Ghodssi R., Lin P., MEMS Materials and Processes Handbook
Подождите немного. Документ загружается.
904 A.G. Darrin and R. Osiander
and aromatic diamines such as 4,4
-diaminodiphenyl ether; (2) condensation prod-
ucts of 3,4,3
,4
-benzophenone tetracarboxylic dianhydride (BTDA) and aromatic
amines; (3) the reaction of BTDA and a diisocyanate such as 4,4
-methylene-
bis(phenylisocyanate); and (4) a polyimide based on diaminophenylindane and
a dicarboxylic anhydride such as carbonyldiphthalic anhydride. Thermoset poly-
imides are produced from condensation polymers that possess reactive terminal
groups capable of subsequent cross-linking through an addition reaction [33].
Polyimides have excellent mechanical and electrical properties and can be used in
a wide range of temperatures. They are widely used in electronics for their excel-
lent high temperature and chemical resistance, but may not be ideal for use in high
temperature curing and high pressure applications.
12.7.3 Polydimethylsiloxane (PDMS)
Polydimethylsiloxane (PDMS) is the most commonly used elastomer in MEMS,
especially in microfluidic systems. Although available in an array of formulations,
polydimethylsiloxanes have limited mechanical properties, but are balanced by good
chemical and thermal degradation resistance. In comparison, polyurethanes can be
either mechanically softer or stiffer than PDMS, offering better adhesion and tear
resistance. These are available in biocompatible formulations that exhibit excellent
chemical resistance. So far, polyurethanes are used as encapsulating structures for
chemical sensors and numerous bio-MEMS applications.
12.7.4 Epoxy
Almost all of these encapsulants use epoxies that are not all that different from the
original ones developed in 1927. Epoxies react with many kinds of molecules, and
those t hat are useful for producing products are called hardeners. Anhydrides are
one of the most important reactants, or hardeners, and are used in many encapsu-
lants and underfills. Epoxies are noted for versatility and balanced properties but
are rather average in general characteristics. Additives are required to bring their
properties to a level where they can function as encapsulants. Flame retardants,
especially bromine-containing epoxies, are added to reduce flammability and to
meet specifications. The average epoxy coefficient of thermal expansion (CTE) is
too high for encapsulation and acceptable levels are achieved by adding low CTE
fillers such as silica. Both liquid and solid epoxies must be thoroughly polymerized
to be useful for package enclosures. Epoxy molding compounds (EMCs) are actu-
ally low melting mixtures of resins and other constituents, and must be polymerized
into non-melting structures that have good mechanical strength and thermal stabil-
ity to survive the solder assembly process. The same is true for liquid encapsulants.
Polymerization, the formation of high molecular weight structures by chemical reac-
tions, occurs when heat is applied to a system that contains epoxy resin, hardener,
12 MEMS Packaging Materials 905
and usually an accelerator to increase the reaction rate. The resulting thermoset is
a 3D structure that is highly cross-linked and non-melting. Excessive heating, how-
ever, can cause the polymer to degrade by thermal decomposition, a present concern
with the increasing temperatures required for many lead-free solders. Epoxies also
absorb significant amounts of moisture, and the explosive release of steam, termed
popcorning, is exacerbated by higher soldering temperatures [34].
12.7.5 Fluorocarbon (Polytetrafluoroethylene)
Polytetrafluoroethylene (PTFE, trade name Teflon) has a wide range of unique and
desirable physical, electrical, and chemical properties. Its tribological properties
are well-suited to anti-stiction applications, and its chemical inertness commends
it as a barrier and passivation layer. However, conventional thin-film techniques are
not suited for depositing Teflon films on microstructures. Spin coating is impos-
sible because of the well-known insolubility of PTFE. Plasma polymerization of
fluorocarbon monomers, ion beam and RF sputtering produce PTFE films that are
deficient in fluorine. Pulsed laser deposition (PLD) use of excimer and Ti:sapphire
lasers is unsatisfactory because UV or near-IR laser ablation “unzips” the PTFE
and high-temperatures are required to re-polymerize the deposited monomeric
film. It has been demonstrated that a completely dry, vapor-phase coating tech-
nique – resonant infrared pulsed laser deposition (RIR-PLD) at a wavelength of
8.26 μm – produces crystalline, smooth Teflon films at low process tempera-
tures. Films deposited on microscale structures show good adhesion, excellent
smoothness, and a high degree of conformability to the structures [35].
