1、NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor ManufacturingRaghavan1Wet Etching and Cleaning:Surface Considerations and Process IssuesDr.Srini RaghavanDept.of Chemical and Environmental EngineeringUniversity of Arizona 1999 Arizona Board of Regents for The University o
2、f ArizonaNSF/SRC Engineering Research Center for Environmentally Benign Semiconductor ManufacturingRaghavan2Outline Etching and cleaning solutions/processes Particle adhesion theory Surface charge and chemistry ContaminationNSF/SRC Engineering Research Center for Environmentally Benign Semiconductor
3、 ManufacturingRaghavan3Etching and Cleaning Solutions HF Solutions Dilute HF(DHF)solutions-prepared by diluting 49%HF with dionized water Buffered HF solutions-prepared by mixing 49%HF and 40%NH4F in various proportions example:Buffered Oxide Etch(BOE)-patented form of buffered HF solution May conta
4、in surfactants for improving wettability of silicon and penetration of trenches containing hydrophobic base nonionic or anionic hydrocarbon or fluorocarbonNSF/SRC Engineering Research Center for Environmentally Benign Semiconductor ManufacturingRaghavan4Etch Rate of SiO2Etch Rate(/min)at constant te
5、mp.Weight%HF0100Etch Rate(/min)NH4F/HF RatiosTemperatureEtch rate of SiO2 increases with increasing weight%of HF in the etch solution,as well as higher ratios of NH4F buffer in BHF solutions.Etch rate also directly increases with increasing temperature.More NH4FLess NH4FNSF/SRC Engineering Research
6、Center for Environmentally Benign Semiconductor ManufacturingRaghavan5Etching and Cleaning Solutions(contd)Piranha H2SO4(98%)and H2O2(30%)in different ratios Used for removing organic contaminants and stripping photoresists Phosphoric acid(80%)Silicon nitride etch Nitric acid and HF Silicon etchNSF/
7、SRC Engineering Research Center for Environmentally Benign Semiconductor ManufacturingRaghavan6Etching and Cleaning Solutions(contd)SC-2(Standard Clean 2)HCl(73%),H2O2(30%),dionized water Originally developed at a ratio of 1:1:5 Used for removing metallic contaminants Dilute chemistries(compositions
8、 with less HCl and H2O2)are being actively consideredNSF/SRC Engineering Research Center for Environmentally Benign Semiconductor ManufacturingRaghavan7Alkaline Cleaning Solutions SC-1(Standard Clean 1)NH4OH(28%),H2O2(30%)and dionized water Classic formulation is 1:1:5 Typically used at 70 C Dilute
9、formulations are becoming more popular Tetramethyl Ammonium Hydroxide(TMAH)Example:Baker Clean TMAH(10%),nonionic surfactant(2%),pH regulators for a range of 8-10,and chelating/complexing agents Could possibly be used with H2O2 to replace SC1 and SC2 sequenceNSF/SRC Engineering Research Center for E
10、nvironmentally Benign Semiconductor ManufacturingRaghavan8Surfactants Alkyl phenoxy polyethylene oxide alcohol Nonionic compounds Alkyl group:8-9 carbons 9-10 ethylene oxide groups Examples:NCW 601A(Wako Chemicals),Triton X-100(Union Carbide)Alkyl phenoxy polyglycidols Nonionic surfactants Example:O
11、lin Hunt Surfactant(OHSR)Fluorinated alkyl sulfonates Anionic surfactants Typically 8 carbon chain Example:Fluorad FC-93(3M)NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor ManufacturingRaghavan9Surfactants(contd)Acetylenic alcohols Unsaturated triple bond in the structur
12、e Nonionic Example:Surfynol 61(APCI)Betaines Zwitterionic in nature Used mostly in alkaline clean Example:Cocoamidopropyl betaineNSF/SRC Engineering Research Center for Environmentally Benign Semiconductor ManufacturingRaghavan10RCA Cleaning Two-step wet cleaning process involving SC-1 and SC-2:1)1:
13、1:5 NH4OH-H2O2-H2O at 70 C Oxidizing ammoniacal solution Ammonia complexes many multivalent metal ions(e.g.CU+)Treatment leaves a thin“chemical”oxide Without H2O2,Si will suffer strong attach by NH4OH 2)1:1:5 HCl-H2O2-H2O at 70 C HCl removes alkali and transition metals(e.g.Fe)NSF/SRC Engineering Re
