Inline coherent imaging of laser micromachining

• Published in: 2010 International Symposium on Optomechatronic Technologies pp. 1 - 4
• Main Authors: Webster, P J L, Yu, J X Z, Leung, B Y C, Wright, L G, Mortimer, K D, Fraser, J M
• Format: Conference Proceeding
• Language: English
• Published: IEEE 01.10.2010
• Subjects: coherent imaging; Laser ablation; Laser beam cutting; Laser beams; Machining; optical coherence tomography; Optical imaging; Optical interferometry; percussion drilling; process monitoring and control
• ISBN: 9781424476848; 1424476844
• DOI: 10.1109/ISOT.2010.5687305

In applications ranging from noncontact microsurgery to semiconductor blind hole drilling, precise depth control of laser processing is a major challenge. Even expensive a priori characterization cannot compensate for material heterogeneity and stochasticity inherent to the ablation process. Here we use in situ depth imaging to guide the machining process in real time. W e image along the machining beam axis at high speeds (up to 300 kHz) to provide real-time feedback, even in high aspect ratio holes. The in situ metrology is based on coherent imaging (similar to optical coherence tomography) and is practical for a wide-range of light sources and machining processes (e.g., thermal cutting or ultrafast nonlinear ablation). Coherent imaging has a high dynamic range (>; 60 dB) and strongly rejects incoherent signals allowing weak features to be observed in the presence of intense machining light and plasmas. High axial resolution (~10 μm) requires broadband imaging light but the center wavelength can be chosen appropriate to the application. Infrared light (wavelength: 1320 ± 35 nm) allows simultaneous monitoring of both surface and subsurface interfaces in non-absorbing materials like tissue and semiconductors. By contrast, silicon based detector technology can be used with near infrared imaging light (805 ± 25 nm) enabling high speed acquisition and low cost implementation.