A LOW PHASE-NOISE VCO FOR MULTI-BAND TRANSCEIVER

USING FULLY PACKAGED MEMS ELECTROSTATIC VARACTORS

Kenichiro Urayama1, Koichiro Akahori1, Nobuyuki Adachi1,
Hiroyuki Fujita2, and Hiroshi Toshiyoshi3
1
Laboratory, Japan Radio Co., Ltd., Japan
2
Institute of Industrial Science, The University of Tokyo, Japan
3
Research Center for Advanced Science and Technology, The University of Tokyo, Japan

ABSTRACT nature of conformal coating of Parylene-C and with the

This paper reports the low-phase noise performance of the high electrical resistance ( = 6 1016 -cm at 23 deg. C,
VCO (Voltage Controlled Oscillator) using the digital 50% RH) and relatively low dielectric permittivity (r =
MEMS varactors with the MIM (Metal-Insulator-Metal) 2.95 at 1 MHz) [7].
capacitors and an electrostatically actuated contact switch.
A packaged 2-bit MEMS varactor was operated at 35 V dc
C3 C4
to alter the capacitor values in four different states ranging
C1 C2
from 0.55 pF to 0.73 pF with a 0.06-pF-step at 1 GHz. A
2-bit MEMS-VCO module was also assembled by using
the MEMS chip on the high frequency PWB (Printed
Wiring Board, 40 mm x 35 mm). Due to the digital
operation, the free-running oscillation frequency was
controlled in between 802 MHz to 822 MHz. A phase noise RF IN RF OUT
level was measured on a 1-bit MEMS VCO module to be
as low as -101 dBc/Hz at a 10 kHz offset from the peak (a)
frequency 776 MHz. OFF-state ON-state

C3 (Air) C4 Air)
INTRODUCTION C1 (Si3N4) C2 (Si3N4) C1 (Si3N4) C2 (Si3N4)
Wideband and low-noise VCOs (Voltage Controlled
RF IN RF OUT RF IN RF OUT
Oscillators) have industrial impact to realize multi-band
transceiver units for mobile phones, where MEMS (b) (c)
varactors of high-Q have found a potential market [1-6]. Dielectric (Parylene-C)

Compactness and multi-bit digital operation are mandatory Suspended

Contact
requirements for MEMS varactors to cover the wide range
of frequency at high accuracy without increasing the
electronics footprint on a printed circuit board. In this Anchor Anchor
paper, we report a VCO module of low phase noise MIM Capacitor
performance using the MIM (metal-insulator-metal) Actuator Actuator
capacitors and electrostatic switches integrated by the (d)
surface micromachining technology. Figure 1: Schematic view of the 1-bit-MEMS varactor. (a)
Close-up view of the contact. (b) Switch state OFF and (c)
DEVICE DESIGN ON. (d) The upper movable contact is electrically isolated
MEMS Varactors from the actuators electrode to suppress stray
Figure 1 is the schematic illustration of the 1-bit MEMS capacitance.
varactor made of plated metal on a Pyrex substrate. A pair
of MIM capacitors has been formed on the RF signal lines Q-factor (Quality factor) of capacitor is expressed by
located underneath the suspended contact bridge (Figure 1
Q= , (1)
1(a)). At OFF-state (Figure 1(b)), they form a series of four 2 f C R s
capacitors including two air gaps in series, thereby where f, C, and Rs are the operation frequency, the value of
resulting in a small capacitance value Clow. At ON-state capacitance, and the internal series resistance, respectively,
(Figure1(c)) with an electrostatic drive voltage over the which implies that small resistance is needed for a high Q
pull-in operation, the MIMs upper electrode is brought value. The series resistance includes the interconnection
into a direct contact with the MIMs top electrodes to form resistance and the dielectric loss (tan) of the insulator
a series of two MIM capacitors, leading to a larger material [8]. We designed the MEMS varactor with a thick
capacitor value Chigh. The suspended contact is isolated electroplated gold to lower the interconnection resistance;
from the actuator electrodes via the Parylene-C hinge, so a small value of interconnection resistance (0.11 ) has
that the parasitic capacitance to the electrostatic actuation been predicted by the Agilent ADS (Advanced Design
is minimized (Figure 1 (d)). The adhesion between the System) simulator on a 1-bit MEMS varactor. For the
Parylene-C and the plated metal is good thanks to the dielectric tan value, on the other hand, we experimentally
studied the performance of MIM capacitors made of three

