Manufacture of MEMS structures in sealed cavity using dry-release MEMS device encapsulation
View Patent ↗The disclosed fabrication methodology addresses the problem of creating low-cost micro-electro-mechanical devices and systems, and, in particular, addresses the problem of delicate microstructures being damaged by the surface tension created as a wet etchant evaporates. This disclosure demonstrates a method for employing a dry plasma etch process to release encapsulated microelectromechanical components.
1. A method of fabricating a microstructure in a sealed cavity comprising the steps of:
providing a substrate having a substantially planar support surface;
depositing a first layer of sacrificial material over said planar support surface;
depositing an etchable layer of structural material over said first layer of sacrificial material;
forming a microstructure on said support surface by etching said layer of structural material, said microstructure contacting said substrate at least at an anchor point;
depositing a second layer of sacrificial material over said microstructure;
depositing a cap layer over said second layer of sacrificial material, said cap layer extending from points on said support surface, whereby said cap layer and said support surface define a capsule about an interior region containing said microstructure and said first and second sacrificial layers;
forming one or more holes in said cap layer;
introducing a dry plasma etchant into said interior region through said one or more holes, wherein said sacrificial material is chosen to have a high etch rate differential with respect to said structural material, so that said dry plasma etchant removes said first and second sacrificial layers while leaving said microstructure and said substrate substantially intact, thereby releasing said microstructure as a movable structure secured at said anchor point to said substrate; and
sealing said one or more holes in said cap layer with a seal layer, thereby forming a sealed cavity that encapsulates said movable microstructure, said sealed cavity being defined by said seal layer and said planar support surface,
wherein said etchant is oxygen plasma, said sacrificial material is photoresist and wherein said structural material is aluminum.
2. The method of claim 1 wherein said substrate is a silicon wafer having a layer of silicon nitride deposited thereon.
3. The method of claim 1 wherein said etchant has a high etch rate with respect to said sacrificial material and a low etch rate with respect to said structural material and with respect to the materials forming said substrate and said cap layer.
4. The method of claim 1 wherein said structural material is resistant to said etchant.
5. The method of claim 1 , wherein the step of introducing said dry plasma etchant into said interior region through said one or more holes is performed using a barrel etcher.
6. The method of claim 1 , wherein said one or more holes are etched into said cap layer so as to establish communication with said first and second layers of sacrificial material.