On the electrostatic actuation of capacitive RF MEMS switches on GaAs substrate

Sensors and Actuators A 232 (2015) 202–207

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Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna

On the electrostatic actuation of capacitive RF MEMS switches on GaAs substrate Anna Persano ∗ , Fabio Quaranta, Maria Concetta Martucci, Pietro Siciliano, Adriano Cola IMM-CNR, Institute for Microelectronics and Microsystems-Unit of Lecce, National Council of Research, Via Monteroni, I-73100 Lecce, Italy

a r t i c l e

i n f o

Article history: Received 4 November 2014 Received in revised form 20 March 2015 Accepted 9 May 2015 Available online 9 June 2015 Keywords: Capacitive RF MEMS switches Double-clamped bridge Electrostatic actuation GaAs technology

a b s t r a c t The electrostatic actuation behaviour of the gold bridge in capacitive radio frequency microelectromechanical system switches, fabricated on GaAs substrate, is investigated. An unconventional imaging technique, based on the out-of-focus reflection, was used to evaluate the topographic profile of the suspended bridge and its lowering as a function of the voltage. Important parameters for the switch actuation, such as the pull-down voltage and the air gap between the bridge and the actuator, are estimated. Capacitance-voltage curves allow to evaluate the capacitance associated to the bridge in the up and down states as well as the dielectric constant of the Si3 N4 layer, which covers the actuator. The experimental values of the pull-down voltage and the dielectric constant are used to extract from the theoretical equations the residual stress of the fabricated gold membrane. Finally, the current through the dielectric Si3 N4 layer was measured as a function of the voltage applied to the actuator, finding that the Poole–Frenkel effect is the dominant conduction mechanism when the switch is actuated. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Microelectromechanical systems (MEMS) are the results of the integration of mechanical and electronic elements on a common substrate. A feature rather common in MEMS is the presence of suspended membranes of different geometry (beams, cantilevers, bridges, etc.), which allows to obtain a unique and very complex functionality [1]. An interesting application of MEMS is the switching of radio frequency (RF) signals for the development of complex circuits, such as phase shifters, varactors, tuneable oscillators, filters, and antennas, taking advantage of the high wide-band linearity, low insertion loss, small volume and low batch fabrication cost of MEMS devices [2]. Among the various actuation methods of RF MEMS switches, the electrostatic one is the most commonly used, thanks to the several benefits it offers, such as the compatibility with the integrated circuits fabrication process and the low power consumption [1,2]. In the electrostatic MEMS, a voltage is applied to the actuator pad situated under the suspended membrane and the movement of this last is driven by the competition between the mechanical force, which tends to restore the membrane to its initial position, and the electrostatic force, which pulls down the membrane on the actuator.

∗ Corresponding author. E-mail address: [email protected] (A. Persano). http://dx.doi.org/10.1016/j.sna.2015.05.008 0924-4247/© 2015 Elsevier B.V. All rights reserved.

When the voltage is lower than a specific value (pull-down voltage), the electrostatic force is smaller than the mechanical force and the membrane remains in the up state (no actuation), whereas, when the voltage is greater than the pull-down voltage, the electrostatic force exceeds the mechanical restoring force and the membrane collapses to the down state (actuation). There are different typologies of electrostatic RF MEMS switches that can differ for circuit configuration (series or shunt) or contact interface (ohmic or capacitive). With respect to the ohmic devices, shunt capacitive RF MEMS switches are characterized by an enhanced reliability and exhibit a higher isolation at the resonance frequency that can be tuned by the manipulation of the geometric parameters of the moveable bridge [1,3]. A great attention is devoted to the fabrication of RF MEMS switches due to the tight correlation of the mechanical/electrical properties of the materials with the device performance and reliability. Silicon-based technology is presently the most consolidated fabrication process of RF MEMS switches. Recently, GaAs-based technology has also attracted a growing interest in view of the monolithical integration of RF MEMS switches in microwave circuits [3–5]. Among the possible metals, which can be used for the fabrication of the membranes in MEMS switches, gold is widespread in both GaAs- and silicon-based technologies. Gold offers the advantages to be highly conductive and chemically inert, thus allowing to reduce the losses and to prevent the contamination and corrosion

