Seeing the Light

The popularity of lasers in industrial, microelectronic, and medical applications is steadily growing. Lasers have established themselves across a wide range of materials processing tasks – among them marking, micro-machining, rapid prototyping, cutting/routing, microvia drilling and embedded passive trimming. These successes have been facilitated by significant progress in laser technology - higher laser powers, improved beam quality, shorter wavelengths, longer component lifetimes - and advanced focusing and handling systems that move and position the laser focus onto the workpiece.

Compared to conventional technologies, laser-based materials processing offers some key advantages. Small heat-affected zones allow precise and stable processes even in combination with very sensitive materials. Furthermore, laser processes are contact-free, and thus provide a degree of flexibility unattainable using tool-based mechanical processes. Non-contact techniques aid in material cleanliness and integrity, and the absence of wear and tear on tools also leads to important cost advantages.

However, these advantages can only be realized if focus quality and positioning onto the workpiece are highly controlled and this necessitates very precise beam positioning and focusing systems. Consider that a mere 15 percent spot variation produces a laser focus power density drop of around 30 percent; such variations strongly influence the quality of processed parts.

One approach to focus control is to maintain a stationary laser focus and to move the workpiece below. This allows a simple, fixed optical path without moving optical elements, but the high positional inertias result in slow part handling and thus impede throughput.

The laser beam can also be positioned onto the workpiece via moving optical elements such as mirrors and lenses while neither the laser nor the workpiece have to be moved. This can be achieved either by movement of the entire optical assembly including mirrors and focusing lenses in systems based on "flying optics" or by rotational movement of mirrors combined with a fixed lens. Typical "flying optics" setups are plotter, gantry or robot-based systems. They allow large working fields to be realized with relatively short focal lengths as the optics can always be positioned close to the workpiece. The positional inertia in such systems is usually less than that of fixed-beam systems, but still relatively high.

Galvanometers are rotational drives with high resolution and acceleration within a certain angular range, to which high reflectance mirrors have been attached. Many galvanometers are equipped with closed-loop position and temperature control and allow very fast and precise positioning of the mirrors mounted on their motor axes. Typically, galvanometers are found in two-axis setups that deflect the laser beam in two dimensions. Unlike their "flying optics" counterparts, galvanometer-based systems usually keep the focusing optics stationary and instead rely on F-Theta objectives. These objectives are specially designed to focus a laser beam deflected by mirrors onto a flat working plane. The position of the focus within that plane is proportional to the angle of the beam on the objective. As the F-Theta objective is usually not moved, a longer focal length is required to achieve a larger field size. To maintain a small spot size this longer focal length is compensated by a larger entrance beam diameter. As only the mirrors, with very small inertias, need to be positioned by galvanometer scanning systems, they allow the highest dynamics and process speeds, as well as the shortest positioning times of all the beam deflection setups described above. This leads to higher throughputs and much more efficient use of costly investments such as UV lasers. Additionally, the high performance of galvanometer systems allows precise control of laser focus speed and positioning, even with small curvature radii and high positioning velocities. This guaranties the best process stability and quality, even in applications demanding maximum processing speeds. Industrial galvanometer based scanning subsystems with optics ranging from the ultraviolet to the far infrared and with input aperture diameters from 7 mm up to 80 mm are commercially available from companies specializing in this technology. This allows systems integrators in most laser based applications to speed up their processes with galvanometers.

Of course both methods – moving workpieces and moving laser beams – can be combined as "on-the-fly" processes in which the movements of workpieces and laser beams are synchronized. A typical example: high-speed laser processing of parts on a moving conveyor belt or on a robot with a galvanometer based scanning system. To synchronize the movements of the workpiece and the scanning system during laser processing, a controller board in realtime calculates the position of the workpiece using positional information obtained, for example, from an external encoder.

Galvanometer scanning systems can also be easily extended in the third dimension to form a three-axis scanning system (Figure 1) capable of processing curved surfaces. This third axis – placed between the laser and the galvanometer - relies on a set of lenses, one of which can be quickly and precisely positioned along the optical axis via a linear motor. In the figure, the motor is placed symmetrically around the lens to achieve tilt-free movement of the lens. This guaranties the smallest laser spot even when lens acceleration is very high. The moving lens allows the focal length of the overall system to adapt to the surface of the part. To match the dynamics of the third axis to the galvanometers, the lens is usually moved only a short distance. Because this small movement needs to produce a much larger focus shift in the working plane, all lens positioning must be precise and tilt-free if process stability and high product quality are to be ensured. Three-axis scan systems can be used with or without F-Theta objectives, depending on the specific requirements.

For controlling such three-axis scan systems, real-time capable PC interface boards are used. Interface boards synchronize the scanning system, part handling and laser pulses, and also use pre-calculated correction tables to take care of optical distortions. They significantly simplify control of even highly sophisticated scanning systems such as three-axis and "on the fly" installations.

State-of-the-art galvanometer scanning systems can deflect laser beams with powers of up to several kilowatts and diameters of up to 80 mm, achieve working fields up to 1.5 x 1.5 m2 or marking speeds up to 1000 characters per second. Of course, they are also suited for precision tasks within small working areas, e.g. micro-machining or via hole drilling applications in which focus diameters of less than 10 µm and resolutions in the µm-range can be reached. For even higher long term and thermal stability they can be equipped with additional sensor systems to compensate for drift effects.

New improvements such as scan heads with digital driver boards will further enhance the performances of these systems and offer new features like real-time monitoring of all important scan head status parameters, for example actual position and speed of the mirrors.

Acknowledgement: I would like to thank Mr. Dominik Brunner of SCANLAB for his valuable input and for Figure 1.

Author: Ronald D. Schaeffer

CircuiTree 9/2003