ELECTRONIC MATERIALS PROCESSING
ELECTRONIC MATERIALS PROCESSING CHE 571
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This 47 page Class Notes was uploaded by Myrna Kunze on Monday October 19, 2015. The Class Notes belongs to CHE 571 at Oregon State University taught by Staff in Fall. Since its upload, it has received 21 views. For similar materials see /class/224475/che-571-oregon-state-university in Chemical Engineering at Oregon State University.
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Date Created: 10/19/15
Crass sc nn 7 an 39 I I r V 5252i522EI52i52I5ZI5EI5IEI5IEI5IEI5225IE2552525u 2522512525 fr g 39I Slurryr Fadi Want enhanced rem va 39quot39I3939I39I 3939 I a quot393939a3939 frva Mh palms m eudaee F Platen ll Prof Milo Koretsky Chemical Engineering Dept Westech 372 Prof Milo Koretsky Chemical Engineering Dept Planarity Requirements Photolithography requirements Need better resolution b for sub 05 um line widths b A quot Increase the numerical aperture NA NA gt Decrease the depth of field 0 A 0 z 2 NA For example 0 z 270 nm for 27 mm X 27 mm stepper Want Global Planarization of 150 nm In Multi Layer Metallization MLM schemes subsequent layers add to the topography of a wafer surface Surface topography leads to resist thickness variations thinner regions getoverexposed Prof Milo Koretsky Chemical Engineering Dept amascene Maul l H J I l5 Level lEiEIEtErlgf Hawaii TEN Ifrfgffffffff fJ and think 1113 Eilirmn l Ilepnsit Midi 39 g Fxyaf layer full g l quot39 39 lth 5m k x fx plasma l Vii etch r EMF till EM Tiltl Prof Milo Koretsky Chemical Engineering Dept Introduction Glow Discharge Plasmas 1 mtorr 1 torr What happens Gas in gt Gas out 1 lonsfand Electrons 50 500 Watts are orme 39 Power Supply 2 The gas between the 39 electrodes begins to glow 39 x on 139 l The glowing region is the Plasma Elecltron Prof Milo Koretsky Chemical Engineering Dept The Plasma Glow Prof Milo Koretsky Chemical Engineering Dept Energy Transfer through Collisions Let s consider how much energy the electrons can transfer to neutrals Before collision u 399 en Prof Milo Koretsky Chemical Engineering Dept Inelastic vs elastic collisions Applying energy and momentum balances Elastic collisions 2 KEM f MiuM f 4meMl KEEJ i 2 2 0052 6 z 2 meuei me Mi Inelastic collisions AU zicosz 6 z Egal39 me Ml39 Prof Milo Koretsky Chemical Engineering Dept Characteristics of a Plasma Fraction of ions amp electrons Pressure Electron mean energy Ionneutral mean energy common frequency Power dissapation Prof Milo Koretsky Chemical Engineering Dept amp Electron and Atom Here an electron hits and excites a neutral which then relaxes and gives light This light is what makes the Plasma glow gt U i wwwvm m C o e at I Characteristic Light w 3 I f m nghSpeed Jquot 39 t A Electron 39 r I h r I x r kw J Neutral Atom Ar Excited Atom Ar Relaxed Atom Ar e39Ar gte39Ar gte39Arhv Prof Milo Koretsky Chemical Engineering Dept Ionization A high speed electron hits an atom hard enough to knock out an electron This forms an ion and another free electron This collision is also inelastic yx x o e 239 q e quot High speed upquot o 0 ElectrorL j giggle o 9 Neutral Atom Ion e39Ar gtAre39e39 Prof Milo Koretsky Chemical Engineering Dept Molecules Dissociation When a molecule undergoes dissociation we get two reactive atoms These atoms are called free radicals High speed Electron a Stable Molecule e39 02 Is this process elastic or inelastic Prof Milo Koretsky Chemical Engineering Dept Rate of Plasma Processes The reaction rate r is given by r knenn f 8 08 k SEswore 8 electron energy where f the electron energy distribution function eedf 08 cross sections of the inelastic process cm2 Prof Milo Koretsky Chemical Engineering Dept Electron Energy Distribution Functions for Te6 eV axwellian Distribution 0 02 Energy elo Prof Milo Koretsky Chemical Engineering Dept Plasma Desmear Electron impact processes Major reactions Neutral generation lon generation CF4 e gtCF3 F2e CF4 e gtCF3 Fe CF4 e gtCF22Fe 02e gtO2e Oze gt20e Oze gtOO Vacuum Pump Feed gas CF4 O2 Pol mer Prof Milo Koretsky Chemical Engineering Dept Deposition by Free Radicals e SiH4 gtSiH2 H H e Silane Bulk I Molecule Sheath Hydrogen 5 Molecules H2 la 139 39 4 A MK I 1 r I l l I x 1 39 Freew V Ragic xx The flux of free radicals on the surface results in an added solid silicon atom on the surface Volatile products hydrogen