Scalable and high-sensitivity readout of silicon quantum devices

Quantum computing is predicted to provide unprecedented enhancements in computational power. A quantum computer requires implementation of a well-defined and controlled quantum system of many interconnected qubits, each defined using fragile quantum states. The interest in a spin-based quantum computer in silicon stems from demonstrations of very long spin-coherence times, high-fidelity single spin control and compatibility with industrial mass-fabrication. Industrial scale fabrication of the silicon platform offers a clear route towards a large-scale quantum computer, however, some of the processes and techniques employed in qubit demonstrators are incompatible with a dense and foundry-fabricated architecture. In particular, spin-readout utilises external sensors that require nearly the same footprint as qubit devices. In this thesis, improved readout techniques for silicon quantum devices are presented and routes towards implementation of a scalable and high-sensitivity readout architecture are investigated. Firstly, readout sensitivity of compact gate-based sensors is improved using a high-quality factor resonator and Josephson parametric amplifier that are fabricated separately from quantum dots. Secondly, an integrated transistor-based control circuit is presented using which sequential readout of two quantum dot devices using the same gate-based sensor is achieved. Finally, a large-scale readout architecture based on random-access and frequency multiplexing is introduced. The impact of readout circuit footprint on readout sensitivity is determined, showing routes towards integration of conventional circuits with quantum devices in a dense architecture, and a fault-tolerant architecture based on mediated exchange is introduced, capable of relaxing the limitations on available control circuit footprint per qubit. Demonstrations are based on foundry-fabricated transistors and few-electron quantum dots, showing that industry fabrication is a viable route towards quantum computation at a scale large enough to begin addressing the most challenging computational problems

Gate-based sensing of silicon quantum dot devices towards 2D scaling

This thesis focuses on using the radio-frequency reflectometry technique for dispersive gate sensing of foundry fabricated silicon nanowire quantum dot devices. I will attempt to answer three questions relating to the scalability of these devices. How do electron and hole spin qubits perform in silicon quantum dots? How do we implement and distribute the placement of dispersive gate sensors in scaled-up quantum dot arrays? And how does a single dopant in the silicon channel affect the gate-defined quantum dot? First, I investigate the difference between electron and hole quantum dots in an ambipolar nanowire device which successfully demonstrated reconfigurable single and double electron and hole quantum dots in the same crystalline environment. I further investigate the effective bath temperature of two-dimensional electron gas and two-dimensional hole gas by performing the thermometry experiment on the same type of device. Secondly, I demonstrate a two-dimensional quantum dot array enabled by a floating gate architecture between silicon nanowires. An analytical model is developed to study the capacitive coupling between remote quantum dots over different distances. Coupling strength under different qubit encodings is also discussed to show the best implementation for neighbour silicon nanowires. Finally, the in-situ dispersive gate sensing allows the measurement of the inter-dot transition between the bismuth donor-dot system. The novel implementation with bismuth donor can open up the possibility of a hybrid singlet-triplet qubit or transferring a coherent spin state between the quantum dot and the donor

