Oxide Wafer

Introduction
• A grown oxide layer occurs on a wafer by providing high-purity oxygen in an elevated temperature environment. A deposited oxide layer is formed on a wafer by reacting an external silicon source with oxygen to deposit a thin film.

Oxide Film
• An amorphous silicon dioxide film grows on a wafer with an atomic structure of a silicon atom surrounded by four oxygen atoms (tetrahedron cell). Thermally grown SiO2 has strong adhesion to silicon and has excellent dielectric properties.
• Silicon dioxide is used in the following applications: 1) device scratch protection and contamination isolation, 2) field isolation (surface passivation), 3) gate dielectric material, 4) doping barrier, and 5) deposited dielectric layer between metal conductor layers.

Thermal Oxidation Growth
• Thermal oxide is grown by a chemical reaction between silicon and oxygen. This is done with dry oxidation or wet oxidation. Wet oxidation has a faster growth rate but the layer is less dense.
• Silicon dioxide grows by consuming silicon. The thickness of silicon consumed is 0.46 of the total oxide thickness. The growth of the oxidation layer is controlled and limited by the movement of oxygen through the oxide at the oxide-silicon interface. This oxide-silicon interface has an incomplete oxidation of silicon that causes an undesirable charge build-up that must be controlled. Use of a chlorine-containing gas during the oxidation process can neutralize the charge accumulation at the interface.
• The rate of oxidation growth has both a linear stage and parabolic stage. The linear stage is the initial growth (up to about 150 angstroms) and consumes silicon on the wafer surface as a linear function of time. This stage is reaction-rate controlled. The parabolic stage is the second phase and is much slower. This second phase is diffusion controlled (limited by the rate the oxygen diffuses through the oxide).
• Factors that affect oxide growth are: heavily doped silicon has increased oxidation, the (111) crystal oxidizes faster in the linear stage than the (100) crystal, increased pressure causes increased growth, and plasma can enhance oxidation.
• The selective oxidation of silicon is done to electrically isolate adjacent devices. The local oxidation of silicon (LOCOS) is the traditional method, but it has excessive lateral growth and is inadequate for 0.25 µm technology and below.

Properties of silicon

Silicon is the chemical element with the atomic number 14 in the periodic table of the elements. Silicon is a classic semiconductor, its conductivity lies between that of conductors and dielectrics. Naturally silicon (from the latin silex/silicis: pebbles) occurs only as oxide: silicon dioxide (SiO2) in form of sand, quartz, or silicate (compounds of silicon with oxygen, metals and others). Thus silicon is a very cheap starting material, whose value is determined with further processing.

On a silicon crystal oxide layers can be produced very easily, the resulting silicon dioxide is an insulator of highest quality which can be fabricated precisly on the substrate. To create similar insulators on germanium or gallium arsenide is very expensive. The possibility to change the conductivity by doping silicon is another big advantage. Other substances are in part very toxic, and compounds with these elements are not as durable and stable as in silicon. Requirement for the use of silicon in semiconductor manufacturing is that the silicon is present in an ultrapure form as single crystal. This means that the silicon atoms in the crystal lattice are regularly arranged and there are absolutely no undefined impurities in the material.