MICROMACHINING

by- NAKUL AGRAWAL

Micro Machining Machining of micro parts is not literally correct.

Removal of material in the form of chips or debris having the size in the range of microns.

Creating micro features or surface characteristics (especially surface finish) in the micro/nano level.

Definition: material removal at micro/nano level with no constraint on the size of the component being machined.

Why Micro Machining? Final finishing operations in manufacturing of precise

parts are always of concern owing to their most critical, labour intensive and least controllable nature.

In the era of nanotechnology, deterministic high precision finishing methods are of utmost importance and are the need of present manufacturing scenario.

The need for high precision in manufacturing was felt by manufacturers worldwide to improve interchangeability of components, improve quality control and longer wear/fatigue life.

Why Micro Machining?Present day High-tech Industries, Design requirements are stringent.

– Extraordinary Properties of Materials (High Strength, High heat Resistant, High hardness, Corrosion resistant etc)

– Complex 3D Components (Turbine Blades)

– Miniature Features (filters for food processing and textile industries having few tens of microns as hole diameter and thousands in number)

– Nano level surface finish on Complex geometries (thousands of turbulated cooling holes in a turbine blade)

– Making and finishing of micro fluidic channels (in electrically conducting & non conducting materials, say glass, quartz, &ceramics)

Different Micromachining Techniques

• Photolithography• Etching• Silicon Micromachining• LIGA• Mechanical Micromachining

Photolithography

Photolithography Process Description

• 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 photoresist (a light• sensitive polymer) on the wafers• Photoresist is exposed to a set of lights through a mask often made• of quartz• •Wavelength of light ranges from 300-500 nm (UV) and X-rays• (wavelengths 4-50 Angstroms)• • Two types of photoresist are used:• – Positive: whatever shows, goes• – Negative: whatever shows, stays

Etching

• Etching is used in micro fabrication to chemically remove layers from the surface of a wafer during manufacturing.

• Etching is a critically important process module, and every wafer undergoes many etching steps before it is complete.

• It is characterized by etch rate , etch selectivity and etch uniformity

Process Variations:

• 1. Wet etching• Etching processes used liquid-phase ("wet") etchants. 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.

• 2. Dry etching• Modern VLSI processes avoid wet etching, and use plasma

etching instead.• plasma etching operates between 0.1 and 5 Torr• 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

Anisotropic and Isotropic

Steps In Wet Etching

• Injection of hole into semiconductor to si+ state

• Attaching –ve charge oh group to positive charge Si

• Reaction between hydrated Si and complex agent in etchant solution

• Dissolution of reaction product

Dry etching

Bulk Micromachinig

• Bulk and surface micromachining are processes used to create microstructures on microelectromechanical MEMS devices.

• While both wet and dry etching techniques are available to both bulk and surface micromachining, bulk micromachining typically uses wet etching techniques while surface micromachining primarily uses dry etching techniques.

• Bulk micromachining selectively etches the silicon substrate to create microstructures on MEMS devices.

Surface Micromaching

• Unlike Bulk micromachining, where a silicon substrate (wafer) is selectively etched to produce structures, surface micromachining builds microstructures by deposition and etching of different structural layers on top of the substrate

• Generally polysilicon is commonly used as one of the layers and silicon dioxide is used as a sacrificial layer which is removed or etched out to create the necessary void in the thickness direction

• The main advantage of this machining process is the possibility of realizing monolithic microsystems in which the electronic and the mechanical components(functions) are built in on the same substrate.

LIGA Process

• An important technology of MST • Developed in Germany in the early 1980s• LIGA stands for the German words – LIthographie (in particular X-ray lithography) – Galvanoformung (translated electrodeposition or

electroforming) – Abformtechnik (plastic molding)

• The letters also indicate the LIGA process sequence

• Apply resist, X-ray exposure through mask,

• remove exposed portions of resist,

• electrode position to fill openings in resist,

• strip resist for (a) mold or (b) metal part

Processing Steps in LIGA

Process steps

• Making an intermediate X-ray absorption mask (IM) with about 2.2 µm high gold absorber structures by electron beam .

• Copying the intermediate mask into a working mask (WM) with about 25 µm high gold absorber structures by X-ray lithography.

• Copying the working mask to 100 µm to 3000 µm high micro structures by deep X-ray lithography

• Electroplating metals like gold, copper or nickel into these structures to form metal micro structures.

• Making a several millimetre thick mould from these structures by nickel electroplating.

• Mass replication of the mould into thermoplastic resin

Advantages of LIGA

• LIGA is a versatile process – it can produce parts by several different methods

• High aspect ratios are possible (large height-to-width ratios in the fabricated part)

• Wide range of part sizes is feasible - heights ranging from micrometers to centimeters

• Close tolerances are possible

Disadvantages of LIGA

• LIGA is a very expensive process– Large quantities of parts are usually required to justify

its application

• LIGA uses X-ray exposure– Human health hazard