Pattern Evolution of NiSi 2 on Si Surface upon High Current Pulsed Ni Ion Implantation
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Pattern Evolution of NiSi2 on Si Surface upon High Current Pulsed Ni Ion Implantation
X.Q.Cheng, H.N.Zhu and B.X.Liu Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, CHINA
ABSTRACT Fractal pattern evolution of NiSi2 grains on a Si surface was induced by high current pulsed Ni ion implantation into Si wafer using metal vapor vacuum arc ion source. The fractal dimension of the patterns was found to correlate with the temperature rise of the Si substrate caused by the implanting Ni ion beam. With increasing of the substrate temperature, the fractal dimensions were determined to increase from less than 1.64, to beyond the percolation threshold of 1.88, and eventually up to 2.0, corresponding to a uniform layer with fine NiSi2 grains. The growth kinetics of the observed surface fractals was also discussed in terms of a special launching mechanism of the pulsed Ni ion beam into the Si substrate.
INTRODUCTION Metal silicides have received much attention for their potential applications in integrated circuits because of their low resisitivity and high thermal stability. Of the metal silicides, NiSi2 attracted both fundamental and practical interests because of its unique structural characteristics and properties, and various techniques have been developed and employed to synthesize NiSi2 [1,2]. It is noted, however, that the resisitivity of NiSi2 fabricated by solid-state reaction technique with a formation temperature at 750 °C is in a range of 30-40 µΩ cm [3], which is about twice of that of CoSi2 or TiSi2, and the high resisitivity restricts its practical application. In the mid-1980s, a new ion source was invented, namely the metal vapor vacuum arc (MEVVA) ion source [4]. Naturally, implantation of high current metal ions into Si wafers can cause a significant temperature rise, resulting in a simultaneous thermal annealing of Si substrates during the implantation process. In the early 90s, MEVVA ion implantation was employed by the authors' group to successfully fabricate some important and applicable metal-silicides, such as C54-TiSi2, CoSi2, β-FeSi2, ZrSi2, and NiSi2 [5-9]. MEVVA implantation has unique characteristics. Firstly, neither external heating during implantation nor post-annealing was required for the formation of metal-silicides. Secondly, metal-ions with high current densities from the MEVVA ion source are extracted in a pulse mode. The current density in a pulse is always the same and the increasing of the average current densities is achieved by increasing the pulse number within a unit time [10]. Thirdly, the ion dose can readily be adjusted, enabling one to trace the detailed growth kinetics of the metal silicides step-by-step. Implantation with a
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MEVVA source is thus not only a unique technique for fabricating various metal-silicides, but also a valuable means for studying the growth of fractal patterns consisting of metal-silicide grains on Si surfaces, because the metal atoms are launched into the Si substrate in a very different way in comparing
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