Design of Dual-Band Uncooled Infrared Microbolometer

Abstract-This paper describes the design and modeling of a smart uncooled infrared detector with wavelength selectivity in the long-wavelength infrared (LWIR) band. The objective is to enhance the probability of detecting and identifying objects in a scene. This design takes advantage of the smart properties of vanadium dioxide (VO
₂
): it can switch reversibly from an IR-transparent to an IR-opaque thin film when properly triggered. This optical behavior is exploited here as a smart mirror that can modify the depth of the resonant cavity between the suspended thermistor material and a patterned mirror on the substrate, thereby altering wavelength sensitivity. The thermistor material used in the simulation is vanadium oxide (VO
_x
). The simulation results show that, when VO
₂
is used in the metallic phase, it reflects IR radiation back to the suspended VO
_x
and enhances IR absorption in the 9.4-10.8-μm band. When the film is switched to the semiconductor phase, it admits most IR radiation, which is then reflected back to the suspended VOχ by a patterned gold thin film under an SiO
₂
spacer layer. The spacer layer is used to increase the resonant cavity depth underneath the microbolometer pixel. Thus, the peak absorption value is shifted to 8-9.4 μm, creating the second spectral band. The detector is designed with a relatively low thermal conductance of 1.71 X 10
^-7
W/K to maximize responsivity (R
_v
) to values as high as 1.27 X 10
⁵
W/K and detectivity (D*) to as high as 1.62 x 10
⁹
cm-Hz
^1/2
/W, both at 60 Hz. The corresponding thermal time constant is equal to 2.45 ms. Hence, these detectors could be used for 60-Hz frame rate applications. The extrapolated noise equivalent temperature difference is 14 and 16 mK for the 8-9.4- and 9.4-10.8-μm bands, respectively. The calculated absorption coefficients in the two spectral bands were 59% and 65%, respectively.