SURFACE MICROMACHINING

NIKITA JAVIA

GURUNATH APTE

What is micromachining?

• Micromachining is used to fabricate three-dimensional

microstructures and it is the foundation of a technology called

Micro-Electro-Mechanical-Systems (MEMS).

• Micromachining is the basic technology for fabrication of micro-

components of size in the range of 1 to 500 micrometers.

• Their need arises from miniaturization of various devices in

science and engineering, calling for ultra-precision manufacturing

and micro-fabrication.

• Bulk micromachining and surface micromachining are two major

categories in this field.

Surface micromachining

• 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.

• Added layers are generally very thin with their size varying from 2-5

Micro metres.

• The size of the substrates can also be much larger than a silicon wafer,

and surface micromachining is used to produce TFTs on large area

glass substrates for flat panel displays.

Advantages• 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.

• The surface micromachined components are smaller in

thickness and mass.

• The expensive silicon wafers can be replaced by cheaper

substrates, such as glass or plastic.

• It is cost effective.

Disadvantages

• Multiple deposition and etching required to build up

structures.

• Vertical dimensions are limited to the thickness of the

deposited layers leading to compliant suspended structures

with tendency to stick support

• Cleanliness is critical at end of process.

• Sawing, packing and testing is difficult.

APPLICATIONS

• Used in manufacturing of flat panel television screen

• Used in production of thin solar cells

• Used in making bimetal cantilever used for monitoring

mercury vapour, moisture, protein conformational changes in

antigen antibody binding

ExamplesSurface Micromachining can be seen in action in the following

MEMS products:

• Surface Micromachined Accelerometers.

• 3D Flexible Multichannel Neural Probe Array.

• Nanoelectromechanical relays

Fabrication Process• Micromachining starts with a silicon wafer or other substrate

and grows layers on top.

• These layers are selectively etched by photolithography and

either a wet etch involving an acid or a dry etch involving an

ionized gas, or plasma.

• Dry etching can combine chemical etching with physical

etching, or ion bombardment of the material.

• Surface micromachining can involve as many layers as is

needed with a different mask on each layer.

• Surface micromachining uses developed technology which is

very repeatable for volume production.

STUCTURAL LAYER• The layer of thin film material with which the microstructures are made of.

• Has physical and chemical properties that are suitable for the desired

application

• Mechanical properties such as high yield and fracture stresses, minimal

creep and fatigue, and good wear resistance

• Polysilicon is usually used as a structural material.

Disadvantages of poly Si (over single crystal Si)

• Lower yield strength

• Lower piezo resistivity

• Stiction.

Sacrificial Layer

• The layer of material used during the fabrication process to

deposit microstructures.

• These are removed towards the end of the fabrication. So, the

layer has no role in the operation of the device.

• Good mechanical properties so that device does not fail while

fabrication

• Good adhesion

• Low residual stresses

Basic Sacrificial Layer Processing

Example: Scanning Probe

Microscopy

• Problems with existing processes

o Etching of positive pyramids

• Difficult tot control etch stop point

• uniformity difficult to obtain

o Bulk etching

• Long etching time involved to etch through

the wafer

A hybrid method to fabricate SPM probes

THANK YOU