Objective  Microbolometers offer advantages such as wide working range, fast response time, and simple device structure. The working principle is that infrared radiation incidents on the absorption layer, generating thermal energy which is then transferred to the thermosensitive material. The thermosensitive material changes its electrical resistance upon absorbing the heat and thus produces a change in output signal detected by the readout circuit. The thermosensitive layer has a significant impact on the performance of microbolometer. Currently, the main industrial materials for the thermosensitive layer are vanadium oxide and amorphous silicon, with a general temperature coefficient of resistance (TCR) of −2%/K. Compared to vanadium oxide and amorphous silicon, manganese cobalt nickel oxide (MCNO) with high TCR coefficients has great potential in uncooled infrared detector applications. Unlike most MCNO thin films prepared by magnetron sputtering of a stoichiometric compound, this paper reports the synthesis of MCNO by combining magnetron sputtering and electron beam evaporation. MnO₂ and Co₂O₃ were co-sputtered initially, followed by e-beam evaporation of Ni. Multilayers of MnO₂-Co₂O₃ oxides and Ni were thus deposited alternatively and subjected to in-situ and/or post-annealing to promote interdiffusion and the formation of MCNO compound. The objective was to investigate the feasibility of the new fabrication method based on solid state synthesis. The prepared thin films showed good crystallinity and negative temperature coefficient of resistance, indicating that the proposed method could be applied to further fine-tuning the compositions of MCNO thin films in the future.

  Methods  Thin films were prepared using a magnetron sputtering-electron beam evaporation hybrid method using a laboratory-developed high vacuum coating unit, where an e-gun and two magnetron sputtering targets coexisted in the same chamber. The sample stage was controlled by a stepper motor for 180-degree rotation, thus enabling alternative up-facing magnetron co-sputtering and down-facing electron beam evaporation in a single fabrication process. Sapphire substrate was cleaned with acetone, ethanol, and deionized water sequentially, dried up with blowing N₂ gas, and pre-heated in the vacuum chamber at 450 ℃ for one hour prior to the deposition. To study the effect of post-annealing on the performances of MCNO thin films, the as-deposited thin film samples were first annealed in-situ in the vacuum chamber for an hour, then annealed in a tube furnace in the air at 750 ℃, 850 ℃, and 950 ℃ respectively. The surface morphology of the resultant MCNO thin films were characterized using scanning electron microscopy (SEM). The crystal phase structure was characterized using X-ray diffraction analysis. The resistance at variable temperature from 220 K to 300 K was measured using a two-probe setup on a UV-THz full-spectrum photoelectric test probe stage. The transmittance and absorbance of the thin films were examined using Fourier-transform infrared spectroscopy (FTIR) in the 2.5-25 μm range.

  Results and Discussions  XRD characterization showed that the thin films exhibited polycrystalline structures with different preferential orientations as the annealing temperature increased. The primary crystal orientation obtained was (111), and the peak intensity increased with increasing annealing temperature. SEM characterization showed that the grain size of the thin film increased with increasing annealing temperature, consistent with the XRD results. FTIR characterization showed that the absorbance of the thin films increased toward the long wavelength in the 6-8.5 μm range and reached 1.50 for the sample annealed at 850 ℃. XPS quantification showed that the ratio of Mn⁴⁺/Mn³⁺ in the film increased with increasing annealing temperature. All thin films exhibited ohmic characteristic curves measured with four-point probe method at room temperature. It was shown that the samples annealed at 750, 850, and 950 ℃ respectively exhibited characteristic negative temperature coefficients, with calculated TCR of −1.95%/K, −4.20%/K, and −4.14%/K accordingly. The high TCR could be attributed mainly to the fabrication method, as solid phase synthesis of MCNO thin films via layer-by-layer deposition could result in metastable states in the thin films, thereby achieving better TCR and absorbance unobtainable in systems of thermal equilibrium.

  Conclusions  This paper explored a magnetron sputtering-electron beam evaporation hybrid method based on solid phase synthesis to fabricate MCNO thin films. The impact of post-annealing on the properties of the thin films was investigated. Thin films prepared with optimized processing parameters showed high absorbance and excellent negative temperature coefficient of resistance. This study demonstrated the feasibility of fabricating MCNO films using the proposed method, offering a new approach for further development of MCNO thin films with optimal composition-property relationship.