Bruno, A.

Kaiser, Ch.

Lisitskiy, M. P.

Siegel, M.

Skacel, S. T.

Ustinov, A. V.

Wünsch, S.

KITopen

 Physics Procedia  36 ( 2012 )  245 – 249 1875-3892  © 2012 Published by Elsevier B.V. Selection and/or peer-review under responsibility of the Guest Editors. doi: 10.1016/j.phpro.2012.06.154 Superconductivity Centennial ConferenceInvestigation of dielectric losses in hydrogenated amorphoussilicon (a-Si:H) thin ﬁlms using superconducting microwaveresonatorsA. Brunoa,b, S.T. Skacelc,d, Ch. Kaiserc, S. Wu¨nschc,M. Siegelc, A.V. Ustinovd, M.P. LisitskiyaaIstituto di Cibernetica ”E. Caianiello”, Consiglio Nazionale delle Ricerche, Via Campi Flegrei 34, I-80078, Pozzuoli (Napoli), Italy.bUniversita` degli studi di Napoli ”Federico II”, Via Cintia 80126 Napoli, Italy.cInstitut fu¨r Mikro- und Nanoelektronische Systeme (IMS), Karlsruher Institut fu¨r Technologie (KIT), Germany.dPhysikalisches Institut (PI), Karlsruher Institut fu¨r Technologie (KIT), Germany.AbstractThe improvement of the coherence times of superconducting qubits depends on the reduction of the dielectric losses inthe insulating materials implemented for the device fabrication. These losses depend on the density of spurious dipolesof diﬀerent nature (two-level systems, TLSs) which couple to phase qubits and, hence, limit their coherence times.Hydrogenated amorphous silicon (a-Si:H), because of its lower loss tangent (tan δ) among conventional dielectrics (a-SiO, a-SiO2, a-SiNx:H), is considered as the best amorphous dielectric for superconducting qubit application. We havedeveloped a reliable method for the direct measurement of dielectric losses in amorphous dielectric thin ﬁlms using anovel design based on four lumped superconducting LC resonators connected in series without coupling capacitors. Thelosses are obtained as tan δ = 1/Q0, where Q0 is the intrinsic quality factor of the resonator, measured at 3 dB abovethe resonance frequency without any ﬁtting procedure. The series type LC resonators with a-Si:H as dielectric werefabricated by the Nb technology. The measurements were done at the conditions of a qubit application (0.5 - 10 GHzfrequency range and low temperatures). The low values of the loss tangent of a-Si:H (up to 2.5 x 10−5 at 4.2 K) haverequired the development of superconducting housing for the resonators in order to eliminate a spurious dependence oftan δ on the microwave power, by reducing losses which were not originated in the dielectric itself. The results of thesimulations agree well with the experiments.c© 2011 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of Horst Rogalla and Peter Kes.Keywords: Microwave resonator, Superconducting qubit, Decoherence, Dielectrics losses, Hydrogenated amorphussilicon1. IntroductionIt is well known that the a major source of decoherence in superconducting qubits based on the Josephsoneﬀect, and of reduced quality factors in superconducting microwave resonators, is the presence of micro-scopic defects in the materials utilized for their fabrication [1].So far, the microscopic origin of these defects has not been fully identiﬁed and they are generally con-sidered as two-level state (TLS) systems. As described by the tunneling model [2], TLSs can couple to theAvailable online at www.sciencedirect.com 2 ublished by Elsevier B.V. Selection and/or pe r-revi w under responsibility of the Guest Editors.Open access under CC BY-NC-ND license.Open access under CC BY-NC-ND license.246   A. Bruno et al. /  Physics Procedia  36 ( 2012 )  245 – 249 qubit’s and to resonator’s electric ﬁeld, extracting their energy, and thus limiting their coherence time T2 andquality factor Q0, respectively. These defect are commonly found in all the amorphous dielectrics involvedin the qubit fabrication [3], including the tunnel barrier, the layers for wiring insulation, and the capacitordielectrics [4], but also the metal oxides on the surfaces of the superconducting ﬁlms [5] and the interfacesbetween diﬀerent materials (e.g. between crystalline substrates and superconducting metal layers).Thus, wiring insulator, superconductor/insulator interfaces, and tunnel barrier are the favorite candidatesto be studied and optimized, in order to reduce decoherence in the next generation superconducting qubits.The transverse relaxation time T2 of a qubit (i.e. the net decay time of quantum information) is relatedwith the energy