The actuation voltage of microelectromechanical system (MEMS) \ud
metal switches was investigated at temperatures ranging from 10 to 290 K. The investigation shows a 50% increase in the actuation voltage at low temperature. A comparison has been made using a published model and showed similar increment of actuation voltage at low temperature

Baek, C.W.

Kim, J.M.

Kim, Y.K.

Lancaster, M.J.

Llamas-Garro, I.

Park, J.H.

Prest, M.

Su, H.T.

English

The actuation voltage of microelectromechanical system (MEMS) metal switches was investigated at temperatures ranging from 10 to 290 K. The investigation shows a 50% increase in the actuation voltage at low temperature. A comparison has been made using a published model and showed similar increment of actuation voltage at low temperature

Su, Hieng

LLamas-Garro, I

Lancaster, Michael

Prest, Martin

Park, JH

Kim, JM

Baek, CS

Kim, YK

University of Birmingham Research Portal

  Performance of RF MEMS Switches at LowTemperaturesSu, Hieng; LLamas-Garro, I; Lancaster, Michael; Prest, Martin; Park, JH; Kim, JM; Baek, CS;Kim, YKDOI:10.1049/el:20062551Citation for published version (Harvard):Su, H, LLamas-Garro, I, Lancaster, M, Prest, M, Park, JH, Kim, JM, Baek, CS & Kim, YK 2006, 'Performance ofRF MEMS Switches at Low Temperatures', Electronics Letters, vol. 42, no. 21, pp. 1219-1221.https://doi.org/10.1049/el:20062551Link to publication on Research at Birmingham portalGeneral rightsUnless a licence is specified above, all rights (including copyright and moral rights) in this document are retained by the authors and/or thecopyright holders. The express permission of the copyright holder must be obtained for any use of this material other than for purposespermitted by law.•	Users may freely distribute the URL that is used to identify this publication.•	Users may download and/or print one copy of the publication from the University of Birmingham research portal for the purpose of privatestudy or non-commercial research.•	User may use extracts from the document in line with the concept of ‘fair dealing’ under the Copyright, Designs and Patents Act 1988 (?)•	Users may not further distribute the material nor use it for the purposes of commercial gain.Where a licence is displayed above, please note the terms and conditions of the licence govern your use of this document.When citing, please reference the published version.Take down policyWhile the University of Birmingham exercises care and attention in making items available there are rare occasions when an item has beenuploaded in error or has been deemed to be commercially or otherwise sensitive.If you believe that this is the case for this document, please contact UBIRA@lists.bham.ac.uk providing details and we will remove access tothe work immediately and investigate.Download date: 01. Feb. 2019Performance of RF MEMS switches atlow temperaturesH.T. Su, I. Llamas-Garro, M.J. Lancaster, M. Prest,J.-H. Park, J.-M. Kim, C.-W. Baek and Y.-K. KimThe actuation voltage of microelectromechanical system (MEMS)metal switches was investigated at temperatures ranging from 10 to290 K. The investigation shows a 50% increase in the actuationvoltage at low temperature. A comparison has been made using apublished model and showed similar increment of actuation voltage atlow temperature.Introduction: Over the past few years, radio-frequency microelectro-mechanical systems (RF MEMS) fabricated using semiconductormicro-fabrication technology have gained significant interest forwireless communication applications owing to their small size,integration capability and superior performance. For instance, usingRF MEMS switches, RF circuits such as variable capacitors, tunablefilters, phase shifters and signal routers have been demonstrated [1].Recently, there has been research interest in the application of thehigh-Tc superconductor (HTS) in combination with RF MEMSdevices. A switched HTS microstrip resonator and a tunable HTSfilter, both incorporating MEMS technology, have been demonstrated[2, 3]. While successful integration of MEMS switches onto HTSdevices is a challenging task, the operational issue of RF MEMSswitches at low temperatures is another key consideration to manymicrowave and system designers. In fact, there has been littleliterature reporting the behaviour of MEMS switches at cryogenictemperature. In this Letter, we report our recent findings on lowtemperature measurements of MEMS metal switches.Design of metal switch: The layout of the metal switch is shown inFig. 