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Benedetto Vigna Paolo Ferrari Flavio Francesco Villa Ernesto Lasalandra Sarah Zerbini Editors Silicon Sensors and Actuators The Feynman Roadmap Silicon Sensors and Actuators Benedetto Vigna • Paolo Ferrari Flavio Francesco Villa (cid:129) Ernesto Lasalandra Sarah Zerbini Editors Silicon Sensors and Actuators The Feynman Roadmap Editors BenedettoVigna PaoloFerrari AnalogMEMS&SensorsR&D STMicroelectronics,AnalogMEMS STMicroelectronics(Italy) andSensorsGroup,MEMSTechnology Cornaredo,Milano,Italy andDesignR&D AgrateBrianza FlavioFrancescoVilla MonzaBrianza,Italy STMicroelectronics,AnalogMEMS andSensorsGroup,MEMSTechnology ErnestoLasalandra andDesignR&D Analog,MEMSandSensorsGroup AgrateBrianza STMicroelectronics(Italy) MonzaBrianza,Italy Cornaredo,Milano,Italy SarahZerbini Analog,MEMSandSensorsGroup STMicroelectronics(Italy) Cornaredo,Milano,Italy ISBN978-3-030-80134-2 ISBN978-3-030-80135-9 (eBook) https://doi.org/10.1007/978-3-030-80135-9 ©SpringerNatureSwitzerlandAG2022 Thisworkissubjecttocopyright.AllrightsarereservedbythePublisher,whetherthewholeorpartof thematerialisconcerned,specificallytherightsoftranslation,reprinting,reuseofillustrations,recitation, broadcasting,reproductiononmicrofilmsorinanyotherphysicalway,andtransmissionorinformation storageandretrieval,electronicadaptation,computersoftware,orbysimilarordissimilarmethodology nowknownorhereafterdeveloped. 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ThisSpringerimprintispublishedbytheregisteredcompanySpringerNatureSwitzerlandAG Theregisteredcompanyaddressis:Gewerbestrasse11,6330Cham,Switzerland Foreword Ihavespentmyentireprofessionalcareerinthesemiconductorindustry,andIhad thepleasureandthelucktoseeinfirstpersontwoimportantturningpointsofthis industry.Inthe1960s,thetechnologicalcommunitystartedtointegratesuccessfully onsiliconmanytransistorsaimingtomimicthefunctionofthebrain,thememory, the nervous system, and the muscle of human beings. In the 1990s, instead, new components, such as variable capacitors and variable resistors, became the core of miniaturized silicon transducers, either sensors or actuators, and we started to integrate into silicon the five senses of human beings. It has been for sure a big revolution. In the last 20 years, the pace of transducer development accelerated thanks to the progresses in microfabrication techniques and successful high-volume market applications. Nowadays, transducers are all around us: in cars, in smartphones, in factories,inprinters,intablets,insatellites,indrones,insmartspeakers,inwatches, andeveninmedicalpatchesandshoes. There is no doubt we are living in interesting times, with many challenges opportunities in front of us. Internet of Things, artificial intelligence, and 5G networks will boost the GDP of many countries and will enable new business models. I really hope these new technologies will help reduce the negative impact ofhumankindonnatureandwillalsohelpreducethegapbetweentherichestand poorestcountriesintheworld. Withintheframeofthisnewblurredworld,micromachinedsiliconsensorswill playevenamoreimportantrolethantoday.Severalparameters,suchasvibrations, sound,atmosphericpressure,pollutionlevel,willreachthedigitalworldwithoutthe need of people typing on a keyboard or moving a mouse or touching an advanced display. The most successful transducers in the market are not exploiting complex quantumphysicseffects.Theiroperationmodeisrelativelyeasytobeunderstood. But under this apparent simplicity, two big challenges reside. On one side, being multidisciplinarydevices,theyrequirethemasteringofmanydisciplines(physics, engineering,electronics,andmaterialsscience)fortheirproperdesign.Ontheother side,therearemanysecretsanddetailstobetailoredduringtheirqualificationand v vi Foreword their production so to reach the high-yield high-reliability low-cost target. In my career, I have been contacted by many professors and startups and I read many papers about innovative transducers. But only few of them reached the market successfully,becausetheytackledproperlyallthedimensions,beyondtheintrinsic beautyofanewsiliconstructure. Physicsandtechnologyofsilicon-basedtransducersarerathercomplex,andthey aretreatedinnumerouspublicationsscatteredthroughouttheliterature.Therefore, aclearneedexistsforabookthatthoroughlyandsystematicallyreviewsthepresent