VARIOUS PACKAGING CHALLENGES AND

SOLUTIONS6.1: Introduction

Micro electro mechanical systems (MEMS) is an exciting field which integrates

concepts of physics, electrical engineering, mechanical engineering, biomedical,

electronics, chemical and material science to create unique low cost and miniature

devices for a variety of applications. Over the past few years RF-MEMS fabricated using

semiconductor, micro-fabrication technology have gained significant interest for wireless

communication applications owing to their small size, integration capability and superior

performance. For instance, using RF-MEMS switches, RF circuits such as variable

capacitors, tunable filters, phase shifters and signal routers have been demonstrated.

RF-MEMS packaging is a promising field with diverse methods being explored to

balance the technological and cost constraints. The packaging cost and expenditure

involved in assembling the final product must be acceptable. Additionally, it must protect

the MEMS during the product life and not degrade its RF performance and should have

low parasitics like resistance, capacitance, or inductance. Integration of RF-MEMS is

another aspect that lowers the number of components needed for an application.

MEMS packages must have the ability to meet at least one or more of the

following criteria:

Isolate non-sensing areas from the sensing areas, often in harsh, corrosive,

or mechanically demanding environments;

Not impede mechanical action, such as tilting, twisting, rotating, sliding,

or vibrating;

Allow the transfer of fluids from one region to another.

Due to many applications, device structures, and needs for MEMS, many

packaging techniques have to be developed to meet the needs of the applications.

Packaging of MEMS is different than packaging of ICs because MEMS need direct

interaction with their environment. So, MEMS cannot be physically isolated from the

environment, it needs selective access too. This creates a problem for the package as it

has to provide simultaneously both the protection from and interaction with the

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environment. Unlike IC die packaging, MEMS dice need to interface with the

environment for sensing, interconnection, and actuation.

MEMS packaging is application specific and the package allows the physical

interface of the MEMS device to the environment. Harsh environments may create

different challenges for the packaging of MEMS. In addition to challenges related to the

environment of the MEMS chip and interfacing it with the environment, challenges also

exist inside the MEMS package with the die handling, die attach, interfacial stress, and

outgassing.

Here, RF-MEMS packaging challenges are described with some solutions and

suggestions. The motivation for overcoming the challenges is the low cost and the

ubiquitous applications of MEMS. MEMS packaging is by now far behind the

capabilities of MEMS designers, and it is the purpose of this chapter to allocate the

challenges of MEMS packaging and create knowledge and a curiosity of MEMS

packaging in the packaging society.

6.2: RF-MEMS Product Integration

An RF-MEMS product is made of three main cost components that are the

MEMS device, the control ASIC and the package. RF function is provided with the

MEMS device such as an ohmic switch, capacitive switch, varactor, resonator, tunable

FBAR, or multi-element circuits. The MEMS devices are incorporated with the ASIC on

the same wafer by an integrated approach or by a hybrid approach.

In the integrated approach, the MEMS and ASIC process can be monolithically

integrated by three basic scenarios: MEMS First, MEMS Last, or embedded MEMS. In

contrast, for the hybrid approach, the MEMS device is fabricated using a MEMS specific

process. Most RF-MEMS devices, such as a switch, varactor, or resonator, are active

elements that respond mechanically to some electrical input signal.

Passive components (like capacitors, resistors, inductors, etc.) are integrated with

active RF electronics in GaAs, SiGe, SOS, or other non- CMOS process. With

appropriate modifications to the materials and other considerations, low contact

resistance switches may be integrated with a CMOS process, or the matter can be

prevented or delayed by subsequent hybrid integration approach.

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6.3: Challenges in integration of RF-MEMS:

Obviously, the goal of IC packaging is to provide physical support and electrical

interface for the chip(s), supply signal, power, and ground interconnections, and allow

heat dissipation; to isolate the chip physically from its environment. But MEMS devices

have to interface with their environment. Another issue is the media compatibility of the

MEMS package. MEMS devices may need to operate in diverse environments like under

automobile hoods, intense vibrations, in salt water, strong acids or other chemicals.

Table 6.1: Current packaging parameters, challenges, and solutions

Packaging

Parameters

Challenges Solutions to these challenges and parameters

Release and

Stiction

Stiction of devices Freeze drying, supercritical CO, drying, roughening

of contact surfaces, non-stick coatings

Dicing Contamination risks Release dice after dicing, flush chip surface to

remove contaminants

Die handling Device failure, top die face is

very sensitive to contact

Fixtures that hold MEMS dice by sides rather than

top face

Stress Performance degradation and

resonant frequency shifts

Low modulus die attach, annealing, compatible

CTE match-ups

Outgassing Stiction, corrosion Low outgassing epoxies, cyanate esters, low

modulus solders, new die attach materials, removal

of outgassing vapors

Testing Possible device failure after

many assembly steps

Post release, pre-package, and/or post package

testing

The package, during operation, must be able to withstand the environment(s).

Another challenge in MEMS packaging is the effect on reliability that packaging

parameters induce. The package is part of the complete system and should be designed as

the MEMS chip is designed, according to the specific application. The chip, package, and

environment all must function together and all must be well-matched.

The materials and design considerations and limitations remain the main issue.

