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