Fracture Of Thin Films Research Articles

Tensile Testing of Polycrystalline Silicon Thin Films Using Electrostatic Force Grip.

In this paper, a new tensile tester for thin films is presented. This tensile tester has a grip that fixes a thin film specimen using electrostatic force. The tester was constructed in a scanning electron microscope (SEM) chamber. Using this tester, the tensile strengths of polycrystalline silicon (poly-Si) thin films have been measured. The tested part of the specimen is 30-300μm long, 5μm wide and 2μm thick. The fracture of the poly-Si thin film was brittle. The mean tensile strength was 2.0-2.6GPa, depending on the length of the tested part. The size of the critical flaw that initiates fracture of the poly-Si thin film is 28-47nm, rather small than the grain size of the poly-Si thin film.

Residual Stress Effects on the Fracture of Tantalum Nitride Films

ABSTRACTIn this study, continuous microscratch testing was used to determine the effects of compressive residual stresses on the fracture of thin tantalum nitride films. The films were sputter deposited at room temperature onto single crystal sapphire substrates to a thickness of 600 nm. Some films were left in the as-deposited condition while the remaining films were vacuum annealed at 300°C. The only discernible change in structure was a surface rearrangement of atoms into parallel arrays of striations on the vacuum annealed samples revealing a high compressive residual stress in the films. These stresses had a strong effect on fracture as shown by the as-deposited films which buckled readily during scratch tests forming uniform-width and circular blisters while the vacuum annealed films exhibited a much lower susceptibility to fracture. Application of mechanics-based models for film buckling defined the levels of compressive residual stress and their effect on interfacial fracture.

Properties and Fracture Resistance of Thin Polycrystalline Aluminum Films on Sapphire Substrates

AbstractWe employed nanoindentation, continuous microscratch testing, and high resolution TEM to determine the effect of structure on the properties and resistance to fracture of thin polycrystalline aluminum films deposited onto single crystal sapphire substrates at 25°C and 250°C. These films had a nominal thickness of 90 nm and a grain size of 160 nm. The elastic and plastic properties were similar for both films. The elastic moduli superimposed, increasing from bulk aluminum values at the surface to sapphire values at the interface. Hardness values also superimposed, but were constant through the film thickness at a value between aluminum and sapphire. In contrast, susceptibility to fracture varied markedly between the films with the 25°C film exhibiting abrupt failure along the film-substrate interface while the 250'C film gave no indication of fracture in the film, along the interface, or in the substrate under the conditions tested.

The Effects of High Temperature Exposure on the Fracture of Thin Tantalum Nitride Films

ABSTRACTContinuous microscratch testing was used in this study to determine the effects of elevated temperature exposure on the adhesion and toughness of thin tantalum nitride films. These films were sputter-deposited at room temperature on sapphire substrates to a nominal thickness of 600 nm with some films heated to 600°C in vacuum while others were heated to 600°C in air. The films heated in vacuum exhibited no changes in composition or structure while the films heated in air completely transformed to tantalum pentoxide. Comparison of the results shows that the interfacial fracture toughness increases from 0.5 MPa-m1/2 for as-sputtered films to 0.8 MPa-m1/2 for films heated in air. However, the toughness increases to more than 3.0 MPa-m1/2 when the films are heated in vacuum. The increase in toughness values follows the reduction in deposition defect content where formation of an oxygen deficient tantalum oxide layer in air from the as-sputtered film increases interfacial toughness slightly while full densification of the tantalum nitride films in vacuum increases toughness to very high levels.

Cracking Mechanisms of Fine Lines by Microwedge Scratch Testing

ABSTRACTThree types of cracks associated with interfacial failure of a tungsten fine line on silicon oxide (SiO2) have been observed in microwedge scratch testing (MWST) of fine lines. They were identified by carefully controlling scratch conditions in a continuous microindenter system1, in conjunction with SEM micrographs. The first type is a hinge crack, which is responsible for nucleating an interfacial crack. The driving force for the hinge crack is the difference in stress conditions in the middle on theline (plane strain) and the outside of the line (plane stress). The second type is the interfacial crack, which is driven by the movement of the microwedge. This crack is modelled by the finite element method (FEM) to obtain an interfacial work of adhesion value. The third type of crack is a spallation crack, and is driven by tensile strains associated with buckling of the thin film once the interfacial crack becomes sufficiently long. Tensile strains in the film at the maximum crack length are calculated by FEM in order to arrive at a value of thin film fracture strength. A brittle failure mechanism has beenconfirmed by a micrograph of a spalled piece exhibiting no perceptible plastic deformation.

