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A creep-immune electrostatic actuator for RF-MEMS tunable capacitor
Sensors and Actuators A 169 (2011) 373–377
Contents lists available at ScienceDirect
Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna
A creep-immune electrostatic actuator for RF-MEMS tunable capacitor Etsuji Ogawa a,∗ , Tamio Ikehashi a , Tomohiro Saito a , Hiroaki Yamazaki a , Kei Masunishi b , Yasushi Tomizawa b , Tatsuya Ohguro a , Yoshiaki Sugizaki a , Yoshiaki Toyoshima a , Hideki Shibata a a b
Device Process Development Center, Corporate Research & Development Center, Toshiba Corporation, 8, Shinsugita-cho, Isogo-ku, Yokohama 235-8522, Japan Mechanical Systems Laboratory, Corporate Research & Development Center, Toshiba Corporation, 1, Komukai-Toshiba-cho, Saiwai-ku, Kawasaki 212-8582, Japan
a r t i c l e
i n f o
Article history: Available online 13 April 2011 Keywords: RF-MEMS Tunable capacitor Electrostatic actuator Creep Brittle material
a b s t r a c t A high creep-immunity MEMS actuator is proposed for RF-MEMS tunable capacitor. The creep-immunity is attained using silicon nitride, SiN, for the spring portions, where the stress is concentrated. Compared with an aluminum spring, the creep-induced deformation is reduced by a factor of 23 at 100 ◦ C. We also confirmed by a billion cycle test that the SiN spring does not develop a brittle fracture. © 2011 Elsevier B.V. All rights reserved.
1. Introduction RF-MEMS tunable capacitors are suitable for tunable components of the future multiband/multimode mobile handsets, because of its low loss and excellent linearity [1,2]. In such RF-MEMS, the use of low-resistivity ductile metals, such as aluminum, is indispensable to attain the low loss. However, the use of such metals at the actuator portion leads to a creep, which is a stress-induced deformation of ductile materials [3–5]. Creep is caused by stress application and increases with total stress time. The creep-induced deformation is currently a major reliability issue hampering the commercialization of the RF-MEMS tunable components. In view of this, we started to find an actuator structure that satisfies low loss and high creep-immunity at the same time. The present paper investigates an actuator structure that uses a brittle material, silicon nitride (SiN), at the high stress concentrations. The brittle fracture issue of the proposed actuator is also addressed in the paper. 2. Creep test of aluminum actuator To analyze the creep of RF-MEMS actuators, we first examined an electrostatic parallel-plate actuator whose movable element is composed purely of aluminum. The structure of the actuator (actuator A) is shown in Figs. 1 and 2. The sample was manufactured by a standard semiconductor process. The aluminum film was formed
∗ Corresponding author. Tel.: +81 45 776 5666; fax: +81 45 776 4104. E-mail address: [email protected] (E. Ogawa). 0924-4247/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2011.04.014
by sputtering method. The top electrode of aluminum has a size of 100 m × 90 m and a thickness of 2 m. The aluminum spring has a width of 5 m and a thickness of 2 m. Biasing to the top electrode is done via the aluminum spring. The bottom electrode is also made of aluminum. The dielectric film, silicon nitride, is formed on the bottom electrode. The gap between the top and bottom electrode is 2.4 m. A creep is known to be affected by total stress time, not by number of switching times. Thus a following test was done to measure the creep deformation. Firstly, a stress was applied to the actuator for a certain period by keeping the actuator in down-state position. The down-state position was attained by applying an actuation voltage difference (30 V) between the top and bottom electrodes. Secondly, the actuation voltage was removed for a short period to measure the up-state capacitance (CUP ). The capacitance was measured using an LCR meter. This procedure was repeated number of times. The total stress time was 10 h. Fig. 3 shows the result of the creep test of the actuator A at 25 ◦ C. The curve plot shows CUP (the shift of CUP ) as a function of the stress time. The increase of CUP during the test is caused by both creep deformation and dielectric charging. As depicted in Fig. 3, the charging contribution can be removed by leaving the actuator unbiased for five days. The remaining contribution ␦CUP corresponds to the permanent deformation caused by the creep. Fig. 4 shows the height of the up-state top electrode before and after the creep test. The height of aluminum surface was measured by an interference microscope. The height difference ␦h in Fig. 4 is the vertical displacement of the top electrode. The top electrode displaces downward mainly due to the deformation at the spring portion. The creep-induced capacitance change ␦CUP was found to
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Fig. 1. Optical-microscope photograph of actuator A with aluminum springs.
