U.S. patent application number 11/189954 was filed with the patent office on 2006-01-26 for method for forming microelectronic spring structures on a substrate.
This patent application is currently assigned to FormFactor, Inc.. Invention is credited to Benjamin N. Eldridge, Stuart W. Wenzel.
Application Number | 20060019027 11/189954 |
Document ID | / |
Family ID | 25124089 |
Filed Date | 2006-01-26 |
United States Patent
Application |
20060019027 |
Kind Code |
A1 |
Eldridge; Benjamin N. ; et
al. |
January 26, 2006 |
Method for forming microelectronic spring structures on a
substrate
Abstract
A method for fabricating microelectronic spring structures is
disclosed. In an initial step of the method, a layer of sacrificial
material is formed over a substrate. Then, a contoured surface is
developed in the sacrificial material, such as by molding the
sacrificial material using a mold or stamp. The contoured surface
provides a mold for at least one spring form, and preferably for an
array of spring forms. If necessary, the sacrificial layer is then
cured or hardened. A layer of spring material is deposited over the
contoured surface of the sacrificial material, in a pattern to
define at least one spring form, and preferably an array of spring
forms. The sacrificial material is then at least partially removed
from beneath the spring form to reveal at least one freestanding
spring structure. A separate conducting tip is optionally attached
to each resulting spring structure, and each structure is
optionally plated or covered with an additional layer or layers of
material, as desired. An alternative method for making a resilient
contact structure using the properties of a fluid meniscus is
additionally disclosed. In an initial step of the alternative
method, a layer of material is provided over a substrate. Then, a
recess is developed in the material, and fluid is provided in the
recess to form a meniscus. The fluid is cured or hardened to
stabilize the contoured shape of the meniscus. The stabilized
meniscus is then used to define a spring form in the same manner as
the molded surface in the sacrificial material.
Inventors: |
Eldridge; Benjamin N.;
(Danville, CA) ; Wenzel; Stuart W.; (San
Francisco, CA) |
Correspondence
Address: |
FORMFACTOR, INC.;LEGAL DEPARTMENT
2140 RESEARCH DRIVE
LIVERMORE
CA
94550
US
|
Assignee: |
FormFactor, Inc.
|
Family ID: |
25124089 |
Appl. No.: |
11/189954 |
Filed: |
July 25, 2005 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
09781833 |
Feb 12, 2001 |
6939474 |
|
|
11189954 |
Jul 25, 2005 |
|
|
|
09710539 |
Nov 9, 2000 |
|
|
|
11189954 |
Jul 25, 2005 |
|
|
|
09364788 |
Jul 30, 1999 |
|
|
|
09710539 |
Nov 9, 2000 |
|
|
|
Current U.S.
Class: |
427/96.8 ;
257/E21.582; 257/E23.014; 257/E23.078; 29/842 |
Current CPC
Class: |
G01R 1/07342 20130101;
H01L 2924/01006 20130101; H01R 13/24 20130101; H01L 2924/01005
20130101; H01L 2924/12042 20130101; H01L 2924/12036 20130101; H05K
3/4092 20130101; H01L 23/4822 20130101; H01L 2924/01042 20130101;
H01L 2924/181 20130101; H05K 7/1069 20130101; H01L 2924/01046
20130101; H01L 2924/01078 20130101; G01R 3/00 20130101; G01R
1/06733 20130101; H01L 2924/01015 20130101; H01L 2924/01075
20130101; H01R 12/52 20130101; H01L 2924/01004 20130101; H01L
2924/12042 20130101; Y10T 29/49147 20150115; H01L 2924/01013
20130101; H01R 13/2407 20130101; H01L 2924/10253 20130101; H01L
2924/3025 20130101; H01L 2924/01033 20130101; G01R 1/06727
20130101; H01L 2924/01049 20130101; G01R 1/0483 20130101; H01L
2924/01082 20130101; H01L 2924/01027 20130101; H01L 2924/01029
20130101; H01L 2924/12036 20130101; H01L 2924/10253 20130101; H01L
2924/01079 20130101; G01R 1/06716 20130101; H01L 2924/01047
20130101; H01L 2924/01011 20130101; H01L 2924/01024 20130101; H01L
2924/181 20130101; H01L 2924/01023 20130101; H01L 2924/01074
20130101; H01L 24/72 20130101; H01L 2924/00 20130101; H01L 2924/14
20130101; H01L 2924/00 20130101; H01L 2924/00 20130101; H01L
2924/00 20130101 |
Class at
Publication: |
427/096.8 ;
029/842 |
International
Class: |
H05K 3/00 20060101
H05K003/00; C23C 28/00 20060101 C23C028/00 |
Claims
1-67. (canceled)
68. A method of forming a molded surface on a substrate using a
stamping tool having at least a portion that is translucent, said
method comprising: pressing said stamping tool into a layer of
moldable material on said substrate; directing a curing stimulus
through at least a portion of said translucent portion of said
stamping tool, wherein at least a portion of said moldable material
is cured;
69. The method of claim 68 further comprising repeating said
pressing and said directing to create a plurality of cured molded
surfaces.
70. The method of claim 69 further comprising forming contact
structures at least in part on said cured molded surfaces.
71. The method of claim 68, wherein: said stamping tool comprises a
tooth, said tooth comprising a first portion and a second portion,
said first portion corresponds to an opening to a surface of said
substrate to be formed in said moldable material, and said second
portion corresponds to a molded surface.
72. The method of claim 71, wherein said stamping tool further
comprises: a plurality of said teeth, each comprising a first
portion and a second portion, and third portion(s) separating
adjacent ones of said teeth.
73. The method of claim 68, wherein said curing stimulus is
ultraviolet light.
74. The method of claim 68, further comprising forming a contact
structure at least in part on said cured portion of said moldable
material.
75. The method of claim 68, wherein said substrate is an electronic
component comprising a plurality of electrically conductive contact
elements.
76. The method of claim 75, further comprising: placing said
electronic component in a mold; and injecting said moldable
material into said mold.
77. The method of claim 75 further comprising planarizing said
layer of moldable material on said electronic component.
78. The method of claim 68, wherein said pressing further comprises
heating said stamping tool.
79. The method of claim 78, wherein said pressing further
comprises, after heating said stamping tool, cooling said stamping
tool while said stamping tool is pressed into said moldable
material.
80. The method of claim 68, wherein said pressing further comprises
cooling said stamping tool while said stamping tool is pressed into
said moldable material.
81. The method of claim 72, wherein at least one of said second
portions comprises a ribbed surface.
82. The method of claim 72, wherein at least one of said second
portions comprises a corrugated surface.
83. The method of claim 72, wherein at least one of said second
portions is selected from a group consisting of a V shape, a U
shape, or a bifurcation.
84. The method of claim 68 further comprising: after pressing said
stamping tool into said moldable material, removing said stamping
tool and depositing a seed layer of conductive material over said
moldable material; patterning a layer of masking material over said
seed layer, wherein patterns in said masking material correspond to
patterns for a plurality of electrically conductive contact
structures; and depositing a contact material onto said seed layer
through said patterns in said masking material.
85. The method of claim 84 further comprising removing said masking
material and said moldable material.
86. The method of claim 68, wherein said stamping tool further
comprises a plurality of reentrant teeth disposed to form a
plurality of lips in said moldable material.
87. The method of claim 86, wherein said moldable material is
elastic.
88. The method of claim 68, further comprising forming a plurality
of contact structures.
89. The method of claim 88, wherein the forming comprises
depositing a contact structure material using an electroless
deposition process.
90. The method of claim 75, wherein said electronic component
comprises a plurality of dies composing an unsingulated
semiconductor wafer.
Description
RELATED APPLICATION
[0001] This application is a continuation-in-part of the co-pending
U.S. patent application Ser. No. 09/710,539, filed Nov. 9, 2000,
entitled "LITHOGRAPHIC SCALE MICROELECTRONIC SPRING STRUCTURES WITH
IMPROVED CONTOURS," by Eldridge and Wenzel (hereinafter the "FIRST
PARENT CASE"), which is a continuation-in-part of co-pending
application Ser. No. 09/364,788, filed Jul. 30, 1999, entitled
"INTERCONNECT ASSEMBLIES AND METHODS," by Eldridge and Mathieu
(hereinafter, the "SECOND PARENT CASE"), which applications are
incorporated herein, in their entirety, by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to electrical contact elements
for electrical devices, and more particularly to
lithographic-scale, microelectronic spring contacts with improved
contours.
[0004] 2. Description of Related Art
[0005] Recent technological advances, such as described in U.S.
Pat. No. 5,917,707 to Khandros et al., have provided small,
flexible and resilient microelectronic spring contacts for mounting
directly to substrates, such as semiconductor chips. 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. It is
further recognized, as described, for example, in U.S. Pat. Nos.
6,032,446 and 5,983,493 to Eldridge et al, that such
substrate-mounted, microelectronic spring contacts can offer
substantial advantages for making electrical connections between
semiconductor devices in general, and in particular, for the
purpose of performing wafer-level test and burn-in processes.
Indeed, fine-pitch spring contacts offer potential advantages for
any application where arrays of reliable electronic connectors are
required, including for making both temporary and permanent
electrical connections in almost every type of electronic
device.
[0006] In practice, 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-scale
microelectronic spring (or contact, or spring contact) structures,
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. Exemplary lithographic-scale spring contacts, and
processes for making them, are described in the commonly owned,
co-pending U.S. Patent Applications "LITHOGRAPHICALLY DEFINED
MICROELECTRONIC CONTACT STRUCTURES, Ser. No. 09/032,473 filed Feb.
26, 1998 by Pedersen and Khandros, and "MICROELECTRONIC CONTACT
STRUCTURES", Ser. No. 60/073,679, filed Feb. 4, 1998 by Pedersen
and Khandros, both of which are incorporated herein, in their
entirety, by reference.
