Sputter-deposited Thin Films Research Articles

On the Deposition Rates of Magnetron Sputtered Thin Films at Oblique Angles

AbstractWe describe here the deposition of thin films using magnetron sputtering at oblique angles. General relations between the deposition rates of the films and experimental parameters, such as gas pressure or substrate tilt angles, are deduced and experimentally tested. The model also permits the direct determination of the thermalization mean free path of the sputtered particles in the plasma gas, a key parameter defining the balance between ballistic and diffusive flows in the deposition reactor. The good agreement between experimental and calculated results supports the validity of our description, which becomes a useful tool to explain the main features of the magnetron sputtering deposition of thin films at oblique angles.

Influence of N2 gas pressure on the structural properties of sputter-deposited GaN thin films

GaN thin films were deposited on Si (100) substrates at room temperature by RF magnetron sputtering of a GaN target in a pure N2 gas whose pressure ranged between 7 and 50 mTorr. The influence of N2 gas pressure on the structural properties of the deposited GaN films was systematically investigated. The crystalline structure in the GaN films changed from wurtzite to zinc-blende as the N2 gas pressure was reduced from 50 to 7 mTorr. The observed change in crystalline structure was found to correlate strongly with the changes in the microstructure, surface topography, and internal stress of the GaN films. The microstructure of the GaN films changed from a voided columnar structure having a rough surface to an extremely condensed structure with a very smooth surface morphology as the N2 gas pressure was decreased. The internal stress of the GaN films was slightly tensile (∼0.34 GPa) at 50 mTorr and became increasingly compressive upon decreasing the N2 gas pressure, reaching a value of ∼3.08 GPa at 7 mTorr. The above experimental results show that the N2 gas pressure plays a crucial role in determining the crystalline structure, microstructure, surface topography, and internal stress of the sputter-deposited GaN films.

Chemical bonding, optical constants, and electrical resistivity of sputter-deposited gallium oxide thin films

Gallium oxide (Ga2O3) thin films were made by sputter deposition employing a Ga2O3 ceramic target for sputtering. The depositions were made over a wide range of substrate temperatures (Ts), from 25 to 600 °C. The effect of Ts on the chemical bonding, surface morphological characteristics, optical constants, and electrical properties of the grown films was evaluated using X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), spectroscopic ellipsometry (SE), and four-point probe measurements. XPS analyses indicate the binding energies (BE) of the Ga 2p doublet, i.e., the Ga 2p3/2 and Ga 2p1/2 peaks, are located at 1118.0 and 1145.0 eV, respectively, characterizing gallium in its highest chemical oxidation state (Ga3+) in the grown films. The core level XPS spectra of O 1s indicate that the peak is centered at a BE ∼ 531 eV, which is also characteristic of Ga-O bonds in the Ga2O3 phase. The granular morphology of the nanocrystalline Ga2O3 films was evident from AFM measurements, which also indicate that the surface roughness of the films increases from 0.5 nm to 3.0 nm with increasing Ts. The SE analyses indicate that the index of refraction (n) of Ga2O3 films increases with increasing Ts due to improved structural quality and packing density of the films. The n(λ) of all the Ga2O3 films follows the Cauchy's dispersion relation. The room temperature electrical resistivity was high (∼200 Ω-cm) for amorphous Ga2O3 films grown at Ts = RT-300 °C and decreased to ∼1 Ω-cm for nanocrystalline Ga2O3 films grown at Ts ≥ 500–600 °C. A correlation between growth conditions, microstructure, optical constants, and electrical properties of Ga2O3 films is derived.

Impact of C-axis Orientation of Aluminium Nitride Thin Films on the Long-term Stability and Mechanical Properties of Resonantly Excited MEMS Cantilevers

In this work, the influence of c-axis orientation of sputter-deposited aluminium nitride thin films on the mechanical properties and long-term stability of resonantly excited, Si-based MEMS cantilevers is investigated. Two AlN films are synthesized at different deposition parameters yielding a strong difference in the degrees of c-axis orientation. It is demonstrated, that the temperature and pressure dependency of the mechanical behaviour is independent of the c-axis orientation of the AlN films. However, a substantial impact of this film parameter on the long-term stability of the devices is observed.