The main scientific challenge is material characteristics. The material properties vary

with their use, processing, and the heat treatments given to the materials. Not all the

materials used, react the same to these parameters, so compromises must be made. Some

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materials may be hard to obtain with R&D production. Low quantities of materials are

used, and suppliers are hesitant to trade small quantities or build up novel few products.

However, the material properties generally get better at the microscale due to a

decrease in the number of defects in the materials. The defect density remains the same as

in macroscale devices. Packaging of MEMS dice is application specific and hence,

desired process steps could vary significantly. Thus, it is important to classify MEMS

dice from the packaging requirements and develop the packaging standards and

concerned knowledge.

Table 6.1 summarizes the current packaging parameters, challenges, and solutions

that can be exercised to minimize or eradicate the different challenges of packaging.

These solutions are incomplete, but are methods that have been exercised and are

guidelines for further research and expansion of RF-MEMS technology. The packaging

parameters and challenges are briefly described below.

6.3.1: Release and Stiction

An important step in MEMS packaging is the release of the MEMS dice. The

polysilicon features are bounded by silicon dioxide that saves the features and stops them

from damaging and contaminating. This oxide must be etched away for the liberation of

the structures. This is done with an HF etch, which is choosy between SiO2 and silicon. It

is done just in a simple and economical way in wafer with a batch process, but this causes

grim impurity risks and harm during the dicing of the released wafer.

The most difficult time is after dicing. Each chip must be released independently

instead of the full wafer in a single time. The MEMS parameters are guarded right

through the potentially fatal dicing phase. In addition, there is also a risk of stiction

during and after the release. Stiction occurs from the capillary action of the evaporated

rinse solution in the crevices between elements like cantilevers and the substrate. This

stiction may cause the MEMS devices of no use after all the assets have by now been

spent in them. Figure 6.1 shows an example of the stiction. The capillary forces have

pulled down the beam-type structure.

Stiction taking place after release is a key issue to be dealt with. It should be

avoided also. Some effective approaches are freeze drying and supercritical CO2 drying.

These methods get rid of the liquid surface tension from the drying process avoiding

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stiction from happening. This, however, does not stop stiction throughout the lifespan of

the device. For preventing stiction all the way through the lifetime of the device, the

surface can be roughened to minimize contact area or non-stick coatings can be employed

to the device surfaces.

Figure 6.1: Beam-type structure being pulled down by the capillary forces

Stiction can also be decreased during the process of designing by employing

dimples in locations of the device that are prone to the stiction. These small protrusions

on the bottom on a structure can very much diminish the contact area between the MEMS

device structure and the substrate. A cantilever beam with dimples on the bottom surface

is illustrated in the figure 6.2.

Figure 6.2: An illustration of dimples used for avoiding stiction.

6.3.2: Wafer Dicing

The dicing of the wafer into the individual dice is another challenge in packaging

of the RF-MEMS. The dicing is characteristically finished with a diamond saw of

thickness about a few mils. This requires the coolant to flow over the surface of the very

sensitive dice along with silicon and diamond particles that are formed during sawing

process. These particles combined with the coolant can contaminate the devices and get

into the crevices of the features resulting in the failure of the devices. A substitute to

wafer dicing is wafer cleaving. Wafer cleaving is frequently done in III-V semiconductor

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lasers and has applications in MEMS. There is no requirement of any coolant in the wafer

cleaving and there is not production of near as many particles as sawing also.

6.3.3: Die Handling

Another area of MEMS system fabrication and assembly is die handling that is at

present not gathering the MEMS dice needs. Because of the fragile surface features of

MEMS, these chips cannot be displaced by employing vacuum pick-up heads, as in

customary IC die assembly. The MEMS dice must be picked up and handled by the edges

that will need novel infrastructure for the automatic handling of MEMS. These new edge

handling techniques would likely be restricted to MEMS since present vacuum pick-ups

will continue to labor for ICs very well.

The MEMS die handling could be performed with the help of fingers or clamps

that carefully handle these MEMS dice by their edges, or collets that fit present pick-and-

place equipment. For handling the MEMS chips, die handling tools and methods that

handle the chips by the edges will become common in intermediate to high volume

MEMS packaging houses. In order to handle chips by the edges is not easier than by the

top surface on account of very much reduced surface area and increased dexterous

requirements of the pick-and-place tool.

Wafer level encapsulation eradicates the need for particular die handling fixtures.

Here, a capping wafer is bonded to the top of a device wafer. These wafers can be bonded

in a vacuum to create a permanent vacuum inside each device chip. These wafers can be

bonded using direct bonding, but the required temperature is about 1000° C. Glass frit or

anodic bonding is more regularly used since the processing temperature is between 450

and 500° C. However, the glass frit may cause stress in the die if the glass is not chosen

with a CTE close to that of silicon.

6.3.4: Stress

Plenty of stress is created in the films, when polysilicon is deposited. This stress

can be annealed out at a temperature of about 1000°C. This is most efficient if the

polysilicon is deposited amorphously and then annealed to shape a polycrystalline

structure. This creates the lowest stress arrangements with the least defects in polysilicon.

The second sources of stress results from the die attach material at the interface between

the MEMS die and the package substrate.