[0007] In general, lithographic processes allow for a great deal of
versatility in design of spring contacts, which in turn permits
numerous improvements over prior art designs. For example, although
prior art lithographically formed structures in general typically
have essentially flat rectangular cross-sections, contoured
non-rectangular cross-sections are desirable for many spring
contact applications. For a given thickness of resilient material,
a lithographic type spring contact can be made stiffer and stronger
by providing it with a suitably contoured, non-rectangular cross
section. Other performance benefits may be realized by utilizing
various other more complex shapes. However, prior art manufacturing
methods are unsuitable for making lithographic type spring contacts
with such suitably contoured, non-rectangular cross-sections, and
other types of more complex shapes. Additionally, prior methods,
for example, as disclosed in the above-referenced U.S. patent
application Ser. Nos. 09/032,473 and 60/073,679, fabricate the
spring structures using a series of lithographic steps, thereby
building up a z-component extension (i.e., extension of the spring
tip away from the substrate surface) with several lithographic
layers. However, the use of multiple layers adds undesirable cost
and complexity to the manufacturing process. Layered structures are
also subject to undesirable stress concentration and stress
corrosion cracking, because of the discontinuities (i.e., stepped
structures) that result from layering processes.
[0008] A need therefore exists for method of making microelectronic
spring structures more quickly and easily by eliminating process
layering steps and the associated costs, while providing springs
with improved properties, such as improved strength, stiffness,
resistance to stress concentration cracking, and elastic range.
Additionally, a need exists for a method of making lithographically
formed, microelectronic spring structures with defined contoured
surfaces and more complex shapes.
SUMMARY OF THE INVENTION
[0009] The present invention provides a method for forming
microelectronic spring structures and methods to address the
foregoing needs, while achieving adequate z-extension without
requiring the use of multiple stepped lithographic layers.
[0010] The present invention provides a method for fabricating
molded microelectronic spring structures, and methods for making
and using such structures, using a molded pre-cursor form. In one
embodiment, a method is provided for making resilient contact
structures. First, a layer of sacrificial material is formed over a
substrate. Then, a contoured surface is developed in the
sacrificial material, preferably by molding the sacrificial
material using a mold or stamp. The contoured surface provides a
mold for at least one spring form, and preferably for an array of
spring forms. If necessary, the sacrificial layer is then cured or
hardened. A layer of resilient material is deposited over the
contoured surface of the sacrificial material, and patterned to
define at least one spring form, and preferably an array of spring
forms. The sacrificial material is then at least partially removed
from beneath the spring form to reveal at least one freestanding
spring structure. A separate conducting tip is optionally attached
to each resulting spring structure, and each structure is
optionally plated or covered with an additional layer or layers of
material, as desired.
[0011] In another embodiment, a method for making a resilient
contact structure using the properties of a fluid meniscus is
disclosed. First, a layer of material is formed over a substrate.
Then, a recess is developed in the material, and fluid is provided
in the recess to form a meniscus. The fluid is cured or hardened to
stabilize the contoured shape of the meniscus. The stabilized
meniscus is then used in the method in the same manner as the
molded surface in the sacrificial material.
[0012] The method according to the present invention is readily
adaptable for use with lithographic manufacturing equipment and
processes that are currently available to make large quantities of
microelectronic spring structures in parallel. The method is
particularly capable for making lithographically formed
microelectronic spring contact structures that are low-aspect ratio
rectangular in cross-section, and have a z-component extension
along a linear or curved slope. The method also provides for
shaping springs in plan view, for example, by providing springs
with tapered triangular shapes. In particular, the method is
capable of forming spring structures over a molded substrate formed
in essentially a single process step, thereby reducing the number
of processing steps required to form springs with desired shapes.
The method additionally provides contoured molding substrates for
forming springs with numerous performance improvements. For
example, the method may be used to readily form structures having a
U-shaped cross-section, a V-shaped cross-section, and/or a rib
running along a length of the spring.
[0013] A more complete understanding of the method for forming
microelectronic spring structures 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
[0014] FIG. 1 is a flow diagram showing exemplary steps of a method
according to the invention.
[0015] FIGS. 2A-2H are cross-sectional views of a substrate and
materials layered thereon during exemplary steps of a method
according to the invention.
[0016] FIG. 3A is a perspective view of a substrate having molding
surfaces impressed thereon as during an exemplary step of a method
according to the invention.
[0017] FIG. 3B is a perspective view of a portion of an exemplary
stamping tool for use in a method according to the invention.
[0018] FIGS. 3C-3G are perspective views of exemplary teeth for use
on a stamping tool in a method according to the invention.
[0019] FIG. 4 is a flow diagram showing exemplary steps for forming
a molding surface according to an embodiment of the invention,
particularly suitable for forming spring structures on uneven
substrates.
[0020] FIGS. 5A-5G are cross-sectional views of a substrate and
materials layered thereon during exemplary steps of an embodiment
of the invention, particularly suitable for forming spring
structures on uneven substrates.
[0021] FIG. 6 is a flow diagram showing exemplary steps for forming
a molding surface according to an embodiment of the invention, also
suitable for forming spring structures on uneven substrates.
[0022] FIG. 7 is a flow diagram showing exemplary steps for forming
a molding surface according to an embodiment of the invention,
using a fluid to shape a molding surface in the form of a fluid
meniscus.
[0023] FIG. 8A is a plan view of a substrate and materials layered
thereon during an exemplary one of the steps shown in FIG. 7.
[0024] FIGS. 8B-8F are cross-sectional views of a substrate and
materials layered thereon during the exemplary steps shown in FIG.
7.
[0025] FIG. 8G is a perspective view of an exemplary spring
structure formed using the exemplary steps shown in FIG. 7.
[0026] FIG. 9 is a flow diagram showing exemplary steps for forming
a spring structure according to an embodiment of the invention
suitable for use with PVD and CVD material deposition
techniques.
[0027] FIGS. 10A-10D are cross-sectional views of a substrate and
materials layered thereon during the exemplary ones of the steps
shown in FIG. 9.
[0028] FIG. 11A is a cross-sectional view of a portion of an
exemplary stamping tool having a re-entrant tooth form for creating
an impression cavity with an overhanging lip.
[0029] FIG. 11B is a cross-sectional view of a typical impression
formed by the stamping tool shown in FIG. 11A.
[0030] FIG. 12A is a perspective view of an exemplary progressive
stamping tool for creating an impression cavity with an overhanging
lip.
[0031] FIG. 12B is a cross-sectional view of a portion of the
stamping tool shown in FIG. 12A.
[0032] FIGS. 12C-12D are cross-sectional views of typical
impressions formed by the stamping tool shown in FIGS. 12A-12B,
during successive steps of a progressive stamping process.
[0033] FIGS. 12E-12F are plan views of exemplary impressions formed
by the stamping tool shown in FIGS. 12A-12B, after completion of a
progressive stamping process.
[0034] FIG. 13 is a flow diagram showing exemplary steps for
forming a spring structure according to an embodiment of the
invention that avoids a masking step by forming a mold cavity with
an overhanging lip.
[0035] FIGS. 14A-14C are cross-sectional views of a substrate and
materials layered thereon during the exemplary ones of the steps
shown in FIG. 13.
[0036] FIG. 14D is a perspective view of an exemplary spring
structure formed by a method as shown in FIG. 13.
[0037] FIG. 15 is a flow diagram showing exemplary steps for
forming a spring structure according to an embodiment of the
invention that avoids a masking step by using a partially
encircling overhanging lip.
[0038] FIG. 16A is a plan view of an exemplary mold cavity with
materials layered thereon during an exemplary one of the steps
shown in FIG. 15.
[0039] FIGS. 16B-16D are cross-sectional views of a substrate and
materials layered thereon during the exemplary ones of the steps
shown in FIG. 15.
[0040] FIG. 17 is a flow diagram showing exemplary steps for
forming a spring structure according to an embodiment of the
invention that uses a radiation-curable substrate.
[0041] FIGS. 18A-18E are cross-sectional views of a substrate and
materials layered thereon during the exemplary ones of the steps
shown in FIG. 17.
[0042] FIG. 18F is a perspective view of exemplary molding surfaces
formed by a method as shown in FIG. 17.
[0043] FIG. 19 is a flow diagram showing exemplary steps for
forming a spring structure according to an embodiment of the
invention using a line-of-sight deposition method for patterning
the resilient material.
[0044] FIG. 20A is a perspective view of a substrate and molded
material during an exemplary step of the method shown in FIG.
19.
[0045] FIGS. 20B-20E are cross-sectional views of a substrate and
materials layered thereon during exemplary ones of the steps shown
in FIG. 19.
[0046] FIGS. 21A-21C are cross-sectional views of a substrate and
materials layered thereon during exemplary steps of the method
shown in FIG. 19, wherein the electroplating step is omitted; and
further shows an embodiment of the invention for forming a spring
structure with an integral redistribution trace.
[0047] FIG. 21D is a perspective view of an exemplary spring
structure with an integral redistribution trace having elevated
bridges.
[0048] FIG. 22 is a perspective view of a plurality of spring
structures having integral redistribution traces, showing an
exemplary configuration thereof.
[0049] FIGS. 23A-23C are perspective views, taken at successively
higher levels of magnifications of an exemplary spring structure
and stop structure formed by a method according to the invention,
wherein the substrate comprises a wafer.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0050] The present invention satisfies the need for a method of
forming microelectronic spring structures, that overcomes the
limitations of previous forming methods. In the detailed
description that follows, like element numerals are used to
describe like elements illustrated in one or more figures.
[0051] Various terms and acronyms are used throughout the detailed
description, including the following:
[0052] "Microelectronic" means pertaining to that branch of
electronics dealing with components of miniature size, such as
integrated circuits. A "microelectronic spring" is not limited to
springs used as electrical contacts, but also includes springs used
as electro-mechanical devices and as purely mechanical springs.
[0053] "Sacrificial layer," "layer of sacrificial material," and
"sacrificial material layer" means a layer of photoresist or
similar material that is deposited on a substrate during formation
of a desired component or structure, such as a microelectronic
spring component, and later removed from the substrate.
[0054] "Sacrificial substrate" means a substrate that is used for
formation of a desired component or structure, such as a
microelectronic spring component, and later removed from the
component or structure.
