A planar refractive X-ray lens made of nano-crystalline diamond

Highly brilliant synchrotron X-ray beams and steady developments in instrumentation are enabling new X-ray experimental techniques. Smaller and smaller beams are available, with increased flux and coherence, for experiments in chemistry, biology, materials science and physics. X-ray analysis of soft and solid matter at the nano-scale is now possible and will be more widespread as the optics and techniques become more reliable. Quality and efficiency of focusing optics play a central role in the design of many synchrotron beamlines. These optics must possess extremely well defined shapes with minimized Figure errors, in order to prevent aberrations, degradation of the beam uniformity and flux losses; small in-line optics such as lenses and zone plates have sub-mm dimensions and sub-µm size features. Zone plates with external zone widths of less than 15 nm have been fabricated,¹ as well as refractive kinoform lenses with vertical walls of 2-4 µm only.^2,3 Refractive lenses and zone plates are now capable of focusing synchrotron light down to 10-50 nm spot sizes^1,4,5. Making these optics out of diamond would be a huge asset both for 3^rd and 4^th generation synchrotrons, as diamond is one of the materials with highest thermal conductivity, and lowest X-ray absorption. Nano-focusing refractive lenses are made of single crystal silicon at the moment,⁴ and for various reasons, including high absorption, they have small effective apertures, of order 30-50 µm. A nano-crystalline diamond lens would provide unprecedented flux for nano-focusing applications over wider apertures compared to silicon lenses. Collaboration between Diamond Light Source, the Micro- and Nano-Technology Centre of the STFC, and the Chemistry Department of the University of Bristol has permitted development of the first nano-diamond micro-focusing lenses, described in this paper.

 

 

Efficiency and aperture of X-ray lenses are limited by absorption in the refractive material. Unlike visible optics lenses, the X-ray lens effective aperture, defined as the aperture that transmits 75% of the full flux from the lens, can be much smaller than the geometrical aperture. The flux transmitted by the lens and the diffraction limited beam size, i.e. the smallest achievable spot, are both determined by the effective aperture. Our experimental data show that the X-ray flux from the lenses we have fabricated increases linearly with the illuminated lens aperture up to an aperture of 200 µm. 

 

Diamond lenses were fabricated in the past decade using plasma etch,^6,7 however lens quality and depth were unsatisfactory due to the fact that etching diamond in a controlled way is incredibly difficult. X-ray in-line focusing optics have to be manufactured with high aspect ratios, therefore other methods like laser ablation and ion beam milling are not good alternatives to plasma etching. 

 

Our new approach to manufacture diamond lenses is described here. The silicon moulds are fabricated using electron beam lithography and plasma dry etch. Diamond deposition is carried out using a microwave plasma CVD reactor at the School of Chemistry of the University of Bristol.^8,9 Prior to deposition, the samples are seeded using electrospray deposition of diamond nano-particles. Uniform layers of nano-crystalline diamond are then grown on the top of the silicon structures and the silicon is finally removed by wet etching. The CVD reactor parameters are adjusted in order to provide uniform diamond deposition without large crystalline domains or poly-crystalline morphology as well as increasing the growth rate. The fabrication process we developed could be adapted to the fabrication of zone plates as well.

 

 

The scanning electron microscope image in Figure 1 clearly shows that this process permits to achieve good material quality and high aspect ratio of the structures. Each array is a planar compound refractive lens (CRL), designed to provide a focal length f = 0.4 m for energy values in the range E = 5 – 20 keV. The CRL can be used to obtain different focal lengths according to the thin lens formula ƒ = R/Nd where N is the number of refractive surfaces, d is the refractive-index real-part decrement and R the radius at the apex of the aspherical surfaces. 

 

 

The B16 Test beamline at Diamond Light Source is a versatile instrument used for a range of experiments in material science, technique and instrumentation development.¹⁰ In this experiment the beamline optical layout includes a double-crystal silicon monochromator and slits to vary the size of the incident beam. The diagnostics system comprises high-spatial-resolution CCD detector, and a photodiode coupled to a piezo system carrying a thin gold wire for precise knife edge measurements of focused spot size. The experiment was located at distance P = 47 m from the bending-magnet source. The lens chip was illuminated by monochromatic X-rays of energy E=18 keV (Lens A), 12 keV (Lens B) and 11 keV (Lens C). The lenses were mounted in vertical focusing geometry as can be seen in Figure 2, and the detector placed at the focal distance Q = 1.1 m (Lens A), Q = 0.6 m (Lens B) or Q = 0.2 m (Lens C). All lenses focused the X-ray beam to micrometer sizes without producing the beam splitting often observed when using single-crystal diamond. A minimum full-width-half-maximum (fwhm) of the focused beam profile is reached of 2.2 µm, 1.6 µm and 0.6 µm for the three different focal lengths, respectively, as summarised in Table 1, and shown in Figure 3. Despite the fact that the lenses are imaging a source with Gaussian shape, the focused beam data have to be fitted by purely Lorentzian profiles: this is a problem often encountered with focusing optics, and we believe that scalloping produced during Si etch is partly responsible for it. High resolution SEM shows that the CVD diamond replicas have high form fidelity; therefore reduced silicon scalloping would result in smoother lenses. Synchrotron radiation has not yet been used to characterise nano-crystalline diamond, therefore we checked the phase content of the lens material using X-ray powder diffraction and no other crystalline form, for instance graphite, was found (even after long exposure to synchrotron radiation). Further studies of the nano-domain distribution are being carried out using small angle X-ray scattering.

 

In summary, diamond micro-focusing lenses have been developed with a novel deposition technique allowing satisfactory control of the lens growth. This technique potentially improves the quality of diamond lenses compared to previous work made using single-crystal diamond. We suggest that growth of nano-diamond is more reliable than etching of single-crystal diamond due to the extreme hardness and the presence of imperfections in single crystals, such as stacking faults. The lenses obtained with deposition have good aspect ratios and uniform efficiency over a wide aperture. Unlike etching, this technique will permit growth of thicker lenses which can actually be used on beamlines. We have produced lenses with a good range of focal lengths in the hard X-ray energy spectrum and demonstrated sub-µm focusing with nano-diamond for the first time. It is expected that this excellent material will allow better focusing efficiency and easier heat load management compared to other micro-fabricated optics.

Lens X-ray energy [keV] Focal length [m] Source demagnification Theoretical focus size fwhm [µm] Measured beam size fwhm[µm]

 

 

Table 1. Summary of micro-focusing lens test results.

 

 

References

 

Funding Acknowledgements

STFC grant ST/F001665/1 and EPSRC grant EP/D074924/1.