Title:
Micromechanical device with gold alloy contacts and method of manufacture
Kind Code:
A1


Abstract:
A MEMS switch device is made using a gold alloy as the switch contact material. The increased mechanical hardness of the alloy compared to the pure gold prevents the contacts of the switch from welding together. A scrubbing action which occurs when the switch closes may allow the contact surfaces to come to rest where their surfaces are complementary, thus resulting in higher contact area and low contact resistance, despite the higher sheet resistance of the gold alloy material relative to the pure gold material.



Inventors:
Carlson, Gregory A. (Santa Barbara, CA, US)
Feierabend, Patrick E. (Santa Barbara, CA, US)
Foster, John S. (Santa Barbara, CA, US)
Grummit, Daryl W. (Santa Barbara, CA, US)
Paranjpye, Alok (Santa Barbara, CA, US)
Rubel, Paul J. (Santa Barbara, CA, US)
Summers, Jeffery F. (Santa Barbara, CA, US)
Thompson, Douglas L. (Santa Barbara, CA, US)
Application Number:
11/785119
Publication Date:
10/16/2008
Filing Date:
04/16/2007
Assignee:
Innovative Micro Technology (Goleta, CA, US)
Primary Class:
International Classes:
F01B29/10
View Patent Images:



Primary Examiner:
CHOUDHRY, MOHAMMAD M
Attorney, Agent or Firm:
Jaquelin K. Spong (Mount Jackson, VA, US)
Claims:
What is claimed is:

1. A micromechanical device, comprising: a first moveable member formed over a substrate surface, the first moveable member having a first contact comprising gold and a first alloying material; and a second contact comprising gold and a second alloying material, wherein the first member moves such that electrical contact is established between the first contact and the second contact, and the first and the second alloying materials are chosen from a group consisting of palladium (Pd), cobalt (Co), copper (Cu), iron (Fe), platinum (Pt), bismuth (Bi), iridium (Ir), silver (Ag), and tungsten (W), rhodium (Rh), ruthenium (Ru), nickel (Ni), cadmium (Cd) and zinc (Zn), at a weight percentage of at least 0.1%.

2. The micromechanical device of claim 1, wherein the first moveable member moves such that surfaces of the first contact and second contact have a component of lateral movement against each other.

3. The micromechanical device of claim 1, wherein the first moveable member is a first thermally actuated cantilevered beam.

4. The micromechanical device of claim 3, wherein the second contact is disposed on a second thermally actuated cantilevered beam formed over the substrate surface.

5. The micromechanical device of claim 1, wherein the first alloying material and the second alloying material comprise palladium with a weight percentage of at most about 10%.

6. The micromechanical device of claim 5, wherein the first alloying material and the second alloying material comprise palladium at a weight percentage of about 2%.

7. The micromechanical device of claim 4, wherein the first thermally actuated cantilevered beam is less stiff than the second thermally actuated cantilevered beam.

8. The micromechanical device of claim 1, wherein the first contact and the second contact have angled adjacent faces which maintain engagement of the first and second contacts.

9. The micromechanical device of claim 4, wherein the first thermally actuated cantilevered beam and the second thermally actuated cantilevered beam each comprises a current-carrying drive loop and a signal-carrying passive beam.

10. The micromechanical device of claim 9, wherein the first thermally actuated cantilevered beam is designed to move about 8 μm when actuated, and the second thermally actuated cantilevered beam is designed to move about 5 μm when actuated.

11. The micromechanical device of claim 9, wherein the first thermally actuated cantilevered beam and the second thermally actuated cantilevered beam comprise at least one of nickel and a nickel alloy.

12. A method of making a micromechanical device, comprising: forming a first moveable member over a substrate surface; forming a first contact on the first moveable member, the first contact comprising gold and a first alloying material; and forming a second contact comprising gold and a second alloying material, wherein the first moveable member is configured to move such that electrical contact is established between the first contact and the second contact, and wherein the first and the second alloying materials are chosen from a group consisting of palladium (Pd), cobalt (Co), copper (Cu), iron (Fe), platinum (Pt), bismuth (Bi), iridium (Ir), silver (Ag), and tungsten (W), rhodium (Rh), ruthenium (Ru), nickel (Ni), cadmium (Cd) and zinc (Zn), at a weight percentage of at least 0.1%.

13. The method of claim 12, wherein forming the first and second contacts comprises electroplating a gold/palladium alloy with the palladium at a weight percentage of about 2%.

14. The method of claim 12, further comprising forming a second moveable member over the substrate surface, wherein the second contact is attached to the second moveable member.

15. The method of claim 14, wherein forming the first moveable member and the second moveable member comprises electroplating a first and a second cantilevered beam of at least one of nickel and a nickel alloy.

16. The method of claim 12, wherein forming the first moveable member comprises forming the first moveable member such that surfaces of the first contact and second contact have a component of lateral movement against each other when the first moveable member moves.

17. The method of claim 14, further comprising forming the first moveable member such that it is less stiff than the second moveable member.

