Anodically bonded silicon membranes for sealed and flush mounted microsensors

This paper reports a novel fabrication process which improves the thermal isolation of silicon membrane structures and allows the resulting devices to be flush mounted in a wall keeping the electronically active side of the membrane protected from the medium to be measured. Silicon wafers are bonded anodically with their processed front to Pyrex glass substrates. After the silicon is etched down to about 15 μm the bonded wafers are diced. The separated devices are glued onto ceramic substrates and electrically connected by wire bonds. The bonding pads of the device are located on the membrane and the wires are applied through holes in the glass substrate. Only the rear of the membrane is exposed to the medium while the electronically active side and the electrical connections are sealed. Important thermal and mechanical properties are derived from finite element models and measurements. Finally, we demonstrate this new approach by the fabrication and characterization of membrane-based thermal flow sensors