In
microfabrication
Microfabrication is the process of fabricating miniature structures of micrometre scales and smaller. Historically, the earliest microfabrication processes were used for integrated circuit fabrication, also known as "semiconductor manufacturing" ...
, thermal oxidation is a way to produce a thin layer of
oxide
An oxide () is a chemical compound that contains at least one oxygen atom and one other element in its chemical formula. "Oxide" itself is the dianion of oxygen, an O2– (molecular) ion. with oxygen in the oxidation state of −2. Most of the E ...
(usually
silicon dioxide
Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is one ...
) on the surface of a
wafer
A wafer is a crisp, often sweet, very thin, flat, light and dry biscuit, often used to decorate ice cream, and also used as a garnish on some sweet dishes. Wafers can also be made into cookies with cream flavoring sandwiched between them. They ...
. The technique forces an oxidizing agent to diffuse into the wafer at high temperature and react with it. The rate of oxide growth is often predicted by the
Deal–Grove model
The Deal–Grove model mathematically describes the growth of an oxide layer on the surface of a material. In particular, it is used to predict and interpret thermal oxidation of silicon in semiconductor device fabrication. The model was first pu ...
.
Thermal oxidation may be applied to different materials, but most commonly involves the oxidation of
silicon
Silicon is a chemical element with the symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic luster, and is a tetravalent metalloid and semiconductor. It is a member of group 14 in the periodic tab ...
substrates to produce
silicon dioxide
Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is one ...
.
The chemical reaction
Thermal oxidation of silicon is usually performed at a temperature between 800 and 1200
°C
The degree Celsius is the unit of temperature on the Celsius scale (originally known as the centigrade scale outside Sweden), one of two temperature scales used in the International System of Units (SI), the other being the Kelvin scale. The ...
, resulting in so called High Temperature Oxide layer (HTO). It may use either
water vapor
(99.9839 °C)
, -
, Boiling point
,
, -
, specific gas constant
, 461.5 J/( kg·K)
, -
, Heat of vaporization
, 2.27 MJ/kg
, -
, Heat capacity
, 1.864 kJ/(kg·K)
Water vapor, water vapour or aqueous vapor is the gaseous p ...
(usually
UHP steam) or molecular
oxygen
Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as wel ...
as the oxidant; it is consequently called either ''wet'' or ''dry'' oxidation. The reaction is one of the following:
:
:
The oxidizing ambient may also contain several percent of
hydrochloric acid
Hydrochloric acid, also known as muriatic acid, is an aqueous solution of hydrogen chloride. It is a colorless solution with a distinctive pungent smell. It is classified as a strong acid
Acid strength is the tendency of an acid, symbol ...
(HCl). The chlorine removes metal ions that may occur in the oxide.
Thermal oxide incorporates silicon consumed from the substrate and oxygen supplied from the ambient. Thus, it grows both down into the wafer and up out of it. For every unit thickness of silicon consumed, 2.17 unit thicknesses of oxide will appear. If a bare silicon surface is oxidized, 46% of the oxide thickness will lie below the original surface, and 54% above it.
Deal-Grove model
According to the commonly used Deal-Grove model, the time ''τ'' required to grow an oxide of thickness ''X
o'', at a constant temperature, on a bare silicon surface, is:
:
where the constants A and B relate to properties of the reaction and the oxide layer, respectively. This model has further been adapted to account for self-limiting oxidation processes, as used for the fabrication and morphological design of
Si nanowires and other nanostructures.
If a
wafer
A wafer is a crisp, often sweet, very thin, flat, light and dry biscuit, often used to decorate ice cream, and also used as a garnish on some sweet dishes. Wafers can also be made into cookies with cream flavoring sandwiched between them. They ...
that already contains oxide is placed in an oxidizing ambient, this equation must be modified by adding a corrective term τ, the time that would have been required to grow the pre-existing oxide under current conditions. This term may be found using the equation for ''t'' above.
Solving the quadratic equation for ''X
o'' yields:
: