MASSE – In-Line Annealing – High Throughput and Controlled Atmosphere

Duration: March 2008 - October 2012
Contracting Authority/ Sponsors: German Federal Ministry for the Environment, Nature Conservation and Nuclear Safety (BMU)
© Fraunhofer ISE
Fig. 1: Prototype of the in-line furnace to anneal crystalline silicon solar cells with an edge length of 125 mm or 156 mm. The furnace has two processing tracks.
© Fraunhofer ISE
Fig. 2: Detail of both processing tracks of the prototype of the in-line furnace to anneal crystalline silicon solar cells with an edge length of 125 mm or 156 mm.
© Fraunhofer ISE
Fig. 3: Distribution of efficiency values for monocrystalline silicon solar cells with passivated rear surfaces, measured before and after an annealing process in inert forming gas. The results of the in-line process are similar to those from single-wafer processing.

Most high-efficiency concepts for silicon solar cells feature dielectrically passivated surfaces and/or novel metallisation approaches. For these structures, thermal treatment at the end of the cell processing is useful or even essential to achieve high efficiency. Up to now, usually quartz tube furnaces with a small volume have been used for the annealing processes. However, these do not offer the throughput needed for industrial applications and in addition, automation is complicated. An in-line process has been developed at Fraunhofer ISE which enables annealing processes to be carried out with a high throughput under a controlled atmosphere, thus meeting the specifications for industrial processes.

Concepts for highly efficient solar cells with dielectrically passivated surfaces are currently being transferred to production. Many of these passivation layers require thermal treatment to activate the surface passivation. Similarly, alternative metallisation procedures, e.g. two-stage metallisation, require an annealing process to improve adhesion and to reduce the contact resistance between the metal and silicon.

In co-operation with Centrotherm, a manufacturer of processing equipment, a prototype facility for in-line annealing (Fig. 1) was installed at Fraunhofer ISE. This demonstrated annealing for a high-throughput, in-line process for the first time. The in-line process avoids time-consuming flushing and loading steps, which has a positive effect on the throughput and the processing costs. The existing facility allows a throughput of app. 700 wafers/h for a plateau duration of 2 minutes. A larger system (five tracks, longer plateau zone) would enable an increase to more than 3000 wafers/h. Gas locks at the inlet and outlet of the furnace effectively isolate the processing atmosphere from the laboratory. By using inert forming gas, an oxygen concentration of less than 1 ppm can be achieved in the processing chamber. To evaluate the prototype system, processes were implemented under identical conditions in the in-line furnace and in a single-wafer reactor as a reference system. The annealing process causes a significant increase in the efficiency value of solar cells with dielectrically passivated rear surfaces (Fig. 2). The in-line process achieves the same level as the reference system.