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1060 IEEE TRANSACTIONS ON MAGNETICS, VOL. 25, NO. 2, MARCH 1989

SNAP STRUCTURES WITH Nb-AIOx-Nb JUNCTIONS FOR MM WAVE RECEIVERS An.B.Ermakov, V.P.Koshelets, I.L.Serpuchenko, L.V.Filippenko,

S.V.Shitov. A.N.Vystavkin Institute of Radio Engineering and Electronics USSR Academy of

Sciences Marx Avenue 18, Moscob GSP-3 103907. USSR

Abstract

High quality small area Nb-AIOx-Nb junctions have been fabricated by Selective Niobium Anodization Process (SNAP). The influence of the preparation conditions on the junctions properties were investigated. The numerical calculations based on IVCs measured by data acquisition system were used to simulate the mixer performanb. It has been shown that the "knee" structure on the IVC of refractory material junctions affects significantly the signal properties of the mixer. The experimental investigation of the signal and noise properties for different types of the integrated SIS mixing elements have been performed in the frequency range of 37-53 GHz; the mixer conversion loss (SSB) was as low as 4 dB at 45 GHz. The mixing elements consist of the odd number of SIS junctions connected in series for Rp and parallel for dc biasing have been tested. The dc bias wiring and IF filters of these structures were designed to provide tuning out of the junctions capacitance and thin film electrodes inductance. The advantages of these mixing elemens on traditional series array have been demonstrated.

Nb-Al%-Nb Junctions

The latest years have seen several attemps to develop a reliable technique for preparation of all refractory material tunnel junctions. It was shown132 that even a very thin sputtered layer of A1 completely covers the fresh surface of Nb, and the tunnel bar- er can be formed by a thermal oxidation of the A1 overlayer. As a result of these investigations the stable low capacitance and high quality all ref- ractory Nb-Al%-Nb junctions have been developed. Three layer Nb-Al-Nb sandwich structure is deposited in one vacuum run without RF etching of the base elect- rode and therefore without film surface damages.

Nb-Al%-Nb junctions in our case were prepared in oil-free vacuum system equipped with DC and RF magnetrons and liquid nitrogen Meisner trap. The background pressure in this configuration was typical- ly 3- 10-7mbar, all layer are deposited on water-cooled substrate. Nb base electrode (d = 100 nm) was deposi- ted by DC magnetron sputtering (v = 1 nm/s). After so- me delay A1 layer ( d 5 - 6 nm) was deposited by RF magnetron sputtering (v = 1 nm/s) A1 layer was oxidi- zed in-situ in pure oxygen for 20 - 30 min. Then the system was pumped for 10 - 15 minutes and Nb counter electrode (d = 35 nm) was sputtered.

=

The base electrode geometry was defined by lift off; that together with small thickness of the base electrode decrease the thermal stress3 in the three layer sandwich structure. The junction area was de- fined by anodization of the counter electrode - SM+; this process was monitored by measuring of the anodiza- tion rate . To decrease the stray capacitance we evapo- rated Si0 layer (d = 250 nm), SNAP photoresist pattern was used as a mask for Si0 layer lift-off (self align- ment contact). The utilization of the photoresist mask for SNAP lead to the "underanodization" ( % 0.4-0.5 U). Thus it is possible to reduce junction area down to 2 - 3 U with usual optic photolitography.

Manuscript received August 22, 1988

The quality of the SIS junctions are usually determined by two parameters: V, = Ic.R(2mV) and smea- ring of the gap 6V . For quasiparticle devices the va- lue of the criticaf current IC is not very important, it can be reduced by some normal layer near tunnel bar- rier or by thermal fluctuations for junctions with high normal resistance Rn ( Rn > l O O n ). Therefore we used ratio R(2mV)IRn to characterize the subgap leaka- ge, and figure Rn'A (where A is the junction area) to define the tunnel barrier thickness. All dc measure- ments were performed by a specially designed data acqu- isition system. The system not only measure the IVCs and calculate the main SIS parameters: jc = Ic/A, Rn'A, R(hV)/Rn, 6Vg (determined on Rn/2 level), V and so forth, but also keep all information in data 'base for the mixer performance simulation and investigation the junction properties dependence on preparation condi- tions.

