The Effect of Carrier Gas in Gate Oxidation on the Gate Oxide Integrity of Thick Gate Oxide for UMOSFETs

Abstract

The effect of carrier gas on the gate oxide integrity ͑GOI͒ of thick gate oxide ͑400 Å͒ for U-trench metal-oxide-semiconductor field-effect transistors UMOSFETs was investigated. Ar or N 2 were selected as the carrier gases during gate oxidation, and X-ray photoelectron spectroscopy was used to identify the nitrogen depth profile. It was observed that GOI of the 400 Å gate oxide grown at 1175°C can be improved by using Ar as the carrier gas instead of N 2 due to the fact that N 2 interacts with the Si surface during high-temperature oxidation.Power metal-oxide-semiconductor field-effect transistors ͑MOSFETs͒ have been widely used in applications such as automatic electronics, wireless communication, desktop, and laptop computers. One of the most popular device structures for low and medium power application is UMOSFET, 1-3 in which a U-shaped trench is etched in the Si substrate and the source and drain are located at opposite sides of the wafer. This vertical structure allows higher current capability, higher package density, and lower onresistance.Power MOSFETs require much thicker gate oxides ͑e.g., 400 Å͒ than submicrometer geometry MOS devices which feature ultrathin gate oxides ͑Ͻ30 Å͒ due to high voltage/power rating. Hightemperature gate oxidation gained interest for processing thick gate oxides, because it has been proven that it can improve gate oxide integrity ͑GOI͒ due to the relaxation of Si-SiO 2 interface stress. 4-7 Moreover, it decreases the oxidation time.Nitrogen gas is the most frequently used carrier gas in semiconductor industry during device processing. However, N 2 interacts with the Si surface at 980-1200°C. 8 Heat treatment of silicon wafers in nitrogen at high-temperature ambient ͑1200°C, 1 h͒ resulted in haziness of the surface, due to the formation of stacking fault nuclei. 9 It was observed that the interaction of N 2 at the Si surface would increase defect density and reduce the dielectric strength of the gate oxide in the range of 70 to 600 Å on planar surfaces. [10][11][12] In this work, the effect of N 2 and Ar carrier gas on the gate oxide integrity of a 400 Å gate oxide, which was grown on the trench surface, was investigated and compared. It is the first attempt to evaluate the GOI of a UMOSFET by comparing Ar with N 2 as the gate oxidation carrier gas. ExperimentalFor X-ray photoelectron spectroscopy ͑XPS͒ measurement, a trench capacitor-like structure was used. Three 150 mm diameter, n-type, ͑100͒ heavily doped Si wafers were used in this experiment. 5 m of lightly doped n-type Si ͑n-Si͒ was grown on the top of the substrate by epitaxy, and the experimental splits are shown in Table I. The trench was etched by reactive ion etching ͑RIE͒ and followed by gate oxidation, and the oxidation time was adjusted to achieve a target thickness of 400 Å for all splits. The location for XPS analysis was carried out at the BPSG mesa between trenches. The instrument settings were as follows: monochromated Al K␣ at 1486.6 eV was used as the X-ray source, with an acceptance angle of ±23°, a...

The Effects of Hydrogen Annealing on Gate Oxide Integrity of U-Shaped Trench MOSFET with 400 Å Gate Oxide

“…Positive stress 15 Additionally, it has been proven that higher oxidation temperature resulted in better GOI due to more relaxation of Si-SiO 2 bonds. [6][7][8][9] In summary, the GOI improvement of 1175°C Ar/O 2 oxidation over 1100°C N 2 /O 2 oxidation is a combined effect of an absence of nitrogen atoms at the Si -SiO 2 interface and higher oxidation temperature.…”

“…[11][12][13][14] The effect of oxidation temperature and carrier gas has been investigated and reported elsewhere. 9,15 Another key element of UMOSFET is the formation of a U-shaped channel, or trench. Due to its anisotropic nature, reactive ion etching ͑RIE͒ is used to perform the trench etching.…”