The present invention relates to micron-scale displacement measurement devices. A first embodiment is a device that includes a substrate and a rigid structure suspended above the substrate to form a backside cavity. Formed in the rigid structure is a reflective diffraction grating positioned to reflect a first portion of an incident light and transmit a second portion of the incident light such that the second portion of the incident light is diffracted. The device also includes a membrane positioned a distance d above the reflective diffraction grating and at least a first photo-detector for receiving interference patterns produced from the first portion of the incident light reflected from the diffraction grating and the second portion of the incident light reflected from the membrane.