Comparison of Gold/Platinum and Gold/Ruthenium 

Contacts on Piezoelectrically Actuated RF MEMS Switches 

by Robert Proie, Daniel Judy, Ronald G. Polcawich, and Jeffrey Pulskamp 

ARL-TR-5218 June 2010 

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Army Research Laboratory 

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ARL-TR-5218 June 2010 

Comparison of Gold/Platinum and Gold/Ruthenium 

Contacts on Piezoelectrically Actuated RF MEMS Switches 

Robert Proie, Daniel Judy, Ronald G. Polcawich, and Jeffrey Pulskamp 

Sensors and Electron Devices Directorate, ARL 

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4. TITLE AND SUBTITLE 

Comparison of Gold/Platinum and Gold/Ruthenium Contacts on 

Piezoelectrically Actuated RF MEMS Switches 

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Robert Proie, Daniel Judy, Ronald G. Polcawich, and Jeffrey Pulskamp 

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13. SUPPLEMENTARY NOTES 

14. ABSTRACT 

Lead zirconate titanite (PZT) radio frequency (RF) microelectromechanical system (MEMS) switches developed at the U.S. 

Army Research Laboratory (ARL) were fabricated with half of the switches on each wafer—approximately 30 switches—

using the typical gold/platinum contacts and using gold (Au)/ruthenium (Ru) contacts for the other half. We measured several 

important parameters using an Electroglass automated probe station, including contact resistance, actuatoion voltage, and bias 

current. The switches were cycled a number of times and the measurements were repeated. In addition, samples of five 

switches of each type were cycled to failure, and the mean output voltage was logged to assess the effect of these contact 

materials on the lifetime. 

15. SUBJECT TERMS

RF MEMS, contact metallurgy, piezoelectric 

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Robert Proie 

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iii 

Contents 

List of Figures iv

List of Tables iv

1. Introduction/Background 1

2. Experimental Procedure 3

3. Results and Discussion 6

4. Conclusion 8

5. References 9

List of Symbols, Abbreviations, and Acronyms 10 

Distribution List 11 
iv 

List of Figures 

Figure 1. Optical profilometer image illustrating the primary components of the tested PZT 

switches. .....................................................................................................................................2

Figure 2. An optical image of the PZT switch tested. This switch is characterized by two 

overhanging clamps that are bridged by the RF contact pad when the device closes. .............3

Figure 3. The test setup for the preparation phase of testing. .........................................................4

Figure 4. The lifetime testing setup used in the study. ...................................................................5

Figure 5. The activation voltage required to make electrical contact before and after device 

poling. ........................................................................................................................................6

Figure 6. The contact resistance observed before and after device current conditioning. ..............7

List of Tables 

Table 1. A summary of the data recorded throughout the testing process. .....................................7
1 

1. Introduction/Background 

Ever since Richard Feynman piqued the interest of researchers with his talk, There’s Plenty of 

Room at the Bottom, there has been a considerable amount of work done in the field of 

miniaturized devices. Due to the increased precision of device fabrication techniques, partially 

driven by the large commercial demand for faster and lower power silicon microprocessors, 

researchers have the ability to design and fabricate devices on the micro- and nano-scales (1). In 

recent years, research regarding these devices, referred to as micro-electro-mechanical systems 

(MEMS), has become increasingly popular. 

The concept of radio frequency (RF) MEMS switches is a specific area that shows a great deal of 

promise. Recently, researchers have demonstrated a number of advantages that RF MEMS 

switches have over the popular alternatives, specifically PiN diodes and field effect transistors 

(FET). These advantages include high isolation, low insertion loss, and very low power 

consumption (2). In addition, these switches have shown near linear operation over the intended 

range of use, from near DC to 50 GHz. This results in a reduction of spurious products during 

switching, which loosens the constraints imposed on other components within the RF system. 

In addition to the general attention of RF MEMS switches within high frequency systems, the 

defense arena has shown a considerable amount of interest in producing phased array antennas 

for both communication and radar systems (3). In reference 4, a Ku-Band, 3-bit phase shifter 

was demonstrated with a maximum insertion loss of 1.23 dB/bit, compared to the 1.6 dB/bit 

insertion loss achieved by a gallium arsenide (GaAs)-FET design (2). 

