Statistical and Engineering Process Control of Phosphorus Diffused Solar Wafers Using Contactless Infrared Reflectometry

For the silicon solar cell diffusion process, statistical process control (SPC) and engineering process control (EPC) are commonly used to ensure that yield and thoughput targets are met during high-volume industrial production. SPC uses control charts to detect out-of-specification (OOS) wafers that need to be reworked (typically those with emitters that are too lightly diffused to be contactable by the screen printed metal electrode)., EPC involves tuning of diffusion furnace parameters based on feedback from the emitter sheet resistance measurements of prior diffusion batches to reduce process variations and the frequency of OOS events. Both of these process control techniques, SPC and EPC, rely on sampled measurements of diffused wafers over each diffusion batch. This paper focuses on best practices and unique advantages of the use of contactless infrared reflectometry (IR) measurements of phosphorus-diffused wafers to implement SPC and EPC.