New glass-to-metal seal is close to perfect

A process that takes place at temperatures to 1,000°C hermetically seals optical glass to an alloy frame. A key point: the glass and alloy have the same coefficient of thermal expansion. The alloy, specially formulated by ceramic engineers and metallurgists, is then manufactured using Metal Injection Technology (MIT) at FloMet LLC, DeLand, Fla. Leak tests on the glass-to-metal seal come in at less than 2 X 10 ¹⁰ atm-cc/sec of helium. This is significantly lower than conventional sealing methods.

The seal goes on optical windows for protecting cameras, photodetectors, and imaging semiconductors such as CCDs, CMOS, and optical MEMS devices used in highdefinition televisions. The oxide-free process, developed with Tekna Seal LLC, Minneapolis, solves quality problems stemming from thermal-expansion mismatches in older methods. These problem manifested themselves as compromises between seal reliability and image quality. The seal also better protects optical equipment from contamination such as hydrocarbons and moisture, thereby extending its useful life.

"Until now, such windows were made by first selecting an alloy then finding a glass that matched its coefficient of thermal expansion (CTE) as closely as possible," says FloMet manager of materials technology Mitchell Gross. "The optical properties of the glass were secondary to matching the CTE of the alloy. A thermal-expansion mismatch between the alloy frame and glass can weaken the bond while placing stress on the glass. Sufficiently high stress distorts the transmitted image," he adds.

Other pluses include sealing a wider range of optical glasses than would be possible with conventional methods, and a significant cost advantage. Also, the MIT method allows making material in smaller batches than those used to make conventional wrought alloys.