**Abstract**

RF (Radio Frequency) MEMS (Micro Electro Mechanical Systems) technology is the application of micromachined mechanical structures, controlled by electrical signals and interacting with signals in the RF range. The applications of these devices range from switching networks for satellite communication systems to high performance resonators and tuners. RF MEMS switches are the first and foremost MEMS devices designed for RF technology. A specialized method for fabricating microsturctures called surface micromachining process is used for fabricating the RF MEMS switches. Die level packaging using available surface mount style RF packages. The packaging process involved the design of RF feed throughs on the Alumina substrates to the die attachment, wire bonding and hermetic sealing using low temperature processes.

**Keywords:** RF MEMS switches, surface micromachining, low temperature packaging

## **1. Introduction**

Due to the reduced size, cost and low power consumption as well as very high precision, MEMS applications have extended from mere pressure and temperature sensors to vast array of applications viz., Aerospace, Automobile, Biotechnology, Consumer products, Defense and the most important and pertinent Telecommunications [1]. Hence RF MEMS devices have the advantage of increased functionality, substantial performance improvements, high agility, modularity and reconfigurability [2]. These devices are applicable to high performance communication systems such as satellite communication and m applications [3].

RF MEMS switches are the first and foremost MEMS devices designed for RF technology. RF MEMS switches compared to their semiconductor counterparts such as FET and PIN diodes show far superior performance. The current–voltage nonlinearity that is the bane of semiconductor devices is non-existent in the case of RF MEMS switches. The power consumed by these switches is far less since most of the switches using electrostatic and piezoelectric actuation require negligible power requirements. They are also not plagued by issues of harmonics and intermodulation of signals. They exhibit very low insertion loss in the range 0.05 to 0.2 dB at a frequency of 40 GHz. They also possess very high isolation in the range of 40 dB at 40 GHz [4, 5]. The only drawback is that their switching speed is far inferior compared to their semiconductor counterparts. However, there are several high performance communication circuits such as in defense and satellite systems where speed may not be the criteria whereas low power consumption and high RF

performance would be the key features required. Due to these features they improve the overall performance of the systems into which they are integrated. Hence, the focus of this work is on RF MEMS switches which are a superior alternative to existing semiconductor switches.

iii. CPW metal layer patterning: Using sputtering and lithography steps

v. Sacrificial layer deposition and patterning: Using Photoresists and

depostion followed by lithography steps.

vii. Top layer release: Using Critical point dryer.

lithography steps

*RF MEMS Switch Fabrication and Packaging DOI: http://dx.doi.org/10.5772/intechopen.95003*

tion of the RF MEMS shunt switches.

ionic contaminants from the wafer surface.

*Steps involved in fabrication of capacitive shunt switches.*

*1.1.2 Oxidation of test wafer*

**Figure 1.**

**255**

*1.1.1 Cleaning of test wafer*

iv. Dielectric deposition and layer patterning: Using PECVD for Silicon Nitride

vi. Top layer deposition and patterning: Using sputtering and lithography steps.

**Figure 1** gives pictorial representation of the process steps followed for fabrica-

The cleaning of the Silicon wafer is the first process employed to removing any organic residue or films on the Silicon wafers. The cleaning process is performed in two parts [12]. The first part of the cleaning process is the famous RCA-1 named after the laboratory at which it was developed. In this process five parts of water is mixed with one part of Ammonium Hydroxide (NH4OH) and one part of Hydrogen Peroxide (H2O2). This mixture is then heated to 75°C on a hot plate. Once the solution bubbles vigorously the Silicon wafer is soaked in this solution for

15 minutes. The wafer is then dipped in a solution made of one part of Hydrofluoric acid (HF) and 50 parts of water for 30 seconds. This solution serves the purpose of etching out the thin oxide layer developed on the wafer. The wafer is again washed with DI water. The next step also called RCA-2 involves the use of Hydrochloric (HCl) acid, Hydrogen Peroxide (H2O2) and DI water in the ratio of 1:1:6. This solution is then heated to a temperature of 75°C for 15 minutes after which the Silicon wafer is placed in this solution. RCA-2 completely removes the traced of

The oxidation of Silicon wafer leads to the formation of a layer of native oxide i.e., Silicon Dioxide on the wafer surface. It is seen that only Silicon material has the

MEMS devices are fabricated by the use of special techniques called micromachining. Micro fabrication or micromachining or micro manufacturing is the use of a set of manufacturing tools based on thin and thick film fabrication techniques commonly used in the electronics industry. It is also a technology for creating small three dimensional structures with dimensions ranging from sub centimeters to sub micrometers. A vast majority of MEMS structures are fabricated using bulk micromachining process. This involves etching of bulk wafer leading to three dimensional structures such as beams, cantilevers and cavities. These processes can be realized on substrates such as Silicon, Glass and Gallium Arsenide etc. The thickness of the structures can range from a few micrometers to 200 mm. The resulting dimensions of microstructures are much larger compared to surface micromachining process. Surface micromachining is a process based on building up of material layers and then selectively retaining or etching by continued processing. The bulk of the substrate remains untouched. LIGA processes combine IC lithography and electroplating and molding to obtain depth. Patterns are created in a substrate and then electroplated to create 3D molds. These molds can be used as the final product, or various materials can be injected into them. This process has two advantages. Materials other than Silicon can be used e.g. metal, plastic and devices with very high aspect ratios can be built [6].

This chapter provides the complete details of the unit step processes used for the fabrication and packaging of RF MEMS switches. The focus is on fabrication of low actuation voltage RF MEMS switches [7–10]. There are several challenges involved in the fabrication of MEMS switches such as, structural deformation, residual stress, non-release of structural layer to name a few. These challenges are overcome and addressed throughout the fabrication process by optimization of several unit processes. The unit processes used is discussed in each section of this chapter.

## **1.1 Fabrication process steps**

Surface micromachining process is used for fabricating the switches. In the present work, fabrication costs were brought down by


The sections below give the detailed description of the fabrication steps followed for successful fabrication of RF MEMS shunt switches.

The test wafers used in this work is P-type {100} low resistivity 4″ wafers with resistance ranging from 1 to 100 Ω. Using low resistivity wafers to fabricate RF MEMS switches has the advantage that integration with CMOS circuits is easier. However, use of low resistivity Silicon wafer leads to higher insertion loss due to inherent parasitics.

The following are the process steps used for fabrication:


**Figure 1** gives pictorial representation of the process steps followed for fabrication of the RF MEMS shunt switches.
