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924 IEEE TRANSACTIONS ON APPLIED …

924 ieee TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 19, NO. 3, JUNE 2009 Millimeter-Wave Lumped Element SuperconductingBandpass Filters for Multi-Color ImagingShwetank Kumar, Anastasios Vayonakis, Henry G. LeDuc, Peter K. Day, Sunil Golwala, andJonas Zmuidzinas, Member, IEEEA bstract The opacity due to water vapor in the Earth s atmos-phere obscures portions of the sub-THz spectrum (mm/sub-mmwavelengths) to ground based astronomical observation. Formaximum sensitivity, instruments operating at these wavelengthsmust be designed to have spectral responses that match theavailable windows in the atmospheric transmission that occurin between the strong water absorption lines. Traditionally, thespectral response of mm/sub-mm instruments has been set usingoptical, metal-mesh bandpass filters [1].

926 IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 19, NO. 3, JUNE 2009 Fig. 3. Spiralinductor schematicdetailing thedimensionsusedin theinductor design. is the length of the th arm of the spiral inductor with for theoutermost arm. and arethe outermostandinnermost dimensionof

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Transcription of 924 IEEE TRANSACTIONS ON APPLIED …

1 924 ieee TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 19, NO. 3, JUNE 2009 Millimeter-Wave Lumped Element SuperconductingBandpass Filters for Multi-Color ImagingShwetank Kumar, Anastasios Vayonakis, Henry G. LeDuc, Peter K. Day, Sunil Golwala, andJonas Zmuidzinas, Member, IEEEA bstract The opacity due to water vapor in the Earth s atmos-phere obscures portions of the sub-THz spectrum (mm/sub-mmwavelengths) to ground based astronomical observation. Formaximum sensitivity, instruments operating at these wavelengthsmust be designed to have spectral responses that match theavailable windows in the atmospheric transmission that occurin between the strong water absorption lines. Traditionally, thespectral response of mm/sub-mm instruments has been set usingoptical, metal-mesh bandpass filters [1].

2 An alternative methodfor defining the passbands, available when using superconductingdetectors coupled with planar antennas, is to use on-chip, super-conducting filters [2]. This paper presents the design and testingof superconducting, lumped element, on-chip bandpass filters(BPFs), placed inline with the microstrip connecting the antennaand the detector, covering the frequency range from 209 416 filters were designed with pass bands 209 274 GHz, 265 315 GHz, 335 361 GHz and 397 416 GHz corresponding to the atmo-spheric transmission windows. Fourier transform spectroscopywas used to verify that the spectral response of the BPFs is wellpredicted by the computer simulations. Two-color operation of thepixels was demonstrated by connecting two detectors to a singlebroadband antenna through two BPFs.

3 Scalability of the design tomultiple (four) colors is Terms Bandpass filters, passive microwave circuits, sub-millimeter INTRODUCTIONTHERE are various existing designs for defining the pass-bands of mm/sub-mm astronomical cameras. Metal-meshfilters [1] have been used in conjunction with Infrared (IR)blocking materials such as teflon and fluorogold to makequasi-optical filters. This approach, however, has the disadvan-tage of preventing multi-color pixel operation, since the entirepixel array is irradiated by the same frequency band. Multicolorpixel operation is desirable for upcoming camera designs [3]Manuscript received August 19, 2008. First published May 27, 2009; currentversion published July 10, 2009. This work was supported in part by the NASAS cience Mission Directorate, Jet Propulsion Laboratory, and the Gordon andBetty Moore Kumar is with IBM Watson Research Center, Yorktown Heights, NY10598 USA (e-mail: He was with the Department ofApplied Physics, California Institute of Technology Pasadena, CA 91125 Vayonakis, S.)

4 Golwala, and J. Zmuidzinas are with the Department ofPhysics, California Institute of Technology, Pasadena, CA 91125 USA G. LeDuc and P. K. Day are with the Jet Propulsion Labora-tory, Pasadena, CA 91109 USA(e-mail: versions of one or more of the figures in this paper are available onlineat Object Identifier increase the overall throughput for a wide field survey bycapturing more photons in different frequency bands. Thishas many advantages [4], including more efficient use of thefocal plane. Further, the quasi-optical mesh filters are typicallydesigned for normal-incidence operation; radiation incident atoblique angles may leak through and load the detector. Properoperation of quasioptical filters also requires care and attentionto detail. For instance, the filters need to be carefully heat sunkin order to prevent detector loading due to radiation emittedby the filters themselves.)