relaxation time T1 as1T2=12T1+1TΦ(1)where 1/TΦ is the pure dephasing rate, which is determined by the low-frequency ﬂuctuations in thecircuit controlling the energy separation between the qubit states, and T1 is inversely proportional to the losstangent tan δ of the dielectrics used to build the superconducting circuit [4]. Therefore, it is clear that theless the dielectric losses (tan δ), the longer the decay time of quantum information of the qubit.So far, the hydrogenated amorphous silicon (a-Si:H) is considered as the best amorphous dielectric forqubit application due to its extremely low tan δ among the conventional dielectrics (such as SiO, SiO2,SiNx) [6]. Both the coplanar waveguide (CPW) and lumped LC superconducting resonators represent twopresent-day technique for experimental measurement of the loss tangent of dielectrics. The latter is a directand reliable tool for the characterization of the loss tangent of any dielectric at the conditions at which aqubit device is typically operated, namely at the lowest cryogenic temperatures, in the 1 - 10 GHz microwaverange, and at extremely low excitation power (10−6 - 10−3 V). In the present work, we optimize the techniqueof multiplexed lumped LC superconducting resonator in such a manner to obtain the experimental conditionsfor reproducible measurements of the low losses of a-Si:H thin ﬁlms.2. ExperimentsAfter our eﬀort to develop a reliable method for the integration of low-defective a-Si:H in the stan-dard fabrication of superconducting circuits based on Nb Josephson junction technology [7], we realizedsuperconducting LC lumped resonators consisting of a superconducting inductive coil and superconductingparallel-plate capacitor containing the dielectric under investigation, as in Fig. 1.Fig. 1. The newest layout with 6 multiplexed resonators and high temperature a-Si:H lift-oﬀ process. Green: base superconductinglayer; yellow: dielectric layer; blue: superconducting wiring layer.The a-Si:H ﬁlms studied in this work were deposited by high-frequency plasma enhanced chemicalvapour deposition (PECVD) [8] on silicon substrates, on which 200 nm thick niobium ﬁlms were alreadybeen deposited and patterned with several geometries of inductors and capacitors. During the PECVD nophotoresist was present on the samples, in order to avoid the contamination of both the deposited a-Si:H andthe reactor chamber, but also because the high temperatures required to obtain the highest quality a-Si:H(∼ 250◦ C) would have permanently burnt any conventional photoresist. A. Bruno et al. /  Physics Procedia  36 ( 2012 )  245 – 249 247The PECVD parameters were optimized during preliminary tests: the silane pressure was set to PS iH4 =200 mTorr at an incident power of 5 W; the plasma was driven at 100 MHz and the substrate temperaturewas set at 250◦ C in order to obtain the lowest defective ﬁlms.After the deposition, the a-Si:H ﬁlms covered uniformly all the sample’s surfaces: it was necessary toopen a contact via through the insulating ﬁlm by means of CF4/O2 reactive ion etching before completingthe capacitor fabrication with the niobium counterelectrode lift-oﬀ deposition.In addition to the conventional RIE process for deﬁnition of the a-Si:H layer, a high temperature lift-oﬀprocess based on the PMGI photoresist was also developed and successfully applied for patterning the a-Si:H ﬁlm. In fact, in order to realize high quality superconducting resonators or qubits, it is preferable toreduce the dielectric coverage of the underlying superconductor [1, 7, 9], limiting the contamination of thesuperconducting surfaces by hydrogen and dielectric inclusions.In the actual experimental setup [10], the resonators under investigation are directly coupled to the mi-crowave cables through microwave SMA connectors and indium contact bonding, i.e. no coupling capacitorare inserted in series on the line. This conﬁguration realizes a stop-band notch ﬁlter, in contrast to the ma-jority of CPW or lumped element superconducting resonators so far realized, which are band-pass ﬁlters[11, 12].Careful electromagnetic simulations lead to a design of 4 and 6 multiplexed resonator in series, withorthogonal resonances and negligible cross talk interference, as shown in the left panel of Fig. 2 in the caseof a 4 series circuit.The lumped LC resonator technique allows to directly measure the loss tangent of the dielectric