Atomic spectroscopy can monitor and control physical vapor deposition process
Researchers at Stanford University (Stanford, CA) and Focused Research (Santa Clara, CA) have developed an atomic-spectroscopy-based monitoring system for physical-vapor-deposition process control of yttrium. Based on frequency-modulation (FM) atomic-absorption spectroscopy, the scheme uses an external-cavity, 670-nm diode laser (New Focus, Santa Clara, CA). The laser beam was passed through an electro-optic phase modulator driven by a radio-frequency source, and its frequency was monitored with a hollow-cathode lamp as reference. Single-mode optical fiber delivered the beam to the vacuum deposition chamber where it was focused onto a wideband photodetector.
Mixing the detector output with the RF signal driving the electro-optic phase modulator resulted in a signal proportional to the first derivative of the absorption profile. The second derivative was used for deposition-rate monitoring and control of the process because it is proportional to the peak absorption. Absorption as low as 10-6 could be measured, and deposition-rate control with an accuracy better than 1% at a rate of 3.5 Å/s was demonstrated, which corresponds to a deposition-rate resolution of 0.03 Å/s. The technique also has applications for understanding thin-film growth and the dynamics of vapor condensation on substrates.