1 Introduction

Advancement in satellite technology and the raise of satellite sectors provides efficient services in Defence, communications, broadcasting, commercial and government sectors [1]. Rapid development in RADAR and Satellite communication increases the demand for reconfigurable devices in modern day communication systems. Among them antennas using MEMS switches have acquired great attention in satellite communications for their reconfigurable capability in frequency, radiation patterns and polarizations for coverage of various geographical locations [2]. These RF MEMS switches has the advantages of nearly Zero (very low) power consumption, good isolation and less insertion losses over FET and PIN diodes which makes them to utilize in high frequency Pico—satellite communication applications (12–35 GHz) [3]. RF MEMS switches provide the flexibility in interconnection between the transmitter and receiver which enhances the capacity of the satellite and these can be utilized in the applications of satellite payload to provide redundancy in communication system. Even though having good flexibility these are not been deployed successfully due to requirement of large switching elements with high speed, less dielectric charging and poor switching topologies [4, 5]. Satellite communications (12–35 GHz) utilizes RF MEMS switches for switching networks, antenna applications, filter banks and phase shifters but the failures occur by contact area degradation due to repetition of impact between the beam and lower electrode, pitting and contaminations which increases insertion and isolation losses. Therefore, there is a need of novel structure RF MEMS switch for low insertion and high isolation with actuation voltage [6].

“As with many Technologies, one design is not appropriate for every application”. Particularly in RF MEMS switches there may be a smaller number of solutions for a specific application. Hence there is a need for development of novel structures for specific applications of RF MEMS switches [7]. A novel switch design with the combination of inner and outer rings, serpentine meanders and comb—drive actuators is integrated in the CPW T—line is designed for antenna switch applications with actuation voltage of 50 V, Less insertion loss of − 0.87 and good isolation 0f − 60 dB is achieved at 40 GHz [8]. A novel cantilever type RF MEMS switches is designed with very thin cantilever beam of 0.5 µm and achieved the pull in voltage of 4.2 V but failed to satisfy the reliability issues [9]. RF MEMS switch based on MIM capacitor is proposed on 400 µm wafer with 0.2 µm thin dielectric layer operates at the frequency of 0.5–6 GHz and is not suitable for higher frequencies [10]. Hence there is high possibility to design novel beam structure which reduces pull—in voltage by having central beam lower to the actuation electrodes of the beam i.e., Step down structure such that the switch also reduces the insertion losses and provides high isolation.

The novel step-down structure of RF MEMS switch beam can be proposed to increase the upstate capacitance reduces the insertion and return losses during the signal transmission and also reduces the actuation voltage to displace the step-down beam downwards and get contact with the dielectric layer. Generally, metals with high conductivity are chosen as the bridge materials among them Gold, Copper, Nickel, Molybdenum, Platinum and Aluminium are used in most of the RF MEMS switches [11]. The decrease in pull down voltage of the RF MEMS switches over the exploitation of buckling effect and bending is reported by considering the new materials such as AlSi4 and Platinum (pt) contains the issues of slow switching and less reliability [12]. The beam material can be selected using Ashby’s approach which is a methodology to select materials by studying its performance indices. It provides a strategy to select material with very less computations [13].

As the number of MEMS materials increases continuously, there is a need of a technique or systematic approach to select the materials for the MEMS switches. In this paper, the performance and material indices have been studied using the Ashby’s approach and the role of beam material on various performance parameters is studied and presented. The paper is organised as follows: in Sect. 2, a detailed description on device and material selection process using Ashby's methodology is stated and illustrated. In Sect. 3, Geometrical and material indices are studied using graphs plotted and the analysis is concluded in the Sect. 4.

