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K-band capacitive MEMS switches on GaAs substrate: Design, fabrication, and reliability
Microelectronics Reliability 52 (2012) 2245–2249
Contents lists available at SciVerse ScienceDirect
Microelectronics Reliability journal homepage: www.elsevier.com/locate/microrel
K-band capacitive MEMS switches on GaAs substrate: Design, fabrication, and reliability A. Persano a,⇑, A. Tazzoli b,c, P. Farinelli d, G. Meneghesso b,e, P. Siciliano a, F. Quaranta a a
Institute for Microelectronics and Microsystems (IMM-CNR), Unit of Lecce, Via Monteroni, 73100 Lecce, Italy University of Padova, Department of Information Engineering, Via Gradenigo 6/B, 35131 Padova, Italy c Carnegie Mellon University, Department of Electrical and Computer Engineering, Pittsburgh, PA 15213, USA d University of Perugia, Department of Electronic and Information Engineering, 06125 Perugia, Italy e Italian Universities Nano-Electronics Team (IUNET), 40125 Bologna, Italy b
a r t i c l e
i n f o
Article history: Received 31 May 2012 Accepted 8 June 2012 Available online 6 July 2012
a b s t r a c t Shunt capacitive RF MEMS switches were developed on GaAs substrate, using a III–V technology process that is fully compatible with standard MMIC fabrication. The switches show an insertion loss lower than 0.8 dB and isolation better than 30 dB with resonance frequencies in K-band, according to the switch geometric parameters. Reliability limits due to dielectric charging were overcome by applying suitable fast bipolar actuation waveforms, making the developed switches good candidates for both redundancy (always on/off) and cycled applications. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction A great interest has been recently devoting to III–V technology for the fabrication of radio frequency microelectromechanical system (RF MEMS) switches. Indeed, compared to traditional siliconbased fabrication process, III–V technology offers a practical way to integrate RF switches with other electronic and photonic devices on GaAs, GaN/Si or GaN/SiC substrates [1]. Among the others, K-band wireless terrestrial and space-oriented applications are making appealing the integration of RF MEMS switches with electronic circuitry, taking advantage of the high wide-band linearity, low insertion loss and power consumption, small volume and low batch fabrication cost peculiar of micromachined devices [2]. In spite of the attractive capabilities, RF MEMS switches reliability is still an open issue, especially for switches fabricated in III–V technology, being this fabrication process less consolidated with respect to the silicon-based one [3]. To date, the major reliability problem encountered in capacitive switches is the charging process of dielectric layers over or around the switch actuator that can cause the electrical parameters shifting, or, when fatal, the device stiction [4–8]. In silicon technology, low temperature silicon dioxide (SiO2) and silicon nitride (Si3N4) deposited by plasma-enhanced chemical vapor deposition (PECVD) are the most common dielectric materials used in RF MEMS switches fabrication. Si3N4 is the standard dielectric material in III–V technology, too. In general, the low temperature SiO2 is less susceptible to surface charging and shows ⇑ Corresponding author. Tel.: +39 0832 422538; fax: +39 0832 422552. E-mail address: [email protected] (A. Persano). 0026-2714/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.microrel.2012.06.008
better breakdown stability compared to PECVD deposited Si3N4 [9]. However, the dielectric constant of SiO2 (typ. 2–4) is much lower than that of Si3N4 (typ. 6 and 7), resulting in a small capacitance ratio and, consequently, in insufficient performance at low GHz frequencies. Hence, in order to both optimize the switch RF performance and guarantee good reliability, proper switch design, fabrication process, and reliability assessment become mandatory. In this work, we developed shunt capacitive RF MEMS switches that are fully compatible with standard GaAs-based MMIC fabrication. Switches with different geometric parameters of the moveable bridge were designed and characterized, obtaining a good isolation (>30 dB) in the K-band. Finally, the reliability of the fabricated switches is addressed, finding that dielectric charging drawbacks can be significantly reduced by applying suitable actuation waveforms consisting of consecutive positive and negative short pulses. 2. Design and fabrication of switches Air-bridge shunt capacitive MEMS switches were designed on GaAs substrate. The switches are in coplanar (CPW) waveguide configuration, with the suspended metal bridge consisting of a plate that is anchored at the two ground planes by two springs on each side. The coplanar central conductor under the suspended bridge is covered with a dielectric layer in order to shunt to ground the RF signal when the bridge is in down state (see Fig. 1a). The switch is actuated by biasing the coplanar central conductor. DC blocks (series capacitors) at input and output ports of about 4.8 pF each were integrated in order to keep decoupled the DC and
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Fig. 1. (a) Layout and (b) equivalent circuit of the designed shunt capacitive RF MEMS switches. The values of CMIM, Z0, and ZL are 4.8 pF, 50 X, and 73 X, respectively.
