Lateral self-diffusion and electromigration in thin metal films

Two techiques which have been developed to study lateral self-transport in thin films are described and compared. Both are non-destructive and involve the determination of the spatial distribution of suitable radioactive tracers incorporated within the films. A sample application of the previously described digital tracer scanner has been made to self-diffusion and electromigration in Sn films; it is found that at 213°C Sn migrates toward the anode with an effective valence Z¹ of -46.7 and with a diffusivity D of 2.91 × 10^-10 cm²/sec. In addition, a new high resolution autoradiography method is described; it is based on the detection of Ag X-rays emanating from exposed photographic emulsions when they are viewed in the scanning electron microscope. Application to Au films provides unambiguous evidence for grain boundary electrotransport which is anode directed.     

Low temperature interdiffusion in the Au-Pd and Au-Rh thin film couples

Interdiffusion profiles in thin polycrystalline multilayer films of Pd-Au and Ti-Rh-Au at temperatures up to 490°C have been measured by Rutherford backscattering. Room temperature grain boundary diffusion of Au into Rh was observed and analyzed to give D_B = 3.5 × 10^-17 cm² sec^-1. The Whipple analysis is applied to our data for the diffusion of Au in Pd; using the lattice diffusivity of Neukam, an activation energy for grain boundary diffusion of 0.9 eV is found. The diffusion of Pd in Au has also been analyzed using the Whipple model, which gives a grain boundary activation energy of 0.6 eV.     

Lattice and grain boundary diffusion of copper in thin aluminum films

The lattice D₁ and grain boundary δD_b diffusivities of Cu in Al thin films at 130–185°C are calculated from measurements employing Auger electron spectroscopy and Ar ion beam etching. The calculated values are D₁ = 0.065 cm² s^-1 × exp(-122 kJ/RT) and δD_b = 4.5 × 10^-9 cm³s^-1 exp(-97.4 kJ/RT). The D₁ value is 3–5 times larger at 130–185°C than that predicted by an extrapolation of radioactive tracer measurements of large grain bulk specimens at 433–652°C. The higher value measured here is attributed to the higher density of subgrain defect structures in the thin film.     

Diffusion kinetics of Au through Pt films about 2000 and 6000 Å thick studied with Auger spectroscopy

The diffusion kinetics of Au through Pt films were studied because of the frequent use of PtAu metallizations in semiconductor technology and because Au is often undesirable in active semiconductor regions etc. that are in contact with the Pt. PtAu couples with approximately 2000 Å and 6000 Å Pt films were heat treated between 250° and 350°C in 1 atm N₂ ambient. Surface composition and depth profiles were measured using Auger spectroscopy and ion milling. Au was found to diffuse initially through thin Pt films (< 6000 Å) by grain boundary migration and more than 10¹⁵ atoms cm^?2 of Au crossed the Pt film when the bulk of the Pt contained very little Au (?1 at.%). For 2250 Å Pt films on Au, the time t(X = 0.5) for half-saturation of the Pt surface with Au was given by t(0.5) = 1.2 X 10^?7X exp (0.96 eV/kT) min, where X is the fractional Au concentration in the first 7 Å of the surface. This diffusion rate is relatively fast; e.g.t(0.5)≈7 min for a 2250 Å Pt film at 350°C. At t(0.5) the bulk of the Pt contained less than 1 at.% Au and t(0.5) was proportional to Pt film thickness near 2000 to 6000 Å.     

Study of NaTl-type IaIIIb Li compounds prepared by the sandwich-type evaporation method

Films of the I_aIII_b intermetallic compounds LiAl and LiIn have been deposited by a new evaporation method named the sandwich-type evaporation method.The method involves evaporating I_a and III_b atoms in the order III_b atoms first, I_a and III_b atoms second and III_b atoms last, to prevent Li atoms diffusing into the glass substrate or becoming exposed to the atmosphere (i.e. the co-deposited I_a and III_b atoms are sandwiched between III_b atoms).The degree of crystallization of the films was investigated by electron diffraction and electrical resistance measurements during annealing.Films of LiIn compound were obtained when the as-deposited films were annealed at substrate temperatures of 200°C or above, and films of LiAl compound were obtained when films deposited at 350°C were annealed for 90 min or more at the same temperature.The resistivity at room temperature was about 5 × 10^?4 Ω cm for LiAl films and 2 × 10^?5 to 5 × 10^?5 Ω cm for LiIn films, which is in good agreement with the resistivity of bulk LiIn measured by Mooser and Junod.     

