Adhesion enhancements and internal stress in MgF2 films deposited with ion beam assistance

We performed adhesion measurements using a dynamically loaded scratch tester to evaluate MgF₂ coatings deposited onto heated and unheated fused silica substrates. Resistance evaporation at 5 Å s^-1 with Ar⁺ ion beam assistance improved the relative adhesive strength of films deposited onto unheated substrates by over an order of magnitude compared with the results for heated substrates without ion beam assistance. Additional experiments were performed to measure internal stress in MgF₂ films on specially masked BK-7 glass substrates using modulated transmission ellipsometry. We found that ion-assisted deposition can reduce internal stress to approximately 450 kgf cm^-2 (tensile) in films 2300 Å thick. The great potential for stress relaxation in MgF₂ films deposited with ion assistance is suggested by these measurements, but additional work with films of higher purity may be required to identify the relative contributions of stress relief, substrate cleaning and chemical bonding to improved adhesion.     

Studies on thin film materials on acrylics for optical applications

Deposition of durable thin film coatings by vacuum evaporation on acrylic substrates for optical applications is a challenging job. Films crack upon deposition due to internal stresses and leads to performance degradation. In this investigation, we report the preparation and characterization of single and multi-layer films of TiO₂, CeO₂, Substance2 (E Merck, Germany), Al₂O₃, SiO₂ and MgF₂ by electron beam evaporation on both glass and PMMA substrates. Optical micrographs taken on single layer films deposited on PMMA substrates did not reveal any cracks. Cracks in films were observed on PMMA substrates when the substrate temperature exceeded 80°C. Antireflection coatings of 3 and 4 layers have been deposited and characterized. Antireflection coatings made on PMMA substrate using Substance2 (H2) and SiO₂ combination showed very fine cracks when observed under microscope. Optical performance of the coatings has been explained with the help of optical micrographs.     

Study of the effect of chopping on the adhesion of codeposited mixed thin films

This paper reports adhesion of codeposited mixed films of Cu-Ag, Cu-Al, MgF₂-ZnS and MgF₂-cryolite and enhancement of adhesion by chopping technique. Results indicate that codeposited mixed films show higher adhesion than single films and chopping improves the adhesion further. The quenching of crystal growth seems to be more effective if two materials are codeposited and chopped. Chopping along with mixing increases the number of nucleation sites and decreases defects in the film. Decreasing defects and modifying the microstructure increases the adhesion of the films. Chopping also seems to increase oxygen affinity of both metallic and dielectric films for growth of interfacial bonding layer.     

Conference Notices

Non-polarizing beam splitters made out of lossless periodic dielectric multilayers, having a basic period of two layers with equal effective optical thicknesses and different refractive indices, are treated theoretically. The results of computer calculations are presented in graphs which simplify the design of nonpolarizing beam splitters and two detailed examples consisting of layers of ZnS and MgF₂ are given.     

Measurement of the Thickness of Deposited Magnesium Fluoride Films by Evanescent-wave Fluorescence: A Critical Test of the General TIRAF Theory

Abstract

We have recently developed a quantitative theory describing the stimulation of fluorescence by an evanescent wave, with a view to the precise measurement of thin films by otherwise conventional light microscope photometry. In our set-up, total internal reflection of a focused laser beam occurs at a glass/water interface and the presence of the evanescent wave is reported by a fluorescent dye dissolved in the water. For convenience, we refer to the method as TIRAF (total internal reflection aqueous fluorescence). We have tested our general TIRAF theory critically by using glass/MgF₂/fluorescein layers such that the evanescent wave established in the glass/MgF₂ interface is, in part, separated from the aqueous fluorescein phase by an accurately and independently measured thickness of deposited MgF₂ film. We show that fluorescence measurements on areas as small as 30 µm² allow precise determination of film thicknesses in the range 2–160 nm for two different decay constants of the evanescent wave.     

Fabrication and characteristics of porous germanium films

Abstract

Porous germanium films with good adhesion to the substrate were produced by annealing GeO₂ ceramic films in H₂ atmosphere. The reduction of GeO₂ started at the top of a film and resulted in a Ge layer with a highly porous surface. TEM and Raman measurements reveal small Ge crystallites at the top layer and a higher degree of crystallinity at the bottom part of the Ge film; visible photoluminescence was detected from the small crystallites. Porous Ge films exhibit high density of holes (10²⁰ cm^?3) and a maximum of Hall mobility at ～225 K. Their p-type conductivity is dominated by the defect scattering mechanism.     