RF signals, providing a good RF signal propagation above 7 GHz. The loss contribution of the DC blocks was estimated to be around 0.5 dB and was not taken into account in the full-wave simulations to reduce the computational time. Fig. 1b shows the equivalent circuit of the designed switches. The shunt bridge is represented by the series of a bridge resistance (Rs), inductance (Ls) and variable capacitance (Con or Coff). The series capacitances CMIM model the two DC blocks, whereas ZL represents the inductive sections added to compensate for the mismatch induced by the off-state capacitance. In order to study the influence of layout geometries (central plate and springs size) on the RF performance, four different switches were designed and fabricated. The main geometric and equivalent circuit parameters of the designed switches are summarized in Table 1. Switches 1 and 2 present identical plate width (D = 100 lm) and spring length (l = 65 lm), whereas they differ in the plate length (L). The calculated values of Coff (Coff-calc) and Con (Con-calc) for the switch 1 and 2 are 75 fF and 3.7 pF, respectively. The bridge width (D = 150 lm) is kept constant also in switches 3 and 4 with a plate length L = 290 lm, whereas the spring length (l) changes in these two switches. The values of Coff-calc and Con-calc for the switches 3 and 4 are of 115 fF and 6.3 pF, respectively, being these values greater than those predicted for switches 1 and 2 due to the greater plate width and, hence, to the larger capacitive area along the RF line (see Fig. 1a). Coff-calc and Con-calc correspond to the parallel plate capacitance between the bridge and the CPW central conductor, i.e. the ideal capacitance calculated without taking into account for surface roughness. The variation in the bridge geometric parameters also causes a change in the resistive (Rs) and inductive (Ls) contributions [10]. The designed switches were fabricated on GaAs substrate with a III–V fabrication process. To this scope, a surface-micromachined approach was followed, using an eight-mask fabrication process.
Table 1 Geometric and equivalent circuit parameters of the designed shunt capacitive switches.
D (lm) L (lm) l (lm) Rs (X) Ls (pH) Coff-calc (fF) Con-calc (pF) Coff-meas (fF) Con-meas (pF) Con-meas/Coff-meas
Switch 1
Switch 2
Switch 3
Switch 4
100 390 65 0.23 22
100 520 65 0.29 27
150 290 94 0.15 14
150 290 145 0.21 19
75 3.7 90 1.1 12.2
The main technological steps for the switch fabrication are shown in Fig. 2. A 500 mn thick Si3N4 layer was deposited by PECVD over all the substrate and it is the same used for the passivation of III–V
(a)
(b)
(c)
(d)
(e)
115 6.3 135 1.3 14.4
2.1 15.5
2.2 16.3
Fig. 2. Process flow with the main technological steps for the central RF line.
A. Persano et al. / Microelectronics Reliability 52 (2012) 2245–2249
devices, thus allowing to not increasing the number of fabrication steps required for the integration of RF MEMS with MMICs. The actuation line was composed by a sputtered 25 nm thick NiCr (50/50) alloy layer and above it the underpass line was fabricated by depositing a metal multilayer of Ti/Pt/Au (30/30/60 nm). Both these layers were defined by lift-off and in between them a 300 nm thick Si3N4 layer was deposited by PECVD (see Fig. 2a). After the underpass line, another 300 nm thick Si3N4 layer. Fig. 2b shows the two Si3N4 depositions along the central RF line. Several works have appeared in the literature, concerning the dependence of dielectric charging in PECVD silicon nitride films on the deposition parameters and, hence, on the material stoichiometry. Indeed, this latter affects the concentration of bulk and interface traps as well as the concentration of dipoles and charge accumulation [11,12]. Following the results presented in [12], a high deposition frequency is preferable for MEMS switches fabrication thanks to the smaller injected charge density and a faster discharging process. In this work, we chose a frequency of 13.56 MHz for the PECVD silicon nitride deposition. The other deposition conditions of the PECVD Si3N4 layers are: power supply of 20 W, substrate temperature of 250 °C, chamber pressure of 350 mtorr, and flow ratio in the SiH4:NH3:N2 gas mixture of 1:4:10. The sacrificial layer for the definition of the air gap under the bridges was made by a 3 lm thick photoresist (Fig. 2c). In order to stabilize this layer and to obtain well-rounded edges of bridges, which are necessary to promote the flatness of the membrane anchors and borders, a hard bake at 200 °C was performed. A metal multilayer of Ti/Au/Ti (5/50/5 nm) was evaporated on the entire surface, to be used as electrical contact film and seed layer for the following electroplating process. The suspended bridge, the anchor points, the CPW lines and the ground pads were 1 lm thick by gold-electroplating deposition (Fig. 2d). The membrane contains a set of holes (diameter of 10 lm, and a distance from each other of 9 lm), with the dual aim of easing the full removal of the sacrificial layer by O2 plasma dry-etching and of fastening the switch actuation time by reducing the air damping. A second 1 lm thick gold-electroplating deposition was performed to thicken the CPW lines and the ground pads. A combination of selective wet and dry etching was used to remove the excess Ti/Au/Ti multilayer among the devices. Finally, the air bridges were released by removing the underneath sacrificial photoresist by a high pressure O2 plasma process performed in a barrel etcher in order to prevent sticking problems (Fig. 2e). The scanning electron microscopy (SEM) image of a fabricated switch #4 is shown in Fig. 3a. A Polytec MSA-500 optical profilometer was used to acquire topographies of the switches and to characterize their dynamic behavior, using the integrated Laser Doppler Vibrometer (LDV). The measurement of the membrane mechanical resonance frequency is shown in Fig. 3b, monitoring the velocity of the membrane central point with the LDV applying a chirp signal to
(a)
