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An electrostatically actuated micro-relay

Posted: 13 May 2002     Print Version  Bookmark and Share

Keywords:power 

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Custom Product Papers and Briefs 4-7

An Electrostatically Actuated Micro-Relay

J. Drake and J.H. Jerman--IC Sensors, Milpitas, CA

B. Lutze and M. Stuber--Brooktree Corporation, San Diego, CA

SUMMARY

An electrostatically actuated micro-relay has been designed

and fabricated for use in automatic test equipment (ATE)

applications. Switch action of the relay is provided by a

polysilicon paddle patterned with a metal shorting bar that

closes the circuit defined by metallization on an opposing

glass cap wafer. Actuation voltages less than 100V have been

obtained depending upon geometrical parameter choices. On-

resistance has been measured to be less than 3 W (most of

which is due to line resistance rather than resistance of the

switch) with standard deviation of the on-resistance less than

100 mW for operation of 1 million cycles, and relays have

been operated for more than 100 million cycles without

failure. Closure times of less than 20 msec have been measured

and the device shows no evidence of "bounce" typical of

conventional electromagnetic relays.

INTRODUCTION

Relays for use in automatic test equipment (ATE) must meet

very stringent requirements for isolation and leakage currents.

Furthermore, in applications requiring the switching of high

frequency signals, a low output capacitance is desired. The

physical contacts of traditional electromagnetic relays provide

very high isolation and, when open, can pass no appreciable

leakage current, factors which account for their popularity in

ATE. However, the traditional electromagnetic relay is

reaching its limit with respect to size of the device and

switching speeds. Solid-state relays are potentially very fast,

quite small, and more reliable than the electromagnetic relay

but they suffer from high "on-resistance" and non-zero

leakage currents. Design tradeoffs for reducing the on-

resistance of solid-state relays tend to increase output

capacitance. Several investigators have begun to apply

micromachining techniques towards the fabrication of small

electromechanical relays.1,2

In this paper a silicon micro-

machined relay (micro-relay) is described that combines the

isolation and low leakage qualities of the electromagnetic relay

with the speed and size advantages of the solid-state relay.

The micro-relay is comprised of silicon and glass chips

bonded together as depicted in Figures 1 and 2. Switch action

is provided by a polysilicon paddle fabricated on the silicon

side of the device. The relay shunt (shorting bar) is patterned

onto the paddle and a cavity is etched into the silicon beneath

the paddle such that, unpowered, the paddle and the shunt

bar are deflected away from the glass cap. On the glass side of

the device is an actuator plate; application of a voltage to this

actuator plate causes the paddle to be attracted

electrostatically towards the glass surface. Upon contact with

the opposing side of the device, the shunt bar shorts the input

and output trace metallizations patterned onto the glass cap

thereby closing the relay.

Figure 1. Top schematic view of micro-relay. Actuator plate not

shown in order to improve clarity.

1.

Taylor, W. P. and Allen, M. A., ISHM '94 Proceedings, (1994) pp. 524-529.

2.

Hosaka, H. et al, Sensors and Actuators A, 40, (1994) pp. 41-47.

Output Line of Relay

Input Line of Relay

Actuator Voltage In

Relay Contact

Polysilicon Paddle

Silicon Contact

Deflection Bump

Shorting Bar (on paddle)

Custom Product Papers

and Briefs

An Electrostatically Actuated Micro-Relay

4-8 Custom Product Papers and Briefs

Figure 2. Schematic cross section of micro-relay.

Initial deflection of the polysilicon paddle away from the

glass side contacts is enforced by deflection bumps patterned

onto both the glass cap and the paddle which are aligned so

that they mate during wafer bonding. The height and position

of the bumps, in concert with other geometrical and material

properties of the paddle, determine the initial deflection of the

paddle.

Flexure arms at either end of the polysilicon paddle

connect it to the silicon substrate. These flexures are narrower

than the paddle to allow for greater actuation flexibility. Also,

they provide electrical contact to the silicon chip through vias

in the passivating silicon dioxide layer. The backside of the

silicon chip is metallized to allow for electrical contact to the

paddle.