978-1-4673-5655-8/13/$31.00 2013 IEEE 737 MEMS 2013, Taipei, Taiwan, January 20 24, 2013
different materials (sputtered Si3N4, SiO2, and Al2O3); the isolation hinge. (d) Finally, the sacrificial copper and the
S-parameters were measured by using the Agilent 8510C chromium are selectively removed by wet etching to
VNA (Vector Network Analyzer) to extract the value of release the suspended structure.
tan by parameter fitting based on the ADS simulation Photo Resist
model. Table 1 compares the tan values of three materials. Au
Due to the smallest loss, we have chosen the sputtered
Si3N4 as a capacitor dielectric. Theoretical calculation (a) Pyrex Glass
based on these experiments predicted a high Q value of 108
so long as the ON-state capacitance is smaller than 6 pF at Au
1 GHz. We used the standard parallel-plate model for the Si3N4
electrostatic operation and estimated maximum drive
voltage of 20 V for the bridge structure of 1-m-thick (b)
plated gold of 480-m-long span suspended over the air
gap of 2 m clearance. Au
Cu
Table 1: Experimentally measured and extracted
(Sacrificial Layer)
parameters of dielectric materials.
Material Sputter Gas tan (c)
Si3N4 Ar 0.006 Parylene-C
Al2O3 Ar 0.009
SiO2 Ar : O 2 = 2 : 1 0.011
(d)
MEMS-VCO Module Figure 3: Fabrication process of the MEMS varactor. (a)
Figure 2 shows the circuit schematic of the MEMS-VCO Gold plating of the bottom electrode. (b) Deposition of the
formed in the Clapp oscillator topology due to the low capacitor dielectric film (sputtered Si3N4) and gold plating
phase noise performance. Oscillation frequency is of the upper electrode. (c) Plating of the sacrificial layer
controlled by tuning the drive voltage applied to the (copper) and the movable electrode (gold). (d) Deposition
MEMS varactor (shown as C_MEMS) used in the LC-tank. and patterning of Parylene-C and removal of the
Oscillation frequency in the 800 MHz band was chosen as sacrificial layer by wet-etching.
in this feasibility study towards mobile phone application.
Vcc
2mm

L2 2mm
R2

C4 C1
C5
R1 Out
D1 C2 R3 C3
L1

(a)
C_MEMS
Figure 2: Circuit schematic of MEMS-VCO
482um

FABRICATION
Figure 3 shows the fabrication process of the MEMS 99um
varactor on a 700-m-thick Pyrex substrate. (a) First, a
seed layer (20-nm-Cr and 50-nm-Au) is sputtered and RF IN
patterned to define the bottom electrode, whose thickness
RF OUT
is increased by additional electroplating of a 2-m-thick
gold. (b) Next, a 220-nm-thick sputtered Si3N4 is deposited,
(b)
followed by the sputtering of 20-nm-Cr and 50-nm-Au
Figure 4: SEM images of the MEMS varactor. (a) 1-bit
seed layer and the electroplating of a 1-m-thick gold,
MEMS varactor on a 700-m-thick Pyrex glass substrate
where a pair of MIM capacitor is formed. (c) Sacrificial
(chip size of 2 mm x 2 mm). (b) Close-up view of a 2-bit-
seed layers of 20-nm-Cr and 100-nm-Cu are sputtered on
MEMS varactor.
the substrate, and a 2-m-thick Cu is plated as a sacrificial
layer. The structural layer of the MEMS varactor is formed
The SEM pictures of the fabricated devices (2 mm x 2 mm)
by first sputtering a barrier 20-nm-Cr and a seed 50-nm-Au
are shown in Figure 4. We have developed a 1-bit varactor
with additional 1-m-thick electroplated gold. A
with a single actuator bridge (Figure 4(a)) and a 2-bit type
1-m-thick Parylene-C was deposited by LPCVD on the
with two parallel bridges suspended over a pair of common
structural gold and patterned by the O2 RIE to form an
input and output RF lines (Figure 4(b)).

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EXPERIMENTAL RESULTS
A 1-port device in Figure 4(a) has been tested to obtain

Voltage
S11-parameters by using the VNA from 0.1 to 5 GHz in
26 V
both ON- and OFF-states, as shown in Figure 5.
Electrostatic drive voltage of 35 Vdc was used to turn on
the varactor. Down-state

Displacement
90 %

1.2 m
UP-state 10 %
S(1,1)

