Supercond. Sci. Technol. 3 (1990) 325-337. Printed in the UK
REVIEW AR"rlCLE
Progress and prospects of superconductor
electronics·
Konstantln K Llkharev
Department of Physics, Moscow State University, Moscow 119899 GSP, USSR
Received 2 February 1990
Abstract. A brief review of the recent progress of superconductor electronics is
presented. Special emphasis is made on the 'intrinsic' development of the field,
including rapid progress of the low-Tc Josephson junction technology and recent
invention of several important devices. In the near future, these developments
promise the advent of several outstanding electronic devices which would be of
practical value, despite the need for helium cooling. Against this background,
introduction of the high-Tc superconductors in the foreseeable future will probably
be restricted to the few simplest electronic devices and components.
Introduction
Recent years have seen a substantial progress in superconductor electronics; an even more spectacular
advance can be expected in the near future. The reason
for this rapid development cannot solely be attributed
to the advent of the high- superconductivity, but also
results from an intrinsic progress in traditional (lowsuperconductor electronics.
That is why, in contrast to recent reviews [1-7], this
review will present a brief glimpse of achievements and
trends of the low- electronics (part I) first. Only then
(part II) will I list major peculiarities of high- superconductors, and try to indicate areas where these new
materials have a chance to compete with traditional
ones.
Because of the limitations of length, much of the literature (especially that concerning the basic physics,
fabrication technologies and particular applications of
the superconductor electronic devices) will not be discussed in detail. I have tried to compensate for this
drawback by making numerous references to relevant
papers.
r.
r.)
r.
r.
PART I. TRADITIONAL SUPERCONDUCTOR
ELECTRONICS: RECENT DEVELOPMENTS AND
FUTURE PROSPECTS
1. The simplest devices and components
1.1. Superconductor resonators and transmission lines
A sharp drop in the surface resistance Rs = Re Z.(w) of
a conductor below its superconducting transition point
can be used to decrease the attenuation in RF tank cir* The paper is based on the invited talk presented at the International
Superconductivity Electronics Conference (lSEC 89 (Tokyo) 20-21
June 1989).
0953-2048/90/070325
+ 13 $03.50 ©
1990 lOP Publishing Ltd
cuits, microwave caVItIes and transmission lines. For
example a classical lowsuperconductor, such as
niobium, allows one to reduce R. to just few nn at
J( =w/2n) '" 1 GHz. This figure compares very favourably with the value R. '" 10- 2 n for the best normal
metals like copper at the same frequency (but at room
temperature). As a result, unique electronic components
such as microwave band resonators with quality factors
in excess of 1011, and microstrip lines capable of carrying picosecond pulses over distances of several centimetres are feasible.
The high cost of helium refrigeration has, however,
prevented a wide use of such components in practice.
Notable exceptions are RF cavities in electron accelerators [8, 9], several unique set-ups in experimental
physics [10-12] and preliminary experiments on analogue microwave processing [13-15] and optoelectronic
sampling [16-17].
r.
1.2. Switches
By switching a superconducting component of a transmission line to its normal state and back, one can effectively control line attenuation and hence the amplitude
of the signal at its output [18]. Switching can be provided by several means, including electric current, laser
illumination, magnetic field and even by the signal itself
(in the latter case the system can work as either a power
limiter or noise discriminator) [18, 19].
The maximum switching speed is limited by the
superconductor gap relaxation time t,\ which is of the
order of 10 - lOS for most practical low- superconductors. Such speed is quite sufficient for most applications.
r.
1.3. Bolometers
Even a slight heating of a superconducting thin film by
incoming radiation leads to a noticeable change of its
325
K K Likharev
resistance R, provided that the operation temperature (T) is close to the critical one (T.:), where the
derivative dRldT is large. This effect is used in superconducting bolometers which can be very sensitive
receivers of infrared radiation (with the noise equivalent
power (NEP) of the order of 10 - 1 S W Hz - 1/2 for T -- 1 K
and the time constant of the order of one second [20]).
This sensitivity is, nevertheless, somewhat worse than
that of the composite semiconductor bolometers [21J,
One possible way. to increase the sensitivity is to
register changes of the kinetic inductance Lk of a superconducting thin-film strip rather than its DC resistance
R [22, 23]. The inductance can be a noticeable function
of temperature even well below T.:, where the film
impedance is almost purely reactive and hence the
Johnson-Nyquist noise is negligibly low. Experiments
with this mode of operation, so far reported [24], are
considered by their authors as 'satisfying for a first try',
but apparently more work is necessary to find out
whether the new type of the signal pick-up can yield
decisive advantages.
DC
2.
SQUIDS
Virtually all other devices of the superconductor electronics make use of either Josephson or quasiparticle
tunnel junctions (see, e.g. [25-27]) as their active elements (the superconductor field-effect transistors are
not yet mature enough for any practical application).
The simplest (and most successful) of the devices are the
celebrated superconducting quantum interference
devices (SQUIDS), which are essentially high-sensitive
low-frequency detectors of the magnetic flux tIl x . Excellent reviews of the SQUID operation and applications are
available (see, e.g. [28-30]), so that we will only
mention some recent trends in their development.
2.1.
