an etching step. By eliminating the electroplating
step, processing time may be substantially reduced for those
applications that do not require a relatively stiff metallic
contact element.
[0077] In the case of layered spring contacts with relatively thin
and flexible metal layers, it may be advantageous to coat a greater
proportion of the compliant surface, up to and including the entire
surface of the compliant pad. An exemplary spring contact 170 with
most of the compliant pad 171 covered by a metallic layer 172 is
shown in FIGS. 12 and 13. Like the other spring contacts described
herein, metal layer 172 comprises a substrate-supported
redistribution portion running between a terminal of the substrate
and the base of the compliant pad 171, a pad-supported portion 176
extending upwards from the base of the pad, and a contact tip 174
at the top of the compliant pad 171. In the exemplary contact 170,
all four sides of the pyramidal pad 171 are covered with the metal
layer 172, except for a relatively small area along the four
corners of the pyramid. Covering a greater proportion of the
compliant pad advantageously lowers the resistivity of the contact
170, and may also help protect the pad from damage. Openings in the
metal layer over the compliant pad may be desirable for stress
relief of the metal layer, to provide room for expansion (bulging)
of the pad when deformed, and to provide ventilation for
outgassing. Stress relief may also be provided without using
openings in the metal layer, such as by providing metal layer 172
of a highly ductile material, such as gold.
[0078] FIG. 14 shows a spring contact 175 configured similarly to
spring contact 170, but with laterally offset openings 177
positioned to provide lateral flexibility for the pad-supported
portions 179 of trace 178. With suitably configured openings 177,
the lateral flexibility of contact 175 may be increased. That is,
contact 175 may be better able to accommodate lateral deflection of
its contact tip relative to its base without tearing of trace 178
or other failure of the spring contact. Lateral deflection forces
may arise from thermal mismatch between the device substrate and a
mating substrate, particularly when contact 175 is soldered at its
tip 174 to a mating substrate.
[0079] FIG. 15A shows a plan view of an exemplary flip-chip device
180 having an array of microelectronic spring contacts 100 on a
surface thereof. An enlarged view of the same device 180 is shown
in FIG. 15B. Each contact 100 is connected to a terminal 114 of the
device 180, as previously described. Device 180 may be a
semiconductor device, such as a memory chip or microprocessor.
Spring contacts 100 may be formed directly on device 180,
preferably prior to singulation from the semiconductor wafer.
Contacts 100 may then be used to connect to the device for both
testing and assembly purposes. Although flip-chip mounting
represents the more compact design, it should be appreciated that
contacts 100 may similarly be incorporated into CSP designs, if
desired.
[0080] FIG. 16 shows a side view of device 180 in contact with a
mating electrical component 184, such as a printed circuit board. A
contact tip of each contact 100 is in contact with a terminal 186
of component 184. A controlled amount of compressive force 182 may
be applied using a mounting frame or other fastening device, if it
is desired to make the installation of device 180 readily
demountable. The compressive force 182 causes deflection of
contacts 100 in a direction perpendicular to substrate 184, and in
a lateral direction parallel to substrate 184. The lateral
deflection of contacts 100 may provide a beneficial wiping action
at the contact tips. Device 180 may be demounted as desired by
releasing the compressive force 182. If contacts 100 are not
soldered to terminals 186, lateral stress from thermal mismatch
between substrate 184 and device 180 may be relieved by sliding
between the contact tips of contacts 100 and terminals 186. If
contacts 100 are soldered in place, it may be desirable to provide
contacts with inherent lateral flexibility.
[0081] For example, contacts 170 of a type as shown in FIGS. 12-14
may be provided on a device 190 that is to be soldered to a
component 184, as shown in FIG. 17. The metallic portions of
contacts 170 are relatively thin and flexible, and may be patterned
for greater lateral flexibility as described elsewhere herein. The
metallic portions of contacts 170 are not self-supporting, and rely
on the compliant pad of each contact for support. Device 190 may be
mounted to terminals 186 using dollops of a solder paste material
192. The compliant pad material used in contacts 170 should be
selected to withstand solder reflow temperatures encountered during
mounting. After being soldered, contacts 170 remain capable of
deflecting laterally at relatively low force levels for relief of
thermal stress. Also, ample space remains between contacts 170 on
device 190 for venting of the spring contact array, so the
likelihood of package failure by gas build-up an elastomer or other
material of the compliant pads may be reduced.
