Confinment Chamber SET-FC300					MEMS devices require hermetic packaging to eliminate sticking of movable parts. In most cases, encapsulation is carried out at wafer level using a wafer bonder. This technique has the advantage of being fast, as all devices are encapsulated at the same time. The main drawback is yield loss, especially when sealing with active caps. In wafer bonding, bad caps can be bonded onto good devices, or good caps onto bad devices. Both cases cause yield loss. Gang chip-to-wafer (C2W) bonding with collective reflow is an alternative hermetic sealing method when capping MEMS devices.

Hermetic vacuum sealing using FC300 Courtesy CEA-Leti

This technique also allows special atmospheres to be encapsulated while bonding the cap. The process flow is illustrated here. The cap is tacked to the substrate. Higher bumps for connection maintain the gap required to ensure gas flow and proper pumping between the inner solder cap ring and the MEMS wafer. Wetting pads are designed so that the cap collapses during reflow, which brings the sealing ring into contact with the corresponding wetting ring on the opposite wafer, capturing the required atmosphere inside the MEMS cavity. After placement of all dice the cover is closed and sealed by the external vacuum action.

Result - Sealed Component			Vacuum is activated to extract the oxygen from the chamber. A flow of acid vapor is then used to reduce the native oxide from the indium bumps. Purging is made by nitrogen flow, before vacuum is applied again. When the vacuum level is correct, the temperature of the wafer is increased. When the indium bumps melt, their surface tension pulls down and self-aligns the cap in X and Y axes, optimizing post-bond accuracy.

This work was carried out in a joint development between CEA Leti and SET, using an FC300 with a process chamber featuring some proprietary features: ultra-fast thermal ramps with controlled plateau temperature, solder oxide reduction, and fluxless soldering in the desired atmosphere or in vacuum.

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