Johansen, Tom Keinicke

Krozer, Viktor

Vidkjær, Jens

In this paper, the influence from substrate effects on the performance of wideband SiGe HBT mixer circuits is investigated. Equivalent circuit models including substrate networks are extracted from on-wafer test structures and compared with electromagnetic simulations. Electromagnetic simulations are also applied to predict short distance substrate coupling effects. Simulation results using extracted equivalent circuit models and substrate coupling networks are compared with experimental results obtained on a wideband mixer circuit implemented in a 0.35 μm, 60 GHz ft SiGe HBT BiCMOS process

Online Research Database In Technology

                       General rights Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.  • Users may download and print one copy of any publication from the public portal for the purpose of private study or research. • You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the public portal   If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim.     Downloaded from orbit.dtu.dk on: Dec 17, 2017Substrate Effects in Wideband SiGe HBT Mixer CircuitsJohansen, Tom Keinicke; Vidkjær, Jens; Krozer, ViktorPublished in:2005 European Gallium Arsenide and other Compound Semiconductors Applications SymposiumPublication date:2005Document VersionPublisher's PDF, also known as Version of recordLink back to DTU OrbitCitation (APA):Johansen, T. K., Vidkjær, J., & Krozer, V. (2005). Substrate Effects in Wideband SiGe HBT Mixer Circuits. In2005 European Gallium Arsenide and other Compound Semiconductors Applications Symposium (pp. 469-472).IEEE.Substrate Eﬀects in Wideband SiGe HBT MixerCircuitsT. K. Johansen, V. Krozer, J. Vidkjær, T. DjurhuusOersted-DTU , Department of Electromagnetic SystemsTechnical University of Denmark — 2800 Kgs. Lyngby, DenmarkAbstract— In this paper the inﬂuence from substrate ef-fects on the performance of wideband SiGe HBT mixer cir-cuits is investigated. Equivalent circuit models includingsubstrate networks are extracted from on-wafer test struc-tures and compared with electromagnetic simulations. Elec-tromagnetic simulations are also applied to predict shortdistance substrate coupling eﬀects. Simulation results usingextracted equivalent circuit models and substrate couplingnetworks are compared with experimental results obtainedon a wideband mixer circuit implemented in a 0.35µm, 60GHz fT SiGe HBT BiCMOS process.I. IntroductionAdvanced SiGe HBT technologies make use of a low-doped p substrate with a resistivity in the range of 10-20Ω · cm. In Si-based monolithic microwave integrated cir-cuits (MMICs) this results in a signiﬁcant high-frequencyloading of circuit nodes involving both transistors and pas-sive elements. The non-zero dielectric constant and theconductivity of the Si-substrate may also lead to unwantedcoupling between components that otherwise are supposedto operate independently [1].Equivalent circuit models suitable for predicting theloading due to substrate eﬀects have been presented in [2],while shielding measures to reduce substrate coupling ef-fects have been discussed in [3]-[4]. In the past, however,investigations of the inﬂuence of substrate eﬀects in high-frequency integrated circuits have mainly been limited tolow-noise ampliﬁers [5]-[6], and voltage-controlled oscilla-tors [7]. The inﬂuence of substrate eﬀects on the frequencyresponse of wideband SiGe HBT mixer circuits has not pre-viously been discussed.In this paper a combined experimental and simulationstudy of the eﬀect of the substrate on wideband SiGe HBTcircuits is described. Part of the study consists of extract-ing suitable equivalent circuit models and substrate cou-pling networks from fabricated on-wafer test structures andelectromagnetic simulation. The equivalent circuit modelsand substrate coupling networks are then used in the sim-ulation of the wideband mixer circuit to investigate theinﬂuence on the frequency response. Especially the im-portance of substrate eﬀects for matching simulations toexperimental results obtained on a wideband mixer circuitimplemented in a 0.35µm, 60 GHz fT SiGe HBT BiCMOSprocess will be discussed.II. Wideband SiGe HBT Mixer DesignA. Circuit DesignThe circuit schematic of the wideband SiGe HBT mixercircuit is shown in Fig. 1. It is based on a modiﬁed Gilbertcell topology with emitter degenerated transconductance50Ω+_RF50Ω50Ω50Ω+_LOVcc+ IFVeeVeeFig. 