RF SQUIDs
RF SQUIDS using single-josephson-junction superconducting quantum interferometers are simpler than their DC
counterparts but have larger noise (a typical energy
resolution is of the order of 10 - 29 J Hz - 1 at signal frequencies above "'" 1 Hz). Nevertheless, they are still produced commercially in both bulk and thin-film versions,
because even this sensitivity is quite sufficient for most
applications in areas such as geophysics [31] and
metrology [32].
with the energy sensitivity of the order of
10 - 30 J Hz - 1 are commercially available.
Nevertheless, some research work is still going on; it
includes search for better junctions with lower 11f noise
[34, 35], optimisation of the interferometer [36, 37],
improvements of the input circuits providing better
SQUID coupling to various signal sources [38, 39], and
design of multichannel SQUID systems [38, 40] which are
especially valuable for biomagnetic applications.
SQUIDS
2.3. Alternative SQUIDs
Several non-traditional types of SQUIDS have been proposed during the last two decades; most of them are not
considered at present competitive. A notable exception
are the relaxation-oscillation (RO)-driven SQUIDS [42-48]
which make use of interferometers with two unshunted
tunnel junctions. A low-ohmic supply circuit (figure I(a))
provides DC biasing of the interferometer to the
unstable branch of its DC J- V curve, so that relaxation
oscillations with frequencies of the order of RN/Ls
(practically, from _10 6 to '" 109 Hz) arise in the circuit.
The amplitude and frequency of the oscillations, as weU
as the DC voltage across the interferometer, are functions of Jc (and hence of tIl,,) and can be used as output
signals.
With the DC voltage pick-up, the flux-to-voltage
transfer coefficients up to 1 mV tIl.;- 1, and energy sensitivities of the order of 3 x 10 - 31 J Hz - 1 have been
demonstrated in the Ro-driven SQUIDS [45-47]. The
former figure is close to, and the latter is only one order
of magnitude worse than, those for the state-of-the-art
conventional DC SQUIDS. Both parameters can be presumably improved further, using the balanced version
of the device (figure l(b) [47,48].
Another alternative SQUID [49] also makes use of the
RF-driven interferometer with unshunted tunnel junctions, but here the RF drive source is external, and the
output signal is picked up in the form of the pulses corresponding to the interferometer switching between its
stable states. Such a pick-up allows one to arrange a
digital processing of the output signal and develop a
negative DC feedback signal on the same chip, thus
2.2. DC SQUIDs
The DC SQUID needs a sensor (interferometer) with two
nearly identical overdamped Josephson junctions: this
is the reason that thin-film sensors with reproducible
externally shunted tunnel junctions are most common.
The basic structure of these devices was accepted quite
a few years ago, and several versions of the low- T.: DC
326
Lx
L
(bl
Figure 1. Equivalent circuit$ of ,(a) the simpiest and (b)
balanced relaxation-osci Ilation-driven
SQUIDs.
Progress and prospects of superconductor electroniCS
avoiding communication with the room-temperature
electronics. An evident drawback of the first version of
the device is the low frequency « 106 Hz) of the RF
drive, resulting in a relatively low sensitivity. The drawback can presumably be circumvented in several ways,
including a use of an internal RF drive generator and
superfast RSFQ logic circuits (see below) instead of the
latching logic gates employed in the first version.
3. Other analogue devices
3.1. Samplers
The Josephson junction sampler [50-54] is a unique
device for measuring short waveforms. Along with a few
picosecond time resolution, it can provide practical sensitivities in the microvolt range, i.e. several orders of
magnitude higher than those of the best semiconductor
samplers. These features have justified a commercial
fabrication of instruments (in particular, reflectometers
[52, 54]) based on the samplers, despite requiring
liquid-helium cooling (note also an original helium-jet
cooling scheme developed for these devices).
Further possible improvements of these samplers
include a use of a balanced version of their heart
element, the current comparator [55, 56], which promises an even higher sensitivity.
3.2. Oscillators and receivers
Microwave and millimetre-wave superconductor
devices include Josephson-frequency oscillators and a
large variety of receivers: amplifiers, mixers and quadratic videodetectors. Most of these receivers can use either
Cooper pair or quasiparticle tunnelling, and operate in
various modes (see, e.g. chapters 12-13 of [27]).
Not all devices have proved to be of practical use.
For example, extremely low-noise microwave parametric amplifiers [57-61] are only suitable for a few
esoteric laboratory experiments at present. For radio
astronomy, the quasiparticle (SIS and SIN) mixers with
external local oscillators have been found most attractive. Millimetre-wave mixers of this kind keep absolute
records of sensitivity (providing noise temperatures as
low as few tens K in the middle of the band) and are
used for a current research in several radiotelescopes
(see, e.g. the recent reviews [62-63]). Major current
trends in the further development of the mixers are
improvements of their coupling to the incoming radiation [64-68], and use of special multijunction arrays
[66] (simple series arrays have not proved to be
effective).
Another considerable improvement of the mixer
would be to supplement it with an on-chip local oscillator with a wide-band electronic tuning of its frequency
and low-noise-wing spectral line. Single-junction
Josephson oscillators [69-71] can only partly meet
these requirements; in particular, their line is too broad,
and high-frequency isolation from the mixer junction
cannot be made sufficient because of a small nominal
power of the oscillator. Both drawbacks can be presumably avoided by using coherent multijunction arrays
whose dynamic has now been well understood [72-74].
