**1. Introduction**

232 Microelectromechanical Systems and Devices

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Lucibello, Emanuela Proietti, Benno Margesin, Flavio Giacomozzi, François Deborgies: *"Dielectric Charging in Microwave Micro-electro-mechanical Ohmic Series and Capacitive Shunt Switches"*; IOP Journal of Applied Physics, Vol. 105, No. 11,

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Charge accumulation in dielectrics solicited by an applied voltage, and the associated temperature and time dependencies are well known in scientific literature since a number of years [1]. The potential utilization of materials being part of a device useful for space applications is a serious issue because of the harsh environmental conditions and the necessity of long term predictions about aging, out-gassing, charging and other characteristic responses [2], [3]. Micro-mechanical Systems (MEMS) for RF applications have been considered for sensor applications as well as for high frequency signal processing during more than one decade [4], [5], [6], [7], [8], [9]. In this framework, RF MEMS switches are micro-mechanical devices utilizing, preferably, a DC bias voltage for controlling the collapse of metalized beams [8]. Magnetic [10], thermal [11] and piezoelectric [12] actuations have been also evaluated, but the electrostatic one seems to be until now preferred for no current flowing, i.e. a virtual zero power consumption, less complicated manufacturing processes and more promising reliable devices [13]. During the last few years, several research activities started to release the feasibility of RF MEMS switches also for Space Applications [14], [15], [16]. The electrostatic actuation of clamped-clamped bridges or cantilevers determines the ON and OFF states depending on the chosen configuration. As well established, RF MEMS switches are widely investigated for providing low insertion loss [8], no or negligible distortion [17], [18] and somehow power handling capabilities [19], [20], [21], [22] for a huge number of structures already utilizing PIN diodes for high frequency signal processing. Actually, redundancy switches as well as single pole multiple throw (SPMT) configurations, [23], [24], matrices [25] true time delay lines (TTDL) [26], [27] and phase shifters [28], [29] for beam forming networks in antenna systems could benefit from their characteristics. On the

<sup>\*</sup> Andrea Lucibello1, Giorgio De Angelis1, Emanuela Proietti1, George Papaioannou2,

Giancarlo Bartolucci1,3, Flavio Giacomozzi4 and Benno Margesin4

*<sup>1</sup>CNR-IMM Roma, Roma, Italy* 

*<sup>2</sup>University of Athens, Athens, Greece* 

*<sup>3</sup>University of Roma "Tor Vergata" – Electronic Eng. Dept., Roma, Italy* 

*<sup>4</sup>Bruno Kessler Foundation, Center for Materials and Microsystems, Povo (TN), Italy* 

Characterization and Modeling of Charging Effects in Dielectrics

for the Actuation of RF MEMS Ohmic Series and Capacitive Shunt Switches 235

one. In order to fabricate micromechanical switches together with integrated resistors and DC blocking capacitors an eight mask process has been developed. Two electroplated gold layers of different thickness are provided for the realization of highly complex moveable bridges and the co-planar waveguides. The substrates are p-type, <100>, 525 μm thick, 5 kΩcm high resistivity silicon wafers. A 1000 nm thick thermal oxide is grown as an isolation layer. Next a 630 nm thick un-doped poly-silicon layer is deposited by low pressure chemical vapour deposition (LPCVD), to be used for the resistors and actuation electrodes obtained by selective dry etching of the poly-silicon layer. Then, tetra-ethyl-ortho-silicate (TEOS) is deposited by a LPCVD process to provide the high isolation needed for the actuation electrodes. Contact holes are then defined and etched by a plasma process. After ashing the photoresist mask, a multilayer underpass metal Ti/TiN/Al/TiN is deposited by sputtering. The total thickness of the multilayer has to be the same of the polysilicon, in such a way that metal underpass and actuation electrodes are at the same level. The wafer front side is then covered with 100 nm of low temperature oxide (LTO) to obtain an insulating layer for capacitive shunt switches. The previous step is un-necessary for series ohmic configurations. The vias in the LTO are defined by masking and dry etching. A Cr/Au layer is defined by lithography and wet etched. The main purpose of this layer is to cover with a noble metal the exposed electrical contacts of the series ohmic switches to get low resistive electrical contacts. The sacrificial layer required for obtaining the air gap is formed by a 3 μm thick photoresist, hard baked at 200 °C for 30 minutes to obtain well-rounded edges. As a seed-layer for electrochemical Au deposition a 10/150 nm thick Cr/Au layer is deposited by PVD. The moveable air bridges are defined using a 4 μm thick positive resist. After an exposure to oxygen plasma at 80 °C a 1.8 μm thick gold layer is selectively grown in a gold sulphite bath. The first plating mask is removed with an appropriate solvent and the CPW lines and anchor posts for the moveable air bridges are defined with 5 μm thick positive resist and then a 4 μm thick gold layer is selectively grown. The last plating mask and the seed layer are then wet removed. At this point a sintering in nitrogen at 190 °C for 30 minutes is performed to provide the gold layers with the appropriate tensile stress. Finally the air bridges of the individual switches are released with a modified plasma ashing

process (20 minutes oxygen plasma at 200 °C) in order to avoid sticking problems.

device CL).

