Question

Diffusion is a key element of integrated circuit fabrication. Describe the two basic heat treatments that are used to diffuse impurities into silicon during this fabrication process.

Answer 1

(i)predeposition-In the predeposition step, impurity atoms are diffused into the silicon, often from a phase, the partial pressure of which is maintained constant. Thus, the surface composition of the impurity also remains constant over time, such that impurity concentration within the silicon is a function of position and time. Predeposition treatments are normally carried out within the temperature range of 900 and 1000°C and for times typically less than one hour.

(ii)drivein diffusion-Drive-in diffusion is used to transport impurity atoms farther into the silicon in order to provide a more suitable concentration distribution without increasing the overall impurity content. This treatment is carried out at a higher temperature than the predeposition one (up to 1200°C), and also in an oxidizing atmosphere so as to form an oxide layer on the surface. Diffusion through this SiO2 layer are relatively slow, such that very few impurity atoms diffuse out and escape from the silicon.