18. The method of claim 14, wherein forming the first contact and the second contact further comprises forming a first and a second contact having angled adjacent faces which maintain engagement of the first and second contacts.

19. A method of using the micromechanical device of claim 7, comprising: energizing the second thermally actuated cantilevered beam; energizing the first thermally actuated cantilevered beam; de-energizing the second thermally actuated cantilevered beam; and allowing the second thermally actuated cantilevered beam to relax against the first thermally actuated cantilevered beam to close the device.

20. A micromechanical device, comprising: a first moveable member formed over a substrate surface, the first moveable member having a first contact; and a second contact formed over the substrate surface, wherein the first member moves such that electrical contact is established between the first contact and the second contact, and moves such that contact surfaces of the first and the second contacts are moved with a component in a lateral direction, parallel to the contact surfaces as electrical contact is established.

Description:

CROSS REFERENCE TO RELATED APPLICATIONS

Not applicable.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

Not applicable.

STATEMENT REGARDING MICROFICHE APPENDIX

Not applicable.

BACKGROUND

This invention relates to microelectromechanical systems (MEMS) devices, and their method of manufacture. More particularly, this invention relates to a contact material for a micromechanical device which forms connections between conductive electrodes.

Microelectromechanical systems (MEMS) are very small moveable structures made on a substrate using lithographic processing techniques, such as those used to manufacture semiconductor devices. MEMS devices may be moveable actuators, valves, pistons, or switches, for example, with characteristic dimensions of a few microns to hundreds of microns. A moveable MEMS switch, for example, may be used to connect one or more input terminals to one or more output terminals, all microfabricated on a substrate. The actuation means for the moveable switch may be thermal, piezoelectric, electrostatic, or magnetic, for example.

FIG. 1 shows an example of a prior art MEMS thermal switch, such as that described in U.S. Patent Application Publication 2004/0211178 A1. The MEMS thermal switch 10 includes two cantilevers, 100 and 200. Each cantilever 100 and 200 contains a passive beam 110 and 210, respectively. A conductive circuit 120 and 220, is coupled to each passive beam 110 and 210 by a plurality of dielectric tethers 150 and 250, respectively. When a voltage is applied between terminals 130 and 140, a current is driven through conductive circuit 120. The Joule heating generated by the current causes the circuit 120 to expand relative to the unheated passive beam 110. Since the circuit is coupled to the passive beam 110 by the dielectric tether 150, the expanding conductive circuit drives the passive beam laterally in the upward direction 165.

Applying a voltage between terminals 230 and 240 causes heat to be generated in circuit 220, which drives passive beam 210 laterally in the direction 265 shown in FIG. 1. Therefore, one beam 100 moves in direction 165 and the other beam 200 moves in direction 265. These movements may be used to open and close a set of contacts located on contacts 170 and 270, each in turn located on tip members 160 and 260, respectively. The sequence of movement of contacts 170 and 270 on tip members 160 and 260 of switch 10 is shown in FIGS. 2a-2d, to close and open the electrical switch 10.

To begin the closing sequence, in FIG. 2a, tip member 160 and contact 170 are moved laterally about 10 μm in the direction 165 by the application of a voltage between terminals 130 and 140. In FIG. 2b, tip member 260 and contact 270 are moved laterally about 17 μm in the direction 265 by application of a voltage between terminals 230 and 240. This distance is required to move twice the 5 μm width of the contacts, a 4 μm initial offset between the contacts 170 and 270, and additional margin for tolerances of 3 μm. In FIG. 2c, tip member 160 and contact 170 are brought back to their initial position by removing the voltage between terminals 130 and 140. This stops current from flowing and cools the cantilever 100 and it returns to its original position. In FIG. 2d, tip member 260 and contact 270 are brought back to nearly their original position by removing the voltage between terminals 230 and 240. However, in this position, tip member 160 and contact 170 prevent tip member 260 and contact 270 from moving completely back to their original positions, because of the mechanical interference between contacts 170 and 270. In this position, contact between the faces of contacts 170 and 270 provides an electrical connection between cantilevers 100 and 200, such that in FIG. 2d, the electrical switch is closed. Opening the electrical switch is accomplished by reversing the movements in the steps shown in FIGS. 2a-2d.

SUMMARY

The tip members 160 and 260 with contacts 170 and 270 are generally made of a material with exceptionally low contact resistance, whereas the cantilevers 110, 120, 210 and 220 may be made of a conductive material which is easy to fabricate and has good mechanical properties. Accordingly, tip members 160 and 260 are generally made of plated gold, whereas cantilevers 110, 120, 210 and 220 are made of plated nickel. However, because gold is a relatively soft material, the contacts in MEMS switch 10 often weld together at the junction of their contacts 170 and 270 upon activation. Such cold welding is particularly problematic when large currents are passed through the junction. Since the forces generated by MEMS thermal switch 10 are relatively small, the welding cannot be overcome by attempting to unlatch the switch. Even if unlatching frees the contacts, the deformation of the contact surfaces from the welding may result in variable and unpredictable values for the contact resistance of the device going forward. Thus, the welding of the contact material of the contact 170 and 270 may render the device non-operational or unusable.

