Low-noise slot antenna SIS mixers - Applied Superconductivity by liuhongmei


                                             VOL. 5, NO. 2, J U N E 1995                                                            3053

                                 Low-Noise Slot Antenna SIS Mixers
                                     J. Zmuidzinas, N. G. Ugras, D. Miller, and M. Gaidis
                                  California Institute of Technology, 320-47, Pasadena, CA 91125

                                                   H. G. LeDuc and J. A. Stern
                                     J e t Propulsion Laboratory, 302-231, Pasadena, CA 91109

   Abstract-We describe quasi-optical SIS mixers op-
erating in the submillimeter band (500-750 GHz)
which have very low noise, around 5 hu/kB for the
double-sideband receiver noise temperature. The mix-
ers use a twin-slot antenna, Nb/Al-Oxide/Nb tunnel
junctions fabricated with optical lithography, a two-
junction tuning circuit, and a silicon hyperhemispher-
ical lens with a novel. antireflectioncoating to optimize
the optical efficiency. We have flown a submillimeter
receiver using these mixers on the Kuiper Airborne
Observatory, and have detected a transition of Hzl'O
at 745 GHz. This directly confirms that SIS junc-
tions are capable of low-noise mixing abwe the gap
frequency.                                                            Fig. 1. The mixer optics include an epoxy-coated hyperhemispheri-
                                                                      cal silicon lens and a plastic lens or a Teflon-coated quartz lens.

                      I. INTRODUCTION
                                                                         The mixer type can either be waveguide (e.g. [4], [6],
   Over the past several years, there have been dra-                                 or
                                                                      [9], and [ll]) quasi-optical (e.g. [3], [5], [7], and [12]-
matic advances in SIS mixers operating in the submil-                 [15]). The waveguide mounts are tunable, highly effi-
                                                                      cient, and were better understood initially. The quasi-
limeter band. The upper frequency limit has been pushed
                                                                      optical mixers are much easier to fabricate, especially at
from around 250 GHz to beyond 700 GHz, with the
best doublesideband receiver noise temperatures scaling               higher frequencies, but are fixed tuned and can suffer from
roughly as 5 h v / l c ~ These SIS receivers are now deployed
                         .                                            low efficiencies. Fortunately, there has been much recent
at a number of astronomical observatories, and the field of           progress in the understanding of planar antennas on di-
submillimeter astronomy is advancing rapidly. References              electric lenses [15]-[17], and there seems to be no fun-
[l], [2] are good review articles on SIS mixers; however,             damental reason that a quasi-optical mixer cannot have
there has been much progress since these were published,              high efficiency and low noise. Indeed, our paper will de-
e.g. [3]-[12].                                                        scribe the quasi-optical mixer we have developed whose
   These advances were made possible through the                      sensitivity exceeds that of any other quasi-optical mixer
development of high quality, high current density                     demonstrated to date, and is fully competitive with the
(10 kA cmV2),small-area (5 1 pm2) Nb/Al-Oxide/Nb                      best waveguide receivers.
tunnel junctions. Furthermore, the RF loss of Nb thin
films appears t o be quite low below the gap frequency (700                                11. MIXEROPTICS
GHz), and so integrated tuning circuits can be fabricated
which match the highly capacitive junction impedance.                   Figure 1 shows the optical layout of our mixer. The
Examples of these tuning circuits can be found in [3]-[9];            SIS chip is mounted on a silicon hyperhemispherical lens,
a discussion of the fundamental limitations of such circuits          which concentrates the incident radiation onto an antenna
is given in [lo].                                                     fabricated lithographically on the SIS chip. Hyperhemi-
                                                                      spherical lenses with planar antennas were introduced by
   Manuscript received October 18, 1994.                              Rutledge and Muha [18],and were first used with SIS mix-
   J. Zmuidzinas, e-mail jonasQtacos.caltech.edu, fax 818-796-8806,   ers by Wengler e t al. [12]. The hyperhemispherical lens
telephone 818-395-6229.                                               simulates a semi-infinite dielectric, and thereby avoids the
   This work was supported by NASA' grants NAG2-744 and
NAGW-107, the NASA/JPL Center for Space Microelectronics              leakage of radiation into substrate modes which occurs
Technology, and a NSF Presidential Young Investigator grant to        when planar antennas are mounted on a finite thickness
J. Zmuidzinas.                                                        substrate. Another benefit of this approach is that the

