Thickness-dependent phase transformation in nanoindented germanium thin films

We investigate the mechanical response of 50-600?nm epitaxial Ge films on a Si substrate using nanoindentation with a nominally spherical (R≈4.3?μm) diamond tip. The inelastic deformation mechanism is found to depend critically on the film thickness. Sub-100?nm Ge films deform by pressure-induced phase transformation, whereas thicker films deform only by shear-induced dislocation slip and twinning. Nanoindentation fracture response is similarly dependent on film thickness. Elastic stress modelling shows that differing stress modes vary in their spatial distribution, and consequently the film thickness governs the stress state in the film, in conjunction with the radius of the nanoindenter tip. This opens the prospect of tailoring the contact response of Ge and related materials in thin film form by varying film thickness and indenter radius.     

Evaluation of the elastic modulus of thin film considering the substrate effect and geometry effect of indenter tip

A method using finite element method (FEM) is proposed to evaluate the geometry effect of indenter tip on indentation behavior of film/substrate system. For the nanoindentation of film/substrate system, the power function relationship is proposed to describe the loading curve of the thin film indentation process due to substrate effect. The exponent of the power function and the maximum indentation load can reflect the geometry effect of indenter and substrate effect. In the forward analysis, FEM is used to simulate the indentation behavior of thin film with different apex angles of numerical conical indenter tip, and maximum indentation load and loading curve exponent are obtained from the numerical loading curves. Meanwhile, the dimensionless equations between the loading curve exponent, the maximum load, elastic properties of film/substrate system and apex angle of indenter are established considering substrate effect. In the reverse analysis, a nanoindentation test was performed on thin film to obtain the maximum indentation load and the loading curve exponent, and then the experimental data is substituted into the dimensionless equations. The elastic modulus of thin film and the real apex angle of indenter can be obtained by solving the dimensionless equations. The results can be helpful to the measurement of the mechanical properties of thin films by means of nanoindentation.     

Effect of surface roughness on nanoindentation test of thin films

By using the two-dimensional quasicontinuum method, the nanoindentation process on a single crystal copper thin film with surface roughness is simulated to study the effect of surface morphology on the measurements of mechanical parameters. The nanohardness and elastic modulus are calculated according to Oliver-Pharr’s method. The obtained results show a good agreement with relevant theoretical and experimental results. It is found that surface roughness has a significant influence on both the nanohardness and elastic modulus of thin films determined from nanoindentation tests. The effect of such factors as the indenter size, indentation depth and surface morphology are also examined. To rule out the influence of surface morphology, the indentation depth should be much greater than the characteristic size of surface roughness and a reasonable indenter size should be chosen. This study is helpful for identifying the mechanical parameters of rough thin films by nanoindentation test and designing nanoindentation experiments.     

Adsorbed multilayer effects on the mechanical properties in nanometer indentation depth

Nanoindentation is a powerful technique for determining the mechanical properties of a material at the nanometer scale. In this study, the changes induced in the mechanical properties of a thin film by the presence of adsorbed multilayers are examined by performing nanoindentation tests with ultra-low indentation depths. The current findings suggest that the results obtained for the mechanical properties of thin films under nanoindentation will be overestimated if the effects of the adsorbed multilayers on reducing elastic recovery between the indenter and the substrate are not taken into consideration.     

Interface stress induced hardness enhancement and superelasticity in polytetrafluoroethylene/metal multilayer thin films

Polytetrafluoroethylene (PTFE)/Al, PTFE/Cu, and PTFE/Ti multilayer thin films have been deposited in order to investigate effects of interface energy on mechanical properties. PTFE, which has a low surface energy of 19.2 mJ/m², was used to introduce a large interface energy into multilayer thin films. PTFE thin film was deposited by rf magnetron sputtering using a PTFE sheet target. Al, Cu, and Ti were deposited by dc magnetron sputtering. The multilayer thin films were fabricated sequentially without breaking vacuum. Substrate used was aluminosilicate glass. The modulation period was changed from 6.7 to 200 nm. The total thickness was about 200 nm for all samples. The internal stress of metal layers changed from tensile to compressive and increased with decreasing modulation period for all of PTFE/Al, PTFE/Cu, and PTFE/Ti. Both hardness enhancement and superelasticity were observed in the results of nanoindentation measurements. The energy dissipated during nanoindentation process (one load and unload cycle) decreased with decreasing modulation period. The minimum value of the ratio of dissipated/loaded energy was < 40%, which is smaller than the values obtained for monolithic PTFE or metal films (about 73% for PTFE and 87% for Al, 72% for Cu, and 71% for Ti, respectively). This meant that the PTFE/metal nano-multilayer thin films became more elastic with decreasing modulation period. The tendency of change in the mechanical properties strongly correlated to internal stress. Mechanisms involved in anomalous behaviors in film hardness and elasticity were discussed based on the relationship to interface energy, interface stress, and internal stress, induced by multilayering of the films. It is concluded that a large compressive stress introduced in the thin films increased the energy needed to deform elastically or plastically the thin film during indentation, resulting in the increase in hardness and elasticity. The nanoindentation analysis of the multilayer thin films emphasized that in PTFE/metal multilayer thin films mechanical properties of the films depend on interface stress induced by the accumulated interface energy, being independent of bulk materials properties composing thin films, resulting in increase in hardness and elasticity.     

