Download materials and fabrication technology for rf-mems

MATERIALS AND FABRICATION TECHNOLOGY
FOR RF-MEMS
3.1: Introduction
In a general way, RF-MEMS is at the same time a objective product and a
technological aspect. As the name implies, “RF” denotes radio frequency that includes
range of frequencies between 0.1 GHz and 300GHz that comprises of applications
concerning microwave and sub-millimeter wave; “Micro” describes the size of the
component; “Electro” refers to both the electricity and electronics in the system; and
“Mechanical” points towards the involvement of a few moving mechanical parts.
In this chapter, we will focus on the material issues for RF-MEMS switches,
tunable capacitors, MEM resonators and film bulk acoustic resonators etc and on the
fabrication technologies. Materials and process technologies are key constraints to the
design of RF-MEMS components, including the package. Materials and process
technology are basic factors for the realization of RF-MEMS products. The choice of
materials and the process flow is evidently governed by the details of the device to be
fabricated.
3.2: Materials used for MEMS fabrication process
Material and mechanical science is being revolutionized and rejuvenated. Polysilicon is still the prevailing material used in both mechanical parts and electronic parts in
MEMS devices. VLSI fabrication technique has advanced to build mechanical
components with electronic components. Just like IC technology in 1970s, MEMS
technology is still immature. Most of the RF-MEMS products are still lab toys.
RF-MEMS are constructed from materials, which have unique reliability results.
Different materials have different responses to failure mechanisms that need to be
understood to increase reliability of device. The materials used in designing and
fabrication of RF-MEMS devices and components are given in the following Figure 3.1.
The choice of the right material for an RF-MEMS component requires knowledge of the
many factors about the application of the material. Apart from availability and cost,
factors guiding in choosing the suitable materials are:
o Fundamental material properties and their controllability.
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o Ease of processing (like etching, deposition, conformal step coverage etc.)
o Processing compatibility (adhesion, diffusion, temperature, packaging, etc.)
o Reliability characteristics (fatigue, creep, dielectric charging, long-term drift, etc.)
Materials for RF-MEMS
Substrates
Packaging
Thin Films
Ceramics like MgO,
Al2O3, and Saphire
Semiconductors like
Si, GaAs, InP, Ge
Polymers
Metals like Al, Au,
Cu, etc. & their Alloys
Polymers
Insulators like SiO2,
Si3N4, and PMMA
Metals
Semiconductors like
Si, GaAs, InP, Ge
Ceramics
Special Materials like
PZT, BST
Glass
Polysilicon
Figure 3.1: Various materials employed for fabrication of RF-MEMS devices.
3.2.1: Single Crystal Silicon
Silicon is a good mechanical material with high precision, high strength, and high
reliability. Like diamond, silicon is a very hard material. It is a tetrahedral covalent
bonded structure. It has better mechanical strength and yield strength than steel, but is
more brittle. Its highest fracture strength makes silicon perfect for many MEMS
structures.
Silicon is the most widely used material for manufacturing RF-MEMS because it
has excellent mechanical and electrical properties. Diamond structure and low
crystallographic index planes of silicon are shown in Figure 3.2. Silicon is resistant to
most acids, except HF or HNO3 and certain bases. Main physical characteristics of silicon
are listed in Table 3.1.
The four factors that have made silicon vital as a mechanical material are:
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(a)
It is available in large quantity, cheap, can be produced and processed
controllably with supreme purity and perfection.
(b)
Silicon processing is based on very thin deposited films which can be
easily miniaturized.
(c)
It can be batch fabricated.
(d)
The device shapes and patterns can be defined and reproduced using
lithographic techniques.
(a)
(b)
Figure 3.2: (a) Crystal structure of silicon and (b) Low crystallographic index
planes of silicon.
Table 3.1: Well defined characteristics of Silicon.
Sr. No.
Characteristics
Value
1.
Melting point
1415
2.
