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6-2014
All-Silicon Technology for High-Q Evanescent
Mode Cavity Tunable Resonators and Filters
Muhammad Shoaib Arif
Birck Nanotechnology Center, Purdue University, marif@purdue.edu
Dimitrios Peroulis
Purdue University, Birck Nanotechnology Center, dperouli@purdue.edu
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Arif, Muhammad Shoaib and Peroulis, Dimitrios, "All-Silicon Technology for High-Q Evanescent Mode Cavity Tunable Resonators
and Filters" (2014). Birck and NCN Publications. Paper 1638.
http://dx.doi.org/10.1109/JMEMS.2013.2281119
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JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 23, NO. 3, JUNE 2014
727
All-Silicon Technology for High-Q Evanescent
Mode Cavity Tunable Resonators and Filters
Muhammad Shoaib Arif, Student Member, IEEE, and Dimitrios Peroulis, Member, IEEE
Abstract— This paper presents a new fabrication technology
and the associated design parameters for realizing compact
and widely-tunable cavity filters with a high unloaded quality
factor ( Q u ) maintained throughout the analog tuning range.
This all-silicon technology has been successfully employed to
demonstrate tunable resonators in mobile form factors in the
C, X, Ku, and K bands with simultaneous high unloaded
quality factors (≥500) and tuning ratios (≥1:2). It is shown that
by employing high-precision micro-fabrication techniques and
careful modeling, the measured RF and tuning performance of
the fabricated device closely match the simulated results. The
capability of monolithic (system-on-chip) integration, low-cost
batch processing, and compatibility with CMOS processing is
some of the key advantages of this 3-D tunable filter fabrication
technology over conventional approaches. This technology also
has the potential to be extended to produce tunable resonators
and filters in the millimeter wave region.
[2013-0123]
Index Terms— Evanescent-mode cavity, MEMS,
tunable resonator, electrostatic, tunable filter, high- Q.
silicon,
I. I NTRODUCTION
T
HE vision of the next-generation software defined radio
(SDR) is to have a single reconfigurable radio with
multi-band and multi-standard compatibility, that can operate
effectively in dense electromagnetic radiation environments
by having the required frequency agility, necessary to communicate using multiple waveforms in rapid succession. The
SDR concept has been under development for several years
in both military and commercial sector [1], [2] but it is still
far from complete. One main area requiring significant innovation is the tunable filters for their reconfigurable RF frontend (RFFE) [3]. The search for a tunable filter technology
that completely fulfills the stringent SDR requirements like
low insertion loss (high-Q), high tuning ratio (T.R), easy
integration, high linearity, high power handling, small size and
low manufacturing cost is ongoing [3]–[5].
Several existing tunable filter technologies have been evaluated for SDR applications. The yttrium-iron-garnet (YIG)
crystal based tunable filters demonstrate a high-Q (≥1 000)
with multiple octave tuning range [6], [7]. However, YIG
filters are not well suited for portable devices due to their
Manuscript received April 23, 2013; revised August 13, 2013; accepted
September 1, 2013. Date of publication September 20, 2013; date of current
version May 29, 2014. This work was supported by DARPA under the
Purdue Microwave Reconfigurable Evanescent-Mode Cavity Filters Study.
Subject Editor D. Elata.
The authors are with the School of Electrical and Computer Engineering,
Birck Nanotechnology Center, Purdue University, West Lafayette, IN 47907
USA (e-mail: marif@purdue.edu; dperouli@purdue.edu).
Color versions of one or more of the figures in this paper are available
online at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/JMEMS.2013.2281119
relatively large size, difficulty in planar integration, high power
consumption, need of controlled temperature environment
for proper operation, and high cost [4]. The planar or 2-D
tunable filter technology is another extensively explored
approach. In these filters, transmission lines are loaded with
lumped tunable elements (varactors) for frequency tuning.
Some of the notable tunable filter demonstrations are based on
solid-state (e.g Schottky or p.i.n diode) [8]–[10], ferroelectric
(e.g Barium-Strontium-Titanate (BST)) [11], [12], and microelectromechanical systems (MEMS) [13]–[15] varactors.
Planar filters have the advantages of small volume, high tuning
ratio, easy on-chip integration and good power handling.
However, their RF performance is suboptimal for SDR due
to their low-Q (≤200) [4], [16].
A third tunable filter family is the non-planar (3-D)
approach, which includes cavity-based technologies with integrated tuners. This approach is rapidly gaining popularity
due to their significantly small size (compared to wavelength)
and weight unlike half-wave cavity resonators, their ability to
achieve a higher Q u as compared to planar approach, wide
tuning, large spurious-free region, high linearity and good
power handling [5], [17]–[26]. Of these 3-D technologies,
the capacitive-post loaded cavity filter technology has demonstrated state-of-the-art performance in the S and C-bands with
a simultaneous high-Q (≥300) and tuning ratio (≥1:2) [5],
[17], [18]. In the capacitive-post loaded filters, the resonant
frequency is highly sensitive to the parallel-plate capacitance
between the post-top and cavity ceiling. By incorporating a
MEMS tuner directly above the capacitive post (as a moveable
cavity ceiling), few micrometers of tuner displacement causes
several gigahertz shift in the resonant frequency. High-Q
resonators/filters with an octave (1:2) tuning range or higher
can therefore be realized in both microwave and millimeter
wave frequencies. Table I summarizes some simulated designs
of such capacitive-post loaded resonators with a maintained
Q u ≥ 500 throughout the frequency tuning.