[0082] For some flip-chip and CSP applications, it may be desirable
to eliminate the need for a compliant pad in the spring contact. A
suitable self-supporting spring contact 300 for providing lateral
resiliency in flip-chip and like applications without need for a
compliant supporting pad is shown in FIG. 18. Spring contact 300 is
an example of a microelectronic spring contact of a type referred
to herein as a horizontal spring contact, meaning that the spring
contact is primarily resilient in a direction parallel to the
surface of the substrate to which it is mounted. Contact 300
comprises a base 306 attached to substrate 116, a cantilevered beam
304 running in a plane substantially parallel to substrate 116 and
having at least one bend along its length, and a contact tip 302
configured for a solder attachment. Contact 300 may be formed from
an integral sheet of resilient and conductive material, such as a
relatively thick nickel alloy trace deposited by a method such as
electroplating. Contact 300 may be coated with an outer layer of a
conductive metal, such as gold, or coated in any other desired
way.
[0083] Various beam shapes may be suitable for horizontal spring
contacts. FIGS. 19 and 20 show plan views of exemplary beam shapes
that may be suitable. Referring to FIG. 19, spring contact 308 has
a serpentine beam 304. Each bend in the beam 304 may add additional
resiliency in the line of direction between base 306 and tip 302.
Referring to FIG. 20, a series of hairpin bends in beam 304 are
used to provide resiliency between base 306 and tip 302 of spring
contact 310. The hairpin design may provide greater horizontal
resiliency in a narrower space between the base and tip. It should
be apparent that numerous other shapes may also be suitable for
beam 304. One skilled in the art may select a suitable shape that
is suitably rigid and self-supporting in the vertical
(perpendicular to substrate) direction while being sufficiently
flexible and resilient in the horizontal direction.
[0084] Exemplary steps of a method 250 for forming horizontal
spring contacts according to the invention are shown in FIG. 21. At
step 252, a first sacrificial layer is deposited over a device
substrate. At step 254, the first sacrificial layer is patterned to
expose the terminals of the device substrate. Additional areas may
be exposed in which structures for supporting the spring contacts
(particularly those with long spans) may be formed. The first
sacrificial layer may be any patternable material, such as a
photoresist material used in the art of photo-lithography. It
should be deposited in a layer of uniform thickness equal to the
desired height of the horizontal springs above the substrate
surface. The first sacrificial layer may then be patterned using a
photo-lithographic technique such as known in the art to expose an
area of the substrate surface including and around the terminals of
the device. The exposed area should be large enough to support the
horizontal spring that is to be constructed against its anticipated
vertical and horizontal loads.
[0085] After the terminals of the device have been exposed, and
while most of the first sacrificial layer remains on the substrate,
at step 256, a seed layer as previously described is deposited over
the first sacrificial layer and exposed terminal areas. At step
258, a second sacrificial layer is deposited over the seed layer.
The second sacrificial layer should also be a photo-patternable
material, and should be deposited to a uniform depth equal to or
greater than the desired thickness of the horizontal spring
material. At step 260, the second sacrificial layer is patterned in
the desired shape of the horizontal springs to be formed. The seed
layer is exposed from each terminal area along a beam running over
the first horizontal layer to a tip, which may be a pad-shaped
tip.
[0086] A layer of conductive material is then deposited in the
patterned second sacrificial layer at step 262, such as by
electroplating a metallic material to the desired thickness. The
conductive material will accordingly be deposited only over the
exposed seed areas to provide a spring contact structure of the
desired shape. The conductive material should be selected according
to the desired structural and electrical properties of the
horizontal spring contacts. For example, a nickel or nickel alloy
material could be selected as the primary structural material for
strength and resiliency, and a secondary layer of a more conductive
material, such as gold, could be applied as a top layer. One
skilled in the art will recognize other suitable materials and
combinations of materials, that may be applied in any number of
layers. After the conductive material or materials have been
deposited, the first and second sacrificial layers are removed at
step 264, such as by dissolution in a suitable solvent, to expose
free standing horizontal spring contacts on the device
substrate.
[0087] A plan view of an exemplary semiconductor device 312
provided with an array 314 of horizontal spring contacts 300 is
shown in FIG. 22. Device 312 may be suitable for use in a flip-chip
mounting application. Each spring contact 300 has a base area 306
adhered to a terminal 316 of device 312, a beam 304 running above
and substantially parallel to the device substrate and having at
least one bend, and a end area 302. End area 302 may be pad-shaped
for accepting a solder ball or dollop of solder paste or other
bonding material. The spring contacts 300 of array 314 are arranged
to provide a pitch-spreading redistribution scheme for terminals
316 of device 312. In the alternative, the contact tips 302 of
contacts 300 may be arranged in a pitch-preserving or
pitch-reducing reducing pattern.
[0088] FIG. 23 shows device 312 in a flip-chip mounting
configuration to an electronic component 184. A solder ball 192 is
used to connect each contact tip 302 to a corresponding terminal
186 of component 184. Beams 304 are generally parallel to the
facing surfaces of device 312 and component 184, while being held
apart from both device 312 and component 184, and free to flex
along their length in a horizontal direction. Stress build-up by
thermal mismatch between device 312 and component 184 may thereby
be mitigated by flexure of the horizontal spring contacts 300. No
elastomer material is needed to isolate the device from the
component, and the horizontal contacts 300 may be used for complete
support of device 312. In the alternative, auxiliary floating
supports (not shown) may be used to support the device 312 above
component 184, in which case contacts 300 may be made even more
flexible.
