Gases & Instrumentation - March/April 2008

Bubbler

Flash

Thermal Vaporizer

Maximum flow-rate

Maintenance

Applicable Pressure range of the

reactor

Cost

Foot print

Effects of bubbles in liquid

precursor

Simplicity of refill system

Required temperature to vaporize

Flow Accuracy

Flow Reproducibility

Flow stability

Particulate source

=“BEST PERFORMANCE”

^{Table 1. Performance table}systemsaremountedhorizontally. Depending Mechanical Considerations upon the chemistry employed, the vaporizer module can run at high temperatures thereby requiring thermal isolation from nearby electronics as well as providing for operator safety. One part of the system that requires special consideration is the chemical delivery system

that provides a flow of precursor to the MFC. These liquids are usually kept in a bulk refill system that supports multiple flash systems. The refill systems deliver the liquid by means of a pressurized vessel or via a pump. Due to the constant pressure required for the source liquid material, flash systems require a very tight control of incoming liquid precursor material pressure. This may lead to a more complex refill system than would otherwise be necessary.

Typical flash systems occupy less than one quarter of a cubic foot (depending upon the manufacturer) and lend themselves to locations much closer to the process chamber. The

Relative Performance

Flash systems are capable of providing (under the correct conditions) stable vapor delivery with a wide variety of materials over a wide variety of temperature ranges. Additionally, flash systems have the smallest footprint of the three systems considered here. However, depending on model and the material to be vaporized flash systems may be prone to stability and maintenance issues. With vaporization taking place on a heated surface and at high temperatures, flash vaporizers are sometimes prone to the build up of materials on the vaporizing surface. This can lead to higher down time for maintenance as

AsH₃

B₂H₆

PH₃

SiH₄

Cl₂

SO₂

ClO₂

VCM

CO₂

COCl₂

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