Development Of Intrinsic Stresses Research Articles

The microstructural and stress evolution in sputter deposited Ni thin films

We report the stress and microstructural evolution for a series of Ni thin films sputter deposited over a range of rates (0.076 and 0.250 nm/s), pressures (0.27, 0.67 and 1.33 Pa), and substrate temperatures (ambient, 100, and 200 °C). In general, as the sputtering pressure increased, the stress-thickness product, measured by wafer curvature, became tensile and trended with an increase in pressure regardless of the deposition rate. However, at the lowest sputtering rate and highest substrate temperature, the films exhibited a compressive growth stress. The collective data was then fitted to a kinetic model that accounts for the stress generation at the grain boundaries, from grain growth, and from the creation of defects within the film. The model's predicted fitted parameters matched well to the experimental measurements except for films deposited at the lowest deposition pressure. These particular films exhibited a bimodal grain size distribution which could not be accommodated by the model's use of a singular grain diameter. Nevertheless, in monomodal grain sizes, the results do provide support for the kinetic model in helping to ascertain the various contributing factors for the intrinsic stress development and their microstructural relationship in context to the Thornton zonal morphology descriptors for thin films and coatings. • First use of a kinetic stress model to Ni sputter deposition • Kinetic model fits yield good agreement to experimental monomodal grain sizes. • Ascertaining energetic-based contributions to stress with varied process parameters.

Benefits of energetic ion bombardment for tailoring stress and microstructural evolution during growth of Cu thin films

We have studied the development of intrinsic stress and microstructure of copper (Cu) films deposited under energetic ion bombardment. Stress evolution during growth of ∼150 nm thick Cu films on Si(001) substrates has been investigated by in situ measurements in a high power impulse magnetron sputtering (HiPIMS) process for different substrate bias voltages (from 0 to −160 V) and benchmarked with conventional direct current magnetron sputtering (DCMS). The microstructure and crystal orientation of the studied films have been examined by various ex situ methods. For HiPIMS, we found that the substrate bias voltage (energy of Cu ions) strongly affects the film continuity during the early growth stages and the compressive stress developed during the post-coalescence stage. Contrarily to common expectations, the stress magnitude can be significantly reduced despite the energy increase of the bombarding particles when using HiPIMS. These results are discussed based on a recent kinetic model taking into account the grain size-dependent defect incorporation due to energetic particle bombardment. Reversible stress relaxations are observed upon growth interrupts, with characteristic time constants of tens of seconds, which suggests that the stress and microstructure development are mainly mediated by surface diffusion processes. In addition, polycrystalline films (111)-textured are obtained for negative bias voltages from 0 to −100 V, while for even higher negative bias voltages (up to −160 V), epitaxial growth of Cu(001) is achieved. For the DCMS samples, there is no significant change in film continuity and crystal orientation when varying the bias voltage.

Experimental and modeling study on the role of Ar addition to the working gas on the development of intrinsic stress in TiN coatings produced by filtered vacuum-arc plasma

The aim of this work is to study and model the effect of argon addition to the N2 working gas on the intrinsic stress of TiN coatings deposited via rectilinear filtered vacuum arc plasma using substrate pulse bias potential. It was found that produced TiN coatings have a NaCl type cubic structure with the [110] axial texture regardless of the concentration of argon in the gas mixture. With increasing the content of argon in the gas mixture, the level of residual stress increased from 7GPa up to 10GPa and the coatings hardness remained at a high level of 27–32GPa. By mathematical modeling based on the modified Davis model, it was shown that increasing the stress level in the TiN coatings with increasing of argon content was due to the intensification of the bombardment process of the coating surface by the inert gas ions.

Real-time curvature and optical spectroscopy monitoring of magnetron-sputtered WTi alloy thin films

WTi thin films are known as potential adhesion promoters and diffusion barriers. WTi thin films were deposited by magnetron sputtering from an alloyed target (W:Ti~70:30at.%). Real-time surface differential reflectance (SDR) spectroscopy and wafer-curvature measurements were performed during deposition to study the growth and the film continuity threshold. SDR measurements during WTi deposition allow the determination of the change in reflectivity of p-polarized light (at Si substrate Brewster's angle) between WTi film and Si substrate in order to monitor layer growth. The comparison between experimental and simulated WTi SDR signals assuming a homogeneous and continuous layer growth shows that film continuity is ensured beyond a thickness of 4.5±0.2nm. Real-time wafer-curvature measurements allow the determination of the intrinsic stress development in the film. Two regimes are noticed during the growth up to the development of a compressive steady state stress. The early stages of growth are rather complicated and divided into sub-regimes with similar boundaries revealed by both in situ techniques. Deposition of an interfacial continuous layer different from WTi bulk is suggested by both in situ techniques below a thickness of 4.5nm.

