The ECR-Plasma Deposition of Silicon Nitride on a Tunnel Oxide
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THE ECR-PLASMA DEPOSITION OF SILICON NITRIDE ON A TUNNEL OXIDE J. C. BARBOUR*, H. J. STEIN* AND C. A. OUTTEN** *Sandia National Laboratories, Albuquerque, NM 87185 **Department of Nuclear Engineering, University of Michigan,
Ann Arbor,
MI
48109.
ABSTRACT Gate dielectric-quality silicon nitride films were deposited on a tunnel oxide from an SiH4 /N 2 gas mixture using an electron cyclotron resonance (ECR) plasma. Electrical characteristics depend not only upon the quality of the nitride film but also upon the state of the interfacial oxide. Quasi-static and 1 MHz capacitance-voltage measurements show that a nitride film deposited at 200°C on 2 run thick thermally-grown oxide (tunnel oxide) on an unbiased Si substrate have an interface state density of 2 l.5xlOII /cm -eV. Time-of-flight elastic recoil detection (TOF-ERD) was used to examine the level of mixing between the tunnel oxide and the deposited silicon nitride as a function of bias voltage. TOF-ERD showed that for an applied bias of -350 V (ion energy = 380 eV), the deposition of a 10 nm thick film was completely mixed to form an oxynitride whereas the tunnel oxide remained intact for an unbiased sample. (Interdiffusion resulting from energetic-beam heating was ruled-out as a possibility for the mixing.)
INTRODUCTION A low-temperature and low-damage process for depositing high-quality thin-film silicon nitride is important for semiconducting electronic and optoelectronic devices. Conventional techniques for depositing SiNx include chemical vapor deposition (CVD) and plasma-enhanced CVD (PECVD). The CVD technique produces good quality Si 3 N4 but CVD is a high temperature process (Z600°C) which is not compatible with all device structures or with compound semiconductors. Where low temperatures are required, RF or DC plasma deposition is commonly used to deposit Si3N4 or Si0 2 films on Si and III-V semiconductors. Yet, most applications of PECVD silicon nitride are for the passivation of surfaces [i] or for diffusion barriers [2], because conventional plasma-deposited nitrides do not exhibit electrical properties sufficient to serve as gate dielectric material in metal-insulatorsemiconductor (MIS) structures. The desirable gate-dielectric properties include: a high dielectric constant, low current leakage, high breakdown voltage, control of fixed charge and charge trapping, and low dielectric/semiconductor interface-state density. For comparison to conventional PECVD, we have begun investigating the use of an electron cyclotron resonance (ECR) plasma for the low temperature deposition of gate dielectric-quality Si 3 N4 . The ECR process allows a decoupling of sample and plasma parameters, an ability which is unattainable in conventional plasma systems. In this way, the influence of individual parameters on the nitride film quality can be determined. The parameters of interest include: substrate temperature, film composition, substrate bias-voltage, microwave power, gas flow ratio, and pressure. Conventional PECVD techniques reduce the deposition temperatur
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