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Drilling down

Precision laser drilling with ultrashort pulse lasers opens up a world of possibilities

The growing demand for inkjet printer systems with high throughput as well as for systems resistant to acidic printing inks and abrasive substances, such as nanoparticles, is driving the need for new inkjet designs composed of highly resistant materials. Contemporary production techniques such as galvanic processes, etching and laser systems with long pulse widths are reaching their limits to fabricate these new inkjet printing jets due to increasing numbers of nozzles per printing head, reduced processing times, new nozzle geometries and associated flow rate throughputs.

This article reviews a sophisticated laser ablation technique that digitally employs ultrashort pulse lasers in the picosecond and femtosecond range (1013 seconds to 1015 seconds) to enable drilling of micro-apertures with well-defined geometries to meet the critical requirements for inkjet production. Compared with conventional production processes, a digital laser procedure enables the highest possible flexibility when the production of freeform jet geometries is required with a variety of jet shapes or flank angles.

Introduction

Drilling of micro-apertures with well-defined geometries is gaining significance in a variety of industries. Laser drilling, in turn, is replacing conventional drilling processes to support many applications, ranging from the setting of micro-drillings in throughput flow filters and sieves to drillings in high-performance solar cells and injection jets in the automotive industry.

Laser techniques are gaining ground in the production of printer inkjets due to the laser’s touchless processing, precise dosing of energy input, minimal heat transmission into the material, precision and repeatability. Laser techniques also provide additional flexibility in defining the drilling geometries. For example, it is possible to generate micro-holes with a high aspect ratio (the relation between the drilling depth and the drilling diameter) or micro-holes with defined taper by varying the machining strategy during the laser process.

The fundamentals

Depending on the application, a variety of lasers can be employed for micro-drilling. While excimer lasers and solid-state lasers working in the UV range are highly suitable for processing polymer substrates, solid-state lasers in the visual or infrared range are used for machining metals. However, selecting the right laser alone is not enough to ensure a successful outcome. Choosing the right drilling technique also plays a decisive role. Well-known among these techniques are percussion drilling and trepanning.

In percussion drilling, the hole is produced by using multiple short-duration laser pulses until the required depth of the hole has been obtained (see Figure 1). The beam guidance in this process is static. Depending on the point of focus, percussion drilling can produce a fixed diameter or variable geometries. It is an extremely rapid drilling method where several hundred or thousand drillings can be obtained per second. However, the process has its limits when high-quality drillings are required.

Trepanning also uses multiple laser pulses to produce the hole. After the initial pilot hole is created, the laser enlarges the pilot hole, moving over the workpiece in a series of increasingly larger circles. The advantages of the trepanning process are the production of micro-holes up to a few millimeters in diameter and greater repeatability as well as the ability to produce special-shaped drillings in addition to round drillings. At the same time, the taper of the micro-hole can be reduced to create a more uniform “straight” hole, which is essential for many applications.

Laser drilling of inkjets

In the production of industrial inkjet printing systems, a variety of jet types are employed. All jet-type working materials are required to be resistant to acidic printing inks or abrasive substances, such as nanoparticles. For this reason, highly resistant working materials such as stainless steel, titanium and glass are preferred. Typical material thickness is around 50 microns. Depending on the requirements of the inkjet printing system manufacturer, a variety of jet geometries, jet forms and flank angles may be needed (see Figure 2).

Typical jet geometries for printer inkjets have input diameters between 50 microns and 100 microns and output diameters between 20 microns and 40 microns. Apart from the production of a suitable jet geometry, the highest priorities in laser drilling are maintaining high surface quality, reproducibility and precision. These are all critical to achieving correct flow behavior of the print mediums and precise distribution of the inks on the substrates because the human eye is capable of recognizing an incorrectly impinged drop of ink produced by a single defective jet.

Due to the high number of jets per printing head, the processing time and associated flow-rate throughput are critical parameters. To address these needs and meet the requirements for industrial manufacturing, 3D-Micromac developed a sophisticated laser drilling process utilizing ultrashort pulse lasers. Based on extensive experience in the manufacturing of OEM laser systems for the drilling of special inkjet printing jets, the modular laser system (see Figure 3)combines high-rate throughput with extremely low production costs. Depending on the customer’s requirements, the system can be equipped with a variety of ultrashort pulse laser sources. The jet and workpiece positioning can achieve accuracies in the nano range.

Laser ablation with pulse durations in the picosecond and femtosecond range (10-13 seconds to 10-15 seconds) is often described as “cold ablation.” However, this only applies to pulse durations less than 10 femtoseconds.

If the laser pulse duration lies above this time interval, an electron-photon interaction and the associated temperature conduction occur in the substrate. Ideally, this restricts itself to less than 100 nm of material width at pulse durations into the picosecond range.

The advantage of the ultrashort pulse laser lies in its ability to impart all of the laser energy to the material within a short time interval. Extremely high power densities up to a few gigawatts per cm² are thereby achieved.

This leads to very good absorption of the laser radiation and the potential for quasi “athermal” and extremely precise processing. The result is qualitatively high-value structures with practically no heat influence or material contamination on the surrounding material.

Real results

To determine whether the new ultrashort pulse laser ablation technique meets the quality and precision requirements for drilling inkjet printing jets, the process was evaluated in the picosecond and femtosecond range. Laser beam sources were employed with wavelengths in the infrared and visual spectrums depending on the jet working material.

The drilling geometries were produced by a galvoscanner. The movement of the complete workpiece was done with a direct-driven XY positioning system (positioning accuracy ±0.002 mm; repeatability ±0.001 mm). An oxygen-free process gas was used to remove the ablated material from the micro-hole as well as prevent oxidation of the metal, further improving the quality of the ablation process.

Drilling results on all working materials were visually flawless and displayed highly uniform jet geometries. Thermal influenced zones, eruptions or working material melts were also practically unrecognizable (see Figure 4). Resulting wall roughness was approximately less than 0.05 microns Ra, which is well within acceptable levels. Further cross-section analysis using a laser scanning microscope confirmed the homogeneity of the jet geometry (Figure 5).

Laser machining with ultrashort pulse lasers is well suited for the next generation of inkjet printing jets in metal materials. Adopting this process enables the production of complex drilling geometries with excellent edge quality and wall roughness.

In addition, the laser drilling technique also opens up new possibilities beyond industrial applications. For example, it can be used to generate freeform channels and cavities for microfluidics applications as well as produce openings for components and interlayer-connections in the display industry, among others.

3D-Micromac AG