1. Design and Analysis of RF
MEMS Varactor for
Extended Tuning Range
Presented by
Pooja Srivastava
IIIT-Allahabad

2. Outline
• Introduction
• MEMS
• Varactor
• Types of Varactor
• Motivation
• Varactor model
• Operation principle
• Varactor structure
• Design parameters
• Simulations and results
• Conclusions
• References

3. 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

4. Varactor Classification
• Solid State Varactor:
PN- junction diodes.
Schottky diode.
Metal oxide semiconductor (MOS) capacitor.
• RF-MEMS Varactor:
Interdigited capacitors.
Zipping Varactor.
Ferroelectric varactors.

5. 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

6. Parallel Plate Configuration
• 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.

7. Varactor Model
•
Basic model of two parallel plate Varactor.

8. •

9. Proposed Varactor Structure
• Variable plate height architecture is used.
Actuation
point
• Top plate (P1): four cantilever beam.
• Material: Ni or Cu.
• Bottom plate: Gold
P1

10. Design Parameters
Actuation electrode (plate 90µm x 90µm x 0.5µm
P2)
Proof mass (plate P1)
Small
Beam
109µm x 109µm x 2µm
directly 50µm x 10µm x 2µm
attached to the plate (for
Actuation)
Beam (For support)
100µm x 10µm x 2µm
Initial gap between upper 2µm
plate
and
actuation
electrode (d1)
Initial gap between upper 3µm
plate and ground plate (d2)

11. 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

12. 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

13. 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

14. 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

15. 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.

16. Thank you