Anisotropy is the structural property of non-uniformity in different directions, as opposed to isotropy. An anisotropic object or pattern has properties that differ according to direction of measurement. For example many materials exhibit very different properties when measured along different axes: physical or mechanical properties.
MEMS is the technology of microscopic devices incorporating both electronic and moving parts. MEMS are made up of components between 1 and 100 micrometres in size, and MEMS devices generally range in size from 20 micrometres to a millimetre, although components arranged in arrays can be more than 1000 mm2. They usually consist of a central unit that processes data and several components that interact with the surroundings.
In integrated circuit manufacturing, photolithography or optical lithography is a general term used for techniques that use light to produce minutely patterned thin films of suitable materials over a substrate, such as a silicon wafer, to protect selected areas of it during subsequent etching, deposition, or implantation operations. Typically, ultraviolet light is used to transfer a geometric design from an optical mask to a light-sensitive chemical (photoresist) coated on the substrate. The photoresist either breaks down or hardens where it is exposed to light. The patterned film is then created by removing the softer parts of the coating with appropriate solvents, also known in this case as developers.
Isotropic etching is a method commonly used in semiconductors to remove material from a substrate via a chemical process using an etchant substance. The etchant may be in liquid-, gas- or plasma-phase, although liquid etchants such as buffered hydrofluoric acid (BHF) for silicon dioxide etching are more often used. Unlike anisotropic etching, isotropic etching does not etch in a single direction, but rather etches in multiple directions within the substrate. Any horizontal component of the etch direction may therefore result in undercutting of patterned areas, and significant changes to device characteristics. Isotropic etching may occur unavoidably, or it may be desirable for process reasons.
Reactive-ion etching (RIE) is an etching technology used in microfabrication. RIE is a type of dry etching which has different characteristics than wet etching. RIE uses chemically reactive plasma to remove material deposited on wafers. The plasma is generated under low pressure (vacuum) by an electromagnetic field. High-energy ions from the plasma attack the wafer surface and react with it.
Dry etching refers to the removal of material, typically a masked pattern of semiconductor material, by exposing the material to a bombardment of ions that dislodge portions of the material from the exposed surface. A common type of dry etching is reactive-ion etching. Unlike with many of the wet chemical etchants used in wet etching, the dry etching process typically etches directionally or anisotropically.
Surface micromachining builds microstructures by deposition and etching structural layers over a substrate. This is different from Bulk micromachining, in which a silicon substrate wafer is selectively etched to produce structures.
Bulk micromachining is a process used to produce micromachinery or microelectromechanical systems (MEMS).
Deep reactive-ion etching (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. It was developed for microelectromechanical systems (MEMS), which require these features, but is also used to excavate trenches for high-density capacitors for DRAM and more recently for creating through silicon vias (TSVs) in advanced 3D wafer level packaging technology. In DRIE, the substrate is placed inside a reactor, and several gases are introduced. A plasma is struck in the gas mixture which breaks the gas molecules into ions. The ions accelerated towards, and react with the surface of the material being etched, forming another gaseous element. This is known as the chemical part of the reactive ion etching. There is also a physical part, if ions have enough energy, they can knock atoms out of the material to be etched without chemical reaction.
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 at a sample. The plasma source, known as etch species, can be either charged (ions) or neutral. During the process, the plasma generates volatile etch products at room temperature from the chemical reactions between the elements of the material etched and the reactive species generated by the plasma. Eventually the atoms of the shot element embed themselves at or just below the surface of the target, thus modifying the physical properties of the target.
Tetramethylammonium hydroxide (TMAH or TMAOH) is a quaternary ammonium salt with molecular formula N(CH3)4+ OH−. It is commonly encountered in form of concentrated solutions in water or methanol. TMAH in solid state and its aqueous solutions are all colorless, but may be yellowish if impure. Although TMAH has virtually no odor when pure, samples often have a strong fishy smell due to presence of trimethylamine which is a common impurity. TMAH has several diverse industrial and research applications.