1. The metal switches were fabricated on a 0.52 mm-thick quartzsubstrate. The switches consist of two electrostatically actuated nickelpads and a small central gold-plated nickel RF contact pad. The RFcontact pad is electrically isolated and physically connected to theactuation pads by a thin dielectric membrane of silicon nitride. Theswitch actuates on a gold 50 O coplanar waveguide (CPW) ofthickness 3 mm, and signal line width 50 mm. Below the RF contactpad of the switch is a 5 mm gap in the signal line. The RF contact padbridges this gap by direct contact when the actuation voltage isapplied. The height of the RF contact pad above the transmissionline, before actuation, is 1.2 mm. The two coplanar ground planes areconnected via air-bridges. The fabrication process of a similar metalswitch has been discussed in [4]. The size of the switch (without thebiasing lines) is about 0.5 0.32 mm.coplanarlinebiasingline actuationpadgroundplane RFcontact padFig. 1 Layout of metal switchMeasurement results: Measurements of the metal switches werecarried out using a cryogenic probe station. Two special 90 micro-wave bent probes [5] with a pitch size of 150 mm were used.Measurements were performed for metal switches on a die. Eachswitch is slightly different in size in terms of the width and length ofthe actuation pads compared with others. S-parameters were measuredusing an Agilent 8722ES network analyser. Calibration wasperformed at 290 K using Cascade Microtech software (WinCal);the calibration technique used was Thru-Reflect-Reflect-Match(TRRM). The measured insertion losses for three of the switches atroom temperature and at 10 GHz were 0.05, 0.16 and 0.14 dB,compared with simulated insertion loss of 0.04 dB. The isolationswere in the range of 20 to 30 dB. Measurements were taken atdifferent temperature intervals. At a particular temperature, biasvoltage was applied to the metal switch through a DC probe.Actuation voltage was recorded when S21 decreased suddenly toclose to about 0 dB (switch closed). Fig. 2 shows the recordedactuation voltages for a number of metal switches over the tempera-ture range 290 to 10 K.0 20 40 60 80 100 120 140temperature, K160 180 200 220 240 260 280 3001015activation voltage, V20253035404550556065C3R9C6R8C7R9C1R7C2R5C3R2(1)Fig. 2 Measured actuation voltages for number of switches againsttemperature and calculated voltages from (1)Note that the actuation voltages of the metal switches were increasedas the temperature reduced. Most of the switches were actuated at a biasvoltage between 30 and 60 V. It was observed that the switches becomemore difficult to actuate at low temperatures. Goldsmith and Forehand[6] published a simple theoretical model for temperature variation ofactuation voltage and reported the same observation of increment in theactuation voltage of MEMS switches as the temperature goes down buttheir measurements only went down to about 50C. They attributedthe increase in actuation voltage to increasing stress in the actuators, asa result of the difference in thermal expansion coefficients between themetal and the quartz substrate.An approximate formula to calculate the actuation or pull-in voltagefor these metal switches is given by [6, 7]:Vp ﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ64gð1 vÞtg3027L2e0ðDaEðTzs  T ÞÞsð1Þwhere the geometrical factor g¼ 1, n is the Poisson’s ratio for nickel(0.31), t is the thickness of the nickel (2.5 mm), g0 is the OFF state gapbetween the actuation pad and the bottom electrode (1.2 mm), L is thelength of the actuation pads (500 mm, assuming they are one piece