International Journal of lnfrared and Millimeter Waves, VoL 17, No. 1, 1996

DESIGN AND RESULTS FOR A 345 GHz SiS FOCAL PLANE ARRAY USING PLANAR TECHNOLOGY

T. H. Legg, 1 M. B. Bell, 1 K. Kornelsen, 2 J. F. Vaneldik, 3 D. Routledge, 3 and M. J. Brett 3

1National Research Council, Ottawa

2Alberta Microelectronic Centre, Edmonton

3Electrical Engineering Department, University of Alberta, Edmonton

Received October 4, 1995

Abstract

We describe a focal plane array using antenna elements and SIS junctions photo-etched on a quartz substrate. The etched antenna is of a new strip- line type in which a two-dimensional corrugated horn is phase-corrected with a strip-line lens. Good beam efficiency is indicated by the measured antenna patterns. A DSB receiver temperature of 225 K was obtained with a number of imperfections still present.

KEY WORDS: Planar, SIS, focal plane, array, sub-mm

Introduction

We report recent results in a program to develop arrays of focal plane heterodyne receivers using SIS junctions (1-5). A principle aim of the work is to use photolithography instead of highly precise machining to provide the very fine tolerances needed in sub-millimeter heterodyne mixers.

Design Features

(6). The array design is based upon a new type of strip-line antenna

The general arrangement of components is sketched in Fig.1.

79

0195-9271/96~I00-0079509~ O 1996 Plenum Publishing Corporation

8o Ix~ a at.

rENNAS, MtX RMOOULE--.

PHOTO-ETCHED ANTENNA E L E M E N T S / ~ ' # ' -

CTIONS

~ I.F. CIRCUIT

CORRUGATED SURFACE ~ ~ ~N~

METAL GR~.)UND-PLA-r'~E. SPACER AND HEAT CONDUCTOR

Fig. 1 The physical arrangement o f the focal plane array. Quartz plates containing photo-etched strip-line antenna elements and SIS junctions are sandwiched between metal plates. The plates have a tapered, corrugated surface at one end to form an open electromagnetic horn which controls the beam shape in the E- plane. The etched antenna elements also have corrugations to give an H-plane pattern o f the same shape.

Antenna elements, Nb/A1Ox/Nb SIS junctions and junction tuning components are photo-etched onto a 60 lam quartz substrate. A quartz superstrate of the same thickness is cemented to the substrate using a very thin coating (a few pm) of a windshield repairing compound made by the Loctite corporation. The broad outside surfaces are aluminized to form a strip-line module. This module controls the shape of the received beam in the plane of the magnetic field. Several identical modules are placed side by side and, in several layers, are sandwiched between brass plates (Fig. 1) to construct a two-dimensional focal plane array. Twenty- eight modules are fabricated on one 3.8 cm (1.5 inch) diameter wafer. An option in constructing the array is to leave several (up to eight) side-by- side mixer modules in one piece. Although this requires that all of the

SIS Focal Plane Array 81

joined mixers be replaced if one fails, we favor this approach because of the ease of assembly.

In the plane of the electric field, the beam shape is controlled by an open electromagnetic horn formed by flaring the metal plates at one end (Fig. 1). Quarter wavelength corrugations are machined in the flared surfaces to give a cosine amplitude weighting across the E-plane aperture. The plates are machined as a single bar of uniform cross-section which is then cut into the required number of shorter sections. A significant advantage of this type of array is that, for 345 GHz for example, enough plates for an array of any practical size can be machined on a standard numerically controlled milling machine in two days. Corrugations would probably not be used for frequencies above 345 GHz where machining becomes progressively more difficult. Instead, a scheme that has been demonstrated at cm wavelengths (7,8) would be used to obtain a cosine amplitude weighting. The tapered surfaces would be covered with dielectric sheets that are coated with photo-etched conducting strips and the sheet thickness chosen to constrain the net electric field to be zero at the surface.

Josephson currents are suppressed in the array junctions by small permanent magnets (Fig. 2). The magnetic field is directed across the junctions by using two iron strips located just above and to either side of the junctions. This arrangement produces a fairly uniform magnetic field over the length of the iron strips and enables the Josephson current to be quenched along a row of several adjacent junctions. The field is adjusted by changing the distance of the strips from the junctions. Compared with superconducting coils, which are sometimes used, permanent magnets have advantages of simplicity, reliability and very small size.

