IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 9, NO. 2, JUNE 1999

Microstructure and RF' Property Correlation in HTS Films

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P. A. Smith, M. A. Bakar, A. Porch and T. W. Button IRC in Materials and School of Electrical & Electronic Eng., University of Birmingham, Edgbaston, Birmingham, U.K.

Abstract-In this paper we report on the assessment of a range of HTS thick and thin films using a small dielectric resonator, operating near 10 GHz, capable of measuring film areas approximately 3 mm in diameter. This is achieved with the utilisation of high permittivity rutile dielectric resonators operating in the "Eols mode which have a permittivity of approximately 105 at 77K. Some HTS thick films have well- defined grains which are larger than the measurement area, and thus the influence of grain boundaries and other microstructural features on the RF properties of the films can be quantified directly. The dielectric loss of the rutile at 8GHz was measured and the housing losses due to the normal copper enclosure were calculated by Superfish, a finite difference programme for the solution of modes with cylindrical symmetry.

I. INTRODUCTION

High temperature superconducting (HTS) films produced by a wide variety of material deposition processes are under active development for a range of applications in both the RF and power engineering fields. The materials range from textured polycrystalline thick films deposited from colloidal inks [ l ] to epitaxial films on single crystal substrates deposited by thin film techniques [2]. More recently, biaxially textured YBCO films are attracting much interest [3]-[4]. A common feature with all these materials is the desire to understand the dependence of the properties on the microstructure.

A dielectric resonator is essentially an electromagnetic resonator that becomes self resonant when either its length or diameter, or both, become one half of a wavelength long. In actuality the resonant frequency depends on the permittivity and metallic enclosure as well as the linear dimensions. When a dielectric resonator (DR) or puck is surrounded by a conducting enclosure the Q of the total structure is a combination of the dielectric losses in the puck and the conductor losses in the walls. When the DR rests on a conducting surface, most of the conductor losses are due to the base plate. Consequently such a structure can be used to measure the high frequency surface resistance of a HTS sample. If the permittivity of the dielectric puck is large then, for a given frequency, the dimensions'of the puck are reduced, and the resonator can be used to measure the surface resistance of very small areas [5].

In this paper we describe the utilisation of high permittivity rutile dielectric resonators operating in the TEols mode for the characterisation of HTS films at frequencies in the range 8- 10GHz. The facility is also useful in that it allows the determination of the homogeneity of large area films. It also

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permits a correlation between the microstructure and morphology of thick films and the surface resistance. The construction of the measurement jig is described, preliminary results and electromagnetic analyses for various HTS films are presented.

11. EXPERIMENTAL

A. Design and Construction of the Test Cavities

The outline of the resonator is shown in Fig.1. In order to measure the dielectric loss of the DR it is supported on a quartz support, marked q, in the centre of a circular copper enclosure. When used to measure the surface resistance of the base plate or HTS film the quartz support is removed so that the DR sits directly on the surface to be characterised. The whole enclosure can be immersed in liquid Nitrogen whilst maintaining a Helium gas atmosphere inside thus enabling measurements to be carried out at 77K. Magnetic coupling was achieved with loops projecting into the cylindrical wall of the enclosure. All RF measurements were carried out using an HP8720 Vector Network Analyser with a resolution of 1Hz.

Two different single crystal dielectric resonators were employed in this work, one made of sapphire and the other of rutile. The sapphire DR was used because it had a much lower dielectric loss at 77K than the rutile and hence it was more sensitive, however the rutile DR had a much larger permittivity than that of sapphire, (105 rather than 9.4), and hence it could characterise much smaller areas. The dimensions of the DR and their enclosures are listed in Table I. The different parameters d , h, D, and H are shown in Fig. 1.

B. Measurement of Dielectric Loss

The Dielectric loss of the rutile DR was measured in the experimental configuration shown in Fig.1. By supporting the rutile upon a quartz spacer 4 mm high the electromagnetic energy is confined almost totally within it and hence the conductor losses on the walls are small. By convention,

D

d 4 .

Manuscript received September 15, 1998 Fig. 1. Dielectric Resonator on a quartz support.

1051-8223/99$10.00 0 1999 IEEE

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T A B L E I DIMENSIONS OF THE DIFFERENT SINGLE CRYSTAL DIELECTRIC

RESONATORS EMPLOYED Parameter Sapphire Rutile

h (mm) 6.32 1.7

H (mm) 14.7 8.0

d (mm) 12.64 3.4

D (mm) 39.0 8.0

however, in order to be able to exclude the effect of the wall losses for materials with permittivity >30, the diameter and length of the surrounding cavity must be at least three times those of the diameter and length of the DR [6]. As this was not this case for the rutile resonator used in this work the contributions due to the wall losses were computed by Superfish, a finite difference programme produced by Los Alamos National Laboratories [7]. A summary of the results is presented in Table 11. The losses of a DR within a conductive enclosure, as in Fig. l., can be sub-divided into the dielectric loss of the material from which the DR is made, together with the conductor losses in the metallic walls of the enclosure. Thus the overall Q can be expressed as

Q Q Dielectric Q w a l i ~ o s s e s

\ - I

where QDielecrric is the Q of the dielectric material from which the DR is made, i.e. UTan 6, and QwU11 Lvsse.7 is the Q due to the conductor losses in the base plate, cylindrical side walls, and top plate. At 8.865GHz the value of QDielecrric was 76,650 but when the DR was on the base plate the frequency of operation became approximately 9.87GHz. Since Q ~ i ~ l ~ ~ ~ ~ i ~ is inversely proportional to frequency its value must therefore be scaled from 8.865 to 9.87GHz. Q ~ i ~ i ~ ~ ~ ~ ; ~ for the sapphire DR was taken to be lo7 [SI and therefore had negligible influence.

