 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.

 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

Mask/reticle Exposure After Development Negative Photoresist UV light Positive Photoresist Substrate Photoresist Negative and Positive Photoresists Substrate Photoresist

 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

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.

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.

 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)/H 2 O 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 N 2 H 4 /H 2 O/IPA at 115ºC

 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

 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.

 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.

 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.