Low temperature interdiffusion in Cu/Ni thin films

Interdiffusion in Cu/Ni thin films was studied by means of Auger electron spectroscopy in conjunction with Ar⁺ ion sputter profiling. The experimental conditions used aimed at simulating those of typical chip-packaging fabrication processes. The Cu/Ni couple (from 10 μm to 60 nm thick) was produced by sequential vapor deposition on fused-silica substrates at 360, 280 and 25 °C in 10^-6 Torr vacuum. Diffusion anneals were performed between 280 and 405 °C for times up to 20 min. Such conditions define grain boundary diffusion in the regimes of B- and C-type kinetics. The data were analyzed according to the Whipple-Suzuoka model. Some deviations from the assumptions of this model, as occurred in the present study, are discussed but cannot fully account for the typical data scatter. The grain boundary diffusion coefficients were determined (for nickel through copper, Q_b = 33.7 kcal mol^-1 (1.46 eV), D_b⁰ = 4.2 × 10^-2 cm²s^-1; for copper through nickel, Q_b = 30.2 kcal mol^-1 (1.3 eV), D_b⁰ = 7.6 × 10^-5 cm²s^-1) allowing calculation of respective permeation distances.     

Electrical properties of silver selenide thin films prepared by reactive evaporation

The electrical properties of silver selenide thin films prepared by reactive evaporation have been studied. Samples show a polymorphic phase transition at a temperature of 403 ± 2 K. Hall effect study shows that it has a mobility of 2000 cm²V^?1s^?1 and carrier concentration of 10¹⁸ cm^?3 at room temperature. The carriers are ofn-type. X-ray diffraction study indicates that the as-prepared films are polycrystalline in nature. The lattice parameters were found to bea= 4.353 Å,b= 6.929 Å andc = 7.805 Å.     

Structural investigation of silicon films chemically vapour deposited onto amorphous SiO2 substrates

The influence of substrate temperature and the silane-to-nitrogen ratio on the structure of silicon films 0.5–0.6 μm thick deposited onto amorphous SiO₂ substrates was investigated by X-ray diffraction. The investigations were carried out for silicon films deposited at various temperatures in the range 500–750 °C and with various silane-to-nitrogen ratios in the range 3.04 × 10^-4-2.84 × 10^-3 by volume. The silicon films deposited at 500 °C were amorphous while the films deposited at 550 °C were randomly oriented polycrystalline. The films deposited in the temperature range 600–700 °C were polycrystalline with a preferred orientation that changed from 〈110〉 through 〈100〉 to 〈111〉. The structure of the films deposited at 750 °C was randomly oriented polycrystalline. Investigations of the influence of the silane-to-nitrogen ratio on the silicon film structure revealed that the structure of films deposited at a substrate temperature of 500 °C was independent of the silane-to-nitrogen ratio. The structure of the films deposited at 600 °C depended on the silane-to-nitrogen ratio and changed from polycrystalline with a 〈110〉 preferred orientation to randomly oriented polycrystalline when the ratio was increased. The structure of films deposited at 700 °C also depended on the silane-to-nitrogen ratio and changed from randomly oriented polycrystalline to polycrystalline with double preferred orientation (〈100〉 and 〈111〉) when the ratio was increased.     

Effect of thermal expansion of the grain boundary reflection coefficient on the temperature coefficient of resistivity of thin polycrystalline films

Numerical values for the exact temperature coefficient of resistivity (TCR) ratio β_Fp/β₀ of thin polycrystalline films were evaluated taking into account the influence of the thermal expansion coefficient of thickness a, the grain size a_g and the grain boundary reflection coefficient r. When the thermal expansion coefficient γ_r related to r is less than (1 ? r) × 10^?5K^?1 we may conclude that for polycrystalline metallic films the TCR dependence on thickness is described with a good agreement by the approximate expression previously derived.     

