(A) study on the thin nickel oxide film for micro-bolometer sensing material

Abstract

In this work, the new bolometer material to replace the conventional bolometer sensing material was proposed. The conventional bolometric materials have demerits such as a reproducibility and difficulty of fabrication though they have a high performance (i.e. ratio of TCR to noise). The most commonly used VO$_x$ (vanadium oxide) has been patented, and a financial support is required for acquiring the right to use patented products. The nickel oxide film developed in this work can be fabricated easily due to a low number of phases of nickel oxide and ease of use of conventional equipment. Therefore, the fabrication process can be simplified, and it is not difficult to secure the reproducibility. Also, developed nickel oxide can be used freely because there is no patent related with micro-bolometer using a nickel oxide film. Nickel oxide film was fabricated by two methods. In the first method, nickel films were deposited by a DC sputtering, and then the deposited nickel films were thermally treated in oxygen ambient at a temperature under 400$^o$C. In the nickel oxidation method, deposited nickel films were changed to nickel oxide films depend-ing on a heat treatment time. acquired 100nm-thick nickel oxide films shows a resistance value around 10MΩ. The bolometric properties of the fabricated thin nickel oxide film were confirmed that a TCR value was -3.2%/$^o$C, which is -1%/$^o$C higher value than that of amorphous silicon used as a one of conventional bolo-metric materials, and the measured 1/f noise parameter k was a 1.3×10$^{-10}$ for a 50um×50um size. In the second method, thin nickel oxide films were deposited by a RF reactive sputtering method in Ar and O$_2$ ambient with a nickel metal target. The electrical property of the deposited nickel oxide films was changed in resistivity range between 0.04Ωcm ~ 7Ωcm. Bolometric properties were confirmed that a TCR was around -2.1%/$^o$C near 1.7Ωcm and the measured 1/f noise parameter k was a 3.6×10${-11}$ for a 50um×50um size. From the results, it was found that the performance of the sputtered nickel oxide film is better that that of the oxidized nickel film. To suppress a 1/f noise property of a sputtered nickel oxide film, various methods were proposed. First, surface treatment methods of oxygen plasma and Ar+ bombardment were introduced to a nickel oxide film. A 1/f noise parameter k of nickel oxide film with oxygen plasma treatment was 1.2×10$^{-11}$, which is 3 times im-proved value than that with no treatment. And nickel oxide film with Ar$^+$ bombardment treatment had a 1/f noise parameter k value of 3×10$^{-12}$ which is about 1 order improved value than the k value of nickel oxide film with no treatment. Based on the results of each surface treatment method, two methods were continuously applied to nickel oxide films, and improved result of 1/f noise suppression was gained as a 1/f noise parameter k value is 2×10$^{-12}$ Second, The nickel electrode was applied to the nickel oxide film. Contrary to an aluminum electrode, the nickel electrode deposited on the nickel oxide film doesn’t take an oxygen from a surface of a nickel oxide film. The property of a nickel that doesn’t take an oxygen prevents to generate a metal oxide with a high resistance property at the interface between nickel and nickel oxide film. Also, the resistance of the surface of a nickel oxide film is kept because nickel oxide doesn’t lose its oxygen. These properties can improve the contact resistance, and decrease a 1/f noise property of the nickel oxide film. A 1/f noise parameter k of the nickel oxide film with the Ar+ bombardment and nickel electrode was 7×10$^{-13}. Third, a heat treatment was applied to a nickel oxide film. For the heat treatment during 15min at 150$^o$C, it was found that the 1/f noise property is additionally reduced. As a result, the smallest 1/f noise parameter k value of 2×10$^{-13}$ was acquired. And, EDS, XPS, SIMS and XRD analysis were conducted for investigating a cause of improved noise characteristic, and the reason about the performance enhancement of a nickel oxide film was described. Additionally, various analyses were conducted about one layer and double-layer nickel oxide films de-posited at various oxygen partial pressures to investigate an electrical property of a sputtered non-stoichiometric nickel oxide film. From the non-stoichiometric characteristic and analysis results, it was found that the nickel vacancy and hole concentration increase as the oxygen partial pressure increases, which leads to an increment of the conductivity of nickel oxide films. And from an additional Hall measurement with a temperature and equations related with an ionization energy, the difference in the number of created nickel vacancy and measured carrier was described. The nickel oxide film developed in this study has the low temperature process, reproducibility and bo-lometric performance similar with the performance of the VO$_x$ film. And if processes of a nickel oxide film are further optimized, a higher performance can be also achieved. Therefore, the reactive sputtered thin nickel oxide film shows a sufficient amount potential as a bolometer sensing material which can replace a VO$_x$ film.