U.S. patent application number 09/731135 was filed with the patent office on 2001-07-19 for article comprising improved noble metal-based alloys and method for making the same.
Invention is credited to Bishop, David John, Jin, Sungho, Kim, Jungsang, Ramirez, Ainissa G., Van Dover, Robert Bruce.
Application Number | 20010008157 09/731135 |
Document ID | / |
Family ID | 27388589 |
Filed Date | 2001-07-19 |
United States Patent
Application |
20010008157 |
Kind Code |
A1 |
Bishop, David John ; et
al. |
July 19, 2001 |
Article comprising improved noble metal-based alloys and method for
making the same
Abstract
A device having electrical contacts formed from an alloy having
improved wear resistance is provided, the alloy being particularly
useful in microrelay devices formed by MEMS technology. In one
embodiment, the alloys are chosen to allow sufficient precipitation
hardening to improve wear resistance, but keep precipitation below
a level that would unacceptably reduce electrical conductivity.
This is achieved by using alloying materials that have very limited
or no solid solubility in the noble metal matrix, e.g., less than 4
wt. % solid solubility. In a second embodiment, an alloy contains a
noble metal matrix and insoluble, dispersoid particles having no
solubility in the matrix, these dispersoid particles offering a
similar strengthening mechanism.
Inventors: |
Bishop, David John; (Summit,
NJ) ; Jin, Sungho; (Millington, NJ) ; Kim,
Jungsang; (Basking Ridge, NJ) ; Ramirez, Ainissa
G.; (Chatham, NJ) ; Van Dover, Robert Bruce;
(Maplewood, NJ) |
Correspondence
Address: |
Lucent Technologies Inc.
Docket Administrator (Room 3C-512)
600 Mountain Avenue
P.O. Box 636
Murray Hill
NJ
07974-0636
US
|
Family ID: |
27388589 |
Appl. No.: |
09/731135 |
Filed: |
December 6, 2000 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09731135 |
Dec 6, 2000 |
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09484627 |
Jan 18, 2000 |
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60161290 |
Oct 25, 1999 |
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60161291 |
Oct 25, 1999 |
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Current U.S.
Class: |
148/678 |
Current CPC
Class: |
H01H 2300/036 20130101;
H01H 1/027 20130101; H01H 1/0036 20130101; H01H 1/023 20130101 |
Class at
Publication: |
148/678 |
International
Class: |
C22F 001/14 |
Claims
What is claimed is:
1. An article comprising at least one electrical contact formed
from an electrically conductive alloy comprising: one or more noble
metal elements; one or more alloying elements, wherein the solid
solubility of each of the one or more alloying elements in each of
the one or more noble metal elements is less than 4 weight percent
and wherein the one or more alloying elements are present in the
alloy in a total amount of 0.1 to 30 weight percent; and
precipitate particles comprising the one or more alloying elements,
wherein the particles have an average diameter less than 500 nm,
and are present in a volume fraction of 0.1 to 30%, wherein the
mechanical hardness of the alloy is at least 30% greater than the
mechanical hardness of the one or more noble metal elements
alone.
2. The article of claim 1, wherein the solid solubility of each of
the one or more alloying elements is less than 2 weight
percent.
3. The article of claim 2, wherein the solid solubility of each of
the one or more alloying elements is less than 0.5 weight
percent.
4. The article of claim 1, wherein the mechanical hardness is at
least 50% greater.
5. The article of claim 1, wherein the one or more noble metal
elements is rhodium and the one or more alloying elements are
selected from the group consisting of C, Ce, Dy, Y, Si, Zr, Al, B,
Bi, Er, Gd, Ge, Pb, Sm, and Yb.
6. The article of claim 1, wherein the one or more noble metal
elements is gold, and the one or more alloying elements are
selected from the group consisting of C, Co, Ho, Lu, Th, Mn, Re,
Rh, Ru, Sb, Yb, Y, B, Bi, Dy, Er, La, P, Pb, Si, Sr, and W.
7. The article of claim 1, wherein the one or more noble metal
elements is platinum, and the one or more alloying elements are
selected from the group consisting of B, Bi, Er, Pb, La, Mo, and
Ti.
8. The article of claim 1, wherein the one or more noble metal
elements is palladium, and the one or more alloying elements are
selected from the group consisting of B, Bi, and Si.
9. The article of claim 1, wherein the one or more noble metal
elements is ruthenium, and the one or more alloying elements are
selected from the group consisting of C, Ce, Hf, Lu, La, Si, Bi,
and Gd.
10. The article of claim 1, wherein the alloy exhibits an
electrical conductivity that is at least 30% of the electrical
conductivity of the one or more noble metal elements alone.
11. The article of claim 10, wherein the alloy exhibits an
electrical conductivity that is at least 50% of the electrical
conductivity of the one or more noble metal elements alone.