In preliminary experiments SIS junctions were pre- pared on 0.35 1110 Si substrates covered by 200 nm pro- tective layer of Al2O3, the quality and the yield of the junctions were high enough. The best figures at T = 4,2 K were Vm = 50 mV, R(2mV)/Rn = 30 - 35, 6Vg = 50 - lOOpV. The spreading of the parameters on the substrate was small, for 10 U square junctions root- mean-square deviations of IC and Rn were as low as 1%. But Si substrates are not the best for microwave appli- cations because of high value of dielectric constant E

and microwave loss. That is why we have tested several low E substrate types: glass, fused and crystalline quartz; the thickness of all the substrates was 0.15 - 0.2 mn. We have found that the quality and yield of the junctions on the crystalline quartz substrates are close to the ones on Si, whereas figures for glass and fused quartz are worse. It can be explained by different thermal conditions during formation of Nb-A1 interface. If the temperature is high enough the diffusion A1 in Nb through grain boundaries take place; as a result the thickness of the normal layer on the interface is increased and tunnel barrier beco- mes nonuniform. We have verified the thermal origin of these phenomena by changine the rate of the Nb base electrode deposition and time between Nb and A1 sput- tering. The increasing of the Nb deposition rate far above 1 nm/s lead to the temperature growth and A1 dif- fusion. This smearing of the boundary is easily seen during anodization process by monitoring the Va/dt on V, dependence6, where Va - is anodization voltage.

Junction capacitance per unit area C/A was determined by nteasuring junction resonances (Fiske modes) in specially prepared long Josephson junction with critical current density jc= 500 A/cm2 (Rn-A = = 300 n-v2 - the' typical value for in microwave applications). The values t/c= 0,27 nm and C/A = 0.032 2 0.005 pF/p2 were calculated from measu- red velocity tr = (1,56 2 O,l)-lO9 cm/s using figures for penetration depth 2A = 100 nm. This value of speci- fic capacitance has been confirmed in two separate experimens: a) the experimental dependence of the "return current" of the resistively shunted Josephson junction5 on @ = (2e/h)-Ic-%2-C guite agree with the theoretical one (see, for example, ) for this value of CIA; b) the resonance voltage in the digital RSFQ curcuits 7 coincide with the calculated figure based on the measured value CIA.

Thus' on low E substrates we have fabricated the small area high quality Nb-AlG-Nb junctions with small specific capacitance by SNAP. As an example, the

junctions used

OO18-9464/89/03oO-1 Oaosol .(NO1 989 IEEE

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IVC of 3 square junction with ratio R(2mV)/Rn = 30 was shown in Fig. 1. These junctions are suitable for microwave appications and low spreading of the para- meter on substrate give a possibility to prepare ar- rays and integrared circuits.

Computer Simulation of the Mixer Performance

The experimental IVCs of all SIS junctions which were measured by data acquisition system were stored in data base as a set of the points ( up to 500 values of I and V for IVC). Then the analytical curve with ten variable parameters:

A1 + A2V A,exp{(V-A6)/A,)2 + A8+ A9V I(V) =

l+exp {-A3( 1 -A4V)2m]+ l+expfi3 ( 1 -AloV) 2n] f

where m, n = 1,2, ..., was fitted to the experimental one by simplex method. The experimental and fitted ana- lytical IVCs are shown in Fig. l by dashed and solid lines respectively. This fitted analytical curve was used subsequently in all computer simulations of mixer performance.

mixer model with very low intermediate frequency (IF), see, for example, 8. Here the incoming signal can be treated as a small differential change in the applied local oscillator (LO) waveform with intermediate frequency. It is assumed that no harmonic voltages appear at the rf terminals of the mixing elements. This model is closely aproximated by the common experimental situation, where SIS junction have relatively large geometrical capacitance ( y = 2n*f.Rn.C > 1) which effectively shorts out any harmonic currents. The large conversion efficiency can be achieved only under the condition that this capacitance is tuned out at the LO frequency.

