Michael W. Johnson

Deposition of contaminants on silicon media during wet processing

The introduction of contaminants on silicon media during processing can reduce the efficiency of photovoltaic (PV) cells and shorten their lifetime. One of the sources of contamination is process baths where media is dipped into water and/or chemicals for etching or cleaning. The sources of bath contaminants are numerous. They can arise from insufficiently pure chemicals or water. The tanks, silicon carriers and handling equipment may be constructed from industrial grade materials that desorb impurities thereby contaminating baths. Alternatively, the impurities could be extracted from common plastics used in industrial grade plumbing components or chemical containers. In this paper, we examine how a contaminated process bath may potentially deposit impurities on silicon media. Upon withdrawing a silicon wafer from a bath, a thin layer of liquid is left on the surface of the silicon. Any contaminants in the liquid layer will be deposited on the silicon when the liquid evaporates. This information will show that the biggest factor affecting deposition of impurities is the concentration of impurities in the bath. Other factors such as removal speed of the wafers from the bath and properties of the liquid including surface tension, density and viscosity are less influential.

Understanding metallic and ionic contamination in photovoltaic wet chemistries from chemical delivery systems

As photovoltaic cell manufacturers strive to improve yield and cell efficiency it is increasingly important to carefully select the materials of construction used in chemical delivery systems. Low purity materials of construction are prone not only to chemical degradation but they also contribute metallic and ionic contaminates into critical high purity process chemistries.