A recent attempt to create a standing-wave oscillator of
this type (figure 2) has given quite encouraging results:
over 1 f.l.V power in an on-chip load has been registered
at a number of discrete frequencies over a band from
340 to 440 GHz [75]. The use of special travelling-wave
structures [72] can presumably improve the continuous
tuning ability of such oscillators radically.
Similar coherent arrays can also revive [75] some
other types of the receivers having an extremely low
noise potential, including Josephson-junction selfoscillator mixers and self-selecting videodetectors [76,
77] which are currently used only in some specific
laboratory experiments (see, e.g. [78]).
3.3, DC voltage standards and multipliers
The multijunction arrays have also become the core of
the recent breakthrough in the field of the Josephson
junction voltage standards. In such arrays, several thousands of nearly identical junctions are connected in
series for DC (for external microwaves, a more complex
connection is used in order to ensure equal Ac-drive
voltages across all junctions-see figure 3). As a result,
DC bias of each junction on a Josephson-Shapiro step
even without a very high number n (-4-6) can provide
a stable DC voltage in the range 1-10 V across the array
[79-82]. Such a voltage can be directly used for comparison with those across most room-temperature
sources, thus avoiding the need for complex voltage
comparators, which limited the accuracy of the former
Quarter -wove
stub ---...
Detector
Junction
Load resistor
biat I,od
Figure 2. Scheme of the distributed standing-wave
multijunction oscillator used for generation of up to 1 IN
power at submillimeter wavelengths [73] (courtesy by J
Lukens. SUNY).
327
K K Ukharev
Figure 3. Photograph of a :20000 junction array used In
the 10 V range DC voltage slandard (84) (courtesy by
J Niemeyer, PTe).
or
single-junction standards. Relative internal instability
the multijunction standards does not exceed a few parts
in 1O- 17(!) [&3]. although thennal EMF changes and
other external effect~ limit the present.day accuracy of
the room-temperature voltage calibration to the level of
a few parts in 10- 10 [81]. Even this figure is a big step
forward compared with the accuracy - 3 )( 10 - 8 of the
single Josephson junction standards., and there is 8 little
doubt that further improvements in the accuracy are
feasible.
The most noticeable drawback of the existing DC
voltage standards is their low output current range
(typically, a few ten ,tIA) making them rather vulnerable
to interferences and load drifts. In order 10 improve this
parameter, an alternative approach to the problem has
been suggested [85, 86]. It is based of the phase locking
of an overdamped Josephson junction which is occurrent-biased slightly below J< ' In this mode, a 27r-lcap
of the Josephson phase across a junction, i.e. a 'single
flux quantum' (sPQ) pulse V(t)
f
V(t) dr = <1>0
~ 2 mV ps
(I)
across it, induces a similar pulse in another junction
which is AC coupled to the first one via either a capa-
34 = V12
vII.: 2V\2
(0)
"A7'"J;J= .;;;~. IV.d'VAI
~--------v~--------~j
!(
--;
vB~
. . .
fb I
;
~ ~D
(VO= L VB)
~------~y~--------~
L
(( )
Figure 4. EQulvalenl circuits of (al the basic FIfO cell capable
of either reproduction or doubling of the input voltage (VI 2)'
and of (b) low-voltage and (e) hlgh·voltage mullipliers using
arrays of the R/O cells [85).
328
citor or a transformer (or both). Connecting these junc:.tions in series for DC, and supplementing them with the
pulse current amplifiers based on the multijunction
interferometers, one gets an elementary cell (figure 4(a))
capable of either reproducing or doubling its input DC
voltage. Connecting the cells in either of the ways
shown in figures 4(b) and (c) one can compose a DC
voltage multiplier.
Now it is sufficient to phase lock the first stage of
the multiplier by an RF reference signal with a well char·
acterised frequency [0 obtain a DC voltage standard
with a wide output current range (up to a few hundred
,tIA). Additional advantages of the new DC voltage
multipliers and standards include possibilities to use a
relatively low reference frequency and to decompose
such a device into many Dc-connected circuits each of a
moderate integration scale. The first experimental
studies of the devices are to be carried out in 1990.
4. Dlglt81 circuits
Since 1983, ways have been found to circumvent the
two major problems encountered in the ill-fated IBM
Josephson computer project. First. a family of technologies of reproducible fabrication of LSI circuits with very
stable all-rigid Josephson junctions has been developed
[&7-91] (note an important recent trend of the technology, planarisation [91]. which pennits fabrication of
more complex circuits for fixed design rules).
4.1. Latcbing logic circuits
This progress has enabled several Japanese teams
(including those of Fujitsu, Hitachi. NEe and ETL). to
design, fabricate and test quite a few outstanding
Josephson junction l.SI digital circuits, in particular:
• 16-bit arithmetic logic unit [92];
• 4-bit data processors [93--95]-see, for example
figure 5;
• I K random access memories [96, 97];
• 4 K random access memory [98].
These circuits exhibit a power cOn!umption of the order
of 10 IJW per logic gate, and can operate at clock frequencies up to ~ I GHz.. Both these figures are better
than those for the best semiconductor (cooled GaAs)
circuits; however. for the operation speed the advantage
can be characterised as marginal (3- to IO-fold).
The reason for the relatively low operation speeds of
the circuits is not the switching speed of the Josephson
junction gates (single-gate switching time as short as
1.5 ps has been demonstrated) but rather a necessity to
avoid the punchthrough effect during the circuit reset
(see, e.g. chapter 5 of [27]), and hard problems of providing the high-frequency AC supply necessary for
periodic reset of the gates. Moreover. the global synchronisation (provided by the AC supply) hinders effective incorporation of the circuits inlO computing
structures.
