Required 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 satisfy. 


Surface specifications that must be checked in wafers include:


1) Flatness - 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.


3) Cracks - fracture that extends to the wafer surface caused by impact of sharp features of fixtures against the wafer.  Crow's feet, 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.


4) Haze - 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 F-154.


5) Saucer Pits - etch pits that are small and shallow.  These microscopic defects are associated with wafer fab processes like chemical etching, 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.


7) Saw Marks - mechanical damage caused by the slicing operation of the saw blade. Saw marks may be removed by lapping.


8) Surface Defects - 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).


For more details on the various electrical, mechanical, and chemical properties required of silicon wafers, please refer to the following links: Electrical Specifications for Si Wafers;  Mechanical Specifications for Si Wafers; Chemical Specifications for Si Wafers


See also:   Wafers for Wafer Fab; Single Crystal Growth


Primary Reference:  Silicon Processing for the Vlsi Era: Process Technology




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