Inductively-Coupled Plasma Nitriding of Fused Silica
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INDUCTIVELY-COUPLED PLASMA NITRIDING OF FUSED SILICA T. K. VETHANAYAGAM AND P. F. JOHNSON College of Ceramics, Alfred University, Alfred, NY 14802
ABSTRACT Plasma nitriding of fused silica has been performed over a temperature range of 750'C to 1300°C in a nitrogen-hydrogen plasma generated by an inductively coupled RF discharge. The plasma is used as both thermal and chemical source. The effects of various process parameters such as surface temperature, gas pressure and treatment time on total nitrogen content have been studied. The advantages and the drawbacks of this direct plasma nitriding technique are briefly discussed.
INTRODUCTION Forming a nitride film on silicon dioxide has been of research interest for nearly 20 years'. Such a film can be achieved either by deposition of the nitride layer or by surface modification of the oxide. Nitrided silica materials cover a wide range of practical and proposed applications. The potential use of such nitride film is as radiation-hardened gate insulator in MOSFETS or as capacitor in VLSI circuits. An oxynitride layer on oxide has proved 2 to be a suitable material for optical waveguides used in integrated circuits. Optical fibers for telecommunication purposes need to be coated with the higher refractive index nitride in order to confine the light rays. The better chemical durability of silicon nitride can be taken advantage of in container industries. Mechanical protection against abrasion, wear, and impact, wherever external damage is a major problem, can also be achieved by nitriding fused silica. Thus, due to the fact that such a treatment has so many attractive applications many processing techniques have been investigated and some of them are even being used in commercial production in the electronics industry. The various techniques that have been investigated include Plasma Reactive Sputtering', 4 Low and Atmospheric Pressure CVD , Atmospheric and High Pressure Direct Thermal 5 7 Nitridation",s, Plasma Enhanced CVD , Plasma Enhanced Thermal Nitridation , Anodic 1 9 Plasma Nitridation , Laser(photon) Induced Deposition °, High Energy(60 to 300 keV) Ion Implantation", and Microwave CVD"'. These techniques have one or more of the following drawbacks; longer treatment time, brittleness of the nitrided layer, presence of pinholes, and lack of purity of the source. Carlson13 attempted ion implantation of nitrogen into fused silica below 300'C and was not successful because such low energy nitrogen ions were neutralised before they reached the surface. Neutral nascent nitrogen can either combine with another and desorb, or structurally combine with the fused silica. The latter was not feasible at such low temperatures. But, higher temperatures should improve the rate of reaction and lead to formation of the nitride. Thermodynamics predicts the formation of silicon nitride below 1480°C and corrosion of silicon dioxide above 1480'C, as shown in fig.1, when nascent nitrogen reacts with fused silica under standard thermodynamic conditions. Burlingame14 0 attempted to n
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