It may be noted from Table I that fabrication tolerances
become critical in realizing repeatable and reliable filters,
especially for millimeter wave applications. Existing implementations are based on cavities machined in the metallic
or ceramic substrates using conventional tooling techniques,
and their tuners are attached using solder, physical pressure
or conductive epoxy [5], [17], [18], [27]. A conventionally
machined cavity suffers from high fabrication uncertainties
(≥ ±20 μm) which inhibits reliable scaling of capacitivepost loaded filter technology beyond X-band (a capacitivepost radius of ≤100 μm is needed with a precision of
≤ ±10 μm). Moreover, the uncertainty due to surface
1057-7157 © 2013 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.
See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.
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TABLE I
T UNABLE C APACITIVE -P OST L OADED R ESONATOR D ESIGN E XAMPLES FOR M ICROWAVE AND M ILLIMETER WAVE
roughness in the metallic or ceramic cavity substrates, and the
uniformity issues inherent with the solder paste or conductive
epoxy attachment like uneven application, shrinkage, and
irregular grain size can cause substantial error in the desired
post-ceiling gap (>2 μm) [5], [18], [27]. These uncertainties
tend to create a significant disparity between the designed and
fabricated performance. For instance, in the designs of Table I,
a 1-μm error in dmin due to epoxy attachment can lead to
>15% reduction in the tuning ratio.
Consequently, a fundamentally different fabrication technology with tighter tolerances is needed for high-frequency
applications. This requirement becomes the premise for
all-silicon capacitive-post loaded tunable filter technology
based on micro-fabrication. Several topologies for high-Q
fixed-frequency (static) resonators and filters using micromachined silicon cavities have already been demonstrated in
the microwave and millimeter-wave regions [28]–[33]. Similarly, single-crystal-silicon has also been extensively explored
as a choice material for MEMS actuators due to its superior
mechanical properties [5], [34]–[37]. The all-silicon tunable
filter technology aims to successfully exploit both the RF and
mechanical advantages of silicon. Two noticeable performance
outcomes demonstrated using this approach include 1) an X to
Ku band capacitive-post loaded cavity tunable resonator with
a tuning range of 9.5 to 17 GHz (1:1.87) and Q u of 650 to
950 using magnetostatic actuation [38], and 2) a C to K-band
electrostatically tunable resonator demonstrating continuous
tuning from 6.1 to 24.4 GHz (1:4) with a Q u varying from
300 to 1000 [39], which is state-of-the-art in the tunable filter
technology for simultaneous high-Q and wide tuning range.
This paper details the groundwork, design methodology and
the fabrication considerations involved in the implementation
of capacitive-post loaded tunable cavity filters using all-silicon
approach. In the design section, simple analytical expressions
are presented to predict and optimize the Q u , tuning range
and biasing voltage. The fabrication section explains the steps
needed to produce high quality tunable filters and to minimize
disparity between the simulated and measured performance.
Some advantages of the all-silicon fabrication technology like
accuracy, high reliability, compatibility with the growing onchip RF technology and low-cost are briefly discussed to
emphasize its potential from commercial viewpoint. Finally,
notable performances of some tunable resonators fabricated
and demonstrated to date are summarized and compared with
some existing tunable microwave filter technologies.
II. A LL -S ILICON T ECHNOLOGY OVERVIEW
In the presented all-silicon approach, silicon is used as the
substrate material for fabricating all constituent components
Fig. 1. (a) The assembly of three individually micro-fabricated constituents
to form an electrostatically-tuned all-silicon capacitive-post loaded cavity
resonator. (b) Schematics and working principle. Tuning is achieved by
applying bias voltage to the electrode, causing a change in capacitive gap
between the cavity post and the Au/Si diaphragm due to electrostatic actuation.
of the tunable resonator. Fig. 1(a) and (b) show the concept
schematics and working principle of an electrostatically actuated all-silicon resonator respectively. The resonator is realized
by combining three individually microfabricated components:
1) A gold-coated cavity with post etched in a bulk silicon
substrate, 2) a gold-coated single-crystal-silicon diaphragm
released by selective etching of a silicon-on-insulator (SOI)
substrate, 3) a micromachined silicon electrode post coated
with a dielectric layer for electric isolation. The diaphragm
SOI is attached to the cavity through gold-gold thermocompression bonding using a gold bonding ring on the SOI
substrate [38]. The thickness of this bonding ring defines the
initial gap between the post-top and the moveable ceiling.