[0055] "Substrate" means a material having a supporting surface for
supporting a desired structure or component. Suitable substrates
upon which microelectronic spring contacts may be formed according
to the present invention include, but are not limited to,
semiconductor materials, such as semiconductor wafers and dice
(with or without integrated circuitry), metals, ceramics, and
plastics; all of the foregoing materials may be in various
geometric configurations and intended for various applications.
[0056] The foregoing definitions are not intended to limit the
scope of the present invention, but rather are intended to clarify
terms that are well understood by persons having ordinary skill in
the art, and to introduce terms helpful for describing the present
invention. It should be appreciated that the defined terms may also
have other meanings to such persons having ordinary skill in the
art. These and other terms are used in the detailed description
below.
[0057] The present invention provides methods for forming
microelectronic springs on a substrate using lithographic
techniques that are readily adoptable by manufacturers of
semiconductor electronic devices. The microelectronic springs
structures are preferably configured as described in the FIRST
PARENT CASE referenced above, although various other configurations
may also be formed using the methods described herein. Various
exemplary methods are disclosed herein, and it should be apparent
that the choice of a preferred method will depend on factors such
as the type of manufacturing equipment available, the
characteristics of the substrate, the desired properties of the
springs, and so forth, that will vary depending on the
circumstances. In some circumstances, two or more methods may be
equally preferable. Additionally, selected steps of the exemplary
methods may be combined in various ways, and optional steps may be
omitted, depending on these and similar variable factors.
[0058] FIG. 1 shows exemplary steps of a generally applicable
method according to the invention, and FIGS. 2A-2H show views of a
substrate 32, and materials layered thereon, during the steps of
the method shown in FIG. 1. In step 102, a layer of sacrificial
material 30 is deposited on a surface of a substrate 32, such as an
upper surface of a semiconductor device, chip, die, or wafer.
Substrate 32 is typically a semiconductor substrate for an
integrated circuit having numerous electrical terminals, one of
which is shown as the contact pad 46 in FIG. 2A. Contact pads, such
as contact pad 46, are typically coupled by conductive traces, such
as trace 44, to internal circuitry within the integrated circuit.
However, the present invention is not limited for use with a
particular type or configuration of substrate. In some embodiments
of the invention, contact pad 46 is electrically and mechanically
coupled to an intermediate conducting layer (not shown), which is
disposed above it. When present, the intermediate layer is
typically a manufacturing artifact of a shorting layer used during
an electroplating step of a process for forming the microelectronic
spring structure. The method 100 may be used to form a spring
structure for conduction of electrical signals and/or power between
a mating substrate, and contact pad 46.
[0059] As known in the semiconductor art, substrate 32 is typically
comprised of numerous layers, such as insulating layers interposed
with conducting and semiconducting layers, and a passivating layer
(not shown) optionally provided on an upper surface of the
substrate 32. The passivating layer may be an insulating layer, a
polysilicon layer, or other layers as known in the art, and is
commonly present on semiconductor devices. When a passivating layer
is present, contact pad 46 is preferably exposed through an opening
in the passivating layer. Prior to depositing subsequent layers, a
passivating layer (if there is one present) may optionally first be
roughened, such as by exposure to an oxygen plasma, to enhance
adhesion of the first subsequent layer. Choice of roughening
techniques and materials suitable for deposition on passivating
layers is as known in the art.
[0060] Referring to FIGS. 1 and 2A, in a preparatory step 102 of a
method for making a contoured spring, a substrate 32, optionally
provided with a contact pads 46 for connecting to a integrated
circuit, is coated with a moldable sacrificial layer 30.
Sacrificial layer 30 may be any number of materials, such as PMMA
(poly methyl methacrylate), which can be coated on a substrate to
the desired thickness, which will deform when pressed with a mold
or stamp, which will receive the resilient material to be deposited
thereon, and which can then readily be removed without damaging the
spring structures formed thereon. Additional candidate materials
for layer 30 include acrylic polymers, polycarbonate, polyurethane,
ABS plastic, various photo-resist resins such as
phenol-formaldehyde novolac resins, epoxies and waxes. The
sacrificial layer 30 preferably has a uniform thickness slightly
greater than the desired height of the contact tips of the finished
spring structure above the substrate 32. For example, if the
desired height is 50 microns (about 2 mils), layer 30 may have a
thickness of 55 microns (2.2 mils). Various methods known in the
art, for example, spin coating, may be used to deposit layer 30
onto substrate 32.
[0061] In an embodiment of the invention, layer 30 comprises
multiple layers, for example a soft material that is in contact
with substrate 32, covered with a hard or brittle material on the
top surface that will cleave or cut cleanly when impressed with
molding tool 34. This type of bi-layer could be formed with
successive addition and curing of wet materials by spin coating or
casting, by successive lamination of dry film polymers, or by
lamination of a dry film that consists of multiple layers. The
brittle layer mentioned above could also be a metallic layer, that
would cleave and eliminate the metal layer deposition step required
to form a conducting surface, such as step 106 shown in FIG. 1. In
yet another embodiment, layer 30 comprises at least one layer of
photo-patternable material and at least one layer of moldable
material that is not photo-patternable. This would provide, for
example, the ability to photo-pattern some regions, then follow the
photo-patterning step with a molding step, or vice versa.
[0062] Also, a stamping tool 34, having a molding face provided
with different molding regions 36, 38, and 42, is prepared for
molding sacrificial layer 30. Various methods may be used to
prepare tool 34. For example, the stamping tool 34 may be formed
from a relatively hard material using a computer controlled,
ultraviolet ("UV") laser ablation process, using an excimer laser,
or a pulsed NdYag laser such as are available from Lambda Physik,
Inc., located in Fort Lauderdale, Fla., or from Heidelberg
Instruments Mikrotechnik GmbH, located in Heidelberg, Germany. In
the alternative, a laser microchemical process, also called laser
assisted etching, available from Revise, Inc. located in
Burlington, Mass., may be used to form the stamping tool. Yet
another alternative is to use a gray-scale photolithography mask,
such as available from Canyon Materials, Inc., of San Diego,
Calif., to form a pattern with a surface profile in a
photopatternable glass or a layer of photoresist (which may be used
as a mold for the stamping tool). The latter method--using a
gray-scale mask to pattern a layer of photoresist--may also be used
to form sacrificial layer 30 directly, but this is less preferred
because it is generally slower than using a stamping tool. All of
the foregoing methods for forming a stamping tool are capable of
defining features with submicron resolution, and thus may be used
for forming spring structures with molded features to about 0.1
micron in size. For example, a spring structure with a cantilevered
beam portion as narrow as about 0.1 microns may be made using
method 100. Maximally protruding molding regions, or "teeth" 36 of
tool 34 are used for deforming the sacrificial layer 30 in the area
of the contact pads 46, where bases of the contact structures will
be formed. Contoured molding regions 38 are used for deforming
layer 30 in a beam region of the contact structures to be formed.
Maximally recessed molding regions 42 are used for receiving excess
material, i.e., "flash," pushed aside by teeth 36. Molding regions
42 also define spacing between adjacent spring structures on
substrate 32. Depending on the choice of materials for sacrificial
layer 30 and stamping tool 34, a layer of mold release material
(not shown) is optionally provided on the molding face of tool 34.
It should be recognized that further layers and materials may be
present on substrate 32 without departing from the method described
herein. For example, a metallic shorting layer (not shown) is
optionally provided between layer 30 and substrate 32, to protect
any integrated circuits embedded in the substrate during processing
operations. In an initial phase of forming and curing step 104, the
stamping tool 34 is applied against substrate 32 with sufficient
pressure to bring the teeth 36 nearly to the surface of substrate
32, and to fully mold layer 30 in all contoured molding regions 48,
as shown in FIG. 2B. To avoid damaging substrate 32, and
particularly because the surface of substrate 32 is typically not
perfectly planar, teeth 36 are preferably not brought into contact
with substrate 32. Tool pressures are preferably relatively low,
such as less than about 7 mega-Pascals ("MPa", about 1000 pounds
per square inch ("PSI")), and more preferably, less than about 0.7
MPa (about 100 PSI). In a preferred embodiment, when teeth 36 have
sunk into layer 30 to the desired depth, flash substantially fills
the maximally recessed regions 42 forming a surface sufficiently
uniform to permit later deposition of a layer of masking material
between the spring structures after the stamping tool 34 is removed
from layer 30. Stamping tool 34 may be heated to assist deformation
of layer 30, and then cooled to harden layer 30 into place. In an
alternative embodiment, layer 30 is selected of a material that is
sufficiently deformable to flow under pressure without application
of heat, and sufficiently viscous to hold its shape after tool 30
is removed. In yet another alternative embodiment, heat, UV light,
or chemical catalysts are used to harden sacrificial layer 30 while
under stamping tool 34, and then tool 34 is removed. In yet another
embodiment, ultrasonic energy is applied by tool 34 to soften layer
30 for molding. Whatever molding technique is used, the cycle times
are preferably relatively short to permit faster manufacturing
throughput.
[0063] FIG. 2C shows the shape of the sacrificial layer 30 after
removal of the stamping tool 34, in a subsequent phase of molding
and curing step 104. A thin layer of residue 51 is shown over the
area of each contact pad 46; however, in some alternative
embodiments, the contact pad 46 is essentially free of residue
after removal of the stamping tool. Negative mold surfaces 48 are
also present, each bearing a negative impression of the desired
contour for the contoured beams to be formed thereon. When present,
it is necessary to remove the residue 51 to expose the substrate 32
in the areas 50 where the bases of the contact structures are to be
formed. To remove the residue 51, the entire substrate with its
molded layer 30 may be isotropically etched by immersion in a bath
of wet etchants, by oxygen plasma, or other methods as known in the
art. Isotropic etching is suitable for relatively flat substrates
for which the residue 51 is of a relatively uniform thickness in
all areas 50 on substrate 32. Preferably, the isotropic etch is
performed so as to remove the residue 51 while at the same time
reducing the thickness of layer 30 to equal the desired height of
the spring structures to be formed. In the alternative, an
anisotropic etching method that etches more rapidly in a direction
perpendicular to the substrate 32, such as reactive ion etching,
may be used. An anisotropic etch is preferably used in cases where
the substrate is relatively uneven, causing non-uniformity in the
thickness of residue 51, or in cases when the lateral dimensions
must be held in close tolerance.