**3. Experimental results** 

The two devices which have been used for the characterization are shown in the photos given in Fig. 1 (series ohmic device, device S1) and in Figure 2 (shunt capacitive switch,

All the measurements have been performed and recorded in a Clean-Room environment, at the temperature T=(231) °C, with a relative humidity RH=(351) %. A nitrogen flux has been used for providing a dry environment for the devices under test. RF measurements have been used as a validation for the state (ON or OFF) of the switches and for their electrical performances before, during and after the voltage application. In particular, after each cycle used for such a measurement, no changes in the electrical performances of the exploited devices has been recorded. A schematic diagram of the measurement bench is shown in Fig. 3. The reliability of the manufactured devices with respect to the charging effects, and specifically the influence of the pulse shape and of the sign of the voltage (positive or negative) on the actuation mechanism, have been studied by using pulse trains where the rise and fall time, as well as the pulse duration and the separation between pulses

other hand, the reliability of this technology has been not yet fully assessed, because of the limitations introduced by: (i) the mechanical response of the single switches [30], (ii) the necessary optimization of the packaging [31], and (iii) the charging mechanisms. In particular, the charging effect is due to the presence of both the dielectric material used for the realization of lateral actuation pads, deposited to control the collapse of bridges and cantilevers far from the RF path, and the dielectric used for the capacitance in the case of shunt connected microstrip and coplanar configurations. Presently, there is a wide literature about the onset of the mechanism [32], [33], [34] and its control by means of uni-polar and bi-polar actuation voltage schemes [35], [36], [37]. Some results give evidence also for the substrate contribution to charging effects [39] and those related to packaging [38]. Specifically, electromagnetic radiation is a serious issue for space applications [40], [41]. Electrostatic discharge has been discussed in [42], and it is clearly influenced by the deposition process [43]. Besides structural dependence of the charging [44], solutions considering the absence of the dielectric material is also considered, giving evidence for a decrease but not for a complete disappearance for such a contribution [45], [46]. Specific aging schemes based on the temperature are also proposed for long term evaluation of the devices [47]. Advanced studies have been also performed by means of the Kelvin Probe Microscopy, for improving the surface resolution of the charging effect detection [48]. Ohmic contact problems have been evaluated in [49]. Different kind of charging mechanisms can influence the reliability of the MEMS devices, as it has been assessed after the study published in [50].

In this chapter, it will be presented the characterization of two configurations of RF MEMS switches, to demonstrate how the actuation voltage is modified by using a uni-polar bias voltage and how it is under control and stable, at least for a limited number of consecutive actuations, if an inversion in the bias voltage is provided. In particular, the measurements recorded for an ohmic series and for a shunt capacitive configuration will be presented and discussed, considering the main source of charging for both devices. Moreover, experiments performed in both MIM and MEMS reveal that the charging process is strongly affected by the temperature [51]. MIM capacitors have been used to assess the material bulk properties with the aid of Thermally Stimulated Depolarization Current (TSDC) method. The charge storage was found to increase exponentially with temperature in both MIM capacitors and MEMS switches. In particular, in the high temperature range the activation energies in MEMS switches were found to have close values with respect to MIMs, and from TSDC experiments in MIM capacitors they have been found to be rather small. Equivalent circuits accounting for the above charging effects can be used as an effective lumped model, useful for circuital simulations of feeding lines and actuation pads [52].

#### **2. Technology**

Suspended bridges have been manufactured in coplanar waveguide (CPW) configuration. The series ohmic switch has been obtained by means of a bridge isolated with respect to the lateral ground planes, closing a capacitive in-plane gap when the proper bias voltage is provided by means of lateral poly-silicon pads. In such a case the bridge is collapsed and the switch passes from the OFF to the ON state. *Vice versa*, the shunt capacitive switch is composed by a metal bridge connecting the lateral ground planes and by a dielectric layer providing a capacitive contribution when the bridge is collapsed. In this case, when the switch is actuated by means of a DC bias voltage, it passes from the ON state to the OFF

other hand, the reliability of this technology has been not yet fully assessed, because of the limitations introduced by: (i) the mechanical response of the single switches [30], (ii) the necessary optimization of the packaging [31], and (iii) the charging mechanisms. In particular, the charging effect is due to the presence of both the dielectric material used for the realization of lateral actuation pads, deposited to control the collapse of bridges and cantilevers far from the RF path, and the dielectric used for the capacitance in the case of shunt connected microstrip and coplanar configurations. Presently, there is a wide literature about the onset of the mechanism [32], [33], [34] and its control by means of uni-polar and bi-polar actuation voltage schemes [35], [36], [37]. Some results give evidence also for the substrate contribution to charging effects [39] and those related to packaging [38]. Specifically, electromagnetic radiation is a serious issue for space applications [40], [41]. Electrostatic discharge has been discussed in [42], and it is clearly influenced by the deposition process [43]. Besides structural dependence of the charging [44], solutions considering the absence of the dielectric material is also considered, giving evidence for a decrease but not for a complete disappearance for such a contribution [45], [46]. Specific aging schemes based on the temperature are also proposed for long term evaluation of the devices [47]. Advanced studies have been also performed by means of the Kelvin Probe Microscopy, for improving the surface resolution of the charging effect detection [48]. Ohmic contact problems have been evaluated in [49]. Different kind of charging mechanisms can influence the reliability of the MEMS devices, as it has been assessed after