Tuning is achieved through electrostatic actuation of the
diaphragm, when a bias voltage is applied to the Si electrode
housed in the etched recess of the SOI handle. The depth of
the electrode post is kept lower than the SOI handle layer
thickness, to create the initial biasing gap. This bias gap
ARIF AND PEROULIS: ALL-SILICON TECHNOLOGY FOR HIGH- Q EVANESCENT MODE CAVITY TUNABLE RESONATORS AND FILTERS
729
(SiNx ) layer. The silicon wet-etching mask is patterned in the
nitride layer by selectively etching it in SF6 plasma using
a photo-resist (PR) mask. The cavity and post of desired
dimensions are formed using a well-characterized silicon wetetching process. The nitride layer is stripped and the cavity is
sputtered with a gold film to provide uniform metal coverage
to the sidewalls and the post.
B. Diaphragm Fabrication
The Au/Si diaphragm fabrication begins with an SOI wafer
of appropriate device layer thickness. First, a gold film is
deposited on the SOI device layer. Then by using a PR
mold, a Au bonding ring is created over the existing gold
film using electroplating or lift-off process. The thickness of
the bonding ring is defined by the initial post-ceiling gap.
Backside lithography is carried out to pattern the PR mask,
required for creating the diaphragm by etching away the SOI
handle substrate using Deep Reactive Ion Etching (DRIE). The
buried oxide of SOI acts as the etch-stop layer for DRIE.
Finally, the oxide layer is removed using Buffered Oxide
Etch (BOE) to release a smooth and flat gold-coated SCS
diaphragm. Major diaphragm fabrication steps are shown in
Fig. 2(b).
C. Bias Electrode Fabrication
In order to get high precision in the electrode/ diaphragm
gap for electrostatic actuation, the bias electrode is also microfabricated. The electrode fabrication process steps are similar
to cavity wet-etching process and are shown in Fig. 2(c). A Si
post of appropriate height and diameter is wet-etched in the
Si substrate using a (SiNx ) mask. The nitride film is removed
and the bias electrode is uniformly coated with an insulation
film like PR to provide electrical isolation.
D. Assembly
Fig. 2. Major fabrication steps for the constituent parts of the proposed allsilicon resonator. (a) Si cavity with post, (b) Au coated single-crystal-silicon
diaphragm and (c) Si bias electrode.
is determined from the desired filter tuning range through
electromechanical analysis of the diaphragm deflection.
Although the fabrication steps are customized based on
specific designs as in [38] and [39], some important fabrication
steps which represent the core of this all-silicon approach are
given below.
A. Cavity Fabrication
As shown in Fig. 2(a), the fabrication starts with a cavity
substrate Si wafer coated with a film of LPCVD silicon nitride
After their fabrication, the cavity and tuner parts are thoroughly solvent-cleaned and UV-treated to remove any debris or
organic contamination on the surface. They are subsequently
aligned and bonded using Au-Au thermo-compression bonding
carried out under high temperature (∼310 °C) and pressure
(∼5 MPa). The assembly of these micro-machined parts to
form a single resonator is shown in Fig. 1(a).
III. A C APACITIVE -P OST L OADED E VANESCENT-M ODE
C AVITY R ESONATOR D ESIGN
Craven [40] proposed that a cutoff guide (which can be
approximated as a lumped inductance), when appropriately
loaded with capacitive obstacles at suitable intervals, acts as a
waveguide equivalent of a lumped element circuit filter. The
capacitive-post loaded evanescent-mode cavity resonators and
filters are designed on the same principle. The advantages
include considerable size and weight reduction compared to
a half-wave resonator with the same resonant frequency and a
large spurious-free region [28]. Fig. 3 shows the cross-section
of a typical capacitive-post loaded cylindrical cavity operating
in evanescent mode. The important design parameters are
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JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 23, NO. 3, JUNE 2014
Fig. 3. Concept schematics of a capacitive post loaded cavity resonator
showing important dimensions and reactive elements of the cavity.
Fig. 5. Field comparison for a cylindrical and micromachined Si cavity:
(a) Electric field, mainly concentrated at the post, (b) Magnetic field, circling
around the post.
Fig. 4. Change in frequency and unloaded quality factor as a function of
post gap.
capacitive post radius (a), cavity radius (b), capacitive post
height (h) and gap (d) between the post-top and cavity ceiling.
The resonant frequency ( f 0 ) can then be approximated by
[32], [41]
1
(1)
f0 ≈
2π L 0 (C0 + C f f + C pp )
0 πa 2
(2)
C pp =
d
where L 0 and C0 are the fixed equivalent cavity inductance and
capacitance respectively. C pp and C f f are the parallel-plate
and fringing-field capacitances between the post and cavity
ceiling. When the post-height approaches the cavity-height
(highly loaded) such that C pp dominates other capacitances,
the gap d becomes the main resonant frequency determining
parameter, as shown in Fig. 4.
Based on the fabrication technology, various cavity and
post shapes have been explored such as cylindrical [5], [18],
square [17] and pyramidal [31], [32], [38], [39]. From the
design viewpoint, a cylindrical cavity and post is relatively
easy to model as it can be closely approximated as a small
segment of coaxial transmission line shorted at one end, and
a series capacitor at the other [5]. Using this analogy the
resonant frequency, tuning range and Q u for a cylindrical
Fig. 6.