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Depending upon the die-attach-material and coefficient of thermal expansion

(CTE) mismatch between the package and the chip, interfacial stress can take place inside

a MEMS package. Reliability is much reduced due to excess stress in the package. This

stress can be caused by stress inducing fabrication processes, CTE mismatch in the die

attach, lid sealing, or shrinkage. The stress may damage the devices; may misalign the

gear teeth, tensile stress may cause the resonant frequency to increase and can also lead to

device fracture, and extra compressive stress results in long beam structures to buckle.

The solders like AuSn or AuSi may put too much stress on the structures and

cause warp or failure the features as well as the die itself. Stress effects also deteriorate as

chip sizes increase. The stress can be minimized by employing smaller modulus die

attach materials that deform with the expansion and contraction of the chip and package.

These low modulus die attach materials may also allow creep over time. Stress relaxation

can be very bad in die attach materials because a change in the stress state will lead to

changes in device performance.

6.3.5: Outgassing

The die attach compounds outgas as they cure when epoxies or cyanate esters are

used. These moisture and organic vapors redeposit on the features, in crevices, and on

bond pads. This results in to stiction of the structure. This also leads to corrosion of the

device. Die attach materials with a low Young’s Modulus, like epoxies; also permit the

chip to shift during ultrasonic wire bonding, causing low bond strength. Solutions to

outgassing challenges are die attach materials with very low outgassing and the removal

of outgassing vapors during curing of the die attach.

6.3.6: Testing

Another significant problem is testing of these devices. With almost every step of

the assembly potentially lethal to the MEMS chips, the chips can fail at any time during

the processing. No one wishes to package an awful chip, it is too costly and too time

consuming. However, no one can have enough money to test the chips after every

assembly step. The packaging engineer must decide when and if any testing of the

devices during packaging would be suitable based on cost and yield figures of the device.

A final test of a completely packaged device may signify that in spite of all earlier tests

and care taken, the part failed during lid seal that makes all earlier efforts to nil.

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6.4: Key Points for the Success of MEMS Packaging

RF-MEMS devices like switches tunable capacitors, varactors, mechanical

resonators and filters contain movable and fragile parts that must be housed for reasons of

protection (such as handling wafer dicing or plastic molding operations) and to ensure

stable and reliable performance characteristics. Due to low mass, low power

consumptions small volume and possible integration with control and sense electronics,

MEMS seem ideal for various applications in different broadband, radio frequency,

wireless systems etc. By shrinking traditional components to the micro-scale, advantages

of MEMS to cost, power and performance can be realized. However, because these

devices are all mechanical in nature and the forces involved are small reliability becomes

a key concern in how quickly the technology can be implemented.

Success of MEMS packaging depends on many factors such as temperature,

contamination, process compatibility, and the use of wafer level encapsulation. Wafer

level encapsulation is part of the product partition from the packaging perspective. We

must consider both 0-level and 1-level packaging, focusing on the technological

capabilities, and product requirements including cost etc. 0-level packaging is defined as

a protective structure that is added to the MEMS devices at the wafer level (wafer level is

favored due to cost and process repeatability).

0- level packaging also simplifies back-end processing and reduces final

packaging costs. This was gifted by assembly of the MEMS die into a 1-level package,

which provided the ultimate environmental protection and electrical connection to higher

level assemblies and systems. This 1-level packaging was more costly because it tries to

satisfy the hermetic environmental needs for the final MEMS product. The assembly

needs for 1-level package requires the MEMS device wafer to be sawn into individual die

that are automatically picked and placed into packages.

The package and die attach must be well-matched with the sacrificial release

process. The benefit of this is that the MEMS device integrity is kept during the assembly

process so that contamination can be reduced during processing, or handling. The batch

processing and compatibility of the packaging materials with the release process is

limited.

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Releasing devices before they are diced or singulated into individual die is also a

solution. Released devices are singulated by the scribe-and-break process that has

drawbacks of location control and the formation of particles. Another solution is coat-

saw-strip process. In it, the wafer is coated with a spin-on organic material (photoresist,

polyimide) after release. The wafers are then sawn and cleaned. Following the dicing

process, the coating material is removed. In a similar manner to the package release,

before stripping and cleaning of the protective material, the die can be assembled into the

packages.

Also, the die can be cleaned in a batch process on dicing tape employing a

stripping process that is compatible with the dicing tape. Depending on the protection

material, the strip and clean process can be a wet or dry process. If it is a wet process,

stiction needs to be avoided. The advantage of this process is that the MEMS die can be

yielded and tested separately before assembly. A key disadvantage is that the die is

released twice and hence, prone to contamination and damage.

An option to coat-saw-strip for sawing released devices is to protect them with a

special dicing tape that has a recess for the devices so that the dicing tape surrounds the

device. The wafer can be sawn from the backside. Ultimately, the die can be pick-and-

placed directly from the tape into the packages for final assembly. Main step of the

singulation processes is to guard the MEMS devices during sawing and other secondary

processing.

0-Level packaging prevents many of these issues by protecting the MEMS from

back-end processing and increasing their environmental toughness through the full life of

the product. This is accomplished by wafer level encapsulation and divided into two

broad categories: wafer bonded encapsulation and thin film encapsulation.

