ICMCTF2000 Session C3: Structure/Property Relationships

Spectrophotometric measurements of reflectance and transmittance, and ellipsometric measurements of psi and delta provide complementary methods for analyzing single-layer dielectric thin films. Both methods can be used to measure the index of refraction of a film as well as a linear variation of the index through the thickness of the film. Recently, it has been shown that ellipsometry is sensitive to the type of index inhomogeneity that would be caused by roughness of the outer surface of the coating.¹

This paper reviews the various measurement and analysis issues associated with extracting index information from ellipsometric and spectrophotometric scans. The spectral characteristics resulting from linear inhomogeneity and surface roughness are compared for the two techniques. The effects of linear inhomogeneity are easily seen in spectrophotometric data as well as in the parameter psi measured by ellipsometry. Surface roughness can be recognized in the shift of the parameter delta measured by ellipsometry. For spectrophotometric data, the contributions from surface roughness are convolved with those from the refractive index. However, by using a simple optical model, information about the surface roughness can be extracted from spectrophotometric data. ¹¹A. V. Tikhonravov, et al., "Sensitivity of the ellipsometric angles Psi and Delta to the surface inhomogeneity," in Advances in Optical Interference Coatings, SPIE Proceedings Series, Vol. 3738, 173-182 (1999).

Since the 80’s the so called low-E (low-emissivity or IR-reflecting) multilayer systems gain in significance concerning the economical and ecological aspects of reducing the heat loss through window panes.

The low-E coatings have to meet the requirements of a high transparency in the region of visible light, a high IR reflection (i.e. a high electrical conductivity) and specific mechanical properties.

A conventional used low-E system consists of a thin silver layer sandwiched between two dielectric layers (e.g. SnO₂ , ZnO , TiO₂ etc.) . Choosing a multilayer system like [glass // SnO₂ // Ag // SnO₂] or [glass // TiO₂ // Ag // TiO₂] the emissivity and the electrical resistance decrease, if an additional thin ZnO layer is inserted, neighbored to the silver layer.

Investigations on the properties of ZnO layers deposited by a reactive pulse sputter process (DMS , Dual Magnetron System) , as well as the resistance reducing effect in a low-E system on glass are presented, as is a first model describing the function of the ZnO layer.

ZrO₂-Y₂O₃ and ZrO₂ films, because of its high refractive index, high dielectric constant and low thermal conductivity properties have many important applications in science and technology. It is well known there are three phases (monoclinic, tetragonal and cubic) for ZrO₂ depending on the temperature environment. The tetragonal phase and the cubic phase can be stabilised in the room temperature by doping with Y₂O₃ . It depends on the doping concentration of the stabiliser for the tetragonal phase or the cubic phase to be formed. ZrO₂-Y₂O₃ films have been reported being prepared by electron beam evaporation, sputtering, ion beam assisted deposition, MOCVD and laser ablation methods. Both the tetragonal phase and the cubic phase are found in these films. But there is no systemically study on the influence of Y₂O₃ concentration on the microstructure and the properties of ZrO₂-Y₂O₃ films so far.

In the present work, we prepared the ZrO₂-Y₂O₃ thin films by rf magnetron reactive sputtering with the Y₂O₃ concentration form 4mol% to 35mol%. All the films have a tetragonal phase structure and a strong preferred orientation along (200) plane. The peak position shift to the low diffraction angle with the increasing of Y₂O₃ concentration. When the Y₂O₃ concentration increases, more Y⁺³ will be introduced in the crystallite of the films which results the changing of the lattice parameters of the films, the d space increases with the increasing of Y₂O₃ concentration. This is the reason of the peak shifting to the low diffraction angle. The crystallite size increases with the increasing of Y₂O₃ concentration. It is found the transmittance of the films increases with the increasing of Y₂O₃ concentration. The refractive index and the extinction coefficient of the films decrease with the increasing of Y₂O₃ concentration. The Y₂O₃ doping concentration has no obviously inference on the residual stress of the films.

Pulsed magnetron sputtering in unipolar or bipolar pulsed mode is the basis for the long term stable run of reactive deposition processes at high deposition rates. In unipolar mode a pulsed voltage is applied between the cathode and the anode of the magnetron discharge. In bipolar mode a voltage with alternating polarity is applied between the two targets of a dual magnetron arrangement that act alternately as cathode and anode.

