In situ plasma process monitoring of BCl$_3$ using quantum cascade laser absorption spectroscopy

BCl$_3$ is used as a source gas in various industrial plasma applications. The online monitoring of its ground state concentration in the plasma process reactor is vital for an improved insight into the plasma chemistry and to increase productivity, reliability and reproducibility of the process. Quantum cascade lasers offer attractive options for the application of mid-infrared absorption spectroscopy for in situ monitoring and control of industrial processes. In this contribution we report on a compact quantum cascade laser measurement and control system (Q-MACS) combined with an infrared-fiber to allow a flexible and completely dust-sealed optical coupling to the reactor chamber of an industrial plasma surface modification system. For different process regimes the absolute concentrations of the precursor BCl$_3$ were measured in a pulsed dc discharge containing Ar/N$_2$/H$_2$ and BCl$_3$ at a pressure of 200 Pa. A synchronization of the Q-MACS with the process control enabled an insight into individual process cycles with a sensitivity of 10$^{-6}$ cm$^{-1}\cdot$ Hz$^{-1/2}$.