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YBCO wafer qualification by surface resistance measurements combined with performance studies of microstrip resonators
Physica C 351 (2001) 25±28
www.elsevier.nl/locate/physc
YBCO wafer quali®cation by surface resistance measurements combined with performance studies of microstrip resonators R. Schwab a,*, E. Gaganidze a, J. Halbritter a, R. Heidinger a, R. Aidam b, R. Schneider b a
Forschungszentrum Karlsruhe, Institute for Materials Research I, P.O. Box 3640, D-76021 Karlsruhe, Germany Forschungszentrum Karlsruhe, Institute for Solid State Physics, P.O. Box 3640, D-76021 Karlsruhe, Germany
b
Abstract High quality double-sided YBCO wafers with ceria buered sapphire substrates are analysed for the lateral homogeneity and ®eld dependence of the surface resistance (RS ) using an open resonator at 145 GHz and a dielectric resonator at 8.5 GHz. In a following step, a set of linear microstrip resonators are patterned on these wafers and are analysed for their high frequency performance at 4 GHz. The results reveal surface resistance analysis of YBCO wafers as a reliable basis for materials quali®cation. At low power levels the high Q-factors of the microstrip resonators (about 30 000) are consistent with the measured RS -values at the considerably higher frequencies. Further the power handling capabilities of the resonators agree with the measured ®eld dependence of RS . Surface resistance mappings at 145 GHz eectively indicates critical ®lm areas for non-optimum microstrip resonator performance. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Surface impedance; Microwave measurement techniques; YBCO ®lms
1. Introduction The successful application of high temperature superconductors in passive microwave devices is based on the reliable control of the microwave properties of epitaxially grown large area YBCO ®lms. The surface resistance (RS ) is generally accepted as the essential material parameter and non-destructive RS -characterisation of the as grown ®lms gives important information on the ®lm quality for both the ®lm producers and component
* Corresponding author. Tel.: +49-721-82-5921; fax: +49721-82-4567. E-mail address: [email protected] (R. Schwab).
designers. Nevertheless, a critical aspect is the transferability of the measured RS -data to the performance of patterned components which for example may be aected by the structuring process. In this paper results of the RS -characterisation of two double-sided 300 -YBCO ®lms (thickness 250±300 nm) on CeO2 -buered sapphire (KarL204 and KarL207) analysed for lateral RS -homogeneity and microwave magnetic ®eld (Hrf ) dependence are compared with the performance of microstrip resonators which are formed on the wafers by patterning one of both ®lms referred as ÔtopÕ ®lm. They are deposited by the ICM-sputtering technique [1] in processing studies of high quality YBCO wafers with controlled and outstanding microwave performance.
0921-4534/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 ( 0 0 ) 0 1 6 8 4 - 1
26
R. Schwab et al. / Physica C 351 (2001) 25±28
2. Non-destructive RS -characterisation The as grown YBCO ®lms are tested for their lateral RS -homogeneity by the open resonator technique at 145 GHz and for their RS (Hrf )dependence with a dielectric resonator at 8.5 GHz. The spatially resolved RS -measurements are obtained by analysing the Q-factor of the fundamental gaussian TEM00q mode of a hemispherical open resonator which tests a ®lm area of about 6 mm diameter with its gaussian intensity pro®le [2]. With the specially designed gas ¯ow cryostat system (cf. Figs. 1 and 2) a two dimensional RS mapping of the ®lm area is obtained at a temperature of 73 K via a radial and rotational degree of freedom. In Fig. 3 the RS -maps of the top ®lms of KarL204 and KarL207 are presented. Whereas the grey scale map of KarL204 shows a uniform RS -distribution, the top ®lm of KarL207 exhibits an inhomogeneous RS -distribution with extended patterns of dierent RS -levels, the highest levels near the centre of the ®lm and the lowest levels in the outer areas. The RS -maps in Fig. 3 are given together with the homogeneity parameters D50 , D10 and D90 quantifying the RS -distribution [3]. These data, which are also given for the bottom ®lms, clearly reveal the higher RS -level (D50 ) and the increased RS -spread (D90 ±D10 ) of the wafer KarL207. Whereas the lateral RS -homogeneity is analysed at extremely low microwave power levels, measurements of the Hrf -dependence are performed
Fig. 1. Open resonator system for RS -homogeneity analysis at 145 GHz.