                                                   1051-8223/95$04.00   0   1995 IEEE

                       Slot Antennas
                                              2000 A   si0

       Ground Plane
                                                                         Fig. 3. A schematic of our two-junction tuning circuit. The circuit is
                                                                         driven antisymmetrically by the two slot antennas because the radial
                                                                         stubs are placed on opposite sides of the slots, and this produces a
                          RF choke                                       virtual ground at the center of the circuit. The SIS junctions are
                        (To IF and DC                                    represented by parallel RC networks.
                        contact pads)

                                                                                         111. TWIN-SLOT
                                                                                                     MIXER             CHIPS
Fig. 2. A diagram of the layout of our mixer chip. The two SIS
junctions are placed symmetrically about the center; their separa-       A. Antenna design
tion along with the width of the microstrip line joining them controls
the tuning inductance. The center of the circuit is a voltage null, or      Our mixer uses a twin-slot antenna (Figure 2) to receive
virtual ground.
                                                                         the radiation. As discussed by Zmuidzinas et al. [15],this
                                                                         antenna has a number of desirable properties, including
                                                                         linear polarization, a symmetric beam with low sidelobes,
                                                                         a low antenna impedance, and an octave bandwidth. The
                                                                         antenna dimensions used for the silicon substrates are as
                                                                         follows: the slot length is L = 0.33X, the width is W =
planar antenna radiates preferentially into dielectric ma-               O.O5L, and the separation is S = 0.17X. Here X is the
terial. The use of high-resistivity silicon ( E , = 11.5; [22])          free-space wavelength at the center frequency, which is
as the dielectric results in a calculated forward efficiency             chosen to be the frequency at which the antenna reactance
(for our antenna design) of 90%, compared to a 9% loss                   vanishes. At the center frequency, the antenna impedance
due to backward radiation into free space. This com-                     is real and has a value of 2, = 330.
pares quite favorably to the 70% forward efficiency quartz                  The antenna properties were calculated using the
(er = 4.5), which we had used in previous generations of                 method described in [15];briefly, this is a spectral-domain
our mixers [3], [15]. However, this improvement would be                 Galerkin moment-method solution which accounts for the
largely canceled by the increased Fresnel reflection at the              mutual interaction of the two slots. This calculation yields
surface of the silicon lens. We have therefore developed                 the antenna impedance as a function of frequency, the ra-
an anti-reflection coating for the silicon lens, which con-              diation pattern, and the fraction of power which is radi-
sists of epoxy loaded with alumina powder [19]. The alu-                 ated into the dielectric (90%). The beam pattern radiated
mina powder is necessary to obtain a dielectric constant                 into the dielectric is slightly asymmetric; the FWHM of
of e, = 4, and the coating has reasonably low loss (< 2%)                the power pattern is 26.5" in the E-plane and 23" in the H-
[20], good adhesion, and is cryogenically recyclable. The                plane. This 15% asymmetry can be reduced by increasing
epoxy is applied and allowed to cure, and is then diamond                the slot separation S , but this also increases the strength
machined [21] to yield a very smooth coating with pre-                   of the Eplane grating sidelobes. With our chosen sepa-
cisely the correct (X/4) thickness. In order to achieve good             ration, the fraction of power radiated into these sidelobes
registration, the spherical surface of the silicon lens is first         is about 1%.
diamond machined, and the coating is subsequently ap-
plied without removing the lens from the diamond-turning                 B. Circuit design
machine. In comparison to uncoated quartz lenses which
have a reflection loss of about 15%, the overall improve-                  The radiation received by the slot antennas is cou-
ment is a factor of 0.9/(0.85 x 0.7) M 1.5. An additional                pled into the SIS junctions with superconducting mi-
improvement of about 10% can be obtained by replac-                      crostrip lines. The properties of the superconducting
ing the polyethylene lens in front of the hyperhemisphere                microstrip lines were calculated using our previously de-
with a Teflon-coated quartz lens as is shown in Fig. 1. We               scribed method [15]. Microstrip radial stubs [15] are used
have obtained such lenses, and have verified the cryogenic               to effect the coupling to the slot antennas, by providing
durability of the Teflon coatings, but we have not yet used              an R F short-circuit between the microstrip and one side
these lenses in a mixer. Another possibility would be to                 of the slot antenna. A novel two-junction circuit [3] is
omit the second lens entirely by using an ellipsoidal lens in            used to tune out the SIS junction capacitance. The op-
place of the hyperhemisphere, but this gives less flexibility            eration of this circuit is shown schematically in Fig. 3.
in the choice of the beam characteristics of the mixer.                  The symmetry of the circuit produces a virtual ground at