Young’s modulus <100>
130 GPa
3.
Poisson’s ratio <100> orientation
0.28
4.
Fracture strength
0.9 MPa m^.5
5.
Thermal conductivity
1.56 W/cm-
6.
Coefficient of thermal expansion
4.2 x 10-6 0C-1
7.
Heat Capacity
20.07 J/mol-K
8.
Breakdown Field
~3 x 105 V/cm
9.
Resistivity
2.3 x 105Ω-cm
10.
Band Gap
1.12 eV
11.
Electron mobility
1500 cm2 /V-s
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0
C
0
C
Electrons in solids can only occupy discrete energy levels, separated into distinct
bands. At the lowest potential, all valence electrons occupy the valence band. The bandgap determines the electrical and optical properties of a device. However, electrical
properties of silicon are sensitive to magnetic fields, stress, heat, and radiation, which is a
characteristic that is beneficial in many solid-state sensors. It exhibits high resistivity of
silicon wafers for RF-MEMS. It is opaque to UV and transparent to infra-red.
Figure 3.3: Energy band structure of silicon and gallium-arsenide.
In direct bandgap semiconductors, like GaAs, conduction band and valence band
are in line. Electrons only need energy to transit levels. When an electron de-excites, a
photo emits. In indirect bandgap semiconductor, like silicon, electrons need to change
both momentum and energy to change levels, as shown in Figure 3.3. Thus, silicon
cannot be used in semiconductor lasers and LEDs. This limit to silicon led to explore new
semiconductors that can emit light.
3.2.2: Gallium Arsenide
After silicon, gallium arsenide is most common semiconductor. It has better
optical and electrical properties. Being a direct band gap semiconductor, it is used in
semiconductor lasers and LEDs. It will help in developing micro-opto-mechanical
devices. The fundamental characteristics of gallium arsenide are mentioned in Table 3.2.
Being brittle and poor mechanical performance, it is not used in the
semiconductor industry. Due to low Young’s modulus, it is weak. It is a poor thermal
conductor and becomes ductile at much lower temperatures than its melting point. So, its
hardness decreases and fatigue increases at high temperatures. GaAs can work at higher
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frequencies than silicon, which makes it ideal for communications applications.
However, there are unstable insulating oxides and nitrides of GaAs that make it difficult
to reproduce passivation layers.
Table 3.2: Fundamental characteristics of Gallium Arsenide.
Sr. No.
Characteristics
Value
1.
Crystal structure
Zinc-blende
2.
Melting point
12370C
3.
Heat capacity
47.02 J/mol-K
4.
Breakdown field
~400000 V/cm
5.
Thermal conductivity
0.46 W/cm-°C
6.
Young’s modulus
85.5GPa
7.
Poisson’s ratio
0.31
8.
Fracture strength
0.44 MPa m^.5
9.
Constant of thermal expansion
6.86 x 10-6 °C-1
10.
Band gap
1.424eV
11.
Resistivity
108 Ω-cm
12.
Electron mobility
8500 cm2/V-s
3.2.3: Polycrystalline Silicon (Polysilicon)
Polysilicon is used as a structural material in the surface micromachining process.
Thin films of silicon are difficult to grow, but that of polysilicon can be grown.
Polysilicon crystallized by metal induced lateral crystallization (MILC) gives improved
performance. It finds many uses in the RF-MEMS industry. The mechanical properties
(Young’s modulus 140-210GPa) of polysilicon films depend greatly upon the deposition
process. Depositing its films at 5900C (transition point between polycrystalline and
amorphous silicon), is useful method of forming an isotropic film of polysilicon.
3.2.4: Silicon Dioxide
Silicon dioxide is an excellent thermal and electrical insulator and commonly
used in integrated circuits. So, it can be used to separate different layers of conductors
and components with little electrical interference effectively in MEMS. It is used recently
as a structural material. Its basic properties are listed in Table 3.3.
