Method for forming a capped micro-electro-mechanical system (MEMS) device

A method of forming a packaged micro-electro-mechanical systems (MEMS) device, wherein the MEMS device is located on a frontside of a device wafer, comprising:

• forming a nitride layer having compressive stress on a frontside of a cap wafer;
• attaching the frontside of the cap wafer to the frontside of the device wafer;
• forming a first aluminum layer having tensile stress on the backside of the cap wafer;
• patterning the first aluminum layer and the cap wafer to form an opening through the cap wafer;
• and
• applying a conductive layer to the backside of the cap wafer, including through the opening to the frontside of the device wafer.

A method of packaging a micro-electro-mechanical systems (MEMS) device, comprising:

• forming the MEMS device on a frontside of a device wafer;
• attaching a frontside of a cap wafer to the frontside of the device wafer;
• performing chemical mechanical polishing on the backside of the cap wafer;
• depositing a first layer of aluminum on the backside of the cap wafer, the first layer of aluminum having tensile stress;
• forming an opening in the first layer of aluminum and the cap wafer;
• and
• depositing a conductive layer on the first layer of aluminum and through the opening to contact the frontside of the device wafer.