A Novel Electrostatic Radio Frequency Micro Electromechanical Systems (RF MEMS) With Prognostics Function

Yunhan Huang, Michael Osterman, and Michael Pecht

Center for Advanced Life Cycle Engineering (CALCE), University of Maryland, College Park, MD 20742, United States

Abstract Radio Frequency Micro Electromechanical Systems (RF MEMS) has emerged as one of the most promising front runners in wireless components market because of their high linearity, high isolation, ultra-low power consumption, and the capability of integrating with integrated circuits for portable wireless communication devices. However, their widespread application in commercial areas is hampered by the relatively poor reliability performance during long-term usage. Among the failure mechanisms of RF MEMS, stiction induced by the charge accumulation in the dielectric layer is the predominant one, accounting for most of the failed components. However, the origin of the accumulated charge, its properties and distribution, and its adverse effect on devices’ electrical performance has not yet been fully understood. In this paper, we propose the design, realization, characterization, and reliability test of a novel RF MEMS capacitive switch which has a high RF performance and low fabrication cost with a capability of predicting its state of health for various applications from phase shifters to tunable antennas. The key characteristic of our design is the introduction of Prognostics and Health Management (PHM) using non-intrusive monitoring method, which allows us to calculate the remaining useful life of our RF MEMS capacitive switches and provide a warning before its onset of failure. We overstress the device using two methods: electrostatic discharge (ESD) and operational voltage waveform. We discovered the difference of RF MEMS behavior and lifetime. We also present the effect of driving voltage polarity on the lifetime of RF MEMS.

1. Introduction The development of the wireless communication has led to the massive growth of radio frequency (RF) and microwave circuits and systems for various applications, including wireless hand-held devices, wireless local area networks, satellite communications and radar. Future hand-held communication devices will require very low weight, volume and power consumption in addition to higher data rates and improved RF functionality, e.g., higher isolation, lower return loss. Improvements in the size and component count have been achieved by increasing the level of integration. There is an urgent need to reduce the size and power consumption of RF frontend circuit to extend the battery lifetime, especially for mobile communication devices or wireless sensor network, while maintaining RF performance. RF-MEMS stands for radio-frequency micro electromechanical systems. They comprise an excellent choice for future wireless communication devices. One of the

most widely researched RF MEMS is electrostatic capacitive RF MEMS switch. Capacitive RF MEMS can provide lower return loss, higher isolation, zero power consumption, and higher linearity than integrated circuit (IC) counterpart technologies, e.g., SOS. They can be used as relays, resonators, tunable capacitors, reconfigurable antenna, phase shifter, and tunable filters [1-4]. Additionally, their capability of integration with IC to form one monolithic RF front-end chip for multiband wireless communication devices can yield a higher space efficiency for multiple bands e.g., GSM, 3G, LTE, Bluetooth, Wi-Fi, FM, etc.. Some drawbacks, such as costly packaging and integration with IC and required high operating voltage that were hampering the commercialization of RF MEMS has been greatly overcome by the development of MEMS packaging and use of integrated charge pump. However, RF MEMS’s relatively poor reliability performance is still the most challenges of the commercialization of RF MEMS. The lifetime of a typical RF MEMS can hardly compare with that of IC counterparts. In order to improve the device’s lifetime, researchers have conducted reliability test and failure analysis and identified the predominant failure is stiction, is a phenomenon that adjacent micro structures get stuck when they come into contact and restoring forces are not great enough to overcome the surface adhesion forces.

2. RF MEMS Design, Fabrication and Characterization As early as 1979 [5], micro-electromechanical switches have been developed and used as a switch to turn on and off low-frequency electrical signals. According to the types of contact between the suspended movable part and the underneath electrodes, RF MEMS switches can be grouped into two types: the capacitive switch and the ohmic switch. In the capacitive switch the two electrodes are electrically separated by a dielectric layer. In this work, we focus on capacitive RF MEMS. The switch designs have utilized cantilevers [6], rotaries [7] and membranes [8] to achieve good performance at RF and microwave frequency range. Because of their intrinsic low loss, low power consumption and lack of intermodulation distortion, RF MEMS switches are an attractive alternative to traditional FET or p-i-n diode switches in applications where microsecond switching speed is sufficient. Membrane-based switches operated by electrostatic force generated by driving voltage have shown excellent performance through 40GHz [8], fast switching speed, and lifetimes in excess of 1 billion cycles, which is one of the most promising configuration of RF MEMS [9]. A schematic of a typical capacitive RF MEMS is shown in Fig. 1.

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Fig. 1 Schematic of a typical capacitive RF MEMS switch consisting of a metal membrane (the bridge structure), a

transmission line underneath (the central line) covered by insulation material (blue area).