Frequency vs. gap plot for different shaped cavities of similar
dimensions.
cavity can be analytically predicted. The unique shape of the
micro-machined Si cavity and post, due to the anisotropic Si
wet-etching, makes it difficult to relate the Q u and resonant
frequency with cavity dimensions analytically. Therefore,
FEM based solvers like Ansys HFSS® [42] are initially used
even for preliminary design parameters. However, a comparison of the simulated electric and magnetic field patterns
of a cylindrical to pyramidal cavity of similar dimensions
(a, b, h and d) show strong similarities (refer Fig. 5). As a
result, the resonant frequency and Q u of these cavities are
also found similar as shown in Fig. 6 and Fig. 7 respectively.
Hence, the analytical expressions obtained from cylindrical
cavity analysis can provide a good starting point for our allsilicon cavity resonator designs.
ARIF AND PEROULIS: ALL-SILICON TECHNOLOGY FOR HIGH- Q EVANESCENT MODE CAVITY TUNABLE RESONATORS AND FILTERS
731
Fig. 7.
Q u vs. frequency plot for different shaped cavities of similar
dimensions.
A. Resonant Frequency and Tuning Ratio
The LC-tank model from (2) can be expanded to relate the
resonant frequency with cavity dimensions (a, b and h) as
follows:
1
(3)
f0 ≈
2π L 0 (C parasit ic + C pp )
where
C parasit ic = C0 + C f f
μ0 h
ln(b/a)
L0 ≈
2π
2hπ0
C0 ≈
ln(b/a)
C f f ≈ K 1 a0ln(h/d)
(4)
(5)
(6)
(7)
C parasit ic is a capacitance that does not contribute to the
sensitivity of resonant frequency f 0 with change in gap d.
K 1 is an empirically fitted constant which is sensitive to
a/b ratio. For a/b ≤ 0.3, this constant has been found to
be 2.78 [43]. The tuning ratio of the resonator can then be
computed by
C parasit ic + 0 πa 2 /dmin
f max
≈
(8)
T.R =
f min
C
+ πa 2 /d
parasit ic
0
max
where dmin and dmax are the minimum and maximum gaps
between post-top and the cavity ceiling respectively, achieved
through ceiling deflection. From (3) and (8), the f0 and T.R
of the resonator depend on 1) cavity shape and dimensions,
2) minimum gap dmin , and 3) ceiling deflection.
1) Cavity Shape and Dimensions: The effects of a, b and
h on the resonant frequency are highlighted in Fig. 8(a),
(b) and (c) respectively. Reducing any of these dimensions,
corresponds to lowering the overall capacitance and therefore causes the resonant frequency to increase. The resonant
frequency is more sensitive to the post radius a than the
Fig. 8.
Simulated resonant frequency and tuning dependance on cavity
dimensions for different frequency ranges. The Q u is ≥400 for all designs.
(a) post radius, (b) cavity radius, and (c) cavity height.
other two dimensions because a directly impacts the dominant
parallel-plate capacitance, whereas b and h affect the parasitic
capacitance. For millimeter-wave applications, a capacitive
post of very small radius (10s of micrometers) with high
precision is needed. This constraint plays a major role in the
choice of suitable fabrication technology for high frequency
applications. A lower parasitic capacitance compared to parallel plate capacitance results in higher tuning range.
2) Minimum Gap (dmin ): The parallel plate capacitance
C pp , which is the dominant capacitance for tuning purposes,
have an inverse relationship with d. Therefore, for the same
ceiling deflection, a higher tuning ratio can be achieved
through lower dmin .
3) Ceiling Deflection: A higher actuation stroke corresponds to better tuning ratio for the same dmin . The resonator
frequency f 0 tends to saturate as the post-ceiling gap increases
such that C pp approaches C parasit ic . The ceiling deflections
beyond this point has negligible impact on the tuning ratio. The
extent of ceiling deflection is determined by the the actuation
mechanism and the stiffness of the ceiling. The stiffness k
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JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 23, NO. 3, JUNE 2014
Fig. 9. Electrostatic actuation schematics of Au/Si diaphragm when voltage
is applied to the bias electrode.
for a circular diaphragm of radius R and thickness t under
uniform loading can be calculated by [9]:
16π Et 3
+ 4πσ t
(9)
3R 2 (1 − γ 2 )
where E is the Young’s modulus, γ is the Poisson’s ratio and
σ is the residual stress in the diaphragm. The k component
of stiffness depends upon material properties and dimensions,
whereas k depends mainly upon the fabrication technology
and is generally the dominant stiffness contributing mechanism
due to difficulty in fabricating low-stress diaphragms. In case
of a Au/Si diaphragm, which is extensively used in this work,
the Au thickness t Au and residual stress σ Au are the dominant
stiffness contributors due to the stress-free nature of crystalline
silicon [5]. Therefore the k part of the stiffness in (9) is
generally several times higher than k and we can approximate
the diaphragm stiffness to be
k = k + k =
k ≈ k = 4πσ Au t Au
(10)
Electrostatic actuation is the most widely used MEMS actuation scheme due to its near-zero power consumption, designsimplicity and high-speed [9]. Fig. 9 shows the schematic
of an electrostatically actuated Au/Si bi-morph diaphragm.
When a voltage V is applied to the bias electrode, the
electrostatic force pulls the diaphragm towards the electrode
by a displacement x. The diaphragm stiffness k creates a
reactionary restoring force proportional to this displacement.
