DIODE-LASER ABSORPTION-SPECTROSCOPY FOR PROCESS-CONTROL - SENSOR SYSTEM-DESIGN METHODOLOGY

A laser absorption spectroscopy (LAS)system has been developed at Lawr ence Livermore National Laboratory for process control. LAS has proven itself to be an accurate and reliable method to monitor both density and composition. In this paper the important features and components o f an industrial LAS diagnostic are described. Application of this appr oach to vaporization processes requires careful selection of the speci es and transitions to be monitored. The relative vapor pressure, hyper fine structure, isotopic frequency shifts, and electronic temperature all effect the selection of a particular transition. In this paper we describe the methodology for choosing the optimal transition or transi tions. Coevaporation of a titanium-niobium alloy is used to illustrate the methodology. In a related paper, T.M. Anklam et al.,'Evaporation rate and composition monitoring of electron beam PVD processes' (Surfa ce and Coatings Technology, 76/77 (1995) 681-686), describe the applic ation of this diagnostic to monitoring and controlling composition in a physical vapor deposition process of industrial interest.