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Hybrid integration of microwave MESFET oscillator and Josephson tunnel junction
Hybrid integration of microwave MESFET oscillator and losephson tunnel junction P. Gutmann, E. Vollmer, J. Kohlrnann, D. Quenter and J. Niemeyer Physikalisch-Technische Bundesanstalt, Postfach 3345, 38023 Braunschweig, Germany Received 11 December 1992
The hybrid integration of a planar X-band oscillator and a superconducting MMIC has been realized for application in Josephson voltage standards and potentiometers. Coupled striplines are used for the microwave transition between both planar circuits. The microwave power coupled to the MMIC is detected by suppression of the critical current of a Josephson tunnel junction
Keywords: cryoelectronics; MESFETs; Josephson tunnel junctions
Superconducting microwave monolithic integrated circuits (S-MMICs) based on series arrays of Josephson tunnel junctions are used for d.c. voltage standards and Josephson potentiometers 1-3. The S-MMICs are operated at a frequency of 24-90 G H z and at a temperature of 4.2 K. They produce quantized d.c. voltages depending only on the order of the constant voltage step and the frequency of the incident microwave. At present the microwave is applied with a klystron or a Gunn oscillator at room temperature and is supplied to the S-MMICs by waveguides. In order to reduce the size and cost of the microwave set-up for Josephson measurement systems, the hybrid integration of the microwave oscillator and a Josephson series array is suggested in reference 4. The complete stripline oscillator is integrated on a separate substrate and the other cryoelectronic parts of the set-up, including the Josephson series arrays, on an S-MMIC. For the microwave transition between both chips a stripline-tostripline transition via a wire or ribbon bond is intended. This paper presents a set-up for and the performance and results of an experiment at the X-band demonstrating a simple method of hybrid integration of a semiconductor microwave oscillator and an S-MMIC with an integrated Josephson tunnel junction. The microwave power is coupled by a section of parallel striplines, one on each chip with a very small gap in between. Both circuits are therefore d.c. decoupled and no bond facilities are required.
Integrated circuit Both circuits, the planar oscillator and the S-MMIC with the tunnel junction, are integrated on a conducting substrate, as shown in principle in Figure 1. The microwave is coupled from the oscillator to the S-MMIC by a short section of coupled striplines at the border of each chip. One line is the 50 f~ stripline on the substrate of the
oscillator and the other is a line section of a high characteristic impedance stripline on the silicon substrate of the S-MMIC (strip width w = 70/~m, characteristic impedance Z = 93 f~). The cross-section of the coupled striplines is shown in Figure lb. The distance s between both strips should be smaller than 0.1 mm to avoid a reduction of the transmission coefficient s2~.
Components An X-band M E S F E T oscillator based on a stripline technique is used for the experiment 5. The oscillator is built on a 0.254 mm thick glass-fibre reinforced teflon substrate (60 x 4 0 m m 2) with 0.017mm copper claddings. The active device is a commercially available packaged MESFET. The load of the oscillator is located at room temperature outside the liquid helium Dewar to check the spectrum of the oscillator. A 50 Q stripline, an abrupt stripline-to-coaxial line transition and a 50 Q coaxial line are used to connect the oscillator to the load. The S-MMIC is built on a 0.300 mm thick silicon substrate (10 x 14mm 2) using thin film techniques 1'6. The in-line Nb/A120 3/Nb Josephson tunnel junction has an area of 30 × 123/~m 2 and a 2 nm thick barrier. The width wj and the length lj of the junction are small compared with the propagation wavelength ,~j of the stripline formed by the junction. For an oscillation frequency of ,~ 11 GHz, 2j is 910 #m and lj/•j ~-0.135. The tunnel junction is integrated into a high characteristic impedance stripline using the 0.300 mm thick silicon substrate as the dielectric layer of the stripline. The strip width w is 35 #m and the resulting characteristic impedance Z is ~93 f~. The strip and the groundplane are made from 200 nm thick niobium layers. The stripline is open-ended, resulting in a standing wave on the stripline. The junction is located at a distance of ~ 3 / 4 of a wavelength from the open end, with a maximum current of the standing wave and for a maximum amplitude dl of
0011-2275/93/101005-03 © 1993 Butterworth-HeinemannLtd Cryogenics 1993 Vol 33, No 10
1005
Hybrid integration: P. Gutmann et al.
a
a
I
I
I
I
I
I
I
s<0,1 mm
2
0
0,2
0,4 I/mA
-0,4
-0,2
0
0,2
0,4 I/mA
1
Figure 1 Principle of hybrid integration of microwave oscillator and S-MMIC with integrated Josephson junction. (a) Stripline layout; (b) cross-section of coupled striplines. 1, Glass-fibre reinforced teflon substrate of oscillator with 50 Q stripline; 2, silicon substrate of S-MMIC with 93 ~ stripline; 3, conducting substrate
the microwave coupled into the junction. The tunnel junction is connected to the d.c. pads by low pass filters.
