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Oxide & Nitride Wafers Further Information

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Diameter of the wafer listed in mm. Typically wafers are talked about in inches; typical sizes are 2”,3”,4”,5”,6”,8”& 12” – with 4”,6” and 8” the most commonly used in industry and academia.

The material is the bulk material of the wafer, typically silicon, but this may vary – some transparent substrates such as glass or quartz are needed for optical devices, and more exotic compound materials such as GaAs or InP for specific band gaps.

Type refers to the electrical behaviour of the wafer. Intrinsic (I), behaves as pure silicon. N-type, dominant charge carriers are electrons. P-type, dominant charge carriers are holes. Whether a wafer is P-type or N-type will affect the electrical response of any device manufactured.

Wafers are grown as single crystals that have an ordered, regular and repeating structure. When they are sliced from the ingot the flat surface is aligned along one of several relative directions, known as the orientation. The orientation is classified by Miller indices, typical indices being (100), (110) and (111). Orientation affects the physical properties of the silicon wafer – how it is etched, ion implantation and how it integrates with other materials.

The dopant is a material that has been deliberately implanted in the silicon to change the TYPE of the silicon. TYPE and DOPANT are linked. Typical N-type dopants are Phosphorus, Arsenic, and Antimony. These all provide an extra electron to the silicon which is then free to carry current. Typical P-type dopants are Boron & Gallium. These have one less electron and so leave a ‘hole’ in the silicon lattice which is free to carry current.

The growth method refers to the process by which the silicon ingot is grown. There are two main techniques: Czochralski Zone (CZ) and Float Zone (FZ).

Grade refers to the variety in the quality of the wafers. Typically these are PRIME, TEST and RECLAIMED. Prime are the highest quality and produced to the highest tolerances on flatness, cleanliness and polish. Test are similar to prime, except with less rigorous specifications to flatness and cleanliness. Reclaimed are wafers that have been stripped and polished of any previous patterning or processing. There are sometimes other grades of Si wafer mentioned but these are either synonyms of the above or have a specific tolerance on a certain parameter.

Resistivity is the measure of the resistance to current flow and the movement of the charge carriers (either holes or electrons) through the silicon. Resistivity is measured in Ohm-cm. The dopant level can be adjusted to reach target resistivity’s, with higher doping lowering the resistivity.

The thickness of the silicon wafer affects the mechanical properties and is typically expressed in µm (microns) and with a tolerance (± 20µm), The tolerance is measured through a total thickness variation (TTV).

Wafer polishing is the final step in the manufacture of silicon wafers, which allows the production of a smooth, super-flat mirrored surface. There are two options for polishing: single side polish (SSP) and double side polish (DSP). SSP: Only one face is polished, the second (the backside) is etched. DSP: Both faces are polished, giving a high flatness to the wafer.

Alignment fiducial refers to the flats or notches used to identify the wafer. Originally flats were used to identify TYPE and well as ORIENTATION, but now there is less convention about what the flats mean, and notches are quite common on 8” (200mm) wafers.

Coating types which typically can be a thermal oxide coating (ATOx) which always coats both sides of the wafer. ATOx stands for atmospheric thermal oxide. Other oxide coating methods include: Dry Oxide. This produces a thinner oxide layer but with a higher uniformity film. PECVD Oxide – produces a coating on a single side of the wafer. Nitride coating. Silicon Nitride (SiN) offers different mechanical and chemical properties to oxide layers. The nitride can be deposited by PECVD, LPCVD or low stress LPCVD. These variants are changes in the method of deposition and alter the final physical and mechanical properties of the film.

Coating thickness, typically µm, nm or Å.

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