Electrical Characterization of Defects in Gate Dielectrics

Defects in gate oxides/insulators have been characterized by many techniques, e.g., electrical, electron beam, ion beam, X-rays, neutron activation analysis, electron spin resonance, and others. These methods played a crucial role during the early MOS development to determine the origin of oxide/interface charges that led to unstable MOS devices. Thermally-grown oxides on silicon are now quite well understood and very well controlled during manufacturing. High-K dielectrics, however, have proven to be less well controlled and more difficult to characterize. I give a brief description of the common defects (oxide, border and interface traps) and then describe many of the more common electrical characterization techniques. 1. OXIDE DEFECTS In this section I will briefly describe the main defects commonly observed in insulators, some in thermally-grown SiO2, others in non SiO2 insulators, and some generated during irradiation of MOS devices. I will also briefly touch on some aspects of oxide breakdown as it pertains to oxide defects. 1.1 FIXED OXIDE CHARGE Fixed oxide charge was identified early during MOS development and was attributed to excess silicon near the SiO2/Si interface in thermally-grown oxides. The model was that as an oxide grows on a Si wafer, oxygen diffuses through the growing oxide to react at the interface forming SiO2. This leaves some excess Si in the oxide near the interface. As the growing oxide front moves into the wafer, the excess Si moves with it and defects associated with this excess Si become positively charged during negative bias stress. The fixed oxide charge is not in electrical communication with the underlying silicon. These positively-charged defects were given the symbol Qss and later Qf. They were originally referred to as slow states and later Deal called them fixed surface states. Qf depends on the final oxidation temperature. The higher the oxidation temperature, the lower is Qf. However, if it is not permissible to oxidize at high temperatures, it is possible to lower Qf by annealing the oxidized wafer in a nitrogen or argon ambient after oxidation. This has resulted in the wellknown "Deal triangle", which shows the reversible relationship between Qf and oxidation and annealing. An oxidized sample may be prepared at any temperature and then subjected to dry oxygen at any other temperature, with the resulting value of Qf being associated with the final temperature. The nature of Qf is still not entirely clear. In one model, some of the trivalent Si interface traps are "tied up" in some form of Si-Si bonds that do not interact with the adjacent oxide network bonding. During H annealing at moderate temperatures (400-500°C), Si-Si bonds break, one terminated with H and the other free. The free bond interacts with a neighboring oxygen to form an over-coordinated oxygen with a fixed positive charge. When the free bond of the Si atom bonds to a neighboring oxygen atom, the electron can tunnel to the adjacent Si substrate leaving behind a positively charged O center. 1.2 MOBILE OXIDE CHARGE Mobile charge in SiO2 is due primarily to the ionic impurities Na, Li, K, and perhaps H. Sodium is the dominant contaminant. Lithium has been traced to oil in vacuum pumps and potassium can be introduced during chemical-mechanical polishing. The practical application of MOSFETs was delayed due to mobile oxide charges in the early 1960s. MOSFETs were very unstable for positive gate bias but relatively stable for negative gate voltages. Sodium was the first impurity to be related to this gate bias instability. By intentionally contaminating MOS capacitors (MOS-Cs) and measuring the gate voltage shift after biastemperature stress, it was shown that alkali cations could easily drift through thermal SiO2 films. Chemical analysis of etched-back oxides by neutron activation analysis and flame photometry was used to determine the Na profile. The drift has been measured with the isothermal transient ionic current, the