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SURFACE MICROMACHININGNIKITA JAVIAGURUNATH APTEWhat 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 micromachiningSurface 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.

AdvantagesThe 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.DisadvantagesMultiple 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 supportCleanliness is critical at end of process.Sawing, packing and testing is difficult.APPLICATIONSUsed in manufacturing of flat panel television screenUsed in production of thin solar cellsUsed in making bimetal cantilever used for monitoring mercury vapour, moisture, protein conformational changes in antigen antibody bindingExamplesSurface Micromachining can be seen in action in the following MEMS products:

Surface Micromachined Accelerometers.3D Flexible Multichannel Neural Probe Array.Nanoelectromechanical relaysFabrication ProcessMicromachining 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 LAYERThe 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 resistancePolysilicon 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 MicroscopyProblems with existing processesEtching of positive pyramidsDifficult tot control etch stop pointuniformity difficult to obtainBulk etchingLong etching time involved to etch through the wafer

A hybrid method to fabricate SPM probes

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