The pressure-dependent performance of a substrate-free focal plane array in an uncooled infrared imaging system

Abstract

Uncooled focal plane arrays (FPAs) are being developed for a wide range of infrared imaging applications. A substrate-free FPA for optical readout infrared imaging is fabricated with a pixel pitch of 120 μm. The pressure dependences of thermal conductance of a FPA with/without substrate are studied by modeling analysis. Infrared imaging experiments are performed to validate the modeling analysis. At atmospheric pressure the total thermal conductance of the substrate-free FPA is only 1/1000 time of the traditional FPA which has a 2 μm air gap between the cantilever beam and the substrate. Room temperature object’s thermal images are obtained even if the FPA is placed in the atmosphere. The air conductance of a substrate-free FPA at atmospheric pressure could be as small as that of a traditional FPA with a substrate in high vacuum (about 1 Pa). The experimental result also shows that the system noise keeps almost unchanged with the pressure. These characters will decrease the vacuum packaging request of the FPA and should be valuable for practical applications.

Uncooled Infrared Imaging Using a Substrate-Free Focal-Plane Array

“…Meanwhile, considering that there are always groups of pixels simultaneously absorbing IR radiation, the temperature change of 1.224 K is a reasonable choice to evaluate the fabricated FPA. Hence, the IR radiation conversion efficiency can be estimated to be 6.81%, which is increased by about 6.2 times compared with the analytical result (0.94%) [8], [9] of the substrate FPA.…”

“…The finite-element analysis indicated that, benefiting from the temperature-variable supporting frame, the temperature of the bimaterial legs in the deformation-magnifying structure is close or even equal to the temperature of the IR absorber. Therefore, the efficiency of the deformation-magnifying structure can be maximized, and our analytical calculation yielded Δθ/ΔT c of 0.2 • /K, which was increased by 33.3% compared with the value of 0.15 • /K of the substrate FPA, where there is a temperature gradient between the supporting frame and the IR absorber [8], [9].…”

“…A three dimensional (3-D) finite-element model of the fabricated FPA but 30 × 30 pixels (microcantilever array; a larger array is unnecessary according to the thermal diffusion effort of the supporting frame) was developed. The conductance of the FPA (including radiation) was analyzed, and because the FPA was placed in a vacuum environment (≤ 1 Pa), the conductance of air could be neglected [9]. The other assumptions used in the finite-element analysis included the incident heat flux of 10 pW/μm 2 , based on which the temperature change of the IR source can be estimated to be 17.97 K, 1 and the boundary temperature of the 3-D finite-element model of 298 K.…”

Microthermomechanical infrared sensors

“…A conducting heat exchange between the paddle of the can− tilever and the substrate's surface through air does not occur. The USTC/CAS group and also Grbovic have suc− cessfully demonstrated thermal imaging with a substrate− −free thermomechanical microcantilever FPA operating at atmospheric pressure [84,110]. However, the image quality of FPAs operating in air is poor, mainly due to the decreased microstructures' Q−factor.…”

Performance of an optimized substrate-free focal plane array for optical readout uncooled infrared detector

“…As in the finite element analysis, the thermal conductance of the FPA ͑including radiative heat exchange͒ was simulated, but air convection was neglected for the vacuum environment ͑Յ1 Pa͒. 13 The other assumptions used in the finite element analysis also included the incident heat flux of 10 pW/ m 2 , according to which the temperature change of IR targets ⌬T S can thus be estimated to be 17.97 K, 14 and the temperature boundary condition was constant at 298 K.…”