Fig. 5. FEM simulation result of stress distribution in down-state for actuator A.
coincide with a capacitance change calculated from the displacement ␦h. 3. Evaluation of a structure using SiN springs 3.1. SiN-spring actuator Fig. 2. Schematic cross section at the dashed line (center line of the top-electrode) in Fig. 1.
Fig. 5 shows a simulation result of the down-state stress distribution of the actuator A. The simulation was performed by FEM analysis method using ANSYS. The simulation indicates two things: (i) the stress is concentrated at the spring portion; (ii) the stress value is low and uniform at the electrode. Based on this observation, we designed an actuator using a brittle material, silicon nitride (SiN), at the spring portion, where the high stress is applied. The structure of the actuator (actuator B) is shown in Fig. 6. The SiN springs, which have a width of 5 m and a thickness of 2 m, are connected to the top electrode and the aluminum anchor. The SiN film was formed by CVD method. The gap between the top and bottom electrode is 2.4 m. Since SiN is an insulator, biasing to the top electrode is done via an aluminum spring. The spring constant of the aluminum spring is negligibly small compared to that of the SiN spring. Therefore, the top electrode position is not affected by a creep of the biasing aluminum spring.
Fig. 3. Shift of up-state capacitance (CUP ) in the creep test at 25 ◦ C for actuator A. The initial value CUP0 is 38 fF.
Fig. 4. Displacement of the top electrode in the creep test for actuator A. The plots are the height at the dashed line (center line of the electrode) in Fig. 1.
Fig. 6. Optical-microscope photograph of actuator B with SiN springs. The schematic cross section at the dashed line (center line of the electrode) is the same as Fig. 2.
E. Ogawa et al. / Sensors and Actuators A 169 (2011) 373–377
Fig. 7. Shift of up-state capacitance (CUP ) at 25 ◦ C for actuator B. The initial value CUP0 is 41 fF.
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Fig. 9. Creep deformation ␦CUP after the creep test for actuators A (aluminum springs) and B (SiN springs) at 25 ◦ C and 100 ◦ C.
3.2. Creep test of SiN-spring actuator The same creep test described previously was carried out for the actuator B having the SiN spring. The results are shown in Figs. 7 and 8. The test was done at 25 ◦ C. As expected, the creepinduced deformation is found to be quite small. The creep tests were performed also at 100 ◦ C for the actuators A and B. The up-state capacitance and up-state top electrode position were measured at 25 ◦ C before and after the creep test. The resulting ␦CUP and ␦h are summarized in Figs. 9 and 10, respectively. As in Fig. 3, the charging contribution is removed from ␦CUP and ␦h by leaving the actuator unbiased for five days. We can see from Figs. 9 and 10 that the creep-induced deformation is reduced drastically by employing the SiN spring. The small amount of deformation observed in the actuator B is considered to originate from creep at the aluminum–SiN joint portion. 3.3. Immunity from brittle fracture While a brittle material does not exhibit creep, it still has a risk of developing a brittle fracture. To see this for the actuator B having the SiN spring, we carried out a switching cycle test. The cycle test was done at 25 ◦ C using a 2 kHz voltage pulse of 30 V amplitude. The voltage pulse is a square wave with a duty cycle of 50%. No structural defects were observed after the 109 cycle test by a microscopic inspection. Fig. 11 shows the shifts of up-state capacitance (CUP ) and the shift of pull-in voltage (VPI ) during the cycle test. The small shifts during the test are mostly due to dielectric charging, as
Fig. 8. Displacement of the top electrode in the creep test for actuator B. The plots are the height at the dashed line (center line of the electrode) in Fig. 6.