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the actuator. A resonant frequency of about 30 kHz was extracted, consistently with other results recently obtained for switches of similar size [13]. 3. RF performance and reliability The RF performance of the fabricated switches were characterized under ambient environmental conditions, using a remotely controlled Agilent E8361A Vector Network Analyzer (VNA), deembedded through short-open-load thru (SOLT) calibration up to 30 GHz, and a Keithley 2612 Source Meter together with a Karl Süss Probe Station equipped with ground-signal-ground RF |Z|probes and DC probes. For each typology, five switches were measured, finding actuation voltages in the range 25-40 V. Fig. 4 shows the typical S21 parameter behavior measured for the different switches in the up and down states. Table 1 summarizes the values of the Con-meas and Coff-meas capacitances and of the ratio between them (Con-meas/Coff-meas), which are obtained by fitting the measured S21 curves. Below 7 GHz, the switch performance is limited by the presence of the DC blocks. All switches showed an insertion loss better than 0.8 dB in almost all the range of measured frequencies. Specifically, a value of Coff-meas larger than the calculated one is extrapolated from the measured values of S21 for all switches. This result is likely due to the fact that ideal DC blocks were considered in the model, and to a down bending of the bridge in the central part, as also confirmed by topography measurements (not reported here). An air gap of 2.5 lm, rather than 3 lm, was then extrapolated by the S21 values measured in the up state. The switch isolation was better than 20 dB in the K-band, with resonance frequencies increasing from 25 GHz, according to the variation of the switch geometric parameters [10], and the peaks of maximum isolation were better than 50 dB. For all switches, a degradation of Con was estimated from measurements with respect to the calculation, indicating a not complete adhesion of the actuated bridge on the CPW central conductor, which can be due to the surface roughness as well as to the bridge form factor [9]. It is worth noting that the more flexible bridges allow for a higher Con/Coff ratios due to a better adhesion of the bridge on the bottom electrode. Specifically, Con-meas varies as the plate or spring length changes, due to the variation of the bridge form factor. Effective Con-meas/Coff-meas ratio values as good as in the range 12.2–16.3 for all switch designs were obtained. Switch reliability was assessed under the application of a variety of unipolar and bipolar polarization waveforms. An arbitrary waveform generator Agilent 33250A and a highfrequency voltage amplifier were used to build the actuation scheme varying the length of the pulses, number of cycles, and the voltage pulses polarity. At least three devices, with identical design, and taken from different wafer area, were tested. The reliability tests shown here were performed on the switch typology
(b)
Fig. 3. (a) SEM image of a fabricated shunt capacitive RF MEMS switch (typology #4). The red mark shows the point at which the Laser Doppler Vibrometer (LDV) measurement was performed. (b) LDV vibration velocity measurement of the suspended membrane central part (10 V periodic chirp signal applied).
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(a)
(b) Fig. 4. S21 parameter measured behavior of the fabricated shunt capacitive switches in the up and down states.
(a) (c)
(b) Fig. 7. S21 parameter evolution under the application of bipolar waveforms consisting of consecutive positive and negative voltage pulses with different durations. The durations of the positive and negative voltage pulses are 480 ms and 20 ms, respectively, in (a); 100 ms and 20 ms, respectively, in (b); and both 100 ms in (c). The different applied polarization waveforms are reported in dashed line in (a), (b), and (c) as guide for eye. A continuous wave signal at 20 GHz, 0 dBm was applied during the test.
Fig. 5. (a) S21 parameter evolution during (actuation) and after (release) the application of a 45 ms long positive voltage pulse. The sketch of the applied polarization waveform is also reported in dashed line as guide for eye. (b) Dependence of the release time (trel) on the duration of the unipolar voltage pulse (tON).
Fig. 8. S21 parameter evolution under the application of a train (1 kHz) of consecutive 20 ls long positive and negative voltage pulses. A continuous wave signal at 20 GHz, 0 dBm was applied during the test. For sake of clearness the applied waveform is reported in blue line. Fig. 6. S21 parameter evolution under the application of a train (1 kHz, 0 dBm) of unipolar 50 ls long voltage pulses. After 6 pulses the switch remained stuck in the down state. A continuous wave signal at 20 GHz, 0 dBm was applied during the test (hot switching).
#4, but similar effects were observed for the other switch typologies. A pulse amplitude of 45 V, which is slightly higher than the measured actuation voltage (40 V), was used for switches #4, in
order to achieve the best trade off between the complete switch actuation and reliability, which is known to reduce with the increasing of the bias voltages [4]. Fig. 5a shows the switch operation under the application of unipolar voltage pulses with a duration of 45 ms. It is observable that the release occurs in 15 s and through intermediate states, which
A. Persano et al. / Microelectronics Reliability 52 (2012) 2245–2249
is symptomatic of a partial movement of the membrane. The dependence of the release time on the duration of the unipolar voltage pulse is reported in Fig. 5b. This time is found to be in the order of 100 ls for unipolar pulses shorter than 10 ms, whereas for longer pulse durations the release time significantly increases, reaching an asymptotic value of 15 s for pulses longer than 30 ms. Moreover, it is worth noting that by applying unipolar voltage pulses, even if with a short duration (<100 ls), the switch undergoes failure after less than 10 actuations (see Fig. 6). The failure accelerates as the time in the down state becomes longer, confirming that the actuation time is critical in the degradation rate of the device. Due to the design of the tested switches (see Fig. 3a), this erratic release can be attributed to the charging of the dielectric layer (approximately 150 lm 200 lm of Si3N4) covering the actuation electrode under the moveable bridge. In fact, it is well known that PECVD Si3N4 layers contain a large density of defects