Oxidation

What is oxidation ?

Oxidation is a process where an atom, molecule or ion loses one or more electrons in a chemical reaction. The first known oxidizing agent was the oxygen. Oxidation´s processes are used in many industries for several purposes.

As an example, we have the heat sink anodizing process, where an oxide is grown on the aluminum heat sink surface. This oxide has the objective to protect the heat sink against natural oxidations, which could lead to a corrosion.

There are a lot of rectifier bridges and AC Switch, where this anodized heat sink is used to protect against the environment exposure.

I think it would be interesting here to remember that, not all the oxidations are desired. Sometimes oxidations can occur even being unwanted. Of course, that should be avoided whenever possible.

Let´s talk about the oxidation process for the semiconductor manufacturing industry.

In the semiconductor industry the oxidation process is mostly used to grow a silicon dioxide layer on a silicon wafer.

Why do we use silicon dioxide? SiO₂ has a few advantages which justifies its use, such as: it is grown naturally from silicon, it is a native oxide capable of being thick or thin as desired and its kinetics and mechanism are relatively well understood.

Where is silicon dioxide used? With which goal is it used?

The silicon dioxide will be used in four basic processes: (a) as a mask to prevent diffusion to occur in some positions of the wafer, (b) to avoid one wafer´s side from metallization deposit, (c) as a gate oxide in MOS and (d) as a mask for ion etching process.

The difference here is basically on the thickness and quality of the silicon dioxide: thick or thin layer.

Thick layers of SiO₂ are grew as a surface mask to avoid metal deposition in one wafer´s side, and to prevent diffusion of a dopant species to occur in some wafer´s position.

Thin layers of SiO₂ are grew as a gate oxide in MOS, as a surface cleaner or as a mask for etching processes.

How can it be grown?

As told previously, the SiO₂ kinetics and mechanism are relatively well understood. It can be formed by various processes. The choice of the process depends on the properties we are looking for this oxide.

Of course, the technique is based on the process chosen. If we were looking for a thin film, then the oxide structure would be of big importance. In cases like this the growth rate would be smaller and very well controlled in terms of atmosphere, time, temperature and purity.

I think I should make a parenthesis here to mention that the oxidation process is influenced by the crystal orientation of the substrate. For polycrystalline structure the oxide quality will be inferior to the monocrystalline structure because of the presence of the grain boundaries.

Now let´s talk a little bit about SiO₂ growth processes!

Silicon dioxide layers on silicon can be formed by various methods. They can be deposited through a vapor- phase reaction (CVD), or they can be formed by anodization, or by plasma reaction, or by thermal reaction.

In a silicon substrate, the technique most used is the thermal oxidation under controlled conditions.

For thermal oxidation there are two options: dry or wet grown oxides. The dry thermal oxidation uses O₂ to react with silicon to produce SiO₂. For the wet thermal oxidation, water is used to react with silicon and obtain SiO₂. The rate of growth of these processes are quite different.

The thermal dry oxidation offers a lower oxide growth tax than the wet oxidation. So, to obtain thin films, with a better structure this is the best choice.

The thermal wet oxidation can offer higher oxide growth rate, and thicker layers with less time.

Which controls are made to verify the characteristics of the silicon dioxide ?

To control the thickness of the oxide layer it is used optical methods, such as ellipsometry or reflectometry, oxide color, or Talystep.

The quality of the oxide can be verified using MOS structure or even SEM.