Laser Micro Structuring

Conventional photo lithographic micro processing of very hard and chemically inert materials is very difficult and requires considerable effort. For such materials direct micro processing using high-power laser radiation can offer an interesting and economic alternative. At the FBH the technique of laser ablation is studied with respect to their potential implementation into the existing process technology. The laser micro processing of silicon carbide and sapphire substrates is particularly interesting for the fabrication of electronic devices on the basis of gallium nitride. The research is focused on laser drilling of micro holes (via holes) in silicon carbide and laser scribing of sapphire, gallium nitride and silicon for subsequent dicing of wafers.

The research is carried out on a commercial laser micro machining system. The laser workstation ILS 500S-Air (InnoLas GmbH) is a unique system that is particularly constructed for materials micro processing in electronics and semiconductor process technology. The Class 1 laser-safe micro machining tool is equipped with a high-power Q-switched solid-state UV laser (Coherent AVIA 355-4500). The diode-pumped 4.5 W laser delivers nanosecond pulses at a wavelength of 355 nm with pulse repetition frequencies of up to 100 kHz.

The laser beam is positioned by a combination of the motion of a CNC controlled XY stage (200x200 mm2) and a galvo scanner (field size 10x10 mm²). Using a telecentric F-theta objective (f=56 mm) the laser beam is focused to a spot size of 10-20 µm in diameter. Powerful image processing and vision system combined with a highly precise air bearing XY stage provide high resolution automated workpiece alignment capability and a beam positioning accuracy of ± 1 µm with respect to an existing pattern on a wafer (Poster, Abstract 1). This accuracy is reached also in cases when the wafer's front side having the alignment marks is flipped down for laser processing of the wafer's back side. Two CCD cameras are placed underneath an open frame wafer chuck (figure) for automated front-to-back side alignment.

Another laser belongs to the set-up of the laser workstation. This excimer laser (GSI Lumonics, IPEX-800) delivers pulses at wavelengths of 193 nm (ArF) and 248 nm (KrF) depending on the excimer gas. The pulse repetition frequency of the nanosecond pulses is 200 Hz at maximum. With the excimer laser the mask projection technique can be used. The mask stage holds 5" masks (typically chromium on fused silica) and the optical system is best suited for direct processing of polymers.

Publication: "UV Laser Processing for Semiconductor Devices", Laser Technik Journal, vol. 5, pp. 26-30 (2013)