Hydrogen cyanide (HCN) aqueous solutions can remove copper (Cu) contaminants from SiO2 surfaces completely even when the cleaning is performed using dilute (e.g., 0.027wt% ) HCN aqueous solutions at 25°C . When pH of the HCN solutions is set at 10, Cu contaminants with a concentration of ∼2×1012atoms∕cm2 can be removed below ∼3×109atoms∕cm2 taking only 10s . The concentrations of Cu+ and [Cu(CN)2]− ions in the HCN solutions drastically decrease with pH, while those of [Cu(CN)3]2− and [Cu(CN)4]3− ions increase. The dissociation probability of HCN exponentially increases with pH.Therefore, the improvement of the cleaning ability with pH is attributable to (i) an increase in the concentration of CN− ions, and (ii) the prevention of readsorption due to the formation of stable Cu–cyano complex ions. Measurements of surface Cu concentrations on the SiO2 surfaces contaminated by the immersion in the CuCl2 -containing HCN solutions clarify the mechanism of Cu removal. The rate-determining step is a reaction between SiO–CuCN on the surface and CN− ions in the solution to form [Cu(CN)2]− complex ions. The equilibrium constant of this reaction is found to be 4.1×105M . This large value shows that desorption of [Cu(CN)2]− proceeds much more easily than adsorption.
We have succeeded in the fabrication of Si-based metal-oxide-semiconductor diodes with an ultrathin ͑i.e., ϳ1.4 nm thick͒ silicon dioxide (SiO 2 ) layer whose leakage current density is lower than those for thermally grown SiO 2 layers with the same thickness. The leakage current density of the as-grown SiO 2 layers formed in concentrated nitric acid (HNO 3 ) solutions ͑i.e., ϳ3 A/cm 2 at the forward gate bias, V G , of 1 V͒ is as high as those for thermal SiO 2 layers. The leakage current density is decreased to ϳ1.5 and ϳ1 A/cm 2 at V G ϭ 1 V by postoxidation annealing ͑POA͒ at 400 and 500°C ͑in hydrogen͒, respectively, and the decreases are mainly attributed to the elimination of interface states and slow states, respectively. The vibrational frequency of longitudinal optical ͑LO͒ phonons of Si-O asymmetric stretching vibration increases with the POA temperature, while that of the transverse optical ͑TO͒ phonons remains constant. From the analysis of these vibrational frequencies, the atomic density of the as-grown SiO 2 layer and that after POA at 500°C are estimated to be 2.34 and 2.56 ϫ 10 22 cm 3 , respectively, i.e., higher than that of crystalline SiO 2 bulk of 2.28 ϫ 10 22 cm 3 . Measurements of the valence band spectra for the Si/SiO 2 structure show that the valence band discontinuity energy at the Si/SiO 2 interface is increased from 4.3 to 4.6 eV by POA at 500°C. Therefore, the other reason for the decrease in the leakage current density by POA is a reduction in the tunneling probability through SiO 2 caused by the increase in the band discontinuity energy. Postmetallization annealing treatment at 200°C in hydrogen performed after POA further decreases the leakage current density, showing that atomic hydrogen passivates defect states more effectively than molecular hydrogen.Low-cost and high-performance large scale integrateds ͑LSIs͒ have been achieved through miniaturization. For further integration, however, several serious and fundamental problems should be solved. Among these problems, an increase in the leakage current density flowing through ultrathin silicon dioxide (SiO 2 ) layers for metal-oxide-semiconductor ͑MOS͒ devices is the most urgent. Namely, in cases where the SiO 2 thickness becomes below ϳ1.5 nm, the leakage current density is likely to exceed the highest allowable limit of 1 A/cm 2 for device operation. [1][2][3] One solution for the leakage current problem is usage of high dielectric constant ͑i.e., high-͒ materials such as HfO 2 , ZrO 2 , TiO 2 , etc. as gate insulators. 4 This idea is based on the fact that the capacitance of the gate insulating layers can be made high with the high-materials, even when their thickness is sufficiently large to prevent a leakage current. However, there exists a fundamental problem in the high-materials: the mobility seriously decreases 5,6 due to, e.g., the remote coulombic potential induced by the high-materials. 7 Other problems may be the formation of an interfacial layer with a low dielectric constant, 8 and crystallization of the high-mat...
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