A noble infrared $lambda/4$ absorbing structure using metal reflector was studied for uncooled infrared sensors. This paper described the design and the fabrication of IR uncooled detectors which were composed of 21 by 21 elements using the surface micromachining technology. The characteristics of the array were investigated in the spectral region of 4.26 ${mu}m$. The fabricated detectors exhibited the thermal mass of $9.75 imes10^{-9}$ J/K, the thermal conductance of $1.31 imes10^{-6}$ W/K, the thermal time constant of 7.4 ms, the responsivity of $1.07 imes10^5$ V/W and the detectivity of $1.04 imes10^9$ $cmHz^{1/2}/W$, at the chopper frequency of 10 Hz and the bias current of 9.22${mu}A$. Finally the absorptance efficiency of $lambda/4$ absorbing structure was about 23.2 % higher than that of absence absorbing structure.