Tolerances For Layer Thicknesses in Dielectric Multilayer Coatings and Interference Filters

A theory is developed for dielectric multilayer coatings in which the layers depart from calculated thickness. The theory is applied to alternating systems of quarter wave layers of ZnS and MgF₂. The effects of thickness errors are: (1) A shift of the wavelength at which maximum reflectance occurs; and (2) a change in phase shift upon reflection. The magnitude of these effects, and also their dependence on various parameters, are determined. Statistical tolerances for layer thicknesses are computed for given tolerances on the multilayer performance. The accuracy required for producing dielectric interference filters is up to about 40 times higher than the accuracy sufficient for the production of dielectric mirrors and beam splitters. Various techniques of experimentally controlling film thicknesses, and their accuracies, are discussed. The production of mirrors and beam splitters deviating from theoretical maximum reflectance by only 1 percent seems to be possible with Dufour’s simple single photocell method of monitoring film thicknesses. With more precise methods, such as those developed by Giacomo and Jacquinot, or Traub, the production of interference filters appears to be possible to within plus or minus one half their half widths.     

Optical excitation of surface plasma waves in an indium film bounded by dielectric layers

The method of attenuated total reflection (ATR) was used to excite optically surface plasma waves (SPWs) in thin indium films. Three successive layers of MgF₂/In/MgF₂ were evaporated onto the base of a glass prism which had a high refractive index. Continous thin films of indium were obtained by maintaining the substrate at liquid nitrogen temperature during deposition. The ATR angular spectra and associated resonant oscillations were studied at three different wavelengths in the visible region and good agreement was obtained between the experimental and the calculated spectra. The symmetric SPW of the indium film was more highly attenuated than the antisymmetric SPW. Owing to the high damping of the plasma in indium only the antisymmetric SPW could be excited via an optical reflectance resonance when the indium slab was isolated by thick dielectric layers. With thinner bounding dielectrics two SPW-type resonances were observed. Complete calculations of the magnetic field oscillations within the variousmedia, as well as calculations of the current distributions and surface charge densities of the indium film, showed resonant oscillations of mixed symmetry.     

The light scattering of dielectric films

The Rayleigh scattering of dielectric films was measured at 441.6 nm wavelength. The total scattering losses of TiO₂, SiO₂, ZnS and MgF₂ films show a close correlation with the evaporation parameters. The light scattering of the TiO₂SiO₂ and ZnSMgF₂ film systems can be explained by scattering at the interfaces. By comparing curves of the light scattering as a function of scattering angle with the results of a numerical computing method the statistical parameters of the interfaces were obtained and were found to be in good agreement with electron microscope results.     

Electron microscope structure and internal stress in thin silver and gold films deposited onto MgF2 and SiO substrates

In this work the internal stress of thin silver and gold films deposited onto MgF₂ and SiO substrate films was measured up to a mean thickness of about 1000 Å. The method used was based on the cantilever beam principle. The internal stress of the metal films was determined by subtracting the contributions due to momentum transfer and heat transfer. From parallel electron microscope investigations of films of various thicknesses it was seen that a close correlation exists between the internal stress generated in the films and their structure. On the basis of these results we propose a model which gives a qualitative explanation of the measured internal stress versus thickness curves and of their dependence on vacuum conditions, evaporation rate and the nature of the substrate.     

Properties of epitaxial, (001)- and (110)-oriented (PbMg1/3Nb2/3O3)2/3-(PbTiO3)1/3 films on silicon described by polarization rotation

Epitaxial (PbMg_1/3Nb_2/3O₃)_2/3-(PbTiO₃)_1/3 (PMN-PT) films with different out-of-plane orientations were prepared using a CeO₂/yttria stabilized ZrO₂ bilayer buffer and symmetric SrRuO₃ electrodes on silicon substrates by pulsed laser deposition. The orientation of the SrRuO₃ bottom electrode, either (110) or (001), was controlled by the deposition conditions and the subsequent PMN-PT layer followed the orientation of the bottom electrode. The ferroelectric, dielectric and piezoelectric properties of the (SrRuO₃/PMN-PT/SrRuO₃) ferroelectric capacitors exhibit orientation dependence. The properties of the films are explained in terms of a model based on polarization rotation. At low applied fields domain switching dominates the polarization change. The model indicates that polarization rotation is easier in the (110) film, which is ascribed to a smaller effect of the clamping on the shearing of the pseudo-cubic unit cell compared to the (001) case.     

