Defect-based testing of LTS digital circuits

A Defect-Based Test (DBT) methodology for Superconductor Electronics (SCE) is presented in this thesis, so that commercial production and efficient testing of systems can be implemented in this technology in the future. In the first chapter, the features and prospects for SCE have been presented. The motivation for this research and the outline of the thesis were also described in Chapter 1. It has been shown that high-end applications such as Software-Defined Radio (SDR) and petaflop computers which are extremely difficult to implement in top-of-the-art semiconductor technologies can be realised using SCE. But, a systematic structural test methodology had yet to be developed for SCE and has been addressed in this thesis. A detailed introduction to Rapid Single-Flux Quantum (RSFQ) circuits was presented in Chapter 2. A Josephson Junction (JJ) was described with associated theory behind its operation. The JJ model used in the simulator used in this research work was also presented. RSFQ logic with logic protocols as well as the design and implementation of an example D-type flip-flop (DFF) was also introduced. Finally, advantages and disadvantages of RSFQ circuits have been discussed with focus on the latest developments in the field. Various techniques for testing RSFQ circuits were discussed in Chapter 3. A Process Defect Monitor (PDM) approach was presented for fabrication process analysis. The presented defect-monitor structures were used to gather measurement data, to find the probability of the occurrence of defects in the process which forms the first step for Inductive Fault Analysis (IFA). Results from measurements on these structures were used to create a database for defects. This information can be used as input for performing IFA. "Defect-sprinkling" over a fault-free circuit can be carried out according to the measured defect densities over various layers. After layout extraction and extensive fault simulation, the resulting information will indicate realistic faults. In addition, possible Design-for-Testability (DfT) schemes for monitoring Single-Flux Quantum (SFQ) pulses within an RSFQ circuit has also been discussed in Chapter 3. The requirement for a DfT scheme is inevitable for RSFQ circuits because of their very high frequency of operation and very low operating temperature. It was demonstrated how SFQ pulses can be monitored at an internal node of an SCE circuit, introducing observability using Test-Point Insertion (TPI). Various techniques were discussed for the introduction of DfT and to avoid the delay introduced by the DfT structure if it is required. The available features in the proposed design for customising the detector make it attractive for a detailed DBT of RSFQ circuits. The control of internal nodes has also been illustrated using TPI. The test structures that were designed and implemented to determine the occurrence of defects in the processes can also be used to locate the position for the insertion of the above mentioned DfT structures

Testability issues in superconductor electronics

An emerging technology for solutions in high-end applications in computing and telecommunication is superconductor electronics. A system-level study has been carried out to verify the feasibility of DfT in superconductor electronics. In this paper, we present how this can be realized to monitor so-called single-flux quantum pulses. As a part of our research, test structures have been developed to detect structural defects in this technology. We also show detailed test results of those structures. It proves that it is possible to detect possible random defects and provide defect statistics for the Niobium-based fabrication process

A superconducting bandpass delta-sigma modulator for direct analog-to-digital conversion of microwave radio

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2003.Includes bibliographical references (p. 291-305).This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Direct analog-to-digital conversion of multi-GHz radio frequency (RF) signals is the ultimate goal in software radio receiver design but remains a daunting challenge for any technology. This thesis examines the potential of superconducting technology for realizing RF analog-to-digital converters (ADCs) with improved performance. A bandpass delta-sigma (AE) modulator is an attractive architecture for digitizing narrowband signals with high linearity and a large signal-to-noise ratio (SNR). The design of a superconducting bandpass AE modulator presented here exploits several advantages of superconducting electronics: the high quality factor of resonators, the high sampling rates of comparators realized with Josephson junctions, natural quantization of voltage pulses, and high circuit sensitivity. Demonstration of a superconducting circuit operating at clock rates in the tens of GHz is often hindered by the difficulty of high speed interfacing with room-temperature test equipment. In this work, a test chip with integrated acquisition memory is used to simplify high speed testing in a cryogenic environment. The small size (256 bits) of the on-chip memory severely limits the frequency resolution of spectra based on standard fast Fourier transforms. Higher resolution spectra are obtained by "segmented correlation", a new method for testing ADCs. Two different techniques have been found for clocking the superconducting modulator at frequencies in the tens of GHz. In the first approach, an optical clocking technique was developed, in which picosecond laser pulses are delivered via optical fiber to an on-chip metal-semiconductor-metal (MSM) photodiode, whose output current pulses trigger the Josephson circuitry. In the second approach, the superconducting modulator is clocked by an on-chip Josephson oscillator.(cont.) These testing methods have been applied in the successful demonstration of a super-conducting bandpass AE modulator fabricated in a niobium integrated circuit process with 1 kA/cm2 critical current density for the Josephson junctions. At a 42.6 GHz sampling rate, the center frequency of the experimental modulator is 2.23 GHz, the measured SNR is 49 dB over a 20.8 MHz bandwidth, and a full-scale (FS) input is -17.4 dBm. At a 40.2 GHz sampling rate, the measured in-band noise is -57 dBFS over a 19.6 MHz bandwidth.by John Francis Bulzacchelli.Ph.D