the midpoint of the circuit; therefore, the short section
of microstrip which connects the two junctions behaves
as two inductive shunts which cancel the junction capaci-           1000          I   ,   .   I    1    1   I   I   .   ,   >   ,   ,   .   I , ,    I    ,   ,   , ,
tances. A quarter-wave impedance transformation section
                                                                                          Twin-Slot S E Mixer Performance
is inserted between the junctions and the slot antennas to                                7/11/94
match the junction resistance t o the antenna impedance.           h          -
However, the length of these sections is not su5cient to           v          -

allow the junctions to meet at the center, so two addi-            9 600 -
tional short sections of microstrip are placed between the         w          -
slots and the X/4 sections in order to obtain a total mi-          V I -
                                                                   4 400      -
crostrip length equal to the slot spacing S. Although this         m          -
                                                                   VI         -
results in an aesthetically pleasing configuration, the per-
                                                                        200   -
formance of the circuit could be better optimized by re-
moving this length constraint, and replacing these “exten-                        _-
sions” with X/4 sections, and taking up the excess length
                                                                          300                     400                500      600                   700                 800
with 90” bends in the microstrip. We plan t o incorporate                                                           FREQUENCY (CHz)
this improvement in our next chip design. The circuit is
analyzed and optimized using the HP/EESOF “Touch-              Fig. 4. Measured DSB receiver noise temperature as a function of
stone” microwave circuit analysis program. The circuit         frequency. The curves with the solid points are the measurements
                                                               for the silicon-based mixers; for comparison, the performance ob-
model includes all of the transmission line sections, our      tained previously with quartz-based mixers i shown by the curves
calculated frequency-dependent antenna impedance, and          with the open points. Each curve represents the frequency response
a parallel RC network representation of the SIS junc-          obtained with a single mixer chip. These measurements have not
tion, but neglects the discontinuity effects associated with   been corrected for the loss and thermal noise associated with the
                                                               beamsplitter used for the local oscillator injection, or other optical
the step changes in microstrip width and the dimensional       losses. The junction areas for the silicon-based mixers are nominally
mismatch between the SIS junction and the microstrip.          1.44 pm2.
For these calculations, we have assumed a junction area-
resistance product of R N A = 20 pm2 and a capacitance
C which gives WRNC 5 at 500 GHz; these are the val-
                                                               For comparison, we also show the performance previously
ues we believe to be representative of Nb/Al-Oxide/Nb          obtained with quartz-based devices [3]; the silicon mix-
                                                               ers outperform the quartz devices by about a factor of 2.
junctions whose current density is J, = 10kAcmP2. The
                                                               The noise temperatures are not corrected for the local os-
optimized circuits typically achieve better than -6 dB re-
                                                               cillator injection beamsplitter (typically 12 pm or 25 pm
turn loss (> 75% coupling efficiency) over 2 140 GHz
                                                               thick mylar film). The local oscillators (LO) used for these
                                                               measurements are Gunn oscillators followed by frequency
                                                               multipliers. Three multipliers were used for these tests:
C. Mixer chip fabrication                                      a 500-580 GHz quadrupler, and x 2 x 3 cascaded units