Fig. 10. Creep deformation ␦h after the creep test for actuators A (aluminum springs) and B (SiN springs) at 25 ◦ C and 100 ◦ C.
can be understood from the plot after the charge removal. The nonexistence of an abrupt change in VPI indicates non-occurrence of a brittle fracture. We may thus conclude that the SiN-spring actuator can be safely used for RF-MEMS. 4. RF-MEMS tunable capacitor Finally, we briefly explain about a creep-immune RF-MEMS tunable capacitor structure. We employed a structure composed of quadruple series capacitors (QSC) [6]. The photograph and
Fig. 11. Plot of CUP and VPI during the switching cycle test at 25 ◦ C. The initial pull-in voltage VPI0 is 25 V.
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that is, the change of up-state capacitance, is negligibly small as in the actuator B. 5. Conclusions
Fig. 12. SEM photograph of RF-MEMS tunable capacitor with QSC structure using SiN springs.
schematic views of the capacitor unit are shown in Figs. 12 and 13, respectively. Two tunable MEMS capacitors and two fixed MIM (metal–insulator–metal) capacitors are connected in series between Port1 and Port2. This structure improves the powerhandling, since the voltage difference between the top and bottom electrodes can be reduced. We confirmed hot-switching at +36 dBm. Actuation of the top electrode is done by applying a voltage difference between the top and bottom electrodes. The top electrode is supported by SiN springs, and the actuation voltage is supplied via the weak aluminum spring having a negligibly small spring constant. Note that the latter spring does not give rise to loss, since the top and bottom electrodes behave as floating nodes for the RF signal, thanks to the Low-pass-filter (LPF). We fabricated a capacitor bank using the capacitor units. The capacitance can be varied from 1.9 pF (all the capacitors in the up-state position) to 5.5 pF (all in the down-state position). The corresponding quality factors extracted from the measured S11 (S parameter) were 114 and 80 at 1 GHz. We also confirmed that the creep deformation,
A high creep-immunity MEMS actuator is proposed. The actuator uses a brittle material, silicon nitride, for the spring portion, where the stress is concentrated. It shows drastically small creepinduced deformation compared with an aluminum-spring actuator. The change of up-state capacitance after the creep test is reduced by a factor of 9 at 25 ◦ C and a factor of 23 at 100 ◦ C. We also confirmed that no brittle fracture occurred by a 109 cycle test. In this respect, the actuator presented in this paper is very promising for use in RF-MEMS tunable capacitor. Based on the investigation of the actuator structure, we fabricated an RF-MEMS tunable capacitor. The proposed structure improves power-handling and is capable of hot-switching under high RF power of +36 dBm. This structure also exhibits very small creep-induced deformation, thanks to the employment of the SiN spring. References [1] G.M. Rebeiz, RF MEMS Theory, Design, and Technology, Wiley Interscience, New Jersey, 2003, pp. 190–191. [2] H.J. De Los Santos, et al., RF MEMS for ubiquitous wireless connectivity: Part 2 – Application, IEEE Microwave Magazine, December 2004, pp. 50–65. [3] Marcel van Gils, et al., Evaluation of creep in RF MEMS devices, in: Proceedings of EuroSimE, 2007, pp. 1–6. [4] R. Modlinski, et al., Creep as a reliability problem in MEMS, Microelectron. Reliab. 44 (9–11) (2004) 1733–1738. [5] R. Modlinski, et al., Creep characterization of Al alloy thin films for use in MEMS applications, J. Microelectron. Eng. 76 (1–4) (2004) 272–278. [6] H. Yamazaki, et al., A high power-handling RF MEMS tunable capacitor using quadruple series capacitor structure, IEEE MTT-S Int. Microwave Symp. Dig. (2010) 1138–1141.
