Cold- and hot-switching lifetime characterizations of ohmic-contact RF MEMS switches

Transcription

1 Cold- and hot-switching lifetime characterizations of ohmic-contact RF MEMS switches Jong-Man Kim 1a), Sanghyo Lee 2, Chang-Wook Baek 3, Youngwoo Kwon 2, and Yong-Kweon Kim 2 1 Department of Nanosystem and Nanoprocess Engineering, Pusan National University, Jangjeon 2-dong, Geumjeong-gu, Pusan , Korea 2 School of Electrical Engineering and Computer Science, Seoul National University, Gwanak P. O. Box 34, Seoul , Korea 3 School of Electrical and Electronics Engineering, Chung-Ang University, 221 Heukseok-dong, Dongjak-gu, Seoul , Korea a) jongkim@pusan.ac.kr Abstract: In this paper, to evaluate the applicability of electrostatically-actuated metal to metal direct-contact RF MEMS switches in practical RF fields, we performed three kinds of lifetime tests for the switches using three different conditions: cold-switching with no power, hot-switching with RF power, and hot-switching with DC current loads. The tested MEMS switch was demonstrating stable operation even after 10 9 cycles when cold-switching with no loads. We also experimentally showed that the tested switches could endure up to 10 6 cycles with 0.5 W of RF power and 10 8 cycles with 30 ma of DC current. Keywords: SCS RF MEMS switch, cold switching, hot switching, lifetime Classification: Micro- or nano-electromechanical systems References [1] J. Ehmk, J. Brank, A. Malczewski, B. Pillans, S. Eshelman, J. Yao, and C. Goldsmith, RF MEMS devices: a brave new world for RF technology, IEEE Emerging Technologies Symposium on Broadband, Wireless Internet Access, [2] Y. Kwon and S. Lee, RF MEMS - enabling technology for millimeterwaves, IEICE Trans. Electron., vol. E89-C, no. 7, pp. 1 3, [3] P.D.Grant,M.W.Denhoff,andR.R.Mansour, Acomparisonbetween RF MEMS switches and semiconductor switches, International Conference on MEMS, NANO and Smart Systems, pp , [4] G. M. Rebeiz, RF MEMS: theory, design and technology, New Jersey: John Wiley & Sons,

2 [5] J. Lampen, S. Majumder, R. Morrison, A. Chaudhry, and J. Maciel, A wafer-capped, high-lifetime ohmic MEMS RF switch, International Journal of RF and Microwave Computer-Aided Engineering, vol. 4, no. 4, pp , [6] J.-M. Kim, J.-H. Park, C.-W. Baek, and Y.-K. Kim, The SiOG-based single-crystalline silicon (SCS) RF MEMS switch with uniform characteristics, J. Microelectromech. Syst., vol. 13, no. 6, pp , Introduction Radio frequency-microelectromechanical systems (RF-MEMS) are an enabling technology to replace all the conventional RF passive devices with micro-miniaturized devices that show low-loss, near-zero power consumption, and superior linear characteristics [1, 2]. RF switches are basic switching components in micro- and millimeter wave RF systems. With recent advances in MEMS technologies, various types of micromachined switches have been successfully demonstrated and their excellent performance compared to their macro-scaled counterparts has been verified [3, 4]. In spite of the diverse merits of the MEMS switches, their practical application has been quite restricted due to their poor reliability. Repeat cycling lifetime is one of the most crucial parameters in determining the reliability of the MEMS switches [5]. The lifetime characteristics of RF switches can generally be evaluated in two ways. The first is the cold-switching test in which the switches are tested by cycling actuations with no RF or DC load applied. The second is the hotswitching test in which the switches experience excessive RF power or DC current during their repeated actuations. In this paper, the cold- and hotswitching lifetime characteristics of the single-crystal silicon (SCS) MEMS switches that were developed by the authors in [6] were experimentally evaluated in order to show the possibility of applying the switches in practical RF areas. The experimental details of the lifetime characterizations of the SCS MEMS switches are described and the test results are discussed. 2 Switch basics The MEMS switches that were used for the lifetime characterizations were previously introduced by the authors in [6] and are basically fabricated by a robust silicon on glass (SiOG) process that combines surface and bulk micromachining technologies. All the movable parts of the developed switch consist of SCS that has superior mechanical and thermal properties, resulting in higher mechanical stability and reliability compared to the conventional metallic plate-based micro switches because the structural deformation problems were alleviated. The switch is actuated by a voltage that creates an electrostatic force between the top and bottom electrodes, and the electrical operation of the switch is controlled by the gold-to-gold direct contact between the top contact part and the bottom signal lines. The fabricated switches show a typical actuation voltage of 30 V with little deviations. 419

3 3 Cold-switching lifetime test Cold-switching means that the switch is repeatedly actuated without applying RF or DC power during the actuations. It is generally known that the cold-switching lifetime of the switch is mainly restricted by structural fatigue, memory effect, stiction of the actuators, and changes in the electrical DC on-resistance, etc. To determine the cold-switching lifetime characteristics of our switch, we observed the actuating behavior in response to repeated actuations with a measurement setup described as follows. First, the test switch was placed on the probe station. A square-wave actuation signal having a magnitude of 35 V and a frequency of 5 khz was applied to the switch through the DC power amplifier connected to the function generator in series. An input sine-wave signal having a magnitude of 10 V p p and a frequency of 100 khz was generated by another function generator, and a multi-meter was connected in parallel to observe the changes of the DC on-resistance after various numbers of actuations. The actuation signal and the signal that was transmitted from the input to the output were observed using a two-channel oscilloscope. With this experimental setup, the switch was continuously tested for 60 hours, which corresponds to over 10 9 cycles. The test was stopped only because of test time limitations, and the switch was still working well even after the test was stopped. The output waveform after the cycling test is shown in Fig. 1 (a), which shows that the switch actuation status was not changed even after repeated actuations of over 10 9 cycles. The results indicate that the tested switch was not influenced by mechanical troubles such as structural fatigue or memory effect. The stiction problem also did not occur during the test procedures because the high restoring force of our switch exceeded the adhesion force. The DC on-resistances were checked every 10 8 cycles to observe the changes due to the repeated cycles, and no significant changes in the DC on-resistance were observed even after 10 9 cycles as shown in Fig. 1 (b). Fig. 1. Measurement results of the cold-switching lifetime test. (a) output waveform, and (b) changes in the DC on-resistance after 10 9 cycle actuations. 420

