Residual Stress In Thin Films Research Articles

Mechanical stress in thin film microstructures on silicon substrate

Analysis of residual stress in thin films and its influence on silicon microstructures was performed. Deflection behaviour of silicon membranes due to built-in stress in deposited thin films was investigated in the scope of different bilayer structures which are commonly met in microstructures such as piezoresistive pressure sensors. Silicon membranes are usually covered by thin films such as thermal oxide, LPCVD or PECVD nitride, as required by the actual fabrication process. In our case, high compressive stress was determined for stoichiometric LPCVD nitride (1.2 GPa) and PECVD nitride (706 MPa) while for thermal oxide tensile stress of 298 MPa was determined. Furthermore, thin film cantilever and bridge microstructures were designed, fabricated and investigated, showing that by a proper combination of thin films in the stacked layer, residual stress can be reduced to a reasonable amount without compromising the dielectric properties of thin films.

Modeling of residual stresses in thin films deposited by electron beam evaporation

The modeling of residual stresses generally deals with the calculation of the thermal ones alone. In the present work, a model is proposed, where intrinsic residual stresses are calculated explicitly. It is based on the Tsui–Clyne model, which was modified in order to describe the deposition of n different deposited layers on the substrate with the same set of equations as in a mono-layer system. For that we introduced the notion of “virtual substrate” and “effective parameters”. The calculated values are compared with experimental results obtained for the following systems: Ta/Si, Mo/Si, Al/SiO x /Si and Pd/SiO x /Si. The depositions were performed by electron beam evaporation onto Si and Si/SiO x (1 0 0) substrates with varying film thickness.

Effect of deposition condition on residual stress of iron nitride thin films prepared by magnetron sputtering and ion implantation

The surface roughness and residual stress development in Fe–N thin films prepared by compound technology—combining magnetron sputtering with plasma based ion implantation were investigated by means of atomic force microscope and synchrotron radiation. The results indicate that the grain size of the thin film increases with the increasing of nitrogen ion implantation time, and the state of residual stress is related closely to the formation mechanism of thin films. With the nitrogen ion implantation time increasing, the residual stress of the thin film changes into tensile stress from initial compressive stress, and the tensile stress decreases with the further increasing of ion implantation time.

Shape and Thermal Effects of Metal Films on Stress-Induced Bending of Micromachined Bilayer Cantilever

In this study, the shape and thermal effects of metal films on the stress-induced bending of micromachined bilayer cantilever are systematically investigated. The cantilever makes use of residual stresses in thin films to induce the bending of microstructures by applying preloads. The characterization of the bilayer cantilever with different Au distribution has been performed to observe out-of-plane deformation. A finite-element model has been established to analyze such a deformation using experimental and theoretical results. It indicates that the commonly used formula for predicting the deformation of the cantilever is not valid for these devices. When the percentage of the area deposited with a metal is increased, deflection angle also increases. Furthermore, the influence of postprocessing temperature on the residual stress of the metal films is examined. Postprocessing temperature and residual stress reveal a close relationship. As postprocessing temperature increases, the residual stress of the metal increases, resulting in a larger out-of-plane deformation of the cantilever. Residual stress increases to a saturation value while the temperature reaches a critical value. This new finding could greatly affect the performance of a stress-induced bilayer cantilever. Finally, a switchable micromachined corner mirror is demonstrated using the bilayer cantilever.

繊維配向したＴｉＮ薄膜中の残留応力の低角入射Ｘ線法および散乱ベクトル法による測定

Titanium nitride (TiN) thin films with the thickness of 0.1, 0.2, 0.5, 1.0, 2.0 and 4.0μm were coated on a carbon steel substrate by the ion beam mixing method. The film had a strong fiber texture with ‹001› axis perpendicular to the film surface. The distribution of the residual stress in thin films was measured by the grazing incidence X-ray diffraction (GIXD) and the scattering vector (SV) methods. The in-plane stress measurement by GIXD was applicable to the thickness down to 0.2μm of TiN films. The stress was a compression of around 2 to 3GPa. The compressive stress was found to increase in the very-near surface region of 20 to 30nm in depth. Thinner films had a steeper increase of the compressive stress in the very-near surface region. The strain distribution measured by the SV method using Cr-Kα radiation was nearly constant over the region of the penetration depth between 0.3 and 0.6μm from the surface, and that by using synchrotron X-rays was also constant over the penetration depth from 0.4 to 1μm. The stress distributions measured by the in-plane measurement and the one-tilt method combined with the surface removal method showed a good agreement with each other. The compressive residual stress was uniform within thin films except very near the surface of about 20 to 30nm in depth.

