1. 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
Radioelektronika 2011

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

3. Motivation: Superconductivity

4. Img: H. K. Onnes, Commun. Phys. Lab.12,120, (1911)
Superconductivity
Josephson effect
RSFQ
Applications
Fabrication
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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)

5. 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

6. Curent conduction: Cooper pair
Superconductivity
Josephson effect
RSFQ
Applications
Fabrication
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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.

7. 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.

8. 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

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

10. Josephson effect, SQUID
II.
Josephson effect, SQUID

11. 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:

12. Josephson junction: IV characteristics
Superconductivity
Josephson effect
RSFQ
Applications
Fabrication
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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

13. 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

14. 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)

15. Superconducting logic RSFQ
III.
Superconducting logic RSFQ

16. 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

17. 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

18. 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

19. 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:

20. 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

21. 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

22. 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

23. 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)

24. Applications: examples
IV.
Applications: examples

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

26. Demonstration 770GHz [*] Toggle- Flip-Flop:
Superconductivity
Josephson effect
RSFQ
Applications
Fabrication
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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

27. FLUX-1 Microprocessor Chip
Superconductivity
Josephson effect
RSFQ
Applications
Fabrication
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FLUX-1 Microprocessor Chip
• 8-bit microprocessor design
• 1-cm chip
• 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 4.5K • 40 GOPS peak computational
capability 20-GHz clock)
• Fabricated in TRW 4 kA/cm2 processin 2002
RCL
Dorojevets, M. IEEE TAS Vol

28. ΔΣ ADC converter Superconductivity Josephson effect RSFQ Applicationns
Fabrication
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ΔΣ ADC converter

29. V.
Fabrication

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

31. Objective: Self-shunted JJ
Superconductivity
Josephson effect
RSFQ
Applications
Fabrication
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Objective: Self-shunted JJ
Img: IPHT Jena - resistivelly shunted JJ

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

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

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

35. 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)