Hermeticity Evaluation of MEMS
SEMI has announced the latest in the series of MEMS standards, MS8— Guide to Evaluating Hermeticity of MEMS Packages, available to users on-line immediately and on CD in early spring. Ron Foster of axept, LLC chaired the committee that drafted and wrote the new standard. “The hermeticity guide is the first to address MEMS packaging,” according to Mr. Foster, “and we anticipate further efforts to standardize the methods used to evaluate MEMS packaging and the types of packages used in an effort to make MEMS more economical for manufacturers, users and consumers.”
MEMS (microelectromechanical systems) are incorporated into products that we already use on a daily basis such as inkjet printer cartridges, automobile airbag deployment and tire pressure sensors, digital light processor projectors and TVs, and game controllers, such as Wii™. MEMS will become even more prevalent in the coming years, enabled by uniform industry standards established to cover non-IP areas. For example, the new guide provides standardization on terminology, categories of use, methods for sealing, seal integrity evaluation and the detection and measurement of hermetic leakage, especially for small volume packages.
One of the goals of the new MS8 guide was to gather and summarize all the hermeticity aspects of MEMS. As more data and MEMS-specific details are gathered, sections of MS8 will be removed, revised, or expanded upon— leading to publication of a specification or test method standard.
One topic that the committee is now looking at more closely involves the permeability of sealing materials, which has a direct effect in determining if a seal is “accepted” or “rejected.” As an example, assuming you have inspected the seal for integrity, the bond quality is good at both interfaces and there are no direct leak paths between the external and internal environments, the next concern would be the actual permeation of gases through the seal. Permeation is dependent on both the seal geometries and material properties. For example, the minimum allowable seal width for a given lifetime expectancy of the enclosed device depends on both the seal thickness and the permeability of the material itself. Although a primary concern for hermeticity, there is little published data on permeability of seal materials used for MEMS, leading the committee to prioritize this area for discussion.
Another issue of concern is the validity of the current test methods utilized for leak testing and residual gas analysis. Current standardized test methods were developed for packages with larger cavity volumes and have proven to be not reliable or not applicable for smaller volume packages, typically
Ideally, as the production volume of commercially available hermetic-packaged MEMS increases, a method that supports high-throughput non-destructive hermetic QC of the MEMS devices will emerge. Perhaps one of the methods discussed in the MS8 guide will become a self-contained standard, but further investigation and input from the MEMS industry is required to develop consensus.
SEMI has published seven other standards related to MEMS and is looking forward to publishing more, as needed by the industry and with its help. The utility of SEMI Standards derives from the insight of the people involved with MEMS on many levels.
About the SEMI MEMS Technical Committee: Formed in 2003, the charter of the MEMS Technical Committee is to develop standards for MEMS devices that cannot be handled by existing technical committees. Current topics include Wafer Bonding Alignment Targets; Step-Height Measurements of Thin, Reflecting Films using an Optical Interferometer; and Ultra High Purity Microscale Fluidic Systems for Use in Scalable Process Environments. The North American MEMS TC will be meeting March 30 in during the 2009 NA Spring Standards Meetings in San Jose, California.
For more information on MEMS Committee activities, please contact: