IMAPS Workshop Report

01/01/2002

Packaging of MEMS and related systems

BY JULIA GOLDSTEIN

IMAPS held its third Advanced Technology Workshop on "Packaging of MEMS and Related Micro Integrated Nano Systems" in early November in Scotts Valley, Calif. The workshop was small, with about 30 attendees representing companies making MEMS devices and those involved in microelectronic packaging.

University Research

The first session was devoted to government-sponsored research linked to universities. Auburn University has an extensive MEMS program, and research associate Robert Dean presented two projects, one on tools for vacuum pickup of MEMS and MOEMS devices, and the other a laminate-based MEMS accelerometer. Dean conceded that laminate technology is not appropriate for all MEMS devices, but in applications in which larger feature sizes can be used it provides a simpler and less expensive alternative to MEMS on silicon. The technique he described uses a prepreg layer that doesn't flow during lamination, allowing computer-controlled laser cutting to provide precision-cut inner layers of the laminate. The 1 x 1-inch accelerometer consists of four capacitors separated by springs that deform in response to acceleration, thereby changing the measured capacitance. Three designs have been made, each to measure a different range of acceleration. Initial testing has used FR4 laminates, but alternatives will be considered in the future for longer shelf life and stable performance.

Michael Krantz (Morgan Research Group) described the progress of a government/industry/university effort working to develop MEMS sensors to monitor environmental conditions during storage of missiles. They are combining packaged MEMS devices and bare MEMS die onto laminate printed circuit boards (PCBs). Krantz discussed the difficulty of finding commercial off-the-shelf (COTS) sensors that meet all the required specifications. Humidity sensors are particularly challenging to package, because the metallization needs to be protected from the environment while the sensor must be exposed to humidity in order to detect it, ruling out hermetic packaging. A humidity sensor die with gold stud bumps was flip chip attached onto a Sn/Ag-coated FR4 or ceramic carrier, using Auburn University's vacuum pickup tool for handling. A Au/Sn eutectic bond is formed between the die and the carrier, using a thermocompression bonder. After underfilling, the packaged device can be attached to a PCB using standard surface mount processes.

Arum Han (Georgia Institute of Technology) gave a talk on microfluidic systems for biochemical analysis. He described methods of producing fluidic channels, patterning conductors, and manufacturing ports and microvalves in a four-layer, biocompatible plastic system.

Developments Across the Globe

Larry Felton (Analog Devices) described the latest developments in accelerometers used in airbags. In the drive to reduce package size and cost, Analog Devices has manufactured an integrated MEMS accelerometer, with the sensor and electronics on the same chip. It is moving to a plastic package, requiring sensor dice to be covered with silicon caps at the wafer level, diced and then packaged.

There was international representation at the workshop, with Dinesh Sood (Royal Melbourne Institute of Technology) describing MEMS research in Australia in a talk titled "MEMS in the 'Outback' and Their Packaging" and Cian Ó Mathúna (National Microelectronics Research Centre [NMRC], University College Cork) presenting work from Ireland. Australian efforts include a micromachine development project working on ultra-low friction suspension devices using magnets and miniature magnetic bearings. The university is also working with Cochlear, an Australian company that makes cochlear implants. A silicone rubber-coated metal wire goes inside the ear, with an external microphone. Using MEMS, researchers hope to produce a microphone that can be fitted inside the ear.

NMRC's focus is on "intelligent environments," including wearable electronics. Ó Mathúna described a 3-D micromodule containing accelerometers, sensors and pressure transducers, and mentioned the trend toward the "penny processor." In response to a comment that MEMS sensors could not be made inexpensively, Ó Mathúna commented that if the sensor costs $2, then the system can be made for $2.01.

Ajay Malshe (University of Arkansas) gave a talk entitled "Micro and Nano Technologies for Integrated Systems: Packaging Challenges and Trends." He said that nanotechnology is needed to integrate bio, micro and informational systems.

Malshe compared MEMS (micromachining processes using selective etching) and nanotechnology (nanometerscale processes manipulating individual atoms). He also discussed the need for application-specific assembly and packaging requirements, questioning whether use of a COTS approach is appropriate. He asked whether anyone in the audience was aware of industry roadmaps for MEMS. The response was that roadmaps did not yet exist, but that they should. Any volunteers?

AP