TimeDomain CVD, Inc.

Tube Reactor Conclusions

Temperature control

Steady state: Radial uniformity is almost automatic, except for the first and last few wafers in the load. 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. Wafers must be loaded slowly, or system temperature ramps used; tens of minutes are required to reach stable temperatures. 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. The powerful influence of total pressure explains the prevalence of LPCVD tube aplications.

Axial film uniformity

Wide variations can occur depending on loading conditions, operating temperature, gas flow rates, and injection scheme.

Productivity advantages

It is easy to achieve large wafers/load by operating at low pressure to allow close wafer spacing: 100 wafers may be processed in a single batch.

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.

Process advantages

Very high purity quartzware can be fabricated; high temperature operation with minimal metallic contamination of films is straightforward in this type of reactor.

Process disadvantages

Difficult to implement plasma reactors in tube configuration.

Tube reactors are commonly employed for "front-end" IC fabrication steps: polysilicon deposition, silicon nitride deposition, and high-temperature silicon dioxide deposition from TEOS, as well as deposition of doped oxides such as BPSG or PSG. This type of reactor is not normally employed for deposition of metals such as tungsten, copper, or TiN due to problems in cleaning.

"Back-end" processes (after metal deposition) require lower temperatures and often employ plasma techniques. Some plasma tube reactors were developed in the early 80's but difficulties with uniformity and maintenance have eliminated this approach in favor of showerhead and high-density plasma approaches, now often in single-wafer configurations.

Reactors: Table of Contents

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Temperature control	Steady state: Radial uniformity is almost automatic, except for the first and last few wafers in the load. 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. Wafers must be loaded slowly, or system temperature ramps used; tens of minutes are required to reach stable temperatures. 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. The powerful influence of total pressure explains the prevalence of LPCVD tube aplications.

Axial film uniformity	Wide variations can occur depending on loading conditions, operating temperature, gas flow rates, and injection scheme.

Productivity advantages	It is easy to achieve large wafers/load by operating at low pressure to allow close wafer spacing: 100 wafers may be processed in a single batch.

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.

Process advantages	Very high purity quartzware can be fabricated; high temperature operation with minimal metallic contamination of films is straightforward in this type of reactor.

Process disadvantages	Difficult to implement plasma reactors in tube configuration.