[0064] The appearance of the molded sacrificial layer 30 after
etching is shown in FIG. 2D, at a later time during forming and
curing step 104. The contact pads 46 are preferably exposed, along
with a surrounding area of substrate 50 sufficient for providing
adhesion of the base of the spring structure to be formed. In
typical semiconductor applications, an exposed area of substrate 32
of between about 10,000 and about 40,000 square microns, most
preferably in excess of about 30,000 square microns, is provided.
After etching, the mold surfaces 48 preferably take on the desired
contoured shape, and the ends of all mold surfaces 48 distal from
substrate 32 are preferably within substantially the same
plane.
[0065] In step 106, a seed layer is sputtered over the surface of
sacrificial layer 30 and exposed base areas 50. The seed layer is
typically a relatively thin layer of uniform thickness, such as
about 4500 .ANG. (Angstroms; or about 0.45 microns) thick, of
sputtered metal, used for electroplating the resilient spring
material. Suitable metals for seed layer 52 include copper, gold,
or palladium; or potentially, titanium-tungsten (Ti--W). Less
preferably, surface modifications of layer 30 and base areas 50,
e.g., plasma treatment, may be used to render them conductive,
thereby creating a seed layer in a surface layer of the
materials.
[0066] In an alternative embodiment, an electrically conductive
mold material, such as a conductive polymer, conductive composite
material, or metal alloy having a low melting point, is used to
form mold layer 30, thereby eliminating the need to deposit a seed
layer in preparation for plating. In such an embodiment, the
resilient spring material can be plated directly onto the
conductive mold material. In addition, the substrate is optionally
covered with a protective shorting layer, as known in the art,
before the layer of conductive mold material is applied the
substrate. The shorting layer, if present, protects any integrated
circuit elements in the substrate, and carries the plating
current.
[0067] In step 108, a patterned layer of a masking material, such
as a photo-resist layer 54, is deposited, to cover areas of the
seed layer where no resilient material is to be deposited. The
photo-resist layer 54 may be selected from various commercially
available resist materials, including wet or dry positive or
negative resists, or wet positive or negative electrophoretic
resist systems. The photo-resist layer may be patterned using any
appropriate method, for example by exposing to UV light through a
mask, except where the spring structures are to be formed, thus
curing it in the exposed areas (in the case of a negative-acting
resist). FIG. 2E shows substrate 32 after application of a seed
layer 52 and a photo-resist layer 54. In FIGS. 2E-2H, the relative
thickness of seed layer 52 is exaggerated. The uncured portions of
photo-resist layer 54 are then dissolved away by a suitable
solvent, as known in the art.
[0068] The masking material 54 is preferably stable in the
environment of subsequent deposition methods. For example, a
typical positive photoresist masking material contains residual
solvent or monomer that can outgas under the high vacuum conditions
present during sputtering operations. Similar difficulties may be
encountered when sputtering over a layer of sacrificial material,
which typically is an organic material that may also contain
residual solvent or other low-molecular-weight constituents. In
preparation for a subsequent deposition step, the masking or
sacrificial material is preferably pre-treated, for example by
baking or by exposure to light, to drive off residual solvent or
cross-link residual monomer, as the case may be, or to otherwise
stabilize the material. A disadvantage of pre-treating is that the
masking or sacrificial material may thereby be made more difficult
to remove later in the process. A suitable sacrificial material and
deposition process can be selected by one skilled in the art.
[0069] After the uncured portions of resist layer 54 are dissolved
away, exposed areas 56 of seed layer 52 are revealed, as shown in
FIG. 2F. Exposed areas 56 have the projected shape, in plan view,
of the desired microelectronic spring structure. For example, if a
triangular beam is desired, the exposed area has a generally
triangular shape, in plan view. In step 110, one or more layers of
resilient material 58 are then deposited onto the seed layer in the
exposed areas 56, using various methods, such as electroplating, as
known in the art. Where the seed layer is covered by resist layer
54, no electroplating will occur. In the alternative, a layer of
resilient material may be built up using a process such as CVD or
PVD selectively applied to areas 56 through a mask (such as a
shadow mask), eliminating the need for the step 106 of depositing a
seed layer. Using any of various deposition methods, a spring
structure 60 comprising an integrally formed base and beam is
formed on the exposed area 56, as shown in FIG. 2G. In step 112,
the layers of sacrificial material 30 and masking material 54 are
removed using a suitable solvent, such as acetone, that will not
attack substrate 32 or the resilient material 58, as known in the
art. Freestanding spring structures 60, as shown in FIG. 2H, are
the result.
[0070] Suitable materials for the resilient material include but
are not limited to: nickel, and its alloys; copper, cobalt, iron,
and their alloys; gold (especially hard gold) and silver, both of
which exhibit excellent current-carrying capabilities and good
contact resistivity characteristics; elements of the platinum
group; noble metals; semi-noble metals and their alloys,
particularly elements of the palladium group and their alloys; and
tungsten, molybdenum and other refractory metals and their alloys.
Use of nickel and nickel alloys is particularly preferred. In cases
where a solder-like finish is desired, tin, lead, bismuth, indium,
gallium and their alloys can also be used. The resilient material
may further be comprised of more than one layer. For example, the
resilient material may be comprised of two metal layers, wherein a
first metal layer, such as nickel or an alloy thereof, is selected
for its resiliency properties and a second metal layer, such as
gold, is selected for its electrical conductivity properties.
Additionally, layers of conductive and insulating materials may be
deposited to form transmission-line-like structures.
[0071] After formation of the spring structures 60, substrate 32 is
optionally coated in a patterned layer with an insulating
encapsulant material over its surface, as further described in the
SECOND PARENT CASE referenced above. The encapsulant layer (not
shown) preferably covers the base areas 50 of the contact
structures, thereby mechanically reinforcing the attachment of the
resilient contact structures to the surface of the substrate. In
addition, spring structures 60 are optionally provided with
separate tip structures. Separate tip structures may be formed on a
sacrificial substrate, and transferred to structure 60, to be
joined adjacent to its free tip, as is further described, for
example, in the commonly owned, co-pending application Ser. No.
09/023,859, filed Feb. 13, 1998, which is incorporated herein, in
its entirety, by reference.
[0072] It should be apparent that method 100, and variations
thereof, may be used to readily form numerous contoured spring
structures on a substrate during a single production cycle. For
example, method 100 may be used to produce tens of thousands of
contoured spring structures on a wafer with multiple dice.
Additionally, each of the tens of thousands of structures so formed
will have a precise size, shape, and location as defined during the
embossing and lithographic manufacturing process. In general,
dimensional errors are anticipated to be on the order of 10 microns
or less. Because so many structures can be formed at the same time,
the cost of forming each structure will be relatively low.
[0073] Furthermore, numerous variations of the above-described
sequence of steps will become apparent to one skilled in the art,
for producing integrally formed spring structures according to the
present invention. For example, a spring contact structure may be
fabricated at an area on a substrate that is remote from a contact
pad to which it is electrically connected. Generally, the spring
contact structure may be mounted to a conductive line (not shown)
that extends from a contact pad of the substrate to a remote
position. In this manner, a plurality of spring contact structures
can be mounted to the substrate so that their tips are disposed in
a pattern and at positions that are not limited to the pattern of
the contact pads on the substrate. Additionally, in an embodiment
of the invention, molds for both the desired spring structures and
the redistribution layer are formed simultaneously, by impressing a
suitably shaped stamping tool into the moldable substrate. In yet
another embodiment, molds for spring contacts are formed on
opposite or adjacent surfaces of substrates, which is useful, for
example, for forming interposer or space-transformer components.
Such molds can be formed either sequentially br simultaneously,
with appropriate tooling.
[0074] For further example, method 100 may further be adapted to
permit the resilient material to be permanently deposited in areas
of the substrate that are not specifically intended for making
interconnections. Generally, any area on the substrate that is not
masked will be plated. This may be useful for, e.g., building
mechanical elements on the face of the die for standoffs. For
example, the edges of the substrate could be plated to provide
stand-offs or stop structures for spring structures 60.
Alternatively, the opposite side of the substrate can be plated
with a shielding or shorting layer. Variations such as the
foregoing may similarly be made in each of the alternative methods
disclosed herein.
[0075] Although various adaptations may be made to the methods
disclosed herein, in general, a molding or other forming process
using a relatively thick layer of sacrificial material, such as
layer 30, is preferred for providing adequate height of the spring
structure without requiring building up of multiple layers of
photo-resist. Additionally, use of a deformable (moldable)
sacrificial material layer facilitates duplication and mass
production of relatively complex, contoured beam shapes.
[0076] Accordingly, in the preferred embodiments of the method, the
entire spring structure (with the exception of optional features
such as separate tips) is definable in a layer of material
deposited (such as by electroplating, CVD, or PVD) on the surface
of a mold form. The resulting spring structures are thus comprised
of an integral sheet, which may comprise a single layer, or
multiple coextensive layers, of resilient, conductive, and/or
resistive material. The integrated sheet may be folded and
contoured, and is preferably essentially free of any overlapping
portion in the direction that the materials are deposited
(typically from above the structure towards a substrate), so it may
be more readily formed by depositing a layer or layers of material
over a molded layer of sacrificial material, according to the
methods described herein. However, substantial overlap may be
achieved using some deposition methods, such as electroplating in
conjunction with a "robber" to drive electrically charged material
under an overhang.
[0077] It should be apparent that the open molding method 100
according to the present invention may be adapted to form contoured
beams for spring structures in a wide variety of shapes and sizes.