Despite these advantages, several hurdles remain before wide-scale adoption takes place. One of 

the primary hurdles is integration. Use of the popular RF-enabling fabrication technologies, such 

as bipolar junction transistor enabled complimentary metal oxide semiconductor technologies 

(BiCMOS), RF-CMOS, and silicon-germanium (SiGe), allows digital CMOS circuitry to be 

fabricated on the same die as the high frequency components. Most MEMS technologies, 

however, have not maturated to the point of a process marriage at that level. This results in a 

large, more complicated, and more costly system, with an increased potential for failure. 

Motivated by that issue, the groundwork has begun to design a CMOS alternative for RF MEMS 

switch systems, as shown in (4). 

The other, more detrimental hurdle results from the reliability of these devices. Numerous 

techniques have been experimented with in order to achieve the high (over 20 billion-cycle) 

reliability required by a defense or commercial product. The remainder of this report will 

examine piezoelectrically actuated RF MEMS switches, and a potential improvement in 

reliability through contract resistance improvement. 
2 

The MEMS devices we tested (see figure 1) used the inverse piezoelectric effect to produce the 

mechanical motion that opened and closed the switch. The inverse piezoelectric effect occurs 

when an applied electric field induces mechanical motion. 

Figure 1. Optical profilometer image illustrating the primary components 

of the tested PZT switches. 

In this specific situation, the switch uses two arms/actuators created from a platinum-PZTplatinum sandwich and a lower structural layer. An electric field is produced by grounding one 

of the platinum electrodes and supplying a bias potential to the other electrode. Upon generation 

of that electric field, the piezoelectric material in the middle, PZT, experiences a compressive 

strain that produces a moment about the neutral axis of the structure, causing it to deflect. With 

proper design of the actuator structure and location of the neutral axis, the cantilever can be 

made to bend up, raising the RF contact pad, which forms the electrical connection between the 

RF input and output lines. 

It is in that contact point that this report is concerned. Specifically, the contact materials are 

altered from a gold (Au)/platinum (Pt) combination to a Au/ruthenium (Ru) combination. It is 

theorized that this change will lead to a much more gradual increase in contact resistance over 

the lifetime of the device. 

A testing procedure is established to verify objectively the performance of each device and the 

criterion for a malfunctioning device in section 2. Section 3 provides and discusses the results 

measured during those tests. Section 4 summarizes those results and draws a conclusion 

regarding the effectiveness of Au/Ru contacts versus Au/Pt contacts. 
3 

2. Experimental Procedure 

This study focused on one specific switch design (see figure 2). This switch varies from the 

version presented in (5) because of the two clamped-clamped strips that are anchored above and 

perpendicular to the movable RF contact pad. We chose this switch over the switch described in 

(5) because it is generally less susceptible to bending during the top metal liftoff process step 

than is the dual cantilever structure. It may also reduce switch bounce and, thus, have a direct 

impact on switch contact lifetime. 

Figure 2. An optical image of the PZT switch tested. This 

switch is characterized by two overhanging clamps 

that are bridged by the RF contact pad when the device closes. 

The device testing was broken into two stages. In stage one—the preparation stage—device 

functionality was examined. Since this study’s purpose was to examine the impact of only the 

contact material on device lifetime, it was important to eliminate devices that showed other, 

unrelated issues. While we had already laid the groundwork for stage, a portion of this study 

was intended to standardize the procedure for future testing. 

All preparation steps were performed on a sample of 13 devices, with contact resistances and 

poling voltages recorded during each step. The preparation effectiveness was then examined in 

order to determine if any changes were necessary before stage two, the lifetime testing. 

The preparation stage testing setup (figure 3) required several pieces of equipment. A DC 

source, connected to the device under test via two DC probes, supplied the actuation voltage. 

The testing procedure required actuation voltages from 0 V to 25 V. Either a network analyzer 

with a bias-t connection (shown) or semiconductor parameter analyzer produced the signal down 

the RF path by way of two RF probes. 
4 

Figure 3. The test setup for the preparation phase of testing. 

The preparation stage was broken into three steps. The first step involved poling each device for 

1 min. It has been shown that poling piezoelectric ceramic materials, specifically PZT in (6), 

with a strong electric field effectively aligns the domains in the direction of the field. The 

electric field was produced by a 20 V potential difference between the two electrodes. This, in 

turn, results in increased mechanical deflection at a given bias voltage. This poling voltage was 

applied for 1 min and the switch was then toggled. 