The linearized equivalent circuits seen by the mixing elements both at the rf and low frequency are shown in the insertion in Fig. 1. Here Ro represents the small signal resistance of the mixing elements under LO power; Cu - is uncompensated part of the junction capacitance, which is partly tuned out on the LO frequency by compensating microwave circuits. For example, in calculations we used the value Cu = 0.1 pF for the junction from Fig. 1, instead of the total capacitance C 0.3 pF. The input data for the program are as follows: fitted to the experiment analytical IVC, source resistance R,, L G Dower and frequency,

We have used the simplified heterodyne

- L-',da

- -,.a

ua.0 - 5 . 0 - -

31.0 - - --9.0

- - . 3 .0 .-.a -

' ' ' - - .7 .0

Fig.1.

v.mv . .O I.O a.- e.0

Experimental (dashed line) and fitted ana- lytical (solid line) IVCs for 3 p square Nb-AIOx-Nb junction. The calculated pumped IVC and corresponding IF output are also shown. The linearized equivalent circuits seen by the mixing elements both at rf and IF are shown in the insertion.

40.0

4.0

-2.0

- 8 . 0

-<q.0

-20.0

t- \ \ I

Fig. 2. Calculated curves of equal conversion ef- ficiency L-l for the junction from Fig. 1. The source resistance Rs is normalized on Rn, the LO power PLO- on Po = hfVg/2eRn. The cross marks the parameters which is used for calcula tions in Fig. 1.

value Cu and load resistance RI. LO voltage Vrf is dependent on vdc; it is calculated from trancendental equations. It was possible to calculate the pumped IVC and conversion loss ( see Fig. 1 ) both for the case of fixed dc bias and for the optimization of the bias voltage.

The calculated curves of equal gain L-l on frequen- cy 46 GHz for the junction from Fig. 1 are shown in Fig. 2. The cross in the zone 0 - 10 dB marks the para- meters which were use for calculation of the pumped IVC and conversion efficiency in Fig. 1. The zone of the large gain appear at noticeable mismatch bet- ween junction and source resistance. This effect can be explained by the fact that at high Rs the Vrf is de- termined by junction impedance, which is dc voltage dependent. When bias voltage is increased the impedan- ce is decreased, as a result the rf voltage Vrf is al- so decreased. It leads to lowering of the dc cur- rent, that is to the growth of the differential resis- tance Rd. The value of Rd determines the junction out- put impedance and conversion efficiency L-l: the higher Rd, the larger gain. Thus some mismatch on mixer input leads to the growth of the Rd and increasing of the conversion efficiency.

The other source of the high and even negative diffferential resistance can be the "knee" on the IVC of the Nb based junctions. It was demonstrated experimentally9 and in numerical simulations that the existence of the "knee" structure leads to increasing of the conversion efficiency both at voltages higher than gap voltage Vg for low LO power and at voltages below gap for higher LO power level. It explains our experimental observation a quantum mixing at the frequencies f where hf/e < AVg.

Microwave Test Apparatus and Mixer DesiRn

Microwave measurements of SIS junctions and arrays were performed in waveguide test system: a schematic diagram of the apparatus used for measuring of the mixer gain and noise is shown in Fig. 3. Continuously tunable 30-55 GHz CW oscillators are used as a source of coherent signal and LO power; they are combined in a directional coupler. Directly on the input of the mixer immersed in liquid He the directional coupler on a crossed waveguides is installed, it attenuates the thermal noise from the top flange. The hot-cold rf loadlo is installed on input flange of the branching shoulder of the directional coupler; it is possible to change the noise signal on the input of the mixer from 2,5 to 40 K .

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f DC bias meter

I "V" ----I 0

Fig. 3. Schedtic diagram of the measuremen: system.

Semirigid coaxial cable is used to connect mixer IF output to the top of the cryostat, the same cable is used for dc bias of the SIS junction. The separation IF arid dc is provided on the top of cryostat. IF signal after room temperature isolator and FET amplifier ( F = 1-1000 MHz, TIF = 150 K ) go through bandpass filters ( F = 980 MHz, It is possible to switch electrically the input of the IF amplifier from the mixer output to the IF hot-cold loadlo, this load is mounted in hellium near mixer. The bidirectional coupler is used together with an external oscillator and detector to measure the coupling efficiency and reflection coefficients of the mixer and load.

A F = 40 MHz).