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Table 3.3: Remarkable characteristics of Silicon Dioxide.
Sr. No.
Characteristics
Value
1.
Density
2.65 g/cm3
2.
Melting point
17280C
3.
Young’s modulus
66 GPa
4.
Tensile strength
69 MPa
5.
Thermal conductivity
1.4 x10-2W/0C-cm
6.
Constant of thermal expansion
7 x 10-6 0C-1
8.
Resistivity
1012Ωcm
9.
Band gap
8 eV
However, due to high anisotropy, it is more difficult to etch than silicon. Silicon
dioxide can be used as the sacrificial layer material in the surface micromachining
process because it is prone to etching with hydrogen fluoride whereas silicon is resistant
to etching. It is a commonly used in glasses. Due to low thermal conductivity, it is a
thermal insulator. Having low tensile strength, it is prone to mechanical fracturing and
rarely used as a mechanical material. Due to the inherent advantages in being able to
integrate such an effective insulator, it has become the semiconductor material of
selection for most applications.
3.2.5: Silicon Carbide
Recently, silicon carbide is being used as a material for RF-MEMS. Its
characteristics depend much upon processing. It offers advantages in hardness and
resistance to harsh environments and high temperatures. It does not have a sharp melting
point (2830 0C). At this temperature, it decomposes into graphite and a silicon rich-melt.
It oxidizes to silicon dioxide above 600 0C.
Several SiC structures are less elastic than silicon, which is useful in certain
MEMS applications. Its Poisson’s ratio varies between 0.183 and 0.192. Its main
drawback is that the wafer growth technology used to grow its wafers results in high
dislocation densities. It weakens its strength that impedes its use in many applications. It
is a wide-bandgap semiconductor with a band-gap around 3eV. It acts as a good insulator.
It has a resistivity of 108 Ω-cm that varies with doping.
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3.2.6: Silicon Nitride
Silicon nitride is a useful masking material for alkaline etch solutions and also as
an insulating thin film. It can also be used to make MEMS membranes, plates and
cantilevers. It does not react well with many etching solutions; hence, it is often used to
stop impurity dispersion and ionic contamination. The silicon nitride films used in most
MEMS devices are amorphous and are either sputtered or deposited by CVD. It has a
bandgap much smaller than that of silicon dioxide, so, it facilitates much less electrical
isolation than that of silicon dioxide. Crucial characteristics of silicon nitride are given in
Table 3.4.
Table 3.4: Crucial characteristics of silicon nitride at room temperature.
Sr. No.
Characteristics
Value
1.
Density
3.1 g/cm-3
2.
Resistivity
1015W-cm
3.
Breakdown field
107 V/cm
4.
5.
Band gap
Thermal conductivity
3.9eV to 4.1 eV
0.28 W /cm-°C
6.
Young’s Modulus
73 GPa
7.
Coeff. of thermal expansion
3×10-5/°C
8.
Fracture strength
460 MPa
9.
Melting point
19000
Silicon nitride (Si3N4) has vital mechanical properties that make it an attractive
material to work with. It is a better thermal insulator than polysilicon, so it is used for
isolating surface micro-machined structures. Also, due to high mechanical strength, it is a
perfect film for dust barriers and friction. Furthermore, it makes a low energy barrier
towards silicon and metals that causes the injection of holes into the dielectric at electric
fields > 2×106 V/cm. So, a designer likes to form insulators with SiO2 and seals it with
silicon nitride.
3.2.7: Other Materials
Metals such as aluminium, gold, copper etc, are used in MEMS as electrical
conductors and occasionally as structural material. The metals used in MEMS are, unlike
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the materials discussed previously, ductile. That means that they will plastically deform if
stressed past the yield strength. Aluminum is commonly used as an electrical conductor
in semiconductor technologies. Aluminum, like most metals, is often alloyed with other
substances to enhance its structural properties. The mechanical properties of aluminum
are considerably poorer than Si and GaAs. Gold is a soft, ductile material that is easily
deformed. Consequently, it is not used as structural material. Instead, it will almost
always be layered on top of a more rigid material or be used in applications that do not
require mechanical motion.
