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Hot-electron bolometer (HEB) mixers are a complex multi-layer thin film structure containing an ultra-thin superconducting film of NbN as a detecting element and a thick normal metal layer as an antenna structure. We have optimized the fabrication process starting with ultra-thin NbN films, Au films for antenna structures and their patterning using e-beam lithography and lift-off. The coupling between normal conducting antenna and NbN detector has been improved by introducing an intermediate NbN film to reduce proximity suppression of superconductivity in the detecting element. A critical temperature of about 9.5 K is reached for NbN films with a thickness between 5 nm and 6 nm. A twofold increase of the film thickness increases the critical temperature to 12 K. We have shown that a 20 nm thick buffer layer of NbN under a much thicker Au layer is sufficient to ensure a critical temperature of the bi-layer of 9 K. This value is close to the critical temperature of 5.5 nm thick HEB devices. The noise temperature of HEB mixer made using improved technology is about 800 K and was measured in a liquid cryogen free system with a quantum cascade laser as 2.5 THz local oscillator.