Temperature dependant dielectric breakdown of sputter-deposited AlN thin films using a time-zero approach

In MEMS (micro electromechanical system) devices, piezoelectric aluminum nitride (AlN) thin films are commonly used as functional material for sensing and actuating purposes. Additionally, AlN features excellent dielectric properties as well as a high chemical and thermal stability, making it also a good choice for passivation purposes for microelectronic devices. With those aspects and current trends towards minimization in mind, the dielectric reliability of thin AlN films is of utmost importance for the realization of advanced device concepts. In this study, we present results on the transversal dielectric strength of 100 nm AlN thin films deposited by dc magnetron sputtering. The dielectric strength is measured using a time-zero approach, using a fast voltage ramp to stress the film up to the point of breakdown. The measurements are performed at different device temperatures. In order to achieve statistical significance, at least 12 measurements are performed for each environment parameter set and the results are analyzed using the Weibull approach. Basically, lower breakdown fields are observed with increasing temperatures up to 300 °C with a characteristic breakdown field strength E0 following the relationship $$\sqrt {E_{0} } \propto T$$E0źT as reported in literature for similar measurements performed at silicon nitride thin films. From the intersection of this linear behavior, the Poole---Frenkel (PF) barrier height źB is determined to 0.54 eV, which is reasonable for AlN thin films. The slope of this relation is similar to values reported for silicon nitride thin films. This allows an estimation of the breakdown field at higher temperatures by extrapolation. Leakage current measurements show a dominant PF type conduction mechanism, verifying the applicability of $$\sqrt {E_{0} } \propto T$$E0źT. No breakdown occurs in negative field direction, which is attributed to the metal---insulator---semiconductor configuration of the sample and hence, the presence of a depletion layer forming in the n-doped silicon and dominating the leakage current behavior.

Impact of doping level on the metal assisted chemical etching of p-type silicon

The impact of doping level on the metal assisted chemical etching of p-type silicon using silver particles is investigated. 12 boron doped wafers with doping levels ranging from 1012 up to 1019cm−3 were used. Silver particles generated from a sputter-deposited thin film via a dewetting procedure act as catalyst in a HF:H2O2 etch mixture. The characteristics of the etch rate as a function of time was investigated, revealing in addition an enhanced material removal at the beginning (i.e. in the range of several seconds) and an undesired etching of silver free regions at longer etch times (i.e. >15min). Furthermore, a strong decrease in etch rate at doping levels exceeding 1016cm−3 is observed. Only at these highly doped samples, a porous sub-structure is detected located adjacent and in-between the pores which are generated by the Ag particles. It is assumed that the injected holes representing majority charge carriers in p-type silicon diffuse at higher doping levels faster and more efficiently away from the metal-semiconductor interface into the silicon sample resulting in a decreased etch rate.

Surface/interface analysis and optical properties of RF sputter-deposited nanocrystalline titanium nitride thin films

Titanium nitride (TiNx) thin films were grown by radio-frequency (RF) magnetron sputter deposition by varying the nitrogen content in the reactive gas mixture over a wide range. The effect of nitrogen gas flow rate on the surface and interface morphology, chemical composition and optical properties of TiN thin films was studied employing atomic force microscopy (AFM), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and spectroscopic ellipsometry (SE). Analysis of the optical properties probed with SE has shown that films deposited at low (0–5sccm) nitrogen flow rates have the highest absorption at energies <2eV. It was also shown that Lorentz oscillators with energy positions lower than 2eV can be distinguished from the Drude oscillator function during parameter fitting. AFM imaging analysis indicates that the roughness decreases and plateaus at approximately 1.5nm with the introduction of a small N2 flow rate, remaining consistent thereafter. SEM cross-sectional imaging analysis indicates the dense, columnar structure for the films grown at lower nitrogen flow rates. XPS analysis of atomic composition and the chemical states indicate that the atomic composition remained nearly constant while the chemical states varied significantly among the samples as a function of N2 flow rate. XPS analyses confirm the presence of TiNx, TiO2 and TiOxNy. These process–property relationships derived could be useful for defining and expanding the range of optical and electronic applications of titanium nitrides and (oxy)nitrides.

