MICROMACHINING

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

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

Mask/reticle

Exposure

After Development

Negative Photoresist

UV light

Positive Photoresist

Substrate

Substrate

Substrate

Photoresist

Negative and Positive Photoresists

SubstratePhotoresist

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

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)/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

BULKMICROMACHINED PARTS

TYPICAL MEMS PARTSSILICON MIRROR ASSEMBLY

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

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

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

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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