Berkovich Nanoindentation on AlN Thin Films
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Berkovich nanoindentation-induced mechanical deformation mechanisms of AlN thin films have been investigated by using atomic force microscopy (AFM) and cross-sectional transmission electron microscopy (XTEM) techniques. AlN thin films are deposited on the metal-organic chemical-vapor deposition (MOCVD) derived Si-doped (2 × 1017 cm−3) GaN template by using the helicon sputtering system. The XTEM samples were prepared by means of focused ion beam (FIB) milling to accurately position the cross-section of the nanoindented area. The hardness and Young’s modulus of AlN thin films were measured by a Berkovich nanoindenter operated with the continuous contact stiffness measurements (CSM) option. The obtained values of the hardness and Young’s modulus are 22 and 332 GPa, respectively. The XTEM images taken in the vicinity regions just underneath the indenter tip revealed that the multiple “pop-ins” observed in the load–displacement curve during loading are due primarily to the activities of dislocation nucleation and propagation. The absence of discontinuities in the unloading segments of load–displacement curve suggests that no pressure-induced phase transition was involved. Results obtained in this study may also have technological implications for estimating possible mechanical damages induced by the fabrication processes of making the AlN-based devices.
KeywordsAlN Nanoindentation Focused ion beam Transmission electron microscopy
The advent and development of microsystems and nanotechnology has its roots in surface and materials science, and in particular, thin films. In the past, many such materials have only been characterized in terms of their electronic, magnetic, and optical properties. Nevertheless, their mechanical and structural characteristics are now just as important. This is because traditional methods such as bulge and tensile testing are impractical and/or unsuitable, since they do not scale well into the micro- and nanoscale. Recently, it has become clear that, in order to fully harvest the unprecedented potential of the emerging nanotechnologies in general, the processes-induced structural and mechanical modifications on the materials might be equally important. For instance, during the past decade, AlN has emerged as an active photonic material for applications in deep ultraviolet emitters and detectors due to its large direct bandgap of 6.2 eV [1, 2]. It has strong chemical bonds, making it highly stable and resistant to degradation as operating under harsh environments/conditions. Because of its high thermal conductivity, large piezoelectronic field, and low electron affinity, AlN also has applications in stable X-ray production, surface acoustic wave devices, and electron emission devices [3–5]. However, the successful fabrication of devices based on the epitaxial AlN thin films requires better understanding of the mechanical characteristics in addition to its electrical performances, since the contact loading during processing or packaging can significantly degrade the performance of these devices. Consequently, there is a growing demand of investigating the mechanical characteristics of materials, in particular in the nanoscale regime, for device applications.
This study addresses the nanoindentation analysis of the mechanical properties of AlN thin films. The nanoindentation technique is especially well suited for the characteristics of small structures [6–8] or thin films and coatings [9–12]. Analysis of the load–displacement curve obtained by nanoindentation permits mainly hardness and Young’s modulus to be obtained without visualizing the indentation. The most usual method of analysis is the one proposed by Oliver and Pharr . However, in the case of thin films, the response after some penetration depth is given not only by the films but also by the substrate and, the mechanical properties obtained are a combination of the true values of the films and the structure. Indentation with contact depths of less than 10% of films thickness is needed to obtain intrinsic film properties and avoid the influence of the substrate . Besides, it is very difficult to obtain meaningful analytical results for indentation depths less than 10 nm because of the equipment limitations. Bearing in mind the above, it is obvious that it is not possible to obtain substrate-independent results for film less than 100 nm thick. Therefore, in order to analyze films less than 100 nm thick, it is essential to monitor the mechanical properties as a function of depth, in order to get an insight on the influence of the substrate. In this study, we use a dynamic approach, termed continuous stiffness measurement (CSM) mode , to continuously monitor the hardness and Young’s modulus values as a function of the indentation depths.
While diamond anvil cell experiments are capable of investigating the mechanical and phase transformation in bulk materials under hydrostatic pressure , the materials behavior under nanoindentation is of more relevance to realistic contact loading conditions. However, the nanoindentation technique itself does not provide the information of subsurface indentation-induced deformation mechanisms and dislocation propagation. In this respect, the focused ion beam (FIB) miller is now widely used for preparing the cross-sections of the locally deformed areas to direct reflect the detailed nanoindentation-induced mechanical responses for a range of materials . In our case here, the cross-sectional observations can provide important information about the in-depth phase distribution and the embedded defect features introduced by contact loading that were impossible to be observed with the plain-view samples.
Herein, in this study, the deformation behaviors of helicon sputtering method derived AlN thin films under contact loading have been investigated using Berkovich nanoindentation, followed by analysis using atomic force microscopy (AFM), FIB, and transmission electron microscopy (TEM) techniques, in order to understand the final structures of the indentation-induced transformation zones observed in experiments.