2 The Proposed Novel Device Structure: Step Structure RF MEMS Switch

The proposed step–down structure RF MEMS switch consists of 550 µm thick substrate in which the 1 µm thin insulating layer is taken on it for low insertion and reflection losses. A coplanar waveguide is constructed over the insulating layer as the transmission line with dimensions 60/100/60 µm to obtain 50 Ω characteristic impedance where the gap between the signal and ground planes is 60 µm and width of the signal line is 100 µm. The biasing pads of width 100 µm and length 110 µm are placed between the signal line and ground planes by creating a recess on the both ground planes and are supplied with the pull in voltage for actuation. The thickness of the biasing pads is constructed in such a way that the gap between the suspended membrane and biasing pads is 2 µm. Above the signal line a 0.1 µm thin dielectric layer made up of HfO2 is considered to develop the capacitance between signal line and suspended membrane. The membrane of the switch is suspended over the signal line with the help of meanders having a gap of 2 µm for electrostatic actuation such that it consists of three sections. The middle part of the membrane is 5 µm lower than the other two sections forming a step-down structure. The other two sections which are present on the either side are considered as actuation an electrode which provides electrostatic actuation with the biasing pads. When the biasing pads are supplied with the necessary pull in voltage the suspended membrane is pulled downwards by the electrostatic force and RF signal is attenuated. The entire switch structure is designed and optimized to transmit the signal in K-band frequency range (Figs. 1 and 2).

Fig. 1
figure 1

Sketch of a typical step radio frequency Microelectromechanical system switch

Full size image
Fig. 2
figure 2

Schematic view of beam layer

Full size image

Beam material plays a key role in RF parameters like pull in voltage, switching speed, spring constant etc. So, selection of beam material is important to have better performance of the switch. The Ashby’s approach is a methodology to select materials by plotting of thermal, mechanical and electrical properties. Plotting of these performance indices is considered as a best way to reduce computations [14]. The component design is specified by three parameters namely geometrical properties, functional requirements and material properties.

The stages laid by in Ashby’s approach with less computation are described as [15].

Stage 1 Extract various materials from all possible ways like (metals, ceramic, polymers etc.).

Stage 2 Analyse the function, constraints and objectives of above-mentioned materials.

Stage 3 Screen out the materials which cannot meet the requirement.

Stage 4 Ranking the material which can meet the high performance of the device.

Stage 5 After ranking process the detail information of the selected materials is evaluated.

Stage 6 After profile information the prime materials are chosen.

Stage 7 In this stage the optimized material was chosen for the beam layer.

The performance of the device is estimated through Functional characteristics, geometrical properties and material indices. It is expressed as

$$P = f[F,G,M]$$
(1)

Here performance of the elements is expressed by individual functions of G, M and F as shown in the Eq. 2. Hence, by reducing the performance index into invidual functions an optimum subset of material can be identified (Fig. 3).

Fig. 3
figure 3

Sketch of a step structured fixed–fixed model

Full size image
$$P = f_{1} (F)f_{2} (G)f_{3} (M)$$
(2)

This optimization of these performance indices is traditionally performed by plotting the material characteristics against each other and bottom left material is chosen as the desired material for beam.

3 Results and Discussions

The step structure RF MEMS switches are mainly used for low pull in voltage and high isolation loss which are used for K band applications. Mainly the performance of the device depends on the materials used for the beam and dielectric layer of the system. Performance parameters like pull in voltage, spring constant and S-parameters are mainly depending on the device dimensions and materials used for the device. The selection of material for beam is useful for maintaining better reliability and high switching speed of the device. The different material properties like electrical resistivity, electrical conductivity, thermal conductivity, young’s modulus, Poisson’s ratio, thermal expansion coefficient, density and heat capacity at constant pressure which are mentioned in Table 1 are utilized to obtain performance indices.

Table 1 Material properties of the consider materials
Full size table

3.1 Performance Indices

3.1.1 Actuation Voltage

In RF MEMS switches actuation voltage is an important parameter which creates an electrostatic force between the membranes and biasing pads such that the voltage required to displace the membrane with in the gap is given by Rebeiz [16]

$$V = \sqrt {\frac{8K}{{27 \in_{0} Ww}}g_{0}^{3} }$$
(3)

where ε0—permittivity of free space. g0—gap between the electrodes, A-overlap area, W-length of the cantilever beam and w-width of the cantilever beam.

