Microelectromechanical systems structures and self-aligned high aspect-ratio combined poly and single-crystal silicon fabrication processes for producing same

A process for fabricating a variable capacitor apparatus, comprising:

• providing a substrate;
• depositing and patterning a silicon nitride layer on the substrate that defines a first etch mask;
• etching trenches into the substrate using a second etch mask to define a first set of fingers of an actuator and a parallel-plate capacitor;
• forming a conformal silicon dioxide layer on the trenched substrate;
• depositing a conductive material in the trenches;
• doping and patterning the deposited conductive material;
• forming a silicon dioxide layer to cover the conductive material and provide a third etch mask;
• etching the silicon nitride layer to form self-aligned trenches in the substrate;
• etching the silicon substrate to form a second set of fingers of the actuator and the parallel-plate capacitor;
• and
• removing the silicon dioxide to release the conductive material from the substrate and form a first and second different size gaps between the conductive material and the substrate and fabricate the capacitor apparatus.