DESIGN OF MICRO-RELAY

Initial design considerations for the micro-relay are based on

an analysis of the mechanical deflection imposed by the

deflection bumps together with an estimation of the

electrostatic action caused by the application of the actuator

voltage.3

The deflection as a function of position along the

polysilicon flexures and paddle is obtained by solving:

where y(x) is the deflection as a function of x, the position

coordinate, along the length of the polysilicon structure, M(x)

is the mechanical moment as a function of x, E is the Young's

modulus for polysilicon (160 GPa), and I(x) is the position

dependent second moment of inertia. Equation 1 is solved for

the half-length model of the polysilicon structure as described

in Figure 3; by symmetry this is also solves Equation 1 for the

other half of the structure.

where: h = polysilicon thickness

hb = deflection bump height

L1 = length of flexure

L2 = half-length of paddle

L = length of entire polysilicon structure

Mo = effective end moment for symmetry

Q = electrostatic load intensity

xb = position of deflection bump

ymax = deflection of paddle at shorting bar

Figure 3. Half-length model for deflection analysis of

polysilicon relay structure.

Equation 1 is solved separately for each of the three

sections indicated in Figure 2 (A, B, C). Boundary conditions

are as follows: (a) at x=0 both the deflection and slope of the

polysilicon flexure vanish, (b) at the boundary between

sections A and B the slope of the flexure is continuous and the

deflection is equal to hb, and (c) at the boundary between

sections B and C deflection continuity applies, and at x=L/2

the slope again vanishes. In sections A and B, I(x) = IF where

IF is the second moment of the flexure. I(x) = IP in section C

where IP applies to the wider paddle. IF is a function of the

width of the flexure (b1) and h; IP depends on h and the

paddle width (b2).

Separate moment equations, M(x), are applied to each

section of the polysilicon structure. In order to account for

the electrostatic force exerted on the paddle of section C, a

constant electrostatic load intensity, Q, is assumed in units of

load per unit length of paddle.

3.

Pisano, A., UC Berkeley, Private Communication.

Silicon Wafer

Glass Cap Wafer

Polysilicon Paddle

Contact

Actuator Plate

Contact

d

2

dx

2

-------- y x( )

M x( )

EI x( )

--------------= [1]

An Electrostatically Actuated Micro-Relay

Custom Product Papers and Briefs 4-9

From the above considerations, the at rest deflection of

the paddle at the position of the shorting bar (ymax) can be

calculated as a function of the geometry of the device. Design

choices are made so as to provide a gap of 1-2 mm between

the shunt bar and the closure contacts prior to the application

of the actuation voltage. Assuming an actuation voltage of

100V, further design choices can be made to allow some of

the actuation voltage to provide the force required to deflect

the paddle to the closed position and the excess is used to

increase the closure force. In developing the micro-relay a

wide parameter space experiment was run to empirically

determine the tradeoffs of certain process and layout choices.

Further design considerations involve other geometry and

processing parameters that influence the on-resistance of the

device. The on-resistance is the sum of the contact resistance

created by the metal to metal closure of the relay and the

resistance of the interconnect metallization, shorting bar, and

package wiring. Contact resistance is minimized by

maximizing the overlap area between the end contacts of the

shorting bar and the contacts on the glass wafer. Here the

overlap area at each end of the shorting bar is on the order of

10 x 10 mm2

. Resistance of the interconnect metallization is

minimized by maximizing its layout area and its deposited

metal thickness.

Prototype chips were fabricated with 12 relays arranged

in a 2 x 6 array that is 2.5 mm x 5.0 mm in size. Simple

integration of the relays can be accomplished by wiring some

of the 12 relays in common configurations (such as SPDT)

but for most industrial applications it is expected that the

required relays will be laid out in the correct circuit

configuration at the chip level.

FABRICATION

The fabrication sequence for the silicon portion of the relay

begins with an passivation oxidation that, ultimately, protects

the underside of the polysilicon from being etched when the

cavity is formed. Contact holes are etched into the silicon

dioxide layer to allow for the polysilicon to contact the silicon

substrate. Wafers are annealed to relieve any residual stress in

the polysilicon layer.

A gold-based metallization is deposited and patterned to

create the deflection bumps and shorting bar. Following the

metallization steps, the etch cavity mask is patterned and the

underlying oxide is removed to expose the silicon substrate

around the polysilicon paddle. Next an anisotropic silicon

etch is performed to create the cavity underneath the

polysilicon structure. The cavity provides the space needed to

allow the paddle to deflect away from the contacts patterned

onto the glass wafer when the actuator voltage is not applied.

Finally the backside of the wafer is metallized to provide

indirect contact to the polysilicon paddle.