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problems. Unfortunately, gold is not an excellent material from the mechanical point of view, and its yield, creep and thermal relaxation properties are often source of high stresses and important non homogeneities in the stress distribution [6]. The prediction of the actual stress in the gold membranes is desirable to improve the MEMS switches performance, but it is not trivial due to the strong dependence of stress on the fabrication process. An important role in the device operation is played by the dielectric layers, especially in capacitive RF MEMS switches, where a dielectric layer is deposited on the actuator. In GaAs technology, Si3 N4 is generally the material used for dielectric layers, whereas SiO2 , is the dielectric material most used in silicon-based technology. Leakage currents and charging phenomena in the dielectric layer still limit the reliability of capacitive RF MEMS switches and are thus subject of a wide literature [3,7–11]. However, most of the reported studies concern silicon-based switches [8–10], whereas there are few reports on Si3 N4 dielectric layers of GaAs-based RF MEMS switches [3,11]. In order to investigate the topography and the mechanical properties of the membrane as well as its electrostatic actuation, different characterization techniques have been used. Mechanical stylus surface profilers can be found in most clean rooms and are thus commonly used for topography measurements in MEMS with the valuation of the stress and Young modulus of the suspended membrane [12,13]. However, some degree of operator skill and care are required to avoid the damage of suspended membrane during the measurements with the stylus profilometer. The optical profilometer, such as the laser Doppler vibrometer, offers the advantage to be a non-contact measurement system, but it generally consists of a sophisticated and expensive experimental setup, mainly based on the light interferometry [14]. Also holographic microscopy has been used to analyse the topography and the actuation of MEMS structures [15]. However, this technique does not provide direct measurements and quite complex numerical models have to be used to reconstruct the phase maps from the recorded holograms. Hence, a relatively simple, inexpensive, and valid characterization technique is still highly desirable. In this work, an unconventional mapping technique was used for the optical investigation of RF MEMS capacitive switches fabricated on GaAs substrate. The topography and displacement of the bridge are valuated as a function of the applied voltage with a vertical sub-micrometer resolution, allowing to evaluate the two most important parameters for the switch actuation, which are the air gap and the pull-down voltage. From the combination of the experimental results and the theoretical equations, the residual stress of the fabricated gold membrane is estimated. Capacitance-voltage curves were measured to extract the capacitance of the switch in the up and down states as well as the dielectric constant of the Si3 N4 layer covering the actuator. Finally, the current through the Si3 N4 layer was measured for the bridge in the up and down states, inferring the conduction mechanisms.

2. Experimental The RF MEMS switches were fabricated on GaAs substrate with an eight-mask surface micromachining process. The actuation line was composed by a sputtered 25 nm-thick NiCr alloy layer and above it the underpass line was fabricated by depositing a metal multilayer of Ti/Pt/Au (30/30/60 nm). A 300 nm-thick Si3 N4 layer was deposited by plasma-enhanced chemical vapour deposition (PECVD) to form the central capacitor along the line. The PECVD deposition parameters were: frequency of 13.56 MHz, 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

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the membrane was made by a 3 ␮m-thick photoresist. In order to stabilize this layer and to obtain well-rounded edges of membranes, 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. A lithographic process was performed to define the membrane area and then a hard bake at 140 ◦ C was performed. The suspended bridge, the anchor points, the CPW lines and the ground pads were 0.97 ␮m-thick and were made by gold-electroplating deposition, using a neutral-based commercial cyanide solution with a plating current density of 0.75 mA/cm2 . The membrane contains a set of holes (diameter of 10 ␮m, and a distance from each other of 9 ␮m), with the dual aim of easing the full removal of the sacrificial layer and of fastening the switch actuation time by reducing the air damping. Another lithographic process was performed for a second 1 ␮m-thick gold-electroplating deposition 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 membranes were released by removing the underneath sacrificial photoresist by a high pressure O2 plasma process performed in a barrel etcher at 75 ◦ C for two hours. More details on the fabrication process of RF MEMS switches are given in Ref. [3]. For mapping measurements, the sensor of nanometer distance “LXS-range” from LMI Sensors 95 was used, which provided the imaging of the suspended membranes under the application of the voltage. In this distance sensor, a laser emitting at the wavelength of 780 nm and at the frequency of 30 kHz was used as light source. The laser was focused (spot size of ∼1 ␮m) at a distance of ∼4 ␮m with an optical density of 260 ␮W/␮m2 . Around the focusing lens, the system is equipped with photodetectors for the detection of the reflected light. These photodetectors provide an out-of-focus signal and the total intensity of the reflected light. These two distinct signals were recorded by a 1 M sample/s data acquisition card. In particular, the out-of-focus signal (in volts) can be converted to a distance from the focus plane, according to the calibration curve that is included in the instrument specifications. The focus plane corresponds to the 0-value of the out-of-focus signal, whereas negative (positive) values of the out-of-focus signal label points which are below (on) the focus plane. The maximum excursion is about 10 ␮m with a resolution of few nanometers, actually this last is much higher being limited by the mechanical vibrations. In order to scan the area of interest, the laser was mounted on a x–y motorized stage with a movement resolution of 100 nm. The vertical displacement, controlled by a piezoelectric stage with a movement resolution of 10 nm, was managed before the measurement in order to measure a convenient value within the working range of the photodetectors when the laser scans the area of interest. A sketch of the optical scanning setup used for the mapping measurements is reported in Fig. 1. For the I–V curves of the dielectric Si3 N4 layer, the membrane was grounded and the 4140B pA-meter/DC voltage source was used to apply the bias voltage to the actuator and to measure the current between the actuator and the membrane. In the capacitance measurements, a HP4284A precision LCR-meter was used in the series circuit mode Cs-Rs, applying a voltage from 0 V to 40 V to the actuator with an oscillating voltage level of 30 mV at the frequency of 1 MHz.

