Properties of Silicon Wafers
As discussed in the article on incoming
wafers for wafer fab, the wafers used for VLSI manufacturing
must meet a multitude of stringent requirements to be able to serve as
reliable starting substrates onto which integrated circuits will be
built. These requirements include electrical, mechanical,
chemical, and surface characteristics that the wafers must
that must be checked in wafers include:
- the maximum deviation from peak to valley exhibited by a wafer as
measured with respect to a reference plane. Flatness is usually
expressed in micrometers (microns) and measured using an
industry-approved method, such as the one defined by ASTM Std-775-83. Depending on the application
of the wafer, a wafer flatness of 1-3 microns is typically acceptable.
2) Particle Density
- the number of particles present per unit area of the wafer.
Detection of particles can be done by visual means, optical microscopy,
or with automatic surface scanners. ASTM Std F-154 and F-24 are
examples of standards used for this requirement. Particle density must
be minimized in wafers.
- fracture that extends to the wafer surface caused by impact of sharp
features of fixtures against the wafer.
which is a special form of crack, is a fracture that resembles a crow's
foot in appearance in <111> material and a cross pattern in <100>
material. A wafer must not have any cracks.
- non-localized scattering of light caused by microscopic irregularities
or roughness on the surface of the wafer. Such surface irregularities
are often caused by crystalline defects and surface imperfections in the
wafer or by mechanical damage produced by external factors. Haze
is usually detected with a narrow beam of tungsten light per ASTM Std
- etch pits that are small and shallow. These microscopic defects
are associated with wafer fab processes like chemical
oxide stripping, etc. Also known as S-pits, their presence
is usually expressed as a defect density. S-pit densities of less
than 100 per sq. cm is tolerable in many applications.
- mechanical damage caused by the slicing operation of the saw blade.
Saw marks may be removed by lapping.
- other defects on the wafer surface due to various causes, e.g.,
stains, streaks, smudges, residues, dimples, mounds, orangepeel,
scratches, etc. (refer to ASTM Std F-154).
details on the various electrical, mechanical, and chemical properties
required of silicon wafers, please refer to the following links:
Specifications for Si Wafers;
Specifications for Si Wafers; Chemical Specifications for
Wafers for Wafer Fab;
Single Crystal Growth
Silicon Processing for the Vlsi Era: Process Technology
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