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Hermetic and Additive Free Glass Bonding Technique for Medical Applications
Keywords: hermetic, additive, free glass bonding
For the past 40 years medical implants have been encapsulated inside titanium. However, using glass as a package material would offer several benefits. The RF transparency of glass enables recharging, data transfer and reprogramming of implants. Transparency to visible light is an advantage in wide range of optical applications: active implants can also have optical components, such as optical sensors or cameras. Glass is also biocompatible. Hermeticity is one of the key factors when reliability is a concern. Military standards, such as MIL-STD-883 and MIL-STD- 750, are commonly used as a guideline for defining test methods, leak rate limits and finally evaluating package hermeticity. Poor hermeticity exposes the device to environmental stresses, such as moisture. Together with temperature it is one of the main ageing or damaging elements in microelectronic devices. In addition, hermeticity may be required to maintain a vacuum inside the cavity. Novel bonding technique enables reliable glass encapsulation offering truly hermetic sealing. Studies have proved that excellent hermeticity can be achieved with this technique: 6,0x10^-12 atmcm3/s Kr-85 (1,1x10^-11 atmcm3/s air). The limit given in military standard for the equivalent volume of the cavity is 5x10^-8 atmcm3/s air. Achieved hermeticity values are the best that can be measured with the available test methods. Robustness of the novel laser bonding technique has been verified in several studies including: thermal, moisture and mechanical stress tests. RGA results for the moisture stressed devices show a remarkable improvement compared to epoxy sealing methods: moisture level of laser bonded devices was 0,6-3,3% when in glued packages moisture of 10-30% was typically detected. This is a meaningful observation since moisture is related to many package failures. Bond strength have been measured with shear strength method: values of 20 MPa have been achieved, however even then the glass material itself fractures before the bonding seam. Using novel glass bonding technology, no additive materials, intermediate layers or pre-handling of the surface is needed. The bonding is implemented at the material interface leaving the surfaces untouched. Additive materials in the intermediate layer can cause outgassing, contamination and problems with hermeticity. In case of optical devices, moisture as well poor gap control can disturb the optical performance. Since the bonding method is based on a laser heat affected zone (HAZ) is extremely small: even organic materials can be successfully encapsulated. Also, bowing or bending of the material can be avoided and the use of ultra-thin materials is possible: all this supports the aim of miniaturization of the packages. In a vacuum applications high electric fields and increase of the cavity pressure during the bonding can be avoided when using Primoceler’s technique. As a conclusion, presented laser bonding technique is a novel solution for glass to glass and glass to silicon sealing. Manufacturing process is repeatable and having high yield which enables also a high-volume wafer-level mass production. Suitable applications are in the areas where high reliability is a necessity such as in: aerospace, micro-optics, optoelectronics, active medical implants, flow cells, and special industrial measuring instruments in harsh environments.
Heidi Lundén, Specialist, Hermetic Glass Packaging
Primoceler Oy

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