Measurement
I
-0,2
I I 25 mm
b
I
-0,4
b
Figure 2 Measured V - I characteristic of Josephson tunnel junction: (a) without and (b) with incident microwave
results 1,0
First the d.c. current-voltage characteristic of the tunnel junction without an applied microwave is sampled, as shown in Figure 2a. The critical current Io of the tunnel junction is ~475 #A and the critical current density 12.9 A cm-2. Then the oscillator is switched on, levelling the output power by changing the drain-source voltage of the FET. The step width AIo of the zero-voltage step can be reduced by the microwave power coupled into the junction. AIo can be completely suppressed by the microwave, as shown in Figure 2b. The measured dependence of Alo/lo on the voltage amplitude al of the incident microwave is shown in Figure 3. Assuming a homogeneous current density distribution, the theoretical dependence
Alo/Io = 21Jo(2e/h x al/f)l
(1)
(where Jo = Bessel function, e = elementary charge, h = Planck's constant and f = microwave frequency) is fitted to the measurement results in Figure 3 as a first approximation. Equation (1) is given in reference 7 for a capacitively shunted Josephson tunnel junction.
1006
Cryogenics 1993 Vol 33, No 10
Alo/lo 0,8 0,6 0,4 0,2 0,0
0
,
,
w
2
4
6
fi,/f in arbitrary units Figure 3 Width of zero voltage step A I o / I o versus microwave amplitude ~ at Josephson tunnel junction [O, measured; --, theoretical value according to Equation (1)] at microwave frequency f = 11 GHz
Hybrid integration: P. Gutmann et al. Stable constant voltage steps have not been observed in the I-V characteristic. The plasma frequency fp = 15.5 GHz of the tunnel junction is small compared with the frequency f of the incident microwave. According to reference 2 for f / f p = 0.72, stable constant voltage steps can only be expected for a much higher microwave power level at the junction than the level achieved here.
Conclusions The hybrid integration of a planar microwave oscillator utilizing low cost microwave technology and an SMMIC in standard niobium silicon technology with a single Josephson tunnel junction has been achieved. A line section of coupled striplines at the border of each chip is used to transmit microwave power from one chip to the other, avoiding d.c. ground loops between both circuits and the need for bonding. This microwave transition could be used for microwave coupling to several d.c. decoupled Josephson tunnel junctions or Josephson series arrays in a Josephson voltage standard. Single junctions can be used for microwave power detection. The junction is integrated into a stripline resonator at a distance of (2n - 1) )./4, with n = 1, 2, 3. . . . . from the open end to increase the microwave current amplitude through the junction above that of a non-resonant matched stripline guiding a travelling wave. This hybrid technique, which is intended for quantum voltage standards and Josephson potentiometers (to be produced in small quantities), has some advantages over
a technique with all the stripline patterns integrated, with the gallium arsenide MESFET mounted on top of the common silicon substrate. Our technique requires less processing steps, allows independent development of the circuits using the most appropriate technology, uses the high dielectric material silicon only when necessary, allows the assembling of different modules and would improve the production yield of 10 V Josephson standards by simple hybrid integration of four less complex 2.5 V chips.