High-temperature tensile behavior of freestanding Au thin films

The mechanical behavior of freestanding thin sputter-deposited films of Au is studied at temperatures up to 340 °C, using tensile testing . Films tested at elevated temperatures exhibit a significant decrease in flow stress and stiffness. Furthermore, the flow stress decreases with decreasing film thickness, contravening the usual notion that “smaller is stronger”. This behavior is attributed mainly to diffusion-facilitated grain boundary sliding .

Second harmonic generation from Ge doped SiO2 (Gex(SiO2)1−x) thin films grown by sputtering

Second-order nonlinear optical properties of sputter-deposited Ge-doped SiO2 thin films were investigated. It was shown that the second-order nonlinearity of SiO2, which vanishes in the electric-dipole approximation due to the centrosymmetric structure, can be significantly enhanced by Ge doping. The observed maximum value of d33 was 8.2 pm/V, which is 4 times larger than d22 of β-BaB2O4 crystal. Strong correlation was observed between the deff values and the electron spin resonance signals arising from GePb centers, suggesting that GePb centers are the most probable origin of the large second-order nonlinearity.

Reducing Optical Losses and Energy-Transfer Upconversion in ${\rm Er}_{x}{\rm Y}_{{2}\hbox{-}{\rm x}}{\rm SiO}_{5}$ Waveguides

Optical losses and energy-transfer upconversion (ETU) in sputter-deposited polycrystalline ErxY2-xSiO5 thin film waveguides are investigated. We find that the high temperature anneal that is necessary for crystallization and optical activation of Er induces grain growth that causes large propagation losses. Through minimizing the grain growth, propagation loss was lowered to 2.2±0.2 dB/cm while maintaining a high confinement factor of 50%. Changing the target material to minimize sputtering of Er clusters reduced the ETU coefficient to an estimated value of 3.1±0.2×10-17 cm3/s at an Er concentration of 7.6×1020 cm-3. The maximum level of population inversion was 0.47±0.02, resulting in signal enhancement of 13.3 dB/cm at 1529 nm.

An (ultra) high-vacuum compatible sputter source for oxide thin film growth

A miniaturised CF-38 mountable sputter source for oxide and metal thin film preparation with enhanced high-vacuum and ultra-high-vacuum compatibility is described. The all home-built sputtering deposition device allows a high flexibility also in oxidic sputter materials, suitable deposition rates for preparation of films in the nm- and the sub-monolayer regime and excellent reliability and enhanced cleanliness for usage in UHV chambers. For a number of technologically important--yet hardly volatile--materials, the described source represents a significant improvement over thermal deposition techniques like electron-beam- or thermal evaporation, as especially the latter are no adequate tool to prepare atomically clean layers of refractory oxide materials. Furthermore, it is superior to commercially available magnetron sputter devices, especially for applications, where highly reproducible sub-monolayer thin film preparation under very clean UHV conditions is required (e.g., for studying phase boundary effects in catalysis). The device in turn offers the usage of a wide selection of evaporation materials and special target preparation procedures also allow the usage of pressed oxide powder targets. To prove the performance of the sputter-source, test preparations with technologically relevant oxide components, comprising ZrO2 and yttrium-stabilized ZrO2, have been carried out. A wide range of characterization methods (electron microscopy, X-ray photoelectron spectroscopy, low-energy ion scattering, atomic force microscopy, and catalytic testing) were applied to demonstrate the properties of the sputter-deposited thin film systems.

Modeling the delamination of amorphous-silicon thin film anode for lithium-ion battery

Sputter-deposited amorphous silicon thin films on metallic copper current collectors are widely studied as lithium-ion anode systems. Electrochemical results indicate these electrodes exhibit near theoretical capacity for first few cycles; however delamination at the thin film–current collector interface causes rapid capacity fade leading to poor cycling performance. Primary reason for this interfacial delamination is the mechanical stress generated due to colossal volume expansion of silicon during lithiation. The focus of the current study is to present a mechanistic understanding of the role of mechanical properties of the current collector on this characteristic delamination behavior during electrochemical cycling. Toward this end, we have developed a computational framework that accounts for the coupled diffusion induced large deformation in silicon, elasto-plastic deformation of the current collector, as well as the nucleation and propagation of interfacial delamination. We have also performed a detailed parametric study to investigate the effect of mechanical properties of the current collector on the delamination of the thin film–current collector interface. We have accordingly determined that current collectors with low elastic modulus such as graphite can completely suppress interfacial delamination. Our analysis thus provides a sound mechanistic approach for designing next generation Si thin film anodes with improved capacity retention.