andmeasuring from end to end), Da is the difference in thermal expansioncoefficient between nickel and quartz (12.8 106 K1), E is theYoung’s modulus for nickel (200 GPa), Tzs¼ 350 K is the effectivetemperature of zero stress in the nickel (dependent on depositiontemperature and as-deposited stress), T is the operation temperatureand e0 is the permittivity of air. Equation (1) assumes that the actuatorsare solid bars and so would tend to overestimate the true value as theactuators have hinged sections to allow easier bending.Fig. 3 shows an example of measured insertion loss and isolation upto 20 GHz at 290 and 80 K. At 290 K, the switch actuated at 29 V, theactuation voltage increased slightly to 30 V at 80 K. The ripples at 80 Kmeasurements are due to calibration as the calibration was performed at290 K.An important design consideration is to know the percentage ofswitches that worked at low temperatures if the switches are to be usedwith superconducting microwave devices. In this experiment, a totalnumber of 28 switches were measured. It was revealed that at 290 Kabout 82% of the switches actuated, and this decreased to 39% at 80 KELECTRONICS LETTERS 12th October 2006 Vol. 42 No. 21and further at lower temperatures. If we consider only working switchesat 290 K, about 48% of these switches worked at 80 K.0 2 4 6 8frequency, GHzinsertion loss, dB10 12 14 16 18 20isolation, dB–60–50–40–30–20–100–20–18–16–14–12–10–8–6–4–20290 K80 Kswitch actuated (ON state)no bias (OFF state)actuation voltage: 29 V(290K)                              30 V(80K)Fig. 3 Example of measured insertion losses and isolations at 290 and80 K (switch C3R2)Conclusion: Low-temperature performance of MEMS metal switcheshas been investigated. Measurements showed an increase in theactuation voltage as the temperature went down to 10 K. It wasfound that the working percentage of these switches decreases atlow temperature.Acknowledgment: This work was supported by the BirminghamUniversity Portfolio Award.# The Institution of Engineering and Technology 200615 August 2006Electronics Letters online no: 20062551doi: 10.1049/el:20062551H.T. Su (School of Engineering, Swinburne University of Technology,Malaysia)I. Llamas-Garro (Large Millimeter Telescope, National Institute forAstrophysics, Optics and Electronics, Mexico)E-mail: llamasi@iee.orgM.J. Lancaster and M. Prest (School of Electronic, Electrical andComputer Engineering, The University of Birmingham,United Kingdom)J.-M. Kim and Y.-K. Kim (School of Electrical Engineering andComputer Science, Seoul National University, Seoul 137-724, Korea)J.-H. Park (LG Electronics Institute of Technology, Seocho-Gu, Seoul137-724, Korea)C.-W. Baek (School of Electrical and Electronics Engineering,Chung-Ang University, Korea)References1 Lucyszyn, S.: ‘Review of radio frequency microelectromechanicalsystems technology’, IEE Proc., Sci. Meas. Technol., 2004, 151, (2),pp. 93–1032 Noel, J., Hijazi, Y., Martinez, J., Vlasov, Y.A., and Larkins, G.L., Jr: ‘Aswitched high-Tc superconductor microstrip resonator using a MEMswitch’, Supercond. Sci. Technol., 2003, 16, pp. 1438–14413 Prophet, E.M., Musolf, J., Zuck, B.F., Jimenez, S., Kihlstrom, K.E., andWillemsen, B.A.: ‘Highly-selective electronically-tunable cryogenicfilters using monolithic, discretely-switchable MEMS capacitor arrays’,IEEE Trans. Appl. Supercond., 2005, 15, (2), pp. 956–9594 Park, J.-H., Lee, S., Kim, J.-M., Kim, H.-T., Kwon, Y., and Kim, Y.-K.:‘Reconfigurable millimeter-wave filters using CPW-based periodicstructures with novel multiple-contact MEMS switches’,J. Microelectromech. Syst., 2005, 14, (3), pp. 456–4635 GGB Industries Inc., Florida (USA) www.picoprobe.com6 Goldsmith, C.L., and Forehand, D.I.: ‘Temperature variation of actuationvoltage in capacitive MEMS switches’, IEEE Microw. Wirel. Compon.Lett., 2005, 15, (10), pp. 718–7207 Pacheco, S.P., Katehi, L.P.B., and Nguyen, C.T.-C.: ‘Design of lowactuation voltage RF MEMS switch’, IEEE MTT-S Int. Microw.Symp. Dig., 2000, pp. 165–168ELECTRONICS LETTERS 12th October 2006 Vol. 42 No. 21