Origins of microstructure and stress gradients in nanocrystalline thin films: The role of growth parameters and self-organization

The development of depth gradients of texture, morphology and stresses in thin nanocrystalline films was experimentally demonstrated for a nanocrystalline CrN film by means of position-resolved synchrotron X-ray nanodiffraction and explained by atomistic processes at the growing film surface and the effect of interfaces, both controlled by the deposition conditions. Controllable changes in the energy of incident particles adjusted by bias voltages ranging from −40 to −120V affect the competitive growth of grains with different orientations, induce disruption of grain growth and thus give rise to structural variations across the film thickness. Subsequent changes in the volume fraction of grain boundaries and film texture were found to be responsible for changes in the residual stress state as defect generation proceeds to different extents in the interior of differently oriented grains and in the interfacial area. While the defect density predominantly affects the development of intrinsic stress, the variation in the number of weakly bonded atoms of grain boundaries determines the thermal stress component. The structural dependence of both stress components thus contributes to the characteristic development of stress gradients in thin nanocrystalline films.

Influence of Al content on the phase formation, growth stress and mechanical properties of TiZrAlN coatings

Quaternary (Ti,Zr)1−xAlxN transition metal nitride films, with Al content x ranging from 0 to 0.37, were reactively sputter-deposited from individual metallic targets under Ar+N2 plasma discharges on Si substrates at Ts=270°C. The influence of Al addition on the crystal structure, phase formation, growth morphology and intrinsic stress development, electrical and mechanical properties was systematically investigated. Three distinct compositional regions were evidenced: i) for 0≤x≤0.07, films develop a columnar structure consisting of cubic TiZr(Al)N grains with (111) and (200) preferred orientation, large compressive stresses up to ~−4GPa and hardness increase from ~20 to ~24GPa, ii) for 0.09≤x≤0.16, Al incorporation favors the growth of nanocomposite films consisting of (200)-oriented cubic TiZr(Al)N nanocrystals surrounded by a highly-disordered matrix, accompanied by a decrease of compressive stress, whereas a maximum hardness H~27GPa and H/E ratio of 0.105 is reached at x~0.12 and x=0.14, respectively, and iii) x>0.16, XRD amorphous films are formed, with reduced mechanical properties. The structure–stress-properties relationship is discussed based on evolutionary growth regimes induced by incorporating a high-mobility metal in a refractory compound lattice.

Structural origins of intrinsic stress in amorphous silicon thin films

Hydrogenated amorphous silicon (a-Si:H) refers to a broad class of atomic configurations, sharing a lack of long-range order, but varying significantly in material properties, including optical constants, porosity, hydrogen content, and intrinsic stress. It has long been known that deposition conditions affect microstructure, but much work remains to uncover the correlation between these parameters and their influence on electrical, mechanical, and optical properties critical for high-performance a-Si:H photovoltaic devices. We synthesize and augment several previous models of deposition phenomena and ion bombardment, developing a refined model correlating plasma-enhanced chemical vapor deposition conditions (pressure and discharge power and frequency) to the development of intrinsic stress in thin films. As predicted by the model presented herein, we observe that film compressive stress varies nearly linearly with bombarding ion momentum and with a (\ensuremath{-}1/4) power dependence on deposition pressure, that tensile stress is proportional to a reduction in film porosity, and the net film intrinsic stress results from a balance between these two forces. We observe the hydrogen-bonding configuration to evolve with increasing ion momentum, shifting from a void-dominated configuration to a silicon-monohydride configuration. Through this enhanced understanding of the structure-property-process relation of a-Si:H films, improved tunability of optical, mechanical, structural, and electronic properties should be achievable.