escape t I Prof Milo Koretsky Chemical Engineering Dept Removal by Free Radicals The neutrals come to the surface The molecule is stable It will not readily react with the polymer The free radicals Oxygen atoms are reactive They will react with the polymer Bulk co2 Carbon Dioxide Sheath H20 Steam Reactive free radical Stable molecule hw eeioo o o o a o Prof Milo Koretsky Chemical Engineering Dept Electrical Structure of Plasmas 1 quot J n 0 Initially e 4 e 9 J9 gtgt Ji 1 Ji T lz39z39 H H lt J gt I At steady state is lt Je 0 J Ji Prof Milo Koretsky Chemical Engineering Dept rf Plasmas self bias 43 if i f 4u Va 0 I lle 41 1 Va e 2 1 21 RV 0 I 1 V t J v 0 1 Vb W 0 39 D Oquotxet ions to 2 steady state Ji Je at 2 b Prof Milo Koretsky Chemical Engineering Dept Ion Bombardment Ions move around slowly in the bulk of a Plasma When they reach the sheath they are strongly l Q J Bulk attracted to the negative surface They hit the surface at a very high speed An ion is a heavy species When it hits a surface at high speed it damages the surface It can also knock off the atoms from that surface 1 Negative Surface Prof Milo Koretsky Chemical Engineering Dept S utter bombards the aluminum surface Displaced power3upply alumInum Aluminum 7 Atoms Target Aluminum Silicon Wafer atom moVIng d ar uquot A target material such as aluminum A39 quotquot m at m5dep s39t is bombarded with Argon ions The displaced atoms of the target material move across the Plasma They are then deposited on a silicon wafer on the wafer and form a film Prof Milo Koretsky Chemical Engineering Dept Chemical Vapor Deposition cvn Free radicals deposit on the wafer surface and chemically combine to form a layer of material Silane SiH4 Molecule Oxygen 02 Molecule 8in Free Radical 0 Free Radical Si02 Formed on Unreacted 0 Free Radical on surface Prof Milo Koretsky Chemical Engineering Dept Etch Process CF4 Plasma Ion bombardment damages surface Physical process Free radical does not react with photoresist AAAAAAAAAAAAA Free radical F moving around Reaction product escapes from surface Free radical reacts with weakened silicon Chemical process Prof Milo Koretsky Chemical Engineering Dept Ion neutral synergy Time lsecondsl Coburn and Writers J Appl Phys 50 3189 1979 Prof Milo Koretsky Chemical Engineering Dept 1 l I l I 1 I I I BO 3 Xer Ga Ar ion Beam xer Gas l Ar on Beam I Only Only 15 E quot 4 60 quot g m MW 8 39 n C 2 40 J c a L a 20 0M 39e o l l 1 l l l l C 200 400 600 800 Plasma Reactor Configurations 1 Diode parallel plate con guration Plasma Etching Reactive Ion Etching P Sputtering 2 Triode 3 Magnetically Enhanced Reactive Ion Etching MERIE 4 High Density Plasmas HDP Transformer Coupled Plasma Inductively Coupled Plasma Electron Cylclotron Resonance 5 Downstream Plasma Prof Milo Koretsky Chemical Engineering Dept Downstream Plasma Microwave Remote plasma rwquot quot chamber Process chamber r r a gt l l l a Wafer With EEO H 0 0 H 0 H 0 WWW W v quot Hong Xiao 470 30quot Heated plate lntroto Semiconductor 39 quot Manufacturing Technology T0 the pump Prof Milo Koretsky Chemical Engineering Dept ICP Plasma flndlucnve coils Ceramic cover Smilce RF I l K r Cl 7 Ci 1 C 1 39 Ci Chamber body ozorf f Wafer Echucli Helium g Hong Xiao Intro to Semiconductor Manufacturing Technology Prof Milo Koretsky Chemical Engineering Dept ECR Plasma Microwave m Magnetic pl 70 i IS EC R Plasma Magnetic field line AWafer Echuck Iquot Hong Xiao i 39 Intro to Semiconductor l 1 hum Manufacturing Technology Prof Milo Koretsky Chemical Engineering Dept Kinetic Theory of Gases Kinetic theory provides us with a model or a way ofthinking about the gas phase processes in Si manufacturing lt invokes atomic theory all matter is made up ofdiscrete atoms or molecules Liquids and solids atoms are closely spaced relative to their size Gases atoms or molecules are far apart There are still lots of molecules in a gas 27 x1019 cm3 at 1 atm Si processing mainly deals with Vacuum processes others at 1 atm can assume ideal gas behavior Ideal gas gases consist of infinitely small rigid spheres which only interact with each other during the period ofdirect collision Kinetic Theory of Gases Prof Milo Koretsky Chemical Engineering Dept Molecular Motion Molecule in motion in a gas colliding with other molecules in its path all other molecules are also in motion Kinetic theory applet Kinetic theory applet ll