[0089] Spring contacts may also be constructed that combine the
characteristics of pad-supported and horizontal spring contacts.
FIG. 24 shows an exemplary combination spring contact 320, having a
metallic trace 322 lain over a prism-shaped compliant pad 329, and
a wiping-type contact tip 324. Beam 326 is shaped in a zig-zag
pattern over pad 329, for greater horizontal flexibility. Various
other horizontally flexible shapes, e.g., serpentine, may also be
used. A substrate-supported terminal portion 328 extends directly
from the base of the prism-shaped pad 329 over substrate 116.
[0090] In an alternative embodiment, a spring contact may be
provided with a horizontally flexible portion extending from above
the base of a compliant pad to a terminal of a substrate. FIGS. 25
and 26 show spring contacts 330, 350 of this general type. A side
view of a terminal 330 having a compliant pad 152 of a truncated
pyramidal shape is shown in FIG. 25. Metallic trace 332 comprises:
a contact tip 334 at the top of the compliant pad 152; a
pad-supported portion 340 connected to the contact tip 334; an
end-supported portion 342 having multiple bends 344 connected to
portion 340 and extending from the compliant pad 152, running above
and free from substrate 116; and a substrate-supported portion 338
connecting portion 342 to a terminal of substrate 116. Because its
contact tip 334 is supported by the compliant pad 152, trace 332
may be made more flexible than might otherwise be possible. Being
thinner and more flexible, end-supported beam portion 342 may
provide greater horizontal flexibility as compared to a
cantilevered structure like spring contact 300 shown in FIG. 18. A
spring contact of the tyke shown in FIG. 25 may thus be especially
preferred for applications requiring greater mitigation of
horizontal thermal stresses and wherein the presence of a compliant
pad is not problematic.
[0091] A similar combination contact 350, utilizing a stepped
pyramidal compliant pad 352, is shown in FIG. 26. The contact tip
334 is provided with a solder ball 192 for subsequent attachment to
a component substrate. Pad-supported trace portion 340 follows the
contours of the pad 352 to a point adjacent to and above its base.
From there, an end-supported portion 342 with two bends 344 extends
to a substrate-supported pad 338 on substrate 116. Spring contact
350 may be made relatively firm and stable in the vertical
direction by its supporting pad 352, while retaining a high degree
of flexibility in a plane parallel to the substrate 116 by its
flexible, end-supported portion 342.
[0092] A second trace portion 356 is also shown in FIG. 26. Second
trace portion 356 runs over a portion of compliant pad 352 to a
second compliant pad and a second contact tip. The second pad and
tip are not shown in FIG. 26, but may be similar to pad 352 and
contact tip 334, or may be differently configured.
[0093] One skilled in the art may construct a spring contact of the
type shown in FIGS. 25-26 by suitably combining the steps of
methods 200 and 250 described herein. For example, the
end-supported portion may be formed by depositing a first resist
layer over a pad (e.g., 152 or 352) and a substrate 116, and then
selectively removing regions of the first resist layer over the pad
and terminal. A seed layer may then be deposited over the first
resist layer and the exposed areas of pad and terminal. Then, a
second resist layer is deposited over the seed layer and patterned
to reveal the seed layer in the pattern of the desired traces. The
traces are then plated onto the exposed seed layer and the resist
layers are removed to reveal a contact like contacts 330, 350.
[0094] Having thus described a preferred embodiment of the layered
microelectronic contact and the horizontal spring contact, it
should be apparent to those skilled in the art that certain
advantages of the within system have been achieved. It should also
be appreciated that various modifications, adaptations, and
alternative embodiments thereof may be made within the scope and
spirit of the present invention. For example, particular shapes of
compliant pads and horizontal spring contacts have been
illustrated, but it should be apparent that the inventive concepts
described above would be equally applicable to other shapes and
configurations of pads and metallic elements having the general
properties described herein.
[0095] As another example, the spring contacts described herein may
be used with any electronic component, including not only
semiconductor devices but (without limitation) probe cards and
other testing devices. As yet another example, additional materials
may be deposited on the spring contact structures described above;
such materials enhancing the strength, resiliency, conductivity,
etc. of the spring contact structures. As still another example,
one or more layers of materials may be formed on the electronic
component prior to or after creating the spring contact structures
as described above. For example, one or more layers of
redistribution traces (separated by insulative layers) may be
formed on the electronic component followed by formation of the
spring contacts on the redistribution layer. As another example,
the spring contacts may first be formed followed by formation of
one or more layers of redistribution traces. Of course, all or part
of the compliant layer (e.g., elastomeric layer) described with
respect to any of the figures may be removed.
* * * * *