A micromachined sensor part for an infrared focal plane array has been designed and fabricated. Amorphous silicon was adapted as a sensing material, and silicon nitride was used as a membrane material. To get a good efficiency of infrared absorption, the sensor was made as a ${lambda}/4$ cavity structure. All the processes were done in $0.5;{mu}m$ iMEMS fab. in the Electronics and Telecommunication Research Institute (ETRI). The processed MEMS sensor structure had a small membrane deflection less than $0.3;{mu}m$. This excellent deflection property can be attributed to the rigorous balancing of the stresses of individual layers. The efficiency of infrared absorption was more than 75% in the wavelength range $8;-;14;{mu}m$.