2-1 RF MEMS Design The basic operation of the capacitive RF MEMS switch discussed here is a co-planar waveguide (CPW) shunt switch as shown in Fig. 1. The RF MEMS switch operates as a tunable capacitor with bi-states. The input signal waveform is a combination of DC bias (square wave) and RF signal (sinusoidal wave). The DC bias frequency should be lower than the mechanical resonance frequency of the membrane, (for most RF MEMS, 10~100 kHz); DC bias voltage should be higher than the pull-down voltage, which is the minimal voltage to pull the membrane down, contacting the underlying dielectric layer. The RF signal frequency is much lower than the resonance frequency of the electrical circuit. With a DC bias that is higher than the pull-down voltage applied, the membrane is pulled down and touches dielectric, a high capacitance is induced due to the reduction of the separation between the membrane and the underlying transmission line. Thus, a low impedance is achieved at RF frequency range and the RF signal is shunted to ground in shunt switch and directed to output in series switch. If only a RF signal is applied without any DC bias, the membrane stays up and does not respond (vibrate) to the RF signal due to that the mechanical resonance frequency of the membrane is lower than the RF signal. Thus, a low capacitance and high impedance is induced, directing the RF signal to the output in a shunt switch and to ground in a series switch. The equivalent circuit of a typical RF MEMS can be modeled as a RLC circuit consisting of a serial resistance Rs, inductance L, and a tunable capacitance C (shown in Fig. 4). The impedence can be written as Z=Rs + jωL + 1/jωC, and Z=1/jωC for f<<f0 , where f0 is the resonance frequency of the circuit that can be expressed as

LCf

2

10 (1)

Fig. 2 Schematic diagram showing the operation of a RF

MEMS switch [10].

Fig. 3 The RF MEMS switch under microscope [10].

Fig. 4 The equvilent electric circuit of a capacitive RF MEMS switch. The tunable capacitve C is formed by the movable

membrane and the transmission line underneath.

Fig. 5 A linear electromechnaical model of capacitive RF

MEMS. The membrane is subjected to an electrostatic force when a bias voltage is applied and a mechanical restoring

force due to the displacement of the membrane.

The schematic of the 1-D linear electromechanical model of our RF MEMS with and without a DC bias is shown in Fig. 2. If we assume the membrane is subjected to a distributed force across the entire membrane, then the spring contant can be found

3)(32L

tEWk (2)

where E is the Young’s Modulus of the material, W, t, and L is the width, thickness, and length of the membrane [11]. When a voltage is applied between the membrane and the signal line, an electrostatic force is induced on the membrane. In order to approximate this force, the membrane over the signal line is modeled as a parallel-plate capacitor. Given that the width of the signal line is W, the parallel plate capacitance is

rtzg

LWC

/0

(3)

where g is the initial gap between the membrane and the dielectric layer, ∆z is the displacement of the membrane. The electrostatic force induced as charge, provides a capacitance, given by [12]

122

2

22

2

)(

2 g

WwV

dg

gdCVFe

(4)

where V is the voltage applied between the membrane and the signal line. Where the force is independent of the voltage polarity. The electrostatic force is evenly distributed across the section of the membrane above the signal line. Therefore, the appropriate spring constant can be used to determine the distance that the membrane moves under the applied force. The pull-down voltage can be found from [13]

WL

kgV eq

p 27

8 3

(5)

where Vp is pull down voltage, k is the spring constant of membrane, g is the equivalent gap between membrane and signal line. When the RF MEMS is in its initial undisplaced position (switch-off), the gap consists of two dielectric materials namely air and SiO2. The dimension of the MEMS switch is shown in Table 1. Component Material Dimension MEMS bridge Aluminum t= 0.5 μm

L=150 μm W=200 μm

Signal line Aluminum Thickness=1 μm W=100 μm

Dielectric layer SiO2 2SiOg =1.4 μm

Air gap Air airg =1 μm

GSG gap Air G=25 μm Table 1 shows the dimension of the MEMS switch.

2-2 RF MEMS Fabrication The fabrication process of the RF MEMS switch is shown in Fig. 6. Arrays of RF MEMS structures were fabricated by first depositing a thin aluminum electrode to serve as the signal line for the switch. A silicon dioxide isolation dielectric layer is deposited on top of the electrode to to enable the RF MEMS switch capacitor when the membrane is pulled down. The membrane is deposited over a sacrificial polymer layer that is released at the end of the surface micromachining process. After sacrificial layer is released, the aluminum membrane is left suspended over the dielectric layer. Its natural state is in the “up” or unactuated position. When a sufficient DC electrical potential is applied between the membrane and electrode, the membrane snaps down into the actuated state. RF MEMS switches were fabricated using dimensions listed in Table 1, and shown in Fig. 2. The metal membrane was fabricated using aluminum because of its high resistance to fatigue and low electrical resistance. A 1 micron, nano-porous SiO2 dielectric isolation layer which separate the membrane and signal line was deposited using a Uni-axis PECD machine . After fabricating the RF MEMS structure, a layer of photoresist was deposited on the surface, to protect the wafer from debris and damage during dicing.