At equilibrium, the electrostatic and restoring forces balance
each other i.e.
Abias V 2
(11)
kx = 0
2(g0 − x)2
where Abias is the bias electrode area and g0 is the initial gap
between the bias electrode and the un-deflected diaphragm.
Using this parallel-plate actuation of diaphragm, a continuous
deflection can be achieved for only one-third of the actuation
gap (g0 /3), followed by the “pull-in” instability. Therefore,
for analog resonator designs, the initial gap between the
diaphragm and bias electrode should be, at least, three times
the required diaphragm deflection to achieve the desired tuning
range. The bias voltage needed to achieve the required deflection therefore depends mainly upon k, A and g0 . From (9)
and (11), the Au/Si diaphragm parameters σ Au , t Au , R and t Si
should be carefully considered in the design process to determine the bias voltage for the required diaphragm deflection as
showin in Fig. 10.
Fig. 10.
Bias voltage vs. deflection for different parameters of Au/Si
diaphragm when g0 is 45 μm. (a) Residual stress in Au film, (b) Au film
thickness, (c) diaphragm radius, and (d) SOI device layer thickness.
B. Quality Factor
The unloaded quality factor of a cylindrical evanescentmode cavity can be analytically approximated using the following equation [44]:
( a1 + b1 ) 2
Rs
1
+
≈
(12)
Qu
2πμf 0
h
ln( ab )
where Rs is the sheet resistance of the cavity sidewalls.
From (12), the Q u of the resonator at a particular frequency
is mainly dependant on: 1) cavity dimensions and 2) sheet
resistance.
ARIF AND PEROULIS: ALL-SILICON TECHNOLOGY FOR HIGH- Q EVANESCENT MODE CAVITY TUNABLE RESONATORS AND FILTERS
Fig. 11. Q u variation with the change in cavity radius b, for two different
post heights at 5 GHz.
1) Cavity Dimensions: Two general rules can be employed
to maximize the quality factor of the resonator on the basis
of cavity dimensions, provided it does not affect the desired
tuning range: 1) increasing b or h results in higher quality
factor because it allows more volume for energy to be stored
in a resonator, as shown in Fig. 11 and Fig. 12 respectively, as
long as b and h are much smaller than the resonant wavelength,
2) when the post radius a is approximately 0.28× the cavity
radius b, which can be mathematically validated from (12).
2) Sheet Resistance: Q u is inversely proportional to the
resistance offered by the cavity walls to the RF current which
is approximated by:
1
ρ
≈
t
σδ
1
δ= √
πμf 0 σ
Rs =
(13)
(14)
where ρ and σ are the cavity metal resistivity and conductivity
respectively. The metal thickness t can be replaced by the
skin depth δ, provided t δ. Therefore highly conductive
metals like Cu, Au and Ag are desirable for high-Q resonator
fabrication.
IV. FABRICATION T ECHNOLOGY
The important considerations in determining the suitability of all-silicon technology for realizing capacitive-post
loaded tunable microwave filters include precision, reliability,
compatibility and cost.
A. Precision
A successful fabrication technology should be accurate
and consistent. In order to realize high frequency capacitivepost loaded tunable filters, precision is mainly needed in
1) dimensions, 2) surface profile (roughness), and 3) integration of constituent parts.
Fig. 12.
733
Simulated Q u variation with cavity height h at 15 and 22 GHz.
1) Dimensional Accuracy: As can be seen from Fig. 8, for
millimeter wave applications the capacitive post dimensions
are in the order of 10s of micrometers, and a small deviation
in post size can cause a major shift in resonant frequency
and tuning performance. The resolution of existing machining
techniques is inadequate for these tight tolerances. Silicon
wet micro-machining, when carried out in a well characterized, temperature and concentration controlled etch-bath,
can provide the precise dimension control necessary for this
application. High-precision characterization tools like laser
confocal microscope or profilometer are required to accurately
monitor to dimensions. In our work, laser confocal microscope
LEXTT M from Olympus® is used for the etch characterization
which has the capability to measure cavity dimensions with
sub-micrometer precision [45]. The etch-depth (Fig. 13(a)) and
post-area (Fig. 13(b)) is measured at regular intervals and
plotted against etch time (Fig. 13(c)), to extract the lateral
and vertical etch rates. These etch rates can then be used for
fabricating cavities with post of desired dimensions with high
precision.
2) Sidewall Roughness: In transmission lines, RF losses
increase with surface roughness and are especially pronounced
at higher frequencies when the roughness is comparable to
skin-depth [46], [47]. Similarly, since the RF current concentrates to the “skin” of the cavity sidewalls and post, the surface
roughness increases the sheet resistance of the cavity and
degrades the Q u . To estimate the impact of roughness on Q u
at different frequencies in a capacitive-post loaded resonator,
we have used HFSS simulations and found a Q u degradation
trend with increased roughness, as expected. The impact of
roughness on Q u is higher at high frequencies as shown in
Fig. 14. Since the purpose of these simulations is to measure
the trend not the actual values, the roughness is approximated
by uniformly spaced ridges in the cavity base, sidewalls, and
post as shown in the inset of Fig. 14. When the cavities are
fabricated using conventional machining, the tooling process
leaves its tracks and irregular marks on the cavity sidewalls and
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JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 23, NO. 3, JUNE 2014
Fig. 13. Cavity with post wet-etch characterization using laser confocal
microscope. (a) Step height measurement with side wall profile, (b) superimposed images of post-area as it reduces with etching time, showing symmetric
etching around the post. (c) Plot of etch depth and post area vs. etch time.
