The effect of longer-range waviness on X-ray reflectivity measurements.

A model for calculating the X-ray reflectivity (XRR) of surfaces to extract both roughness and waviness features is presented. Expressions of reflectivity intensity are derived as a function of root-mean-square (RMS) roughness σ, RMS waviness σL, and the cut-off frequency between the features ω0. Experiments were conducted at the Advanced Light Source at Lawrence Berkeley National Laboratory, beamline 8.3.2, on BK7 glass manufactured with a multi-step polishing process to validate the model, and were compared with atomic force microscopy (AFM), Fizeau interferometry and surface profilometry measurements. The parameter results and their deviations for XRR measurements were σ = 2.9 ± 0.2 nm and σL = 14.6 ± 0.5 nm with a wavelength cut-off of 1/(18 ±â€…2) µm-1, while the results from the AFM, Fizeau and profilometry measurements were σAFM = 3.4 ± 0.4 nm, σL,Fizeau = 21.6 nm, σprof = 4.0 ± 0.1 nm, and σL,prof = 21.4 ± 0.1 nm with cut-offs for the profilometry and Fizeau measurements limited to frequencies of (1/16) µm-1 to (1/4) mm-1.

Radiopaque Resists for Two-Photon Lithography To Enable Submicron 3D Imaging of Polymer Parts via X-ray Computed Tomography.

Two-photon lithography (TPL) is a high-resolution additive manufacturing (AM) technique capable of producing arbitrarily complex three-dimensional (3D) microstructures with features 2-3 orders of magnitude finer than human hair. This process finds numerous applications as a direct route toward the fabrication of novel optical and mechanical metamaterials, miniaturized optics, microfluidics, biological scaffolds, and various other intricate 3D parts. As TPL matures, metrology and inspection become a crucial step in the manufacturing process to ensure that the geometric form of the end product meets design specifications. X-ray-based computed tomography (CT) is a nondestructive technique that can provide this inspection capability for the evaluation of complex internal 3D structure. However, polymeric photoresists commonly used for TPL, as well as other forms of stereolithography, poorly attenuate X-rays due to the low atomic number (Z) of their constituent elements and therefore appear relatively transparent during imaging. Here, we present the development of optically clear yet radiopaque photoresists for enhanced contrast under X-ray CT. We have synthesized iodinated acrylate monomers to formulate high-Z photoresist materials that are capable of forming 3D microstructures with sub-150 nm features. In addition, we have developed a formulation protocol to match the refractive index of the photoresists to the immersion medium of the objective lens so as to enable dip-in laser lithography, a direct laser writing technique for producing millimeter-tall structures. Our radiopaque photopolymer resists increase X-ray attenuation by a factor of more than 10 times without sacrificing the sub-150 nm feature resolution or the millimeter-scale part height. Thus, our resists can successfully replace existing photopolymers to generate AM parts that are suitable for inspection via X-ray CT. By providing the "feedstock" for radiopaque AM parts, our resist formulation is expected to play a critical role in enabling fabrication of functional polymer parts to tight design tolerances.