Inline phosphoric acid thickness & uniformity measurement system

Increasingly, manufacturers of silicon solar cells are doping wafers by misting, spraying, or rolling on phosphoric acid. No inline system exists for measuring the mass and uniformity of the deposited film to provide process control feedback in real-time. We show that an optical infrared measurement system can calculate film mass and uniformity inline. Using a parametric optical model, we calculate the expected amount of reflected infrared light as a function of film thickness, acid concentration, incidence angle, and different material optical properties. The model results suggest that the technique is applicable over a wide range of process conditions. Measurements taken of a 20% concentration film for varying film thickness correlate well with the model. We believe this confirms the overall utility of this in-line system for a range of differing process variables.