Preparation of P-type indium phosphide films by close space vapor transport

InP films have been grown by close space transport employing 0.8 mol% PCl₃ in H₂. For deposition on InP single crystals, 700C source and 650C substrate temperatures produced epitaxial films on (100), polycrystalline films on (111)A, and powdery layers on (111)B. Growth rates are 6 to 10 |Gmm/hr on (100) InP and ~50 μm/hr on (111)A InP. Regardless of InP source doping, deposits exhibit net donor concentrations of 5×10¹⁷ to 1×10¹⁸cm^?3. Zn doping with 0.02 to 0.5 mol% Zn(C₂H₅)₂ in the gas phase resulted in partially compensated p-InP with net acceptor concentrations up to 7×10¹⁸cm^?3. Polycrystalline films have been grown on Mg-coated carbon or molybdenum substrates at 700C source and 590C substrate temperatures. Growth rates lie between 40 and 50 μm/hr. Substantial recrystallization and grain growth are observed after 2 day anneals at 950C under 5 atm of phosphorus.     

Investigation of electrochromic properties of nanocrystalline tungsten oxide thin film

Thin films of tungsten oxide were grown by organometallic chemical vapor deposition (OMCVD) using tetra(allyl)tungsten, W(η³-C₃H₅)₄. X-Ray diffraction (XRD) analyses showed amorphous films at substrate temperatures (T_s) <350°C and polycrystalline films at T_s>350°C. Atomic force microscopy (AFM) and scanning electron microscopy (SEM) revealed grain sizes in the range 20–40 nm. In situ electrochemical reduction of WO_3.2/ITO (2.0 M HCl) produced a faint blue color in less than 1 s. The maximum coloration efficiency (CE) was found to be 22 cm²/mC at 630 nm. The density of the films decreases from 4.53 to 4.29 g/cm³ after annealing. An optical bandgap (E_g) of ∼3.2 eV was estimated for both as-deposited and annealed films.     

Microstructural evolution in nanostructured gold films

Thin Au films in the thickness range t = 1.5-126 nm were coated by DC sputtering on SiO_x/Si substrates at room temperature inside a vacuum chamber with a base pressure of about 1 × 10^− 2 mbar (1 Pa). The film structure, nanograin characteristics, and the surface roughness as a function of thickness were analyzed using X-ray diffraction, scanning tunneling microscopy and transmission electron microscopy. The results reflect the microstructural evolution with film thickness. They help us to understand the mode of grain growth, which is monomodal-normal columnar as well as spherical. By determination of the dynamic scaling exponent derived from power law dependence of the mean grain size and film thickness, the prevailed mechanism of grain growth is deduced to be the diffusion of mobile Au atoms in grain boundaries. The surface roughness increases with the film thickness following a power law: R_rms ~ t^b. The linear fitted value for b is 0.60.     

The electrical model of polycrystalline CdSe Thin films

Studies of the electrical properties of polycrystalline CdSe thin films evaporated onto ruby substrates show that the mobility of charge carriers depends on the longitudinal electric field in some ranges of the field intensity. A structural model of a polycrystalline thin film has been developed by other authors. A diode model for the boundary regions has also been used.By analogy with other work we decided to construct an electrical model of the CdSe films that were studied. The proposed model contains two kinds of elements—diodes and resistors. This model was analysed numerically to verify its static electrical characteristics. The qualitative resemblance of the I-U, U_b-U and U_m-U characteristics of the model and the real sample (where U_b is the total voltage drop on all grain boundary regions and U_m the total voltage drop on all microcrystallites) is shown. It seems that after development the proposed electrical model can be used with advantage in the analysis of the electrical properties of semiconductor polycrystalline thin films.     

Effet Hall dans les films d'argent déposés par voie chimique

Study of the Hall effect on silver films prepared by chemical reduction shows an electronic conductivity. At 25°C the Hall coefficient R_H is ?(12 ± 1) × 10^?11 m³ C^?1 and the number of conduction electrons n = 0.89 electrons per atom agrees within 10% of the result obtained from measurement of the conductivity.The mobility of the electron transport in the annealed films, μ_Hr(25°C) = 4.85 × 10^?3 m²V^?1s^?1 is six to seven time greater than that of the same unannealed films, μ_Hi(25°C) = 0.75 × 10^?3 m² V^?1 s^?1.     

