UNIT 2INTEGRATED CIRCUIT FABRICATION

(OXIDATION)

E5163 – IC DESIGN

LEARNING OUTCOMES

At the end of this topic, student should be able to:

• Explain the oxide layer in IC layout.

• Elaborate on the thin oxide layer formation on wafer in natural surroundings.

• List five functions of oxide layer.

ROLE OF OXIDATION1. It is used for surface passivation (creating protective layer on the wafer

surface).

2. It protects the junction from moisture and other atmospheric contaminants.

3. It serves as an insulator on the wafer surface. Its high relative dielectric constant, which enables metal line to pass over the active silicon regions.

ROLE OF OXIDATION4. SiO2 acts as the active gate electrode in MOS device structure.

5. It is used to isolate one device from another.

6. It provides electrical isolation of multilevel metallization used in VLSI.

Oxidation layer as insulator in doping / diffusion process.

OXIDATIONIn oxidation:

1. Wafer is exposed to oxygen.

2. Oxygen molecules diffuse into the wafer.

3. A chemical reaction occurs between oxygen and silicon (wafer).

4. A layer of oxide grows on the wafer surface.

The critical parameters or factor in controlling the growth rate during oxidation are:

1. temperature.

2. the length of exposure to oxygen.

3. the amount of moisture.

growth

diffuse

45% diffuse into silicon and 55% growth on silicon

LEARNING OUTCOMES

At the end of this topic, student should be able to:

• Explain wet oxidation and dry oxidation.

• Draw the furnace of each method.

• Write equation for each method.

LEARNING OUTCOMES

At the end of this topic, student should be able to:

• Discuss the factors that can affect the reaction with the aid of suitable graphs.

• Compare both methods in terms of reaction time, reaction temperature, layer thickness and layer quality.

OXIDATION• Oxidation / Thermal oxidation refers to an oxide growth

process occurring at high temperatures.

• Growing oxides at high temperatures increases the oxide growth rate, making oxidation cost effective.

• During thermal oxidation, silicon is exposed to the oxidants at temperatures between 900 and 1,200oC. Consequently, the rate of oxide growth increases dramatically.

• With thermal oxidation, oxides can be grown under both wet and dry conditions.

OXIDATION TECHNIQUES1. Cleaned wafers are placed in the wafer load station where dry nitrogen (N2) is

introduced into the chamber. The nitrogen prevents oxidation from occurring while the furnace reaches the required temperature.

2. Once the specified temperature in the chamber is reached, the nitrogen gas flow is shut off and oxygen (O2) is added to the chamber. The source of the oxygen can be gas or water vapor depending upon the dry process or wet process.

• After the oxidation is complete and the oxide layer is the correct thickness, nitrogen is reintroduced into the chamber to prevent further oxidation from occurring.

• The wafers are then removed from the chamber. After inspection, they are ready for further processing.

• Thermal oxidation can be either a dry or a wet process.

• http://matec.org/ps/library3/secure/modules/033/LA2/M033Animation.swf

DRY OXIDATION1. During dry oxidation, dry oxygen gas is introduced into the

process tube where it reacts with silicon. Since dry oxidation is a slow process, it is only used to grow thin oxides.

2. In dry oxidation, the amount of water in the processing tube is kept at a minimum. If the water level exceeds 25-50 parts per million (ppm), the oxidation rate increases and a thick layer of poor quality oxide is produced.

3. The chemical reaction for dry oxidation is:

DRY OXIDATION IN SEMICONDUCTOR MANUFACTURING

• Growing thin oxides is important in the manufacture of MOS transistors, MOS gates, and dielectric components of devices. High quality thin oxides are difficult to grow because under normal manufacturing conditions, the oxidation growth rate is too fast to control.

• In order to grow a high quality thin oxide, the oxidation process must be adjusted to slow down the oxide growth rate. This can be done by introducing hydrochloric acid (HCl), trichloroethylene (TCE), or trichloroethane (TCA) with the oxygen in the oxidation tube.

• Other adjustments, such as reducing the pressure level, or lowering the temperature while increasing the pressure, are also used to slow down the oxide growth rate and improve the quality of the oxide.

WET OXIDATION1. The silicon dioxide growth rate is faster during wet oxidation. Wet

oxidation is therefore the preferred method to grow thick oxides.

2. During wet oxidation, water vapor is introduced into the heated oxidation tube. Since water molecules are smaller in size than oxygen molecules, they diffuse into silicon dioxide faster and the oxide growth rate increases.

3. As the silicon dioxide forms, it traps the hydrogen atoms within it. These hydrogen atoms are released in subsequent processing steps and do not affect the quality of the oxide.

4. The wet thermal oxidation reaction is:

WET OXIDATION TECHNIQUESThere are several different wet oxidation techniques:

1. Bubblers

2. Flash system

3. Dryox

BUBBLERS TECHNIQUES1. A glass flask, or bubbler, containing deionized water is

attached to the oxidation tube. The water is heated (90-99OC) and water vapor (steam) forms. A carrier gas, nitrogen, passes through the vapor, it becomes saturated with water (steam). The vapor travels into the oxidation tube. With additional heating, it turns into steam and oxidation occurs.

2. A consistent oxide growth rate is hard to maintain with the bubbler method because of the difficulties involved in controlling both the amount of water vapor entering the oxidation tube and the temperature of the water. The risk of contamination is also high.

3. Today, bubblers are found more educational and R & D facilities than in manufacturing facilities.

WHAT IS DEIONIZED WATER?• Deionized water is a type of with mineral ions

(salts) removed. These mineral ions include sodium, calcium, iron, copper, chloride, and bromide. Deionized water is created by taking conventional water and exposing it to electrically charged resins that attract and bind to the salts, removing them from the water. Because most of the impurities in water are mineral salts, deionized water is mostly pure, but it does still contain numerous bacteria and viruses, which have no charge and therefore are not attracted to the electrified resins.

FLASH SYSTEM TECHNIQUES1. A flash system is similar in design to a bubbler. A small amount of

deionized water is dropped onto a heated quartz surface where it instantly turns into steam.

2. A carrier gas moves the steam into the heated oxidation chamber. As with the bubbler, it is very difficult to achieve a constant rate of oxide growth. Unlike a bubbler, however, the flask is never opened in a flash system so the risk of contamination is low.

DRYOX TECHNIQUES1. In a Dryox system, oxygen and hydrogen directly enter a heated oxidation tube. In the

heated oxidation tube, the two gases mix and form water as steam.

2. Mass flow controllers are used to regulate the gas flow into the tube. This ensures uniform oxide growth can be precisely controlled.

3. Contamination of the oxide is limited since the gases are clean.

4. One major disadvantage of Dryox system is the explosive nature of hydrogen at high temperatures. Safety precautions must be taken to minimize the risk of a hydrogen explosion.