Tunable RF MEMS Capacitor for Wireless Communication
Xiuhan Lia, Yu Xiab, Jian Liub, Li Yina, Yu Liub, Dongming Fangb, Haixia Zhangb∗
Beijing Jiaotong Univ. School of Electronics and Information Engineering, Beijing, China, 100044;
b
National Key Laboratory of Nano/Micro Fabrication Technology, Institute of Microelectronics,
Peking University, Beijing, China,100871;
a
ABSTRACT
High quality tunable MEMS parallel plate capacitors have been widely used in phase shifters, oscillators and tunable
filters for wireless communication. As electrostatic actuation and air dielectrics have led to devices with low power
consumption and high quality factors, an electrostatically actuated MEMS tunable capacitor with two flexible plates have
been designed and fabricated in this paper. According to the measurement results, tunable capacitor shows good
performance in the frequency range of 0GHz-2GHz. The pull-in voltage for the capacitor is 13 V. The tuning ratio of the
capacitor is very high, which can reach 320%.
Keywords: RF MEMS, tunable capacitor, electrostatic actuation
1. INTRODUCTION
High performance RF MEMS tunable capacitors have the potential to replace conventional varactor diodes in many RF
circuits such as phase shifters [1], oscillators [2] and tunable filters [3]. The capacitance can be varied either by changing
the gap spacing [4-6] or by altering the overlap area of the combs [7]. Many driven method such as thermal, piezoelectric
and electrostatic have been used for tunable capacitor. Because electrostatic driven devices shows good performance in
low power consumption and high-quality factors, a lot of electrostatic driven tunable capacitors have been fabricated [811]. Such capacitors often consist of suspended top plate that can be electro-statically actuated via applied voltage to
change the capacitance between the plates. A well-known phenomenon of the electrostatic actuators or micro-beams is
pull-in. In order to achieve equilibrium, the electrostatic force must balance the spring force generate by the elastic beam.
However, as the applied voltage increases or the initial gap decrease, the electrostatic force increase much faster than the
linear spring force. The equilibrium is broken and pull-in occurs when the applied voltage exceeds a threshold value.
Tuning range is of first important for tunable capacitors. In order to improve the tuning range, many high tuning range
electrostatic tunable capacitors, including two movable plates and comb-driven tunable capacitors, are fabricated and
studied [12]. The comb-driven tunable capacitors also have disadvantages, such as high control voltage [13], being not
compatible with IC process [14], low reliability [15], and high complexity [16].
It is important to note that an optimal capacitor solution would need to exhibits not only good performance, but also low
cost, simple fabrication process to save time, and easily integrated with IC. In this paper, a low-voltage-controlled
tunable capacitor by means of simple MEMS technology is designed and fabricated, which is compatible with IC
process. Because electrostatic actuation[17] and air dielectrics have contributed to devices with low power consumption
and high performance, an electro-statically actuated MEMS tunable capacitors with two flexible plates have been put
forward in this paper. The MEMS tunable capacitor with high performance was fabricated through the use of simple
MEMS technology. According to the measurement results, tunable capacitor shows good performance in the frequency
range of GHz2GHz. The pull-in voltage for the capacitor is 13 V. The tuning ratio of the capacitor is very high, which
can reach 320%.
∗
Contact information: lixiuhan@bjtu.edu.cn; Phone: 86-138100-45932; Corresponding author: Haixia Zhang
2009 International Conference on Optical Instruments and Technology: MEMS/NEMS Technology and Applications,
edited by Zhaoying Zhou, Toshio Fukuda, Helmut Seidel, Xinxin Li, Haixia Zhang, Tianhong Cui, Proc. of SPIE
Vol. 7510, 751002 · © 2009 SPIE · CCC code: 0277-786X/09/$18 · doi: 10.1117/12.845903
Proc. of SPIE Vol. 7510 751002-1
2. MECHANICAL DESIGN AND FABRICATION
2.1 Mechanical design
The two dimension RF MEMS tunable capacitor model was given in Fig.1. It consists of two parallel plates, four Tshaped beams to suspend the top plate; Air is between the top plate and bottom plate. When DC voltage is applied to the
two pads, and the top and down plate will attract each other because of electrostatic force. Figure 2 showed a schematic
diagram of the cross section of the tunable capacitor. The capacitor was composed of two movable plates with an
insulation dielectric layer (Si3N4) on top of the bottom plate. If the two plates are flexible, it will be possible for them to
attract each other and to decrease the distance before reaching the pull-in voltage. In addition, the capacitor demonstrated
an extended tuning range even after the two plates touched each other. In theory, the suspended plate can be pulled down
at most by 1/3 of the original gap, corresponding to a 300% capacitance increase.