Size effect of thermal expansion and thermal/intrinsic stresses in nanostructured thin films: Experiment and model

The thermal expansion coefficient of nanocrystalline materials and its dependence on grain size was investigated in two different model systems, soft metallic Cr and hard ceramic CrN thin nanocrystalline films, both composed of grains having a cubic structure and sizes ranging between 10 and 30 nm. The dominant contribution to the enhancement of thermal expansion in nanocrystalline materials with respect to their coarse-grained counterparts was identified in the interfacial area containing weakly bonded atoms. Based on the experimental results a model is proposed which aids in understanding the thermal expansion of nanocrystalline solids and of the origin and development of thermal stress in thin nanocrystalline films. This model was experimentally validated by the correlation between the thermal expansion coefficient and the material grain size, which was controllably varied by the film deposition process, coating architecture and thermal treatment. The number of atoms in grain boundaries and their bonding character were, in addition, found to be crucial for the development of intrinsic stress. Its increase with increasing volume fraction of grain boundaries is attributed to the enhanced diffusional flux of weakly bonded surface adatoms into this area and enhanced defect generation due to the higher sensitivity of grain boundary atoms to displacement by incident particles.

Thermo-mechanical evolution of multilayer thin films: Part I. Mechanical behavior of Au/Cr/Si microcantilevers

MEMS microcantilever test structures were utilized to examine the microstructural evolution of Au/Cr/Si thin films subject to annealing. Curvature evolution of the micron-sized structures was measured in response to anneals at various times and temperatures. Particular emphasis was placed on the accelerated annealing condition of 225 °C for 24 h. The thermo-mechanical response of the microcantilevers consisted of both linear-elastic and inelastic regimes. The temperature at which the thermo-mechanical profile deviates from linear thermo-elasticity is influenced by the stress, curvature and/or the microstructure of the specimens. Stress analysis suggests that microstructural evolution, not plastic yielding, controls the inelastic portion of the thermo-mechanical profile. Maximum stress increases of 146.3 and 202.9 MPa (i.e. 500% relative to the as-deposited state) were observed in the gold layer of the microcantilevers of different silicon thickness, as the result of the inelastic strain at elevated temperature. Increasingly greater curvature change is observed for specimens as annealing temperature is increased up to 150 °C, whereas the magnitude of curvature change is diminished as annealing temperature is increased above 150 °C. A complex curvature evolution is observed at 225 °C over a 24-h timeframe. Curvature evolution during isothermal hold occurs in response to the development of intrinsic stress within the metals. Use of a nitrogen atmosphere or nano-thickness alumina surface coatings was seen to alter the stability of the curvature evolution at 225 °C. The critical thickness for a protective alumina passivation occurs between 6.5 and 32.5 nm. Thermo-mechanical behavior is discussed here, while the corresponding microstructural evolution is discussed in the second part of this paper.

Intrinsic stress development and microstructure evolution of Au/Cr/Si multilayer thin films subject to annealing

Au/Cr/Si microcantilevers were studied in their as-deposited condition and annealed state, with emphasis on a thermal treatment of 225 °C for 24 h. Change in beam curvature was monitored during isothermal hold as a function of time. Secondary grain growth was observed in the gold, which contained non-uniformly distributed twins and dislocation defects. Diffusional transport of the chromium layer was observed during annealing. Nodules arranged in a “rolling hill” topography were observed at the free surface, both before and after annealing. Nanometer thick coatings of alumina grown by atomic layer deposition improved the uniformity of both microstructure evolution and curvature evolution during high-temperature annealing.

Intrinsic stresses and mechanical properties of Ti-containing hydrocarbon coatings

A detailed examination of the intrinsic stress development within and mechanical properties of Ti containing hydrocarbon (Ti–C:H) coatings deposited in an inductively coupled plasma assisted hybrid chemical vapor deposition/physical vapor deposition environment has been carried out, combining in situ substrate curvature measurements with plasma probe measurements, ex situ electrical resistivity measurements, and instrumented nanoindentation measurements. Intrinsic stresses within Ti–C:H have been found to be compressive over wide ranging compositions and plasma parameters. The intrinsic compression within Ti–C:H was found to depend significantly on the Ti composition, and was related to a percolation type transition in the nanoscale structure. The intrinsic compression within Ti–C:H has further been shown to be significantly influenced by the energy of ionic species bombarding the substrate during growth. Measured stress–thickness history was discussed in terms of possible mechanisms contributing to intrinsic stress generation. Although there are likely multiple mechanisms influencing intrinsic stress development, our present results suggest that ion bombardment plays a significant role in intrinsic stress generation within Ti–C:H, and is likely to influence stress development in other low temperature deposited amorphous hydrocarbon based ceramic nanocomposite coatings.

