Axial uniformity is achieved using multi-zone heaters. Quartz hardware allows operation at temperatures in excess of 1000 C, although at very high temperatures migration of sodium through the furnace tube becomes a concern for MOS device fabrication. Transient temperature control is much more challenging.
Modern reactors employ model-based temperature control to minimize heatup and cooldown ramp times.
Radial film uniformity Radial uniformity is excellent if pressure is low and surface reaction rate slow. Wide variations can occur depending on loading conditions, operating temperature, gas flow rates, and injection scheme. Productivity disadvantages Low rates required to maintain uniformity, and slow thermal response, imply long process times. Large batch sizes necessary to achieve reasonable throughput put a large number of wafers at risk in each run should a problem occur. The complex cantilever supports, specialized wafer boats, and possibly also cages, are expensive and require periodic cleaning.