## 3 Large Arrays Fabrication
### Absorbing Layer  
An absorbing layer that is thermally well connected to the TES is an essential component of an X-ray microcalorimeter. The absorbers can be directly attached to a small area of the TES slice, or can be place over/near the TES films via thermal coupling structures [94]. For imaging detectors, the absorber is placed directly on top of the TES to maximize the filling factor and thereby defines generally the pixel size. Several absorbers can also be connected to a single TES via thermal links of different length, as it is the case of the emerging class of detectors discussed in Section 5.4. It is important that the absorber has a high intrinsic thermal conductance to guarantee a fast thermalization, after a photon hit, and to minimize potential internal fluctuation noise. The absorbers are employed to enhance the quantum efficiency of the devices over the target energy band. Thus, providing sufficient stopping power over the large detection area is of primary importance. Therefore, materials of high atomic number with good thermal conductivity are desired. In the soft X-ray field, semi-metals like Bi or normal metal such as Au and Cu have been widely used. In Figure 7b, we show a microphotograph of an X-ray absorber made of a 2.3 \(\upmu\)m thick Au film, which provides a quantum efficiency of 83 % for a 6 keV photon. It is possible to achieve a quantum efficiency as close as 100 % by increasing the thickness of Au up to 7 \(\upmu\)m at the cost of an increase of the heat capacity and a degradation of the detector sensitivity. To overcome this counter-effect, Bi, which has a very low specific heat at low temperature, can be deposited directly onto a thinner normal metal absorbing layer like Au. The absorbing layers can be deposited by using sputter deposition, evaporating, or electroplating. Any of these can be adopted to form a uniform large array, but electroplated thin films have been shown to achieve higher Residual-Resistivity Ratio (RRR), usually defined as the ratio of the resistivity of a material at room temperature and at mK, and better thermal characteristics [104; 105]. Furthermore, electroplating is preferable, especially for the thicker layers, to minimize the thermal loading during deposition and the stress in the films. On top of this, evaporating thick layers of Au is far too expensive. In Figure 9, we reproduce the scanning electron micrographs of evaporated and electroplated bismuth  
Figure 8: (**a**) SEM micrograph of an array of ultra-low-noise TES bolometers on suspended SiN structures with diagonal long legs (340 \(\upmu\)m \(\times\) 0.5 \(\upmu\)m \(\times\) 0.25 \(\upmu\)m) design. In the middle of each detector there is a TES and an absorber on a SiN membrane [101]. (**b**) Microphotograph of an array of thousands of square pockets etched by using a Si deep-RIE process. The pocket size is 190\(\times\)190 \(\upmu\)m\({}^{2}\) and the pitch is 250 \(\upmu\)m.  
studied at NIST to better understand the non-Gaussian spectral response observed in X-ray absorbers [105]. To achieve the required high QE however, a careful study needs to be performed on the thickness and the quality of the Au/Bi bilayer. It has been recently shown [106] that the roughness, the edge profile, the angles of incident of the X-ray photons, and the filling factor needs to be known well to obtain an accurate estimate of the QE. To minimize the effect of the absorption of the stray light in the detector, it is also desirable to increase the reflectivity of the absorber for long-wavelength infrared radiation. For an Au/Bi absorber, this can be done for example by adding a Au capping layer on top of the Bi, with a thin Ti adhesion layer. A quantum efficiency of 90% has been reported for TES-X-ray calorimeters designed for Athena/X-IFU [106] with an increase of reflectivity, measured at room temperature, from 45% to up to 80% after adding a 40 nm Au capping layer.