The pull in voltage of the switch occurs when the step-down membrane displaces 2/3rd of the gap and it can be reduced by reducing the parameter such as spring constant and initial gap between the biasing pads and membrane and also it can be reduced by increasing the overlapping area between the membrane and electrodes. Hence the reducing the mechanical spring constant of the switch is a feasible method to decrease pull in voltage (Table 2).

Table 2 Geometrical and Material properties of the consider materials
Full size table

The spring constant mainly depends on the young’s modulus of the membrane and meanders stiffness and is given by

$$k = 4E\left( {\frac{t}{L}} \right)^{3}$$
(4)

The spring constant mainly depends on the E young’s modulus of the material used for the bridge, therefore from the Eqs. 3 and 4, the actuation voltage is directly proportional to the square root of the young’s modulus. The actuation voltage is directly proportional to the square root of the young’s modulus and density of the membrane. Therefor the material indices (MI) related to the actuation voltage is

$${\text{MI}}_{1} = \sqrt E$$
(5)
$${\text{MI}}_{2} = \sqrt \rho$$
(6)

The spring constant also depends on the Poisson’s ratio and thermal residual stress of the material of beam which is given by

$$k = \frac{{8\gamma (1 - \nu )(\sigma_{0} - \Delta \sigma )tw}}{L}$$
(7)

From the above equation the terms \(\gamma\) is geometrical factor which is consider as the heat capacity at constant pressure CP for different beam materials. \(\Delta \sigma\) is define as the change in thermal residual stress \(\nu\) is Poisson’s ratio and \(\sigma_{0}\) is thermal residual stress at reference temperature. From the Eqs. (3) and (7) it is observed that the third material index and the first geometrical indices can be derived from the actuation voltage.

$${\text{MI}}_{3} = \nu$$
(8)
$${\text{GI}}_{1} = {\text{ C}}_{\text{p}}$$
(9)
$$\Delta \sigma = E\Delta \alpha \Delta T$$
(10)

The variation in temperature also results the residual stress caused by thermal of the beam as shown in Eq. 10, where \(\Delta T\) is the temperature change of the beam, \(\Delta \alpha\) is the difference in the thermal expansion co-efficient. It is also observed that the thermal expansion coefficient is directly proportional to the thermal residual stress. From the Eqs. 7 and 10, it can be exhibited that low spring constant can be achieved by higher value of the thermal expansion co-efficient. Hence the fourth material indices is

$${\text{MI}}_{4} = \alpha$$
(11)

From the all above equation the performance indices related to the actuation voltage is

$${\text{PI}}_{1} = {\text{f}}({\text{E}},\alpha ,\nu )$$
(12)
$${\text{GI}}_{1} = {\text{f}}\left( {{\text{C}}_{\text{P}} } \right)$$
(13)

3.1.2 S-parameters

Second performance index is related to the radio frequency loss that can be reduced by choosing proper material for the bridge of radio frequency switch which have good conductivity. The dissipated power in the beam is give as

$${\text{P}}_{\text{loss}} = {\text{I}}^{2} \cdot {\text{R}}$$
(14)

From the above equation the current produced in the beam of the switch is represented as I and the resistance offered in the beam in represented as R.

The resistance R can also expressed as

$$R = \frac{\beta \rho L}{4tw}$$
(15)

where \(\beta\) is related to current crowding into membrane which is constant and the electrical resistivity of the beam can be represent as \(\rho\). From the Eqs. 14 and 15 it is inferred that power loss of the beam structure is proportional to the electrical resistivity of the material bridge. Hence the fifth material index is based on resistivity of the bridge material.

$${\text{MI}}_{5} = \rho$$
(16)

Therefore the second performance index is the function of resistivity and is expressed as

$${\text{PI}}_{2} = {\text{f}}(\rho )$$
(17)

3.1.3 Thermal Residual Stress

The change in thermal residual stress is due to the self-heating for large radio frequency signal in the microelectromechanical bridge structure.

