rf mems capacitive switch with leaky nanodiamond dielectric film
TRANSCRIPT
Diamond & Related Materials 20 (2011) 546–550
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Diamond & Related Materials
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RF MEMS capacitive switch with leaky nanodiamond dielectric film☆
Changwei Chen a, Yonhua Tzeng a,⁎, Erhard Kohn b, Chin-Hung Wang c, Jun-Kai Mao c
a Institute of Microelectronics, Department of Electrical Engineering, National Cheng Kung University, One University Road, Tainan 701, Taiwanb Institute of Electron Devices and Circuits, Ulm University, Ulm, Germanyc Industrial Technology Research Institute — South, Tainan 709, Taiwan
☆ Presented at NDNC 2010, the 4th International ConNano Carbons, Suzhou, China.⁎ Corresponding author. Tel.: +886 62757575x34001
E-mail address: [email protected] (Y. Tzen
0925-9635/$ – see front matter © 2011 Elsevier B.V. Adoi:10.1016/j.diamond.2011.02.008
a b s t r a c t
a r t i c l e i n f oAvailable online 12 February 2011
Keywords:NanodiamondRF MEMS switchesDielectric charge trapping
RF MEMS capacitive switches using leaky nanodiamond as a dielectric film are studied and compared withthose using Si3N4. Characteristics of dielectric charging and discharging are analyzed at temperatureranging from −196 °C to 150 °C. Electrical resistivity of leaky nanodiamond is measured to be lower thanthat of Si3N4 by 3 to 6 orders of magnitude at room temperature. Trapped charges in leaky nanodiamonddielectric discharge much more quickly than those in Si3N4 while the power dissipation of nanodiamondbased switches remains low. As a result, charge trapping induced shift in electrostatic actuation voltage isgreatly reduced compared to that with Si3N4 and becomes non-detectable under the reported conditions.RF MEMS capacitive switches based on leaky nanodiamond dielectric are, therefore, more reliable thanthose with Si3N4.
ference on New Diamond and
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ll rights reserved.
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
Switches with high radiation hardness, excellent isolation andlinearity, and low insertion loss are desirable for RF applications suchas phase-array radars and wireless communication [1–3]. Micro-Electro-Mechanical-Systems (MEMS) switches [4–6] exhibit afore-mentioned superior characteristics compared to solid-state devicesand have attracted a great deal of interest. An RF MEMS capacitiveswitch makes use of a dielectric layer of high dielectric constant andbreakdown strength to achieve high capacitance and low impedancewhen two electrodes are closed.
SiO2 and Si3N4 are commonly used dielectric materials for thisapplication. These dielectric films have very high electrical resistivityand suffer from charge trapping after repetitive switching on and off.Electrostatic actuation of MEMS switches is commonly applied due toits simplicity and low power consumption. Electrostatic forcecontributed by trapped charges results in the shift of the actuation-voltage. Trapped charges also increase the force needed to restore themovable electrode when the switch is turned off. When theelectrostatic force due to trapped charges is too strong, the movableelectrode is stuck to the dielectric film [3–6]. Alternate voltage hasbeen applied in an effort to reduce the extent of dielectric chargetrapping but still could not eliminate the charge trapping completelydue to non-equal charging and discharging speeds when the dielectricfilm is under alternate positive and negative biases [7]. Leaky dielectric
materials with optimized electrical resistivity for quick escape oftrapped charges in the off-state of the switch while maintaining lowpower consumption in the on-state of the switch are desired.
Nanodiamond of a few nanometers to 100 nm in sizes containsdefects and graphitic phases in the grains and grain boundariesallowing electrical resistivity to be fine tuned [8,9]. Nanodiamond ofgood crystalline quality possesses high hardness, low coefficient offriction, high young's modulus, high thermal conductivity, excellentchemical stability [10]. Sp2-bonded non-diamond phases in grain andgrain boundaries allow electron transport by hopping and quickescape of trapped charges [11,12].
