BELT FURNACE FOR SILICON SOLAR CELL DRYING AND FIRING

The firing process, also referred to as sintering, is one of the key steps with which the front-metal contact is formed in a silicon solar cell. In this process, the thick film paste is dried at about 150°C to remove much of the solvents. The presence of solvents can cause excessive out gassing which can lead to cracks and voids. The dried substrates are then fired inside a firing furnace. The firing process consists of four primaries. The first step is the initial temperature ramp up where the paste solvents are volatilized. The second step is the burn out. The objective of the burn out phase is to remove all of the organic binder that was used in paste formation. The burn out phase is carried out at 300-400°C. The third step is the sintering, or the firing process, which is done between the ranges of 700-800°C. During this process, the Ag metal forms a bond with the underlying silicon substrate to form metal contact. The final step in the firing process is the wafer cool down phase.

The mechanism behind contact formation in a fire through contact is complex. the process starts by evaporating the solvent between 100-200°C and then burning out the polymer between 200°C and 400°C. Later, from 400-600°C, the glass frit melts and the sintering of the Ag particles take place. Furthermore, from 600-800°C, molten glass with some amount of dissolved Ag etches the silicon nitride anti reflection coating and then reaches the Si surface. Here, it reacts and etches a very thin layer of Si. Ag in the glass will then precipitate onto the Si surface in the form of crystallites. An ideal temperature profile for the firing process is illustrated as below.

The quality of the contact influences the shunt resistance, series resistance and junction leakage current, which have a significant impact on the efficiency of the solar cell.

Furnace for Firing Operation

Our HSH series furnace is a specially designed infrared furnace that caters to the needs of the photovoltaic metallization firing requirements. The heating in this furnace is achieved with the help of short wave infrared lamp heaters. The fast response of the IR lamps allow for quick heating. The furnace is rated at 1000°C and can operate very well in the 750-800°C range required for sintering of front contact metallization. The belt width comes in various standard sizes, including, 250mm, 300mm and 380mm to match with the requirements of the wafer size. Cooling can be achieved through forced air, as well as, water per requirement. The presence of a muffle helps to control the atmosphere within the furnace, as well as, prevent the external atmosphere from entering. In short, the muffle design helps maintain a cleaner furnace atmosphere. As a standard feature, this furnace is equipped with a steel brush and helps in the cleaning process of the conveyor belt. Ultrasonic belt cleaning is available as an extra option as well.

A microprocessor based PID controller controls the furnace. Type K thermo-couples are used for determining the zone temperatures. Controls are located on the right hand side and can be viewed from the entrance of the furnace. The central processing unit (CPU) is mounted under the exit table. The furnace is controlled by a microprocessor based controller system and the CPU is loaded with a Windows operating system that allows for easy computing. The computer system comes with a pre-installed program for controlling the Confurnace parameters including the belt speed and the zone temperatures. Temperature profiles can be stored and retrieved for future purposes. Thermocouple ports are located at the entrance table for connecting the profiling thermocouple directly to the microprocessor. This feature allows the monitoring and recording of the actual temperature experienced by the part. Software is also provided with the computer to capture, display, printout and store the furnace profile. 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.

A Typical Silicon Solar Cell Processing Furnace