Wafer bonded encapsulation requires at least one lid wafer in addition to the

device wafer. The device wafer is made of silicon, glass, GaAs, or ceramic. The

encapsulation wafer is made of silicon, glass, or ceramic and is usually patterned and

etched to form cavities and seal rings. The encapsulation process continues with the

alignment and bonding process that are conducted in specific wafer bonding equipment

now.

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After the release of the device, it is sealed by a seal layer of polymers (polyimide,

BCB), dielectrics (SixNy), or metals. Also, other barrier layers are added to increase the

hermeticity of the encapsulation. The hermeticity must keep a definite environment inside

the cavity and must avoid moisture from entering the cavity. The cavity internal

environment will define the performance and the reliability of the encapsulated devices.

6.5: Typical MEMS Design, and Manufacturing Process

Figure 6.3 illustrates a typical design and manufacturing process for a MEMS

device. During the design solid modeling is required since electro-thermal-mechanical

coupling is essential to the functions of most of MEMS devices. A fabrication often

involves deposition and etching of micron-thick layers with controlled mechanical

electrical properties. Generally, after completion of the fabrication process, the sacrificial

materials are removed by etching in order to release the device for mechanical

movements. This release process is usually the first step in MEMS packaging.

The released device shown in the figure represents a configuration of pressure

sensors of accelerometers or an element of an array for optical micromirrors and RF-

switches. After release the devices can be tested on the refer label, followed by dicing.

The released, diced device is assembled and sealed in a package. These testing dicing,

assembly and sealing step are very challenging. Without proper protection the micro-

scale movable features could be damaged easily during these steps. As a result, it is

always desirable to replace the process illustrated here by wafer-level-packaging.

Hundreds of MEMS devices have been demonstrated and the number of their

applications is growing. For example:

a) Inertial sensors: accelerometers for measuring acceleration for launching air

bags.

b) Pressure sensors: for sensing air pressure and fuel pressure to decrease

emission and fuel consumption; for measuring blood pressure.

c) Optical MEMS: micro-mirror for projection displays; optical switches for

wavelength division multiplex switches;

d) Radio frequency MEMS: micro-resonators for integrated RF transceiver chips;

RF-switches for millimeter-wave systems.

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e) Power MEMS: On-chip power generations and energy storage for portable

systems.

f) MEMS-based data storage: micro-positioning and tracking devices for

magnetic, optical, thermal, or atomic force data tracks; micro-mirrors for

optical beam steering.

g) Micro-surgical instruments; for noninvasive techniques, intra-vascular devices.

Figure 6.3: Block Diagram of MEMS Design, Fabrication and Packaging.

MEMS provide micro-scale components for a number of applications. This

miniaturization will significantly reduce the size, power dissipation and cost for future

compact wireless communication systems. Most of MEMS application listed above are

emerging and will stimulate more novel designs for future engineering and medical

systems.

Packaging is a core technology and critical aspect of realizing most of MEMS

applications. Packaging requirements increases the cost substantially and severely limits

the integration of the device with other components. Some of the MEMS packaging

challenges have been identified and proper solutions have been developed. In the

following section, we will discuss some of them. However, it should be noted that

MEMS packaging challenges are not fully understood and are usually application

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dependent. It is impossible to define common challenges applied to all the MEMS-based

systems. Most discussion should be treated as examples to help us understand and

appreciate these emerging fields.

6.6: Overview of Packaging of MEMS

Packaging of MEMS devices begins after the completion of the micro-fabrication

of the devices. When the fabrication process of MEMS devices is complete, the

sacrificial material is removed to release the devices for mechanical movement. This

release process is usually first step in MEMS packaging. The subsequent processes

involved in packaging may be packaging/ substrate fabrication, assembly, testing and

reliability assurance. However, it is sometimes difficult to distinguish MEMS fabrication

from packaging.

For example, in wafer level packaging, device fabrication and packaging

processes are fully integrated. One of the performance measures of any MEMS device is

reliability. Reliability and also the performance of any MEMS device are strongly

affected by the package. Reliability assurance is considered as a packaging activity since

packaging engineers rather than fabrication engineers usually conduct environmental

process, burn-ins, and accelerated test to ensure the production of a reliable MEMS

device.

MEMS packaging is application specific and the package allows the physical

interface of the MEMS device to the environment. Packaging of MEMS devices is a

complex process as packages serve to protect from the environment, while also enabling

interaction with that environment to measure or affect the desired physical or chemical

properties. In case of fluid mass flow control sensor, the fluid flows in and out of the

package.

This would cross more harsh challenges for MEMS packaging. There is large

verity of basic building blocks in MEMS. Sensors and actuators can be composed of

pyro-electric, resistive, thermo-electric, magnetic, acoustic, chemical, optical elements

etc. There is no standard package process for a particular building block. Although there

is no generic package for MEMS device, the package should:

• Provide protection and be robust enough to withstand its operating environment.

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• Allow for environmental access and connection to physical domain (optical fibers

etc.)

• Minimize electric interference effects from inside and outside the device.

• Dissipate generated heat and withstand high operating temperatures (where

necessary).

• Minimize stress from external loading i.e. provide mechanical support.

• Handle power from electrical leads without signal disruption.

• Serves to integrate all of the components required for a system application in a

manner that minimizes size, cost, mass and complexity.

