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 siliconsubstrates to produce silicon dioxide.
The chemical reaction
Thermal oxidation of silicon is usually performed at a temperature between 800 and 1200°C, resulting in so called High Temperature Oxide layer (HTO). It may use either
water vapor(steam) or molecular oxygenas 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(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. Conversely, if a bare silicon surface is oxidized, 46% of the oxide thickness will lie below the original surface, and 54% above it.
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.
Solving the quadratic equation for "Xo" yields::
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 quartzrack (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.
Vertical furnaces stand higher than horizontal furnaces, so they may not fit into some microfabrication facilities. However, they help to prevent
dustcontamination. 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.
Vertical furnaces also eliminate an issue that plagued horizontal furnaces, uniformity of grown oxide across the wafer. Horizontal furnaces typically have convection currents inside the tube which causes the bottom of the tube to be slightly colder than the top of the tube. As the wafers lie vertically in the tube the convection and the temperature gradient with it causes the top of the wafer to have a thicker oxide than the bottom of the wafer. Vertical furnaces solve this problem by having wafer sitting horizontally, and then having the gas flow in the furnace flowing from top to bottom, significantly dampening any thermal convections.
Vertical furnaces also allow the use of load locks to purge the wafers with nitrogen before oxidation to limit the growth of native oxide on the Si surface.
Wet oxidation is preferred to dry oxidation 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- densityoxide, with lower dielectric strength.
The long time required to grow a thick oxide in dry oxidation 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.
metal ions can degrade performance of MOSFETs ( sodiumis of particular concern). However, chlorinecan immobilize sodium by forming sodium chloride. Chlorine is often introduced by adding hydrogen chlorideor trichloroethyleneto the oxidizing medium. Its presence also increases the rate of oxidation.
* 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
crystalaffects 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 depositionof oxide resulting in Low Temperature Oxide layer (reaction of TEOSat about 600 °C). However, the high temperatures required to produce High Temperature Oxide (HTO) restrict its usability. For instance, in MOSFETprocesses, 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.
* [http://www.ee.byu.edu/cleanroom/OxideTimeCalc.phtml Oxide growth time calculator]
*Online calculator including deal grove and massoud oxidation models, with pressure and doping effects at: http://www.lelandstanfordjunior.com/thermaloxide.html
Wikimedia Foundation. 2010.
Look at other dictionaries:
thermal oxidation — šiluminis oksidavimas statusas T sritis radioelektronika atitikmenys: angl. heat oxidation; thermal oxidation vok. thermische Oxydation, f; thermische Oxydierung, f rus. термическое окисление, n; термическое оксидирование, n pranc. oxydation… … Radioelektronikos terminų žodynas
Rapid Thermal Oxidation — Rapid Thermal Processing (dt.: schnelle thermische Bearbeitung) ist ein Überbegriff für die Bearbeitung von Wafern in einem Hochtemperaturprozess, bei dem eine sehr rasche Erhitzung des Wafers mit Halogenlampen erzielt wird. Inhaltsverzeichnis 1… … Deutsch Wikipedia
Thermal desorption — is an environmental remediation technology that utilizes heat to increase the volatility of contaminants such that they can be removed (separated) from the solid matrix (typically soil, sludge or filter cake). ExplanationThermal desorption is not … Wikipedia
Thermal barrier coating — Thermal barrier coatings are highly advanced material systems applied to metallic surfaces, such as gas turbine or aero engine parts, operating at elevated temperatures. These coatings serve to insulate metallic components from large and… … Wikipedia
Thermal degradation of polymers — is molecular deterioration as a result of overheating. At high temperatures the components of the long chain backbone of the polymer can begin to separate (molecular scission) and react with one another to change the properties of the polymer.… … Wikipedia
Thermal analysis — is a branch of materials science where the properties of materials are studied as they change with temperature. Techniques include: *Differential scanning calorimetry *Dynamic mechanical analysis *Thermomechanical analysis *Thermogravimetric… … Wikipedia
Thermal spraying — techniques are coating processes in which melted (or heated) materials are sprayed onto a surface. The feedstock (coating precursor) is heated by electrical (plasma or arc) or chemical means (combustion flame). Thermal spraying can provide thick… … Wikipedia
oxidation–reduction reaction — ▪ chemical reaction Introduction also called redox reaction any chemical reaction in which the oxidation number of a participating chemical species changes. The term covers a large and diverse body of processes. Many oxidation– reduction… … Universalium
Thermal grease — From left to right: Arctic Cooling MX 2 and MX 3, Tuniq TX 3, Cool Laboratory Liquid Metal Pro( Liquid Metal based), Shin Etsu MicroSi G751, Arctic Silver 5, Powdered Diamond. In background Arctic Silver grease remover … Wikipedia
Thermal Via — Eine Leiterplatte (auch Leiterkarte, Platine) ist ein Träger aus isolierendem Material mit festhaftenden leitenden Verbindungen. Sie dient der mechanischen Befestigung und elektrischen Verbindung elektronischer Bauteile. Oben: Bestückungsseite… … Deutsch Wikipedia