MEMS devices and methods for forming same

A method for forming a microelectromechanical system (MEMS) device, the method comprising:

• forming a MEMS wafer comprising:
• forming a first dielectric layer on a first side of a first substrate;
• forming a first polysilicon layer over the first dielectric layer;
• forming a bonding layer over the first polysilicon layer;
• and
• exposing a first portion of the first polysilicon layer by removing a portion of the bonding layer;
• bonding a carrier wafer to the MEMS wafer using the bonding layer, the carrier wafer having an opening aligned with the first portion of the first polysilicon layer, the opening and the removed portion of the bonding layer forming a first cavity;
• patterning the first substrate, the patterning exposing portion of the first dielectric layer;
• forming a second dielectric layer on a second side of the first substrate, the second side being opposite the first side;
• forming a second polysilicon layer over the second dielectric layer;
• patterning the second polysilicon layer to expose portions of the second dielectric layer;
• removing portions of the first and second dielectric layers to form a second cavity;
• forming a first plurality of conductive bonds on the second polysilicon layer, at least one of the first plurality of conductive bonds sealing the second cavity;
• forming a second plurality of conductive bonds to a cap wafer;
• bonding the cap wafer to the MEMS wafer by bonding the second plurality of conductive bonds to the first plurality of conductive bonds, the bonding forming a third cavity;
• and
• thinning a backside of the carrier wafer, the thinning exposing a portion of the opening in the carrier wafer.

A method comprising:

• forming a microelectromechanical system (MEMS) wafer having a first cavity and a second cavity, the first cavity having a first pressure and the second cavity having a second pressure;
• bonding a carrier wafer to a first side of the MEMS wafer, the bonding forming a third cavity, the third cavity having a third pressure, the third pressure being greater than the first pressure and the second pressure;
• and
• bonding a cap wafer to a second side of the MEMS wafer, the second side being opposite the first side, the bonding forming a fourth cavity, the fourth cavity having a fourth pressure, the fourth pressure being greater than the first pressure and the second pressure and less than the third pressure.