• Support and protect the device from thermal and mechanical shock, vibration,

high acceleration, particles and other physical damage during storage and

operation of the inner parts.

Packaging is regarded as a very critical block in the successful commercialization

of micro-systems. Packaging has been and continues to a major challenge. The packaging

cost is about 50-90% of the total cost of the MEMS product. The main reasons for their

high costs are the following shown in the Figure 6.4.

Figure 6.4: Percentage cost involved in different processes of MEMS.

6.7: More Packaging Challenges

Microsystems consist of more complex three-dimensional structures. Moving

mechanical parts often requires an encapsulated cavity with a controlled atmosphere

(hermetic package). Each MEMS design is unique and compared to electronic devices

where the packaging is less dependent on the chip functionality, the package has to be

individually developed for each micro-system. However, the electronics manufacturing

industry has robust and viable infrastructure, direct application of electronics packaging

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techniques to most MEMS parts is not feasible due to the complexities of their

operational structure and domain.

For example, packaging should allow some moving parts to interact with other

components through optical, electrical, thermal, mechanical or chemical interfaces. As a

result, many MEMS packaging problems are new to most of the electronic packaging

engineers. For example, vacuum packaging is needed when viscous damping is

important, die-attachment may create severe thermal stresses, thermal strains may affect

the performance of membrane devices, moisture can cause stiction etc. In a national

science foundation (NSF) workshop, several MEMS packaging and reliability challenges

have been identified and discussed. They are summarized as below:

1. Environment

The primary function of packaging is to protect the devices from the environment.

MEMS devices need to operate in diverse environments such as under automobiles

hoods, intense heat, intense vibration and shocks, in salty water, strong acids or other

chemicals, alkaline or organics solutions. The package while performing detection or

actuation must be able to withstand the environments. The protection includes resistance

to chemicals, resistance to ingress of moisture or undesired gases, insulation from heat

sources, and insulation from mechanical loading. However, it is impossible to completely

isolate the packaged devices from external environmental loads.

2. Functional Interfaces

The package provides functional interfaces between the MEMS device and the

environment. These interfaces are directly related to the application. In different

applications a wide range of different functional interfaces are needed like thermal

(radiation, conduction and convection), radio frequency, optical fluids (liquid or gases),

mechanical, magnetic etc. Due to the variety of interfaces, there exists no standard

package. The core issue is to identify and develop standard package for each functional

interface such that cost, performance etc of the product substantially reduce.

3. Reliability

Fracture and fatigue, mechanical wear and tear, due to frequency and humidity,

and shock and vibration effects are the main causes of MEMS failures. During the last 15

years, MEMS reliability has been improved significantly. The package is a part of

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complete system and should be designed as the MEMS chip is designed with specific and

many times custom package. It is necessary for the chip package and environment to

function together and must be compatible with each other. This determines which

materials and what design considerations and limitations become significant.

One of the major challenges is the issue of material properties. The most reliable

MEMS devices are hermetically packaged single-point contact or no-contact devices.

However, hundreds of novel MEMS devices demonstrated in laboratories demand

contacts. For example, surface impact is desirable to achieve a high capacitance ratio, e.g.

Con/Coff. = 100, which is critical to the performance of RF-MEMS. With surface

contact, reliability with trillions of impacting or rubbing cycle is needed.

4. Modeling

The package is usually an integral part of the device. Both the device and the

package have to be designed at the same time. In order to have a one pass design,

physical and semi-empirical models have to be developed. With such diverse

applications, a MEMS computer aided design (CAD) tool needs to cover every

engineering discipline: electrical, thermal, mechanical, optical, electromagnetic wave and

chemical. The integration of all the existing tools with innovative interface solutions will

be challenging.

5. Integration

MEMS packaging and reliability are strongly related to the application. In

addition, packaging and reliability are strongly related to the device fabrication. For each

MEMS product, there is always a critical integration issue to be considered: where and

how the fabrication and the packaging processes are integrated. Such an integration

consideration also provides us an opportunity to create new concepts or technologies for

low-cost, high-performance MEMS. For example, wafer level packaging can be

completed in the same fabrication facility, which may eliminate a packaging step. Such a

packaging approach will result in low cost and compact MEMS and is the main

development target for most of the MEMS packages being manufactured today.

On the other hand, packaging technologies can be used to fabricate MEMS

devices and to eliminate another packaging step. Flexible circuit board technologies have

been used to develop paper movers and RF-MEMS switch. Co-fired ceramics

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technologies are very popular in the development of micro chemical plants and high

temperature MEMS. In addition, fabrication and packaging technologies can be

integrated to form new MEMS. For example, solder technologies have been developed to

self-assemble MEMS. The combination of the planar fabrication and the solder self-

alignment enables us to develop three-dimensional, complex MEMS without demanding

complicated fabrication processes.

6. Thermal issues

Thermal loading and dynamic loads require special attention during the design of

the device, because the device will respond to this energy. The effects of temperature

manifest themselves because of the thermal and elastic mismatch between device-

substrate, substrate-package, package-board, and board-module.

The performance drift can be linked to the temperature dependent curvature for

capacitive switches or electrostatic actuators. This sensitivity and failure modes are

assessed by measuring functional performance over temperature and by measuring failure

rates due to temperature (thermal shock, temperature cycling, and high/low temperature

storage).