Fig. 6 The diagram showing our fabrication process. (1) Deposition and patterning of coplanar waveguide. (2) Depositon and patterning of silicon dioxide. (3) Spin coating and patterning of sacrificial layer—photoresistor polymer. (4) Deposition and patterning of a aluminum bridge. (5) Removal of the sacrificial layer and relasing the membrane [10].

A chemical composition analysis was performed to check if there was any carbon element (photoresistor) left out in between the membrane and the dielectric layer. If there is photoresistor, it may serve as an adhesive to cause the membrane stick to the dielectric, which is refered to as stiction.

Fig. 7 Energy dispersive spectroscopy (EDS) analysis of the top surface of dielectric and the bottom surface of membrane

after the membrane is peeled off.

2-3 RF MEMS Characterization The electromechanical and RF signal properties of the MEMS is characterized after fabrication. A ground-signal-ground (GSG) probe having 250 micro pitch with appropriate Bias Tee and a Network Analyzer are used to make time domain reflectometry (TDR) analysis of the RF MEMS to extract S11 insertion loss parameters and paracitics (Fig. 8). The Bias Tee is used to apply DC bias power to the signal line. We found that the pull-down voltage required to change the capacitance of the switch was around 30 volts. The

Al

Al SiO2

123

resonance frequency of our RF MEMS is measured 23.75 kHz.

Fig. 8 The GSG probe was connected to a RF MEMS switch

[10].

Fig. 9 (a) Mechanical resonance frequency of RF MEMS measured by laser vibrometer. (b) the modal analysis at 1st

resonance frequency.

3. Reliability of RF MEMS Switches and PHM Implementation

In order to estimate the state of health of RF MEMS, we conducted two types of overstress tests: (1) ESD test and (2) operational voltage test. We characterized the RF MEMS switches before any test, then we subject those switches to either ESD (Human body model) pulses or 60V unipolar square waveform. We measured capacitance-voltage (C-V) curve intermittently and looked at the membrane with a non-contact surface profiler.

Fig. 10 The overstress of RF MEMS using ESD and 60V

square waveform. (a) is the ESD simulator that generates a ESD pulse shown in (c). (b) is the controllable power supply generates square waveform shown in (d).

We tested RF MEMS under different ESD (Human body model) voltage level and found that pulses higher than 2kV can cause burnout of both membrane and dielectric layer.

Fig. 11 Scanning electron microscope (SEM) image shows

the burnout of RF MEMS after ESD tests.

For a lower ESD voltage, the primary failure is charge-induced stiction.

Fig. 12 The shift of C-V curve indicates the charge

accumulation in dielectric after several ESD pulses.

We used surface profiler to prove that the stiction occurred.

a)  b)

c) d)

a)

b) 

a)

124

Fig. 13 surface profiler data shows after 20 ESD pulses, the

membrane was stuck to the dielectric.

For operational voltage tests using 60V, 0.5 Hz, square waveform, the primary failure is stiction shown by surface profiler. No burnout was observed.

-30 -20 -10 0 10 20 300

0.1

0.2

0.3

0.4

0.5

0.6

Bias Voltage (V)

Cap

acita

nce

(pF)

initialafter 2min (60 cycles)after 10min (300 cycles)after 60min (1800 cycles)

Fig. 13 The shift of C-V curve indicates the charge

accumulation in dielectric after operating cycles. The black curve shows stiction occurs.

By using the data of surface profiler, we can measure the displacement of membrane of RF MEMS. Since the displacement is caused by electrostatic force due to dielectric charge, the amount of charge can be estimated, and in turn, the state of health and remaining useful life of RF MEMS can be predicted. The algorithm to predict the onset of failure will be presented at ECTC.

Fig. 14 The shift of C-V curve of RF MEMS overstressed under ESD test (a) and operational voltage test (b). ESD causes C-V shift even after one pulse, whereas in operational voltage tests the shift of C-V curve is exponential with the number of cycles.

By analyzing the shift the C-V curve, the accumulated charge density in the dielectric layer can be found.

4. Conclusions In this paper we proposed the design, realization, characterization, and reliability test of a novel RF MEMS capacitive switch. We developed a non-intrusive monitoring method using surface profiler to assess the amount of charge trapped in RF MEMS switches. The key characteristic of our design is the introduction of Prognostics and Health Management (PHM) that allows us to calculate the remaining useful life of our RF MEMS capacitive switches and provide a warning before its onset of failure. We verified the method by overstressing our devices under both electrostatic discharge (ESD) and operational voltage waveform.

Acknowledgments The authors thank the Center for Advanced Life Cycle Engineering (CALCE) at the University of Maryland for supporting this reserach. CALCE provides equipment and resources to support the study of energy harvesting technology. More than 100 national and international organizations and companies support the Center. Also, the authors would like to acknowledge Dr. Ravi Doraiswami, Prof. Reza Ghodssi, Prof. Balakumar Balachandra for their valuable help.

b) a)

b)

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