Oxidation [1] – VLSI Technology

- deepak khushalani

What is Oxidation ?

• Oxidation is defined as the interaction between oxygen molecules and all the different substances they may contact, from metal to living tissue.

• A freshly-cut apple turns brown, a bicycle fender becomes rusty and a copper penny suddenly turns green. What do all of these events have in common? They are all examples of a process called oxidation.

Oxidation and Reduction…

• Technically, however, with the discovery of electrons, oxidation came to be more precisely defined as the loss of at least one electron when two or more substances interact.

• And Incidentally, the opposite of oxidation is reduction — the addition of at least one electron when substances come into contact with each other.

Oxidation leads to…??

• In the case of iron, the oxygen creates a slow burning process, which results in the brittle brown substance we call rust.

• When oxidation occurs in copper, on the other hand, the result is a greenish coating called copper oxide.

• The metal itself is not weakened by oxidation, but the surface develops a patina after years of exposure to air and water.

Oxidation as Unit Process Step in VLSI

• Oxidation refers to the chemical process of reaction of silicon with oxygen to form silicon dioxide(Sio2).

• Its necessity is throughout the fabrication of integrated circuits but its primary uses are :– It serves as mask against implant or diffusion atoms

into silicon.– It is used for surface passivation.– To isolate one device/layer from other.– It serves as gate oxide during fabrication of MOSFET.

Oxidation Growth Techniques

• Wet anodization vapour phase(silicon + nitricacid)

• Plasma anodization of silicon• Thermal Oxidation

– These techniques are developed to obtain high quality oxide layers but before we understand them we need to understand the basic oxidation method.

Thermal Oxidation Basics• A way to produce a thin layer

of oxide (usually silicon dioxide) on the surface of a wafer.

• 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 we will only consider oxidation of silicon substrates 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 vapour (usually UHP steam) or molecular oxygen as the oxidant; it is consequently called either wet or dry oxidation.

• The reaction is one of the following:

The chemical reaction (cont…)• Sio2 formation involves sharing of valence

electrons between silicon and oxygen (Covalent bond).

• Sio2 grows in both directions upwards as well as downwards approximately with same thickness.

Crystalline Structure

Oxide Kinetics…

• The Deal–Grove model mathematically describes the growth of an oxide layer on the surface of a material.

• In particular, it is used to analyze thermal oxidation of silicon in semiconductor device fabrication.

• The model was first published in Applied Physics Vol 36, 1965 by Bruce Deal and Andrew Grove, of Fairchild Semiconductor.

Model• Cg = concentration of oxidizing

species in the bulk of gas• Cs = concentration of oxidizing

species adjacent to the oxide surface

• Co= concentration of oxidizing species in the outer interface between gas and sio2

• Ci = concentration of oxidizing species in the inner surface between sio2 and silica

• flux is defined as the rate of flow of a property per unit area, which has the dimensions

[quantity]/([time]·[area]).

Derivation

• F1 = hg(Cg – Cs)– To understand the equilibrium Concentration C* -

we use Henrys law

• C0 = Hps and C* = Hpg

• Hence we have F1 = h(C* - C0) – where h = hg/Hkt

Derivation (Cont….)

• F2 = {D(C0 – Ci)}/d0

– D = diffusion Coefficient, d0 = oxide thickness

• And finally F3 = ksCi

– Ks = rate constant of chemical surface reaction

• After F1 = F2 = F3 in steady state configuration we can achieve the equations for Ci and C0; but limiting cases arise in two possibilities that D is either very small or very large….

Derivation (Cont….)

• Ci = 0 and C0 = C* we have a diffusion controlled case– Where D is small hence oxidation rate depends on

the supply of oxidants through the oxide.

• Ci = C0 we have a reaction controlled case– Where D is large hence oxidation rate depends on

rate constant ks as supply of oxidants is abundant.