Degradation in Oxide Reliability Due to the Presence of Nitrogen in the Oxidation Ambient
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ABSTRACT Ultrathin rapid thermal oxides have been formed in oxygen with varying levels of nitrogen incorporated into the oxidation ambient. Metal-oxide-semiconductor capacitors and MOSFET devices were subsequently fabricated and tested. Device reliability was degraded by the addition of nitrogen into the oxidation ambient. Time-independent catastrophic breakdown measurements showed a large increase in the number of extrinsic breakdowns in devices formed in higher levels of nitrogen. Device performance was measured by interface trap density, subthreshold slope, channel mobility and threshold voltage. A small increase in the interface trap density was observed for increasing levels of nitrogen in the oxidation ambient. However, no trends were observed for MOSFET devices in terms of subthreshold slope, channel mobility or current drive. No improvement in the interface state generation rate due to nitrogen incorporation in the oxidation ambient was observed in this study. X-ray photoelectron spectroscopy detected no nitrogen in the oxides indicating less than 1% nitrogen incorporation. INTRODUCTION In conventional furnace oxidation processes the concentration of nitrogen in the oxidation ambient is relatively high due to ramping-up the temperature in a nitrogen ambient and diffusion of nitrogen into the hot wall furnace from the external environment. Therefore it is difficult to study the effects of low levels of nitrogen in the oxidation environment using a hot wall furnace. Rapid thermal oxidation (RTO) has been utilized as a means of forming ultrathin MOS gate insulators in a controllable manner. RTO reactors can have significantly lower levels of nitrogen since they are cold wall systems capable of supporting a vacuum allowing better control of the gas ambient. As a result, it is advantageous to use RTO to study the effects of low concentrations of nitrogen in an oxidation ambient. In this study, the electrical characteristics of oxides formed in various oxygen/nitrogen mixtures have been investigated. EXPERIMENTAL DESCRIPTION Capacitors were fabricated on 0.1 a-cm (100) p-type, four inch silicon wafers. After patterning of a 2000A field oxide, wafers were cleaned by a conventional five minute, 750C, NH 4OH:H 20 2 :H20 (1:1:5) bath followed by a five minute, 75'C, HCI:H 20 2:H20 (1:1:5) bath. Immediately prior to gate oxide formation the wafers were dipped in 10% HF and rinsed in 645 Mat. Res. Soc. Symp. Proc. Vol. 318. ©1994 Materials Research Society
ultra-pure deionized water followed by a five minute UV/0 3 exposure. A 65A gate oxide was formed by rapid thermal oxidation at 1050'C for 60 seconds at a pressure of 740 Torr with varying concentrations of nitrogen in the oxygen ambient. Rapid thermal LPCVD polysilicon was then deposited without breaking vacuum and subsequently doped by POCI 3 diffusion at 900*C for 30 minutes after removal from the RTP system. As a final metallization step, a 10ooA film of titanium and an 8000A film of aluminum were deposited. The devices were then annealed in forming gas at 400°
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