Grain boundary self-diffusion in Fe films with a stable nanostructure

Grain boundary self-diffusion in Type-C regime was investigated in nano-crystalline bcc iron at low temperatures between 100 and 250 °C using neutron reflectometry in combination with [^natFe(7 nm)/⁵⁷Fe(3 nm)]₁₀ isotope multilayers. The method allows to determine diffusivities in a stable nanostructure without concurrent grain growth. An activation enthalpy of diffusion of (0.9 ± 0.3) eV is found, comparable to the value of coarse-grained Fe.     

Composition and structure of chemically deposited CulnSe2 thin films

The composition, morphology and crystallographic structure of CuInSe₂ films grown at 50°C and 90°C by the chemical method are discussed. Characterization includes EDS, X-ray and transmission electron diffraction and optical absorption spectroscopy. Nearly stoichiometric CuInSe₂ thin films are obtained with chalcopyrite structure and with thickness in the range 1–3 μm and the grain size in the range 0.2–1.5 μm, band gap near 0.9 eV and absorption coefficient α ≅ 10⁵ cm⁻¹.The effect of deposition mixture temperature on film orientation has been studied by X-ray and electron diffraction. Preferred orientation along [112] direction occurs at a deposition-mixture temperature of 90°C.     

Nickel diffusion in samarium sulfide

The diffusion of nickel atoms in samarium monosulfide (SmS) has been studied for the first time. Using the sequential layer removal technique, it was found that the coefficient of diffusion of the ⁶³Ni radioactive isotope at a temperature of T = 1050°C amounts to D ～ 1.8 × 10^?10 cm²/s in SmS single crystals and to 5.3 × 10^?9 and 1.2 × 10^?10 cm²/s for the fast and slow diffusion components, respectively, in polycrystalline SmS. The process of nickel diffusion in thin polycrystalline SmS films was studied using an X-ray diffraction technique. The coefficient of diffusion at T = 400°C in thin-film samples according to these data is ～10^?13 cm²/s.     

Effect of the substrate temperature on the structural, optical and electrical properties of spray-deposited CdS:B films

Boron doped CdS films have been deposited by spray pyrolysis method onto glass substrate temperature in the range of 350–450 °C. And the effect of substrate temperature (T _s) on the structural, electrical and optical properties of the films were studied. The structural properties of boron doped CdS films have been investigated by (XRD) X-ray diffraction techniques. The X-ray diffraction spectra showed that boron doped CdS films are polycrystalline and have a hexagonal (wurtzite) structure. By using SEM analysis, the surface morphology of the films was observed as an effect of the variation of substrate temperature. The substrate temperature is directly related with the shift detected in the band gap values derived from optical of parameters and the direct band gap values were found to be in the region of 2.08–2.44 eV. The electrical studies showed that the film deposited at the substrate temperature 400 °C had high carrier concentration and Hall mobility and minimum resistivity. This resistivity value decreased with increase in temperature up to 400 °C indicating the semiconducting nature of B- doped CdS films. The lattice parameter, grain size, microstrain and dislocation densities were calculated and correlated with the substrate temperature (T _s ).     

Electrotransport in copper alloy films and the defect mechanism in grain boundary diffusion

Among the polycrystalline thin film conductors of Al, Cu, Ag, Au and their alloys, conductors made of Cu alloyed with Be exhibit the longest lifetimes under similar conditons of electromigration testing. Data have been analyzed for a Cu-Al sample tested at 2×10¹⁶ A cm^-2 and 265°C for 1400 h and for a Cu-Be sample tested at 3×10⁶ A cm^-2 and 291°C for 13 000 h. In both cases the transport of Cu occured from the negative to the positive terminal. The rate of grain boundary diffusion of Cu in Cu-Be is more than fifteen times smaller than that in Cu-Al.Observations of mass transport effects due both to electromigration and to compressive stresses in Cu-Al samples indicate that the atomic flux in grain boundary diffusion is accompanied by a counter flux of defects, presumably vacancies.