A compact MEMS thermal switch is disclosed herein which has contacts made from a gold alloy rather than the plated gold used in the MEMS switch 10 shown in FIG. 1. In one embodiment, the gold alloy is gold/palladium, with about 2% by weight of palladium. Accordingly, the switch described herein may have performance advantages relative to the switch shown in FIG. 1, in that the contacts achieve very low contact resistance but still resist contact welding.

The compact MEMS thermal switch is one embodiment of the more general micromechanical device. The micromechanical device may comprise a first moveable member formed over a substrate surface with a first contact comprising gold with a first alloying material, and a second contact comprising gold with a second alloying material, wherein the first member moves such that electrical contact is established between the first contact and the second contact, and the first and second alloying material are chosen from the group consisting of palladium (Pd), cobalt (Co), copper (Cu), iron (Fe), platinum (Pt), bismuth (Bi), iridium (Ir), silver (Ag), and tungsten (W), rhodium (Rh), ruthenium (Ru), nickel (Ni), cadmium (Cd) and zinc (Zn), at a weight percentage of at least 0.1%. The moveable member may move such that surfaces of the first contact and second contact have a component of lateral motion against each other, and therefore “scrub” the contact surfaces somewhat, which substantially reduces the contact resistance.

In one embodiment, this scrubbing action may achieved by allowing a second thermally actuated, laterally moving cantilevered beam to relax against a first thermally actuated, laterally moving cantilevered beam, which is then held in place by frictional forces. The first thermally actuated, laterally moving cantilevered beam may move a greater distance than the second thermally actuated, laterally moving cantilevered beam, to activate the device by engaging the first contact with the second contact. The contacts disposed on the cantilevered thermal actuators may be tip members made with the gold/palladium alloy.

In the quiescent state, the contacts on the tip members of the cantilevered beams may be directly adjacent to one another. To close the MEMS switch, the second stiffer cantilever swings away to clear the adjacent contact of the tip member of the first cantilever. The first cantilever then deflects into a flexed position, whereupon the second cantilever relaxes to approximately ⅔ of it stroke causing the two contacts to touch thus closing the switch. The relaxation of the second cantilever produces some lateral motion, causing the contact surfaces to move laterally with respect to one another. The second cantilever then holds the first cantilever in the displaced position, despite the restoring force acting upon the first cantilever, thus the switch is latched.

In one exemplary embodiment, frictional forces keep the cantilevers from becoming unlatched. In another exemplary embodiment, the contact surfaces of the cantilevers are angled to prevent unlatching, even in the situation where no friction is present.

Because the switch closes with a lateral motion of the tip members, the contact surfaces may scrub each other slightly, and thus settle where their surfaces are complementary. This may increase the contact area of the tip members, and thus reduce their contact resistance, despite the higher resistance of the bulk alloy compared to pure gold.

These and other features and advantages are described in, or are apparent from, the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

Various exemplary details are described with reference to the accompanying drawings, which however, should not be taken to limit the invention to the specific embodiments shown but are for explanation and understanding only.

FIG. 1 is a schematic view of a prior art MEMS thermal switch;

FIGS. 2a-2d are diagrams illustrating the sequence of movements required to close the switch illustrated in FIG. 1;

FIG. 3 is a schematic view of an exemplary MEMS thermal switch;

FIGS. 4a-4d are diagrams illustrating an exemplary sequence of movements;

FIG. 5 is a schematic view showing greater detail of an exemplary shape of the angled contacts;

FIG. 6 is a plot of the contact resistance of gold contacts of a switch such as that shown FIG. 3;

FIG. 7 is a plot of the contact resistance of gold/palladium contacts of a switch such as that shown in FIG. 3;

FIG. 8 illustrates a first step in the fabrication of the compact MEMS switch with gold/palladium contacts;

FIG. 9 illustrates a second step in the fabrication of the compact MEMS switch with gold/palladium contacts;

FIG. 10 illustrates a third step in the fabrication of the compact MEMS switch with gold/palladium contacts;

FIG. 11 illustrates a fourth step in the fabrication of the compact MEMS switch with gold/palladium contacts; and

FIG. 12 illustrates a fifth step in the fabrication of the compact MEMS switch with gold/palladium contacts.

DETAILED DESCRIPTION

Although the devices and methods described herein are applied to an MEMS cantilevered thermal switch, it should be understood that this is only one embodiment, and that the devices and methods may include any number of micromechanical switch devices requiring electrical contacts. It has been determined that an alloy of gold, for example, gold and palladium may make a particularly advantageous composition for the electrodes of a micromechanical switch, particularly a switch in which the surfaces of the electrodes are scrubbed against each other when the switch is opened or closed. The gold alloy may have increased mechanical hardness, and thus may resist the tendency of the gold contacts to weld together. The increased electrical resistance of the gold alloy contact material may be mitigated in applications wherein the switch contacts are moved laterally across each others' surfaces, in a scrubbing action, which may actually increase the contact area of the switch. Thus, little or no penalty may be paid in terms of increased contact resistance by using the harder gold alloy.