12.7.6 Acrylic (PMMA)
Poly(methyl methacrylate) (PMMA) is a thermoplastic and transparent plastic.
Chemically, it is the synthetic polymer of methyl methacrylate. Acrylic coatings
have numerous benefits from a high volume and cost effectiveness perspective.
Acrylics tend to be hard, rigid, and extremely tough. Combined with excellent
electrical properties, acrylics can be considered for commercial high volume appli-
cations. As opposed to epoxy, acrylics exhibit little shrinkage during the curing
process. They have excellent moisture resistance and can endure weak acids and
bases. Decreased dielectric constants with increasing frequencies make them good
candidates for high frequency applications. However, acrylics offer poor abrasion
resistance and can be attacked by strong solvents and acids.
12.7.7 Parylene
Parylene is the tradename for a variety of chemical vapor deposited poly(p-xylylene)
polymers used as moisture barriers and electrical insulators. Among them, Parylene
906 A.G. Darrin and R. Osiander
C is the most popular due to its combination of barrier properties, cost, and other
manufacturing advantages. Parylene’s low permeability to moisture and gases and
hydrophobic nature makes it advantageous in electrical applications. Parylene is
often used as a conformal coating in critical applications. Parylene C is an inert,
hydrophobic, optically clear biocompatible polymer coating material used in a wide
variety of industries, and, because it is deposited by vapor deposition, Parylene pro-
vides a conformal coating on virtually any substrate which is not achievable by
other means. Its lubricity is close to that of Teflon but can be applied uniformly in
much thinner coatings, making them ideal for MEMS and bio-MEMS-based appli-
cations. Parylene’s pinhole free conformal coating protects coated substrates from
moisture, chemicals, and electric charge. While Parylene is typically used to coat
circuit boards or immobilize particles, it can also be removed from the original
surface and used as a microstamp allowing MEMS patterns to be reproducibly trans-
ferred to polymers with various properties (Parylene properties are available from
the manufacturer at www.scscookson.com/parylene_knowledge/index.cfm).
12.7.8 Liquid Crystal Polymer
Liquid crystal polymers (LCPs) are a class of aromatic polyester polymers. They are
unreactive, inert, and highly resistant to fire. LCP has been identified as an excel-
lent candidate to control the humidity in nonhermetic packaging and is used in RF
MEMS. LCPs have the following advantages [36]:
(1) Near-hermetic permeability to moisture keeping the MEMS dry for months
even in humid environments;
(2) Low RF loss properties allowing excellent RF performance after packaging;
(3) Excellent moldability allowing high-speed glob-top encapsulation.
12.8 Electrical and Thermal Requirements
12.8.1 Electrical Considerations
Electrical interfaces, based on the long history of the microelectronics industry, are
the most standardized. For hermetic components feedthroughs are glass matched,
glass compression, or ceramic. Glass matched seals rely on glass and metal com-
binations with similar coefficients of thermal expansion to form an oxide bond that
results in a hermetic seal. Compression seals rely on glass and metal combinations
that have greater thermal expansion coefficients, thus allowing the creation of a com-
pression seal providing hermeticity. Al
2
O
3
ceramic seals are used as an alternative
12 MEMS Packaging Materials 907
to glass. Integrated ceramic feedthroughs that are sometimes referred to as co-planar
or strip-line connectors, can contain conductors designed for RF, DC, and/or ground
signals.
Standard leadframes are used primarily for flatpacks although some machined
housings utilize them as well. The standard leadframe consists of co-planar par-
allel leads connected to a common tie bar that is used for electroplating contact.
Typically, the lead portion of the leadframe is 0.010
thick and 0.015
wide and
has an overall length of either 0.743
or 1.000
(including the 0.125
wide tie bar).