12. The article of claim 1, wherein the contact is formed on an
adhesion promoting layer that is formed on a substrate
13. The article of claim 1, wherein the alloy comprises Rh and Si,
Rh and Al, Rh and Zr, Rh and Y, Rh and Sm, Au and Si, Au and Dy, Au
and La, Pt and La, Pd and Si, Ru and Hf, Ru and Si, or Ru and
La.
14. The article of claim 1, wherein the article comprises a
microrelay device comprising at least two contacts formed from the
electrically conductive alloy.
15. The article of claim 14, wherein the microrelay device further
comprises a base structure having one of the contacts located
thereon and a cantilever structure having another of the contacts
located thereon.
16. The article of claim 1, wherein the precipitates are present in
a volume fraction of 0.5 to 5%.
17. A process for fabricating an article, comprising the steps of:
providing a substrate; and depositing on the substrate an alloy
comprising one or more noble metal elements, and one or more
alloying elements, wherein the solid solubility of each of the one
or more alloying elements in each of the one or more noble metal
elements is less than 4 weight percent and wherein the one or more
alloying elements are present in the alloy in a total amount of 0.1
to 30 weight percent, wherein the substrate is heated to at least
100.degree. C. during the deposition such that, upon deposition,
particles comprising the one or more alloying elements precipitate,
the particles having an average diameter less than 500 nm and being
present in the alloy in a volume fraction of 0.3 to 30%.
18. The process of claim 17, wherein the alloy comprises Rh and Si,
Rh and Al, Rh and Zr, Rh and Y, Rh and Sm, Au and Si, Au and Dy, Au
and La, Pt and La, Pd and Si, Ru and Hf, Ru and Si, or Ru and
La.
19. A process for fabricating an article, comprising the steps of:
providing a substrate; depositing on a substrate an alloy
comprising one or more noble metal elements, and one or more
alloying elements, wherein the solid solubility of each of the one
or more alloying elements in each of the one or more noble metal
elements is less than 4 weight percent and wherein the one or more
alloying elements are present in the alloy in a total amount of 0.1
to 30 weight percent; and heat treating the deposited alloy at a
temperature of at least 100.degree. C. subsequent to deposition,
such that particles comprising the one or more alloying elements
precipitate, the particles having an average diameter less than 500
nm and being present in the alloy in a volume fraction of 0.3 to
30%.
20. The process of claim 20 wherein the alloy comprises Rh and Si,
Rh and Al, Rh and Zr, Rh and Y, Rh and Sm, Au and Si, Au and Dy, Au
and La, Pt and La, Pd and Si, Ru and Hf, Ru and Si, or Ru and
La.
21. The process of claim 19, wherein the heat treatment increases
the electrical conductivity, compared to the electrical
conductivity of the deposited alloy, by at least 30%.
22. An article comprising at least one electrical contact formed
from an electrically conductive alloy comprising: one or more noble
metal elements; and one or more types of insoluble dispersoid
particles, wherein the insoluble dispersoid particles are present
in the alloy in a total amount of 0.1 to 30 weight percent, and
wherein the mechanical hardness of the alloy is at least 30%
greater than the mechanical hardness of the one or more noble metal
elements alone.
23. The article of claim 22, wherein the dispersoid particles range
in size from 2 to 500 nm.
24. The article of claim 23, wherein the dispersoid particles range
in size from 2 to 50 nm.
25. The article of claim 22, wherein the noble metal elements are
selected from the group consisting of gold, rhodium, platinum,
palladium, ruthenium, and silver, and the dispersoid particles are
selected from the group consisting of oxides, nitrides, carbides,
and fluorides.
26. The article of claim 22, wherein the alloy exhibits an
electrical conductivity that is at least 50% of the electrical
conductivity of the one or more noble metal elements alone.
27. The article of claim 26, wherein the alloy exhibits an
electrical conductivity that is at least 80% of the electrical
conductivity of the one or more noble metal elements alone.
28. The article of claim 22, wherein the article comprises a
microrelay device comprising at least two contacts formed from the
electrically conductive alloy.
29. The article of claim 28, wherein the microrelay device further
comprises a base structure having one of the contacts located
thereon and a cantilever structure having another of the contacts
located thereon.
30. The article of claim 22, wherein the contact is formed on an
adhesion promoting layer that is formed on a substrate.
31. A process for forming an article, comprising the step of
forming at least one electrical contact from an electrically
conductive alloy that comprises one or more noble metal elements
and one or more types of insoluble dispersoid particles, the
insoluble dispersoid particles being present in the alloy in a
total amount of 0.1 to 30 weight percent, and the mechanical
hardness of the resultant alloy being at least 30% greater than the
mechanical hardness of the one or more noble metal elements
alone.
32. The process of claim 31, wherein the step of forming the at
least one contact comprises: providing a substrate; and forming a
film on the substrate by co-sputtering from a first target
comprising the one or more noble metal elements and from a second
target comprising a dispersoid material insoluble in the one or
more noble metal elements.