Biographies Etsuji Ogawa received the B.S. degrees in Metallurgical Engineering from the University of Tokyo, Japan, in 1987. He joined Toshiba Corporation in 1987. Since 2004, he has been engaged in the research and development of MEMS devices. Tamio Ikehashi received the Ph.D. degree in Theoretical Physics from University of Tokyo in 1995. In 1995, he joined Semiconductor Device Laboratory, Toshiba Corporation, Yokohama, Japan, where he has been engaged in the development of semiconductor memories. Since 2004, he has been engaged in the research and development of MEMS devices. Tomohiro Saito was born in Yokohama, Japan, in 1969. He received the B.S. and M.S. degrees in Physics from Waseda University in 1992 and 1994, respectively. In 1994, he joined ULSI Research Center, Toshiba Corporation, Kawasaki, Japan, where he engaged in the development of device technologies for ULSI. Since 2000, he joined Process & Manufacturing Engineering Center, Semiconductor Company, Toshiba Corporation, Yokohama, Japan, where he engaged in the research and development of advanced CMOS process. Since 2007, he joined Center for Semiconductor Research & Development, and he has been engaged in the research and development of MEMS devices. Hiroaki Yamazaki received the B.S. and M.S. degrees in Electronic Engineering from Tohoku University, Sendai, Japan in 2003 and 2005, respectively. He joined Toshiba Corporation in 2005, where he has been engaged in the research and development of RF MEMS devices. Kei Masunishi received the B.S. and M.S. degrees in Mechanical Engineering from Yokohama National University in 1996 and 1998, respectively. He joined NEC Corporation in 1998, where he has been engaged in the research and development of ink-jet printer head. In 2002 he switched company to PENTAX Corporation. In 2003 he switched company to Toshiba Corporation, where he has been engaged in the research and development of MEMS devices. He is currently engaged in the research and development of Energy Harvesting devices, too.
Fig. 13. Schematic top view and cross section of the tunable capacitor with QSC structure.
Yasushi Tomizawa received his B.S. and M.S. degrees in Mechanical Engineering from the University of Tokyo in 1996 and 1998, respectively, and joined to the HDD development department in Toshiba Corporation. While working as a mechanical engineer of HDD he studied MEMS in the University of Southern California from 2002 to 2004 and received his second M.S. degree in Electrical Engineering there. In 2007 he was transferred to the R&D center in Toshiba and since then he is engaged
E. Ogawa et al. / Sensors and Actuators A 169 (2011) 373–377 in development of MEMS devices. Currently he is a member of the Japanese national project for MEMS research called “BEANS project”. Tatsuya Ohguro was born in Aichi, Japan, on August 23, 1963. He received the B.S. and M.S. degrees in Physics from Hokkaido University, Sapporo, Japan, in 1987, 1989, respectively. He joined Toshiba Corporation in 1989, where he has been engaged in the research and development of advanced logic CMOS devices, mixed signal and RF CMOS, passive elements and advance device-process such as Ni salicide technology, epitaxial channel structure, elevated source and drain and so on. Mr. Ohguro is a member of IEEE. Yoshiaki Sugizaki received the B.E. and M.E. degrees in Chemistry from Chiba University, Chiba, Japan in 1985 and 1987, respectively. He joined Toshiba Corporation in 1987, where he has been working on the research and development of semiconductor package and MEMS. Mr. Sugizaki is a member of Japan Institute of Electronics Packaging. Yoshiaki Toyoshima received B.E degree in Electronic Engineering from Waseda University, Tokyo, Japan in 1981. He joined the Semiconductor Device Engineer-
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ing Laboratory, Toshiba Corporation, Kawasaki, Japan, in 1984, where he worked on the advanced CMOS device design and process development for ULSI. Main research area has been scaled CMOS technology including device reliability, gate stack technology and shallow junction technology. In 2002, he participated the joint development of high speed SOI CMOS technology with IBM Corporation and Sony Corporation as a project leader from Toshiba. Since 2006, he is managing the Advanced CMOS Technology Department, Toshiba Corporation, Yokohama, Japan. Hideki Shibata received the B.S. degree in Metallurgical Engineering and the Ph.D. degree in Electrical and Computing Engineering from Nagoya Institute of Technology, Nagoya, Japan, in 1982 and 1995, respectively. In 1982, he joined the Semiconductor Device Engineering Laboratory, Toshiba Corporation, Kawasaki, Japan. Since 1995, he has been engaged in the development of advanced interconnects integration technology for high-performance logic devices and high density flash memories. He is currently the Senior Fellow, Corporate Research and Development Center, Toshiba Corporation, Yokohama, Japan.
