Table Of ContentMEMS Materials and Processes Handbook
MEMS Reference Shelf
SeriesEditors:
StephenD.Senturia RogerT.Howe
ProfessorofElectrical ProfessorDepartment
Engineering,Emeritus ofElectricalEngineering
MassachusettsInstitute StanfordUniversity
ofTechnology Stanford,California
Cambridge,Massachusetts
AntonioJ.Ricco
SmallSatelliteDivision
NASAAmesResearchCenter
MoffettField,California
Forothertitlesinthisseries,goto:
www.springer.com/series/7724
·
Reza Ghodssi Pinyen Lin
Editors
MEMS Materials
and Processes Handbook
123
Editors
RezaGhodssi PinyenLin
DepartmentofElectricalandComputer TouchMicro-systemTechnologyCorp.
Engineering Taoyuan
InstituteforSystemsResearch Taiwan
MEMSSensorsandActuatorsLaboratory and
UniversityofMaryland
WalsinLihwaCorporation
CollegePark,Maryland
Taipei
USA
Taiwan
[email protected]
[email protected]
ISSN1936-4407
ISBN978-0-387-47316-1 e-ISBN978-0-387-47318-5
DOI10.1007/978-0-387-47318-5
SpringerNewYorkDordrechtHeidelbergLondon
LibraryofCongressControlNumber:2011921730
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Foreword
Thefieldthatisaffectionatelyknownas“MEMS”(anacronymforMicro-Electro-
Mechanical Systems) is a descendant of the integrated circuits industry, but a
descendent that has developed in ways and directions never anticipated by its par-
ent.Nowahighlyspecializeddisciplineinitsownright,MEMSdrawsnotonlyon
allofconventionalmicroelectronicsbutalsoonnovelfabricationmethodsanduses
ofnon-microelectronicmaterialstocreatedevicesthataremechanical,orfluidic,or
biochemical, or optical, many without any transistors in sight. The key words are
sensors and actuators, sometimes combined with (or without) microelectronics to
create complete microsystems. MEMS devices and microsystems are now found
everywhere – in automobiles, in ink-jet printers, in computer games, in mobile
telephones, in forensic labs, in factories, in sophisticated instrumentation systems
launchedintospace,intheoperatingroomandintheclinic.Thegenieisoutofthe
bottle.MEMSdevicesareeverywhere.
Because of this immense diversity, no single book can capture the essence of
the entire field. But all MEMS devices represent highly specific answers to two
criticalquestions:“HowshallImakeit?AndfromwhatmaterialsshallImakeit?”
Processesandmaterials.Materialsandprocesses.Becausethesetwochallengesare
common to all MEMS devices, it makes sense to gather the wisdom of the least
severaldecadeson“howtomakeit”and“fromwhatmaterials”intoasingledata-
rich,process-detail-richcompendium.Thatistheraisond’etreforthisvolume,and
thatisitsgoal:todocumentMEMSprocessesandmaterialsatasufficientlevelof
detailtobeofsignificantpracticaluse.
Icongratulatetheco-Editors,RezaGhodssiandPinyenLin,aswellasourcon-
sultingeditorsandallofthecontributors,fortheirdiligence,persistence,care,and
skill in bringing this material to published form, and I invite the MEMS commu-
nity world-wide to benefit from the knowledge, wisdom and cumulative expertise
gatheredintothesepages.
Brookline,Massachusetts StephenD.Senturia
June2010
v
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Preface
Throughout the relatively short history of microelectromechanical systems
(MEMS), there have been numerous advances and inventions directly related to
devicefabrication.FromhumblebeginningsusingborrowedandmodifiedICfabri-
cationtechniquestocurrentMEMS-specifictoolssuchasdeepreactiveionetching
(DRIE) using inductively coupled plasma (ICP) sources, MEMS researchers have
continually advanced and augmented the capabilities of wafer-based fabrication
technologies. These advances have been instrumental in the demonstration of new
devicesandapplications–TexasInstruments’DigitalMicromirrorDevice,theMIT
microturbine, Analog Devices’ accelerometers – and even in the creation of new
fields of research and development: bioMEMS, microfluidic devices, and optical
MEMS.
