Advances in the application of high Tc superconductors to microwave devices for analog signal processing

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Advances in the application of high Tc superconductors to microwave devices for analog signal processing

J. Mage, B. Marcilhac, M. Mercandalli, Y. Lemaître, S. Barrau, B.

Dessertenne, D. Mansart, J. Castera, P. Hartemann

To cite this version:

J. Mage, B. Marcilhac, M. Mercandalli, Y. Lemaître, S. Barrau, et al.. Advances in the application of high Tc superconductors to microwave devices for analog signal processing. Journal de Physique III, EDP Sciences, 1994, 4 (7), pp.1285-1294. �10.1051/jp3:1994190�. �jpa-00249183�

J. Phys. III Franc-e 4 (1994) 1285-1294 _JULY 1994, _PAGE 1285

Classification Physic-s Abstracts

74.30G 74.70V 74.75

Advances in the application of high l~ superconductors to

microwave devices for analog signal processing

J. C. Mage, B. Marcilhac, M. Mercandalli, Y. Lemaitre, S. Barrau, B. Dessertenne, D. Mansart, J. P, Castera and P. Hartemann

Thomson-CSF/LCR, 914040rsay, France

(Received J5 July 1993, revised 23 Mat-c-h 1994, ac<.epted12 April 1994)

Abstract. During the last five years, the surface resistance R~ ^lo GHS, 77 K) of YBCO thin films has decreased from _a value of about lo _to 20 milliohms I-e- the _{same as} cooled pure copper or as bulk YBCO down _to values lower than 200 micro-ohms, close to the theoretical value of loo micro-ohms obtained by _a straightforward calculation from BCS theory. This

improvement of R~ ^{is due} to a better quality of the material from random grain ceramics _to quasi epitaxial films. These highly textured films _can be obtained by _many deposition methods

sputtering, laser ablation, co-evaporation, ^{molecular beam} epitaxy, MOCVD, using heating sample holders in order to obtain iii situ crystal oriented layers. The value of the surface resistance is about _one hundred times lower than that of usual metallizations, which

can be used either to

improve the specifications ^of _some components by two order of magnitude such _as high

Q 3D _resonators (Q ~10~ _{for low} phase noise oscillators) and high Q inductances (Q _~ 10~ for

circuit matching of _antennae in the MHz range) _or to reduce the size of voluminous devices such _as filter banks for multiplexing or spectral analysis.

1, Introduction.

The expected applications of high _T~ superconductors are numerous, in many fields from heavy electrotechnics _to submicronic micro-electronics (or _« nano-electronics _» high field magnet coils, _motors, transformers, magnetic bearings and levitation, high speed logic, active microwave components (detectors, mixers, oscillators, transistors.. ), and passive microwai>e

<.omponents (iesonatois, filters, delay lines~ ).

Many of these applications require ^material specifications which _{are not} yet ^mastered high J~ wires, vortex pinning, single crystal films, true Josephson junctions, ^while passive

miciom~aite devices _are the closest _to opeiationa/ applications since they require only quasi epitaxial films which _are already available in several laboratories, and since many demonstrator

devices have already been made.

This paper is _{not an} exhaustive review of all the processing methods and all the potential applications of high _T~ superconductors. ^{It is} just concerned by the methods _we have opeTated by the applications to analog signal processing.

2. The deposition methods,

In Thomson LCR, several methods except MOCVD (Tab. I) have been operated. All of

them have been carried _out with heating sample holders (600 to 800 °C), using either silver

paint or clamping jig to fix the substrate, in order _to produce quasi epitaxial films. This is _a sine qua ^non condition in order _to obtain the quality required for microwave applications (and for most of the applications). Indeed _room temperature deposited films _are amorphous and

electrically insulating. Post annealing results in granular superconducting films with many native uncontrolled Josephson junctions which _can be used in _some applications (bolometers, detectors, _« random _» SQUIDS...) but which lead to very poor electrical properties (low J~, high _R~, I/f noise.. ). Thus epitaxy _on heated substrates is required for _our purpose.

