Use of excimer lasers for materials processing

by Dave Taylor

Excimer lasers are characterised by short wavelengths, high intensities and short pulse durations. These characteristics mean that a single photon is capable of breaking a chemical bond. The majority of laser materials processing techniques are essentially thermal processes in which absorption of a large number of photons heats the material to enable cutting, welding or surface modification operations to be performed.

Excimer lasers were first demonstrated in 1975, some time after many of the other laser sources, and are now fairly well established in their niche applications. The term 'excimer' stands for 'excited dimer', where 'dimer' refers to a diatomic molecule such as O ₂ or N ₂ . This is not strictly a correct term, as the two atoms that make up the molecules used in excimer lasers can be different. The most important molecules are rare gas halides such as F ₂ , ArF, KrCl, KrF, XeCl and XeF. These do not exist in nature but can be produced by passing an electrical discharge through a suitable gas mixture. This means that excimer lasers generate ultraviolet energy over a range of wavelengths, depending on the gas mixture used (e.g. 157nm for F ₂ and 351nm for XeF).

Typical average output powers are in the range from less than 1 watt up to around 200 watts. This is two orders of magnitude less than the more traditional Nd:YAG or CO ₂ lasers which operate in the infrared part of the spectrum. The high intensity beam of an excimer laser is the product of pulse energy (10 - 1000mJ), spot size (governed by focusing optics) and pulse duration (around 10ns).

Applications of excimer laser are primarily in machining of materials such as plastics, paper, ceramics, glasses, crystals, composites and biological tissue. When illuminated with an excimer laser, the relatively weak organic bonds are broken down. This creates a pressure rise and subsequent shock wave that removes material, with little heat transfer to the surrounding material, in a process called 'ablation'. This processing is usually most efficient when carried out using a mask with an image of the required feature. Excimer laser machining is used for its precision, producing features down to approximately 40µm in resolution, but with virtually no heat affected zone.

Excimer laser processing using step-and-repeat mask projection

Some research has been carried out in welding and cutting of sheet metals, but showed no significant advantages over CO ₂ or Nd:YAG laser which are available in much higher average powers. Excimer lasers do also lend themselves to more 'niche' applications, such as surface modification of metals and glass for strengthening adhesive bonding and smoothing of machined surfaces to increase wear resistance of components such as camshafts and pistons. As well as increasing wear resistance, this process is being studied as a means of increasing corrosion resistance through production of a thin, amorphous layer on the surface of the material.