Diffusjonstrinn

Diffusjonstrinn is a term used in semiconductor fabrication to describe a step in which dopants are introduced into a silicon wafer by diffusion at elevated temperature. The process alters the electrical properties of specific regions, creating doped areas that form devices such as pn junctions, transistors, resistors, or contact regions. Diffusion can occur from solid sources, vapors, or dissolved species, with masking layers (such as oxide or photoresist) defining the patterns.

During a diffusjonstrinn, the wafer is prepared and placed in a diffusion furnace or subjected to rapid thermal processing. The dopant source, temperature, ambient atmosphere, and diffusion time determine the depth and concentration profile. Typical temperatures range from roughly 800 to 1050 degrees Celsius, and diffusion times can span from seconds to hours, depending on the desired dopant distribution. Dopants commonly used include boron for p-type regions and phosphorus or arsenic for n-type regions. The resulting dopant profile is often approximated as a diffusion curve, with depth-dependent concentration that is shaped by the masking pattern.

Variants of diffusjonstrinn include drive-in diffusion, where an implanted wafer is further diffused to deepen the doped region, and diffusion from liquid or gaseous sources such as POCl3. Faster processes are available through rapid thermal diffusion, which provides shorter, high-temperature pulses. Diffusion lengths scale with the square root of the diffusion coefficient and time, L ≈ sqrt(Dt), giving designers a handle on junction depth.

In practice, diffusjonstrinn has become less dominant for many junction formations in contemporary CMOS processes, which increasingly rely on ion implantation followed by annealing. However, diffusion remains relevant for certain low-cost, large-area, or specialized diffusion steps and legacy manufacturing.