X.-A. Zhao

Transition‐metal silicides formed by ion mixing and by thermal annealing: Which species moves?

The moving species during the formation of Pt2Si, Ni2Si, and CrSi2 by both ion mixing with 300–600 keV Xe ions and thermal annealing is identified with inert markers using backscattering spectrometry. Samples of metal-on-silicon and silicon-on-metal have been used, evaporated on SiO2 substrates with two very thin markers (Mo for Pt2Si, W for Ni2Si and CrSi2) placed at the metal–silicon interface, and at the bottom interface with the SiO2 substrate. Monitoring the separation of the two markers as a function of the amount of silicide formed determines the ratio of atomic transport through the growing silicide layer. The results establish that the dominant moving species in both silicide formation processes is the same for the refractory metal-silicide CrSi2, e.g., Si, whereas different atomic transport ratios are found in the case of the near-noble metal silicides Pt2Si and Ni2Si. This outcome is discussed in terms of high-temperature effects during thermal formation of transition-metal silicides.

Reactive sputtering of RuO2 films

Abstract The growth rate, resistivity and intrinsic stress of RuO 2 films deposited by reactive sputtering with an O 2 Ne or an O 2 Ar gas mixture were investigated as a function of the following sputtering parameters: gas composition, gas pressure, sputtering power and substrate bias. We found that stoichiometric RuO 2 films could be obtained over a wider range of sputtering conditions in the O 2 Ne mixture than in the O 2 Ar mixture.

Correlation between the cohesive energy and the onset of radiation-enhanced diffusion in ion mixing

A correlation between the cohesive energy of elemental solids and the characteristic temperature Tc for the onset of radiation-enhanced diffusion during ion mixing is established. This correlation enables one to predict the onset of radiation-enhanced diffusion for systems which have not yet been investigated. A theoretical argument based on the current models of cascade mixing and radiation-enhanced diffusion is provided as a basis for understanding this observation.

WxN1−x alloys as diffusion barriers between Al and Si

Reactively sputtered tungsten nitride (WxN1–x) layers are investigated as diffusion barriers between Al overlayers and Si shallow n + -p junctions. Both amorphous W80 N20 and polycrystalline W60 N40 films were found to be very effective in preserving the integrity of the n + -p diodes for 30-min vacuum annealing up to 575 °C. Diode failure at higher temperatures is caused by localized penetration of Al into through the WxN1–x barriers. The effectiveness of the barrier decreases for polycrystalline W90 N10 and is worse for pure W.

Reaction of thin metal films with crystalline and amorphous Al2O3

We have investigated the thermal reaction between thin transition metal films and sapphire, alumina or amorphous Al2O3 using backscattering spectrometry and x-ray diffraction. Thin films of Y, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, and Pt were deposited on the substrates by rf sputtering in an Ar gas ambient. The samples were subsequently annealed in vacuum at 800–900 °C for 20 min to 4 h duration. We found that only films of Y, Ti, and Hf react, regardless of the type of substrate, by forming aluminides near the substrate and oxides on the surface. The other metal films do not react with the Al2O3 substrates. Our results agree with thermodynamic consideration based on the heats of reactions between metals and Al2O3.

TiAl3 formation by furnace annealing of Ti/Al bilayers and the effect of impurities