Fig. 15. Surface, sidewalls and capacitive-post profiles from a conventionally
machined cylindrical cavity in bulk copper (Top) and anisotropic wet-etched
Si cavity (Bottom). Both cavities are 6-mm wide with a post-top diameter of
200-μm.
Fig. 14.
Simulated effect of surface roughness on the resonator Q u
at different frequencies. (Inset) Roughness is approximated in HFSS by
introducing uniformly spaced ridges of roughness height and width.
post, which are extremely difficult to polish in small cavities
(few millimeters) with a fragile rough post in their center.
Depending upon the equipment precision, this roughness can
be in the order of 2–20 μm. Using the wet chemical etching of
silicon, cavity/ posts sidewalls with mirror-like surface finish
and sub-micrometer roughness can be obtained. Fig. 15 shows
sidewall roughness comparison of a conventionally machined
cavity with post in bulk copper to a micro-machined Si cavity
of similar dimension.
3) Cavity-Ceiling Attachment: High conductivity, uniformity and accuracy are three essential requirements in the
cavity-ceiling integration. A highly conductive attachment will
offer less resistance to the RF current whereas a uniform
attachment will result in symmetric current distribution. Both
of these factors impact the resonator Q u . In the ceramic
cavities, the cavity-tuner attachment is done using conductive
silver epoxy [5], [18] with an approximate conductivity of
2 × 106 S/m [48]. The Q u of a resonator with Cu or Au as
the cavity metal suffers a substantial degradation (more than
30% simulated for Au vs. epoxy) in Q u due to this bonding
metal conductivity variation (Fig. 16). Another disadvantage
ARIF AND PEROULIS: ALL-SILICON TECHNOLOGY FOR HIGH- Q EVANESCENT MODE CAVITY TUNABLE RESONATORS AND FILTERS
735
Fig. 18. Fabricated parts of 15.5 GHz static resonator (a) cavity with post,
(b) static ceiling with feed lines and electroplated gold bonding ring.
Fig. 16.
Impact of bonding material conductivity on the resonator Q u .
Fig. 19.
Simulated and measured performance of the all-silicon static
filter designed for 15.5 GHz. The black dashed curve shows the post
measurement simulation to match the measured resonant frequency for Q u
and d0 comparison.
Fig. 17. (a) Fabrication uncertainty in d0 and C pp due to cavity and ceiling
surface roughness and flatness. All imperfections assumed on cavity side for
simplicity. (a) Imperfect cavity-ceiling attachment due to surface roughness
only, (b) post roughness and (c) non-parallel mating surfaces.
thermo-compression bonding (σ Au ≈ 4.1 × 107 S/m). Au-Au
thermo-compression bonding has been successfully employed
in several micromachined static resonators and filters [28],
[29], [31], [32], and [49]. The role of a bonding ring in this allsilicon approach is very important in accurately establishing
the initial-gap (d0 ). The thickness of the bonding ring can be
accurately controlled using well-characterized metal deposition methods commonly used in microfabrication, like e-beam
evaporation, sputtering and electroplating.
B. Reliability
of epoxy based cavity-ceiling attachment with rough cavity
surfaces, is the high-uncertainty in determining the post-ceiling
gap (d0 ) and parallel-plate capacitance (C pp ). This is explained
in Fig. 17. In an all-silicon approach, we eliminate the issue
of roughness of mating surfaces by exploiting the smoothness
and flatness of the Si wafers which are received polished and
leveled to sub-micrometer accuracy at the foundries. With Au
as the cavity metal, several precise and conductive attachment
methods are available including the highly conductive Au-Au
The reliability of a contact based MEMS (DC and capacitive
switches) has been challenged by problems like stiction,
fatigue and creep and dielectric charging [9]. Special techniques like use of novel contact material, modified actuation
waveform and proprietary fabrication processes have been
developed to mitigate these failure mechanisms and currently
several switch technologies have reliably demonstrated over
a billion switching cycles [50]–[53]. Contact-less MEMS are
preferred in the tunable filter applications where analog tuning
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Fig. 20. The measured tuning response (S21 ) of a weakly coupled, magnetostatically tuned all-silicon resonator designed for 8−18 GHz applications [38].
The inset shows the schematics of the measured device.
Fig. 22. The measured tuning response (S11 ) of a weakly coupled, electrostatically tuned all-silicon resonator designed for 12−24 GHz applications.
The inset shows the schematics of the measured device.
Fig. 21. Fabricated parts and assembly of electrostatic all-silicon tunable
resonator.
range is desired. In contact-less MEMS, creep and hysteresis
are the two main failure mechanisms that can reduce the device
repeatability and reliability [5]. The piezoelectric actuation
of a copper clad lead-zirconate-titanate (PZT) diaphragm as
used in [18] suffer from substantial hysteresis. Whereas electrostatic actuation of gold coated single-crystal-silicon (SCS)
diaphragm used in this work has been demonstrated to be
almost creep-free with no hysteresis in the analog tuning
range [5].
