Extended abstract of a paper presented at MC 2007, 33rd DGE Conference in Saarbrücken, Germany, September 2 – September 7, 2007

Nucleation can play an important role during the formation of silicides, especially when the difference in Gibbs free energy ΔG between the existing and newly formed phase is small. In this work, it is shown that the addition of elements that form a solid solution with either the existing or nucleating phase influences the entropy of mixing and thus changes ΔG. In this way, the height of the nucleation barrier may be controlled, thus controlling the nucleation temperature. The influence of mixing entropy on silicide nucleation is illustrated by experiments for two ternary systems: Co–Mn–Si and Ni–Pd–Si. It is shown that the nucleation temperature of CoSi2 is increased by the addition of Mn, the nucleation temperature of MnSi1.7 is increased by the presence of Co, the nucleation temperature of NiSi2 is increased by the addition of Pd, and the nucleation temperature of PdSi is decreased by the addition of Ni. In all four cases, the effect of the alloying element on the nucleation temperature can be explained by a model on the basis of the concept of mixing entropy.

We have studied the kinetics of NiSi agglomeration and NiSi2 phase formation during heating of NiSi on Si, using simultaneous in situ measurements of resistance, light scattering and x-ray diffraction. NiSi is a desirable contact to Si because of its low resistivity, limited Si consumption and low formation temperature. However, the formation of the higher resistivity phase NiSi2 must be avoided for device applications. Ni thin films 5 to 30 nm thick were deposited on substrates of poly-Si and silicon-on-insulator (SOI) and were studied using heating rates from 0.3 to 27 °C/s. At low heating rates and for the thinnest films studied, NiSi agglomeration precedes NiSi2 nucleation by as much as 350°C. The agglomeration temperature decreases with decreasing film thickness and linewidth. Once the film is agglomerated, the formation of NiSi2 is delayed to higher temperature by its low nucleation site density and decreased contact area. We conclude that agglomeration is the primary failure mechanism limiting the morphological stability of NiSi as a contact material to Si devices.

The results are presented of a study on grain growth in magnetron sputter deposited Cu films. Similarly to the well-known case of electroplated Cu, we observed significant changes in the microstructure of sputter deposited films during storage at room temperature, as evidenced by a decrease in sheet resistance (up to 40% decrease in less than 6 hours). Direct evidence for grain growth was observed by FIB and SEM and by a strong decrease in the XRD peak width. Moreover, a decrease in the total defect content of the film was observed as a function of storage time from positron annihilation experiments, probably reflecting the decreasing number of grain boundaries in the film. It is well known that the microstructure of an as-deposited film is strongly dependent on the deposition parameters. This relationship is summarized in the Thornton structure-growth zone diagram [7]. We have studied the kinetics of room temperature grain growth as a function of Ar pressure and substrate temperature for Cu films with a thickness between 50 nm and 1 micrometer. It is found that spontaneous grain growth during storage at room temperature occurs mainly for a zone T microstructure (at low Ar pressure and a substrate temperature below 40°C), while it does not occur for zone I (high Ar pressure, low substrate temperature) or zone II (substrate temperature > 100°C) microstructures.

A reactive Ti capping layer is needed to getter oxygen contamination and to make the cobalt silicidation reaction a more robust process. However, the presence of a Ti capping layer induces two other effects (in addition to the beneficial gettering of oxygen impurities): the formation temperature of CoSi2 is increased and the CoSi2 layer has a strong (220) preferential orientation. Because of the current technological importance of the Ti/Co/Si system, we have made a detailed investigation of the influence of several process parameters (annealing temperature, selective etching, layer thickness) on the nucleation of CoSi2 in the Ti/Co/Si system. Moreover, it is shown that the addition of Ni (i.e. a Ti/Co/Ni/Si or Ti/Ni/Co/Si structure) causes a decrease of the CoSi2 nucleation temperature.

The silicide formation for Ni/Pd and Pd/Ni bilayers on Si(100) substrates was investigated. X-ray diffraction and photoelectron spectroscopy (XPS) depth profiling have been applied to study the phase formation of the silicide. We found that with addition of Pd into Ni/Si, a uniform layer of ternary Ni1−xPdxSi layer formed and kept stable for a wide temperature range. The lattice parameter of Ni1−xPdxSi as a function of Pd addition was calculated. The nucleation temperature of NiSi2 was delayed due to the addition of Pd. The higher the Pd addition, the larger the increase in NiSi2 nucleation temperature. We also studied the effect on the addition of Ni to the Pd/Si reaction. For pure Pd/Si reaction PdSi nucleated from Pd2Si at 750°C or above. For Ni/Pd/Si reaction, Pd2Si changed to Ni1−xPdxSi at temperature lower than 750°C due to the incorporation of Ni. The phenomena were explained by classic nucleation theory taking into account the effect of mixing entropy effect.

