Inline coherent imaging of laser micromachining

Abstract

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.