Following device-poling, we used current conditioning to improve the contact resistance of each 

device. As indicated by (7), currents as high as 100 mA have been shown to break up 

contamination on electrical contacts, thus providing reduced contract resistance. In these 

devices, the RF line provided a direct electrical path to the contact. In order to apply current 

along the RF line, a bias-t setup (see figure 3) was first connected; the DC source used in that 

configuration supplied 2 V with a maximum current of 75 mA. Then, the actuation voltage was 

swept from 0 V to 25 V. The current was limited to 75 mA in order to avoid damaging the 

contacts. 
5 

The final preparation step was to measure the device’s new minimum actuation voltage and 

contact resistance. These measurements were taken with 0 V DC supplied to the bias-t. At this 

point, devices were binned into one of four categories: no electrical contact (stuck open), shorted 

(stuck closed), contact resistance below 5 , and contact resistance greater than 5 . We only 

used devices with a contact resistance below 5  for the second stage of testing. 

After verification of the testing preparation stage, an electroglass probe station was used to 

automate the process. In that setup, the network analyzer and DC power supply were replaced 

by a semiconductor parameter analyzer. A computer was used to control the probe station and 

record the data obtained by the parameter analyzer. 

The second stage of testing, termed lifetime testing, involved cycling the switch until failure. A 

failed switch was defined as a switch where the average voltage dropped below 50% of the 

expected average for more than 10 samples. 

The setup (figure 4) required multiple pieces of testing equipment. First, we used a function 

generator and amplifier sequence to supply the actuation voltage—a 15 Vpp square wave at 1000 

cycles per second with a 50% duty cycle. Another function generator supplied a voltage along 

the RF path. Two square wave pulses were supplied per actuation pulse. The first pulse 

occurred sufficiently after the visible switch bounce subsided and lasted for 0.25 ms. The second 

pulse occurred after the removal of the actuation voltage and lasted for 0.25 ms. We used this 

technique, called cold switching, to avoid electrical arcs that could occur during state transitions. 

Figure 4. The lifetime testing setup used in the study. 

In addition to the function generators, a multimeter and an 18 K load resistor were placed in 

series with the switcher’s output. The multimeter was used to read the mean voltage output from 

the switch, and an attached computer recorded that value once every second. Finally, an 

oscilloscope was used to allow visual verification of correct operation for each of the signals. 

This setup was used to record the lifetime of 10 Pt/Au contact switches and 10 Ru/Au switches. 
6 

3. Results and Discussion 

As mentioned previously, the first step of the preparation stage was to pole the devices for 1 min. 

Figure 5 shows a comparison between the actuation voltage required before poling and 

afterwards. As is shown, before poling occurred, a majority of the devices required an actuation 

voltage greater than 12 V. After poling, all of the devices were successfully actuated under 8 V. 

Figure 5. The activation voltage required to make electrical contact before and after device poling. 

Step 2 of the preparation stage was current conditioning. The pre-conditioning and postconditioning contact resistances are presented in figure 6. Before this step, a majority of the 

devices had a contact resistance over 200  and some were over 1 K. After conditioning, the 

contact resistance of each device was under 5 . 

0

2

4

6

8

10

<2<4<6<8

<10<12<14<16<18

More

Number of Devices

Measured Vact (Volts)

Comparison of Vp Required for Electrical

Contact Before and After Polling

Before Polling

Post Polling
7 

Figure 6. The contact resistance observed before and after device current conditioning. 

The results we obtained were sufficient to provide confidence in the preparation method. 

Following that verification, the automated preparation system aforementioned was used to 

examine a complete wafer composed of half Au/Ru devices and half Au/Pt devices. Table 1 

shows a summary of these results. 

Table 1. A summary of the data recorded throughout the testing process. 

