Processability and electrical characteristics of glass substrates for RF wafer-level chip-scale packages

Various types of glass substrates have been compared with respect to their suitability as a low-loss substrate in wafer-level chip-scale packaging for RF applications. Processability has been evaluated by fabrication of shallow and deep recesses using wet etching in HF (/H3PO4) solutions. Electrical characteristics (dielectric constant and attenuation) have been extracted from measurements on coplanar wave guides (CPWs) up to 10 GHz. Results show that Coming Pyrex #7740 glass provides optical quality of wet-etched deep recesses, but exhibit about 2 times higher electrical attenuation than Hoya SD-2. Pyrex #7740 and SD-2 are thermally matched to silicon and due to some alkali content, they are suitable for anodic bonding. The relatively high content of Al2O3 (similar to20%) in SD-2 is favorable for its electrical properties, but makes wet etching of deep recesses more difficult. The non-alkaline AF45, with CTE almost 2 times of Si (CTEAF45=4.5x10(-6) K-1), is suitable for adhesive bonding and is difficult to pattern using wet etching. Its electrical attenuation is close to that of SD-2. The measured dielectric, constants (at 6 GHz) for SD-2, Pyrex #7740 and AF45 are 4.7, 5.9 and 6.1, respectively.