33. The process of claim 31, wherein the step of forming the at
least one contact comprises: providing a substrate; and forming a
film onto the substrate by sputtering from a target that comprises
the one or more noble metal elements and a dispersoid-forming
material, wherein the sputtering is performed in an atmosphere
comprising one or more gases reactive with the dispersoid-forming
elements, such that the insoluble dispersoid particles are formed
in the deposited film.
34. The process of claim 33, wherein the atmosphere comprises at
least one gas selected from the group consisting of
oxygen-containing gases, nitrogen-containing gases, and
fluorine-containing gases.
35. The process of claim 31, wherein the step of forming the at
least one contact comprises: providing a substrate; forming on the
substrate a film comprising the one or more noble metal elements
and one or more dispersoid-forming elements selected from the group
consisting of Al, Ti, Si, Zr, and rare earth elements; and heat
treating the film in an oxidizing atmosphere to induce formation of
oxide dispersoid particles from the dispersoid-forming elements.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority of Provisional application
Ser. No. 60/161290 which was filed Oct. 25, 1999 and Provisional
application Ser. No. 60/161291 which was filed Oct. 25, 1999.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The invention relates to devices comprising electrical
contact materials, in particular, high-speed switching relays,
including relays based on microelectromechanical systems
(MEMS).
[0004] 2. Discussion of the Related Art
[0005] High-speed switching relays are useful in a variety of
applications in automatic test equipment, automotive technologies,
and telecommunications technologies--e.g., signal routers,
impedance matching networks, and tunable filters. Today,
microrelays can be manufactured by microelectromechanical systems
(MEMS) technology. In this technology, devices and components are
fabricated from silicon using well-developed techniques related to
integrated circuit processing. Microelectromechanical relays are
receiving increased attention because of their advantages over
conventional relays--i.e., smaller size and less power consumption.
In addition, their potential is being realized in the integration
of these systems with electronics, where large numbers of relays
can exist on a single chip that may contain other electronics as
well.
[0006] The advantages of MEMS microrelays over semiconductor
switches include lower on-resistance, higher off-resistance, higher
dielectric strength, lower power consumption, and lower cost. (S.
Hannoe et al., Proceeding of the International Symposium on
Microsystems, Intelligent Materials and Robots 7th Symposium,
173-176 (1995)) Specifically, conventional switches using
transistors have relatively low breakdown voltage (e.g., 30 V) and
relatively low off-resistance (50 kilo-Ohms at 100 MHz), as
discussed in U.S. Pat. No. 5,578,976. Solid state switches tend to
exhibit large on-state loss and poor off-state isolation. In
addition to replacing conventional relays, microrelays have the
potential to replace traditional solid state devices in significant
markets. In addition, complex switching arrays and devices designed
to accommodate high-frequency signals with low loss are a natural
extension for such MEMS relays.
[0007] Switches for telecommunications applications require a large
dynamic range between on-state and off-state impedance, in the RF
regime. Achieving a large on/off impedance ratio requires good
electrical contact with minimal resistance when the switch is on
(closed circuit) and low parasitic capacitive coupling when the
switch is off (open circuit). Mechanical switches with
metal-to-metal contact are still preferred where low insertion loss
and high off-isolation are required, particularly in cost-sensitive
applications. A key to the success of these relays is reliable,
wear-resistant contact materials.
[0008] Historically, gold alloys have been successful in electrical
contact materials for reed switches (R. G. Baker and T. A. Palumbo,
Plating and Surface Finishing, 70, 63-66 (1983)). This is
especially true for applications requiring low-force, low voltage
contacts that cannot tolerate the buildup of tarnish films that can
modify contact resistance. However, microrelay switches are
expected to experience billions of on-off cycles. This design
expectation renders soft materials, like pure gold, ineffective,
e.g., because of local welding and stiction problems caused by
friction and wear. As a result, new materials must be developed to
provide good electrical contact and wear-resistance. Noble metal
contact materials and their alloys, while possessing desirably low
electrical resistivities and oxidation resistance, are not always
suited for the sliding low-voltage low-current electrical contacts
used in MEMS microrelay switches. In particular, wear resistance is
often questionable. Thus, improved noble metal alloys are desired
as contact materials for reliable microrelay switches.
SUMMARY OF THE INVENTION
[0009] The invention provides a device having electrical contacts
formed from an alloy having improved wear resistance. The contact
alloy is particularly useful in microrelay devices formed by MEMS
technology.
