Combined Effect of Rapid Thermal Annealing and Crystal Nature on the Gate Oxide Reliability of Czochralski Silicon

Abstract

Internal gettering (IG) of metallic impurities (MI) in Czochralski silicon (CZ-Si) is one of the key techniques in modern microelectronic engineering to manufacture high-performance devices with high process yield. Various silicon substrates such as EPI wafers with heavily boron-doped substrates, ion-implanted wafers, and rapid-thermal annealing (RTA) treated wafers have been used to enhance IG ability [1-2] and retain denuded zone (DZ) for the active area of devices. Especially, RTA-treatment is an easy way to form IG and DZ simultaneously with a rapid heat process that injects the intrinsic point defects, mainly vacancies, into the bulk. However, it was reported that vacancy clusters, or ‘nano voids’ whose size was less than 50nm [2-3] could be formed in the near-surface region after RTA process via vacancy self-trapping [4], which can possibly degrade the performance and yield of device. In this study, we investigated the effect of RTA treatment on the gate oxide reliability and their relation to the crystal nature, i.e., as-grown point defect distribution in silicon wafers.As shown in Fig. 1 a), we performed the Cu-contamination method [5] on the prepared samples to verify the crystal nature of each sample. After that, we separated the samples into two groups whether they treated by RTA or not, and carried out time-zero dielectric breakdown (TZDB) reliability tests on the patterned poly-Si/oxide/Semiconductor capacitors with the gate oxide thickness of 10 nm. As shown in Fig. 1 b), distribution of TZDB failed cell on the wafer was closely related to the crystal nature. Moreover, it was revealed that a vacancy-dominant region tends to have lower breakdown field than an interstitial-dominant region. These phenomena were only found in RTA-treated wafers and entire fail rate was drastically increases when RTA effect was combined with vacancy-dominated region, which was shown in Fig. 1 b). We assumed that as-grown vacancies contained in the vacancy-dominated region tend to enhance the supersaturation of vacancy beneath the oxide-semiconductor interface, so the formation of nano voids was accelerated and the breakdown field was lowered due to the presence of such defects. A detailed mechanism of the experimental result will be discussed. Figure 1. a) Crystal nature measured by Cu contamination method and b) TZDB frequency with or without RTP