Silicon Encapsulation

As a wafer packaging approach, sputtered silicon encapsulation is viable for MEMS devices requiring isolation. Devices that do not require interaction with the sur­roundings to function, such as accelerometers, RF switches, inductors, and filters can be fully encapsulated at the wafer level after fabrication. In one example, a MEMSTech 50 g capacitive accelerometer was used to demonstrate a sputtered encapsulation technique. Encapsulation with a very uniform surface profile was achieved using spin-on glass (SOG) as a sacrificial layer, SU-8 as a base layer, RF sputtered silicon as the main structural layer, eutectic gold-silicon as a seal layer, and liquid crystal polymer (LCP) as the outer encapsulant layer. SEM inspection and capacitance tests indicated that the movable elements were released after encap­sulation [12]. SU-8 is a commonly used epoxy-based negative photoresist. It is a very viscous polymer that can be spun or spread over a thickness ranging from 0.5 micrometer up to 2 mm and still be processed with standard contact lithography. SU-8 was originally developed as a photoresist for the microelectronics industry to provide a high resolution mask for fabrication of semiconductor devices. SU-8 is frequently used in the fabrication of microfluidics and MEMS parts. It is also a biocompatible material and is often used in bio-MEMS. Other examples include the use of epitaxially deposited polysilicon as an encapsulation structure for piezoresis-tive accelerometers [13]; plasma enhanced chemical vapor deposition of polysilicon [14]; and the use of permeable polysilicon to fabricate a vacuum shell over movable elements of a MEMS resonator [15]; and the use of a silicon cap bonded with glass frit over an accelerometer as shown in the case studies in Section 12.11.1. With such methods, use of an outer encapsulant should be considered to strengthen the initial encapsulation structure to withstand the subsequent transfer molding process, and care should be taken to assure that using any additional glob top does not further induce stress, especially during the hardening and curing steps.

Date: 2015-02-28; view: 1072