Contents lists available at ScienceDirect
Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna
A creep-immune electrostatic actuator for RF-MEMS tunable capacitor Etsuji Ogawa a,∗ , Tamio Ikehashi a , Tomohiro Saito a , Hiroaki Yamazaki a , Kei Masunishi b , Yasushi Tomizawa b , Tatsuya Ohguro a , Yoshiaki Sugizaki a , Yoshiaki Toyoshima a , Hideki Shibata a a b
Device Process Development Center, Corporate Research & Development Center, Toshiba Corporation, 8, Shinsugita-cho, Isogo-ku, Yokohama 235-8522, Japan Mechanical Systems Laboratory, Corporate Research & Development Center, Toshiba Corporation, 1, Komukai-Toshiba-cho, Saiwai-ku, Kawasaki 212-8582, Japan
a r t i c l e
i n f o
Article history: Available online 13 April 2011 Keywords: RF-MEMS Tunable capacitor Electrostatic actuator Creep Brittle material
a b s t r a c t A high creep-immunity MEMS actuator is proposed for RF-MEMS tunable capacitor. The creep-immunity is attained using silicon nitride, SiN, for the spring portions, where the stress is concentrated. Compared with an aluminum spring, the creep-induced deformation is reduced by a factor of 23 at 100 ◦ C. We also confirmed by a billion cycle test that the SiN spring does not develop a brittle fracture. © 2011 Elsevier B.V. All rights reserved.
1. Introduction RF-MEMS tunable capacitors are suitable for tunable components of the future multiband/multimode mobile handsets, because of its low loss and excellent linearity [1,2]. In such RF-MEMS, the use of low-resistivity ductile metals, such as aluminum, is indispensable to attain the low loss. However, the use of such metals at the actuator portion leads to a creep, which is a stress-induced deformation of ductile materials [3–5]. Creep is caused by stress application and increases with total stress time. The creep-induced deformation is currently a major reliability issue hampering the commercialization of the RF-MEMS tunable components. In view of this, we started to find an actuator structure that satisfies low loss and high creep-immunity at the same time. The present paper investigates an actuator structure that uses a brittle material, silicon nitride (SiN), at the high stress concentrations. The brittle fracture issue of the proposed actuator is also addressed in the paper. 2. Creep test of aluminum actuator To analyze the creep of RF-MEMS actuators, we first examined an electrostatic parallel-plate actuator whose movable element is composed purely of aluminum. The structure of the actuator (actuator A) is shown in Figs. 1 and 2. The sample was manufactured by a standard semiconductor process. The aluminum film was formed
∗ Corresponding author. Tel.: +81 45 776 5666; fax: +81 45 776 4104. E-mail address: [email protected] (E. Ogawa). 0924-4247/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2011.04.014
by sputtering method. The top electrode of aluminum has a size of 100 m × 90 m and a thickness of 2 m. The aluminum spring has a width of 5 m and a thickness of 2 m. Biasing to the top electrode is done via the aluminum spring. The bottom electrode is also made of aluminum. The dielectric film, silicon nitride, is formed on the bottom electrode. The gap between the top and bottom electrode is 2.4 m. A creep is known to be affected by total stress time, not by number of switching times. Thus a following test was done to measure the creep deformation. Firstly, a stress was applied to the actuator for a certain period by keeping the actuator in down-state position. The down-state position was attained by applying an actuation voltage difference (30 V) between the top and bottom electrodes. Secondly, the actuation voltage was removed for a short period to measure the up-state capacitance (CUP ). The capacitance was measured using an LCR meter. This procedure was repeated number of times. The total stress time was 10 h. Fig. 3 shows the result of the creep test of the actuator A at 25 ◦ C. The curve plot shows CUP (the shift of CUP ) as a function of the stress time. The increase of CUP during the test is caused by both creep deformation and dielectric charging. As depicted in Fig. 3, the charging contribution can be removed by leaving the actuator unbiased for five days. The remaining contribution ␦CUP corresponds to the permanent deformation caused by the creep. Fig. 4 shows the height of the up-state top electrode before and after the creep test. The height of aluminum surface was measured by an interference microscope. The height difference ␦h in Fig. 4 is the vertical displacement of the top electrode. The top electrode displaces downward mainly due to the deformation at the spring portion. The creep-induced capacitance change ␦CUP was found to
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Fig. 1. Optical-microscope photograph of actuator A with aluminum springs.
