Characteristics and applications of silicon direct bonded interfaces

In this work we have used a range of electrical and physical characterisation techniques to assess the joined interface for hydrophobic direct silicon to silicon fusion bonding and its suitability for a range of applications. The physical characteristics of the bonded interface were analysed using spreading resistance profiling (SRP) measurements, transmission electron microscopy (TEM), and secondary ion mass spectroscopy (SIMS). Simple pn and PIN diode structures were manufactured and forward and reverse bias diode characteristics analysed. Different cleaning regimes used for silicon direct bonding were compared and related to the proposed applications in terms of electrical quality, oxygen gettering, and interface defects. Samples were prepared by joining polished silicon device wafers to polished silicon handles. The joined pairs were annealed at temperatures between 600°C and 1200°C, and the device wafers thinned by grinding and polishing. The results showed that a high quality bonded interface with good electrical quality and low leakage current at the interface can be obtained with hydrophobic cleaning. Hydrophilic cleaning is easier to implement but limits the electrical quality of the interface due to large oxide islands and thermal donors from interface oxygen. Applications of discrete power, photo-detectors and MEMS devices are discussed. MEMS processing requires the formation of released structures for operation and wafer bonding can provide an alternative to surface micro machining. Cavity bonding and novel structures formed by heavily doped buried layers and different orientations are demonstrated.