Laser techniques for micromachining and other forms of material processing, such as texturing, scribing and surface cleaning, have been moving into industrial processes increasingly over the past few years.
The recent advances in femtosecond laser micromachining have prompted a lively debate, both in the literature and at international conferences, on the subject of the most appropriate type of laser to carry out these processes in the industrial environment, whilst bearing in mind the importance to industrial users of ruggedness, reliability, cost and speed, as well as the quality of the finished part.
Some of the protagonists in this debate have accepted invitations to contribute papers to this Focus Issue of Optics Express.
Two prominent groups in Germany from the Laser Zentrum, Hannover and the Institute of Applied Physics, Friedrich Schiller University, Jena, have jointly contributed a paper (see Korte et al.) on making holes in thin metal films with diameters below the diffraction limit, using femtosecond infra-red pulses. Their technique exploits the steep threshold ablation response of the metallic film so that only at the centre of the Gaussian focal spot is the threshold for ablation exceeded. However, while the clear aperture of the holes thus produced can be as small as 0.5 µm, the hole is surrounded by a ‘doughnut-like’ deformation of the metal surface which, as the authors say, can play a crucial role in possible applications of the technique for the repair of photolithographic masks. The same authors also report on another use for femtosecond lasers in which the high intensity achieved at focus produces effects that cannot be replicated with conventional nanosecond lasers. They demonstrate the production of waveguide structures in crystalline quartz using a focused femtosecond laser beam to create damage in the form of a sub-surface track of modified refractive index that can form a waveguide for visible light in the bulk of the material.
Two papers from UK industrial companies report on high precision micromachining with nanosecond pulsed lasers. Oxford Lasers Ltd (see M.R.H. Knowles) show examples of their micro-hole drilling in gasoline fuel injectors, inkjet printers and medical devices using high repetition rate (2–50 kHz) copper vapour lasers which emit pulsed visible (511nm and 578nm) beams. Holes as small as 1.5 µm have been drilled and the laser is capable of achieving tolerances better than ±0.25 µm in holes with diameters in the range 5 to 200 µm. Examples are also shown of micro cutting of diamond and ceramics, as well as surface processing including sub-micron surface patterning of polished steel and direct writing of microcircuit tracks. Exitech Ltd (see M.C. Gower) present examples of micromachining of plastics and other organic materials using a variety of pulsed lasers including hybrid processes for the drilling of blind microvias in printed circuit boards. In this process a Nd: YAG laser trepans a hole in the top conductive layer that then acts as a contact mask for the ablation of a blind hole of the same size into the fiber reinforced layer below using 10.6mm CO2 laser radiation. A variety of laser mask projection techniques are used to machine arrays of holes in plastic materials for inkjet printers and medical devices. Using micromachining with KrF lasers and orthogonal mask dragging, a variety of micro-optical surfaces have been produced, such as moths-eye and laser beam shaping devices.
The laser group at the Department of Engineering, University of Liverpool (see Lee and Watkins) address the problem of oxide layers and small particles on copper surfaces, which affect the quality of solder connections of printed circuit boards and cause loss of production yield during fabrication of micro-electronic devices. They describe a simple technique for detecting the completion of the removal of copper oxide from a copper surface by monitoring the sound made when the pulse from a Nd: YAG laser is focused to a small spot on the surface. They also describe how tilting the incident laser beam to within 10 degrees of grazing incidence renders the process of ablating copper particles adhered to a metallic surface significantly more efficient than simply irradiating at normal incidence.
This Focus Issue, which aims to provide a snapshot of the state-of-the-art, therefore carries the debate on potential and current industrial uses of lasers for ablation and micro-machining beyond arguments over whether femtosecond lasers are necessary for production of high-quality micromachining results into specific examples of lasers used in real industrial situations.
Paul French, Imperial College of Science
Colin Webb, University of Oxford