that may act as positive or negative charge traps as well as contribute to the formation of dipoles, leading all these factors to the charging of the dielectric layer [6,7]. The observed failure was found to be no permanent, in agreement with the reversible nature of the charging processes, but it clearly makes the devices not usable in a real application. In order to overcome the dielectric charging issues, a negative voltage pulse was applied right after the positive pulse. Thanks to the rapid change (10 ns) of the sign of the actuation signal and the mechanical inertia of the bridge, the switch was observed to remain constantly in the down state. Fig. 7a shows the switch response to bipolar waveforms with consecutive positive and negative pulses long 480 ms and 20 ms, respectively. In the first cycle, the erratic switch operation was observed to reduce, being the release faster with respect to the case of the application of only positive pulses (141 ms rather than 15 s). However, even in this case, the release is observed to be non-instantaneous and occurring through intermediate states, likely due to the fact that the duration of the negative pulse is not enough to allow the complete discharge of the charge trapped during the positive pulse. With the increasing of cycles, the release time becomes longer until, after three cycles, no complete release occurred. Fig. 7b shows the switch response to bipolar waveforms with consecutive positive and negative pulses long 100 ms and 20 ms, respectively. Under this polarization conditions, the duration of the negative pulse is observed to be long enough to allow the complete release of the charge trapped during the positive pulse, leading to a correct switch release (in 100 ls). On the other hand, with 100 ms long positive and negative pulses, the switch is observed to not completely release because of the charge trapped during the negative pulses (see Fig. 7c). Fig. 8 shows the S21 parameter evolution under the application of a train of successive positive and negative voltage pulses (duration of each pulse = 20 ls, duty cycle of 50%) during the nominal down state. The switch correctly operated with no variation in the scattering parameters after 1 million of cycles under the application of 1 kHz signal, modulated with bipolar pulses (20 ls long, duty cycle of 50%) during the ON-state. The switch remained in the down state during the entire polarization pulse and it quickly released (in 100 ls) when the zero voltage was applied. Similar
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results were observed in all the tested switches (3 samples per typology, for a total of 12 samples). 4. Conclusions Shunt capacitive RF MEMS switches operating in K-band were developed with a GaAs III–V fabrication process that is fully compatible with standard MMIC production. Insertion losses better than 0.8 dB, isolations greater than 30 dB with resonance frequencies in K-band, and Con/Coff ratios between 12 and 16 were measured for switches with different geometric parameters. A correct switch operation as well as an improvement of the lifetime (from few cycles to more than 1 million of cycles) were achieved by applying fast bipolar voltage pulses, significantly reducing dielectric charging related problems. Acknowledgement This work was partially supported by Apulia Region (Italy) under the Regional Project ‘‘SENS&MICRO LAB’’. References [1] Malmqvist R, Samuelsson C, Simon W, Rantakari P, Smith D, Lahdes M, et al. Design, packaging and reliability aspects of RF MEMS circuits fabricated using a GaAs MMIC foundry process technology.In: Proc. 40th Europ. micro. conf, Paris; 2010. p. 85–8. [2] Pacheco S, Nguyen CT.-C, and Katehi LPB. Micromechanical electrostatic Kband switches. In: Proc. IEEE MTT-S International microwave symposium, Baltimore, Maryland; 1998. p. 1569–72. [3] Persano A, Tazzoli A, Cola A, Siciliano P, Meneghesso G, Quaranta F. Reliability enhancement by suitable actuation waveforms for capacitive RF MEMS switches in III–V technology. J Microelectromech Syst 2012;21:414–9. [4] Goldsmith C, Ehmke J, Malczewski A, Pillars B, Eshelman S, Yao Z, et al. Lifetime characterization of capacitive RF MEMS switches. In: Proc. IEEE MTT-S Int. microw. symp. dig, Phoenix, AZ; 2001. p. 227–30. [5] Van Spengen WM, Puers R, Mertens R, De Wolf I. A comprehensive model to predict the charging and reliability of capacitive RF MEMS switches. J Micromech Microeng 2004;14:514–21. [6] Yuan X, Cherepko S, Hwang J, Goldsmith CL, Nordquist C, and Dyck C. Initial observation and analysis of dielectric-charging effects on RF MEMS capacitive switches. In: IEEE MTT-S Int. microwave symp. dig, Forth Worth, TX; 2004. p. 1943–6. [7] Papaioannou G, Exarchos M-N, Theonas V, Wang G, Papapolymerou J. Temperature study of the dielectric polarization effects of capacitive RF MEMS switches. IEEE Trans Microwave Theory Technol 2005;53:3467–73. [8] Massenz A, Barbato M, Giliberto V, Margesin B, Colpo S, Meneghesso G. Investigation methods and approaches for alleviating charge trapping phenomena in ohmic RF-MEMS switches submitted to cycling test. Microelectron Reliab 2011;51:1887–91. [9] Peng Z, Palego C, Hwang JCM, Forehand DI, Goldsmith CL, Moody C, et al. Impact of humidity on dielectric charging in RF MEMS capacitive switches. IEEE Microw Wireless Compos Lett 2009;19:299–301. [10] Rebeiz G. RF MEMS theory, design, and technology. New York: WileyInterscience; 2003. [11] Zaghloul U, Papaioannou GJ, Bhushan B, Wang H, Coccetti F, Pons P, et al. Effect of deposition gas ratio, RF power, and substrate temperature on the charging/ discharging processes in PECVD silicon nitride films for electrostatic NEMS/ MEMS reliability using atomic force microscopy. J Microelectromech Syst 2011;20:1395–418. [12] Zaghloul U, Papaioannou GJ, Wang H, Bhushan B, Coccetti F, Pons P, et al. Nanoscale characterization of the dielectric charging phenomenon in PECVD silicon nitride thin films with various interfacial structures based on Kelvin probe force microscopy. Nanotechnology 2011;22. 205708-205708. [13] Tazzoli A, Barbato M, Mattiuzzo F, Ritrovato V, Meneghesso G. Study of the actuation speed, bounces occurrences, and contact reliability of ohmic RF MEMS switches. Microelectron Reliab 2010;50:1604–8.
