Table Of ContentBenedetto 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
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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