[0010] The alloy contains at least one noble metal, such as
rhodium, platinum, palladium, gold, or ruthenium. (Noble metals
includes Au, Ag, Pt, Pd, Ir, Rh, Ru, Os.) Noble metals are known to
be useful as contact materials due to high oxidation- and
corrosion-resistance and reasonable electrical conductivity. But
because the contact materials in microrelay devices typically
undergo billions of cycles of contact involving friction and wear,
the contact materials must meet extremely high standards for wear
resistance. By selecting particular alloying elements, along with
providing a particular microstructure, it is possible to attain
such higher wear resistance and still maintain a suitable
electrical conductivity.
[0011] In one embodiment, the alloys are chosen to allow sufficient
precipitation hardening to improve wear resistance, but keep
precipitation below a level that would unacceptably reduce
electrical conductivity. This is achieved by using alloying
materials that have very limited or no solid solubility in the
noble metal matrix, e.g., less than 4 wt. % solid solubility. The
low solubility reduces the extent to which the conductivity will be
compromised by conventional alloying effects, i.e., deterioration
of the electrical conductivity due to solute atoms. And the low
solubility also reduces the extent to which the second phase
(precipitates) are able to coarsen (gow) during heat treatment,
repeated contact operations, and similar processes. Yet, it is
still possible to form sufficient precipitates in the noble metal
matrix to achieve the desired mechanical strengthening and wear
resistance. The resulting alloy exhibits a mechanical hardness at
least 30% higher than the noble metal matrix alone.
[0012] In a second embodiment, an alloy contains a noble metal
matrix and insoluble, dispersoid particles, which offer a similar
strengthening mechanism--e.g., mechanical hardness at least 30%
higher than the noble metal matrix alone. (Insoluble indicates a
material having less than 0.01 wt. % solubility in the noble metal
matrix.) Examples of dispersoid particles include oxides, nitrides,
and carbides. Because the dispersoids have essentially no
solubility in the matrix, the particles remain in the matrix to
impede the motion of dislocations and grain boundaries, thereby
strengthening the material.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] FIG. 1 illustrates an embodiment of a microelectromechanical
(MEMS) based microrelay.
[0014] FIG. 2A illustrates a two component temperature-composition
phase diagram, reflecting limited solid solubility of the alloying
element.
[0015] FIG. 2B illustrates a two component temperature-composition
phase diagram with essentially no solid solubility of the alloying
element, reflecting the creation of an intermetallic phase upon
annealing, according to a first embodiment of the invention.
[0016] FIG. 3A schematically illustrates the microstructure of a
supersaturated noble-metal based alloy of one embodiment of the
invention, in an as-deposited state.
[0017] FIG. 3B schematically illustrates the microstructure of a
noble-metal based alloy of one embodiment of the invention, after
precipitation heat treatment.
[0018] FIG. 4 schematically illustrates the microstructure of a
dispersoid containing alloy according to a second embodiment of the
invention.
[0019] FIG. 5 illustrates a process useful for forming an alloy
according to the second embodiment of the invention.
[0020] FIG. 6 illustrates another process useful for forming an
alloy according to the second embodiment of the invention.
[0021] FIG. 7 illustrates a further process useful for forming an
alloy according to the second embodiment of the invention.
DETAILED DESCRIPTION OF THE INVENTION
[0022] Microelectromechanical relays offer the advantage of being
smaller and consuming less power than conventional larger-scale
relays. These relays perform an electrical function of connecting
electrical paths for current to traverse, by actuation and bending
of a cantilever-type switching arm such that circuit connection
occurs at contacts made of specialized contact materials. Repeated
contact at these contact points and the continual emission of
current demand careful selection and fabrication of contact
materials. The invention involves use of particular contact
materials containing alloyed noble metals, the alloying and
associated microstructure allowing attainment of greater mechanical
strength with a lower loss of conductivity as compared to
conventional alloying.
[0023] FIG. 1 schematically illustrates a microelectromechanical
microrelay device 10 of the invention. In this device, an
electrostatic voltage is applied between the beam electrode 12 and
the actuator electrode 14, which induces an attractive force and
closes the gap between the cantilever 16 and the substrate 18. This
movement of the cantilever 16 connects the electrical circuit by
bringing the two contact pads 20 together. The low-resistivity
contact pads 20, formed from particular wear-resistant alloys of
the invention, provide relatively low power loss and time-dependant
degradation. In this embodiment, the switch is fabricated on a
silicon substrate using conventional microfabrication techniques
such as masking, etching, lift-off and deposition. (See, e.g., M.
J. Madou, Fundamentals of microfabrication, (1997) ISBN
0-8493-9451-1.) The switch components are formed by thin-film
deposition buildup or by etching away surrounding material. The
actuating part is composed of a cantilever arm 16, typically formed
from semiconducting or insulating material, such as silicon,
silicon dioxide or silicon nitride, for example. The contact pads
20 are deposited on part of the substrate as well as on part of the
mating cantilever. According to the invention, one or both of the
pads 20 are formed using a thin film of the noble-metal alloy, as
described in more detail below.
