Method and structure for CMOS-MEMS thin film encapsulation

A method of forming a micro-electromechanical system (MEMS) device, the method comprising:

• forming an electrical component in a substrate;
• forming an insulating layer over the substrate;
• forming, in a MEMS device region of the insulating layer, a dummy interconnect structure in an upper portion of the insulating layer, wherein the dummy interconnect structure comprises a first plurality of conductive vias and a first plurality of metal features interleaved and connected with the first plurality of conductive vias, wherein an uppermost layer of conductive features of the dummy interconnect structure is exposed at an upper surface of the insulating layer distal from the substrate;
• forming, in an interconnect region of the insulating layer, an interconnect structure in the insulating layer, wherein the interconnect structure is electrically coupled to the electrical component, wherein the interconnect structure comprises a second plurality of conductive vias and a second plurality of metal features interleaved and connected with the second plurality of conductive vias, wherein an uppermost layer of conductive features of the interconnect structure extends below the upper surface of the insulating layer;
• after forming the dummy interconnect structure and the interconnect structure, forming a metal layer over the upper surface of the insulating layer, wherein the metal layer contacts the uppermost layer of conductive features of the dummy interconnect structure, wherein the metal layer is separated from the uppermost layer of conductive features of the interconnect structure by the insulating layer;
• after forming the metal layer, performing a wet etching process, wherein the wet etching process removes the metal layer and the dummy interconnect structure, wherein after the wet etching process, void regions are formed in the insulating layer at spaces left by the removal of the dummy interconnect structure;
• and
• forming sealing structures over the upper surface of the insulating layer to seal the void regions.

A micro-electromechanical system (MEMS) device comprising:

• a substrate;
• an insulating layer over the substrate, wherein the insulating layer comprises first layers and second layers interleaved with the first layers;
• a void region in the insulating layer, wherein the void region comprises first set of openings in the first layer and second set of openings in the second layers, wherein the first set of openings are connected with the second set of openings;
• a seal structure over the insulating layer, wherein the seal structure seals the void region and separates the void region from ambient air;
• an active membrane embedded in a first portion of the insulating layer, wherein a lower surface of the active membrane facing the substrate is covered by the first portion of the insulating layer;
• and
• a reference membrane embedded in a second portion of the insulating layer under the first portion of the insulating layer, wherein an upper surface of the reference membrane distal from the substrate is covered by the second portion of the insulating layer, wherein the void region extends between, and separates, the active membrane and the reference membrane.