07-29-2009, SCHOTT North America, Inc.
MEMS tightly sealed with SCHOTT HermeSTM
NEC SCHOTT Components Corporation is developing a new 8” high via density wafer that features hermetically sealed electric feedthroughs for micro-electro-mechanical systems and is intending to launch this product in 2010.July 29, 2009 (Singapore) - Micro-electro-mechanical systems or MEMS, for short, are finding their way into more and more applications. They are being used as acceleration, pressure and gyro sensors in automobiles, to switch on light in telecommunications networks with the help of tiny mirrors or to eject ink from printing heads. Many other applications could also benefit from MEMS; however the difficulties involved in packaging these tiny helpers have prevented them from being put to widespread use in mass markets. The problem is that sensitive electronic and mechanical components must be hermetically sealed and protected against environmental influences and yet, at the same time, electric signals need to be able to enter and exit them. SCHOTT HermeSTM, a new technology for manufacturing glass substrates that contain embedded metal feedthroughs that has been developed by NEC SCHOTT Components Corporation (NSC), part of SCHOTT’s Electronic Packaging business group and a global leader in glass-to-metal seals for electronic applications, is capable of meeting both of these demands.
Instead of relying on complicated sealing solutions in which the connecting wires run through a sealing ring and occasionally cause leaks, NSC is now using a completely new approach called ‘HermeS’ (derived from “hermetic seal”) in which the connector vias are baked into glass. These are then joined together with the contacts of the MEMS on the silicon wafer that is the same size using a soldering or pressure technique. In this way, glass-to-metal feedthroughs can be manufactured using a one-step process and then be connected with hundreds of MEMS during a second step – an ideal prerequisite for cost-effective mass production.
Due to the fact that the complete lengths of the metal feedthroughs used in ‘HermeS’ are melted inside glass and do not run through sealing rings that are prone to leaking, the MEMS in ‘HermeS’ are hermetically sealed much more tightly than with any other approach. During tests with helium, the leakage rate was less than 10-9 pa•m3/sec – this is true even over decades, as SCHOTT Electronic Packaging has been able to prove for over 50 years with other glass-to-metal feedthroughs. ‘HermeS’ also achieves peak values with respect to electric insulation, but also with its low dielectric constant. Borofloat 33, which has the same thermal expansion as silicon from room temperature to over 300 degrees Celsius, is used as the glass substrate. Even when subjected to severe heat, for instance inside the engine compartment of an automobile, no cracks occur between the glass substrate and the MEMS.
‘HermeS’ is currently available as 4-inch and 6-inch wafers. The metal vias made of tungsten can be 100 micrometers in thickness, but must be kept at least 250 micrometers apart from each other. Correspondingly, thicker vias require more space. The positioning accuracy is approximately ±50 micrometers. Theoretically, this allows for substrates with several 10,000s of metal feedthroughs.
Starting from 2010, NSC plans to offer 8-inch wafers as well. With even smaller via diameters and pitch distances, these wafers are most appropriate for high volume industrial applications especially in the field of sensors and optical devices. For the customers, it means that such wafers can enable a high level of integration and miniaturization of the MEMS packaging designs while maintaining the same superior hermeticity performance and electrical properties. In addition, as glass is transparent, ‘HermeS’ also offers entirely new possibilities with respect to quality control. For instance, even after it has been packaged, visual inspections can be performed on the MEMS or it can be adjusted using laser light. This wouldn’t be possible with metal housings.
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