Performance of RF MEMS Switches at Low Temperatures

H.T. Su

I. Llamas-Garro

M.J. Lancaster

M. Prest

J.-H. Park

J.-M. Kim

C.-W. Baek

Y.-K. Kim

MUCC (Crossref)

Performance of RF MEMS switches at low temperatures

  University of BirminghamPerformance of RF MEMS Switches at LowTemperaturesSu, Hieng; LLamas-Garro, I; Lancaster, Michael; Prest, Martin; Park, JH; Kim, JM; Baek, CS;Kim, YKDOI:10.1049/el:20062551Citation for published version (Harvard):Su, H, LLamas-Garro, I, Lancaster, M, Prest, M, Park, JH, Kim, JM, Baek, CS & Kim, YK 2006, 'Performance ofRF MEMS Switches at Low Temperatures', Electronics Letters, vol. 42, no. 21, pp. 1219-1221.https://doi.org/10.1049/el:20062551Link to publication on Research at Birmingham portalGeneral rightsUnless a licence is specified above, all rights (including copyright and moral rights) in this document are retained by the authors and/or thecopyright holders. The express permission of the copyright holder must be obtained for any use of this material other than for purposespermitted by law.•	Users may freely distribute the URL that is used to identify this publication.•	Users may download and/or print one copy of the publication from the University of Birmingham research portal for the purpose of privatestudy or non-commercial research.•	User may use extracts from the document in line with the concept of ‘fair dealing’ under the Copyright, Designs and Patents Act 1988 (?)•	Users may not further distribute the material nor use it for the purposes of commercial gain.Where a licence is displayed above, please note the terms and conditions of the licence govern your use of this document.When citing, please reference the published version.Take down policyWhile the University of Birmingham exercises care and attention in making items available there are rare occasions when an item has beenuploaded in error or has been deemed to be commercially or otherwise sensitive.If you believe that this is the case for this document, please contact UBIRA@lists.bham.ac.uk providing details and we will remove access tothe work immediately and investigate.Download date: 01. Mar. 2020Performance of RF MEMS switches atlow temperaturesH.T. Su, I. Llamas-Garro, M.J. Lancaster, M. Prest,J.-H. Park, J.-M. Kim, C.-W. Baek and Y.-K. KimThe actuation voltage of microelectromechanical system (MEMS)metal switches was investigated at temperatures ranging from 10 to290 K. The investigation shows a 50% increase in the actuationvoltage at low temperature. A comparison has been made using apublished model and showed similar increment of actuation voltage atlow temperature.Introduction: Over the past few years, radio-frequency microelectro-mechanical systems (RF MEMS) fabricated using semiconductormicro-fabrication technology have gained significant interest forwireless communication applications owing to their small size,integration capability and superior performance. For instance, usingRF MEMS switches, RF circuits such as variable capacitors, tunablefilters, phase shifters and signal routers have been demonstrated [1].Recently, there has been research interest in the application of thehigh-Tc superconductor (HTS) in combination with RF MEMSdevices. A switched HTS microstrip resonator and a tunable HTSfilter, both incorporating MEMS technology, have been demonstrated[2, 3]. While successful integration of MEMS switches onto HTSdevices is a challenging task, the operational issue of RF MEMSswitches at low temperatures is another key consideration to manymicrowave and system designers. In fact, there has been littleliterature reporting the behaviour of MEMS switches at cryogenictemperature. In this Letter, we report our recent findings on lowtemperature measurements of MEMS metal switches.Design of metal switch: The layout of the metal switch is shown inFig. 