Reactions of Ti/Al couples induced by furnace annealing were investigated (at elevated temperature) using large-grained Al substrates and vacuum-evaporated bilayers of both sequences. 4 He MeV backscattering spectrometry was principally used to monitor the reactions. Profiles of oxygen impurity were obtained by elastic 16 O(alpha,alpha)16 O resonant scattering. In the range of 460–515 °C, TiAl3 forms as a laterally uniform layer at the Ti/Al interface. The thickness of this compound layer increases as (annealing time)1/2. The activation energy is 1.9–2.0±0.1 eV. For evaporated bilayers on an oxidized Si substrate, the sequence of the bilayers does not effect the growth mechanism of TiAl3 , but the growth rate of samples with the Ti on top is lower than that of samples with Al on top, that is, oxygen in Ti/Al samples can reduce the reaction rate by decreasing the pre-exponential factor. Oxygen already contained in the Ti film and oxygen from the annealing ambient are incorporated in the growing TiAl3 compound during thermal annealing. In addition, a TiAl3 layer also forms at the free Ti surface during vacuum annealing when the oxygen-containing contaminants in the ambient are minimized. So far, we succeeded in accomplishing this only for large-grained Al substrates. We conclude that the formation of the TiAl3 compound at the surface is controlled by nucleation and depends sensitively on the condition of the surface layer of the Ti film.

Kinetics of NiAl3 growth induced by steady-state thermal annealing at the Ni- interface

Substrates of large grained aluminum crystals were prepared by the strain annealing technique, and Ni films were vacuum evaporated on these substrates after an in situ sputter cleaning process. Upon thermal annealing of samples in vacuum, a laterally uniform growth of NiAl3 is observed, starting from 330 °C, without any indication of boundary diffusion effects. The aluminide phase grows as (duration)1/2 after an initial incubation period with an activation energy of 1.4 eV, i.e., K=x2/t=0.387 (cm^2/s)exp(–1.4 eV/kT) for 600 K<T<650 K. Impurities, either at the interface or inside the Ni film, retard this reaction.

Cosputtered W75C25 thin film diffusion barriers

Abstract The performance of cosputtered W75C25 thin films as diffusion barriers between an Si〈111〉 substrate and metallic overlayers of silver, gold or aluminum is investigated. Backscattering spectrometry and X-ray diffraction are used to detect metallurgical interactions, and four-point probe measurement of resistance is used to monitor the electrical stability of the metallization schemes upon thermal annealing in vacuum for 30 min at temperatures ranging from 500 to 700 °C. It is found that the W75C25 layer prevents metallurgical interdiffusion and reaction between gold or silver overlayers and the Si〈111〉 substrate up to 700 °C, and between an aluminum overlayer and the Si〈111〉 substrate up to 450 °C. In addition, the W75C25 film as a diffusion barrier on GaAs was also investigated and found to be very effective metallurgically between silver overlayers and GaAs substrates for annealing up to 700 °C.

Growth kinetics of NiAl3 formation on large-grained 〈Al〉 substrates

Abstract To minimize the effect of grain boundaries, Al substrates with single-crystalline grains on the order of up to 1 cm in diameter have been prepared by the strain-annealing technique. An in-situ sputter cleaning procedure has been successfully applied to remove the native oxide layer from the Al surface for an experimental investigation of the growth kinetics of the NiAl 3 layer that forms upon furnace annealing of a thin Ni film deposited on these substrates. Reaction of Ni films with the Al crystals yields laterally uniform growth of Al 3 Ni without any indication of grain boundary diffusion effects. In the temperature range 330–380°C, assuming a stoichiometric NiAl 3 phase of a density of 1.707× 10 22 formula units/cm 3 , the growth of this first aluminide phase is given by x 2 = Kt , where x is the thickness of the compound, t is the annealing time after an initial incubation period and K = 0.387 ×exp(-1.4 eV / kT )cm 2 / s .

Study of the initial aluminide phase growth in Al/Pt couples

Abstract The thermal reaction of thin platinum films with large-grained aluminum substrates is studied with emphasis on the growth kinetics of the initial aluminide phase. In situ d.c. sputter cleaning is performed on the large-grained aluminum substrates before the platinum film is evaporated to ensure a clean interface. Uniform layer-by-layer growth of Pt2Al3 is observed and monitored by 2 MeV4He+ backscattering. The thickness x of this initial aluminide phase grows parabolically with the annealing duration t as x2 = Kt. After the incubation period, the growth constant K is given by K = x 2 t = 8.76 exp − 1.45 eV k B T ( cm 2 s −1 ) for 255°C