Table I. Comparison of deposition methods (LCR results e~<ecpt MOCVD).

CRITERION laboratory film quality ^industrial

(surface resistance) implementing

DEPOSITION METHOD _cost (large area)

on axis sputtering simple fair limited

(oxygen atmosphere) inexpensive

off axis DC magnetron sputtering simple good simple _up to 2"

(facing targets) inexpensive

inverted cylindrical cathode simple good simple _up to 3"

DC sputtering (diameter ⁵⁰ mm) inexpensive

laser ablation simple good complex if

~ l"

excimere laser (KrF 248 nm) expensive (scanning)

molecular beam epitaxy complex fair complex

Dy _vs. Y ₊ BaF~ ₊ atomic oxygen (low yield)

MOCVD very complex fail- simple _up to 4"

medium

It is interesting to note that _a method like MOCVD which has been very difficult _to operate in the laboratory- because of precursor problems becomes very competitive from _an

industrial point of view, while the laser deposition- _very simple to operate ^{in the}

laboratory seems relatively inadequate for industrial applications.

The _nature of the substrate is very important for the quality of the deposited film and for the

device _to be eventually pattemed. Several kinds of substrates have been tested (Tab. II).

The deposition _parameters (substrate temperature, pressure, cooling down, in situ annealing)

have been optimized in search of the minimal value of the surface resistance R~. ^This major parameter ^{for both microwave} applications and absolute quality criterion is somewhat

correlated with other parameters : . J~ higher than 4 MAmps/cm2

. sharp DC transition lo §b-90 §b _narrower than I K) _;

N° 7 SUPERCONDUCTING THIN FILMS AND MICROWAVE DEVICES 1287

Table II. Comparison of substrate materials (LCR results).

SUBSTRATE SrTiO~ Y-ZrO~ LaAIO~ LaGaO~ MgO A120~

CRITERION

lattice matching good _poor good good fuzzy fuzzy

microwave properties poor poor good ^fair good good

dielectric constant _~ lo 000 22.8 23.6 24.4 9.67 9.8

dielectric loss 77 K 4 _x 10-~ ₂

x 10-3 ₃

x 10-6 12 _x 10-6 4 _x 10-6 _x

inter diffusion low low low low moderate high

mechanical properties fair good fair good _{poor very} good

crystal quality good _poor twinned good good _very good

. normal resistivity lower than 300 micro-ohms-cm at 300 K _:

. slope ratio R (300 K )/R ( loo K

~ 3 (DC resistance) _;

but the correlation is _not loo 9b _ans it is necessary ^to have _a simple and _accurate

R~ measuring method.

At the present time, the lowest value we have obtained is

. R~ (10 GHz, 77 K)

=

190 micro-ohms

. by ^{the inverted} cylindrical cathode sputtering _on LaAl03 substrates.

Other deposition methods are typically within the 400 to 500 micro-ohms range.

It must be noted that very good results (150 micro-ohms at lo GHz and 77 K) have also been obtained by co-evaporation on MgO substrates [1, 2].

3. The measurement of the surface resistance.

The measurement of such _a low R~ value has confirmed the accuracy of the dielectric resonator method (Fig. Ii that _we have used (for R, measurement) since 1988 [31. It appeared to be _more

accurate than many other _ones such _as

. rectangular waveguide _{: very} ^limited by ^low Q value

. cylindrical _copper cavity operating in the mm wave range where superconductors are

close _to copper (as _to R~)

. parallel plate resonator which requires 2 samples and radiation correction

. pattemed lines _or rings _: destructive method, moderate Q values _more appropriate for etching test

. the superconducting high Q niobium cavity is the only method which _can reach _a higher

accuracy but it must be operated at liquid heliuin temperature ^{and it is much heavier than the} dielectric resonator method.

We _{use a} rutile resonator with _a permittivity of 105 (perpendicular to c-axis) _at 77 K and _a dielectric loss tangent ^of 10-5 _at lo GHz.

