Growth of Ultrathin Silicon Oxides by Wet Oxidation

There is increasing interest in the growth of ultrathin oxides of silicon because the fast down-scaling of device dimensions requires a corresponding decrease in gate oxide thickness to <3 nm. The growth of these ultrathin oxides imposes stringent demands on oxidation techniques to achieve the required thickness uniformity and high electrical quality. Wet oxide growth has been reported to produce superior oxide quality, but dry oxidation has remained the main process used for obtaining ultrathin oxides for gate and tunnel oxide use. This is because wet oxidation at 1 atm water vapor pressure results in a high growth rate and because there is a large density of electron trapping centers present in the wet oxide.

Researchers at the Indian Institute of Technology (Madras, India) have grown ultrathin oxides of silicon by wet oxidation at 900° C at very low (0.04 atm) water vapor pressure. The low water vapor pressure enables good control of the oxide thickness to be achieved in the ultrathin region.

The wafers used were single-crystal, n-type silicon, (100) orientated and with resistivity of 1-10 W.cm. After pre-cleaning by the sacrificial oxide growth method, the wafers were dipped into dilute (5%) hydrofluoric acid for 10 sec, rinsed in deionized water for 1 min and blow-dried in pure nitrogen gas. Ultrathin oxide layers were grown on the wafers by the wet oxidation technique. Good control of the oxide thickness in the ultrathin regime was achieved by maintaining the bubbler water temperature at 30°C so that the water vapor pressure was 0.04 atm.

The researchers investigated the electrical properties of the ultra-thin oxides grown in this way by using them to fabricate Al/thin SiO₂/n-Si MOS tunnel diodes with an area of 0.002 cm². The C-V characteristics were studied with an automated LCR meter over the 10 kHz to 1 MHz frequency range. The ultrathin oxide thickness was determined from measured C-V characteristics at 10 kHz where the effect of the series resistance on the C-V characteristic is negligible.

In each case, at least 10 devices were characterized and the results were found to be reproducible to within ±5%. The characteristics of MOS tunnel diodes formed on oxides grown for 3, 5, 6, 8 and 11 min showed that the capacitance decreases with an increase in the oxidation time due to an increased oxide thickness. The C-V characteristics show that there is an increase in the tunneling current in the thinner oxides. The oxide growth rate was linear, indicating that the growth kinetics was reaction-limited.

The I-V characteristics showed that, in the region where direct tunneling occurred, the tunnel current density increased by more than four orders of magnitude as the oxide thickness fell from 4.5 to 2.5 nm. The researchers claim that the tunnel current density of the thinnest of the wet oxides investigated is more than an order of magnitude less than that of dry oxides reported in the literature. This explains the good electrical properties of the wet oxides studied.

The charge trapping characteristics were studied by using the constant current stress technique. This involves passing a constant stressing current through the diode and monitoring the voltage across it as a function of the stress time. The workers used a stress current density of 25 µA/cm².

For diodes with different oxide thicknesses, the gate voltage always decreased from its initial value as the stress duration was increased. This was attributed to positive charge trapping in ultra-thin oxide films. The charge trapping decreases with a decrease in oxide thickness.

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