Micro-fabricated alkali vapor cells sealed at low temperature using asymmetric Au-In transient liquid phase (TLP) bonding

Vapor cells as the key part of chip-scale atomic clocks determine their volume stability and power consumption. Micro-fabrication, especially the cavity packaging, is challenging. In this paper, micro-fabricated rubidium vapor cells are fabricated and the Au-In TLP bonding with asymmetric metal configuration is proposed and designed to package wafer-scale cell chips at a low processing temperature of 200 °C. Tests show that the designed intermetallic compounds AuIn₂ and AuIn are uniformly formed in the asymmetric TLP bonding interlayer, indicating that the bonded interlayer can endure a post-processing temperature as high as 490 °C. The TLP bonded surface exhibits shear strength of 31.68 MPa, and the upper limit for the leak rate of the sealed vapor cell is 5 × 10^-10 mbar • L s^-1 which remains stable after undergoing a high temperature post-treatment. Meaning the proposed Au-In TLP bonding technique is applicable at low-temperature, hermetic and reliable wafer-scale packaging for micro-fabricated alkali vapor cells.