A metalloid Ti13Cu87 target was sputtered by reactive DC magnetron sputtering at various substrate temperatures in an Ar-N2 mixture ambient. The sputtered species were condensed on Si (111), glass slide and Potsssium bromide (KBr) substrates. The as-deposited films were characterized by X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, Scanning electron microscope (SEM), energy dispersive X-ray spectroscopy (EDX), optical spectrophotometry and four point probe technique. The as-deposited films present composite structure of nano-crystallite cubic anti-ReO3 structure of Ti inserted Cu3N (Ti:Cu3N) and nano-crystallite face centre cubic (fcc) structure of Cu. The titanium atoms and sequential nitrogen excess form a solid solution within the Cu3N crystal structure and accommodate in crystal lattice and vacant interstitial site, respectively. Depending on substrate temperature, unreacted N atoms interdiffuse between crystallites and their (and grain) boundaries. The films have agglomerated structure with atomic Ti:Cu ratio less than that of the original targets. A theoretical model has been developed, based on sputtering yield, to predict the atomic Ti:Cu ratio for the as-deposited films. Film thickness, refractive index and extinction coefficient are extracted from the measured transmittance spectra. The films’ resistivity is strongly depending on its microstructural features and substrate temperature.
Various researches demonstrated the possibility of obtaining a large variety of non-equilibrium microstructure and phase compositions in thin films produced by DC magnetron sputtering. Particularly, several studies were focused on the production of metal alloys made of partly miscible or immiscible elements, deposited in chemically inert (Ar) or in reactive (e.g. Ar-N2) atmosphere. The potential interest in magnetron sputter process rely on the possibility of producing thin films with new properties, markedly different from those of corresponding equilibrium bulk phases when one considers a nano-composite that can be formed, e.g. by combining nano-crystalline phases or by embedding nano-crystalline domains in an amorphous matrix. It is now demonstrated that co-deposition of elements (from different targets or from a mixed target) whose phase diagram presents a wide miscibility gap is a prerequisite for forming non-equilibrium nanocrystalline or amorphous phases. This tendency to form non-crystalline phase is further increased by sputter deposition in a reactive atmosphere [1-9].
Copper nitride has been attracted much attentions due to its low decomposition temperature and sequential applications as write-once optical recording media [10-12], as micrometric conductive lines [
A sintered Ti-Cu target (13 at.%Ti, 76 mm in diameter and 4.5 mm thickness) was reactively sputtered at Ar-N2 gas mixture, and the sputtered species were deposited on ultrasonically pre-cleaned Si (111), glass slide and KBr substrates. The deposition conditions were listed in
The films structure was studied by X-ray diffraction (XRD) in scan mode with CuK radiation (1.5418Å). The crystallites size was estimated from the full width at half maximum (FWHM) of strongest XRD peak using Scherrer formula. The chemical bonding was characterized by Fourier transform infrared (FTIR) spectroscopy (FTIR spectrometer Bruker Tensor 27). Films morphology and their chemical composition were determined using scanning electron microscope and energy dispersive X-ray spectrometer (SEM/EDX, Philips XL 30). Also, an estimation of the atomic ratio was made in according to sputtering yield. Optical studies were performed by measuring transmittance in the wavelength region 360 - 1100 nm using spectrophotometer (Shimadzu, UV-1700 Pharma Spec) at room temperature. These measurements allowed obtaining refractive index, extincttion coefficient and meaning film thickness. The electrical resistivity of the films at room temperature was deduced from measurement using four point probe method.