At high temperatures a poorly designed MEMS switch may open itself or its

contacts may weld closed preventing the switch from subsequent opening. With careful

design, these may be avoided if the thermal path through the package is sufficient to keep

a reasonable MEMS temperature. The MEMS process must be designed to convey the

heat to the package and the package must be designed to take away the heat to the PCB

quickly.

7. Mechanical Issues

The best device design will mechanically isolate the device from the mechanical

deformations that would be delivered through the package. The performance degradation

can be improved by increasing the in-plane spring coefficient of the device (i.e. folded

spring designs). The shock or vibrations are translated to the MEMS device through the

whole package. During qualification the packaged devices are tested for swept-sine

vibration, random vibration, and shock or drop.

The effect of vibration and shock on the packaged MEMS device can be

understood through a system model of the device. The system model would include the

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mass, spring, and damping behavior of the device, substrate, package, board, and module.

Damping is a dissipation element that dissipated the applied mechanical energy and is

very low for high Q devices.

MEMS packaging challenges are often application dependent. In order to discuss

the challenge in more detail, we will focus on specific devices. In the following section,

we will present one representative MEMS packaging case. We will here present RF-

MEMS switches which are representative of the physical MEMS applications.

6.8: Case study of RF-MEMS Switches

MEMS fabrication technologies have existed since 1970 in the form of sensors,

however RF devices have been here. The most common radio frequency device is the

MEMS switch MEMS microwave switch technique existed since 1980 by Lassy Lessons

at Hughes Labs. RF-MEMS switches are the devices open circuit in RF transmission line.

There are designed for low loss applications that do not need fast switching rates such as

in airborne and satellite communication. Many RF-MEMS switches have been

developed, tested and intensively published because of their attractive performances.

Figure 6.5 shows three dimensional diagram of a packaged RF-MEMS Switch in

an LCP enclosure. Figure 6.6 shows prototypes of the packaged RF-MEMS Switch using

liquid crystal polymer (LCP) capping. LCP is adhered onto silicon to form an enclosure

for packaging an RF-MEMS Switch without the use of adhesives. LCP is an emerging

low cost dielectric material that is commercially available as a single sheet or laminated

substrates that have low moisture absorption, adjustable coefficient of thermal expansion

(CTE), low coefficient of hydroscopic expansion (CHE), low dielectric constant and low

dielectric losses. This unique combination of excellent electrical and mechanical

properties for harsh environment and economical considerations make LCP as a serious

candidate for multi-chip module (MCM) and advance packaging technology.

Lamination of LCP onto silicon to form enclosure for package also has advantage

of low temperature processing (below ~3150C) as compared to metalling or glass bonding

(~4000C). The MEMS cavity is formed in LCP using laser ablation to the copper lid. This

cavity acts as a hermatic enclosure formed by the copper lid and SCP walls. The

commercially available LCP films have a melting temperature from 240° to 315°, which,

for robustness of process, is thermally well below any temperature that may impact the

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MEMS Switch. Inert gas can be injected into the cavities to help improve the switch

performance during the lamination process. Out-gassing is a major barrier in using

polymer materials for packaging RF-MEMS.

Figure.6.5: Diagram of a packaged RF-MEMS switch in an LCP enclosure.

During the processing of polymer in RFMEMS packaging, polymer tend to

release gas partials that would degrade the reliability of the RF-MEMS switch. LCP is

polymer material with negligible out-gassing and hence it is switchable for RF-MEMS

switch packaging. It is well known that polymer materials are usually unsuitable for

hermetic packaging because of their high permeabilities which cause failure during fine

leak testing. LCP has been reported to have a permeability 2.19X10-11 cm2/s. Package

hermeticity is quantitatively analyzed by using the diffusion leak rate closed form

approximation equation.

Leak Rate, R = K. A. ∆Pd (1)

Where K is the permeability, A is the exposed package area, ∆P is the Pressure

difference and d is the package wall thickness. LCP films are able to adhere to other

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materials without the use of external adhesives in a lamination process. This feature not

only simplifies the packaging process, but also reduces the electric loss that is associated

with loosy adhesive materials. The application of MEMS Switches covers a very broad

range.

Figure 6.6: Prototypes of the packaged RF-MEMS switch using LCP CAPPING.

These are intended to switch the propagation path of an analog signal with very

high signal purity. This function might be used to connect or disconnect an electric

potential or to select signals e.g. incoming and outgoing signals of antenna or to recon

figurate a subsystem. Typical applications are switching of filter banks, tuning of filter by

switching capacitors or inductors, switching of delay lines in phase shifters, impedance

matching by switching stub lines or capacitors, and configuration of antenna patterns or

frequencies by switching antennas. Depending upon frequency specifications either

capacitive or metal contact switches are preferred for certain applications.

To switch DC or RF signals required in high quality test and measurement

equipment, the most demanded property is very large bandwidth from DC to a few GHz.

The switching speed and power consumption are less important. Metal contact switches

are good candidates for these applications. Microrelays are intended for applications

where DC to low frequency currents has to be switched. Capacitive shunt switches are

usually designed for frequencies above one GHz and small bandwidths. These switches

are used in wireless equipments where low power consumption and small size are

required.

