6Wafer Lapping and Edge Grind Basic Process StepsCrystal GrowthShapingWafer SlicingWafer Lapping and Edge GrindEtchingPolishingCleaningInspectionPackagingPurification
73. Crystal Trimming and Diameter Grind PolysiliconSeed crystalHeaterCrucible1. Crystal Growth2. Single Crystal Ingot3. Crystal Trimming and Diameter Grind4. Flat Grinding5. Wafer Slicing6. Edge Rounding7. Lapping8. Wafer Etching9. Polishing10. Wafer InspectionSlurryPolishing tablePolishing head
8Purification of Silicon Common quartz sand is mainly silicon dioxide, which can react with carbon at high temperatures.Carbon used doesn't need very high purity; it can be in the form of coal, coke or even pieces of wood.At a high temperature carbon starts to react with SiO2 to form carbon mono or dioxide.
9Purification of Silicon This process generates polysilicon with about 98% to 99% purity called crude silicon or MGS.MGS has high impurities that makes it inconvenient for electronic applications.To purify MGS, the crude silicon is ground into fine powder. Then the powder is introduced into a reactor to react with HCl vapor, forming any of a number of SiHCl.MGS: Metallurgical-Grade Silicon
10Purification of Silicon The chemical reaction can be expressed as:TCS (SiHCl3) vapor then goes through a series of filters, condensers and purifiers to get ultrahigh-purity liquid TCS. (9s!)TCS now has less than one impurity per billion atoms.
11Purification of Silicon Purified polysilicon is obtained from TCS which is purified earlier by fractional distillation, in a large CVD reactor.The high purity polysilicon is called electronic-grade silicon, or EGS.
13Crystal PullingThe EGS which is obtained from CVD has polycrystalline structure whereas Si which is used in fabrication of electronic devices is single crystal.The resulting polysilicon may be broken up into pieces to load into crucibles for Czochralski crystal growth or the poly rod itself could be used as the starting material for float-zone crystal growth.
20Czochralski Crystal Growth The only significant drawback to the CZ method is that the silicon is contained in liquid form in a crucible during growth and as a result, impurities from the crucible are incorporated in the growing crystal.
21Czochralski Crystal Growth Oxygen and carbon are the most significant contaminants.To avoid additional impurities from the ambient, the growth is normally performed in an argon ambient.
22Float-Zone (cont.) Gas inlet (inert) Chuck Polycrystalline rod (silicon)Molten zoneRFTraveling RF coilSeed crystalChuckInert gas out
24Float-ZoneNot to use any crucible in the FZ method impurity levels particularly oxygen is much lowered in theresulting crystal. And itmakes easier to growhigh-resistivity material.Thus the FZ process isused when only highresistivity, low oxygencontent or both is required.
25GrindingThe boule is placed in a lathelike machine to grind with a diamond wheel into a perfect cylinder.After the boule is ground to an appropriate diameter one or more “flats” are normally ground along its length.
32LappingThe lapping operation is done under pressure using a mixture of alumina (Al2O3), water and glycerine to improve the flatness of the wafer to about ±2 µm, removing most of the taper and bow that results from the sawing operation.This process removes about 50 µm from both sides of the wafer
34EtchingTo remove any particles and damages that many still remain from sawing and lapping steps chemical etching is done as a batch process, with the wafers are loaded into cassettes and immersed in a mixture of nitric, hydrofluoric and acetic acids.
36Chemical Mechanical Polishing As the wafers are need to have one mirror finish at least, CMP is the next step.Upper polishing padLower polishing padWaferSlurry
37Chemical Mechanical Polishing The slurry consists of a suspension of fine silica particles in an aqueous solution of NaOH.The rotation and pressure generate heat that drives a chemical reaction in which OH¯ from the NaOH oxidize the silicon. The SiO2 particles abrade the oxide away.