Fig. 5. FEM simulation result of stress distribution in down-state for actuator A.
coincide with a capacitance change calculated from the displacement ␦h. 3. Evaluation of a structure using SiN springs 3.1. SiN-spring actuator Fig. 2. Schematic cross section at the dashed line (center line of the top-electrode) in Fig. 1.
Fig. 5 shows a simulation result of the down-state stress distribution of the actuator A. The simulation was performed by FEM analysis method using ANSYS. The simulation indicates two things: (i) the stress is concentrated at the spring portion; (ii) the stress value is low and uniform at the electrode. Based on this observation, we designed an actuator using a brittle material, silicon nitride (SiN), at the spring portion, where the high stress is applied. The structure of the actuator (actuator B) is shown in Fig. 6. The SiN springs, which have a width of 5 m and a thickness of 2 m, are connected to the top electrode and the aluminum anchor. The SiN film was formed by CVD method. The gap between the top and bottom electrode is 2.4 m. Since SiN is an insulator, biasing to the top electrode is done via an aluminum spring. The spring constant of the aluminum spring is negligibly small compared to that of the SiN spring. Therefore, the top electrode position is not affected by a creep of the biasing aluminum spring.
Fig. 3. Shift of up-state capacitance (CUP ) in the creep test at 25 ◦ C for actuator A. The initial value CUP0 is 38 fF.
Fig. 4. Displacement of the top electrode in the creep test for actuator A. The plots are the height at the dashed line (center line of the electrode) in Fig. 1.
Fig. 6. Optical-microscope photograph of actuator B with SiN springs. The schematic cross section at the dashed line (center line of the electrode) is the same as Fig. 2.
E. Ogawa et al. / Sensors and Actuators A 169 (2011) 373–377
Fig. 7. Shift of up-state capacitance (CUP ) at 25 ◦ C for actuator B. The initial value CUP0 is 41 fF.
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Fig. 9. Creep deformation ␦CUP after the creep test for actuators A (aluminum springs) and B (SiN springs) at 25 ◦ C and 100 ◦ C.
3.2. Creep test of SiN-spring actuator The same creep test described previously was carried out for the actuator B having the SiN spring. The results are shown in Figs. 7 and 8. The test was done at 25 ◦ C. As expected, the creepinduced deformation is found to be quite small. The creep tests were performed also at 100 ◦ C for the actuators A and B. The up-state capacitance and up-state top electrode position were measured at 25 ◦ C before and after the creep test. The resulting ␦CUP and ␦h are summarized in Figs. 9 and 10, respectively. As in Fig. 3, the charging contribution is removed from ␦CUP and ␦h by leaving the actuator unbiased for five days. We can see from Figs. 9 and 10 that the creep-induced deformation is reduced drastically by employing the SiN spring. The small amount of deformation observed in the actuator B is considered to originate from creep at the aluminum–SiN joint portion. 3.3. Immunity from brittle fracture While a brittle material does not exhibit creep, it still has a risk of developing a brittle fracture. To see this for the actuator B having the SiN spring, we carried out a switching cycle test. The cycle test was done at 25 ◦ C using a 2 kHz voltage pulse of 30 V amplitude. The voltage pulse is a square wave with a duty cycle of 50%. No structural defects were observed after the 109 cycle test by a microscopic inspection. Fig. 11 shows the shifts of up-state capacitance (CUP ) and the shift of pull-in voltage (VPI ) during the cycle test. The small shifts during the test are mostly due to dielectric charging, as
Fig. 8. Displacement of the top electrode in the creep test for actuator B. The plots are the height at the dashed line (center line of the electrode) in Fig. 6.
