Abstract

Real-time monitoring of etch depth during laser micromachining is useful if not essential to control the dimension of the etched part. Chen et al.¹ showed that the amplitude of the low frequency acoustic signal detected by a microphone in a photoacoustic cell is linearly related to the intensity of the laser generated plume during ablation. Recently, Sell et al.² demonstrated the possibility of using photoacoustic laser beam deflection technique to measure etch depth. Both methods have different types of drawbacks. (1) The requirement of placing the sample inside a photoacoustic ceil severely limits the possibility of using this method in mass production. (2) Because of the limited frequency response of the microphone, the repetition rate of the laser must be kept low. (3) Furthermore, error may be generated by the deposition of the ablated material onto the surface of the microphone and the photoacoustic cell window. Photoacoustic laser beam deflection technique does not have the aforementioned drawbacks. Probe beam alignment can be very critical, however, with the results sensitive to the air temperature in the region between the sample and the probe beam, and this temperature is strongly affected by the ablation process.

© 1991 Optical Society of America