Improved Thermal Matching of Materials for Hermetic Optical Windows
- Friday, 01 July 2005
A wider range of glass materials can be bonded when custom alloys are used to create the frame.
A family of iron-nickel and ironnickel- cobalt alloys made using metal injection technology (MIT) has been designed to match the coefficient of thermal expansion (CTE) of select optical glass materials more closely than commonly available wrought alloys of similar composition. Using these glass and metal combinations, custom windows can be developed with improved optical properties, greater hermeticity levels, and lower residual stress.
Window assemblies of this type can be used to protect detectors, cameras, and imaging semiconductors (e.g., CCD or CMOS) from contamination, such as hydrocarbons or water, extending the life of such devices. A common approach to providing a hermetic window is to create a bond between a glass window and a metallic frame. The table above shows a comparison of methods of creating this bond. The metallic frame can be subsequently bonded to the device package by welding, soldering, or adhesives (see Figure 2).
MIT CTE can be modified to match the CTE of the optical glass to the nearest 0.1 ppm. MIT alloys are produced by admixing elemental metal powders in small batches (relative to wrought alloys). The MIT alloy composition can be modified to the 0.1 percent for any given element in the alloy.
After the metal powders are produced, they are combined with the binders into a feedstock. The binders allow the metal particles to flow during the injection molding process. This feedstock is injection molded into a green part. Then the part is thermally processed to remove the binders. Following debinding, the parts are sintered, which transforms the particles into a homogenous hermetic microstructure with no open porosity and less than 2 percent closed porosity.
This approach to produce windows was tested on two glasses — Corning 7056 and Ohara S-BAL35. A custom alloy was designed to match the CTE of each glass, and MIT frames were produced from each alloy. The glass was then sealed into the MIT frame to produce the optical window. After sealing, the windows were ground and polished to attain flatness and finish. This was followed by testing for hermeticity and residual stress.
The helium leak rate of the windows was measured to be less than 2.0 × 10-10 atm cc/s, or two orders of magnitude better than the hermeticity standard (1 × 10-8 atm cc/s) for the industry. Polarization techniques used to evaluate the stress birefringence showed little or no residual stresses apparent in the optical window that used the MIT frame and the oxide-free glass-to-metal sealing process.
This work was done by Doug McCarron of Tekna Seal LLC and Mitch Gross of FloMet LLC.
The information contained within this document has been disclosed in a pending patent filed by Tekna Seal LLC with the U.S. Patent Office.