$$\Delta \sigma = T_{u} \cdot \Delta \alpha \cdot P_{loss} \cdot R_{TH}$$
(18)

where RTH is the thermal resistance and Ploss is the power loss during transmission of the signal. Tu is the uniform distribution of temperature and \(\Delta \alpha\) is the difference in the thermal expansion co-efficient. The thermal resistance and electrical resistivity of the material of bridge is due to the self-heating in the microelectromechanical system bridge. From the above Eqs. (18) and (14) there the sixth material index can be given as

$${\text{MI}}_{6} = {\text{R}}_{\text{TH}} \cdot {\text{R}} = \frac{1}{K}\rho$$
(19)

Therefore, third performance index that related to the beam structure and thermal residual stress is

$$PI_{3} = f\left( {\frac{1}{K}\rho } \right)$$
(20)

The performance of the RF MEMS switch bridge material can be selected by analyzing material and performance indices and the graphs illustrates the trade-off between various parameter.

The possible variation between the Poisson’s ratio and thermal expansion co-efficient is observed from the Fig. 4 and the graph concludes that the high values of Poisson’s ratio and thermal expansion coefficient results in low actuation voltage. Hence it is observed that SU-8 and PTFE are promising materials.

Fig. 4
figure 4

Thermal expansion coefficient versus Poisson’s ratio

Full size image

It is observed that the material having low young’s modulus requires low actuation voltage as shown in Fig. 5. Hence from the graph, SU-8 is most suitable material for the bridge material followed by gold and aluminum (Fig. 5).

Fig. 5
figure 5

Young’s modulus versus Poisson’s ratio

Full size image

The material having low thermal conductivity and high electrical resistivity will provide minimum thermal residual stress, Hence from the Figs. 6, 7, 8 and 9, It is observed that SU-8 is good material followed by gold and aluminum for bridge material selection (Figs. 8 and 9).

Fig. 6
figure 6

Thermal conductivity versus electrical resistivity of polymers

Full size image
Fig. 7
figure 7

Thermal conductivity versus electrical resistivity of metals

Full size image
Fig. 8
figure 8

Thermal conductivity versus electrical conductivity of polymers

Full size image
Fig. 9
figure 9

Thermal conductivity versus electrical conductivity of metals

Full size image

The material having low thermal conductivity and low electrical conductivity will provide minimum thermal residual stress. Therefore, Su-8 is good material followed by gold and aluminum for bridge material selection. From the Figs. 10 and 11 we can conclude that the material having low Young’s modulus will reduce the RF losses. So SU-8 is good material followed by gold and aluminium for bridge material selection.

Fig. 10
figure 10

Young’s modulus versus electrical conductivity of polymers

Full size image
Fig. 11
figure 11

Young’s modulus versus electrical conductivity of metals

Full size image

The geometrical effect of the device is discussed for high reliability in the form of heat capacity at constant pressure which can be represents as CP.

The material having low Young’s modulus will reduce the RF losses due to larger displacement of the beam which increases down state capacitance. Hence by observing the Figs. 12, 13, 14 and 15, SU-8 is good material followed by gold and aluminum for bridge material selection.

Fig. 12
figure 12

Young’s modulus versus electrical resistivity of metals

Full size image
Fig. 13
figure 13

Young’s modulus versus electrical resistivity of polymers

Full size image
Fig. 14
figure 14

Young’s modulus versus Heat capacity at constant pressure

Full size image
Fig. 15
figure 15

Square root of young modulus with square root of Poisson’s ratio by electrical resistivity

Full size image

4 Conclusions

Novel design based on the bridge structure and its material selection of RF MEMS shunt switch pertinent for present and future satellite communication application is the major research performed in this paper. The dimensions of the switch were optimized for material selection high performance of the switch. This paper reports a beam material selection approach for step-down structure RF MEMS switch used for satellite communication applications.

As there are wide range of material available to design a beam in RF-MEMS switches, Ashby’s approach is preferred to select more appropriate possible material for beam. The performance indices like actuation voltage, S-parameters and residual stress by heat are plotted for selection of material. Among the different materials SU-8, PTFE followed by aluminum and gold are the more suitable materials for beam material, but SU-8 is not much feasible for the fabrication and PTFE have some dis-advantages such as high thermogenic which is also suitable for fabrication. The Aluminum is chosen as the beam material due to its low cost than gold.