1. Circuit schematic of wideband SiGe HBT mixer based on amodiﬁed Gilbert Cell topology.stage and a shunt feedback stage for the load circuit assur-ing wideband operation at all mixer ports. The principleof strong impedance mismatch is followed throughout thedesign in order to minimize the inﬂuence from parasiticseﬀects. For the wideband SiGe HBT mixer circuit in Fig.1 certain interconnection lines can be identiﬁed as criticalfor the performance and needs careful consideration [8]. Es-pecially, the interconnection lines providing access to themixer ports are typical lengthy and needs accurate mod-eling due to their potential strong eﬀect on the matchingproperties of the circuit.B. Interconnection Line ModellingIn SiGe HBT BiCMOS technology, the metal-insulator-semiconductor interconnect line structure results in largecapacitance to the substrate. The loss in the substratecan not be neglected at high frequencies and should betaken into account in the modelling of the interconnectlines [9]. Even though the interconnect line in principle isa distributed structure, it is assumed here that a simpleπ-type model as shown in Fig. 2 is suﬃciently accurate tomodel the behavior of the interconnect line.The equivalent circuit consist of series resistance and se-ries inductance (RLF and LLF ) representing the low fre-quency ohmic loss and self-inductance of the line. This isfollowed by a latter network which models the skin eﬀect.The series impedance is shunted by oxide capacitors to thesubstrate (Cox), and substrate resistances (Rsub). Addingshunt capacitors (Csub) across the substrate resistances be-comes necessary at higher frequencies to take the transitionfrom the slow-wave mode to the quasi-TEM mode into ac-count [10]. The elements for the interconnect line modelare extracted from the admittance parameters for an on-wafer test structure of a thru line de-embedded for padAuthorized licensed use limited to: Danmarks Tekniske Informationscenter. Downloaded on January 22, 2010 at 06:32 from IEEE Xplore.  Restrictions apply. RsubCsub CsubCox CoxRsubRLF LLFa)S11S21b)Fig. 2. a) Interconnect line equivalent circuit model. b) Measured(-), modelled (-o-), and simulated (-x-) S-parameters in the frequencyrange from 45 MHz-40 GHz.parasitics. The thru line have length 367µm and width10µm and is implemented in the third metal layer locatedapproximately 2.5µm over the substrate. At low frequen-cies the series resistance (RLF ) and the series inductance(LLF ) are extracted asRLF = {−1Y12}(1)andLLF =1ω{−1Y12}(2)respectively. The remaining elements of the latter networkis ﬁttet to the real and imaginary part of the frequencydependent series impedance. The oxide capacitance (Cox)and substrate resistance (Rsub) are extracted at suﬃcientlylow frequencies (ω  1/(CsubRsub)) asCox =−1ω{1Y11+Y12} (3)andRsub = {1Y11 + Y12}(4)respectively. The substrate capacitor Csub is extracted asCsub =√C2ox{Y11 + Y12}Rsub−1ω2R2sub− Cox. (5)TABLE IExtracted elements for interconnect line model.As shown in Fig. 2b) the extracted equivalent circuitmodel accurately predicts the measured frequency depen-dent lossy behavior of a interconnection line structure inthe frequency range from 45 MHz-40 GHz. The intercon-nect line structure was also simulated with the electromag-netic simulation tool Momentum in Agilent ADS. As shownin Fig. 2b) the results from the electromagnetic simulationaccurately predict the performance of the interconnect line.In Table. I the elements for the interconnect line model areextracted as described above based on either measurementor electromagnetic simulation using Momentum. The ex-cellent agreement between the two approaches shows thatthe development of scalable models for interconnect linescan conﬁdentially be based upon electromagnetic simula-tion.C. Substrate Coupling ModellingAs the frequency of operation rises the coupling into thesubstrate increases. This may lead to coupling betweencomponents which otherwise are supposed to operate inde-pendently. It is diﬃcult to design on-wafer test structuressuitable for