Fig. 2. Dielectric resonator for power dependence analysis at 8.5 GHz.
Fig. 3. RS -homogeneity analysis of the wafers KarL204 and KarL207 at 145 GHz and 73 K.
R. Schwab et al. / Physica C 351 (2001) 25±28
with a dielectric resonator arrangement [4] (cf. Fig. 2). In a closed housing a sapphire cylinder (diameter 16 mm, 8 mm high) is arranged between two YBCO ®lms testing a central area of both YBCO ®lms of about the sapphire diameter at 8.5 GHz in the TE011 mode. Cooled directly in LN2 the Q-factor is analysed up to a ®eld strength of 10 mT using the decay method with pulses below 200 ls to avoid thermal eects on RS . Data evaluation for RS and Hrf is based on numerical simulations using the MAFIA code. From the measured Q-factor the individual RS of the sample ®lm can be ®xed using a well known reference ®lm. The measured RS (Hrf )curves (cf. Fig. 7) show substantially ®eld independent RS -data at a low RS -level for the top and the bottom ®lm of wafer KarL204. In contrast, wafer KarL207 shows higher RS -levels already at low ®elds and a pronounced nearly linear ®eld dependence with dierent slopes for top and bottom ®lm.
27
Fig. 4. Arrangement of the microstrip resonators. Dashed lines trace linescans at 145 GHz.
3. Correlation with microstrip resonator performance After the non-destructive RS -characterisation of the as grown ®lms a set of six linear microstrip resonators for KarL204 and of eight for KarL207 with a resonance frequency of about 4 GHz are patterned on the top ®lms using wet chemical etching [5]. The resonators are arranged along the central line as sketched in Fig. 4. The bottom YBCO ®lm is covered with Au in a pattern that avoids additional losses by leakage rf ®elds in the Au layer but gives sucient ohmic and thermal contact to the housing which is cooled directly by liquid nitrogen. S-parameter measurements with a HP8720C vector network analyser provide the unloaded Q-factor (Q0 ) as a function of the oscillating power Posc which is the adequate quantity to compare the power handling capability of the microstrip resonators (cf. Fig. 5). Evidently, the signi®cant dierences in wafer quality which are established by the non-destructive quali®cation go along with the microstrip resonator performance where KarL204 provides resonators with high and constant Q0 -values up to an oscillating power of 5 W in contrast to KarL207.
Fig. 5. Unloaded Q-factor of the microstrip resonators on KarL204 and KarL207.
The level of Q0 is quantitatively related with the conductor losses which are proportional to 1=Q0 if the contribution of substrate and housing can be neglected. The RS of the top ®lm is dominant even though the RS of the bottom ®lm contributes to an eective RS . This approach is applied in Fig. 6 where RS -linescans along the central line of the top ®lm are plotted together with the 1=Q0 data of the microstrip resonators operated in the low power regime. Clearly the RS -homogeneity analysis is able to describe the microstrip resonator data such as the considerably reduced Q0 -values and their signi®cant position dependence for the resonators formed on KarL207. Accordingly the low and homogeneous RS -values for KarL204 predict the consistently high Q0 -data of the microstrip resonator set.
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R. Schwab et al. / Physica C 351 (2001) 25±28
Fig. 6. RS -linescans along the central line ( , ) compared with 1=Q0 data of microstrip resonators for KarL204 ( ) and KarL207 ( ).
the ®eld range covered by the non-destructive method, it matches perfectly the eective RS -level and the ®eld independence of the central resonators (3 and 4) on wafer KarL204. Also it predicts for the central resonators (d and e) on wafer KarL207 the observed quasi linear dependence of RS (Hrf ). In this case, the eective RS -values from the microstrip resonator performance exceeds the values of the non-destructive characterisation. This may be due to uncertainties in the conversion factors and the applied frequency scaling exponent. Such deviations in absolute scaling are also apparent in the spatially resolved low power characterisation (cf. Fig. 6).
