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MICROMACHINING
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Refers to techniques for fabrication of 3D structures on the micrometer scale
Most methods use silicon as substrate material Some of process involved in this are
photolithography, etching, bulk machining, laser machining etc.
Unit metal removal rate and equipment precision are key factors to micro machining.
INTRODUCTION
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The wafers are chemically cleaned to remove particulate matter, organic, ionic, and metallic impurities
High-speed centrifugal whirling of silicon wafers known as "Spin Coating" produces a thin uniform layer of photo resist (a light sensitive polymer) on the wafers.
Photoresist is exposed to a set of lights through a mask often made of quartz
Two types of photoresist are used: – Positive: whatever shows, goes – Negative: whatever shows, stays
PHOTOLITHOGRAPHY
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Mask/reticle
Exposure
After Development
Negative Photoresist
UV light
Positive Photoresist
Substrate
Substrate
Substrate
Photoresist
Negative and Positive Photoresists
SubstratePhotoresist
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Etching is used in micro fabrication to chemically remove layers from the surface of a wafer during manufacturing.
For many etch steps, part of the wafer is protected from the etchant by a "masking" material which resists etching.
In some cases, the masking material is a photo resist which has been patterned using photolithography.
Other situations require a more durable mask, such as silicon nitride.
These are of two types. They are 1)dry etching 2)wet etching In wet etching these are of two types 1)isotropic 2)anisotropic
ETCHING
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WET ETCHING: The wafer can be immersed in a bath of etchant,
which must be agitated to achieve good process control. For instance, buffered hydrofluoric acid (BHF) is used commonly to etch silicon dioxide over a silicon substrate.
A perfectly isotropic etch produces round sidewalls. A perfectly anisotropic etch produces vertical
sidewalls. Red: masking layer; yellow: layer to be removed Anisotropic wet etching: Some wet etchants
etch crystalline materials at very different rates depending upon which crystal face is exposed.
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DRY ETCHING: Modern VLSI processes avoid wet etching, and
use plasma etching instead. The plasma produces energetic free radicals,
neutrally charged, that react at the surface of the wafer. Since neutral particles attack the wafer from all angles, this process is isotropic.
In this plasma contains a source gas which are rich in chlorine and fluorine such as carbontetraflouride in between electrodes and the wafer is etched.
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Uses anisotropic etching of single crystal silicon Bulk micromachining is a process used to
produce micro machinery or microelectromechanical systems (MEMS).
Common chemicals used in anisotropic etching processes are:
Potassium hydroxide (KOH)/H2O solutions, sometimes with ispropyl alcohol (IPA) additive at 65-85°CEthylene diamine pyrocatechol (EDP), diluted with water at 115ºC
Tetra methyl ammonium hydroxide (TMAH) and water at 90ºC
Hydrazine N2H4/H2O/IPA at 115ºC
BULK MICROMACHINING
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BULKMICROMACHINED PARTS
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TYPICAL MEMS PARTSSILICON MIRROR ASSEMBLY
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High-power laser pulses are used to evaporate matter from a target surface
A supersonic jet of particles (plume) is ejected normal to the target surface which condenses on substrate opposite to target.
The ablation process takes place in a vacuum chamber- either in vacuum or in the presence of some background gas
Excimer and Femto lasers are power source in this techniques
When high intensity laser is incident on the work piece it will remove the material by vaporization
LASER MICROMACHINING
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Highly flexible. Few processing steps. Capable serial or batch mode production. High quality products are produced. Good surface finish. Applicable to any type of material like polymer,
glasses, crystals. Material removal rate is minimum which is main
advantage for the machining at micrometer scale.
Ability to machine flat and contoured surfaces.
ADVANTAGES
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Expensive equipment required in laser micro machining
Surface ripples due to shock waves are formed in laser micromachining.
Maintaining of high quality and high accuracy is difficult
Maintaining high pressure, high degrees of thermal variability makes micromachining costly
There is high potential radiation damage on the surface of wafer in bulk machining.
Disadvantages
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Manufacturing of MEMS parts. In Biochip manufacture using laser machining. Laser ablation is commonly used for patterning
thin films to produce small features or isolate regions within a device.
Applications of laser ablation of thin films includes production of photovolatic cells.
Drilling fine pitch vertical probe cards for semiconductor testing.
Laser machining of the holes in fuel injector nozzles guarantees accuracy and high quality .
Applications
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THANK YOU