New Insights into Nanoindentation-Based Adhesion Testing
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Nanoindentation, or instrumented indentation, is a versatile technique that is most often used to measure the elastic modulus and hardness of thin film systems. It can also be employed to measure thin film adhesion energies by producing well-defined areas of delamination. When combined with the proper mechanics-based model and characterization of the failing interfaces, nanoindentation-induced delamination is a powerful tool to quantify interfacial fracture. This article highlights new improvements to the technique that build off the work of Marshall and Evans in the 1980s. Indentation-induced delamination in systems with brittle films or substrates can be a balance between causing delamination and causing through-thickness or bulk fracture. Focused ion beam cross-sectioning and confocal laser scanning microscopy were used to characterize failing interfaces, additional fracture events were observed in the load–displacement curves, and the adhesion energy was determined using not only symmetric, ideally shaped buckles, but also irregular-shaped and half-delaminated buckles.
Thin film adhesion has been investigated since the early days of fabrication in the fields of microelectronics and protective coatings. Early pioneers such as Mittal, Weaver, and Chapman1, 2, 3 helped define the field of thin film adhesion and brought testing methods to the forefront during the late 1970s and 1980s. These early adhesion tests were mostly qualitative or semiquantitative measurements and included peel tests, tape tests, scratch and lap shear tests. In the late 1980s and 1990s, a new generation of materials scientists and mechanics researchers introduced indentation-based techniques, stressed overlayers, four point bending, and bulge testing with their appropriate models to quantify the adhesion energy of an interface.4, 5, 6, 7, 8, 9, 10, 11 These methods and combinations of methods have now become commonplace for those researchers working in the thin film adhesion area.12, 13, 14, 15, 16
Having well-adhering films for micro and nanoelectronics, hard coatings, and flexible electronics is still a challenge as well as a fruitful area of research and development. Films are becoming both thinner and thicker, more chemically complex, and substrates are becoming more diverse (metals, ceramics, polymers, etc.). There is continued growth in technique development to quantify interface adhesion and to tackle the new interfaces and film systems. Some use indentation,17 , 18 whereas others use complex micro-mechanical bending geometries and perform the experiments in situ with pico-indenters in the scanning electron microscope (SEM) or transmission electron microscope (TEM).19 , 20 In the flexible electronics area (films on compliant polymer substrates), tensile-induced delamination is prominent.21 , 22 Nanoindentation, focused ion beam (FIB) milling, and confocal laser scanning microscopy (CLSM) are bringing more insight to adhesion testing as well as increasing the imaging areas compared with other available methods (atomic force microscopy (AFM) or profilometry). Nanoindenters are now a basic measurement tool at most research institutes, and their practicality for these tests increases when scratching and imaging are used in conjunction with the more common indenting procedures. FIB allows for site-specific cross-sectioning at the microscale to quickly identify a failing interface and additional fracture events that may have occurred with an indent or a scratch. FIB is also useful to create transmission electron microscopy samples using the lift-out method for the examination of a film microstructure and interface structure. CLSM is a 3D surface imaging technique that is ten times faster than AFM and has a higher resolution compared with a profilometer. With this technique, larger delaminations, both in height and width, can be analyzed and used to measure adhesion.23
This study will demonstrate the use of nanoindentation-based techniques to measure the adhesion of barrier layers. Barrier layers provide chemical stability to conductive metallizations for microelectronic devices. A prime example is silicon nitride, Si3N4, which is used as an ion-barrier material, oxidation barrier, insulator, or etch mask. By using a Tungsten-Titanium (WTi) stressed overlayer combined with nanoindentation, well-defined areas of delamination can be produced. The produced delaminations were measured with AFM and CLSM, whereas FIB cross-sectioning was used to identify the failing interface and additional fracture and deformation events present in the load–displacement curves. The combination of the two characterization techniques will be shown to improve the understanding of the evolution of the buckles under indentation loading.
Materials and Experiment
The samples investigated consisted of silicon wafers (725 µm thick) with 800 nm of borophosphosilicate glass (BPSG) deposited using plasma-enhanced chemical vapor deposition (PECVD), followed by 400 nm of PECVD silicon nitride (Si3N4). To act as an adhesion and diffusion barrier layer, a 300 nm thick Tungsten-Titanium (WTi) film was sputter deposited on the Si3N4 where the tungsten film contained 20 at.% of Ti. The WTi film was deposited under conditions that induced a compressive residual stress of about 1.5 GPa (measured with x-ray diffraction).