Todate,anumberofbookshavebeenwrittenaboutthesenewfabricationtech-
nologies and materials in general, but discussion of their relationship to MEMS
design has been minimal. As a particularly diverse and multidisciplinary area of
research, the field of MEMS offers a vastly different set of challenges relative to
typicalICfabricationanddesign.Mucheffortisoftenfocusedoncharacterization
runs and developing in-house recipes and specific processes to develop and man-
ufacture MEMS structures, each time at the risk of wasting research efforts and
“reinventing the wheel.” A wealth of knowledge exists in the MEMS community,
butmuchofthisexpertiseismostreadilyaccessedbyinformal,nonmethodological
meanssuchasdiscussionswithcolleaguesatconferences.Theauthorsofthisbook
have observed an unnecessarily steep learning curve for the development of com-
mon MEMS processes, and believe the time spent traversing this curve would be
better spent brainstorming new ideas and uncovering new applications. This book
wasconceivedandbornofthisbelief.
A fundamental and comprehensive MEMS-focused reference book will be an
important asset for current and future research scientists and engineers. It was
decided early in the brainstorming sessions for this book to include materials as
well as processes in the discussion, as MEMS utilizes a wide variety of each in
commonapplications.Weintendthisbooktoprovidethereaderwiththebasicsof
MEMSmaterialsandprocesses,butbeyondthisgoal,weintendforittogiveprac-
ticalinsightintotheworkingsandstandardprocedurescarriedoutinresearchlabs
vii
viii Preface
andproductionfacilitiesonadailybasis.Tothisend,eachchapterhasanextended
section with case studies, giving step-by-step examples and recipes prepared by
experts in industry and academia. Particularly, the effect of processing conditions
on material properties are covered where applicable, illustrating the interdepen-
dence and multidisciplinary nature of MEMS fabrication. The chapters are meant
to be a springboard of sorts, providing basic information about each topic, with a
largenumberofclassicandcontemporaryliteraturereferencestoprovidein-depth
knowledge.Ultimately,itisourgoaltoprovideausefuldesignreferencevolumefor
theseasonedresearcherandtheMEMSnewcomeralike.Wehopethisbookconsol-
idatesimportantinformationforreadersandtherebyspursthecreationofmanynew
devicesandprocesses.
MEMS devices are essentially microsystems that have structures and empty
space built together. The authors of this book view the materials and processes
as the fundamental building blocks for making those structures and empty spaces.
Keepingthisinmind,thebookisdividedintotwomainsections:Chapters2,3,4,
5, and 6 covering materials and Chapters 7, 8, 9, 10, 11, 12, and 13 covering fab-
ricationtechniques.ThesetwogeneralthrustsarebookendedbyChapter1,which
discussesgeneralMEMSdesign,andChapter14,whichdealswithMEMSprocess
integration.
Chapter1providesabasicframeworkforthedesignofMEMSsystemsandpro-
cesses, which we highly recommend reading before diving into the materials and
process sections of the book. Chapter 2 presents an overview of the recipes and
methods used in the deposition of semiconductor and dielectric thin-films, partic-
ularly those most commonly used in the fabrication of MEMS. The basics here
includechemicalvapordeposition,epitaxy,physicalvapordeposition,atomiclayer
deposition, and spin-on techniques. Additive processes for depositing metal films
are discussed in detail in Chapter 3, where particular attention is paid to thick
metaldepositionwithsignificantcoveragedevotedtoelectrochemicalandelectro-
lessplatingprocessesthatareoftenrequiredforMEMSfabrication.Theentiretyof
Chapter4isdevotedtotheuseofpolymericmaterialsforMEMS.Polymers,suchas
polydimethylsiloxane(PDMS),areimportantmaterialsforavastarrayofdevices,
as encapsulants for tactile sensors and as an integral enabling technology for the
emerging field of bioMEMS. The piezoelectric films detailed in Chapter 5 are an
importantpartofMEMStechnology,servingasbothsensorandactuatorelements.
