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Large tuning ratio high aspect ratio variable capacitors using leveraged bending

Abstract

Variable capacitors are a key component in Radio Frequency Micro Electro-Mechanical Systems (RF MEMS). They comprise fixed and flexible electrodes. Deformation, or actuation, of the flexible electrode changes the capacitance of the capacitor. This way, electrical properties of high frequency circuits can be modified. Traditionally, variable capacitors are based on a planar layout architecture, while a newer, vertical-wall, quasi three-dimensional approach theoretically enables increased device performance. Such devices depend on high aspect ratios, i.e. relatively high micro structures with very thin walls and gaps. A few vertical-wall variable capacitors made of nickel or gold have been fabricated to date, using deep X-ray lithography and subsequent electroplating (Achenbach et al. 2006; Klymyshyn et al. 2007, 2010) as the fabrication approach. They feature, amongst others, excellent quality factors of Q ≤ 95 at 5.6 GHz with 50 Ω reactance, but suffer from a very limited tuning range of the capacitance value (tuning ratio of, e.g., 1.38:1). The devices presented here exploit the same architecture and materials selection, resulting in similar, excellent Q-factors, but feature a different electrode layout approach, referred to as leveraged-bending. This layout is based on pulling a flexible electrode sideways, towards a fixed electrode, increasing the capacitance when actuating the variable capacitor. The leveraged bending approach theoretically enables infinitely high tuning ratios for components with perfect structure accuracy. To date, a significantly increased tuning ratio of 1.9:1 has been demonstrated. Limiting factors are an electrically non-ideal layout geometry chosen as a compromise to increase the fabrication yield, and structure deviations of ~1.6 μm from CAD layout to the electroplated component. Electrostatic actuation requires voltages between 0 and 72 V for capacitance values on the order of C = 0.3 pF at device dimensions of about 1.5 mm overall length, 5–10 μm gap and wall widths, and 100 μm metal height. Device performance measured with a vector network analyzer is in 97 % agreement with simulation results based on two-dimensional electrostatic-structural coupling (ANSYS Multiphysics) and three-dimensional electromagnetic field simulations (Ansoft HFSS). These simulations also indicate that an optimized gap geometry will allow to reduce the actuation voltage required by up to 40 %.

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References

  1. Achenbach S, Klymyshyn D, Haluzan D, Mappes T, Wells G, Mohr J (2006) Fabrication of RF MEMS variable capacitors by deep X-ray lithography and electroplating. Microsyst Technol 13:343–347

    Article  Google Scholar 

  2. Bakri-Kassem M, Mansour RR (2009) Linear bilayer ALD coated MEMS varactor with high tuning capacitance ratio. J Microelectromech Syst 18:147–153

    Article  Google Scholar 

  3. Davis JR (1998) “Metals Handbook: Desk Edition”, Materials Park, OH: ASM International, 2nd edn

  4. De Los Santos HJ (1999) Introduction to Microelectromechanical (MEM) Microwave Systems. Artech House, Boston

    Google Scholar 

  5. Dec A, Suyama K (1998) Micromachined electro-mechanically tunable capacitors and their applications to RF IC’s. IEEE Trans Microw Theory Tech 46:2587–2596

    Article  Google Scholar 

  6. Haluzan D “Electrostatically actuated LIGA-MEMS structures with high aspect ratio beams for RF applications and mechanical property extraction”, Ph.D. Thesis, University of Saskatchewan, Saskatoon, Canada, 2012

  7. Haluzan D, Klymyshyn D, Achenbach S, Börner M, Mohr J (2012) VM-TEST: mechanical property measurement using electrostatically actuated vertical MEMS test structures fabricated in thick metal layers. Microsyst Technol 18:443–452

    Article  Google Scholar 

  8. Hung ES, Senturia SD (1999) Extending the travel range of analog-tuned electrostatic actuators. J Microelectromech Syst 8:497–505

    Article  Google Scholar 

  9. Jou J, Liu C, Schutt-Aine J (2001) Development of a wide tuning range two-parallel plate tunable capacitor for integrated wireless communication systems. Int J RF Microw Comput Aided Des 11:322–329

    Article  Google Scholar 

  10. Klymyshyn DM, Haluzan DT, Börner M, Achenbach S, Mohr J, Mappes T (2007) High aspect ratio vertical cantilever RF-MEMS variable capacitor. IEEE Microwave Wirel Compon Lett 17:127–129

    Article  Google Scholar 

  11. Klymyshyn DM, Börner M, Haluzan DT, Gono Santosa E, Schaffer M, Achenbach S, Mohr J (2010) Vertical high-Q RF-MEMS devices for reactive lumped-element circuits. IEEE Trans Microw Theory Tech 58:2976–2986

    Article  Google Scholar 

  12. Osterberg PM (1995) “Electrostatically actuated microelectromechanical test structures for material property measurements”, Ph.D. Thesis, Massachusetts Institute of Technology, Cambridge, Massachusetts

  13. Osterberg PM, Senturia SD (1997) M-TEST: a test chip for MEMS material property measurement using electrostatically actuated test structures. J Microelectromech Syst 6:107–118

    Article  Google Scholar 

  14. Ruzzu A, Matthis B (2002) “Swelling of PMMA-structures in aqueous solutions and room temperature Ni-electroforming”. Microsyst Technol 8(2–3):116–119

    Article  Google Scholar 

  15. Young DJ and Boser BE (1996) “A micromachined variable capacitor for monolithic low-noise VCOs”, in Solid-State Sensor and Actuator Workshop (Hilton Head, SC), pp 86–89

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Correspondence to S. Achenbach.

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Achenbach, S., Haluzan, D.T., Klymyshyn, D.M. et al. Large tuning ratio high aspect ratio variable capacitors using leveraged bending. Microsyst Technol 20, 1807–1813 (2014). https://doi.org/10.1007/s00542-013-1940-7

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Keywords

  • Actuation Voltage
  • Capacitance Electrode
  • Variable Capacitor
  • Auxiliary Structure
  • Tuning Ratio