# THz Range Low-Noise SIS Receivers for Space and Ground-Based Radio Astronomy +
## 2 Materials and Methods  
### Nb/AlO\({}_{x}\)/Nb and Nb/AlN/NbN SIS Mixers  
The implementation of quantum-limited characteristics requires SIS tunnel junctions with an extremely low leakage current below a gap voltage \(V_{g}\) and a minimum spreading of the energy gap \(\delta V_{g}\). This is especially important for low-frequency devices (\(f<300\) GHz), since the \(\delta V_{g}\) should be much smaller than the quasiparticle step size \(\mathit{lf}/e\), and the leakage current at a bias voltage around \(V_{g}\)-\(\mathit{hf}/2e\) (in the middle of this step) determines the noise of the mixer. The well-developed technology of the Nb-AlO\({}_{x}\)-Nb tunnel junctions is based on the fact that a very thin Al layer can completely cover the base Nb electrode [25; 26; 27; 28], effectively planarizing the column-like microstructure of the Nb film [27]. This Al layer is subsequently oxidized and the top Nb electrode is deposited on the oxidized layer to form a so-called three-layer structure. The selective niobium etching and anodization process (SNEAP) [26] was used to fabricate the SIS tunnel junctions from the Nb/Al-AlO\({}_{x}\)/Nb three-layer structure circuits [28; 29; 30] on 24 \(\times\) 24 mm quartz substrates. To prevent etching of the substrate at the junction definition process and to simplify photolithography on a transparent substrate, a \"monitor\" Nb layer with a thickness of approximately 100 nm was applied to the substrate using DC magnetron sputtering. The next step is the deposition of a three-layer structure, which was carried out using DC magnetron sputtering in one vacuum cycle. First, a 200 nm thick lower niobium layer is deposited together with a 5 nm thick barrier aluminum layer. After that, the aluminum surface is oxidized in an atmosphere of pure oxygen, and a layer of upper Nb is deposited with a thickness of 100 nm. The geometry of the base electrode is formed by lift-off. SIS junctions are formed by reactive ion etching (RIE) in CF\({}_{4}\); the top layer of the Nb three-layer structure is removed in accordance with a mask from the photoresist defining the geometry of the junction. The AlO\({}_{x}\) barrier layer acts as a stop layer, preventing further etching of the structure. After RIE, light anodization is carried out up to 10 V using the same photoresist mask, then, an insulating SiO\({}_{2}\) layer with a typical thickness of 250 nm is deposited using RF magnetron sputtering. The opening of the contacts to the SIS junctions is carried out by lift-off. The upper electrode is also formed by the lift-off process from the deposited niobium layer with a thickness of 350-400 nm. The gold contact pads of the base and top electrodes are formed in a similar manner. Usually, the well-pronounced knee-like feature arises on the IVC at voltages slightly higher than \(Vg\). This feature is due to the presence of the aluminum layer near the tunnel barrier; the IVC of the Nb/Al-AlO\({}_{x}\)/Nb (S-S'/I/S) tunnel junction depends on the quasiparticle density of states in the S' layer (Al). This density of states for the Nb/Al bilayers was calculated using the microscopic proximity effect model [31]. The model assumes a short electron mean-free path (dirty limit conditions) in both S (Nb) and S' (Al) materials. Experimentally, the dependence of this effect on the parameters of the tunnel structure was investigated in [28]. The presence of the knee structure results in the negative differential resistance on the quasiparticle steps that in turn leads to the mixer instability and nonlinear operation at some frequencies. It was demonstrated that the knee structure can be completely suppressed by introducing an additional Al layer in the base electrode. The current-voltage characteristic (IVC) of an Nb-Al-Nb/Al-AlO\({}_{x}\)/Nb SIS mixing element (area of approximately 1 \(\upmu\)m\({}^{2}\)) with an additional Al layer in the bottom electrode, measured in the voltage-bias mode, is shown in Figure 1; the SIS junction critical current is suppressed by a magnetic field.