Journal of Biosensors, Biomarkers and Diagnostics

3. Switching Time (t_s): the minimum time taken by the switch to complete one cycle is known as switching timein RF MEMS Switch it completely depends on pull in voltage (V_p), Applied Voltage (V_s) and is formulated as
3.67 (Vp / Vsω0)

4. Damping (𝝃): How mutch time the structure is taking to come to stability can be conclude using damping. If 𝝃=0 the structure will take more time to achive stability, elseif 𝝃 = 1.5 stability is achived quickly by the structure. The general damping is formulated as
\[\frac{b}{{2\sqrt {Km} }}\] Where: b-Damping Coefficient

5. Quality Factor: The quality factor is one of the important parameter of RF MEMS Switch and can be explained as the ratio of the energy stored to the energy dissipated per cycle at the resonant frequency. High Q indicates a lower rate of energy loss relative to the stored energy of the resonant circuit.
Quality Factor (Q) = K/bω0

Electrical Properties:This paper mainly discuss the electrostatic based switches, so the main electrical properties listed below

1. Electrostatic force: when a significant voltage is applied in between electrodes some electrostatic force will create and it can be expressed as
\[\frac{1}{2}\frac{{{\varepsilon _0}A{V^2}}}{{{{\left( {{g_0} + \frac{{{t_d}}}{{\varepsilon r}} - \delta } \right)}^2}}}\] 2. Pull-in Voltage: The basic principle of electrostatic RF MEMS Switch is, some amount of voltage need to apply to get the deformation in the beam for switching. And for the recangular bassed beams the expected pull-in voltage is
\[\sqrt {\frac{{8K}}{{27A{\varepsilon _0}}}{g_0}{{\left( {\frac{{{t_d}}}{{{\varepsilon _r}}}} \right)}^3}} \] 3.Pull-up Voltage: After applying the pull in voltage the beam will deforms and because of elastic property of the beam it will try to get the stability. At waht voltage level the bram try to get the initial position is said to be pull up voltage.and the pull up voltage is formulated as
\[\sqrt {\frac{{2K}}{{A{\varepsilon _0}}}{g_0}{{\left( {\frac{{{t_d}}}{{{\varepsilon _r}}}} \right)}^2}} \] 4. Capacitance: The capaciatnce in RF MEMS Switches is divided as Upstate capacitance and downstate capacitance.

The bridge based capacitive type switch has the Upstate Capacitance (C_u) = C_p + C_pp+C_f, and Down State Capacitance (Cd) = C_p+1.5C_pp+C_f, where C_p-Parasitic capacitance, Cpp-Parallel plate Capacitance (𝞮0Ww / _g0), and C_f- Fringing Field Capacitance. Fringing field capacitance which is around 10%-20% of C_pp where: ε₀-Free space permittivity = 8.854 x 10⁻¹² F/m [2].

Finite Element Method (FEM) tools are very helpful to design and simulate the RF MESM Switches. Comsol Multiphysics, Coventorware, Intellisuite are the well-known FEM Tools. In this paper as an example, an electrostatic actuated, lateral deflective, bridge based, series capacitive RF MEMS Switch is designed and simulated using Comsol Multiphysics FEM tool.

As shown in the (figure 4), a cantilever based switch with geometrical dimensions length l = 90 μm, width w = 20 μm, thickness t = 1 μm, gap between the electrodes g_o=0.1μm. and the dielectric thickness t_d =0.9 μm and the width of the bottom electrode W = 20 μm. Gold (Au) is used as a cantilever material, and Silicon Nitride (Si₃N₄) with dielectric constant 7.5 is used as dielectric material. And the working principle of the switch is, initially the input applied voltage is 1mv there is no deformation in the switch under this condition input is equal to output, but the second case in addition a 5v is added to 1mv then there is some electrostatic force is created between the electrodes then the cantilever will deform and touches to ground under this condition the output is zero.

With the above mentioned dimensions and working principle an electrostatically actuated capacitive switch is designed and simulated using Comsol Multiphysics FEM tool, the results are shown in (Figure 5).

The micro fabrication from top to bottom involves different important processes, and are listed below

A. Material Selection:

1) Wafer: Most of the MEMS devices fabrication takes the substrate as silicon, GaAs, Quartz, as active substrates and polymers, Plastic, Ceramics, are as passive substrates.

Additionally LaAlO3 [8] can be used as substates.

2) Beam: The frequently used materials as membrane are Gold(Au), Aluminum(Al), polymers. From list polymers are preferable because of their light weight the pull-in voltage can be minimize.