Optical analysis of absorbing double layers by combined reflection and transmission ellipsometry

A method known as combined ellipsometry, which utilizes the simultaneous interpretation of the ellipsometric parameters determined for light reflected and transmitted by the air side and by the substrate side of thin films, was used to obtain an optical analysis of absorbing double layers deposited onto a non-absorbing substrate. It is shown that the use of this method enables the evaluation of all the optical parameters characterizing absorbing double layers consisting of a non- absorbing and an absorbing thin film. Combined ellipsometry can be used for analysing the double layers formed by two highly absorbing thin films in a reasonable way if this method is complemented by a precise auxiliary method, e.g. the Fizeau method. These conclusions are demonstrated by experimental results obtained for samples of the following double layers: MgF₂/Al, Al/MgF₂, TiO/Au and Au/Ni. The ellipsometric parameters were measured at a wavelength λ of 632.8 nm.     

Stress measurements and calculations for vacuum- deposited MgF2 films

Previous in situ measurements of the intrinsic stress in thin crystalline dielectric films have been interpreted in terms of a stress mechanism due to forces at the grain boundaries. The same stress interpretations were given for metal films by Hoffman and coworkers several years ago.In the present work it is shown that the tensile stress is a decreasing function of impurity concentration in the MgF₂ films. A similar decrease occurs through water vapour adsorption. Assuming that the impurity is located mainly on the crystallite surface, as with adsorbed water molecules, this effect can also be interpreted using the grain boundary model which predicts a decreasing tensile stress with increasing crystal size. The results of stress measurements of MgF₂ films deposited onto heated substrates support this prediction.     

Crystal orientation in CuSn-alloy coatings obtained by magnetron sputtering

Crystal orientation in copper-tin alloy coatings of different chemical composition as a function of substrate temperature has been studied using the X-ray texture goniometer technique. The coatings were obtained by magnetron sputtering an alloy target in argon at a gas pressure of 6 × 10⁻¹ Pa using two kinds of substrate—one of ferritic stainless steel and the other of a glass ceramic material. It has been established that Cu-Sn 7.8% alloy films, which represent an α-solution of Sn in Cu are textured. At low substrate temperature (710–770 K) the α-phase crystals have a mixed 〈100〉 and 〈110〉 orientation and at temperatures in the range from 870 to 1000 K—〈110〉 orientation. No orientation was detected when the condensation proceeded by the mechanism vapour→liquid→solid. The η-Cu₆Sn₅ and Sn phase crystals in CuSn 74% alloy films were not oriented.     

Abnormal morphology of amorphous germanium films in contact with palladium

The fractal and dense branching morphologies of amorphous germanium films in contact with palladium have been investigated by TEM. The experimental results suggest that the production of fractal morphology in Pd/a-Ge bilayer films was easier than that in a-Ge/Pd bilayer films. An island-like fractal morphology can be formed at the step and crevice hole areas annealed at higher temperatures. It is difficult for co-evaporated Pd-Ge films to realize fractal morphology. The formation of fractal morphology can be explained by a RSN model. The SAED patterns suggest that the dense branching morphology cannot be completely demonstrated by the Pd, Ge, Pa₂Ge, PdGe and Pd₂₅Ge₉, maybe it is a new phase.     