[0024] Noble metals, such as rhodium, platinum, palladium, gold,
ruthenium, and silver, are useful as contact materials because
these elements offer high oxidation- and corrosion-resistance and
reasonable electrical conductivity. For instance, these materials
have been used successfully in the reed switch industry. However,
according to the invention, improved contact materials are obtained
by alloying and processing such noble metals in a particular
manner. Rhodium, for example, exhibits relatively good wear
resistance, reasonable electrical conduction properties, relatively
high mechanical strength and a high melting temperature
(1966.degree.C.). While these properties all indicate the
usefulness of rhodium for contact materials, the contact materials
in microrelay devices typically undergo billions of cycles of
contact involving friction and wear. Increased wear-resistance is
therefore desired.
[0025] In a first embodiment, the invention addresses this need by
alloying and precipitation hardening of such noble metals. The
resulting alloys exhibit high mechanical strength due to the
precipitation hardening, yet with relatively low conductivity loss
due to solid-solution-induced effects. (As known in the art,
creation of a solid-solution tends to reduce conductivity of the
matrix material.) In particular, the alloying materials for use
with noble metals have very limited or no solid solubility in the
noble metal matrix. The advantages of selecting alloys with very
low solubility limits are two-fold. First, the low solubility
reduces the extent to which the conductivity will be compromised by
conventional alloying effects, i.e., substantial deterioration of
the electrical conductivity of elemental metals due to addition of
solute atoms. Second, the low solubility reduces the extent to
which the second phase (precipitates) are able to coarsen during
heat treatment, repeated contact operations, local heating,
exposure to ambient temperature rise, and similar environmental
changes. Such coarsening is undesirable because of potential
deterioration of mechanical hardness and wear resistance.
[0026] For example, FIGS. 2A and 2B illustrate phase diagrams for
alloys that provide the desired properties. In FIG. 2A, which
reflects, for example, the rhodium-silicon system, there is very
limited solid solubility of the alloying element around room
temperature. In FIG. 2B, which reflects, for example, the
rhodium-aluminum, or rhodium-boron system, there is essentially no
solid solubility of the alloying element in the rhodium matrix
around room temperature and up to several hundred degrees Celsius.
As reflected in the phase diagrams, there exist higher temperature
regions where it is possible to decompose a metastable solid
solution and therefore form a second phase with moderate heating
schemes. Within these temperature regions, the features of the
alloys can be easily tailored.
[0027] Several metals with phase diagrams similar to that depicted
in FIGS. 2A and 2B exist. The desired solid solubility of alloying
elements in the noble metal matrix is less than 4 weight percent,
advantageously less than 2 weight percent and more advantageously
less that 0.5 weight percent, at or near room temperature (about
25.degree.C.). Typical compositions of the noble metal-based
contact alloys contain 0.1-30 weight percent of the alloying
element, advantageously 0.5-10 weight percent, and more
advantageously 1-5 weight percent, with the particular amount
varying depending on, among other things, the particular solubility
and the desired degree of alloy strengthening. Control samples are
easily prepared to allow one to tailor the properties to a
particular application.
[0028] Rhodium-based alloy systems suitable for the wear-resistant
contact materials of the invention, in which alloying elements have
limited solubility in rhodium, include Rh--C, Rh--Ce, Rh--Dy,
Rh--Y, Rh--Si, Rh--Zr, Rhodium-based alloy systems in which the
alloying elements have essentially no solubility in rhodium include
Rh--Al, Rh--B, Rh--Bi, Rh--Er, Rh--Gd, Rh--Ge, Rh--Pb, Rh--Sm, and
Rh--Yb. In these systems, the precipitates formed are typically
intermetallic compounds according to the phase diagrams. (See,
e.g., Binary Alloy Phase Diagrams, 2nd Ed., ASM International
(1990).) Alloying systems to be avoided include R--Co, R--Cr,
R--Ir, R--Cu, R--Fe, R--Hf, R--Mn, R--Mo, R--Nb, R--Ni, R--Os,
R--Pd, R--Ru, R--Sb, R--Sn, R--Ta, R--Ti, R--V, R--W, as these
alloying elements have substantial residual solubility in rhodium
even after precipitation, and thereby deteriorate the electrical
conductivity of the rhodium.
[0029] For gold (Au) based systems, suitable alloying systems with
limited solubility include Au--C, Au--Co, Au--Ho, Au--Lu, Au--Th,
Au--Mn, Au--Re, Au--Rh, Au--Ru, Au--Sb, Au--Yb, Au--Y. Systems with
essentially no solubility include Au--B, Au--Bi, Au--Dy, Au--Er,
Au--La, Au--P, Au--Pb, Au--Si, Au--Sr, Au--W. Alloy systems to be
avoided include Au--Ag, Au--Al, Au--Fe, Au--In, Au--Li, Au--Mg,
Au--Nb, Au--Ni, Au--Pd, Au--Pt, Au--Sn, Au--Ta, Au--Ti, Au--V,
Au--Zn, Au--Zr.
