Disclosed herein is a process flow for forming a MEMS IMU including an accelerometer and a gyroscope each located in a separate sealed cavity maintained at a different pressure. Formation of the MEMS IMU includes the use of a first vHF release to etch a sacrificial layer underneath a structural layer containing the accelerometer and gyroscope and capping the device under formation to set both cavities at a first pressure. The floor of one of the cavities is formed to including a gas permeable layer. Formation further includes forming a chimney underneath the gas permeable layer and then performing a second vHF release to etch through the gas permeable layer and expose the cavity containing the gas permeable layer so that its pressure may be set to be different than that of the other cavity when the chimney is sealed.