C. Calculation of Surface Resistance

When the DR rests on the base plate the conductor wall losses, Qw,ll Lu,7.res can be sub-divided into

where QBp, Qcy and Qrp are the conductor Q ’ s associated with the base plate, cylindrical wall and top plate respectively. These three quantities can all be calculated by Superfish, Hence together with QD;p[ectric, Q c y and QTP it is

T A B L E I1 RESULTS FOR THE DIELECTRIC LOSS OF THE RUTILE DR MEASURED

AT 71K AND 8.865GHZ SUPPORTED ON QUARTZ Q Qwar, ~nssei QDic/emic Q D ~ ~ I ~ ~ , ~ ~ at 9.87GHz (scaled)

At 8.865GHz

53620 179,240 76,650 68,830

possible to calculate QBP when the DR rests on a HTS base plate. Once QBP is known then together with the: geometric factor, G again calculated by Superfish, the surface resistance R, can be calculated from

G R, =- QBP

The conductive losses for the metallic housing (of each Dli were calculated and expressed as QConducror, i.e. including the top plate and cylindrical wall losses. QConducror was 800,000 and 583,000 for the rutile and sapphire DR respectively. The geometric factor for the base plate was 175 and 570 for th.e rutile and sapphire DR respectively.

D. Characterisation of HTS Films

The sensitivity, accuracy and repeatability of thle measurement system incorporating the rutile resonator was investigated using a HTS thin film sample supplied by DERA Malvern. A range of HTS thick film samples were subsequently investigated using both the sapphire and rutile resonators in order to demonstrate the feasibility of measuring the microstructural dependence of the RF properties of these materials. The calculated error in the surface resistance measurements of the thick film materials was Itlo%, other errors are as shown in the tables. The YBCO thick film samples were prepared by a melt- processing route on polycrystalline zirconia substrates which is well documented elsewhere [9].

111. RESULTS AND DISCUSSION

A. Low Power Surface Resistance values

The results for a range of different base plate materials are shown in Tables I11 and IV for the rutile and sapphire DR.s respectively. Previous measurements on thin films produced by DERA suggest an R,7 of about 0.150 m a at 9.87 GHi!, [lo], which is in reasonable agreement with the results obtained here. It should be noted that the results presented here for the thin film sample are subject to an error margin of about 50% because the conductor losses in the cylindrical walls of the rutile test jig considerably limit the sensitivity that can be attained. However in future test systems this could be overcome by spacing the side walls further away. The

TABLE I11 SURFACE RESISTANCE MEASUREMENTS FROM THE RUTILE. DR AT 9.81

GHZ ON VARIOUS SUBSTRATES Film Q Q S O X R,(mQ) -

HTS Thin Film 60,692 143,1200 0.12W.06 Copper 11,840 14,560 12.w1.0 TF2408 29,390 54,815 3.20

TF2349D 27,493 48,560 3.61 TF2596 27,340 48,077 38.64

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TABLE IV SURFACE RESISTANCE MEASUREMENTS FROM THE SAPPHIRE DR AT

8.89GHZ ON YBCO SUBSTRATES

TF2408 171290 242550 2.35 TF2349D 149400 200900 2.80 TF2596 177700 255600 2.23

Film Q QBlW R, (ma)

theoretical value for the surface resistance of the copper used was approximately 1 0 d so this also agreed reasonably well with the value measured with the rutile DR.

It is well documented that YBCO thick film samples have a higher surface resistance than the good thin film samples [l] and this is also demonstrated by our results. However, both thick film samples, TF2349D and TF2408, have a higher measured surface resistance with the rutile resonator than with the Sapphire resonator. This is to be expected because the rutile resonator operates at 9.87GHz as opposed to 8.89GHz for the Sapphire resonator. Our data is in reasonable agreement with the dependence of surface resistance on frequency being approximately proportional to the square of the frequency. This is shown in Table V where the surface resistance values obtained from the Sapphire DR at 8.89GHz have been scaled up to those at 9.87GHz, at which the rutile DR operates.

It should be also be noted that the RF magnetic fields used for these measurements are different for the two resonators. In the rutile DR the RF magnetic field is approximately 30Nm whereas that for the sapphire DR was only about 10Nm. Consequently in addition to a difference in operating frequency there was superimposed upon this a difference in effective RF magnetic field, which may cause an increase in the measured frequency dependence. The dependence of the surface resistance upon elevated RF magnetic field is discussed in further detail below.

