Introduction to VLSI Fabrication Technologies

1 IntroductiontoVLSI FabricationTechnologies27/09/2005 EmanueleBaravelli2 Organization MaterialsUsedin VLSI Fabrication VLSI FabricationTechnologies Overviewof FabricationMethods Devicesimulation3 MainCategoriesof MaterialsMaterialscan beclassifiedintothreemaingroupsregarding theirelectricalconductionproperties: Insulators Conductors Semiconductors4 ConductorsConductorsare usedin IC design forelectricalconnectivity. The followingare goodconductingelements: Silver Gold Copper aluminum Platinum5 InsulatorsInsulatorsare usedtoisolate devicesand Capacitorsrelyon choiceof the insulators(and the conductors)in IC design dependsheavilyon howthe materialsinteractwitheachother, especiallywiththe The basic semiconductor material usedin devicefabricationisSilicon The success of thismaterial isdue to.

2 Phisicalcharacteristics Abundance in nature and very low cost Relatively easy process Reliable high volume Fabrication Othersemiconductors( GaAs) are usedforspecial applications7 Organization MaterialsUsedin VLSI Fabrication VLSI FabricationTechnologies Overviewof FabricationMethods Devicesimulation8 Overviewof Processing TechnologiesAlthougha numberof processing technologiesareavailable, the majority of the production isdonewithtraditionalCMOS. Otherprocessesare limitedtoareaswhereCMOS isnotverysuitable(likehigh speedRF applications)Bipolar:2%SOI: 1%GaAs: 2%CMOS: 90%BiCMOS.

3 5%9 CMOS technology An Integrated Circuit (IC)is an electronic network fabricated in a single piece of a semiconductor material The semiconductor surface is subjected to various processing steps in which impurities and other materials are added with specific geometrical patterns The Fabrication steps are sequenced to form three dimensional regions that act as transistors and interconnects that form the network10 SimplifiedViewof MOSFET11 CMOS ProcessThe CMOS processallowsfabricationof nMOSand pMOStransistorsside-by-sideon the MaterialsUsedin VLSI Fabrication VLSI

4 FabricationTechnologies Overviewof FabricationMethods Devicesimulation13 Fabrication process sequence Silicon manifacture Wafer processing Lithography Oxide growth and removal Diffusion and ion implantation annealing Silicon deposition Metallization Testing Assembly and packaging14 Single CrystalGrowth(I) Pure siliconismeltedin a pot (1400 C) and a smallseedcontainingthe desiredcrystalorientationisinsertedintom oltensiliconand slowly(1mm/minute) CrystalGrowth(II) The siliconcrystal(in some casesalsocontainingdoping) ismanufacturedasa cylinder(ingot) witha diameterof 8-12 inches(1 = ).

5 Thiscylinderiscarefullysawedintothin( mm thick) diskscalledwafers, whichare laterpolishedand (I)Lithography: process used to transfer patterns to each layer of the ICLithography sequence steps: Designer: Drawing the layer patterns on a layout editor Silicon Foundry: Masks generation from the layer patterns in the design data base Printing: transfer the mask pattern to the wafer surface Process the wafer to physically pattern each layer of the IC17 Lithography(II) : the surface to be patterned is spin-coated with a light-sensitive organic polymer called (exposure): the mask pattern is developed on the photoresist, with UV light exposure depending on the type of photoresist(negative or positive), the exposed or unexposed parts become resistant to certain types of : the soluble photoresistis chemically removed The developed photoresistacts as a mask for patterning of underlying layers and then is Photoresist coatingSiO2 PhotoresistSubstrate3.

6 DevelopmentSubstrateSubstrateMaskUltra violet lightOpaqueExposedUnexposed2. Exposure18 OxideGrowth/ OxideDeposition Oxide can be grownfrom silicon through heating in an oxidizing atmosphere Gate oxide, device isolation Oxidation consumes silicon SiO2is depositedon materials other than silicon through reaction between gaseous silicon compounds and oxidizers Insulation between different layers of XFOXS ilicon waferSilicon surfaceField oxide19 Etching Once the desired shape is patterned with photoresist, the etching process allows unprotected materials to be removed Wet etching.

7 Uses chemicals Dry or plasma etching: uses ionized gases20 Diffusionand IonImplantationDoping materialsare addedtochangethe electricalcharacteristicsof siliconlocallythrough: Diffusion: dopantsdeposited on silicon move through the lattice by thermal diffusion (high temperature process) Wells Ion implantation: highly energized donor or acceptor atoms impinge on the surface and travel below it The patterned SiO2serves as an implantation mask Source and Drain regions21 AnnealingThermal annealingis a high temperature process which: allows doping impurities to diffuse further into the bulk repairs lattice damage caused by the collisions with doping ions22 Silicon Deposition and Metallization Films of silicon can be added on the surface of a wafer Epitaxy.

8 Growthof a single-crystal semiconductor film on a crystalline substate Polysilicon: polycrystalline film with a granular structure obtained through depositionof silicon on an amorphous material MOSFET gates Metallization: deposition of metal layers by evaporation interconnections 23 Advanced CMOS processes Shallow trench isolation source-drain halos (series resistance) Self-aligned silicide(spacers) ..n-wellp+p+n+n+p-dopingn-dopingSilicide Oxide spacern+ polyp+ polyShallow-trench isolationp-type substrateSource-drainextension24 Organization MaterialsUsedin VLSI Fabrication VLSI FabricationTechnologies Overviewof FabricationMethods Devicesimulation25 MOS device simulation 2D m n-channelMOS, tox= 4 nm Id(Vds) simulation, Vgs= 8 refinementcycles Refinementon , n , p26 Researchactivities Geometricalissues: Sophisticated 2D geometries(2nd generation wavelets) 3D geometries Simulationissues.

9 Advancedmodels(hydrodynamic, quantum effects)27 Links external/training/tech/special/ELEC2002