Polymers are useful in sensing of chemical gases because of their absorption and
adsorption properties and polymethylmethacrylate (PMMA) is a useful polymer in the
LIGA process for forming high aspect ratio structures. Plastic micro devices are useful
for micro fluidic applications for biosensors. As no batch process fabrication techniques
are recognized for plastics, the University of Michigan is finding surface and bulk
micromachining of plastics and the embedding of silicon devices into cast plastic
substrates. Harvard University is also exploring new organic polymers and processes
such as micro molding and micro printing to produce structures from them.
Ceramics are also employed for RF-MEMS for some particular applications.
Ceramic pressure sensors have been developed for pressure computation of the high
temperature surroundings. Moreover, some functional ceramics like ZnO, PZT, etc, have
also been incorporated in the RF-MEMS devices. The fabrication processes for ceramic
MEMS are screen printing, micro-moulding, micro-stereo-lithography, Sol-Gel, etc.
Research in new materials for MEMS is also active. For example:

Amorphous alloys can be extruded and forged using micro-dies and so,
these assure as future materials for MEMS;

Poly-caprolactone is a biodegradable polymer that makes it beneficial for
implantable biomedical micro-devices;

Tantalum oxide can be active as a protective coating for sensors; and

Polycrystalline silicon-germanium can be deposited and annealed to
trigger dopants (impurities added to affect the semiconductor properties)
at much lower temperatures than polysilicon and becomes a good
contender for structural material.
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3.3: RF-MEMS Fabrication Techniques
Obviously, MEMS are the integration of electronic and mechanical components
on a same silicon chip using microfabrication processes. The mechanical parts (actuators
and sensors) are manufactured by micro-fabrication approaches. Electronics and
electrical circuits are used to make signal processing, control units etc. The electrical and
electronics is fabricated by IC process like CMOS. In this way, the entire system can be
fabricated on a single chip as shown in the Figure 3.4. In this figure cross-sectional view
of the Sandia MM or CMOS integrated MEMS technology is represented. The
mechanical structure fabricated prior to the electronics is embedded down inside a
planarized well.
Micromachining is the combination of design and fabrication tools that precisely
make microstructures and components at the microscale. Micromachining is the basis for
RF-MEMS devices fabrication. Normally, in micromachining, parts of the silicon wafer
are selectively etched away or new structural layers are added to make the mechanical
and electromechanical devices. Several processes and tools used in the design and
fabrication of RF-MEMS devices are taken from IC technology.
Figure 3.4: Cross-section representation of the Sandia MM/CMOS integrated
MEMS technology.
Common fabrication techniques that are used for machining structures include
bulk micromachining, wafer-to-wafer bonding, surface micromachining, and high-aspect
ratio micromachining (like LIGA). Flip chip technique is employed when silicon is not
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used as substrate. High-aspect ratio micromachining is a new technique that facilitates
the fabrication of layers (generally thickness lies between hundreds of microns to a few
centimeters), precisely, high-aspect ratio MEMS structures (structures with almost
vertical sides).
3.3.1 Bulk Micromachining
The most mature and familiar micromachining fabrication technique is the bulk
micromachining.
This micromachining is employed to fabricate micromechanical
structures inside the bulk of a wafer by selectively etching material of the wafer. Bulk
micromachining involves the selective removal of considerable amount of silicon from
any part of the bulk substrate. It is a subtractive process that uses wet anisotropic etching
or a dry etching method such as reactive ion etching (RIE), to create three dimensional
microstructures such as large pits, grooves, membranes, trench holes, cavities, nozzles,
cantilevers, channels, etc. different bulk micromachining microstructures are shown in
the Figure 3.5. Materials typically used for wet etching include silicon and quartz, while
dry etching is typically used with silicon, metals, plastics, ceramics etc.
