Device Isolation Techniques in Integrated Circuit Fabrication

This lecture, authored by Dr. G. S. Virdi, Ex-Chief Scientist, CSIR–Central Electronics Engineering Research Institute, Pilani, provides a detailed exploration of device isolation methods used in semiconductor and VLSI fabrication. The lecture explains the need for isolation between components in monolithic ICs to prevent parasitic effects, leakage currents, and unwanted conduction paths.

It systematically covers major isolation techniques including Junction Isolation, Dielectric Isolation (Etched Field Oxide and LOCOS), and Trench Isolation (STI), highlighting their working principles, process steps, and advantages and limitations such as the Bird’s Beak problem in LOCOS.

The presentation offers valuable insights for engineering and science students, as well as research scholars, aiming to understand modern isolation processes that enable high-performance, densely packed microelectronic devices.

1. DeviceIsolationTechniques
Dr.G.S.Virdi
Ex.Chief Scientist
CSIR-Central Electronics Engineering Research Institute
Pilani—33303 1,India

2. Objectives
•Tounderstandwhatisdeviceisolation
•Toappreciatetheneedfordeviceisolationin
micro-fabrication
•Tostudythevariousdeviceisolation
techniques,and
•Toidentifymeritsanddemeritsofeach

3. DeviceIsolationTechniques
WhatistheneedforDeviceIsolation?
•VLSIconsistsofseveralactiveandpassive
componentsinterconnectedwithinamonolithic
blockofsemiconductormaterial.Each
componentmustbeelectricallyisolatedfrom
eachothertoallowdesignflexibility.
•InMOS,source-drainregionsofadjacentMOS
transistorstogetherwithinterconnectionmetal
linesmayconstituteparasiticMOStransistors
unlesstheyareisolatedfromeachother.

4. 4
DeviceIsolationisalsonecessary:
•Topreventundesiredconductingpaths;
•Toavoidcreationofinversionlayersoutsidethe
channels;
•Toreduceleakagecurrents.
DeviceIsolationTechniques
WhatistheneedforDeviceIsolation?

5. DeviceIsolationTechniques
•JunctionIsolationTechnique
•DielectricIsolationTechnique–
•EtchedFieldoxideIsolation
•LOCOS
•TrenchIsolationTechnique–
•STI

6. JunctionIsolationTechnique
•Inolderdaybipolarjunctiontransistortechnology,
theisolationwasprovidedbyadeepPNjunctions
<=Junctionisolationtechnique
•Thereversebiasedpnjunctionwouldactasthe
electricalisolationbetweenadjacentdevices.

7. JunctionIsolationTechnique
Demerits:
Asthedevicedimensionswerereducedandaswe
wantedfasterandfasterdevices,junctionisolation
becamenon-viabletechnology,because:
•thediffusionwillalwayshavesomelateral
spread;therefore,asthedevicedimensions
becomesmaller,itbecomesmoredifficultto
controltheisolationtubwidthand
•thecapacitanceassociatedwiththese
junctions,hindersthespeedofthetransistors

8. DielectricIsolationTechnique
Themostcommondielectric,silicondioxideis
usedforisolation.
•EtchedFieldoxideIsolation
•LOCOS

9. EtchedFieldoxideIsolation
•Devicesarecreatedindedicatedregions
calledactiveareas.
•Eachactiveareaissurroundedbythickoxide
barriercalledfieldoxide.
•Thickoxideisgrownoncompletesurfaceof
thechipandthenselectivelyetchedtodefine
activeareas.
•Thicknessofoxideleadstolargeoxidesteps
attheboundariesofactiveareasand
isolationregion.

10. LOCOSTechnique
•LOCOS=localoxidationofSilicon
•Localoxidationthatisoxidationiscarriedoutat
someregionsofthesemiconductor;theother
regionsarepreventedfromgettingoxidizedby
usingasiliconnitridemask.
•Siliconnitridehasaninterestingbehaviorthatit
does notallowoxidationtoproceed
underneath.ThishasbeenusedinLOCOS.

11. •Theactiveregionisprotectedbyadepositingalayer of
siliconnitride->Actsasoxidebarrier
•Underneath,thereisalayerofthinoxidecalledpad
oxide.
•Padoxideisneededbecauseotherwisesiliconnitride will
notadhereverywelltothesilicon.Therewillbelot of
stressinduced.Bydepositingthesiliconnitrideon topof
the padoxide,the stresswillbereduced.
LOCOSTechnique
1

12. •Thepatterntransferisperformedby
photolithography.Afterlithographythe
patternisetchedintothenitride.
•Theactiveareasfortheoxidationprocess
arethusdefined
LOCOSTechnique
2

13. •ThenextstepisthemainpartoftheLOCOS
process,thegrowthofthethermaloxide.
3
LOCOSTechnique

14. •Aftertheoxidationprocessisfinished, the
laststepistheremovalofthenitride layer.
4
LOCOSTechnique

15. TheadvantagesofLOCOSfabricationare:
•thesimpleprocessflowand
•thehighoxidequality,becausethewhole
LOCOSstructureisthermallygrown.
LOCOSTechnique-Merits

16. •But,asthedevicedimensionswentonbecoming
smaller,theLOCOStechniquealsostartedtoshow
itslimitationandwhataretheselimitations?
•Bird’sBeakProblem
LOCOSTechnique-Demerits

17. •Therewillbealittlebitofencroachmentofoxide
underneathsiliconnitride.
•Thisisbecauseoxidationisproceedingnotmerelyin
the,onthehorizontalsurface,butalsoatthe
verticalsidewalls.
•Theslightencroachmentunderthesiliconnitrideis
shapedlikeabird’sbeak.,hencethisiscalled
bird'sbeakproblem.
•Aswereducethedevicedimensionsi.eIfthis
entireactivetransistorareaitselfisverysmall,then
evenasmallencroachmentwillbeaconsiderable
percentageofthetotalactivearea.
LOCOSTechnique-Demerits

18. TrenchIsolationTechnique
•Inatrenchisolation,Atrenchiscutin
thesemiconductorandthenthetrench
isfilledwithnon-conductingmaterial.
•Inshallowtrenchisolation,insteadof
thermallygrowingoxide layer,the
silicondioxidelayerisdepositedby
CVD.

19. ShallowTrenchIsolationTechnique
Step1:Nitridedeposition
•Intheshallowtrenchisolationprocess,atfirstathin
layerofoxideisgrownbythermalprocessand
subsequentlyathinlayerofsiliconnitrideis
depositedbyCVD.

20. Step2.CreateaTrench
•Usingphotolithographyandetching,wherever
electricalisolationisneeded,atrenchismadein
silicon(i.e.thesiliconnitride,silicondioxideand
thenSiareremovedbydryetchinginthose
areas).
ShallowTrenchIsolationTechnique

21. Step3:TrenchOxideDeposition
•Thensilicondioxideofsufficientthicknessis
depositedbyCVD.Usuallythisisdoneunder
highdensityplasma(HDP)conditions.
ShallowTrenchIsolationTechnique

22. Step4:ChemicalMechanicalPlanarisation
•Inthenextstep,theexcessoxideisremoved by
chemicalmechanicalplanarization.Here, care
mustbetakentoensurethatonlythe oxideis
removedand thenitrideisnot removed.
ShallowTrenchIsolationTechnique

23. Step5:Nitrideremoval
•Thenthenitrideisremovedbywetetching
usingphosphoricacid.
ShallowTrenchIsolationTechnique

24. ShallowTrenchIsolationTechnique

25. Thank You…
Dr.G.S.VIRDI