Formation Of Co2Si Research Articles

Stress evolution in Co/Ti/Si system

The total force per unit width ( F/ W) in the film during isochronal annealing of the Co/Si and Co/Ti/Si systems were measured using a laser scanning method for substrate curvature measurements. During isochronal annealing of the sample at a ramp rate of 5 °C min −1, several abrupt changes of F/ W were observed. The correlation between the phase transformation of cobalt silicides and the abrupt changes of F/ W was found. For these two systems, formation of Co 2Si and transition of Co 2Si to CoSi induce compressive changes in F/ W, while transformation of CoSi to CoSi 2 induces tensile changes in F/ W. With Ti interlayer increasing from 0 to 9 nm, the CoSi 2 formation temperature is raised from 509 to 673 °C and the magnitude of the maximum change in F/ W is reduced from 160 to 80 N m −1.

Reaction sequence of Co/Ni/Si(001) system

The influence of a Ni interlayer on the formation of cobalt silicides is investigated. By substrate curvature measurements, x-ray diffraction, and Auger electron spectroscopy, a detailed reaction sequence for the Co(90 nm)/Ni(9 nm)/Si(001) system annealed at a ramp rate of 5 °C/min is revealed. For the relatively thick Ni interlayer, the reaction sequence began with the formation of Ni2Si and was followed by the formation of Co2Si on top of the Ni2Si. As the temperature rose, the layer of Ni2Si transformed into NiSi and the formation of CoSi occurred quickly. After, the occurrence of the (NixCo1−x)Si2 phase began at the interface of NiSi/CoSi. The initial phase formation of cobalt silicides occurred via the diffusion of Si through the layer of nickel silicides. It is only when the (NixCo1−x)Si2 phase formed that both Co and Si became mobile in the layer and a CoSi2–(NixCo1−x)Si2–CoSi2 structure developed. The structure exhibits a preferred (400) orientation for the bottom CoSi2 layer.

Effect of Kr +-mixing and thermal treatment on Co-silicide formation

Silicide formation in the Co/Si system during ion beam mixing with Kr + at elevated temperatures and subsequent annealing has been studied as a function of Kr + dose, implantation temperature and annealing conditions. It was found that CoSi is formed during ion beam mixing even at room temperature. The formation of Co 2Si is only detected for mixing at a target temperature of 350°C. During the additional thermal treatment at temperatures between 600°C and 1000°C the whole Co-layer reacts with silicon and polycrystalline CoSi 2 grows. The grain size of CoSi 2 was found to depend on Kr + dose, implantation temperature and substrate orientation.

Silicide formation in the Co-Si system by rapid thermal annealing

Rapid thermal annealing was used to obtain additional information on the phases of the Co-Si system formed during the silicidation reaction between cobalt thin films and silicon substrates. The phase sequence, their stability and their coexistence were investigated by X-ray diffraction and sheet resistance. At an appropriate temperature all three phases are likely to coexist for short times, although the most common observation is the sequential formation of Co 2Si, CoSi and CoSi 2 as reported earlier. Cursory determination of the activation energy for CoSi 2 formation by sheet resistance measurements is 241.1 kJ mol -1.

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 restriction is expected to relax due to enhanced atomic mobilities under ion irradiation; the ion-induced reactions should then be driven by thermodynamics; i.e., growth of the phase with the largest ΔHf is favored. In this work, phase formation induced thermally and with ion mixing in the Co-Si system was investigated using Rutherford backscattering spectrometry and x-ray diffraction (Read camera). It was found that in thermal annealing, Co2Si is the first phase to form and Co is the dominant moving species in the formation of Co2Si, in agreement with previous results. In ion mixing, both CoSi and Co2Si are observed to form. At low temperatures, the formation of CoSi dominates. As the substrate temperature is increased, the formation of Co2Si becomes more significant. Co and Si are found to be the dominant moving species in the ion-beam-assisted formation of Co2Si and CoSi, respectively. By introducing the concept of an effective heat of formation, initial phase formation in both ion mixing and thermal annealing can be rationalized in terms of the thermodynamical driving force.

I n s i t u resistivity measurement of cobalt silicide formation

I n situ resistivity measurements have been utilized to study the reaction and silicide formation between cobalt and amorphous silicon thin films from room temperature to 800 °C. In conjunction, structure and composition changes were analyzed by x-ray diffraction and Rutherford backscattering spectrometry. Formation of Co2Si, CoSi, and CoSi2 were observed. Interfacial reaction to form Co2Si occurs at approximately 400 °C. In bilayers of excess silicon, CoSi forms at approximately 520 °C and, if free silicon is still present, CoSi2 forms at about 550 °C. In the case of excess cobalt, Co2Si forms first and is followed by a cobalt-rich solid solution. Co3Si silicide was not observed.

Kinetics and moving species during Co2Si formation by rapid thermal annealing

We have investigated the growth kinetics and identified the moving species during Co2Si formation by rapid thermal annealing (RTA). For the kinetics study, samples which consisted of a thin Co film on an evaporated Si substrate were used. To study which species moves, samples imbedded with two very thin Ta markers were employed. Upon RTA, only one silicide phase, Co2Si, was observed to grow before all Co was consumed. The square root of time dependence and the activation energy of about 2.1±0.2 eV were observed during the Co2Si formation up to 680 °C. The marker study indicated that Co is the dominant mobile species during Co2Si formation by RTA. We conclude that Co2Si grows by the same mechanisms during RTA and conventional thermal annealing.