B. Correlation of low power surface resistance results with microstructure

Melt processed YBCO thick films commonly possess a spherulitic grain morphology, with the scale of the spherulites ranging from 1-lOmm [ l l ] . In order to investigate any correlation between the microstructure and RF properties of thick film samples two films were chosen which had different spherulite sizes. Film TF2596, shown in Fig.2., had spherulites that were of the order of 5mm in diameter and thus, with the rutile DR of diameter 3.4mm, it was possible to examine the surface resistance of individual spherulites and

TABLE V FREQUENCY SCALING OF THE SURFACE RESISTANCE RESULTS FROM

THE TWO DRs

Film RS R, (mQ) (mQ) Scaled from 8.89GHz

at 9.87GHz using Rpcfreq’ TF2408 3.20 2.89

TF2349D 3.61 3.45 TF2596 7-64 2 I5

Fig.2. Optical micrograph of film TF2596 showing spherulites -5mm in diameter

inter-spherulitic regions. These measurements were compared with those from sample TF2408 which had much smaller spherulites around Imm in diameter. The three positions A, B, and C marked on Fig.2. correspond to the centre of a spherulite, the boundary between two spherulites and the. boundary between several spherulites respectively. The surface resistance values measured at these three positions are listed in Table VI.

Thus there appears to be a strong positional dependence of the surface resistance values measured for TF2596, with the centre of a spherulite possessing the lowest Rs which then increases as the number of inter-spherulitic boundaries in the measurement area is increased. Repeated measurements at various positions on film TF2408 revealed no such correlation. However, as the diameter of the rutile DR is much larger than the average diameter of the spherulites in this sample, all measurements would incorporate some boundary regions. It is interesting to note that the higher surface resistance values measured for the inter-spherulitic regions of sample TF2596 were much higher than the values measured using the larger sapphire DR on this sample. This may again be due to the different RF magnetic field levels of the two measurements.

C. High Power Surface Resistance Results

Other investigators have found that the measurement of the non-linearity of a HTS film is a more discriminatory technique for examining film quality. In order to try and study the effect of spherulite boundaries upon the non linear performance of HTS thick .films, samples TF2408 and

TABLE VI CORRELATION BETWEEN SURFACE RESISTANCE AND MICROSTRUCTURE AS MEASURED BY THE RUTILE DR Film Position Rr (mQ)

TF2408 Centre 3.20 TF2596 A 3.64 Tf2596 B 5.04 TF2596 c 9.08

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0 100 200 300 400 RF Magnetic Field (Nm)

Fig.3 Power Dependence graph measured with the rutile DR. The curves marked T, Cu and D are those of the HTS Thin film, copper and thick film base plate TF2408 respectively. The curves marked A, B, and C are those of the thick film sample TF2596, measured in the middle of a spherulite, on a boundary, and several boundaries respectively.

TF2596 were examined at high RF power levels in the rutile DR. These results are presented in Fig.3, which also shows similar measurements on the DERA thin film sample, and a polycrystalline copper plate.

One significant problem with performing power dependence measurements using a rutile DR is the very large temperature dependence of the permittivity of the rutile [12]. Since the Dielectric Q of the rutile is relatively low approximately half of the total dissipated power is lost in the form of dielectric heating in the rutile. This leads to frequency shifts due to the change in permittivity of the rutile and also due to changes in the penetration depth of the HTS as it is warmed up by the rutile DR resting on it.

It is believed that the apparent power dependence displayed by the thin film sample and the copper is caused by the thermal effects mentioned earlier. Pulsed power measurements would have allowed the performance of samples to be examined at high RF fields without generating thermal effects, but unfortunately such facilities were not available. Sample TF2596 showed considerable variation in power dependence depending upon its location. The lowest degree of power dependence was displayed in the centre of a grain and then steadily worsening performance depending upon the number of boundaries which were present beneath the rutile DR. It is also interesting to note that much larger differences are displayed in RF performance at high RF fields than were evident at lower fields. Although the performance of TF2596 is poor such behaviour is not displayed in the smaller grained film TF2408 and indeed is not found in the overwhelming majority of thick films previously tested.

IV. CONCLUSIONS

We have demonstrated RF test systems based on single crystal sapphire and rutile dielectric resonators. Surface

resistance measurements in both systems have been shown to be comparable. Due to the high permittivity of the rutile DR, the system operating around lOGHz has been used to characterise small areas of HTS films. Significant positional variations in the surface resistance and power dependence of some thick samples have been shown. It is not posisible at this stage to draw any conclusions about the type of surface morphology required for good RF performanoe in these materials. However, the rutile DR seems well suited to studying the surface resistance of small areas of HTS films and further work on development of the measurement system and the correlation of microstructure with RF performance is in progress.

ACKNOWLEDGEMENT

The authors are grateful to Professor R Humphreys of DERA Malvern for the provision of thin film HTS samples, and to Dr J. Powell for useful discussions. The authors are also grateful to Mr C.Meggs and Mr G.Dolman for their assistance in the experimental work.

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