 Platinum/Gold Ruthenium/Gold 

Initial Conditions 

 Average Vact 15.33 V 17.32 V 

 Average Rc 2518.90  14285.14 

 Working Devices 21 19 

Post-Preparation Conditions 

 Average Vact 7.52 V 9.29 V 

 Average Rc 9.61  14.45 

 Working Devices 21 24 

Lifetime 

 Average Lifetime 2.2 Million Cycles 6.2 Million Cycles 

 Minimum Lifetime 37,000 Cycles 29,000 Cycles 

 Maximum Lifetime 10 Million cycles 33 Million Cycles 

0

2

4

6

8

10

12

14

<5

<20<50

<100<200<500

<1000

More

Number of Devices

Contact Resistance (Ω)

Comparison of Rc Before and After Current

Conditioning

Before Current

Conditioning

Post Current

Conditioning
8 

The wafer contained 31 Au/Pt devices and 28 Au/Ru devices. The average Vact is the average 

actuation voltage required to achieve electrical contact, and the average Rc was the contact 

resistance recorded for the device. Both numbers only take into account those devices that 

achieved electrical contact without being stuck closed. 

For both types of contacts, the actuation voltage required after preparation was approximately 

half of the initial Vact. The contact resistance also decreased substantially. Note that the Rc 

shown includes the resistance of the cables used to take the measurements. 

One point of interest is that five Au/Ru devices were unable to make electrical contact initially 

due to a combination of a high Rc and Vact. After conditioning, these devices were able to 

function at a much more acceptable level. 

The final row in table 1 shows the lifetime results obtained for a sample of 20 devices—10 Au/Pt 

devices and 10 Au/Ru devices. Devices with Ru contact material lasted approximately three 

times longer than the Pt version. In both situations, several devices failed relatively quickly. Of 

the top six performing devices, four of them used the Ru/Au contact combination, including the 

best performing device. 

4. Conclusion 

This study has shown the validity of a method that objectively prepares and separates functional 

devices from those that are not working as intended. In addition, the contact material 

combination of Au/Ru was shown to produce devices that lasted three times longer than the 

Au/Pt alternative. 

Throughout the course of this study, humidity was observed to negatively impact the lifetime of 

these devices. For several days, the humidity was over 50% within the laboratory due to a 

malfunctioning air conditioner. Measurements taken at that time were discarded since they 

introduced an unintended variable into the study. All future measurements were conducted in an 

enclosed space, with nitrogen added to reduce the humidity. The container averaged a humidity 

level of approximately 7%–10%. We now believe that given more time for the wafer to adjust to 

the low humidity environment, the device lifetime could have improved substantially; a future 

study is needed to confirm this theory. 

 
9 

5. References 

1. Ojha, A. Nano-electronics and Nano-Computing: Status, Prospects, and Challenges. Proc. 

of IEEE Region 5 Conference: Annual Technical and Leadership, 2004. 

2. Rebeiz, G. RF MEMS Switches: Status of the Technology. The 12th International 

Conference on Solid State Sensors, Actuators and Microsystems, 2003. 

3. Chung; Polcawich, R.; Judy, D.; Pulskamp, J.; Papapolymerou. A SP2T and a SP4T Switch 

Using Low Loss Piezoelectric MEMS. 2007. 

4. Judy, D.; Polcawich, R.; Pulskamp, J. Low Voltage Complementary MEMS Logic Using 

Piezoelectric Actuators. Physical Sensors, Actuators, or Systems, 2008. 

5. Polcawich, R.; Judy, D.; Pulskamp, J.; et al. Advances in Piezoelectrically Actuated RF 

MEMS Switches and Phase Shifters. IEEE MTT-S Microwave Symposium June 2007, 2083–

2086. 

6. Wang, H.; Cheng, S.; Wang, C. Optimization of Poling Process for Piezoelectric PZT 

Ceramics. IEEE/CHMT 1989. 

7. Slade, P. Electrical Contacts: Principles and Applications. CRC 1999. 

 
10 

List of Symbols, Abbreviations, and Acronyms 

ARL U.S. Army Research Laboratory 

Au gold 

BiCMOS bipolar junction transistor enabled complimentary metal-oxide-semiconductor 

technology 

CMOS complementary metal-oxide-semiconductor 

FET field effect transistors 

GaAs gallium arsenide 

MEMS microelectromechanical system 

Pt platinum 

PZT zirconate titanate 

Rc contact resistance 

RF radio frequency 

Ru ruthenium 

SiGe silicon-germanium 

Ti titanim 

Vact actuation voltage 

Zr zirconium 
11 

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