2. Experimental
An RF MEMS capacitive switch and its schematic diagram [13–16]are shown in Fig. 1. The co-planar waveguide (CPW) made of a 1 μmthick tungsten film is the bottom electrode. Si3N4 or nanodiamond of300 nm thick serves as the dielectric film and is patterned by reactiveion etching (RIE). Si3N4 and nanodiamond are deposited by RF plasmaenhanced CVD and microwave plasma enhanced CVD processes,respectively. Photoresist is used as an etch mask and a sacrificial layerfor forming the bridge and the gap between the movable electrodeand the dielectric film.
Current versus electric field (I–E) relationships of Si3N4 andnanodiamond based metal–insulator–metal (MIM) capacitors aremeasured to reveal the dielectric charging and discharging processes.The bottom electrode and top electrode (500 μm×500 μm) of theMIM capacitors are made of tungsten (0.5 μm) and aluminum(0.5 μm), respectively. To deposit nanodiamond, the substrate isinitially seeded by immersion in a solution of suspending
Fig. 1. Schematic diagram of a fabricated RF MEMS capacitive switch and its opticalmicrograph.
Fig. 2. SEM micrograph and a Raman spectrum of a nanodiamond film deposited ontungsten.
Fig. 3. Discharging current of nanodiamond capacitors under different applied voltages(□: 10 V; ○: 15 V; △:20 V; dotted line: calculated).
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nanodiamond particles of 5–10 nm under ultrasonic agitation. A gasmixture of Ar/H2/CH4 is used with flow rates of 190/8/1.5 standardcubic centimeters per minute (sccm). The gas pressure duringdeposition of nanodiamond is 160 Torr. The substrate holder isheated by a resistive heater and the processing plasma to 650 °C asmeasured by an optical pyrometer. Microwave of 2.45 GHz has itspower set between 600 W and 750 W. The CVD process lasts for 1 h.The thickness of deposited nanodiamond film is about 300 nm [17-22]. Aluminum is sputter deposited as a hard mask on nanodiamondfor subsequent patterning by oxygen plasma [23].
3. Results and discussion
Capacitance–voltage (C–V) curves are measured by an Agilent4284 LCR meter to monitor the capacitance of a switch during the on–off transition and in its on- and off-states [24]. Themetal bridgemovesfrom the off-state (separate from the dielectric) to the on-state (incontact with the dielectric film) after a voltage larger than the pull-involtage is applied between these two electrodes. The switch reachesits on-state capacitance (Con) and allows a low impedance coupling ofRF signal to the ground.When the applied voltage is zero, themovableelectrode becomes separated from the dielectric causing the overallcapacitance, denoted as Coff, including the series connection of the aircapacitor and the solid-state dielectric capacitor to decrease and theimpedance to increase [25]. The pull-in voltage of fabricated switchesis about 22 V with the Con/Coff ratio ranging from 6 to 10. The effectivespring constant of the bridge varies with the shape, material, andmechanism of its support, and can be optimized to achieve a desiredVpull-in [26].
Table 1Calculated discharging time constants after 10 V is applied.
Dielectric materials τD1 (s) τD2 (s) τD3 (s)
Si3N4 28 145 990Nanodiamond: Ar/H2/CH4=190/8/1.5 sccm;no heater; microwave power 750 W; 160 Torr
23 81 478
Nanodiamond: Ar/H2/CH4=150/6/1.5 sccm;no heater; microwave power 650 W; 180 Torr
18 92 552
Nanodiamond: Ar/H2/CH4=80/3/1 sccm;resistively heated to 550 °C; microwavepower 400 W; 100 Torr
21 115 666
Nanodiamond:Ar/H2/CH4=190/8/1.5 sccm;resistively heated to 650 °C; microwavepower 650 W; 160 Torr
18 101 990
Fig. 4. Leakage current of MIM capacitor structures under 10 V applied voltage.
Fig. 5. I–E characteristics and electrical breakdown of a nanodiamond film (gas flow rate:Ar/H2/CH4=190/8/1.5 sccm; resistive heater: 650 °C; microwave power: 650W).
