Effect of Initial Al Contamination on Ultrathin Gate Oxides

Abstract

As the semiconductor industry pushes toward increased transistor density and continues to develop faster, smaller, and less expensive microchips, it is necessary to continually meet significant technical challenges. The next generation technology will require more stringent tolerances on the composition, structure, and uniformity of silicon wafer surfaces and on the insulating and conducting layers that make up ultralarge scale integrated circuits (ULSIs). Specifically, ultrathin gate oxides used in these technologies are sensitive to surface contaminants and impurities that may be introduced during silicon wafer manufacturing and ULSI fabrication. Therefore, the understanding, optimization, and control of preoxidation cleaning sequences continue to be an important area of research.Al is ubiquitous in semiconductor fabrication processes. It is known that Al does not degrade the lifetime of silicon, does not increase leakage current, and does not lead to a drop in breakdown voltages of thermal oxides when it exists on the Si surface. However, negative effects of Al contamination have been reported on gate oxide properties. For example, Al contamination increased the n ϩ /p junction leakage current, decreased breakdown voltages due to the increase in SiO 2 /Si interface state density, 1 and generated fixed negative charge. 2 For an ultrathin gate oxide, such undesirable effects of Al on device performance can be exacerbated. Also as the feature size of ULSI circuits decrease, the gate oxide thickness must scale down, and its control becomes important. DeLarios et al. investigated the effect of Al on oxidation kinetics and reported that Al contamination on Si surfaces decreases the oxidation rate when oxide thickness is greater than 200 Å. 3,4 In their work, the oxidation rate degrades by 15ϳ20% for 1000 Å oxides. In this case, Al exists on the Si surface and is deposited from a "reverse RCA" clean. Such a large difference of oxidation rate may not be tolerable for ultrathin gate oxides. The deleterious effect of Al contamination on the oxide properties has to be elucidated. Therefore, in this study we show the effect of Al contamination on the oxidation kinetics and the corresponding dielectric breakdown trends for ultrathin oxides.Experimental p-Type silicon (100) wafers ( ϭ 5 ϳ 20 ⍀ cm) were used in this study. The Si wafers were first treated with the standard prediffusion clean used at Stanford's Center for Integrated Systems (CIS). The wafers were dipped into Piranha solution (H 2 SO 4 :H 2 O 2 ϭ 4:1) at 90ЊC for 10 min and followed by a deionized water rinse. Then they were immersed in 49% HF:H 2 O ϭ 1:50 solution at room temperature for 30 s and went though the deionized water rinse. They were dipped into the SC2 solution (HCl:H 2 O 2 :H 2 O ϭ 1:1:5) at 80ЊC for 10 min and rinsed in deionized water. The Si wafers were then immersed in an SC1 solution (NH 4 OH:H 2 O 2 :H 2 O ϭ 1:1:5) made from ultrapure chemicals (Kanto Co), which were spiked with 0, 0.1, 1, and 10 parts per billion of Al in solution. Al in 1...

Investigation of the initial deposition steps and the interfacial layer of Atomic Layer Deposited (ALD) Al2O3 on Si

“…However, it is not evident if this oxidation is in-herent to the ALD process or if it is due to subsequent oxidation after the sample exposure to air. In both cases, this result shows that the Al deposition enhances Si oxidation, even at low Al surface concentration [30,31]. Frank et al [30] also showed that after the first TMA pulse, subsequent D2O exposures lead to subsurface oxidation of Si [30].…”

“…The ALD film starts forming on those defects. Then, subsequent deposition of Al on the surface catalyzes further Al2O3 deposition on nearby sites, as well as localized substrate oxidation, under and around the deposited Al2O3 [30,31].…”

“…The proposed mechanism for the Si oxide formation is the diffusion of O species from the deposited Al2O3 film [29], leading to Si oxidation. However, the reaction of H2O with surface Si groups during the island growth where the surface is not fully covered by the ALD film, catalyzed by the presence of Al [30,31], has also been suggested as a mechanism for the interfacial oxide growth [24,30]. Naumann et al [24] reported that the OH groups formed during the initial island growth, lead to the formation of SiOH species.…”

Ιn situ N2-NH3 plasma pre-treatment of silicon substrate enhances the initial growth and restricts the substrate oxidation during alumina ALD

“…This oxidation can be due to the sample exposure to the atmosphere after the deposition, or to oxidation from H2O. The presence of Al on the surface is known to catalyze the Si oxidation 17,31 . Hence, a small amount of Al deposited on the surface could lead to the oxidation of nearby Si.…”

Characterization of the Humidity Calibration Chamber by Numerical Simulations

“…Furthermore, the redeposition of Al on Si has been shown to be directly proportional to the amount of contaminant Al in SC-1. 13 ALD alumina has already displayed potential as a thin-film material with a low etch rate in a variety of solutions including SC-2; 14 however, in general, the as-deposited ALD Al 2 O 3 films tend to be amorphous 15 and unstable in many solutions. For example, as-deposited ALD Al 2 O 3 has been reported to be susceptible to water corrosion.…”