RF MEMS Extended Tuning Range Varactor and

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RF MEMS Extended Tuning Range Varactor and Varactor Based True Time Delay Line Design Yaping Liang1 , C. W. Domier2 , and N. C. Luhmann, Jr.2 1

Department of Electronics and Information Engineering, Hangzhou Dianzi University, China 2 Department of Applied Science, University of California, Davis, USA

Abstract— MEMS varactors are one of the important passive MEMS devices. Their applications include use in VCOs, tunable impedance matching networks, tunable filters, phase shifters, and true time delay lines. The shunt capacitive structure has been employed in most of the conventional MEMS varactor designs because of its simplicity. However, the capacitance ratio of this conventional shunt capacitive MEMS varactor is limited to 1.5 because of the MEMS PullIn effect, which happens when the deflection between the MEMS top and bottom metal plates increase beyond 1/3 of the airgap between the two metal plates. At that time, the top metal plate will quickly snap down. This effect is the major limitation in MEMS varactor designs and can cause nonlinearity and mechanically instability. In order to eliminate this Pull-In effect, the author employed the so-called MEMS extended tuning range structure. This structure utilizes a variable height top metal beam with separate actuation parts. The airgap between the center part of the top beam and the bottom plate has been designed to be less than 1/3 of the airgap between the top beam and the bottom actuation pads. When DC bias is applied to the actuation parts, the entire top beam will move down together. Consequently, before the Pull-In effect happens at the actuation parts, the center part has already traveled through its entire tuning range, which means that the capacitive ratio of this kind of MEMS varactor can go to infinity. A fabrication process employing a GaAs substrate has been designed based on surface micromachining technology. The maximum capacitance ratio of the designed MEMS extended tuning range varactor is 5.39 with a Cmax value of 167 fF. Based on this MEMS varactor design, a Ka-band MEMS varactor based distributed true time delay line has been designed. This distributed true time delay line includes a high impedance CPW transmission line with 70 Ω unloaded impedance at 28 GHz and eight MEMS extended tuning range varactors based on the varactor design periodically loaded on the CPW line. The testing results show that a 56◦ phase delay variation has been achieved at 28 GHz. The measured insertion loss at 28 GHz is −1.07 dB at the up-state and −2.36 dB at the down-state. The measured return losses, S11 and S22 , are both below −15 dB at 28 GHz and below −10 dB over the entire tested frequency range of 5 GHz to 40 GHz.

1. INTRODUCTION

MEMS varactors are one of the important passive MEMS devices. They have considerable advantages compared with other semiconductor devices, including low loss, very high Q at mm-wave frequencies, high power handling capability, low power consumption, and high IIP3. The RF MEMS varactor can be employed in a phase shifter or true time delay line design to replace the GaAs Schottky varactor diode for low-loss, broadband, and high frequency applications in modern communication, automotive and defense applications. It can also be used in low loss tunable circuits including matching networks, tunable filters, and low noise oscillators. 2. RF MEMS EXTENDED TUNING RANGE VARACTOR

Conventional RF-MEMS varactors usually employ a shunt parallel plate capacitor whose capacitance is determined by the spacing between a fixed bottom plate and a movable suspended top plate. Electrostatic actuation occurs when an electrostatic force is created by applying a DC voltage between the capacitor plates, thereby displacing the movable plate toward the fixed plate. However, this shunt capacitance MEMS varactor structure suffers from the so-called Pull-In effect [1]. It happens when the displacement between the two metal plates exceeds 1/3 of the entire airgap. At that moment, the electrostatic attraction force loses balance with the mechanical restoring force and that causes the two metal plates to quickly snap into contact. The Pull-In effect is the major limitation in MEMS varactor designs. It will cause nonlinearity and mechanical instability

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Figure 1: Model of a MEMS extended tuning range varactor structure.

of the MEMS varactors. In order to avoid the snap down, the designed capacitance ratio of the conventional MEMS capacitive varactor is usually set to 1.2 to 1.5 [2]. In order to eliminate this Pull-In effect, one approach is to employ the so-called MEMS extended tuning range structure [3]. This structure, as shown in Figure 1, utilizes a variable height top metal beam E1 with separate actuation parts E3 . The airgap between the center part of the top beam E1 and the bottom plate E2 has been designed to be less than 1/3 of the airgap between the top beam E1 and the bottom actuation pads E3 . When DC bias is applied to the actuation parts, the entire top beam E1 will move down together. Consequently, before the Pull-In effect happens at the actuation parts, the center part has already traveled through its entire tuning range, which means that the capacitive ratio of this kind of MEMS varactor can theoretically approach infinity. A MEMS extended tuning range varactor has been designed at 28 GHz on a GaAs substrate by using the Ansoft HFSS and Agilent ADS simulation tools. Figure 2 shows the designed five-mask fabrication process. The most important and difficult step in building this extended tuning range

Figure 2: Fabrication process steps.