1. The metal switches were fabricated on a 0.52 mm-thick quartzsubstrate. The switches consist of two electrostatically actuated nickelpads and a small central gold-plated nickel RF contact pad. The RFcontact pad is electrically isolated and physically connected to theactuation pads by a thin dielectric membrane of silicon nitride. Theswitch actuates on a gold 50 O coplanar waveguide (CPW) ofthickness 3 mm, and signal line width 50 mm. Below the RF contactpad of the switch is a 5 mm gap in the signal line. The RF contact padbridges this gap by direct contact when the actuation voltage isapplied. The height of the RF contact pad above the transmissionline, before actuation, is 1.2 mm. The two coplanar ground planes areconnected via air-bridges. The fabrication process of a similar metalswitch has been discussed in [4]. The size of the switch (without thebiasing lines) is about 0.5 0.32 mm.coplanarlinebiasingline actuationpadgroundplane RFcontact padFig. 1 Layout of metal switchMeasurement results: Measurements of the metal switches werecarried out using a cryogenic probe station. Two special 90 micro-wave bent probes [5] with a pitch size of 150 mm were used.Measurements were performed for metal switches on a die. Eachswitch is slightly different in size in terms of the width and length ofthe actuation pads compared with others. S-parameters were measuredusing an Agilent 8722ES network analyser. Calibration wasperformed at 290 K using Cascade Microtech software (WinCal);the calibration technique used was Thru-Reflect-Reflect-Match(TRRM). The measured insertion losses for three of the switches atroom temperature and at 10 GHz were 0.05, 0.16 and 0.14 dB,compared with simulated insertion loss of 0.04 dB. The isolationswere in the range of 20 to 30 dB. Measurements were taken atdifferent temperature intervals. At a particular temperature, biasvoltage was applied to the metal switch through a DC probe.Actuation voltage was recorded when S21 decreased suddenly toclose to about 0 dB (switch closed). Fig. 2 shows the recordedactuation voltages for a number of metal switches over the tempera-ture range 290 to 10 K.0 20 40 60 80 100 120 140temperature, K160 180 200 220 240 260 280 3001015activation voltage, V20253035404550556065C3R9C6R8C7R9C1R7C2R5C3R2(1)Fig. 2 Measured actuation voltages for number of switches againsttemperature and calculated voltages from (1)Note that the actuation voltages of the metal switches were increasedas the temperature reduced. Most of the switches were actuated at a biasvoltage between 30 and 60 V. It was observed that the switches becomemore difficult to actuate at low temperatures. Goldsmith and Forehand[6] published a simple theoretical model for temperature variation ofactuation voltage and reported the same observation of increment in theactuation voltage of MEMS switches as the temperature goes down buttheir measurements only went down to about 50C. They attributedthe increase in actuation voltage to increasing stress in the actuators, asa result of the difference in thermal expansion coefficients between themetal and the quartz substrate.An approximate formula to calculate the actuation or pull-in voltagefor these metal switches is given by [6, 7]:Vp ﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ64gð1 vÞtg3027L2e0ðDaEðTzs  T ÞÞsð1Þwhere the geometrical factor g¼ 1, n is the Poisson’s ratio for nickel(0.31), t is the thickness of the nickel (2.5 mm), g0 is the OFF state gapbetween the actuation pad and the bottom electrode (1.2 mm), L is thelength of the actuation pads (500 mm, assuming they are one piece andmeasuring from end to end), Da is the difference in thermal expansioncoefficient between nickel and quartz (12.8 106 K1), E is theYoung’s modulus for nickel (200 GPa), Tzs¼ 350 K is the effectivetemperature of zero stress in the