3A-2 – RF–MEMS, NEMS
MNE’06 Micro- and Nano- Engineering
Fabrication of a Tunable RF MEMS Inductor on Silicon in a sub-150°C Process
Stella Chang and Siva Sivoththaman
Centre for Integrated RF Engineering, Department of Electrical and Computer Engineering
University of Waterloo, Waterloo, N2L 3G1 Canada
phone: 519–888–4567 e-mail: stella@alumni.uwaterloo.ca
Key words: inductor, PECVD amorphous silicon, RF MEMS,
One of the major challenges of successfully integrating radio frequency integrated circuits (RFIC)
on silicon (Si) continues to be the poor performance of inductors. The low resistivity Si substrate is a
cost-effective solution, however it is a major source of energy loss and limits the quality factor (Q) of
inductors at high frequency. Conventional oxides introduce considerable stress due to the difference
in thermal expansion coefficient (TEC) with Si and stoichiometric SiO2 requires high temperature
processing. Plasma enhanced chemical vapor deposition (PECVD) amorphous Si (a-Si) is a low
temperature candidate for improving the isolation of RF devices on Si [1]. It has been explored as an
alternative to polysilicon due to comparable mechanical properties at a considerably lower deposition
temperature for the potential of microelectromechanical system (MEMS) integration [2].
MEMS Inductors have been fabricated using surface and bulk micromachining to elevate and
suspend the coil from the lossy Si substrate. Tunable inductors would further benefit wireless
communication circuitry where accurate impedance matching and frequency tuning would
increase the flexibility and reliability of the RF system.
In this work, a tunable RF MEMS inductor based on the bimorph effect of an amorphous silicon
and aluminum structural layer is presented. The inductor is fabricated in a sub-150°C surface
micromachining process outlined in figure 1.(a) wafer preparation of bare c-Si substrate (8-12 Ω cm
resistivity), (b) PECVD 2 µm a-Si at 150°C, (c) sputter 1 µm Al and wet etch to form contacts, (d)
spin-on sacrificial layer (non-photosensitive resist) and dry etch vias to access contacts, (e) PECVD 1
µm a-Si at 150°C and dry etch with SF6, (f) sputter 2 µm Al and wet etch to form inductor coil, and
(g) remove the sacrificial layer by wet etching and CO2 drying. When the device is released, the
inductor coil warps due to the intrinsic stress of the two thin films. The tunability of the inductor is
based on the difference in TEC of the a-Si/Al layer. When voltage is applied, the temperature of the
bimorph increases due to joule heating and the structure deforms. From scanning electron microscopy
(SEM) analysis shown in figure 2, the outer turns of the inductor coil reach a vertical height of 450
µm when the sacrificial layer is removed. Due to the symmetry of the layout, the released inductor is
also symmetric and agrees well with thermomechanical simulations that were performed in
Coventorware.
The performance of the inductor is extracted from the measured S-parameters from 50 MHz to 20
GHz. Figure 3 shows a tuning range of 32% with high inductance (5.6-8.2 nH) is achieved by the
application of a DC voltage. The structure is completely flat when 2 V is applied across the terminals
of the two port device. With no actuation, the peak quality factor is 15 at 4 GHz and the selfresonance frequency is 7 GHz as illustrated in figure 4. When voltage is applied, the Q rolls off at a
lower frequency because the distance between the inductor and the lossy Si substrate is reduced. We
present a tunable inductor with significant tuning range and reasonable Q. The device is fabricated on
Si in a low temperature (150°C) process which enhances the potential for system integration where a
low thermal budget is desired.
[1] S. Chang, S. Sivoththaman, Can. J. Elect. Comput. Eng., 30, (2005) 69-72.
[2] S. Chang, S. Sivoththaman, J. Micromech. Microeng. 16 (2006) 1307-1313.
3A-2 – RF–MEMS, NEMS
MNE’06 Micro- and Nano- Engineering
Fabrication of a Tunable RF MEMS Inductor on Silicon in a sub-150°C Process
Figure 1. Fabrication process of tunable RF MEMS inductor on Si.
Figure 2. SEM image of fabricated RF MEMS tunable inductor, viewed at 75° tilt.
The vertical height of the outer turns of the inductor reaches hundreds of microns.
Figure 3. Measured inductance values of
fabricated RF MEMS tunable inductor with
0 to 2 V applied across the terminals. A 32%
tuning range of the low frequency
inductance is achieved (5.6 to 8.2 nH).
Figure 4. Measured quality factor of
fabricated RF MEMS tunable inductor with
0 to 2 V applied across the terminals. A peak
Q of 15 at 4 GHz is measured when no
voltage is applied to the device.