In this paper anode effects on energetic particle bombardment of the substrate in unipolar and bipolar pulsed sputtering are discussed. The measured values of thermal substrate load, ion current density onto a floating substrate and difference between the plasma and substrate potential differ by up to one order of magnitude for the different pulse modes. The experiments show that the location of the anode relative to the magnetic flux lines of the magnetron is the major cause for these results. The investigated influence of the energetic substrate bombardment on film properties of alumina is also presented.

The use of pulsed DC power at the substrate is a recent development in the magnetron sputtering field. Pulsing the substrate bias voltage in the mid-frequency range (20-350kHz) has been found to significantly increase the ion current drawn at the substrate. In magnetron systems, the fluxes of ions and electrons incident at the substrate are controlled by the substrate bias voltage. It is normally found that the current drawn at the substrate saturates at bias voltages of the order of -100V. Further increases in bias voltage do not lead to a further increase in current. It is generally assumed that the saturation current is an ion currrent, as electrons approaching the substrate will be repelled at this voltage. Recent experiments, though, have shown that if the bias voltage is pulsed, not only is the magnitude of the saturation current greater than for the DC bias case, but the current drawn at the substrate continues to increase as the bias voltage is increased. In addition, both of these effects become more marked as the pulse frequency is increased. The exact mechanism causing these effects is not yet clear. However, it is known that plasmas generated by oscillating fields are more energetic (i.e., have higher plasma densites and electron temperatures) than DC plasmas.

Pulsing the substrate bias voltage, therefore, offers a novel means of controlling the ion current drawn at the substrate. Consequently, the variation in ion current with pulse frequency and bias voltage, and the associated substrate heating effects, have been studied for an unbalanced magnetron sputteirng system. The influence of these variables on film propeties is also reported.

ZrO₂ coatings in the thickness range between 100 and 150 nm were deposited by reactive Pulsed Magnetron Sputtering (PMS). The process parameters sputtering power, substrate-target distance and sputtering pressure were varied.

The structure of the coatings was investigated by grazing angle XRD. Atomic force microscopy investigations were used to characterize the topography and the surface roughness. With increasing sputtering pressure a higher surface roughness of the coatings is observed.

The mechanical properties of the coatings are drastically influenced by the sputtering pressure. With increasing sputtering pressure the intrinsic stresses are changed from very high compressive values of about -1800 MPa to low tensile stresses of about 150 MPa. After a storage time only small changes in the stress values are observed, independent on the sputtering pressure.

Additionally the hardness and Young's modulus were determined by nanoindentation technique with the application of the continuos stiffness measurement. With increasing sputtering pressure the hardness is decreased from about 12,3 to 6,2 GPa and the Young's Modulus is decreased from about 173 GPa to 150 GPa respectively.

Titanium dioxide is an insulator well known for its interesting optical properties (high refractive index). High hardness and corrosion resistance are in favour of multiple industrial applications. However, such coatings have limited applications with regards to their relative brittleness and weak fatigue behaviour. In this paper, titanium dioxide films of 0.2 to 1 µm are deposited by DC or pulsed DC reactive magnetron sputtering from titanium targets in different argon oxygen gaseous mixtures, on glass slides and high speed steel (M2) substrates. Deposition rates and optical properties are in situ controlled by optical emission spectroscopy with an optical fiber located behind a glass substrate in order to perform a real time control of the transmittance of the growing film. With this technique it is possible to determine the refraction index, the extinction coefficient and the thickness of the as deposited films by using a simple simulation, developed on a MATLAB software.

Optical and mechanical properties are investigated in relation to the film structure and composition, which depend on the adopted sputtering conditions. In particular, the effects of the sputtering pressure (working pressure and oxygen partial pressure), the discharge power, the pulse frequency and the substrate bias are investigated in detail.

Films structure and composition are assessed by standard metallurgical techniques such as Scanning Electron Microscopy (SEM), grazing angle X-Ray Diffraction (XRD) and Electron Probe Micro Analysis (EPMA).

Mechanical and tribological properties are also investigated using depth sensing nano hardness testing as well as monopass and multipass microscratch testing under progressive and constant loads.