Formation of Ultra-Thin Silicon Oxynitride Films by Low-Energy Nitrogen Implantation.
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ABSTRACT
Oxynitrides (SiOxNy) have been used as gate insulators for submnicron devices [1]. The present work reports the oxynitride formation at SiO 2/Si structure by N 2' implantation at low energies. Si substrates were implanted with N 2 ÷ion beams (energy = 5.6 keV and dose -lx 1015 ions/cm 2), annealed at 950 0C for 30 min in N2 ambient, oxidized at 950 0C in 02 + 1% TCE environment and annealed at 950 0C for 30 min in N 2. After these process steps, the oxynitride formation was investigated by FT1R, SIMS and ellipsometric analysis. These physical characterizations revealed the presence of Si-O and Si-N bonds. The film thicknesses and refractive indexes were 7 nm and 1.62, respectively. The dielectric constant = 4.39 and effective charge density = 7x 1010 cm- 2 were determined by C-V, indicating that the SiO•Ny films formed are suitable gate insulators for MOS devices.
INTRODUCTION Ultra thin oxynitride films (SiO×Ny) grown in NH3, N 2 0 or NO ambients are utilized as gate insulators for submicron MOS devices [1] due to their high radiation hardness [3,4], low gate threshold shifts, high dielectric constant and low impurity difusion [3,5]. These properties are due to nitrogen incorporation at the Si0 2/Si interface. The Si-N bonds replace the strained Si-O bonds at the Si0 2 /Si interface, decreasing the interface strain [6]. However, RTN or conventional thermal nitridation of Si0 2 film require high-temperature treatments, incompatible for ULSI technology [1,3]. Otherwise, low-temperature depositions in NH 3/SiH 4 ambients [2,31 incorporate hydrogen and introduce a large number of electron traps in the oxynitride film [3]. As alternative for these processes, medium-energy and high dose nitrogen ion implantation through the poly-Si gate MOS structure was used to improve the gate oxide integrity [2]. Also, nitrogen ion implantation have been used to produce silicon nitride films for local oxidation [7,8,9,13]. It was found that the high dose nitrogen implantions degrade the quality of the films [2,7,16], resulting in high values of effective charge density [2]. The present work reports the first experimental results of the oxynitride formation by low-energy and medium dose nitrogen implantation in Si substrates prior to thermal oxidation. Using TRIM program [10], the implanted nitrogen distribution in the Si wafer was simulated. Ellipsometry was used to determine the thickness and refractive index of the SiOxNy films formed. Chemical bonding characteristics of the SiOxNy films were determined by FTIR analyses and the nitrogen profile at the SiONy/Si structure was measured by SIMS. C-V measurements [11] were performed to evaluate the insulator-semiconductor interface and bulk properties. 249 Mat. Res. Soc. Symp. Proc. Vol. 396 0 1996 Materials Research Society
EXPERIMENT The silicon oxynitride layers were formed on p-type single-cristal Si( 100) wafers with resistivities ranging from 4.9 to 9.1 Q.cm. These substrates were cleaned by a RCA method, implanted with molecular nitrogen ions (GA4204/EATON CORPORATIO
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