This invention relates to the sealing of microelectromechanical systems (MEMS) devices in an enclosure and the method of manufacture of the sealed enclosure. In particular, this invention relates to the formation of a hermetic seal between a fabrication wafer supporting the MEMS devices, and a lid wafer.
Microelectromechanical systems (MEMS) are devices often having moveable components which are manufactured using lithographic fabrication processes developed for producing semiconductor electronic devices. Because the manufacturing processes are lithographic, MEMS devices may be batch fabricated in very small sizes. MEMS techniques have been used to manufacture a wide variety of sensors and actuators, such as accelerometers and electrostatic cantilevers.
MEMS techniques have also been used to manufacture electrical relays or switches of small size, generally using an electrostatic actuation means to activate the switch. MEMS devices often make use of silicon-on-insulator (SOI) device wafers, which are a relatively thick silicon “handle” wafer with a thin silicon dioxide insulating layer, followed by a relatively thin silicon “device” layer. In the MEMS switches, a thin cantilevered beam of silicon is etched into the silicon device layer, and a cavity is created adjacent to the cantilevered beam, typically by etching the thin silicon dioxide layer to allow for the electrostatic deflection of the beam. Electrodes provided above or below the beam may provide the voltage potential which produces the attractive (or repulsive) force to the cantilevered beam, causing it to deflect within the cavity.
Because the MEMS devices often have moveable components, such as the cantilevered beam, they typically require protection of the moveable portions by sealing the devices in a protective cap or lid wafer, to form a device cavity. The lid wafer may be secured to the device wafer by some adhesive means, such as a low outgassing epoxy. FIG. 1 shows an embodiment of an exemplary epoxy bond in a MEMS assembly 1. To achieve the epoxy bond, a layer of epoxy 20 is deposited on a cap or lid wafer 10, or on the fabrication wafer 30, around the perimeter of the MEMS device 34. The assembly 1 is then heated or the epoxy otherwise cured with wafer 10 pressed against the fabrication wafer 30, until a bond is formed between the cap or lid wafer 10 and the fabrication wafer 30. The bond forms a device cavity 40 which surrounds the MEMS device 34. The assembly 1 may then be diced to separate the individual MEMS devices 34.