**3. Design of RF feed throughs on Alumina substrate**

There are several choices of substrates for packaging like Quartz, Silicon, Aluminum nitride (AlN) and Alumina (Al2O3) to name a few. Ceramic substrates such as Aluminum Nitride and Alumina are most commonly used packaging materials for MEMS. Alumina is the primary choice because it combines economic, physical and electrical advantages [20]. Also, Alumina is readily available in sizes that range from tiny chips to large ceramics in thicknesses from 0.25 mm to 1.5 mm and in a variety of shapes and designs. The finished substrate can be drilled or cut with diamond tools and lasers.

aged adhesion on substrates like Alumina and it's comparatively low cost. The three steps involved in the formation of CPW lines on the Alumina substrate is as given

i. **Scribing:** Scribing is basically designing on the substrate using laser. The blank substrate is first divided into a number of regions by scribing. The laser used in this process is a combination of Nitrogen, Carbon Dioxide (CO2) and helium gases. At higher temperatures, the valence electrons

4.Small thermal influence by optimized uv treatment

After the scribing process the plates are subjected to the de-burring process. De-burring is done to remove the ceramic particles that accumulate on the surface due to laser penetration. De-burring is done using another ceramic plate. The plates are cleaned in de-ionized water and then dried in an oven

ii. **Screen-Making:** This is the preliminary process for printing. Here, the stainless steel mesh is first stretched with hydraulic force. The frames are then attached to the mesh. The chromo difloro film, is first stuck on to the wet screen and dried in the oven after which they are exposed to UV light with the respective photo film layer. The film is developed using water. After screen making process is over, printing is performed on the Alumina

below.

**Figure 19.**

• Scribing

• Screen-Making

• Printing, Drying and Firing

combine to produce laser light.

*CPW layout on alumina substrate. (1) Front view. (2) Back view.*

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

1.High edging steepness

2.Small edge roughness

3.No micro cracks

at 120°C.

substrate.

**269**

The advantages of laser scribing are

5.Contact free material processing

6.High precision and positional accuracy.

Some of the key properties of Alumina are as given below:


Having chosen Alumina as the substrate material, the RF feedthroughs on the Alumina substrate had to be designed. The generic CPW line is as shown in **Figure 18**. The design of the CPW lines on the Alumina substrate was based on many parameters such as trace (S) and ground line (G) lengths, permittivity of Alumina (€r), material properties of conductor and the operating frequency. The designed layout for CPW lines on the Alumina substrate is as shown in **Figure 19**.

Silver Palladium paste (7474 Ag/Pd) is used to form the CPW conductors on the Alumina substrate. This paste is chosen for its excellent solderability and excellent

**Figure 18.** *Schematic of CPW line.*

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

#### **Figure 19.**

The wafer to be diced was mounted on the dicing platform with the alignment set. The wafer is then diced into unit chips or dies with high precision. The unit dies obtained were observed under a microscope. It was visually confirmed that the RF MEMS switches were undamaged. The samples of diced chip are as shown in

There are several choices of substrates for packaging like Quartz, Silicon, Aluminum nitride (AlN) and Alumina (Al2O3) to name a few. Ceramic substrates such as Aluminum Nitride and Alumina are most commonly used packaging materials for MEMS. Alumina is the primary choice because it combines economic, physical and electrical advantages [20]. Also, Alumina is readily available in sizes that range from tiny chips to large ceramics in thicknesses from 0.25 mm to 1.5 mm and in a variety of shapes and designs. The finished substrate can be drilled or cut with

**3. Design of RF feed throughs on Alumina substrate**

*Nanofibers - Synthesis, Properties and Applications*

Some of the key properties of Alumina are as given below:

• Resistance to strong acid and alkali attack at high temperatures

• Excellent dielectric properties from DC to GHz frequencies

Having chosen Alumina as the substrate material, the RF feedthroughs on the

Alumina substrate had to be designed. The generic CPW line is as shown in **Figure 18**. The design of the CPW lines on the Alumina substrate was based on many parameters such as trace (S) and ground line (G) lengths, permittivity of Alumina (€r), material properties of conductor and the operating frequency. The designed layout for CPW lines on the Alumina substrate is as shown in **Figure 19**. Silver Palladium paste (7474 Ag/Pd) is used to form the CPW conductors on the Alumina substrate. This paste is chosen for its excellent solderability and excellent

• compatibility with thick film resistors and dielectrics

• Excellent adhesion with thick film conductors

**Figure 17**.

**Figure 18.**

**268**

*Schematic of CPW line.*

diamond tools and lasers.

