Bow of Silicon Wafers After In-Line High-Rate Evaporation of Aluminum

Contacting silicon solar cells through in-line high-rate evaporation of aluminum leads to thermomechanical stresses and, thus, to bowing of the solar cells. Understanding the formation of the cell bow is essential for improving the deposition process. We deposit 2 μm-thick aluminum layers onto 230 μm-thick planar p-type silicon wafers of edge lengths of 100, 125, and 156-mm and measure the wafer bow after the deposition. The bow is proportional to b²d/W², where d is the aluminum layer thickness, W the wafer thickness, and b the wafer edge length. We measure a lower bow than expected by the linear elastic stress theory and show this to be caused by plastic deformation in the Al layer. Due to plastic deformation, only the first 70 K of temperature change actually causes a bow.