Diode-laser-based in-situ atomic absorption spectroscopy for deposition rate control

In-situ atomic absorption spectroscopy with diode lasers has been performed for monitoring and study of the deposition process in electron beam evaporation. The combination of the wavelength-modulation spectroscopy with external-cavity diode lasers and the balanced detection scheme guarantees the high sensitivity and reliability of the system. Direct measurements of atomic flux in e-beam evaporated yttrium and barium, which are components in YBCO superconducting thin films, have been demonstrated. Atomic number density and velocity were measured through absorption and Doppler shift measurements, respectively, to monitor the atomic flux. The measured velocities show that the e-beam evaporated atoms are in a non-thermal equilibrium state, dependent on source conditions, implying that the flux measurement rather than a simple density measurement for rate control is necessary. Comparison with quartz crystal monitors shows that the present scheme, employing two laser beams counterpropagating at an angle to the substrate surface for measuring the velocity component normal to the substrate surface, can provide a pressure-independent flux measurement. In yttrium, which has an additional significantly populated metastable level, results show that pressure-independent flux measurement requires measurements at both the ground state and the metastable levels. Real-time curve-fitting for in- situ flux measurement was demonstrated. In addition, sticking and re-evaporation characteristics of barium was also investigated, demonstrating the feasibility of in-situ sticking coefficient measurement.