Wolfram Kaiser, Stefan Krankenhagen and Kerstin Poehls, Exhibiting Europe in Museums. Transnational Networks, Collections, Narratives and Representations

Wolfram Kaiser, Stefan Krankenhagen and Kerstin Poehls, Exhibiting Europe inMuseums. Transnational Networks, Collections, Narratives and Representations, NewYork/Oxford: Berghahn Books, 2016 [hardback 2014], paperback £24.00, pp. viii+238

Investigating adhesion of polyimide in semiconductor devices with cross-sectional nanoindentation

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Strain rate influence on the thermo-mechanical deformation behavior of Aluminum thin films

Thin metal films used as top metallization in power semiconductor applications may repetitively undergo rapid temperature changes with heating rates reaching 106 K/s. It is well known that the mismatch of the coefficients of thermal expansion between metal and substrate causes stresses in the films, and this effect may lead to their thermo-mechanical fatigue. The stress vs. temperature behavior of such film-on-substrate combinations is mostly analyzed using X-ray diffraction or wafer-curvature-based methods. Both classes of methods can generally only be applied for analyzing materials undergoing slow temperature changes, either due to experimental constraints, e.g. measurement times in the XRD, or due to problems with stress calculation, e.g. Stoney formula being only valid for homogeneously heated specimens. It is questionable if the material response at low and high heating rates is comparable; hence, the development of methods to monitor the material behavior in a situation close to the application conditions is needed. A novel setup which allows measuring wafer curvature during rapid temperature changes has recently been developed 1, allowing the rapid heating of the tested metallizations using Joule heating and the simultaneous measurement of specimen curvature using either a high speed camera or laser scanning Doppler vibrometer. Using this setup, heating rates between 102 and 105 K/s can be utilized, to study the effect of cyclic heating with various temperature amplitudes and repetition rates on the metallization behavior. The stress-temperature behavior measured in such films is compared to the results obtained by standard wafer curvature experiments conducted at heating rates of less than 1 K/s. When comparing films cycled at 10² K/s and 10-1 K/s, the measurement results show that below 85 °C the coatings deform elastically and an identical deformation behavior is observed. The good comparability of the material behavior in the elastic regime proves that the novel setup is able to correctly measure curvature at high heating rates. At temperatures above 85 °C, where plastic deformation sets in, significant differences are seen in the specimens, which are caused by the influence of the different time-dependent relaxation mechanisms active at such temperatures. Microstructural changes in the films undergoing cycling at various heating rates are monitored using scanning electron microscopy and confocal laser scanning microscopy. Finally, the advantages and disadvantages of the application of fast temperature cycling to measure the stress-temperature behavior are discussed. 1 T. Islam, J. Zechner, M. Bernardoni, M. Nelhiebel, and R. Pippan, Rev. Sci. Instrum. 88, 24709 (2017)

Deformation and fatigue behavior measurement of thin films undergoing thermomechanical loading at high strain rates – A novel test setup

The deformation and fatigue behavior of thin films on substrates undergoing either single or repetitive thermo-mechanical loading has been investigated extensively in the last years due to the high relevance for industrial applications, ranging from cutting tools to microelectronic devices. A popular method to evaluate the stresses occurring in the thin film during temperature cycling is to measure the change in the curvature of the thin film on the substrate during heating and cooling. From the change in curvature, knowing the elastic constants of the substrate and the film and substrate thickness, the stresses in the film can be calculated using Stoney’s formula. Such wafer-curvature measurements are generally conducted at slow heating rates lower than several 10 K/s. This is mainly due to experimental constraints, but also originates in the fact that Stoney’s formula is only valid if substrate and thin film have a homogeneous temperature, which will not be the case for high heating rates. In power semiconductors, short high power pulses cause material heating with rates in the range of 10e5 - 10e6 K/s. It is questionable if the material response at low and high heating rates is comparable, which necessitates the development of methods to monitor the material behavior at heating rates comparable to the ones occurring during usage. Therefore, a new wafer curvature measurement setup has been developed, where the curvature is measured from the reflection of incident parallel laser beams using a high speed camera, allowing much faster data acquisition rates than with conventional cameras. Thin metallization films deposited onto polysilicon and single crystalline Si are heated by Joule heating using a pulse generator allowing to vary pulse shape, length, repetition rate and power. This can be used to vary the heating rate between 10e2 and 10e5 K/s and can be utilized to study the effect of cyclic heating with various temperature amplitudes and frequencies on the metallization behavior. Besides the description of the test setup, an overview on the stress evaluation procedure, necessitating the use of finite element modeling is presented. Changes in material response are deduced from changes in the stress-temperature behavior of the thin films after either one temperature cycle, or after thermo-mechanical fatiguing. Microstructural and morphological changes in the coatings are investigated using SEM, FIB cross-sections and surface roughness measurements