Stress anisotropy compensation of the sputter-deposited metal thin films by variable bias voltage

We introduce a new technique for compensating stress anisotropy across the thickness of sputter-deposited metal films. Our technique balances the film vertical stress gradient by altering the substrate bias during sputtering and by controlling the ion flux and energy that bombards the growing film. Sputter-deposited metal films are appealing materials for microfabrication of freestanding and out-of-plane structures, especially because of their low thermal budget. These microstructures extend the design space of micro-electro-mechanical systems (MEMS)-based devices, and they overcome some of the limitations of in-plane processing. Unfortunately, most elemental metals and alloys when sputter deposited have a substantial stress gradient across their thickness that can deteriorate their mechanical properties and severely distort the shape of the fabricated freestanding microstructures. The stress gradient across the thickness of a sputtered film can be compensated (balanced) by embedding a layer in the film with the opposite stress polarity compared to that of the bulk of the film. The force exerted by the stress mismatch between this layer and the bulk of the film easily overcomes the film's vertical stress gradient. This compensating force guarantees that a released freestanding structure remains flat and does not curl upward. This virtual layer is introduced to the growing film by altering the substrate bias voltage during the sputtering process. The substrate bias voltage controls the ion flux and energy that bombards the film, and it enables tailoring the film stress parameters. This technique has enabled us to fabricate freestanding microstructures up to 500 µm long with negligible stress-related deformation.

Improving thin-film zinc-oxide surface acoustic wave device insertion loss using a grooved reflective grating structure

A novel silicon groove reflective grating structure (RGS) to reduce effectively the insertion loss of a high C-axis (002) thin-film zinc oxide (ZnO) surface acoustic wave (SAW) device is proposed. The insertion loss (S 21 parameter) of three types of SAW devices was investigated and compared; one type had no RGS design, and the other two types had either a conventional metal electrode RGS or a novel etched silicon groove RGS. The wave propagation modes, the harmonic response, and the displacement amplitude profile of the proposed SAW devices were simulated by finite element analysis software (ANSYS). The implemented basic SAW device consists of a sputter-deposited high-quality ZnO thin film on the surface of a Si/SiO 2 /Si 3 N 4 /Cr/Au substrate. The insertion loss of the proposed SAW device with silicon groove RGS can be reduced to −19.49 dB , which is much smaller than the insertion losses of the SAW devices with a metal electrode-type RGS (−33.15 dB ) and without an RGS (−47.9 dB ). The resonant frequencies of the SAW devices studied (47.37 to 48.05 MHz) are in close agreement with the simulated results (46.8 to 47.55 MHz).

Lithium diffusion in sputter-deposited lithium iron phosphate thin-films

Thin films of lithium iron phosphate (LiFePO4, LFP), with a thickness between 80 nm and 320 nm are prepared by ion beam sputter deposition. X-ray diffraction, transmission electron microscopy, and electron energy loss spectroscopy show that the films exhibit the desired structure, morphology, and chemical composition. Chrono potentiometry and cyclic voltammetry are carried out to determine the electrochemical behavior of the LFP films. Within these electrochemical measurements, a well-defined lithium intercalation/deintercalation reaction at around 3.45 V vs. Li/Li+ and a specific capacity of 104 mAh g−1 are observed. Moreover, a quite high cycling stability is found: Measurement data at 1C rate indicate a capacity fading of only 20% after ≈1700 charge–discharge cycles. In addition, the diffusion of lithium within the LFP thin films is studied by cyclic voltammetry and galvanostatic intermittent transition technique. Within these measurements it is observed that the lithium kinetics in the sputter-deposited thin films is about two orders of magnitude faster, compared to the powder material and thin films deposited by pulsed laser deposition. This behavior may be explained by the strong texture of the sputtered films.

Reflectance Spectroscopy of Functional Ag-Cu Thin Films: Correlation of Reflectivity with Cu Content

We have investigated the optical properties of sputter-deposited silver-copper alloy thin films on silicon substrates at room temperature. We found that alloying copper with silver can significantly improve the metal’s surface morphology, keeping its infrared reflectivity same as that of pure silver film. However at the same time, it worsens in the blue parts of the spectrum. The reflection in Ag-Cu thin films is interpreted in terms of a simple model describing its trend due to alloying. The experimental results and the speculations reveal a prospect for the improvement and optimization of solar reflectors.

