Processing CIGS in a belt furnace

The fabrication of CIGS solar cells within a production environment contains the deposition of copper, indium and gallium on the selected substrate material while annealing them at elevated temperatures in controlled atmospheres. First, the substrate material is cleaned and then heated to an elevated temperature. Afterwards, copper, indium, gallium and selenium are deposited through a sputtering process and then the deposited parts are selenized in an elevated temperature profile. The selenization process involves ramping up from room temperature to 450°C in about 4 minutes. The samples are soaked at this temperature for 7 minutes and then elevated to 550°C for approximately 4 minutes. The samples are then held at 550°C for another 7 minutes and then cooled down. Finally, while held at an elevated temperature, hydrogen sulphide gas is introduced to sulphurization. A detailed profile is illustrated as follows.

Belt Furnace for CIGS processing

A furnace that processes CIGS solar cells should be capable of operating to 650°C or higher. The aim is for a wider belt with superior cross belt uniformity so that larger substrates can be contained in the future. More over, a muffle is required to ensure a cleaner operating environment because various processes in CIGS processing, such as sulpharization, involves the introduction of different gasses at different time periods. The muffle needs to have the capability to control the gas type and gas flow in each zone. With all of this in mind, Torrey Hills Technologies has designed a furnace that is well suited to match the requirements for thin film solar applications.

The HSA series belt furnace uses ceramic heater boards in order to achieve elevated temperatures. Aside from the standard belt size of 350 and 650mm, wider belts have been designed as well to accommodate wider glass substrates. While a 700mm wide belt has been successfully engineered, efforts are underway to build a belt as wide as 1000mm. As a standard feature, this furnace is equipped with a steel brush for cleaning the conveyor belt, however, Ultrasonic belt cleaning is available as an extra option.

A microprocessor based PID controller is what controls the furnace. Type K thermocouples are used in determining the zone temperatures and the controls are located on the right hand side of the furnace which can be viewed from the entrance. The central processing unit (CPU) is mounted under the exit table and the CPU is primed with a Windows operating system for easy computing. The computer system is pre-installed with a program for controlling the furnace parameters, including the belt speed and the zone temperatures. Temperature profiles can be stored and retrieved as well for future purposes. Thermocouple ports are located at the entrance table for connecting the profiling thermocouple directly into the microprocessor. This feature allows for the monitoring and recording of actual temperatures experienced by the part. Software is also included with the computer to capture, display, printout and store the furnace profile. Additionally, the furnace is equipped with a redundant overheat safety protection system which incorporates an additional type K thermocouple in the center of each controlled zone and the multi-loop alarm. The specification of a HSA 7503 belt furnace is listed in the following table.