mems varactor paper 257
TRANSCRIPT
Design and Analysis of RF MEMS Varactor for
Extended Tuning Range
Presented by
Pooja Srivastava
IIIT-Allahabad
Outline• Introduction
• MEMS• Varactor• Types of Varactor
• Motivation• Varactor model
• Operation principle
• Varactor structure• Design parameters• Simulations and results• Conclusions• References
Introduction• MEMS
• Micro-Electro-Mechanical-System• Integration of electrical unit, mechanical unit, sensor and actuator on the
single substrate.
• Varactor• Variable capacitor.• Capacitance varies with DC voltage.
• MEMS Varactor: Advantages• Low power• High sensitivity• Robust device
• Solid State Varactor: PN- junction diodes.
Schottky diode.
Metal oxide semiconductor (MOS) capacitor.
• RF-MEMS Varactor:Interdigited capacitors.
Zipping Varactor.
Ferroelectric varactors.
Varactor Classification
Motivation • RF-MEMS device• Applications (High Frequency)
• VCO• Frequency selective circuits• Amplifiers
• Earlier (Solid State varactors)• PN-junction diode• Schottky diode• MOS capacitor
• Disadvantages:• small tuning range• lower quality factor• Difficult ON-Chip realization
• Limitations• Pull-in-Voltage
• Maximum tuning range= 1.5
• Technique to increase the tuning range• Two gap capacitors.• Three parallel plate configurations. • Digital varactors by using a bank of MEMS switches.
• Lateral comb structures in place of parallel plate.
• In this paper extended tuning range structure is constructed and analyzed.
Parallel Plate Configuration
Varactor Model• Parallel plate capacitor.• Electrostatically actuated.• Operation:
Capacitance between two plates can be given as
C = ɛ
Change in capacitance
C = ɛ
And the tuning range is defined as
Tuning range =
If upper plate further goes beyond the distance d/3, because of the applied voltage then due to the increased electrostatic force the two plates will snap down and the tuning range is limited to 50%
Basic model of two parallel plate Varactor.
• When a DC bias is applied, an attractive electrostatic force is generated between the plates and it is given as:
= • An effective spring constant ke for the electrostatic force can be defined as the
• At the equilibrium condition the magnitude of both the forces and are equal, so
=
• The expression of in terms of is
• The pull-in voltage is given as the
• Vp limits the tuning range to 50%
Proposed Varactor Structure• Variable plate height architecture is used.
• Top plate (P1): four cantilever beam.• Material: Ni or Cu.
• Bottom plate: Gold
P1
Actuationpoint
Design Parameters
Actuation electrode (plate
P2)
90µm x 90µm x 0.5µm
Proof mass (plate P1) 109µm x 109µm x 2µm
Small Beam directly
attached to the plate (for
Actuation)
50µm x 10µm x 2µm
Beam (For support) 100µm x 10µm x 2µm
Initial gap between upper
plate and actuation
electrode (d1)
2µm
Initial gap between upper
plate and ground plate (d2)
3µm
Simulation And Results• Electromechanics Physics.• Voltage range:0-20 Volts• Pull in voltage: 17.8 Volts
3D plot of the deformed suspended top plate after applying DC voltage
C-V characteristics• Capacitance change for different applied voltages.• Voltage: 0-18 volts• Capacitance: 0.037 pF – 0.2417 pF• Cmax: 0.2417 pF @ 17.6 Volts• Tuning range: 6.53• Capacitance ratio: 6.53:1
Displacement vs. Voltage Plot• Displacement between plate P1 and P2 is a function of applied
voltage.• Initial plate gap: 2 µm• Gap before pull in: 1.6 µm
Conclusion
• MEMS Varactor model for extended tuning range is developed.
• Model is simulated to obtain tuning range greater than 1.5.
• Sudden change in the measured capacitance is around 17.8 Volts.
• Structure pull-in voltage: 17.8 volts.
• Change in capacitance: 0.037pF to 0.2417pF
• Plate displacement: 0.4µm (from 2µm to 1.6µm)
• Cmax: 0.2417 pF
• Cmin: 0.037 pF
• Tuning range achieved: 6.53
References
• J. Iannacci, A. Faes, B. Margesin, "MEMS Technology for RF Passive Components", Proc. of the 4th Int. Symposium on Applied Sciences in Biomedical and Communication Technologies (2011).
• M.Rahimi ,S.S.Jamuar , M.N.Hamidon, M.R.Ahmad and S.A.Mousavi, "The Design and Simulation of an Optimized MEMS Varactor with High Factor for RF Circuits", IEEE Int. Conf. on semiconductor Electronics, pp:161-165, 2008.
• A. Gallant and D Wood, "The role of fabrication techniques on the performance of widely tunable micro machined capacitors", International Journal of Sensors and Actuators, pp: 423-431, 2004.
• J. Z. Chang Liu, J. S. Aine, J. Chen, and S.M. Kang, “Development of a Wide Tuning Range MEMS Tunable Capacitor for, Wireless Communication Systems”, Int. conf. on Electron Devices Meeting, Technical Digest, pp: 403-406, 2000.
• C. L. Goldsmith, A. Malczewski, Z.J. Yao, S.Chen, J. Ehmke and D. R. Hinzel, '' RF MEMS Variable Capacitors for Tunable Filters," Int. J. RF and Microwave CAE, vol. 9, pp: 362-374, 1999.
• J. Yao, S. Park, and J. DeNatale, "High tuning ratio MEMS based tunable capacitors for RF communicationsapplications", Tech Dig Solid-State Sensors Actuators Workshop, pp: 124-127, 1998.
• Rebeiz, G. M., RF MEMS Theory, Design, and Technology, John Wiley & Sons, Inc., Hoboken, New Jersey, 2003.
Thank you