In one exemplary embodiment described below, a gold/palladium alloy material, having about 2% by weight of palladium, is used in a micromechanical switch device, wherein the moveable members move in such a way as to scrub the electrode surfaces laterally. The device makes use of the harder gold/palladium alloy to reduce the incidence of contact welding, while maintaining, or even reducing, the contact resistance of the switch. Although the systems and methods are described with respect to a gold/palladium embodiment, it should be understood that the system and methods may also be applied to gold with other alloying materials, such as cobalt (Co), copper (Cu), iron (Fe), platinum (Pt), bismuth (Bi), iridium (Ir), silver (Ag), tungsten (W), rhodium (Rh), ruthenium (Ru), nickel (Ni), cadmium (Cd) and zinc (Zn).

FIG. 3 is a schematic view of an exemplary laterally moving compact MEMS switch 1000 which uses a scrubbing action to engage the electrodes. Compact MEMS-switch 1000 is described more fully in co-pending U.S. patent application Ser. No. 11/263,912 (Attorney Docket No. IMT-ThermalSwitch) incorporated by reference herein in its entirety. Like the MEMS switch 10 illustrated in FIG. 1, the compact MEMS switch 1000 includes two cantilevers. The first cantilever will be hereinafter referred to as the “latch” cantilever 300, and the second cantilever will be referred to as the “spring” cantilever 400. The latch cantilever 300 and the spring cantilever 400 both include a cantilevered passive beam 310 and 410, respectively, and a drive loop circuit 320 and 420, respectively. The passive beams 310 and 410 and the drive loops 320 and 420 each have proximal and distal ends, with the proximal end of the passive beams 310 and 410 coupled to anchor points 317 and 417, respectively, and the proximal ends of the drive loops 320 and 420 coupled to terminals 330 and 340, and 430 and 440, respectively.

In the embodiment depicted in FIG. 3, the passive beams 310 and 410 will carry the signal being switched. Therefore, the passive beams 310 and 410 may be made of any suitably conductive material. In one exemplary embodiment, nickel or a nickel alloy is chosen because it is a straightforward material to plate, as will be described below. However, in order to isolate the signal carrying passive beams 310 and 410 from the drive loop circuits 320 and 420, the drive loop circuits 320 and 420 will be tethered to the passive beams 310 and 410 by dielectric tethers 350 and 450, respectively, which are electrically insulating but mechanically rigid materials. The drive loop circuits 320 and 420 may also be made of nickel or a nickel alloy, and formed at the same time as the cantilevered passive beams 310 and 410.

The dielectric tethers 350 and 450 may be made of any convenient, non-conducting material which couples the drive loops 320 and 420 to cantilevered passive beams 310 and 410 mechanically, but not electrically. In one embodiment, dielectric tethers 350 and 450 may be made from an epoxy-based photoresist such as SU-8, a negative photoresist developed by IBM of Armonk, N.Y.

As can be seen in FIG. 3, the compact MEMS switch 1000 also includes two tip members 360 and 460, to which two contacts 370 and 470 are affixed. Like the prior art switch 10, the tip members 360 and 460 and the two contacts 370 and 470 may be made of different material than cantilevered passive beams 310 and 410. The material of tip members 360 and 460 and contacts 370 and 470 may be made of a material which has a low contact resistance relative to the material of cantilevered passive beams 310 and 410. In one embodiment, the material of tip members and contacts 360, 460, 370 and 470 is gold/palladium, however, other gold alloy materials may be used as well. The addition of the alloying material to the gold in the tip members 360 and 460 may serve to reduce the tendency of the tip members 360 and 460 to weld together upon contact, by increasing the hardness of the alloy relative to the pure gold metal. Often, the addition of the alloying material also raises the resistivity of the material compared to pure gold. However, as will be described further herein, the addition of some alloying materials to the gold of tip contact 360 and 460 does not appreciably raise the contact resistance of the electrodes, despite the known rise in sheet resistance of gold alloy compared to that of pure gold. It is hypothesized that this advantageous characteristic is the result of the scrubbing action between the contacts of the switch which increases the contact area of the junction, thereby decreasing the contact resistance.

In contrast to MEMS switch 10, in the quiescent state, the two contacts 370 and 470 of compact MEMS switch 1000 are located adjacent to each other, rather than one in front of the other as was the case with the prior art arrangement of the contacts 170 and 270 shown in FIG. 1. The initial position of contact 370 relative to contact 470 is shown schematically in FIG. 4a. By being “adjacent” to each other, it should be understood that the spring contact 470 and latch contact 370 have one dimension longer than the other, and in a quiescent state, surfaces having the shorter dimension are located a minimum distance apart.

