1.

Introduction

High-sensitivity X-ray detectors are affected by out-of-range radiation [microwave, infrared (IR), visible (VIS), and ultraviolet (UV)], which can deteriorate their performance. The presence of filters to shield the detectors is thus necessary for the correct operation of the detector, yet their X-ray transparency must be maximized to guarantee a high detection efficiency.

Charge-coupled devices, silicon drift detectors, and depleted p-channel field effect transistor arrays, as baselined for the wide field imager¹ (WFI) instrument of the Athena mission, are sensitive to UV and visible photons with energy larger than the Si band gap. If these photons hit such a detector, electron-hole pairs are formed, which add to those generated by the X-ray photon, causing a spectral resolution degradation, and a shift of the energy scale by a few eV. An optical blocking filter (OBF) is thus required to remove UV and visible radiation which is typically brighter than X-rays in most astrophysical sources.² OBFs are particularly important for experiments observing the EUV and X-ray emission from the chromosphere and solar corona, in this case the much brighter UV/VIS emission from the photosphere must be fully blocked. A few such examples include the X-ray telescope experiment³ on Hinode, the extreme ultraviolet imaging telescope⁴ experiment on Solar and Heliospheric Observatory, and the extreme ultraviolet imager experiment⁵ on Solar Orbiter.

Filters are usually made of a shielding layer, usually a low-Z metal such as aluminum for keeping a low X-ray attenuation, and a supporting membrane, which is needed because the low thickness needed to allow for limiting the X-ray attenuation does not ensure sufficient mechanical strength.

Silicon nitride is a good candidate as a filter support material since both silicon and nitrogen are low-Z chemical elements, silicon nitride (SiN) is known to be thermodynamically stable ^{6 ,7} and, additionally, the related technological processes are well developed because this compound is extensively employed in the fabrication of integrated circuits. The most used materials to fabricate filters for X-rays missions are polymers, in particular polyimide, because their excellent mechanical properties. However, this class of materials contains a very large amount of carbon resulting in high absorption around the C K-edge.

Filters based on SiN are in general less transparent than most of used polymeric thin films like polyimide as it can be seen in Fig. 1 (calculated curves). To be competitive with polyimide the SiN membrane thickness shall be at least half the thickness of the polymer. In addition, the Si L edge at around 100 eV is quite pronounced significantly reducing the transmission up to the N K edge at around 400 eV.

On the other hand, being SiN carbon free, it may have niche applications when high transparency is required around the C K-edge region.

Performing a spectroscopic characterization of sandwiches composed of silicon nitride and aluminum is a fundamental step for designing filters with these materials; since their spectroscopic properties strongly depend both on their intrinsic properties and on their thickness and morphology.⁸

In this paper, we show a multi-technique investigation [UV-VIS-NIR spectroscopy, X-ray photoelectron spectroscopy (XPS), X-ray transmission spectroscopy, and atomic force microscopy (AFM)] of Al-coated ultra-thin silicon nitride membranes. Furthermore, a selection of these were irradiated with different proton doses, and then characterized by spectroscopy and AFM to assess any potential modifications in their properties. We measured the X-ray transmission of these materials and recovered the thicknesses of all the present layers by modeling. The analysis of X-ray photoelectron spectra allowed us to derive the thickness of the native aluminum oxide that reduces the nominal amount of metallic aluminum. We also studied these filter samples by UV/VIS/NIR spectroscopy to estimate the attenuation capability in this band, which is relevant in space when such filters are used to protect EUV/X-ray detectors from Sun light and out-of-band optical load from other astrophysical sources.

2.

Materials and Methods

3.

Results and Discussion

All the Al 2p XPS spectra shown in Fig. 3 are composed of two main signals: a major contribution at higher binding energy attributed to the ${\mathrm{Al}}^{3+}$ (oxidized aluminum) component and a minor signal at lower energy assigned to the ${\mathrm{Al}}^0$ component (metal aluminum). For the ${\mathrm{Al}}^0$ is possible to resolve the doublet due to the spin-orbit coupling which corresponds to the two possible states having distinguishable binding energies, attributed to $2{\mathrm{p}}_{1/2}$ and $2{\mathrm{p}}_{3/2}$ Al peaks.¹⁶

Figure 3 shows that as the kinetic energy (KE) of the ejected electrons increases, the relative contribution of metallic aluminum increases with respect to the aluminum oxide. As the KE increases, the investigated thickness is larger, as the effective attenuation length (EAL) of the photoemitted electrons increases with the photon energy, thus more signal is detected from the deep metallic aluminum substrate with respect to the superficial oxide layer.

Assuming a flat homogeneous oxide layer on top of a homogenous Al substrate, the thickness of the aluminum oxide, $t$, can be calculated using the following equation:¹⁷

The EALs are computed using the NIST EAL database¹⁸ and the TPP- 2M predictive formula.¹⁹ The values of the photoionization asymmetry parameter were taken from Ref. 20. The uncertainties of the EAL are considered for Al and ${\mathrm{Al}}_2{\mathrm{O}}_3$ 10.9 and 17.4 % is considered, respectively.¹⁸

For the ${\mathrm{Al}}_2{\mathrm{O}}_3$ overlayer, we assume the density of $3.3\mathrm{g}{\mathrm{cm}}^{-3}$, band gap energy 7.0 eV²¹ and 24 valence electrons per molecule. The ratio between the atomic densities was set at 1.5 and the areas of the Al 2p components were obtained by fitting the Al 2p peaks. An example of peak analysis is reported in Fig. 4.

