Laser focus positioning method with submicrometer accuracy

Abstract

Accurate positioning of a sample is one of the primary challenges in laser micromanufacturing. There are a number of methods that allow detection of the surface position; however, only a few of them use the beam of the processing laser as a basis for the measurement. Those methods have an advantage that any changes in the processing laser beam can be inherently accommodated. This work describes a direct, contact-free method to accurately determine workpiece position with respect to the structuring laser beam focal plane based on nonlinear harmonic generation. The method makes workpiece alignment precise and time efficient due to ease of automation and provides the repeatability and accuracy of the surface detection of less than 1 μm.

A highly sensitive laser focus positioning method with sub-micrometre accuracy using coherent Fourier scatterometry

“…It is thus not recommended to use such a large diameter for focus-searching purposes with materials of low reflection. The obtained results are comparable to those of the focus searching methods used in micro-machining applications such as that based on a nonlinear harmonic generation [23]:…”

Single-lens dynamic $$z$$-scanning for simultaneous in situ position detection and laser processing focus control

“…While other mechanical, resistance-, and capacitancebased methods exist, the majority of surface detection methods are based on the optical detection of backreflected light. The focus error or defocusing (z s À z f ) is determined from the position (triangulation) 5 , intensity 13 , beam size 14,15 , astigmatism 16,17 of a reflected probing beam, or chromatic aberrations 10,11 , nonlinear harmonics 18 of process-generated light. Other back-reflected detection methods also adopt differential optical paths 19,20 , interferometry 21,22 , diffractive beam samplers 23 , spatial light modulators 24 , dynamic mechanical scanning 3,25,26 , digital image correlation 12 , or commercial displacement sensors 7,9 .…”

“…Other back-reflected detection methods also adopt differential optical paths 19,20 , interferometry 21,22 , diffractive beam samplers 23 , spatial light modulators 24 , dynamic mechanical scanning 3,25,26 , digital image correlation 12 , or commercial displacement sensors 7,9 . However, most optical metrology with automatic focus alignment still requires the mechanical motion of a z stage attached to the sample 6,14,18,19,22 or the focusing lens 3,8,10,[23][24][25] as the second 'movement' module in its feedback loop. Even with the advance of the fast mechanical stages and scanning galvomirrors, adjusting z position can be up to three orders of magnitude slower than the lateral speed of adjusting x and y 27 due to mechanical acceleration/deceleration.…”

In-Situ Real-Time Focus Detection during Laser Processing Using Double-Hole Masks and Advanced Image Sensor Software

“…More importantly, some other studies have already improved the optical setup to explore the focus on a non-planar sample based on diffractive beam samplers [ 12 , 13 ] or the macro/micro dual-drive principle [ 14 ]. Furthermore, many auto-focusing devices have been developed for laser direct writing [ 14 , 15 , 16 ]; laser ablation [ 17 ]; automatic microscopy and measurement [ 18 ]; laser material processing [ 19 ]; two-photon photo-polymerization (TPP)-based micro-fabrication [ 20 ]; and several other applications [ 21 , 22 , 23 , 24 , 25 , 26 , 27 , 28 , 29 , 30 ] in Shack-Hartmann wave-front sensor fabrication [ 21 ], parallel data processing [ 22 ], photonic force microscopy [ 23 ], controllable mirror-lens retrofocus objective [ 24 ], tunable lens focal offset measurement [ 25 ], laser micromachining [ 26 ], remote sensing [ 27 ], automated optical inspection [ 28 ], auto-focusing infinity corrected microscopes [ 29 ], and direct imaging technology [ 30 ] with detailed theoretical models. However, these focus detection systems are complicated, expensive, and inadequate for the mass production of 3D patterns [ 21 , 22 , 23 , 24 , 25 , 26 , 27 , 28 , 29 , 30 ].…”