Experimentally, AlN thin films used in this study were deposited on Si-doped (2 × 1017 cm−3) GaN template, prepared by metal-organic chemical-vapor deposition (MOCVD) , using the helicon sputtering system which with an average thickness of about 400 nm. The detailed growth procedures of AlN thin films can be found elsewhere .
The mechanical properties of AlN thin films were characterized by means of an MTS NanoXP° (MTS Cooperation, Nano Instruments Innovation Center, TN, USA). The nanoindentation measurements, using a three-side pyramidal Berkovich diamond indenter of 40 nm radius (faces 65.3° from vertical axis), were conducted under the continuous stiffness measurement (CSM) procedures , which was accomplished by superimposing a small oscillation on the primary loading signal and analyzing the resulting response of the system by using a lock-in amplifier. Prior to real measurement, the indenter was loaded and unloaded three times to ensure that the tip was properly in contact with the surface of AlN thin films and that any parasitical phenomenon is released from the measurement. At the fourth time, the indenter was loaded at a strain rate of 0.05 s−1 until reaching an indent depth of 50 nm and was held for 10 s. Then, it was withdrawn with the same strain rate until 10% of the peak load was reached. At least 10 indents were performed on each sample. Each indentation was separated by 50 μm to avoid possible interferences between neighboring indents. We also followed the analytic method proposed by Oliver and Pharr  to determine the hardness and Young’s modulus of AlN thin films from the load–displacement results. In this way, hardness and Young’s modulus are obtained as a continuous function of penetration depth.
In addition, in order to reveal the role played by the nucleation and propagation of dislocations in indentation-induced deformation, cyclic nanoindentation tests were also performed in this study. These tests were carried out by the following sequences. First, the indenter was loaded to some chosen load and then unloaded by 90% of the previous load, which completed the first cycle. It then was reloaded to a larger chosen load and unloaded by 90% for the second cycle. Figure 2 illustrated a typical cyclic indentation test repeated for 2 cycles. It is noted that in each cycle, the indenter was hold for 30 s at 10% of its previous maximum load for thermal drift correction and for assuring that complete unloading was achieved. The thermal drift was kept below ± 0.05 nm/s for all indentations considered in this study. The same loading/unloading rate of 10 mN/s was used. At least 20 indents were performed on AlN thin films. The nanoindentations were sufficiently spaced to prevent from the mutual interactions.
The cross-sectional transmission electron microscopy (XTEM) samples were prepared from the indents by means of a dual-beam focused ion beam (FIB, Nova 220) station with Ga ions at 30 keV. Prior to milling, the FIB was used to deposit an ~1 μm thick layer of Pt to protect the AlN thin films surface. The details of FIB produces in preparing XTEM sample can be found elsewhere . The XTEM lamella was examined in an FEI TECNAI G2 TEM operating at 200 kV.
Results and Discussion
The inset of Fig. 3 shows the typical AFM image for an indented surface obtained with an indentation load of 50 mN. There is no evidence of either dislocation activity or crack formation in the area of the indented surface. Therefore, if the dislocation nucleation and subsequent propagation are indeed the primary mechanism for the observed multiple pop-ins, it should prevail underneath the indented surface. It is also interesting to check if there is any pressure-induced phase transformation involved.
Furthermore, to further confirm whether phase transformation occurred beneath the indent, SAD analysis of the deformed area (region I) was shown. We do not observe any halo rings in SAD result; therefore, we can say that AlN thin film did not undergo the amorphization in Berkovich nanoindentation. It can also be expected that the distortion of diffraction spots is more significantly than the pristine one (not shown here), demonstrating a nanoindentation load effect on the plastic deformation intensity. According to the above-mentioned results, we can conclude that although significant plastic deformation occurs in AlN thin film beneath the Berkovich indenter, no phase transformation is induced.
To summarize, a combination of nanoindentation, FIB, and TEM techniques was used to investigate the contact-induced structural deformation behaviors in the AlN thin films.
The load–displacement curves show the multiple “pop-ins” during nanoindentation loading. No evidence of either nanoindentation-induced phase transformation or formation of micro-cracking is observed in AlN thin film by AFM and XTEM. Also, XTEM observation revealed that the primary deformation mechanism in GaN thin film is via propagation of dislocations on both basal and pyramidal planes. Finally, as displayed in SAD result, the distortion of diffraction spots, however, does indicate severe deformation of indented AlN thin film resulting from the nanoindentation load. The CSM technique was used to determine the hardness and Young’s modulus of AlN thin films. Furthermore, analysis of the load–displacement data reveals that the values of hardness and Young’s modulus of AlN thin films are 22 and 332 GPa, respectively.
This study was partially supported by the National Science Council of Taiwan, under Grant No.: NSC97-2112-M-214-002-MY2. Author likes to thank Dr. Y.-S. Lai and Dr. P.-F. Yang for their technical support (Central Product Solutions, Advanced Semiconductor Engineering, Taiwan), and Dr. M.-R. Chen and Prof. H.-L. Kao (Department of Electronic Engineering, Chung Yuan Christian University, Taiwan) for their support in AlN thin films.
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