Hrem Study of Fracture and Deformation Behavior of Nanostructured Thin Films

ABSTRACTNanocrystalline gold and nickel thin films were prepared by both ion beam sputter deposition and electron beam evaporation techniques. The grain sizes were between 8–60 nm, depending on the processing. The deformation and fracture behavior of these nanostructural thin films were then investigated in a high resolution electron microscope (HREM) with atomic resolution. The behavior was a strong function of grain size. During slow strain rate deformation of small grain size materials (< 25 nm), nanopores formed and grew at grain boundary triple junctions in the front of crack and eventually linked with the main crack. The coalescence of the main crack with the growing nanopore, along with the elimination of the ligament between the two by a diffusive process, both indicated that diffusion played a role in deformation and fracture of these nanocrystalline thin films. In nickel films with larger grain sizes (> 25 nm), the cracks propagated in a mixed mode which was partially intergranular and partially transgranular. The transgranular propagation was crystallographic in nature, showing very fine, distinct crystallographic facets on the crack faces. It is suggested that a transition from intergranular propagation to a “cleavage-like” mechanism of propagation occurs with the increase of grain size.

Three-dimensional effects in thin film fracture mechanics

This paper presents the results of a three-dimensional finite element analysis of the mechanics of crack growth and decohesion in a highly compliant thin film bonded to a rigid substrate. Essential features of the model are a surface-breaking initial crack which has penetrated to the film-substrate interface, and a remote biaxial residual stress field in the film. Computational analyses show that, in the absence of decohesion, the stress intensity factor along the leading edge of the crack reaches a steady state value when the crack is about twice the film thickness. This steady state condition gives rise to a nearly parabolic crack front along the leading edge. The computed results also confirm the existence of high interface stresses that are the driving force for film decohesion. When decohesion is permitted, the stress intensity factor is observed to increase with debond opening angle. The equilibrium state between crack propagation and decohesion may be determined from the respective stress intensity factors and fracture toughness of the film and the interface.

A new in situ technique for studying deformation and fracture in thin film ductile/brittle laminates

A new technique for studying deformation and fracture of thin film ductile/brittle laminates is described. The laminates are prepared by sputtering a brittle coating on top of an electropolished TEM thin foil. The composites are then strained in situ in the TEM. In this preliminary investigation, the composites consisted of a ductile aluminum substrate and a brittle silicon coating. Cracks in the brittle film grew discontinuously in bursts several micrometers in length. The crack opening displacement initiated plastic deformation in the ductile film, thus dissipating energy and allowing crack arrest. The interface was well bonded, and delamination was not observed. Due to the good interfacial bond and the crack opening behind the crack tip, it was possible to study very large plastic deformations and ductile fracture in the aluminum in situ, without buckling of the foil. The possibility of micromechanical modeling of the fracture behavior is briefly discussed.

Characterisation of Pointed Indentation Fracture of Thin Silica Films on a Ductile Substrate.

ABSTRACTObservations are presented of the indentation of amorphous silica films of different thickness on mild steel substrates. The films were prepared by sol—gel dip coating and sintering at 500° C. Indentation of the films with a Berkovich indenter using an ultra micro—indentation hardness system (UMIS) revealed a critical load for the fracture of the thin films. The variation of the fracture load was characterised by a Weibull analysis that gave a mean fracture load and a Weibull modulus. The latter was found to be nearly independent of film thickness whereas the former varied with the square of the thickness.

Scanning tunneling microscopy of the topology of vapour-deposited and electro-deposited metal films

The fracture and subsequent spontaneous delamination of thin metal films under the influence of excessive internal stresses, resulting from low temperature deposition and processing, can be extremely deleterious to the long term serviceability of IC devices. The conditions for fracture are determined by the magnitude of the residual stress, which can be measured by x-ray diffraction techniques, the fracture toughness of the thin film material and the size of any internal or sinface flaws that may be present.In order to establish the size and depth of microcracks at the surface, high resolution scanning (SEM) and transmission electron microscopy (TEM) and scanning tunneling microscopy (STM) are being used to define the topology of the surfaces of vapour-deposited films for comparison with electroplated films of the same metal. Direct knowledge of the surface relief is required to predict the onset of through-thickness cracking or to derive the fracture energy of the film material by detecting the critical conditions for cracking. Detailed knowledge of surface structures also is useful for understanding the mechanisms of film growth, including information on the crystallography of surface facets and growth steps.