Processing of the glass cap wafer is simpler than that of

the silicon wafer. Essentially only two steps are required: 1)

etch depressions are created to give clearance for surface

topography on the silicon wafer and 2) a gold-based

metallization is deposited and patterned onto the wafer to

provide the interconnects and the actuator plate.

After fully fabricating the silicon and glass wafers, the two

wafers are aligned and then bonded together using a

proprietary metal sealing technique. Care must be taken to

insure that the deflection bumps on the glass wafer line up

well with the deflection bumps on the silicon wafer.

Following the bonding routine, the die are separated using a

conventional dicing saw.

Figure 4. Micro-relay as viewed through the glass cap.

Figure 4 is a photograph of a completed microrelay as

viewed with green light through the glass chip. In this

particular design the actuator plate metallization was

intentionally omitted so as to provide viewing access to the

polysilicon paddle. The contours are interference fringes

which indicate deflection increments of 0.25 mm. This

particular relay has a paddle width (b2) of 100 mm and a

paddle half-length (L2) of 70 mm.

An Electrostatically Actuated Micro-Relay

4-10 Custom Product Papers and Briefs

DEVICE PERFORMANCE

The ambient environment in which the relay is operated has a

major influence on the performance of the device specifically

with respect to stiction upon relay closure. Various ambients,

including Ar, N2, N2H2, and room air, were evaluated for

their influence on the micro-relay with the result that an

ordinary room air displayed the least stiction. This is

consistent with the findings of other investigators in that

some water vapor in the ambient is desired in order to prevent

sticking at the contacts.4

A critical performance criteria for the relay is that it

survive through many millions of cycles without failure where

definitions of failure include contact stiction, high or unstable

on-resistance, and breakage of the polysilicon structure. Early

in the development of this device, we found the that its

performance over many contact cycles follows the form

described in Figure 5.

Figure 5. Lifetime characteristics of the micro-relay.

As shown in Figure 5, the micro-relay tends to exhibit an

early period of unstable performance, a middle range of

useful, stable life, and then a period of instability preceding

failure of the device. "Bounce" upon initial actuation was not

observed for the micro-relay. In the development of the

micro-relay we have succeeded in bringing the range of useful

life above 100 million cycles. Figure 6 shows lifetime data of

an actual device that began to display instability after 104

million cycles.

Figure 6. Lifetime test of a micro-relay.

Actuation of the relay shows some hysteresis as indicated

in Figure 7. For this particular relay, actuation occurred at 82

V and drop-out, where the paddle fell back into the open

condition, occurred at about 76 V.

Figure 7. Actuation characteristics of the micro-relay.

On-resistance of the micro-relay during its stable period is

as shown in Figure 8. This relay displayed an average on-

resistance of 2.34 W and a standard deviation of about 10 mW

over the course of 20 thousand cycles. Analysis of the on-

resistance indicates that on the order of 0.5 W is due to

contact resistance and the rest is due to the resistance the

shorting bar and in the trace metallization on the glass wafer.

4.

Augis, J. A. and Hines, L. L., IEEE Transactions on Components, Hybrids, and Manufacturing Technology, Vol. CHMT-1, No.1,

(March 1978) pp 46-53.

Number of Cycles

Resistance

An Electrostatically Actuated Micro-Relay

Custom Product Papers and Briefs 4-11

Figure 8. On-resistance of micro-relay as

measured over 20 thousand cycles.

CONCLUSIONS

A micro-relay has been developed that represents an

advancement in the use of silicon micromachining to fabricate

very small and fast electromechanical relays for use in ATE

applications. Switching action is provided by electrostatic

actuation of a polysilicon paddle which closes a circuit

fabricated onto an opposing glass cap. The relay has shown

lifetimes in excess of 100 million cycles, actuation voltages in

the range of 50-100 V, and stable on-resistances of less than

3 W. The micro-relay has been designed with the intent of

using it for custom applications where a variety of switch

circuits and relay layouts can be realized.

ACKNOWLEDGEMENTS

The authors would like to acknowledge the contributions of

several people whose work helped to make this development

effort successful. Ken Barnes and Chris James were

instrumental in performing the characterization of the relay.

Al Pisano of the University of California at Berkeley assisted

with the initial design effort. Also, we'd like to thank Robin

Stevens for her efforts in fabricating the device.

An Electrostatically Actuated Micro-Relay

4-12 Custom Product Papers and Briefs





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