Nanoindentation was conducted with a Keysight G200 nanoindenter. A 90° conical diamond tip with a 1-µm tip-diameter and a load range between 100 mN and 500 mN was used to generate indentation-induced delamination. Fifteen indents were made per maximum load in this range, which was increased in intervals of 50 mN. The indents were set in a grid being 250 µm apart from each other to avoid any interaction of the formed blisters, indent plastic zones, or fracture events. After indentation, all resulting delaminations were imaged with an AFM (Veeco Dimension DI3000) or CLSM (Olympus LEXT OLS 4100). The buckle measurements were made from the AFM and CLSM images using Gwyddion,25 and the model of Hutchinson and Suo10 was modified for a bi-layer film using the theory of Kriese et al.9 to calculate film stresses and adhesion energies. The elastic modulus of WTi was determined from nanoindentation experiments using the continuous stiffness method and a well-calibrated Berkovich tip at E = 171.8 GPa. The Poisson’s ratio of WTi was estimated using a simple rule of mixture with ν = 0.288. The modulus and Poisson’s ratio of Si3N4 were taken from Vlassak et al.26 where these properties E = 222 GPa and ν = 0.27 were measured by bulge testing.
Cross sections were made using a femtosecond laser and FIB. A femtosecond pulsed laser, which provides an ablation rate four to six orders of magnitude higher than a Ga+ ion beam,27 was used to reduce the time needed for the rough cut of the buckle cross section. The use of a femtosecond pulsed laser allows structuring of materials with ideally no heat affected zone as a result of the ultrashort pulse duration, but the shock wave of the ablation process can lead to the injection of dislocations. The amorphization of Si, or periodic surface structures in the range of a few hundred nanometers in depth, are generated when using a laser in the ultrashort pulse regime.28 For the investigation of the failing interfaces, these modifications needed to be removed, thus, requiring a polishing step with the FIB. In this study, a recently developed prototype, which combines the high material removal rate of a femtosecond pulsed laser with the high precision of a FIB, was used.29
The FIB cross sections revealed that during indenting, multiple forms of cracking occurred in the film stack. For the indents at the 250 mN interface, delamination was not produced and the cross section shows no observable interface fracture (Fig. 4a). Nevertheless, cracks in the Si3N4 film can be observed directly under the indent, which can be linked to the pop-in event in the load–displacement curve. Of interest is that the indent deforms the lower BPSG layer and the WTi and Si3N4 thicknesses remain constant under the indent. At 300 mN, multiple interfaces have separated as shown in Fig. 4b and c. Interface cracks develop between the Si3N4 and the BPSG as well as between the BPSG and the Si. The interface crack between BPSG and Si originates directly under the indenter tip, extends for a few micrometers, and eventually kinks up to the Si3N4 interface (Fig. 4b). This type of fracture under the indenter has been observed in other film systems.30 Once the interface crack has kinked, it propagates along the Si3N4-BPSG interface until it either extends for another 30 µm to become a small buckle (Fig. 4b) or it grows a further 70 µm into a large buckle (Fig. 4c). It can be seen from the cross section in Fig. 4b that the small buckle is a result of two interfaces separating (Si3N4-BPSG and BPSG-Si) and the kinking of the crack rather than a single interface separation. This tortuous crack path influences the calculation of the adhesion energy using the buckle in Fig. 4b.
