Thermal and Acoustic Techniques for Monitoring Pulsed Laser Processing

A variety of thermal and/or acoustic techniques have been developed over the past few years to monitor and study laser initiated processes used in the microelectronics industry. A review of some of these will be given with examples from the work of the author and his collaborators, as well as from other laboratories. In many processes, lasers are valuable tools because they are capable of delivering controlled amounts of energy at specific times to localized regions of space. Processes such as hole drilling and laser enhanced etching or photochemical ablation involve a variety of complex physical and chemical processes. The understanding of these requires knowledge of the energy balance between chemical excitation (bond breaking, chemiluminescence, combustion, etc.), kinetic energy of expelled particles, heat in the substrate, and heat transmitted to the surroundings through radiation or conduction. Information concerning this energy balance can be obtained using steady state and time resolved calorimetry, infrared detection of thermal radiation, and steady state and time resolved photoacoustic techniques, and through the study of high frequency ultrasonic waves generated in the substrate. The advantages and disadvantages of each technique will be discussed within the context of specific examples. A better fundamental understanding of the processes involved is achieved as well as the development of practical process monitors.