What is a top hat Beam?
A Top hat Laser beam is a beam who’s intensity profile is flat and uniform, with sharp edges where the energy drops rapidly to zero. The top hat beam shape can be a square, rectangle, line, circle or any other shape.
There are generally two types of top hat laser beams, collimated top hats and waist (Focus) top hats. Collimated top hats are beams that propagate some distance as a flat- top beam distribution, but if focused or propagated to the far field, will not generate a Top hat spot (but rather a Bessel-like central lobe). These beams are usually the output of either a top hat laser or a special optical module such as Holo/Or’s collimated top hat module
These collimated top hat beams cannot be used at the focus of a lens, requiring imaging at high ratios, thus are usually used as free beams (unfocused) for applications such as wafer inspection or sensing.
The more interesting type of top hat beams, at least for industrial high power laser applications, are waist Top hats. These are beams that are generated by a diffractive top hat beam shaper and are then focused to a Flat top spot that appears only around the beam waist, the focal plane of the lens.
This focusing behavior makes focused top hat beams ideally suitable for applications in laser microelectronics production processes, where the work is often done at the focal plane of a scanning system.
Advantages of top hat beams used in microelectronics laser processing
Most microelectronics laser processes are threshold processes, including micro machining such as the removal of metal from selected areas by laser ablation, drilling of via holes in printed circuit boards (PCB) or glass, contacting by laser induced forward transfer and other laser processes. Threshold processes are characterized by a certain threshold laser energy density that must be reached for a certain area to be processed. For such processes, a flat top energy distribution has advantages that make it highly desirable.
Unlike the Gaussian beam profile of most laser beams, where the width of the Gaussian spot (above the process threshold) is directly proportional to the energy, a top hat energy profile is characterized by having almost no dependence between the pulse energy and spot size above the Threshold.
For microelectronics, this means that the processed area has sharp edges, with a well-defined size that is almost independent of the laser energy. This adds flexibility to a laser machine, enabling it to handle for example different material thicknesses while still maintaining the correct feature dimensions. Thus, microelectronics laser processing is a leading top hat application, in addition to similar industrial applications in solar panel manufacturing and semi-conductors.
Integration of diffractive top hat beam shapers into laser microelectronics processing
Diffractive top hat beam shapers are often easy to integrate into existing laser machines for microelectronics processing. Such machines often employ a combination of a galvo-scanner with F-theta scan lenses to steer the spot on the work surface. This spot can be shaped by adding a suitable diffractive flat top beam shaper before the scanner. This is a single, flat, windows like component, making integration straightforward.
The main requirements for successful use of the top hat laser shaper are:
- Single mode input beam, M2<1.3
- Beam diameter +-5% of designed beam shaper input diameter
- Clear aperture of scanner, lens and all other optics at least X2 times the input beam size.
To further aid integration, Holo/or has written an installation guide and application notes.