Copper nitride has a cubic anti-ReO3 type structure and exhibits a vacant site at the centre of the cell [
The evolution of the film lattice constant is due to the variation in nitrogen stoichiometry [24-26]. However, there is no information about the position of excess nitrogen in the Cu3N unit cell [
chemical composition to characterization method, lattice constant of Cu3N revealed good qualitative criterion for composition determination. The samples prepared at substrate temperature of 70˚C, 110˚C, 180˚C are N rich or over-stoichiometric, but the sample prepared at substrate temperature of 150˚C is N poor or substoichiometric. Addition of titanium to Cu3N films does not change the cubic anti ReO3 structure of Cu3N (
Using the broadening of the peaks, it is possible to determine mean crystallite size, from Scherrer formula (Warren) [
(1)
where, is the FWHM of the diffraction peak, λ is the wavelength of the incident CuK X-ray and is Bragg diffraction angle. b is standard instrumental broadening (0.08˚). The mean crystallite size of Ti:Cu3N is calculated by using strongest peak namely (100) peak (
The presence of N neutrals reflected [
Surface morphology of the nitrided Ti-Cu thin films
deposited at substrate temperature are shown in
In order to estimate the atomic Ti:Cu ratio in films, we make the following assumptions:
• difference in throw distance of any sputtered component in target-substrate spacing;
• difference in nitriding kinetics on target surface of any component;
• difference in sticking coefficient of any sputtered component on substrate;
• re-sputtering contribution in altering the chemical composition due to the reflected N neutral and the sputtered Ti and Cu atoms collide with the growing film are excluded. The atomic Ti:Cu ratio can roughly be calculated by [
where is Ti concentration in target surface. is relative partial pressure of jth gas, j = N2 and Ar. is sputtering yield of ith atom due to jth species, I = Ti, Cu.
We assume ion bombardment acts as same as two separate N+ ions with half energy. Sputtering yield is depending on energy and angular such as
where E is the energy of incident ions and is approximately equal to 0.75 eVd [
q, Eth, and, which are material dependent parameters, listed in
Angular distribution of sputtered atom is proposed by Yamamura et al. [
where is a fitting parameter. The fitting parameter depends on the mass and binding energy of the target material, mass and ion energy. It may be expressed as
where Mt is the mass of the sputtered atom, Esb is binding energy of the sputtered material (
The atomic Ti:Cu ratio is shown in
it is more difficult to nitride Cu than Ti due to weaker Cu-N bonding. After being sputtered for some time the Ti:Cu ratio on the target surface will reach an equilibrium so that the ratio of the sputtered yield of the two materials are less than their ratio in the bulk target. Thus, it is likely that the difference between the composition in the target and film is caused by their different throw distance, nitriding kinetics, angular distribution of any sputtered component and different absorption rates on the film surface.
There is an easy method for determination of optical constants, which depends on single transmittance measurement. The refractive index n and the extinction coefficient k as well as the thickness d of polycrystalline nitrided Ti-Cu thin films on thick quartz substrate were studied. They were determined from transmittance data only using PUMA approach and code described by Birgin et al. [
The experimental transmittance data are compared with theoretical values, in PUMA code. The difference between the two values is minimized until a best solution is achieved for the refractive index n, the extinction coefficient k and the film thickness d. Deposition rate (R) has been calculated from the average film thickness at given deposition time (
The Ti:Cu3N lattice constant (
There have been reports that the resistivity of Cu3N and Cu are 20 μΩcm - 2103 cm and 1.75 - 2 μΩcm, respectively [
Nitrided Ti-Cu thin films were grown on Si (111), glass slide and KBr substrates by reactive DC magnetron sputtering of a sintered Ti-Cu bi-component target. The asdeposited films on Si (111) substrate have composite structure of Ti:Cu3N nano-crystallite and metallic Cu nano-particle. The titanium atoms are positioned in the Cu3N crystal lattice and not segregated in other phases. The Ti addition causes to nitrogen excess; located at vacant interstitial site of Ti:Cu3N crystal structure, trapped at nano-crystallite and grain boundaries. The films have granular structure with clear grain boundaries. The atomic Ti:Cu ratio of films is less than that of the original target due to various differential sputtering yields, different throw distance and various sticking coefficient of any component. Refractive index, absorption coefficient and resistivity of the films are strongly dependent on substrate temperature and microstructural properties.
The corresponding author would like to acknowledge financial support of Iranian nanotechnology initiative. Also, he thanks Mohammad Safi Shalmazari from faculty of chemistry, university of Tabriz, for invaluable help. Also, he appreciates Department of Solid State and Electronics, Faculty of Physics, University of Tabriz, Tabriz, Iran.