1. thermal oxidation
1.1. purpose of thermal oxidation process
1.1.1. is a chemical process, where silicon dioxide (SiO_2) is grown in an ambient with elevated temperatures.
1.1.1.1. dry oxidation
1.1.1.1.1. Si (silicon) react with O_2 (oxygen gas) produce SiO_2 (Silicon dioxide)
1.1.1.2. wet oxidation
1.1.1.2.1. Si (silicon) react with H_2 O (water) produce SiO_2 (Silicon dioxide) and hydrogen gas
1.1.2. The oxide is used to provide insulating and passivative layer
1.2. Basic Concepts
1.2.1. oxidation involves volume expansion ~2.2x
1.3. Working Principle
1.3.1. Furnace tube preheated: 800-1000 degree celcius
1.3.2. Wafers are placed in Silica tube
1.3.3. Dry Oxygen gas and water is flowed into the tube at atmospheric pressure and flow out at the other end
1.3.4. Timing, temperature, gas flow are controlled to ensure the quality of Silicon dioxide (SiO_2)
2. Diffusion
2.1. Purpose of Diffusion Concepts
2.1.1. Process of doping
2.1.1.1. dopants are gradually transported from the high concentration region near the surface into the substrate through diffusion,
2.2. Basic Concepts
2.2.1. Si wafer is exposed to solid, liquid or gaseous source containing desired impurity
2.2.1.1. Solid sources
2.2.1.1.1. placed in the silica tube upstream from the sample or in a separate heating zone of the furnace
2.2.1.2. gaseous sources
2.2.1.2.1. can be metered directly into the gas flow system
2.2.1.3. liquid sources
2.2.1.3.1. liquid source inert carrier gas is bubbled through the liquid before being introduced into the furnace tube
2.2.2. a reaction at wafers surface establish a supply of dopant atoms immediately adjacent to Si crystal
2.2.3. at elevated temperature atom diffuse in the region Si is not protected by oxide
2.2.4. Surface doping concentration is up to 10^21/ cm^3
2.2.5. diffusion in SiO_2 is relatively low
2.2.6. SiO_2 protects Si for a limited time depending on
2.2.6.1. oxide thickness
2.2.6.2. temperature
2.2.6.3. background droping
2.2.7. The diffusivity of dopants in solids, D, has a strong Arrhenius dependence on temperature, T. It is given by D = Do x exp- (E_A/kT), where Do is a constant depending on the material and the dopant, and E_A is the activation energy.
2.2.8. there is some point in the sample at which the introduced acceptor concentration just equals the background donor concentration in the originally n-type sample. This point is the location of the p-n junction