basicknowledgeonthesedevices.Myenthusiasticwelcome,therefore,goestothis enlighteningbook,SiliconSensorsandActuators,writtenwiththecontributionsof morethaneightyauthorswithdifferentbackgroundsandeducation(technologists, physicists,andelectronic,mechanical,andbiotechengineers).Attheoriginofthis book and its scientific contents, there is a fantastic adventure, which began about 25 years ago in STMicroelectronics, a mature hi-tech semiconductor multibillion- dollar company. All these people, with the support of whole organization of STMicroelectronics and the one of Academic and research centers all over the world,havebeenabletoconceive,design,qualify,produce,andsellmorethan20 billiontransducerstomanycustomersbigandsmall. This book provides a complete and up-to-date overview of these devices, includingindustrialization’sbiggestchallengesrelatedtoreliability,packaging,and engineering. I believe that students, researchers, or engineers involved in silicon- based sensor and actuator research and development will find a wealth of useful information in this book, thanks to the proven track of record of the authors. The readerwillbeabletoacquireasolidtheoreticalandpracticalbackgroundthatwill allowthemtoanalyzethekeyperformanceaspectsofthesedevices,criticallyjudge afabricationprocess,andconceiveanddesignnewonesforfutureapplications.This bookanditsgreatcollectionofachievementsrepresentsamilestonefortechnicians andpassionatesupportersofthefield.Ibelieveitcanhelptostimulatethefantasy and creativity of the readers so as to generate new devices that can enhance the brilliantthinkingnatureofHomoSapiens,suchasweare. Ihadtheopportunityandthepleasuretoworkwithsomeofthesehighlytalented people. Twenty-five years are gone, but without any doubt the challenges of these transducers’developmentandindustrializationhelpedallofusstayyounginspirit. Ireallyhopeyouwillenjoyreadingthisbook. BrunoMurari STTechnicaladvisorandformerdirector ofDivisionalResearchDevelopment Center–Cornaredo(Mi),Italy Introduction Fourteen was the number of the Third Industrial Revolution, also known as the DigitalRevolution,andFourteenisthenumberoftheFourthIndustrialRevolution. Fourteen is the number of electrons around a nucleus composed by 14 protons and 14 neutrons. These elementary particles all together compose the beautiful atom of silicon, discovered in 1824 by the Swedish chemist Berzelius. Today, withoutsilicon,wewouldnothaveanyofthekeyblocksoftheDigitalRevolution (computers, mobile phones, the World Wide Web) that nowadays we all take for granted; we also couldn’t think about the present and future waves of artificial intelligence. Inthesecondhalfofthetwentiethcentury,thesilicontransistor,inventedin1947, boosted the pace of innovation, and consequently the worldwide gross domestic product was as never seen before in the history of mankind. Silicon started to be usedforitssemiconductorpropertiesinanaloganddigitalcircuits,indiscreteand integrated chips, and it drove the third industrial revolution of the 1970s. The first operational amplifier, the first microcontroller, the first memory, the first analog to digital converter were all using silicon as their base material. Nowadays, the semiconductorindustryhasavalueofabout400B$anditemploysseveralmillion peoplearoundtheworld.Asoftenhappensduringtheprogressofmankind,military applications drove the development of this strategic industry, and at the time in which this book is being written, it is still creating tense geopolitical frictions betweentheUSAandChina. WhattriggeredtheSiliconRevolutionwasasimplesilicontransistor,whichwas realized on 1” wafer, replacing the mature and reliable vacuum tube thanks to its semiconductor properties. Moreover, during the past 60 years, lithographic pitch reduction has been driving and aligning all the players of this industry along the guidelinesofthewell-knownMoore’sLaw. In parallel, in the last four decades, in a few research laboratories scattered throughouttheUSAandEuropeandinahandfulofalmost-obsoletemanufacturing plants, a few visionary and brave pioneers have been exploiting other physical propertiesofsilicon(seePartIofthisbook).Thesepeopleweretheactorofwhat Iliketocallthe‘’SecretMEMSRevolution.”Theseresearchersdidn’tappreciate vii viii Introduction deepsubmicrontechnologiesrunningin12”highcapital-intensivewaferfabs,they didn’t