Laser processing
by using diffractive optical laser beam shaping

Application note No. AN1012IL01

Gediminas Račiukaitis1, Evaldas Stankevičius1, Paulius Gečys1, Mindaugas Gedvilas1, Christian Bischoff2, Erwin Jaeger2, Udo Umhofer2 and Friedemann Volklein3.
1 Center for Physical Sciences and Technology, Savanoriu Ave. 231, LT-02300 Vilnius, Lithuania
2 TOPAG Lasertechnik GmbH, Nieder-Ramstaedter-Str. 247, D-64285 Darmstadt, Germany
3 Institut für Mikrotechnologien, Hochschule RheinMain, Am Brückenweg 26, D-65428 Rüsselsheim
Keywords: diffractive optics, beam shaper, laser processing, flat-top

Abstract
Laser beam shaping and homogenization techniques are substantial to optimize a large number of laser-material processing applications and laser-material interaction studies. Diffractive optical elements (DOE) play important role in provision of the process-adapted laser beam shaping. In this paper, we present an approach for laser beam shaping by using innovative DOE. The circular Gaussian beam was transformed to a square flat-top intensity profile in the focal plane of the objective lens by using novel DOE from TOPAG Lasertechnik GmbH. About 95% of the laser energy was coupled in the main beam with a nearly perfect rectangular shape. Experimental results have been achieved by applying the shaped beam for laser microfabrication. The picosecond and nanosecond lasers with moderate beam quality (M2=1.5) were used in the experiment. The shaped laser beam was applied for direct laser ablation of metal film on the glass substrate, drilling in a silicon wafer, scribing of thin-film solar cells and other technical materials. This technique allows creating a rectangular-shaped flat-top intensity profile in the focal plane that shows distinct advantages in laser material processing.
1. Introduction
A Gaussian beam is a well-known and most convenient spatial laser beam distribution. It preserves its distribution when propagating and is excellent in focusing to a diffraction-limited spot. However, the spot area limited by a beam diameter (1/e2 level) contains only 86.5% of the laser beam power and intensity at the boundary is only 13.5% of the peak intensity. The energy in wings is lost or even causes several damage of surrounding material as well as a high-intensity central peak. Because of threshold behavior of laser ablation the use of Gaussian beam profile is not favorable and most of laser material processing applications require uniform irradiation within a limited area with sharp edges on boundaries. This is especially true for thin–film structuring in electronics and photovoltaics, where high selectivity of the processing is top-most important. A uniform irradiance is further of interest for laser welding, laser micro-fabrication, laser radar, laser scanning and optical processing.
The laser beam with uniform intensity distribution over the cross-section of the working plane is called a flat-top beam. The problem of shaping a laser beam arose nearly together with invention of a laser. Frieden published the first paper on shaping of the laser beam to one with uniform irradiance in 1965. Beam shaping can be implemented by integration (averaging) and deterministic transformation. There exist several laser beam shaping techniques:

  • Use of apertures when only the central flat portion of the beam is selected resulting in high energy losses;
  • Multi-aperture beam integrators (homogenizers, micro-lens arrays, etc) ;
  • Field mapping transformation with refractive optics to get a uniformly distributed beam in the output plane, applicable to well-defined single mode laser beams. Lossless transformations of the Gaussian beam to uniform irradiance use refractive, reflective or diffractive optics.

Refractive and reflective optical elements of a special shape are arranged usually in beam expander geometry. More intensive central part of the beam is spread over a larger area, while weak wings of the lateral beam distribution are concentrated closer to the center. Considerations about description of flat-top profiles are given in.
Transforming the beam irradiance profile (Gaussian to more uniform) by retaining the wave-front shape requires 2 beam shaping elements. Examples of laser beam shaping optics are:

  • refractive optical systems with at least two aspheric elements;
  • single bi-aspheric element;
  • reflective optical systems;
  • binary diffractive optics;
  • 1D beam shaping with adjustable phase slit .

A laser beam shaper consisting of two aspheric lenses provides a fixed geometrical radial remapping of the optical power entering the input aperture to the output aperture. It must be mounted and aligned carefully in order to use it most efficiently. The shaper requires that the angular misalignment be less than about 140 µrad with respect to the input beam, and translationally centered on the input beam to less than 25 µm. If the input laser beam has a poor profile, this will be echoed in the output beam profile. Only beams with a high quality (M2 < 1.10) can give acceptable output profile flatness.
Flat-top intensity distribution can be realized through transformation of a single transverse mode Gaussian laser beam using an optical beam shaper. A low cost technique to fabricate the beam shaper that can convert a circular Gaussian beam into a circular flat-top beam was developed by. The input beam size and laser wavelength should match its design specifications within 10% tolerances, and fine alignment between the input beam and the shaper within the 10 µm tolerance is required. Flat-top size is altered by changing focal length of the objective. In general, an accurate continuous surface profile is hard to achieve by etching process, leading to performance degradation of the fabricated flat-top beam shapers. Complicated beam shaping systems can afford very high shaping efficiency (95% of energy in area of 90% intensity), however they require an ideal Gaussian beam and precise alignment. Sensitivity to misalignment and manufacturing inaccuracy can diminish and hide marginal improvement in performance from simple to advanced diffractive optics elements. Using simplified binary phase steps can greatly increase the beam shaper fabrication tolerance to low-cost flat-top beam shaper prototyping and production.
Processing, e.g. scribing thin films, with lasers with a circular Gaussian beam causes formation of non-smooth edges and damage or excess modification in overlapping areas. A flat-top square-shaped beam is proposed to be ideal for laser processing. Use of a square-shaped laser beam prevented cracking of the thin films in a-Si solar cells. A picosecond laser system with a special beam shaping optics was utilized for optimization of structuring processes of CIGS thin film solar cells. Optimized top-hat optics with high straightness and steepness of the edges was developed using free-form refractive optics. Processing results are demonstrated for various laser-based processes in thin film as well as crystalline Si solar cell production. The “free-shape” optics is difficult to fabricate and that limits cost–effective application of the technology.
New diffractive optics technique called Fundamental Beam-mode Shaping (FBS) was applied to transformation of a circular Gaussian beam at the input plane of FBS into rectangular beam with nearly flat-top (top-hat) distribution at the focal plane of the focusing objective. FBS represents kind of a phase plate with a continuous and smooth phase relief profile to transform a beam to one of sin(x)/x-type in front of a focusing lens and which was realized by lithographic methods.

The FBS-shaped beam was applied in laser microfabrication experiments using picosecond and nanosecond lasers with moderate beam quality (M2=1.5). The shaped laser beam was applied to direct laser ablation of metal film on the glass substrate, drilling in a silicon wafer, scribing of thin-film solar cells and other technical materials.

EKSPLA note: due to the continuous product improvements, laser models were replaced respectively: NL640 and NL15100 were replaced by Baltic and BalticHP series, PL10100 was replaced by Atlantic series.