References 1 P6pel, R., Niemeyer, J., Fromknecht, R., Meier, W. et al. Nb/AI203/Nb Josephson voltage standards at 1 V and 10 V IEEE Trans (1991) IM-40 298-300 2 Hamilton, C.A., Kautz, R.L., e t a / . A 24-GHz Josephson array voltage standard IEEE Trans (1991) IM-40 301 304 3 Kohlmann, J., Gutmaan, P., L6hr, K., Weimann, T. et aL Ratio standard for dc resistance measurements using a second generation of Josephson junction arrays Proc SQUID 91 (Ed Koch, H.), Springer-Verlag Berlin, Germany (1992) 4 Vollmer, E., Gutmann, P. and Niemeyer, J. Hybrid integration of a microwave oscillator with a Josephson series array Proc CPEM'92 Paris, France (1992) 368 5 Vollmer, E. and Gutmann, P. X-band GaAs MESFET oscillator for cryogenic application at 4.2K Electron Lett (1991) 27 (24) 2210-2211 6 Qnenter, D. Mikrowellenexperimente an Serienschaltungen von Josephson-Tunnelkontakten, Diplomarbeit, Universit/it Ttibingen, Germany (1990) 7 Solymar, L. Superconductive Tunnelling and Application Chapman and Hall Ltd, London, UK (1972)
Cryogenics 1993 Vol 33, No 10
1007
The hybrid integration of a planar X-band oscillator and a superconducting MMIC has been realized for application in Josephson voltage standards and potentiometers. Coupled striplines are used for the microwave transition between both planar circuits. The microwave power coupled to the MMIC is detected by suppression of the critical current of a Josephson tunnel junction
Keywords: cryoelectronics; MESFETs; Josephson tunnel junctions
Superconducting microwave monolithic integrated circuits (S-MMICs) based on series arrays of Josephson tunnel junctions are used for d.c. voltage standards and Josephson potentiometers 1-3. The S-MMICs are operated at a frequency of 24-90 G H z and at a temperature of 4.2 K. They produce quantized d.c. voltages depending only on the order of the constant voltage step and the frequency of the incident microwave. At present the microwave is applied with a klystron or a Gunn oscillator at room temperature and is supplied to the S-MMICs by waveguides. In order to reduce the size and cost of the microwave set-up for Josephson measurement systems, the hybrid integration of the microwave oscillator and a Josephson series array is suggested in reference 4. The complete stripline oscillator is integrated on a separate substrate and the other cryoelectronic parts of the set-up, including the Josephson series arrays, on an S-MMIC. For the microwave transition between both chips a stripline-tostripline transition via a wire or ribbon bond is intended. This paper presents a set-up for and the performance and results of an experiment at the X-band demonstrating a simple method of hybrid integration of a semiconductor microwave oscillator and an S-MMIC with an integrated Josephson tunnel junction. The microwave power is coupled by a section of parallel striplines, one on each chip with a very small gap in between. Both circuits are therefore d.c. decoupled and no bond facilities are required.
Integrated circuit Both circuits, the planar oscillator and the S-MMIC with the tunnel junction, are integrated on a conducting substrate, as shown in principle in Figure 1. The microwave is coupled from the oscillator to the S-MMIC by a short section of coupled striplines at the border of each chip. One line is the 50 f~ stripline on the substrate of the
oscillator and the other is a line section of a high characteristic impedance stripline on the silicon substrate of the S-MMIC (strip width w = 70/~m, characteristic impedance Z = 93 f~). The cross-section of the coupled striplines is shown in Figure lb. The distance s between both strips should be smaller than 0.1 mm to avoid a reduction of the transmission coefficient s2~.
Components An X-band M E S F E T oscillator based on a stripline technique is used for the experiment 5. The oscillator is built on a 0.254 mm thick glass-fibre reinforced teflon substrate (60 x 4 0 m m 2) with 0.017mm copper claddings. The active device is a commercially available packaged MESFET. The load of the oscillator is located at room temperature outside the liquid helium Dewar to check the spectrum of the oscillator. A 50 Q stripline, an abrupt stripline-to-coaxial line transition and a 50 Q coaxial line are used to connect the oscillator to the load. The S-MMIC is built on a 0.300 mm thick silicon substrate (10 x 14mm 2) using thin film techniques 1'6. The in-line Nb/A120 3/Nb Josephson tunnel junction has an area of 30 × 123/~m 2 and a 2 nm thick barrier. The width wj and the length lj of the junction are small compared with the propagation wavelength ,~j of the stripline formed by the junction. For an oscillation frequency of ,~ 11 GHz, 2j is 910 #m and lj/•j ~-0.135. The tunnel junction is integrated into a high characteristic impedance stripline using the 0.300 mm thick silicon substrate as the dielectric layer of the stripline. The strip width w is 35 #m and the resulting characteristic impedance Z is ~93 f~. The strip and the groundplane are made from 200 nm thick niobium layers. The stripline is open-ended, resulting in a standing wave on the stripline. The junction is located at a distance of ~ 3 / 4 of a wavelength from the open end, with a maximum current of the standing wave and for a maximum amplitude dl of
0011-2275/93/101005-03 © 1993 Butterworth-HeinemannLtd Cryogenics 1993 Vol 33, No 10
1005
Hybrid integration: P. Gutmann et al.
a
a
I
I
I
I
I
I
I
s<0,1 mm
2
0
0,2
0,4 I/mA
-0,4
-0,2
0
0,2
0,4 I/mA
1
Figure 1 Principle of hybrid integration of microwave oscillator and S-MMIC with integrated Josephson junction. (a) Stripline layout; (b) cross-section of coupled striplines. 1, Glass-fibre reinforced teflon substrate of oscillator with 50 Q stripline; 2, silicon substrate of S-MMIC with 93 ~ stripline; 3, conducting substrate
the microwave coupled into the junction. The tunnel junction is connected to the d.c. pads by low pass filters.