Figure 3.5: Different structures obtained by using bulk micromachining process.
Bulk micromachining is based on etching single-crystal silicon either chemically
(wet etching) or using plasma etching or reactive ion etching (RIE) techniques (dry
etching). Bulk micromachining involves the various etching procedures that selectively
remove material with a chemical enchant whose etching properties depend on the
crystallographic structure of the bulk material. Wet etching removes material by
immersing it in a liquid bath of a chemical etchant. These etchants can be isotropic or
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anisotropic. HNA, a mixture of hydrofluoric acid (HF), nitric acid (HNO3) and acetic
acid (CH3COOH), is the most common isotropic silicon etch.
Dry etching involves vapor phase or plasma-based methods of etching using
suitable reactive gases or vapors at high temperatures. The most used dry etching for
MEMS is reactive ion etching (RIE) that uses an additional energy (radio frequency
power) to start the chemical reaction. Energetic ions are accelerated towards the material
to be etched within a plasma phase. Irrespective of crystal planes RIE can etch any shape
with vertical walls like deep trenches and pits. Deep Reactive Ion Etching (DRIE) is a
much higher-aspect-ratio etching method that involves an alternating process of highdensity plasma etching and protective polymer deposition to get higher aspect ratios of
the microstructures.
Figure 3.6:
Bulk micromachining along crystallographic planes.
Figure 3.6 shows the steps of a typical bulk micromachining technique. Here,
anisotropic wet etching is used to make microstructures from the silicon substrate. Bulk
micromachining has a few basic shortcomings. For example, the orientation of crystal
planes of wafer determines the highest attainable aspect ratios. The more aspect ratios
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need bigger sizes as compared to the other micromachining processes. Obtaining
complex microstructures by employing this process is also very difficult.
3.3.2 Surface Micromachining
Surface micromachining involves processing on the substrate that is used as a
support for the desired structure. It was initiated in the 1980’s to produce MEMS.
Material is added to the substrate in the shape of layers of thin films on the surface of the
substrate. These layers can either by structural layers or act as spacers or sacrificial layers
(that are removed later). Hence, the process typically involves films of two different
materials: a structural material (popularly polysilicon, silicon nitride and aluminium) out
of which the free standing structure is built and a sacrificial material (popularly an oxide),
deposited wherever either an open area or a free standing mechanical structure is vital.
The desired structure is shaped by removing the sacrificial layer and releasing the
structure.
(a)
(d)
(b)
(e)
(c)
Figure 3.7:
(f)
Various processing steps involved in typical surface micromachining.
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In surface micromachining (SMM), layers of structural and sacrificial material are
deposited alternatively and etched to get the desired shape. Surface micromachining
facilitates the construction of free-shape, complex and multi-fabricate systems without
limitations on materials, geometry, assembly and integration that are the source for the
richness and depth of MEMS applications. The various steps involved in the fabrication
of microstructures using surface micromachining are shown below in the Figure 3.7.
Surface micromachining involves the fabrication of the devices from thin films
deposited on the surface of a silicon wafer. Hence, this technique needs a well- matched
set of sacrificial layer, structural layer and etchants. All these materials must be proper
for the application. The size of the SMM structures can be of much lower than structures
got from bulk micromachining. These are also easy to integrate in IC circuits. These
structures are normally planar.
The success of the surface micromachining process
depends on the ability to remove all of the sacrificial layers effectively so that the
structural elements become free for actuation. The key benefits of surface
micromachining are the use of standard CMOS fabrication and the ease of the process.
3.3.3: Wafer-to-wafer Bonding
There is a fundamental limitation to bulk micromachining that it cannot produce
complex three dimensional microstructures. This problem is overcome by fabricating and
then assembled altogether the micro-components of a complex structure. In wafer
bonding technique, different wafers are almost integrated together without any fault. It
can be divided into three well-known types: intermediate layer assisted bonding, anodic
bonding, and direct bonding.
