Soft, comfortable polymer dry electrodes for high quality ECG and EEG recording

Conventional gel electrodes are widely used for biopotential measurements, despite important drawbacks such as skin irritation, long set-up time and uncomfortable removal. Recently introduced dry electrodes with rigid metal pins overcome most of these problems; however, their rigidity causes discomfort and pain. This paper presents dry electrodes offering high user comfort, since they are fabricated from EPDM rubber containing various additives for optimum conductivity, flexibility and ease of fabrication. The electrode impedance is measured on phantoms and human skin. After optimization of the polymer composition, the skin-electrode impedance is only similar to 10 times larger than that of gel electrodes. Therefore, these electrodes are directly capable of recording strong biopotential signals such as ECG while for low-amplitude signals such as EEG, the electrodes need to be coupled with an active circuit. EEG recordings using active polymer electrodes connected to a clinical EEG system show very promising results: alpha waves can be clearly observed when subjects close their eyes, and correlation and coherence analyses reveal high similarity between dry and gel electrode signals. Moreover, all subjects reported that our polymer electrodes did not cause discomfort. Hence, the polymer-based dry electrodes are promising alternatives to either rigid dry electrodes or conventional gel electrodes

UV cure of oxycarbosilane low-k films

status: publishe

Mechanical integrity of nano-interconnects; the impact of metallization density

In this study the impacts of two main design parameters, namely the metal and via densities on the mechanical integrity of nano-interconnects were investigated. To this aim, analytical modelling was used in order to derive the effective mechanical properties of nano-interconnect layers which were subsequently used as material properties in a layer-specific finite element model of nano-interconnects exposed to bump level mixed mode loading. The energy release rates for nano-interconnect cracks were used to determine the impact of metal and via density variations on the mechanical integrity of nano-interconnects. Using a parametric study, the key parameters that determine the mechanical integrity of nano-interconnects under chip package interaction (CPI) loads were identified to be the via densities in the intermediate layers with ultra low-k dielectric (ULK) and the via and metal densities of the top stiff group of layers often referred to as the “Z” group. Increasing the effective stiffness of the “Z” group by maximizing its via and metal density mitigated the energy release rate at the ULK pre-cracks in the intermediate via layers by means of elastic stress shielding. In addition, increasing the via density of the via layers integrated with ULK, increases the effective critical fracture energy of the via layer (i.e. the effective via layer adhesion), thereby improving the mechanical integrity. Keywords: Nano-interconnect, Mechanical integrity, Delamination, Chip package interaction (CPI), Metal density, Via densit

Nanoindentation study of thin plasma enhanced chemical vapor deposition SiCOH low-k films modified in He/H-2 downstream plasma

The effect of He/H-2 downstream plasma (DSP) on the mechanical properties of plasma enhanced chemical vapor deposition SiCOH low-k films was studied using nanoindentation (NI) with the continuous-stiffness measurement technique. Furthermore, the main requirements for reliable NI measurements on plasma-modified low-k films are discussed. The results show that the mechanical properties of these films are intimately linked with their porosity and that exposure to He/H-2 DSP causes a change in both the porosity and the mechanical properties of the films. This change is related to the removal of porogen residue formed during the ultraviolet curing of the low-k film.status: publishe

Modeling the substrate effects on nanoindentation mechanical property measurement

Nanoindentation technique is commonly used to characterize the mechanical properties of thin films. However, the validity of the measurements is greatly affected by the indentation depth and the substrate properties. The purpose of this study is to understand the influence of the substrate properties, the thin film thickness and the mechanical properties of the thin film itself on force-displacement curves obtained by nanoindentation. The experimentally obtained force versus indentation depth curves of single and bi-layer thin film systems on silicon wafers were simulated using Finite Element Modeling (FEM). The materials properties of the thin film layers were extracted by fitting the load-displacement curves obtained by FEM and experiments. The experimental part of this study includes nanoindentation tests performed on low-k films deposited on a silicon substrate. These films were treated during different periods of time (0s, 20s, 35s, 70s, 140s, 350s and 700s) with He/H2 downstreamplasma (DSP) at a fixed temperature of 280°C, thereby resulting in a double layered structure with the same total film thickness, but with different thickness and mechanical properties of the “modified” top layer. The results of this work provide considerable insight for the determination of the mechanical properties of layered systems.status: publishe

Determining the physical properties of EpoClad negative photoresist for use in MEMS applications

In this paper, the internal stress and Young modulus of EpoClad are investigated using different methods. The swelling behavior when submerged in common processing liquids is also measured and reported. Different on-wafer micro electromechanical structures are used, together with nanoindentation measurements. The experimental results yield an internal stress that is strongly dependent on the processing conditions, the ambient and the submersion liquid. Young’s modulus is found to be 4.4 ± 0.2 GPa.status: publishe

Extraction of elastic modulus of porous ultra-thin low-k films by two-dimensional finite-element simulations of nanoindentation

Continuous scaling of integrated circuits has led to the introduction of highly porous low dielectric constant (low-k) materials, whose inferior mechanical properties raise concerns regarding the reliability of integrated circuits. Nanoindentation is proven to be a straightforward method to study mechanical properties of films. However, in the case of low-k, the measurement and analysis are complex due to the porous nature of the films and reduced film thicknesses which give rise to substrate effects. A methodology that combines nanoindentation experiments with finite-element simulations is proposed and validated in this study to extract the substrate-free elastic modulus of porous ultra-thin low-k films. Furthermore, it is shown that imperfections of the nanoindentation probe significantly affect the finite-element results. An effective analytical method that captures the actual nanoindenter behavior upon indentation is proposed by taking both tip radius and conical imperfections into account. Using this method combined with finite element modeling, the elastic modulus of sub-100 nm thick low-k films is successfully extracted. Standard indentation tests clearly overestimated the actual modulus for such thin films, which emphasizes the importance of the proposed methodology.status: publishe

Characterisation of the mechanical behaviour of a polyurethane elastomer based on indentation and tensile creep experiments

In this study a comparison between the characterisation of material properties on micro and macro scale was made. Spherical indentations were performed at the micro scale while tensile experiments represent the macro scale behaviour. The samples for both types of experiments were made of the same polyurethane elastomer sheet film. The experimental test setups were load controlled and consisted of a loading phase followed by a holding phase in which the material exhibited a creep response under a fixed load. In order to visualise and quantify the occurring strain levels during the indentation experiment, a FE-model was build. Based on the outcome of this FE-model, the macro tensile test setup was established. According to this procedure, the material experienced a similar level of deformation during both types of testing. As described by M. Oyen (J. Mater Res., 20, 2094-2100, 2005) the standard linear solid model was applied to capture the material behaviour during indentation testing. The model was adapted for macro tensile testing and applied to fit the experimental data. This fitting procedure was executed through the use of different scripts and revealed quantitative values for all components (spring and time constants) of the standard linear solid model. The dependence of the results on different loading rates and hold values was investigated. The results of these experiments indicate an overall stiffer response of the material during the nanoindentation tests compared to the macro tensile tests. This observation can be explained by the obstruction of the poisson effect during the indentation test. An extra boundary condition like biaxial tensile tests could be introduced in the future in the macro scale tests. The influence of the loading rate and hold value was minimal and validated the use of a linear viscoelastic analysis.status: publishe