The Future of Silicon Technology
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To paraphrase Heraclitius ofEphesus(535BC-475BC), "The onlyconstantischange."Thismantra has driven the electronics worldsincetheinception ofthe tubein the early 1910's.Thelatest advancein3Dchips may be aglimpse into the nextwaveforelectronics.
Weusuallyhearaboutthe active part of the systemaswhatdrivestheadvanceofelectronics, but the interconnectshaveplayedjustasvital arolein increasing functional density inthelastnearly100years.Itcan be argued that theinterconnectshaveevenleadadvances intheactivecomponents.
Thevacuumtubesuseddiscrete wiring. They evolvedintothetransistor.Theprintedcircuitboard was created toincreasethefunctionaldensityofdiscretetransistors.Theintegratedcircuitminiaturizedthecircuit boardinterconnectsandenabledordersofmagnitudeincreases in functionaldensityandlaunchedthetreadmillofMoore's Law.
Inthisphase,weevolved an alphabet soupofindividualpackagestyles:DIP,PQFP,PGA, LGA, BGA, CSP.Circuitboardtechnologyevolvedtomicroviasand HDI to keep upwiththeinterconnectdensitydemandsof the CSPintroduced in theearly1990s.I've writtenthreebooksandnumerous articlesonthesetechnologies.
Packagingtechnology took thefunctionaldensityleadwiththeintroductionof the multi-chippackage.Onepackagedcomponenthadthefunctional density of achiptwoorthreesilicon-generationsaway.ICtechnologyfollowedinpackaging'sfootsteps withthesystem-on-a-chip(SoC)andmulti-coreprocessors.Andmulti-chipmodules changeditsnametosystem-in-package (SiP)tosoundcooler.
Howeverwonderfulthe advances of SoC andSiParetoadvancefunctionaldensity, theywill always live inaplanarworld,limitedto one orat most, twosurfaces.Packagingtechnologyallowedcloseto 100%packagingefficiency, but itcouldnever exceedwhatthesiliconwascapable.
Packagingtechnology took the lead in breakingtheboundsofthe2Dworld,with stacked chips. Starting withflashandSRAMsforcellphones,there are no fundamentallimitstostackeddiemodules.Thepackagingefficiencyexceeds400%insomeapplications.
Whilesilicontechnology has madesmallforaysintothe3Dworld,with laminatedsilicon layersand through-silicon-viassuch as the IBM illustration shown in Figure 1, none of themhave shown legs. Until this recent announcement of 3D integratedcircuits.
Thefirst3Dchipisrunning at 1.4 GHz at the UniversityofRochester.Thisisnotacollection of stackedlayers;itisanintegrateddesignspecifically optimized for3Dwithallkeyprocessesfunctioningverticallythroughmultiplelayersofprocessors.
"Icallitacubenow,because it's not just achipanymore,"saysEbyFriedman,DistinguishedProfessorofElectricalandComputerEngineering atRochester andco-creatoroftheprocessor."This isthe waycomputing is going tohave to bedoneinthe future.Whenthe chipsare flush against eachother, theycandothings youcouldnever dowith a regular 2Dchip."
Ofcoursetherearechallenges ahead, but that's exactlywhatwassaidatthebeginningof the IC era. Functionaldensityin2Dwillalwaysoperate at themost cost-effectivepointoftheintegrationvsyieldcurve. Thisdefinesthelargestchippossible.Packagingtechnology will alwaysgivethesilicon thenextboostinfunctional density. But now, thereisaglimpseofhowsilicontechnology might be able to jumptothenextleveloffunctionaldensity bygrowinginthethirddimension.
"Arewegoingtohitapoint where wecan'tscaleintegratedcircuitsanysmaller?Horizontally, yes,"saysFriedman."But we'regoingtostartscaling vertically, and thatwillnever end.At leastnotinmylifetime. Talk to my grandchildrenaboutthat."
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