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Magnetic modelling, analysis and on-chip shielding of SFQ Circuits

Bakolo, Simeon Rodwell (2018-03)

Thesis (PhD)--Stellenbosch University, 2018.

Thesis

ENGLISH ABSTRACT: Single Flux Quantum (SFQ) based electronic circuits are susceptible to failure if exposed to external magnetic fields, even in very small quantities. Operating margins of SFQ circuits have shown to decrease significantly in the presence of magnetic fields as small as 5 μT. This challenge makes SFQ circuits infeasible for the normal environment in which other technologies, such as CMOS, operate. Sources of magnetic fields include SFQ circuit’s own bias lines, the Earth, which can produce magnetic fields up to 65 μT and any other electromagnetic interference sources. Without protective measures, most SFQ circuits cannot work in the open environment. By using the tools available in the inductance extraction tool, InductEx, a method for projecting uniform magnetic fields, of varying orientations on SFQ cells, strides have been made to make SFQ circuits more tolerant to magnetic fields. On-chip analysis of SFQ cells was done in the presence of magnetic fields from the x, y and z orientations. In addition, by varying currents in 3-D coils, the orientation of the magnetic fields can be varied. This way, it is now possible to analyse bias, parameter and operating field margins of SFQ cells in any direction of the magnetic field. Two on-chip shielding solutions were analysed and developed. The conventional continuous superconductor layer shield, dubbed the solid shield, and the grid shield. The grid shield resembles a Faraday cage and it is implemented by laying out bars of 2.5 μm width using the topmost layer in the Hypres0 4.5 kA/cm2 process. The solid shield is more effective against perpendicular (z-directed) magnetic fields than against those in-plain (x and y-directed). In addition, the solid shield’s inclusion in SFQ cells results in the reduction of circuit inductance by up to 25 %. The grid shield is a very effective approach against in-plane magnetic fields. However, its effectiveness is inversely proportional to the spacing between grid bars. Compared to the solid shield, the grid shield has less effect on circuit inductance with a typical reduction of 8 % at a grid bar spacing of 5 μm. The large reduction in inductance can be overcome by making the inductors thinner and shorter. Shielding effectiveness of on-chip shields is enhanced by making ground contact vias from the shield layer to the ground plane. So far, uniformly grounded shields have shown to be the most effective approach. The solid shield improved the operating field margin of a DFF cell against a z-directed magnetic field from 30 μT to 531 μT, while the grid shield, of 5 μm grid spacing, improved the margins from 68 μT to 290 μT against an x-directed magnetic field. In the DC-SFQ cell, the operating field margin was improved with a solid shield from 47 μT to 464 μT, against a z-directed magnetic field, while with a grid shield, the improvement was from 211 μT to 381 μT, against an x-directed fields. To further enhance the magnetic field tolerance of SFQ circuits, design tenets that target specific components, such as inductors, were analysed. Thin and narrow inductors have shown less coupling to external magnetic fields. In addition, moats have shown to influence OFM results depending on the orientation of magnetic fields. This work has delivered design and analysis methods for magnetic field tolerant SFQ circuits.

AFRIKAANSE OPSOMMING: Elektroniese stroombane gebaseer op Single Flux Quantum (SFQ) tegnologie is sensitief vir selfs klein eksterne magnetiese velde. Werksbereik van SFQ stroombane is reeds gedemonstreer om beduidend te verklein wanneer die bane vloeddigtheid so laag as 5 μT ervaar. Dit is een van die uitdagings wat SFQ bane onprakties maak in die tipe werksomgewings waar ander tegnologieë soos CMOS normaalweg opereer. Magnetiese veldbronne sluit die voorspanningslyne van die SFQ bane en die geomagnetiese veld in. Laasgenoemde kan vloeddigtheid van tot 65 μT bedra. Ander elektromagnetiese steurnisse kan ook die werking van SFQ bane beïnvloed, sodat SFQ bane nie sonder beskermende maatreëls in ‘n oop omgewing kan funksioneer nie. Deur die gebruik van die gereedskap beskikbaar in die induktansie-onttrekkingsmodule InductEx, naamlik ’n metode om uniforme magneetvelde met veranderende oriëntasie oor SFQ selle aan te lê, is groot vordering gemaak om SFQ stroombane meer vas te maak teen magnetiese velde. Analise van SFQ selle soos dit op ’n mikroskyfie geïmplementeer word, is simulasiegewyse in die teenwoordigheid van magnetiese velde met x-, y- en z-oriëntasie verrig. Sodoende is dit nou moontlik om voorspanning-, parameter- en veld-werksbereik van SFQ selle to analiseer vir enige veldrigting. Twee afskermingsoplossings vir geïntegreerde skyfie uitlegte is ontwikkel en geanaliseer: die konvensionele kontinue supergeleier afskermingslaag – of solide afskerming – en rooster-afskerming. Rooster-afskerming benader ’n Faraday kou, en is geïmplementeer deur die uitleg van stroke met 2.5 μm wydte in die boonste supergeleierlaag van Hypres se 4.5 kA/cm2 proses. Soliede afskerming is meer effektief teen loodregte magnetiese velde (in die z-rigting) as teen velde in die xy-vlak. Soliede afskerming verlaag egter stroombaaninduktansies met ongeveer 25 %. Rooster-afskerming is ’n baie effektiewe metode teen xygeöriënteerde magnetiese velde, maar effektiwiteit is omgekeerd eweredig aan die spasiëring tussen stroke. Vergeleke met soliede afskerming, het die rooster-afskerming ’n kleiner effek op stroombaaninduktansie, met tipies slegs sowat 8 % vermindering wanneer strookspasiëring 5 μm is. Afskermingseffektiwiteit word verbeter deur grondkontakte van die afskermingslaag na die grondvlak. Eweredig-gegronde afskermings is aangetoon om die mees effektiewe benadering te wees. Soliede afskerming verbeter die veld-werksbereik van ’n DFF sel in z-gerigte magneetvelde vanaf 30 μT tot 531 μT, terwyl rooster-afskerming met 5 μm strookspasiëring die veld-werksbereik in x-gerigte magneetvelde vanaf 68 μT tot 290 μT verbeter. In die GS-na-SFQ sel is die veld-werksbereik met soliede afskerming verbeter vanaf 47 μT tot 464 μT in z-gerigte magneetvelde, terwyl dit in x-gerigte magneetvelde vanaf 211 μT tot 381 μT verbeter is met rooster-afskerming. Vir verdere verbetering van die magnetiese veldtoleransie van SFQ bane, is ontwerpsmetodes vir spesifieke komponente, soos induktore, ontwikkel. Dun en nou induktore is aangetoon om swakker te koppel met eksterne magneetvelde, terwyl gragte in die grondvlak aangetoon is om veld-werksbereik te beïnvloed. Hierdie werk stel ontwerps- en analisemetodes daar vir magnetiese veldvaste SFQ bane.

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