Progress and prospects of superconductor electronics
4.2.
Figure 5. Photograph of a 1 ns cycle 4-bit Josephson data
processor chip containing 8454 all Nb Josephson junctions
[95] (courtesy by Yu Hatano, Hitachi).
In terms of the Josephson junction dynamics [27]
these drawbacks are a high cost for the use of the 'latching' logics. In these circuits the digital data presentation
is similar to that accepted in the semiconductor logics:
the binary zero is presented by the zero-voltage
(superconducting) state of the gate, while the binary
unity, by the resistive state with V ~ V& = 2tJ.(T)/e ~
2-3 mY. Note, however, that the latter state corresponds to a generation of the Josephson oscillations (i.e.
of the SFQ pulses, equation (1)) with an extremely large
frequency of the order of 10 12 Hz. In the latching logics
this generation is not employed and, moreover, presents
an implicit source of the punch through effect. Utilisation of the relatively slow latching logics was apparently the second major problem which has led to the
failure of the IBM cryogenic computer project.
RSFQ circuit family
In an attempt to avoid the drawbacks of the latching
logics, a new concept nicknamed rapid single flux
quantum (RSFQ) logic was advanced in 1985 [99]. In this
concept, the binary unity is presented by an SFQ pulse
(1) rather than by a DC voltage. The binary zero is
encoded by absence of such a pulse during a certain
time interval. In the current versions of the RSFQ logic
circuits, this interval is determined by two successive
pulses of a quasiperiodic train of the similar SFQ pulses
arriving from a special clock circuit (figure 6).
Such presentation of the digital data is very convenient because the overdamped Josephson junctions and
the simple circuits using such junctions can readily generate, amplify/reproduce and detect/memorise the SFQ
pulses (figure 7). Since 1986, all major components of
the RSFQ circuits have been designed, fabricated (using
simple 10 fJ-m and 5 fJ-m all Nb technologies) and tested
to operate within large parameter margins at clock frequencies up to '" 100 GHz [100, 102]. A natural intrinsic memory of each RSFQ gate allows one to organise its
effective local self-timing (instead of the global synchronisation typical of the latching logic families) and as a
result to maintain high operation speeds for arbitrary
complex digital circuits [103, 104]. Moreover, with a
transition to 1 fJ-m design rules the clock frequency can
presumably be raised to '" 300 GHz [105].
Other important advantages of the RSFQ circuits
include their low energy consumption (of the order of
TJL1L1L
I
., •
1-
>
S
1
---,
1
1
I
'0' ~1
1I
1
I
1
I
I
I
t
I
I
1
1
:
_:'t"'~
t
f---- 't'---I
Figure 6. Scheme of presentation of the binary data in the
RSFO logic circuits: S is the voltage across the Signal line,
while T is that across the clock (timing) line. Area of each
pulse is given by equation (1).
Table 1. Estimates of basic parameters of 32 x 32-bit fixed-point multipliers based on various digital circuit technologies
(according to [103]).
Circuit type; fabrication
technology (design rules)
Serial; JJ RSFO
(2.5,um)
Integration scale (Josephson/p-n
junctions per Chip)
1500
Productivity (bi Ilion
operations per second)
0.5
Time delay (ns)
2
Parallel pipel ines; JJ RSFO
(2.5,um)
40000
Parallel; JJ latching
(2.5,um)
70000
0.5
2
Parallel; GaAs
(0.5,um)
100000
0.15
6
Parallel pipelined; Si-MOS
(1.0,um)
200000
0.2
30
2
150
329
K K Likharev
~
:2.8mY
TIME, t /"0
TIME. II ..,
(a)
(b)
Figure 7. The basic components of the RSFQ logic family: (a)
the buffer/amplifier stage and (b) the RS flip-flop. and
results of simulation of their dynamic for a low-Tc high-fc
Josephson junction technology [103].
10- 18 Joule/bit for the helium temperatures) and DC
power supply. Conservative estimates [105J show that
the RSFQ logic/memory family enables one to fabricate
VLSI digital circuits which could operate at least 30
times faster than similar circuits using the latching
logics. leave alone the semiconductor integrated circuits
(table 1).
5. AD and DA converters
Both high operation speed and natural quantisation of
the magnetic flux make the Josephson junction circuits
quite promising for the analogue-to-digital (AD) and
digital-to-analogue (DA) conversion. Several types of the
devices have been proposed and (partly) tested.
5.1. Parallel AD converters
In contrast to traditional parallel converters requiring
at least 2n comparators (where n is the bit number, i.e.
e = 2 -. is the relative accuracy of the converter), the
Josephson junction devices can be restricted to only n
comparators. Their first versions [106-108J, however,
required an impractically narrow (of order e) margin of
parameters of all their components including the analogue signal divider and the Josephson junction comparators.
Later suggestions [109-111] have opened the way
for a drastic increase in the parameter margins of the
comparators. Nevertheless, in high-precision analogy
dividers the problem of the parallel comparators still
remains, and the rapid progress of their series counterparts has presumably made the latter devices more preferable.