Because of the location of contacts 370 and 470 adjacent to one another, contacts 370 does not need to be retracted as was shown in FIG. 2a. Instead, the sequence of motion for the compact MEMS switch 1000 is shown in FIGS. 4b-4d. In FIG. 4b, spring drive loop 420 is energized by applying a voltage to terminals 430 and 440. The resulting current flowing through spring drive loop 420 causes the drive loop 420 to rise in temperature and expand. Because spring drive loop 420 is tethered to spring passive beam 410 by dielectric tethers 450, the expansion of spring drive loop 420 causes the spring passive beam 410 to arc in the direction 465 shown in FIG. 3. This moves the spring tip member 460 along with spring contact 470 as shown in FIG. 4b. However, in contrast to the motion depicted in FIG. 2b, the distance that the spring cantilever 400 must move is only the width of the contact 370, about 5 μm. This has a substantial impact on the mechanical characteristics of the spring beam. Specifically, because the travel distance of spring beam 400 is relatively small, the relative stiffness of spring beam may be made large compared to the stiffness of the latch beam. In one exemplary embodiment, the spring cantilever stiffness is about 140 N/m.

After the spring beam has moved about 5 μm, the latch drive loop 320 is energized by applying a voltage to terminals 330 and 340. This resulting current flowing through latch drive loop 320 causes the latch drive loop to rise in temperature and expand. Because the latch drive loop 320 is tethered to the latch passive beam 310 by dielectric tethers 350, the expansion of latch drive loop 320 causes the latch passive beam 310 to arc in the direction 365 shown in FIG. 3. This moves the latch tip member 360 along with latch contact 370 as shown in FIG. 4c, a total distance of about 8 μm. Because the latch cantilever 300 moves a larger distance than the spring cantilever 400, it may be made relatively flexible.

Several features may contribute to the greater displacement capability of the latch cantilever 300, compared to spring cantilever 400. For example, the latch beam 310 may be made with a narrower hinge portion 315 where it is anchored to the substrate. Rear serpentines 325 in both the outer portion 322 and inner portion 324 of the drive loop 320 again decrease the stiffness of the latch cantilever 300 at its base. Each of these features is designed to increase the displacement of the latch cantilever for a given temperature, at the expense of latch stiffness. However, as will be described further below, the latch beam is not required to have much stiffness, as the contact force for the contacts 370 and 470 will be provided by the spring cantilever 400, rather than the latch cantilever.

The latch cantilever made according to the design shown in FIG. 3 may have a cantilever stiffness of about 14 N/m, compared to the spring cantilever stiffness of about 140 N/m as set forth above. Accordingly, the latch beam generates about 1/10 of the force of the spring beam. In one exemplary embodiment, in the latched position, the latch beam may generate a force of about 60 μN, whereas the spring beam may generate a force of about 500 μN.

For a spring cantilever 400 made according to the design shown in FIG. 3, the displacement of the contact 470 is about 5 μm for a drive current of 180 mA. For a latch cantilever 300 made according to the design shown in FIG. 3, including the drive loop serpentine features and narrowed hinge portion 315, the displacement of the latch contact 370 of the latch cantilever 300 is about 10.5 μm for a 180 mA drive current. This corresponds to an angular deflection of between about 1 and 3 degrees for cantilevers 300 and 400.

In FIG. 4d, the switch is closed by allowing the spring cantilever 400 to relax to at least about 80 percent of its initial displacement. This is achieved by removing the voltage applied to terminals 430 and 440. As the current ceases to flow, the spring drive loop 420 cools, and shrinks, pulling the spring passive beam 410 back to nearly it original position. However, the presence of the latch contact 370 prevents the spring contact 470 from moving further than the position shown in FIG. 4d, in which it is resting against, and engaged with, the latch contact. In this position, the latch contact and the spring contact form an electrical connection, such that an electrical signal is allowed to pass from the latch cantilever 300 to the spring cantilever 400, thereby closing the switch. Opening the MEMS switch 1000 is accomplished by energizing the spring cantilever 400, which releases the latch cantilever 300. The latch cantilever 300 then returns to its initial position shown in FIG. 4a, because of the restoring force of the latch cantilevered passive beam 310.

A comparison of the sequence of motion for the compact MEMS switch 1000 shown in FIGS. 4b-4d with the sequence of motion for the prior art MEMS switch 10 shown in FIGS. 2a-2d reveals that the sequence of motion is one step shorter for the compact MEMS switch 1000 than for the prior art MEMS switch 10. This is because the first retraction step shown in FIG. 2a is absent in the sequence for the compact MEMS switch 1000.

As can be seen in FIGS. 4a-4d, the spring cantilever, and rather than the latch cantilever, provides the force necessary to close the switch. Furthermore, in the closed state, the latch cantilever is held in the deflected position, under tension, by the spring cantilever. In fact, with the design shown in FIGS. 4a-4d, the only force keeping the switch closed is friction between the latch contact 370 and the spring contact 470. For many situations, this may provide a satisfactory and reliable switch. Because the switch is maintained in the closed position by friction, there is also, necessarily, some lateral motion inherent between the surfaces of the contacts 370 and 470 as the switch closes. This lateral motion may be an important element in achieving the low contact resistance, as discussed below.