Superconducting single photon detectors for quantum information processing

Single photon detectors are a vital part of many emerging technologies which harness the quantum properties of light to benefit the fields of communication, computation and sensing. Superconducting nanowire single photon detectors (SNSPDs) offer high detection efficiency, low dark count rates, low timing jitter, and infrared sensitivity that are required by the most demanding single photon counting applications. This thesis presents SNSPDs fabricated and tested at the University of Glasgow that are integrated with optical structures which enable enhanced detection efficiency and integration with waveguide circuit technology. The monolithic integration of waveguide circuit components presents a route towards realisation of an optical quantum information processor that has the stability and scalability to perform the demanding tasks of quantum computation. A novel process is introduced for incorporating superconducting detectors with single mode gallium arsenide waveguides and quantum dot single photon sources. Together these elements would enable the generation of quantum states of light which could be manipulated and detected on a single chip. Detectors are patterned in NbTiN thin superconducting films on to suspended nanobeam waveguides with better than 50 nm alignment accuracy. Low temperature electrical and optical testing confirms the detectors’ single photon sensitivity under direct illumination as well as to waveguide coupled light. Measured detectors were found to have internal registering efficiencies of 6.8 ± 2.4%. Enhancing absorption of photons into thin superconducting films is vital to the creation of high efficiency superconducting single photon detectors. Fabricating an SNSPD on a dielectric mirror creates a partial cavity that can be tailored to enhance detection of light at specific wavelengths. Devices have been fabricated and tested in this thesis with enhanced detection efficiency at infrared and visible wavelengths for quantum cryptography, remote sensing and life science applications. Detectors fabricated in NbTiN on GaAs/AlGaAs Bragg mirrors exhibited a system detection efficiency of 1.5% at 1500 nm wavelength for the best device measured. SNSPDs were also fabricated in NbN on aperiodic dielectric mirrors with a range of different bandwidths. A peak system detection efficiency of 82.7% at 808 nm wavelength was recorded

Microwave Superconductivity

We give a broad overview of the history of microwave superconductivity and explore the technological developments that have followed from the unique electrodynamic properties of superconductors. Their low loss properties enable resonators with high quality factors that can nevertheless handle extremely high current densities. This in turn enables superconducting particle accelerators, high-performance filters and analog electronics, including metamaterials, with extreme performance. The macroscopic quantum properties have enabled new generations of ultra-high-speed digital computing and extraordinarily sensitive detectors. The microscopic quantum properties have enabled large-scale quantum computers, which at their heart are essentially microwave-fueled quantum engines. We celebrate the rich history of microwave superconductivity and look to the promising future of this exciting branch of microwave technology.Comment: 18 page

