# XCT and DLW: Synergies of Two Techniques at Sub-Micrometer Resolution
## 1 Introduction  
DLW via two-photon polymerization (2 PP) can fabricate 3D objects with high resolution [1]. The technique is also called direct laser lithography or multiphoton lithography and utilises the absorption of two or more photons to create a polymerisation in a confined volume (the so-called voxel) of the photoresist [2]. Depending on the used DLW setup and photoresist, resolutions of about \(50\,\mathrm{nm}\) lateral and \(180\,\mathrm{nm}\) longitudinal were shown [3, 4]. Hybrid polymers such as ORMOCERs [5] or SZ2080 [6, 7] are used in DLW as they offer a comparatively low shrinkage together with a high precision and small feature size. Described applications range from micro-optical lenses [8, 9, 10, 11] to microfluidic arrangements [12, 13] and scaffolds for tissue engineering [14, 15]. Mechanical properties such as Young's modulus [16] or optical properties, e.g., laser-induced damage threshold (LIDT) [17, 18] and refractive index [19], have been described in the literature. To broaden the usage of DLW in industry, a clear set of tools for process control is needed as lined out by [PERSON] and [PERSON] [20]. Due to the small sizes of the fabricated structures, the physical or optical analysis is challenging, and most standard imaging methods such as scanning electron microscopy (SEM) or light microscopy (LM) are not suitable to examine the complete geometry and their inner structure in detail. DLW, with described feature sizes below \(100\,\mathrm{nm}\)[4] and the ability to construct structures in an entire three-dimensional way, optical microscopy, as well as other spectroscopy methods, comes to its limit. For light microscopy, the optical resolution is given by the Abbe diffraction limit:\[d=\frac{\lambda}{2\cdot NA} \tag{1}\]  
where \(d\) is the limit, \(\lambda\) is the wavelength of the light source and \(NA\)--the numerical aperture of the optical system used. For a 40\(\times\) objective with a \(NA\) of 0.65 and a light source with 550 nm, the diffraction limit \(d\) is around 423 nm for a conventional optical microscopy setup. With an oil immersion objective that has a typical \(NA\) of 1.4, the limit can be lowered to 193 nm. The SEM resolution limit is below the resolution limit of the DLW technique. However, the problem remains that with LM as well as SEM, internal structures of the fabricated objects cannot be observed. For SEM, polymer structures fabricated with DLW profit from an additional sputter-coating step to avoid charging the sample. However, Raman spectroscopy can be applied to analyse the chemical composition of the polymer, e.g., the degree of conversion, as described by [PERSON] et al. [21]. White Light Interferometry Microscopy has an axial resolution in the nanometer range, but it is limited to the Abbe diffraction limit in lateral resolution; additionally, only the surface of the structure can be examined. Internal structures and undercuts cannot be examined. XCT and XRM are non-destructive testing methods with a basic spatial resolution of several hundreds to tens of nanometers [22; 23], depending on the device used, and is the only technique available to study the quality of micrometer-sized 3D structures in detail. XRM includes additional lenses based on the reflective or diffraction properties of X-rays, for instance, a capillary condensor tube or a Fresnel lens [24]. Such lenses are not used in XCT setups, and the resolution limit is purely based on the X-ray tube focal spot size and/or scintillator thickness of the detector [22]. The resolution limit of XCT and XRM differs by a factor of about 10, with today's available techniques. Note that the nominal 3D resolution of both techniques is identical to the actual 2D resolution of the acquired radiographies. Resolution, by means of the detectability of smallest sample features, is also limited by the sample/feature material, sample/feature size, an applied X-ray spectra filter, image noise, X-ray beam path opening angle and the corresponding achieved imaging setup sharpness and/or feature contrast [25].