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Military and space applications also favor MEMS components because of their

robustness against external influences such as radiation, temperatures and strong

electromagnetic fields and because of their high shock resistance. Switch reliability and a

large number of switching cycles of the order of tens of billions are equally important for

most applications. The metal contacts are the most crucial part of the MEMS Switches

because they determine on-state resistant and the current handling capabilities and are the

source of the most dominant failure mechanisms in metal contact switches.

Packaging problems are thought to be one of the main reasons why RF-MEMS

Switches are still not available on a commercial base. Besides the general MEMS

packaging problems, the packaging of RF-MEMS Switches is additionally confronted

with the following packaging challenges:

1. Electrical interconnection lines, penetrating through the hermetic package,

should have low signal attenuation and low reactions. Thus, metal sealing with isolated

signal lines through or underneath the sealing ring, might not be appropriate.

2. Hermetic packages are required for reliability and performance stability,

especially for metal contact switches, to avoid surface absorption of contaminants and

moisture uptake in isolation layers, causing stiction.

3. Especially, when targeting wireless applications, the chip size is very

important. Thus, wafer bonding used for many high-volume hermetic MEMS packages

might not be possible without additional thinning of at least one of the substrates.

4. All involved materials, including substrates passivation layers and mounting

frames, should have good RF properties.

5. Standard packaging approaches and methods are often not suitable for RF-

MEMS Switches.

6.9: Challenges related to RF Performance

The high RF performance of RF-MEMS devices brings corresponding challenges

to the packaging. Key aspects include power loss, inductive and capacitive parasitics,

non-linearity, power handling, and interactions between control and RF circuits. Main

factors related to power loss include the resistivity of the metal used in the packaging, the

surface finish of the metal, adhesion layers, die substrate losses, package dielectric losses

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and intermetallic junctions. The length of conductors must be minimized, both for loss

and reactive parasitics.

At RF frequencies, the skin depth of currents flowing in a good conductor

(copper, silver and gold) is on the order of the thin films, but much less than the metals

thickness used in packaging.. Also, any adhesion layers exposed to the RF signals must

be much thinner than a skin depth. The dielectrics used in the package must be low loss.

Mounting methods that avoid solder are preferred. Conductive epoxies should be

avoided. Main factors related to reactive parasitics are physical length of signal paths and

their dielectric loading. It is best if the package is built using low dielectric-constant

insulators such as polymers.

To minimize the length of interconnects and avoiding substrate interactions, flip-

chipping of the RF-MEMS die is often a necessity. Wafer-level packaging for high

performance flip chipping prevents contamination of the MEMS during the attachment.

Sometimes, the length of interconnect between a MEMS package and another component

may degrade performance. So, integrated module, where the MEMS die is hybrid

integrated with other RF components should be used. The best performance is got with

3D integration using die stacking etc.

Linearity is another factor to increase RF systems progress. But packaging

degrades the linearity due to the formation of intermetallic junctions. So, metals and

solder compositions must be carefully chosen. Power handling is driven by the power

dissipation in the device and package, the temperature tolerance of the device and the

thermal path to the environment. The power dissipation is reduced only during the basic

design for performance. Coupling between the MEMS control signals and RF signals

depends strongly on the MEMS and package design.

The MEMS control signals for electrostatic operation are at high voltage. These

voltages must be kept separate from the RF signals, partly to prevent interference and

partly to prevent unintentional modulation of other devices. Conversely, RF signals

coupled to the high voltage lines may radiate or provide parasitic RF paths that degrade

RF responses. Monolithically integrated control ASICs simplify this challenge as the high

voltages are confined to the chip and the number of control lines is significantly reduced.

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DC and control lines must be separated from the RF lines with ground shields or using

bypass capacitors on the DC and control lines.

6.10: Industrial Challenges

Some of the major challenges facing the MEMS industry include:

(1). Access to Foundries

MEMS companies today have very limited access to MEMS fabrication facilities,

or foundries, for prototype and device manufacture. In addition, the majority of the

organizations expected to benefit from the technology currently do not have the required

capabilities and competencies to support MEMS fabrication. For example,

telecommunication companies do not currently maintain micromachining facilities for the

facilities are crucial for the MEMS commercialization.

(2). Design, Simulation and Modeling

Due to the highly integrated and interdisciplinary nature of MEMS, it is difficult

to separate device design from the complexities of fabrication. Consequently, a high level

of manufacturing and fabrication knowledge is necessary to design am MEMS device.

Furthermore, considerable time and expense is spent during this development and

subsequent prototype stage.

In order to increase innovation and creativity, and reduce unnecessary ‘time-to-

market’ costs, an interface should be created to separate design and fabrication. As

successful device development also necessitates modeling simulation, it is important that

MEMS designers have access to adequate analytical tools. Currently, MEMS devices use

older design tools and are fabricated on a ‘trial and error’ basis. Therefore, more powerful

and advanced simulation and modeling tools are necessary for accurate prediction of

MEMS device behavior.

(3). Packaging and Testing

The packaging and testing of devices is probably the greatest challenge facing the

MEMS industry. As previously described, MEMS packaging presents unique problems

compared to traditional IC packaging in that MEMS package typically must provide

protection from an operating environment as well as enable access to it. Currently, there

are no generic MEMS packaging solution, with each device requiring a specialized

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format. Consequently, packaging is the most expensive fabrication step and often makes

up to 90% (or more) of the final cost of a MEMS device.