Characterization of interconnect interfacial adhesion by cross-sectional nanoindentation

The technique of cross-sectional nanoindentation (CSN), as applied to thin films used in microelectronic devices, is reviewed. The technique was developed to characterize interfacial adhesion of interconnect thin films. A Berkovich diamond indenter is used to initiate fracture in a silicon substrate on which multilayer thin film structures are deposited. The cracks propagate to the weakest interface in the system. Both dielectric-dielectric and metal-dielectric interfaces are studied. The technique produces a qualitative measure of interfacial adhesion for blanket and patterned thin films. Quantitative results are obtained for blanket thin films through the application of analytic and finite element modeling. Fracture energy release rates obtained with CSN are in good agreement with results obtained with the four-point bending technique.     

Finite element analysis of the indentation stress characteristics of the thin film/substrate systems by flat cylindrical indenters

The indentation stress characteristics of thin film/substrate systems by the flat cylindrical indenters have been simulated by means of the finite element method (FEM). The emphasis was put on the stress distribution ahead of the indenters. The influences of the friction coefficient between the indenter and the thin film, the thickness and hardening modulus of the thin film have been considered. It is found that the stress distribution was not affected by the friction coefficient. But the influence of the thickness and hardening modulus of the thin film on the stress distribution was obvious. At small indentation depth, the plastic deformation occurs at the edge of the indenter only, and the zone will propagation both vertically and laterally with the indentation depth increasing. When the indentation depth reaches a certain value, the thin film at the interface will occur the deformation plastic zone for the case studied in this paper. At lager depths, the two plastic zones will connect, and then the plastic zone propagates along the lateral direction. Beside, it is also found that the maximum of the Mises stress and the shearing stress on the interface occur at 0.8r and r(r is the radius of the indenter), respectively.     

Effect of residual stresses on mechanical properties and interface adhesion strength of SiN thin films

Residual stresses play a significant role in the mechanical reliability of thin films. Thus in this study, the mechanical properties and interface adhesion strengths of SiN thin films containing different residual stresses have been investigated by using nanoindentation and nanoscratch tests. With varied residual stresses from compressive to tensile, the penetration depth of nanoindentation tests shifted to a higher value. The hardness and elastic modulus decreased from 11.0 and 95 GPa, respectively, for the film containing a compressive stress of 235 MPa to 9.6 and 84 GPa for the film with a tensile stress of 86 MPa. With decreasing compressive stress and increasing tensile stress, the interface adhesion energy decreased from 1.8 to 1.5 J/m². Compressive stresses were expected to blunt crack tips and inhibit crack propagation, while tensile stresses enlarged crack opening and facilitated crack propagation, thus changing the mechanical properties of the SiN thin films.     

An elasto-plastic contact model applied to nanoindentation

This paper is devoted to the finite element modeling of the nanoindentation problem. The frictional contact between the Berkovitch indenter and the very thin elasto-plastic film is treated by the bi-potential method. The elasto-plastic constitutive equation is integrated by means of the radial return mapping algorithm and the consistent tangent operator is explicitly derived. Numerical results show the validity of the model.     

A model for the evaluation of indentation crack arrest fracture toughness of supported films

A simple model is proposed for the evaluation of crack-arrest fracture toughness K _Ic0 of thin films by Vickers indentation. This approach applies to films thinner than the penetration depth of the Vickers indenter. The model equations are provided in closed form, even though they are so complex that they must be integrated numerically in practical applications. The problem of the evaluation of K _Ic0 for thin films and substrates is derived in general form and applied to three cases: (i) evaluation of K _Ic0 for the film in the case that the depth of the crack in the film is smaller than the film thickness, (ii) evaluation of K _Ic0 for the film in the case that the crack emanating from the film either crosses the film/substrate interface or is stopped by it, (iii) evaluation of K _Ic0 for the substrate in the case that the crack emanating from the film crosses the film/substrate interface. The model was tested with original and literature experimental data: (i) revision K _Ic0 values of electroless Ni-P thin films were re-evaluated, (ii) K _Ic0 of electroless Ni-P thin films of various thickness with various loads were measured (original data) and computed, (iii) K _Ic0 of electroless Ni-P substrates coated with electrodeposited Au-Cu were measured (original data) and computed.     

