Lasers etch sub-micron structures into sapphire

21 Mar 2007

Micro-optic and microfluidic applications could benefit from a laser-based patterning technique being developed in Germany and Greece.

Sub-micron period structures can be written into sapphire using a technique known as laser induced backside wet etching (LIBWE), say researchers in Greece and Germany. The approach could be useful micro-optic and microfluidic applications as well as the development of Bragg reflectors, high efficiency diffractive elements and computer generated holograms (Optics Express 15 1428)

Sapphire is a very hard and chemically stable material which makes it hard to process. Although a variety of grinding and polishing approaches as well as direct laser patterning have been tried in the past, each has inherent disadvantages. For example, chemical wet etching involves the use of highly aggressive and toxic chemicals such as hydrofluoric acid.

LIBWE is a laser-based patterning technique, which according to Klaus Zimmer from Germany's Leibniz-Institute for Surface Modification offers a number of benefits over existing methods.

"The main advantages of LIBWE are direct, single step patterning; no masking is required; local processing; simple alteration of patterns; rapid prototyping and no vacuum processing," Zimmer told optics.org. "Compared with other laser patterning techniques LIBWE has less or no deposition of debris; less roughness during etching and higher vertical precision."

How it works

Zimmer's set-up creates a periodic pattern on the back surface of a 0.5 mm thick sapphire wafer. A fused silica phase mask measuring 25 x 10 mm is placed close to the front surface of the sapphire while the back surface forms the fourth wall in a chlorobenzene cell.

"The chlorobenzene absorbs most of the energy of the laser pulse near the solid surface within a layer of about a few microns," explained Zimmer. "The heat transfer from the hot liquid enhances the temperature of the sapphire. When the surface temperature exceeds a critical temperature, the sapphire melts or softens. Due the simultaneously induced transient large pressure of the liquid, the molten/soften layer is expelled."

The team uses the fourth harmonic (266 nm) of an Nd:YAG laser emitting 150 ps pulses. The phase masks splits the laser beam creating an interference pattern of high quality fringes on the back surface of the sapphire sample. The researchers say that the best results in terms of grating depth were obtained for an energy density of 260 mJ/cm².

"You can etch an area 2 mm in diameter without moving the laser spot and this takes between 1 and 50 seconds depending on the number of pulses and the repetition rate," said Zimmer. "The maximum grating depth was, on average, 80 nm although structures as deep as 100 nm were obtained. Our method has great potential for inscribing ultra-short period (around 200 nm) Bragg reflectors in sapphire wafers."

Zimmer and colleagues are now tuning the processing conditions to achieve the highest yield and optimum surface structuring. "We are structuring sapphire to develop compact Ti:Sapphire distributed feedback waveguide and photonic crystal lasers," he commented. "Closer collaboration with potential users will also help us to develop the LIBWE technique to meet the specific requirements of industry."

This research was carried out by Klaus Zimmer and Rico Böhme from the Leibniz-Institute for Surface Modification, Germany and Stavros Pissadakis from the Institute of Electronic Structure and Laser, Greece.