When used for consumer products, the thermal imaging device must operate for 5-10 years, depending on the targeted application, before its performance deteriorates. To prolong the lifetime of the device, the vacuum in micro-cavities must be sustained. Several steps must be implemented in order to achieve and maintain proper operation conditions: i) a hermetic sealing of the cavity should be realized to minimize gas leakage from surrounding environment; ii) degassing of the system before seal-off is needed to dislodge and release trapped gases within the materials; and if necessary, iii) incorporating a non-evaporable getter which acts as a chemical pump to rid the system of residual gases during the device lifetime.
In this presentation, we provide an overview of current challenges in hermetic wafer-level packaging of microbolometers, which is a crucial step in the large scale production of uncooled thermal cameras. The presentation will address advantages and limitations of different bonding processes. Special focus will be made on fluxless AuSn solder and its use to achieve hermetic wafer-level packaging. Results of temperature-dependent surface analysis studies including in situ and ex situ spectroscopic and microscopic techniques (LEEM/PEEM, XPS, SEM-NanoAuger, and TEM) will be discussed to highlight key mechanisms in wafer preprocessing (e.g. patterning of the seal rings) and establish key parameters controlling the quality of the bonding. The role of pre-processing at elevated temperatures in degassing the cavity from non-getterable gases and its influence on Sn and Ti surface diffusion will be discussed. This surface diffusion yields to the formation surface oxide, which is detrimental to the bonding process.