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some of the common MEMS fabrication techniques. To begin with let's understand what is MEMS
fabrication, MEMS fabrication is a technique that use semiconductor manufacturing processes such
as iron etching diffusion oxidation sputtering etc. in combination with specialized micro
machining techniques, this machining occurs in the range of one to one hundred micrometers
in size where both the mechanical parts and the electronics that control them are built in
the same piece of silicon. MEMS fabrication consists in the application of the following steps
normally several times during the manufacturing the process starts with a polished silicon the
substrate wafer that undergoes these steps, such as thin film growth or deposition doping
lithography and etching and micromachining. Deposition in MEMS technique refers to deposition
of thin films of material onto the substrate and has a thickness ranging from few nanometers to
100 micrometers. MEMS deposition can be broadly classified into physical vapor deposition
PVD and chemical vapor deposition CVD. PVD can be further classified into evaporation
PVD and sputtering PVD. CVD is classified into atmospheric pressure CVD, low pressure CVD and
plasma enhanced CVD. evaporation PVD system consists of a high vacuum chamber with a crucible
containing an evaporant to be deposited that is heated by reactive inductive or by electron beam
eating techniques, the component to be coated is supported within the chamber, the system may
also have shutter systems to control deposition. Sputtering PVD employs plasma formed by large
voltage in low pressure across closely spaced electron pair, the target material at cathode is
bombarded by energetic ions from an inert gas, the sputtered target atoms are deposited
onto the substrate, sputtered atoms have higher mobility and hence better step coverage,
gases with higher atomic number is preferred, hence argon is the frequently used sputtering gas,
sputtering doesn't depend on substrate temperature though higher temperature is preferred, because
it can improve adhesion and prevent film cracking. CVD is used to deposit non-volatile solid film on
substrate by reaction of a vapor phase chemical, the desired gases are introduced into the
chamber at high temperature and low pressure, the gases moves over the substrate and the
substrate adsorbs reactants onto its surface, the film is formed by surface reactions such
as decomposition and surface mitigation, the byproducts is then desorbed and removed from
the chamber by forced convection. Lithography is one of the most common semiconductor
and MEMS fabrication technique process, it is the process of transferring desired shapes
on a photo mask to a thin layer of photoresistive material commonly PMMA or Novolac over the
surface of the silicon wafer or substrate. Optical lithography is most widely used but
for sub-micron resolution, e-beam lithography, x-ray lithography, ion beam lithography etc are
employed and the most popular light source for photolithography is high-pressure mercury arc
lamp. Photolithography is broadly categorized according to the basic exposure method into three,
namely contact printing, proximity printing, and projection printing. Contact printing has
high resolution but damages both the mask and resist layer as they are in contact.
Proximity printing was adopted to eliminate the damage done due to contact between mask and
resist layer but resulted in lower resolution due to diffraction. Projection printing evolved
to accomplish high resolution and eliminate damage due to contact between mask and resist layer.
Steps in photolithography. Wafer substrate such as silicon wafer is cleaned chemically to
remove any organic ionic or metallic impurities, deposition of barrier layer onto surface of wafer
such as sio2, adhesion promoter and photoresist applied by spin coating to form uniform thin
layers, the coated wafer is pre-baked at about 100 degrees Celsius. Photo mask aligned
over substrate and is exposed to UV light, then the substrate is placed in developer
to remove soluble area and post baked at 120 degrees Celsius. Pattern formed on photoresist is
followed by either dry or wet etching of exposed barrier layer, the resist is finally stripped
off using solvent. Bulk micromachining, here bulk material of substrate are selectively removed
in the fabrication of micromechanical structures, depending on the etchant used, it is classified
into wet etching that is use of chemical solution, and dry etching that is use of gas phase
etchants in plasma. Bulk micromachining can be classified into anisotropic or isotropic
depending on direction of etching of the layer, isotropic etchants etch the material irrespective
of direction while anisotropic etchants etch faster in one direction than the other. Surface
micro machining, in surface micro machining microstructures are fabricated using deposited
structural and sacrificial layers on substrate, steps can be summarized as sacrificial layer
deposited on substrate, and patterned structural layer deposited over sacrificial layer and
patterned sacrificial layer is removed by etchant to form freestanding microstructures. we're
touching here the wafer is immersed in chemicals and the exposed areas are etched and washed
away where touching process involves chemical reaction to etch the material and
produce water-soluble by-products dry etching it employs use of gas-phase etchants
in plasma to avoid undercutting and achieve ultra-large-scale integration dry etching is also
called plasma etching and can be classified into physical which is unselective and chemical based
which is isotropic dry etching is characterized as an isotropic in nature have high resolution
less undercutting better process control and less use of chemicals the basic steps in
the process are etchant species or reactants are generated in plasma and then add a sobbed on wafer
surface surface chemical reaction takes place on surface to facilitate etching volatile by-products
desorbed diffused and pumped out after etching reactive ion etching RIE is a good compromise
between anisotropy and selectivity SF6 gas is used to etch silicon the gas flows through a vacuum
chamber which is applied with RF voltage across two electrodes that facilitates some of the gases
to get ionized generating plasma wafer held on RF powered cathode while the ground chamber acts as
anode the RF voltage accelerates electrons to high kinetic energy and these high energy electrons
etch the substrate deep reactive ion etching DRIE is employed to achieve high aspect ratio
structures and deep groove with vertical cider walls by high density plasma base dry etching
the DRIE works similar to RIE except that a protective layer is deposited between etching
to achieve higher aspect ratio most commonly used deposition of protective layer on side walls
are by polymerization between the etching process LIGA a german acronym for lithography
galvanoformang ab for mung this process is used to create high aspect ratio structures using x-rays
or relatively low aspect ratio structures using UV rays the steps involves deposition of thick resist
layer PMMA on the metal surface PMMA is then exposed to mast x-rays and developed followed by
metal electrode deposition onto primary substrate PMMA is then removed to expose freestanding
structure finally plastic injection molding takes place and repeated if you have found the video
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