|Looking at Transistor Gate Oxide Formation in Real Time|
The oxide gate layer is critical to every transistor, and present-day layer thicknesses are in the 10–20 Å range (1–2 nm). However, little information exists on the oxidation process at this thickness. Available results are either for thicker layers grown under high-pressure conditions or for only the first couple of monolayers studied under high-vacuum conditions. Now, for the first time, a group of researchers has obtained real-time oxidation results for this elusive range. Using the ambient-pressure x-ray photoelectron spectroscopy (APXPS) endstation at ALS Beamline 9.3.2, they examined oxidation of Si(100) at pressures up to 1 torr and temperatures up to 450 ºC. The Si 2p chemical shifts allowed determination of oxide thickness as a function of time with a precision of 1–2 Å. The initial oxidation rate was very high (up to ~234 Å/h). Then, after an initial oxide thickness of 6–22 Å was formed, the rate decreased markedly (~1.5–4.0Å/h). Neither rate regime can be explained by the standard Deal-Grove (D-G) model for Si oxidation. These results are a significant step toward developing a better understanding of this critical thickness regime.
Silicon oxidation has been greatly studied via various characterization techniques. This research is generally divided into two categories: that which looks at oxide thicknesses of 100 angstroms or more, for which a D-G model can be used to predict the oxide growth kinetics, and more fundamental surface science studies at roughly the single monolayer level. However, a gap in our knowledge of kinetics exists in the thickness regime of current oxide gate layers, for which the D-G model does not apply. Real-time surface science studies of oxidation kinetics using photoelectron spectroscopy have also been limited to oxidation pressures of <10–5 torr, far below the multi-torr regime of real semiconductor processing.
The research team studied the oxidation rate of a Si(100) surface via chemical-state-resolved APXPS. They monitored Si 2p core levels in real time during oxidation, with spectra having high sensitivity to the surface and, via chemical shifts in binding energy, the individual oxidation states created during oxidation.
The Si 2p core-level spectrum of oxidized Si exhibits a bulk-silicon component (Si0) and various oxide components that demonstrate a higher binding energy of up to about 4.4 eV. The peak at the lowest binding energy is accepted as that of elemental Si (Si0) and the peak at the ca. 4.4 eV higher binding energy as that of stoichiometric oxide (Si4+). From the measured ratio of Si4+ intensity to Si0, the thickness of stoichiometric SiO2 as a function of time can be calculated with 1–2 Å precision.
Si 2p spectra were obtained at 30-second intervals over a two-hour period at pressures of 0.01–1 torr and temperatures of 300–530 ºC. The time evolution of the oxide thickness reveals two kinetic regimes: a rapid regime at the beginning of oxidation and a subsequent slow regime in which the growth is much reduced. The oxide thickness of the break point dividing the two regimes is between about 6 and 22 angstroms and strongly depends on oxidation temperature and pressure. These drastic changes in rate curves cannot be explained by the D-G model. The oxidation rates in the rapid and slow regimes are 64–234 Å/h and 1.5–4.0 Å/h under test oxidation conditions, respectively, and are much larger than those deduced from the D-G model.
These rate curves should provide a unique test of alternative kinetic models in this important parameter regime. In addition to this study, oxidation of silicon with both oxygen and water present, as is often the case in industrial processing, has been examined, providing additional first-of-a-kind data for this important process. Future studies are planned at higher pressures and of more complex oxide combinations that are replacing simple SiO2.
Research conducted by Y. Enta (Hirosaki University, Japan); B.S. Mun (Hanyang University, Korea, and ALS); M. Rossi, and Z. Hussain (ALS); P.N. Ross Jr. (Berkeley Lab); C.S. Fadley (University of California at Davis and Berkeley Lab); and K.-S. Lee and S.-K Kim (Seoul National University, Korea).
Research funding: the U.S. Department of Energy; the Humboldt Foundation (Germany); the Helmholtz Association (Germany); the Jülich Research Center (Germany); the University of Hamburg (Germany); Overseas Advanced Education and Research Program from the Ministry of Education, Culture, Sports, Science, and Technology of Japan; Creative Research Initiatives of MOST/KOSEF (Korea).
Publications about this research: Y. Enta, B.S. Mun, M. Rossi, P.N. Ross, Jr., Z. Hussain, C.S. Fadley, K.-S. Lee, and S.-K. Kim, "Real-time observation of the dry oxidation of the Si(100) surface with ambient pressure x-ray photoelectron spectroscopy," Appl. Phys. Lett. 92, 012110 (2008); M. Rossi, B.S. Mun, Y. Enta, C.S. Fadley, K.S. Lee, S.-K. Kim, H.-J. Shin, Z. Hussain, and P.N. Ross, Jr., "In situ observation of wet oxidation kinetics on Si(100) via ambient pressure x-ray photoemission spectroscopy," J. Appl. Phys. 103, 044104 (2008).