The mask used in photo-etching the strip-line antenna and junction tuning circuit is shown in Fig. 3. Dark areas of Fig. 3 indicate regions where the etching leaves the quartz substrate covered with metal. This forms the center conductor of the strip transmission line. The width of the strip-line antenna aperture is 4 wavelengths at 345 GHz. This dimension governs the number of antenna-mixer elements that can be fabricated on one wafer. An incoming wave enters the edge of the quartz in a "parallel-plate" mode in which electric fields above and below the center conductor are in phase. However, the leading edges of the

82 Legg et at.

Fig. 2 An antenna-mixer block for testing three adjacent array elements at 345 GHz. Iron strips for Josephson current suppression, with small permanent magnets between them, can be seen protruding from the top right side.

superstrate and substrate are offset to give a different refractive index, and different phase length, above and below the center conductor. The wave is thereby converted to the strip-line mode with antiphase fields on either side of the center conductor.

The sides of the strip-line horn are corrugated with quarter wavelength stubs. These constrain the net field to be zero at the corrugation boundary and, like the machined corrugations of the E-plane horn, result in a cosine amplitude weighting over the aperture of the horn. The pattern of square holes etched in the center conductor (Fig. 3) forms a two dimensional lens which corrects the shape of the phase front. A plane wave incident on the front edge of the quartz is converted into a

SIS Focal Plane Array 83

II |

Fig 3. An enlarged portion o f the photo-lithographic mask used in etching one H-plane antenna element and junction tuning structure. The triangularly shaped area, with quarter- wavelength corrugations at the edge, is a new type o f antenna, a two-dimensional strip-line horn. The full-scale size o f the square area at the bottom left, a bonding pad, is one mm on a side.

cylindrical wave centered on the horn apex. The particular pattern of etched holes shownAn Fig. 3 increases the effective refractive index by a factor of 1.15. The two lines of circular holes near the lens are positioned to cancel reflections caused by the change in refractive index at the lens boundaries. So far as is known, this is the first use of a strip- line lens. With water-free quartz as the dielectric, loss in the complete strip-line antenna is estimated to be about 0.32 db at 345 GHz.

The strip-line horn tapers to a 50 tam wide transmission line which is increased in width over its last quarter wavelength to provide an impedance transformation The SIS junction (not discernible in Fig. 3) is

s4 ~ et at.

located on the top surface of the transformer and is connected to a radial stub. This stub, which can be seen just below the transformer in Fig. 3, provides a virtual ground for the junction and leads the I.F. signal into an R.F. filter and a rectangular bonding pad. Also on the top surface of the transformer is a thin insulating layer of SiO covered by a metallized region. This structure forms an inductive length of microstrip transmission line and radial stub which are used to resonate the junction capacitance in a manner similar to that first described by R~iisanen et al (9). Because the dimension of fringing fields is comparable to some of the conductor dimensions, mathematical and scale modelling of this circuit was needed to ensure a good match between the antenna impedance and the SIS junction. Our mask incorporates several design options specifically to match junctions of 12, 25 and 35 ohms. However, with a wider choice of dimensions, the strip-line structure is capable of matching impedances from about 5 to over I00 ohms, a much larger range than is possible with a waveguide mixer.

Relatively small antenna elements (4~.), with relatively large beamwidths (-17°), are used to fit as many elements as possible onto a quartz wafer. In early experiments with array elements parts of this wide beam were obstructed by the dewar window. More recently an ellipsoidal mirror has been used inside the dewar to reduce the width of the beam at the window. With this new arrangement the E and H-plane beams outside of the dewar have different widths because of a displacement of 1.1cm between the phase axes of the E and H-plane antennas. The difference in width can be removed by installing a mirror that is suitably astigmatic, rather than ellipsoidal, and it is hoped that this will be done in time for the next series of experiments.

It has recently been realized that a simpler alternative to an astigmatic mirror is to give the two-dimensional strip-line lens a shorter focal length. The lens would be made such that the apex of the strip-line horn and the aperture of the E-plane flared horn were at conjugate foci. If the H-plane aperture and the E-plane horn apex coincided (for example), then the wavefronts in the E and H planes (for the transmitting case) would have identical curvatures at the E-plane aperture. With this arrangement, a strip-line array of the present design would, so far as the optics are concerned, be interchangeable with a waveguide array.

