Download various packaging challenges and solutions

VARIOUS
PACKAGING
CHALLENGES
AND
SOLUTIONS
6.1: 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
Challenges
Solutions to these challenges and parameters
Stiction of devices
Freeze drying, supercritical CO, drying, roughening
Parameters
Release and
Stiction
Dicing
of contact surfaces, non-stick coatings
Contamination risks
Release dice after dicing, flush chip surface to
remove contaminants
Die handling
Stress
Outgassing
Device failure, top die face is
Fixtures that hold MEMS dice by sides rather than
very sensitive to contact
top face
Performance degradation and
Low modulus die attach, annealing, compatible
resonant frequency shifts
CTE match-ups
Stiction, corrosion
Low outgassing epoxies, cyanate esters, low
modulus solders, new die attach materials, removal
of outgassing vapors
Testing
Possible device failure after
Post release, pre-package, and/or post package
many assembly steps
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 CO 2 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-andplace 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 coatsaw-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-andplaced 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 RFswitches. 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 microscale 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 selfalignment 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 devicesubstrate, 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 RFMEMS 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. ∆P
(1)
d
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 RFMEMS 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, flipchipping 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-tomarket’ 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 multimillion 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 precompetitive 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 welltrained 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 highperformance 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 RFMEMS 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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