Development of a low-debris laser driven soft x-ray source for lithographic applications
Publisher: Imperial College London
Published: 2016
Place of publication: London, U.K.
Description: This work comprehensively describes the design, build and characterisation of a low-debris laser driven soft x-ray source for a variety of applications in particular lithography, in combination with the optimized multilayer structures in order to use the source output as efficiently as possible. 
 The aim of this work was to study the debris emission from different target materials and to minimise or eliminate debris from laser irradiated thin tapes used in multi-shot and long run-time applications. VHS video tape is used as the primary test target in this work and is made of a Mylar (C10H8O4) carrier film coated with a fine magnetic powder of both Fe2O3 and chromium dioxide CrO2 together with a backing layer of carbon black for static control and a binding agent (polyester-polyurethane). VHS video tape is inexpensive, readily available and amenable for use in multi-hour experiments at high repetition rate.
 The x-ray source described here is built around a 1064 nm Nd:YAG laser, frequency doubled to 532 nm (green) or tripled to 355 nm, with a pulse length of ~800 ps and a repetition rate up to 50 Hz. A versatile cubic target chamber was designed to accommodate the source and a set of computer controlled stage motors are used to allow positioning of the x-ray emission point. A glass plate between the focusing lens and the target prevents the lens from being coated with debris and the use of a low pressure N2 buffer gas (2-6 mbar) was explored as collisions of atomic size debris particles with gas molecules reduces their kinetic energy and consequently their adhesion to the surrounding surfaces. The chamber can also be continually pumped close to the laser-tape interaction point to ensure continuous removal of debris particles. In the VHS video tape target, the source emitted a range of x-ray wavelengths between 0.19 and 2.3nm relevant to kα line emission of both Fe (7.1 keV) and O (0.5 keV) resolved using the mica crystal spectroscopy. In copper target, a 0.13nm wavelength of k edge (8.9 keV) was resolved, and a 0.79nm x-ray light of Al k edge (1.5 keV) was obtained in aluminium tape target. The measured flux of x-ray photons under vacuum was (7.3 × 109 Photons/s) at 100mJ laser energy and the calculated efficiency of the system (laser energy in versus x-rays out) was (1.1 × 10-6). 
 C/Cr multilayer mirrors of variable layer numbers (N= 30- 200) were designed using the IMD software. A magnetron sputtering technique was used to fabricate the multilayers which were then probed using a hard x-ray diffraction method (Cu Kα radiation, λ = 0.154 nm) to characterize their reflectivity, bi-layers structure and surface roughness. Atomic force microscope was used to determine the surface topography and to analyze the surface structure imperfections such as roughness and stress induced damage.
 The design developed and refined over the course of this work has been shown to be better at reducing target debris than other mitigation strategies described in the literature. A reduction of "large" and potentially damaging particles ranging from 140 to 5 microns in size by a factor of 28 was achieved, and a 10% reduction in the flux of small particles (~5 microns) was observed if only a buffer gas was used. A reduction of 50% in the flux of these smaller particles was seen if both buffer gas and pumping strategies were employed. This should provide longer life time for the delicate optics used in close proximity to a long run time laser plasma x-ray source, and at the same time reduces the cost of running such sources for a range of different applications.