3. Results and discussion The reflection map recorded while the laser scans all the switch area with no applied voltage is reported in Fig. 2(a). All the device surface is covered by the electrodeposited gold layer, appearing thus, as bright region in the reflection map. No reflection arises

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Fig. 1. Sketch of the optical scanning setup used for the mapping measurements.

from the regions surrounding the device, which consist of the GaAs substrate covered by the Si3 N4 layer. The suspended membrane is characterized by the highest reflected light intensity due to the up position and, hence, to the lower distance from the laser photodetectors with respect to the other device regions. The membrane has the geometry of a fixed–fixed bridge with a perforated central plate and two anchors on each side, which connect the bridge to the ground pads. The bridge is suspended on the actuator covered by the dielectric layer and situated in the central part of the metal line aimed to the transmission of the RF signal. Fig. 2(b) shows the distance map extrapolated from the out-of-focus signal, which was recorded when the laser scans a bridge and the surrounding area with no applied voltage. In order to measure the height of the bridge, the vertical displacement was managed to provide a 0 outof-focus signal in the regions with no metal depositions, which are around the bridge. The bridge is observed to have a height of ∼4 ␮m, in agreement with the up position expected for V = 0 V. In particular, regions of colour slightly different can be noted inside the bridge, pointing out that the bridge is not perfectly flat. Out-of-focus signal maps with a scan step of 1 ␮m were recorded in the central region (30 × 165 ␮m2 ) of the bridge. The distance maps extrapolated from these scans under the application to the actuator of V = 0 V and V = 35 V are shown in Fig. 2(c) and (d), respectively. The height of the bridge reduces from ∼4 ␮m to ∼1 ␮m, with increasing the applied voltage from 0 V to 35 V, due to the electrostatic actuation. Fig. 1(e) shows the height profiles, for V = 0 V and V = 35 V, extracted from the distance maps along a line parallel to the anchors and passing through the holes, as shown in Fig. 2(c) and (d). For V = 0 V, the central part of the bridge is obtained to have a height of ∼3.5 ␮m with respect to the underwent actuator. This height is consistent with the bridge gold thickness of 0.97 ␮m and the air gap of ∼2.5 ␮m, which was extrapolated by the S-parameters modelling of these MEMS switches [3]. The lateral regions of the bridge show an outof-plane of ∼0.5 ␮m with respect to the centre, probably due to the residual stress of the gold [6]. For V = 35 V, the height of the entire bridge is obtained to be around 1 ␮m, which is close to the thickness of the gold electrodeposition, pointing out that the bridge was collapsed on the actuator. By recording the out-of-focus signal maps for different voltages applied to the actuator underwent the bridge show in Fig. 2(b), the dependence of the air gap(g) under the bridge on the voltage applied to the actuator was extrapolated (Fig. 3). With no applied voltage, the air gap is 2.5 ␮m (g0 ), slowly reducing with increasing the voltage until the sharp bridge actuation for V = 30 V. Specifically,

we can note that the last value (for V = 27 V) of the air gap before the actuation is equal to 1.8 ␮m, which is near to 2/3 g0 (=1.7 ␮m). This result is consistent with the fact that for g = 2/3 g0 , the increase of the electrostatic force is greater than the increase of the mechanical restoring force, resulting in the collapse of the bridge to the down-state position [1]. For a fixed–fixed beam in the capacitive configuration, the pull-down voltage is given by [1,16]:

