Oxidation in IC Fabrication

Oxidation

The process in which a thin layer of silicon dioxide (SiO₂) formed on a surface of silicon wafer using thermal growth technique is called oxidation. In the planar process it is essential to protect certain regions of surface of the wafer so that the dopant atoms may be driven into other selective regions during the processes such as diffusion or ion implantation. For such shielding purpose, silicon dioxide (SiO₂) is best suited. Some of the important properties of SiO₂ are as follows.

i) It acts as mask or shield against implant or diffusion of dopant into silicon wafer.

ii) It provides surface passivation.

iii) It acts as a component in MOS structures.

iv) It provides electrical isolation between multilevel interconnected layers.

v) The most commonly used silion dopants such as phosphorus, boron, arsenic and antimony diffuse with difficulty in SiO₂. In other words, all these dopants have low diffusion coefficient in SiO₂. Hence SiO₂ acts as shield against all the above mentioned impurities.

Following are the different techniques developed for forming oxide layers.

i) Thermal oxidation.

ii) Wet anodization or oxidation.

Iii Vapour-phase technique (CVD).

iv) Plasma anodization or oxidation.

Thermal oxidation is the basic process in the fabrication of semiconductor devices. When the interface between oxide and silicon requires a low charge density level, this type of oxidation is preferred.

When the oxide layer is required on the top of a metal layer such as in multilevel metallization structure, the vapour phase technique i.e. chemical vapour deposition (CVD) is used. 

Plasma oxidation is a low temperature vacuum process carried out in a pure oxygen discharge. This process allows to grow high quality oxides on the surface of silicon wafer as compared with other techniques. The plasma oxidation can grow thick oxide layers at comparitively lower temperatures with higher growth rate. The growth rate depends on temperature of substrate, plasma density and dopant concentration in substrate.

1. Thermal Oxidation

The oxidation processes is called thermal oxidation because to grow the oxide layer, the temperature maintained is high.

The selection of the oxidation technique depends upon the thickness and the oxide properties required. For thin oxides with low charge density at interface, the oxides are grown in dry oxygen. This is also called dry oxidation explained by the reaction given below.

For thick layers of oxides, steam or water vapour is used at high pressure for oxidation. This is called wet oxidation. The chemical reaction for wet oxidation is as follows.

In the most commonly used oxidation technique, the wafers are stacked vertically in a slotted paddle or boat which is open ended. This is made up of quartz and it is placed in quartz tube. This quartz tube is slowly passed through resistance heated horizontal furnace. Typically the temperature maintained is 700 °C to 1200 °C. The oxidizing agent used may be dry oxygen or a mixture of oxygen and water vapour. The schematic for the thermal oxidation equipment is as shown in the Fig. 1.7.1.

Basically an oxidation is high temperature process. Due to the oxidation process, the layer departs from its original location. To overcome this undesired result, the oxidation process is carried out at low temperatures. But this increases the growth time. To overcome this, pressure is increased because an increase by 1 atm pressure, decreases temperature by 20 °C for same growth rate. Thus high pressure oxidations with pressures upto 25 atm are used in the industrial applications at the temperatures in the range of

Initially the oxide growth formation rate is very fast and then gradually it slows down, as oxygen atoms have to diffuse through the oxide to reach the interface between silicon substrate and SiO2. As compared to dry oxidation process, wet oxidation process is faster at a given temperature. Typically to grow 1 pm thick oxide layer, dry oxidation process takes 2 hour 30 minutes, while wet oxidation takes only 1 hour and 30 minutes. Eventhough the wet oxidation process is time saving, it has a drawback of higher impurity contents of the oxides. Generally MOS ICs require a very pure oxide for reliable performance. For this purpose specially dry oxidation process is preferred.

The process is known as thermal oxidation because the oxide layer is formed on silicon wafer using high temperatures. The silicon surface has very high affinity for oxygen, an oxide layer rapidly forms when silicon is exposed to a oxidising agent even at room temperature. During the oxidation process, the Si-SiO₂ interface pierces into the silicon substrate. It is observed that after completion of oxidation process, the oxide of thickness d consumes 0.44 d thickness of silicon. This is illustrated in the Fig. 1.7.2.

Review Questions

1. Explain importance of SiO₂ layer.

2. Describe the step of oxidation in IC Fabrication.

3. Write a note on different techniques of oxidation.

4. With the help of neat diagram, explain thermal oxidation process. Write chemical reaction.