// Semiconductor and thinfilm
Physical and Chemical Methods for Thin-Film Deposition and Epitaxial Growth
Thin film deposition is the process of adding a thin layer of one material on another in order to modify the properties of the initial underlying material, i.e., to increase hardness, to change electrical conduction, or to alter the optical properties of the initial material referred to as the substrate. Thin film deposition is primarily method for improving the structural and electronic properties of surfaces in order to prepare these surfaces for other experiments. For many experiments the desired surface will be that of a highly perfect single crystal.
Three major family of methods:
Chemical Vapor Deposition (CVD) – host of techniques would include:
Plasma Enhanced CVD (PECVD)
Low Pressure CVD (LPCVD)
Microwave assisted CVD
Physical Vapor Deposition (PVD) – host techniques would include:
Pulsed laser deposition (PLD)
Atomic Layer Deposition (ALD)
For Epitaxial Growth – host of techniques would include both chemical and physical vapor deposition like:
Metal Oxide CVD (MOCVD)
Vapor Phase Epitaxial (VPE) systems
Molecular Beam Epitaxial (MBE) system
Laser MBE basically PLD with k-cells/effusion cells for epitaxial growth.
For tutorials of PVD Systems we suggest following tutorial links from AJA International Corporation USA:
Sputtering is a technique used to deposit thin films of a material onto a surface (substrate). By first creating a gaseous plasma and then accelerating the ions from this plasma into some source material (a.k.a. "target"), the source material is eroded by the arriving ions via energy transfer and is ejected in the form of neutral particles - either individual atoms, clusters of atoms or molecules. As these neutral particles are ejected they will travel in a straight line unless they come into contact with something - other particles or a nearby surface. If a "substrate" such as a Si wafer is placed in the path of these ejected particles it will be coated by a thin film of the source material.
Thermal Evaporation is one of the simplest of the Physical Vapor Deposition (PVD) techniques. Basically, material is heated in a vacuum chamber until its surface atoms have sufficient energy to leave the surface. At this point they will traverse the vacuum chamber, at thermal energy (less than 1 eV), and coat a substrate positioned above the evaporating material (average working distances are 200 mm to 1 meter).
The pressure in the chamber must be below the point where the mean free path is longer than the distance between evaporation source and the substrate. The mean free path is the average distance an atom or molecule can travel in a vacuum chamber before it collides with another particle thereby disturbing its direction to some degree. This is typically 3.0 x 10-4 Torr or lower. The main reason to run at the high end of the pressure range is to allow an ion beam source to be employed simultaneously for film densification or other property modification.
Belongs to family of PVD now method or source being used is ebeam evaporation technique. E-Beam evaporation is a physical vapor deposition (PVD) technique whereby an intense, electron beam is generated from a filament and steered via electric and magnetic fields to strike source material (e.g. pellets of Au) and vaporize it within a vacuum environment. At some point as the source material is heated via this energy transfer its surface atoms will have sufficient energy to leave the surface. At this point they will traverse the vacuum chamber, at thermal energy (less than 1 eV), and can be used to coat a substrate positioned above the evaporating material. Average working distances are 300 mm to 1 meter.
Ion Milling is a physical etching technique whereby the ions of an inert gas (typically Ar) are accelerated from a wide beam ion source into the surface of a substrate (or coated substrate) in vacuum in order to remove material to some desired depth or underlayer. It is easily visualized as "atomic sandblasting", or more accurately "ionic sandblasting".
Pulsed Laser Deposition
Pulsed laser deposition (PLD) is a PVD technique where a high-power pulsed laser beam is focused inside a vacuum chamber to strike a target of the material that is to be deposited. This material is vaporized from the target (in a plasma plume) which deposits it as a thin film on a substrate. This process can occur in UHV or in the presence of a background gas, such as oxygen which is commonly used when depositing oxides to fully oxygenate the deposited films.
While the basic setup is simple relative to many other deposition techniques, the physical phenomena of laser-target interaction and film growth are quite complex. When the laser pulse is absorbed by the target, energy is first converted to electronic excitation and then into thermal, chemical and mechanical energy resulting in evaporation,ablation and plasma formation and even exfoliation. The ejected species expand into the surrounding vacuum in the form of a plume containing many energetic species including atoms, molecules, electrons, ions, clusters, particulates and molten globules, before depositing on the typically hot substrate.
Atomic Layer Deposition (ALD)
Molecular beam epitaxy (MBE)
Molecular beam epitaxy (MBE) is an epitaxy method for thin-film deposition of single crystals. It was invented in the late 1960s at Bell Telephone Laboratories by J. R. Arthur and A;fred Y. Cho. MBE is widely used in the manufacture of semiconductor devices, including transistors and it is considered one of the fundamental tools for the development of the nanotechnologies.
Chemical Vapor Deposition CVD
Chemical vapor deposition (CVD) is a chemical process used to produce high quality, high-performance, solid materials. The process is often used in the semiconductor industry to produce thin films. In typical CVD, 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. Frequently, volatile by-products are also produced, which are removed by gas flow through the reaction chamber.
Microfabrication processes widely use CVD to deposit materials in various forms, including: microcrystalline, poly crystalline, amorphous, and epitaxial. These materials include: Silicon oxide (SiO2), germanium. Carbide, nitride oxynitride, carbon (fiber, nanofibrers, nanotubes, diamond and graphene), fluorocarbons, filaments, tungsten, TiN and various high-k dielectric materials.
Low Pressure CVD
LPCVD is a CVD technology that uses heat to initiate a reaction of a precursor gas on the solid substrate. This reaction at the surface is what forms the solid phase material. Low pressure (LP) is used to decrease any unwanted gas phase reactions, and also increases the uniformity across the substrate. Since LPCVD helps to grow thin films at lower temperature so internal stress within the thinfilm is reduced.
Plasma Enhanced CVD
PECVD is a CVD process used to deposit thin films from a gas state (vapor) to a solid state on a substrate. Chemical reactions are involved in the process, which occur after creation of a plasma of the reacting gases. The plasma is generally created by radio frequency (RF) alternating current (AC)) frequency or direct current (DC) discharge between two electrodes, the space between which is filled with the reacting gases.
Metalorganic vapour phase epitaxy (MOVPE), also known as organometallic vapour phase epitaxy (OMVPE) or metalorganic chemical vapour deposition (MOCVD), is a CVD method used to produce single or polycrystalline thin films. It is a highly complex process for growing crystalline layers to create complex semiconductor multilayer structures. In contrast to MBE the growth of crystals is by chemical reaction and not physical deposition. This takes place not in a vacuum, but from the gas phase at moderate pressures (10 to 760 Torr). As such, this technique is preferred for the formation of devices incorporating thermodynamically metastable alloys, and it has become a major process in the manufacture of optoelectronics.