Contents lists available at SciVerse ScienceDirect
Microelectronics Reliability journal homepage: www.elsevier.com/locate/microrel
K-band capacitive MEMS switches on GaAs substrate: Design, fabrication, and reliability A. Persano a,⇑, A. Tazzoli b,c, P. Farinelli d, G. Meneghesso b,e, P. Siciliano a, F. Quaranta a a
Institute for Microelectronics and Microsystems (IMM-CNR), Unit of Lecce, Via Monteroni, 73100 Lecce, Italy University of Padova, Department of Information Engineering, Via Gradenigo 6/B, 35131 Padova, Italy c Carnegie Mellon University, Department of Electrical and Computer Engineering, Pittsburgh, PA 15213, USA d University of Perugia, Department of Electronic and Information Engineering, 06125 Perugia, Italy e Italian Universities Nano-Electronics Team (IUNET), 40125 Bologna, Italy b
a r t i c l e
i n f o
Article history: Received 31 May 2012 Accepted 8 June 2012 Available online 6 July 2012
a b s t r a c t Shunt capacitive RF MEMS switches were developed on GaAs substrate, using a III–V technology process that is fully compatible with standard MMIC fabrication. The switches show an insertion loss lower than 0.8 dB and isolation better than 30 dB with resonance frequencies in K-band, according to the switch geometric parameters. Reliability limits due to dielectric charging were overcome by applying suitable fast bipolar actuation waveforms, making the developed switches good candidates for both redundancy (always on/off) and cycled applications. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction A great interest has been recently devoting to III–V technology for the fabrication of radio frequency microelectromechanical system (RF MEMS) switches. Indeed, compared to traditional siliconbased fabrication process, III–V technology offers a practical way to integrate RF switches with other electronic and photonic devices on GaAs, GaN/Si or GaN/SiC substrates [1]. Among the others, K-band wireless terrestrial and space-oriented applications are making appealing the integration of RF MEMS switches with electronic circuitry, taking advantage of the high wide-band linearity, low insertion loss and power consumption, small volume and low batch fabrication cost peculiar of micromachined devices [2]. In spite of the attractive capabilities, RF MEMS switches reliability is still an open issue, especially for switches fabricated in III–V technology, being this fabrication process less consolidated with respect to the silicon-based one [3]. To date, the major reliability problem encountered in capacitive switches is the charging process of dielectric layers over or around the switch actuator that can cause the electrical parameters shifting, or, when fatal, the device stiction [4–8]. In silicon technology, low temperature silicon dioxide (SiO2) and silicon nitride (Si3N4) deposited by plasma-enhanced chemical vapor deposition (PECVD) are the most common dielectric materials used in RF MEMS switches fabrication. Si3N4 is the standard dielectric material in III–V technology, too. In general, the low temperature SiO2 is less susceptible to surface charging and shows ⇑ Corresponding author. Tel.: +39 0832 422538; fax: +39 0832 422552. E-mail address: [email protected] (A. Persano). 0026-2714/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.microrel.2012.06.008
better breakdown stability compared to PECVD deposited Si3N4 [9]. However, the dielectric constant of SiO2 (typ. 2–4) is much lower than that of Si3N4 (typ. 6 and 7), resulting in a small capacitance ratio and, consequently, in insufficient performance at low GHz frequencies. Hence, in order to both optimize the switch RF performance and guarantee good reliability, proper switch design, fabrication process, and reliability assessment become mandatory. In this work, we developed shunt capacitive RF MEMS switches that are fully compatible with standard GaAs-based MMIC fabrication. Switches with different geometric parameters of the moveable bridge were designed and characterized, obtaining a good isolation (>30 dB) in the K-band. Finally, the reliability of the fabricated switches is addressed, finding that dielectric charging drawbacks can be significantly reduced by applying suitable actuation waveforms consisting of consecutive positive and negative short pulses. 2. Design and fabrication of switches Air-bridge shunt capacitive MEMS switches were designed on GaAs substrate. The switches are in coplanar (CPW) waveguide configuration, with the suspended metal bridge consisting of a plate that is anchored at the two ground planes by two springs on each side. The coplanar central conductor under the suspended bridge is covered with a dielectric layer in order to shunt to ground the RF signal when the bridge is in down state (see Fig. 1a). The switch is actuated by biasing the coplanar central conductor. DC blocks (series capacitors) at input and output ports of about 4.8 pF each were integrated in order to keep decoupled the DC and
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A. Persano et al. / Microelectronics Reliability 52 (2012) 2245–2249
Fig. 1. (a) Layout and (b) equivalent circuit of the designed shunt capacitive RF MEMS switches. The values of CMIM, Z0, and ZL are 4.8 pF, 50 X, and 73 X, respectively.
