INTEGRIRAN SISTEM ZA SAMODEJNO UMERJANJE OBČUTLJIVOSTI MAGNETNEGA SENZORJA

GRADIŠEK, MIHA (author), Trontelj, Janez (mentor)

Doktorsko delo obravnava integriran senzorski mikrosistem za merjenje magnetnega polja z implementacijo samodejnega umerjanja občutljivosti senzorja v temperaturnem območju od -40 ℃ do 125 ℃. Jedro mikrosistema, realiziranega v standardni 0,35 μm CMOS (ang. Complementary Metal Oxide Semiconductor) tehnologiji, predstavlja Hallov element. Ena poglavitnih pomanjkljivosti magnetnih senzorjev, baziranih na Hallovih elementih, predstavlja lezenje občutljivosti senzorja z nepredvidljivimi in nepovratnimi dejavniki, kot so staranje, mehanske oz. piezoelektrične sile, vlaga in kot glavni vpliv temperatura, saj je Hallov element implementiran v temperaturno dokaj nestabilnem aktivnem področju dopiranega N-otoka (ang. N-well). Poleg tega je občutljivost celotnega senzorja podvržena fluktuaciji temperature, ki predstavlja glavni vpliv na variacijo karakteristik elektronskih sklopov za obdelavo zajetih signalov. Običajni pristopi kompenzacije občutljivosti za naštete vplive ne morejo biti uporabljeni v procesu proizvodnje, saj je treba zagotoviti umerjanje, katerega delovanje se aktivno prilagaja glede na obratovalne razmere tekom celotnega življenjskega cikla integriranega senzorja. Samodejno umerjanje izbranega pristopa izhaja iz uravnavanja občutljivosti glede na povratnozančno ovrednotenje temperaturno spreminjajočega se referenčnega signala, referenciranega na znano in stabilno referenčno magnetno polje, generirano z integrirano tuljavico na samem integriranem vezju. Integracija tuljavice na štirih metalnih nivojih CMOS procesa je izvedena v μm razredu nad Hallovim elementom, kar prispeva k večji učinkovitosti tuljavice, saj bi izvedba tuljavice zunaj integriranega vezja močno zmanjšala jakost magnetnega polja na razdalji, kjer se nahaja Hallov senzor [1]. Učinkovitost integrirane tuljavice se lahko izboljša z manjšimi dimenzijami, saj je generirano magnetno polje obratno sorazmerno s premerom integrirane tuljavice. Kljub izboljšani učinkovitosti lahko premajhno dimenzionirana tuljavica na mestu Hallovega elementa generira negativno magnetno polje in na splošno prispeva k dokaj neuniformno razporejenemu magnetnemu polju, ki neposredno vpliva na referenčni signal, uporabljen v procesu umerjanja. Predstavljen je pristop večkriterijske optimizacije 3D modela integrirane tuljavice z namenom iskanja optimalne geometrije tuljavice ob predpostavki čim večje jakosti enakomerno razporejenega magnetnega polja in minimalne upornosti njenih ovojev. Izvedba integrirane tuljavice z minimalno upornostjo zmanjša temperaturne izgube in posledično prispeva k preciznejšemu umerjanju občutljivosti celotne signalne poti magnetnega senzorja. Optimizacija elektromagnetnih karakteristik integrirane tuljavice je izvedena v programskem okolju Comsol Multiphysics in glede na tehnološke podatke uporabljene CMOS tehnologije parametrizirana s 3D modelom, podprtim znotraj načrtovalskega orodja Solidworks. Optimiziran model integrirane tuljavice z 11 ovoji na štirih metalnih nivojih ima učinkovitost 376 mT/A. Simulacije celotnega električnega vezja s pomembnejšimi podsklopi, začenši z modelom Hallovega elementa, čelnim vezjem za zajem signala, delta-sigma modulatorja, DA pretvornika in izhodnim filtrom so opravljene s simulatorjem vezij HSPICE. Celotna signalna pot od zajema signala preko obdelave do izhodnega signala je realizirana diferencialno, s čimer se zmanjša vpliv neželenih sofaznih motenj. Dani pristop ne zmanjšuje le vpliva variacij občutljivosti Hallovega elementa, ampak celotne signalne poti, zaradi česar ni treba skrbeti za temperaturno konstantno ojačenja posameznih stopenj v procesu obdelave signalov. Simulacijski rezultati nakazujejo izboljšano umerjanje integriranega senzorja ob uporabi večbitnega pristopa v primerjavi z 1-bitnim kvantizatorjem in 1-bitnim DA pretvornikom v povratni zanki modulatorja za tipično približno 4,2-krat za celotno temperaturno območje od -40 ℃ do 125 ℃, medtem ko se umerjanje nekoliko poslabša za celoten nabor možnih procesnih parametrov izdelave, a kljub temu predstavljeni pristop v dobršni meri odpravlja tudi vpliv procesnih variacij na samo umerjanje. Ovrednotenje celotnega mikrosistema je izvedeno preko meritve izhodne analogne napetosti, dani pristop pa omogoča nadaljnjo obdelavo digitalnega signala bitnega-toka v procesu DSP (ang. Digital Signal Procesing) obdelave, kar ni tema tega dela. Umerjanje občutljivosti integriranega magnetnega senzorja v celotnem temperaturnem področju kažejo neželene motnje, a v dobršni meri opravljajo funkcionalnost predstavljenega pristopa, in sicer s točnostjo umerjanja občutljivosti senzorja 85 ppm/℃. Učinkovitost realizirane tuljavice je 131 mT/A, vplivi posameznih ovojev na superpozicionirano magnetno polje v ravnini Hallovega elementa pa se dobro skladajo s simulacijskimi rezultati.

