Anodic Bonding between LTCC Substrate and Si Substrate with Electrical Connections

Abstract

This paper describes metal-metal electrical connection simultaneously established with anodic bonding between a LTCC (low temperature cofired ceramic) substrate and a Si substrate. Metal pads are composed of Sn on Cu. Sn melts during anodic bonding, absorbing the height margin of the metal pads to ensure good contact between the LTCC substrate and the Si substrate. This study first investigated formic acid vapor treatment before anodic bonding to remove an oxide layer on the Sn surface. The removal of the oxide layer proceeds at a process temperature of 150°C or higher. By the treatment at 250°C, the surface of the Sn layer is smoothed due to reflow effect, but the multilayer structure of the metal pads does not significantly change after 5 min treatment. The bonded metal pad is almost uniform in both structure and composition throughout its thickness. The composition of the bonded metal pads is approximately Sn : Cu = 1 : 1 in atomic ratio, and might have a remelting temperature of ca. 415°C, which is much higher than a reflow temperature in device mounting process.