Fig. 23. Tuning enhancement from 12−24 GHz (1:2) to 6.1−24.4 GHz (1:4)
for the same resonator design by using Parylene-N dielectric spacer film [39].
substrate [38]. In addition, coaxial pin feed for filters has
also been successfully investigated and demonstrated in this
all-silicon approach [39], to show the compatibility of this
technology with the modular RF designs.
C. Ease of Integration
D. Cost
The monolithic integration of the RF components minimizes
fabrication complexity and interconnect losses [28], [29],
[32], [49], and [54]. Silicon has become the most highly
used RF substrate with the popularity of CMOS RFICs,
therefore the compatibility of filter fabrication with CMOS
processes is highly desirable. The all-silicon filter technology
has this unique edge over the previous cavity based tunable
filter approaches that it allows system-on-chip integration with
CMOS and SOI CMOS RFICs. Two variations of planar
integration have been demonstrated where the RF feed can
either be on the SOI diaphragm (Fig. 1) or on the cavity
All-silicon filters can be reliably and economically produced
using standard microfabrication equipment and processes.
Unlike conventional filter fabrication methods which required
individual machining and assembling of each filter, it is a
wafer-level batch fabrication approach which is fast, consistent
and cost-effective.
V. P ERFORMANCE A NALYSIS
We summarize the results of three different all-silicon resonator designs, to demonstrate the versatility of this approach
along with its performance.
ARIF AND PEROULIS: ALL-SILICON TECHNOLOGY FOR HIGH- Q EVANESCENT MODE CAVITY TUNABLE RESONATORS AND FILTERS
737
Fig. 24. All-Si tunable resonators performance comparison with other tunable resonator/filter technologies [5], [14], [17], [18], [22], [23], [38], [39], [56]–[58].
The comparison is based on the highest Q u reported at their highest f 0 and the tuning ratio.
A. 15.5 GHz Static Resonator
A proof-of-concept resonator is fabricated to check the feasibility of our all-silicon approach for making high frequency
capacitive-post loaded cavity filters with high accuracy and
desired performance. The resonator is designed for a fixed
resonant frequency of 15.5 GHz with a Q u of 780, when the
post-ceiling gap is fixed to 10-μm. It is excited with a planar
short-ended CPW feed on the cavity ceiling, that enters the
cavity via feed-through slots etched on cavity top sidewalls.
The fabricated cavity and ceiling parts are shown in Fig. 18.
The measured device demonstrated a Q u of 700 at 15.71 GHz
which corresponds to 90% of the simulated Q u with ≤2%
deviation from targeted frequency, as shown in Fig. 19. The
extracted d0 from simulations is 10.6 μm which corresponds
to ≤1 μm fabrication uncertainty in dmin .
B. 8−18 GHz Resonator With Magnetostatic Actuation
In [38], the all-silicon tunable filter technology is demonstrated for the first time using magnetostatic MEMS actuation.
The cavity and diaphragm are fabricated and assembled using
the procedure described earlier in section II. However, to
implement magnetostatic actuation of the diaphragm, magnetic
field interactions between a PCB mounted current carrying
coil and a small permanent magnet attached to the back of
the diaphragm are exploited. Using bi-directional magnetostatic actuation, the diaphragm is designed to actuate between
5–40 μm from the post to achieve 8 to 18 GHz tuning with
a Q u of 950–1500. The resonator is fed by an open-ended
microstrip line patterned at the back of the cavity. The field
is coupled through a rectangular slot etched inside the cavity
gold. A coplanar waveguide (CPW) to microstrip transition is
incorporated to perform on-wafer probing. Fig. 20 shows the
measured transmission (S21 ) parameters of a weakly coupled,
magnetostatically actuated all-silicon resonator. The resonator
tuned from 9.5 to 17 GHz with the Q u of 650–950 when the
coil current was changed between ± 70 mA.
C. 12−24 GHz Resonator With Electrostatic Actuation
The electrostatic actuation of a Au/Si diaphragm is
employed for a 12–24 GHz all-silicon resonator. A diaphragm
deflection from 2–15 μm from the post is required to achieve
the desired tuning range, with a simulated Q u of 600–1350.
The cavity is excited using a coaxial pin inserted through
a via etched in Si cavity bottom. The fabricated parts and
resonator assembly is shown in Fig. 21. From the measured
reflection (S11 ) parameters presented in Fig. 22, the resonator
continuously tuned from 12.3 to 23.7 GHz when the bias
voltage is ramped to 370 V. A further increase in voltage
resulted in a discrete frequency jump to 25 GHz due to
electrostatic pull-in effect. An unloaded quality factor of
510–1020 is extracted through data post-processing using
reflection method [55].
