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Micro/Nano Lithography

Nanosecond lasers micromachine glass

High-quality microfabrication, selective metallization, and color marking can be performed on transparent materials using laser-induced plasma-assisted ablation.
10 September 2006, SPIE Newsroom. DOI: 10.1117/2.1200608.0334

The need for precision micromachining of transparent materials is increasing in various industries. To fulfill the demand for a simple and cost-effective method for glass micromachining, we developed the laser-induced plasma-assisted ablation (LIPAA) technique. This can perform microfabrication on transparent materials.1,2 In this method, the glass substrate must be transparent at the laser wavelength, so that the laser beam first goes through the substrate, then irradiates a metal target (see Figure 1). At a laser fluence above the ablation threshold for the target and below the damage threshold for the substrate, the plasma generated from the target quickly propagates to the rear surface of the substrate. If the target and substrate are separated by several hundred microns and if the laser's pulsewidth is several tens of nanoseconds, then strong interactions occur between the laser beam, the plasma, and the substrate.3 This results in high-quality ablation, with neither cracks nor severe distortions on the rear side of the substrate. Additionally, if we adjust the irradiation conditions, then we can deposit a thin metal film on the ablated surface. Therefore, we can use this ablation method and successive chemical plating to metallize selected parts of the transparent material.

Figure 1. This schematic diagram of the laser-induced plasma-assisted ablation technique shows how the plasma generated from the target, coupled with the laser light, micromachines the back side of a transparent material.

We have developed a prototype LIPAA system for practical industrial use. The system consists of a 532nm pulsed laser and two optical components: an objective lens for high-resolution microfabrication and galvano scanner optics for high-speed patterning of complicated microstructures. Figure 2 shows examples of a glass slide ablated using these components. After ablation, a cleaning process using an acidic solution—such as HCl—can remove metallic residue. Our method can be applied not only to surface microstructuring of glass materials, but can also dice glass substrates.1Figure 2(c) shows glass chips cut by LIPAA. Any shape can be cut from a glass substrate using multiple scans of the laser beam.

Figure 2. Microscopic images of glass samples prepared using LIPAA show how the method can be used on glass to (a) form grids and lines and (b) deposit color marks (such as the brown words). (c) The method can also cut small pieces of glass.

After LIPAA, metal from the target deposits on the ablated surface as a thin film. Using this unique feature, we attempted the selective metallization of ablated areas in LIPAA-processed transparent substrates. Figure 3(a) shows one result: a glass sample treated by electroless copper plating after ablation. Both the LIPAA method and metallization can be applied to soft materials, too, as shown in Figure 3(b) and 3(c).2 For polyimide substrates, we modified the system in two ways: we used a 1064nm pulsed laser as a light source, because polyimide has an excellent transparency at that wavelength; and, instead of a solid metal target, we used a thin film of gold (500nm Au and 50nm Cr) on glass. The thin film offers a lower ablation threshold than the polyimide. By using this technique, we also demonstrated selective metallization of polyethylene terephthalate (PET) film. The PET films with grid patterns of narrow metal lines less than 20μm wide are useful as transparent electromagnetic shield films for plasma displays. A technique for metallizing film areas bigger than 900 × 500mm2 is now under development for industrial use.

Figure 3. Selective metallization using LIPAA processed (a) glass and (b) polyimide. (c) An enlargement of (b) shows fine detail on the polyimide substrate.

Another application of the LIPAA is color marking. By changing the target material, we can deposit materials with different colors on the substrate. Figure 4 shows line and spacing color marking on a glass substrate using the different targets.

Figure 4. By using targets of different materials, LIPAA can mark substrates with a variety of colors.

In summary, we have demonstrated high-quality and high-efficiency microfabrication of both hard and soft transparent materials such as glass, polyimide, and PET films using our LIPAA method. A single nanosecond-long pulse leads to effective ablation of the transparent materials by coupling plasma generated from a metal target by the same laser pulse. We used LIPAA to microstructure, cut, color mark, and selectively metallize several kinds of transparent materials. This method shows great potential for cost-effective microfabrication of transparent materials in various industries.

Yasutaka Hanada, Koji Sugioka, Katsumi Midorikawa
Laser Technology Laboratory
RIKEN—The Institute of Physical and Chemical Research
Wako-shi, Saitama
Dr. Yasutaka Hanada is a special postdoctoral researcher working at RIKEN (The Institute of Physical and Chemical Research). He recently received his PhD from the Tokyo University of Science and is currently working on laser microfabrication of transparent materials.