The brass mixer block (see Fig. 4) consists of two parts: one includes the input flange and adjustable lateral backshort in the wide side of the waveguide, the other part contains a section of full height waveguide with sliding backshort. Both backshorts are non-contacting type, their position can be adjusted with accuracy better than 10 m. The quartz substrate (10 mm x 1,5 nnn x 0 . 2 nnn) with SIS junction and rf choke filters are installed in a special sample holder (see Fig. 4). This holder is clamped between two parts of the mixer block so that one electrode of the junction is connected to the ground by small In tab. The substrate is situated close to one wall of the waveguide; it is equivalent to the reduction of a waveguide impedance relatively to rf terminals of a SIS junction. The length of the first stage of rf fil- ters is chosen so that its capacitive impedance serial- ly connected with SIS junction, tuned outll inductance of junction thin film electrodes.

Mixer tuning, optimization of the dc bias and LO power have been provided on small signal level noticeably below SIS saturation (output IF signal 5 -100 dBm); the mixer conversion loss (SSB) as low as 4 dB have been measured. After tuning the noise signal from the variable temperature rf load was applied to the mixer input, and difference in the output signal

m

Fig. 4 . Mixer block (a) and SIS junction mounting con- figuration (b): 1- rear full height waveguide with sliding backshort; 2 - sample holder; 3- substrate with SIS junction and rf filters; 4- semirigid coaxial cable.

corresponding to the changing of the 'load temperature at mixer input (usually between 4,2K and 30K) was measured. The mixer output have been calibrated by hot-cold IF load. Preliminary experiments with single SIS junction were performed and conversion efficiency L-1 (SSB) = 0,25 2 0,05

mate that the mixer noise temperature is smaller than 30 K. It is because of the thermal noise from a room temperature isolator is reflected from a junction and hot-cold load. It leads to a large and uncertain contribution to the output IF signal siderably rements.

have been achieved. In our system up to this moment we can only esti-

and decrease con- the accuracy of the noise temperature measu-

SIS Arrays with Tuning Circuits

Arrays with several number of SIS junctions have been used in a number of mixer realizations. It have been demonstratedl2,13that for N junctions arrays the dynamic range is extended and the array input impedance increases proportionally N at the same optimal value of Y = 3 - 5. But for series arrays the indentity of the junctions is very important, this requirement imposes limitations on the lowering of the junctions area and on increasing the junctions number N. The array with parallel dc bias have been introduced133 14 which overcomes the above mentioned limitations. We also proposed13~1~ the three electrodes circuit including two-junction array with parallel dc bias, where both rf electrodes are directly connected to a waveguide and dc bias circuit is used to tune out the junction capacitance.

Series array of SIS junctions tuned by inductive elements inte rated close to the array have been describedl2,15. Impedance measurements on large-scale

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Thus our previous results99 13* l4 and all foregoing investigations demonstrate that the Nb-AIOx-Nb parallel DC bias arrays with tuning out circuits are very promising as m wave mixing elements.

References

[l] J.M.Rowel1, M.Gurwitch and J.Geerk, "Modification of Tunnelling Barrier on Nb by a Few Monolayers of Al", Phys.Rev:B, Vol. 24, pp.2278-2281, 1981.

[2] H.A.Huggins and M.Gurwitch, "Preparation and Characteristics of Nb-Al-Oxide-Nb Tunnel Junc- tions", J.Appl,Phys., vol. 57, pp.2103-2109, 1985.

[3] M.Yuda, K.Kuroda and J.Nakamo, "Small Nb/Al-Oxide/Nb Josephson Junction Fabrication

Fig. 5. Schematic diagram of the parallel dc bias array with tuning out circuits. The counter electrode is hatched; the additional shorts from a soft material are shown by dot-dashed lines.

model demonstrated15 the noticeable influence of the stray thin film inductance. The two individually tuned SIS junctions have been suggested1& which overcomes the difficulty associated with the series inductance of inductively tuned array.

We propose the parallel dc bias Nb-Al0,-Nb array (see Fig. 5) with tuning circuits based on shortened two-wire lines. It is possible to use the additional shorts from the soft material (shown by dashed lines in Fig. 5) and change the inductance by partial remo- ving of the shorts. Without additional shorts the length of the two-wire lines are close to quarte wave- length, so impedance of the lines is much higher than the junction impedance, and this array works as a usu- al series array. The investigation of the large scale model of the array on the frequencies 100-500 MHz demonstrated that this system of coupled oscillatory circuits with N degree of freeedom have a set of normal modes. For realizing the in-phase oscillations with equal rf current amplitude in any array section the additional capacitors with the same value as junction capacitance were designed. These capacitors were formed by bottom and counter electrodes with 250 nm Si0 interlayer and were placed near all junctions except the outer ones. Characteristic impedance p of the two-wire lines was estimated from large-scale measurements as 140 ? 5n at strip width and separation between strips 10 pm for 0,15 mm glass substrate.

