by Jacopo Iannacci, Ph.D.
Researcher, Fondazione Bruno Kessler
In developing packaging solutions for MEMS and RF MEMS devices, one must take several considerations into account. First, the main task of the package is to offer adequate protection to the devices from harmful factors, such as mechanical shocks as well as moisture, dust particles, and various contaminants. Depending on devices types and on the operational environment, the packaged part should be robust with respect to the harmful external factors. Depending on the application, different materials can be utilized such as silicon, glass, and ceramics. For each type of material, one needs to apply different techniques to set up the electrical signal interconnection scheme as well as the necessary space to accommodate the devices to be packaged.
Utilization of RF MEMS devices and networks implies their integration and interfacing with other functional blocks in a certain system. The packaging represents a delicate step also in the sense of making easier the final mounting of a chip including MEMS devices onto a board. Within the MEMS and device packaging community, the definitions of zero-level and first-level packaging have been introduced. Zero-level packaging refers to a packaging solution performed at the device level after the manufacture of MEMS or RF MEMS structures (i.e., encapsulation of naked devices). The electrical interconnect scheme is made available for the final mounting of the MEMS chip. This is based, for instance, on a ball grid array, a dual in-line package, a pin grid array, or a leadless chip carrier. On the other hand, in first-level packaging, a chip within which a packaged MEMS or RF MEMS has already been included is packaged/integrated in a more complex system, and so on, obtaining increasing architectural and functional complexity of the subsystem.
Among the various zero-level packaging solutions for MEMS and RF MEMS devices, a widely known technique is referred to as wafer-level packaging. This means that an entire device wafer has to be bonded to the package. The wafer-to-wafer bonding can be performed, for instance, by means of solder reflow, anodic bonding, or via the use of adhesive materials such as SU-8. After the bonding step, the capped wafer must be singulated, which means sawing it into dies of smaller size (e.g., 1 x 1 cm2). Finally, the single die must be made ready for final on-board mounting, for example, by wire-bonding it to a carrier chip provided with leads for a dual in-line package or solder balls for flip-chip mounting. In other words, a zero-level solution is an intermediate packaging step which requires additional steps to get a chip ready for standard surface-mount technologies, such as the ones mentioned above. This is necessary when dealing with RF MEMS devices as they need to be protected immediately after fabrication (e.g., during wafer handling and singulation). Another solution, called thin film capping, involves fabricating a package at zero level. In this case, the package is not based on a second substrate that has to be processed first and then wafer-bonded to the device wafer, but is processed directly onto the MEMS or RF MEMS device to be protected, by means of additional fabrication steps performed after the MEMS device processing (post-processing).
For the more advanced integration of RF MEMS devices, it is worth investigating additional functionalities that the package might provide. One of the most interesting ideas is the possibility to interface CMOS circuitry with a passive RF MEMS part directly on-chip by exploiting the cap to provide a proper housing for the CMOS chip, along with an interconnect scheme for interfacing the signals. This allows one to get a complete functional block (e.g., an oscillator based on a MEMS resonator and a CMOS sustaining circuitry) packaged and ready for surface-mount technologies. Because the RF MEMS and CMOS parts are obtained via different incompatible technologies, this solution is called hybrid packaging.
Another important characteristic can be enabled by the package. This is the hermeticity and the vacuum sealing which are possible depending on the particular solution chosen for the bonding of the capping part to the MEMS device. It has been demonstrated that a MEMS lateral comb-drive resonator exhibit Q factors of 25-30 when operated in air. The Q factor for the same device rises to about 50,000 when it is operated in a vacuum. Typically, it is very difficult to maintain the vacuum condition in the packaged cavity over a long period of time, as leakages through the sealing may occur as outgassing of the materials within the chamber (e.g., the sealing material or the MEMS device itself) may take place. This leads to a drift (i.e. degradation) over time of the performance of the capped MEMS devices. An effective solution leading to a drastic reduction of this problem is offered by the use of so-called getters. Such materials exhibit very selective absorption properties with respect to certain species which might jeopardize the vacuum condition in the package. Getter materials are usually arranged in thin films, so they can be easily accommodated in the cavity in which the vacuum condition must be enhanced and maintained. They are obtained by sintering different materials to achieve the desired sensitivity to the gaseous species that must be trapped. The effectiveness of getters has been demonstrated, proving that the Q factor of a MEMS resonator operated in vacuum conditions is more stable over time when a getter layer is added within the sealed cavity hosting the device.
Dr. Jacopo Iannacci is Researcher in MEMS technology at the Fondazione Bruno Kessler (FBK) in Trento, Italy, where he focuses on compact modeling, design, optimization, integration, packaging and testing for reliability of MEMS and RF MEMS devices and networks for sensors/actuators, energy harvesting and telecommunication systems. He received the MSc Degree in Electronic Engineering (2003) and the Ph.D. in Information Technology (2007) from the ARCES Research Center at the University of Bologna, Italy, and worked as Visiting Researcher (2005-2006) at the DIMES Technology Center of the Technical University of Delft, the Netherlands, on development of packaging and integration solutions for RF MEMS devices. Dr. Iannacci has authored and co-authored numerous scientific contributions for international journals and conference proceedings, as well as books and several book chapters in the field of MEMS and RF MEMS technology.
Reprinted with permission from "Practical Guide to RF MEMS", © 2013 Wiley-VCH Verlag GmbH & Co. KGaA.