4 From all the test results, we concluded that the developed SCS MEMS switch shows excellent cold-switching lifetime characteristics. 4 Hot-switching lifetime test The lifetime of the switch is really restricted more by hot-switching than by cold-switching because most of the signals that are transmitted through the switch have high power loads in real cases. Therefore, the hot-switching characteristics of the switch are very important in determining the switch s reliability. In this paper, the hot-switching lifetimes of the SCS MEMS switches were tested in two different ways by using two kinds of loads. The first test was performed using a high level of RF power, while second test was performed using a high DC current. 4.1 Hot-switching lifetime test using a high RF power level The hot-switching lifetime using RF power through the switch was measured using the measurement setup described as follows. The input power signal was applied to the test switch using a power amplifier that was being driven by the signal source. The coupler connected to the output port of the power source has the role of producing a scaled version of the input power signal and we observed the input power level by measuring this signal with the power meter. The square-wave actuation signals having a magnitude of 35 V and a frequency range of Hz were independently applied to the test switches using the function generator connected to the DC power amplifier. The power transmitted by the switch actuations was observed using the spectrum analyzer. For the test, we first selected five identical switches showing similar actuating voltage characteristics of 30.4 ± 2.6 V, and then the hot-switching cycling tests for the switches were performed for different power levels of 0.5, 0.75, 1, 1.5, and 2 W. The test results are shown in Fig. 2. All the tests were stopped when the switches did not return to the initial up-positions when Fig. 2. Measured hot-switching lifetime characteristics for various applied RF power levels. 421

5 the actuation signals were removed. This means that the top contact parts of the switches were permanently stuck to the bottom signal lines, which is caused by micro-welding between the contacts. The micro-welding phenomenon can occur because of the elevated temperature due to the repeated opening and closing of the contacts while applying the high RF power signals. With the increased input power level, the hot-switching lifetime was exponentially reduced. When the input power was 2 W, the switch was permanently shorted out after only 480 cycles. This means that the application of higher power levels leads to more rapid micro-welding at the contact region, so the lifetime of the switch is accordingly shortened. However, our switch did not reveal any lifetime limitations up to 10 6 cycles of repeated actuations when the applied RF power was 0.5 W. 4.2 Hot-switching lifetime test using a high DC current The measurement setup for the hot-switching test using DC current was basically the same as the setup for the cold-switching test, except that a series resistance was used to apply a DC current to the test switch. A square-wave actuation signal having a magnitude of 35 V and a frequency range of khz was applied to the test switches by the function generator through the DC power amplifier. In this case, the actuation signal was adjusted with a DC offset, so signal levels of 0 V (switch-off) or 35 V (switch-on) were applied to the switch. The series resistance was located next to the function generator when applying the actuation signal in series so the DC current flowed through the contacts when the switch was turned on. The actuation status due to cycling while using the high DC current was also evaluated by observing the output signal waveform with the oscilloscope. For this test, four identical SCS switches having similar actuation voltages were prepared. Four different current levels of 30, 50, 70, and 100 ma were employed to evaluate the high current-based hot-switching lifetime characteristics of the prepared switches. The measured results are shown in Fig. 3. Once again, high current levels caused the contacts to become microwelded. As a result, the switch did not return to the off-state when the actuation signal was removed. The high temperature generated during repeated contacts under high current conditions made permanent connections between the contacts. The trace of the measured lifetimes with high DC currents in the switches is similar to that of the switches under high RF power conditions. As the current level was increased, the lifetimes of the switches were significantly shortened. When using a current level of 100 ma, the switch was permanently shorted out after only 230 cycles because of the abrupt increase in the local contact temperature. Nevertheless, the switch showed good and stable actuation behavior for over 10 8 cycles with a current level of 30 ma. 422

6 Fig. 3. Measured hot-switching lifetime characteristics for various DC current level. 5 Conclusions In this paper, we evaluated how the reliability of our SCS MEMS switches was affected by the power level in the switch, which is one of most influential factors that affect the lifetime of the switch. The tests addressed three categories of switching: cold-switching with no load, hot-switching with high RF power levels, and hot-switching with high DC currents. In the cold-switching lifetime test, the test switch maintained its mechanical actuation status even after 10 9 cycles without any structural fatigue, memory phenomenon, stiction problems, or significant changes in the DC on-resistance thanks to the robust SCS-based actuators. Through the hot-switching lifetime tests consisting using two kinds of loads, we confirmed that the switches lifetime characteristics under high power loads were mainly limited by micro-welding problems due to the high temperature that was generated at the contact regions. However, the SCS switches endured over 10 6 cycles of repeated actuations when using an RF power level of 0.5 W, and 10 8 cycles when using a DC current level of 30 ma. These results clearly show that it is feasible to use the developed SCS MEMS switches in practical RF applications. 423