MICROELECTROMECHANICAL SYSTEMS MEMS HARIKRISHNA SATISH T Introduction MEMS

MICROELECTROMECHANICAL SYSTEMS ( MEMS ) HARIKRISHNA SATISH. T

Introduction: • MEMS stands for Micro-electromechanical systems, a • • • manufacturing technology that enables the development of electromechanical systems using batch fabrication techniques similar to those used in integrated circuit (IC) design. They can range in size from micrometers to millimeters MEMS integrate mechanical elements, sensors, actuators and electronics on a silicon substrate using a process technology called micro fabrication. Micro electro mechanical System (MEMS) is making and combining of miniaturized mechanical and electrical components on a common silicon substrate through micro fabrication technology.

How MEMS work? • The sensors gather information by measuring mechanical, thermal, biological, chemical, magnetic and optical signals from the environment. • The microelectronic Ic’s act as the decision-making piece of the system, by processing the information given by the sensors. • Finally, the actuators help the system respond by moving, pumping, filtering or somehow controlling the surrounding environment to achieve its purpose.

Manufacturing process:

• The MEMS materials: • Primarily silicon its compounds • other materials are quartz crystal, glass, metals such as aluminum, • titanium, tungsten and copper polymers such as photo resist. • What is Micromachining ? • Micromachining is a parallel (batch) process in which dozens to tens • of thousands of identical elements are fabricated simultaneously on the same wafer. Divided into three major categories: basic, advanced, and nonlithographic processes.

• Basic process flow in micromachining: Figure : Illustration of the basic process flow in micromachining: Layers are deposited; photoresist is lithographically patterned and then used as a mask to etch the underlying materials. The process is repeated until completion of the microstructure.

BASIC PROCESS TOOLS Deposition Process Epitaxy: Epitaxy is a deposition method to grow a crystalline silicon layer over a silicon wafer, but with a differing dopant type and concentration Impurity dopants [As. H 3 ; PH 3] (Controlled) Silicon Contain source gas [Si. H 4/Si. Cl 4/Si. H 2 Cl 2] Wafer Scale p-n junction for Controlled electrochemical etching High Temperature ( ) [Controlled]

Oxidation: High-quality amorphous silicon dioxide is obtained by oxidizing silicon in either dry oxygen or in steam at elevated temperatures (850º– 1, 150ºC) Si + O 2/ Steam Si. O 2 It is Depend on Temperature , Oxidizing environment and Time. Sputter Deposition:

Evaporation Al , Si, Ti, Cr , Al 2 O 3 Heated at high Temperature (Scanning at high voltage electronic beam) Evaporation (Water cooling of the target and shielding from X-ray) Condenses on a substrate to form a film

CVD (Chemical Vapour Deposition) Ø Deposition Of Polysilicon : 25 p. a -150 p. a In CVD Chamber Si. H 4 Si + 2 H 2 Polysilicon may be grown directly with doping. Ø Deposition of Silicon dioxide : Si. H 4 + O 2 Si. Cl 2 H 2 + 2 N 2 O Si. O 2 Si(OC 2 H 5)4 + 2 N 2 O Si. O 2 + + 2 H 2 2 N 2 + 2 HCl + byproduct High temperature Oxide Doped Impurity P 2 O 5 Semiconductor

Ø Deposition of Silicon Nitrides 3 Si. H 4 + NH 3 Si 3 N 4 + 12 H 2 3 Si. Cl 2 H 2 + 4 NH 3 Si 3 N 4 + 6 HCl + 6 H 2 2 Si. H 4 + N 2 2 Si. NH + 3 H 2 NH 3 Si. NH + 3 H 2 Si. H 4 + • Silicon nitride is common in the semiconductor industry for the passivation of electronic devices because it forms an excellent protective barrier against the diffusion of water and sodium ions

Photolithography Photosensitive material Selectively expose to Light Non-exposed part (remain same in physical properties) Exposed part (physical property change) Perform Etching Deposition of metal or other thin film deposition

Ø Etching: In etching, the objective is to selectively remove material using imaged photoresist as a masking template. Wet Etching : Anisotropic Wet Etching: Dipped into Substrate KOH Solution Removal of material Depending on the Crystallographic orientation

Electrochemical Etching

Ø Reactive Ion Etching ( RIE ) § Process of reactive ion etching. Placed into Substrate Reactor contain several gas Plasma is struck in the mixture Gas molecule into ions React with the surface of the material and etched

Ø Deep Reactive Ion Etching ( DRIE ) SF 6 isotopic Etching (SF 6 & O 2 etches the substrate) C 4 F 8 deposition SF 6 anisotropic etching for floor cleaning

Using these processes some micromachining methods are applied on silicon materials are: • A) Bulk micromachining: Bulk micromachining designates the point at which the bulk of the Si substrate is etched away to leave behind the desired micromechanical elements. The methods commonly used to remove excess material are wet and dry etching, allowing varying degree of control on the profile of the final structures. • B) Surface Micromachining: The MEMS fabrication process based on standard CMOS microelectronic processes. MEMS structures are photo lithographically patterned in alternating layers of deposited polysilicon and silicon dioxide, and then are "released" by dissolving away the silicon dioxide layers.

Applications of MEMS : • • • inkjet-printer cartridges, accelerometer, miniature robots, micro engines, inertial sensors, micro actuators, optical scanners, fluid pumps, chemical, pressure and flow sensors. Application in Radio frequency (RF)

Reference : • By Maluf Nadim and Williams Kirt. An Introduction to Microelectromechanical Systems Engineering. • By committee on Advanced Material and Fabrication. Microelectromechanical Systems. • The MEMS Handbook.

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