Electrical properties of CoSi2 precipitates in cobalt-implanted silicon: a conducting atomic force microscopy study

Abstract

Ion beam synthesis of cobalt disilicide CoSi2 in silicon has been the subject of a number of studies [1±3]. The peculiar properties of CoSi2 which make it a suitable candidate for wide application in microelectronic devices such as contact materials, gate electrodes or interconnects are its low resistivity, its high thermal stability and its cubic CaF2 structure with a lattice constant only 1.2% smaller than that of Si. The buried CoSi2 thin ®lm with high quality can be formed after post-implantation annealing due to the latter factor. For as-implanted samples, a large amount of CoSi2 precipitates distribute on the surface and in the bulk of silicon; the details of the distribution pro®le depend on dose, energy and substrate temperature during implantation. Previous investigations were focused mainly on classi®cation and identi®cation of the type of CoSi2 precipitates, formation and characterization of thin buried CoSi2 layer, and electrical properties such as Hall coef®cient and temperature-dependent resistance. The invention of scanning tunnelling microscopy (STM) in 1981 and subsequently atomic force microscopy (AFM) stimulated the development of many novel scanning probe microscopes [4]. Combined investigation of surface morphology and simultaneous electrical imaging can be realized by modifying the commercial AFM, i.e. adding a preampli®er and using a conducting probe tip. Conducting AFM has been used to study the delineation of semiconductor doping [5], local electrical properties of metal surfaces [6] and quantization of the conductance of alkane layers on graphite [7]. Recently, preliminary results of mapping the conductance of Ni±SiO2 composite have been reported by using conducting AFM [8]. A substantial change of conductance for the Ni composition below and above the critical value was observed. However, owing to its low resolution, it is impossible to probe the electron transport of a single Ni cluster or connected conducting channel. Consequently, the conductance from the experiment is the sum of surface metal particles that are in contact with the probe. We report here the ®rst mapping of the CoSi2 precipitates in silicon media, and their electron transport properties on the nanometre scale by using conducting AFM. Commercial silicon nitride cantilevers coated with Ti=Au or Cr=Au by evaporation were used. These tips show good conductivity and reproducibility during scanning. The Co implantation was performed with a metal vapour vacuum arc (MEVVA) ion source into (1 0 0) p-type silicon wafers with resistivity 10±20 Ucm at an extraction voltage of 70 kV to a dose of 2 3 10 cmy2. The substrate temperature during the implantation ranged from 210 to 700 8C. Details of the implantation process and characterization of asand post-implanted samples by transmission electron microscopy (TEM) and electrical measurements have been reported elsewhere [2]. TEM and high resolution electron microscopy (HREM) revealed that Aand B-type CoSi2 precipitates distribute from the surface down to 280 nm in the bulk. The density of the precipitates is correlated with the depth pro®le of the Co centres at low substrate temperature, and the large density position shifts to the surface with an increase in substrate temperature due to coarsening and coalescence effect. At suf®ciently high substrate temperature (above 650 8C) a continuous layer is formed near the surface. Generally the typical size of the precipitates increases with the substrate temperature. When an external voltage is applied between the conducting tip and the sample, there is a current owing in the electrical circuit. The tip may be in contact with the silicon surface or a CoSi2 precipitate on the surface during scanning. (A thin silicon dioxide layer was present on the silicon surface because no chemical treatment was performed before the measurement.) A signi®cant current increase is expected when the probe tip is in contact with the surface precipitates due to different contact resistance between Au=CoSi2 and Au=SiO2 contact. The main transport mechanism in the sample is via tunnelling since a barrier of about 0.6 eV exists between CoSi2 and the surrounding silicon media. Of course, the electron may transport directly through the silicon media. There is also the possibility for the electron to transport via precipitate networks formed, especially for samples implanted at higher dose. The physical mechanism of the system is similar to that of metal±insulator composite, which is beyond the scope of this letter. The surface morphology and simultaneous current image for a sample at substrate temperature of 210 8C with bias V ˆ 100 mV is presented in Fig. 1. The surface is rather at, with root-mean-square