Interface charge control of directly bonded silicon structures

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Abstract
The influence of interface states and charges on the properties of Si/Si and Si/SiO2 interfaces prepared by wafer bonding, using the direct bonding technique, has been investigated. Surface potentials of Si/Si interfaces with all combinations of doping type (n-n,p-p,p-n) are dependent on surface and heat treatments in the bonding procedure and on wafer dopant concentration. In earlier reported works, hydrophilic wafer surface properties have been reported as necessary for a good mechanical bonding. We find that wafer treatment in HF giving hydrophobic surfaces not only gives good mechanical properties, but also better electronic properties as well. For all combinations of doping type, lower magnitudes in surface potential were measured in samples prepared from wafers pretreated in HF in order to etch off the native oxide layer, normally present on silicon surfaces. If a native oxide is present when the bonded interface is prepared, the current through the interface will be influenced by an energy barrier due to the presence of charged interface states. The amount of charge trapped at the interface has been found to be dependent on the applied bias. A theoretical description is made for the Si/Si interfaces, and predictions are compared to results obtained from electrical measurements. Based on this theory, using data from the current-voltage characteristic, an interface state density in the region 5×1010–1012 cm−2 eV−1 at the bonded interface has been deduced for different samples. Bonded Si/SiO2 interfaces with interface state densities of about 1011 cm−2 eV−1 and low flat-band voltages have been achieved. No influence of different chemical pretreatments, used in this paper, on properties of bonded Si/SiO2 interfaces, as seen in flat-band voltage and interface state density, was found, although the bonding process is critical in the preparation of the Si/SiO2 interfaces.