)

The proposed Nb-AIOx-Nb arrays have a number of advantages in comparison with traditional series arrays: - lowering of the junction nonuniformity influence13 (i.e. increasing the junction parameters tolerance) ;

- optimization of the array output impedance by choice the junction nimber N l3 providing the rf matching simultaneously;

- decreasing the stray inductance of the tuning cir- cuits;

- possibility to provide the frequency tuning; - absence of any dc/IF block (like decoupling tance CB or quarte-wave transmission line stuby5).

ca aci-

The investigation of these arrays is underway, but preliminary results prove the possibility to tune out the junction capacitance in the array. It have been demonstrated by coincidence of the experimental pumped IVC with the theoretical one for negligibly small capacitance. It should be noted that for array with compensation circuits only one external tuning is necessary and it works more smoothly.

Using Lift-off Process" 26, pp. L166-L168, 1987.

Jap.J.of Appl.Phys., vol.

[4] H.Kroger, L.N.Smith and D.W.Jillie, "Selective Niobium Anodization Process for Fabricating Josephson Tunnel Junctions", Appl.Phys.Lett., V01.39, pp. 280-282, 1981.

[5] Yu.E.Zhuravlev, V.P.Koshelets, A.N.Matlashov, I.L.Serpuchenko, L.V.Filippenko, "DC-Squid Pre- amplifier for DC-Squid Magnetometer", presented at the ASC-88, San-Francisco, August 21-25, 1988.

[61 K.K.Likharev, "Dynamics of Josephson Junctions and Circuits", N.Y., Gordon and Breach Science Publishers, 1986, Ch.4, pp.96-99.

[71 V.K.Kaplunenko, M.I.Khabipov, V.P.Koshelets, K.K. Likharev, O.A.Mukhanov, V.K.Semenov, 1.L.Serpu- chenko, A.N.Vystavkin, "Experimental Study of the RSFQ Logic Elements", presented at the ASC-88, San-Francisco, August 21-25, 1988.

[81 J.R.Tucker, M.J.Feldman, Quantum Detection at Millimeter Wavelengths", Rev. of Modern Physics, vol. 57, pp. 1055-1113, 1985.

[ 9 I V. P. Koshelets, S. A. Kovtonyuk, G.A. Ovsyannikov, I.L.Serpuchenko, S.V.Shitov, A.N.Vystavkin, "Re- fractory Material Superconducting Structures for mm Wave Receivers", Extended Abstracts of ISEC'87, August 28-29, 1987, pp. 111-113.

[lo] W.R.McGrath, A.V.Raisanen and P.L.Richards, "Variable-Temperature Loads for Use in Accurate Measurements of Cryogenically-Cooled Microwave . - Amplifiers and Mixers", 1nt.J.Infrared and MM Waves, vol. 7, pp. 543-553, 1986.

[ll] A.V.Raisanen, D.G.Grete, P.L.Richards and F.L. Lloyd, "Wide Band Low Noise mm-Wave SIS Mixer with Single Tuning Element", 1nt.J.Infrared and MM Waves, vol. 7, pp. 1835-1852, 1986.

[12] P.L.Richards, "Progress in the Development of SIS vol. Quasiparticle Mixers", IEEE Trans. on Magn. ,

MAG-23, pp. 1247-1253. 1987.

[13] V.Yu.Belitsky, V.N.Gubankov, V.P.Koshelets, G.A. Ovsyannikov, I.L.Serpuchenko, S.V.Shitov, M.A.Tarasov, A.N.Vvstavkin. "Refractory Material SIS Junction Structures", IEEE Trans. on Magn., vol. MAG-23, pp. 684-687, 1987.

[ 141 V.Yu.Belitsky, V. P.Koshelets, G. A.Ovsyannikov, S .V. Shitov, "Microwave Detector", a.c. No1270869, 1985.

[151 A.R.Kerr, S.K.Pan and M.Y.Feldman, "Integrated Tuning Elements for SIS Mixers", 1nt.J.of Infrared and MM Waves, vo1.9, pp. 203-212, 1988.