Characterizing the stress and electrical properties of superconducting molybdenum films

Abstract

In the process of minimizing stress in sputtered Molybdenum (Mo) films for fabricating transition-edge sensor (TES) devices, we have investigated correlations between the stress and film deposition parameters. At a fixed sputtering power, the tensile stress of our film samples decreases toward both low and high ends of Ar pressure, suggestive of two physical mechanisms at work: an “atomic peening” effect at low Ar pressure and the development of voids at high Ar pressure. We have also carried out correlative studies of the stress and electrical properties (including superconducting critical temperature and residual resistivity) of the film samples, and found that the results are complex. We have made extensive comparisons with the published results, and attempted to explain the discrepancies in terms of film deposition techniques, sample preparation and treatment, and dynamical ranges of measurements. It is fairly clear that the microscopic properties, including porosity and disorder, of Mo films may have significant impact on the correlations.

Study of long-term stability of Mo–Cu bilayer films

“…The Mo and Cu diffraction peaks are clearly separated, except for Mo (211)/Cu (220) and Mo (222)/Cu (400). Previously, we have established that the dominant crystalline phase of our Mo films is Mo (110) [11], which is also seen here. Therefore, for the Cu layer, Cu (220) may be as dominant as Cu (111).…”

“…The results show that the surface roughness of the Cu layer increases with its thickness, indicating increasing grain size, which is confirmed by our measurements with an atomic force microscope. On the other hand, the inter-layer surface roughness (as indicated by σ Mo in the figure) shows no apparent correlation with the thickness of the Cu layer, but is comparable to that of our bare Mo films [11]. These results, combined with the GID measurements (as shown in figure 1), suggest that the Mo-Cu interface is clean for fresh bilayer samples, free of oxidation.…”

“…Briefly, a Mo layer is first deposited on a Si wafer, which is covered by Si 3 N 4 films of thickness 500 nm on both sides, at room temperatures, until its target thickness is reached. The details of the process, as well as the characteristics of the Mo films, have been described in a previous work [11]. A Cu layer is then immediately deposited on top of the Mo layer, to maintain a clean bilayer interface.…”

Development of TES microcalorimeters for the HUBS Mission

“…6 Briefly, the process of growing Mo and Mo/Cu superconducting films is now fairly mature, and the films produced are of high quality, as measured in terms of, e.g., residual resistance ratio (RRR) and internal stress, with good repeatability in terms of superconducting properties. 7,8 The bilayer films have been monitored for long-term stability in their properties, and have been subjected to MeV proton irradiation for assessing their suitability for applications in space environment. 9,10 Based on the Mo/Cu bilayer films, the TES arrays of different sizes have been fabricated and tested.…”

Transition-edge sensors using Mo/Au/Au tri-layer films

“…[6] Mo/Au and Mo/Cu are typical material combinations for TESs because both bilayer systems are robust with regard to diffusion and the formation of an intermetallic phase. [7][8][9] A TES microcalorimeter usually requires a low normal-state resistance to reduce internal thermal fluctuation noise. It is straightforward to grow a Cu film with low resistivity via sputtering.…”