The Quantum Socket: Three-Dimensional Wiring for Extensible Quantum Computing

Quantum computing architectures are on the verge of scalability, a key requirement for the implementation of a universal quantum computer. The next stage in this quest is the realization of quantum error correction codes, which will mitigate the impact of faulty quantum information on a quantum computer. Architectures with ten or more quantum bits (qubits) have been realized using trapped ions and superconducting circuits. While these implementations are potentially scalable, true scalability will require systems engineering to combine quantum and classical hardware. One technology demanding imminent efforts is the realization of a suitable wiring method for the control and measurement of a large number of qubits. In this work, we introduce an interconnect solution for solid-state qubits: The quantum socket. The quantum socket fully exploits the third dimension to connect classical electronics to qubits with higher density and better performance than two-dimensional methods based on wire bonding. The quantum socket is based on spring-mounted micro wires the three-dimensional wires that push directly on a micro-fabricated chip, making electrical contact. A small wire cross section (~1 mmm), nearly non-magnetic components, and functionality at low temperatures make the quantum socket ideal to operate solid-state qubits. The wires have a coaxial geometry and operate over a frequency range from DC to 8 GHz, with a contact resistance of ~150 mohm, an impedance mismatch of ~10 ohm, and minimal crosstalk. As a proof of principle, we fabricated and used a quantum socket to measure superconducting resonators at a temperature of ~10 mK.Comment: Main: 31 pages, 19 figs., 8 tables, 8 apps.; suppl.: 4 pages, 5 figs. (HiRes figs. and movies on request). Submitte

RF MEMS-Based Frequency Dependent Power Limiters

Microwave power limiters are passive devices that are widely used to protect receivers from large interfering signals. Conventional radio frequency (RF) power limiters attenuate both desired and undesired signals in the entire frequency band. The need for frequency-dependent power limiters (FDPLs) arises in cases where the system dynamic range is frequency-dependent and the threshold level is dependent on the frequency band. RF micro-electro-mechanical systems (RF MEMS) switches self-actuate under high RF power and are well-known for their linearity and very low insertion loss. A FDPL can benefit from all the advantages of RF MEMS switches, particularly the linearity. In this study, a novel approach to FDPLs is proposed. RF MEMS switches are integrated with bandpass filters to form a power limiter where the output RF power is limited to specific levels that can vary with the frequency band. Both non-planar and planar versions of FDPLs are presented using circulators and hybrids, and RF MEMS-based power limiters are analyzed theoretically and experimentally for one frequency band. The limiter attenuates the high power signal only within the bandwidth of the integrated filter. The design of the proposed power limiter is expanded to achieve power limiting for various frequency bands. The flatness of the threshold level can be set to the desired value by controlling the return loss of the filters used in the FDPL circuit. Measured results for an FDPL circuit are presented, demonstrating that the limiting power level can be controlled by adjusting the dc bias of the MEMS switches. The commercially available electrostatically actuated switches OMRON and Radant are employed for the realization of the FDPL. Additionally, tunable FDPL circuits are fabricated and measured, demonstrating the feasibility of realizing adaptive frequency-dependent power limiters. In order to improve the controllability of the proposed MEMS-based FDPL circuits, both electrostatically-actuated and thermally-actuated switches are investigated theoretically and experimentally for use in power limiter applications. It is concluded that MEMS thermally- actuated switches provide better controllability of the self-actuation RF power level through adjustment of the bias voltage to the thermal actuator. Additionally, a novel concept for realizing an RF power limiter for protecting superconductor digital receivers is proposed. A lumped-element niobium-based filter is used as a protection circuit. It consists of lumped-element resonators formed using spiral inductors and metal- insulator-metal capacitors integrated on a multilayer niobium process. The circuit operates as a bandpass filter at low power levels, allowing RF signals to pass, and as a reflector at high power levels. The lumped-element filter circuit is studied in detail to explain the performance of the filter at high power levels. It is concluded that some of the lumped-element inductors switch from being inductors when operating at low power levels to being capacitors when operating at high power levels. When the lumped-element inductors switch to capacitors, the filter circuit that consists of L-C resonators switches to a circuit that consists of capacitors, causing the input power to be reflected back. Both theoretical and experimental results are presented to verify this phenomenon. In addition to applications in power limiters, the concept can be employed to realize transmit/receive (T/R) switches in order to isolate the transmit circuit from the receive circuit

Fluxon readout for superconducting flux qubits

This thesis presents experimental studies on developing of a novel fluxon readout for superconducting flux qubits. It is based on a scheme for detecting the microwave radiation of an oscillating fluxon in an annular Josephson junction (AJJ). The readout was implemented for a superconducting flux qubit coupled as a current dipole to the AJJ. An energy spectrum of the flux qubit was measured via detecting a frequency shift of the fluxon oscillations versus a flux bias through the qubit