predicting this type of coupling. Instead thesubstrate coupling between nearby components can be pre-dicted by electromagnetic simulations. In this work, thesubstrate Green’s function as calculated by Agilent ADSMomentum have been used to predict the short-distancesubstrate coupling. In the Green’s function approach theactive areas on-chip are abstracted into equipotential sub-strate ports. Substrate coupling networks between sub-strate ports can then be extracted [1].As an example, consider the coupling between a p+guard ring and the collector-substrate junction for a SiGeHBT device as shown in Fig. 3. The layer structure sim-p-substrateVip+ guard ringfield implant layerFig. 3. Example of substrate coupling between p+ guard ring andthe collector-substrate junction for a SiGe HBT device.Parameter Extracted Value Extracted Value(Measurement) (Momentum)RLF [Ω] 1.6 1.8LLF [nH] 0.38 0.42Cox [fF] 19.4 18.0Rsub [Ω] 251.6 315.4Csub [fF] 13.6 9.0Authorized licensed use limited to: Danmarks Tekniske Informationscenter. Downloaded on January 22, 2010 at 06:32 from IEEE Xplore.  Restrictions apply. 0 10 20 30 400246810mag(Yout) [mS]Frequency [GHz]020406080100120140160180phase(Yout) [Degrees]Fig. 4. Inﬂuence from substrate coupling network on output admit-tance for a SiGe HBT device model. Measurements (-), modelled withno substrate coupling network included (-o-), modelled with substratecoupling network included (-x-).ulated consists of substrate ports on top of a thin p dopedﬁeld implant layer with resistivity 1Ω − cm followed by a710µm thick p− substrate with resistivity of 19Ω− cm. Inthe simulation setup using Momentum, a voltage source ex-citation Vi is applied to the substrate port representing thecollector-substrate junction and the guard ring is groundedusing a ground reference port. The admittance parametersimulated can be represented as a shunt RC network. Theextracted resistance is around 802Ω and the capacitance isaround 3fF in good agreement with the values extractedfrom on-wafer measurements using the method reported in[11]. As a further veriﬁcation of the above described ap-proach, Fig. 4 shows the improvement in the small-signaloutput admittance modelling of a 8x0.35µm2 area SiGeHBT device when including the substrate coupling networkfound from electromagnetic simulation.III. Experimental ResultsThe circuit was realized in a 0.35µm, 60 GHz fT SiGeHBT BiCMOS process. The photograph of the fabricatedwideband SiGe HBT mixer circuit is shown in Fig. 5. Thecurrent consumption was 61 mA from a ±2.5 V supply.The characterization of the circuit were performed usingon-wafer measurements. In order to investigate the inﬂu-ence from substrate eﬀects on the frequency response ofthe wideband SiGe HBT mixer circuit two levels of simula-tion complexity are compared. The ﬁrst level is a nominalsimulation including only SiGe HBT devices, poly resistorsand MIM capacitors. This level of simulation complex-ity corresponds to what is normally possible with standardSiGe HBT foundry process design kits. In the nominalsimulation the substrate networks for all components areneglected. The second level of simulation complexity takesthe pads, critical interconnect lines, and substrate couplingnetworks associated with SiGe HBT devices, poly resis-tors, and MIM capacitors into account. The SiGe HBTdevices are modelled using the VBIC95 model with thesubstrate terminals interconnected through the substratecoupling network found from electromagnetic simulation.As shown in Fig. 6 the high frequency roll-oﬀ observedFig. 5. Photograph of wideband SiGe HBT mixer (0.9x0.9mm2).in the measured conversion gain is more closely matchedby simulations when the pads, critical interconnect lines,and substrate coupling networks are properly taken intoaccount. The remaining diﬀerence is believed to be due tothe accumulated inﬂuence from interconnection lines notyet accounted for in the simulation. Fig. 7 shows the dou-ble sideband noise ﬁgure for the wideband active mixer.It is observed that the nominal simulation underestimatesthe noise ﬁgure at higher frequency while the complex sim-ulation more accurately captures the increase in noise. Itis observed however, that the noise ﬁgure seems to be lesssensitive to substrate eﬀects than the frequency responseof the conversion gain.2 4 6 8 10 12 14 16 1805101520Conversion Gain [dB]RF Frequency [GHz]Fixed IF=LO−RF=0.1 GHzRF Power=−30 dBmLO Power=−5 dBm Fig. 6. Comparing measured (-) conversion gain with nominal sim-ulation (-o-) and simulation with substrate networks included (-x-).Authorized licensed use limited to: Danmarks Tekniske Informationscenter. Downloaded on January 22, 2010 at 06:32 from IEEE Xplore.  