Many compound materials exhibit polymorphism, that is they can exist in different structures called polymorphs. Silicon carbide (SiC) is unique in this regard as more than 250 polymorphs of silicon carbide had been identified by 2006,with some of them having a lattice constant as long as 301.5 nm, about one thousand times the usual SiC lattice spacings. The polymorphs of SiC include various amorphous phases observed in thin films and fibers,as well as a large family of similar crystalline structures called polytypes Regions of the wafer crystallography which are polycrystalline or of a different polytype material than the remainder of the wafer, such as 6H mixed in with a 4H type substrate. Foreign polytype regions frequently exhibit color changes or distinct boundary lines, and are judged in terms of area percent under diffuse illumination.
They are interfaces where crystals of different orientations meet. A grain boundary is a single-phase interface, with crystals on each side of the boundary being identical except in orientation. The term "crystallite boundary" is sometimes, though rarely, used. Grain boundary areas contain those atoms that have been perturbed from their original lattice sites, dislocations, and impurities that have migrated to the lower energy grain boundary.
A scratch is dened as a singular cut or groove into the frontside wafer surface with a length-to-width ratio of greater than 5 to 1, and visible under hight intensity illumination.
Crystalline solids exhibit a periodic crystal structure. The positions of atoms or molecules occur on repeating fixed distances, determined by the unit cell parameters. However, the arrangement of atom or molecules in most crystalline materials is not perfect. The regular patterns are interrupted by crystallographic defects Striations in silicon carbide are dened as linear crystallographic defects extending down from the surface of the wafer which may or may not pass through the entire thickness of the wafer, and generally follow crystallographic planes over its length.
A cumulative subtraction of all noted defect areas from the frontside wafer quality area within the edge exclusion zone. The remaining percent value indicates the proportion of the frontside surface to be free of all noted defects (does not include edge exclusion).
Often shortened to roughness, is a measure of the texture of a surface. It is quantified by the vertical deviations of a real surface from its ideal form. If these deviations are large, the surface is rough; if they are small the surface is smooth.
A micropipe, also referred to as “micropore”, “microtube”, “capillary defect “or “pinhole defect”, is a crystallographic defect in a single crystal substrate.It is a important parameter to manufacturers of silicon carbide (SiC) substrates which are used in a variety of industries such as power semiconductor devices for vehicles and high frequency communication devices. However, during the production of these materials, the crystal undergoes internal and external stresses causing growth of defects, or dislocations, within the atomic lattice. A screw dislocation is a common dislocation that transforms successive atomic planes within a crystal lattice into the shape of a helix. Once a screw dislocation propagates through the bulk of a sample during the wafer growth process, a micropipe is formed. The presence of a high density of micropipes within a wafer will result in a loss of yield in the device manufacturing process. Micropipes and screw dislocations in epitaxial layers are normally derived from the substrates on which the epitaxy is performed. Micropipes are considered to be empty-core screw dislocations with large strain energy (i.e. they have large Burgers vector); they follow the growth direction (c-axis) in silicon carbide boules and substrates propagating into the deposited epitaxial layers. Factors which influence formation of micropipes (and other defects) are such growth parameters as temperature, supersaturation, vapor phase stoichiometry, impurities and the polarity of the seed crystal surface. Micropipe density (MPD) is a crucial parameter for silicon carbide (SiC) substrates that determines the quality, stability and yield of the semiconductor devices built on these substrates. The importance of MPD is underscored by the fact that all existing specifications for 6H- and 4H-SiC substrates set upper limits for it.
Wafers are grown from crystal having a regular crystal structure, with silicon having a diamond cubic structure with a lattice spacing of 5.430710 Å (0.5430710 nm).When cut into wafers, the surface is aligned in one of several relative directions known as crystal orientations. Orientation is defined by the Miller index with [100] or [111] faces being the most common for silicon.Orientation is important since many of a single crystal's structural and electronic properties are highly anisotropic. Ion implantation depths depend on the wafer's crystal orientation, since each direction offers distinct paths for transport.Wafer cleavage typically occurs only in a few well-defined directions. Scoring the wafer along cleavage planes allows it to be easily diced into individual chips ("dies") so that the billions of individual circuit elements on an average wafer can be separated into many individual circuits. In silicon carbide,the growth plane of the crystalline silicon carbide. Orientations are described using Miller Indices such as (0001) etc. Different growth planes and orientations have different arrangements of the atoms or lattice as viewed from a particular angle.
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