Superconducting RSFQ Logic: Towards 100GHz Digital Electronics Vratislav MICHAL, Emanuele BAGGETTA, Mario AURINO, Sophie BOUAT, Jean-Claude VILLEGIER IINAC, UMR-E CEA /UJF, CEA-Grenoble 38054, Grenoble, France vratislav.michal@stericsson.com Radioelektronika 2011

Outline Superconductivity: introduction Josephson effect, SQUID RSFQ Applications Fabrication 2/35 Outline Superconductivity: introduction Josephson effect, SQUID Superconducting logic RSFQ Application, examples Fabrication, process

Motivation: Superconductivity

Img: H. K. Onnes, Commun. Phys. Lab.12,120, (1911) Superconductivity Josephson effect RSFQ Applications Fabrication 4/35 Brief history 1908: liquefaction of 4He 1911: Superconductivity in mercury 1925: Prediction of Bose-Einstein condensation 1927: Superfluidity 4He 1933: Meissner effect 1950: Ginzburk-Landau theory 1957: BCS theory 196: Josepshon effect, SQUID 1986: HTC 1985: RSFQ Img: H. K. Onnes, Commun. Phys. Lab.12,120, (1911)

Superconductivity Josephson effect RSFQ Applications Fabrication 5/35 Superconductivity Superconductivity is a fundamental macroscopic state, occurring at the transition temperature TC (phase transition) New phenomena occurs in the superconducting core. Amongst most important: Resistivity drop to zero Magnetic field screening: Meissner effect (magnetic induction has to be a constant in time, i.e. dB/dt = 0 Magnetic flux quantization

Curent conduction: Cooper pair Superconductivity Josephson effect RSFQ Applications Fabrication 6/35 Curent conduction: Cooper pair Below TC, the electrons condense into pairs called Copper pairs. the crystal lattice is deformed by electrons, causing local positive polarization, attracting another electron thorough an exchange of the virtual phonon, with the crystal lattice (atoms) The bounded electrons (Cooper pairs) travel in the crystalline lattice without energy loss and keeps the phase coherence.

Macroscopic Coherence Superconductivity Josephson effect RSFQ Applications Fabrication 7/35 Macroscopic Coherence The Cooper pairs are the particles with integer spin (boson), condensating in the single ground state, close to Fermi surface (do not obey the Pauli exclusion principle). Due to the long distance coherence length ξ, wave function Ψi of Copper pairs overlap, and all electron gas can be described by the single wave function: The phase coherence is maintained by the gap energy, exceeding the electrons coulomb repulsive force.

Magnetic flux quantization Superconductivity Josephson effect RSFQ Applications Fabrication 8/35 Magnetic flux quantization The long-distance phase coherence results in 2πn phase drop around an closes superconducting loop: This make appear the shielding supercurrents, quantizating the flux inside the loop: The value of Ф0 is the magnetic flux quanta: nФ0

RF property of the superconductor Superconductivity Josephson effect RSFQ Applications Fabrication 9/35 RF property of the superconductor

Josephson effect, SQUID II. Josephson effect, SQUID

Superconductivity Josephson effect RSFQ Applications Fabrication 11/35 Josephson junction Two isolated superconductors keeps the phase coherence across a normal-state (non-superconducting) barrier. Effect of the Cooper-pairs tunneling is referred to as Josephson effect Wave function of the electrodes: Josephson phase: The current/voltage and the Josephson phase are related by the Josephson equations:

Josephson junction: IV characteristics Superconductivity Josephson effect RSFQ Applications Fabrication 12/35 Josephson junction: IV characteristics Barrier type (or external shunt resistance): the I/V characteristic can be hysteretic, or single-valued. The JJ with a DC voltage below the superconducting gap behave as the oscillator with frequency f = 483 597.9GHz/V Demonstration: Shapiro steps

SQUID: Superconducting Quantum Interference Device Superconductivity Josephson effect RSFQ Applications Fabrication 13/35 SQUID: Superconducting Quantum Interference Device B = 0: IJ1=IJ2 B > 0: IJ1<IJ2

Application: magnetometer Superconductivity Josephson effect RSFQ Applications Fabrication 14/35 Application: magnetometer Most-sensitive magnetic detector: Linearization by magnetic feedback COMPARISON: Threshold for SQUID: 1 fT Magnetic field of heart: 50,000 fT Magnetic field of brain: a few fT Img: Gross, R. Applied Superconductivity (lectures)

Superconducting logic RSFQ III. Superconducting logic RSFQ

Electrical model of Josephson junction Superconductivity Josephson effect RSFQ Applications Fabrication 16/35 Electrical model of Josephson junction 2nd Josephson relation: Josephson junction behaves as nonlinear RLC circuit: MODEL RCSJ

Junction Dynamics Mc Cumber parameter: Switching time optimum: Superconductivity Josephson effect RSFQ Applications Fabrication 17/35 Junction Dynamics Plasma period of LC circuit Characteristic time of RL circuit: Nb: 0.15ps, NbN: 0.07ps Characteristic time of RC circuit Mc Cumber parameter: Switching time optimum: (modified by the shunt resistance) Another parameters are important to optimize the JJ’s circuits: product RNIc superconducting gap 2Δ/e Current density JC (~kA/cm²) Critical temperature

RSFQ logic physics 2.07 µV/GHz Quantum Flux Superconductivity Josephson effect RSFQ Applications Fabrication 20/35 RSFQ logic physics Damped JJ Current Voltage SFQ pulses generation: over dumped Josephson junction 2.07 µV/GHz Quantum Flux

