Pixel size reduction in an uncooled infrared detector plays a crucial role in determining significant attributes such as size, weight, and cost. Without a loss in sensitivity, however, it is challenging to develop an uncooled infrared detector that has the pixel pitch of 12 μm and below. Especially, there has been a limitation of an uncooled focal plane array (FPA) with a single-level design, which has to accomplish both functions of absorption and heat conduction in a single layer and thus compromise the sensitivity of FPA. A selective etching process of a resistive material, titanium oxide, on the connecting legs in the microbolometer FPA has been developed to overcome the limitation and increase the sensitivity of the single-level design FPA. Furthermore, structural stress modulation has been applied to ensure the mechanical robustness of the developed FPA. Here, we present a 12 μm single-level design FPA using a titanium oxide as a resistive material of microbolometer FPA. High-resolution detectors with an array size of 640 x 480 pixels (VGA format) and 1024 x 768 pixels (XGA format) have been developed, and the noise equivalent temperature difference (NETD) and time constant for the VGA detector are 40.5 mK and 8.3 msec, respectively. Thermal image obtained by the 12 μm XGA detector shows excellent image quality and fine resolution.
Heat-sensitive material is one of the most essential parts of microbolometer fabrication. Vanadium oxide (VOx) and amorphous silicon (a-Si) are widely accepted materials for commercialized focal plane arrays. Meanwhile, there are a lot of efforts for finding alternative materials having better performance, lower process cost and higher yield. In this study, reactively sputtered titanium oxide (TiO2-δ) films were investigated for heat sensitive material. Microbolometer device was also fabricated by using the TiO2-δ film as a heat sensitive material.
It is well known that the TiO2-δ can have several phases according to film deposition condition. Properties of TiO2-δ film could be largely varied by controlling the deposition condition. Resistivity of the fabricated TiO2-δ film was ranged from 10-2 Ω•cm to 10 Ω•cm. Negative TCR(temperature coefficient of resistance) value up to 2.8 %/K was obtained. 1/f noise of the TiO2-δ film was comparable to that of VOx film. From the fabrication result of microbolometer device, feasibility of the reactively sputtered TiO2-δ film was demonstrated. NETD(Noise equivalent temperature difference) of the 50μm-pitch simple single-level membrane structure microbolometer was 34mK with conditions of 1V bias and 30Hz operation frequency.