For the purpose of microelectronic spring contact structures,
certain sizes and structural properties are preferred, as further
described in the FIRST PARENT CASE referenced above. However,
method 100 is capable of forming structures both smaller and larger
than in the preferred ranges. Current available techniques for
forming stamping tools place a lower limit on feature size at about
0.1 micron. While there is no clearly defined upper limit on
feature size, above a certain feature size, for example, features
that require forming the sacrificial layer 30 to depths of greater
than about 10,000 microns (about 1 cm or 400 mils), prior art
fabrication methods, such as sheet metal forming, are likely to be
more economically feasible.
[0078] A perspective view of an exemplary impression formed by a
stamping tool is shown in FIG. 3A. A similar view of an exemplary
portion of a stamping tool used for making the impression is shown
in FIG. 3B. It should be appreciated, however, that the impression
need not define or correspond to the plan shape of the desired
spring structure, because the desired plan shape may be defined
using a pattern mask. The impression need only define the desired
contour in the z-direction for the spring structure to be formed.
In alternative embodiments of the invention, the plan shape of the
impression--e.g., a recess formed in the moldable substrate--may be
used to define the spring shape. Exemplary ones of these
embodiments are described in more detail later in the
specification.
[0079] As shown in FIG. 3B, a plurality of teeth 36 are arranged on
a face 35 of stamping tool 34, each having an identical contoured
surface 38, corresponding to a molding surface 48 formed in layer
30 on substrate 32. The teeth 36 may be arranged in a rectangular
array, or in any pattern desired on face 35. Teeth 36 may be made
substantially identical to each other, or may comprise various
different shapes on the same stamping tool 34, depending on the
desired spring structures to be formed. Exemplary tooth shapes
include a tooth 36 with a ribbed surface, for forming a ribbed beam
of a spring structure, shown in FIG. 3C; a tooth 36 with a
corrugated surface 64 shown in FIG. 3D, for forming a corrugated
beam; and a tooth 36 with a V-shaped surface 66 shown in FIG. 3E,
for forming a V-shaped beam. Teeth may additionally be shaped
differently to form spring structures having various shapes in plan
view. For example, FIG. 3C shows a tooth for forming a spring
structure with a beam and base that are both rectangular in plan
view; FIG. 3D shows a tooth for forming a rectangular beam and a
semi-elliptical base; and FIG. 3E shows a tooth for forming a
triangular beam and semi-elliptical base. An exemplary tooth 36 for
forming a structure with a beam that is U-shaped in plan view is
shown in FIG. 3F; and an exemplary tooth 36 for forming a
bifurcated beam with parallel arm is shown in FIG. 3G. Advantages
and characteristics of various contoured spring structures are
described in the FIRST PARENT CASE referenced above. It will be
apparent that the desired shape of tooth 36 will be defined by the
molding counterpart of the desired spring structure shape.
[0080] Furthermore, although a specific configuration of stamping
tool 34 is shown in FIG. 3B, it should be apparent that tool 34
(and therefore, the impression made by it) may be provided in
various configurations, without departing from the scope of the
invention. For example, tool 34 may comprise as few as a single
embossing tooth. In the alternative, tool 34 may comprise a
plurality of embossing teeth 36, disposed in a pattern. In such
case, the embossing teeth may be positioned for forming molds on an
entire surface of a substrate, or on a selected portion of a
substrate surface. In a tool 34 having a plurality of embossing
teeth 36, all of the teeth may have the same size and shape.
Alternatively, teeth on the same tool may have various different
sizes and shapes, depending on the application requirements. The
embossing teeth 26 may be disposed in the same plane, or disposed
in different planes, or disposed on a curved surface, such as a
cylinder. For example, a cylindrical stamping tool may be used to
form molding surfaces by rolling over a substrate, which may be
useful, for example, for forming spring structures on continuous
webs of material.
[0081] In many cases, the upper surface of the silicon substrate
will have substantial irregularities (non-planarities) that will
transfer to the upper surface of a uniform layer of sacrificial
material, such as a spin-coated layer. The tips of the spring
structures formed by the foregoing molding method will accordingly
not be in substantially the same plane. If the irregularities are
larger than about 10% of the tip height of the spring structures
above the substrate, an array of spring structures on the substrate
will be unsuitable for making contact with another planar
substrate. Also, because mating substrates will also have
non-planar surfaces, it is desirable to reduce non-planarities in
the tips of the spring structures to avoid errors from tolerance
build-up. Accordingly, the present invention provides a method 400
for making spring structures with tips in substantially the same
plane, relative to surface irregularities in a substrate.
[0082] Exemplary steps of the method 400 are shown in FIG. 4, and
cross-sectional views of a substrate and layered materials during
steps of the method are provided in FIGS. 5A-5G. In an initial step
402, a substrate 42, having an irregular upper surface 33, is
mounted in a mold 71, comprising a cover plate 68, spacers 70, a
mounting surface 74, and an injection port 72. An inner surface 77
of cover plate 68 is planarized to the desired tolerance and
polished to the desired surface finish. Substrate 32 is mounted to
mounting surface 74, for example, a wafer chuck, so that the upper
surface 33 of substrate 32 is substantially parallel to the inner
surface 77. The depth of the sacrificial layer 30 to be formed in
mold 71 is controlled by the thickness of spacers 70.
[0083] In step 404, a moldable material (for forming sacrificial
layer 30) is injected through port 72 to fill the interior of mold
71. The moldable material may be any suitable moldable material,
including the materials previously described for forming a coated
sacrificial layer. In step 406, the material is cooled or cured to
the desired hardness. In step 408, the cover plate 68 is removed
from the substrate 32 with the adhered layer 30, as shown in FIG.
5C. After the molding process, the upper surface 78 of layer 30 is
substantially planar relative to the irregular upper surface 33 of
substrate 32. In step 410, contoured molding surfaces are formed in
sacrificial layer 30, using a stamping tool 34, as shown in FIG.
5D. Details of step 410 are essentially the same as step 104 of
method 100, described above. In the alternative, features for
forming the contoured molding surfaces 48 can be machined directly
into inner surface 77 of cover plate 68, and step 410 may be
omitted. The appearance of the molded sacrificial layer after
forming the molding surfaces 48 is shown in FIG. 5E. The upper
surface of residue 51 over base areas 50 are located at a uniform
depth h from the reference plane defined by the inner surface 77 of
cover plate 68. The reference plane is itself located a distance
d.sub.1 from the mounting plane 82 of substrate 32, where d, is
longer than h.
[0084] In step 412, the substrate is exposed at the base areas 50,
preferably by etching the sacrificial layer 30 using an anisotropic
etch 76, as previously described. The etch 76 is continued until
all substrate areas are exposed, as shown by the dotted lines in
FIG. 5E. Conventional end-point detection techniques can be used to
determine the ending point for the etching process. After etching,
the base areas 50 are disposed on the irregular upper surface 33,
and thus are no longer disposed at an equal depth from the
reference surface. However, the upper surface of layer 30 is still
within essentially the same plane, disposed at a distance d.sub.2
from the mounting plane 82 of substrate 32, where d.sub.2 is less
than d.sub.1. A layer of resilient material is then deposited over
the sacrificial layer and patterned, and the sacrificial layer is
removed from the substrate 32, as previously described in
connection with method 100. The resulting spring structures 60 have
their tips 80 disposed in substantially the same plane, located a
distance d.sub.2 from the mounting plane 82 of substrate 32. The
distance d.sub.2 is preferably constant, but may vary in a regular
fashion across any straight section of the substrate (that is, the
plane of the spring structure tips need not be exactly parallel to
the mounting plane of substrate 32), within limits of about 20% of
the average tip height of the spring structures 60 above substrate
32.
[0085] Similar results may be obtained using an alternative method
600, exemplary steps of which are shown in FIG. 6. In step 602, a
sacrificial layer is deposited on the substrate as previously
described in connection with step 102 of method 100. In step 604, a
molding surface is formed in the sacrificial material layer, as
previously described in connection with step 104. Then, at step
606, the upper surface of the sacrificial material layer is made
planar, using a process known in the art, such as
chemical-mechanical polishing. As previously described in
connection with method 400, the upper surface of the sacrificial
layer is thus made to be disposed within a plane that is
substantially parallel to, or slightly inclined to, the mounting
plane of substrate 32. The remaining steps of method 600 are
essentially the same as previously described in connection with
steps 106-112 of method 100.
[0086] In some circumstances it may be advantageous to avoid
forming the molding surfaces on a sacrificial layer by a method
that requires a stamping tool and ancillary equipment. The present
invention provides a method 700 for forming contoured molding
surfaces in a sacrificial layer without the need for a stamping
tool. Exemplary steps of method 700 are shown in FIG. 7. Related
views of a substrate with layered materials during steps of the
method 700, and a view of an exemplary resulting spring structure,
are shown in FIGS. 8A-8G. In an initial step 702, a layer of
sacrificial material 30 is deposited on a substrate 32. Sacrificial
layer 30 is preferably deposited in a layer of uniform thickness,
using any of the methods previously described. In step 704, the
layer of sacrificial material is patterned to form one or more
recesses 86, as shown in FIG. 8B, extending to the surface of
substrate 32 in at least a portion of the recess 86. Various
methods as known in the art, such as photo patterning, may be used
to form recess 86. In plan view, such as shown in FIG. 8A, recess
86 has the shape of the spring structure to be formed, which may be
any of the shapes previously described, or any other suitable
shape. For example, in an embodiment of the invention, the beam
shape is triangular in plan view, and the base area is rectangular,
as shown in FIG. 8A.
[0087] In step 706, the surfaces of the recess 86, and in
particular, the sidewalls, are preferably treated to alter their
wetting properties as desired. The wetting properties can be
modified by various techniques as known in the art, such as
silanization. For further example, exposure to plasmas of oxygen,
nitrogen/hydrogen, and other gases can change surface wetting
properties. Further, increasing the surface roughness will
generally increase the wetability of the surface. The sidewalls of
the recess 86 are treated to alter the surface energy, which
determines wetability, relative to the chosen wetting fluid. If a
concave meniscus is desired, the surface energy of the sidewalls is
decreased (if necessary) such that the chosen wetting fluid will
cling to the sidewalls and form a concave meniscus in the recess
86. Conversely, if a convex meniscus is desired, the sidewalls are
treated to repel the wetting fluid, thereby causing the fluid to
form a bead having a convex meniscus. In the preferred embodiment
of the invention, the selection of the sacrificial material,
wetting fluid, and recess shape are such that no surface treatment
of the recess 86 is needed to achieve the desired meniscus shape.
