C.A. Hewett

On the thermodynamical driving force during ion mixing of the Co-Si system

The relative importance between the thermodynamical driving force and kinetics in thermal annealing and ion mixing in the thermally activated regime has not been clarified. To probe the role of the thermodynamical driving force in reactions between metals and silicon, the Co‐Si system was chosen for investigation. In general, three silicide phases are formed during thermal annealing of samples consisting of Co thin films deposited on Si substrates, i.e., Co2Si (the first phase to form with a heat of formation, ΔHf=−9 kcal/g atoms), CoSi (ΔHf=−12 kcal/g atoms), and CoSi2 (the last phase to form, with ΔHf=−8.2 kcal/g atoms). Previous experiments have shown that annealing a sample of Si/CoSi/Co converts CoSi into Co2Si instead of a continuous growth of CoSi. This type of reaction is apparently unrelated to the magnitude of the thermodynamical driving force since ΔHf of CoSi is significantly larger than those of Co2Si and CoSi2, but is kinetically restricted instead. Under ion mixing conditions the kinetic re...

An x‐ray photoemission spectroscopy investigation of oxides grown on AuxSi1−x layers

X‐ray photoemission spectroscopy was used to analyze the oxide grown at low temperature on AuxSi1−x films. It was found that the oxide is stoichiometric SiO2, but is structurally distinct from oxides grown on Si at high temperatures (950 °C). Also, unoxidized Au‐Si atom inclusions were observed in the oxide. The composition of the inclusions is dependent on the initial bulk AuxSi1−x composition.

The effect of boundary conditions in ion mixing of multilayered NiSi samples

Abstract In ion mixing of metal-semiconductor systems, bilayer samples initially lead to the formation of equilibrium phases. However, in multilayered samples where the average composition is fixed, ion irradiation at high doses leads to metastable (or amorphous) phase formation. The metastable phase appears to form at a critical ion dose, apparently without the formation of any compounds at subcritical doses (at least not reported in the literature). The objective of this study is to investigate the effect of individual layer thickness of NiSi multilayered samples on ion induced reactions. Four types of NiSi samples were used: (i) Ni( ~ 700 A)/Si〈100〉, (ii) two layers of Ni( ∼350 A) interposed with two layers of Si( ∼ 360 A) on SiO 2 , with an average composition of Ni 1.65 Si (this composition is in the two phase region of the phase diagram), (iii) three layers of Ni( ∼ 240 A) interposed with three layers of Si( ∼ 260 A) on SiO 2 , and (iv) five layers of Ni( ∼140 A) interposed with five layers of Si( ∼145 A) on SiO 2 . It was found that the mixing efficiency increases with the number of layers in the samples. Amorphous phase formation was observed only in samples with six and ten individual layers at high doses. An equilibrium phase of Ni 2 Si does appear to be detectable by X-ray diffraction in all types of samples at low doses. Based on this observation, and to a first order approximation, the boundary condition imposed by multilayered samples is similar to that imposed by bilayer samples for ion mixing at low doses. As mixing continues under ion irradiation the boundary conditions start to differ for these two types of samples. For bilayer samples, Ni 2 Si grows with dose, as reported in the literature. For multilayer samples, amorphous phase formation is possible when uniform mixing is achieved since the average composition of these samples is in a two phase region of the phase diagram. The relationship between layer thickness, mixing efficiency and amorphous phase formation is discussed.

On the temperature dependence and the moving species during ion mixing

Abstract In this paper, we review the experimental observations of the temperature dependence and the moving species in ion mixing, emphasizing the metal-semiconductor systems. Ion mixing is the combined effect of two components. One component is temperature independent and is primarily due to events in the prompt regime; the other component is temperature dependent and has the characteristics of the associated thermal reactions. The moving species during ion mixing are influenced by collisional effects, either due to secondary recoils, or due to local hot spots, or both. The secondary recoil concept is consistent with experimental observations that the motion of the lighter element in a bilayer sample is enhanced. There is ample evidence that while the athermal regime is caused by particle-solid interactions, thermodynamical forces are important in deciding the magnitude of mixing. In the thermally activated regime, the ion induced reaction product should be influenced by the heats of formation of various compounds. We also indicate areas where satisfactory explanations are not available at present.

