Microstructures And Properties Of Nanocrystalline Copper And Cuprous (Cupric) Oxide Films

Copper has been increasingly used as an alternate material for aluminum-based interconnects in ultra-large-scale-integration (ULSI) metallization, because of its high resistance to electromigration and low electrical resistivity. Electroless plating has been proposed as an potential technology for the preparation of copper seed layer in ULSI metallization. The microstructures and properties of this seed layer have important effects on the properties of subsequent electrodeposited layers. Therefore, it is essential to fabricate electroless copper film with good properties by optimizing conventional bath composition and processing parameters. Copper oxides including cuprous oxide and cupric oxide are p-type semiconductor. Thermal oxidation under controlled conditions has been used to prepare copper oxide thin films. Especially, CuO nanowires (nanorods) can be synthesized on the copper substrate by this method, without any catalyst and template. At present, interests on the nanowires (nanorods) are focused on the fabrication technique. Electrodeposition is attractive method to synthesize oxide and sulphide. Cu₂O thin films synthesized by electrodeposition have wide applications in solar cells. The structural characterizations of Cu₂O thin film are found to influence the properties of solar cells.Based on above methods, nanocrystalline copper films on the glass have been fabricated by electroless plating with the addition of surfactant, sodium dodecyl benzene sulfonate (SDBS). Cu₂O and CuO thin films as well as CuO nanowires (nanorods) have been synthesized by the thermal oxidation of the as-prepared nanocrystalline copper thin films. Cu₂O thin films with different microstructure have been synthesized on indium-doped tin oxide (ITO) glass by potentiostatically electrodeposition, with cationic surfactant, cetyl trimethylammonium chloride (CTAC) as an additive in cupric acetate electrolyte. Microstructures and properties of these thin films are investigated by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscope (FESEM), atom force microscope (AFM), transmission electron microscope (TEM), fourier-transform infrared transmission (FTIR), UV-vis spectrophotometer, etc. The main results are shown as follows:1. Nanocrystalline copper thin films have been synthesized by electroless plating. Microstructure analysis show that an obvious (111) texture exits in the copper thin films. The grain size and the intensity of (111) texture of copper thin films increases with increasing deposition time. The transition from a random grain growth to one preferred on (111) plane and the expense of the (111) orientated grains are responsible for this texture development in the Cu film. Morphologies observations reveal that 1min deposition is able to achieve a continuous copper thin film. Nanocrystalline copper thin films are composed of small nodules which composed of several grains and the growth of them follows the Volmer-Weber model. 2. The improvement of RMS with the increasing deposition time is due to the increasing number of nodules and the growth in three dimensions. A dense copper thin film can be formed with the addition of SDBS which improves the quality of surface. A good adhesion strength exits between the Cu film and the glass substrate. The results of electrical resistivity show that it decreases rapidly with the increasing of film thickness when the film thickness is smaller than 100nm. The increased electrical resistivity arises from the enhancement of the surface and grain boundary reflections. The grain boundary reflection coefficient (R) which was calculated based on a combination model and the resultant value of R varies in a range of 0.4-0.75. It was showed that grain boundaries have more important contribution to R than surfaces. The increasing grain boundaries are the main factor that increased the electrical resistivity in the nanocrystalline copper thin films.3. The oxidation behaviors of electroless copper thin films, 100nm thick in the range of 100°C-600°C have shown that Cu₂O can be formed at the initial oxidation stage of 250°C. While at the temperature of 300°C, a mixture of Cu₂O and CuO is formed and Cu₂O evolves into pure CuO above 350°C. The oxidation temperature and time have played a key role in the microstructure and surface morphologies. CuO nanowires with the diameter of about 10-15nm and the length of 200-300nm have been observed at 300°C for 8h. Copper oxide films have higher transmittance in the visible and IR region, while almost completely opaque in the UV region. The variation of optical transmittance spectra depends on the composition and microstructure of oxide films. The band gaps are 1.93-1.98eV and 2.2-2.4 eV for pure CuO and the mixture of Cu₂O and CuO, respectively. 4. It can be found that the optimum temperature is between 300oC and 400oC for a 100nm thick thin film at the oxidation time of 1h. No nanowires are observed when the oxidation temperature is lower or higher than this range of temperature. The number of nanowires decreases with the prolonging of oxidation time at 350oC. However, few nanowires could be seen when the thin film was oxidized at 400oC. The increase of film thickness is benefit to the formation of nanowires. The formation and growth of these nanowires is probably due to vapor-solid (VS) mechanism. The transmittance of CuO nanowires increases with the increasing of introduced wavelength and decreases with the increase of film thickness. The band gaps of CuO nanowires are in the range of 1.94eV-2.03eV.5. Cu₂O thin films with different microstructures and morphologies have been fabricated on ITO glass by potentiostatically electrodeposition, with CTAC as an additive of electrolyte. The electrochemical analysis shows that the formation of copper can be restrained and the formation of Cu₂O can be accelerated by the addition of CTAC. It is seen that the deposition of Cu₂O favors the higher pH value. The peaks according to Cu₂O are slightly shifted to positive potential with increasing the temperature. The deposition current increases with the increase in electrolyte concentration. The deposition potential is the lowest at the concentration of 1.0 C0. The XRD results indicate that CTAC has great effect on the microstructure of Cu₂O thin films. The diffraction intensity of Cu₂O thin films with the CTAC of 3.0mM is higher than that of others. With the concentration of CTAC increased, the flowers of Cu₂O turn into spheres with shagginess and eventually another type of flowers can be formed. It is found that the increase of electrolyte concentration is propitious to the formation of flowers for Cu₂O and it gradually becomes much larger. The temperature has no obvious effects on the morphologies of Cu₂O. However, the density of Cu₂O particles increases, while the particle size decreases with the increased temperature. The effect of CTAC on the particle size and morphologies of Cu₂O thin films might be attributed to the adsorption of CTAC, which in turn change the surface energies of different crystal face and affect the growth kinetics. The addition of CTAC has no obvious effect on the chemical state, and the binding energies of Cu 2p3/2, Cu 2p1/2 and O 1s are 932.6eV, 952.5eV and 530.4ev, respectively. The variation of optical transmittance varied with the introduced wavelength depends on the variation of CTAC. Moreover, the increase of concentration and temperature tend to enhance the optical transmittance of Cu₂O thin films. The band gaps of electrodeposited Cu₂O thin films are in the range of 1.70-2.02 eV.