A Big Deal About Almost Nothing – Vacuum Encapsulation of MEMS Devices by Wafer Bonding

by Eric Pabo, Business Development Manager, MEMS, EV Group

Aligned wafer bonding for wafer-level capping was an enabling technology for the amazing growth of inertial MEMS devices, such as accelerometer and gyroscopes, during the last decade. Today, the vacuum level in the cavity of the packaged MEMS device has a significant effect on power consumption for devices such as gyroscopes and on the device performance for microbolometers. However, until today, the high-volume process systems for aligned wafer bonding have done most of the pre-processing and all of the wafer handling and wafer-to-wafer alignment in an ambient atmosphere. The bond chamber was the only process step where a vacuum environment was possible.

This handling and alignment in ambient atmosphere imposed two significant constraints. The first is that any surface modification performed had to be compatible with an ambient atmosphere. For example, a pre-processing step that removes aluminum oxide from an aluminum metal layer would be useless because a native oxide layer would start growing as soon as the aluminum metal was exposed to ambient atmosphere. The second constraint is that any pre-processing to desorb water vapor and other molecules from the surface of the wafers that was performed prior to alignment and bonding would be rendered useless by exposure to the ambient atmosphere. The effectiveness of baking out the aligned and clamped wafer pair separated by spacers of 50-100 microns inside the bond chamber is limited by the space between the wafers. In addition, it is very difficult to bake out the wafers at different temperatures, since this limits the ability to protect thermally sensitive wafers as well as to fully activate the getters (materials that are used to improve and maintain the vacuum level in a cavity or device). Both the issues of preventing pre-treated surfaces from being exposed to ambient atmosphere and the need to bake out wafers effectively for high-vacuum applications can be resolved by doing all of the wafer handling and alignment in a high vacuum.

Previously, the need for high-vacuum encapsulation of MEMS devices was met by device-level packaging and there was no impetus for solving the technical challenges associated with designing and operating the necessary equipment—such as optical wafer-to-wafer alignment systems—in a high-vacuum environment. However, device-level packaging is expensive and not suitable for high-volume manufacturing, therefore driving the need for aligned wafer bonding in a high-vacuum cluster tool configuration.

A vacuum cluster tool base that enables handling in a high vacuum can be configured with the appropriate processing modules for the desired process. One configuration, which is configured for surface pre-treatments, can have a load lock for moving wafers in and out of the vacuum environment, a surface treatment module capable of removing surface oxides and activating the surface, a vacuum aligner, and a bond chamber. The configuration optimized for high-vacuum encapsulation of MEMS devices would replace the surface treatment module with a programmable bake-out module for desorption and getter activation. Such a high-vacuum cluster tool does not necessarily displace existing bonding systems but will enable low-temperature covalent bonding, low-temperature bonding of aluminum, and the encapsulation of high vacuum levels in device cavities while encapsulating the devices at the wafer level.

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Process Flow for Aligned Wafer Bonding in High Vacuum Cluster Tool