Figure 4c shows that the interface crack between the Si3N4 and the BPSG extends much farther (70 µm in width) and that the buckle height is increased (2–3 µm); the interface crack then propagates until it kinks through the Si3N4 film as shown in the inset of Fig. 4c. Hence, the whole buckle is the result of this interface separation and is indicated in the load–displacement curve in Fig. 4c, where a large pop-in occurred during the hold time at a load of 300 mN. The cross section also shows the fracture under the indenter tip extended through the BPSG and into the Si substrate as a single vertical crack. When the load was increased to 350 mN film, failure occurred in two ways. The interface crack propagates similar to the case of a 300 mN load, forming a large buckle with a diameter of about 70 µm, or the interface crack kinks toward the surface during interface crack propagation and spallation occurs, as shown in Fig. 4d. Both of these events are indicated in the load–displacement curve by a large pop-in event between 300 mN and 350 mN, as shown in Fig. 4d for a spalled buckle. From the pop-in events alone, it is not possible to differ between buckle formation and spallation (compare load–displacement curves of Fig. 4c and d). Some buckles produced were irregularly shaped and with localized spallation and chipping which created half-buckles (box in Fig. 3). Additionally, although the small buckles show no sign of radial cracking around the indent, the large buckles do and the extent increases with increasing load (Fig. 4c), which aids the spallation of the buckle (Fig. 4d). In this particular system, the WTi film acts as a stressed overlayer with its large residual compressive stress, helping to control the delamination of the Si3N4 barrier layer as well as supporting this brittle film to prevent it from spalling from the BPSG. As seen in Fig. 4d, the Si3N4 film cracks at the base of the buckles causing spallation before adhesion could be measured. The FIB cross sections also confirm that the indentation buckles have an unpinned geometry because the indents were not connected to the substrate at the center of the indent.
A range of loads and number of indents is necessary to understand how delamination and fracture events transpire. As demonstrated in Fig. 4, pop-ins do not always indicate interface fracture and can relate to fracture events of the underlying films or substrate. The FIB cross sections also identified that the WTi, Si3N4, and BPSG films co-deformed, most likely as a result of the low modulus of the BPSG (E = 70 GPa). With this hard-on-soft system, Marshall and Evans should not be applied as the volume of the indent would be challenging to determine. Large pop-ins correlated to the interface fracture and occurred at approximately the same load in this system (around 300 mN). Yet not all film-substrate systems will behave in such a repeatable manner and should be carefully characterized.
Quantifying the Adhesion Energy
Summary of Average Half-Buckle Widths (b), Buckling Stresses (σ b), and Mixed-mode Adhesion Energies (Γ(Ψ)) from the Buckles Produced by Nanoindentation Shown in Figs. 3 and 4. Standard Deviations are Based on the Number of Measurements of Each Buckle Type (about 15–20 Measurements on 4–10 Buckles)
It should be noted that nanoindentation-induced delamination has a few limitations. For example, the technique should only be applied to film systems on rigid substrates. Nanoindentation-induced delamination will not work for films on compliant/polymer substrates or with compliant layers. Some plastic deformation is necessary to induce the delamination and compliant substrates, at the loads typically used, only elastically deform. Also, typically large indentation loads are needed; therefore, one would need a nanoindenter capable of at least 100 mN and higher loads. Post-analysis of the delaminating interface is necessary, but FIB does not have to be used. A simple peel test or careful removal of a buckle can be used to determine the failing interface.33 Nevertheless, this does not help for the separation of fracture events from the interface fracture when multiple layers or pop-ins are present in the load–displacement curves. Finally, it is best to make several indents at various loads to understand fully the film deformation, film fracture, and interface fracture processes.
The adhesion energy of a Si3N4-BPSG interface has been determined through the use of nanoindentation-induced delaminations and a stressed overlayer. As a result of the support of the compressive WTi stressed overlayer, nanoindentation can be employed to delaminate and subsequently buckle this interface. The elastic deformation of the metal film induces the necessary stress into the system to cause interface separation and forces the rigid Si3N4 film to buckle as a circular buckle. The development of interface separation and buckling falls into a narrow load range between 300 mN and 350 mN for this tip geometry. Loads of 300 mN and greater led to the development of Si3N4-BPSG interface separation, indicated by a large pop-in. The adhesion energy of the interface calculated from the large and half indentation buckles are in good agreement and show that the adhesive strength of this interface is low and comparable with other ceramic–ceramic interfaces. A brittle film can buckle under the right experimental conditions when supported by an elastic compressively stressed overlayer, making this mechanical test method viable for adhesion testing for film systems in microelectronics. Because every multilayered thin film system may behave differently, complete characterization of the failing interface and load–displacement curves should be carried out to calculate the adhesion energies properly. Nevertheless, nanoindentation-induced delamination is a versatile technique that can be easily implemented for a variety of interfaces, especially when augmented by FIB, CLSM, and AFM characterization techniques.
This work was jointly funded by the Austrian Research Promotion Agency (FFG: Project No. 846579) and the Carinthian Economic Promotion Fund (KWF, Contract KWF-1521/26876/38867). Additional financial support from the Austrian Federal Government (837900) within the framework of the COMET Funding Program is appreciated.
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