3) Dielectric: Silicon Dioxide(Sio₂), Silicon Nitride (Si₃N₄), Polymethyle Methacrylate (PMMA).

4) Sacrificial Layer: the material used as sacrificial layer must and should consist some mechanical properties like adhesion and low residual stress to avoid failures in the switch. So the selection of structural material and the sacrificial layer combination is very important. Few combinations are shown in (Table 1).

a. Cleaning: After completion of each fabrication step, followed by there is cleaning process will present to remove dust particles. Piranha cleaning is famous for wafer cleaning after completion of each step.

b. Piranha Cleaning: The combinaation of sulfuric acid (H₂SO₄):Hydrogen peraxide (H₂O₂) is used as cleaning solution for wafer and some tiems used to remove photoresist.

c. Masks preparation:Generally RF MEMS Switch fabrication require transmission layer mask, dielectric mask, sacrificial layer mask, beam or membrane mask, overall minimum four masks require for lithography [8].

d. Photo Lithography: This process is used to remove the unwanted portion from the deposited film. In this process fist a photoresist layer is uniformly formed on the surface of the film with the help of spin coater. Generally PMMA, PMGI are used as photoresist material. Next UV light is projected on the photoresist through a mask, and after by using developers the exposed (Positive) or unexposed (negative) photoresist is removed by using different developers as shown in (Figure 6).

e. E. UV Light: Generally PMMA, PMGI, DQN are used as positive photoresist and Bis (aryl)azide rubber, KTRF are used as negative resist materials. Positive resist materials are more sensitive to UV light, and negative resist materials are more sensitive to electron beam.

f. Deposition Techniques:In micro fabrication, different type of materials like semicondutors, metals, poly silicon, dielectrics need to diposit on the wafers for that there different dipocition techniques are available. The diposition techniques are classification is shown in the (Table 2).

g. Etching: Removal of specified portion with the help of chemicals or gases is known as etching in micro fabrication process. Etching processes classification is shown in the (Table 3).

To fabricate MEMS Based RF Switches there are deferent Micromachining methods. They are elaborated below,

B. Bulk Micromachining: In bulk micromachining the entire structure of the element is created in the substrate as shown in the (Figure 7).

Cantilev: In this section the fabrication of a SiO₂ cantilever on silicon wafer using bulk micromachining is explained.

(a) As the basic step, the wafer is cleaned using piranhasolution (H₂SO₄:H₂O₂ in 9:1 ratio) to remove the organic and chemical contaminations on the wafer surface, rinse with DI water and blower with nitrogen. After cleaning the wafer samples are forwarded to thermal oxidation to form the SiO₂ on the Silicon wafer surface. As the next step in photolithography, first the samples are dehydrated at 250°C for 10 minutes. Next a photoresist (PMMA) is formed on the surface of the samples by using a spin coater with 4000 rpm for 10 sec. Soft baking is the intermediate process of keeping the samples at 95°C for 2 minutes to evaporate the solvents in the photoresist. (b) And next UV light is exposure through a cantilever mask for 2 sec. (c) After UV exposure the samples are placed in a special developing solution (NAOH) which will dissolve the exposed photoresist, rinse with DI water and blower with nitrogen processes will follow. (d) Now the exposed SiO₂ is removed using BHF based wet etching. (e) Acetone Solution is used to remove the leftover photoresist and again the piranha cleaning using H₂SO₄:H₂O₂ in 3:1 ratio, DI water clean nitrogen blow is done to remove the dust particles. (f) Next the exposed silicon is etched isotropic ally using KOH.

J. Surface Micromachining: In surface micromachining the entire structure of the element is created on the top of the substrate as shown in the (Figure 9).

h. Cantilev: In this section the patterning of a simple Silicon cantilever beam using Silicon on Insulator (SOI) wafer by following surface micromachining process is explained.