330
5.2. Higb-frequency series AD converters
A core of such a device is a 'ripple counter' [112] which
records the number of the SFQ voltage pulses (1) generated by a superconductor quantum interferometer
under the action of an external magnetic flux which is
induced by the input analogue signal. Figure 8 shows
an advanced version of this device capable of counting
the SFQ pulses coming from both Josephson junctions of
the interferometer and hence of processing a signal with
an arbitrary sign in its time derivative [113J. The main
problem in the development of such converters is a need
for a fast periodic read-out of the contents of the binary
counter stages without hindering the counting process.
Such read-out circuits were designed and partly tested
recently in the course of development of the RSFQ logic
family [103].
Moreover, the RSFQ circuits can be used for a superfast on-chip digital processing (e.g. filtering) of the
output signal of the converter, enabling one to improve
its accuracy substantially. As a result of these suggestions, development of the converters with the resulting
accuracy of -20 correct bits at the signal frequency
-10 MHz (or, alternatively, -15 bits at -100 MHz),
has become quite feasible even using present-day
Josephson junction technology. To the best of my
knowledge, these figures are far superior to those promised by devices based on other physical principles.
5.3. Low-frequency DA and AD CODverters
Unfortunately, an accuracy in excess of ,..., 20 bits can
hardly be obtained in the converters discussed above,
because of a finite dynamic range of their input
(analogue) components. This limitation can be apparently circumvented by design of the compensation-type
converters of the type shown in figure 9 [85, 86]. This
device includes a DC voltage multiplier (discussed in
section 3.3) driven by the SFQ pulses. The roomtemperature semiconductor code-to-number converter
ensures that the number of the pulses per time unit corresponds to the digital integrator output. (Note that this
part of the device forms a low-frequency DA converter
with an accuracy limited practically only by that of the
RF reference.)
RSFQ
DIGITAL
PROCESSING'
~------~
FIgura 8. A version of the series A.D converter based on the
'ripple oounter' formed by the two-junction (J 1 , J;)
interferometer. After amplification by the junction arrays
J:2' ...• J m and J:2' ...• J'", • the SFQ pulses generated by the
oounter are 'normalised' (quantlsed In time) by the N
circuits and thus become suitable for digital processing in
the RSFQ logic circuitry [113).
Progress and prospects of superconductor electronics
DIGITAL
OUTPUT
... RF
REFERENCE
BINARY COOEPULSE NUMBER
COfilvt:RTER
T : 4,2 K
lOR 77 K I
v
I
-
IX
DC VOLTAGE
MULTIPLIER
J,
I_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
I
..JI
Figure 9. Block circuit of a high-accuracy low-frequency AD
converter making use of the DC voltage multiplier (figure 4)
[85].
An auxiliary low-bit, but low-noise, AD converter
(say, just a SQUID) closes the negative-feedback loop of
the whole device by balancing the signal voltage V", and
the DA converter output voltage Vo. As a result one
obtains a low-frequency AD converter which is virtually
free of the dynamic range limitations and promises an
extremely high accuracy at low signal frequencies (at
least - 30 bits below 0.1 kHz). Such a device could be
extremely valuable for solving metrological problems.
(60-80 K). Transition to this range from the 'helium'
temperatures « 20 K) allows, primarily, a drastic
reduction of the refrigeration costs (due to both the
-102-fold lower cost per litre, and the 160-fold larger
latent heat of evaporation of nitrogen). Note that for
on-board space applications, a possibility to reach the
nitrogen temperatures by a simple radiative cooling can
be even more important.
Another positive effect of the transition is a higher
heat flow which can be removed from a substrate. (If
one restricts oneself to simply placing the substrate into
the cooling liquid, undesirable sheet boiling starts at
-10 W cm- 2 for LN [114J and at -0.3 W cm- 2 for
LHe [90].)
One should remember, however, that operation at
-77 K also produces some negative effects. The most
important of them is a 20-fold increase of the thermal
fluctuations which are especially hazardous for the
Josephson junction devices. In fact, a proper operation
of each particular device is possible only within certain
limits [27J of the dimensionless parameter
21tkB T
Y=--·
<1>0 Ie
(2)
It means that in order to compensate for the larger
6. Conclusion to part I
The low- ~ superconducting devices discussed in the
previous sections are still awaiting practical realisation,
and one can imagine that some unexpected problems
will be met on the way. Nevertheless, the stream of new
ideas in the superconductor electronics (especially concerning circuits of the large integration scales) has
become so wide that there is little doubt that the union
of these ideas with the modern LSI low-~ Josephson
junction technologies can bring outstanding results in
the near future. In particular, one can expect that such
single-chip devices as
with on-chip feedback loops;
•
SIS mixers with on-chip local oscillators;
• interference-proof DC voltage multipliers, potentiometers and standards;
• high-precision AD converters; and
• superfast specialised microprocessors
•
DC SQUIDS
will become commercially available in a few years time.
If this is really the case, we will have a firm basis for
development of the more complex devices and systems,
in particular, general-purpose digital-signal processors
and eventually computers.
PART II. HIGH-Tc SUPERCONDUCTORS IN
ELECTRONIC DEVICES
1. Operating at the nitrogen temperatures
The main advantage of high- ~ superconductors is their
ability to operate in the 'nitrogen' temperature range
thermal fluctuations, one should raise the critical
current Ie of the Josephson junctions by a factor of the
order of 20. Consequences of this increase include:
• larger power P dissipated by the junctions (in most
devices P IX. Ie' but sometimes P IX. I:), which can result
in a forbidding self-heating of quite a few LSI circuits;
• smaller junction impedance Z.(w) (at a fixed Ie RN
product, Z.(w) scales as I e- 1), especially harmful for
most microwave receivers;
• lower inductances L of the superconducting
quantum interferometers and other junction interconnects (L IX. 1;1).