The basic properties of these materials and the physics of operation are described
indetailaswellaspracticaldepositionandfabricationmethods.Chapter6focuses
on the fabrication and integration of shape memory alloy (SMA) materials, which
providehigh-forceandhigh-displacementactuatormechanismsforMEMS.
Chapter7beginsthesectiononprocessingofmaterialsforMEMSapplications
by covering the very important area of dry etching methods (including DRIE),
particularly the influence of different parameters on the etch recipe development
process. Complementing the coverage of dry etching, wet etching processes for
MEMSmicromachiningarecoveredinChapter8withacomprehensiverecipeand
referencelistincludedinthischaptertoaidinfindingetchratesandetchselectivities
forawiderangeofmaterialsfromsilicontoIII–Vcompoundsemiconductors.
Preface ix
Chapter 9 describes the technology of lithography and related techniques, cov-
ering traditional contact lithography, projection and X-ray lithography, and more
exoticdirect-writeandprintinglithographictechniques.Dopingprocessestypicalin
andforMEMSapplicationsforelectricalpurposesandetchingcontrolarereviewed
in Chapter 10, along with diagnostic techniques for these methods. Wafer bond-
ing, a crucial fabrication technique for silicon MEMS encapsulation and structure
fabrication, is covered in detail in Chapter 11 with emphasis placed on direct and
intermediatelayerbondingmethods.
Chapter 12 discusses the still-evolving field of MEMS packaging, pointing out
differences with current microcircuit packaging techniques; this chapter in partic-
ular highlights how MEMS devices present very unique challenges as compared
totraditionalmicrocircuits.SurfacetreatmentsforMEMSdevicesarediscussedin
Chapter 13, covering antistiction and planarization coatings, functionalization of
surfacesforbiologicalandopticalapplications,andchemicalmechanicalpolishing
(CMP).Chapter14concludesthebookwithadiscussionoftheintegrationofany
number of the above processes and materials into a compatible and efficient pro-
cess flow, referred to here as process integration. The final chapter also discusses
economic and practical aspects of process integration, citing some commercially
successfulexamplesofMEMSdevices.
Thisreferencevolumewouldnothavebeenpossiblewithoutthehelpofmanyof
ourcolleaguesintheMEMSfields,frombothacademiaandindustry.Wewouldlike
to extend words of thanks and gratitude to Stephen (Steve) Senturia for providing
thevision,support,andguidancetoourteamoverthepastfiveyearsinnavigating
thecompletionofthechaptersandforreviewingthemdiligently,andtotheSeries
Associate Editors Roger Howe and Antonio (Tony) Ricco for carefully reviewing
thechapters,providinghelpfulcomments,andforrecommendingprospectivecon-
tributingauthors.WethankSteven(Steve)Elliot,fromSpringer,forhispatiencein
dealingwiththirty-fiveuniqueandindependentexperts,andhispersistenceincon-
tactingeachofusinordertodevelopthelogisticsforthebookpublishingprocess.
Last but not least, we acknowledge all thirty-five contributing authors, who self-
lesslygavetime,expertise,effort,andcreativitytomakethisbookaone-of-a-kind
contribution to the current and future MEMS community, including industry and
governmentprofessionals,academicfacultyandstaff,andstudents.
The idea for this book was born at the Transducers 2005 Conference in Seoul,
SouthKorea,anditwasfinalizedandfinishedatTheHiltonHead2010Workshop
on Hilton Head Island, South Carolina. The five years of cooperative activity that
culminated in this handbook prove that great ideas can become reality when col-
leagues work collaboratively to achieve a common goal. This message, which we
havetriedtoconveybywritingthisbook,iswhatthegreaterMEMScommunityis
allabout.
CollegePark,Maryland RezaGhodssi
Taoyuan,Taiwan PinyenLin