The oxide thickness is calculated as the slope of the linear fit of Eq. (3), reported in Fig. 5

The oxide thicknesses computed for all analyzed spectra are reported in Table 6.

Table 6

Oxide thicknesses obtained from XPS analysis of the Al 2p peak for the TO8-C1-2, TO8-C2-2#2, TO8-C2-6, and TO8-C2-2 samples.

Such values are in good agreement with the thickness of native oxide on bulk Al(111) and Al(100) crystals¹⁶ and with the oxide thickness that was found in witness samples of the current baseline filters for the Athena X-ray integral field unit and WFI detectors,²² while it is very different to the case of the XMM back-up filters monitored over time.²³ In the last case, the aluminum oxide thickness was found to be 5.9 nm, thus nearly twice that measured for the samples investigated in this paper.

The main purpose of the X-ray transmission measurements performed at BESSY II was to model the transmission curve of each filter, derive the areal density $\rho x$ of the filter material and the thickness of each layer, with its associated uncertainties. Two fit functions were used, one involving the single atoms (elements) and disregarding the stoichiometry,²⁴ and the other using the chemical formula of each layer of the filter material.

The fit using elements has up to four parameters, namely each atomic areal density $\rho x_{i\text{-}\mathrm{th}}$

The fit function is based on the product of the transmission due to the i-th element $T_i$, and has four free parameters, the areal density $\rho {\mathrm{x}}_i$ of each element. The quantity $\mu /\rho$ is the mass attenuation coefficient of the $i$’th chemical element and is calculated from the atomic scattering factors by Henke et al.²⁵ Hydrogen was not included since it does not contribute to the transmission of the filter in the investigated energy range. In principle, all layer thicknesses can be estimated, introducing two quantities a posteriori: material density and stoichiometry [see Eqs. (5) and (6)]. From $\rho x_{\mathrm{Al}}$ one can approximately calculate the thickness of the Al layer by dividing it for the Al bulk density of $2.7\mathrm{g}/{\mathrm{cm}}^3$ (in so doing, one considers only the aluminum in bulk form). To estimate the silicon nitride thickness, one can calculate the equivalent thickness, $x_N$ within the ${\mathrm{SiN}}_y$ using a stoichiometry $y=4/3$

The fit using materials has up to three parameters, namely each material thickness

The second material is a layer of aluminum oxide, whose presence is a well-established fact in the literature and is proven by the XPS measurements reported above. The advantage of using materials instead of atoms lies in that it is possible to directly obtain each layer thickness (using the following densities: ${\rho }_{\mathrm{Al}}=2.7\mathrm{g}/{\mathrm{cm}}^3$, ${\rho }_{\mathrm{Al}2\mathrm{O}3}=3.3\mathrm{g}/{\mathrm{cm}}^3$, and ${\rho }_{\mathrm{Si}3\mathrm{N}4}=3.44\mathrm{g}/{\mathrm{cm}}^3$) and make a comparison with the nominal thicknesses. A correction needs to be introduced for the filters supported by a mesh, to consider the OA and the mesh material, which is not generally opaque and could be transparent to X-rays at the higher end of the investigated energy spectrum. To this end, two more parameters were considered, namely the mesh material areal density and the OA, as seen by the beam spot (which is generally different from the nominal value)

Two best fits, one using elements and one using materials, were obtained for all the filter samples. For filters with a mesh, the additional parameters of mesh areal density and OA were derived. The areal densities and OA of the best fit on elements and the derived thicknesses are summarized in Table 7, whilst the layer thicknesses, mesh thickness and OA obtained from the best fit on materials are reported in Table 8.

Table 7

Areal densities of each element (Al, Si, O, N, and Si mesh) and OA with 3σ statistical uncertainty.

The measured X-ray transmission spectra of the investigated filters are shown in Figs. 6 Fig. 7 Fig. 8 Fig. 9–10 along with the best fits and details at the absorption edges of the involved elements. In general, all fits are globally good, also at the Al, Si, and N edges, but discrepancies are visible at the O edges, partly related to the low amount of O.

As can be seen in Table 8, the sum of the Al and the ${\mathrm{Al}}_2{\mathrm{O}}_3$ thicknesses is in agreement, within the uncertainties, with the equivalent thicknesses of the Al layer, calculated from the areal densities in Table 7. In most cases, it is slightly higher than the nominal Al thickness. Furthermore, for the samples with the thin silicon nitride substrate, the ${\mathrm{Al}}_2{\mathrm{O}}_3$ thickness varies from sample to sample, probably due to the fit being less accurate around the O K-edge. The obtained thickness of the SiN substrate itself deviates only a few percent from the nominal values. The obtained OA from both fits is also close to the nominal value. Estimating the mesh thickness is trickier, because both the OA and the mesh areal density are interdependent parameters and difficult to disentangle for the optimization algorithm.

