In this work, a novel bolometer structure has been proposed to achieve high detectivity. For the purpose, it is desirable to lower the bolometer’s thermal conductance. However, the thermal conductance of a traditional metal bolometer structure is usually larger than $1 x 10^{-7} W/K$ in a two-level structure due to the use of metal wires, located on supporting legs, for reading signal. In the proposed bolometer structure, movable bridges were employed to lower the thermal conductance of the bolometer. Therefore, its thermal conductance was unaffected by that of the metal wire by separating the metal wire and the supporting leg into two parts such as a bridge and a post, respectively. The movable bridges were actuated by electrostatic force applied between the bottom electrode and the bridges.
Analytical calculations and simulations of the bolometer to optimize design parameters were performed and bolomters were fabricated using the optimized parameters.
First, mechanical properties were investigated. In order to lower the pull-in voltage, we used a method that the aluminum film was first sputtered at high temperatures more than room temperature and then the polyimide curing process followed, thereby producing the pull-in voltage of about 8 V. The switching time of the signal bridge was less than 1 ms. The flat membrane was obtained by depositing stress-controlled films and using the thermal annealing method for adjusting the stress of the titanium film.
Secondly, thermal and optical properties were investigated. The TCR value of the resistor was -0.275 %/K. Measured thermal conductance of the two legs was $4.13 ＼times 10^{-8}$ [W/K], which is lower than those of traditional metal bolometers. Thermal response time was measured to be 10 ms enough for a bolometer with a frame rate of 30 Hz. Taking Johnson, 1/f, temperature fluctuation, and background fluctuation noise into consideration, total noise voltage was 15.57 mV. As a result, the responsivity and the detectivity were estimated to be $4.65 ＼times 10^4$ [V/W] and $4.23 ＼times 10^9 [cmHz^{1/2}/W]$, respectively.
It is considered that because the proposed bolometer structure has a high detectivity, it has promise for use as a new structure for bolometers in the near future. However, optimizing fabrication processes such as polyimide removing and aluminum patterning process is still required to improve electrical property and reliability.
