Elsevier

Thin Solid Films

Volume 414, Issue 1, 1 July 2002, Pages 13-17
Thin Solid Films

Properties of nitrogen doped silicon films deposited by low pressure chemical vapour deposition from disilane and ammonia

https://doi.org/10.1016/S0040-6090(02)00434-0Get rights and content

Abstract

Nitrogen doped silicon films have been deposited by low pressure chemical vapour deposition from disilane Si2H6 and ammonia NH3. Deposition kinetics is investigated, pointing out the influences of the deposition temperature, the total pressure and the gas flow rates. According to the Bruggeman theory, variations of the NH3/Si2H6 gaseous ratio allow for a wide range of the SiNx stoichiometry as well as a good control of the film nitrogen doping. The different behaviours of the nitrogen atom in silicon films are discussed and an overview of the nitrogen doped silicon physical properties (optical, mechanical and electrical) is proposed for the development of boron-doped polysilicon gates.

Introduction

For the past few decades, miniaturization allows for the improvement of the performances of submicronic MOSFET transistors. As the threshold voltage is linked to the technological process, the control of this non-scalable parameter is of paramount importance in the CMOS structure. The introduction of nitrogen atoms during the oxidation or post-oxidation anneal using ammonia NH3, nitrous oxide N2O or nitric oxide NO gaseous sources improves the behaviour of the silicon oxide as a barrier against boron penetration from the polysilicon gate [1], [2], [3], [4], [5]. However, the presence of nitrogen at the Si/SiO2 interface reduces the boron concentration in the substrate, but not in the gate oxide. In order to prevent boron penetration into the oxide, one solution consists in the deposition of a thin nitrogen doped silicon (NIDOS) layer at the gate/oxide interface. The development of such layers, either implanted [6] or in-situ doped [7], [8], has led to the study of metal/NIDOS/oxide/silicon or polysilicon/NIDOS/oxide/silicon capacitive structures. However, if NIDOS films are to be successfully used in advanced polysilicon gates, the nitrogen atom behaviour in silicon films must be understood and the varied NIDOS film properties must be studied and optimised accordingly.

The aim of this work is to study the deposition kinetics of nitrogen doped silicon films deposited by low pressure chemical vapour deposition (LPCVD) from disilane Si2H6 and ammonia NH3, and to characterise the various physical properties of NIDOS films (optical, electrical and mechanical) for their future use in advanced polycrystalline gate structures.

Section snippets

Experiments

Growth experiments were carried out in a conventional hot-wall, horizontal, low pressure chemical vapour deposition (LPCVD) furnace. Nitrogen doped silicon films were deposited from disilane Si2H6 and ammonia NH3 on 10 cm, (111), oxidised (approx. 120 nm thermal oxide) silicon wafers. The deposition temperature T and the NH3/Si2H6 gaseous ratio R ranged from 450 to 480 °C and from 0 to 6, respectively, while the total pressure P was set constant at approximately 200 mtorr. A full load of 12

Deposition properties of NIDOS films

The deposition kinetic of NIDOS film has been represented on Fig. 1. For NH3/Si2H6 gaseous ratios R ranging from 0 to 1, the activation energy Ea can be considered as constant, roughly equal to 2.25 eV. In fact, the ammonia flow increase is only responsible for the decrease of the deposition rate for a given temperature. In order to characterise this influence, deposition rates Vd have been represented as a function of the ratio between the ammonia flow and the total gas flow, i.e. as a

Conclusion

A complete study of the LPCVD deposition of nitrogen doped silicon (NIDOS) from disilane Si2H6 and ammonia NH3 has been done. Since ammonia has been shown to act as a neutral gas, the influences of the different deposition parameters (temperature, total pressure,…) have been clarified, pointing out the major influence of the NH3/Si2H6 gaseous ratio. Thus, a wide range and a good control of the film nitrogen doping (according to Bruggeman theory, of the film refractive index) have been obtained

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