Integrated thin-film diaphragm; backside etch

A process for fabricating a thin film microsensor for air flow comprising the steps of:

• providing a single crystal (100) silicon wafer having a front planar surface, a backside surface and having a <110>
• direction;
• depositing and delineating a rectangular area of a thin film layer of sacrificial selectively etchable material on the silicon front surface;
• depositing a thin film of silicon nitride on the selectively etchable material and the rest of the silicon front surface, the film being thicker than the thickness of the sacrificial material;
• depositing and delineating a thin film of electrically resistive material on the silicon nitride to form desired circuit elements such as detector resistors, circuit resistors and a heater resistor, said heater resistor and detector resistors being located over said sacrificial material area;
• depositing an additional thin film of silicon nitride over the resistive material and the previous silicon nitride;
• depositing a thin film of silicon nitride over the backside surface;
• opening a cut through the silicon nitride on the backside to expose an area of the backside silicon surface, the cut being rectangular and orthogonal to the <110>
• direction;
• introducing an anisotropic etch to the backside silicon surface at said nitride cut to anisotropically etch a first anisotropic pit through the silicon wafer until the layer of sacrificial selectively etchable material is reached;
• introducing selective etch through the first anisotropic pit to etch out all of the sacrificial selectively etchable material leaving a thin cavity in its place;
• and,
• introducing the anisotropic etch through the first pit and the thin cavity to anisotropically etch the silicon exposed by the removal of the sacrificial material and form a second anisotropic etch pit.