## EUV and SXR Full Field Microscopes Based on a Double Stream Gas-Puff Target Source
### SXR Contact Microscope  
The microscope described in this section is based on the same principle of the full field SXR setup, with the difference that the source is based on Ar plasma. In such a system, the Ar ionization is enough  
Figure 11: (**a**) SXR tomography setup and (**b**) internal components of the microscope. In (**c**,**d**) a scheme and a picture of the tip- and slit-holder are depicted, respectively. Figure modified, based on [59].  
to emit radiation in the \"water window\" spectral range. The samples are placed in contact with a 500 nm thick PMMA photoresist, deposited onto a silicon wafer, which acts as a detector to record the images of the samples. The photoresist irradiation by SXR and its subsequent chemical development causes local modification of the structure. The X-ray irradiation energy absorbed by the photoresist is converted into a relief-like structure, where the height difference will be directly proportional to the deposited radiation dose. Such difference is afterward converted to an image, typically by atomic force microscope (AFM).  
The system, firstly developed for imaging of dried samples [67], was then optimized for imaging of living cells [81]. In this last configuration, the sample exposure is quite easy, not being required the sample encapsulation between two membranes, typically needed with other techniques, such as transmission electron microscopy (TEM) [82; 83] or synchrotron-based microscopes [84]. Moreover, this configuration does not require the evacuation of the sample chamber, avoiding consequential problems related to eventual sample degassing due to incorrect sealing.  
The setup allows irradiation of dried samples (in a vacuum) or wet samples (in a gaseous environment). For irradiation in a vacuum, the samples are mounted onto a rotational carousel, hosting up to 8 samples. For irradiation in a gaseous environment, the sample is mounted in a small cylindrical chamber filled with He, placed outside the source chamber. The two chambers are separated by a 200 nm thick Si\({}_{3}\)N\({}_{4}\) membrane, \(\sim\)16 mm from the plasma. A sample holder enables to load the PMMA-coated Si wafers, so that only a few mm gap between the sample and the separating membrane is filled with He gas at a pressure of \(\sim\)1 bar. The He gas facilitates the removal of the air from the sample chamber, which, consequentially, efficiently attenuates the SXR radiation.  
Sensitivity, contrast, and roughness depend on PMMA development conditions. The proper combination of those factors allows to achieve a high spatial resolution and, at the same time, the preservation of the biological structures investigated, avoiding affecting the region of the sample unexposed to SXR radiation [10].  
A scheme of the SXR contact microscope is shown in Figure 12a. The employment of this technique does not require monochromatic radiation. Due to the employment of the Si\({}_{3}\)N\({}_{4}\) filter, indeed, most of the required energy (2.8-4 nm) resides inside the \"water window\" spectral range (Figure 12b). Indeed, the K-edge of the filter blocks the lower energetic photons. Moreover, due to the limited energy of the laser pulse, and due to the limited gas-puff density, the generated plasma is not sufficiently hot to produce higher energy photons. The produced photon flux, measured by an AXUV 100 photodiode (Opto Diode Corporation, CA, USA), was \(\sim\)\(8\times 10^{8}\) photons/pulse at the PMMA surface, corresponding at an energy of 48.34 nJ. The experimental system is depicted in Figure 12c. More details can be found in references [85; 86].  
Figure 12: (**a**) Scheme of the SXR contact microscope setup. (**b**) Spectrum of Ar emission acquired directly from the plasma (continue, purple-line) and at the sample plane (dashed-green line). Image modified, based on [86]. (**c**) Picture of the SXR contact microscope system.