A Novel And Efficient Packaging Technology For RF-MEMS Devices

Abstract

RF MEMS technology shows great promise for wireless communication and other RF applications. However, the packaging of RF MEMS devices becomes one of the most important issues in MEMS device fabrication and presents challenges due to the unique requirements. Special precautions are not only required to allow assembly processes after releasing the delicate structures but also to ensure the structures to operate in a stable atmosphere during its entire lifetime. These requirements come on top of the other wellknown requirements for cell phone components: cost, size and height. NXP Semiconductors has developed an efficient packaging technology for RF-MEMS devices in close cooperation with the Fraunhofer Institute in Itzehoe (ISiT). This paper presents a summary of several major aspects of our development activities in order to generate this new packaging technology.

MEMS packaging and reliability: An undividable couple

“…For instance, for the BCB chip capping [16] typically a curing step of several minutes at about 250°C is required. Similar temperature budgets are required for solder bonding, e.g., several minutes at around 300°C for the Au/Sn soldering [13] and at around 250°C for Cu/Sn diffusion soldering [15]. It is preferred to carry out the packaging at low temperatures (the closer to room temperature the better).…”

“…Chip capping as opposed to thin-film capping has the advantage of a large flexibility in processing temperatures and control of the cavity ambient (pressure and gas composition), but on the other hand leads to a rather large form factor and profile. The capping is either done as a die-to-wafer (D2W) capping [12], or as a wafer-to-wafer (W2W) capping [13,5]. The end result is in essence the same, but from the view of process complexity and cost, W2W is the preferred route for small caps (<2 mm 2 ), whereas D2W is preferred for larger cap sizes.…”

“…This for instance excludes using specific flux agents (as routinely used for conventional soldering) during the micro-soldering of a chip cap onto the MEMS substrate. For this reason special fluxless soldering processes have been developed [12][13][14][15].…”

Die fracture probability prediction and design guidelines for laminate-based over-molded packages

“…Obviously, MEMS devices with cavities are vulnerable to the packaging loads subjected to it, especially the molding pressure. One of the critical issues is the cracking of die and cap induced during the molding process [2,3].…”

“…In most situations, hermetic cavities are needed in the design to allow the movement of the moving parts. So far, packaging is the limiting factor in getting MEMS devices into low-cost commercial products [2]. To achieve that, MEMS packages should be fabricated using standard available or slightly modified assembly processes and tools.…”