Etude d'un convertisseur analogique-numérique <br />à très grande dynamique à base de portes logiques supraconductrices

RSFQ (Rapid Single Flux Quantum) superconducting logic is suitable to process very high speed digital data with very low power dissipation and with performances well beyond what should be possible with CMOS technology in the next decades. The RSFQ circuit technology, based on superconducting niobium nitride (NbN), presently developed at CEA-G, involves NbN/Ta_{X}N/NbN internally shunted Josephson junctions with high critical current density and high maximum switching frequency close to 1 THz at 10 K, as required by ultra-fast RSFQ electronics. The purpose of this work is to apply the NbN technology to an ADC (Analog-to-Digital Converter) for space telecommunications. An ADC with sigma-delta architecture was studied oversamplingat 200 GHz clock frequency a signal with a bandwidth of 500 MHz modulated over a carrier frequency of 30 GHz. In particular a clock, a comparator and some other logic gates were studied at 200 GHz as well as a third order sigma-delta band-pass modulator whose the SNR and SFDR performances, after optimization, should satisfy the specifications. Decimation filter architecture complexity was analyzed. Some basic components of the filter, such frequency dividers and shift registers, were studied and designed, and in the end some possible methods to test the modulator were also proposed. The implementation of NbN circuits in a multi-levels technology was treated including the complete fabrication of two wafers. For some technology problems clarified afterwards, these two lots couldn't reach the ADC logic gate test. However the RSFQ gate margins were determined thanks to junctions, SQUIDs and microwaves filters (resonators) characterization. Finally, we compare some circuits based on self-shunted NbN junctions and other similar circuits in Nb foundry technology using Nb/AlO_{X}/Nb externally shunted junctions operating at 4 K. This comparative study shows all the advantages given by the NbN technology.La logique supraconductrice RSFQ (Rapide Single Flux Quantum) est une solution très attractive pour le traitement des données à très haute fréquence avec une dissipation très faible et des performances nettement supérieures à ce que la technologie CMOS pourra offrir dans la prochaine décennie. La technologie RSFQ en nitrure de niobium (NbN) en cours de développement au CEA-G est basée sur des jonctions Josephson NbN/Ta_{X}N/NbN auto-shuntées qui présentent une fréquence d'oscillation maximum proche du THz jusqu'à 10 K. L'objectif de cette recherche a été d'appliquer cette technologie NbN 9K à un CAN (Convertisseur Analogique-Numérique) adaptable aux télécommunications spatiales. Une architecture de type CAN sigma-delta a été étudiée, sur-échantillonnant à 200 GHz de fréquence d'horloge un signal avec une bande de 500 MHz et modulé sur une porteuse de 30 GHz. En particulier une horloge, un comparateur et différents portes logiques ont été étudiés et conçus pour opérer à 200 GHz ainsi qu'un modulateur sigma-delta passe-bande du troisième ordre dont les performances SNR, SFDR, devraient après optimisation satisfaire les objectifs visés. La complexité de l'architecture du filtre de décimation a été analysée. Certains composants de base du filtre, des diviseurs de fréquence et des registres à décalage, ont été étudiés et dessinés, enfin quelques méthodes de test du modulateur sont proposées. Le travail d'implémentation de circuits NbN en technologie multi-niveaux a été traité conduisant à la réalisation complète de deux lots de circuits qui pour des raisons technologiques clarifiées ensuite n'ont pu aboutir au test des portes logique du CAN. Cependant, les marges de fonctionnement des portes logiques NbN ont été déterminées grâce à la caractérisation de jonctions, SQUIDs et de filtres (résonateurs) micro-ondes. Finalement, une étude comparative entre des circuits à jonctions NbN auto-shuntées opérant à 9K en réfrigération allégée et des circuits similaires obtenus en fonderie Nb basés sur des jonctions Nb/AlO_{X}/Nb shuntées en externe opérant à 4K, démontre tous les avantages qu'on peut espérer attendre de la technologie NbN