Microfabrication techniques
Introduction
Microfabrication is a process used to fabricate miniature structures of micrometre scales and smaller. Historically, the earliest microfabrication processes were used for integrated circuit fabrication, also known as "semiconductor manufacturing" or "semiconductor device fabrication". In the last two decades microelectromechanical systems (MEMS), microsystems (European usage), micromachines (Japanese terminology) and their derivatives have re-used, adapted or extended microfabrication methods. Flat panel displays and solar cells are also using similar techniques.
Techniques
Microfabrication techniques can be divided into two categories: additive and subtractive. In additive processes, layers of materials are added to the wafer, while in subtractive processes, layers are removed to create the desired pattern.
Additive Processes
Additive processes include physical vapor deposition (PVD), chemical vapor deposition (CVD), electrochemical deposition (ECD), molecular beam epitaxy (MBE) and more recently, atomic layer deposition (ALD) among others.
Physical Vapor Deposition (PVD)
PVD is a method of depositing thin films by the condensation of a vaporized form of the desired film material onto various workpiece surfaces (e.g., onto semiconductor wafers). The coating method involves purely physical processes such as high-temperature vacuum evaporation with subsequent condensation, or plasma sputter bombardment rather than involving a chemical reaction at the surface to be coated as in chemical vapor deposition.
Chemical Vapor Deposition (CVD)
CVD is a chemical process used to produce high-purity, high-performance solid materials. The process is often used in the semiconductor industry to produce thin films. In a typical CVD process, the wafer (substrate) is exposed to one or more volatile precursors, which react and/or decompose on the substrate surface to produce the desired deposit.
Electrochemical Deposition (ECD)
ECD is a plating process in which metal ions in a solution are moved by an electric field to coat an electrode. The process can be used to build up thickness on undersized parts, or to form objects by electroforming.
Molecular Beam Epitaxy (MBE)
MBE is a method of depositing single crystals. MBE is widely used in the manufacture of semiconductor devices, including transistors for cellular phones and WiFi.
Atomic Layer Deposition (ALD)
ALD is a thin film deposition technique based on the sequential use of a gas phase chemical process; it is a subclass of chemical vapor deposition. The majority of ALD reactions use two chemicals, typically called precursors. These precursors react with the surface of a material one at a time in a sequential, self-limiting, manner.
Subtractive Processes
Subtractive processes include reactive-ion etching (RIE), deep reactive-ion etching (DRIE), plasma etching and wet etching.
Reactive-Ion Etching (RIE)
RIE is an etching technology used in microfabrication. It uses chemically reactive plasma to remove material deposited on wafers. The plasma is generated under low pressure (vacuum) by an electromagnetic field.
Deep Reactive-Ion Etching (DRIE)
DRIE is a highly anisotropic etch process used to create deep penetration, steep-sided holes and trenches in wafers/substrates, typically with high aspect ratios.
Plasma Etching
Plasma etching is a form of plasma processing used to fabricate integrated circuits. It involves a high-speed stream of glow discharge (plasma) of an appropriate gas mixture being shot (in pulses) at a sample.
Wet Etching
Wet etching is a material removal process that uses liquid chemicals or etchants to remove materials from a wafer. It can be either isotropic or anisotropic. Isotropic etching removes materials with the same rate in all directions while anisotropic etching removes materials with a different rate in each direction.
Applications
Microfabrication is the key technology behind integrated circuits, microsensors, microactuators and microsystems, collectively known as MEMS. The leading fields in the application of microfabrication include electronics, optoelectronics, magnetic devices, and Bio-MEMS.