(a) As the basic step, the SOI is cleaned using piranha solution (H₂SO₄:H₂O₂ in 9:1 ratio) to remove the organic and chemical contaminations on the wafer surface, rinse with DI water and blower with nitrogen. A Positive Photoresist Material (PMMA) is formed on the surface of the SOI by using a spin coater with 4000rpm for 10 sec. Soft baking is the intermediate process of keeping the samples at 95°C for 2 minutes to evaporate the solvents in the photoresist. (b) Next the coated SOI is forwarded to photolithography; here first the samples are dehydrated at 250°C for 10 minutes and UV light is exposure through a cantilever mask for 2 sec. (c) After UV exposure the samples are placed in a special developing solution (NAOH) which will dissolve the exposed photoresist, rinse with DI water and blower with nitrogen processes will follow. (d) Now the exposed Si is removed using dry etching like anisotropic plasma etching. (e) Piranha cleaning to remove the leftover photoresist using H₂SO₄:H_2O2 in 3:1 ratio, DI water clean nitrogen blow is done to remove the dust particles. In the cleaning process petri dishes are used to carry the solutions. (f) Next the samples are place in wet etch BHF solution to remove the exposed SiO₂ and to release the cantilever. Finally the samples are dried up to critical point to avoid the stiction and to prevent the structural collapse.

i. UV Light: Alternatively there exist one more fabrication method i.e. LIGA Process, but it is costly and complex compared to Surface and bulk micromachining processes.

After fabrication to improve the reliability characterization is important. Microscopes are used to characterize the Micro and Nano devices. General microscopes used to characterize the micro and Nano elements materials are Optical microscope

Field Emission Scanning Microscope (FESM), Scanning Electron Microscope (SEM), Tunneling Electron Microscope (TEM), and Atomic Force Microscope (AFM). Finally the structure frequency is characterized by applying some voltage using Laser Doppler Vibrometer (LDV). The Micro & NANO devices can characterize by finding device materials surface properties, device input impedance, practical reliability, and estimation of lifetime.

Hermetic packaging is used for RF MEMS Packaging because the devices are more sensitive. Seam, Roller or Laser sealing are used in hermetic packaging which is composed of materials like ceramic, beryllium oxide, aluminum, Aluminum Nitride. The packaging of RF MEMS Switches is most expensive it is around 30$-50$ per unit. Packaging costs are currently high, and the packaging technique itself may adversely affect the reliability of the MEMS switch [1]. Improving the yield of MEMS device after packaging. Even though the device could yield up to 90%, after packaging, the yield could be as low as 20% [3].

Last few decades, an excellent research is forwarded in the area of RF MEMS Switches still the challenges are showing the scope of research. Few important challenges in electrostatically actuated RF MEMS switches are, the switching speed of most MEMS switches is around 2-40 μs. Certain communication and radar systems require much faster switches. Most MEMS switches cannot handle more than 20-50 mW. MEMS switches that handle 0.2-10 W with high reliability simply do not exist today. Electrostatic MEMS switches require 20-80 V for reliable operation, and this necessitates a voltage up-converter chip when used in portable telecommunication systems. The reliability of mature MEMS switches is 0.1-10 billion cycles. However, many systems require switches with 20-200 billion cycles. Also, the long-term reliability (years) has not yet been addressed. MEMS switches need to be packaged in inert atmospheres (nitrogen, argon, etc.) and in very low humidity, resulting in hermetic or near-hermetic seals. Packaging costs are currently high, and the packaging technique itself may adversely effect the reliability of the MEMS switch [1]. Improving the yield of MEMS device after packaging. Even though the device could yield up to 90%, after packaging, the yield could be as low as 20% [3]. MEMS Devices are operated at 1-100mw, so linearity is a critical parameter to prevent distortion or inter channel interference [2]. Alleviate the stiction phenomenon significantly increasing the device lifetime [7]. Reliability of MEMS is a real challenge, narrowing of the actuation and release voltages during long period of continuous actuation [7]. Electrostatically actuated switches do not effectively consume dc power; they suffer stiction and dielectric charging problems due to humidity [9]. Thermal actuators are not sensitive to those problems [6]. At higher temperatures the beam stress becomes compressive and leads to buckling of the beam and device failure [10]. Typical reliability problems of the contact type switches are listed in (Table 4) [11].

This paper reviewed the merits and demerits in the realization of different RF MEMS Switches. In the review, major challenges are noticed at different levels of realization. Based on the review, for high frequency applications prefer capacitive shunt switches and for low frequency applications prefer the series resistive switches. The main challenge in the RF MEMS Switches is more pull-in voltage, to reduce the pullin voltage the structure must be light weight and more elastic for that polymers are the best materials as a cantilever. To fabricate the switch, both surface and bulk micromachining are giving good results,

and LIGA Process is much costly compared to Surface and Bulk micromachining. Packaging the switch is the biggest challenge, which is playing major role in the switch reliability.

Other than the packaging the major challenges are reduction of pull-in voltage, humidity and temperature tolerance, stiction, lifetime, Structure Viscoelastic Problems.

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