fall in love with Moore’s Law. Inspired by the motivating words of the Noble Prize winner R. P. Feynman’s legendary talk (“There’s Plenty of Room at theBottom,”1959),theambitionofthosepioneerswasverysimple:tomanufacture low-cost energy-efficient miniaturized sensors and actuators on silicon wafers to help as many people as possible. Later, those miniaturized devices became more known as micro-electromechanical Systems (MEMS): millimeter-sized systems where not only electrons are moving, but also fluids, cantilevers, and membranes. Themanufacturingprocessesrequiredtorealizesuchdevicesarewelldescribedin PartIIofthisbook. All those explorers, out of the mainstream silicon technology development roadmap and with limited support by the Semiconductor Industry Association, have gone through many theoretical and practical challenges to bring their ideas tothemarket.Onlyafewinnovativecompanieswithbraveandresilientteams,who daredtochallengethemselveswithaclearvisionthatbeyondMoore’sLawthereis anotherbusinessworthworld,havebeensuccessful.STMicroelectronicsisamong the few successful semiconductor companies thanks to its early investment in 8” MEMSmanufacturinglinein2005andtoagreatteamwhosewordsyoucanread inthisbook. ToolmakinghasalwaysdifferentiatedourspeciesfromallothersonEarth.Four hundredthousandyearsago,HomoSapienscarvedaerodynamicallyshapedwooden spearstokillanimalstofeedthemselvesproperlyandallthemembersoftheirtribe. SincetheCognitiveRevolutionthirty-fivethousandyearsago,HomoSapienshave beenabletoimagineanewworldandinfluenceit.HomoSapiensareequippedwith fivesenses(smell,touch,sight,taste,hearing),twocouplesofactuators(armsand legs), and, most importantly, the best brain in nature able to integrate the signals gatheredbysensorsandthenmovingtheinnateactuators.Overtime,ourancestors realizedthattheyneededmoreprecisesensorsandmoreefficientactuatorstobetter mastertheworldaroundthem,andthustheystartedtousetheirbrainstoconceive, develop,andmanufacturesensorsandactuatorstoaugmentthelimitedcapabilities oftheirbody. Timehasbeenthefirstvariablethatmankindhasbeeninterestedtomeasure.In the ancient world, time measurement was important for many different purposes: from knowing the exact hour during which to hold religious rites to the time slot allocated to the defense lawyers in ancient Rome’s courts and then to the paid time slot allocated to prostitutes’ customers in many brothels all over the world. The Egyptians used large obelisks to track the movements of the sun and to measure the passing of time; they also developed water clocks, later used by both the Greeks (a.k.a. clepsydrae) and the Chinese. With the exception of a few other instruments (like the compass invented in China, used for divination first and later for navigational orienteering around 1050 AD), we will need to wait for the Scientific Revolution of the seventeenth century, whose foundational Galilean scientific method required the development of many more instruments to challenge the dogmatism of the pre-modern era. Barometers, accelerometers, thermometers, and other bulky and expensive sensors were all invented starting Introduction ix fromtheseventeenthcentury.Theirwidespreadusehasbeenlimitedbytheirhigh price, also linked to the amount of raw material used to realize those sensors. Only in the last 25 years, a strong boost to the massive adoption of sensors came fromtheautomotivemarket(airbags, ...)and,later,fromtheconsumer(Nintendo WiiConsole)andpersonalelectronicsmarket(computers,mobilephones,watches, etc.), thanks to the sensors’ increased reliability, optimized power consumption, miniaturized size, and, most importantly, more affordable cost (see Parts III, VI, andVIIofthisbook). Almost eight thousand years ago, Homo Sapiens started using cows for tilling the land for more efficient farming, and later began riding horses to move more quicklyfromonepointtoanother.Muchlater,intheeighteenthandthenineteenth centuries,J.Watt’ssteamandtheelectromagneticfieldsofT.EdisonandN.Tesla, respectively, ignited the First and the Second Industrial Revolution in the Old Continent. All these innovations were meant to offset the natural limit of our legs andarmsandsustaintheeconomicanddemographicgrowthofmankindthanksto anincreaseinproductivity. Today,thesesensorsandactuatorssurroundus,andweinteractwiththemdaily. We can find several types of