• Language: Slovenian
• Keywords: magnetni senzor, samodejno umerjena občutljivosti, optimizacija integrirane tuljavice, temperaturna odvisnost občutljivosti
• Work type: Doctoral dissertation
• Organization: FE - Faculty of Electrical Engineering
• Year: 2021
• PID: 20.500.12556/RUL-125011
• Publication date in RUL: 01.03.2021

Secondary language

• Language: English
• Title: INTEGRATED SYSTEM FOR AUTOCALIBRATION OF MAGNETIC SENSOR SENSITIVITY

Abstract

In the thesis integrated sensor microsystem for magnetic field measurement with implemented self-calibrated sensitivity technique in the temperature range from –40 ℃ to 125 ℃ is addressed. The core of the microsystem, realized in a standard 0.35 μm CMOS (Complementary Metal Oxide Semiconductor) technology, represents a Hall element. One of the main drawbacks of this type of sensor is the drift of sensitivity in the presence of unpredictable and irreversible effects, such as aging, mechanical or piezoelectric forces, humidity and, above all, the influence of temperature, since the Hall element is implemented in a temperature unstable N-well region. In addition, temperature affects the sensitivity of the entire sensor since the characteristics of the electronics which composes the signal processing path are susceptible to the temperature effect. Conventional approaches of sensitivity calibration during the production line could not be applied for the above mentioned distortion influences, but should be handled with an active on the fly calibration technique that adapts based on the operating conditions throughout the lifetime of the integrated sensor. The self-calibration presented in this paper is based on the adjustment of the sensitivity with respect to the evaluation of the temperature-dependent reference signal related to the known and stable reference magnetic field generated by the integrated coil in the integrated circuit. The coil integration on four metal layers of the CMOS process is realized in the range of μm dimensions above the Hall element, which contributes to improved coil efficiency, since the external coil at the position of the Hall element would generate a much lower intensity of the magnetic field [1]. The efficiency of the integrated coil could be improved at smaller dimensions since the intensity of the magnetic field is inversely proportional to the diameter of the integrated coil. Despite the improved efficiency, a coil with too small dimensions could generate a negative magnetic field at the location of the Hall element and generally contribute to a rather unevenly distributed magnetic field that directly affects the reference signal used in the process of calibration. With the aim of realizing an optimal geometry of the integrated coil that produces the highest possible magnetic field intensity with uniform distribution and reduction of coil winding resistance, the multi-objective optimization of the 3D model of the integrated coil is performed. The realization of the integrated coil with minimum resistance promises lower thermal losses and consequently contributes to the more accurate calibration of the sensitivity of the overall magnetic sensor signal path. The optimization of the electromagnetic properties of the integrated coil is performed in the Comsol Multiphysics environment, which interchanges 3D model parameters based on the data of the used CMOS technology with the Solidworks environment. The optimized model with 11 turns at four metal layers promises the efficiency of 376 mT/A. Simulations of the entire electrical circuit with the main structures starting with the Hall model, the front-end circuit to acquire the signal, the delta-sigma modulator, the DA converter and the output filter are performed using the simulator HSPICE. The entire signal path from acquisition to signal processing to output is implemented differentially, which improves the suppression of common mode distortion. The given approach mitigates not only the effect of Hall element sen sitivity variations but also that of the entire signal path, which relaxes the requirements for the design of signal processing stages with temperature independent gains. Simulation results show improved calibration in the temperature range from -40 ℃ to 125 ℃ by a factor of 4.2 when multibit quantization is used instead of 1-bit quantization in combination with the 1-bit DA converter. Over the entire set of process parameters, the overall calibration performance is slightly degraded, but not significantly, so the presented approach also mitigates the effect of process variations on the calibration to a good extent. The evaluation of the whole microsystem is performed with the measurement of the analog output signal, but the approach allows further processing of the digital bit-stream signal in a process of DSP (Digital Signal Processing), which is not the subject of this work. The calibration of the sensitivity of the integrated magnetic sensor shows some distortions, but it fulfils the functionality of the presented approach with an accuracy of the calibration of the sensor sensitivity of 85 ppm/℃. The efficiency of the integrated coil is 131 mT/A, and the influence of the specific coil turn on the overall magnetic field in region of the Hall element is in a good correlation with the simulation results.

• Keywords: magnetic sensor, self-calibrated sensitivity, integrated coil optimization, temperature dependence of the sensitivity