An analysis shows [7J that most Josephson junction
circuits can nevertheless be made to operate at the
nitrogen temperatures, but only if sufficient care and
skill are used at their design.
2. Properties of the hlgh.Tc superconductors
Studying possible applications of the new materials in
electronics, one should take into account their following
peculiarities (see, e.g. the reviews [1-7, 115-117J and
recent works [118, 119J):
• high anisotropy of all transport properties and
hence a necessity to use highly-oriented (quasi-monocrystalline) thin films in most electronic circuits;
• low normal conductivity (even along the Cu-O
planes) and as a consequence a relatively high surface
resistance R. at microwave and millimetre-wave frequencies (figure 10);
331
K K Likharev
10.1
/ 10
3.2. Resonantors, transmission lines and interconnects
1
/
/
/
/
e.c:
0
..,
N
"0
10 1
1(j3
e
.3-
"'."
I'
0<
lIj3
OJ
0<
05
'---'----''---'-..L.J.._-'-------' 1
1012
f (Hz)
Figure 10. Surface impedance ReZ s and effective
penetration depth Red of a typical high-To superconductor
(YBaCuO) and one of the best normal conductors (Cu), both
for 77 K. lines show results of calculations using simple
two-fluid model with parameters extracted from
independent measurements [7], while pOints show the best
experimental results for high-Tc (quasi)monocrystailine thin
films [120-122].
e;
• small coherence length
• vulnerability of the superconducting properties to
even small distortions of the atomic composition and
structure; and
• high chemical activity at the surfaces, and interfaces
with most metals and insulators [123-125].
3. The simplest devices and components
At first, let us survey the simplest devices, considered in
part I of the paper, as well as those which can be practical only with the high- T" superconductors.
3.1.
EMF shielding
The most evident example of such new possibilities is
the shielding of electromagnetic fields. It is enough to
cover a volume (say, containing sensitive electronics) by
a film only a few )'L thick to provide a virtually perfect
shielding of the interior from external EMF interferences
at all frequencies from DC to ultraviolet. For monocrystalline thin films of the high- 1;, superconductors of
the proper orientation (the Cu-O planes parallel to the
surface) the necessary thickness d is as small as ...... 1 J1.m,
while the critical power density Pc is as high as
...... 106 W cm - 2 [7]. For the granular films, the necessary d is larger, and Pc lower, but nevertheless their use
seems quite possible here. (Note that for most other
applications the critical current density of the films
should be well over 104 A cm'- 2, so that the granular
films can be of hardly any use.)
The superconducting shielding can be especially
valuable at relatively low frequencies (below, say,
I kHz) where the normal-metal shields would be too
thick.
332
A relatively large surface resistance Rs of the known
high- 1;, superconductors (figure to) hardly makes them
competitive with the normal metals for applications in
high-Q resonators and low-attenuation transmission
lines in the short-microwave and millimetre-wave bands
at present. Nevertheless, the virtual independence of the
effective penetration depth Re a = 1m Z.(w)jWJ1.o on the
signal frequency wallows a use of the superconductors
for fabrication of microstrip lines with a propagation
speed independent of the frequency up to _10 12 Hz.
Such lines are capable of transmitting very short pulses
by distances in excess of 1 cm [142-145J, and can be
very useful for intrachip communication in VLSI digital
circuits.
This remarkable property, however, cannot be
referred to the semiconductor-transistor integrated circuits. In fact, simple estimates show [1, 4-7, 147-149]
that use of the superconducting on-chip interconnects
would yield practically no change of the operation
speed of the semiconductor devices, because it is limited
by high intrinsic impedance of the transistors rather
than that of the normal-conductor wiring.
On the other hand, optoelectronic devices are
believed to be the best candidates for long-distance
communication in the computers of the future. This is
why the only possible niche for the high- 1;, interconnects in the semiconductor computers seems to be the
intermediate-range (e.g. on-board) communications,
although a lot of engineering and technological problems are to be solved in order to really occupy this
niche.
The last remark also concerns the prospects of using
the high- 1;, superconductor interconnects in the projected ultra-high-density (_10 10 gates/cm 2 ) 'single-electron
logic' (SEL) integrated circuits [150J based on the recently discovered effect of the correlated single-electron
transfer in ultrasmall tunnel junctions [151, 152].
3.3 Switches
Estimates and first experiments [19] show that the
high-1;, thin films are quite capable of replacing their
low-1;, counterparts in switches discussed in part I. At
77 K, the order parameter relaxation times should be
very short (_10- 12 s or less), so that the switches can
be probably introduced to some new electronic systems
which require large switching speeds.
3.4. Bolometers and mixers
Preliminary estimates [7, 153] show that NEP of the
nitrogen-cooled high-1;, thin-film bolometers can be in
the range (10- 12 -10- 13 ) W Hz- 1 12 , provided that additional noise sources like the flux creep are avoided.