Intermediate layer assisted bonding needs an intermediate layer that can be metal,
glass, polymer, solder etc to fulfill bonding between wafers. Anodic bonding is
developed normally between a sodium glass and silicon for MEMS. A voltage is applied
between the glass and silicon, and a heater also maintains bonding temperature around
180-500 ºC simultaneously. A strong hermetic chemical bond is formed between silicon
and glass during the bonding.
Direct bonding (or silicon fused bonding) is used for silicon wafer to silicon wafer
bonding, so, also termed as. The basis of this type of bonding is the chemical reaction
between the hydroxyl groups present on the surface of native silicon or developed oxide
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films covering the wafers involved. Wafer-bonding continues to progress with vertically
integrated micromachined filters which are demonstrated at frequencies ~ 10GHz. In
wafer bonding, bulk-micromachined substrates are bonded together.
3.3.4: LIGA (High-Aspect-Ratio Micromachining)
High-aspect-ratio micromachining (HARM) is a process that involves
micromachining as a tooling step followed by injection moulding and, if required, by
electroforming to replicate microstructures in metal from molded parts. It is one of the
most attractive technologies for replicating microstructures at a high performance-to-cost
ratio. LIGA is an important tooling and replication method for high-aspect-ratio
microstructures. LIGA products are high aspect ratio fluidic structures such as molded
nozzle plates for inkjet printing. The materials that can be used are electro-formable
plastics (including acrylate, polycarbonate, styrene, polyimide) and metals.
LIGA (Lithographie Galvanoformung Abformung – German acronym for
lithography, electroplating and molding) is a process for producing high aspect ratio
microstructures (HARMs). Unlike usual optical lithography, X-rays are used to produce
the mould made from a polymer resist (e.g. polymethylmethacrylate (PMMA)) and a thin
metal base. Electroplating is used to fill the mould with a metal and the remainder of the
resist is removed, leaving high aspect ratio microstructures. X-ray lithography allows the
creation of HARMs with aspect ratios of greater than 100; however, the use of collimated
X-rays from synchrotrons makes this process expensive. LIGA is used to produce HARM
structures in Ni-Co and PZT (lead zirconate titanate).
The technique employs X-ray synchrotron radiation to expose thick acrylic resist
of PMMA under a lithographic mask. The exposed areas are chemically dissolved and, in
areas where the material is removed, metal is electroformed, thereby defining the tool
insert for the succeeding moulding step. LIGA is capable of creating very finely defined
three dimensional microstructures having heights of many hundred micrometers, which is
impossible with other micromachining techniques using silicon. Various metal ceramics,
plastics, or their combinations can be used to obtain difficult and complex
microstructures. An illustration representing the LIGA technique is shown in Figure 3.8.
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(a)
(e)
(b)
(f)
(c)
(g)
(d)
(h)
Figure 3.8: Various steps involved in a typical LIGA process.
LIGA is limited by the need to have access to an X-ray synchrotron facility. A
compromise which combines some features of LIGA with surface micromachining
eliminating the need for exposure to X-rays has been developed and is known as SLIGA
(Sacrificial LIGA). It replaces the thick PMMA photoresist with polyimide as the
electroplating mould, thus enabling compatible conventional IC batch processing. HARM
production methods have provided radically new ways to produce micromachined parts
for MEMS devices at relatively low cost. In particular, techniques such as SLIGA enable
the production of MEMS components with much lower manufacturing infrastructures in
terms of investment, facilities and access to advanced materials and technology.
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3.3.5: Flip Chip Method
In the flip chip method, a chip is mounted on a substrate with an interconnect
material by employing methods like tape-automated bonding, fluxless solder bumps,
metallurgy bumps, wire interconnects, pressure contacts, and compliant bumps,
conductive polymers, anisotropy conductive adhesives, etc. The components used in flip
chip are mainly semiconductor devices. With the progress in this technology, now-adays, MEMS devices have started replacing in the flip chip shape.
