Metallization

The contact firing and metallization application is used in the manufacturing of silicon solar cells. It involves the process by which thick film conductive inks (usually silver and aluminum) are applied to the front and back side of the solar cell. The material is usually screen printed or spot printed onto the cell. The paste will be dried and subsequent layers are added using the same method. The final step is performed in the metallization or firing furnace. The firing furnace actually integrates four process steps into one tool: drying to remove the last solvent, burnout to remove the binder, firing to form the electrical contact, and finally the cool down. Firing is the most critical step. The solar cells are rapidly heated to a peak temperature, ranging from 780°C to 900°C followed by rapid cooling.

Very fast ramp rates are required to optimize contact formation on silicon wafers. Rapid heating ensures proper fire-through to deliver excellent contact to n-Si layer and also to improve the aluminum back surface field, while fast cooling is required to prevent the diffusion of silver into the junction. Rapid ramp rates are constantly pushing for faster belt speed, which in turn drives the overall length of the firing furnace. Uptime, reliability, and Cost of Ownership are also becoming increasingly important. In the meantime, cell producers strive to reduce wafer thickness for higher material efficiency, which will pose more stringent requirements on next generation firing furnace.

BTU offers the PVD and Tritan™ lines of near infrared lamp furnaces to handle the rapid heating tasks of this application. Both are in-line continuous furnaces designed for rapid thermal processing.