These figures are comparable with those for photovoltaic semiconductor detectors (operating at the same
temperatures), but can be obtained at longer wave-
Progress and prospects of superconductor electronics
lengths including the far infrared band where no competitive devices at present exist [154, 155]. The figure
NEP ~ 4 X 10- 11 W Hz- 1 / 2 achieved in the first quantitative experiments with a composite bolometer [156]
(see also less quantitative measurements [157-165]) is
evidently a preliminary one and can hardly be considered as contradicting the theoretical estimates.
Moreover, a potentially fast photoresponse of the
high- ~ superconducting thin films gives one a hint to
their possible application in mixers, although performance limits of such devices is still to be estimated.
4. Hlgh-T. Josephson Junctions
The final three features mentioned in section 2 mean
that fabrication of reproducible Josephson and tunnel
junctions of the high- ~ materials presents a difficult
problem (for a recent review [126]). In spite of some
recent success in this direction [127-132] it is believed
that the present-day technologies would hardly allow
one to fabricate artificial Josephson junction with the
I. RN products higher than those of the natural intergrain junctions « 300 j.lV at 77 K [133, 134]). Only
more advanced technologies (possibly, consequent epitaxial growth in situ of layered structures with subsingle-atomic-layer monitoring of the thickness and
sub-one-per-cent stoichiometry control of each layer)
could possibly enable one to fabricate the Josephson
and quasiparticle tunnel junctions with parameters
close to the values following from the BCS theory and its
extensions ([see, e.g. [25-27]).
This is why three possible scenarios of the development of the high- ~ Josephson junction electronics will
be considered below.
A. (Very unfavourable). All attempts to fabricate
reproducible junctions fail, and we are left with
(irreproducible) intergrain junctions with I. RN ~ 300
j.lV and P. < 1 (the junctions can be readily singled out
by various lithographic techniques [133-141]).
B. (Most probable). In a couple of years we will be
able to establish a technology of reproducible fabrication of the SNS-type junctions with the similar values of
I.RN and P•.
C. (Extremely favourable). By some technological
miracle, reproducible Bcs-scaled tunnel junctions (with
I.RN ~ 20-30 mY) do become available.
5. Scenario A
There is virtually only one type of the Josephson junction devices, namely SQUIDS, which can employ irreproducible (e.g. intergrain) Josephson junctions. (DC SQUID
interferometers with 1.1 ~ 1.2 can be obtained by either
using a single intergrain boundary for both junctions, or
by a mere selection from samples with random I.J
N = N1 N2
ib)
Figure 11. Equivalent circuits of (a) usual and (b)
series-type DC transformers for the SQUIDs.
Estimates using the well established theory of DC
SQUIDS (see, e.g. chapter 7 of [27]) show [3, 7, 166] that
the energy sensitivities of the order of 10 31 J Hz- 1 can
be achieved at frequencies in the white-noise range, provided that the interferometer inductance is low enough
(::5 30 pH).
The
best
experimental
figure,
-10- 30 J Hz- 1 [134] for a DC interferometer (but not
yet for the SQUID as a whole!) is not very far from these
predictions. Such a sensitivity is already sufficient for
most present-day applications of the SQUIDS. In addition, one can expect that lower refrigeration costs and
simpler cryogenic techniques will make the high- ~
SQUIDS applicable in some new fields of science and
technology.
The most obvious problem for practical high- ~
SQUIDS is the need for an effective coupling of the signal
source to the interferometer with its higher I. and lower
inductance. The series connection of several superconducting DC transformers (figure 11) which was only
occasionally used in the low- ~ SQUIDS; could become a
necessity.
6. Scenario B
Reproducible, though low-I. R N , junctions would pave
the way only for SQUIDS, but also for several other
Josephson junction devices.
6.1. Samplers
Elementary estimates show [7] that the high- ~ superconductor samplers can retain their present-day time
resolution and, somewhat unexpectedly, their sensitivity. The reason is that at present sensitivity is limited
by certain technical factors [7] rather than by fundamental thermal fluctuations.
Reduction of the refrigeration costs is likely to make
these devices quite attractive in a variety of fields, in
particular for testing superfast digital devices.
6.2. Receivers
Signal properties (gain, bandwidth, etc) of the Josephson junction receivers would not suffer a lot from transition to the overdamped high- ~ junctions with
333
K K Likharev
leRN "" 0.3 mV (unfortunately, no decent SIS mixers are
possible with such junctions). The reason is that
although the best low- Tc tunnel junctions do have considerably higher Ie RN products, they are intrinsically
underdamped (Pc ~ 1). In this situation, the signal
properties are limited by the plasma frequency wp of the
junctions, and Ie RN product is effectively replaced by
Vp = (1i/2e)wp [27]. (For typical low-jc junctions with
the area S"", 10 x 10 JlIll2 and critical current density
je ,.., 103 A cm - 2, v;, is of the order of 0.3 m V.)
On the contrary, the noise of these devices scales
almost exactly as T. Practically it means that using well
designed coherent multijunction arrays one can hope to
build, for example (results of [75-77] have been used to
make these estimates):
• parametric amplifiers with noise temperatures TN 20 K at the signal frequencies of the order of 30 GHz;
• Josephson junction mixers with the double-sideband
noise temperature of the order of 300 Kat"'" 100 GHz;
• quadratic videodetectors with the fluctuation sensitivity _10- 2 K atf':::'. 300 GHz.
These figures compare quite favourably with other
nitrogen-cooled receivers (say, the Schottky-diode
mixers [154, 167]), but one should not forget one
serious drawback of the Josephson junction devices,
their low saturation power p. [27]. Even with the
arrays, one could hardly increase p. over 10- 8 W.