[0030] For platinum (Pt) based systems, suitable alloys with
limited or no solubility include Pt--B, Pt--Bi, Pt--Er, Pt--Pb,
Pt--La, Pt--Mo, Pt--Ti (B and Ti having limited solubility and the
others having essentially no solubility). Systems to be avoided
include Pt--Al, Pt--Nb, Pt--Ni, Pt--Os, Pt--Mn, Pt--Mo, Pt--V.
[0031] For palladium--based (Pd) systems, suitable alloys include
Pd--B, Pd--Bi, Pd--Si (B having limited solubility and the others
having essentially no solubility). Systems to be avoided include
Pd--Fe, Pd--Al, Pd--C, Pd--Hf, Pd--Mn, Pd--Ir, Pd--Y, Pd--Dy,
Pd--Ho, Pd--Ta, Pd--Mo, Pd--Nb, Pd--Ni, Pd--Pb.
[0032] For ruthenium--based (Ru) systems, suitable alloys include
Ru--C, Ru--Ce, Ru--Hf, Ru--Lu, Ru--La, Ru--Si, Ru--Bi, Ru--Gd (C,
Ce, and Hf having limited solubility, and the others having
essentially no solubility). Systems to be avoided include Ru--Co,
Ru--Cr, Ru--Mo, Ru--Ti, Ru--Fe, Ru--Ir, Ru--Nb, Ru--Ni, Ru--Os,
Ru--Re.
[0033] When such alloys, for example, Au--B, Rh--Al or Rh--Si, are
deposited as a thin film at an ambient or low temperatures, the
alloy tends to form as a metastable, supersaturated solid solution,
well beyond the thermodynamically-allowed solid solubility limit.
Due to this instability, the forced-in alloying element
precipitates out whenever thermal movement of atoms is allowed, and
the desired alloy decomposition, i.e., precipitation, therefore
occurs. It is possible for this decomposition to occur during thin
film deposition, e.g., by heating the substrate, or alternatively
during a post-deposition heat treatment. The presence of
precipitates provides a strengthening mechanism--the precipitates
impede the movement of dislocations and grain boundaries, and the
alloys are therefore harder than the elemental noble metal itself
or solid-solutioned noble metal alloys. The contribution of the
precipitates to improving the mechanical strength of the films also
improves the wear resistance of the films. An additional advantage
of using such a forced-in alloy system is that the noble metal
matrix becomes depleted with the alloying element as alloy
decomposition occurs. Thus, the electrical conductivity of the
alloy improves after such precipitation, to a relatively high value
that is often comparable to the conductivity of the noble metal
itself.
[0034] In fact, the techniques for providing alloy strengthening
and improved wear resistance according to the invention are able to
provide a relatively low loss of electrical conductivity, contrary
to conventional alloying practices. In particular, the electrical
conductivity of the alloy of this embodiment is typically at least
15% of the conductivity of the pure element noble metal,
advantageously at least 30% of the conductivity of the pure
element, more advantageously at least 50% of the conductivity of
the pure element. The mechanical hardness of the alloy should be at
least 30% greater than the hardness of the pure element,
advantageously at least 50% greater than the hardness of the pure
element, more advantageously at least 100% greater than the
hardness of the pure element. The hardness of the alloy should be
at least 15% greater than the hardness of the same alloy but in
single phase form, advantageously 30% greater, and more
advantageously at least 60% greater.
[0035] The microstructure of the alloys of this first embodiment
contain a fine distribution of precipitate particles, the particles
typically less than 500 nm in average diameter, advantageously less
than 150 nm, more advantageously less than 50 nm. The precipitate
particles are typically present in a volume fraction of 0.1% to
30%, advantageously 0.5% to 5%. (Greater than 30% tends to
deteriorate the conductivity and cause embrittlement.)
[0036] This microstructure is generally obtained by one of two
processing techniques. The first is to create such a structure in
situ by film deposition (e.g., sputtering) with the substrate
temperature sufficiently high (typically greater than 100.degree.
C., advantageously greater than 200.degree. C.) to drive the
segregation of the second phase precipitate during the deposition.
In this case, no post-deposition annealing process is needed.
[0037] The second approach is to prepare the thin film with the
supersaturated alloying element as a solid solution in the noble
metal matrix, and then, after deposition of the film, perform a
post-deposition heat treatment to decompose the alloy and create
the desired precipitates. Upon heating, the alloy will be in a
two-phase region and precipitates will form. Continuous heating and
cooling, or isothermal holding at a constant temperature, are both
possible. Typical decomposition temperatures are 100-800.degree.
