Micro electro mechanical system

1. Presented by
Shubham Kumar Gupta
Roll no. 13/ME/143
Department of Mechanical Engineering
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2. Micro-electromechanical systems (MEMS) is a process technology used to
create tiny integrated devices or systems that combine mechanical and
electrical components. They are fabricated using integrated circuit (IC)
batch processing techniques and can range in size from a few micrometers
to millimetres. These devices (or systems) have the ability to sense, control
and actuate on the micro scale, and generate effects on the macro scale
MEMS takes advantage of either silicon’s mechanical properties or both its
electrical and mechanical properties. In the most general form, MEMS
consist of mechanical microstructures, microsensors, microactuators and
microelectronics, all integrated onto the same silicon chip. MEMS can be
found in systems ranging across automotive, medical, electronic,
communication and defence applications. If semiconductor
microfabrication was seen to be the first micromanufacturing revolution,
MEMS is the second revolution.
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3.  SUBSTRATE OR WAFER MATERIAL
The most common substrate material for micromachining is silicon. It has been
successful in the microelectronics industry and will continue to be in areas of
miniaturization for several reasons:
a) silicon is abundant, inexpensive, and can be processed to unparalleled purity.
b) silicon’s ability to be deposited in thin films is very amenable to MEMS .
c) high definition and reproduction of silicon device shapes using photolithography are
perfect for high levels of MEMS precision.
d) silicon microelectronics circuits are batch fabricated (a silicon wafer contains
hundreds of identical chips not just one).
Other crystalline semiconductors including germanium (Ge) and gallium arsenide
(GaAs) are used as substrate materials due to similar inherent features, but silicon is
distinguished from other semiconductors in that it can be readily oxidized to form a
chemically inert and electrically insulating surface layer of SiO2 on exposure to steam.
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4.  ADDITIVE FILMS AND MATERIALS
The range of additive films and materials for MEMS devices is
much larger than the types of possible substrates and includes
conductors, semiconductors and insulators such as:
• silicon - single crystal, polycrystalline and amorphous
• silicon compounds (SixNy, SiO2, SiC etc.)
• metals and metallic compounds (Au, Cu, Al, ZnO, GaAs, IrOx,
CdS)
• ceramics (Al203 and more complex ceramic compounds)
• organics (diamond, polymers, enzymes, antibodies, DNA etc.)
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5. Deposition:
o Physical deposition :
Physical deposition consists of a process in which a material is removed
from a target, and deposited on a surface
o Chemical deposition :
Chemical deposition techniques include chemical vapor deposition, in
which a stream of source gas reacts on the substrate to grow the material
desired.
Patterning:
Patterning of MEMS is the transfer of a pattern into a material. Lithography is
widely used process in patterning of MEMS.
Examples of lithography are– Photolithography, Electron beam lithography
Ion beam lithography, Ion track technology, X-ray lithography.
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6. ETCHING:
o Wet etching :
Wet chemical etching consists in selective removal of material by
dipping a substrate into a solution that dissolves it.The chemical
nature of this etching process provides a good selectivity.
o Dry etching :
Dry etching relies on vapour phase or plasma-based methods of
etching using suitably reactive gases or vapours usually at high
temperatures. The most common form for MEMS is reactive ion
etching (RIE) which utilizes additional energy in the form of radio
frequency (RF) power to drive the chemical reaction.
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8. Bulk Micromachining
High Aspect Ratio Micromachining
(HARM)
Surface Micromachining
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9. Bulk micromachining involves the removal of
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 large pits, grooves and channels.
Materials typically used for wet etching include
silicon and quartz, while dry etching is typically
used with silicon, metals, plastics and ceramics.
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10. WET ETCHING:
Wet etching describes the removal of material through the immersion of a
material (typically a silicon wafer) in a liquid bath of a chemical etchant.
These etchants can be isotropic or anisotropic.
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11. DRY ETCHING:
•Dry etching relies on vapour phase or plasma-
based methods of etching using suitably reactive
gases or vapours usually at high temperatures.
•The most common form for MEMS is reactive ion
etching (RIE) which utilizes additional energy in
the form of radio frequency (RF) power to drive the
chemical reaction.
• Energetic ions are accelerated towards the
material to be etched within a plasma phase
supplying the additional energy needed for the
reaction; as a result the etching can occur at much
lower temperatures (typically 150º - 250ºC,
sometimes room temperature) than those usually
needed (above 1000ºC).
•RIE is not limited by the crystal planes in the
silicon, and as a result, deep trenches and pits, or
arbitrary shapes with vertical walls can be etched. FIG.:Deep Reactive Ion
Etching 11

12. Surface micromachining involves processing above the substrate, mainly using
it as a foundation layer on which to build. Material is added to the substrate in
the form of layers of thin films on the surface of the substrate (typically a
silicon wafer). These layers can either by structural layers or act as spacers,
later to be removed, when they are known as sacrificial layers. Hence the
process usually involves films of two different materials: a structural material
out of which the free standing structure is made (generally polycrystalline
silicon or polysilicon, silicon nitride and aluminium) and a sacrificial material,
deposited wherever either an open area or a free standing mechanical structure
is required (usually an oxide).
These layers (or thin films) are deposited and subsequently dry etched in
sequence, with the sacrificial material being finally wet etched away to release
the final structure. Each additional layer is accompanied by an increasing level
of complexity and a resulting difficulty in fabrication.
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13. A typical surface micromachined cantilever beam is shown in figure . Here, a
sacrificial layer of oxide is deposited on the silicon substrate surface using a
pattern and photolithography. A polysilicon layer is then deposited and patterned
using RIE processes to form a cantilever beam with an anchor pad. The wafer is
then wet etched to remove the oxide (sacrificial) layer releasing and leaving the
beam on the substrate. More complex MEMS structures can be achieved using
structural polysilicon and sacrificial silicon dioxide, including sliding structures,
actuators and free moving mechanical gears.
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14. High-aspect-ratio micromachining (HARM) is a process that
involves micromachining as a tooling step followed by injection
moulding or embossing and, if required, by electroforming to
replicate microstructures in metal from moulded parts. It is one of
the most attractive technologies for replicating microstructures at
a high performance-to-cost ratio and includes techniques known
as LIGA. Products micromachined with this technique include
highaspect-ratio fluidic structures such as moulded nozzle plates
for inkjet printing and microchannel plates for disposable
microtitreplates in medical diagnostic applications. The materials
that can be used are electroformable metals and plastics,
including acrylate, polycarbonate, polyimide and styrene.
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15. Lithographie, Galvanoformung, Adformung
LIGA:
LIGA is a German acronym for Lithographie, Galvanoformung, Adformung
(Lithography, Electroplating and Molding) that describes a fabrication
technology used to create high-aspect-ratio microstructures. 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 microstructures up to 1000 µm high. LIGA is limited by the
need to have access to an X-ray synchrotron facility
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16. • X-ray radiation is casted on sensitive PMMA
onto a suitable substrate.
• A special x-ray mask is used for the selective
exposure of the PMMA layer using x-rays.
• The PMMA is then developed and will be
defined with extremely smooth and nearly
perfectly vertical sidewalls.
• The penetration depth of the x-ray radiation
into the PMMA layer is quite deep and allows
exposure through very thick PMMA layers, up
to and exceeding 1 mm.
• After the development, the patterned PMMA
acts as a polymer mold and is placed into an
electroplating bath and Nickel is plated into the
open areas of the PMMA.
• The PMMA is then removed, thereby leaving
the metallic microstructure
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