Scientific direction Development of key enabling technologies
Transfer of knowledge to industry

PhD : selection by topics

Engineering science >> Metrology
2 proposition(s).

Study and design of an integrated system for the automatic calibration of dispersions within a transducers array and application to a PMUT array

Département Architectures Conception et Logiciels Embarqués (LIST-LETI)

01-09-2019

SL-DRT-19-0293

gwenael.bechet@cea.fr

The purpose of this thesis is to study and design an integrated electronic system dedicated to the automatic and continuous compensation of dispersions within a MEMS (Microelectromechanical Systems) array. With the dissemination and the continual expansion of Internet of Things (IoT) and Cyber-Physical Systems (CPS), man-machine and machine-machine interfaces require increasingly efficient and sophisticated sensors. In addition to advantages in cost, reliability, size and power consumption, MEMS based transducers enable sensors to integrate more and more intelligence in their front-end electronics. They also allow innovative topological configurations giving access to measurement ranges that are not addressable by their discrete counterparts. Arrays of MEMS based transducers enable the spatial discretization of the transduction surfaces and improve the measurements yields and accuracies (gas detector, mass spectrometry, pressure distribution, etc.). They also enable the resolution improvement of electromagnetic and acoustic beams (location, navigation, communication, etc.). Despite the considerable technological advancements that MEMS are continually enjoying, some application requirements are beyond the transducers intrinsic performances. It is then necessary to implement calibration systems to correct the transducers biases introduced during manufacture or evolving with the operating conditions. The evaluation and compensation of these errors requires costly calibration process in a dedicated test laboratory, that are not compatible with massive production. The aim of this thesis is to achieve an integrated electronic diagnostic alternative, an electromechanical BIST (Built-In Self-Test) specific to transducers arrays, combined with an automatic correction system, which will operate in coexistence with the main functions of the sensor interface. The proposed use-case is that of PMUT (Piezoelectric Micromachined Ultrasonic Transducer) arrays. These devices offer alternatives and complementary solutions to electromagnetic sensors for detection and localization [1], gesture recognition [2] or wake-up signals detection [3]. For most applications, these resonant transducers operate in transmit / receive modes (TX / RX) and need to be actuate at their resonance frequency to optimize the transmission power. The emitted and received beam is focused and steered by phase control. Errors and dispersion in the PMUT characteristics generates biases in their resonant frequency, gain and quality factor, leading to losses and distortions in the emitted and received beams. For example, a few percent of dispersions on the mechanical stiffness of the transducers can lead to several tens of percent loss on the acoustic power transmitted to a target. As a first step, the doctoral student will get familiar with the quantities and physical phenomena characterizing PMUT arrays. Based on an analytical model developed within the host laboratory, he will be able to understand the sensitivities to dispersions and their impact on the beam power and directivity. He will then define the electronic methods and architectures that will allow the system to converge towards the optimal operating conditions, for example by identifying the average resonance frequency of the array the required phase and gain correction coefficients to allocate to each transducer. The architecture and implementation choices must allow the system to adapt itself according to dispersions and drifts in a continuous and autonomous way, without disrupting the main measurement functions. The chosen solution will be implemented and validated in a mixed design environment in order to result in a functional demonstrator. [1] Przybyla, R. J., Tang, H. -., Guedes, A., Shelton, S. E., Horsley, D. A., & Boser, B. E. (2015). 3D ultrasonic rangefinder on a chip. IEEE Journal of Solid-State Circuits, 50(1), 320-334. [2] Ling, K., Dai, H., Liu, Y., & Liu, A. X. (2018). Ultragesture: Fine-grained gesture sensing and recognition. Paper presented at the 2018 15th Annual IEEE International Conference on Sensing, Communication, and Networking, SECON 2018, 1-9. [3] Yadav, K., Kymissis, I., & Kinget, P. R. (2013). A 4.4-µ W wake-up receiver using ultrasound data. IEEE Journal of Solid-State Circuits, 48(3), 649-660.

Interaction mechanisms of hydrogen with defects of silicon bulk and at the interfaces of passivated contacts in PV cells

Département des Technologies Solaires (LITEN)

Laboratoire Matériaux et Procédés Silicium

01-10-2019

SL-DRT-19-0595

raphael.cabal@cea.fr

Although fluctuating, the photovoltaic market is still dominated by silicon technologies occupying ~94%. The most promising homo-junction cell architectures systematically integrate a so-called "passivated" contact through a stack of polycrystalline silicon on tunnel oxide. The hydrogenation of such structures makes it possible to achieve very efficient yields >25%. However, the introduction of hydrogen can also lead to layer delamination or resistive losses through accumulation effects at the interfaces, significantly degrading the efficiency of the final device. To avoid its effects and develop this type of structure with associated yields, it is essential to understand the interactions of hydrogen involved and to understand its role in passivation phenomena. However, hydrogen is an extremely difficult element to characterize by its very nature. Its characterization therefore represents a real challenge, to which are added the difficulties related to the textured surface state of solar silicon and the configuration of poly-Si/Si/SiOx/Si interfaces. To meet this challenge, the work proposed here will be to implement and correlate characterization techniques, allowing both to locate and quantify hydrogen in the volume of silicon and at the interfaces of passivated contact structures. The implementation of a characterization methodology will lead to the main objective of the thesis, which is to propose mechanisms of hydrogen interaction with defects and its role in the quality of passivated contacts. This will open up opportunities for the development and optimization of passivated contact structures. This study will benefit from the infrastructure for the realization of samples from CEA-LITEN in INES and the means of characterization of the nano-characterization platform with its expert environment.

Voir toutes nos offres