1858 光導波路の伝播特性に及ぼす薄膜残留応力の影響(J09-4 有限要素解析と強度信頼性評価,J09 電子情報機器,電子デバイスの熱制御と強度・信頼性評価)

1858 光導波路の伝播特性に及ぼす薄膜残留応力の影響(J09-4 有限要素解析と強度信頼性評価,J09 電子情報機器,電子デバイスの熱制御と強度・信頼性評価)

Stress reduction in tungsten films using nanostructured compliant layers

The residual stress in thin films is a major limiting factor for obtaining high quality films. We present a strategy for stress reduction in sputter deposited films by using a nanostructured compliant layer obtained by the oblique angle deposition technique, sandwiched between the film and the substrate. The technique is all in situ, does not require any lithography steps, and the nanostructured layer is made from the same material as the deposited thin film. By using this approach we were able to reduce stress values by approximately one order of magnitude in sputter deposited tungsten films. These lower stress thin films also exhibit stronger adhesion to the substrate, which retards delamination buckling. This technique allows the growth of much thicker films and has enhanced structural stability. A model is developed to explain the stress relief mechanism and the stronger adhesion associated with the presence of the nanostructured compliant layer.

Residual stress analysis of thin films and coatings through XRD 2 experiments

Residual stresses are one of the crucial parameters determining the performances of structural as well as functional materials. In the case of coatings and films, the substrate and the deposition process may determine very high residual stress fields which can affect both performances and surface integrity, since adhesion or cracking resistance can be strongly altered. The accurate and reliable assessment of residual stress is thus mandatory for the evaluation of these materials. In this paper we applied a new approach to evaluate the residual stress by means of the analysis of a single 2D-diffraction image collected by a laboratory X-ray microdiffractometer equipped with an image plate detector. The residual stress in thin films of LaCoO 3 was calculated and correlated to cubic-to-rhombohedral phase transformation.

Development of Control Method for Residual Stress in Thin Film by Substrate Vibration

The prevention of residual stress of thin film syntheses is very important in processing engineering surfaces. In this paper, details of residual stress control by the vibration of the substrate using PZT at frequency of 102-106Hz and voltage of 0-100V are presented. The thin films deposited are crystalline TiN, Ti, Cu, Al films and amorphous C, Si films. The residual stresses are measured using Stoney's method. The results of our experiment show that for Ti, TiN and Cu thin films, compressive stress are changed to tensile stresses while the Al films show only a decrement of the compressive stresses. The amorphous thin films on the other hand did not show any change in the residual stress. XRD and Auger spectroscopy results verify that the composition and crystalline structure of the treated and untreated crystalline thin films to be the same. But it is shown that grain size in the film tend to decrease with increasing the vibration amplitude. From this microstructure change of the film, a model for the control of residual stress is proposed.

Factors limiting the measurement of residual stresses in thin films by nanoindentation

Experimental results and finite element simulations are presented to examine whether a recently developed nanoindentation method for measuring residual stresses in bulk, monolithic materials can be applied to thin films. The method is based on indentation with spherical indenters in the elastic–plastic transition regime. Experiments were performed on 7 μm copper films and 5 μm chromium films deposited on (100) silicon substrates using indenters with radii of 30, 69 and 122 μm. The biaxial stresses in the films, which were controlled by processing, were verified by X-ray techniques. Nanoindentation measurement of the film stresses proved difficult due to practical considerations arising from surface roughness, substrate effects and problems in producing an appropriate reference specimen for comparison. Finite element simulations showed that the substrate problems can be alleviated by using an indenter with a radius of the order of (or smaller than) the film thickness. However, the other difficulties pose serious obstacles to the practical implementation of the method to thin film residual stress measurement.

Impact of flip-chip packaging on copper/low-k structures

Copper/low-k structures are the desired choice for advanced integrated circuits (ICs). Nevertheless, the reliability might become a concern due to the considerably lower strength and greater coefficient of thermal expansion (CTE) of the low-k materials. To ensure successful integration of the new chips within advanced packaging products, it is essential to understand the impact of packaging on chips with copper/low k structures. In this study, flip-chip die attach process has been studied. Multilevel, multiscale modeling technique was used to bridge the large gap between the maximum and minimum dimensions. Interface fracture mechanics-based approach has been used to predict interface delamination. Both plastic ball grid array (PBGA) and ceramic ball grid array (CBGA) packages were evaluated. Critical failure locations and interfaces were identified for both packages. The impact of thin film residual stresses has been studied at both wafer level and package level. Both PBGA and CBGA packaging die-attach processes induce significantly higher crack driving force on the low-k interfaces than the wafer process. CBGA die-attach might be more critical than PBGA die-attach due to the higher temperature. During CBGA die-attach process, the crack driving force at the low-k/passivation interface may exceed the measured interfacial strength. Two solutions have been suggested to prevent catastrophic delamination in copper/low-k flip-chip packages, improving adhesion strength of low-k/barrier interface or adding tiles and slots in low-k structures to reduce possible area for crack growth.

