We discuss the design, fabrication and optical performance of a broadband form-birefringent quarter-wave plate for the 3.5 to 5 µm wavelength region. Rigorous coupled wave analysis (RCWA) was used to design the requisite subwavelength grating for silicon substrates in ambient air. Fabricated samples yield a measured phase retardation of 89° to 102° over the desired wavelength range.