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Application of MEMS Tunable Capacitor for RF VCOs
Soheil BigdeliDepartment of Electrical Engineering &
Computer Science
University of California, Berkeley
Abstract
In this work, a voltage controloscillator is used to
characterize aMEMS tunable capacitor. The upper
plate of the capacitor rises above thesurface of the substrate
and as aresult, the gap between the two platesof the parallel-plate
capacitor gets
bigger or smaller. Therefore thecapacitance changes. A brief
description of fabrication process for the variable capacitor is
also
presented. HSIPCE is used tomeasure Q, and the result shows
thatthe capacitor presented in this paper is not suitable for VCO
design.
Introduction
Variable RF capacitors are major control components of
highfrequency receivers and transmitters.Also, variable capacitors
are largelyused for designing voltage controloscillators, which
themselves are
main components of phase lock loops(PLLs). If the output
frequency of anoscillator varies by a voltage, then thecircuit is
called VCO [5]. Almostall VCOs use off-chip tank capacitances to
control the frequency.The schematic of a tank capacitance
is shown in figure 1. Having a tank capacitance means that some
off-chipcircuitry is required. Therefore morefabrication is
required and the overallcost of the design increases. Also,having
off-chip circuitry adds to thecomplexity of the design and thereare
reliability issues that need to beaddressed. In addition, many of
thetodays passive RF components have
big sizes [4]. Therefore a monolithicdesign for passive devices
can help to
Figure 1: Tank Capacitance
reduce the negative effects mentionedabove.
Theory and Design
Capacitance can be varied by either changing the dielectric
thickness or bychanging the overlapping area betweenthe two
parallel plates. This is inaccordance with the equation
(E1)assuming a constant dielectric.
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C= A/t (E1)
The design used in this work is basedon the Micro-Elevator for
Self-Assembly (MESA) [4]. In thismethod, the dielectric (air for
thisdesign) thickness is changed in order to make a variable
capacitor. Threesurface micro-machined plates areconnected to each
other using hingesas shown in figure 2 (the other twosides of the
middle plate are alsoconnected to two plates for better support but
they are not shown in
figure 2 for simplicity). Twoelectrostatic actuators
(comb-drives)are biased such a way that they movetowards each other
with the samespeed. As a result, the top plate of the capacitor
rises above the surface(max height is 250 m from surface)of the
silicon and consequently thegap between the two plates widens.The
bottom plate of the capacitor is
stationary. The entire process isshown in figure 2.
Figure 2 : The variable capacitance process
Fabrication Process
The standard MUMPS process will be used to fabricate the
variablecapacitor. MUMPS is a three-layer
polysilicon surface micro-machining process.
Results and Conclusion
The designed capacitor is used in thecircuit shown in figure 3.
HSPICEsimulation shows that the Q value of 15 at 950MHZ is
achieved. Theobtained for value of Q is very lowcompare to other
designs [1], [7].
Figure 3: Design of a VCO with currentmirrors
Lateral actuation of the comb-drives,which is transferred to the
upper
plates vertical movement, causes theQ to drop. Also, contact
resistance
between the hinges reduces the Q [4].The main problem with this
design isthe variation of the capacitance.Because the inverse
relationship
between the capacitance and thedielectric thickness, the
capacitancevariation is huge (500fF-25fF) [4].Consequently the
variablecapacitance is not suitable for fine-tuning. Fine-tuning is
required for the voltage control oscillator.
Some changes to the design willimprove the value of Q. For
example, using thermal actuator instead of comb-drive
theoreticallyimproves the Q. Since thermalactuator can be used to
move theupper plate up without any lateral
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movement, the hinges can beeliminated from the design. As
aresult, the hinge resistance iseliminated from the design. Also,
byusing a 2-D actuator, instead of changing the dielectric
thickness, theoverlapping area between the two
parallel plates can be changed.Referring to (E1), area is
linearly
proportional to the capacitance;therefore huge variation of
capacitance can also be eliminated.Other designs have
beendemonstrated for the variablecapacitor [1], [2], [3], [7].
AcknowledgementThe author would like to thank Professor Roger T.
Howe for hisencouragement and support.
Reference
1. A. Mohamed, H.Elsimary,M.Ismail, Design of MEMS
Tunable Capacitor all MetalMicrostructure for RFWireless
Application, AnalogVLSI Lab, Ohio StateUniversity
2. K.F.Harsh, W.Zhang,V.M.Bright, Y.C.Lee, Flip-Chip Assembly
for Si-BasedRF MEMS, Department of
Mechanical Engineering,
University of Colorado,Boulde.
3. C.L.Goldsmith,A.Malczewski, Z.J.Yao,S.Chen, J.Ehmke,
D.H.Hinzel,RF MEMS VariableCapacitor for Tunable Filters,Raytheon
SystemsCorporation, Dallas, Texas
4. L.Fan, R.T.Chen, A.Nespola,M.C.Wu, Universal MEMSPlatforms
for Passive RFComponents: Suspended
Inductor and VariableCapacitors, Department of Electrical
Engineering,University of California, LosAngeles
5. B.Razavi, RFMICROELECTRONICS,Prentice Hall
CommunicationEngineering and Engineering
Technologies Series, 1998
6. D.J.Young, B.E.Boser, AMicromachined VariableCapacitor for
Monolithic Low-
Noise VCOs, Solid-StateSensor and Actuator Workshop, Hilton
HeadIsland, SC, June 3-6, 1996