Table 8

Layer thickness of each material (Al, Al2O3, SiN, SiO2, and Si mesh) and OA with 3σ statistical uncertainty and nominal thickness for all filters.

Transmission spectra acquired in the UV/VIS/NIR range are reported in Figs. 11 and 12. The reported spectra show that these filters are quite transparent in the UV region (190 to 400 nm), partly due to the constructive interference effects generated by the bilayer of aluminum, while in the visible portion of the spectra the rejection of the radiation is more efficient.

The shape of the transmission curves of Fig. 12 does not look affected by the irradiation treatment, suggesting that these multilayers are quite resistant when proton irradiation occurs. Although a blueshift of the main peak (close to 300 nm) must be noticed when the Q.F. increases, this peak moves from 312 nm (pristine filter, 3.97 eV) to 302 nm (1 Q.F. sample, 4.11 eV) and to 296 nm (10 Q.F., 4.19 eV).

Zhang et al. showed that the position of the aluminum absorption band close to 4 eV is size-dependent: if the size of the nanoparticle increases, the peak redshifts.²⁶ Then the observed blueshift in our work might be ascribed to a slight reduction of the size of aluminum nanoparticles assembled to create the metal coating.

AFM images were analyzed using the data analysis software Gwyddion²⁷ and are reported in Fig. 13. The zero-height reference is located in the lowest point measured by the probe.

Since in literature it is found that ion irradiation can affect the aluminum surface^{28 ,29} and that proton irradiation may create ripples and hillocks changing the roughness of the surfaces,³⁰ both the roughness and the average heights are computed.

The roughness, $R$, of the samples was calculated as reported in the following equation:

The height data, obtained from the z-profiles [Figs. 13(d)–13(f)], are reported in three histogram plots [Figs. 13(g)–13(i)], and fitted using a Gaussian curve. The average heights correspond to the maxima of the fits. The obtained average heights for the samples TO8-C2-1, TO8-C2-6, and TO8-C2-3 are 12.75 nm, 13.78 nm, and 9.32 nm, respectively. The AFM analysis suggests that the average heights of the pristine samples are larger than the irradiated filter, this may hint at the possibility of an erosion process due to the proton exposure as suggested by Sznajder et al.³¹

4.

Conclusions

The analyses of the XPS spectra we reported show that Al/SiN/Al is undoubtedly oxidized on the surface. The thickness values that we find for the surface oxide agree with other results, already reported in the literature, for Al-coated polymeric filters.^{20 ,21} The irradiation treatment has no relevant effect on the surface oxide thickness up to 10 Q.F. On the other hand, the amount of oxide revealed by the X-ray transmission measurement is larger than the one obtained by XPS, because this technique is sensitive to the bulk, consequently, the whole amount of the present aluminum oxide is detected: both the two layers on the two external coating surfaces and any interfacial Al oxide between the silicon nitride and the aluminum coating. Moreover, the amount of Al oxide computed by the X-ray transmission model might be affected by the presence of ${\mathrm{Si}}_{\mathrm{x}}{\mathrm{O}}_{\mathrm{y}}$, which can be found on the silicon mesh and/or the silicon nitride.

Both surface roughness and UV/VIS/NIR transmission are not significantly affected by proton irradiation. The samples we studied are rather transparent below 400 nm; this can be ascribed to a total small amount of aluminum (20 nm) of which at least 20% is oxidized. To design an effective OBF for use in high energy astrophysical space missions based on SiN, the amount of Al must be increased to at least 30 nm possibly deposited on a single side to reduce the aluminum oxidation. However, this choice would cause a low transmission in the soft X-rays, that for the investigated samples is already $<70\%$ at 0.5 keV. These materials can be proposed in those cases where the presence of carbon atoms might be an issue although the capability to manufacture very thin large area membranes and their mechanical performances under differential pressure and dynamic launch stresses needs to be demonstrated.

Disclosures

The authors have no financial interests in the manuscript and no other potential conflicts of interest to disclose.

Code and Data Availability

The data that support the findings of this article are not publicly available due to privacy. They can be requested from the author at luisa.sciortino@inaf.it.

Acknowledgments

We acknowledge Elettra Sincrotrone Trieste for providing access to its synchrotron radiation facilities and for financial support, we received the Italian full support for both the proposals (Grant Nos. 20180369 and 20190214). E.M., S.N., and I.P. acknowledge funding from EUROFEL project (RoadMap ESFRI). The research leading to these results received funding from the European Space Agency for the project "Large Area high-performance Optical Filters for X-ray instrumentation – LAOF" (Grant No. 4000120250/17/NL/BJ), and from the European Union’s Horizon 2020 Program for the project AHEAD2020 (Grant No. 871158).

Luisa Sciortino (PhD in chemistry, Italy, in 2012) is a permanent staff member of the INAF-OAPA, Palermo, Italy. She has been working on the development of thermal and optical blocking filters for high-energy astrophysical mission. Her interest is focused on the spectroscopic and morphological characterization of materials.

Biographies of the other authors are not available.