Fig. 10. Creep deformation ␦h after the creep test for actuators A (aluminum springs) and B (SiN springs) at 25 ◦ C and 100 ◦ C.
can be understood from the plot after the charge removal. The nonexistence of an abrupt change in VPI indicates non-occurrence of a brittle fracture. We may thus conclude that the SiN-spring actuator can be safely used for RF-MEMS. 4. RF-MEMS tunable capacitor Finally, we briefly explain about a creep-immune RF-MEMS tunable capacitor structure. We employed a structure composed of quadruple series capacitors (QSC) [6]. The photograph and
Fig. 11. Plot of CUP and VPI during the switching cycle test at 25 ◦ C. The initial pull-in voltage VPI0 is 25 V.
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that is, the change of up-state capacitance, is negligibly small as in the actuator B. 5. Conclusions
Fig. 12. SEM photograph of RF-MEMS tunable capacitor with QSC structure using SiN springs.
schematic views of the capacitor unit are shown in Figs. 12 and 13, respectively. Two tunable MEMS capacitors and two fixed MIM (metal–insulator–metal) capacitors are connected in series between Port1 and Port2. This structure improves the powerhandling, since the voltage difference between the top and bottom electrodes can be reduced. We confirmed hot-switching at +36 dBm. Actuation of the top electrode is done by applying a voltage difference between the top and bottom electrodes. The top electrode is supported by SiN springs, and the actuation voltage is supplied via the weak aluminum spring having a negligibly small spring constant. Note that the latter spring does not give rise to loss, since the top and bottom electrodes behave as floating nodes for the RF signal, thanks to the Low-pass-filter (LPF). We fabricated a capacitor bank using the capacitor units. The capacitance can be varied from 1.9 pF (all the capacitors in the up-state position) to 5.5 pF (all in the down-state position). The corresponding quality factors extracted from the measured S11 (S parameter) were 114 and 80 at 1 GHz. We also confirmed that the creep deformation,
A high creep-immunity MEMS actuator is proposed. The actuator uses a brittle material, silicon nitride, for the spring portion, where the stress is concentrated. It shows drastically small creepinduced deformation compared with an aluminum-spring actuator. The change of up-state capacitance after the creep test is reduced by a factor of 9 at 25 ◦ C and a factor of 23 at 100 ◦ C. We also confirmed that no brittle fracture occurred by a 109 cycle test. In this respect, the actuator presented in this paper is very promising for use in RF-MEMS tunable capacitor. Based on the investigation of the actuator structure, we fabricated an RF-MEMS tunable capacitor. The proposed structure improves power-handling and is capable of hot-switching under high RF power of +36 dBm. This structure also exhibits very small creep-induced deformation, thanks to the employment of the SiN spring. References [1] G.M. Rebeiz, RF MEMS Theory, Design, and Technology, Wiley Interscience, New Jersey, 2003, pp. 190–191. [2] H.J. De Los Santos, et al., RF MEMS for ubiquitous wireless connectivity: Part 2 – Application, IEEE Microwave Magazine, December 2004, pp. 50–65. [3] Marcel van Gils, et al., Evaluation of creep in RF MEMS devices, in: Proceedings of EuroSimE, 2007, pp. 1–6. [4] R. Modlinski, et al., Creep as a reliability problem in MEMS, Microelectron. Reliab. 44 (9–11) (2004) 1733–1738. [5] R. Modlinski, et al., Creep characterization of Al alloy thin films for use in MEMS applications, J. Microelectron. Eng. 76 (1–4) (2004) 272–278. [6] H. Yamazaki, et al., A high power-handling RF MEMS tunable capacitor using quadruple series capacitor structure, IEEE MTT-S Int. Microwave Symp. Dig. (2010) 1138–1141.
