The RDGT - Integration of Micromechanics and Electronics by Plasma Assisted Wafer Bonding

Doctoral thesis, 2002

A high-resolution capacitive sensing technique is presented in this thesis: the Resonant Double Gate Transistor (RDGT). The major advantage of the RDGT compared to "pure" capacitive sensing techniques is its low output impedance because of the direct capacitance to current conversion of the transistor. The important difference between the Resonant Gate Transistors (RGT) reported earlier and the RDGT is that in addition to the oscillating gate there is also a fixed gate. The additional gate provides long- term stability and makes possible a choice of the working point of the transistor independent of the bias applied to the oscillating gate. According to simulations the RDGT provides a spatial resolution of the order of 10-13 m Hz-1/2 with an air gap of 2 µm and a DC voltage of 10 V on the oscillating gate. First measurements on fabricated RDGT´s resulted in a spatial resolution of 2 x 10-11 m Hz-1/2. Oxygen plasma assisted wafer bonding is evaluated as a tool for integrating micromechanics with electronics such as the RDGT. Our oxygen plasma enhanced wafer bonding using ICP RIE results in high surface energies after storage for 24 hours at room temperature. The surface energies achieved are comparable to the surface energies obtained when wet chemical cleaned and bonded wafers are annealed at 800 °C. Investigations were carried out to understand the plasma assisted bonding mechanism. We propose that a porous silica network resulting from the plasma treatment makes faster diffusion of water through a porous network possible. This makes it easier for hydroxyl groups to come into contact for reaction, which gradually increases the surface energy at room temperature. The first electrical characterisation of ASIC´s with standard electronic components and circuits after oxygen plasma treatment using an ICP RIE showed no degradation of their performance. Based on the measurement results it is very likely that our oxygen plasma assisted room temperature bonding is CMOS compatible. Finally, a technology with the potential of integrating the high resolution detection method for oscillating structures, the RDGT, with bulk micromachined devices by room temperature oxygen plasma assisted wafer bonding using RIE is presented.