The diameter of the resonator is 7 _mm and its thickness is I _mm for _a resonant frequency ^of

Generator De4ector

Coupllng Loops

3

Copper

~ates

SAMPLE

i

ColdHead

1IQ ₌ A-tan 6 + Bi.Rsi + 82.Rs2 ₊ 83.Rs3

Thick mwnator -> tan 6

~et rewnetor -> Rsi

Fig, I. Measurement of the surface resistance by the dielectric _resonator method.

lo GHz. This unusually flat shape for _a dielectric resonator has been specially designed to

improve the accuracy on R~,

The sample _to be measured, with the dielectric resonator laid _on it, is enclosed in _a hermetic copper box which is either soaked in _a liquid nitrogen ^{bath for fast} measurement at 77 K or

attached to the cold finger of _a helium closed cycle cryocooler in order to plot ^the curve

R~ itersus T between lo K and T~.

The R~ ^{is calculated from the} quality factor of the TE 011 mode, using a 2D Finite Element Method for axisymmetrical objects. The _use of the TE 011 mode is very important for the

reproducibility of the measurement, because this mode is quite insensitive to any gap between

the resonator and the film (contrary to TM modes and hybrid modes).

The maximal Q

= loo 000 _at lo GHz and 77 K is limited by the dielectric loss of the

resonator. The typical value for _a 0.4 mohm film is Q _~ ^{50 000 and the ultimate resolution is}

in the micro-ohm range with accurate correction from the dielectric loss.

4. Bandpass filters,

The interest of planar superconducting filters is _to get ^the same performances as cavity filters

by simpler reproducible _means, within _a much smaller volume. This last advantage always

holds _as far _as cryogeny is not considered. When the volume of cryogeny is accounted for, this

advantage can still hold only if _a cryogenic system ^is already available within the system ^for other purposes (infrared detectors, low noise amplifiers.. or if _a large number of filters is

concerned (typically more than ten).

This last case concerns for instance the channelizers. Systems involving _up to one hundred

narrow band filters have been considered. The modelling of such filters is _{not as} simple _as it

N° 7 SUPERCONDUCTING THIN FILMS AND MICROWAVE DEVICES 1289

could _{seem, not} because of the properties of superconductors themselves but because usual

microwave CAD codes _are neither appropriate to model _a bandwidth _narrower than 9b _nor

capable of calculating the S-parameters of _a circuit when the permittivity of the substrate is

higher than 20. Most of the pusblished results _concern few ill bandwidth filters which _are easier _to be modelled. Problems _occur when _a bandwidth

~ l fb is forwarded (and when high permittivity substrates _are used), ^{which is the} interesting point for high resolution multi channel systems [41.

The figure 2 shows the transfer function of _a 3 pole filter made in Thomson LCR

. structure : microstrip ;

. central frequency

=

9.45 GHz (for lo GHz calculated)

. bandwidth

=

loo MHz _;

v

CEMTER 9.350000000 GHz

WAN 2.O@O@@O@@@ GHz

al

WMmR 9.330@@0@OO l*~z

EW'~14 Z,00@0@@@@O l*~z

b)

Fig. 2. Superconducting filter characteristics a) Sj~ ^lo dB/div,), b) Sj~ (5 dB/div.).

. insertion loss less than 0.5 dB

. rippling I dB

. return losses lower then 15 dB

. rejection out of the band higher than 50 dB

. volume (unpackaged)

=

15 _x 15 _x

= 225 mm3.

This demonstrator has allowed _{us to} define the real problems of modelling which _are

responsible for instance of _too large a rippling in the band and shifted central frequency

. analytical formulae of usual CAD _{are not accurate} enough for low coupling and fail for

e ~ 20

. 2D Finite Element Methods _cannot account for the finite length of the poles _;

. 3D Finite Element Methods require much heavier hardware _are rather slow and less

accurate than expected _;

. Boundary Element Methods could constitute the best compromise as far _as the substrate is

isotropic.