SIS Focal Plane Array 85

The quartz antenna-mixer units are made at the Alberta Microelectronic Centre, Edmonton. Substrates are prepared by bonding two lmm thick quartz disks together and then thinning the top one to the required 60 pm thickness. The back surface of the thinned wafer is metallized prior to the bonding and thinning. Niobium junction processing takes place using this bonded sandwich to prevent breakage. Nb/A1Ox/Nb trilayers are deposited using a Perkin-Elmer 4410 sputter system. The Nb and AI layers are deposited in the main sputter chamber while the load lock is used for both the oxidation step and deposition of a capping gold layer to prevent oxidation of the top Nb surface. Processing is a three mask process, with the first mask defining the antenna, junction "pad" areas, I.F. filter circuit, connecting pads and alignment marks. The second mask is used to isolate the tunnelling junction area in a three step process. First, the Nb surrounding the tunnelling area is removed in an RIE etch, then 2000 Angstroms of SiO is evaporated for electrical isolation, and finally the top of the junction mesa is exposed by dissolving away the photoresist. The final mask defines the junction tuning structure and connects the junction to the I.F. filter.

Results

Because of difficulties in junction processing, we so far have made only one set of noise temperature measurements with a tuned SIS junction. The measurements were made in March, 1995 and are shown in Fig. 4. At that time part of the beam was still being blocked by the dewar window, the junction quality was relatively poor and the junction temperature fairly high (4.6K). The best DSB receiver temperature measured was 225K near 300 GHz, indicating that the junction tuning needed to be adjusted upwards toward 345 GHz. The low tuning frequency was consistent with measurements made on several junctions of the same batch using the Josephson current resonance in the direct current I-V characteristic as described by Lehnert et al (10). Lower receiver noise temperatures are expected with the beam focussed by an internal mirror, a better quality junction and a lower junction temperture.

An ellipsoidal mirror was mounted inside the dewar by the time that beam patterns were measured but the junction available then was not suitable for further noise temperature measurements. The H-plane pattern,

86 Legg et al.

1500

1000

5OO

, / , i I , i ~ , , i i , , i , , , i

/ oo o,, J

i I I i i , I i i , I ~ , i I

300 320 340 360 F r e q u e n c y (GHz)

0 I I | I 1 1 280 380

Fig. 4 Receiver temperature measurements made in March, 1995 using a photo-etched antenna and a 1.2xl.2 (l~m) 2 SIS junction. A DSB receiver temperature of 225 K was measured near 300 GHz where the junction capacitance evidently resonates with an inductive tuning stub. The local oscillator used in the test could be tuned no lower than 290 GHz.

shown on a linear scale of power in Fig. 5, was measured in May 1995 with the dewar, local oscillator and mylar diplexer fixed. A Gunn oscillator and quadrupler, serving as a signal, were moved through the beam on a linear track. The small features near the peak of the pattern vary too rapidly with angle to be produced by the mirror aperture; they are caused by reflective interference in the measuring apparatus. Side lobes are estimated to be 1% of the main beam or less.

Measurement o f the E-plane pattern, Fig. 6, required a different physical arrangement with a 45 ° mirror diverting the beam from the horizontal to the vertical. A network analyzer was used to provide a

SiS Focal Plane Array 87

"K 1.2

E 0

v

L . 6 D

.4

.2

0

, I , , , , i , , i i I

t I , , , , l ~ , J , I -10 0 10

Angle (degrees)

I I _

I !

Fig. 5 The H-plane pattern produced by a photo-etched antenna on a quartz substrate illuminating an ellipsoidal mirror. Detected IF. power is shown on a linear scale. The small, rapidly varying feature near the peak is caused by reflections in the measuring setup.

logarithmic scale of power which shows more clearly a side-lobe level of less 1%. As mentioned above, the difference in angular width between the E and H-plane patterns is the result of an offset of 1.1 cm between phase axes. The E and H-plane patterns agree with earlier cm wavelength measurements made on large scale models of the antenna elements. The patterns demonstrate that the received emission is efficiently concentrated into a single well-shaped beam. The array antennas should provide good beam efficiency on a telescope.

Discussion

The antenna elements of the array are similar in performance to

88 h~gg ~t,~.

