**1.2 Packaging of RF MEMS shunt capacitive switches**

The main objectives of packaging of MEMS devices are to protect the actual functioning of the device from external environmental influences like chemicals, temperature, electromagnetic influences. The packaging forms a foundation on which the actual device is mounted thus giving much needed mechanical support. Packaging also helps in routing of interconnections of the chip with the outside world.

**Figure 14.** *Tousimis Samdri critical point dryer at CeNSE, IISc, Bangalore.*

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

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

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

substrate.

Critical point drying.

*Nanofibers - Synthesis, Properties and Applications*

visible without any PR residues.

world.

**Figure 14.**

**264**

**1.2 Packaging of RF MEMS shunt capacitive switches**

*Tousimis Samdri critical point dryer at CeNSE, IISc, Bangalore.*

The main objectives of packaging of MEMS devices are to protect the actual functioning of the device from external environmental influences like chemicals, temperature, electromagnetic influences. The packaging forms a foundation on which the actual device is mounted thus giving much needed mechanical support. Packaging also helps in routing of interconnections of the chip with the outside

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 listed below:


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 coatings and hermetic sealing.

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 following concerns.


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

#### *1.2.1 Die level packaging*

This is a type of packaging used for low volume requirements. Die level packaging is also called 1-level of packaging. The 1-level package usually consists of a prefabricated metal can/ceramic/plastic package with leads for connecting to the outside circuits or systems. These packages come with the base as well as the lid. For both ceramic as well as metal packages the cavity formation in the base of the package is an established method. The MEMS chip is attached to the base package using low temperature solder based epoxies and baked for removal of gaseous by products of the solder or epoxy. The next step involves the placement of the top cover over the base package in a vacuum or nitrogen atmosphere. Next, hermetic sealing is done along the package rim which is performed using localized heating.

• Die bonding to package base

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

Wafer dicing is the process by which the individual unit of dies are separated from the wafer. This process may be carried out by using mechanical sawing, scribing, breaking or laser cutting. Of the several issues and challenges of RF MEMS packaging, dicing is one of the foremost challenges. In the case of ICs, the resultant contaminants or debris due to the dicing process, on the surface of the die can be easily removed by a post-dicing cleaning process, however, in the case of MEMS devices the fragile mechanical structures on the die may get damaged by these contaminants. Dicing methods such as mechanical sawing, scribing and breaking lead to debris from the dicing process which may scatter on to the die leading to buckling or breaking of the delicate MEMS structures. Therefore, the choice of the

In order to obtain least residues from the dicing process, Chicago Laser System (CLS 960) Neodymium-doped Yttrium Aluminum Garnet (Nd:YAG) laser has been employed. This is shown in **Figure 16**. Nd:YAG lasers are one of the most common types of laser used in cutting and welding steel, semiconductors and various alloys. These lasers typically emit light with a wavelength of 1064 nm, in the infrared.

dicing process is of utmost importance in the case of RF MEMS devices.

• Wire bonding

**2. Dicing of wafer**

**Figure 16.**

**Figure 17.**

**267**

*Chicago laser system (CLS 960) laser dicer.*

*Diced chips as seen under a microscope.*

• Hermetic sealing

This method of packaging is expensive and is suitable for telecommunication base stations, satellites and defense systems but not for high volume applications like mobile phone handsets. Furthermore, the additional costs are mainly due to the great care with which the MEMS chips are to be handled after their release. Furthermore, standard scribing procedures cannot be used for dicing the wafer into chips since there is a high possibility of introduction of contaminants on wafer surface. These contaminants cannot be removed by mere cleaning. This cleaning will furthermore require a critical point drying for every chip which would further escalate the costs. A generic 1-level packaging is as shown in **Figure 15**.

In this thesis the focus is on die level packaging using available surface mount style RF packages. However, the whole packaging process is performed under low temperature in order to free the MEMS structures of thermally induced stress which otherwise would affect the performance of the switch. The details of the packaging process starting from the design of RF feed throughs on the Alumina substrates to the die attachment, wire bonding and hermetic sealing are discussed in details in the following sections.

#### *1.2.2 Packaging of RF MEMS shunt capacitive switches*

The packaging of RF MEMS switches involves the following steps:


**Figure 15.** *Simplified one level RF MEMS packaging flow.*

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


*1.2.1 Die level packaging*

*Nanofibers - Synthesis, Properties and Applications*

following sections.

• Dicing of wafer

**Figure 15.**

**266**

*Simplified one level RF MEMS packaging flow.*

This is a type of packaging used for low volume requirements. Die level packaging is also called 1-level of packaging. The 1-level package usually consists of a prefabricated metal can/ceramic/plastic package with leads for connecting to the outside circuits or systems. These packages come with the base as well as the lid. For both ceramic as well as metal packages the cavity formation in the base of the package is an established method. The MEMS chip is attached to the base package using low temperature solder based epoxies and baked for removal of gaseous by products of the solder or epoxy. The next step involves the placement of the top cover over the base package in a vacuum or nitrogen atmosphere. Next, hermetic sealing is done along the package rim which is performed using localized heating. This method of packaging is expensive and is suitable for telecommunication base stations, satellites and defense systems but not for high volume applications like mobile phone handsets. Furthermore, the additional costs are mainly due to the great care with which the MEMS chips are to be handled after their release. Furthermore, standard scribing procedures cannot be used for dicing the wafer into chips since there is a high possibility of introduction of contaminants on wafer surface. These contaminants cannot be removed by mere cleaning. This cleaning will furthermore require a critical point drying for every chip which would further

escalate the costs. A generic 1-level packaging is as shown in **Figure 15**.

The packaging of RF MEMS switches involves the following steps:

*1.2.2 Packaging of RF MEMS shunt capacitive switches*

• Design of RF feed throughs on Alumina substrate

• Attachment of the base package to Alumina substrate

In this thesis the focus is on die level packaging using available surface mount style RF packages. However, the whole packaging process is performed under low temperature in order to free the MEMS structures of thermally induced stress which otherwise would affect the performance of the switch. The details of the packaging process starting from the design of RF feed throughs on the Alumina substrates to the die attachment, wire bonding and hermetic sealing are discussed in details in the • Hermetic sealing