The influence of surface roughness on elastic nanoindentation measurements

The characterization of mechanical properties of layered thin-film structures is an important issue with respect to the better understanding and for improving the design of microelectronic devices. Due to the small investigated volume and the easy implementation of the measurement procedure, nanoindentation is an appropriate method for determination of the mechanical properties of thin films systems. Chudoba and Schwarzer et al. [1] developed an analytical approach that allows to derive values of Young´s modulus from load-displacement curves measured within the elastic range of interaction. This analytical approach together with nanoindentation using spherical indenter geometries is employed in this study. Preliminary investigations have been conducted on Fused Silica (FS) standard samples with known values of surface roughness and Young’s modulus (E=72 GPa). Different surface roughness values were adjusted by different times of etching the samples with hydrofluoric acid (HF). It could be shown that the roughness has a strong influence on the statistics of the measured load-displacement curves as well as on the derived Young’s modulus values (see Figure 1). Therefore, in the current study the influence of surface roughness shall be investigated in a more detailed way. This is done by applying a model that was developed for contact stiffness measurements using AFM-based methods. The model takes into account the contact stiffness of the indenter tip and the investigated sample as well as the contact stiffness of the multiasperity contact, arising from the roughness of the sample and the indenter. The aim of the study is to combine the analysis approach used for AFM data with the nanoindentation measurements and thereby proof the AFM model on a bigger length scale. Please click Additional Files below to see the full abstract

High-temperature small-scale fracture mechanics and plasticity of a hardcoating system

Forging and cutting tools for high-temperature applications are often protected using hard nanostructured ceramic coatings. While a moderate amount of knowledge exists for material properties at room temperatures, significantly less is known about the system constituents at the elevated temperatures generated during service. For rational engineering design of such systems, it is therefore important to have methodologies for testing these materials to understand their properties under such conditions. Additionally, small-scale mechanical testing is of inherent importance for thin-films systems and materials subject to surface modification or treatment as for plasma nitrided steels. In this work, we present results on both the hard ceramic coating and the nitrided steel substrate using in situ micro-mechanical measurements at temperatures to 500 °C. The fracture and plastic yield behavior of FIB milled micro-pillars of plasma-nitrided tool steel was first investigated using in situ compression experiments. It was found that the yield strength of nitrided steel is particularly sensitive to temperatures within the service range. Elevated temperature led to significant softening of the nitrided steel and transition from slip-based to more ductile plastic flow. A 70% reduction in yield stress was observed when transitioning from room-temperature to 500 °C, which was then recovered upon cooling back to RT indicating a mechanistic activation at high temperature. The fracture toughness behavior of a hard CrN coating was then investigated using various micro-geometries and notching parameters. Toughness measurements at high temperatures highlighted the profound effect of the notching ion during small-scale fracture measurements. It was found that gallium ion implantation led to significant toughening of CrN, based on gallium dosage experiments and alternative notching using both xenon and helium sources. The effect of different notching ions was additionally understood through Monte Carlo simulations of energetic ion interactions in a dense ceramic matrix

In-situ deformation monitoring of thin electrochemically deposited copper lines during thermo-mechanical pulsing

In semiconductor industry, the development of the last years led to smaller and smaller devices in order to maximize efficiency and minimize costs. As a result, a miniaturization of the test structures is required as well as a proper method to monitor gradual deformation processes during repetitive thermal cycling. Thin metal films, e.g., Cu are commonly used in power semiconductor devices. Rapid temperature changes combined with a mismatch in thermal expansion coefficients of the different materials in the layer stack lead to thermo-mechanical stresses and as a result to deformation of the metallization. In order to realize high heating rates (up to 106 K/s) and to be able to observe deformation on the metallization surface, polyheater structures are used. There, a polysilicon layer works as a heating plate (Joule heating) for the Cu layer above, allowing repetitive heating and cooling on short timescales. The temperature of the system is measured using an integrated sensor. Since the deformation features, e.g. slip bands and extrusions, are on the sub-micron length scale, a scanning electron microscope (SEM) is necessary for in-situ deformation monitoring. This novel approach provides the possibility to observe the gradual deformation of metallizations under variable test parameters at high magnification and in vacuum. As test structures, 20x20x300 µm³ Cu lines with different types of copper on top of the polysilicon were chosen to be able to observe the surface as well as the side walls of a metallization structure. It is revealed, that different Cu grain microstructures lead to differences in deformation behavior during thermo-mechanical cycling. Videos of the deformation process and EBSD images are presented to demonstrate the method

Capturing the Dynamics of Ti Diffusion Across Ti _x W _1−x /Cu Heterostructures using X‐Ray Photoelectron Spectroscopy

Interdiffusion phenomena between adjacent materials are highly prevalent in semiconductor device architectures and can present a major reliability challenge for the industry. To fully capture these phenomena, experimental approaches must go beyond static and post-mortem studies to include in situ and in-operando setups. Here, soft and hard X-ray photoelectron spectroscopy (SXPS and HAXPES) is used to monitor diffusion in real-time across a proxy device. The device consists of a Si/SiO2/TixW1−x(300 nm)/Cu(25 nm) thin film material stack, with the TixW1−x film (x = 0.054, 0.115, 0.148) acting as a diffusion barrier between Si and Cu. The interdiffusion is monitored through the continuous collection of spectra whilst in situ annealing to 673 K. Ti within the TiW is found to be highly mobile during annealing, diffusing out of the barrier and accumulating at the Cu surface. Increasing the Ti concentration within the TixW1−x film increases the quantity of accumulated Ti, and Ti is first detected at the Cu surface at temperatures as low as 550 K. Surprisingly, at low Ti concentrations (x = 0.054), W is also mobile and diffuses alongside Ti. By monitoring the Ti 1s core level with HAXPES, the surface-accumulated Ti was observed to undergo oxidation even under ultra-high vacuum conditions, highlighting the reactivity of Ti in this system. These results provide crucial evidence for the importance of diffusion barrier composition on their efficacy during device application, delivering insights into the mechanisms underlying their effectiveness and limitations