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Wet Oxidation

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1. Mechanism of oxidation process

In an oxidation process of silicon that usually takes place at very high temperature (thermal oxidation), silicon (Si) reacts with either water vapor (H20) or oxygen (O2) to form silicon dioxide,SiO2 on the silicon surface. The reaction is represented by following equations:

Dry oxidation: Si + O2 → SiO2 Wet oxidation: Si + 2H2O → SiO2 + 2H2

The oxidation process can be implemented through diffusion of either water or an oxygen species through the oxide already formed up which then reacts with the silicon at the Si-SiO2 interface. At high temperature, thermal energy makes oxygen molecule moving much faster, which can drive them to diffuse across an existing oxide layer and react with Si to form SiO2. As the oxidation continues, the interface moves into the silicon and a new, clean silicon surface is produced.

The thicker of the film, the lower of the growth rate when the temperature is higher, oxygen molecules moving faster and hence, the quicker the oxide film grown. The oxide film quality is better than that grown when the temperature is low.

2. Steps of wet oxidation

1. There are two nitrogen sources in an oxidation system, one for process application, with higher purity, and another with lower purity for the chamber purge. 2. Nitrogen is used because it is a stable gas and will not react with silicon even at 900oc. 3. For wet oxidation, high purity oxygen gas is used to oxidize silicon. 4. Since the quartz tube starts to sag when temperature is above 1150oC, the dry oxidation process usually operates at about 900oC. 5. When the silicon dioxide is forming on the single-crystal silicon surface, there is an abrupt change at the silicon-silicon dioxide interface. Post oxidation annealing in N2/H2 ambient is a common technique to reduce the interface state charge. 6. During the system idle time, the furnace is always kept at a high temperature, such as 850oC, so that it does not need too much time to ramp up the temperature to the reduced process temperature. 7. Before wafer loading, process N2 gas starts to flow into the process tube and fill it with high purity nitrogen. 8. When the wafer boats are placed in the process tube, the temperature starts to ramp up, with ramp up rate about 10oC/min. 9. After the furnace reaches the setting temperature required by the process, a few minutes of temperature-stabilizing with N2 flow are required to allow to temperature oscillation, to die down and make the furnace reach the steady state of the setting temperature.
10. Since the system is now ready for oxidation, turning on the oxygen and anhydrate hydrogen chloride flows and turning off the nitrogen flow, makes oxygen react with silicon to form a thin layer of silicon dioxide on the silicon wafer surface.
11. After the required oxide thickness is reached, the O2 and HCI flows are stopped and N2 flow is resumed.
12. The wafer stays at high temperature for a while to anneal the oxide so that the quality of silicon dioxide, makes it denser, reduces the interface state, and increase the breakdown voltage.

3. Max-Min Uniformity (%) of the SiO2 thickness

Using the formula:

| |
|Non-uniformity(%)={(Max value-Min Value)/2 x average}* 100% |

4. Parameters that affects the oxidation rate

i. Temperature ii. Pressure iii. Humidity iv. Crystal orientation v. Silicon orientation vi. Dopant concentration vii. Surface preparation viii. Ambient type ix. Chlorine addition x. Nitridation xi. Oxidant pressure xii. Time

5. Estimated thickness of SiO2 using colour chart

Colour turn from brown to tan. Before oxidation process thickness wafer is 601.4 Ao which is near to value 700 Ao-brown colour. After oxidation process thickness wafer is 560.1 Ao which is near to value 600 Ao-tan colour.

6. Growth rate of SiO2 Oxide thickness growth rate: Average oxide thickness/time Ao/sec

Oxidation: < 1 0 0 > Silicon Dry Oxidation

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