This filter _can be the first step ^towards a loo channels multiplexer covering the 5 to 15 GHz

band with _a loo MHz resolution. For 6 poles, the volume of _a 5 GHz filter would be

000 mm3 and the volume of the 15 GHz filter would be loo mm3, _so that the volume of the

whole intraconnected but unpackaged multiplexer would be less than 50 cm3. Including a

specific cryocooler (volume

=

000 cm3), the packaged system ^{would be} really competitive

versus a classical cavity _or dielectric _resonator multiplexer, the volume of which would be _so

larger that such _a system ^could hardly be considered.

This example shows clearly that the _use of superconductors

. requires specific modelling (independently of their complex conductivity)

. does not only supply improved components ^but can imply _to reconsider the whole architecture of _a system.

5. Low phase noise oscillator,

When operating our R~ measuring system (Fig. I), using different dielectric _resonators (different materials and different shapes), _we often observed Q values of several hundred

thousands. Thus _we planned _very early to use this kind of _set up ^{to make} high

Q resonators.

First attempts were operated with lanthanum aluminate _resonator. The permittivity of 23.6 led _to compact resonators, but the Q was limited _to 300 000 by the dielectric loss of the

available crystals. Then _we used magnesia the intrinsic dielectric loss of which is very low, but the surface of this material is very sensitive to machining, polishing and cleaning (due to

magnesium hydroxide). The measured Q value is usually lower than expected, the best results

were obtained with _a cubic cleaved _resonator. Finally _we turned _to sapphire which is much

more chemically stable and is well-known for its very low dielectric loss.

The association of a sapphire dielectric _resonator and _two superconducting films allowed _us

to make _a resonator with _a quality factor Q

=

000 000, at lo GHz (Fig. 3). In spite of the

relatively low permittivity of alumina which implies large fringing fields _out of the resonator and thus parasitic loss, two 25 _mm diameter films with R~ ₌ ^0.5 milliohm have been sufficient to get ^this value, so that Q _up to 3 _x 106 _can be planned with lower R~ ^and larger diameters

(Fig. 4). This value is still limited by the R~ ^of the films since the intrinsic Q of sapphire is much higher than lo?-

It must be noted that the _non linear effects due to high reactive J~ can hardly be observed up

to mW.

N° 7 SUPERCONDUCTING THIN FILMS AND MICROWAVE DEVICES 1291

0 .S dB'

Q=1.@25 E+G

CENTER 10~l092@335@ l*~z

Fig. 3. Experimental result of _a hybrid YBCO/sapphire resonator.

Q

30WOW

2 OW COO

i m coo

__,

;""

~,.

"''

i coo coo ^""

sooooo

'~~~

lo 20 3O W 5O

YBCO film diameter

Ra1mD Rsffyo ^Rs25@JD

Fig. 4, Q modelling of _a hybrid resonator.

Such high Q hybrid resonator can easily be operated in the 4 to 20 GHz range. For lower

frequencies a different compact structure can be designed down to I GHz with similar Q values thanks _to the square law of R~ versus frequency. At lower frequencies, LC circuits

can be considered with lower but still outstansing Q values.

These values represent a gap versus the state of the art, since usual _room temperature

resonators such _as copper or silvered cavities and dielectric resonators are limited to

Q values of 30 000 (TE 011 mode). Higher values up ^to 200 000 have been obtained

only with oversized _resonators such as Pdrot-Fabry mirror _{resonator or} Whispering Gallery

Modes sapphire resonators which _are rather voluminous below lo GHz _so that they cannot

really compete ^with superconducting resonators (even including the cryogeny).

Such high Q resonators are very useful to design low phase noise oscillators. Operating at 77 K implies _a gain of 6dB for the floor level of white noise far from the carrier. The usual

increase of the noise _near the carrier _starts from the point f/2 Q _" 5 kHz for _our resonator.