On the moving species in ion-induced metal-semiconductor interactions

We present evidence in this study that the moving species of metal-semiconductor (metal-Si and metal-Ge) systems under ion mixing conditions are affected by the implantation damage distribution in the sample in accordance with the inverse Kirkendall effect. The direction of thermal annealing atomic transport appears to play a role in ion mixing as well. When these two factors are in the same direction, only one dominant moving species is observed. When these two factors are in opposite directions, both metal and the semiconductor can contribute to the atomic transport in ion mixing.

Phase transformations in ion‐irradiated silicides

The phase transformations in a number of ion‐implanted and subsequently annealed silicides have been investigated. The electrical resistance change as a function of 28Si+ ion implantation has been found to correlate with the presence of a disordered state in the silicide. Epitaxial silicides such as CoSi2 were found to regrow in a layer by layer manner when implanted such that the top region was amorphous but with a single‐crystal seed remaining at the bottom of the original layer. Recrystallization temperatures (defined as the temperature at which one half of the silicide has transformed) were determined by in situ electrical measurements as well as by x‐ray diffraction studies. Recrystallization temperatures were found to be approximately 1/3 of the silicide melting point. Both cosputtered and implanted refractory metal silicides were also found to be governed by this rule. Under the assumption that recrystallization can be described by the Avrami equation, it was found that n, the mode of transformatio...

A simple method to prepare carbon substrates for RBS analysis

Abstract We have investigated the possibility of forming 2 μm thick low Z (carbon, Z = 6) layers on Si wafers via simple and conventional semiconductor processing techniques. These carbon substrates were prepared by spinning a layer of photoresist (Shipley AZ1350J or 1400-33) onto Si substrates and then heating at temperatures of up to 900°C. The resulting carbon layer is sufficiently thick to shift the RBS signal due to the Si to energies below that corresponding to C on the surface. As an example we show the use of these carbon film in the investigation of the SiAu system. In this paper, we present a discussion on the preparation of the carbon substrate and application of these substrates in the investigation of oxidation of the AuSi system and other systems commonly encountered in thin film investigation. The objective of this investigation is to show that very simple techniques commonly available in semiconductor laboratories can be used to fabricate carbon substrates suitable for backscattering analysis.

Moving species during ion mixing in GexSi1−x/metal systems

The origin of the motion of semiconductors during ion mixing was investigated by studying both the temperature and the atomic mass dependence of moving species in the GexSi1−x/Ni and the GexSi1−x/Pd systems. Ion mixing was performed with 280‐keV Ar ions at temperatures between 30 K and room temperature. The atomic mass of the GexSi1−x alloy was adjusted by changing the concentration of Ge in the alloy. In thermally induced reactions, no preferential motion of Si or Ge was observed. During ion mixing, the atomic flux of Si was observed to be enhanced compared to that of Ge. The atomic flux of the sum of Si and Ge to metal decreases with increasing substrate temperature during mixing and with increasing Ge concentration in the GexSi1−x alloy. From the strong atomic mass dependence of the moving species during ion mixing it is concluded that the origin of the motion of semiconductors under ion mixing conditions is due to the effects of secondary recoils.

Improved uniformity of PtSi Schottky barrier diodes formed using an ion mixing scheme

Low dose ion implantation through the Pt/Si silicon interface prior to annealing has been shown to yield a more planar PtSi/Si interface after annealing. The electrical characteristics of these ion mixed diodes have also been shown to be more uniform from device to device as well as more nearly approaching theoretical performance than conventionally prepared diodes.