nickel (dependent on depositiontemperature and as-deposited stress), T is the operation temperatureand e0 is the permittivity of air. Equation (1) assumes that the actuatorsare solid bars and so would tend to overestimate the true value as theactuators have hinged sections to allow easier bending.Fig. 3 shows an example of measured insertion loss and isolation upto 20 GHz at 290 and 80 K. At 290 K, the switch actuated at 29 V, theactuation voltage increased slightly to 30 V at 80 K. The ripples at 80 Kmeasurements are due to calibration as the calibration was performed at290 K.An important design consideration is to know the percentage ofswitches that worked at low temperatures if the switches are to be usedwith superconducting microwave devices. In this experiment, a totalnumber of 28 switches were measured. It was revealed that at 290 Kabout 82% of the switches actuated, and this decreased to 39% at 80 KELECTRONICS LETTERS 12th October 2006 Vol. 42 No. 21and further at lower temperatures. If we consider only working switchesat 290 K, about 48% of these switches worked at 80 K.0 2 4 6 8frequency, GHzinsertion loss, dB10 12 14 16 18 20isolation, dB–60–50–40–30–20–100–20–18–16–14–12–10–8–6–4–20290 K80 Kswitch actuated (ON state)no bias (OFF state)actuation voltage: 29 V(290K)                              30 V(80K)Fig. 3 Example of measured insertion losses and isolations at 290 and80 K (switch C3R2)Conclusion: Low-temperature performance of MEMS metal switcheshas been investigated. Measurements showed an increase in theactuation voltage as the temperature went down to 10 K. It wasfound that the working percentage of these switches decreases atlow temperature.Acknowledgment: This work was supported by the BirminghamUniversity Portfolio Award.# The Institution of Engineering and Technology 200615 August 2006Electronics Letters online no: 20062551doi: 10.1049/el:20062551H.T. Su (School of Engineering, Swinburne University of Technology,Malaysia)I. Llamas-Garro (Large Millimeter Telescope, National Institute forAstrophysics, Optics and Electronics, Mexico)E-mail: llamasi@iee.orgM.J. Lancaster and M. Prest (School of Electronic, Electrical andComputer Engineering, The University of Birmingham,United Kingdom)J.-M. Kim and Y.-K. Kim (School of Electrical Engineering andComputer Science, Seoul National University, Seoul 137-724, Korea)J.-H. Park (LG Electronics Institute of Technology, Seocho-Gu, Seoul137-724, Korea)C.-W. Baek (School of Electrical and Electronics Engineering,Chung-Ang University, Korea)References1 Lucyszyn, S.: ‘Review of radio frequency microelectromechanicalsystems technology’, IEE Proc., Sci. Meas. Technol., 2004, 151, (2),pp. 93–1032 Noel, J., Hijazi, Y., Martinez, J., Vlasov, Y.A., and Larkins, G.L., Jr: ‘Aswitched high-Tc superconductor microstrip resonator using a MEMswitch’, Supercond. Sci. Technol., 2003, 16, pp. 1438–14413 Prophet, E.M., Musolf, J., Zuck, B.F., Jimenez, S., Kihlstrom, K.E., andWillemsen, B.A.: ‘Highly-selective electronically-tunable cryogenicfilters using monolithic, discretely-switchable MEMS capacitor arrays’,IEEE Trans. Appl. Supercond., 2005, 15, (2), pp. 956–9594 Park, J.-H., Lee, S., Kim, J.-M., Kim, H.-T., Kwon, Y., and Kim, Y.-K.:‘Reconfigurable millimeter-wave filters using CPW-based periodicstructures with novel multiple-contact MEMS switches’,J. Microelectromech. Syst., 2005, 14, (3), pp. 456–4635 GGB Industries Inc., Florida (USA) www.picoprobe.com6 Goldsmith, C.L., and Forehand, D.I.: ‘Temperature variation of actuationvoltage in capacitive MEMS switches’, IEEE Microw. Wirel. Compon.Lett., 2005, 15, (10), pp. 718–7207 Pacheco, S.P., Katehi, L.P.B., and Nguyen, C.T.-C.: ‘Design of lowactuation voltage RF MEMS switch’, IEEE MTT-S Int. Microw.Symp. Dig., 2000, pp. 165–168ELECTRONICS LETTERS 12th October 2006 Vol. 42 No. 21