RF signals, providing a good RF signal propagation above 7 GHz. The loss contribution of the DC blocks was estimated to be around 0.5 dB and was not taken into account in the full-wave simulations to reduce the computational time. Fig. 1b shows the equivalent circuit of the designed switches. The shunt bridge is represented by the series of a bridge resistance (Rs), inductance (Ls) and variable capacitance (Con or Coff). The series capacitances CMIM model the two DC blocks, whereas ZL represents the inductive sections added to compensate for the mismatch induced by the off-state capacitance. In order to study the influence of layout geometries (central plate and springs size) on the RF performance, four different switches were designed and fabricated. The main geometric and equivalent circuit parameters of the designed switches are summarized in Table 1. Switches 1 and 2 present identical plate width (D = 100 lm) and spring length (l = 65 lm), whereas they differ in the plate length (L). The calculated values of Coff (Coff-calc) and Con (Con-calc) for the switch 1 and 2 are 75 fF and 3.7 pF, respectively. The bridge width (D = 150 lm) is kept constant also in switches 3 and 4 with a plate length L = 290 lm, whereas the spring length (l) changes in these two switches. The values of Coff-calc and Con-calc for the switches 3 and 4 are of 115 fF and 6.3 pF, respectively, being these values greater than those predicted for switches 1 and 2 due to the greater plate width and, hence, to the larger capacitive area along the RF line (see Fig. 1a). Coff-calc and Con-calc correspond to the parallel plate capacitance between the bridge and the CPW central conductor, i.e. the ideal capacitance calculated without taking into account for surface roughness. The variation in the bridge geometric parameters also causes a change in the resistive (Rs) and inductive (Ls) contributions [10]. The designed switches were fabricated on GaAs substrate with a III–V fabrication process. To this scope, a surface-micromachined approach was followed, using an eight-mask fabrication process.
Table 1 Geometric and equivalent circuit parameters of the designed shunt capacitive switches.
D (lm) L (lm) l (lm) Rs (X) Ls (pH) Coff-calc (fF) Con-calc (pF) Coff-meas (fF) Con-meas (pF) Con-meas/Coff-meas
Switch 1
Switch 2
Switch 3
Switch 4
100 390 65 0.23 22
100 520 65 0.29 27
150 290 94 0.15 14
150 290 145 0.21 19
75 3.7 90 1.1 12.2
The main technological steps for the switch fabrication are shown in Fig. 2. A 500 mn thick Si3N4 layer was deposited by PECVD over all the substrate and it is the same used for the passivation of III–V
(a)
(b)
(c)
(d)
(e)
115 6.3 135 1.3 14.4
2.1 15.5
2.2 16.3
Fig. 2. Process flow with the main technological steps for the central RF line.
A. Persano et al. / Microelectronics Reliability 52 (2012) 2245–2249
devices, thus allowing to not increasing the number of fabrication steps required for the integration of RF MEMS with MMICs. The actuation line was composed by a sputtered 25 nm thick NiCr (50/50) alloy layer and above it the underpass line was fabricated by depositing a metal multilayer of Ti/Pt/Au (30/30/60 nm). Both these layers were defined by lift-off and in between them a 300 nm thick Si3N4 layer was deposited by PECVD (see Fig. 2a). After the underpass line, another 300 nm thick Si3N4 layer. Fig. 2b shows the two Si3N4 depositions along the central RF line. Several works have appeared in the literature, concerning the dependence of dielectric charging in PECVD silicon nitride films on the deposition parameters and, hence, on the material stoichiometry. Indeed, this latter affects the concentration of bulk and interface traps as well as the concentration of dipoles and charge accumulation [11,12]. Following the results presented in [12], a high deposition frequency is preferable for MEMS switches fabrication thanks to the smaller injected charge density and a faster discharging process. In this work, we chose a frequency of 13.56 MHz for the PECVD silicon nitride deposition. The other deposition conditions of the PECVD Si3N4 layers are: power supply of 20 W, substrate temperature of 250 °C, chamber pressure of 350 mtorr, and flow ratio in the SiH4:NH3:N2 gas mixture of 1:4:10. The sacrificial layer for the definition of the air gap under the bridges was made by a 3 lm thick photoresist (Fig. 2c). In order to stabilize this layer and to obtain well-rounded edges of bridges, which are necessary to promote the flatness of the membrane anchors and borders, a hard bake at 200 °C was performed. A metal multilayer of Ti/Au/Ti (5/50/5 nm) was evaporated on the entire surface, to be used as electrical contact film and seed layer for the following electroplating process. The suspended bridge, the anchor points, the CPW lines and the ground pads were 1 lm thick by gold-electroplating deposition (Fig. 2d). The membrane contains a set of holes (diameter of 10 lm, and a distance from each other of 9 lm), with the dual aim of easing the full removal of the sacrificial layer by O2 plasma dry-etching and of fastening the switch actuation time by reducing the air damping. A second 1 lm thick gold-electroplating deposition was performed to thicken the CPW lines and the ground pads. A combination of selective wet and dry etching was used to remove the excess Ti/Au/Ti multilayer among the devices. Finally, the air bridges were released by removing the underneath sacrificial photoresist by a high pressure O2 plasma process performed in a barrel etcher in order to prevent sticking problems (Fig. 2e). The scanning electron microscopy (SEM) image of a fabricated switch #4 is shown in Fig. 3a. A Polytec MSA-500 optical profilometer was used to acquire topographies of the switches and to characterize their dynamic behavior, using the integrated Laser Doppler Vibrometer (LDV). The measurement of the membrane mechanical resonance frequency is shown in Fig. 3b, monitoring the velocity of the membrane central point with the LDV applying a chirp signal to