Restrictions apply. 2 4 6 8 10 12510152025Noise Figure [dB]RF Frequency [GHz]Fixed IF=LO−RF=0.1 GHzRF Power=−30 dBmLO Power=−5 dBm Fig. 7. Comparing measured (-) double-sideband noise ﬁgure withnominal simulation (-o-) and simulation with substrate networks in-cluded (-x-).IV. ConclusionIn this paper an investigation of substrate eﬀects onthe frequency response of wideband SiGe HBT mixer cir-cuits was reported. As part of the investigation, equivalentcircuit models and substrate coupling networks were ex-tracted from on-wafer test structures and electromagneticsimulation. Nominal simulations fails to predict the roll-oﬀrate in the conversion gain and the increase in double side-band noise ﬁgure at high frequencies. Inclusion of parasiticeﬀects comprised of the extracted substrate networks werefound increasingly important at higher frequencies for theprediction of measured results with simulations.References[1] R. Singh, Y. V. Tretiakov, J. B. Johnson, S. L. Sweeney, R. L.Barry, M. Kumar, M. Erturk, J. Katzenstein, C. E. Dickey, andD. L. Harame, “Parasitic modeling and noise mitigation in ad-vanced RF/mixed-signal Silicon Germanium processes,” IEEETrans. Electron Devices, vol. 50, pp. 700–715, Nov. 2003.[2] M. Pfost, H. M. Rein, and T. Holzwarth, “Modeling substrateeﬀects in the design of high-speed Si-bipolar IC,” IEEE J. Solid-State Circuits, vol. 31, pp. 1493–1501, Oct. 1996.[3] M. Pfost, H. M. Rein, W. Steiner, and A. Sturmer, “Simulationof substrate coupling with special regard to shielding in high-speed Si/SiGe-bipolar IC,” in Proc. 29th European MicrowaveConference, Munich, Germany, Oct. 1999, pp. 133–136.[4] M. Pfost, P. Brenner, T. Huttner, and A. Romanyuk, “An exper-imental study on substrate coupling in Bipolar/BiCMOS tech-nologies,” IEEE J. Solid-State Circuits, vol. 39, pp. 1755–1763,Oct. 2004.[5] J. T. Colvin, S. S. Bhatia, and K. O. Kenneth, “Eﬀect of sub-strate resistances on LNA performance and a bondpad structurefor reducing the eﬀect in a silicon bipolar technology,” IEEE J.Solid-State Circuits, vol. 34, pp. 1339–1344, 1999.[6] A. Raghavan, U. Jalan, S. Chakraborty, C. Lee, J. Lasker,E. Chen, J. Lee, J. D. Cressler, G. Freeman, and A. Joseph,“A millimeter-wave linear low noise ampliﬁer in SiGe HBT tech-nology with substrate parasitic model,” in Proc. European Gal-lium Arsenide and other Compound Semiconductors Applica-tion Symposium, Amsterdam, The Netherlands, Oct. 2004, pp.399–402.[7] A. Tasic and W. A. Serdijn, “Eﬀects of substrate on phase-noiseof bipolar voltage-controlled oscillators,” in IEEE InternationalSymposium on Circuits and Systems, 2002, pp. 819–822.[8] H.-M. Rein and M. Mo¨ller, “Design considerations for very-high-speed Si-bipolar IC’s operating up to 50 Gb/s,” IEEE J. Solid-State Circuits, vol. 31, pp. 1076–1090, Aug. 1996.[9] J. Zheng, Y. Hahm, V. K. Tripathi, and A. Weisshaar, “CAD-oriented equivalent-circuit modeling of on-chip interconnects onlossy Silicon substrate,” IEEE Trans. Microwave Theory Tech.,vol. 48, pp. 1443–1451, 2000.[10] H. Hasegawa, M. Furukawa, and H. Yanai, “Properties of mi-crostrip line on Si-SiO2 system,” IEEE Trans. Microwave The-ory Tech., vol. MTT-19, pp. 1443–1451, 1971.[11] T. K. Johansen, J. Vidkjær, and V. Krozer, “Substrate eﬀectsin SiGe HBT modeling,” in Proc. European Gallium Arsenideand other Compound Semiconductors Application Symposium,Munich, Germany, Oct. 2003, pp. 879–882.Authorized licensed use limited to: Danmarks Tekniske Informationscenter. Downloaded on January 22, 2010 at 06:32 from IEEE Xplore.  Restrictions apply. 

Substrate Effects in Wideband SiGe HBT Mixer Circuits

http://orbit.dtu.dk/ws/files/4101008/Johansen.pdf

Substrate Effects in Wideband SiGe HBT Mixer Circuits

Abstract

Similar works

Full text

Available Versions

Online Research Database In Technology