58 research outputs found
Single flux quantum circuits with damping based on dissipative transmission lines
We propose and demonstrate the functioning of a special Rapid Single Flux
Quantum (RSFQ) circuit with frequency-dependent damping. This damping is
achieved by shunting individual Josephson junctions by pieces of open-ended RC
transmission lines. Our circuit includes a toggle flip-flop cell, Josephson
transmission lines transferring single flux quantum pulses to and from this
cell, as well as DC/SFQ and SFQ/DC converters. Due to the desired
frequency-dispersion in the RC line shunts which ensures sufficiently low noise
at low frequencies, such circuits are well-suited for integrating with the
flux/phase Josephson qubit and enable its efficient control.Comment: 6 pages incl. 6 figure
High Density Fabrication Process for Single Flux Quantum Circuits
We implemented, optimized and fully tested over multiple runs a
superconducting Josephson junction fabrication process tailored for the
integrated digital circuits that are used for control and readout of
superconducting qubits operating at millikelvin temperatures. This process was
optimized for highly energy efficient single flux quantum (ERSFQ) circuits with
the critical currents reduced by factor of ~10 as compared to those operated at
4.2 K. Specifically, it implemented Josephson junctions with 10 uA unit
critical current fabricated with a 10 uA/um2 critical current density. In order
to circumvent the substantial size increase of the SFQ circuit inductors, we
employed a NbN high kinetic inductance layer (HKIL) with a 8.5 pH/sq sheet
inductance. Similarly, to maintain the small size of junction resistive shunts,
we used a non-superconducting PdAu alloy with a 4.0 ohm/sq sheet resistance.
For integration with quantum circuits in a multi-chip module, 5 and 10 um
height bump processes were also optimized. To keep the fabrication process in
check, we developed and thoroughly tested a comprehensive Process Control
Monitor chip set.Comment: 10 pages, 5 figures, 1 tabl
Advanced Fabrication Processes for Superconducting Very Large Scale Integrated Circuits
We review the salient features of two advanced nodes of an 8-Nb-layer fully
planarized process developed recently at MIT Lincoln Laboratory for fabricating
Single Flux Quantum(SFQ) digital circuits with very large scale integration on
200-mm wafers: the SFQ4ee and SFQ5ee nodes, where 'ee' denotes the process is
tuned for energy efficient SFQ circuits. The former has eight superconducting
layers with 0.5 {\mu}m minimum feature size and a 2 {\Omega}/sq Mo layer for
circuit resistors. The latter has nine superconducting layers: eight Nb wiring
layers with the minimum feature size of 350 nm and a thin superconducting MoNx
layer (Tc ~ 7.5 K) with high kinetic inductance (about 8 pH/sq) for forming
compact inductors. A nonsuperconducting (Tc < 2 K) MoNx layer with lower
nitrogen content is used for 6 {\Omega}/sq planar resistors for shunting and
biasing of Josephson junctions. Another resistive layer is added to form
interlayer, sandwich-type resistors of m{\Omega} range for releasing unwanted
flux quanta from superconducting loops of logic cells. Both process nodes use
Au/Pt/Ti contact metallization for chip packaging. The technology utilizes one
layer of Nb/AlOx-Al/Nb JJs with critical current density, Jc of 100
{\mu}A/{\mu}m^2 and minimum diameter of 700 nm. Circuit patterns are defined by
248-nm photolithography and high density plasma etching. All circuit layers are
fully planarized using chemical mechanical planarization (CMP) of SiO2
interlayer dielectric. The following results and topics are presented and
discussed: the effect of surface topography under the JJs on the their
properties and repeatability, critical current and Jc targeting, effect of
hydrogen dissolved in Nb, MoNx properties for the resistor layer and for high
kinetic inductance layer, technology of m{\Omega}-range resistors.Comment: 10 pages, 12 figures, 1 table, 27 references. The paper was presented
on September 8, 2015 at the 12th European Conference on Applied
Superconductivity, EUCAS 2015, 6-10 September 2015, Lyon, France, IEEE
Transaction on Applied Superconductivity, 201
Beyond Moore's technologies: operation principles of a superconductor alternative
The predictions of Moore's law are considered by experts to be valid until
2020 giving rise to "post-Moore's" technologies afterwards. Energy efficiency
is one of the major challenges in high-performance computing that should be
answered. Superconductor digital technology is a promising post-Moore's
alternative for the development of supercomputers. In this paper, we consider
operation principles of an energy-efficient superconductor logic and memory
circuits with a short retrospective review of their evolution. We analyze their
shortcomings in respect to computer circuits design. Possible ways of further
research are outlined.Comment: OPEN ACCES
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