In general, it is preferred that the surface of the recesses 86 be
easily wetted, to avoid difficulties with filling multiple recesses
with a uniform quantity of fluid.
[0088] In step 708, recess 86 is partially filled with a suitable
wetting fluid 84. A suitable fluid is one with a low enough
viscosity and surface tension to wet recess 86, which may be
solidified without significant shrinkage or otherwise distorting
the desired meniscus shape, and which may be later removed from the
substrate 32 dissolution along with layer 30. In an embodiment of
the present invention, fluid 84 is a photo patternable material
such as photoresist (e.g., SU8-25 or SU8-2). Several methods may be
used to get a specified volume of fluid 84 into the recess 86.
Generally the recesses 86 are small, for example about 250 microns
wide, 250 microns deep, and 1000 microns long. The volume of a
"manhattan" (rectangular) cavity with these dimensions is 62.5
nanoliters, and special techniques must be used to accurately
deposit a specified volume of liquid, which is preferably less than
the recess volume. In one embodiment, a substrate having recesses
86 with a volume of less than about 100 nanoliters is spin coated
with a fluid 84. The spin-coating process leaves a small amount of
fluid 84 in each cavity, the volume of which depends on the fluid
viscosity, surface wetting properties of the fluid 84 and recess
86, the shape of recess 86, and spin process parameters such as
rotational velocity and acceleration, and radial distance from the
axis of rotation. Fluid 84 may be applied by directing (such as by
spraying) a fluid mist onto a rotating substrate, or by immersion.
A portion of the fluid 84 is also removed by the spin coating
process from recess 86, so that the fluid 84 only partially fills
the recess 86, as shown in cross-section in FIG. 8C.
[0089] The relative surface energies of fluid 84 and the sidewalls
of recess 86 are such that the fluid 84 has a meniscus having a
first contoured shape 88 in the length direction of the recess 86,
and a second contoured shape 89 in the width direction, as shown in
FIGS. 8C and 8D, respectively. Where recess 86 is narrower, such as
toward the point of the triangle shown in FIG. 8A, the surface
tension of fluid 84 preferably causes surface 88 to rise, as shown
in FIG. 8C. Across the width of recess 86, surface tension pulls
the surface 89 into a concave U-shape.
[0090] In step 710, after the fluid 84 partially fills recess 86,
the fluid is solidified, for example, by curing with a chemical
catalyst or UV light, by heating to drive out solvents, or by
cooling below its melting point. The solidified fluid 92 may then
be further patterned to define a mold for the spring structure. For
example, as shown in FIG. 8E, a portion of the solidified fluid 92
may be removed in a base area 50, by exposing the solidified fluid
92 to an anisotropic etch 76 through mask 90. The remaining
solidified fluid 92 defines a contoured molding surface 48 and
exposed base area 50 as shown in FIG. 8F, upon which a suitable
resilient material may be deposited according to the previously
described method 100, or other suitable method. The resulting
spring structure has a beam with a U-shaped contour across its
width, as shown in FIG. 8G.
[0091] Each of the foregoing fabrication methods may be used to
define a spring structure having a defined contoured shape. In
general, one of the advantages of contouring the beam of a spring
structure is that a contour can be used to reduce the thickness of
material that is needed to achieve a beam of adequate stiffness for
use as a microelectronic spring contact. Accordingly, alternative
deposition techniques, such as physical vapor deposition ("PVD") or
chemical vapor deposition ("CVD"), can be used to deposit the
resilient spring material over the molding surface. For example,
PVD and CVD are generally less suitable than electroplating for
depositing layers more than 5 microns thick, which is a suitable
range of thickness for contoured springs. The present invention
accordingly provides a method 900 for forming a microelectronic
spring structure using an alternative material deposition
technique, as shown in FIG. 9. Views of a substrate and materials
layered thereon during exemplary steps of method 900 are shown in
FIGS. 10A-10D.
[0092] Steps 902 and 904 of method 900, for depositing a
sacrificial layer 30 on substrate 32, and forming the molding
surfaces, are substantially the same as corresponding steps 102 and
104 of method 100, as previously described. Other methods, such as
method 400 also described herein, may also be used to form a
molding surface in the sacrificial material. At step 906, the
surface of the sacrificial layer 30 is coated with a layer of
resilient material 58 using a process such as CVD or PVD, to a
uniform thickness of at least about one micron, and preferably
about five microns. To achieve a thickness greater than about 5
microns, it is preferred to deposit the resilient material 58 by
electroplating, after first depositing a seed layer, as described
in connection with method 100. A cross-section of the substrate
after the deposition process is shown in FIG. 10A. In step 908, a
patterned layer of masking material, such as a photo-resist layer
54, is applied to cover the resilient material in areas where
spring structures are to be formed, as shown in FIG. 10B. At step
910, the excess (unmasked) resilient material is removed using a
etching process as previously described, resulting in the layered
materials as shown in FIG. 10C. At step 912, the sacrificial layer
30 and masking layer 54 are removed in a suitable solvent, leaving
the spring structure 60, comprised of resilient material 58,
adhered to substrate 32. Spring structure 60 is then typically
post-processed, for example, by plating with gold and/or by
adhering a separate tip structure (not shown), as further described
herein and in the co-pending applications referenced herein.
[0093] The steps required to pattern layers of resilient materials
and/or seed layers may be reduced or eliminated by providing at
least a portion of the molding surfaces and base areas with an
overhanging lip. Such techniques may be generally applied to the
methods previously described to reduce manufacturing costs. An
overhanging lip may be provided using a mold tooth of suitable
form, such as the re-entrant tooth 98 provided on tool 34 and shown
in FIG. 11A. When re-entrant tooth 98 is pressed into a layer of
sacrificial material, the recess formed thereby is provided with an
overhanging lip 96. It should be apparent that, to remove tooth 98
from layer 30 after being fully impressed therein without damaging
lip 96, it is helpful for the layer of sacrificial material 30 to
be a visco-elastic material. A visco-elastic material will deform
sufficiently to permit removal of tooth 98 without damaging lip 96,
but will recover its shape after the tooth is removed. Similar
benefits may be realized if layer 30 is formed from a soft, elastic
material that does not adhere to tool 34. Generally, layer 30
should comprise a solid material with a low shear modulus, i.e., a
gel. The gel may have a viscous component, making it visco-elastic,
or it may be more purely elastic, e.g., a soft elastic
material.
[0094] As an alternative to using a re-entrant tooth, progressive
stamping tools may be used to form an overhanging lip. FIG. 12A
shows an exemplary progressive stamping tool, having a primary
tooth 36 and a secondary tooth 37. Primary tooth 36 is shaped as
previously described. Secondary tooth 37 is shaped as a relatively
shallow ring that partially or fully encloses the perimeter of the
recess formed by tooth 36. A cross-sectional view of primary tooth
36 and a representative portion of secondary tooth 37 are shown in
FIG. 12B. The primary and secondary teeth are designed to be
sequentially impressed on substrate 30 by first impressing the
primary tooth 36, lifting the tool 34 from the sacrificial material
30, relocating the stamping tool 34 so that secondary tooth 37 is
positioned over the recess formed by the primary tooth, and
impressing the tool a second time. In the alternative, the primary
and secondary teeth may be provided on separate stamping tools (not
shown) which are then applied in sequence to the sacrificial layer
30. It should be apparent that progressive stamping is not limited
to use with two progressive tools, and any number of sequential
impression tools may be used without departing from the scope of
the invention.
[0095] The resulting impressions formed by sequential impression of
the primary and secondary teeth are shown in FIGS. 12C-12F. FIG.
12C shows a cross-section of an exemplary layer of sacrificial
material 30 after being impressed with primary tooth 36. FIG. 12D
shows the same exemplary material layer 30, after the progressive
stamping tool 34 is shifted a distance and re-impressed upon the
material, forming an overhanging lip 96 around the perimeter of the
molding surface 48 and base area 50. The sequence may be repeated
to provide the next recess formed by the primary tooth 36 with an
overhanging lip, and so forth, as the tool 34 progresses across the
surface of the material layer 30. A plan view of an exemplary
triangular/rectangular recess 86 with an overhanging lip is shown
in FIG. 12E, and a similar rectangular recess 86 is shown in FIG.
12F.
[0096] A fully enclosing overhanging lip, as shown in FIGS. 12E and
12F, may be used to pattern a layer of resilient material according
to the method 1300, shown in FIG. 13. Cross-sectional views of the
substrate and layered materials during steps of the method 1300 are
shown in FIGS. 14A-14C. In an initial step 1302, a layer of
conductive material 53 is deposited on substrate 32, to serve as a
shorting layer, according to methods known in the art. The
conductive layer 53 may be titanium-tungsten (Ti--W) alloy, a
chrome-gold (Cr--Au) bi-layer, or any other appropriate conductive
precursor layer, typically deposited by sputtering to a thickness
between about 300 and 10,000 .ANG.. The shorting layer 53
substantially conforms to and contiguously covers the surface of
the substrate 32, and any contact pads or other features that may
be present on the substrate. Alternatively (but less preferably for
the purposes of method 1300), shorting layer 53 can be deposited in
a pattern of multiple, non-contiguous regions. Patterning the
shorting layer 53 is generally for the purpose of defining a
redistribution layer between contact pads on the substrate 32 and
the spring structures to be formed.
