Gas Delivery Monitoring and Control

• Precise control of process gas delivery to deposition process tools is critical to the growth of high quality epitaxial films.
• Traditional gas generation and delivery systems, such as bubbler, sublimator or direct liquid injection (DLI) have less than ideal repeatability, accuracy, stability and reproducibility of chemical delivery to the process tools.

Traditional Bubbler Delivery System
	Advantages: Excellent short term stability at low carrier gas flow rates, i.e., when no splashing. Good ability to handle low mass transfer rates. As a rule, MFC’s are not wetted with liquid process chemical. Disadvantages: High sensitivity to temperature variations. High sensitivity to system and/or atmospheric pressure changes. Some sensitivity to liquid level particular at lower levels. Temperature of liquid is an unknown function of position in liquid volume. Splashing (creation of aerosol droplets) occurs at high carrier gas flow rate needed for high rate of process chemical mass transfer unless bubbler becomes large, e.g., 55 gallon drum.
Traditional Sublimator Delivery System
	Advantages: Excellent short term stability at low carrier gas flow rates. Some sources exist only as solids. Good ability to handle low mass transfer rates of process chemicals. As a rule, MFC’s are not wetted with liquid process chemical. Disadvantages: High sensitivity to the carrier gas temperature. High sensitivity to system and/or atmospheric pressure changes. Sensitivity to surface area of solid source particular when near depletion.
Traditional DLI Delivery System
	Advantages: Ability to handle higher mass transfer rates of process chemical in small footprint. Low sensitivity to temperature variations Disadvantages: LMFM's exhibit short and long term drift which is exacerbated by the direct contact with the chemical. Nitrogen/Helium "pushing-gas" for process chemical can come out of solution creating bubbles that cause large perturbation to liquid flow when a bubble passes through the LMFM. Vaporization process is chaotic as liquid droplets randomly strike hot surface producing “noisy” concentration. (Generally will average out through mixing & diffusing before reaching process chamber.) Possibility of dumping large volume of liquid into process chamber if problem with liquid control valve. (Also true for bubbler with auto refill.)
Conclusions
	Traditional chemical delivery systems accurately control the mass transfer rate of the carrier gas while leaving the mass transfer rate of the process gas uncontrolled and thus free to drift with changes in bubbler temperature, pressure and liquid level. Use of PIEZOCON® concentration gas sensor and control system reverses this situation by sensing variations in concentration and adjusting the carrier gas MFC set-point to restore desired mass transfer rate (grams/min) of the process chemical to chamber. The PIEZOCON® Sensor System measures the concentration of a mixture of two gases (binary mixture of a carrier gas and process chemical in vapor form) and controls the grams/min of process chemical delivered to the chamber by continuously adjusting the MFC’s set-point
Result
	Piezocon greatly improved the uniformity of TCS delivery on Centura, in terms of material thickness, compared to process without Piezocon feedback control. (Piezocon is installed on Chamber A only)

Film Thickness Histogram of Chamber A, with Piezocon Control	Film Thickness Histogram of Chamber B, without Piezocon Control	Film Thickness Histogram of Chamber C, without Piezocon Control

Resistivity Histogram of Chamber A, with Piezocon Control	Resistivity Histogram of Chamber B/C, without Piezocon Control

For more information, please check out Silicon International web site for the full report Deposition Rate Control During Silicon Epitaxy.