Intrinsic stress development in Ti–C:H ceramic nanocomposite coatings

The development of intrinsic stresses within titanium-containing hydrocarbon (Ti–C:H) nanocomposite coatings was monitored during growth by in situ substrate curvature measurements using a multibeam optical sensing technique. Stress as a function of coating thickness was measured in a wide range of specimens, from nearly pure amorphous hydrocarbon (a-C:H) to nearly pure titanium carbide (TiC). The intrinsic stress within the nanocomposite coating was found to vary significantly in magnitude, and to depend systematically on the Ti composition. The observed stress variation as a function of the Ti composition correlates well with a previously reported percolation-type transition in the coating microstructure.

Growth of cubic boron nitride films by ibad and triode sputtering: development of intrinsic stress

Two methods are employed to evidenced the stress behavior in c-BN films. On the one hand, in depth stress profile of c-BN film, deposited by ion beam assisted evaporation, was performed by recording infrared spectra and substrate curvature after reactive ion etching (RIE) steps. It shows a peak of stress up to −17 GPa in the h-BN basal layer and a stress relaxation when the cubic phase appears. On the other hand, dynamic stress profiles of c-BN films deposited by a triode sputtering system, are obtained by recording infrared spectra and substrate curvature after various c-BN deposition times, with the same experimental conditions. Likewise, a peak of stress of −12 GPa is unmistakably observed in the h-BN basal layer followed by a stress release during c-BN nucleation, where an average value of −12 GPa is observed in the c-BN film volume. These results provide a support for the stress model proposed by McKenzie even if along with a minimum stress a high level of densification of the layer is needed.

Dynamic stress investigations for cubic boron nitride films deposited by triode sputtering technique

In the present work c-BN films have been deposited on unheated silicon substrates by the triode sputtering technique. The films were characterized by infrared spectroscopy. High-resolution cross-sectional TEM is used to confirm the FTIR results. Films with c-BN content up to 70% were obtained at temperature of 300°C in a pure nitrogen discharge. The addition of argon in the discharge increases the c-BN content up to 90%. It is well known that the development of intrinsic stress hinders the deposition of thick c-BN films and consequently limits their applications. In order to understand the behavior of the stress within the film better, we have recorded infrared spectra and substrate curvature after various deposition times, with the same experimental conditions. The results give the dynamic stress profile during deposition and, therefore, enable a better understanding of the behavior of the stress and its relationship to the transition from the hexagonal phase to the cubic one. Therefore, the deposition conditions have been changed during film growth, in order to reduce the stress level and to try to improve the adhesion.

Intrinsic Stress Measurements in CVD Diamond Films

AbstractDiamond films were grown over Si substrate at 1253K by the hot filament chemical vapor deposition method using CH4/H2 gas mixture, and intrinsic stresses in the film were deduced from the ex-situ curvature measurements. In order to account for the creep deformation of the Si substrate, an elastic/plastic stress and strain analysis were conducted. Results showed that intrinsic stresses were generally several times larger than the average film stresses and always positive increasing with the film thickness. For the film thickness larger than 10μm, stress relaxation by creep of the substrate became significant, and must be considered for the accurate assessment of the film stress in diamond. Later, an analysis based on the grain growth accounted for the development of intrinsic stresses reasonably well.

Intrinsic Stress Measurements in CVD Diamond Films

ABSTRACTDiamond films were grown over Si substrate at 1253K by the hot filament chemical vapor deposition method using CH4/H2 gas mixture, and intrinsic stresses in the film were deduced from the ex-situ curvature measurements. In order to account for the creep deformation of the Si substrate, an elastic/plastic stress and strain analysis were conducted. Results showed that intrinsic stresses were generally several times larger than the average film stresses and always positive increasing with the film thickness. For the film thickness larger than 10μm, stress relaxation by creep of the substrate became significant, and must be considered for the accurate assessment of the film stress in diamond. Later, an analysis based on the grain growth accounted for the development of intrinsic stresses reasonably well

Growth of aluminum nitride thin films on Si(111) and Si(001): Structural characteristics and development of intrinsic stresses

We have grown aluminum nitride thin films by ultrahigh vacuum reactive sputter deposition on Si(111) and Si(001) substrates. We show results of film characterization by Raman scattering, ion beam channeling, and transmission electron microscopy, which establish the occurrence of epitaxial growth of wurtzitic aluminum nitride thin films on Si(111) at temperatures above 600 °C. In contrast, microstructural characterization by transmission electron microscopy shows the formation of highly oriented polycrystalline wurtzitic aluminum nitride thin films on Si(001). Real-time substrate curvature measurements reveal the existence of large intrinsic stresses in aluminum nitride thin films grown on both Si(111) and Si(001) substrates.