The major problem in high-k materials is that they are yet to meet the electrical characteristics that SiO2 or most recently SiON, can offer. Figure 6 shows the transistor characteristics of different n-channel devices with similar EOTs. As can be seen, the electrical characteristics of devices with HfO2 and hafnium silicates, still must be significantly improved. The fundamental electrical property that determines the application of these metal oxides in CMOS devices is their charge trapping characteristics. Charge trapping can alter the threshold voltage of transistors over time and subsequently affect the defect nature of the dielectrics. In addition to charge trapping in the bulk high-k layer, the contribution from the interface traps present in between the high-k layer and the interfacial layer and in between the substrate and the interfacial layer, is significant. As far as the deposition method is concerned, charge trapping in high-k dielectrics is almost identical in both ALD and MOCVD deposited HfO2 due to the large number of preexisting traps. As described earlier, the origin of the preexisting traps is tied to the way these dielectrics are formed. The energy levels of the traps in the high-k dielectric bandgap are deep and help in trap assisted tunneling. These deep traps can be annealed but an increase in shallow traps may appear due to crystallization of the high-k film.
Defects or leakage current have always been at the forefront with the evolution of new technologies or introduction of new materials in CMOS technology. To match the properties of high-k dielectrics with those of conventional silicon oxide, the existence of various defects and how they affect the reliability through trapping of charge at the defect sites must be looked at. In bulk high-k devices, shallow electron traps at the conduction bandedge have been found to be inherent in the most recent studies. These traps are closely related to charge transport and charge redistribution during stress. The shallow traps, considered to be the root cause of trapping, especially for low voltage stress, also control charge detrapping and redistribution during relaxation after removal of stress. There are reports that permanent trap creation may not be possible for low stress levels in high-k materials and charge trapping is reversible by applying stress of opposite polarity. But Houssa et al.11 report neutral trap and positive charge generation in the bulk high-k material and at the interfacial layer, respectively, under high negative stress voltage. Fig. 7 shows the effect of stress as a function of temperature on the leakage current through a TiN/HfSixOy/SiO2 gate stack. Electron trapping and neutral trap generation in the bulk of high-k rather than at the interfaces seem to be dependent on the stress voltage and temperature.
Several misconceptions are possible in reliability studies of high-k gate materials. For example, when the devices are subjected to an electrical stress, poststress leakage current is typically slightly higher than the prestress leakage at low positive and negative gate biases, i.e., within the range of trap-assisted tunneling regime. But at comparatively high positive/negative gate bias, poststress leakage current can be lower than the prestress value. This is a typical example of stress-induced leakage current, which is attributed to neutral trap generation. But, one must be careful with high-k dielectrics as shallow traps with energy levels lying within 0.3-0.8 eV below the conduction bandedge give rise to extensive electron trapping during stress, which distorts the internal electric field. Significant relaxation induced detrapping takes place after the stress. Detrapping from shallow traps may also occur during poststress current-voltage measurement, which increases poststress leakage even in the trap-assisted tunneling regime. Therefore, adequate time for relaxation must be provided before a poststress current-voltage measurement is made. Besides, dominance of negative charge trapping near the injection side that causes electric field distortion can help the injected electrons see a triangular barrier, the effect being, poststress leakage is lower than the prestress value and the shift is parallel.
Systematic studies of charge trapping and trap generation characteristics of various high-k gate stacks, especially for high stress levels and at elevated temperatures, and interpretation in terms of spatial and energy level distribution of these traps are currently being carried out by many groups. The use of high-k dielectrics will become routine when many of these reliability problems are solved.