C. Van Bockstael

Phase formation and thermal stability of ultrathin nickel-silicides on Si(100)

The solid-state reaction and agglomeration of thin nickel-silicide films was investigated from sputter deposited nickel films (1–10 nm) on silicon-on-insulator (100) substrates. For typical anneals at a ramp rate of 3 °C/s, 5–10 nm Ni films react with silicon and form NiSi, which agglomerates at 550–650 °C, whereas films with a thickness of 3.7 nm of less were found to form an epitaxylike nickel-silicide layer. The resulting films show an increased thermal stability with a low electrical resistivity up to 800 °C.

Pt redistribution during Ni(Pt) silicide formation

We report on a real-time Rutherford backscattering spectrometry study of the erratic redistribution of Pt during Ni silicide formation in a solid phase reaction. The inhomogeneous Pt redistribution in Ni(Pt)Si films is a consequence of the low solubility of Pt in Ni2Si compared to NiSi and the limited mobility of Pt in NiSi. Pt further acts as a diffusion barrier and resides in the Ni2Si grain boundaries, significantly slowing down the Ni2Si and NiSi growth kinetics. Moreover, the observed incorporation of a large amount of Pt in the NiSi seeds indicates that Pt plays a major role in selecting the crystallographic orientation of these seeds and thus in the texture of the resulting Ni1−xPtxSi film.

Epitaxial Formation of a Metastable Hexagonal Nickel–Silicide

The growth of epitaxial layers of hexagonal {theta}-nickel-silicide on Si(100) and Si(111) substrates is reported. They form at 370 C on Si(100) and 360 C on Si(111), from codeposited Ni/Si mixtures, containing 37 to 42 atom % Si and the equivalent of a 50 nm Ni layer. These codeposited layers model the Ni/Si mixing layer at the interface in sputter-deposited films. The occurrence and stability at room temperature conflict with the phase diagram for bulk Ni/Si. Congruent crystallization is shown to initiate the growth of this metastable phase.

Influence of a transient hexagonal phase on the microstructure and morphological stability of NiSi films

The morphological stability of NiSi is investigated when 40% of Si is mixed into an as deposited 10 nm Ni film. When annealing at 3 °C/s, scanning electron microscopy images and in situ sheet-resistance measurements show that NiSi agglomeration is delayed by more than 100 °C. In situ x-ray diffraction reveals that NiSi grows from an unusual transient hexagonal θ-nickel-silicide phase. The significant improvement of the NiSi film’s morphological stability can be related to its microstructure, with large grains and a strong texture. This peculiar microstructure is compared to the microstructure of the θ-nickel-silicide precursor by electron backscattering diffraction and pole figures.

The influence of Pt redistribution on Ni1−xPtxSi growth properties

We have studied the influence of Pt on the growth of Ni silicide thin films by examining the Pt redistribution during silicide growth. Three different initial Pt configurations were investigated, i.e., a Pt alloy (Ni+Pt/⟨Si⟩), a Pt capping layer (Pt/Ni/⟨Si⟩) and a Pt interlayer (Ni/Pt/⟨Si⟩), all containing 7 at. % Pt relative to the Ni content. The Pt redistribution was probed using in situ real-time Rutherford backscattering spectrometry (RBS) whereas the phase sequence was monitored during the solid phase reaction (SPR) using in situ real-time x-ray diffraction. We found that the capping layer and alloy exhibit a SPR comparable to the pure Ni/⟨Si⟩ system, whereas Pt added as an interlayer has a much more drastic influence on the Ni silicide phase sequence. Nevertheless, for all initial sample configurations, Pt redistributes in an erratic way. This phenomenon can be assigned to the low solubility of Pt in Ni2Si compared to NiSi and the high mobility of Pt in Ni2Si compared to pure Ni. Real-time RBS furt...

Effect of Pt addition on growth stress and thermal stress of NiSi films

We have studied the effect of the addition of Pt on the growth stress and thermal stress of NiSi films. Platinum was added in the form of an interlayer (Ni/Pt/Si), capping layer (Pt/Ni/Si), and as an alloying element within the as-deposited Ni film (Ni–Pt/Si). The evolution of film stress during the solid-state reaction was monitored using in situ curvature measurements. The large transient compressive growth stress observed during the reaction of pure Ni with Si was significantly reduced for samples with a Pt interlayer or alloy. Based on in situ XRD measurements, this reduction in the compressive growth stress could be related to the disappearance of Ni rich phases from the phase sequence and an overall increase in the silicidation temperature during ramp anneals. Pt was also found to affect the buildup of thermal stress while cooling down the sample after Ni1−xPtxSi formation. The presence of Pt as a solute in the monosilicide causes a significant increase in the stress relaxation temperature, and ther...