This is intrinsically ^{better than acoustic} resonators such _as SAW (surface acoustic wave), BAW (bulk acoustic wave), HBAR (high overtone bulk acoustic resonator) which _are limited to operating frequencies below I GHz and thus require multiplication by n ₌ lo _to 000 with the consequent multiplication extra noise (20 Log n). The comparison between fundamental

superconducting ^lo GHz oscillators and multiplied acoustic oscillators clearly shows that _an

improvement of 20 _to 30 dBc for the phase noise at I kHz from the carrier _can be expected (Fig. 5).

dB#Hz

48o ^'

oo

ioo

ii o

120

13o

14o

isc

iw

170

O.1 O.3 3 10 3O 100 300 IOOO

kHz

DRO x1 SAW x10 BAW x30 HBAR _x8 HTSC x1

Fig. ^5. -State of the art of phase noise for R-T- oscillators and expected phase noise of _a HTSC oscillator.

N° 7 SUPERCONDUCTING THIN FILMS AND MICROWAVE DEVICES 1293

Such _an improvement also represents a technological _{gap versus state} of the _art. Its

application _to communication systems ^{and earth} observation radars _can require _{a new} design of the signal generation sub-system.

Resonators _are operational in several laboratories [5, 61. The problem is _{now to} make oscillators which verify the theoretical model. The design of the oscillator and the choice of the low noise active element _{are a} real challenge for the next years.

6. Antennae.

6.I VHF/uHF ANTENNAE. In this frequency range, there is _an interest in _antennae which

can be made small _versus the wavelength. Then the impedance of such _{an antenna} is constitued of _a small real part (few ohms) due to the radiation resistance and _a large imaginary _part due to the low value of the capacitance for _an electric monopole _or dipole. In order to avoid _a reflection of the microwave energy towards the generator, ^{it is} necessary ^to conveniently

match the impedance of the _{antenna to} the characteristic impedance (typically 50 ohms) of the emitter (or receiver).

As the mismatch is large, the matching network requires high values of inductances. If normal metal inductances _are used, most of the energy is dissipated in the matching network.

Thus low loss and high self _resonant frequency inductances _are required. Superconducting

planar spirals _are ideally suited for this purpose. Q values higher than 104 have been obtained for _a 5 turn spiral _{on a} 14 x14 _mm LaAIO~ substrate _at the self _resonant frequency of 300 MHz. Using lower permittivity substrates will allow _{us to} increase the resonant frequency

for _a given _geometry and theoretical considerations let expect ^much higher Q ^values.

Nevertheless such _an inductance is sufficient _{to tum a} low efficiency _antenna (less than 5 §b of radiated energy) into _a high efficiency antenna (more than 90 9b of radiated energy).

This is the first step ^towards matched, efficient small _antennas

. the _use of superconductors _concerns mainly the matching circuit

. radiators _can be either normal _or superconductor depending on the convenience of the

design ^of the cryogenic system ^without any major effect _on the efficiency of the _antenna (I.e.

90 to 95 §b).

Receiving antennae are operational while emitting antennae are power limited by the large reactive currents in the inductances which will require _a careful design in order _to avoid _non linear effects. The limit power will be proportional to the width of the line and thus _to the size of the inductance.

6.2 MILLEMETRE wAvE ANTENNAE. Planar arrays of _resonant patches are very convenient

to get high directivity antennae, but when the number of patches is increased, the length of the

feeding circuit increases and the attenuation of the lines, couplers ^and transformers becomes prohibitive. The _use of _a superconducting distribution network _can avoid this problem and is

quite adequate in the 20 to 50 GHz range (according to the today state of the _art I-e. 50 _mm diameter samples). The increase of the gain of the _antenna is typically ₊ 3 dB which is

interesting _{as a} demonstration but is hardly sufficient to operate ^such an antenna.

The proof of the advantage of superconductors will require at least loo _mm diameter device

using _{one or} several low permittivity substrates. Nevertheless the feasibility of such _a device with the problem of efficiently cooling a large radiating surface is questionable. A

multi-layer design with normal metal radiating patches can help to solve this problem.

Up to now, only receiving antennae have been considered because of possible _non linear effects, but few _watts emitting antennae can also be reasonably envisaged because there is _no

large reactive power in this kind of antenna (except ^{in the} resonant patches ^which can

eventually be made of normal metal).