   The mixer chip fabrication was performed at the JPL         covering 580-660 GHz and 650-750 GHz. The envelope of
Center for Space Microelectronics Technology, using their      the lowest noise temperatures measured is well described
standard Nb/Al-Oxide/Nb trilayer process. This is es-          by the line TRX = 5 h v / k ~ note that this corresponds to
sentially the same process which was used in our previous      about 55 K (DSB) at 230 GHz. However, the noise tem-
generation chips [3], [15], and uses only optical lithogra-    peratures rise rather steeply above about 700 GHz, which
phy. A 2000w thick S i 0 insulating film was used for junc-    is the gap frequency of niobium. We believe that this
tion isolation and for the microstrip dielectric. A total of   behavior is caused by the large increase in circuit losses
720 chips are fabricated on a single 50 mm diameter, 250       due to the onset of pair-breaking above the gap frequency.
pm thick high-resistivity silicon wafer. There are a total     It may be possible to overcome this problem by fabricat-
of 30 different chip designs, corresponding t o two antenna    ing Nb/Al-Oxide/Nb junctions with normal-metal tuning
center frequencies (550 and 650 GHz), three junction ar-       circuits.
eas (1.0, 1.44, and 2.25 pm2), and five variations of the        We have attempted to use the shot-noise method to sep-
tuning circuit.                                                arately determine the mixer noise temperature and con-
                                                               version loss; however, we do not trust the results since
               Iv.   MIXERPERFORMANCE                          the IF impedance of the pumped SIS mixer is typically
                                                               a factor of 10 higher than the normal-state resistance.
  Figure 4 shows the double-sideband receiver noise tem-       The results we obtain with this method typically indicate
perature as a function of frequency which we measured          rather high IF noise temperatures (- 20 K) and low mixer
for two of the mixer chips. The measured noise tem-            conversion losses (- 6 dB). To obtain reliable data, we
peratures represent averages over our 1-2 GHz IF band.         plan to make measurements with a calibrated IF system.

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   The silicon-based mixers have been used in a submil-                  “A low noise receiver for millimeter and submillimeter wave
limeter receiver system which flies aboard the NASA                     lengths”, Int. J. IR and MM Waves, vol. 6, pp. 697-706, 1985.
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February, 1994, we detected the 745 GHz transition of                   submillimeter astronomy”, ZEEE Trans. Microwave Theory
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                                                                        SIS mixers”, IEEE Trans. Microwave Theory Tech., vol. 40,
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                         ACKNOWLEDGMENT                                 tennas”, Proc. IEEE, vol. 80, pp. 1748-1770, Nov. 1992.
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                                                                        slot antennas on extended hemispherical and elliptical silicon
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Phillips, .R. Schoelkopf, and J. Ward for their contribu-               41, pp. 1738-1749, 1993.
tions t o the laboratory work and for helpful discussions.              D. B. Rutledge and M. S. Muha, “Imaging antenna arrays”,
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We are particularly grateful to D. Kaneshiro and all the                Stycast 2850FT, Emerson and Cuming, Gardena CA.
employees of Janos Technology Inc. for their excellent                  M. Halpern, H. P. Gush, E. Wishnow, and V. De Cosmo, “Far
work in the development of the diamond-machined coat-                   infrared transmission of dielectrics at cryogenic and room tem-
ings for the silicon lenses.                                            peratures: glass, Fluorogold, Eccosorb, Stycast, and various
                                                                        plastics”, Appl. Opt., vol. 25, pp. 565-570, February 1986.
                                                                        Janos Technology Inc., HCR #33, Box 25, Route 35, Town-
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