C., advantageously 100-400.degree. C. Typical decomposition time is
0.01-100 hours, advantageously 0.1-20 hours. A schematic
illustration of the microstructures of the metal before and after
decomposition heat treatment is illustrated in FIGS. 3A and 3B. In
FIG. 3A, the as-deposited (but not heat treated) microstructure 30
contains forced-in solute atoms, but no precipitates. In FIG. 3B,
the post-heat treatment microstructure 40 contains the desired
precipitates 42. The heat treatment generally improves the
electrical conductivity, compared to the as-deposited alloy, by at
least 30%, due to movement of solute atoms from the matrix to the
precipitates.
[0038] The alloys of this embodiment are formed into thin films
appropriate for contacts by any suitable technique, including
sputtering, thermal evaporation, electron-beam evaporation, laser
ablation, glow discharge, ion plating, ion-beam assisted
deposition, ion-cluster beam techniques, chemical vapor deposition,
electrolytic deposition, and electroless plating.
[0039] In a second embodiment, the improved contact alloy is a
composite structure containing a noble metal or alloy matrix and
insoluble, non-coarsening dispersoid particles. The composite
structure provides high mechanical strength with substantially no
solid-solution-induced loss of electrical conductivity.
[0040] The materials selected for the dispersoids have essentially
no solid solubility in the noble metal matrix. FIG. 4 illustrates a
typical microstructure for inventive composite-structured contact
metal with desirable microstructural characteristics. The matrix 50
is a noble elemental metal or alloy within which nano-scale,
insoluble dispersoids particles 52 are distributed. The dispersoid
particles impede the motion of dislocations or grain boundaries,
thus increasing the contact metal strength and wear resistance. The
insolubility reduces the possibility that the conductivity will be
compromised, in contrast to conventional alloying in which the
electrical conductivity generally deteriorates due to solute atom
additions. The insolubility also reduces the extent to which the
second phase (dispersoid particles) can coarsen (grow) during heat
treatment, repeated contact operations, local heating, exposure to
ambient temperature rise, and similar actions, which is desirable
since such coarsening typically causes a deterioration in strength
and wear resistance.
[0041] It is possible for the noble metal matrix to be a pure
element, an alloy of several noble metal elements, or multilayer
thin film structure of such elements. Dispersoid particles include
oxides, nitrides, carbides, sulfides, and fluorides, as well as
other insoluble stable compounds such as oxynitrides or
oxycarbides. Such materials are capable of being produced by
reaction of various elements with reactive gases, e.g., oxygen,
air, or water in the case of oxides; nitrogen or ammonia in the
case of nitrides; methane, acetylene, or propane in the case of
carbides; H.sub.2S in the case of sulfides; and HF or CF.sub.4 in
the case of fluorides. Particular dispersoid materials include
Al.sub.2O.sub.3, SiO.sub.2, TiO.sub.2, ZrO.sub.2, HfO.sub.2, MgO,
Y.sub.2O.sub.3, In.sub.2O.sub.3, SnO.sub.2, GeO.sub.2,
Ta.sub.2O.sub.5, AIN, BN, Si.sub.3N.sub.4, TiN, ZrN, TaN, WC, SiC,
TaC, TiC, ZrC, CdS, CuS, ZnS, and MgF.sub.2, CaF.sub.2, CeF.sub.3,
ThF.sub.4. Oxycarbides and oxynitrides of Ti, Ta, Al, and Si, along
with arsenides, amorphous phases, intermetallics, fullerenes, and
nanowires (e.g., carbon nanotubes) are also capable of acting as
dispersoid material. The dispersoid particles typically range in
size from 1 to 5000 nm, advantageously 2 to 500 nm, more
advantageously 2 to 50 nm. The dispersoid particles are typically
present in the alloy in an amount ranging from 0.1 to 30 volume
percent, advantageously 0.5 to 5 volume percent. Too high a volume
fraction, for example in excess of 30%, tends to deteriorate the
electrical conductivity of the noble metal, and also tend to lead
to undesirable embrittlement of the contact material.
[0042] The alloys of this second embodiment are able to be
fabricated into a thin film configuration by any suitable
technique, e.g. sputtering, thermal evaporation, electron-beam
evaporation, laser ablation, glow discharges, ion plating, ion-beam
assisted deposition, ion-cluster beam techniques, chemical vapor
deposition, electrolytic deposition or electroless plating with
simultaneous trapping during deposition of dispersoid particles
mixed in the liquid.