sensors in cars, smartphones, pacemakers, drones, smartspeakers,washingmachines,andmanyotherequipment.Theirusesarevery widespread.Theycanhelpusinteractinaneasierwaywithcomplexdigitaldevices or they can make our cars greener, smarter, and safer. These sensors are all made in silicon, and a detailed description of these sensors can be found in Part III of this book. The sensors that we find all around us are much smaller than their macroscopic counterpart of the past. As an example, let’s consider the example of the gyroscope, a sensor able to measure angular rates of the system where it is mounted.Wecanfinditinallmediumandhigher-endcarsand,since2010,alsoin medium-high-end smartphones of several brands. The silicon gyroscope occupies a volume of few cubic millimeters, it has a weight of few milligrams, and it is muchsmallerthantheFoucault’spendulum(year1851)usedtomeasuretheEarth’s rotation,thankstoasuspended28-kilogrambrass-coatedleadbobattachedtoa67- meterlongwire! Today,weareusedtoprintingmanydocumentseasily,remaininginthecomfort of our homes and offices. These printers are much smaller and cheaper than the original press machine of Gutenberg (year 1453). That machine was about three cubicmeterswithaweightabout200kg!Today,ink-jetprintersaremuchsmaller thankstoamicromachinedthermalorpiezoelectricactuatorabletoejectaccurately andquicklypicoliter-sizedropletsofink.(seePartIV). This field of sensors and actuators is so diverse and multidisciplinary that it is difficult for any single person to follow up all its activity. Thus, I asked my colleagues, expert in sensors and actuators, to join me in writing all the relevant andspecifictopicsthatconcernthisvastfield.Theresultisthisnewbookonsilicon sensorsandactuators. Thisbookisintendedforpracticingengineers,scientists,andadvancedgraduate students who seek a broader understanding on important subjects regarding the micro-sensor and micro-actuators field. The topics in this book are arranged in x Introduction logicalorderintheformofeightparts.Besidesthisintroductionandapartrelated to the silicon properties, five other important areas are covered: micromachining technology, device modeling and required circuitry, assembly and calibration techniques,reliabilitytests,andpresentandfuturedeviceapplications. The first part provides a good overview of the silicon properties. The second part describes the different micromachining technologies and the varied materials usedtorealizesensorsandactuators.PartsIIIandIVdescribeindetailthetheory and working mode of the transducer element of different type of sensors and actuators, while Part V addresses the challenges of the related electronic circuitry. PartVIfocusesontheimportanceofassemblyandcalibrationontheperformances and high-volume manufacturability of MEMS, while the seventh part addresses reliability, a very important, but often-forgotten topic. In the last part, we take a quickglanceatpotentialfutureapplicationsofsensorsandactuators. In this book, we purposely decided not to address the topic of CMOS image sensors, since they require 12” factories and, like microprocessors and memory chips,followmorecloselytheMoore’slaw. Most of the 80 writers of this book are coming from STMicroelectronics. We alsoreceivedvaluablecontributionsfromcolleaguesatPolitecnicodiMilano,and from uSound and Polight, two European startups with which we cooperate. The reader of this book will have the pleasure to see all the theoretical and industrial challengesexplainedindetailbyaverytalentedteamwhichhasbeenabletoscale up the production of several MEMS products, from few low-yield prototypes to high-volume high-yield production scale. Together, this team acquired in 25 years thousands of patents and thousands of years of experience in MEMS, through successesand,mostimportantly,throughfailures.Thisistheteambehindthetwenty billionunitsofsiliconsensorsandactuatorsdeployedinthemarketinthelasttwo decades.Thisteamhasbeenabletogrowsuchbusinessthankstostrongteamwork, high talent, and obviously some luck. “Audaces Fortuna Iuvat,” old Romans were saying. Eachpartofthisbookisself-contained,andreadersinterestedinasubjectwillbe abletofindtheneededinformationeasily.Itismysincerewishthatthecombination ofthevarietyanddepthofthetopicsandindustrialexperiencesharedinthisbook willmakeitavaluablereferenceaswellasausefulteachingtext. BenedettoVigna