Position effect of cylindrical indenter on nanoindentation into Cu thin film by multiscale analysis

The quasi-continuum multiscale method (QC) is applied to investigate position effect of cylindrical indenter on nanoindentation into Cu thin film. Load–displacement responses reflect that indenter position influences the critical load and critical displacement distinctly. The microscopic deformation mechanism shows that it is the retarding of dislocation nucleation beneath the indenter that actually leads to a relative lower critical load. Once the plastic deformation is retarded, the load–displacement curve will undulate several times until dislocations nucleate at the retarded region. It is found that the critical loads periodically change with indenter positions. The period equals to the distance between two adjacent atomic planes in [1 1 1] direction. An improved elastic model is proposed to predict the critical load in consideration of the effect of indenter position. The agreement between QC simulation and the present model has shown the effectiveness of the improved model.     

线接触作用下超薄膜-基底界面力学分析

高副接触的摩擦部件中广泛使用1 μm左右的超薄膜,其界面失效对部件的工作寿命具有重要影响。该文针对轴承钢基底表面1 μm厚度的硬质和软质超薄膜所构成的膜基系统,建立了线接触载荷作用下的界面力学分析模型。采用复变函数镜像法求解了单元点力的格林函数解,并积分获得了界面应力的分布状态。利用匀质模型完成了退化对比验证,以DLC和MoS₂两种硬软固体薄膜的线接触为例进行了计算和分析。该方法可用于机械部件表面沉积超薄膜的膜基界面分析与设计。     

Extracting thin film hardness of extremely compliant films on stiff substrates

A previously reported method for extracting the thin film hardness from nanoindentation into a film on an elastically mismatched substrate was applied to four different cases of extreme mismatch in elastic properties: Parmax, Ultem, Polysulfone and Perfluorocyclobutyl polymer thin films on Si substrates. All of these cases represent extremely compliant films on a stiff substrate, where the ratio of film shear modulus to substrate shear modulus ranged from 0.008 to 0.036. Analyzing the nanoindentation data into these film/substrate systems poses a significant limitation when using the Oliver and Pharr method as the hardness increases rapidly with indentation depth. Therefore, a method involving the measured contact stiffnesses to more accurately determine the correct contact areas was used to extract the true hardness of the polymer thin films. The results indicate that our method is able to remove the substrate effects as well as the complications arising from pile-up and surface roughness to yield a wide plateau in hardness despite the extreme elastic mismatch conditions.     

Influence of thickness on nanomechanical behavior of Black Diamond™ low dielectric thin films for interconnect and packaging applications

In the present study, we have investigated the thickness dependence of mechanical properties of the Black Diamond? (SiOC:H, BD, Low-k) films, which are of great interest in current Cu/low-k Back End of the Line (BEOL) interconnect/packaging technologies. For this investigation the BD thin films of six different thicknesses 100, 300, 500, 700, 1,000 and 1,200 nm were deposited on the 8″ Si wafer by using plasma enhanced chemical vapor deposition (PECVD) technique. Nanoindentation and nanoscratch tests of the BD films were performed by using the Nano Indenter^® XP (MTS Corp. USA). In nanoindentation testing of the BD films, significant differences in the elastic modulus of the BD films were observed. In nanoscratch testing, it is found that the critical load (Lc) and scratch width increases as the thickness of the film increases. Cross-sectional analysis of residual nanoindentation impressions was carried out using atomic force microscopy (AFM) to study the deformation behavior. The nanoindentation and nanoscratch responses of the BD thin films of six different thicknesses are different and they are expected mainly due to the molecular reorganization in thin/ultra thin films.     

Tip-radius effect in finite element modeling of sub-50 nm shallow nanoindentation

An accurate representation of the indenter geometry is essential for correct finite element simulations of shallow indentations of less than 50 nm using a 90° cube-corner indenter. A nonlinear regression method for estimating the tip radius of an indenter is presented, which takes into account that initially the contact is only with the spherical surface of the tip and subsequently also includes contact with the equivalent conical surface. The tip radius of a Berkovich indenter is estimated by a finite element modeling (FEM) best-fitting method. Using the estimates of tip radii, the yield strength of gold in the film of a gold/silicon system was estimated from the best fit between FEM simulations and nanoindentation experiments using the 90° cube corner indenter, which compared favorably with an FEM simulation and nanoindentation data using a Berkovich indenter.     

