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Beam shaping expands laser processing potential

16 Nov 2007

Lens array beam shaping is extending the scope of the laser materials processing market. Vitalij Lissotschenko and Paul Harten of LIMO explain why direct diode applications such as welding and cutting are in line to benefit.

High-power laser sources are used in a large variety of materials processing applications. Today, the most common include welding, soldering, cutting, drilling, laser annealing, micromachining, ablation and microlithography. As well as choosing the right laser source, suitable high-performance optics that generate the appropriate beam profile are critical.

Adequate design software is a priority when you are developing the optics that are best suited to these applications. At LIMO, we have developed our own software that solves Maxwell's equations and takes all of the important physical aspects of the beam-shaping task into account.

Various beam-shaping principles, such as phase shifting for singlemode lasers and beam mixing for multimode lasers, are applied when we design industrial beam-shaping solutions. The most widely used illumination geometries are squares, rectangles and lines, and these profiles – as well as other customized solutions – can be implemented efficiently using microlens arrays with asymmetric lens profiles.

Free-form microlens surfaces can be structured cost-effectively on a wafer using LIMO's unique production technology in tandem with computer-aided design. LIMO can structure theoretically optimized surfaces into any material with high precision, including fused silica, BK7 and calcium fluoride for DUV applications and silicon, germanium and zinc selenide for CO2 lasers.

Micro-optics beam shaping improves the performance of many varieties of laser: gas, solid-state, fibre and diode for example. The beam-shaping optics can be pre-aligned in compact and robust modules with well-defined interfaces and integrated into production systems for use in harsh environmental conditions. Such modules can also be enhanced using accessories ranging from simple telescopes up to complex measurement systems. The complete system is then placed into the collimated input beam and generates the required intensity profile at the target.

Using this approach, we can obtain homogeneous light fields or light lines with dimensions spanning nine orders of magnitude from the micrometre to the kilometre scale. The working distance can also vary from 100 µm up to several kilometres. The aspect ratio of lines (the width compared with the length of the line) can vary between 1:1 and more than 1:5000.

New applications are becoming feasible and the business cases for existing applications are improving thanks to lens array beam shaping. Applying this technology to industrial diode laser systems with excellent beam quality, combined with falling dollar-per-watt prices creates new opportunities, particularly in areas such as direct-diode welding and cutting, and silicon thin-film crystallization.

Direct diode applications
Compact and robust high-power diode lasers are reliable tools for materials processing. Fibre-coupled devices with more than 1 kW of output power, delivered via fibres with diameters ranging from 200 to 600 µm, are readily available for use in applications such as plastics welding, heat conduction welding of stainless steel and soldering in semiconductor, automotive or electronics.

But the potential scope of direct diode laser processing is much larger. New applications such as microwelding of sensor housings or electrical contacts in PCB production; fine cutting of thin metal sheets; and soldering and annealing processes in semiconductor and display production illustrate this point.

Fibre-coupled and line-focus industrial ultrahigh-brightness diode lasers using simple interfaces and application-specific beam shaping and delivery create opportunities by increasing throughput and easing integration into OEM production equipment. In-line process speed and reliability monitoring, as well as pre-production process verification in applications test beds virtually eliminate all risk of component or process failure.

Numerous applications require a uniform illumination of the processed area. Intensity peaks in the profile of the laser light might destroy the workpiece and intensity dips below the threshold of the process may leave the workpiece unfinished. Homogenizer modules based on micro-optic lens arrays generate homogeneous intensity profiles with fluctuations of less than 1% even at multi-kilowatt laser power levels.

Heat removal technology must also be considered when deciding on the appropriate laser system for your application. For diode lasers, there are two principal alternatives: microchannel-based "active" and pure heat conduction-based "passive" heat sink architectures. Passive cooling can withstand fail-safe tap-water-based operation without deionized water. Actively cooled systems may carry a lower initial price tag, but in the long term can cause a higher cost of ownership.

Beam shaping and delivery designs that have a high optical efficiency translate directly into lower operating currents and extend the lifetimes of diodes to 30,000 h and beyond, according to ISO17526:2003(E). A low operating current also reduces the thermal load and allows compact cooling technologies to be used. When you couple all of these factors together, the end result is a reduction in capital expenditures, operating cost savings and more efficient use of limited production floor space.

Direct diode welding and cutting
A good illustration of how improved beam quality via lens array beam shaping translates into economical processing is the conductive welding and cutting of small workpieces. In the past, pulsed lasers have been the sources of choice for materials processing. For example, pulsed laser sources with high repetition rates and nanosecond pulse durations show a good penetration into highly reflective materials.

However, many applications do not require a high peak pulse power. In conductive welding, a high peak pulse power turns out to be a disadvantage as the resulting weld seam can suffer from spilling and pores.

High-brightness, high-power diode lasers in combination with the right accessories can provide weld seams that are difficult to distinguish from the regular finish of the metal surface. A small beam spot size on the workpiece results in a high-power density and a high process speed in either continuous- or quasi-continuous-wave modes of operation. The process is assisted by the good absorption of diode laser light between 808 and 980 nm in most metals. The tact times of the two fuel valve welds (see figure 1) are only 3.7 s.

Figure 2 shows the point welding of two steel springs. The microwelding process requires only three laser shots, each 4 µs in duration. A pulsed constant current supply controls the laser power accurately yielding a stable output power with less than 1% power fluctuations. This means that the stable joining of the steel springs is reproducible over the whole device life of the laser tool.

Choosing the correct process gas and gas flow avoids temper colours. In addition, the tensile strength of the joined parts is comparable with the strength of the material itself. Hydraulic measurements show a tensile strength of more than 28 kN. Both welds, rod-to-ball and ball-to-disc, are done simultaneously by one rotation of the device. After optimizing the process parameters at a pre-production test bed within the LIMO Applications Centre, the process was transferred straight into production.

To date, fusion cutting has been carried out using Nd:YAG and CO2 lasers. Again, today's ultrahigh-brightness diode lasers are level in performance with these commonly used sources and have demonstrated quality cutting of thin metal sheets and foils successfully.

Figure 3 shows a laser fusion cut of a 0.2 mm-thick kovar plate. The process has a small thermal influence zone and a sharp kerf of only 100 µm. The corresponding cutting head is shown in figure 4.

Industrial diode laser system accessories include sensors to increase the process stability and reliability. For processes with high-power stability, it is necessary to have online power and temperature control that is integrated into the processing head (see figure 5). A sample rate of 1 kHz allows the laser power to be adjusted every millisecond. A vision system with an integrated camera is deployed in the processing heads to position the laser beam on the workpiece.

Crystallization of thin films
Industrial crystallization of silicon thin films has traditionally been dominated by excimer laser-based processes. The arrival of ultrahomogeneous line beams of up to several metres in length combined with the design freedom provided by lens array beam shaping creates new opportunities in this application. Falling diode prices continue to improve the business case for very large systems with several tens of kilowatts of power.

Researchers at IPhT in Jena and CIS in Erfurt, both Germany, have used a maintenance-free 350 W diode line laser to treat thin-film solar cells. Micro-optic lens arrays are integrated into the laser head to shape the processing line making it scalable to several metres. Crystallization of 200 and 500 nm a-Si films on a SixNy diffusion barrier on glass has been demonstrated. Large crystallites of more than 100 µm size are achieved at a scanning speed of 33 mm/s. The result is shown in figure 6.

• This article originally appeared in the November 2007 issue of Optics & Laser Europe magazine.

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