*1.1.7 Top layer release*

will result in uniform coating of thick resist on the wafer. In the second step the speed is ramped upto 1000 rpm within a time of 30 sec. A solid film of the photoresist is formed with the complete evaporation of the solvent. This step decides the thickness and uniformity of the photoresist. The third step consists of the spin coater speed set at 2000 rpm for 40 sec. This last step ensures that any leftover solvent is completely evaporated. The complete cycle of spin coating is as shown in **Figure 11**. Using a Dektak optical profiler the thickness of this layer was

**Sacrificial layer patterning**: The patterning of the sacrificial layer photoresist is processes by first depositing one more layer of positive PR S1813 on this layer. This was achieved by the spin coater speed set to 500 rpm for 30 seconds, followed by a ramp up of 1000 rpm for 30 sec and 200 rpm for 40 sec. After soft baking the PR is exposed to UV rays through a mask aligner at a proximity of 30 μm and energy of 75 mJ. The mask used for generating the pattern for this layer is as shown in **Figure 12**. The PR is then developed using the developer AZ 351B for 30–60 seconds. Next, the wafer is hard baked on an oven at 90°C for 30 minutes. The PR layer thickness

The top layer or beam formation defines the performance of the RF MEMS switch. The top layer designs were simulated using Coventorware™. These designs have been chosen due to their lower pull-in voltages. Gold is the choice for the top layer due to its favorable characteristics such as, its high conductivity, non-tarnishing property, high Young's Modulus and compatibility with micromachining processes. The top metal layer deposition and patterning is described in the following sections.

a. **Gold layer deposition:** The deposition of this layer was carried out using the TECPORT sputtering equipment. It may be recalled that the bottom layer has the composition of Cr/Au/Cr. This composition would lead to excellent adhesion of the top layer anchors with the previously deposited Chrome layer. Several Iterations were carried out in order to sputter the top Gold layer without residual stress. Several parameters such as temperature, rate of deposition were optimized in order to arrive at top layers without buckling

Finally, with the optimized parameters setting temperature and rate of deposition a stress free top layer was arrived at. The stress free top layer is of critical importance for reduction in actuation voltage. The process parameters of the sputter coater were set at a base pressure of 5x10<sup>6</sup> Torr, deposition

shrunk from 3 μm to 2.09 μm after development and baking.

*1.1.6 Top layer deposition and patterning*

*Nanofibers - Synthesis, Properties and Applications*

after release process.

**Figure 12.** *Mask 3 for PR layer.*

**262**

confirmed to be 3 μm.

The release of the top switch membrane is the most crucial step in the whole fabrication process. There are many methods to release the top layer without deformation and stiction. The first step in the top layer release is to etch the sacrificial layer. This could be achieved by using dry etching or wet etching. In wet etching, conventional liquid solvents are used to completely remove the sacrificial layer followed by drying. The drying could be through the process of air drying or through critical point drying.

**Figure 13.**

*Four top layer designs for RF MEMS switch. (a) Fixed-fixed beam switch. (b) Fixed-fixed Flexure switch. (c) Fixed-Fixed Single Flexure switch. (d) Crab leg Flexure switch.*

Critical Point Drying (CPD) was found to be the best method for MEMS devices [19]. In this work the wet etch was followed by CPD to release the top layer. PR layer first stripped by using Piranha solution. The Piranha solution is prepared by mixing Sulfuric Acid and Hydrogen Peroxide in the ratio of 3:1. This is an extremely strong oxidizing agent which removes organic residues and especially PRs from the substrate.

The most critical factor for the successful commercialization of micro level devices is packaging. With the maturity gained in IC (integrated circuits) fabrication over the past many years, the packaging of ICs also has gained great maturity and sophistication. The same cannot be said about MEMS packaging. Although some of the advancements of IC packaging can be applied to meet the requirements of MEMS devices, some specialized techniques are required for MEMS packaging. Packaging of MEMS devices is much more complex and expensive than conventional IC packaging. This is because MEMS devices usually consist of three dimensional structures with free movement. This leads to the requirement of encapsulated cavities. Microsystem packaging also involves, bonding, interconnecting, and assembly of micro scale component to form a microsystem product. Packaging is the last and crucial step in the lifecycle of MEMS devices and may cost anywhere between 20–90% of the total device cost. Important functions of packaging are

• Environment invulnerability against temperature, electromagnetic

• Assimilation of multiple chips to form a multifunctional system

In the case of MEMS devices the requirement of hermetic sealing may vary from device to device since some of the MEMS devices need an exposure to the environment in which they work and some other devices do not. It is also necessary to note that the packaging needs are special and case specific due to the micro mechanical structures. MEMS packaging involves key design and packaging considerations such as wafer thickness, wafer dicing, thermal issues, stress effects, isolation, protective

The packaging for RF MEMS devices has to meet more stringent specifications due to the high frequency range of interest. Also, the demand is for high performance, low cost strategies which is usually a challenge. Furthermore, apart from the general MEMS packaging issues, the packaging of RF-MEMS devices has the

1.Hermiticity of the packages should be ensured to provide high reliability RF MEMS devices since their operation depends on the ambient conditions under

2. Interconnects, package substrates and passivation layers through the package

The packaging of RF MEMS devices can be classified into two broad categories, one, wafer level packaging and the other, die level packaging. This work focuses on

to the outside world should offer low loss and low intermodulation.

3.Footprint of the total packaged device must be small, keeping with the requirement of miniaturization and high component densities especially for

die level packaging hence the following paragraphs will focus on this.

satellite and wireless communication systems.

listed below:

• Mechanical reinforcement and ruggedness

aberrations, chemical reactions

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

• Interfacing with outside world

• Hermetic sealing

coatings and hermetic sealing.

which they perform.

following concerns.

**265**

Critical point drying.

There was the requirement of a drying technique wherein surface tension could be reduced to zero and a continuity of state of the liquid could be obtained. It was found that if the temperature of the liquefied gas is increased the resulting pattern of the meniscus is flat indicating a reduction in surface tension. This results a very low surface area of the liquid which in turns leads to the evaporation of the liquid. This is called the critical point of the liquid. The critical phenomena can be utilized as a drying technique as it achieves a phase change from liquid to dry gas without the effects of surface tension and is therefore suitable for delicate biological specimens. MEMS devices. Of all the gases that were tested for the critical point, Carbon Dioxide (CO2) remains the most common medium for the CPD procedure and is termed the 'Transitional Fluid'. However, CO2 is not miscible with water and therefore water has to be replaced in the specimen with another fluid which is miscible with CO2, this is termed the 'Intermediate Fluid'. IPA is solvable in CO2 and hence most of the MEMS devices are place in this liquid for CPD process.

The critical point dryer used in this work was the Tousimis Samdri® line of Supercritical Point Drying machine as shown in **Figure 14**. The wafer after the Piranha dip was placed with great care in a petri dish containing IPA. This was then carefully transferred to the CPD equipment. Once the release cycle was finished, the Switches were inspected under a microscope and then using Scanning Electron Microscope (SEM) and were found to be free of residual stress on the top beam. Also, the gap between the top membrane and the bottom electrode was clearly visible without any PR residues.