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structure is to form the variable height top metal beam E1 . Here, it has been realized by spinning two layers of photoresist continuously with different masks to pattern, see Figure 2(c) and (d). The first step (Figure 2(a)) is to evaporate 0.7 µm of gold to form the signal lines and actuation pads using a gold lift-off process. The second step (Figure 2(b)) is to use PECVD to deposit 3000 ˚ A of Si3 N4 and to use dry Reactive Ion Etching (RIE) to form the dielectric layer between the bottom and top metal beams. The third step (Figure 2(c)) is to spin 1 µm thick photoresist and pattern the anchor points. The fourth step (Figure 2(d)) is to spin another 2 µm thick photoresist layer and pattern the anchor points and the center lower beam E2 . The fifth step (Figure 2(e)) is to electroplate 2 µm of gold and use photolithography to form the upper beam E1 . The final step (Figure 2(f)) is to use a dry etch to remove the sacrificial layer and release the whole structure.

Top beam

CPW transmission line

Figure 3: SEM picture of a MEMS extended tuning range varactor.

Figure 4: On-wafer C-V testing results.

Figure 3 shows an SEM picture of one of the fabricated MEMS varactors. On-wafer measurements by using an HP 4279A C-V meter have been employed and the results show that the maximum capacitance ratio is 5.39 with a Cmax value of 167 fF (see Figure 4).

sLline sCline

CVaractor

s Figure 5: SEM picture of a MEMS varactor based true time delay line.

Figure 6: Equivalent circuit of unit section LC ladder network.

3. RF MEMS VARACTOR BASED TRUE TIME DELAY LINE

RF MEMS varactor based true time delay line technology employs a distributed LC ladder structure by parallel loading the MEMS varactors on high impedance coplanar waveguide (CPW) transmission lines. Figure 5 shows an SEM picture of a portion of one of the fabricated MEMS extended tuning range varactor based true time delay lines. The unit section equivalent circuit of the distributed LC ladder network is shown in Figure 6. When the operation frequency is far below the

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Bragg cutoff frequency of the LC ladder network, the group velocity remains essentially constant as the frequency is varied [4]. This MEMS varactor based true time delay line comprises 8 MEMS extended tuning range varactors loaded on a 70 Ω CPW transmission line operated at 28 GHz. The on-wafer testing results show that the insertion loss at 28 GHz is −2.36 dB in the down-state; the return loss, S11 and S22 , are both below −15 dB at 28 GHz (see Figure 7). The measurement phase delay is 56◦ at 28 GHz (see Figure 8).

S22

S11

Freqency (GHz)

Figure 7: Measured down-state S-parameters of the MEMS varactor based true time delay line.

Insertion Loss (dB)

Return Loss (dB)

S21

Freqency (GHz)

Figure 8: S21 phase delay.

4. CONCLUSIONS

A novel RF MEMS extended tuning range varactor structure has been employed to eliminate the Pull-In effect of the conventional MEMS varactor designs. On-wafer measurement results show that the maximum capacitance ratio is 5.39 for the extended tuning range MEMS varactors. A 28 GHz proof-of-principle MEMS varactor based true time delay line design employed the MEMS extended tuning range varactor structure. The maximum phase delay is 56◦ with a usable range extending from 5 to 40 GHz over which the line has demonstrated both low insertion loss and high return loss. ACKNOWLEDGMENT

The authors are grateful to Miao Lu, Xiaodong Hu, and Yongjun Yan, of the Hebei Semiconductor Research Institute in China, for fabricating our MEMS varactors and delay lines. The authors would also like to thank Mehmet Ozgur and Michael Huff of the MEMS and Nanotechnology Exchange, funded by DARPA, for fabricating additional MEMS devices and circuits. This work was supported in part by the U.S. Department of Defense under Grant No. NBCH1050014 and by the U.S. Department of Energy under Grant No. DE-FG02-99ER54531. REFERENCES

1. Senturia, S. D., Microsystem Design, Kluwer Academic Publishers, Boston, MA, 2001. 2. Rebeiz, G. M., RF MEMS Theory, Design, and Technology, John Wiley & Sons, Inc., Hoboken, New Jersey, 2003. 3. Zou, J., C. Liu, et al., “Development of a wide tuning range MEMS tunable capacitor for wireless communication system,” Technical Digest of International Electron Devices Meeting, 403–406, 2000. 4. Hsia, R. P., “Nonlinear transmission lines and applications,” PhD Dissertation, UC Davis, 1997.