However, an alternative embodiment for the latch contact 370 and the spring contact 470 which does not rely on friction is the angled compact MEMS switch 2000, depicted in FIG. 5. Angled compact MEMS switch 2000 has a contact junction that forms an angle with respect to the direction of motion of the spring cantilever 610 and the latch cantilever 510. Like the embodiment illustrated in FIGS. 4a-4d, the angled contacts start in the initial position shown in FIG. 4, in which the angled contacts are in an adjacent arrangement, however, in this embodiment, the angle formed on the contact surfaces resists the tendency of the tip members 360 and 460 to separate. However, this angled embodiment also includes some lateral motion of the contacts 370 and 470, upon closure and opening of the switch.

However, an alternative embodiment for the latch contact 370 and the spring contact 470 which does not rely on friction is the angled latch contact 570 and angled spring contact 670. Like the embodiment illustrated in FIGS. 4a-4d, the angled contacts 570 and 670 start in the initial position shown in FIG. 5, in which the angled contacts 570 and 670 are in an adjacent arrangement.

In the closed position, the spring contact 670 holds the latch contact 570 in the deflected position. However, when the switch is closed, the angled contacts form a contact surface 580, which is disposed at an angle with respect to the tip members 560 and 660. The angled contact surface 580 may retain the engagement of the tip members 560 and 660, without relying on friction.

An important feature of compact MEMS switch 1000 may be the motion of the one contact 370 against the other contact 470 when the switch opens or closes. In compact MEMS switch 1000, there is a lateral component of motion, in a direction parallel to the contact surfaces, of the contacts against each other as the switch closes. In the prior art MEMS switch 10, the only motion of the switch upon closure is perpendicular to the contact surfaces 170 and 270. The lateral motion in contact MEMS switch 1000 may accomplish a scrubbing action, which may lower the contact resistance between the surfaces. Accomplishing a scrubbing action in prior art MEMS switch 10 requires additional motion which be must be added to the basic motion of closing the switch, by programming the software controlling the switch accordingly. In compact MEMS thermal switch 1000, this scrubbing action is an inherent feature of the switch closure motion. By “scrubbing action”, it should be understood that the surfaces of contacts 370 and 470 move laterally some amount against each other, with a component in a direction parallel to the contact surface of contacts 170 and 270. This scrubbing action may reduce the contact resistance of the compact MEMS switch by removing debris or by allowing the surfaces to find a position in which their topography is complementary.

It has been determined that the scrubbing action in MEMS switch 1000 may be particularly advantageous to the use of alternative alloy contact materials. For example, palladium may be added to the gold contact material to discourage welding of the contact surfaces. Without the palladium, the malleable gold contact material may weld the contacts together, such that the relatively weak unlatching force generated by MEMS switches such as that shown in FIGS. 3 and 4 are unable to pull the contacts apart, rendering the switch non-functional. Accordingly, systems and methods are described herein for providing a gold alloy, for example, gold/palladium (AuPd) material for use in micromechanical device using electrical contacts.

The addition of an alloying material, such as palladium, to gold to form a conductive alloy material for a micromechanical device is not obvious, because most alloying materials such as palladium are known to raise the electrical resistivity of gold. For example, the addition of 2% by weight of palladium to gold in the form of a 5 μM thick sheet was measured to raise the sheet resistance of the material from about 4.4 milliohms/square for a pure gold sheet to about 6.1 milliohms/square for the 2% gold/palladium alloy, for a rise of about 39%. This negatively impacts the ability of the switch to carry an electrical signal without excessive losses. For many micromechanical switches, the contact resistance is required to be well under 1 ohm, so that using such a gold alloy as the contact material may cause unacceptably high yield losses for excessive contact resistance.

However, the addition of some alloying materials, such as palladium, to gold improves some mechanical properties, in particular, the alloy may form a harder, less malleable material which resists contact welding. Table 1 summarizes the hardness measurements carried out on a film of gold/palladium, compared to a film of pure gold. The applied force for the measurements summarized in Table 1 was about 0.01 kgf (0.098 N), and the Knoop Hardness was calculated according to the standard formula:


KH=(Cp)Force/lengtĥ2

where Cp is a correction factor related to the shape of the indenter, in this case, Cp=1450E3, when the length is measured in microns and the applied force in Newtons.

TABLE 1
Hardness of AuPd compared to Au
Calculated
platedCalibrated lengthKnoop
sampleBaththicknessmicronsHardness
Au—Pd2% wt Au—Pd3.8 um27.6187.1
28.1180.2
27.0194.3
average27.6187.1
Au—Pd2% wt Au—Pd  8 um26.0210.2
26.3206.1
26.0210.2
average26.1208.5
AuECF63 High6.7 um34.3120.6
PH
30.4153.6
30.9148.4
average31.9139.9
AuES 256.7 um30.7151.0
31.7141.2
35.4113.7
average32.8132.4

As can be seen in Table 1, the addition of 2% palladium by weight to the gold increases the Knoop hardness by over 30%.