1) Tuning Range Enhancement: The tuning range of this
electrostatic is enhanced by introducing a thin (0.5 μm) lowloss Parylene-N (r = 2.65, tan δ = 0.0006 at 1 MHz)
spacer film between the post and the diaphragm as demonstrated in [39]. This Parylene-N film accurately allows submicrometer gaps between the cavity and the post, not easily
possible with an air-gap. Moreover, the high relative permittivity of Parylene-N further enhances the capacitive density at
small deflections. With this modified approach, the fabricated
resonator demonstrated a tuning range from 6.1 to 24.4 GHz
(see Fig. 23) with a Q u of 300–1000. To the best of authors’
knowledge, this is the highest reported tuning ratio (1:4) to
date in the MEMS based tunable filter technology.
A comparison has been made between the existing tunable
filter technologies with the all-silicon approach, based on the
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reported performance in terms of highest operating frequency,
unloaded quality factor and tuning ratio in Fig. 24. It can be
seen that the performance of all-silicon resonators is superior
to all other approaches demonstrated to date.
VI. C ONCLUSION
The capacitive-post loaded 3-D filter technology has great
potential for realizing compact, high-Q (≥500) and widely
tunable (≥1:2) filters suitable for microwave and millimeter
wave SDR applications. However, this performance is strongly
dependant on the fabrication technology due to tight design
tolerances. Conventional fabrication methods used for realizing capacitive-post loaded filters have suboptimal precision
which impedes reliable scaling of these filters beyond X-band.
The presented all-silicon technology utilizes microfabrication tools and techniques to achieve the desired accuracy.
This paper focused on design, optimization and fabrication
techniques for the all-silicon capacitive-post loaded cavity
resonators to simultaneously achieve high-Q and tuning ratio.
Three different RF feed variations are presented to show the
diversity in the all-silicon approach. Some notable technology
demonstrations include 1) a 9.5–17 GHz (1:1.87) magnetostatically tunable all-silicon resonator with a Q u of 650 to 950,
and 2) an electrostatically tunable 12.3–23.7 GHz (1:1.93) resonator with a Q u of 510 to 1020. The tuning ratio of the electrostatically actuated resonator has been doubled to 6.1 to 24.4
GHz (1:4) by introducing a low-loss dielectric film between
the post and diaphragm to achieve a sub-micrometer capacitive
gap. This two octave (C to K-band) tunable resonator with
a high-Q of 300 to 1000 is state-of-the-art in the present
tunable filter technology. With this supreme RF performance
and advantages like high reliability, compatibility with the onchip RF technology and a low cost batch production, it is
believed that the all-silicon filters will have a significant contribution in the realization of next-generation reconfigurable RF
front-ends.
ACKNOWLEDGMENT
The authors would like to thank N. Ragunathan,
Dr. H. Sigmarsson, and Dr. X. Liu for their technical help. The
views expressed are those of the author and do not reflect the
official policy or position of the Department of Defense or the
U.S. Government. Approved for Public Release, Distribution
Unlimited.
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Muhammad Shoaib Arif (S’09) received the bachelor’s degree in aeronautical engineering from the
National University of Science and Technology,
Islamabad, Pakistan, in 1999, and the master’s
degree in electrical and computer engineering from
Purdue University, West Lafayette, IN, USA, in
2009.
He is currently pursuing the Ph.D. degree in electrical and computer engineering at Purdue University. His current research interests include novel RF
MEMS devices and micro-machined high-Q tunable
filters for reconfigurable RF front-ends.
Dimitrios Peroulis (S’99–M’04) received the Ph.D.
degree in electrical engineering from the University
of Michigan, Ann Arbor, MI, USA, in 2003. He
has been with Purdue University, West Lafayette,
IN, USA, since August 2003, where he is currently
leading a group of graduate students on a variety
of research projects in the areas of RF MEMS,
sensing, and power harvesting applications, as well
as RF identification (RFID) sensors for the health
monitoring of sensitive equipment. He has been a
Principle Investigator (PI) or a co-PI in numerous
projects funded by government agencies and industry in these areas. He
is currently a Key Contributor in two Defense Advanced Research Project
Agency (DARPA) projects at Purdue, which focus on very high quality
(Q ≥ 1000) RF tunable filters in mobile form factors (DARPA Analog
Spectral Processing Program, Phases I, II, and III) and on developing
comprehensive characterization methods and models for understanding the
viscoelasticity/creep phenomena in high-power RF MEMS devices (DARPA
M/NEMS ST Fundamentals Program, Phases I and II). Furthermore, he leads
the experimental program on the Center for the Prediction of Reliability,
Integrity, and Survivability of Microsystems (PRISM) funded by the National
Nuclear Security Administration. In addition, he heads the development of
the MEMS technology in a U.S. Navy project funded under the Technology
Insertion Program for Savings program focused on harsh-environment wireless
microsensors for the health monitoring of aircraft engines. He has authored
or coauthored more than 110 refereed journal and conference publications in
the areas of microwave integrated circuits and antennas.
Dr. Peroulis was the recipient of the 2008 National Science Foundation
CAREER Award. His students have been the recipients of numerous Student
Paper Awards and other student research-based scholarships. He has been
the recipient of Eight Teaching Awards including the 2010 HKN C. Holmes
MacDonald Outstanding Teaching Award and the 2010 Charles B. Murphy
Award, which is Purdue University’s highest undergraduate teaching honor.