Measurement
I
-0,2
I I 25 mm
b
I
-0,4
b
Figure 2 Measured V - I characteristic of Josephson tunnel junction: (a) without and (b) with incident microwave
results 1,0
First the d.c. current-voltage characteristic of the tunnel junction without an applied microwave is sampled, as shown in Figure 2a. The critical current Io of the tunnel junction is ~475 #A and the critical current density 12.9 A cm-2. Then the oscillator is switched on, levelling the output power by changing the drain-source voltage of the FET. The step width AIo of the zero-voltage step can be reduced by the microwave power coupled into the junction. AIo can be completely suppressed by the microwave, as shown in Figure 2b. The measured dependence of Alo/lo on the voltage amplitude al of the incident microwave is shown in Figure 3. Assuming a homogeneous current density distribution, the theoretical dependence
Alo/Io = 21Jo(2e/h x al/f)l
(1)
(where Jo = Bessel function, e = elementary charge, h = Planck's constant and f = microwave frequency) is fitted to the measurement results in Figure 3 as a first approximation. Equation (1) is given in reference 7 for a capacitively shunted Josephson tunnel junction.
1006
Cryogenics 1993 Vol 33, No 10
Alo/lo 0,8 0,6 0,4 0,2 0,0
0
,
,
w
2
4
6
fi,/f in arbitrary units Figure 3 Width of zero voltage step A I o / I o versus microwave amplitude ~ at Josephson tunnel junction [O, measured; --, theoretical value according to Equation (1)] at microwave frequency f = 11 GHz
Hybrid integration: P. Gutmann et al. Stable constant voltage steps have not been observed in the I-V characteristic. The plasma frequency fp = 15.5 GHz of the tunnel junction is small compared with the frequency f of the incident microwave. According to reference 2 for f / f p = 0.72, stable constant voltage steps can only be expected for a much higher microwave power level at the junction than the level achieved here.
Conclusions The hybrid integration of a planar microwave oscillator utilizing low cost microwave technology and an SMMIC in standard niobium silicon technology with a single Josephson tunnel junction has been achieved. A line section of coupled striplines at the border of each chip is used to transmit microwave power from one chip to the other, avoiding d.c. ground loops between both circuits and the need for bonding. This microwave transition could be used for microwave coupling to several d.c. decoupled Josephson tunnel junctions or Josephson series arrays in a Josephson voltage standard. Single junctions can be used for microwave power detection. The junction is integrated into a stripline resonator at a distance of (2n - 1) )./4, with n = 1, 2, 3. . . . . from the open end to increase the microwave current amplitude through the junction above that of a non-resonant matched stripline guiding a travelling wave. This hybrid technique, which is intended for quantum voltage standards and Josephson potentiometers (to be produced in small quantities), has some advantages over
a technique with all the stripline patterns integrated, with the gallium arsenide MESFET mounted on top of the common silicon substrate. Our technique requires less processing steps, allows independent development of the circuits using the most appropriate technology, uses the high dielectric material silicon only when necessary, allows the assembling of different modules and would improve the production yield of 10 V Josephson standards by simple hybrid integration of four less complex 2.5 V chips.
References 1 P6pel, R., Niemeyer, J., Fromknecht, R., Meier, W. et al. Nb/AI203/Nb Josephson voltage standards at 1 V and 10 V IEEE Trans (1991) IM-40 298-300 2 Hamilton, C.A., Kautz, R.L., e t a / . A 24-GHz Josephson array voltage standard IEEE Trans (1991) IM-40 301 304 3 Kohlmann, J., Gutmaan, P., L6hr, K., Weimann, T. et aL Ratio standard for dc resistance measurements using a second generation of Josephson junction arrays Proc SQUID 91 (Ed Koch, H.), Springer-Verlag Berlin, Germany (1992) 4 Vollmer, E., Gutmann, P. and Niemeyer, J. Hybrid integration of a microwave oscillator with a Josephson series array Proc CPEM'92 Paris, France (1992) 368 5 Vollmer, E. and Gutmann, P. X-band GaAs MESFET oscillator for cryogenic application at 4.2K Electron Lett (1991) 27 (24) 2210-2211 6 Qnenter, D. Mikrowellenexperimente an Serienschaltungen von Josephson-Tunnelkontakten, Diplomarbeit, Universit/it Ttibingen, Germany (1990) 7 Solymar, L. Superconductive Tunnelling and Application Chapman and Hall Ltd, London, UK (1972)
Cryogenics 1993 Vol 33, No 10
1007