Biographies Etsuji Ogawa received the B.S. degrees in Metallurgical Engineering from the University of Tokyo, Japan, in 1987. He joined Toshiba Corporation in 1987. Since 2004, he has been engaged in the research and development of MEMS devices. Tamio Ikehashi received the Ph.D. degree in Theoretical Physics from University of Tokyo in 1995. In 1995, he joined Semiconductor Device Laboratory, Toshiba Corporation, Yokohama, Japan, where he has been engaged in the development of semiconductor memories. Since 2004, he has been engaged in the research and development of MEMS devices. Tomohiro Saito was born in Yokohama, Japan, in 1969. He received the B.S. and M.S. degrees in Physics from Waseda University in 1992 and 1994, respectively. In 1994, he joined ULSI Research Center, Toshiba Corporation, Kawasaki, Japan, where he engaged in the development of device technologies for ULSI. Since 2000, he joined Process & Manufacturing Engineering Center, Semiconductor Company, Toshiba Corporation, Yokohama, Japan, where he engaged in the research and development of advanced CMOS process. Since 2007, he joined Center for Semiconductor Research & Development, and he has been engaged in the research and development of MEMS devices. Hiroaki Yamazaki received the B.S. and M.S. degrees in Electronic Engineering from Tohoku University, Sendai, Japan in 2003 and 2005, respectively. He joined Toshiba Corporation in 2005, where he has been engaged in the research and development of RF MEMS devices. Kei Masunishi received the B.S. and M.S. degrees in Mechanical Engineering from Yokohama National University in 1996 and 1998, respectively. He joined NEC Corporation in 1998, where he has been engaged in the research and development of ink-jet printer head. In 2002 he switched company to PENTAX Corporation. In 2003 he switched company to Toshiba Corporation, where he has been engaged in the research and development of MEMS devices. He is currently engaged in the research and development of Energy Harvesting devices, too.
Fig. 13. Schematic top view and cross section of the tunable capacitor with QSC structure.
Yasushi Tomizawa received his B.S. and M.S. degrees in Mechanical Engineering from the University of Tokyo in 1996 and 1998, respectively, and joined to the HDD development department in Toshiba Corporation. While working as a mechanical engineer of HDD he studied MEMS in the University of Southern California from 2002 to 2004 and received his second M.S. degree in Electrical Engineering there. In 2007 he was transferred to the R&D center in Toshiba and since then he is engaged
E. Ogawa et al. / Sensors and Actuators A 169 (2011) 373–377 in development of MEMS devices. Currently he is a member of the Japanese national project for MEMS research called “BEANS project”. Tatsuya Ohguro was born in Aichi, Japan, on August 23, 1963. He received the B.S. and M.S. degrees in Physics from Hokkaido University, Sapporo, Japan, in 1987, 1989, respectively. He joined Toshiba Corporation in 1989, where he has been engaged in the research and development of advanced logic CMOS devices, mixed signal and RF CMOS, passive elements and advance device-process such as Ni salicide technology, epitaxial channel structure, elevated source and drain and so on. Mr. Ohguro is a member of IEEE. Yoshiaki Sugizaki received the B.E. and M.E. degrees in Chemistry from Chiba University, Chiba, Japan in 1985 and 1987, respectively. He joined Toshiba Corporation in 1987, where he has been working on the research and development of semiconductor package and MEMS. Mr. Sugizaki is a member of Japan Institute of Electronics Packaging. Yoshiaki Toyoshima received B.E degree in Electronic Engineering from Waseda University, Tokyo, Japan in 1981. He joined the Semiconductor Device Engineer-
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ing Laboratory, Toshiba Corporation, Kawasaki, Japan, in 1984, where he worked on the advanced CMOS device design and process development for ULSI. Main research area has been scaled CMOS technology including device reliability, gate stack technology and shallow junction technology. In 2002, he participated the joint development of high speed SOI CMOS technology with IBM Corporation and Sony Corporation as a project leader from Toshiba. Since 2006, he is managing the Advanced CMOS Technology Department, Toshiba Corporation, Yokohama, Japan. Hideki Shibata received the B.S. degree in Metallurgical Engineering and the Ph.D. degree in Electrical and Computing Engineering from Nagoya Institute of Technology, Nagoya, Japan, in 1982 and 1995, respectively. In 1982, he joined the Semiconductor Device Engineering Laboratory, Toshiba Corporation, Kawasaki, Japan. Since 1995, he has been engaged in the development of advanced interconnects integration technology for high-performance logic devices and high density flash memories. He is currently the Senior Fellow, Corporate Research and Development Center, Toshiba Corporation, Yokohama, Japan.