Miniaturized Optical Probes for Near Infrared Spectroscopy

RÉSUMÉ L’étude de la propagation de la lumière dans des milieux hautement diffus tels que les tissus biologiques (imagerie optique diffuse) est très attrayante, car elle offre la possibilité d’explorer de manière non invasive le milieu se trouvant profondément sous la surface, et de retrouver des informations sur l’absorption (liée à la composition chimique) et sur la diffusion (liée à la microstructure). Dans la gamme spectrale 600-1000 nm, également appelée gamme proche infrarouge (NIR en anglais), l'atténuation de la lumière par le tissu biologique (eau, lipides et hémoglobine) est relativement faible, ce qui permet une pénétration de plusieurs centimètres dans le tissu. En spectroscopie proche infrarouge (NIRS en anglais), de photons sont injectés dans les tissus et le signal émis portant des informations sur les constituants tissulaires est mesuré. La mesure de très faibles signaux dans la plage de longueurs d'ondes visibles et proche infrarouge avec une résolution temporelle de l'ordre de la picoseconde s'est révélée une technique efficace pour étudier des tissus biologiques en imagerie cérébrale fonctionnelle, en mammographie optique et en imagerie moléculaire, sans parler de l'imagerie de la durée de vie de fluorescence, la spectroscopie de corrélation de fluorescence, informations quantiques et bien d’autres. NIRS dans le domaine temporel (TD en anglais) utilise une source de lumière pulsée, généralement un laser fournissant des impulsions lumineuses d'une durée de quelques dizaines de picosecondes, ainsi qu'un appareil de détection avec une résolution temporelle inférieure à la nanoseconde. Le point essentiel de ces mesures est la nécessité d’augmenter la sensibilité pour de plus grandes profondeurs d’investigation, en particulier pour l’imagerie cérébrale fonctionnelle, où la peau, le crâne et le liquide céphalo-rachidien (LCR) masquent fortement le signal cérébral. À ce jour, l'adoption plus large de ces techniques optique non invasives de surveillance est surtout entravée par les composants traditionnels volumineux, coûteux, complexes et fragiles qui ont un impact significatif sur le coût et la dimension de l’ensemble du système. Notre objectif est de développer une sonde NIRS compacte et miniaturisée, qui peut être directement mise en contact avec l'échantillon testé pour obtenir une haute efficacité de détection des photons diffusés, sans avoir recours à des fibres et des lentilles encombrantes pour l'injection et la collection de la lumière. Le système proposé est composé de deux parties: i) une unité d’émission de lumière pulsée et ii) un module de détection à photon unique qui peut être activé et désactivé rapidement. L'unité d'émission de lumière utilisera une source laser pulsée à plus de 80 MHz avec une largeur d'impulsion de picoseconde.----------ABSTRACT The study of light propagation into highly diffusive media like biological tissues (Diffuse Optical Imaging) is highly appealing due to the possibility to explore the medium non-invasively, deep beneath the surface and to recover information both on absorption (related to chemical composition) and on scattering (related to microstructure). In the 600–1000 nm spectral range also known as near-infrared (NIR) range, light attenuation by the biological tissue constituents (i.e. water, lipid, and hemoglobin) is relatively low and allows for penetration through several centimeters of tissue. In near-infrared spectroscopy (NIRS), a light signal is injected into the tissues and the emitted signal carrying information on tissue constituents is measured. The measurement of very faint light signals in the visible and near-infrared wavelength range with picosecond timing resolution has proven to be an effective technique to study biological tissues in functional brain imaging, optical mammography and molecular imaging, not to mention fluorescence lifetime imaging, fluorescence correlation spectroscopy, quantum information and many others. Time Domain (TD) NIRS employs a pulsed light source, typically a laser providing light pulses with duration of a few tens of picoseconds, and a detection circuit with temporal resolution in the sub-nanosecond scale. The key point of these measurements is the need to increase the sensitivity to higher penetration depths of investigation, in particular for functional brain imaging, where skin, skull, and cerebrospinal fluid (CSF) heavily mask the brain signal. To date, the widespread adoption of the non-invasive optical monitoring techniques is mainly hampered by the traditional bulky, expensive, complex and fragile components which significantly impact the overall cost and dimension of the system. Our goal is the development of a miniaturized compact NIRS probe, that can be directly put in contact with the sample under test to obtain high diffused photon harvesting efficiency without the need for cumbersome optical fibers and lenses for light injection and collection. The proposed system is composed of two parts namely; i) pulsed light emission unit and ii) gated single-photon detection module. The light emission unit will employ a laser source pulsed at over 80MHz with picosecond pulse width generator embedded into the probe along with the light detection unit which comprises single-photon detectors integrated with other peripheral control circuitry. Short distance source and detector pairing, most preferably on a single chip has the potential to greatly expedites the traditional method of portable brain imaging

Silicon Nanodevices

This book is a collection of scientific articles which brings research in Si nanodevices, device processing, and materials. The content is oriented to optoelectronics with a core in electronics and photonics. The issue of current technology developments in the nanodevices towards 3D integration and an emerging of the electronics and photonics as an ultimate goal in nanotechnology in the future is presented. The book contains a few review articles to update the knowledge in Si-based devices and followed by processing of advanced nano-scale transistors. Furthermore, material growth and manufacturing of several types of devices are presented. The subjects are carefully chosen to critically cover the scientific issues for scientists and doctoral students

