Miniaturized Two Axis Acceleration Sensor Using Direct and Anodic Wafer Bonding for an Implantable Sensor Unit
Authors: Wiemer Maik , Wuensch Dirk , Schroeder Tim , Baum Mario , Otto Thomas 
Published: February 2020

The packaging of MEMS (Micro-Electro-Mechanical-Systems) such as acceleration, pressure and optical sensors requires a permanent and hermetically sealed bond interface of two or more wafers. In the area of system integration more and more new and heterogeneous materials have to be integrated and bonded at wafer level.
In particular, silicon fusion and anodic bonding has a major advantage within the MEMS packaging. Both techniques enables joining without additional intermediate layers and with minimal technological effort.
The bond mechanism of the fusion bonding at room temperature (pre-bond) is caused after contacting the two wafers by hydrogen bonds between the water molecules located on the opposite wafer surfaces. After the pre-bond a heat treatment (annealing) takes place, in order to form covalent siloxane bonds. Consequently, the interface water is removed by diffusion, whereby the bond strength is significantly increased. To activate these diffusion processes, surface plasma activation prior to the bonding process is one possibility. The pre-treatment of the substrate surfaces with plasma causes different interface and surface effects, such as the removal of contaminants on the wafer surface, an increase in the amount of silanol groups, an improvement of the diffusivity of water and gas trapped at the interface and an enhancement of viscous flow. Due to these mechanisms, the bonding and annealing process is supported.
The anodic bonding process itself is characterized by the bond voltage, temperature and current limitation. This technique uses an electrostatic field (200-1000 V) caused by the atomic contact of two clean and smooth surfaces at temperatures of 400-500°C. By application of heat the alkali ions in the glass become free and moveable. The electrical field cause positive alkali ions to move to the electrode. A space charge region is formed between glass and silicon. The space charge region forces the materials into molecular contact. Oxygen and silicon are then forming a chemical bond between substrates.
Within this work, we report the hermetic encapsulation of a two-axis acceleration sensor using fusion and anodic bonding processes. In order to achieve the required miniaturization, the acceleration sensor was developed in a MEMS technology. The MEMS sensor technology is based on previously reported approaches of the so-called BDRIE technology. BDRIE stands for Bonding and Deep Reactive Ion Etching. Both Si-Si-glass as well as Si-Si-Si configurations required six lithography steps with the sensor manufacturing on 6-inch wafer level and pre-etched Si or glass cap wafer.
For wet etching of 40 µm deep cavities in the glass cap wafer, a masking layer stack consisting of Cr, Au and photoresist is used. The subsequent wet etch process is carried out in a spin-etch tool. The hermetic encapsulation with a glass cap wafer is accomplished with an anodic bonding process performed at 400°C with a voltage of -250 V in a standard bonding equipment. The pressure in the bond chamber can be adjusted according to the requirements of the application. In order to minimize damping, low vacuum conditions was chosen. For hermetic encapsulation with a pre-etched Si wafer, a fusion bonding process including an additional plasma activation step has been applied. Pre-bonding is done in the standard bonding equipment at a tool pressure of 2 bar. Afterwards, an annealing step at 800°C for 5 h in nitrogen contributes to increase the bond strength.
The MEMS core die (sensor area) has a dimension of 1 x 1 mm². The overall die size is 1.2 x 1.5 mm². The maximum height of the sensor chip is 0.65 mm. The Figure shows the miniaturized xy acceleration sensors (Si-Si-glass variant).
First characterization tests showed that both the Si-Si-glass and Si-Si-Si system is working and the results are quite promising. The sensitivity of at least 20 measured sensors (Si-Si-glass variant), obtained by 4-wire measurement, is 125±14 fF/g (x-axis) and 88±14 fF/g (y-axis). These values are even slightly higher than the target value (75 fF/g). Further work is focused on the comparison of the sensing properties using glass or silicon cap wafer.
The final system is a implantable sensor unit. Besides the acceleration, the implantable sensor module detects pressure, temperature, voltage, and impedance. Except for the MEMS and pressure sensor all other sensor functions are integrated in an ASIC (application-specific integrated circuit). The three individual components (pressure, acceleration, ASIC) are mounted on a ceramic interposer module.

Figure 1