In the flip chip process, gold transfer bumps and the anchor bump are placed
properly on the transfer pads that are made on the MEMS host substrate. These transfer
pads are merely connected to the host substrate with the silicon dioxide sacrificial layer.
These transfer pads and hence, the MEMS devices are entirely disconnected from the
host substrate when the silicon dioxide sacrificial layer is dissolved in the hydrofluoric
acid. There is permanent connection between the anchor and the host substrate. Due to
this reason, the anchor prevents the contact between the host substrate and the device
after release. In addition to it, it also suspends the substrate to act as a shield during the
drying and the release processes.
Then the gold bumps are pressed against a smooth glass substrate. This pressing is
done before bonding the MEMS device to other substrates. The pressing is needed to
compress any trailing wires from the top of the bump left by the wire bonding machine.
Moreover, the pressing of the bumps facilitates improved planarity of the bumps
throughout the actual bonding. Then the complete structure MEMS flip chiplet bonded to
a target substrate such as coplanar waveguide (CPW) using thermal sonic bonding. The
silicon substrate is anchored on an anchor bump. Then silicon dioxide is removed with
the help of HF (hydrofluoric acid). Then, the host substrate is removed with the help of
clamps to crack the anchor without any harm to the MEMS device.
After that, HF is prevented from doing more etching inside the devices by
washing the flip chip devices thoroughly with methanol so that the HF is removed. The
devices are rinsed in the methanol for some time and then these devices are well-dried
properly with the help of a special drying chamber. This chamber uses liquid carbondioxide to displace the methanol. This step is needed because, otherwise, methanol would
pull devices below and would make contact with the substrate and therefore, the desired
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devices would be damaged. In the end, the MEMS device can be transferred to the other
substrate effectively. The schematic diagram of flip chip technique is shown in the Figure
3.9.
This technology has benefits of highest electrical performance, smallest size (due
to no requirement of package and bond wires, the desired area get reduced), more
reliability, greatest input and output density, lower cost, and more flexibility as compared
to other technologies. In addition to these benefits, there is better availability of devices,
materials, and apparatus employed for flip chip technology. Also, flip chips are solid tiny
blocks of cured epoxy when finished with adhesive; hence, these are the most rugged
structures mechanically.
(a) MEMS Chip with Gold bumps.
(b) Pressing Gold bumps on a smooth glass substrate.
(c) Bonding MEMS chip to CPW substrate by thermal sonic bonding.
(d) Removing Silicon Dioxide using Hydrofluoric acid etchant.
(e) Removing MEMS host substrate.
Figure 3.9: Various steps involved in a typical flip chip technique.
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The MEMS organizations and industries are attracted by the flip chip technology,
due to its ability to integrate a system on a chip by packaging many dies on the same
substrate. Also, many MEMS fabrication techniques only limited structural layers due to
problems relating to price and time. But, in this process, many structural layers are
presented to the MEMS designers. This technology provides more space and many
releasable layers to manufacture the designs. MEMS devices can be easily transferred to
the other substrates. Moreover, flip chip technology is also employed for the production
of extremely advanced MEMS that are much appropriate for the applications concerning
to radio frequency, microwave, sub-millimeter wave, and optical frequency.
3.4: Computer Aided Design
Computer Aided Design (CAD) is, in general, used in RF-MEMS for the design
of photolithographic masks. This is a simple process as MEMS structures are relatively
large in comparison to the sub-micron structures usually associated with the silicon chip
components. In addition to the use of CAD for mask designing, CAD and finite element
analysis (FEA) are important simulation tools for the designing MEMS applications.
Unluckily, so far there is a short of acceptable extremely developed software based
design tools to model, analyze and simulate MEMS microstructures completely. This has
acted as a stumbling block to the advancement of RF-MEMS devices and components.