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the actuator. A resonant frequency of about 30 kHz was extracted, consistently with other results recently obtained for switches of similar size [13]. 3. RF performance and reliability The RF performance of the fabricated switches were characterized under ambient environmental conditions, using a remotely controlled Agilent E8361A Vector Network Analyzer (VNA), deembedded through short-open-load thru (SOLT) calibration up to 30 GHz, and a Keithley 2612 Source Meter together with a Karl Süss Probe Station equipped with ground-signal-ground RF |Z|probes and DC probes. For each typology, five switches were measured, finding actuation voltages in the range 25-40 V. Fig. 4 shows the typical S21 parameter behavior measured for the different switches in the up and down states. Table 1 summarizes the values of the Con-meas and Coff-meas capacitances and of the ratio between them (Con-meas/Coff-meas), which are obtained by fitting the measured S21 curves. Below 7 GHz, the switch performance is limited by the presence of the DC blocks. All switches showed an insertion loss better than 0.8 dB in almost all the range of measured frequencies. Specifically, a value of Coff-meas larger than the calculated one is extrapolated from the measured values of S21 for all switches. This result is likely due to the fact that ideal DC blocks were considered in the model, and to a down bending of the bridge in the central part, as also confirmed by topography measurements (not reported here). An air gap of 2.5 lm, rather than 3 lm, was then extrapolated by the S21 values measured in the up state. The switch isolation was better than 20 dB in the K-band, with resonance frequencies increasing from 25 GHz, according to the variation of the switch geometric parameters [10], and the peaks of maximum isolation were better than 50 dB. For all switches, a degradation of Con was estimated from measurements with respect to the calculation, indicating a not complete adhesion of the actuated bridge on the CPW central conductor, which can be due to the surface roughness as well as to the bridge form factor [9]. It is worth noting that the more flexible bridges allow for a higher Con/Coff ratios due to a better adhesion of the bridge on the bottom electrode. Specifically, Con-meas varies as the plate or spring length changes, due to the variation of the bridge form factor. Effective Con-meas/Coff-meas ratio values as good as in the range 12.2–16.3 for all switch designs were obtained. Switch reliability was assessed under the application of a variety of unipolar and bipolar polarization waveforms. An arbitrary waveform generator Agilent 33250A and a highfrequency voltage amplifier were used to build the actuation scheme varying the length of the pulses, number of cycles, and the voltage pulses polarity. At least three devices, with identical design, and taken from different wafer area, were tested. The reliability tests shown here were performed on the switch typology
(b)
Fig. 3. (a) SEM image of a fabricated shunt capacitive RF MEMS switch (typology #4). The red mark shows the point at which the Laser Doppler Vibrometer (LDV) measurement was performed. (b) LDV vibration velocity measurement of the suspended membrane central part (10 V periodic chirp signal applied).
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(a)
(b) Fig. 4. S21 parameter measured behavior of the fabricated shunt capacitive switches in the up and down states.
(a) (c)
(b) Fig. 7. S21 parameter evolution under the application of bipolar waveforms consisting of consecutive positive and negative voltage pulses with different durations. The durations of the positive and negative voltage pulses are 480 ms and 20 ms, respectively, in (a); 100 ms and 20 ms, respectively, in (b); and both 100 ms in (c). The different applied polarization waveforms are reported in dashed line in (a), (b), and (c) as guide for eye. A continuous wave signal at 20 GHz, 0 dBm was applied during the test.
Fig. 5. (a) S21 parameter evolution during (actuation) and after (release) the application of a 45 ms long positive voltage pulse. The sketch of the applied polarization waveform is also reported in dashed line as guide for eye. (b) Dependence of the release time (trel) on the duration of the unipolar voltage pulse (tON).
Fig. 8. S21 parameter evolution under the application of a train (1 kHz) of consecutive 20 ls long positive and negative voltage pulses. A continuous wave signal at 20 GHz, 0 dBm was applied during the test. For sake of clearness the applied waveform is reported in blue line. Fig. 6. S21 parameter evolution under the application of a train (1 kHz, 0 dBm) of unipolar 50 ls long voltage pulses. After 6 pulses the switch remained stuck in the down state. A continuous wave signal at 20 GHz, 0 dBm was applied during the test (hot switching).
#4, but similar effects were observed for the other switch typologies. A pulse amplitude of 45 V, which is slightly higher than the measured actuation voltage (40 V), was used for switches #4, in
order to achieve the best trade off between the complete switch actuation and reliability, which is known to reduce with the increasing of the bias voltages [4]. Fig. 5a shows the switch operation under the application of unipolar voltage pulses with a duration of 45 ms. It is observable that the release occurs in 15 s and through intermediate states, which
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is symptomatic of a partial movement of the membrane. The dependence of the release time on the duration of the unipolar voltage pulse is reported in Fig. 5b. This time is found to be in the order of 100 ls for unipolar pulses shorter than 10 ms, whereas for longer pulse durations the release time significantly increases, reaching an asymptotic value of 15 s for pulses longer than 30 ms. Moreover, it is worth noting that by applying unipolar voltage pulses, even if with a short duration (<100 ls), the switch undergoes failure after less than 10 actuations (see Fig. 6). The failure accelerates as the time in the down state becomes longer, confirming that the actuation time is critical in the degradation rate of the device. Due to the design of the tested switches (see Fig. 3a), this erratic release can be attributed to the charging of the dielectric layer (approximately 150 lm 200 lm of Si3N4) covering the actuation electrode under the moveable bridge. In fact, it is well known that PECVD Si3N4 layers contain a large density of defects that