Laser Micro Strukturing

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 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.

The workstation is equipped with a fast endpoint-detection system (LTB Lasertechnik Berlin GmbH) that allows to automatically interrupt laser processing between two consecutive laser pulses up to 20 kHz pulse repetition frequency. The detector analyzes the optical emission of the laser-induced micro plasma and stops the laser drilling when the desired interface is reached, i.e. when the emission line of the target element appears in the spectrum (Abstract 2). It is possible to automatically stop the laser drilling process of silicon carbide as soon as the interface of the gallium nitride layer is reached. The plasma detector system allows automatic interruption of the drilling process in between two consecutive laser pulses at laser pulse repetition frequencies of up to 20 kHz.

A second laser extends the basic 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. The two additional wavelengths in the UV range combined with the mask projection technique significantly extend the variability and potential applicability of the laser workstation. The mask stage holds 5" masks (typically chromium on fused silica) and the optical system is best suited for direct processing of polymers.