Advanced silicon etching (ASE) is a deep reactive-ion etching (DRIE) technique to etch deep and high aspect ratio structures in silicon. ASE was created by Surface Technology Systems Plc (STS) in 1994 in the UK. STS has continued to develop this process with faster etch rates. STS developed and first implemented the switched process, originally invented by Dr. Larmer in Bosch, Stuttgart. ASE consists in combining the faster etch rates achieved in an isotropic Si etch (usually making use of an SF6 plasma) with a deposition or passivation process (usually utilising a C4F8 plasma condensation process) by alternating the two process steps. This approach achieves the fastest etch rates while maintaining the ability to etch anisotropically, typically vertically in Microelectromechanical Systems (microelectromechanical systems (MEMS)) applications.
In semiconductor electronics fabrication technology, a self-aligned gate is a transistor manufacturing approach whereby the gate electrode of a MOSFET is used as a mask for the doping of the source and drain regions. This technique ensures that the gate is naturally and precisely aligned to the edges of the source and drain.
A hardmask is a material used in semiconductor processing as an etch mask instead of a polymer or other organic "soft" resist material.
Buffered oxide etch (BOE), also known as buffered HF or BHF, is a wet etchant used in microfabrication. Its primary use is in etching thin films of silicon dioxide (SiO2) or silicon nitride (Si3N4). It is a mixture of a buffering agent, such as ammonium fluoride (NH4F), and hydrofluoric acid (HF). Concentrated HF (typically 49% HF in water) etches silicon dioxide too quickly for good process control and also peels photoresist used in lithographic patterning. Buffered oxide etch is commonly used for more controllable etching.
Tokyo Electron Limited, or TEL, is a Japanese electronics and semiconductor company headquartered in Akasaka, Minato-ku, Tokyo, Japan. The company was founded as Tokyo Electron Laboratories, Inc. in 1963.
The Wright etch is a preferential etch for revealing defects in <100>- and <111>-oriented, p- and n-type silicon wafers used for making transistors, microprocessors, memories, and other components. Revealing, identifying, and remedying such defects is essential for progress along the path predicted by Moore's law. It was developed by Margaret Wright Jenkins (1936-2018) in 1976 while working in research and development at Motorola Inc. in Phoenix, AZ. It was published in 1977. This etchant reveals clearly defined oxidation-induced stacking faults, dislocations, swirls and striations with minimum surface roughness or extraneous pitting. These defects are known causes of shorts and current leakage in finished semiconductor devices should they fall across isolated junctions. A relatively low etch rate at room temperature provides etch control. The long shelf life of this etchant allows the solution to be stored in large quantities.
Photolithography is a process in removing select portions of thin films used in microfabrication. Microfabrication is the production of parts on the micro- and nano- scale, typically on the surface of silicon wafers, for the production of integrated circuits, microelectromechanical systems (MEMS), solar cells, and other devices. Photolithography makes this process possible through the combined use of hexamethyldisilazane (HMDS), photoresist, spin coating, photomask, an exposure system and other various chemicals. By carefully manipulating these factors it is possible to create nearly any geometry microstructure on the surface of a silicon wafer. The chemical interaction between all the different components and the surface of the silicon wafer makes photolithography an interesting chemistry problem. Current engineering has been able to create features on the surface of silicon wafers between 1 and 100 μm.
Metal Assisted Chemical Etching is the process of wet chemical etching of semiconductors with the use of a metal catalyst, usually deposited on the surface of a semiconductor in the form of a thin film or nanoparticles. The semiconductor, covered with the metal is then immersed in an etching solution containing and oxidizing agent and hydrofluoric acid. The metal on the surface catalyzes the reduction of the oxidizing agent and therefore in turn also the dissolution of silicon. In the majority of the conducted research this phenomenon of increased dissolution rate is also spatially confined, such that it is increased in close proximity to a metal particle at the surface. Eventually this leads to the formation of straight pores that are etched into the semiconductor. This means that a pre-defined pattern of the metal on the surface can be directly transferred to a semiconductor substrate.
Vapor etching refers to a process used in the fabrication of Microelectromechanical systems (MEMS) and Nanoelectromechanical systems (NEMS). Sacrificial layers are isotropically etched using gaseous acids such as Hydrogen fluoride and Xenon difluoride to release the free standing components of the device.