5 These issues may be addressed usingon-chip designs for the filters. Although we chose to uselumped-element designs, on-chip bandpass filters may also beimplemented using distributed circuit elements [5]. However,using distributed circuit elements causes the bandpass filters toleak at higher harmonic frequencies requiring a cascaded low-pass filter. This adds to the complexity of the circuit and takesup extra real estate on the chip. The lowpass filter may insteadbe implemented using a metal mesh filter as long as the issuesmentioned above are addressed. The lumped-element, on-chipbandpass filters presented in this paper offer a relatively simpleand compact solution to the problem of multicolor mm/sub-mmimaging. We will now describe the detailed filter design andthe measurement BANDPASSFILTERDESIGNA. Lumped-Element Circuit DesignWe use a three-element Butterworth lowpass filter prototype[Fig.]

6 1(a)] with cutoff atand terminated in a 1load as a basis for design of our bandpass filters. This nor-malized lowpass filter has element values,[Fig. 1(a)]. It is converted to a lumped-element bandpass filterusing the usual transformations [6]: replacing the series inductorby a series combination of inductorandcapacitor, and shunt capacitor by a par-allel combination of inductorand capac-itor[Fig. 1(b)].is the characteristicimpedance of the filter, chosen to be equal to the character-istic impedance of the microstrip transmission line which bringssubmillimeter power from the antenna and is equal to the design bandwidth of the BPFandis the center frequency given by the geo-metric mean of the 3 dB cutoff frequencies of the the upper and lower cutoff frequencies of the band-pass filter, respectively. For the ease of fabrication of the cir-cuit, shunt inductors to ground should be avoided.

7 For suffi-ciently narrow bandwidth (nominally 10 20%), impedance in-verters can be used to replace a parallel shunt admittance1051-8223/$ 2009 IEEEA uthorized licensed use limited to: CALIFORNIA INSTITUTE OF TECHNOLOGY. Downloaded on August 10, 2009 at 12:47 from ieee Xplore. Restrictions apply. KUMARet al.: MILLIMETER-WAVE LUMPED ELEMENT SUPERCONDUCTING BANDPASS FILTERS925 Fig. 1. Steps for lumped-element bandpass filter circuit design. (a) 3-pole But-terworth low-pass filter prototype. and are normalized inductor and ca-pacitor, respectively. (b) Bandpass filter with series LC and shunt parallel LCresulting from the prototype low-pass filter. (c) Bandpass filter with impedanceinverter blocks and no shunt inductors. (d) Final lumped-element bandpass filtercircuit with no shunt 1 CIRCUITELEMENTVALUES FORFILTERS (D)WITHPASSBANDDEFINED BYATMOSPHERICTRANSMISSIONWINDOWSby a series impedancefor two-port networks [7] [Fig.]

8 1(c)].Using the identity, where,, andis the impedance inverter, weget the values forand series capacitanceusingand. The impedance inverter is im-plemented using a two-port T network with the series branchescontaining capacitorsand shunt branch containing capac-itorwhere[Fig. 1(c)]. The negative capacitancein the series branches can be absorbed intoandto givethe final circuit [Fig. 1(d)]. Here,,, lumped circuit element values derived using this synthesismethod are further optimized using Agilent ADS [8], a circuitsimulation software package, to give a power transmission ofat most40 dB in stop band, unity transmission in passbandwith sharpest possible rolloff. These optimized values for var-ious lumped components for the four bandpass filters are listedin Table Circuit Layout and FabricationThe schematic of the filter layout is shown in Fig.

9 2. The in-tegrated inline BPF circuit was fabricated on chip with the otherpixel components using photolithography. The inductors wereimplemented using spiral geometry and the capacitors in theparallel plate configuration. The bottom layer is 200 nm thickniobium film deposited on the silicon substrate using DC mag-netron sputtering. The ground plane, spiral inductors (Fig. 2shown in dark grey) and lower plate of the parallel plate ca-pacitors [not visible in Fig. 2(a)] are patterned on this layerusing photoresist andInductively Coupled Plasma (ICP)etching. Following this, a 400 nm thickdielectric layeris deposited on the bottom layer by RF magnetron sputteringwith substrate bias. Subsequently, a 200 nm thick layer of nio-bium is deposited using DC\ magnetron sputtering and patternedusing ainductively coupled plasma (ICP) etch toFig.

10 2. SONNET layout for the bandpass filters: (a) top view, (b) side view. Thetop microstrip is made of niobium (light grey). Below it is a layer of (whitehatched with black lines, not visible in the top view), then niobium ground plane(dark grey). The series inductors and capacitors are fabricated on top of thesilicon substrate (white) directly in holes in the ground the wiring layer of microstrip geometry (Fig. 2, shown inlight grey). The dielectric is then removed using aICP etch. The input microstrip brings the broadband submil-limeter power from the slot antenna to the filter (Fig. 8). In orderto avoid having to make conducting via contacts between thebottom niobium layer and the top niobium layer, the first se-ries capacitor is divided into two series capacitors each havingdouble the design capacitanceand placedon either side of the spiral inductor as shown in Fig.


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