Micro-ablation with extreme ulta-violet laser light

Microelectromechanical systems (MEMS) comprising components of 1 – 100 µm size are constructed in industry using a range of technologies including direct laser ablation where ultra-violet to infra-red lasers ablate features limited to sizes > 1 µm. We are engaging in a study currently at Technology Readiness Level (TRL) 1-2 with the aim of extending laser ablation in manufacture to the extreme ultra-violet where sub-micron size features can be produced. Such short wavelength radiation interacts with material via fundamentally different physics, which may, in addition, enable significantly faster ablation of material.

When a visible or infra-red high power laser pulse is focused onto a solid target, the first photons in the leading edge of the pulse produce free electrons by multi-photon processes, with the bulk of the laser energy interacting with plasma expanding normally to the target surface. Consequently, laser light only penetrates through the expanding plasma plume up to a critical density, which is typically a factor 100 – 1000 less than the solid density. Dropping the laser wavelength into the extreme ultra-violet (EUV) (to <60 nm) causes the critical density to be greater than the electron density produced at solid density. The laser light can now interact through a solid target over an attenuation length defined by the level of photo-ionisation. It is possible to focus to a focal width of f X wavelength, where f is the f-number of the focussing optic which means that due to the short wavelength of the EUV laser, deep (multi-micron) features of just 100 nm in width can in principle be cut into the material.