[0097] At step 1304, the sacrificial material layer 30 is deposited
according to a method previously described. At step 1306, a molding
surface 48 with an overhanging lip 96 is formed in the layer of
sacrificial material, preferably using a re-entrant tooth or
progressive stamping tool, as previously described. At step 1308, a
seed layer 52 and 55 is deposited on the surface of the sacrificial
layer, using a process such as sputtering (especially ionized
physical-vapor deposition (I-PVD)), or similar line-of-sight
deposition process. It will be apparent that the overhanging lip 96
shields the perimeter of the molding surface from deposition of the
seed layer, resulting in a first portion 52 of the seed layer
disposed over the molding surface 48 and base area in recess 86,
and a second portion 55 of the seed layer over the surrounding area
of the sacrificial material layer, as shown in FIG. 14A. If will
further be apparent that, so long as the overhanging lip 96 fully
encloses the recess 86, the first portion 52 of the seed layer will
be connected to the shorting layer 53, and the second portion 55
will be isolated from the shorting layer 53 and from the first
portion 52.
[0098] Then, in step 1310, the substrate is electroplated with a
resilient material using shorting layer 53 to apply a plating
potential to the first portion 52. The resilient material 58 thus
is selectively plated on the first portion 52 of the seed layer,
and does not cover the second portion 55. Then, at step 1312, the
sacrificial material layer and second portion 55 of the seed layer
are removed by dissolving the sacrificial material in a suitable
solvent, as previously described. It should be noted, however, that
even if the resilient material 58 is incidentally plated over the
second portion 55, this unwanted plated material can later readily
be removed without harming the desired spring structures, so long
as it is not continuous with the resilient material 58 plated over
the first portion 52. In either case, a separate, free-standing
spring structure results from application of the method 1300, an
exemplary one of which is shown in FIG. 14D, without the need for
any separate patterning step.
[0099] A similar process may be used, utilizing a partially
enclosing overhanging lip, according to the method 1500 shown in
FIG. 15. No shorting layer is needed for method 1500, however, an
additional step is needed to separate the resilient material of the
spring structures from the surrounding material. A plan view of the
substrate during a step of the method is shown in FIG. 16A, and
cross-sectional views of the substrate and materials layered
thereon during steps of the method are shown in FIGS. 16B-16D. In
step 1502, a layer of sacrificial material is deposited according
to one of the previously described methods. In step 1504, a molding
surface is formed as described above, except that the overhanging
lip 96 does not completely enclose the molding surfaces within
recess 86. As shown in FIG. 16A, the overhanging lip 96 is formed
to enclose the recess 86 on three sides, and no lip is formed on
the side adjacent to the top of the sacrificial layer, where the
tip of the spring structure will be formed. In step 1506, a seed
layer 52 is deposited over the surface of the sacrificial layer 30,
using a line-of-sight method, as previously described. Because
recess 86 is not completely enclosed by the overhanging lip, the
seed layer 52 is electrically connected to the deposited seed layer
everywhere else on the surface of layer 30, as shown in FIG. 16A.
Seed layer 52 can thus be used for electroplating the resilient
material 58, and no shorting layer is needed for this purpose
(although one may optionally be present, for other reasons).
[0100] The appearance of the substrate after deposition of the
resilient material layer is shown in FIG. 16B. As is also clear
from FIG. 16A, the layer of resilient material 58 will be parted on
all sides of the recess 86, where no seed layer was deposited,
except for near the surface of layer 30, where it is connected to a
more generally extending layer. It is necessary, therefore, to
remove the excess resilient material, which is done is step 1510,
by any suitable precision machining method, such as
chemical/mechanical polishing. At the same time, the surface of
layer 30 is preferably planarized, so the tips of the spring
structures will reside in the same plane, for the reasons discussed
previously. A cross-section of the substrate after step 1510 is
shown in FIG. 16C. The next step is to remove the remaining portion
of sacrificial layer 30, using any of the methods described herein,
to leave the free standing spring structure 60, as shown in FIG.
16D.
[0101] In some cases it may be advantageous to form a plurality of
microelectronic spring contacts by replicating a single mold tooth
(or a relatively small group of teeth), instead, of by using a
stamping or molding tool having a plurality of teeth covering a
relatively large area, such as the area of an die or wafer. The
present invention provides a "one-up" method 1700, exemplary steps
of which are shown in FIG. 17, for such cases. For example, method
1700 may be advantageous for small production runs, or runs
involving atypical "custom" positioning of the spring structure,
because it avoids the need for an intricate stamping tool having
many teeth. FIGS. 18A-18E show cross-sectional views of a substrate
32 and layered materials during steps of the method 1700. FIG. 18F
shows a perspective view of exemplary molding surfaces 48 which may
be formed using method 1700, for molding spring structures, or for
use as a stamping tool with many teeth. In an initial step 1702, a
layer of sacrificial material 30 is deposited on substrate 32. In
an embodiment of the invention, the layer 30 is a material that may
be cured (hardened) by exposure to a radiation, such as by exposure
to UV light, or to an electron beam. FIG. 18A shows the sacrificial
layer after deposition during step 1702. Also shown is an exemplary
single-tooth stamping tool 34, having a tooth 36. Tooth 36 is as
previously described; however, in an embodiment of the invention,
tooth 36 is additionally provided with a radiation-transparent
portion 39 and an opaque portion 41.
[0102] A process loop, comprising steps 1704 through 1708, is then
performed. In a first cycle of the loop, a single contoured molding
surface is formed using tooth 36, at step 1704. FIG. 18B shows the
substrate 32, layer 30, and tooth 36 during step 1704, with tooth
36 fully impressed into substrate 30. Flash 49 is evident on either
side of tooth 36. In step 1706, while tooth 36 is in position,
molding surface 48, which is under transparent portion 39 of tooth
36, is preferably selectively cured. In an embodiment of the
invention, UV light is beamed through the tooth 36 to cure portion
31. The opaque portion 41 preferably prevents the sacrificial layer
30 from being cured in the area of the base, so that the substrate
may be more readily exposed to a layer of resilient material there.
Steps 1704 and 1706 are repeated until the desired number of
molding surfaces 48 have been defined, as indicated by decision
step 1708. The appearance of the substrate during a second cycle of
the process loop is shown in FIG. 18C, and the appearance of the
substrate after the second cycle is shown in FIG. 18D. Two cured
portions 31 are shown, surrounded by uncured areas of flash 49.
These uncured portions are readily removed in step 1710, by
dissolving in a suitable solvent, leaving only the molding surfaces
48 comprised of cured portions 31. The molding surfaces may be used
for forming spring structures as previously described. In the
alternative, the molding surfaces 48 may be used as the teeth of a
stamping tool. It should be apparent that stamping methods using a
transparent tooth, such as method 1700, are readily adaptable to
methods using tools with a plurality of transparent teeth separated
by opaque regions, which may be used, for example, to form a
plurality of spring structures in parallel at single-die,
multiple-die and wafer scales.
[0103] A similar "one-up" method may be used to form molding
surfaces for spring contacts, using plunge EDM. According to a
plunge EDM method, a suitable plunge EDM tool is shaped like, and
replaces, the transparent stamping tooth 36 discussed above with
respect to method 1700. Instead of embossing a deformable
substrate, the plunge EDM tool is used to form molding surfaces in
a substantially non-deformable, electrically conductive substrate.
Candidates for molding surfaces include metals and polymers filled
with conductive particles or fibers. The surface so formed may be
used as a mold form for spring contacts, or as a multi-toothed
forming tool, depending on the characteristics of the conductive
substrate and the desired objective.
[0104] In yet another embodiment of the invention, a spring
structure is formed on a molded substrate utilizing the properties
of a line-of-sight material deposition technique, such as
sputtering or evaporation, so as to eliminate certain process
steps. Exemplary steps of a method 1900 using a line-of-sight
deposition technique are shown in FIG. 19. Exemplary views of a
substrate and layered materials during method 1900 are shown in
FIGS. 20A-20E. At step 1902, a substrate 32 is provided, typically
having at least one exposed contact pad 46. At step 1904,
dielectric layer 43 is optionally deposited and patterned as known
in the art. At optional step 1906, a shorting or adhesion layer 53,
such as a layer of titanium, titanium-tungsten, or chromium, is
deposited over layer 43 and contact pad 46 as known in the art. The
purpose of layer 53 is to facilitate the subsequent optional
plating step 1916. If step 1916 is to be omitted, step 1906 is
preferably omitted also. At step 1908, a sacrificial layer of
moldable material 30 is deposited on substrate 32, and formed, such
as by embossing with a stamping tool, to provide a mold for a
microelectronic spring. Any suitable moldable material, such as
described herein, may be used. At step 1910, any residual moldable
material 30 covering contact pad 46 is removed, such as by using a
suitable anisotropic etch process. A layer of metallic material 52
is then deposited over the moldable layer 32 using a line-of-sight
process, such as sputtering or evaporation, at step 1912.
[0105] Exemplary views of a substrate and layered materials after
completion of step 1912 are shown in FIGS. 20A and 20B. A recess 86
having vertical or relatively steep sidewalls 87 has been provided
in layer 30, such as by a stamping tool having a suitably shaped
embossing tooth. For the purposes of method 1900, "steep" means
inclined (positively or negatively) less than about 45.degree. from
vertical, and preferably, less than about 30.degree. from vertical.
Still more preferably, sidewalls 87 are inclined between about
0.degree.-5.degree. from vertical. A bottom surface of recess 86
comprises a molding surface 48 for defining the shape of a
microelectronic spring structure. Molding surface 48 is isolated
from the top surface 57 of the moldable layer 30 by the sidewalls
87 which preferably surround the entire periphery of recess 86,
thereby separating molding surface 48 from the upper surface 57 of
layer 30. As shown in FIG. 20C, because of the properties of
line-of-sight deposition, the thickness "t.sub.1" of the layer 52
is substantially greater on the molding surface 48 of layer 30 than
the thickness "t.sub.2" on the sidewalls 87. In particular, if
sidewalls 87 are substantially vertical, or overhang the molding
surface 48 (that is, are inclined with respect to the line of
deposition of the line-of-sight deposition method so as to not
present a face for deposition of material thereon), no material
will be deposited on the sidewalls. Although the upper surface 57
of layer 30 is shown as being substantially horizontal and planar,
the shape and inclination of surface 57 is not critical, and may
have a variety of different shapes, so long as sidewalls 87 are
present and inclined so as to isolate surface 57 from molding
surface 48.