Miniaturized Transistors, Volume II

In this book, we aim to address the ever-advancing progress in microelectronic device scaling. Complementary Metal-Oxide-Semiconductor (CMOS) devices continue to endure miniaturization, irrespective of the seeming physical limitations, helped by advancing fabrication techniques. We observe that miniaturization does not always refer to the latest technology node for digital transistors. Rather, by applying novel materials and device geometries, a significant reduction in the size of microelectronic devices for a broad set of applications can be achieved. The achievements made in the scaling of devices for applications beyond digital logic (e.g., high power, optoelectronics, and sensors) are taking the forefront in microelectronic miniaturization. Furthermore, all these achievements are assisted by improvements in the simulation and modeling of the involved materials and device structures. In particular, process and device technology computer-aided design (TCAD) has become indispensable in the design cycle of novel devices and technologies. It is our sincere hope that the results provided in this Special Issue prove useful to scientists and engineers who find themselves at the forefront of this rapidly evolving and broadening field. Now, more than ever, it is essential to look for solutions to find the next disrupting technologies which will allow for transistor miniaturization well beyond silicon’s physical limits and the current state-of-the-art. This requires a broad attack, including studies of novel and innovative designs as well as emerging materials which are becoming more application-specific than ever before

Microlenses for optical microsystems

Tese de doutoramento (Programa Doutoral em Líderes para as Indústrias Tecnológicas)Lenses have been used by mankind for thousands of years for innumerous different reasons and applications. More recently, lenses in the micro scale dimension, so called microlenses (MLs), have been designed and fabricated using semiconductor technology. These new lenses are used for collimation, focusing or imaging and are an appealing alternative for applications where miniaturization and alignment simplicity are important requirements. Moreover, they also opened a large number of new applications for optical structures and, at the same time, reducing the mechanical and electrical complexity of the existing systems. In this context, the presented thesis has as main purposes, the design and development of a process that allows the fabrication of different sized plano-convex MLs with minor intervention on the process parameters. The MLs were fabricated using a photoresist, the AZ4562, through classical photolithography and the thermal reflow process. Another achievement was the fabrication of MLs directly on the surface of a silicon die containing complementary metal–oxide–semiconductor (CMOS) photodiodes (PDs) for quantifying the differences in their photocurrents generation capacity. The MLs’ optimum fabrication process was achieved when a 128k dots per inch (dpi) super high-resolution chrome on soda lime glass 3×3-0.060” photomask was employed. This photomask allows the design pattern to be transferred into the photoresist with very high precision. Nevertheless, for actually obtaining the desired lens profile, it is necessary to apply a thermal treatment to the fabricated microstructures. When the photoresist is submitted to a temperature higher than its glass transition temperature, it softens allowing the shape change to occur. For MLs, the major external force acting during this process is the surface tension. The fabricated MLs were structurally characterized using a profilometer and scanning electron microscope (SEM) images. For measuring the focal length, an optomechanical alignment system was assembled and a difference of just 4% was found between the measured and the theoretical values. An additional improvement was achieved by introducing a rehydration step in the fabrication process. The prebake stage used during the fabrication serves for evaporating the solvent off the photoresist but also, all of its water content. As a result, it was demonstrated that the AZ4562 needs rehydration in order to obtain excellent results by preventing structural damages in the MLs which are crucial for achieving efficient optical properties. The main advantage of this new optimized process is the further improvement of well-established standard microfabrication processes, i.e., photolithography combined with photoresist thermal reflow. Then, three approaches for integrating the MLs with the photodetecting substrate were tested. The first was using a polydimethylsiloxane (PDMS) intermediate layer for controlling the thickness between the MLs and the photodetecting substrate for allowing different focal lengths to be used depending on the application. The second one is setting the MLs’ focal length within the photodetectors’ depletion region using a 150 μm thin glass substrate for demonstrating that the current generation is enhanced for the same active area. Finally, the third approach consists on a setup composed by a MLs array fabricated directly on top of the PDs and in this approach, two solutions are presented. One is the fabrication of a ML on a square PD with the side measuring 24 μm. This setup enables the capture of light that would otherwise fall outside the photodiodes’ active area resulting in an overall photocurrent generation gain. The other is the fabrication of a MLs array using the same photomask but on a square PD