The analytical solution of the contact problem of spherical indenters on layered materials: application for the investigation of TiN films on silicon

Utilising an analytical solution of the contact problem of spherical indentation into layered materials we investigated TiN layers of various thicknesses on single-crystal silicon. We used Heaviside functions to describe the discontinuity of the mechanical parameters at the interface of the TiN-silicon compound. A spherical 5 μm diamond indenter was used to obtain the penetration depth-force curves. In spite of this small indenter we only obtained good utilizable results for film thicknesses of more than 0.8 μm. For thinner films the influence of failure effects such as roughness of the film, not an exactly known film thickness and substrate parameters, deviation of the shape of the diamond indenter from the ideal spherical one and the failure caused by the indentation apparatus themselves becomes too influential. Thus, here we show an investigation of a 1.4 and an 0.8 μm TiN layer on silicon including calculation of the films Young's modulus and the stresses, and for an 0.4 and an 0.2 μm TiN layer on silicon a stress calculation only using estimated Young's moduli from the thicker films. We paid special attention to the investigation of the non-elastic behaviour of the compounds utilising the von-Mises criterion.     

Characterisation of nanolayered aluminium/palladium thin films using nanoindentation

Structure, hardness, and elastic modulus of nanolayered aluminium/palladium thin films, with individual layer thickness varying from 1 nm to 40 nm, were investigated using transmission electron microscopy (TEM) and nanoindentation. TEM micrographs indicated a sharp but not flat Al-Pd interface. With just 6.5% (v/v) Pd a hardness enhancement of ~ 200% was observed for nanolayered Al/Pd compared to the hardness of pure Al film. A maximum hardness enhancement of up to 350% was observed for nanolayered Al/Pd samples compared to the hardness of pure Al film when bilayer thickness was 2 nm and Pd was 50% (v/v). Modulus enhancement was also observed for the nanolayered thin films.     

Microstructural characteristics and mechanical properties of magnetron sputtered nanocrystalline TiN films on glass substrate

Nanocrystalline TiN thin films were deposited on glass substrate by d.c. magnetron sputtering. The microstructural characteristics of the thin films were characterized by XRD, FE-SEM and AFM. XRD analysis of the thin films, with increasing thickness, showed the (200) preferred orientation up to 1·26 μm thickness and then it transformed into (220) and (200) peaks with further increase in thickness up to 2·83 μm. The variation in preferred orientation was due to the competition between surface energy and strain energy during film growth. The deposited films were found to be very dense nanocrystalline film with less porosity as evident from their FE-SEM and AFM images. The surface roughness of the TiN films has increased slightly with the film thickness as observed from its AFM images. The mechanical properties of TiN films such as hardness and modulus of elasticity (E) were investigated by nanoindentation technique. The hardness of TiN thin film was found to be thickness dependent. The highest hardness value (24 GPa) was observed for the TiN thin films with less positive micro strain.     

Tribological characteristics of thin films and applications of thin film technology for friction and wear reduction

Thin film deposition technologies based on chemical and physical vapor deposition are now well established to improve the wear resistance of cutting tools. Although severe tribological conditions occur here, coated tools perform exceptionally well. Despite this, the growth in use of thin film deposition technology is slow in the general mechanical industry. In part, this is due to lack of data on the tribological characteristics of bodies coated with thin films.In this paper, tribological test data gathered in a systematic program of sliding and rolling contact tests are presented to demonstrate the usefulness of thin film coatings for wear protection and friction reduction. Samples coated by reactive magnetron sputtering were tested in sliding and rolling contact tests at low and high speeds and at varying contact stresses. These results are presented and used to identify the primary consideration guiding the choice of coating materials, coating properties, film thickness needed etc. A newly developed film-to-substrate adhesion strength test is also briefly described and the results obtained are cited. It is shown that film material selection and coating process selection have a reasonable physical basis.     

多尺度模拟研究纳米凸体几何形貌对初始塑性的影响

研究金属表面微凸体的力学行为对深入理解摩擦、磨损和设计微纳米机电器件有很大帮助.采用准连续方法探索了纳米压痕作用在薄膜(001)表面的纳米微凸体几何形貌对铝和铜薄膜初始塑性的影响规律.结果显示,相较于平坦表面,微凸体的存在显著地降低了薄膜的屈服应力.矩形微凸体横纵比对屈服应力的影响不大.随着底角α的增大,梯形微凸体的屈服应力呈现降低的趋势,尤其是α＞54.7°.同时,在纳米尺度限制全位错形成的条件下,铝中可能容易形成挛晶结构.