4. Conclusions The combination of open resonator and dielectric resonator methods provide spatially resolved and ®eld dependent RS -datasets for large area high quality YBCO wafers. The comparison with microstrip resonators subsequently manufactured prove that these datasets are adequate to predict component performance with respect to quality factors and power handling capability. Consequently these techniques oer an ideal combination for non-destructive ®lm quali®cation. Fig. 7. Power handling capability in the centre of KarL204 and KarL207 characterised by dielectric and microstrip resonator measurements.
The power dependent datasets Q0 (Posc ) for the microstrip resonators can be transferred into effective datasets RS (Hrf ) using conversion factors estimated by approximate formulas for this resonator type [5]. The dielectric resonator technique provides these RS (Hrf )-data non-destructively at 8.5 GHz for the ®lm area covered by the two central resonators. The quantitative comparison is done using a frequency scaling according to RS / xb with b 1:7 (cf. Fig. 7). Clearly within
References [1] J. Geerk, F. Ratzel, H. Rietschel, G. Linker, R. Heidinger, R. Schwab, Trans. Appl. Supercond. 9 (1999) 1543. [2] R. Schwab, R. Heidinger, R. Sp orl, Conf. Digest, 23rd International Conference on Infrared and Millimeter waves, Colchester, UK, 1998, pp. 379±381. [3] R. Heidinger, R. Schwab, Proc. EUCASÕ 99 Inst. Phys. Conf. Ser. No. 167, (2000) 89±92. [4] E. Gaganidze, R. Schwab, J. Halbritter, R. Heidinger, R. Aidam, R. Schneider, Appl. Supercond. Conf. ASCÕ 2000, 17±22 Sept. 2000, Virginia Beach, USA, IEEE Trans. Appl. Supercond., to be published. [5] R. Schneider, R. Aidam, A. Zaitsev, J. Geerk, G. Linker, F. Ratzel, R. Smithey, Physica C 351 (2001) 21, these Proceedings.
www.elsevier.nl/locate/physc
YBCO wafer quali®cation by surface resistance measurements combined with performance studies of microstrip resonators R. Schwab a,*, E. Gaganidze a, J. Halbritter a, R. Heidinger a, R. Aidam b, R. Schneider b a
Forschungszentrum Karlsruhe, Institute for Materials Research I, P.O. Box 3640, D-76021 Karlsruhe, Germany Forschungszentrum Karlsruhe, Institute for Solid State Physics, P.O. Box 3640, D-76021 Karlsruhe, Germany
b
Abstract High quality double-sided YBCO wafers with ceria buered sapphire substrates are analysed for the lateral homogeneity and ®eld dependence of the surface resistance (RS ) using an open resonator at 145 GHz and a dielectric resonator at 8.5 GHz. In a following step, a set of linear microstrip resonators are patterned on these wafers and are analysed for their high frequency performance at 4 GHz. The results reveal surface resistance analysis of YBCO wafers as a reliable basis for materials quali®cation. At low power levels the high Q-factors of the microstrip resonators (about 30 000) are consistent with the measured RS -values at the considerably higher frequencies. Further the power handling capabilities of the resonators agree with the measured ®eld dependence of RS . Surface resistance mappings at 145 GHz eectively indicates critical ®lm areas for non-optimum microstrip resonator performance. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Surface impedance; Microwave measurement techniques; YBCO ®lms
1. Introduction The successful application of high temperature superconductors in passive microwave devices is based on the reliable control of the microwave properties of epitaxially grown large area YBCO ®lms. The surface resistance (RS ) is generally accepted as the essential material parameter and non-destructive RS -characterisation of the as grown ®lms gives important information on the ®lm quality for both the ®lm producers and component
* Corresponding author. Tel.: +49-721-82-5921; fax: +49721-82-4567. E-mail address: [email protected] (R. Schwab).
designers. Nevertheless, a critical aspect is the transferability of the measured RS -data to the performance of patterned components which for example may be aected by the structuring process. In this paper results of the RS -characterisation of two double-sided 300 -YBCO ®lms (thickness 250±300 nm) on CeO2 -buered sapphire (KarL204 and KarL207) analysed for lateral RS -homogeneity and microwave magnetic ®eld (Hrf ) dependence are compared with the performance of microstrip resonators which are formed on the wafers by patterning one of both ®lms referred as ÔtopÕ ®lm. They are deposited by the ICM-sputtering technique [1] in processing studies of high quality YBCO wafers with controlled and outstanding microwave performance.