(4). Standardization

Due to the relatively low number of commercial MEMS devices and the pace at

which the current technology is developing, standardization as been very difficult. To

date, high quality control and basic for standardization are generally only found at multi-

million dollar (or billion dollars) investment facilities. However, in 2000, progress in

industry communication and knowledge sharing was made through the formation of a

MEMS trade organization. Based in Pittsburgh, USA, the MEMS industry group

(MEMS-IG) with founding members including Xerox, Corning, Honeywell, Intel and

JDS Uniphase, grew out of study teams sponsored by DARPA that identified a need for

technology road-mapping and a source for objective statics about the MEMS industry.

In addition, a MEMS industry roadmap, sponsored by Semiconductor Equipment

and Materials International organization (SEMI), has also been identified to share pre-

competitive information on the processes, technology, application and markets for

MEMS. The networking of these smaller companies and organizations on a global scale

is extremely important and necessary to lay the foundation for a formal standardization

system.

(5). Education and Training

The complexity and interdisciplinary nature of MEMS require educated and well-

trained scientists and engineers from a diversity of fields and backgrounds. The current

numbers of qualified MEMS-specific personnel is relatively small and certainly lower

than present requirements.

6.11: Summary

RF-MEMS (Radio Frequency-Micro Electro Mechanical Systems) are made up of

moveable and fragile components (membranes, beams, cantilevers) that must be enclosed

in for protection and for stable performance characteristics. Packaging is an important

technology and critical aspect for the advancement of RF-MEMS. This chapter elaborates

the various RF-MEMS packaging challenges in the context of environment, modeling

reliability, integration, stiction etc. All these challenges are application dependent;

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therefore case study on RF-MEMS switches with liquid crystal polymer (LCP) enclosure

is also presented for an in-depth illustration.

RF-MEMS have the potential to have a tremendous impact on various fields such

as wireless communication, defense, aerospace, radars, satellite etc. Packaging engineers

are trying to overcome the packaging issues. They are trying to develop economical high-

performance and highly reliable packaging solutions. The package plays a key role in

ensuring the long term reliability and performance of a MEMS device.

The package has to provide the interface between the chip and the world. It

should protect the chip during its working condition with very little attenuation or

distortion of the signal in the given environment and do so at a low cost. In the coming

times, MEMS assemblies will need for MCM style packaging schemes in which several

MEMS chips are placed in the same package and are modular.

We need to develop application specific MEMS systems that are marketable, each

possibly requiring a unique and separate packaging scheme. We are to see the same

revolution with low cost, batch fabricated MEMS based sensors and actuators monitoring

and controlling our worlds, though there is a lot of packaging work ahead if the same

revolution is to take place. The major issue is to produce the knowledge base for the

packaging and integration of MEMS, which is essential for the success of MEMS

systems.

The packaging, assembly, and the materials used are the integral parts in the

MEMS system. Established packaging become blurred and united in the development of

the system. MEMS packaging breaks the paradigms of customary packaging, and is a

thrilling area for extensive research and development of novel applications.

Packaging of MEMS devices is challenging since achieving signal distribution

and environmental protection requires careful design & fabrication. However, there are

large numbers of packaging issues for RF-MEMS. Several radio frequency MEMS

packaging challenges have been listed and discussed here in this paper. Since RF-MEMS

Switches are used in wide range of applications, their required specifications and

packaging challenges are application specific. Also, the packaging approach for RF-

MEMS strongly affects the device performance especially in the case of devices with

moving parts.

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The various critical packaging issues belong to environment, reliability,

integration, modeling, functional interfaces, stiction, dicing, out-gassing etc. In addition

to general discussion about packaging challenges of RF-MEMS devices, the case study

on RF-MEMS Switches has been presented. Despite many benefits, RF-MEMS Switches

are not yet seen in commercial products due to various packaging challenges like

reliability issues, functional interfaces with environment, modeling and integration

problems etc. Also, the actuation voltages are typically too high for electronics

applications and require additional complicated drive circuitry. It is critical to solve the

packaging problems and overcome the barriers in order to make such an impact real.

As marvelous progress has been made to bring RF-MEMS from the laboratory

into products, many challenges still remain to be overcome. Solutions to all of these are

under development and should be available in the next few years.

(a). RF-MEMS processes; integrated RF-MEMS processes; hermetic, wafer

level MEMS encapsulation with RF-MEMS or CMOS processes are not

mature much.

(b). Launching of initial products needs different sources. RF-MEMS process,

CMOS process, and hermetic wafer level packaging are not available from

a single source.

(c). Low temperature, hermetic area-efficient wafer level encapsulation for

MEMS is not yet available.

(d). Wafer level RF-MEMS encapsulation not fully compatible with standard

flip-chip and redistribution technology.

(e). Insufficient broad concerted efforts to develop fully integrated RF-MEMS

technology. Low cost solutions are particularly difficult because of the

large Technology Development investment required.

(f). MEMS device from the environment should be isolated thermally and

mechanically through the package. Such provision is not mature.

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