Apparently this figure is too low for most communication and radar systems. It is satisfactory for the radioastronomy, but in this field there are typically no serious
problems with helium cooling, so that the ultra-Iownoise low- T" superconductor receivers look preferable.
Thus, the only apparent field where the high- T"
superconductor receivers could be used with a success
seems to be the space instrumentation where the simple
radiative cooling can be a decisive advantage over the
helium refrigeration.
6.3. DC voltage standards and multipliers
Overdamped high- T" junctions would hardly allow
practical De voltage standards based on the simple
series multijunction arrays [79], and one's only hope in
this case are the recently suggested devices based of the
SFQ pulse locking [85, 86] (see part I). Estimates show
that although increase of the thermal fluctuations and
the resulting necessity to increase Ie and reduce L
(section 1) present rather a problem, it can be solved via
a careful design of the circuit layout.
Note, however, that these integrated circuits are
already so complex that their fabrication (rather than
refrigeration) is likely to be the major concern. If such a
unique device as an interference-proof De voltage standard of the 10 V range is fabricated and is functioning
properly, it would be evidently useful at many working
places even if the helium cooling is needed.
Thus, one can expect that the low- T" superconductor devices of this kind will be fabricated and intro334
duced at first, and that their transfer to the high· T"
superconductors would take place only later (if ever).
6.4. Digital and analogue-digital devices
All conclusions of the previous section are even more
justified for such LSI digital and analogue-digital devices
like the RSFQ circuitry (note that no latching logics can
be operative with the overdamped junctions). Here the
low- T" superconductor circuits with their well devel·
oped technology will probably dominate in a foreseeable future.
7. Scenario C
The course of events when the 'perfect' (Bes scaled)
tunnel junctions become available seems somewhat
improbable presently, so that in this section I will
restrict myself to few general remarks.
(i) The Bes scaling concerns the Ie RN product but
not the plasma frequency wp of the Josephson junctions.
There is no ground to believe that the practical critical
current density of the high- T" Josephson junctions can
be higher than that of the best present-day junctions (a
few tens kA cm - 2) and that their specific capacitance
CIS can be much lower than the standard figure of a
few p.F cm - 2. Hence the plasma frequency can also
hardly be larger than few 10 12 s -1.
It means that if a Josephson junction should be
overdamped for a particular application (SQUIDS, ripple.
counter-based devices, RSFQ logic circuits,) it should be
externally shunted until its effective I eRN product
reaches the very modest level of ~ ':::'. 0.3-1.0 m V. Such
a junction would not substantially differ in its dynamics
and noise from those considered in scenario B. In other
words, all our toil and trouble fabricating advanced
junctions would be in vain.
(ii) In the applications requiring underdamped junctions, their (relatively large) capacitance will nevertheless limit the device performance seriously, For example,
the signal bandwidth of most microwave receivers
cannot be extended below '" (RN C) - 1, The normal
resistances RN are typically limited by the standard
wave resistance p - 10 2 Q of the waveguides available,
so that the bandwidth will be of the same order as in
the low- devices, typically not more than a hundred
GHz. While the operation frequency is of the same
order (as it is now), this limitation presents no problem,
but it will be a very serious obstacle for attempts to
increase the signal frequencies proportionally to Ie R N ,
i.e. to the 10 THz range.
(iii) The Bes-type tunnel junctions would make the
latching logic circuits possible in principle. The power
dissipated by these circuits, however, scales as T2, and
would be considerably larger than 1 milliwatt per gate.
This power consumption is larger than that of the
present-day nitrogen-cooled semiconductor gates, and
makes operation of the VLSI circuits virtually impossible
r.
Progress and prospects of superconductor electronics
because of their self-heating. Hence it seems at present
that the energy-saving RSFQ logic circuits are the last
chance for the digital signal and processing using
high- Tc superconductors.
Conclusion
Recent intensive advances of superconductor electronics
were mainly due to its 'internal' development rather
than the discovery of the high- Yo superconductivity. As
a result of the progress in the low- Yo Josephson junction
technology and invention of new electronic circuits,
their union promises the fabrication of quite a few outstanding electronic devices for processing of analogue
and digital signals in the near future. These devices
seem to have a chance to be quite useful in many scientific and technological fields, despite needing helium
cooling.
The high-Yo superconductors apparently could be
soon introduced to the simplest electronic devices and
components (such as EMF shields, switches, bolometers
and SQUIDS) for which the refrigeration costs limit the
application range of their low- Yo versions. Introduction
of the high-Yo Josephson junctions to more complex
devices based on the LSI and VLSI circuits will depend on
the success of their fabrication technology, but will evidently proceed at a much slower pace.
Thus we still have a chance to use some of the exciting helium-cooled superconducting electronic devices in
the forthcoming millennium.
Acknowledgments
Many colleagues, both from the USSR and other countries have been extremely kind to respond to my request
to send me information of their recent work, some of it
prior to publication. I am extremely grateful to them,
and very sorry that it is impossible to list all of them
here. It is also my pleasure to thank the Department of
Physics, Chalmers University of Technology, for hospitality during my visit to Goteborg, Sweden, where work
on this paper was started. The work was supported by
the Soviet Scientific Council on the high-Yo Superconductivity Problem under the Grant no 42.
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