0921-4534/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 ( 0 0 ) 0 1 6 8 4 - 1
26
R. Schwab et al. / Physica C 351 (2001) 25±28
2. Non-destructive RS -characterisation The as grown YBCO ®lms are tested for their lateral RS -homogeneity by the open resonator technique at 145 GHz and for their RS (Hrf )dependence with a dielectric resonator at 8.5 GHz. The spatially resolved RS -measurements are obtained by analysing the Q-factor of the fundamental gaussian TEM00q mode of a hemispherical open resonator which tests a ®lm area of about 6 mm diameter with its gaussian intensity pro®le [2]. With the specially designed gas ¯ow cryostat system (cf. Figs. 1 and 2) a two dimensional RS mapping of the ®lm area is obtained at a temperature of 73 K via a radial and rotational degree of freedom. In Fig. 3 the RS -maps of the top ®lms of KarL204 and KarL207 are presented. Whereas the grey scale map of KarL204 shows a uniform RS -distribution, the top ®lm of KarL207 exhibits an inhomogeneous RS -distribution with extended patterns of dierent RS -levels, the highest levels near the centre of the ®lm and the lowest levels in the outer areas. The RS -maps in Fig. 3 are given together with the homogeneity parameters D50 , D10 and D90 quantifying the RS -distribution [3]. These data, which are also given for the bottom ®lms, clearly reveal the higher RS -level (D50 ) and the increased RS -spread (D90 ±D10 ) of the wafer KarL207. Whereas the lateral RS -homogeneity is analysed at extremely low microwave power levels, measurements of the Hrf -dependence are performed
Fig. 1. Open resonator system for RS -homogeneity analysis at 145 GHz.
Fig. 2. Dielectric resonator for power dependence analysis at 8.5 GHz.
Fig. 3. RS -homogeneity analysis of the wafers KarL204 and KarL207 at 145 GHz and 73 K.
R. Schwab et al. / Physica C 351 (2001) 25±28
with a dielectric resonator arrangement [4] (cf. Fig. 2). In a closed housing a sapphire cylinder (diameter 16 mm, 8 mm high) is arranged between two YBCO ®lms testing a central area of both YBCO ®lms of about the sapphire diameter at 8.5 GHz in the TE011 mode. Cooled directly in LN2 the Q-factor is analysed up to a ®eld strength of 10 mT using the decay method with pulses below 200 ls to avoid thermal eects on RS . Data evaluation for RS and Hrf is based on numerical simulations using the MAFIA code. From the measured Q-factor the individual RS of the sample ®lm can be ®xed using a well known reference ®lm. The measured RS (Hrf )curves (cf. Fig. 7) show substantially ®eld independent RS -data at a low RS -level for the top and the bottom ®lm of wafer KarL204. In contrast, wafer KarL207 shows higher RS -levels already at low ®elds and a pronounced nearly linear ®eld dependence with dierent slopes for top and bottom ®lm.
27
Fig. 4. Arrangement of the microstrip resonators. Dashed lines trace linescans at 145 GHz.