[0043] A particularly useful process for preparing the alloy of
this second embodiment is co-deposition or co-sputtering from two
separate targets--one containing the noble metal matrix material
and the other containing the dispersoid material, as illustrated in
FIG. 5. The Figure shows sputtering, e.g., dc-magnetron sputtering,
of a noble metal target 60 and sputtering, e.g., rf-sputtering, of
an oxide target 62 being simultaneously performed to obtain a
composite structure having dispersoid particles 66 within a noble
metal matrix 68. It is possible for the co-deposition to be carried
out on a heated substrate 64, e.g. heated to a temperature of 50 to
500.degree. C., to accelerate the formation of stable dispersoid
compound during the film deposition. Alternatively, it is possible
to first form a composite film at room temperature and then perform
a post-deposition heat treatment to aid in the formation of stable
dispersoid particles of a desired size, e.g., by heating at 100 to
500.degree. C. for 1 to 10,000 minutes. Such post-deposition heat
treatment is advantageously carried out either in an inert
atmosphere (such are argon, helium or nitrogen), in a reducing
atmosphere (such as H.sub.2, H.sub.2+N.sub.2, CO), or in a vacuum,
to reduce or avoid oxidation of the contact metal film surface.
[0044] Another technique for forming the alloy of this second
embodiment is an in-situ reactive deposition technique, such as
reactive sputtering or evaporation in an oxygen-containing
atmosphere, as reflected in FIG. 6. The sputtering target 70
contains both the noble metal 72 and the reactive
dispersoid-forming metal 74, for example, an Rh-Al alloy target or
a target having separate areas of Rh and Al. During deposition, the
Al atoms react with oxygen atoms in the atmosphere and form
Al.sub.2O.sub.3 particles 76, which are trapped and incorporated in
the unreactive Rh metal film 78 deposited on the substrate 80. A
variety of dispersoid-forming metals are possible, including Al,
Ti, Si, Mg, Zr, W, Ta, and Y. These elements are capable of
reacting with oxygen, nitrogen or fluorine atoms in a deposition
chamber to form the corresponding oxide, nitride or fluoride. It is
also possible to vary this technique to deposit films in a
oxidizing plasma, i.e., use an rf-excited discharge and an oxygen
pressure of less than one Torr to generate oxide particles within
the matrix material.
[0045] Another technique suitable for preparing the composite alloy
of this second embodiment is an internal oxidation process. As
illustrated in FIG. 7, a pure noble metal or noble metal alloy film
90 containing one or more strongly oxidizing elements such as Al,
Ti, Si, Zr, or rare earth elements is deposited onto a substrate
92, with atoms of these oxidizing elements metastably incorporated
in the noble metal matrix. The film 90 is then subjected to a heat
treatment in a chamber 94 filled with an oxidizing atmosphere such
as O.sub.2, O.sub.3 or Ar+O.sub.2, such that sufficient oxygen
atoms diffuse into the deposited film 90 to react with the
oxidizable element atoms and form oxide dispersoid particles
96.
[0046] The alloys of this second embodiment exhibit a relatively
low loss of electrical conductivity, versus the matrix metal alone,
while offering improved wear resistance. The conductivity of the
alloy is typically at least 50% of the conductivity of the noble
metal matrix alone, advantageously at least 80%, more
advantageously at least 90%. The mechanical hardness of the alloy
is typically at least 30% greater than that of the noble metal
matrix alone, advantageously at least 50% greater, more
advantageously at least 100% greater.
[0047] The alloys of the invention, when used as contact materials,
e.g., in MEMS relays, are typically formed as a film having a
thickness of 1 to 10,000 nm, advantageously 1 to 1000 nm. However,
noble metals, because of their inert nature, tend to exhibit poor
adhesion to substrates, particularly when deposited as a thin film,
and often delaminate. To improve the adhesion between the noble
metal alloys of the invention and substrate surfaces, e.g., in
microrelay devices, it is possible to first deposit one or more
adhesion-promoting layers onto the substrate. Useful metals for
such adhesion-promoting layers include chromium, titanium,
tantalum, zirconium and alloys containing such metals. To provide
smooth transition from the adhesion-promoting layer to the contact
alloy, it is possible to add a small amount of these adhesion
promoting elements to the alloy, e.g., 0.1 to 5 weight percent,
advantageously 0.1 to 1 weight percent. The typical thickness of
the adhesion-promoting layer is 1 to 1000 nm, advantageously 1 to
100 nm.
[0048] To simplify fabrication of the contacts, it is often
desirable to use alloying systems that do not require such
adhesion-promoting measures. It is possible to attain both
self-adherence and wear resistance by selection of alloying
elements. Useful matrix-alloying element combinations for providing
these properties include Rh--Si, Rh--Al, R--Zr, R--Y, R--Sm, A--Si,
A--Dy, A--La, P--Ti, P--La, P--Si, R--Hf, R--Si, and R--La. It is
notable that several of these alloying elements are suitable for
forming precipitates according to the first embodiment of the
invention, or insoluble dispersoids according to the second
embodiment, depending on the formation process. Alloys containing
Si are particularly useful for MEMS relay contacts, as the
substrate surface in MEMS devices typically are silicon dioxide
(SiO.sub.2) or silicon (Si), onto which contact alloys containing
silicon bond well.
[0049] Other embodiments of the invention will be apparent to those
skilled in the art from consideration of the specification and
practice of the invention disclosed herein.
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