In situ Study of the Formation of Silicide Phases in Amorphous Ni-Si Mixed Layers

In this paper, we investigated Ni silicide phase formation when Si is added within an as deposited 50 nm Ni film. A series of 22 samples with a Si content varying from 0 to 50 at. % was prepared and systematically investigated with in situ x-ray diffraction. The inert oxide substrate was used to identify the phases which first crystallize in an amorphous Ni–Si mixture of a given concentration. The noncongruent silicides Ni3Si and Ni3Si2 are never observed to crystallize readily out of the mixture. A remarkable observation is the initial crystallization at low temperature of a hexagonal Ni-silicide, observed over a broad mixed layer composition [35–49%Si]; this hexagonal phase nucleates readily as a single phase [39–47%Si] or together with Ni2Si [35–38%Si] or NiSi [49%Si]. This low-temperature phase is related to the high temperature theta-phase, but covers a wide composition range up to 47%Si. For the same Ni–Si films deposited on Si(100), the initial nucleation of the Ni(Si) mixture is similar as for the samples deposited on SiO2, such that the complex sequence of metal-rich Ni-silicide phases typically observed during Ni/Si reactions is modified. For samples containing more than 21%Si, a simpler sequential phase formation was observed upon annealing. From pole figures, the phase formation sequence was observed to have a significant influence on the texture of the technologically relevant NiSi phase. For mixture composition ranging from 38% to 43%Si, the initial transient theta-phase appears extremely textured on Si(100). The observed transient appearance of a hexagonal phase is of importance in understanding the phase formation mechanisms in the Ni–Si system.

On the growth kinetics of Ni(Pt) silicide thin films

We report on the effect of Pt on the growth kinetics of δ-Ni2Si and Ni1−xPtxSi thin films formed by solid phase reaction of a Ni(Pt) alloyed thin film on Si(100). The study was performed by real-time Rutherford backscattering spectrometry examining the silicide growth rates for initial Pt concentrations of 0, 1, 3, 7, and 10 at. % relative to the Ni content. Pt was found to exert a drastic effect on the growth kinetics of both phases. δ-Ni2Si growth is slowed down tremendously, which results in the simultaneous growth of this phase with Ni1−xPtxSi. Activation energies extracted for the Ni1−xPtxSi growth process exhibit an increase from Ea = 1.35 ± 0.06 eV for binary NiSi to Ea = 2.7 ± 0.2 eV for Ni1−xPtxSi with an initial Pt concentration of 3 at. %. Further increasing the Pt content to 10 at. % merely increases the activation energy for Ni1−xPtxSi growth to Ea = 3.1 ± 0.5 eV.

Fundamentals of Intrinsic Stress during Silicide Formation

Silicides are a very useful group of materials which can be used to make electrical contacts to circuits in electronic devices with an extremely high performance. The stress in thin films is an increasingly important technological issue from the standpoint of reliability and performance in IC processing. Manufacturers of micro electronic devices have to control the stress levels in the contact films to avoid device failures. Phase transitions such as silicidation or even a simple rearrangement of atoms like relaxation in the metal film cause a difference in the volume of the film from its starting value. This volume change produces stress inside the film. In this work we analyzed the stress evolution during the silicidation reaction of some metals such as W and Mo by using a home built in situ stress system at the University of Ghent.

Phase formation and stability of Ni silicide contacts - scaling to ultra-thin films

We discuss the effect of scaling of the Ni thickness on the formation and stability of nickel silicide contacts. When scaling the Ni thickness from 15nm down to 5nm, the start of the agglomeration of the resulting NiSi layer is gradually shifted towards lower temperature. The influence of the microstructure of the polycrystalline NiSi layer on the agglomeration behavior, and methods to stabilize the NiSi film will be discussed. For an as deposited Ni film with a thickness less than 5nm, a completely different behavior is observed, where annealing results in the formation of a Ni silicide phase that exhibits an epitaxial orientation with respect to the Si substrate. Based on sheet resistance measurements and scanning electron microscopy, this ultra-thin film is surprisingly stable during post-annealing at high temperatures.