A Chemical Route to Monolithic Integration of Crystalline Oxides on Semiconductors

This work demonstrates the growth of crystalline SrTiO₃ (STO) directly on germanium via a chemical method. After thermal deoxidation, the Ge substrate is transferred in vacuo to the deposition chamber where a thin film of STO (2 nm) is deposited by atomic layer deposition (ALD) at 225 °C. Following post‐deposition annealing at 650 °C for 5 min, the STO film becomes crystalline with epitaxial registry to the underlying Ge (001) substrate. Thicker STO films (up to 15 nm) are then grown on the crystalline STO seed layer. The crystalline structure and orientation are confirmed via reflection high‐energy electron diffraction, X‐ray diffraction, and transmission electron microscopy. Electrical measurements of a 15‐nm thick epitaxial STO film on Ge show a large dielectric constant (k ≈ 90), but relatively high leakage current of ≈10 A/cm² for an applied field of 0.7 MV/cm. To suppress the leakage current, an aluminum precursor is cycled during ALD growth to grow crystalline Al‐doped STO (SrTi_1‐x­Al_xO_3‐δ) films. With sufficient Al doping (≈13%), the leakage current decreases by two orders of magnitude for an 8‐nm thick film. The current work demonstrates the potential of ALD‐grown crystalline oxides to be explored for advanced electronic applications, including high‐mobility Ge‐based transistors.     

Effect of nanoparticles on ferroelectric and electrical properties of novel PMNT thin-films

This work studies the impact of adding nanoparticles to high-k PMNT (lead magnesium niobate-lead titanate, Pb(Mg_0.33Nb_0.67)_0.65Ti_0.35O₃) thin films. PMNT thin films were grown on Pt(111)/TiO₂/SiO₂/Si substrate using a sol-gel technique. Ligand stabilised PMNT nanoparticles were added to the sol-gel material with the aim of seeding the crystallization process. The measurements show that use of nanoparticles in PMNT thin films influences the remanent polarization, coercive field and dielectric constant. These characterization results support the ongoing investigation of the material ferroelectric and electrical properties which are necessary before the novel dielectric can be used in silicon applications.     

Microstructural evolution of oxides and semiconductor thin films

This review article introduces the preparation methodologies and the microstructural characteristics of semiconductor thin films, including SnO₂ thin films, Au/Ge bilayer films, and Pd-Ge alloy thin films, and metal oxides, including SnO, SnO₂, Mn₂O₃ and Mn₃O₄ nanocrystals which can be in the form of nanoparticles, nanowires, nanorods, and nanofractals. Firstly, the preparation methodologies and the microstructural characteristics of tin oxides have been investigated in detail and described in Section 2. Secondly, the crystallization of amorphous Ge, and the formation of nanocrystals and compounds developed with improved micro- and nanostructured features are described in Section 3. Thirdly, a novel selective synthesis route for various morphologies of manganese oxides nanocrystals, including nanoparticles, nanorods and nanofractals, and their unique microstructural characteristics are presented in Section 4. Intricate fundamental properties of manganese oxides nanocrystals are studied in detail. To sum up, it is expected that the fabrication methodologies developed and the knowledge of microstructural evolution gained in semiconductor thin films, including SnO₂ thin films, Au/Ge bilayer films, and Pd-Ge alloy thin films, and metal oxides, including SnO, SnO₂, Mn₂O₃ and Mn₃O₄ nanocrystals in the forms of nanoparticles, nanowires, nanorods, and nanofractals, will provide an important fundamental basis underpinning further interdisciplinary (physics, chemistry and materials science) research in this field leading to promising exciting opportunities for future technological applications involving these oxide and thin film materials.     

Quenched aging of thin film deposits by chopping

We studied the aging of thin films of MgF₂, of cryolite and of mixtures of these compounds prepared by codeposition with and without chopping of the incident beams. Ellipsometric measurements indicated that chopping of the films during deposition reduces the aging of the films. Spectrophotometric data showed that the transmittances of the single films change owing to aging much more than those of mixed films.     

Influence of deposition parameters on the optical and structural properties of TiO2 films produced by reactive d.c. plasmatron sputtering

We investigated the variations in the structure and optical properties of TiO₂ films produced by reactive d.c. plasmatron sputtering with the most important deposition parameters.Over a wide range, the phase composition (ratio of rutile to anatase) and the grain size of the TiO₂ films can be influenced in a controlled manner by variations in the substrate temperature and the oxygen partial pressure.Because of their high refractive index and low light-scattering losses, plasmatron-sputtered TiO₂ films are of great interest in the field of optical interference coatings, e.g. for dielectric multilayer stacks.