People suffering from certain types of arrhythmia may benefit from the implantation of a cardiac pacemaker. Pacemakers artificially stimulate the heart by applying short electrical pulses to the cardiac tissue to restore and maintain a steady heart rhythm. By adjusting the pulse delivery rate the heart is stimulated to beat at desired pace. The stimulation pulses are transferred from the pacemaker to the heart via an electrode, which is implanted into the cardiac tissue. Additionally, the electrode must also sense the cardiac response and transfer those signals back to the electronics in the pacemaker for processing. The communication between the electrode and the tissue takes place on the electrode/electrolyte (tissue) interface. This interface serves as the contact point where the electronic current in the electrode is converted to ionic currents capable to operate in the body. The stimulation/sensing signals are transferred across the interface via three electrochemical mechanisms: i) non-faradaic charging/discharging of the electrochemical double layer, ii) reversible and iii) irreversible faradaic reactions. It is necessary to study the contribution of each mechanism to the total charge transferred to evaluate the pacing/sensing performance of the pacemaker electrode. In this thesis, the electrochemical properties and performance of stimulation/sensing electrodes for pacemaker applications have been investigated by electrochemical impedance spectroscopy, cyclic voltammetry and transient electrochemical techniques. All measurements were performed in synthetic body fluid with buffer capacity. Complementary surface analysis was performed with scanning electron microscopy, energy dispersive spectroscopy and X-ray photoelectron spectroscopy. The results reveal different interfacial behaviour and stability for electrode materials such as Pt, TiN, porous carbon, conducting oxides (RuO2 and IrO2 and mixed oxides) and porous Nb2O5 oxide. The influence of the charge/discharge rate on the electrode characteristics also has been evaluated. Although the rough and porous electrodes provide a high interfacial capacitance, the maximum capacitance cannot be fully employed at high charge/discharge rates because only a small part of the effective surface area is accessible. The benefit of pseudo-capacitive material properties on charge delivery was observed. However, these materials suffer similar limitations at high charge/discharge rate and, hence, are only utilising the surface bound pseudo-capacitive sites. Porous Nb2O5 electrodes were investigated to study the performance of capacitor electrodes. These electrodes predominantly deliver the charge via reversible non-faradaic mechanisms and hence do not produce irreversible by-products. They can deliver very high potential pulses while maintaining high impedance and, thus, charge lost by faradaic currents are kept low. By producing Nb oxide by plasma electrolysis oxidation a porous surface structure is obtained which has the potential to provide a biocompatible interface for cell adherence and growth. This thesis covers a multidisciplinary area. In an attempt to connect diverse fields, such as electrophysiology, materials science and electrochemistry, the first chapters have been attributed to explaining fundamental aspects of the respective fields. This thesis also reviews the current opinion of pacing and sensing theory, with special focus on some areas where detailed explanation is needed for the fundamental nature of electrostimulation/sensing. / QC 20101014

Au vu des limitations rencontrées par la miniaturisation des circuits microélectroniques, l’augmentation de performances des systèmes repose largement aujourd’hui sur la fabrication d’empilements de couches minces complexes et innovants pour offrir davantage de compacité et de flexibilité. L’intérêt grandissant pour la réalisation de structures innovantes temporaires, i.e. permettant de réaliser des circuits sur les deux faces d’un même film, nous a mené à évaluer les potentialités d’une technologie combinant le transfert de films minces monocristallins, i.e. la technologie Smart Cut™, et un procédé de de porosification partielle du silicium afin de mettre au point une technologie de double report de film monocristallin. En ce sens, des substrats de silicium monocristallin ont été partiellement porosifiés par anodisation électrochimique. La mise en œuvre de traitements de substrats partiellement poreux a nécessité l’emploi de techniques de caractérisation variées pour dresser une fiche d’identité des couches minces poreuses après anodisation et évaluer l’évolution des propriétés de ces couches en fonction des différents traitements appliqués. Les propriétés chimiques, structurales et mécaniques des couches de Si poreux ont ainsi été étudiées via l’utilisation de différentes techniques de caractérisation (XPS-SIMS, AFM-MEB-XRD, nanoindentation, technique d’insertion de lame, etc.). Ces études ont permis d’appréhender et de décrire les mécanismes physiques mis au jeu au cours des différents traitements et de déterminer les caractéristiques {porosité, épaisseur} optimales des couches poreuses compatibles avec les séquences de la technologie proposée. La technologie Smart Cut™ a ainsi été appliquée à des substrats partiellement porosifiés menant à la fabrication réussie d’une structure temporaire de type Silicium-sur-Isolant avec une couche de silicium poreux enterrée. Ces structures temporaires ont été « démontées » dans un second temps par collage polymère ou collage direct et insertion de lame menant au second report de film mince monocristallin par rupture au sein de la couche porosifiée et donc fragile. Les structures fabriquées ont été caractérisées pour vérifier leur intégrité et leurs stabilités chimique et mécanique. Les propriétés cristallines du film mince de Si monocristallin, reporté en deux temps, ont été vérifiées confirmant ainsi la compatibilité des structures fabriquées avec des applications microélectroniques telles que les applications de type « Back-Side Imager » nécessitant une implémentation de composants sur les deux faces du film. Ainsi une technologie prometteuse et performante a pu être élaborée permettant le double report de films minces monocristallins et à fort potentiel pour des applications variées comme les imageurs visibles ou le photovoltaïque. / As scaling of microelectronic devices is confronted from now to fundamental limits, improving microelectronic systems performances is largely based nowadays on complex and innovative stack realization to offer more compaction and flexibility to structures. Growing interest in the fabrication of innovative temporary structures, allowing for example double sided layer processing, lead us to investigate the capability to combine one technology of thin single crystalline layer transfer, i.e. the Smart Cut™ technology, and partial porosification of silicon substrate in order to develop an original double layer transfer technology of thin single crystalline silicon film. To this purpose, single crystalline silicon substrates were first partially porosified by electrochemical anodization. Application of suitable treatments of porous silicon layer has required the use of several characterization methods to identify intrinsic porous silicon properties after anodization and to verify their evolution as function of different applied treatments. Chemical, structural and mechanical properties of porous silicon layers were studied by using different characterization techniques (XPS-SIMS, AFM-MEB-XRD, nanoindentation, razor blade insertion, etc.). Such studies allowed comprehending and describing physical mechanisms occurring during each applied technological steps and well determining appropriated {porosity, thickness} parameters of porous silicon layer with the developed technological process flow. The Smart Cut™ technology was successfully applied to partially porosified silicon substrates leading to the fabrication of temporary SOI-like structures with a weak embedded porous Si layer. Such structures were then “dismantled” thanks to a second polymer or direct bonding and razor blade insertion to produce a mechanical rupture through the fragile embedded porous silicon layer and to get the second thin silicon film transfer. Each fabricated structure was characterized step by step to check its integrity and its chemical and mechanical stabilities. Crystalline properties of the double transferred silicon layer were verified demonstrating the compatibility of such structures with microelectronic applications such as “Back-Side Imagers” needing double-sided layer processing. Eventually, a promising and efficient technology has been developed to allow the double transfer of thin single crystalline silicon layer which presents a high potential for various applications such as visible imagers or photovoltaic systems.