In this work, AFM and sheet resistance measurements are used to characterize the thermal stability of thin CoSi2 films. We were particularly interested in the influence of different multilayer structures on the topography and surface roughness. Four different multilayer structures (Co/Si, TiN/Co/Si, Ti/Co/Si and Co/Ti/Si) were investigated. Thermal degradation of CoSi2 formed from a standard Co/Si structure is found to have an activation energy of about 4.2 eV, independent of layer thickness (in agreement with previous results by Alberti et al.). A TiN capping layer is shown to improve the thermal stability. However, if the TiN layer is too thick (e.g. 50 nm), a new failure mode is observed: although the TiN prevents grain boundary grooving of the silicide, the thermal stress induced by the TiN causes the CoSi2 layer to crack. For a Ti capping layer, a strong increase of the thermal stability of the CoSi2 layer is observed, even if the Ti capping layer is removed by a selective etching step after the first RTP annealing. The presence of a very thin Ti-O-N containing layer on the CoSi2surface seems to strongly decrease surface diffusion and in this way reduce the tendency for grain boundary grooving.

Evidence is presented that the nucleation of CoSi2 can be influenced by the presence of small amounts of other elements. The presence of trace amounts of Ti in the CoSi (originating from either a Ti capping layer or interlayer) causes an increase in the CoSi2 nucleation temperature. Moreover, the presence of Ti in the CoSi induces a preferential orientation of the CoSi2 : for an increasing amount of Ti, we observed a transition from polycrystalline CoSi2 over preferential (220) orientation towards epitaxial (400) CoSi2. We observed similar effects for other elements (e.g. Ta, W, C, Mo, Cr). We were able to explain these findings based on the heterogeneous nucleation of CoSi2.

It is generally known that nucleation effects strongly influence the CoSi to CoSi2 phase transition. According to classical nucleation theory, the small difference in Gibbs free energy between the CoSi and CoSi2 phase is responsible for the nucleation barrier. Adding elements that are soluble in CoSi and insoluble in CoSi2 will influence the entropy of mixing, and thus change ΔG. In this way, the height of the nucleation barrier may be controlled.

By depositing Fe or Ge (respectively replacing Co and Si in the CoSi lattice) in between the Co and the Si substrate, we were able to increase the nucleation barrier. In the presence of Ni, the nucleation barrier is lowered, and low-resistive disilicide is formed at lower temperatures.

We have studied CoSi2 formation in the presence of a Cr or Mo interlayer or capping layer. We shall show that, contrary to what was previously reported, Cr and Mo may be used as interlayers to grow epitaxial CoSi2. However, unlike for Ti, the thickness of the interlayer is very important. If the Cr or Mo interlayer is too thick (> 5 nm), polycrystalline CrSi2 or MoSi2 are formed first and epitaxial growth of CoSi2 becomes impossible. However, both XRD and random/channeling RBS results indicate that for a 2-3 nm interlayer of Cr or Mo, CoSi2 forms epitaxially on Si(100). For thinner interlayers, there is a preferential (220) and (400) orientation. This can be explained by the presence of Cr or Mo on the CoSi grain boundaries, which will affect the heterogeneous nucleation of CoSi2.

Ultra-thin epitaxial CoSi2 films formed by Co(3∼5nm)/Ti(1 nm)/Si(100) and Co(3∼5nm)/Si(lnm)/Ti(Inm)/Si are studied. The multilayers are deposited by ion-beam sputtering. Rapid thermal annealing (RTA) is used for silicidation. XRD, RBS, TEM, AFM, four-point probe, I-V and C-V measurements are carried out for characterization. The XRD spectra show the CoSi2 film formed by Co/Ti/Si or Co/Si/Ti/Si solid phase epitaxy has, epitaxial characteristic. XTEM shows that the film is continuous. RBS/Channeling shows that the formed CoSi2 has sharp interface with a minimum channeling yield of Co signal of 40%. AFM shows that the surface of ultra-thin CoSi2 film is smooth with a roughness of nearly 0.7 nm. The Rs∼T relationship shows that the CoSi2 films formed by Co/Si/Ti/Si reaction have the best thermal stability (stable up to 900°C). Those formed by Co/Ti/Si reaction are stable up to 850°C, while those formed by Co/Si reaction are only stable up to 750°C. By fitting the experimental I-V and C-V curves of the epitaxial CoSi2/Si Schottky diodes, barrier heights of around 0.6 eV and close to unity ideality factors are obtained.

Ballistic electron emission microscopy (BEEM) and deep level transient spectroscopy (DLTS) have been used to study the effects of substrate damage introduced by an ion-milling process in PtSi/n-Si Schottky contacts. Argon ions with well-defined energies of 300, 500, 700, 1000, 1500 eV were used to sputter n-type Si substrates in an ion beam sputtering system before metal deposition and silicide formation. Histograms of the PtSi/n-Si Schottky barrier height (SBH) measured by BEEM show that the mean SBH decreases with increasing ion energy, which can be explained as a result of donor-like defects that are introduced by the ion milling treatment. From DLTS measurements, we found direct evidence for the presence of such defects.

In this work, we will show that a reactive Ti capping layer can reduce oxide layers that are present at the cobalt-silicon interface. We have used a thin, chemically grown oxide to have a well-controlled form of interfacial oxide layer. To study the silicidation reaction, isochronal annealing was done at various temperatures. Different sets of samples were made in order to investigate the influence of the thickness of the Co layer and its Ti cap. Titanium capping strongly influenced the silicidation reaction. We have found that already at moderate temperatures, Ti can diffuse towards the Co/Si interface, and can reduce the interfacial oxide there present, enabling CoSi2 formation. The CoSi2 thus formed has a preferential epitaxial orientation, and the process by which this epitaxy is attained may be explained by a combination of OME (Oxide Mediated Epitaxy) and TIME (Titanium Mediated Epitaxy).