FZ and CZ crystal growth: Cost driving factors and new perspectives

Abstract

In the field of monosilicon crystal growth, the Czochralski (CZ) as well as the floating zone (FZ) method have grown to a mature technology over the last 60 years and, until today, the costs have been continuously reduced. The cost driving factors for CZ and FZ are reviewed with respect to further potential improvements. Generally, the cost for the feedstock material is still dominanting the cost of ownership for both methods despite the dramtic decrease of the poly silicon price over the last years. This is particularly true for FZ where notably more than 50% of the production costs are related to the polysilicon feed rods which are much more expensive as compared to the chunk polysilicon as used for CZ. As a consequence, the FZ method is only used for applications where silicon crystals with very low oxygen concentration are required. Due to the strong influence of the feedstock material on cost, the main focus of the development work has been on the increase of crystal yield and the identification of factors impacting the yield. However, for most electronic and, in particular, for solar application, the crystal yield has reached a level where significant improvements cannot be expected anymore. With the potential change from p- to n-type silicon in the solar industry, previously developed techniques, like continuously recharged CZ, which were finally stopped for electronic applications, are reconsidered and will be discussed with respect to their future potential. Special attention will be given to the advantages and disadvantages of the FZ method. It will be shown that most of the current drawbacks of FZ can be remedied by a new technique which avoids the use of expensive polysilicon feed rods and enables the FZ method to grow dislocation free crystals with inexpensive silicon granules.