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Dry vs Wet Oxidation: Key Differences Explained

Both are thermal oxidation processes that oxidize silicon at high temperature to form SiO2, but they use different oxidants. Dry oxidation uses dry oxygen (O2). It grows slowly, but produces a dense, high-quality film with few interface states, so it has been used for thin gate oxides. Wet oxidation uses water vapor (H2O). Because H2O dissolves in SiO2 far more than O2 does, the oxide grows several to about 10 times faster. That is why wet oxidation is used for thick films such as LOCOS field oxides and masks.

AspectDry oxidationWet oxidation
OxidantDry oxygen (O2)Water vapor (H2O). It is commonly produced by burning hydrogen and oxygen at the furnace inlet (pyrogenic oxidation)
ReactionSi + O2 → SiO2Si + 2H2O → SiO2 + 2H2
Growth rateSlow. Example: about 70 nm in 1 hour at 1000 °CFast (several to about 10 times faster than dry). Example: about 400 nm in 1 hour at 1000 °C
Why it is fast or slowO2 dissolves only a little in SiO2H2O dissolves in SiO2 several hundred times more than O2, so much more oxidant reaches the interface
Film quality and densityDense, with few interface states and high resistance to dielectric breakdownSlightly less dense than dry oxide; hydrogen and OH groups tend to remain in the film
Temperature (typical range)About 800–1200 °CAbout 800–1200 °C. Because it is fast, the same thickness can be grown at a lower temperature or in a shorter time
Thickness controlSlow growth makes it easy to precisely control thin films from a few nm to tens of nmFast growth makes it unsuitable for precise control of thin films
Treatment in the Deal-Grove modelDescribed by x² + Ax = B(t + τ). Initial oxidation below tens of nm is faster than the model predicts, so the initial thickness (about 25 nm) is converted into a time τ as a correctionSame equation; it fits the model well. τ ≈ 0 is often assumed. The rate constants B and B/A are much larger than for dry oxidation
Typical thicknessA few nm to tens of nmHundreds of nm to about 1 µm
Typical usesGate oxides, tunnel oxides, thin pad oxidesLOCOS field oxides, masks for ion implantation and diffusion, thick insulating films

Figures are typical orders of magnitude and vary by process generation and product.

When to use which

Dry oxidation
When the film can be thin but interface quality and thickness accuracy come first: for example, MOSFET gate oxides and the thin pad oxide placed under a nitride film.
Wet oxidation
When you need a thick oxide in a short time: for example, LOCOS field oxides, diffusion and implantation masks, and thick insulating films around devices.
Combined
The two are sometimes combined in sequence: dry oxidation to form the interface, wet oxidation to build up thickness, and a final dry oxidation to finish.

FAQ

Why is wet oxidation faster than dry oxidation?

Oxidation takes place at the SiO2/silicon interface, so the oxidant must diffuse through the film to reach it. H2O dissolves in SiO2 several hundred times more than O2, so the oxidant concentration in the film is higher and more of it reaches the interface. In the Deal-Grove model, this shows up as larger rate constants B and B/A.

Why has dry oxidation been used for gate oxides?

Dry oxide is dense, has few interface states at the silicon interface, and is highly resistant to dielectric breakdown. Its slow growth also makes it easy to control a thickness of a few nm precisely. Advanced gates have moved to high-k films, but the very thin SiO2 interfacial layer beneath them is still important.

Why does only dry oxidation need a correction in the Deal-Grove model?

In dry oxidation, the film is known to grow faster than the model predicts while it is thinner than tens of nm. To correct for this, the fast initial growth is treated as having formed about 25 nm of oxide, and that amount is added to the equation as a time τ. Wet oxidation fits the model well, so τ ≈ 0 is often assumed.

How much silicon is consumed in thermal oxidation?

The same for both dry and wet oxidation: silicon equal to about 44% of the resulting oxide thickness is consumed. The oxide grows about 56% above the original surface and about 44% below it.

Check your understanding

Which correctly compares dry oxidation (O2) and wet oxidation (H2O) in the thermal oxidation of silicon?

  1. ADry oxidation grows oxide faster
  2. BWet oxidation grows oxide faster
  3. CBoth grow oxide at the same rate
Show answer and explanation

Answer: B. Wet oxidation grows oxide faster

Wet oxidation uses steam; H2O dissolves in SiO2 several hundred times more than O2, so it grows oxide several to ten times faster than dry oxidation—at 1000 °C for an hour, roughly 70 nm dry versus 400 nm wet. It suits thick isolation and masking oxides. Dry oxidation is slower but yields dense, low-interface-state films with precise thickness control, so it was used for thin gate oxides.

According to the Deal-Grove model, where does thermal oxidation take place?

  1. AAt the outermost surface of the oxide, where silicon atoms react with the oxidant
  2. BSilicon atoms diffuse to the oxide surface and react there
  3. CThe oxidant diffuses through the oxide and reacts with silicon at the Si/SiO2 interface
Show answer and explanation

Answer: C. The oxidant diffuses through the oxide and reacts with silicon at the Si/SiO2 interface

The Deal-Grove model (1965) treats oxidation as oxidant entering the oxide surface from the gas, diffusing through the film and reacting with silicon at the interface to form new oxide. Oxide therefore grows at the interface, which is always freshly formed, and growth slows as the diffusion path lengthens. Setting the three fluxes equal yields a single equation encompassing both the linear and parabolic regimes.

In the Deal-Grove model, what is the relationship between oxide thickness and time early in thermal oxidation, while the oxide is still thin?

  1. AThickness is proportional to the square root of time
  2. BThickness is roughly proportional to time (increases linearly)
  3. CThickness stays constant regardless of time
Show answer and explanation

Answer: B. Thickness is roughly proportional to time (increases linearly)

While the oxide is thin, oxidant passes through it easily and the interface reaction limits growth, so thickness increases nearly linearly with time (rate constant B/A). As the oxide thickens, diffusion through it becomes limiting and thickness follows the square root of time (rate constant B). The crossover occurs around a thickness of A/2—tens of nm to about 100 nm for dry oxidation.

  1. Once the oxide has become sufficiently thick in thermal oxidation, how does its thickness relate to oxidation time?
  2. Which chemical reaction occurs in the dry thermal oxidation of silicon?
  3. Which generally describes the thickness relationship between the field oxide used for isolation such as LOCOS and the gate oxide of a MOSFET?

Practice Oxidation and diffusion questions

Key terms explained

Dry oxidation
Thermal oxidation with dry oxygen. Growth is slow, but it produces dense, high-quality oxide, used for gate oxides and the like.
Wet oxidation
Thermal oxidation in an atmosphere containing water vapor. It grows faster than dry oxidation and is used to make thick oxides.
Deal-Grove model
A model of thermal oxidation in which the oxidant diffuses through the oxide and reacts with silicon at the Si/SiO2 interface. Thickness grows linearly with time for thin oxides and with the square root of time for thick ones.
Thermal oxidation
A method of forming an SiO2 film on silicon by heating it in oxygen or water vapor. There are dry and wet oxidation.
LOCOS
A method that isolates devices by thickly oxidizing only areas not covered by nitride. It forms a beak-like encroachment at the edges (the bird's beak) and has been replaced by STI.

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