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Wafer-level packaging for ultra-thin glasses using hermetic room temperature welding technology
Keywords: ultra-thin glass, hermetic, room temperature
The aim of the study was to develop a hermetic package using ultra-thin glass wafers. A novel glass welding technology, with a minimal heat load, was used to construct the encapsulations. Industry requirements of the miniaturization of the electronic components, for example in medical implants and consumer electronics and opto-electronics, challenge the conventional manufacturing technologies and package materials [1]. Low-cost glass interposers and packages have been research by GeorgiaTech at their PRC industry program. Glass packaging can offer even ten times affordable option than using silicon [2]. During the last few years, the advancements in the glass manufacturing technologies have enabled a cost effective production of the ultra-thin glass wafers and panels. Laborious glass grinding process from thick material to ultra-thin is no longer necessary since the fusion forming process can be used. Ultra-thin glass makes it possible to reduce the package size and weight [3]. Typically, glasses thickness of 300 m or less are considered as ultra-thin material. However, even as thin as 25 m glass is commercially available [4]. Several methods are used for glass joining including: anodic, fusion, and adhesive bonding [5]. During the recent years increasing number of laser based techniques are applied to glass packaging. Numerous studies have concentrated on frit bonding [6, 7]. Elementary study of the direct laser joining without any additive layers has been demonstrated by Miyamoto et al. [8]. Glass welding method has been further investigated in several researches [9, 10]. The package is constructed of three ultra-thin glass wafers: base and lid, thicknesses of 200 m and spacer, thickness of 100 m. Commercially available 6 borosilicate, D263T, wafers are used which were welded together by using novel laser welding technology. Welding is implemented on material interface without using any additive materials or coating layers. Glass surfaces are left untouched and optical quality is retained. Welding was performed on a wafer level and the single packages were cut after the final welding. Hermeticity test was performed to ensure the welding quality. Radioisotope leak test with krypton 85 was performed at Oneida Research Services. The results showed an excellent hermeticity: leak rate less than 6,0x10-12 atmcm3/s Kr-85 was achieved. Limits set in the MIL-standard are easily reached. The study proves that novel glass welding technology can be applied to wafer-level packaging with ultra-thin glasses. Technique eliminates the need of additive materials, and due to the minimal heat load bending and warping of the material can be avoided. Also, glass offers effortless visual inspection through the entire lifetime of the device.
Heidi Lundn, Materials Engineer
Primoceler Oy
Tampere, Pirkanmaa

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