In these days, MEMCAD is most familiar, successful and commercially
obtainable software design tool. Microcosm Technologies in North Carolina, USA, has
evolved this package. The MEMCAD package would be capable of quickly solving
thermal, mechanical, magnetic, electrostatic, fluidic, and optical and radio frequency
solutions in a united manner. The MEMCAD system specifies layout and process of a
device, and then constructs the three dimensional geometry of the device, assembles a
comprehensive three dimensional model and analyses performance of device. Besides all
these activities, this MEMCAD system also performs the variations in the design
manufacturing and sensitivity of a device.
MEMS Pro is another package from Tanner Research in California. It provides
RF-MEMS designers to simulate the development or deposition, implantation or
dispersion. It also facilitates simulation of any of the etch step concerned within a MEMS
fabrication technique. However, the problems wanting solutions in the MEMS are much
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diverse and hence need many more solvers and a more sophisticated CAD tool. Without
the standard CAD tool, it is difficult to provide cost minimized solutions guaranteed to
work in the present systems.
However, the complete simulation of the MEMS that are integrated with the
circuits is a challenge. A novel model is generated each time the geometry is altered.
Now-a-days, a concerted step is being done to grow methods to decompose an integrated
MEMS design into a hierarchical design of fundamental MEMS elements. Then, such a
design can be completely integrated into the IC CAD tool. Then, IC and MEMS solutions
can be solved at the same time by the same tool and variations in the size of the
fundamental MEMS elements can be computed at once. Existing solvers do the balanced
solutions by a lengthy iterative method.
3.5:
Summary
After many decades of progress, the RF-MEMS field is growing exponentially,
due to the high volume commercial markets and innovative research fields enabled by
novel functions made possible by MEMS technology. In this chapter, several materials
employed to fabricate the RF-MEMS devices, components or systems are described. The
prominent, well-defined characteristics or parameters are also given or tabulated.
Materials and process technology are basic factors for the realization of RF-MEMS
products. Silicon is still the prevailing and widely used material in both mechanical parts
and electronic parts in MEMS devices; because it is easily and economically available
with excellent mechanical and electrical properties.
After silicon, gallium arsenide is most common semiconductor. Gallium arsenide
finds most of its uses due to its better optical and electrical properties. Polycrystalline
silicon (polysilicon) is used as a structural material in the surface micromachining
process. Since it is difficult to grow thin films of single crystal silicon, thin films of
polysilicon are grown. Silicon dioxide can be used as the sacrificial layer material in the
surface micromachining process because it is prone to etching with hydrogen fluoride
acid whereas silicon is resistant to etching. Silicon nitride is a useful masking material for
alkaline etch solutions and also as an insulating thin film. Silicon nitride (Si3N4) has vital
mechanical properties that make it an attractive material to work with. It is a better
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thermal insulator than polysilicon, so it is used for isolating surface micromachined
structures.
The most common fabrication techniques employed for microelectromechanical
systems such as bulk micromachining, wafer-to-wafer bonding, fusion bonding, surface
micromachining, the LIGA process and flip chip technique are briefly described here.
The most mature and familiar micromachining fabrication technique is the bulk
micromachining.
This micromachining is employed to fabricate micromechanical
structures inside the bulk of a silicon wafer by selectively etching material of the wafer.
In surface micromachining, thin films are sequentially deposited and patterned on
top of the substrate, with the final removal of the sacrificial layer to release the movable
structures. The LIGA process involves X-ray lithography of thick photo-resist on
conductive substrate, plating through the developed resist, and injection molding with the
plated components. The flip chip technology has benefits of more electrical performance,
smallest size, more reliability, greatest input/output density, lower cost, and more
flexibility as compared to other technologies. The progress of material technology and
fabrication techniques are the key to the success of novel RF-MEMS systems.
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