via theintrinsic quality factor Q0 = 1/ tan δ, using no ﬁtting parameters. The measurement of Q0 is performedby means of a microwave network analyzer, by taking the width of the resonance peak at 3 dB above theminimum of the transmissivity |S 12| data (right panel of Fig. 2).3. Results and DiscussionThe present analysis of a-Si:H resonators came right after an extensive survey of commonly used di-electrics: a ﬁrst set of measurements involved Nb2O5 resonators, where the anodic niobium oxide wasgrown under diﬀerent conditions and chemistries in order to determine the dependence of the quality factoron the oxide preparation; then SiO, and SiNx resonators were characterized, showing the expected increasein the quality factor due to their less defective dielectrics [10]. All these measurements were done in brasssample housings at IMS. With such conﬁguration, the frequency dependencies of the dielectric loss of suchinsulating materials were measured with great reproducibility at T = 4.2 K.Fig. 2. Experimental data from a 4-multiplexed a-Si:H resonators. Left graph: all 4 resonances measured in a single frequency sweep(0 -12 GHz). Right graph: detail of the gray-box area from previous graph, showing the loaded quality factor QL and, in its inset, the-3 dBm method to determine the intrinsic quality factor Q0 for the selected resonance.248   A. Bruno et al. /  Physics Procedia  36 ( 2012 )  245 – 249 Dielectric losses could be written as tan δ ∼ χ′′ where χ′′ is the imaginary part of the electric suscepti-bility [13]. The tan δ data recorded for the diﬀerent materials at 4.2 K and at diﬀerent resonant frequencieswere ﬁtted by the ”Universal Law” χ′′ ≈ ωn−1 with 0 < n < 1 and ω being the resonant frequency. Theexponent n is indicative of the mutual interactions between TLS dipoles: for n = 0 there is no interaction,for n ∼ 0.5 nearest-neighbor interaction is prevalent, and for n > 0.6 many body interaction is the maindefect-defect one.In the case of the above mentioned materials it was measured 0.63 < n < 0.68, indicating that the TLSinteraction could be described by strong dipole many-body interaction model.As soon as we started to measure the resonators with a-Si:H as capacitor dielectric, it appeared evidentthat the measured quality factors were somehow ”limited” to Q0 values lower than 9000, by unknownreasons, instead of the expected unloaded quality factor Q0 ∼ 30000 for such resonators at 4.2 K. Moreover,for these resonators, the Q0 were also strongly and anomalously dependent on the applied microwave power,unlike the case of all the perviously measured resonators.Further analysis, and accurate simulations using Sonnet [14] Electromagnetic Field Solver pointed outthat the limiting factor had to be found in the sample housing and, in particular, in the choice of its constitut-ing material. Simulations of the housing inﬂuence on quality factors showed that superconducting housingreduces the housing inﬂuence to an absolute minimum. In fact, it came out that the on-chip microstrip lunch-ers, used to connect the SMA connectors to the lumped element resonator, although being well matched (50Ω) to groundplane by means of its width and tapered design, induces currents in the groundplane. The latter,in the brass-housing experimental setup, is the lossy and rough brass-housing surface underneath the chipsubstrate. Moreover, under the superconducting lumped inductor, there is no need of a groundplane, andideally there should not be at all. So having a lossy ground plane is the most undesirable condition in ourexperimental setup.Housing material Sonnet Q0 Experimental Q0Brass 9255 8391Copper 13724 12740Lead 29000 19598Niobium 29000 26400Fig. 3. Left: photo of the samples boxes with installed SMA connectors and bonded samples (for the niobium housing, the Nb cover isalso installed). Right: simulated and measured quality factors for a ∼ 4.5 GHz a-Si:H resonator varying the sample housing material.For the above reasons, we realized new versions of the sample housing, as shown in Fig. 3. We havebeen able to measure very low losses in the a-Si:H resonators (as low as tan δ = 2.5 x 10−5 at the highestfrequencies, and as expected from amorphous silicon at 4.2 K) initially by means of housings made ofcopper, then of superconducting lead, and ﬁnally of superconducting niobium.Thus, in order to measure the dielectric losses of an extremely