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Encyclopedia > Thermal oxidation

In microfabrication, thermal oxidation is a way to produce a thin layer of oxide (usually silicon dioxide) on the surface of a wafer (semiconductor). 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. Thermal oxidation may be applied to different materials, but this article will only consider oxidation of silicon substrates to produce silicon dioxide. Microfabrication is the collective term for the technologies used to fabricate components on a micrometer-sized scale. ... An oxide is a chemical compound of oxygen with other chemical elements. ... R-phrases R42 R43 R49 S-phrases S22 S36 S37 S45 S53 Flash point non-flammable Supplementary data page Structure and properties n, εr, etc. ... See Wafer (cooking) for the original meaning of the word. ... General Name, Symbol, Number silicon, Si, 14 Chemical series metalloids Group, Period, Block 14, 3, p Appearance as coarse powder, dark gray with bluish tinge Atomic mass 28. ... R-phrases R42 R43 R49 S-phrases S22 S36 S37 S45 S53 Flash point non-flammable Supplementary data page Structure and properties n, εr, etc. ...

Contents

The chemical reaction

Thermal oxidation of silicon is usually performed at a temperature betwen 800 and 1200°C. It may use either water vapor (steam) or molecular oxygen as the oxidant; it is consequently called either wet or dry oxidation. The reaction is one of the following: Celsius relates to the Celsius or centrigrade temperature scale. ... Water vapor, also aqueous vapor, is the gas phase of water. ... General Name, Symbol, Number oxygen, O, 8 Chemical series Nonmetals, chalcogens Group, Period, Block 16, 2, p Appearance colorless (gas) very pale blue (liquid) Atomic mass 15. ...

The oxidizing ambient may also contain several percent of hydrochloric acid (HCl). The chlorine removes metal ions that may occur in the oxide. The chemical compound hydrochloric acid is the aqueous (water-based) solution of hydrogen chloride (HCl). ...


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. Conversely, 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

Main article: Deal-Grove model

According to the commonly-used Deal-Grove model, the time t required to grow an oxide of thickness Xo, at a constant temperature, on a bare silicon surface, is:

where the constants A and B encapsulate the properties of the reaction and the oxide layer, respectively.


If a wafer 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. See Wafer (cooking) for the original meaning of the word. ...


Solving the quadratic equation for Xo yields:

Oxidation technology

Most thermal oxidation is performed in furnaces, at temperatures between 800 and 1200°C. A single furnace accepts many wafers at the same time, in a specially designed quartz rack (called a "boat"). Historically, the boat entered the oxidation chamber from the side (this design is called "horizontal"), and held the wafers vertically, beside each other. However, many modern designs hold the wafers horizontally, above and below each other, and load them into the oxidation chamber from below. A furnace is a device for heating air or any other fluid. ... Quartz is one of the most common minerals in the Earths continental crust. ...


Vertical furnaces stand higher than horizontal furnaces, so they may not fit into some microfabrication facilities. However, they help to prevent dust contamination. Unlike horizontal furnaces, in which falling dust can contaminate any wafer, vertical furnaces only allow it to fall on the top wafer in the boat. After just three years of use dust has blocked this laptop heat sink, making the computer unusable Dust is a general name for minute solid particles with diameters less than 500 micrometers (otherwise, see sand or granulates) and, more generally, for finely divided matter. ...


Oxide quality

Wet oxygen is preferred to dry oxygen for growing thick oxides, because of the higher growth rate. However, fast oxidation leaves more dangling bonds at the silicon interface, which produce quantum states for electrons and allow current to leak along the interface. (This is called a "dirty" interface.) Wet oxidation also yields a lower-density oxide, with lower dielectric strength. Covalently bonded hydrogen and carbon in a molecule of methane. ... A quantum state is any possible state in which a quantum mechanical system can be. ... Density, or volumic mass (ISO 31), is a measure of mass per given unit volume. ... In physics, the term dielectric strength has the following meanings: Of an insulating material, the maximum electric field strength that it can withstand intrinsically without breaking down, , without experiencing failure of its insulating properties. ...