14、search Center for Environmentally Benign Semiconductor ManufacturingRaghavan11Problems with SC1 Clean Some metals(e.g.Al)are insoluble in this oxidizing,highly basic solution and tend to precipitate on the surface of Si wafers High Fe contamination of the wafer surface after a SC1 clean Rough surfac
15、e after cleaning SC1 solutions with lower ammonia content(X:1:5,X RF,particle deposition is favorableNSF/SRC Engineering Research Center for Environmentally Benign Semiconductor ManufacturingRaghavan17Particle Deposition Model Parameters controlling deposition zeta potential of wafers size and zeta
16、potential of particles ionic strength and temperature of solution Transport of particles towards the wafer requires diffusion through a surface boundary layer(particles move along the flow in the solution and deposit by diffusion).Along the flowDiffusion layerSubstrateNSF/SRC Engineering Research Ce
17、nter for Environmentally Benign Semiconductor ManufacturingRaghavan18Surface Charge and Surface Electricity Development of surface charge Adsorption of H+and OH-ions(oxides)Selective adsorption of positive or negative ions(hydrophobic materials)Ionization of surface groups(polymers such as nylon)Fix
18、ed charges in the matrix structure exposed due to counter ion release example:positively charged modified filters used in DI water purificationNSF/SRC Engineering Research Center for Environmentally Benign Semiconductor ManufacturingRaghavan19Surface Charge Development on SiO2Immersed in Aqueous Sol
19、utions-O-Si.-Si-O.-Si-O.-O-Si.-O-Si-OH2+-O-Si-OH2+-O-Si-OH-O-Si-O-O-Si-O-O-Si-OHBulk SolidSolutionBulk SolidSolutionH+OH-Bulk SiO2Aqueous SolutionAcidic Solutions(low pH)Basic Solutions(high pH)H+OH-NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor ManufacturingRaghavan20P
20、oint of Zero Charge(PZC)of MaterialsPZC =the solution pH value at which the surface bears no net charge;i.e.surf=00surf(microcoulombs/cm2)pH20-20PZCMaterialpHPZCSiO22-2.5TiO25.5-6Al2O39Si4Ny lon6 Development of+or-charge at a given pH depends on the nature of the metal-oxygen bond and the acid/base
21、character of the surface MOH groups.Acidic oxides have a lower PZC than basic oxides.NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor ManufacturingRaghavan21Surface Potential(o)and Zeta Potential()SolidLiquid+-+-+-+-+-0oSurface Potential(o):Not experimentally measurable O
22、xides immersed in aqueous solns,o=0.059(PZC-pH)voltsZeta Potential():Potential in the double layer at a short distance(typically the diameter of a hydrated counter ion)from the solid surface Experimentally measurable through electrokinetic techniques Decreases(more negative)with increasing pHNSF/SRC
23、 Engineering Research Center for Environmentally Benign Semiconductor ManufacturingRaghavan22Zeta Potential Electrophoretic MethodEK =dielectric constant of liquid=viscosity of liquidK=constant dependent on particle size 1/or 1/(1/is the electrical double layer thickness)Technique useful for particl
24、es suspended in aqueous or non-aqueous mediaENSF/SRC Engineering Research Center for Environmentally Benign Semiconductor ManufacturingRaghavan23Zeta Potential from Streaming PotentialV(+)and(-)chargesLIQUID INPLIQUID OUT Generation of an electrical potential due to the flow of liquid past a charged
25、 surface Potential generated =streaming potential(Estr),which is related to zeta potential 4kEPs,and k are viscosity,dielectric constant,and conductivity of solution;Es/P is the slope of the streaming potential vs.pressure drop.NSF/SRC Engineering Research Center for Environmentally Benign Semicondu
26、ctor ManufacturingRaghavan24Streaming Potential CellSchematic Sketch-6”wafersBlockCellLIQ OUTLIQ INChannelElectrodeLIQ INLIQ OUTElectrodeNSF/SRC Engineering Research Center for Environmentally Benign Semiconductor ManufacturingRaghavan25Zeta Potential vs.pHOxide Wafer-Activation Etch0(-)Zeta Potential,mVpHNSF/SRC Engineering Research Center for Environmentally Benign Semiconductor ManufacturingRaghavan26Contamination Mechanisms Liquid film draining(liquid/air interface)Bulk deposition from liquids Contaminant pick-up from airALHydrophilicHydrophobicAL(OR)