Mitigation of residual film stress deformation in multilayer microelectromechanical systems cantilever devices

An approach to compensate for the residual thin film stress deformation of multilayer microelectromechanical systems (MEMS) devices is presented based upon analytical and numerical modeling and in-process thin film characterization. Thermal and intrinsic deposition stresses can lead to the warping of released MEMS structures. This detrimental phenomenon in many cases can prevent proper device operation. Ellispsometric and laser wafer bow measurements yield thickness and film stress values that are used to update the deflection model during device fabrication, allowing for the compensation of the fabrication process variability. The derivations of linear and nonlinear residual film stress induced deflection models are presented. These models are based upon Bernoulli–Euler beam theory and are thus restricted to the associated geometric constraints. The models are initially validated by comparison with surface micro-machined sol–gel lead–zirconate–titanate cantilever structures; with initial experimental results agreeing well with both.

A method for in situ measurement of the residual stress in thin films by using the focused ion beam

A new method for the in situ measurement of the residual stresses, σ R, in thin films, thickness h, is described. It is based on the combined capability of the focused ion beam (FIB) imaging system and of high-resolution strain mapping software ( vic-2d). The method can be used for any film material (whether amorphous or crystalline) without thinning the substrate. In the method, a region of the film surface is highlighted and scanning electron images of that region taken before and after a long slot, depth a, is introduced using the FIB. The mapping software evaluates the displacement of the surface normal to the slot, u x , by using naturally occurring features on the film surface. Numerical results relate the displacements to σ R. The accuracy of the method has been assessed by performing measurements on thin (sub-micron) films of diamond-like carbon on glass substrates. Independent measurements of the residual stress exist for this system. The new method determines σ R to within 5% of the magnitude determined independently. New possibilities enabled by the method are discussed, such as the ability to perform local measurements and to ascertain through thickness gradients.

Crystal orientation and residual stress in TiN film by synchrotron radiation

Abstract The residual stress in a TiN film, the thickness of which is less than 1 mm, deposited by arc ion plating was measured using the SPring-8 synchrotron radiation facility installed at Japan Synchrotron Radiation Research Institute (JASRI). Films thicker than 300 nm have a {111} texture, whereas those with a thickness of 100 nm have a relatively random orientation. Instead of the conventional laboratory X-ray equipment, ultrabright synchrotron radiation can be used to precisely measure the residual stress in thin films with a thickness of 100 to 800 nm. The residual stress in thicker TiN films is about 7 GPa in compression and it is not affected by the film thickness. The slightly small compressive stress in a film of 100 nm in thickness may be due to the different crystal orientations in the film.

Stress and elastic-constant analysis by X-ray diffraction in thin films

Residual stresses influence most physical properties of thin films and are closely related to their microstructure. Among the most widely used methods, X-ray diffraction is the only one allowing the determination of both the mechanical and microstructural state of each diffracting phase. Diffracting planes are used as a strain gauge to measure elastic strains in one or several directions of the diffraction vector. Important information on the thin-film microstructure may also be extracted from the width of the diffraction peaks: in particular, the deconvolution of these peaks allows values of coherently diffracting domain size and microdistortions to be obtained. The genesis of residual stresses in thin films results from multiple mechanisms. Stresses may be divided into three major types: epitaxic stresses, thermal stresses and intrinsic stresses. Diffraction methods require the knowledge of the thin-film elastic constants, which may differ from the bulk-material values as a result of the particular microstructure. Combining an X-ray diffractometer with a tensile tester, it is possible to determine X-ray elastic constants of each diffracting phase in a thin-film/substrate system, in particular the Poisson ratio and the Young modulus. It is important to notice that numerous difficulties relative to the application of diffraction methods may arise in the case of thin films.

Effect of sputtering pressure on residual stress in Ni films using energy-dispersive x-ray diffraction

A procedure for determining the residual stress in thin films using energy dispersive x-ray diffraction was investigated. The effect of the sputtering pressure on the residual stress in dc magnetron sputtered Ni films was studied in greater detail using this approach. The behavior reported suggested the possibility of controlling or influencing the sign and/or magnitude of the residual stress. In addition, the stress variation with increasing negative bias voltage is also presented. In the range studied, between −15 and −150 V, residual stress is always tensile.

