The thermal-oxidation kinetics of silicon are examined in detail. Based on a simple model of oxidati?n which takes into account the reactions occurring at the two boundaries of the oxide layer as well as t~e dIffusion process, the general relationship x02+:4xo=B(t+r) .is derived. This relationship is ~hown}o be m ~x cellent agreement with oxidation data obtamed over a ;VIde range of temperature (7~ -1300 C), partial pressure (0.1-1.0 atm) and oxide thickness (300-20000 A) for both oxygen and water. oXI~ants. T~e p~ram eters A, B, and r are shown to be related to the physico-chemical.constant.s of the o~datlOn reaction m the predicted manner. Such detailed analysis also leads t? .f~rther mfor~tlo~ regardmg the nature of the transported species as well as space-charge effects on the Imtlal phase of OXidatIOn.

The nature of the surface-state charge (Qss) associated with thermally oxidized silicon has been studied experimentally using MOS structures. The effects of oxidation conditions, silicon orientation, annealing treatments, oxide thickness, and electric field were examined, as well as the physical location of the surface-state charge. The results indicate that the surface-state charge can be reproducibly controlled over a range 1010-1012 cm -2, and that it is an intrinsic property of the silicon dioxide-silicon system. It appears to be due to an excess silicon species introduced into the oxide layer near the silicon during the oxidation process.In much of the early work on the properties of semiconductor surfaces, experimental results were interpreted within the framework of two quantities: fast and slow surface states (1). "Fast surface states" are electronic states within Che forbidden gap of the semiconductor, located at the surface, which are in good electrical communication with the semiconductor bulk. Because of this, they can act as surface recombination centers. Their density per unit area for both clean germanium and silicon surfaces has been generally found to be of the same order as the surface atom density. If the semiconductor is covered by an adsorbed layer, or an oxide, the density of fast states has been found to decrease to 1011-101~ cm-2. "Slow states," in contrast, have been attributed to ionic contamination within an oxide covering the semiconductor surface. Because of their relatively large distance from the semiconductor, they are in poor electrical communication with it. Their density is a strong function of the ambient and surface treatment of the sample, but generally ranges in the neighborhood of 1012-1013 am-2 In the past several years, the thermally oxidized silicon surface has been investigated very intensively, partly due to its extreme technological importance and partly to the ease with which its characteristics can be studied (2). As a result of these investigations, a detailed picture of this system has emerged which is not explainable strictly within the framework of fast and slow surface states. Vol. 114, No. 3 SURFACE-STATE CHARGE OF OXIDIZED Si 267An idealized representation of the current picture of the thermally oxidized silicon system is depicted in Fig. 1. Fast surface states have been identified, but in many cases their density has been found to be less than 5 x 10 TM cm-2 (3), considerably lower than in other systems, thus leading to the low surface recombination velocity of passivated silicon devices (4). While slow surface states, in the customary sense, have not been evident, ionic space charges within the oxide have been observed. These were found to result from either contamination by relatively mobile ions such as sodium (5, 6), or from exposure to ionizing radiation (7). They are evidently in poor electrical contact with the underlying silicon even when located very near the oxide-silicon interface.These charges within the oxide, which have general...

It is shown that changes in the capacitance-voltage characteristic of a metal-insulator-semiconductor structure provide a powerful tool for the observation of ion motion in thin insulating films. Using this method, a detailed study of the kinetics of alkali ion migration in thermally grown silicon dioxide films has been made. Alkali ions were initially deposited at the metal-oxide interface and their transport through the oxide was studied as a function of time, temperature and applied voltage. When the metal is biased positively the number of ions accumulated at the oxide-silicon interface is initially proportional to the square root of time and then approaches a saturation value. The temperature dependence is exponential and leads to an activation energy for the diffusion coefficient of 32 kcal/mole for Na and 22 kcal/mole for Li. A simple model is developed which is based on the division of the insulator into two regions: a thin boundary layer near the metal-insulator interface in which ion transport is by diffusion, and the remainder of the insulator where field transport dominates. It is shown that most features of this model are in excellent agreement with the experimental results.

Interface states and electron spin resonance centers have been observed and compared in thermally oxidized (111) and (100) silicon wafers subjected to various processing treatments. The ESR Pb signal, previously assigned to interface ⋅Si≡Si3 defects on (111) wafers, was found to have two components on (100): an ⋅Si≡Si3 center oriented in accord with (100) face structure, and an unidentified center consistent with ⋅Si≡Si2O. The quantitative proportionality of Pb spin concentration to midgap interface trap density Dit is maintained on (100), and both are lower by a factor of about 3 compared to (111). This correlation persists over the range of oxidation temperatures 800–1200°C, for both n- and p-doped silicon, cooled by fast pull in oxygen, and cooled or annealed in nitrogen or argon. The correlation is independent of doping level. In samples with different oxide thickness, neither Pb nor Dit varied significantly over the range 100–2000 A, but Pb was smaller at 50 A. In general, ESR is judged to offer promise for further studies of specific interface features.

The ESR Pb center has been observed in thermally oxidized single-crystal silicon wafers, and compared with oxide fixed charge Qss and oxidation-induced interface states Nst. The Pb center is found to be located near the interface on (111) wafers. Its g anisotropy is very similar to that of known bulk silicon defects having SiIII bonded to three other Si atoms; the Pb unpaired electron orbital, however, is exclusively oriented normal to the (111) surface. The Pb center cannot be identified with any other known defect in Si or SiO2; in particular, it is totally unlike the common E′ center of SiO2. In contrast to Qss, both Pb and Nst were found to be greatly reduced by steam oxidation and hydrogen annealing. Both Pb and Nst may be regenerated by subsequent N2 anneals at 500 °C. In a graded series of samples, Pb and Nst are found to be proportional and nearly equal in concentration. This possible confirmation of SiIII at the interface, and correlation with Nst, support the theoretical indication of an SiIII band-gap energy level. The E′ center is unobservable, and if present, exists only in a concentration well below that of Qss. Thus, in addition to a lack of strong correlation with Pb, Qss is evidently not due to E′ centers in their normal charge state. Overall, ESR is judged to be a useful technique for research on silicon wafer defects.