Thermal Oxidation of Silicon PDF

Thermal Oxidation of Silicon Surface of silicon wafer oxidizes to form Silicon Dioxide upon exposure to oxygen. Different thickness of the silicon dioxide layer produce different colours.  Thermal Oxidation of Silicon Uses of silicon dioxide:  Gate or Dielectric ox...

Thermal Oxidation of Silicon Surface of silicon wafer oxidizes to form Silicon Dioxide upon exposure to oxygen. Different thickness of the silicon dioxide layer produce different colours.  Thermal Oxidation of Silicon Uses of silicon dioxide:  Gate or Dielectric oxide  Electrical insulator  Barrier material during diffusion (introduce dopants into a region of the silicon wafer) SiO2 n p  Oxidation Process Thermal oxidation is achieved by heating the silicon wafer to a high temperature, usually 900 to 1200C. Wet oxidation Dry oxidation Si + 2H2O → SiO2+2H2 Si + O2 → SiO2  Oxidation Growth Growth rate of and silicon wafer is different.  Silicon wafers are classified into or orientations to be covered in Advanced Wafer Fabrication Technology  MOS devices are usually fabricated using silicon because it yields lower interface traps while BJT devices are fabricated using silicon  Oxidation Growth Wet oxidation is faster than Dry oxidation. The longer the time the thicker is the oxide. The higher the temperature the thicker is the oxide. The growth rate is different at different thickness. Oxide thickness calculations to be covered in Advanced Wafer Fabrication Technology  Oxidation Growth As new oxide is being formed, the newest layer is always at the bottom as the oxygen molecules travel through the oxide to react with the silicon to form new oxide. Growth rate is therefore different for different oxide thickness & slower with thicker oxide as the oxygen molecules takes longer time to travel through the Faster Slower oxide.  Oxide Quality & Applications Wet oxidation is faster but the density of oxide is lower. Dry oxidation is slower but the density of oxide is higher. Therefore wet and dry oxide are used for different applications.  Wet oxidation is much more feasible & used to grow thick masking or barrier oxide.  One of the most important properties of SiO2 is its ability to mask or block dopants during high-temperature diffusion to form p or n regions. Oxide Quality & Applications  Thick oxide for isolation are also formed using wet oxidation.  To achieve better quality oxide at the silicon to oxide interface, a wet followed by dry oxidation sequence is employed. Wet Oxide Dry Oxide Newest oxide grows at silicon-oxide interface  Oxide Quality & Applications Thin gate oxide of MOS devices are formed using dry oxidation:  Better quality oxide  Difficult to control the fast rate of wet oxidation for thinness Dielectric for capacitors are formed using dry oxidation. A typical & more reasonable shorter process time for thick oxide oxidation cycle usually consists of a dry-wet-dry sequence so that the poorer but thick quality oxide is sandwiched by good quality oxide. Oxidation Technology  Summary Dry Oxidation, good quality but slow, good for gate oxide or dielectric Simply Si + O2 → SiO2 Wet Oxidation, fast but bad quality, for isolation and diffusion masking Si + 2H2O → SiO2 + 2H2