Evaluation of Thin-Film Residual Stress Using Nano-Indentation Combined with an Atomic Force Microscope

Thin-film stress changes the shape of nanoindentation loading curve. The change in the indentation load is treated as the effect of the residual stress on the indenting deformation. A residual-stress-induced normal load is defined as a multiple of contact area and plastic deformation-sensitive deviator stress component extracted from the equi-biaxial thin-film stress. A final equation for the residual stress is derived from the residual-stress-induced normal load by considering an integration along a depth-controlled stress-relaxation route. This proposed model is applied to the analyses of the nanoindentation curves for diamond-like carbon thin films. The residual stresses from the nanoindentation analyses were consistent with the values obtained by the conventional curvature method.

Residual stress measurement in thin carbon films by Raman spectroscopy and nanoindentation

The reliability of hermetic carbon-coated optical fibers is affected by residual stresses in the coating created during the fiber draw process. Thermally induced residual stresses are caused by differences in the coefficient of thermal expansion (CTE) of the coating and the optical fiber. This mismatch creates shear stresses at the interface that can delaminate the film. This work presents and validates a surface residual-stress measurement technique using Raman spectroscopy. Since select Raman peaks for carbon films exhibit a wavenumber shift in proportion to the magnitude of residual stress, Raman spectra can be correlated to a theoretical model to obtain its residual stress. The model, validated with nanoindentation, shows an equi-biaxial stress field through the depth of the film. Nanoindentation also provides an accurate measure of residual stress in thin films with unknown material properties. The approach presented in this study is a non-destructive and non-intrusive method for measuring residual surface stress in thin films, and is ideal for small curved-surface specimens such as carbon-coated optical fibers.

ＣＶＤ法で被覆したＴｉＣ膜の残留応力と強度評価に関するＸ線的研究

Thin films of titanium carbide (TiC) were coated on various substrates such as cemented carbide (WC-Co) with different cobalt contents, ferritic and austenitic steels by the chemical vapor deposition (CVD) method. Thin films did not have strong texture. The residual stress in thin films was measured by the sin2Ψ method of X-ray stress measurement. The residual stress was tensile for WC-Co substrates, while compressive for steel substrates. The residual stress was nearly uniform within the film thickness for the cases of WC-Co substrates, while it had rather steep distribution for the cases of steel substrates. The magnitude of the residual stress increased in proportion to the mismatch of the coefficient of thermal expansion (CTE). The measured stress was more compressive than the prediction by CTE mismatch. Under the uniaxial applied stress, the stress in the thin film was biaxial because of the mismatch of Poisson's ratio, and increased in proportion to the applied strain as predicted from the elasticity relation of coated layers. At high applied strains, the stress in the thin film measured by X-rays did not increase because of cracking. The tensile fracture strength of TiC films on WC-Co substrates was found to be around 750 to 800MPa. This value is about twice the fracture strength of bulk materials.

繊維配向を有するＴｉＮおよびＴｉＣＮ薄膜の熱による残留応力変化にともなう機械的強度への一影響

TiN and TiCN thin films are used as antiwear layer on parts such as metal cutting tools. It is reported that the thin films that are deposited ceramics and metals have preferred orientation, and that the residual stresses generate by the difference in the coefficients of thermal expansion between the thin films and the substrate. Tools coated with TiN and TiCN films are heated to high temperature during service due to the frictional heat. Under this condition, the texture and the residual stress should change due to the heat. In the material engineering, it is well known that the residual stress in the material affects the mechanical strength of the industrial products. The purpose of this study is to examine the effect of the residual stress on the mechanical strength of TiN and TiCN thin films having preferred orientation.In this study, specimens that are deposited TiN and TiCN respectively by Physical Vapor Deposition (PVD) were annealed in a furnace at temperature of 573K, 798K, 843K and 893K. Using X-ray diffraction technique, the crystallite orientation was evaluated by the pole figure and the crystallite orientation distribution function (ODF). The films exhibited {111} fiber texture. After that, the residual stress of thin films was measured using CoKα radiation with the two-exposure method at Ψ=39° and 75°. Since the relationship between the mechanical strength and the residual stress has not been revealed about the thin film materials, the effect of the residual stress on the mechanical strength of the TiN and TiCN thin films having preferred orientation was investigated by the dynamic hardness (DH) and the scratch test.As a result, although the ODF little changed due to the heat treatment, the residual compressive stress relaxed at higher temperatures than 573K. As for the hardness of thin films, the residual stress did not affect the hardness. However, the residual stress influenced the behavior of the scratch test. The abrasive wear and the micro cracks were decreased by highly residual compressive stress.