 Vp =

8k d ] [g0 + 270 A r

(1)

where k is the total spring constant, 0 is the vacuum dielectric constant, A is the area of overlap between the bridge and the actuator, d and r is the thickness and the relative dielectric constant of the dielectric layer, respectively. In order to estimate the relative dielectric constant of the Si3 N4 layer, the capacitance-voltage curve was performed for the bridge shown in Fig. 2(b). Fig. 4 shows the variation of the capacitance signal as a function of the voltage applied to the actuator. An evident increase of the capacitance is observable for V = 30 V, due to the switch actuation. In the up state, a capacitance value (Cup ) of ∼0.4 pF is measured, whereas the capacitance increases to ∼1.8 pF in the down state (Cdown ). The capacitance value was also measured for fixed bridges, which were deposited directly on the dielectric layer covering the actuator without the deposition and the consequent removal of the sacrificial layer. For fixed bridges, the capacitance value of 4.4 pF was measured. In this last case, the material system, bridge-dielectric-actuator can be modelled as a metal-insulator-metal (MIM) structure with a constant capacitance given by: CMIM = 0 r A/d. By replacing in this equation CMIM = 4.4 pF, d = 300 nm, and A = 24.819 ␮m2 [17], the relative dielectric constant of the Si3 N4 layer, r, is found to be 6, in agreement with the value reported for Si3 N4 films deposited by PECVD [7]. In principle, the capacitance value CMIM for the bridges deposited directly on the dielectric layer is coincident with the value of Cdown for the moveable bridges in the down state. Actually, Cdown is lower than CMIM and provides an effective dielectric constant (re ) value of 2.45, according to the equation Cdown = 0 re A/d. The observed reduction of Cdown with respect to CMIM is consistent with the presence of a residual air gap (gres ) between the bridge in the down state and the dielectric layer, likely due to the roughness of the contact surface, which avoid the bridge to perfectly conform on the dielectric layer.  A According to this interpretation, Cdown is given by: Cdown = res0 d g

+

r

and gres is found to be 72 nm. This value is close to that reported

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205

Air gap (µm)

3 2 1 0

0

10

20

30

40

50

60

Voltage (V) Fig. 3. Dependence of the air gap on the voltage applied to the actuator, which was extrapolated from the out-of-focus signal in the centre of the bridge shown in Fig. 2(b).

5

Capacitance (pF)

4

fixed bridge

3 bridge down

2 1 0

bridge up

0

5

10

15

20

25

30

35

40

Voltage (V) Fig. 4. Typical capacitance-voltage curve for the bridge shown in Fig. 2(b) and for a fixed bridge.

in the up and down states and the capacitance values expected from the air gaps, measured by the mapping technique in the central point of the bridge, leads to a considerable degrade of the capacitance ratio. This is not surprising due to several factors, which are the fringing field capacitance, the parasitic capacitance to ground of the MEMS structure and of the input/output t-line and the nonplanar capacitance surface, caused by the a warp of the bridge in the length and width directions [1]. The total spring constant, k, is given by [1]: k = k + k

(2)

where k

is due to the stiffness of the bridge, which accounts for the elastic properties of the material and is proportional to the Young’s modulus of the structure, while k is determined by the biaxial residual stress within the beam and is generally a results of the fabrication process. In the case of a rectangular beam, clamped at both ends, as the bridges studied here, k is given by [1]: Fig. 2. (a) Map (1450 × 1847 ␮m2 ) of all the switch area, obtained recording the total reflected intensity with a scan step of 10 ␮m. The length and the width of the central moveable bridge is 1 mm and 100 ␮m, respectively. (b) Distance map (630 × 230 ␮m2 ) of a moveable bridge, extrapolated from the acquisition of the outof-focus signal with a scan step of 5 ␮m. The anchors are 145 ␮m-long and 20 ␮mwide, whereas the length and the width of the central plate is 290 ␮m and 150 ␮m, respectively. (c) and (d) Zoom of the region (30 × 165 ␮m2 ) indicated by the yellow square in (b), obtained recording the out-of-focus signal with a scan step of 1 ␮m under the application of V = 0 V and V = 35 V, respectively. (e) Height profiles for V = 0 V (red squares) and 35 V (blue circles) along the line drawn in red blue and blue color in (c) and (d), respectively. All the spatial and colour scales are in ␮m, except for the colour scale in (a) that is in arbitrary units. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