3. Correlation with microstrip resonator performance After the non-destructive RS -characterisation of the as grown ®lms a set of six linear microstrip resonators for KarL204 and of eight for KarL207 with a resonance frequency of about 4 GHz are patterned on the top ®lms using wet chemical etching [5]. The resonators are arranged along the central line as sketched in Fig. 4. The bottom YBCO ®lm is covered with Au in a pattern that avoids additional losses by leakage rf ®elds in the Au layer but gives sucient ohmic and thermal contact to the housing which is cooled directly by liquid nitrogen. S-parameter measurements with a HP8720C vector network analyser provide the unloaded Q-factor (Q0 ) as a function of the oscillating power Posc which is the adequate quantity to compare the power handling capability of the microstrip resonators (cf. Fig. 5). Evidently, the signi®cant dierences in wafer quality which are established by the non-destructive quali®cation go along with the microstrip resonator performance where KarL204 provides resonators with high and constant Q0 -values up to an oscillating power of 5 W in contrast to KarL207.
Fig. 5. Unloaded Q-factor of the microstrip resonators on KarL204 and KarL207.
The level of Q0 is quantitatively related with the conductor losses which are proportional to 1=Q0 if the contribution of substrate and housing can be neglected. The RS of the top ®lm is dominant even though the RS of the bottom ®lm contributes to an eective RS . This approach is applied in Fig. 6 where RS -linescans along the central line of the top ®lm are plotted together with the 1=Q0 data of the microstrip resonators operated in the low power regime. Clearly the RS -homogeneity analysis is able to describe the microstrip resonator data such as the considerably reduced Q0 -values and their signi®cant position dependence for the resonators formed on KarL207. Accordingly the low and homogeneous RS -values for KarL204 predict the consistently high Q0 -data of the microstrip resonator set.
28
R. Schwab et al. / Physica C 351 (2001) 25±28
Fig. 6. RS -linescans along the central line ( , ) compared with 1=Q0 data of microstrip resonators for KarL204 ( ) and KarL207 ( ).
the ®eld range covered by the non-destructive method, it matches perfectly the eective RS -level and the ®eld independence of the central resonators (3 and 4) on wafer KarL204. Also it predicts for the central resonators (d and e) on wafer KarL207 the observed quasi linear dependence of RS (Hrf ). In this case, the eective RS -values from the microstrip resonator performance exceeds the values of the non-destructive characterisation. This may be due to uncertainties in the conversion factors and the applied frequency scaling exponent. Such deviations in absolute scaling are also apparent in the spatially resolved low power characterisation (cf. Fig. 6).
4. Conclusions The combination of open resonator and dielectric resonator methods provide spatially resolved and ®eld dependent RS -datasets for large area high quality YBCO wafers. The comparison with microstrip resonators subsequently manufactured prove that these datasets are adequate to predict component performance with respect to quality factors and power handling capability. Consequently these techniques oer an ideal combination for non-destructive ®lm quali®cation. Fig. 7. Power handling capability in the centre of KarL204 and KarL207 characterised by dielectric and microstrip resonator measurements.
The power dependent datasets Q0 (Posc ) for the microstrip resonators can be transferred into effective datasets RS (Hrf ) using conversion factors estimated by approximate formulas for this resonator type [5]. The dielectric resonator technique provides these RS (Hrf )-data non-destructively at 8.5 GHz for the ®lm area covered by the two central resonators. The quantitative comparison is done using a frequency scaling according to RS / xb with b 1:7 (cf. Fig. 7). Clearly within
References [1] J. Geerk, F. Ratzel, H. Rietschel, G. Linker, R. Heidinger, R. Schwab, Trans. Appl. Supercond. 9 (1999) 1543. [2] R. Schwab, R. Heidinger, R. Sp orl, Conf. Digest, 23rd International Conference on Infrared and Millimeter waves, Colchester, UK, 1998, pp. 379±381. [3] R. Heidinger, R. Schwab, Proc. EUCASÕ 99 Inst. Phys. Conf. Ser. No. 167, (2000) 89±92. [4] E. Gaganidze, R. Schwab, J. Halbritter, R. Heidinger, R. Aidam, R. Schneider, Appl. Supercond. Conf. ASCÕ 2000, 17±22 Sept. 2000, Virginia Beach, USA, IEEE Trans. Appl. Supercond., to be published. [5] R. Schneider, R. Aidam, A. Zaitsev, J. Geerk, G. Linker, F. Ratzel, R. Smithey, Physica C 351 (2001) 21, these Proceedings.