Question: What are the basic reactions in the formation of SiO2 in dry oxidation & wet oxidation? Explain where these methods are used during MOSFET fabrication process.
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Subject: Electronics Engineering

Topic: IC Technology

Difficulty: Medium / High

ic technology • 299 views
 modified 4 months ago  • written 4 months ago by Sayali Bagwe • 1.9k
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The oxidation takes place under pure oxygen atmosphere. The silicon and oxide react to form silicon dioxide:

$Si+ O_2 → SiO_2$

This process is done at 1000 to 1200 °C actually. To create a very thin and stable oxide the process can be done at even lower temperatures of about 800 °C.

Characteristic of the dry oxidation:

1. slow growth of oxide
2. high density
3. high breakdown voltage

Dry oxidation has lower growth rate than wet oxidation, although the oxide film quality is better. Therefore thin oxides such as screen oxide, pad oxide and especially the gate oxide uses the dry oxidation processes.

The oxidation rate is low and so the oxide thickness can be controlled accurately.

Wet Oxidation:

It is a form of hydrothermal reaction. The oxidation is done by involving the dissolved components in H2O with oxygen as an oxidizer. When air is used, it is referred as Wet Air Oxidation.

The oxidation is given by:

$Si+ 2H_2 O → SiO_2+ 2H_2$

This process is done by 900 to 1000 °C. The characteristics if wet thermal oxidation are:

1. fast growth even on low temperatures
2. less quality than dry oxides

In order to avoid excess evaporation of water, the system is maintained under pressure.

The growth rate is much higher for wet oxidation than dry oxidation. Thus wet oxidation is applied for thick oxides in insulation and passivation layers.

Mainly wet oxidation is to grow thick oxides such as masking oxides, blanket field oxide etc.