On the multiple fracture of low-elongation thin films deposited on high-elongation substrates

The multiple fracture of brittle, low-elongation thin films adhering to high-elongation plastic substrates subject to uniaxial strain is addressed. Results are presented for Permalloy on Kapton. It is observed that crack density in the film increases dramatically immediately after crack onset and subsequently approaches an asymptotic value with substrate strain. Thicker films experience crack onset earlier, and their asymptotic crack densities are lower than with thinner films. To provide some theoretical basis for the observed phenomena, the relevant literature is summarized. An empirical model is developed that fits the data obtained in this study and agrees with the results obtained by others as well. An attempt is made to mechanistically model the problem using a modified shear-lag theory developed initially for bulk composite materials. It is shown that the shear-lag model has potential application to this problem and needs further investigation.

Delamination and fracture of thin films : Erik Klokholm. IBM J. Res. Dev.31, 585 (1987)

Delamination and fracture of thin films : Erik Klokholm. IBM J. Res. Dev.31, 585 (1987)

Fracture Strength of Aluminum Oxide Ceramic Thin Coating Films

Static and cyclic fracture tests were performed on amorphous aluminum oxide thin films coated on Fe-42% Ni alloy base. The thin films were subjected to the static and cyclic tensile strains using a four-point bending test apparatus, and the initiation and propagation of cracks in the thin films were observed by an optical microscope. The effects of coating thickness and surface finishing of the metal, such as mirror polishing or machining, on the static and cyclic fracture strengths of Al2O3 thin coating films were investigated. Under cyclic loadings, the fracture of Al2O3 thin film was observed at a strain below the static fracture strain. The both static and cyclic fracture strengths were found to decrease with increasing coating thickness. It was also found that the surface finishing of base metal affected the fracture strength in such a way that the mirror polishing increased it.

Delamination and fracture of thin films

The fracture and delamination of thin films is a relatively common occurrence, and prevention of these mechanical failures is essential for the successful manufacture of thin-film devices. Internal elastic stresses are an inherent part of the thin-film deposition process, and are largely responsible for the mechanical failures of thin films. However, it is not the magnitude of the film stress S which governs film fracture or delamination, but the elastic energy U stored in the film. It is the intent of this presentation to show that the mechanical stability of the film and the substrate requires that U be less than a critical value U <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</inf> and that U <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</inf> is dependent upon the surface energy γ.

A simple model for elastic fracture in thin films

A simple model for the cracking of thin films has been developed and investigated. The film is represented by a network of bonds and nodes which initially form a triangular lattice in which each node is at the junction of six bonds. The nodes are also attached to a rigid substrate by bonds which are not broken but have a relatively small force constant. Cracking is simulated by a sequence of thermally activated bond breaking events followed by mechanical relaxation. If the stretching force constant associated with the bonds in the surface layer is large, linear cracks propagate rapidly and, at a later stage, are connected by secondary cracks which become less and less linear as the strain in the surface layer is relaxed by the cracking process. Many of our simulations exhibit a distint “initiation” period in which the bond breaking rate is low, followed by a period of rapid crack propagation in which a large fraction of the surface strain energy is released. This period is then followed by a second period of relatively slow bond breaking. During the first period isolated defects are formed, followed by linear cracks in the second period. During the third period non-linear cracks connect the linear cracks, and structures which resemble shear bands are formed. Results for the effects of finite relaxation rates for the elastic network (visco-elastic effects) are also presented.

Delamination Fracture Toughness of Graphite and Aramid Epoxy Composites

The delamination fracture toughness of graphite and aramid-epoxy composite laminates was determined as a function of loading rate for unidirectional and woven reinforcements. In addition, the in-situ fracture toughness of the epoxy matrix was obtained by determining the crack energy release rate during the delamination fracture of thin epoxy films. The fracture surfaces were investigated using scanning electron microscopy. The results show that increasing the loading rate and the use of woven reinforcements increase the fracture toughness. A model was used to estimate the relative contributions from the fiber-matrix interface and the matrix to the overall delamination crack energy release rate.