Photolithography-free fabrication of organic light-emitting diodes for lighting applications

We investigate the photolithography-free fabrication of organic light-emitting diodes (OLEDs) for lighting applications with an attempt to embed the deposition and patterning process of an indium–tin–oxide (ITO) anode and insulating layer into an in-line-type organic evaporation system. This scheme inevitably brings in leakage current induced by the spike-like surface of ITO. To suppress it, we cover the ITO edges with three different insulation materials (i.e. sputter-deposited inorganic Al2O3 thin film, monomer (polymer) thin film deposited by organic acrylate evaporation or thermally evaporated organic insulation layer (tris-(8-hydroxyquinoline) aluminum (Alq3))). Although small-molecule organic insulation materials that can be thermally evaporated are the most suitable for such a cost-effective fabrication process, yet their insulation capability is low due to the carrier transporting property. In this paper, we demonstrate that it can be boosted to a great extent with an increase of their thickness. It is likely that pinholes existing on the Al2O3 thin film act as leak channels, degrading the device performance. We also verify that the insulation capability of polymer fabricated by organic acrylate evaporation is just comparable with that of polyimide (PI) insulator patterned using a standard photolithography process.

Synthesis of Vertically Aligned CuO Nanorods by Thermal Oxidation

A simple thermal oxidation of Cu thin films deposited on planar substrates established a growth of vertically aligned copper oxide (CuO) nanorods. DC sputter-deposited Cu thin films with various thicknesses were oxidized in environments of various oxygen partial pressures to control the kinetics of oxidation. This is a method to synthesize vertically aligned CuO nanorods in a relatively shorter time and at a lower cost than those of other methods such as the popular hydrothermal synthesis. Also, this is a method that does not require a catalyst to synthesize CuO nanorods. The grown CuO nanorods had diameters of ~100 nm and lengths of 1~25μm. We examined the morphology of the synthesized CuO nanorods as a function of the thickness of the Cu films, the gas environment, the oxidation time, the oxidation temperature, the oxygen gas flow rate, etc. The parameters all influence the kinetics of the oxidation, and consequently, the volume expansion in the films. Patterned growth was also carried out to confirm the hypothesis of the CuO nanorod protrusion and growth mechanism. It was found that the compressive stress built up in the Cu film while oxygen molecules incorporated into the film drove CuO nanorods out of the film.

Tensile behavior of Al1−xMox crystalline and amorphous thin films

The exceptional strength and distinct deformation physics exhibited by pure ultrafine-grained and nanocrystalline metals in comparison to their microcrystalline counterparts have been ascribed to the dominant influence of grain boundaries in accommodating plastic flow. Such grain-boundary-mediated mechanisms can be augmented by additional strengthening in nanocrystalline alloys via solute and precipitate interactions with dislocations, although its potency is a function of the changes in the elastic properties of the alloyed material. In this study, we investigate the elastic and plastic properties of Al1−xMox alloys (0⩽x⩽0.32) by tensile testing of sputter-deposited freestanding thin films. Isotropic elastic constants and strength are measured over the composition range for which three microstructural regimes are identified, including solid solutions, face-centered cubic and amorphous phase mixtures and body-centered cubic (bcc)/amorphous mixtures. Whereas the bulk modulus is measured to follow the rule of mixtures over the Mo composition range, the Young’s and shear moduli do not. Poisson’s ratio is non-monotonic with increasing Mo content, showing a discontinuous change at the onset of the bcc/amorphous two-phase region. The strengthening measured in alloyed thin films can be adequately predicted in the solid solution regime only by combining solute strengthening with a grain boundary pinning model. The single-step co-sputtering procedure presented here results in diversity of alloy compositions and microstructures, offering a promising avenue for tailoring the mechanical behavior of thin films.

Stress tuning in sputter-deposited MoOx films

The evolution of the compressive stress in Mo/MoOx sputter-deposited thin films has been followed in situ during deposition as a function of two parameters: the oxygen flux and the negative voltage bias applied to the substrate. In addition the microstructure of the films has been characterized. Oxygen accumulation at the interface was observed. We describe the origins of the stress in terms of ion bombardment and oxygen incorporation.