University of Birmingham Research Archive, E-prints Repository

Performance of RF MEMS switches at
low temperatures
H.T. Su, I. Llamas-Garro, M.J. Lancaster, M. Prest,
J.-H. Park, J.-M. Kim, C.-W. Baek and Y.-K. Kim
The actuation voltage of microelectromechanical system (MEMS)
metal switches was investigated at temperatures ranging from 10 to
290 K. The investigation shows a 50% increase in the actuation
voltage at low temperature. A comparison has been made using a
published model and showed similar increment of actuation voltage at
low temperature.
Introduction: Over the past few years, radio-frequency microelectro-
mechanical systems (RF MEMS) fabricated using semiconductor
micro-fabrication technology have gained significant interest for
wireless communication applications owing to their small size,
integration capability and superior performance. For instance, using
RF MEMS switches, RF circuits such as variable capacitors, tunable
filters, phase shifters and signal routers have been demonstrated [1].
Recently, there has been research interest in the application of the
high-Tc superconductor (HTS) in combination with RF MEMS
devices. A switched HTS microstrip resonator and a tunable HTS
filter, both incorporating MEMS technology, have been demonstrated
[2, 3]. While successful integration of MEMS switches onto HTS
devices is a challenging task, the operational issue of RF MEMS
switches at low temperatures is another key consideration to many
microwave and system designers. In fact, there has been little
literature reporting the behaviour of MEMS switches at cryogenic
temperature. In this Letter, we report our recent findings on low
temperature measurements of MEMS metal switches.
Design of metal switch: The layout of the metal switch is shown in
Fig. 1. The metal switches were fabricated on a 0.52 mm-thick quartz
substrate. The switches consist of two electrostatically actuated nickel
pads and a small central gold-plated nickel RF contact pad. The RF
contact pad is electrically isolated and physically connected to the
actuation pads by a thin dielectric membrane of silicon nitride. The
switch actuates on a gold 50 O coplanar waveguide (CPW) of
thickness 3 mm, and signal line width 50 mm. Below the RF contact
pad of the switch is a 5 mm gap in the signal line. The RF contact pad
bridges this gap by direct contact when the actuation voltage is
applied. The height of the RF contact pad above the transmission
line, before actuation, is 1.2 mm. The two coplanar ground planes are
connected via air-bridges. The fabrication process of a similar metal
switch has been discussed in [4]. The size of the switch (without the
biasing lines) is about 0.5 0.32 mm.
coplanar
line
biasing
line actuation
pad
ground
plane 
RF
contact pad
Fig. 1 Layout of metal switch
Measurement results: Measurements of the metal switches were
carried out using a cryogenic probe station. Two special 90 micro-
wave bent probes [5] with a pitch size of 150 mm were used.
Measurements were performed for metal switches on a die. Each
switch is slightly different in size in terms of the width and length of
the actuation pads compared with others. S-parameters were measured
using an Agilent 8722ES network analyser. Calibration was
performed at 290 K using Cascade Microtech software (WinCal);
the calibration technique used was Thru-Reflect-Reflect-Match
(TRRM). The measured insertion losses for three of the switches at
room temperature and at 10 GHz were 0.05, 0.16 and 0.14 dB,
compared with simulated insertion loss of 0.04 dB. The isolations
were in the range of 20 to 30 dB. Measurements were taken at
different temperature intervals. At a particular temperature, bias
voltage was applied to the metal switch through a DC probe.
Actuation voltage was recorded when S21 decreased suddenly to
close to about 0 dB (switch closed). Fig. 2 shows the recorded
actuation voltages for a number of metal switches over the tempera-
ture range 290 to 10 K.
0 20 40 60 80 100 120 140
temperature, K
160 180 200 220 240 260 280 300
10
15
a
ct
iv
at
io
n 
vo
lta
ge
, V
20
25
30
35
40
45
50
55
60
65
C3R9
C6R8
C7R9
C1R7
C2R5
C3R2
(1)
Fig. 2 Measured actuation voltages for number of switches against
temperature and calculated voltages from (1)
Note that the actuation voltages of the metal switches were increased