100um
350um
100um
700um×700um
50um
20um
Fig. 1. Two dimensional RF MEMS capacitor structure configuration
Top plate Nickel
Spring ku
x1
d
Spring kb
C
V(t)
x2
Bottom plate (Poly, Nitride)
Spring kb
Silicon substrate
Fig.2. A schematic diagram of the RF tunable MEMS capacitor (cross section).
According to electrostatic field theory, the electronic energy between the two plates is:
E=
ε AV 2
1
CV 2 = 0
2
2d
And the electrostatic force is derived as follow:
Proc. of SPIE Vol. 7510 751002-2
(1)
ε 0 AV 2
∂E
Fe1 =
=
∂x 2(d − x1 − x2 ) 2
(2)
ε 0 AV 2
∂E
Fe 2 =
=
∂x 2(d − x1 − x2 ) 2
The restoring force for the up and bottom plate is:
Fm1 = k u x1
(3)
Fm 2 = k b x2
In the equilibrium state, we can derive the following equation:
⎧ Fm1 = k u x1 = Fm 2 = k b x2
⎨
⎩ Fe1 = Fm1 = Fe 2 = Fm 2
From the above equations we can deduce the displacement of the up and bottom palates: x1 =
x2 =
(3)
2k b d
,
2(k u + k b )
2k u d
, therefore the tunable capacitance can reach:
2(k u + k b )
C' = 3
ε0 A
d
= 3C0
(4)
4k u k b
d
9ε 0 A(k u + k b )
(5)
The pull in voltage is:
V pull −in =
Here, ku and kb are the effective spring constant of the four T-shaped beams and the bottom plate, ku can be written as
k up = 4 k eq
keq =
2k1k 2
k1 + 2k 2
(6)
(7)
3
EWiTi
ki =
(i = 1,2)
L3i
(8)
where keq is the equivalent spring constant of each T-shaped beams, Li, Wi, and Ti are the length, width and thickness of
the beams (see Fig. 3), E is the Young’s modulus of the metal material. In our work, the thickness of the beams and top
plate is 20um, the metal we adopted is nickel, and E=200GPa at room temperature [18].
Proc. of SPIE Vol. 7510 751002-3
Fig.3. Simplified diagram and a spring model of the T-shaped suspension.
While for the bottom plate, the spring constant kb can be calculated from:
⎛t ⎞
kb = 32 Ec w⎜ ⎟
⎝l ⎠
3
(9)
Because the bottom plate is composed of three stacked layers ie Si3N4, Polysilicon and Si3N4. The young’s module for
the multilayer Ec can be gained from:
E poly t 2 E SiN t3
Ec t c
E t
= SiN 21 +
+
2
1 − ν c 1 − ν 1 1 − ν 22 1 − ν 32
(10)
ν c t c = ν 1t1 + ν 2t 2 + ⋅ ⋅ ⋅ + ν n t n
Here, ESiN and Epoly are the young’s module of low stress Si3N4 and Polysilicon film separately, ti is the thickness of each
film, vi is the poisson’s ratio of each layer. Since the ANSYS software can synthesize the multiple factors, so we use
ANSYS to simulate the top plate’s displacement (caused by electrostatic forces) versus the applied voltage. Fig. 4 shows
the deformation shape of the top plate and the T shape beams, as the applied voltage is 13V. In the figure, the
displacement of the top plate does not fully achieve 1/3 of the original gap, only the center of the top plate can be
achieved. Figure 5 show the simulation results by ANSYS, the max displacement of the bottom plate is 0.144um under
the DC voltage of 10V. It can be observed that the max displacement gains at the center of the bottom plate.
Fig.4. Deformation shape of the upper plate with the actuated voltage13V simulated by ANSYS
Proc. of SPIE Vol. 7510 751002-4
Fig. 5. Deformation shape of the bottom plate with the actuated voltage10V simulated by ANSYS
2.2 Fabrication method
The tunable RF MEMS capacitor is fabricated using two structural layers, three sacrificial layers, and two insulating
layers of nitride. The top plate is fabricated from electroplating nickel with a thickness of 20um covered by a gold layer
of 2um thickness, and the bottom plate is made of polysilicon covered by a nitride layer with a thickness of 0.35um.
