SCHOTT solutions no. 1/2010 > Electronic Packaging

HermeS® wafers are available in 4-inch and 6-inch diameters. As of 2010, SCHOTT is planning to offer 8-inch wafers too. Photo: SCHOTT/B. Lammel

Perfectly Packaged

NEC SCHOTT Components Corporation has developed glass substrates with hermetically sealed feedthroughs for MEMS.

Bernd Müller

Many of today’s modern technologies would be unthinkable without micro-electro-mechanical systems (MEMS). These are 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 benefit from MEMS, however, the difficulties involved in packaging these tiny helpers have prevented them from being put to widespread use in mass markets. For instance, sensitive electronic and mechanical components must be hermetically sealed and protected against environmental influences and yet electric signals and power supply connections need to be able to enter and exit them. A new housing concept called ”HermeS®” (derived from ”Hermetic Substrate”) meets both of these demands. It was developed by NEC SCHOTT Components Corporation (NSC), a joint venture of the Electronic Packaging business unit of SCHOTT AG and the Japanese electronics conglomerate NEC.

Conventional housings rely on metal caps and sometimes several sealing rings through which the electric wires from the MEMS are fed. However, this poses the risk of leaks and can lower the lifespan. With HermeS®, on the other hand, the conductor feedthroughs are melted into a glass wafer and joined with the contacts of the MEMS on the silicon wafer by soldering or applying pressure. Thousands of glass-to-metal feedthroughs can be manufactured in this manner 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.

The feedthroughs of HermeS® are directly melted into the glass and do not go through sealing rings. Hence, the MEMS in HermeS® are hermetically sealed much more tightly than with any other approach. The experience that SCHOTT has accu­mulated since 1941, in the manufacturing of glass-to-metal feedthroughs, such as packages for optoelectronics and airbag igniters, clearly indicates this. Tests using helium have revealed that the leakage rate of HermeS® was less than 10-9 mbar · m3/sec. Besides, HermeS® also achieves top scores in the area of electrical insulation and as a low dielectric constant.
Micro-electro-mechanical systems (MEMS) need to be protected from environmental influences and yet still be able to exchange electrical signals – demands that HermeS® meets. These are actually glass wafers with integrated conductor feedthroughs that are manufactured using simple processes and connected to thousands of MEMS to seal them tightly for decades. HermeS® was developed by the NEC SCHOTT Components Corporation.
Borofloat® 33, a glass that displays 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, inside the engine compartment of an automobile, for instance, no cracks occur between the glass substrate and the MEMS. When it comes to the metal that is used for the feedthroughs, tungsten is the material of choice because it is very stiff and can be manufactured to have exact dimensions and be extremely thin. In addition, it suits the thermal expansion of Borofloat® 33 just perfectly and maintains the pressure that is exerted on the wire after it has been melted in.
The most important quality criterion for the glass wafer is the exactness with which the feedthroughs can be placed. With HermeS®, this is ± 20 micrometers and is measured optically. This allows for substrates with several 10,000s of metal feedthroughs.

HermeS® wafers are available in 4-inch and 6-inch sizes. As of 2010, SCHOTT is planning to offering 8-inch wafers too. With even smaller diameters and pitch distances, these wafers are most appropriate for high volume industrial applications, especially in the field of sensors and optical devices. This means that customers are able to combine a high level of integration and miniaturization of the MEMS packaging designs with superior electrical properties and hermeticity.

Yet another advantage of packaging components in glass is that, unlike metal, it is transparent and visual inspections can be performed on the MEMS and adjustments made using laser light, even after it has been packaged.
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