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Fig. 2 shows an SEM image and a Raman spectrum of a leakynanodiamond film deposited on tungsten. The average grain size isapproximately 50 nm. A 532 nm laser is used for the Raman scatteringanalysis. Two broad bands near 1350 cm− 1 and 1580 cm− 1
are observed. On top of the broad D-band near 1350 cm−1 italso shows a small diamond peak at 1332 cm−1. The G-band near1580 cm−1 is due to sp2 bonded carbon. Two bands centered around1140 cm−1 and 1480 cm−1 are often observed in Raman spectra ofnanodiamond films and are attributed to trans-polyacetylene [27,28].Roughness of the nanodiamond film is 7.2 nm according to AFMmeasurements.
3.1. Transient current measurements
A DC voltage is applied to a metal–insulator–metal (MIM)structure. After 500 s, the charging circuit is grounded and thedischarge current is recorded for 1000 s using Agilent 4155C. Thefollowing two equations from Ref. [29] are applied to extract chargingand discharging time constants:
IC = qAdQdt
= qA∑JQ J
τ JC
exp −tON = τJC� �
ð1Þ
ID = qAdQdt
= qA∑JQ J
τ JD
exp −tOFF = τJD
� �ð2Þ
where QJ is the steady-state charge density; tON and tOFF are the on-time and off-time of the switch corresponding to the charging timeand the discharging time, respectively; τC and τD are charging and
Table 2Resistivity of Si3N4 and nanodiamond films.
Dielectric materials Resistivity (Ω-cm)
Si3N4 5×1014
Nanodiamond: Ar/H2/CH4=190/8/1.5 sccm; noheater; microwave power 750 W; 160 Torr
4×1011
Nanodiamond: Ar/H2/CH4=150/6/1.5 sccm;no heater; microwave power 800 W; 180 Torr
8×1010
Nanodiamond: Ar/H2/CH4=80/3/1 sccm; resistivelyheated to 550 °C; microwave power400 W; 100 Torr
9×108
Nanodiamond: Ar/H2/CH4=190/8/1.5 sccm;resistively heated to 650 °C; microwave power650 W; 160 Torr
1.8×1010
discharging time constants. We apply 10 V, 15 V, and 20 V tonanodiamond MIM capacitors and measure the discharging currentas shown in Fig. 3 to extract discharging time constants of 53 s, 31 s,and 59 s, respectively, which vary not much with the applied voltage[30].
Fig. 6. C–V curves of switches with different dielectric materials. (a) Si3N4, (b)nanodiamond.□: before stressing;○: after stressing;△: 10 min of idling after observingactuation voltage shift.
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The discharge current is divided into three time slots forcalculating the discharging time constants as follows:
τD1
: tOFF
b 20s
τD2
: 20s b tOFF
b 200s
τD3
: 200s b tOFF
b 1000s
Table 1 shows the calculated discharging time constants when10 V is applied to MIM capacitors. It clearly reveals that nanodiamondMIM capacitors have smaller discharging time constants than that ofSi3N4. Nanodiamond allows quick escape of trapped charges from thedielectric and the prevention from the undesired shift of the pull-involtage of an RF MEMS capacitive switch.
3.2. DC analysis
Shown in Fig. 4 is the leakage current through nanodiamond andSi3N4 when 10 V is applied. The current is measured by an ammeter(Agilent 4155C). The electrical resistivity of nanodiamond is found tobe lower than that of Si3N4 by 3 to 6 orders of magnitude at roomtemperature as shown in Table 2. Fig. 5 shows that the 300 nm thicknanodiamond film has a dielectric strength of 2.2 MV/cm, which issufficient to withstand the applied high electric field during the on-
Fig. 7. I–V measurements of MIM capacitors with 300 nm thick dielectric films at roomtemperature (□) and 150 °C (○). (a) Si3N4, (b) nanodiamond. Gas flow rate: Ar/H2/CH4=190/8/1.5 sccm; microwave power: 650W; pressure: 160 Torr; resistively heated:650 °C.
state of the switch. The tunability of resistivity by CVD processparameters and the high dielectric strength make nanodiamond apromising leaky dielectric material for RF MEMS switches.
3.3. Accelerated dielectric charging tests
Accelerated dielectric charging is done by applying a continuousDC voltage to a switch in the on-state. 40 V DC, which is higher thanthe minimum pull-in voltage, is applied to a switch. After repetitivestressing for 80 s in each cycle, C–V measurements are measured.Fig. 6 shows C–V curves measured before stressing, immediately afterstressing, and 10 min after an idling time following the stressing forallowing the discharging of trapped charges [31].
As shown in Fig. 6(a), the switch with Si3N4 as the dielectric filmshows a shift in the C–V curve towards a lower pull-in voltageimmediately after stressing by a positive voltage. It indicates thatcharges are trapped in the dielectric film. Ten minutes after thestressing, the shift in the C–V curve disappears and the C–V curve isrestored, indicating that trapped charges have escaped from Si3N4. Onthe contrast, there is no shift in the pull-in voltage as measured by theC–V curve shown in Fig. 6(b) for a switch with nanodiamond. Trappedcharges in the nanodiamond dielectric film discharge quickly throughresistive paths. The capability of nanodiamond in minimizing the shiftin actuation voltage and increasing the reliability of RF MEMScapacitive switches have, thus, been demonstrated.
Fig. 8. I–V measurements of MIM capacitors with 300 nm thick dielectric films at roomtemperature (□) and when immersed in dry ice (△). (a) Si3N4, (b) nanodiamond. Gasflow rate: Ar/H2/CH4=190/ 8/1.5 sccm; microwave power: 650 W; pressure: 160 Torr;resistively heated: 650 °C.
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3.4. Temperature effects
In order to confirm that nanodiamond is a superior dielectricmaterial for RF MEMS capacitive shunt switches in a wide range oftemperature, MIM capacitors are wire-bonded for measurements attemperatures ranging from 150 °C to that of liquid nitrogen. Whendielectric films are heated to 150 °C, we measure the leakage currentthrough a Si3N4 MIM capacitor and compare it with what is measuredat room temperature. Fig. 7 shows measured I–V curves. The leakagecurrent increases with temperature by about two orders of magnitudefor Si3N4. The leakage current increases by less than one order ofmagnitude for nanodiamond. Therefore, at the elevated temperatureof 150 °C, nanodiamond MIM capacitors are still superior to that ofSi3N4 in terms of lower resistivity for easier and quicker discharging oftrapped charges.
Si3N4 and nanodiamond capacitors are also compared at reducedtemperatures by immersing them in dry ice. Fig. 8 shows that theleakage current for nanodiamond decreases with temperature morethan that for Si3N4. Nevertheless, at the temperature when thesamples are immersed in dry ice, nanodiamond is still moreconductive and, therefore, a superior leaky dielectric material for RFMEMS shunt switches. The same test is carried out by immersing thepackaged MIM capacitors in liquid nitrogen. As shown in Fig. 9,nanodiamond still has better leaky characteristics than Si3N4 at theliquid nitrogen temperature.
4. Conclusions
RF MEMS capacitive shunt switches are fabricated for studyingcharge trapping and discharging in nanodiamond in comparisonwith
Fig. 9. I–Vmeasurements of MIM capacitors with 300 nm thick dielectric films at roomtemperature (□) and 77 K (△): (a) Si3N4, (b) nanodiamond. Gas flow rates: Ar/H2/CH4=190/8/1.5 sccm; microwave power: 650 W; pressure: 160 Torr; resistivelyheated: 650 °C.
Si3N4. Nanodiamond exhibits dielectric strength of greater than2 MV/cm. The electrical resistivity of nanodiamond is found to belower than that of Si3N4 by 3 to 6 orders of magnitude at roomtemperature. Accelerated dielectric charging tests reveal that Si3N4
switches exhibit reduced pull-in voltage subsequent to excessivecharge trapping. Pull-in voltage shift due to charge trapping is notobserved for nanodiamond based switches. Leaky nanodiamond is,therefore, a superior dielectric at temperatures ranging from−196 °C to 150 °C for applications to RF MEMS shunt capacitiveswitches.
Acknowledgments
We are grateful to the financial support by NSC-Taiwan undergrants 98-3114-M-006-001, 99-2120-M-006-004, and 96-2221-E-006-286-MY3. Technical assistance in device fabrication by NationalNano Device Laboratory and C–V measurements by Laboratory ofProfessor W.C. Liu, Institute of Microelectronics, National Cheng KungUniversity are appreciated.
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