Micromachined capacitor structure and method for making

A method for forming travel limit stops in a micromachined capacitor structure comprising the steps of:

• providing a substrate having a top surface;
• covering the top surface with a first silicon nitride layer;
• forming a first polysilicon layer on the first silicon nitride layer,
• patterning the first polysilicon layer to form a first anchor and a lower capacitor plate, wherein portions of the first silicon nitride layer are exposed by the patterning step;
• covering the first patterned polysilicon and the exposed portions of the first silicon nitride with a first sacrificial layer comprising silicon oxide;
• covering the first sacrificial layer with a second silicon nitride layer;
• covering the second silicon nitride layer with a second sacrificial layer comprising silicon oxide;
• etching a first hole in the second sacrificial layer, the second silicon nitride, and the first sacrificial layer to expose the first anchor;
• etching a second hole in the second sacrificial layer over the lower capacitor plate;
• forming a second polysilicon layer that fills the first and second holes and covers the second sacrificial layer;
• patterning the second polysilicon layer to form a middle capacitor plate that is mechanically attached to the anchor, has a portion spaced from the lower capacitor plate by the thickness of the first and second sacrificial layers together with the second silicon nitride layer, and a travel stop portion which is spaced from the lower capacitor plate by the thickness of the first sacrificial layer and the second silicon nitride layer;
• and
• performing a sacrificial etch of the first and second sacrificial layers using an isotropic etchant that etches the silicon oxide faster than silicon nitride, wherein only a portion of the second silicon nitride layer which is covered by the travel stop portion remains after the sacrificial etching.