The l eads are spaced either 0.050
or 0.100
center-to-center. Special leadframes
with varying dimensions such as thinner cross sections, longer overall lengths, non-
standard lead spacings, etc., may be obtained. Round pins, sometimes called straight
pins, are used for all package styles but are most commonly used in plug-in type
packages. Round pins are typically 0.018
in diameter but other sizes are avail-
able. Pin length can range from 0.100
to as long as 1.000
. When round pins are
used in flatpacks, they usually will have one end flattened to create a small area for
wire bonding. High power leads are used to transmit high current signals through
sealed conductors. These usually have copper-core under a sealing alloy like Kovar
or Alloy 51. There are many materials that are able to conduct high current but many
are not suitable for glass sealing.
12.8.2 Thermal Considerations
For independent small signal circuits, the temperature of the device junction does
not increase substantially during operation, and thermal dissipation from the MEMS
is not a problem. However, with highly integrated System-in-Package applications,
MEMS devices share thermal paths with other components, such that the temper-
ature rise in the device junctions can be substantial causing the circuits to operate
in an unsafe region. Clearly, such distributed thermal dissipation requirements for
such highly integrated packages can place severe design constraints on the package
design.
Three thermal resistances that must be minimized: the resistance through the
package substrate, the resistance through the die-attach material, and the resistance
through the carrier or package base. Furthermore, the thermal resistance of each is
dependent on the thermal conductivity and the thickness of the material. A package
base made of metal or metal composites has very low thermal resistance and does
not add substantially to the total resistance. When electrically insulating materials
are used for bases, metal-filled via holes are routinely used, under the MEMS, to
provide a thermal path to the heat sink. Although thermal resistance is a consider-
ation in the choice of the die attachment material, adhesion and bond strength are
even more important. To minimize the thermal resistance through the die-attachment
material, the material must be thin, void-free, and the two surfaces to be bonded
should be smooth [37].
908 A.G. Darrin and R. Osiander
12.9 Hermeticity and Getter Materials
12.9.1 Hermeticity and Pressurized Packaging
The definition of hermeticity comes from the United States Department of Defense
specification for Microelectronic Packaging (MIL-STD-883) stating that a seal that
prevents the entry of contaminants or reactive gasses into the internal cavity is
deemed hermetic. In practice, small gas molecules may enter the cavity over long
time scales through diffusion or permeation. Table 12.12 comes from the Military
Standard and defines the leak rate limit by package volume. To test to this specifi-
cation, the package is placed in a pressurized Helium atmosphere for the specified
time followed by a dwell time between pressure release and measurement. The unit
is then placed in a calibrated Helium mass s pectrometer that measures its helium
leak rate.
Table 12.12 Leak rate testing to cavity size from MIL-STD-883
Pkg Volume, cm
3
PSIG Exposure time, h Dwell, h
Reject limit
atm-cm
3
/s He
V <0.40 60±-2 2 +0.2,–0 1 5×10
−8
V. ≥0.40 60±22+0.2,–01 2×10
−7
V.≥0.40 30±24+0.4,–01 1× 10
−7
12.9.2 Hermeticity and Vacuum Packaging
Some special applications require wafer-level packaging with hermetic cavities
under vacuum, creating the need for bonding equipment that can be used in high- or
ultra-high-vacuum environments. These include:
• High-accuracy accelerometers and gyroscopes;
• MEMS switches and oscillators;
• High-frequency resonators; and
• Optical switches and infrared (IR) imaging sensors
A wafer bonding technique developed to accommodate these requirements uses
getter materials to ensure suitable vacuum (total pressure < 1×10
3
mbar) and long-
term stability in MEMS devices. Gettering is essential for removing unwanted gases
out of the MEMS package to create the desired final atmosphere.
