Thesis (Ph.D)--Biomedical Engineering, Georgia Institute of Technology, 2009. / Committee Co-Chair: Shawn Hochman; Committee Co-Chair: Stephen P. DeWeerth; Committee Member: Lena Ting; Committee Member: Robert J. Butera; Committee Member: Robert Lee; Committee Member: Vivian K. Mushahwar. Part of the SMARTech Electronic Thesis and Dissertation Collection.

Thesis (M.S.)--State University of New York at Binghamton, Department of Psychology, 2008. / Includes bibliographical references.

L’objectif principal du projet est d’étudier les techniques d’analyses de défaillances des circuits intégrés VLSI basées sur l’emploi de laser. Les études ont été effectuées sur l’équipement à balayage laser MERIDIAN (DCGSystems) et le testeur Diamond D10 (Credence) disponible au CNES. Les travaux de thèse concernent l’amélioration des techniques dynamiques dites DLS comme « Dynamic Laser Stimulation ». Les techniques DLS consistent à perturber le fonctionnement d’un circuit intégré défaillant par effet photoélectrique ou effet photothermique, en fonctionnement dynamique, à l’aide d’un faisceau laser continu balayant la surface du circuit. Un faisceau laser modulé avec des impulsions supérieures à la nanoseconde et de façon synchrone avec le test électrique à l’aide d’un signal TTL peut être également avantageusement utilisé pour localiser des défauts non accessibles par des techniques purement statiques (OBIRCh, OBIC etc.). L’analyse de la réponse des paramètres électriques à la perturbation laser conduit à une identification de l’origine de la défaillance dynamique. L’optimisation des techniques DLS actuelles permet d’augmenter le taux de succès des analyses de défaillance et d’apporter des informations difficilement accessibles jusqu’alors, qui permettent la détermination de la cause racine de la défaillance.Dans un premier temps, le travail réalisé a consisté en l’amélioration du processus d’analyse des techniques DLS par l’intégration étroite avec le test de façon à observer tout paramètre électrique significatif lors du test DLS. Ainsi, les techniques de « Pass-Fail Mapping » ou encore les techniques paramétriques de localisation de défauts ont été implémentées sur le banc de test constitué du Meridian et du D10. La synchronisation du déroulement du test opéré par le testeur avec le balayage laser a permis par la suite d’établir des méthodologies visant à rajouter une information temporelle aux informations spatiales. En effet, en utilisant un laser modulé nous avons montré que nous étions capable d’identifier avec précision quels sont les vecteurs impliqués dans le comportement défaillant en modulant l’éclairement du faisceau laser en fonction de la partie de la séquence de test déroulée. Ainsi nous somme capable de corréler la fonction défaillante et les structures du CI impliquées. Cette technique utilisant le laser modulé est appelée F-DLS pour « Full Dynamic Laser Stimulation ». A l’inverse, nous pouvons connaitre la séquence de test qui pose problème, et par contre ne pas connaitre les structures du CI impliquées. Dans l’optique de rajouter cette l’information, il a été développé une technique de mesure de courant dynamique. Cette technique s’est avérée efficace pour obtenir des informations sur le comportement interne du CI. A titre d’exemple, prenons le cas des composants « latchés » où les signaux sont resynchronisés avant la sortie du composant. Il est difficile, même avec les techniques DLS actuelles, d’avoir des informations sur une dérive temporelle des signaux. Cependant l’activité interne du composant peut être caractérisée en suivant sur un oscilloscope l’évolution du courant lorsque le circuit est actif, sous la stimulation laser. L’information sur la dérive temporelle peut être extraite par observation de cette activité interne.Enfin, ces techniques de stimulation laser dynamique, ont également prouvé leur efficacité pour l’étude de la fiabilité des CI. La capacité de ces techniques à détecter en avance d’infimes variations des valeurs des paramètres opérationnels permet de mettre en évidence l’évolution des marges de ces paramètres lors d’un processus de vieillissement accéléré. L’étude de l’évolution de la robustesse des CI face aux perturbations externes est un atout majeur qu’apportent les techniques DLS à la fiabilité.Les méthodologies développées dans cette thèse, sont intégrées dans les processus d’analyse et de caractérisation de CI au laboratoire. / The principal objective of the project is to investigate laser based techniques for failure analysis of VLSI integrated circuits. The investigations will be performed on the DCGSystems’ Meridian laser scanning microscope coupled with the Credence’s Diamond D10 tester available at CNES. This study was interested more specifically in the improvement of dynamic laser stimulation techniques said DLS like Dynamic Laser Stimulation. DLS techniques consists in modifying the operation of a dynamically failing integrated circuit by photoelectric effect or photothermal effect using a continuous laser beam sweeping the surface of the circuit. A laser beam modulated in the nanosecond range synchronously with the electrical test through a TTL signal can also be advantageously used. Analysis of the electrical parameters response to the laser disturbance leads to an identification of the dynamic failure origin. The optimization of current DLS techniques will increase the failure analyses success rate and bring information hardly accessible by other means, which allows determining the failure root cause. The work performed was the improvement of the DLS process flow by closely integrating the test to monitor any relevant electrical parameters upon DLS. The « Pass-Fail Mapping » technique and the parametric techniques were implemented on the test tools combining the D10 and the Meridian. The synchronization of the test with the laser scan allows establishing methodologies and techniques in order to add timing information to the defect localisation. Indeed, by modulating the laser beam depending on the test pattern sequences, we show our capability to identify precisely which are the vectors responsible for the IC defective behaviour. We are able now to correlate the defective IC functions with the IC structures involved. This technique is known as F-DLS for Full Dynamic Laser Stimulation.In some cases, we know when the failure occurs in the test pattern but we ignore which IC structures are involved. So, we also developed a dynamic current measurement under laser stimulation technique. This technique proved to be efficient to obtain information about the internal IC behaviour. As an example, for the latched component which signals are synchronised just before the outputs, it is hard to measure shift in the signal propagation. Nevertheless, the IC internal activities can be characterized by monitoring on a scope the current variations under laser stimulation when the IC is activated. The information about the shift in the signal propagation could be extracted then by observing of the IC internal activities.Finally, these DLS techniques proved their efficiency for device qualification for reliability issues. Their accuracy allows early detection of operational parameter tiny variations. This is used to highlight electrical parameter margin evolutions during accelerated aging process. DLS techniques demonstrate their potential to deal with the IC robustness evolution facing external perturbation for reliability purposes.The techniques and methodologies developed during this work have been successfully integrated in the IC analysis and characterisation process in the laboratory. We exposed these techniques but the main case studies remain confidential.