Figure 6 illustrates the cross section of the process steps used to fabricate the capacitor with the MetalMUMP’s process
[19]. First, a layer of 2um of oxide is deposited on the silicon substrate to form an isolation layer, as illustrated in Fig.
6(a). A 0.5um oxide is deposited and patterned, as illustrated in Fig. 6(b). This oxide layer defines the area that will be
used to etch a trench in the silicon substrate. The first nitride layer of 0.35um thickness is deposited and patterned, as
illustrated in Fig. 6(c). This nitride layer forms the bottom cover of the polysilicon layer and acts as a part of the tunable
capacitor’s bottom plate. On the top of the first nitride layer, a layer of polysilicon with the thickness of 0.5um is
deposited and patterned to form the bottom conductive plate of the tunable capacitor, as shown in Fig. 6(d). The second
nitride layer is deposit on the top of the polysilicon layer to form the isolating layer that prevents any electrical contact
between the two plates, thus eliminating the sticktion problem, as shown in Fig. 6(e). A 1.1um layer of second oxide
(PSG) is then deposited and patterned, as illustrated in Fig. 6(f). The last layer is the metal layer, which is composed of
20um of nickel with 2um of gold on top of it, as shown in Fig. 6(g). After thick photoresist patterning, the nickel layer is
deposited by Ni electroplating, then a 2um of gold layer was deposited. The capacitor is then released using a wet etchant
and the trench is formed by etching the silicon substrate, and the sacrificial layers is also etched out, as well as to etch a
trench in the silicon substrate. The trench etch of the substrate is defined by the first oxide layer. The silicon substrate is
etched to form a trench of a depth of 25um.[20]Figure 7 shows the SEM image for our MEMS tunable capacitor.
Proc. of SPIE Vol. 7510 751002-5
a
e
b
f
c
g
d
h
Fig.6. Fabrication method for RF MEMS tunable capacitor (cross section)
.
Fig.7. SEM image for RF MEMS tunable capacitor (top view)
3. MEASUREMENT RESULTS
The change of the capacitance as a function of the DC applied voltage is illustrated in Fig.8. It has been measured on the
Analog C-V instrument. When the applied voltage is 0V, the measured capacitance is about 3 pF, while when the applied
voltage is 13V, the measured capacitance is 9.8 pf, which indicates that the tuning ratio of the capacitor is 3.2:1. Figure 9
shows the measured capacitance variation with the increase of the frequency in the context of no DC voltage. The
capacitance of the capacitor showed stable performance in the frequency range of 2GHz-10GHz. The self-resonant
frequency of the tunable capacitor is around 2 GHz. Figure 10showed the series resistance under different applied DC
voltage.
Proc. of SPIE Vol. 7510 751002-6
Capacitance(pF)
50
40
30
20
10
0
0
5
10
15
20
25
30
DC voltage(V)
Fig.8. Measurement results for C-V curve
0.4
0.2
0.0
-0.2
-0.4
Cs
-0.6
-0.8
Driven voltage 0V
-1.0
-1.2
0 1 2 3 4 5 6 7 8 9 1011121314151617181920
Frequency(GHz)
Fig.9. Measurement results for capacitance in the frequency range of0GHz-20GHz by network analyzer
Proc. of SPIE Vol. 7510 751002-7
13
12
Rs(ohm)
11
Driven voltage 0V
10
9
8
7
0
5
10
15
20
Frequency(GHz)
Fig.10. Measurement results for Rs in frequency range of 0GHz-20GHz under driven voltage of 0V
4. CONCLUSIONS
Tunable MEMS parallel plate capacitors have been widely used in phase shifters, oscillators and tunable filters for
wireless communication. As electrostatic actuation and air dielectrics have led to devices with low power consumption
and high quality factors, an electrostatic actuation MEMS tunable capacitors with two flexible plates have been designed
and fabricated in this paper.
The low-voltage-controlled (below 15 V) tunable capacitors have been designed and fabricated with simple Metal
MUMP process. The tuning ratio is 3.2:1 and the self-resonant frequency is more than 2 GHz. Because the fabrication
process can fabricate RF MEMS inductors at the same time, it is promising to fabricate high-performance low-voltagecontrolled oscillators (VCOs) or tunable filters using this MEMS fabrication technology.
ACKNOLOGMENT
This paper was supported by National Science Foundation of China (60706031), Chinese National 863 Plan (Project code:
2006AA04Z359), Project supported by the Beijing Jiaotong University, Project supported by the “Talents Project” of
Beijing Jiaotong University and China Postdoctoral Science Foundation(200902004).
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