In this chapter, it will be presented the characterization of two configurations of RF MEMS switches, to demonstrate how the actuation voltage is modified by using a uni-polar bias voltage and how it is under control and stable, at least for a limited number of consecutive actuations, if an inversion in the bias voltage is provided. In particular, the measurements recorded for an ohmic series and for a shunt capacitive configuration will be presented and discussed, considering the main source of charging for both devices. Moreover, experiments performed in both MIM and MEMS reveal that the charging process is strongly affected by the temperature [51]. MIM capacitors have been used to assess the material bulk properties with the aid of Thermally Stimulated Depolarization Current (TSDC) method. The charge storage was found to increase exponentially with temperature in both MIM capacitors and MEMS switches. In particular, in the high temperature range the activation energies in MEMS switches were found to have close values with respect to MIMs, and from TSDC experiments in MIM capacitors they have been found to be rather small. Equivalent circuits accounting for the above charging effects can be used as an effective lumped model, useful

Suspended bridges have been manufactured in coplanar waveguide (CPW) configuration. The series ohmic switch has been obtained by means of a bridge isolated with respect to the lateral ground planes, closing a capacitive in-plane gap when the proper bias voltage is provided by means of lateral poly-silicon pads. In such a case the bridge is collapsed and the switch passes from the OFF to the ON state. *Vice versa*, the shunt capacitive switch is composed by a metal bridge connecting the lateral ground planes and by a dielectric layer providing a capacitive contribution when the bridge is collapsed. In this case, when the switch is actuated by means of a DC bias voltage, it passes from the ON state to the OFF

for circuital simulations of feeding lines and actuation pads [52].

the study published in [50].

**2. Technology** 

one. In order to fabricate micromechanical switches together with integrated resistors and

DC blocking capacitors an eight mask process has been developed. Two electroplated gold layers of different thickness are provided for the realization of highly complex moveable bridges and the co-planar waveguides. The substrates are p-type, <100>, 525 μm thick, 5 kΩcm high resistivity silicon wafers. A 1000 nm thick thermal oxide is grown as an isolation layer. Next a 630 nm thick un-doped poly-silicon layer is deposited by low pressure chemical vapour deposition (LPCVD), to be used for the resistors and actuation electrodes obtained by selective dry etching of the poly-silicon layer. Then, tetra-ethyl-ortho-silicate (TEOS) is deposited by a LPCVD process to provide the high isolation needed for the actuation electrodes. Contact holes are then defined and etched by a plasma process. After ashing the photoresist mask, a multilayer underpass metal Ti/TiN/Al/TiN is deposited by sputtering. The total thickness of the multilayer has to be the same of the polysilicon, in such a way that metal underpass and actuation electrodes are at the same level. The wafer front side is then covered with 100 nm of low temperature oxide (LTO) to obtain an insulating layer for capacitive shunt switches. The previous step is un-necessary for series ohmic configurations. The vias in the LTO are defined by masking and dry etching. A Cr/Au layer is defined by lithography and wet etched. The main purpose of this layer is to cover with a noble metal the exposed electrical contacts of the series ohmic switches to get low resistive electrical contacts. The sacrificial layer required for obtaining the air gap is formed by a 3 μm thick photoresist, hard baked at 200 °C for 30 minutes to obtain well-rounded edges. As a seed-layer for electrochemical Au deposition a 10/150 nm thick Cr/Au layer is deposited by PVD. The moveable air bridges are defined using a 4 μm thick positive resist. After an exposure to oxygen plasma at 80 °C a 1.8 μm thick gold layer is selectively grown in a gold sulphite bath. The first plating mask is removed with an appropriate solvent and the CPW lines and anchor posts for the moveable air bridges are defined with 5 μm thick positive resist and then a 4 μm thick gold layer is selectively grown. The last plating mask and the seed layer are then wet removed. At this point a sintering in nitrogen at 190 °C for 30 minutes is performed to provide the gold layers with the appropriate tensile stress. Finally the air bridges of the individual switches are released with a modified plasma ashing process (20 minutes oxygen plasma at 200 °C) in order to avoid sticking problems.

The two devices which have been used for the characterization are shown in the photos given in Fig. 1 (series ohmic device, device S1) and in Figure 2 (shunt capacitive switch, device CL).
