Towards low-temperature anodic bonding for RbN3-filled MEMS atomic vapor cells and defect inspection by OCT

From the different methodologies to fill microfabricated alkali-metal vapor cells, the rubidium azide (RbN₃) decomposition by UV radiation is a cost-effective solution to produce rubidium (Rb) and nitrogen (N₂). The typical fabrication of the vapor cells is based on silicon and glass bonding, in which the substrates are heated to temperatures between 300 and 450 ºC along with a high electrostatic field (400 – 1000 V) to establish the solid-state connection. However, the RbN₃ compound has been reported to undergo thermal decomposition within the temperature range of 355–395 ºC. In this work, a systematic variation of the bonding temperature (from 200 to 300 ºC) in silicon cavity-based vapor cells is presented to prevent the RbN3 decomposition during the cell fabrication. Considering that the anodic bonding process can be well represented by a simplified equivalent electrical circuit model, we report a maximum bond strength of 6.05 MPa and a lowest time constant τ of 180.03 s for 300 ºC with a simple electrode configuration. A total transferred charge of ≥0.250 mC.mm⁻² for temperatures above 225 ºC are indicative of a good quality bond. Interestingly, distinct differences in the failure mode of bonding are observed, in which undamaged bonded interfaces are only observed for temperatures of 275 ºC and above. As a result, a 3 mm diameter vapor cell was successfully fabricated using anodic bonding at 275 ºC. Moreover, and for the first time, optical coherence tomography (OCT) was implemented as an effective and novel technique to investigate the glass-silicon-glass bonding in a MEMS vapor cell, providing a cross-sectional image of the device, in a non-destructive and contactless manner, to ensure the production of reliable and defect-free devices.