While the increase in hardness of the contact material is generally an advantageous feature, gold/palladium alloys are not used because the sheet resistance measurements lead to the expectation of a higher contact resistance for gold/palladium contacts. However, it has been determined that the addition of palladium to the gold in a gold/palladium alloy may not result in a higher contact resistance switch, particularly for a switch which uses a scrubbing action when opening and/or closing. In this situation, the addition of palladium to the gold may result in a contact surface with a slightly rougher topography. When the surfaces are rubbed together laterally, the mating surfaces may come to rest in a position wherein their topography is complementary, which may actually increase the area of contact between the contact surfaces. Because the palladium prevents adhesion of the surfaces, the stiction between them does not rise to an unacceptable or inoperative level, despite the larger contact area. Thus, the contact surfaces resist welding, yet still mate with very low contact resistance.

FIG. 6 is a histogram of the contact resistance of a population of gold contact electrodes, such as 360 and 460 shown in FIG. 3 and FIGS. 4a-4d. The histogram shows that the population of switches had a mean value of contact resistance of about 0.44 ohms. In contrast, as shown in FIG. 7, a population of gold/palladium contacts actually had a lower contact resistance of only about 0.41 ohms. Not only did the contact resistance not rise as would be expected based on the sheet resistance measurements, the contact resistance of the gold/palladium alloy was actually lower than that of pure gold. It is conjectured that the lower value of the contact resistance is a result of the scrubbing action of the switch, as described above.

An exemplary method for fabricating the MEMS switch 1000 with gold/palladium contacts will be described next. Although the method is described with respect to the gold/palladium embodiment, it should be understood that the method may be applied to other gold alloys, such as gold/cobalt, and using other switch designs, rather than that shown in FIG. 3.

The compact MEMS switch 1000 may be fabricated on any convenient substrate 620, for example silicon, silicon-on-insulator (SOI), glass, or the like. Because in FIGS. 8-12, the compact MEMS switch is shown in cross section, only one of the two cantilevered beams of the compact MEMS thermal switch is shown. However, it should be understood that the second cantilever 300 or 510 may be formed at the same time as, and using identical processes to those used to form the first cantilever 400 which is depicted in the figures.

FIG. 8 illustrates a first exemplary step in the fabrication of the compact MEMS switch 1000. The process begins with the deposition of a seed layer 630 for later plating of the MEMS switch cantilever 400, over the substrate 620. The seed layer 630 may be chromium (Cr) and gold (Au), deposited by chemical vapor deposition (CVD) or sputter deposition to a thickness of 100-200 nm. Photoresist may then be deposited over the seed layer 630, and patterned by exposure through a mask. A sacrificial layer 680, such as copper, may then be electroplated over the seed layer. The plating solution may be any standard commercially available or in-house formulated copper plating bath. Plating conditions are particular to the manufacturer's guidelines. However, any other sacrificial material that can be electroplated may also be used. In addition, deposition processes other than plating may be used to form sacrificial layer 680. The photoresist may then be stripped from the substrate 620.

A second exemplary step in fabricating the compact MEMS switch 1000 is illustrated in FIG. 9. In FIG. 9, the substrate 620 is again covered with photoresist (not shown), which is exposed through a mask with features corresponding to gold/palladium pads 640 and 645 and a gold/palladium tip member 460. The photoresist is then developed to open regions in which the gold/palladium is to be plated. Gold/palladium may be used for the tip members 360 and 460 because it may have lower contact resistance than the material that will form the cantilever 400. As described above, the addition of palladium to the alloy also reduces the tendency of the electrode material to weld together. Although not shown in this view, it should be understood that the features for contacts 370 and 470 may also be formed in this step. The features 460 and 640 will subsequently be plated in the appropriate areas opened in the photoresist. The gold/palladium features 640 may form a bonding ring, which may eventually form a portion of a hermetic seal which may bond a cap or lid wafer (not shown) over the substrate 620 and switch 1000. Thus, the gold alloy material may also be chosen to participate in a metal alloy hermetic seal, such as described in co-pending U.S. patent application Ser. No. 11/211,622 (Attorney Docket No. IMT-Preform), incorporated by reference herein in its entirety. One of the gold/palladium features 645 may also be an external access pad that will provide access to the MEMS switch 1000 electrically, from outside the hermetically sealed structure.