[0106] At step 1914, if present on sidewalls 87, layer 52 is etched
isotropically to remove all of layer 52 adhering to sidewalls 87
while leaving it substantially intact on molding surface 48 and top
surface 57. That is, the etching step 1914 preferably is halted as
soon as sidewalls 87 are free of deposited metallic material, at
which point layer 52 on molding surface 48 will preferably be of a
desired thickness. After step 1914, layer 52 on molding surface 48
will remain electrically connected to shorting layer 53. An
isolated portion 55 of layer 52, on the upper surface 57 of layer
30, will be physically isolated from layer 52 on molding surface
48, and preferably, also electrically isolated from shorting layer
53. Layer 52 is thus patterned to define a spring structure, by
separation of the molding surface 48 from upper surface 57 by
sidewalls 87, and elimination of metallic (or resilient) material
from the sidewalls. It should be apparent that if there is no
metallic layer 52 on sidewalls 87 after deposition step 1912 (such
as if sidewalls 87 are vertical or overhanging) step 1914 will be
unnecessary and may be omitted.
[0107] At optional step 1916, a layer of resilient material 58 is
electroplated onto the portion of layer 52 on molding surface 48.
Preferably, no additional material will be plated onto the isolated
portion 55, because it is preferably not connected to shorting
layer 53, through which the plating current flows. A view of the
substrate and layered materials after completion of step 1916 is
shown in FIG. 20D. Note that resilient layer 58 does not contact
the isolated portion 55. Hence, isolated portion 55 and sacrificial
moldable layer 30 are readily removed at step 1918, such as by
dissolution in an etchant, without damaging the resilient material
58 deposited on molding surface 48. It should further be apparent
that if metallic layer 52 is sufficiently thick to provide the
desired strength and stiffness, plating step 1916 may be omitted.
In particular, where the spring structure is to be provided with
stiffening features, such as a contoured or ribbed cantilevered
portion, plated layer 58 (which may be used to provide strength and
stiffness) is less likely to be necessary. FIG. 20E shows a
cross-sectional view of the resulting spring structure 60 after
removal of the sacrificial moldable layer 30 at step 1918. Isolated
portion 55 and exposed portions of shorting layer 53 are also
removed at step 1918. A plurality of microelectronic spring
structures, such as structure 60, may thus be formed in parallel
using method 1900, without the need for any pattern-masking
step.
[0108] Other structures may be formed on the surface of a substrate
at the same time as, and using the same processes as used for
forming a microelectronic spring. In particular, redistribution
traces, bridges, and bumps may be formed with a spring structure
according to the present invention. FIGS. 21A-21D show a substrate
and layered materials during exemplary steps of a method for
forming a redistribution trace 45 and bridges 59 with a spring
structure 60. Although method 1900 is adapted for this purpose to
illustrate an application thereof that omits the plating steps, any
other suitable method described herein may also be used to form
features in parallel with a spring structure. FIG. 21A shows a
substrate having a contact pad, dielectric layer, and moldable
layer 30, as described above in connection with method 1900. After
preparation of moldable layer 30, a stamping tool 34 is used to
define a molding surface 48, a trace-defining portion 63 for
molding for a redistribution trace, and bumps 61.
[0109] FIG. 21B shows the substrate with stamping tool 34 fully
impressed into moldable layer 30. Bumps 61 may be any suitable
shape, and have a height less than the tip height of the spring
structure to be formed. In an embodiment of the invention, bumps 61
have a height and shape suitable for acting as stop structures,
i.e., structures capable of preventing over-compression, for their
companion spring structure. For example, suitable shapes include
those with arched, semi-circular, triangular, or rectangular cross
sections, having a height above the substrate sufficient to prevent
over-compression of the spring structure. Bumps 61 may be
configured as connected to trace-defining portion 63, or as
isolated from it.
[0110] Residue 51 is typically present on the substrate 32 after
the tool 34 is removed. Such residue is removed to reveal the
contact pad 46 and dielectric layer 43 at the bottom of recess 86
in the area of the redistribution race and base for the spring
structure. By suitable design of tool 34, recess 86 is surrounded
by steep sidewalls 87 which separate molding surface 48 and the
bottom of recess 86 from the upper surface 57 of moldable layer 30,
as previously described herein. A layer of resilient material is
deposited generally on the substrate, including over the bottom of
recess 86 and over molding surface 48, using a line-of-sight
deposition technique. FIG. 21C shows the molded resilient material
52 after deposition of resilient layer 58. In this example, layer
58 is sufficiently thick so that no additional resilient layer is
needed.
[0111] Moldable layer 30 is then removed, revealing a spring
structure 60 with an integral redistribution trace, as shown in
FIG. 21D. In this example, spring structure 60 has a contoured beam
for enhanced stiffness. Bridges 59 correspond to bumps 61 formed by
tool 34. Bridges 59 may serve to provide stress relief to trace 45,
particularly if trace 45 is relatively lengthy. Bridges 59 may also
serve as stop structures for spring structure 60. Additional
bridges (not shown) may additionally be provided, that are
electrically isolated from any contact element, and therefore
perform a purely mechanical function, such as a mechanical stop.
Thus, a complete contact system, including a plurality of spring
contacts, associated redistribution traces, and stop structures,
can be made using relatively few process steps. To further
illustrate an application of the method, FIG. 22 shows an exemplary
two of many contact structures with integral redistribution traces,
for performing a pitch spreading function from a relatively fine
pitch "p1" at the contact pads, to a coarser pitch "p2" at the tips
of the spring elements. A wide variety of geometric configurations
for pitch spreading and other redistribution purposes are possible,
without departing from the scope of the invention.
[0112] In an alternative embodiment, a separately formed stop
structure, as further described in the co-pending application Ser.
No. 09/364,855, filed Jul. 30, 1999, entitled "INTERCONNECT
ASSEMBLIES AND METHODS," by Eldridge and Mathieu, which is hereby
incorporated herein by reference, is provided to prevent
over-compression of the microelectronic spring structures under
application of a contact force, according to methods described
therein. Perspective views of a substrate 32 with an array of
contoured, microelectronic spring contacts 60, and provided with
stop structures 47, are shown in FIGS. 23A-23C. The substrate is
shown at a wafer level in FIG. 23C. A view of a single die 97 on
the wafer, showing an array of spring structures 60 on the die, is
shown in FIG. 23B. A detailed view of a single contoured spring
structure 60 and surrounding stop structure 47 is shown in FIG.
23C. It should be apparent that the spring structures may be
disposed in any desired pattern on the substrate. In particular,
spring structures may be disposed at locations on the substrate
remote from underlying contact pads and vias, by creating an
intermediate redistribution layer between the contact pads or vias
and the spring structures, as further described in the co-pending
application Ser. No. 09/364,855 referenced above.
[0113] It should be appreciated that the contoured microelectronic
spring structures 60 described herein, such as shown in FIGS.
23A-23C may also be used for other types of interconnect
assemblies, such as probe card assemblies, interposers, and other
connection systems where electrical contact to or through a
substrate is desired. In particular, such spring structures may be
used both for making high-temperature, temporary connection during
a wafer or chip level burn-in process, and subsequently, for making
a more permanent, ambient temperature connection between the
substrate and an electronic component such as a printed circuit
board. It is anticipated that the low cost and versatility of the
spring structures will greatly reduce the costs associated with
high-temperature testing by permitting testing at higher
temperatures, and thus achieving higher throughput than is possible
using methods according to the prior art.
[0114] The methods of the present invention are further illustrated
by the following example:
EXAMPLE
[0115] A silicon wafer with a 0.5 micron surface oxide layer was
selected for a prototype substrate. A layer of chrome was sputtered
on a surface of the substrate, followed by a layer of gold, to
provide a shorting layer. A 4.0 mil (100 micron) thick layer of
negative dry-film photoresist was applied to the sputtered gold
layer using a vacuum laminator. A second 3.0 mil (75 micron) thick
layer of the same type of photoresist was applied over the first
layer. The substrate was placed on a hot plate and heated until the
photoresist was soft. An embossing tool with protruding triangular
teeth contoured to produce the desired spring shape was pressed
into the photoresist laminate while the laminate was soft. The
substrate was cooled and the embossing tool was removed. A
photolithography mask and UV light were used to expose (and thus
cross link) the photoresist laminate everywhere except in the area
of the spring base contact. The photoresist was developed using a
spray developer with standard sodium carbonate developer solution,
which removed the unexposed photoresist from the spring-base
contact. The spring-base contacts were then cleaned using an oxygen
plasma descum for ten minutes. A seed layer of metal
(palladium/gold) for a subsequent electroplating step was sputtered
over the entire surface of the photoresist laminate and exposed
base area. A 4.0 mil layer of dry-film photoresist was applied over
the sputtered layer using a vacuum laminator at 80.degree. C. The
photoresist was exposed with UV light using a photolithography mask
to shield the resist over the molding surface, where the spring was
to be formed. The photoresist was then developed to remove in the
area of the molding surface, and then a plasma descum was used to
clean the molding surface as before. A resilient spring metal
(nickel) was deposited in the mold form by electroplating for 20
minutes at about 50 ASF current density. The substrate was removed
from the electroplating solution and immersed in a solution of RD87
negative resist stripper to remove all layers of photoresist. A
free-standing spring structure remained on the substrate having a
thickness of 12 microns (about 0.5 mil), a cantilevered beam that
was triangular in plan view and that extended about 180 microns (7
mils) from the surface of the substrate.
[0116] Having thus described a preferred embodiment of a method for
forming microelectronic spring structures, it should be apparent to
those skilled in the art that certain advantages of the invention
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, a method for forming microelectronic spring contact
structures has been illustrated, but it should be apparent that the
inventive concepts described above would be equally applicable to
form similar structures for other purposes. For example,
electromechanical spring contacts, such as relays, or purely
mechanical springs could be formed on a variety of substrates for
various applications using the methods described herein.
Additionally, other lithographic type structures comprising open
contoured sheets of materials, such as channels, funnels and
blades, may be made at microscopic scales by suitably adapting the
methods herein. The invention is further defined by the following
claims.
* * * * *