in literature for RF MEMS switches with a gold bridge on a silicon nitride dielectric layer [18]. On the other hand, the capacitance measured for the bridge in the up state (0.4 pF) is greater than the capacitance value (0.1 pF) of a MIM structure with the insulating layer composed of the Si3 N4 film and the initial measured air gap of 2.5 ␮m. The discrepancy between the measured capacitance values

k = 4Ew

 t 3 l

,

(3)

where E is the Young’s modulus of the bridge gold (=78 GPa [1]), w and l are the width and the length of the anchors, and t is the bridge thickness. For the geometrical parameters of the bridges studied here [see Fig. 2(b)] and assuming a reduction of 25% of the Young modulus due to the holes [19], k is calculated to be 1.5 N/m. Due to the capacitive configuration of the investigated bridge, the electrostatic force is distributed over the portion of the beam on the central conductor (a) and the component of the spring constant due to the biaxial residual stress is given by [1]: t 1  , k = 8(1 − v)W ( ) L 3−2 x

(4)

L

where  is the residual stress,  the Poisson ratio of gold (=0.44), W and L are the width and the length of the entire bridge, and x = (L+a)/2. Using Eq. (1) with the values of Vp = 30 V and r = 6, the total spring constant k of 40.2 N/m is calculated. By replacing the values of k and k in Eq. (2), k is obtained to be 38.7 N/m that is

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reported for dielectric layers in actuated capacitive MEMS switches [10]. 4. Conclusions

Fig. 5. Typical current through the dielectric Si3 N4 layer as a function of the voltage applied to the actuator underwent the bridge shown in Fig. 2(b). The inset shows the plot ln(J/E) vs. E1/2 for V > 30 V.

a dominant value with respect to that of k (k /k ∼26), as it is expected [1]. By replacing this value of k in Eq. (3), a residual stress ␴ of 55.3 MPa is obtained. This stress value is mainly the result of the interplay of the initial stress ( in ) of the bridge gold and the possible thermal stress ( th ) induced by the temperature changes during the fabrication process. From measurements made on blank wafers, the initial stress of 1.8 ␮m-thick gold layer fabricated by electrodeposition has been found to be around 125 MPa tensile [6]. The thermal component of the stress can be estimated by the simple formula [20]: th =

E␣T (1 − v)2

(5)

where ˛ is the thermal expansion coefficient difference between gold and GaAs and T is the temperature difference. In the case studied here, being the thermal coefficients of GaAs and gold 6 × 10−6 K−1 and 14.2 × 10−6 K−1 [1], respectively, and T = 50 ◦ C due the barrel etching for the removal of the sacrificial layer, a compressive stress of 76.5 MPa is calculated by Eq. (5). Hence, from the expected initial tensile stress of ∼125 MPa and the calculated compressive thermal stress of 76.5 MPa, a residual tensile stress of ∼48.5 MPa is obtained. This value agrees with the residual stress of 55.3 MPa, obtained by replacing the experimental values of Vp and r in Eq. (1) and by using Eqs. (3)–(4). The small discrepancy between the two values, 48.5 MPa and 55.3 MPa, of residual stress is likely due to the fact that the compressive thermal stress can be reduced by other temperature dependent components of gold internal stress, like grain growth or reorganization, which normally produces tensile stresses [20]. The values obtained here for the residual stress are comparable to those reported for electrodeposited gold beams typically used for MEMS switches [12]. The current through the dielectric Si3 N4 layer was also measured with the bridge in the up and down states (Fig. 5). A sharp increase of the leakage current was observed for V = 30 V (E = 1 MV/cm), due to the switch actuation. Before actuation (E < 1 MV/cm), the current through the dielectric increases approximately linearly with the field and the conductivity is very low. For these low electric field values, the current in dielectric films is expected to be due to the hopping conduction mechanism [21]. After actuation, the electric field applied to the dielectric Si3 N4 layer becomes greater than 1 MV/cm and the increase of the current with voltage becomes no linear, which is indicative that a conduction mechanism different from hopping dominates. Specifically, we found that the plot ln(J/E) vs. E1/2 shows a linear dependence (inset of Fig. 5). This result points out that Poole–Frenkel effect dominates the conductivity when the membrane is actuated, as already