While three decades of intensive cortical electrophysiology using a variety of sustained visual stimuli has made a significant contribution to many aspects of visual function, it has not supported the existence of intracortical circuit operations in cortical processing. This study investigated cortical processing by a comparison of the response of primary visual cortical neurones to transient electrical and strobe-flash stimulation. Experiments were performed on 74 anaesthetised Long Evans rats. Standard stereotaxic and extracellular electrophysiological techniques were employed. Continuous (on-line) raster plots and peri-stimulus time histograms (PSTHs) of the extracellular spikes from 81 visual cortical and 55 lateral geniculate nucleus (LGN) neurones were compiled. The strobe-flash stimuli (0.05 ms) were applied to the contralateral eye while the monopolar or bipolar electrical stimuli (0.2 ms, 80-400 μA) were applied to the ipsilateral LGN. 60 of the 81 (74%) tested cortical units were found to be responsive to visual stimuli. A distinct and consistent difference in the cortical response to the two types of transient stimuli was found: (a) Electrical stimulation evoked a prolonged period (197 ± 61 ms) of inhibition in all cortical neurones tested (n=20). This was the case even in those cortical units that were completely unresponsive to visual stimulation. The protracted inhibition was usually followed by a 100-200 ms phase of rebound excitation. (b) Flash stimulation evoked a prominent excitatory discharge (5-30 ms duration) after a latency of 30-60 ms from the onset of the stimulus (n = 59). This was followed by either moderate inhibition or return to a firing rate similar to control activity, for a maximum of 40 ms. Thereafter, cortical neurones showed a sustained increased level of activity with superimposed secondary excitatory phases. The duration of this late re-excitatory phase was 200-300 ms. In 17 of 20 (85%) tested units, the temporal profile of the cortical response to flash stimulation was modulated by small changes in the level of background illumination. In 16 of the 17 units, this sensitivity was reflected primarily as an emergence of a brief secondary inhibitory phase at the lowest level of background illumination (0 lux). Only 1 of the 17 cortical units displayed a flash-evoked primary inhibitory phase at O lux. We explored the possibility that neurones in the lateral geniculate nucleus (LGN) of the thalamus were responsible for the late phase of cortical reexcitation. 49 of the 55 (89%) LGN neurones could be classified as either of the "ON type" i.e. excited by visual stimuli, or the "OFF type" i.e. inhibited by visual stimuli. The response of ON-like LGN neurones to strobe-flash stimulation of the contralateral eye was characterised by a primary excitatory or early discharge (ED) phase after a latency of 25-40 ms. Thereafter, a 200- 400 ms period of inhibition was observed. In 57% of the sample, a rebound excitatory or late discharge (LD) phase completed the response. OFF-like LGN neurones were inhibited by the strobe-flash stimuli after a latency of 30- 35 ms. This flash-evoked inhibition was maintained for 200-400 ms. The sensitivity of the flash-evoked LGN response to the level of background illumination was tested in 11 ON-like and 10 OFF-like neurones. No sustained secondary excitatory events, as observed in visual cortical neurones, were found in any of the ON- and OFF-like LGN neurones, irrespective of the level of background illumination. In conclusion, the data show that the late re-excitatory phase evoked in cortical neurones upon strobe-flash stimulation, is not due to sustained LGN (thalamic) input. Rather, it suggests that these re-excitatory phases are due to intracortical processing of the transient stimuli. These findings emphasize the independent role of the cortex in computing the response to visual stimuli, and cast doubt on traditional theories that have emphasised the role of the thalamus in shaping cortical responses. The difference in the flash and electrically evoked cortical response suggests that even though substantial inhibition is available to the cortex, only a small fraction of this inhibitory capacity is utilised during natural stimulation.

Theta-burst stimulation (TBS) is a form of repetitive transcranial magnetic stimulation (TMS) developed to induce neuroplasticity. TBS usually consists of 50 Hz bursts at 5 Hz intervals. When applied intermittently, it can lead to facilitation of motor evoked potentials (MEPs), although these effects can be variable between individuals. Here, we aimed to determine whether a version of intermittent TBS (iTBS) consisting of 30 Hz bursts at 6 Hz intervals would produce less variable modulation. Nineteen healthy adults underwent single-pulse TMS to assess corticomotor excitability at baseline as reflected in MEP amplitude. 30 Hz iTBS was then administered and MEP amplitude was reassessed at 5-, 20- and 45-mins after the iTBS protocol. Compared to baseline, MEPs were significantly facilitated up to 45-min post-iTBS and most participants exhibited the expected facilitation. These observations suggest that 30 Hz/6 Hz iTBS may provide a sound alternative to induce consistent neuromodulatory effects over the commonly used 50 Hz/5 Hz protocol.

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