Embedded permeable polysilicon layer in mems device for multiple cavity pressure control

A method of forming a package including a first instrument positioned within a first cavity maintained at a first pressure and a second instrument positioned within a second cavity maintained at a second pressure different than the first pressure, the method comprising:

• forming an oxide layer on a substrate;
• depositing a permeable layer on the oxide layer, the permeable layer being gas permeable but vapor etch resistant;
• depositing an insulator layer on the permeable layer and exposed portions of the oxide layer;
• depositing a barrier layer on the insulator layer;
• depositing a sacrificial layer over exposed portions of the insulator layer;
• etching the sacrificial layer and insulator layer at portions thereof overlying the permeable layer so as to expose portions of the permeable layer;
• forming a structural layer over the sacrificial layer and exposed portions of the permeable layer;
• forming the first and second instruments in the structural layer;
• performing a first vapor etching through open portions of the structural layer to remove exposed portions of the sacrificial layer, the first vapor etching also passing through the permeable layer at the exposed portions thereof to thereby remove portions of the oxide layer underlying the permeable layer to expose portions of the substrate;
• bonding a cap wafer to the structural layer at bonding points to thereby:
• form the first cavity as being sealed by the cap wafer, portions of the barrier layer underlying the structural layer adjacent the first instrument, and the bonding points;
• and
• form the second cavity as being sealed by the cap wafer, portions of the barrier layer, underlying the structural layer adjacent the second instrument, and the exposed portions of the substrate;
• wherein the bonding of the cap wafer to the structural layer is performed at a first pressure, thereby setting pressure within the first and second cavities to the first pressure;
• forming a chimney opening extending through an entire thickness of the substrate to reach the permeable layer;
• performing a second vapor etching through the chimney opening and permeable layer to remove portions of the oxide layer under the permeable layer and portions of the insulator layer stacked on the permeable layer, thereby creating a fluid connection extending from the chimney into the second cavity;
• setting the pressure inside the second cavity to a second pressure different than the first pressure;
• and
• forming a plug in the chimney opening to thereby seal the second cavity.