The gold/palladium features 640, 645 and 460 may then be electroplated in the areas exposed by the photoresist using the underlying copper sacrificial layer as a seed layer for the plating, to form gold/palladium features 640, 645 and 460 and any other contact structures needed. The electroplating bath may consist of a solution of commercially available gold sulfite chemistry with palladium salts added to give the desired concentration in the plated film. In this embodiment, a 40 C, pH 8.8, ethylene diamine tetra-acetic acid (EDTA) buffered, 9.8 g/L gold, 0.8 g/L palladium solution was used (ECF-63 and Pallaspeed concentrate from Technic, Inc. Anaheim, Calif.), with a current density of 0.9 mA/cm2. After plating the features 640, 645 and 460, the photoresist may then be stripped from the substrate 620. The thickness of the gold/palladium features 640, 645 and 460 may be, for example, 1 μm. Although a plating method of deposition is described here, it should be understood that other methods may be used to form the gold alloy tip member 460, as well as the other alloy features 640 and 645, such as ion beam deposition.

FIG. 10 illustrates a third step in fabricating the compact MEMS switch 1000. In FIG. 10, photoresist (not shown) is once again deposited over the substrate 620, and patterned according to the features in a mask. The exposed portions of the photoresist are then dissolved as before, exposing the appropriate areas of the seed layer 630. The exposed seed layer 630 may then be electroplated with nickel or a nickel alloy, such as the NiMn alloy described in U.S. patent application Ser. No. 11/386,733 to form the passive beam 410 and drive loop 420 of the cantilever 400 of the compact MEMS switch 1000. For simplicity, both the passive beam 410 and the drive loop 420 are depicted in FIG. 9 as a single beam 400. The tip member 460 will be affixed to the cantilevered passive beam 410 by the natural adhesion of the gold/palladium to the nickel, after deposition. Although nickel is chosen in this example, it should be understood that any other conductive material that can be electroplated may also be used. In addition, deposition processes other than plating may be used to form conductive cantilever 400. The photoresist may then be stripped from the substrate 620. Although for simplicity, only a single beam 400 is depicted in FIG. 9, it should be understood that the second cantilevered beam 300, with its constituent parts passive beam 310 and drive loop 320, may be fabricated at the same time and using the same or similar process steps as those used to form the first cantilevered beam 400.

FIG. 11 illustrates a fourth step in the fabrication of the MEMS switch 1000. In FIG. 11, a polymeric, non-conducting material such as the photoresist SU-8 is deposited over the substrate 620, and cantilevered beam 400, including passive beam 410 and drive loop 420. The photoresist is then cross-linked, by for example, exposure to UV light. The unexposed resist is then dissolved and removed from the substrate 620 and structure 400 in all areas that the dielectric tether is absent. This step forms the dielectric tether 450, that tethers drive loop 420 to cantilevered passive beam and 410. The photoresist may then be cured by, for example, baking.

Although not shown, it should be understood that dielectric tethers 350, which tether passive beam 310 and drive loop 320, may be formed in a manner similar to that described above for dielectric tether 450, tethering passive beam 410 and drive loop 420.

FIG. 12 illustrates a fifth step in the fabrication of the MEMS switch 1000. In this step, the cantilever 400 and tip member 460 may be released by etching the sacrificial copper layer 680. Suitable etchants may include, for example, an isotropic etch using an ammonia-based Cu etchant. The Cr and Au seed layer 630 is then also etched using, for example, a wet etchant such as iodine/iodide for the Au and permanganate for the Cr, to expose the SiO2 surface of the substrate 620. The substrate 620 and MEMS switch 1000 may then be rinsed and dried.

The resulting compact MEMS device 1000 may then be encapsulated in a protective lid or cap wafer. Details relating to the fabrication of a cap layer may be found in co-pending U.S. patent application Ser. No. 11/211,625, (Attorney Docket No. IMT-Interconnect) incorporated by reference herein in its entirety.

It should be understood that one gold/palladium feature 645 may be used as an external access pad for electrical access to the MEMS switch 1000, such as to supply a signal to the MEMS switch 1000, or to supply a voltage the terminals 430 or 440 in order to energize the drive loops of the switch, for example. The external access pad 645 may be located outside the bond line which will be formed upon the bonding of a cap layer to the substrate 620. Alternatively, electrical connections to MEMS switch 1000 may be made using through-wafer vias, such as those disclosed in co-pending U.S. patent application Ser. No. 11/211,624 (Attorney Docket No. IMT-Blind Trench) and U.S. patent application Ser. No. 11/482,944 (Attorney Docket No. IMT-RPP Vias) incorporated herein by reference in its entirety.

While various details have been described in conjunction with the exemplary implementations outlined above, various alternatives, modifications, variations, improvements, and/or substantial equivalents, whether known or that are or may be presently unforeseen, may become apparent upon reviewing the foregoing disclosure. While the embodiment described above relates to a thermally actuated microelectromechanical switch, it should be understood that the techniques and designs described above may be applied to any of a number of other microelectromechanical devices. Furthermore, details related to the specific design features and dimensions of the compact MEMS switch are intended to be illustrative only, and the invention is not limited to such embodiments. Finally, although the systems and methods have been described with respect to a gold/palladium embodiment, it should be understood that the systems and methods may be applied to other gold alloys. Accordingly, the exemplary implementations set forth above, are intended to be illustrative, not limiting.