as the temperature reduced. Most of the switches were actuated at a bias
voltage between 30 and 60 V. It was observed that the switches become
more difficult to actuate at low temperatures. Goldsmith and Forehand
[6] published a simple theoretical model for temperature variation of
actuation voltage and reported the same observation of increment in the
actuation voltage of MEMS switches as the temperature goes down but
their measurements only went down to about 50C. They attributed
the increase in actuation voltage to increasing stress in the actuators, as
a result of the difference in thermal expansion coefficients between the
metal and the quartz substrate.
An approximate formula to calculate the actuation or pull-in voltage
for these metal switches is given by [6, 7]:
Vp ﬃ
ﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ
64gð1 vÞtg30
27L2e0
ðDaEðTzs  T ÞÞ
s
ð1Þ
where the geometrical factor g¼ 1, n is the Poisson’s ratio for nickel
(0.31), t is the thickness of the nickel (2.5 mm), g0 is the OFF state gap
between the actuation pad and the bottom electrode (1.2 mm), L is the
length of the actuation pads (500 mm, assuming they are one piece and
measuring from end to end), Da is the difference in thermal expansion
coefficient between nickel and quartz (12.8 106 K1), E is the
Young’s modulus for nickel (200 GPa), Tzs¼ 350 K is the effective
temperature of zero stress in the nickel (dependent on deposition
temperature and as-deposited stress), T is the operation temperature
and e0 is the permittivity of air. Equation (1) assumes that the actuators
are solid bars and so would tend to overestimate the true value as the
actuators have hinged sections to allow easier bending.
Fig. 3 shows an example of measured insertion loss and isolation up
to 20 GHz at 290 and 80 K. At 290 K, the switch actuated at 29 V, the
actuation voltage increased slightly to 30 V at 80 K. The ripples at 80 K
measurements are due to calibration as the calibration was performed at
290 K.
An important design consideration is to know the percentage of
switches that worked at low temperatures if the switches are to be used
with superconducting microwave devices. In this experiment, a total
number of 28 switches were measured. It was revealed that at 290 K
about 82% of the switches actuated, and this decreased to 39% at 80 K
ELECTRONICS LETTERS 12th October 2006 Vol. 42 No. 21
and further at lower temperatures. If we consider only working switches
at 290 K, about 48% of these switches worked at 80 K.
0 2 4 6 8
frequency, GHz
in
se
rti
on
 lo
ss
, d
B
10 12 14 16 18 20
is
ol
at
io
n,
 d
B
–60
–50
–40
–30
–20
–10
0
–20
–18
–16
–14
–12
–10
–8
–6
–4
–2
0
290 K
80 K
switch actuated (ON state)
no bias (OFF state)
actuation voltage: 29 V(290K)
                              30 V(80K)
Fig. 3 Example of measured insertion losses and isolations at 290 and
80 K (switch C3R2)
Conclusion: Low-temperature performance of MEMS metal switches
has been investigated. Measurements showed an increase in the
actuation voltage as the temperature went down to 10 K. It was
found that the working percentage of these switches decreases at
low temperature.
Acknowledgment: This work was supported by the Birmingham
University Portfolio Award.
# The Institution of Engineering and Technology 2006
15 August 2006
Electronics Letters online no: 20062551
doi: 10.1049/el:20062551
H.T. Su (School of Engineering, Swinburne University of Technology,
Malaysia)
I. Llamas-Garro (Large Millimeter Telescope, National Institute for
Astrophysics, Optics and Electronics, Mexico)
E-mail: llamasi@iee.org
M.J. Lancaster and M. Prest (School of Electronic, Electrical and
Computer Engineering, The University of Birmingham,
United Kingdom)
J.-M. Kim and Y.-K. Kim (School of Electrical Engineering and
Computer Science, Seoul National University, Seoul 137-724, Korea)
J.-H. Park (LG Electronics Institute of Technology, Seocho-Gu, Seoul
137-724, Korea)
C.-W. Baek (School of Electrical and Electronics Engineering,
Chung-Ang University, Korea)
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ELECTRONICS LETTERS 12th October 2006 Vol. 42 No. 21


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Performance of RF MEMS switches at low temperatures

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