12.10 Quality and Reliability
As was discussed in the first section and shown in Table 12.1, the flat world view
of integrated circuits and microcircuits does not fully address the MEMS packaging
12 MEMS Packaging Materials 909
schemes. However, failure mechanisms seen in the microcircuit world may be evi-
dent in the MEMS realm. Common failure modes seen in microcircuit packaging
are:
(a) Die and passivation cracking
(b) Delamination between the die, die attach, die pad, and plastic passivation
(c) Fatigue failure of interconnects
(d) Fatigue fracture of solder joints
(e) Warping of printed circuit board
Most of the failures as described above are due to the following sources:
(a) Mismatch of coefficients of thermal expansion between the attached materials.
(b) Fatigue fracture of materials due to thermal cycling and mechanical vibration.
(c) Deterioration of material strength due to environmental effects such as mois-
ture.
(d) Intrinsic stresses and strains from microfabrication processes such as thermal
oxidation, diffusion, and depositions [38].
Suffice it to say that the MEMS packaging engineer must consider all of the
above and then more.
12.10.1 MEMS Packaging Reliability Concerns
As discussed in earlier chapters, virtually all MEMS devices require micro-
fabrication techniques such as surface micromachining. All future processing steps
including bonding depend on proper surface treatment prior to actual bonding. Any
improper treatment in surface conditioning would leave voids at the bonded inter-
faces. These imperfections can act as crack initiators or stress concentrators for
subsequent delamination of the interfaces [38]. Any improper selection of materials
will be apparent in the temperature dependent mismatch failures. Thus, precision
control of all processes e.g. temperature, contact pressure and applied voltages in
anodic bonding process is fundamental to successful packaging scenarios. Once
packaged, the device sees reliability and quality assurance screens and applica-
tion stresses. Any imperfections at the interfaces and delamination of the bonding
surfaces will be exacerbated and may lead to failure.
At all fabrication s teps, the concerns of material compatibilities need to be
addressed. During packaging of MEMS, stresses will be distributed within the die
attachment, die and substrate the reliability of the packaging structure. Numerous
studies in the literature discuss stress during processing steps [39].
Outgassing may cause environmental contamination that leads to clogging or
material build up. The device may then become inoperable depending on its
910 A.G. Darrin and R. Osiander
function. Contamination binding and build up have been found to cause device
failures in strategic active areas [40].
The inner surfaces of micro-conduits used for electro-osmotic and electrophore-
sis pumping require coats of special polymers, allowing for the release of ions
under the influence of electrical fields. These ions interact with those released from
the contacting fluids via electromigration and thus results in fluid capillary flow.
These delicate thin film coatings, often a few nanometers thick, can be attacked by
free ions, reducing their effectiveness over long electric field exposure times. This
reliability issue is difficult to resolve [38].
Multiple stresses may be more detrimental to reliability than the effects of a
single factor. In the design process, both design factors and test criteria must con-
sider both individual and/or combined life cycle stresses to produce the robustness
needed to withstand the hazards identified in the system profile. The synergistic
effects of typical combined environments can be illustrated in a matrix relationship,
which shows combinations where the total effect is more damaging than the cumu-
lative effect of each environment acting independently. For instance, an item may be
exposed to a multitude of environmental factors such as temperature, humidity, alti-
tude, shock, and vibration while it is being transported. Demonstrating adequate end
of life service must include combined effects. For example, many delicate MEMS
device components, such as the thin silicon diaphragms used in micro pressure sen-
sors, are in contact with corrosive or reactive gases whose is to be sensed. Many of
these gases, such as the exhaust gases from internal combustion engines, are hot and
contain corrosive chemical compounds. Extended exposure to these hot media may
cause serious damage to delicate components [38].
12.10.1.1 Thermal Effects
High temperatures impose a severe stress on most electronic devices including
MEMS, often causing mechanical failure or resulting in deterioration due to chem-
ical effects. MEMS design inherently requires small sizes with high part densities.
This generally requires a cooling system to provide a path of low thermal resis-
tance from heat-producing elements to a heat sink. Adequate life with such thermal
stresses usually demand the use of heat dissipation devices, cooling systems, thermal
insulation, and heat-resistant materials.