A low-cost and unconventional imaging technique was demonstrated to be powerful for the optical characterization of the suspended double-clamped bridge in RF MEMS capacitive switches. The devices were developed on GaAs substrate, using a technology which is compatible with MMIC fabrication. The topographic profile of the bridge and its displacement, under the application of voltages in the range 0–60 V, were evaluated with a sub-micrometer resolution. A pull-down voltage of 30 V and air gap values ≤2.5 ␮m under the moveable bridge were extracted as a function of the voltage applied to the actuator. The actuation behaviour was also observed in the capacitance-voltage curve, allowing the dielectric constant of 6 to be estimated for the Si3 N4 layer deposited on the actuator. By combining the experimental values with the theoretical equations, the residual stress of 55.3 MPa was obtained for the studied gold bridge. Finally, the I–V curve was performed to evaluate the current through the dielectric Si3 N4 layer with the bridge in the up and down states. When the membrane actuates, a current of around 200 pA was measured through the Si3 N4 layer and the Poole–Frenkel effect is found to be the dominant conduction mechanism. Acknowledgment This work was partially supported by MIUR under Project 02876 “TASMA” of the National Operative Program (PON). References [1] G.M. Rebeiz, RFMEMS, Theory, Design, and Technology, John Wiley & Sons, Inc., 2003, 2015. [2] G.M. Rebeiz, K. Entesari, I.C. Reines, S.-J. Park, M. El-Tanani, A. Grichener, A.R. Brown, Tuning in to RF MEMS, IEEE, Microw. Mag. 10 (2009) 55–72. [3] A. Persano, A. Tazzoli, P. Farinelli, G. Meneghesso, P. Siciliano, F. Quaranta, K-band capacitive MEMS switches on GaAs substrate: design, fabrication, and reliability, Microelectron. Reliab. 52 (2012) 2245–2249. [4] D. Hyman, J. Lam, B. Warneke, A. Schmitz, T.Y. Hsu, J. Brown, J. Schaffner, A. Walston, R.Y. Loo, M. Mehregany, J. Lee, Surface-micromachined RF MEMS Switches on GaAs Substrates, Int. J. RF Microw. Comput. Aided Eng. 9 (4) (1999) 348–361. [5] R. Malmqvist, C. Samuelsson, W. Simon, P. Rantakari, D. Smith, M. Lahdes, M. Lahti, T. Vähä-Heikkilä, J. Varis, R. Baggen, Design, packaging and reliability aspects of RF MEMS circuits fabricated using a GaAs MMIC foundry process technology, in: Proc. 40th Europ. Micro. Conf., Paris, September 28–30, 2010, pp. 85–88. [6] V. Mulloni, F. Giacomozzi, B. Margesin, Controlling stress and stress gradient during the release process in gold suspended micro-structures, Sens. Actuators A 162 (2010) 93–99. [7] T. Lisec, C. Huth, B. Wagner, Dielectric material impact on capacitive RF MEMS reliability, in: Proceedings of the 12th GAAs Symposium, Amsterdam, September 2004, 2015, pp. 471–474. [8] W.M. van Spengen, R. Puers, R. Mertens, I. De Wolf, A comprehensive model to predict the charging and reliability of capacitive RF MEMS switches, J. Micromech. Microeng. 14 (2004) 514–521. [9] C.L. Goldsmith, J. Ehmke, A. Malczewski, B. Pillans, S. Eshelman, Z. Yao, J. Brank, M. Eberly, Lifetime characterization of RF MEMS switches, IEEE MTT-S Int. Microwave Symp. Dig. 1 (May (20–24)) (2001) 227–230. [10] G. Papaioannou, F. Giacomozzi, E. Papandreou, B. Margesin, Floating electrode microelectromechanical system capacitive switches: a different actuation mechanism, Appl.Phys. Lett. 99 (2011) 73501. [11] A. Persano, A. Tazzoli, A. Cola, P. Siciliano, G. Meneghesso, F. Quaranta, Reliability enhancement by suitable actuation waveforms for capacitive RF MEMS switches in III–V technology, J. Microelectromech. Syst. 21 (2) (2012) 414–419. [12] V. Mulloni, S. Colpo, A. Faes, B. Margesin, A simple analytical method for residual stress measurement on suspended MEM structures using surface profilometry, J. Micromech. Microeng. 23 (2013) 025025. [13] M.W. Denhoff, A measurement of Young’s modulus and residual stress in MEMS bridges using a surface profiler, J. Micromech. Microeng. 13 (2003) 686–692.

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