Likharev approach: SFQ pulses Superconductivity Josephson effect RSFQ Applications Fabrication 18/35 Likharev approach: SFQ pulses Main idea: the logical bits: presence or absence of the SFQ pulse IJ < IC : Junction in superconducting state IJ > I0 : phase increase linearly. IJ > I0 : (short time): SFQ pulse generation:

SFQ pulse flow: JTL and SQUID Superconductivity Josephson effect RSFQ Applications Fabrication 19/35 SFQ pulse flow: JTL and SQUID SFQ pulses flow is mediated by L, and JJ, biased close to Ic Two cases: Meta-stable JTL Memory cell: SQUID

Example: D-type flip-flop Superconductivity Josephson effect RSFQ Applications Fabrication 21/35 Example: D-type flip-flop Operating phases: An SFQ pulse arrive to D: penetrate into the SQUID loop Circulating current occurs, shifting IcJ1 and IcJ2 values SFQ pulse at CLK commutate J2 liberating the SFQ pulse  Circuit transmitted logical 1  the overall time consumed by operation is one clock cycle

Img: pavel.physics.sunysb.edu Superconductivity Josephson effect RSFQ Applications Fabrication 22/35 Basic RSFQ cells: Josephson Transmission Line (JTL): allowing the transfer of the SFQ pulses over long distances. In some circumstances, the JTL allows to shape and amplify the SFQ pulses. Asynchronous components: e.g. merger or splitter, allow merging/reproducing the SFQ pulses. Logical gates: such as the OR, AND etc. Flip-flop: Logical block such as the previously presented D-type flip-flop, having two stable states (memory cells). Special purpose circuit: as the mentioned SFQ/DC or DC/SFQ converters, allowing an easy to handle output for laboratory purposes. « OR » « D » Img: pavel.physics.sunysb.edu

Gross, R. Applied Superconductivity (lectures) Josephson effect RSFQ Applications Fabrication 23/35 Performances Operating frequency : Tens of GHz, 770GHz demonstrated Gate delay ~ ps Power consumption 10-18 J per bit. Operating voltage ~3mV DC bias current Ic × Junction count (Amperes) Operating temperature 4.2K (Nb), 9k (NbN) Process ~1µm² Gross, R. Applied Superconductivity (lectures)

Applications: examples IV. Applications: examples

Applications: state of the art Superconductivity Josephson effect RSFQ Applications Fabrication 25/35 Applications: state of the art

Demonstration 770GHz [*] Toggle- Flip-Flop: Superconductivity Josephson effect RSFQ Applications Fabrication 26/35 Demonstration 770GHz [*] Toggle- Flip-Flop: Relate the input and output voltage throughout the Josephson relation. Input part: DC/SFQ converter, output: low-pass filter. IC = 0.5 and 2.5 mA/µm², TC = 1.8K and 4.2K Simple JJ Nb/AlOx/Nb, 2Δ/e = 2mV (fc = 950GHz) [*] W. Chen et al. IEEE TAS 1999

FLUX-1 Microprocessor Chip Superconductivity Josephson effect RSFQ Applications Fabrication 27/35 FLUX-1 Microprocessor Chip • 8-bit microprocessor design • 1-cm chip • 8 - 20 Gb/s TRX • FLUX-1 chip redesigned, fabricated, partially tested • 1.75 μm, 4 kA/cm2 junction Nb technology • 20 GHz internal clock • 5 GByte/sec inter-chip data transferlimited by μP architecture • 63 K junctions, 5 Kgate equivalent • Power dissipation ~ 9 mW @ 4.5K • 40 GOPS peak computational capability (8-bits @ 20-GHz clock) • Fabricated in TRW 4 kA/cm2 processin 2002 RCL Dorojevets, M. IEEE TAS Vol. 13 2003

ΔΣ ADC converter Superconductivity Josephson effect RSFQ Applicationns Fabrication 28/35 ΔΣ ADC converter

V. Fabrication

Superconductor IC: Simpler Than CMOS Superconductivity Josephson effect RSFQ Applications Fabrication 30/35 Superconductor IC: Simpler Than CMOS

Objective: Self-shunted JJ Superconductivity Josephson effect RSFQ Applications Fabrication 30/35 Objective: Self-shunted JJ Img: IPHT Jena - resistivelly shunted JJ

Circuit realization: CEA Superconductivity Josephson effect RSFQ Applications Fabrication 32/35 Circuit realization: CEA Elaboration of texture controlled NbN SNS junction on Si-200mm wafers (350 ºC): compatible with C-MOS platform

NbTiN Ground plane, NbN/TaN/NbN JJ Superconductivity Josephson effect RSFQ Applications Fabrication 33/35 NbTiN Ground plane, NbN/TaN/NbN JJ

E. Baggetta, PhD Thesis CEA Grenoble (2008) Superconductivity Josephson effect RSFQ Applications Fabrication 34/35 Realization example Frequency divider: more compact on NbN E. Baggetta, PhD Thesis CEA Grenoble (2008)

Villegier J.C. et al. IEEE TAS (to be published) Superconductivity Josephson effect RSFQ Applications Fabrication 35/35 Fabrication: Outcome NbN-TaN-NbN trilayer crossection: fabrication using UV-248 stepper I-V-T characteristics 4×4μm, NbN-TaN-NbN, Jc = 4,3kA/cm2 Villegier J.C. et al. IEEE TAS (to be published)