We have stacked subwavelength gratings (SWGs) on a single substrate to create a compact, integrated circular polarization filter. The SWGs consist of a wire grid polarizer and a broadband form-birefringent quarter-wave plate (QWP). Rigorous coupled-wave analysis was used to design the QWP for operation over the 3.5-5.0-mum wavelength range. The fabricated silicon broadband QWP exhibited a phase retardance of 82-97 degrees across this wavelength range. Two stacked structures are presented, each with a different wire grid polarizer fabricated on an organic planarization layer (SU-8) that is deposited on a QWP grating. Transmittance measurements of the first structure when illuminated with nominally right- and left-circularly polarized light indicate a circular extinction ratio (CER) limited by the low linear extinction ratio of the polarizer. Use of a wire grid polarizer with a higher extinct ratio led to a stacked SWG structure that demonstrated CERs of 10-45 across the 3.5-5.0-mum wavelength range.