S(1,1)
77 s 28 s

-0.2 -0.1 0.0 0.1 0.2

Time (ms)
Figure 6: Switching behavior of the MEMS varactor.
freq (100.0MHz to 5.000GHz) freq (100.0MHz to 5.000GHz)
(a) (b) After dicing the developed varactor chip, it was die-bonded
onto an alumina interposer substrate (4 mm x 4 mm) and
Figure 5: Measured S11 of the MEMS varactor from 0.1 to gold-wire bonded as shown in Figure 7(a). The chip was
5 GHz. (a) OFF (up)-state (Vdc= 0 V). (b) ON (down)-state then hermetically sealed with a plastic cap in atmosphere,
(Vdc= 35 V). and surface-mounted onto an evaluation circuit board by
using silver paste as shown in Figure 7(b).
Capacitor performance was also calculated by using the Subsequently, high frequency performance such as 2-port
following equations: S-parameters was measured by using the VNA. Figure 8
1 Re(S11 ) 2 Im(S11 ) 2 shows the frequency dependence of the capacitors a 2-bit
Rs = Re(Z in ) = Z 0 ,
2
(2) MEMS varactor operated at 35 V. Four different digital
(1 Re(S11 ) ) + Im(S11 )
2
states (00, 01, 10, and 11) were found to set the capacitor
values ranging from 0.55 pF to 0.73 pF at 1 GHz with a
2 Im( S11 ) (3)
X = Im(Z in ) = Z 0 ,
2
0.06-pF-step with a relatively flat performance within the
(1 Re( S11 ) )2
+ Im( S11 ) target frequency range, which was found to be in good
agreement with our theoretical design.
1
C= , (4)
2 f X

Im( Z in )
Q= , (5)
Re( Z in )

where Zin is the input impedance of the measured device,

and Z0 (=50) is the characteristic impedance. Measured
and designed capacitor parameters are compared in Table 2,
2mm 2mm
where a good agreement is seen. However, the measured
Q-factor was degraded to the 3/8 of the designed value.
TEG (Test-Element-Group) patterns prepared on the same
wafer were tested to find that the major contribution came
from the metal-metal contact resistance rather than the tan (a)
of the MIM insulator or the interconnection resistance. 4mm Cap MEMS

Table 2: Electrical properties of the MEMS varactor at 1 Silver paste Gold wire
4mm
GHz.
C_off C_on Rs
Q Alumina substrate
(pF) (pF) () Evaluation Board
Designed 0.01 0.89 1.19 160
Measured 0.13 0.86 3.10 60 (b)
Figure 7: SEM and optical microscope images of
The switching speed of the developed device was packaged device. (a) A chip after die-bonding onto an
measured by using the LDV (Laser Doppler Vibrometer). alumina substrate and gold wire-bonding. (b) A chip after
Figure 6 shows the transient behavior measured with a 26 hermetic cap and surface assembly onto an evaluation
V square wave voltage at 3.7 kHz, indicating OFF-to-ON circuit board.
(up-to-down) and ON-to-OFF (down-to-up) responses of
77 s and 28s, respectively.

739
 0.80
bit1=35V, bit2=35V
0.75
Capacitance (pF)

10.00 kHz
bit1=0V, bit2=35V -101.1 dBc/Hz
0.70

0.65 bit1=35V, bit2=0V

0.60
bit1=0V, bit2=0V
0.55

0.50
0.0 0.5 1.0 1.5 2.0
Frequency (GHz) Figure 11: Measured phase noise of a 1-bit MEMS-VCO
Figure 8: Experimentally measured frequency response of (ON-state). Phase noise of -101 dBc/Hz has been defined
a packaged 2-bit-MEMS varactor. by the power at a 10 kHz offset from the peak 776 MHz.

A MEMS-VCO module was assembled by using the CONCLUSIONS

MEMS chip on the high frequency PWB (Printed Wiring We have developed electrostatically actuated MEMS
Board, 40 mm x 35 mm), as shown in Figure 9. Due to the varactors to realize a high-Q tunable microwave
digital operation, the oscillation frequency was controlled component. A thick gold plated within the Parylene-C
in between 802 MHz and 822 MHz as shown in Figure 10. hinges was used as a structural layer of low contact
From a separate RF measurement on a 1-bit MEMS resistance, and a dielectric Si3N4 layer of small tan was
varactor, we confirmed the phase noise as low as -101 used to lower the RF insertion loss. The device fabricated
dBc/Hz at a 10 kHz offset from the peak frequency 776 on a Pyrex substrate by the surface micromachining
MHz as shown in Figure 11. process was die-bonded onto an alumina interposer
substrate and gold-wire bonded. The packaged device
covered by the plastic cap was implemented the Clapp
oscillator assembled on a high frequency PWB using
discrete components. A 2-bit MEMS-VCO module was
controlled in between 802 MHz to 822MHz. From a
separate RF measurement on a 1-bit MEMS-VCO module,
we confirmed the phase noise as low as -101 dBc/Hz at a
10kHz offset from the peak frequency 776 MHz.

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a PWB (40 mm x 35 mm). digital/analog tuning capabilities, 2008 IEEE MTT-S
Int. Microwave Symp. Dig., pp. 1283-1286.
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