Applications of Artificial Microcavities in Wafer Bonded Silicon

Doctoral thesis, 1996

A novel sensor structure has been investigated and extensively studied. The device can be used as a pressure sensor, a voltage measuring device and as an optical modulator. The sensor was made using thermal bonding and micromachining techniques. It consisted of two silicon wafers bonded together and included a silicon membrane (or boron doped silicon membrane) with a silicon dioxide (or pure silicon) step as the wall of the air-filled cavity. Three different structures were investigated. The boron doped silicon-dioxide-silicon (BSDS) structure included a very thin boron doped silicon membrane (about 3-5 µm) which formed one of the walls of an air cavity in silicon dioxide. The pn junction (pn) structure consisted of n-type and p-type silicon joined together surrounding a cavity with a certain depth. The silicon-dioxide-silicon (SDS) structure was built with a thick silicon membrane and silicon dioxide as distance material. When a varying external pressure bends the membrane a corresponding variation in the light flux leaving the cavity due to interference can be measured. A Fabry-Perot analysis can be applied locally to describe the interference phenomena. Numerical calculations based on a finite element method (FEM) were performed to simulate the membrane bending and the light flux from the structure at different pressures. A simplified analytical treatment has also been developed and compared with FEM calculation. For the thicker membranes numerical calculations based on FEM must be performed in order to simulate the light flux from the structures at different pressures. Application of a voltage between the two wafers produces an attractive electrostatic force between the membrane and the underlying silicon wafer resulting in a deflection of the membrane. This changes the depth of the cavity and thus the conditions for optical interference. As a result the intensity of light transmitted through or reflected from the structure is changed and becomes a measure of the applied voltage. The pn structure can be used as an optical modulator. Light from one light source can be modulated by a second light source.