with the side measuring 240 μm for determining the level of photocurrent generation. Moreover, two light sources (red and white lights) were used for evaluating the light acquisition enhancement capacity. From the results that were obtained under different integration solutions, the direct fabrication of MLs on PDs was the one with the better results concerning photocurrent generation by improving it by more than 14% and 2% for red and white lights, respectively. The red light has the ideal penetration depth in silicon for achieving the most prominent enhancement in photocurrent generation presented in this thesis. The MLs that were designed and fabricated, as well as their integration solutions with a photosensitive substrate, show interesting potential in applying them on industry standard fabrication processes for optical microsystems, from light-acquisition enhancement applications to image sensors.Desde há milhares de anos que a Humanidade tem usado lentes por inúmeras razões e para diferentes aplicações. Mais recentemente, têm sido desenvolvidas e fabricadas lentes de microdimensões, também designadas de microlentes (MLs), utilizando a tecnologia dos semicondutores. Este novo tipo de lentes é normalmente utilizado para colimar, focar ou criar imagens, e é uma alternativa apelativa para aplicações onde a miniaturização e simplicidade de alinhamento são requisitos importantes. Além disso, elas também deram origem a um conjunto de novas aplicações para estruturas óticas reduzindo, ao mesmo tempo, as complexidades mecânicas e elétricas dos sistemas existentes. Nesta perspetiva, a presente tese tem como principais objetivos o desenho e desenvolvimento de um processo que permita o fabrico de MLs plano-convexas de diferentes tamanhos com intervenção mínima nos parâmetros do processo. As MLs foram fabricadas utilizando um polímero fotosensível (PF), o AZ4562, através de fotolitografia e refluxo térmico. Outro objetivo foi o fabrico de MLs diretamente na superfície de um die de silício, que contém fotodíodos (FDs) em tecnologia complementary metal–oxide semiconductor (CMOS), para quantificar as diferenças na sua capacidade de gerar fotocorrente (FC). O processo de fabrico ótimo de MLs foi alcançado quando uma fotomáscara (FM) de crómio de super alta-resolução de 128k dots per inch (dpi) foi usada. Esta FM permite que o desenho-padrão seja transferido para o PF com elevada precisão. No entanto, para se obter o perfil de lente, é necessário aplicar um tratamento térmico à microestrutura fabricada. Quando o PF é submetido a uma temperatura mais alta do que a sua temperatura de transição vítrea, este amolece permitindo assim que a sua forma se altere. No caso das MLs, a principal força responsável para que essa mudança ocorra durante este processo térmico é a tensão superficial. As MLs fabricadas, foram estruturalmente caracterizadas usando um perfilómetro e imagens de scanning electron microscope (SEM). Para medir a distância focal (f), foi concebido um sistema de alinhamento opto-mecânico e verificou-se que existe uma pequena diferença de 4% entre o valor medido e o calculado. Foi conseguida ainda uma melhoria adicional com a introdução de uma fase de reidratação no processo de fabrico. A fase de prebake utilizada no fabrico serve para evaporar os solventes do PF mas, todavia, retira também todo o seu conteúdo de água. Por isso, foi demonstrado que o AZ4562 necessita de ser reidratado para se conseguir excelentes resultados prevenindo danos estruturais nas MLs que é fundamental para a obtenção de propriedades óticas eficientes. A maior vantagem neste novo processo otimizado é a melhoria conseguida nos processos de microfabricação standard estabelecidos, i.e., fotolitografia combinada com o refluxo térmico do PF. Em seguida, foram testadas três formas para integrar as MLs num substrato fotossensível. A primeira consistiu em utilizar uma camada intermédia de polidimetilssiloxano (PDMS) para controlar a espessura entre as MLs e o substrato fotodetetor e assim, permitir a utilização de diferentes f dependendo da aplicação. A segunda foi colocar f dentro da região de depleção do FD usando um substrato de vidro com 150 μm de espessura demonstrando que a geração de FC é aumentada para a área ativa. Por último, a terceira abordagem foi o desenvolvimento de um setup composto por um array de MLs fabricado diretamente sobre os FDs e duas soluções são apresentadas. Uma delas é o fabrico de uma ML num FD quadrado com 24 μm de lado. Este setup permite a captura de luz que não iria incidir na área ativa do FD resultando num aumento de geração de FC. O outro é o fabrico de um array de MLs usando a mesma FM, mas num FD quadrado com 240 μm de lado, para determinar o nível de geração de FC. Nestes testes, recorreu-se a duas fontes de luz (vermelha e branca) para avaliar a capacidade de aumentar a aquisição de luz. Relativamente à geração de FC, o melhor dos resultados obtidos nas várias soluções de integração propostas, foi conseguido com o fabrico direto de MLs nos FDs com aumentos superiores a 14% e 2% para as luzes vermelha e branca, respetivamente. A luz vermelha tem a penetração ideal no silício para atingir os resultados mais proeminentes no que concerne aos ganhos obtidos na geração de FC apresentado nesta tese. As MLs que foram desenhadas e fabricadas, bem como as soluções propostas de integração num substrato fotossensível, demonstram um potencial interesse de aplicação em processos industriais de fabrico standard para microsistemas óticos, desde aplicações de aumento de aquisição de luz, até sensores de imagens