low lossy material, like a-Si:H, withour microstrip-coupled lumped element resonators, one need to ensure that the dielectric losses have tobe the dominant losses in the measurement setup. Furthermore, the diﬀerence between the measurementsperformed in lead and in niobium housings (right table in Fig. 3) could be indicative of the higher lossesexpected from lead oxides compared to niobium oxides.Having minimized the spurious losses present in our experimental setup by using superconducting hous-ings, we proceeded to correctly characterize the a-Si:H material, and we extracted a parameter n = 0.57,showing that the nature of the TLS interaction in such material seems to be closer to the nearest-neighborone, which is indicative of a lower density of TLS defects.Additionally, we have investigated resonators having the same resonance frequencies, but realized withdiﬀerent combination of the inductor length and capacitor area, demonstrating that the quality factor Q0 is A. Bruno et al. /  Physics Procedia  36 ( 2012 )  245 – 249 249independent of the resonator geometry. Moreover, for all the investigated materials, no dependence of thelosses on the dielectric ﬁlm thickness was found.4. ConclusionsThe obtained loss tangent results at T=4.2 K and preliminary measurements at 300 mK (not reportedin this work) conﬁrm the suitability of fabricated a-Si:H ﬁlms to be employed as dielectric layers for therealization of qubits with longer coherence times.Furthermore, having minimized the spurious losses of our setup, we showed that the improved lumpedLC lumped resonator method is a reliable direct measurement of low temperature dielectric loss and thuswe opened the door for a new survey of a number of ultra-low loss amorphous (such as a-SiC:H, DLC, etc.)or single crystalline [15] dielectric candidates, in a quick and accurate way.As a last remark, it could be very interesting to characterize losses of LC resonators and coherence timesof qubits in the same measurement, to correlate the two phenomena at the same experimental conditionsand in the same deposited material, by means of co-fabricated circuits. In this context, the developedlift-oﬀ process is capable to integrate the high temperature PECVD deposition of a-Si:H with existing NbJosephson technology, fulﬁlling the above mentioned requirements of reduced coverage of the underlyingsuperconducting ﬁlms.References[1] J.M. Martinis, K.B. Cooper, R. McDermott, M. Steﬀen, M. Ansmann, K.D. Osborn, K. Cicak, S. Oh, D.P. Pappas, R.W. Sim-monds, and C.C. Yu, Phys. Rev. Lett. 95, 210503 (2005).[2] C. Enss, S. Hunklinger, Low-Temperature Physics, Springer Verlag, Berlin Heidelberg (2005).[3] J. Gao, M. Daal, A. Vayonakis, S. Kumar, J. Zmuidzinas, B. Sadoulet, B.A. Mazin, P.K. Day, and H.G. Leduc, Appl. Phys. Lett.92, 152505 (2008).[4] R. McDermott, IEEE Trans. Appl. Supercond. 19, 2 (2009).[5] J. Gao, M. Daal, J.M. Martinis, A. Vayonakis, J. Zmuidzinas, B. Sadoulet, B.A. Mazin, P.K. Day, and H.G. Leduc, Appl. Phys.Lett. 92, 212504 (2008).[6] J.M. Martinis, Quantum Information Processing 8, 81 (2009).[7] A. Bruno, P. Mengucci, L.V. Mercaldo, and M.P. Lisitskiy, in preparation for Appl. Phys. Lett. (2011).[8] P. Delli Veneri, L.V. Mercaldo, and C. Privio, Renewable Energy 33, 42 (2008).[9] R. Barends, H.L. Hortensius, T. Zijlstra, J.J.A. Baselmans, S.J.C. Yates, J.R. Gao, and T.M. Klapwijk, Appl. Phys. Lett. 92,223502 (2008).[10] Ch. Kaiser, S.T. Skacel, S. Wu¨nsh, R. Dolata, B. Mackrodt, and A. Zorin, M. Siegel, Supercond. Sci. Technol. 23, 075008 (2010).[11] P. Macha, S.H.W. van der Ploeg, G. Oelsner, E. Il’ichev, H.G. Meyer, S. Wu¨nsch, and M. Siegel, Applied Physics Letters 96,062503 (2010).[12] H. Paik, and K.D. Osbornm, Appl. Phys. Lett. 96, 072505 (2010).[13] K.L. Ngai, A.K. Jonscher, and C.T. White, Nature 277, 185 (1979).[14] Sonnet c©, 3D Planar High-Frequency Electromagnetic Software, http://www.sonnetsoftware.com[15] S. Weber, K.W. Murch, D.H. Slichter, R. Vijay, and I. Siddiqi, Appl. Phys. Lett. 98, 172510 (2011).

Investigation of dielectric losses in hydrogenated amorphous silicon (a-Si:H) thin films using superconducting microwave resonators

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Investigation of dielectric losses in hydrogenated amorphous silicon (a-Si:H) thin films using superconducting microwave resonators

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