• Good thermal conductivity

• High strength and stiffness

• Excellent size and shape capability

*CPW layout on alumina substrate. (1) Front view. (2) Back view.*

aged adhesion on substrates like Alumina and it's comparatively low cost. The three steps involved in the formation of CPW lines on the Alumina substrate is as given below.

	- i. **Scribing:** Scribing is basically designing on the substrate using laser. The blank substrate is first divided into a number of regions by scribing. The laser used in this process is a combination of Nitrogen, Carbon Dioxide (CO2) and helium gases. At higher temperatures, the valence electrons combine to produce laser light.

The advantages of laser scribing are

1.High edging steepness


After the scribing process the plates are subjected to the de-burring process. De-burring is done to remove the ceramic particles that accumulate on the surface due to laser penetration. De-burring is done using another ceramic plate. The plates are cleaned in de-ionized water and then dried in an oven at 120°C.

ii. **Screen-Making:** This is the preliminary process for printing. Here, the stainless steel mesh is first stretched with hydraulic force. The frames are then attached to the mesh. The chromo difloro film, is first stuck on to the wet screen and dried in the oven after which they are exposed to UV light with the respective photo film layer. The film is developed using water. After screen making process is over, printing is performed on the Alumina substrate.

These are the following precautions to be observed in screening:

diced MEMS chip/die since the die contains fragile mechanical structures. The dies have to be picked from the wafer either using manual methods or by automatized grippers. They have to be then placed on the base package cavity. The choice of die bonding process depends upon package sealing strategy, operating conditions and environmental and reliability requirements. The die attach can be achieved through

Eutectic bonding uses a die bonding technique with an intermediate metal layer (Au/Al) which would result in a eutectic system. The most important feature of this type of bonding is that the eutectic temperature can be much lower than the melting temperature of individual elements. Solder Attach is the most preferred type of die bonding since the solder provides for good thermal conductivity. But this type of die bond would lead to large amount of heat generation during the attachment process which may lead to a large thermal stress on the mechanical structure in the

Epoxy bonding is achieved by attaching die to the substrate by using epoxy glue. A drop of the epoxy is first dispensed on the substrate and the die is placed on it. In order to cure the epoxy the substrate or package may need to be heated. Most commonly used adhesives are polyimide, epoxy and silver filled glass. Epoxy bonding has the following important features such as low curing temperature, used for wide range of die sizes and can be reworked easily [21, 22]. Epoxy is used for die

The bare die is attached to the Base Package using non- conductive epoxy (H74 epoxy) and curing at room temperature of 25°C for 48 hours, keeping in mind the low temperature requirement for packaging in this work [23]. H74 epoxy is a thermally conductive epoxy designed for hybrid circuit assembly including die attach. The outstanding feature of this epoxy is that its curing process is fast even at low temperatures and also has a built in color change when the adhesive is cured. The adhesion of the dies is good and is confirmed by the non-destructive pull test (NDPT) and die shear test. It is passing the NDPT of greater than 16 grams and the die shear strength is greater than 6.55 kgs. The tested samples are as shown in the

the following bonding methods:

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

3.epoxies, silver filled glass or polymide

1.eutectic bonding

2. solder attach

case of a MEMS device.

attachment in this work.

**Figure 21**.

**Figure 21.** *Die shear test.*

**271**


#### **4. Attachment of base package to alumina substrate**

Surface mount packages are used for packaging the diced chips. The current packaging methodology proposes the use of surface mounted plastic packages supplied by Elecsys technologies, USA. These packages are suitable for DC to 18 GHz range which is also the frequency of interest of the RF MEMS switches. These packages also have their leads to be co-planar compatible. These packages have a conductive metal base attached to an Alumina ceramic ring frame with a cup shaped lid with a b-stage epoxy preform for sealing. **Figure 20** shows the layout details of the SMX series packgae used for packaging the RF MEMS shunt switches. **Figure 20(a)** shows the base of the package with leads made of Copper and Gold. **Figure 20(b)**shows the top view of the base package showing the jutting leads.

The attachment of Base Package to the substrate is achieved by using a nonconductive epoxy named 8700 K. This epoxy is a high thermal conductivity, low temperature curing, microelectronics grade adhesive. It is then cured at 150°C for 2 hours. In order to connect the base package pins to the CPW conductor lines, a conductive epoxy 84–1 LMINB1 is used. This conductive epoxy used, is a high purity silver filled die attach adhesive ideal for application by automatic dispenser.

**Figure 20.** *Layout details of the SMX series package. (a) inside view (b) top view.*

#### **5. Die bonding**

Die bonding or die attach is one of the most crucial steps in the packaging process especially in the case of MEMS devices. This requires careful handling of the

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

diced MEMS chip/die since the die contains fragile mechanical structures. The dies have to be picked from the wafer either using manual methods or by automatized grippers. They have to be then placed on the base package cavity. The choice of die bonding process depends upon package sealing strategy, operating conditions and environmental and reliability requirements. The die attach can be achieved through the following bonding methods:


These are the following precautions to be observed in screening:

2.The screen tension should be within the specification of

iii. **Printing:** In this process, the conductors are printed on the substrate. The conductor paste is (7474 –Palladium Silver) is screen printed. It is then dried at 150o C for 15–20 minutes in order to remove the solvents and then fired at 850<sup>o</sup> C in a fast firing furnace. At this temperature sintering takes place with a dwell time of 10 min and then ramp down takes place after which the paste starts behaving like a conductor. At the end of this process

Surface mount packages are used for packaging the diced chips. The current packaging methodology proposes the use of surface mounted plastic packages supplied by Elecsys technologies, USA. These packages are suitable for DC to 18 GHz range which is also the frequency of interest of the RF MEMS switches. These packages also have their leads to be co-planar compatible. These packages have a conductive metal base attached to an Alumina ceramic ring frame with a cup shaped lid with a b-stage epoxy preform for sealing. **Figure 20** shows the layout details of the SMX series packgae used for packaging the RF MEMS shunt switches. **Figure 20(a)** shows the base of the package with leads made of Copper and Gold. **Figure 20(b)**shows the top view of the base package showing the jutting leads. The attachment of Base Package to the substrate is achieved by using a nonconductive epoxy named 8700 K. This epoxy is a high thermal conductivity, low temperature curing, microelectronics grade adhesive. It is then cured at 150°C for 2 hours. In order to connect the base package pins to the CPW conductor lines, a conductive epoxy 84–1 LMINB1 is used. This conductive epoxy used, is a high purity silver filled die attach adhesive ideal for application by automatic dispenser.

Die bonding or die attach is one of the most crucial steps in the packaging process especially in the case of MEMS devices. This requires careful handling of the

*Layout details of the SMX series package. (a) inside view (b) top view.*

1.The screen should be free of foreign particles.

CPW lines on the Alumina substrate were formed.

**4. Attachment of base package to alumina substrate**

workmanship.

*Nanofibers - Synthesis, Properties and Applications*

**5. Die bonding**

**Figure 20.**

**270**

3.epoxies, silver filled glass or polymide

Eutectic bonding uses a die bonding technique with an intermediate metal layer (Au/Al) which would result in a eutectic system. The most important feature of this type of bonding is that the eutectic temperature can be much lower than the melting temperature of individual elements. Solder Attach is the most preferred type of die bonding since the solder provides for good thermal conductivity. But this type of die bond would lead to large amount of heat generation during the attachment process which may lead to a large thermal stress on the mechanical structure in the case of a MEMS device.

Epoxy bonding is achieved by attaching die to the substrate by using epoxy glue. A drop of the epoxy is first dispensed on the substrate and the die is placed on it. In order to cure the epoxy the substrate or package may need to be heated. Most commonly used adhesives are polyimide, epoxy and silver filled glass. Epoxy bonding has the following important features such as low curing temperature, used for wide range of die sizes and can be reworked easily [21, 22]. Epoxy is used for die attachment in this work.

The bare die is attached to the Base Package using non- conductive epoxy (H74 epoxy) and curing at room temperature of 25°C for 48 hours, keeping in mind the low temperature requirement for packaging in this work [23]. H74 epoxy is a thermally conductive epoxy designed for hybrid circuit assembly including die attach. The outstanding feature of this epoxy is that its curing process is fast even at low temperatures and also has a built in color change when the adhesive is cured. The adhesion of the dies is good and is confirmed by the non-destructive pull test (NDPT) and die shear test. It is passing the NDPT of greater than 16 grams and the die shear strength is greater than 6.55 kgs. The tested samples are as shown in the **Figure 21**.

**Figure 21.** *Die shear test.*