The long time required to grow a thick oxide in dry oxygen makes this process impractical. Thick oxides are usually grown with a long wet oxidation bracketed by short dry ones (a dry-wet-dry cycle). The beginning and ending dry oxidations produce films of high-quality oxide at the outer and inner surfaces of the oxide layer, respectively.


Mobile metal ions can degrade performance of MOSFETs (sodium is of particular concern). However, chlorine can immobilize sodium by forming sodium chloride. Chlorine is often introduced by adding hydrogen chloride or trichloroethylene to the oxidizing medium. Its presence also increases the rate of oxidation. Hot metal work from a blacksmith In chemistry, a metal (Greek: Metallon) is an element that readily forms positive ions (cations) and has metallic bonds. ... An ion is an atom or group of atoms that normally are electrically neutral and achieve their status as an ion by loss or addition of one or more electrons. ... The metal-oxide-semiconductor field-effect transistor (MOSFET, MOS-FET, or MOS FET), is by far the most common field-effect transistor in both digital and analog circuits. ... General Name, Symbol, Number sodium, Na, 11 Chemical series alkali metals Group, Period, Block 1, 3, s Appearance silvery white Atomic mass 22. ... General Name, Symbol, Number chlorine, Cl, 17 Chemical series halogens Group, Period, Block 17, 3, p Appearance yellowish green Atomic mass 35. ... Sodium chloride, also known as common salt, table salt, or halite, is a chemical compound with the formula NaCl. ... R-phrases , S-phrases , , , , Flash point non-flammable Supplementary data page Structure and properties n, εr, etc. ... The chemical compound trichloroethylene is a chlorinated hydrocarbon commonly used as an industrial solvent. ...


Other notes

  • Thermal oxidation can be performed on selected areas of a wafer, and blocked on others. Areas which are not to be oxidized are covered with a film of silicon nitride, which blocks diffusion of oxygen and water vapor. The nitride is removed after oxidation is complete. This process cannot produce sharp features, because lateral (parallel to the surface) diffusion of oxidant molecules under the nitride mask causes the oxide to protrude into the masked area.
  • Because impurities dissolve differently in silicon and oxide, a growing oxide will selectively take up or reject dopants. This redistribution is governed by the segregation coefficient, which determines how strongly the oxide absorbs or rejects the dopant, and the diffusivity.
  • The orientation of the silicon crystal affects oxidation. A <100> wafer (see Miller indices) oxidizes more slowly than a <111> wafer, but produces an electrically cleaner oxide interface.
  • Thermal oxidation of any variety produces a higher-quality oxide, with a much cleaner interface, than chemical vapor deposition of oxide. However, the high temperatures that it requires restrict its usability. For instance, in MOSFET processes, thermal oxidation is never performed after the doping for the source and drain terminals is performed, because it would disturb the placement of the dopants.

Silicon nitride (Si3N4) is hard, solid substance, that can be obtained by direct reaction between silicon and nitrogen in high temperatures. ... For the connotation of the term relating to chemistry, see Solvation. ... A dopant, also called doping agent and dope, is an impurity element added to a semiconductor lattice in low concentrations in order to alter the optical/electrical properties of the semiconductor. ... In heat transfer analysis, thermal diffusivity (symbol: ) is the ratio of thermal conductivity to heat capacity. ... For other senses of this word, see crystal (disambiguation). ... In geometry, a Miller index is used to describe sets of planes in a crystal. ... DC plasma (violet) enhances the growth of carbon nanotubes in this laboratory-scale PECVD apparatus. ... The metal-oxide-semiconductor field-effect transistor (MOSFET, MOS-FET, or MOS FET), is by far the most common field-effect transistor in both digital and analog circuits. ...

References

  • Jaeger, Richard C. (2002). “Thermal Oxidation of Silicon”, Introduction to Microelectronic Fabrication. Upper Saddle River: Prentice Hall. ISBN 0-201-44494-7.

 
 

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