Conversely, low temperatures experienced by MEMS can have a reliability
impact. These problems usually are typically associated with mechanical system
elements. They include mechanical stresses produced by differences in the coef-
ficients of expansion (contraction) of metallic and nonmetallic materials, embrit-
tlement of nonmetallic components, mechanical forces caused by freezing and
expansion of entrapped moisture, stiffening of fluid constituents, etc. Typical exam-
ples include cracking, delaminations, binding of mechanical linkages, and excessive
viscosity of lubricants. Reliability improvement techniques for such low tempera-
ture stresses include the use of heating devices, thermal insulation, and cold-tolerant
materials.
12 MEMS Packaging Materials 911
Additional stresses are produced when MEMS are exposed to sudden changes
of temperature or rapidly changing thermal cycling conditions. These conditions
generate large internal mechanical stresses in structural elements, particularly when
dissimilar materials are involved. Effects of thermal shock-induced stresses include
cracking of seams, delamination, loss of hermeticity, leakage of fill gases, separation
of encapsulating materials from components and enclosure surface leading to the
creation of voids, and distortion of support members.
A thermal shock test may be specified to check the integrity of solder joints since
such a test creates large internal forces due to differential expansion. Repetitive
stress may create segregation effects in solder alloys leading to the formation of
lead-rich zones, which are susceptible to cracking effects.
12.10.1.2 Shock and Vibration
MEMS often are subjected to environmental shock and vibration during both normal
use and testing. Such environments can cause physical damage when deflections
cause mechanical stresses that exceed the allowable working stress of the constituent
parts.
Natural frequencies of the MEMS parts are important parameters that must be
considered in the design process since a resonance can result if any such natural
frequency is within the vibration frequency range encountered. The resonance con-
dition will greatly amplify subsystem deflections and may increase stresses beyond
the safe limit.
The vibration environment can be particularly stressful for electrical sliding con-
nections, since it may cause relative motion between members of the connector. In
combination with other environmental stresses, this motion can produce frit corro-
sion which can generate wear debris and cause large variation in contact resistance.
Mitigation techniques for vibrational stress include the use of stiffening, control of
resonance, and reducing freedom of movement.
12.10.1.3 Humidity
Humidity can cause degradation of MEMS as discussed previously. Mitigation tech-
niques for humidity and salt environments include use of hermetic sealing, moisture-
resistant materials, dehumidifiers or desiccants, protective coatings/covers, and
reduced use of dissimilar metals.
Deleterious effects are often exacerbated with high humidity. For example, pack-
age crack growth has a dependence on moisture that is well documented [41]. Also,
electrical performance may change as moisture condenses in gaps causing surface
tension that may induce a piezoresistive stress effect [42]. Perhaps best known is
the relationship of adhesion and friction of polycrystalline silicon MEMS [43]. This
dependence is reduced, but not eliminated, when molecular coatings are applied to
the surfaces. Such anti-stiction coatings have the ability to penetrate into the intri-
cate side wall and under-surface spaces in three dimensions. Thus, these coatings
extend the operating life of MEMS devices by reducing stiction [44].
912 A.G. Darrin and R. Osiander
12.10.2 MEMS Packaging and Quality Assurance
We return to our first table of the chapter discerning differences in the microcircuit
and MEMS arenas in respect to packaging. The hermetic packaging segment of the
United States grew out of the US military’s use of hermetic packaging. The two
documents that serve as a starting point for developing qualification and assurance
tests for MEMS packaging are MIL-PRF-38535 Integrated Circuits (Microcircuits)
Manufacturing, General Specification and MIL-STD-883 Test Method Standard.
MIL-PRF-38535 specification establishes general reliability requirements for
integrated circuits or microcircuits. The quality and reliability assurance require-
ments must be met for their acquisition. The intent of this specification is to allow
the device manufacturer the flexibility to implement best commercial practices to
the maximum extent possible while still providing a product that meets military
performance needs. MIL-STD-883 Microcircuit Test Methods establishes uniform
methods, controls, and procedures for testing microelectronic devices suitable for
use within military and aerospace electronic systems including basic environmen-
tal tests. These tests determine robustness to deleterious effects of natural elements
and conditions surrounding military and space operations; mechanical and electri-
cal tests; and workmanship and training procedures. This standard applies only to
microelectronic devices. However, MEMS devices in microcircuit packages may be
tested in accordance with MIL-STD-883.
Package tests should be defined and tailored in respect to the final applica-
tion, usage, life expectancy, shelf life time, and criticality. Table 12.13 offers some
suggested tests to be considered for screening MEMS packages.
Whether following the above prescriptive assurance specifications or deriving
new specifications, it is important to consider the application and to then apply the
appropriate test. As an example, Analog Device have developed stress tests called
“Random Drop” and “Mechanical Drop”. “Random Drop” is the random-orientation
batch drop of packaged devices from a height of 1.2 m onto a marble surface. The
drop is repeated for 10 times, with a basic functionality check done between each
drop. In a “Mechanical Drop” test, devices are dropped one by one from a height of
0.3 m onto a marble surface, first in the X-axis, then the Y-axis, and finally the Z-axis
directions. An electrical screen is performed, and the drop test procedure repeated
from a height of 1.2 m [45]. This work by Analog Devices
TM
is an excellent example
of the need to tailor test plans to achieve a reliable program.
12.11 Case Studies
It is rather difficult to show packaging cases as examples, since most MEMS devices
require their own individual package design, matching, for example, the application,
the interface, or the environment it will be used. There can be hundreds of examples
found in the literature for MEMS packages, each one unique. In this section, we will
discuss some of the commercial s uccess stories: the Analog Devices accelerometer,
12 MEMS Packaging Materials 913
Table 12.13 Mechanical, electrical and environmental packaging tests for tailoring
Mechanical considerations
Hermeticity Specify leak rate of helium (cc/s) at certain pressure
differential (atm)
Pin pull test Specify (lbs, kg, Newtons, or ounces) with direction of
force applied
Pin bend test Specify an angle of bend, direction of bends, no. of cycles.
Where to support the pin (P = distance of pivot point
from glass) during the test must be defined
Pressure Specify in (psi, bars, or atm) at certain temperature with
pressure differential
Torque Specify in (in-lbs, ft-lbs, N-cm); definition of support the
body and length of engagement of torque wrench to the
part is required
Mechanical shock Specify with a profile of pulse (g) and time (milliseconds)
Vibration Specify a spectrum and power density
Durability Specify in no. of mate/un-mate cycles
Thermal cycling Specify hot (+
◦
Cor
◦
F) and cold temps (−
◦
Cor
◦
F),
holding time at each temperature (hours or minutes),
dwell time between hot and cold zones, and no. of cycles
Thermal shock Specify with hot and cold temperatures, and holding time at
each temperature
Drop test Specify height, surface, orientation and no. of drops
Electrical considerations
Insulation resistance (IR) Specify in ohms () at a certain voltage level (volts)
Dielectric withstanding voltage
(DWV or HI-POT)
Specify current leakage (milliamps) at certain voltage level
(volts)
Contact resistance Specify in (micro-ohms)
Impedance Specify in ohms () at appropriate frequencies
Conductivity Typically known as current carrying capacity and specified
in (amps or milliamps)
Environmental considerations
Solderability Refers to a military or industry standard, which may require
steam aging before testing for specific period of time (h)
Corrosion resistance Refers to a military or industry standard, which specifies
exposure to test chamber for a period of time (h) and salt
content (%)
Plating adhesion Specify as peel-off tape test or bend test. Acceptance
criteria must be defined such as “must not peel off” or
“must not crack”. Baking step may be required before the
adhesion test at temperature (
◦
ForC)foraperiodof
time (hours or minutes)
Wire bond strength Specify in (grams, lbs, or ounces) with a specific wire size
and bond surface material
Outgassing Applies to all components except glass. Glass does not
outgas. Specify the type of gases exposed in and around
the parts
Ozone exposure Applies on space applications. Specified exposure time (h),
ozone concentration (%)