may act as positive or negative charge traps as well as contribute to the formation of dipoles, leading all these factors to the charging of the dielectric layer [6,7]. The observed failure was found to be no permanent, in agreement with the reversible nature of the charging processes, but it clearly makes the devices not usable in a real application. In order to overcome the dielectric charging issues, a negative voltage pulse was applied right after the positive pulse. Thanks to the rapid change (10 ns) of the sign of the actuation signal and the mechanical inertia of the bridge, the switch was observed to remain constantly in the down state. Fig. 7a shows the switch response to bipolar waveforms with consecutive positive and negative pulses long 480 ms and 20 ms, respectively. In the first cycle, the erratic switch operation was observed to reduce, being the release faster with respect to the case of the application of only positive pulses (141 ms rather than 15 s). However, even in this case, the release is observed to be non-instantaneous and occurring through intermediate states, likely due to the fact that the duration of the negative pulse is not enough to allow the complete discharge of the charge trapped during the positive pulse. With the increasing of cycles, the release time becomes longer until, after three cycles, no complete release occurred. Fig. 7b shows the switch response to bipolar waveforms with consecutive positive and negative pulses long 100 ms and 20 ms, respectively. Under this polarization conditions, the duration of the negative pulse is observed to be long enough to allow the complete release of the charge trapped during the positive pulse, leading to a correct switch release (in 100 ls). On the other hand, with 100 ms long positive and negative pulses, the switch is observed to not completely release because of the charge trapped during the negative pulses (see Fig. 7c). Fig. 8 shows the S21 parameter evolution under the application of a train of successive positive and negative voltage pulses (duration of each pulse = 20 ls, duty cycle of 50%) during the nominal down state. The switch correctly operated with no variation in the scattering parameters after 1 million of cycles under the application of 1 kHz signal, modulated with bipolar pulses (20 ls long, duty cycle of 50%) during the ON-state. The switch remained in the down state during the entire polarization pulse and it quickly released (in 100 ls) when the zero voltage was applied. Similar
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results were observed in all the tested switches (3 samples per typology, for a total of 12 samples). 4. Conclusions Shunt capacitive RF MEMS switches operating in K-band were developed with a GaAs III–V fabrication process that is fully compatible with standard MMIC production. Insertion losses better than 0.8 dB, isolations greater than 30 dB with resonance frequencies in K-band, and Con/Coff ratios between 12 and 16 were measured for switches with different geometric parameters. A correct switch operation as well as an improvement of the lifetime (from few cycles to more than 1 million of cycles) were achieved by applying fast bipolar voltage pulses, significantly reducing dielectric charging related problems. Acknowledgement This work was partially supported by Apulia Region (Italy) under the Regional Project ‘‘SENS&MICRO LAB’’. References [1] Malmqvist R, Samuelsson C, Simon W, Rantakari P, Smith D, Lahdes M, et al. Design, packaging and reliability aspects of RF MEMS circuits fabricated using a GaAs MMIC foundry process technology.In: Proc. 40th Europ. micro. conf, Paris; 2010. p. 85–8. [2] Pacheco S, Nguyen CT.-C, and Katehi LPB. Micromechanical electrostatic Kband switches. In: Proc. IEEE MTT-S International microwave symposium, Baltimore, Maryland; 1998. p. 1569–72. [3] Persano A, Tazzoli A, Cola A, Siciliano P, Meneghesso G, Quaranta F. Reliability enhancement by suitable actuation waveforms for capacitive RF MEMS switches in III–V technology. J Microelectromech Syst 2012;21:414–9. [4] Goldsmith C, Ehmke J, Malczewski A, Pillars B, Eshelman S, Yao Z, et al. Lifetime characterization of capacitive RF MEMS switches. In: Proc. IEEE MTT-S Int. microw. symp. dig, Phoenix, AZ; 2001. p. 227–30. [5] Van Spengen WM, Puers R, Mertens R, De Wolf I. A comprehensive model to predict the charging and reliability of capacitive RF MEMS switches. J Micromech Microeng 2004;14:514–21. [6] Yuan X, Cherepko S, Hwang J, Goldsmith CL, Nordquist C, and Dyck C. Initial observation and analysis of dielectric-charging effects on RF MEMS capacitive switches. In: IEEE MTT-S Int. microwave symp. dig, Forth Worth, TX; 2004. p. 1943–6. [7] Papaioannou G, Exarchos M-N, Theonas V, Wang G, Papapolymerou J. Temperature study of the dielectric polarization effects of capacitive RF MEMS switches. IEEE Trans Microwave Theory Technol 2005;53:3467–73. [8] Massenz A, Barbato M, Giliberto V, Margesin B, Colpo S, Meneghesso G. Investigation methods and approaches for alleviating charge trapping phenomena in ohmic RF-MEMS switches submitted to cycling test. Microelectron Reliab 2011;51:1887–91. [9] Peng Z, Palego C, Hwang JCM, Forehand DI, Goldsmith CL, Moody C, et al. Impact of humidity on dielectric charging in RF MEMS capacitive switches. IEEE Microw Wireless Compos Lett 2009;19:299–301. [10] Rebeiz G. RF MEMS theory, design, and technology. New York: WileyInterscience; 2003. [11] Zaghloul U, Papaioannou GJ, Bhushan B, Wang H, Coccetti F, Pons P, et al. Effect of deposition gas ratio, RF power, and substrate temperature on the charging/ discharging processes in PECVD silicon nitride films for electrostatic NEMS/ MEMS reliability using atomic force microscopy. J Microelectromech Syst 2011;20:1395–418. [12] Zaghloul U, Papaioannou GJ, Wang H, Bhushan B, Coccetti F, Pons P, et al. Nanoscale characterization of the dielectric charging phenomenon in PECVD silicon nitride thin films with various interfacial structures based on Kelvin probe force microscopy. Nanotechnology 2011;22. 205708-205708. [13] Tazzoli A, Barbato M, Mattiuzzo F, Ritrovato V, Meneghesso G. Study of the actuation speed, bounces occurrences, and contact reliability of ohmic RF MEMS switches. Microelectron Reliab 2010;50:1604–8.