0

v

O ta,

-10

- 2 0

I

I J i l l " " " l l l l l l l l

-5 0 5 Angle (degrees)

! I

10

I |

Fig. 6 The E-plane pattern of the open electromagnetic horn of the array illuminating an ellipsoidal mirror. I.F. power is shown on a logarithmic scale. The angular scale differs by almost a factor two from that of Fig 5.

good quality waveguide antennas. Side-lobe levels and beam efficiency are better than those obtained with diagonal horns. In retrospect, however, we should have incorporated a mirror within the dewar at

an earlier stage. This would have allowed smaller antenna elements by a linear factor of about 1.5 and made possible the fabrication of some 60 or more antenna-mixers on one quartz wafer. At times we have observed poor H-plane beam patterns and have concluded that this is due to a gap between superstrate and substrate which allows non-TEM waves to propagate (12). This problem has been solved by cementing the superstrate and substrate with the Loctite windshield-repairing compound mentioned earlier. The cementing also makes assembly of a mixer easier.

SIS Focal Plane Array 89

The noise temperature measurements were made with the test mixer not optimized in several respects. Part of the beam was blocked by the dewar window, the junction quality was relatively poor, the junction temperature higher than necessary, and the quartz-vacuum interface and the dewar vacuum window were not matched. We estimate that if these shortcomings are dealt with, DSB temperatures of about 100K should be reached at 345 GHz.

The present antenna-mixer design has a number of features to recommend it for a focal plane array and possibly for a single beam receiver. Antenna components fabricated with the same planar technology as the SIS junctions are very inexpensive compared to more conventional waveguide antennas for sub-mm wavelengths. This advantage becomes even more important at frequencies above 345 GHz. Our experience has shown that it is possible to work with quartz thinned to 301am or less which would allow scaling of the design to 690 GHz or above. The ability of strip-line to match very low impedances is important in allowing larger, lower impedance junctions to be matched. The strip-line structure also has an inherently wide-bandwidth capability. Though this has not been exploited with the present simple design, some work has been done on wide bandwidth matching (11) and an octave bandwidth mode-converter for the H-plane antenna has been designed but not built.

This work was supported in part by the James Clerk Maxwell Telescope Development Fund. J.F.V., D.R. and M.J.B. acknowledge financial support from the Canadian Natural Sciences and Engineering Research Council, and the assistance of the Alberta Microelectronic Centre..

REFERENCES

1. Dotan, G.J., Phillips, T.G., and Woody, D.P., "Low-noise 115 GHz mixing in superconducting oxide-barrier tunnel junctions", Appl. Phys. Lett.., vol. 34, pp347-349, 1979.

2. Richards, P.L., Shen, T.M. Harris, R.E. and Lloyd, F.L., "Quasiparticle heterodyne mixing in SIS tunnel junctions", Appl. Phys. Lett.,vol. 34,

90 l.egg et ai.

pp345-347, 1979.

3. Sutton, E.C., "A superconducting tunnel junction receiver for 230 GHz", IEEE Trans Microwave Theory Tech., vol. MTT-31, pp589-592, 1983.

4. Tucker, J.R. and Feldman, M.J., "Quantum detection at millimeter wavelengths', Rev. Mod. Phys., vol. 57, pp1055-1113, 1985.

5. Wengler, M.J., "Submillimeter-wave detection with superconducting tunnel diodes", Proc. IEEE, vol. 80, pp1810-1826, 1992.

6. Legg, T.H., "Quasi-optical strip-line devices", U.S. patent number 5 117237, 1992.

7. Lier, E. and Schaug-Pettersen, T., "The strip-loaded hybrid-mode feed horn", IEEE Trans. Ant. Prop., vol. AP-35, pp 1086-1088, 1987.

8. Rodrigues, S. R., Mohanan, P., and Nair, K.G., "A strip-loaded feed- horn antenna", IEEE Microwave Guided Wave Lett.,vol 1, p1086, 1991.

9. R~iis/inen, A.V., McGrath, W.R., Richards, P.L., and Lloyd, F.L., "Broad-band RF match to a millimeter-wave SIS quasi-particle mixer", IEEE Trans. Microwave Theory Tech.. vol. MTT-33 pp 1495-1499, 1985.

10. Lehnert, T., Sch/ifer, F., and Gundlach, K. H., "Resonance effects in Josephson tunnel junctions with integrated tuning structures", J. Appl. Phys., vol 74 (2), p1403, 1993.

11. Legg, T.H., "SIS junctions as artificial line elements: a wide band matching scheme", Int. J. IR & mm Waves, vol.14, pp 997-1000, 1993.

12. Nghiem, D., Williams, J.T., Jackson, D.R., and Oliner, A.A., "Proper and improper dominant mode solutions for a stripline with an airgap", Radio Science, vol. 28, pp1163-1180, 1993.