Kinetic39l39heoryof Prof Milo Koretsky Chemical Engineering Dept Kinetic Theory of Gases Perpetual motion of collisions on a atomic scale The degree of motionis determined by the macroscopic variable Temperature ZLmC2 ikT Through elastic collisions the speeds ofthe molecules become randomized ie different molecules have different speeds While the speed ofa given molecule may change via collision the entire population of molecules obtains a fixed distribution of velocities or equivalently energiesat equilibrium This distribution is characterized by the MaxwellBoltzmann distribution function Pv 32v eXp 2 Tv2 Kinetic Theory of Gases Prof Milo Koretsky Chemical Engineering Dept MaxwellBoltzmann Distribution most probable velocity Argon at 300 K cm 2 In El EIUEIDZ average mean velocity 0000015 Z Pv C 727 El IZIUIJDi mean square velocity El 300005 I 3kT Crms 02 Z 7 39 20000 40000 60000 30000100000 v cms Kinetic Theory of Gases Prof Milo Koretsky Chemical Engineering Dept MaxwellBoltzmann Distribution u i SiH4 at 300 K i n mtm l 250000 500000 v cms 14 SIH4 at 1500 K 5 10 14 4 1014 3 1011 2 1014 0000003 1 10 F3 0000005 I 437500 4 9000 0000004 1 0000002 i 250000 500000 V OmS Kinetic Theory of Gases Prof Milo Koretsky Chemical Engineering Dept P I V 139 JA Pressure P The force per unit area on the walls of a container associated with the elastic bouncing of molecules from a wall PV NkT Collision frequency v The number of collision a molecule undergoes per unit time Mean free path 7 The distance on average a molecule goes between collisions 63 of collisions occur within k 36 occur between X and 5 k Impingement Rate or Flux JA molecules A that hit a surface per area pertime JA lnA 4 Kinetic Theory of 351595 Prof Milo Koretsky Chemical Engineering Dept Ar at 300 K P torr v 5391 9 cm 1 67X106 6X10quot3 1000 67109 6X 10396 10393 67 X 103 6 10396 67 6000 Prof Milo Koretsky Chemical Engineering Dept Kinetic Theory of Gases Class Exercise What is the impingement rate flux to a surface of Ar at 300 K and 1 torr How many monolayer equivalents strike the surface in 1 sec Prof Milo Koretsky Chemical Engineering Dept Kinetic Theory of Gases Vacuum System A typical vacuum system is made of four parts 1 Gas supply 2 Reaction chamber 3 Pumping system 4 Gauges and control Gas Supply Gauge System Exhaust Kinetic Theory of Gases Prof Milo Koretsky Chemical Engineering Dept Gas Flow Viscous Flow kltltd collisions between atoms dominate collisions with wall There are two main types Laminar Flow characteristic of lowers flow rates where uid stream lines move parallel to each other typical for vacuum systems Turbulent Flow characteristic of higher ow rates where eddies form Molecular Flow kgtgtd collisions with wall dominate collisions between atoms Characteristic of high vacuum Transition Flow kd collisions with wall roughly equal to collisions between atoms Knudsen Number Kn Kn 2 ampIN Kinetic Theory of Gases Prof Milo Koretsky Chemical Engineering Dept Pumping Speed amp Pressure Pumping speed Pumping speed refers to the gas ow rate through the system Larger pumps give higher pumping speed Choice of pump size and type determines pumping speed of gas Ultimate and operating pressure Different types of pumps are able to achieve different pressures Choice of pump type determines the ultimate and operating pressure Pumping speed depends on pump size and type Ultimate amp operating pressure depend on pump type Kinetic Theory of Gases Prof Milo Koretsky Chemical Engineering Dept Vacuum Pumps Pumps used to achieve low vacuum 750 to 10393 torr are called low vacuum pumps The dry mechanical pump and the Roots blower are examples of low vacuum pumps Pumps used to achieve high vacuum 1 to 1010 torr are called high vacuum pumps The turbo pump and the cryo pump are examples of high vacuum pumps 760 torr 103 torr 10quotquot torr High Vacuum Atmospheric pressure Kinetic Theory of Gases Prof Milo Koretsky Chemical Engineering Dept Viscous Flow Viscous flow is possible when there is a lot of gas high pressure If part of the bulk is moved in one direction the remaining bulk comes in to ll its space It is like pumping water Tank B Kinetic Theory of Gases Vacuum Systems Prof Milo Koretsky Chemical Engineering Dept Molecular Flow Molecular ow occurs at high vacuum At high vacuum there are so few molecules in the gas that it does not behave like a bulk The molecules behave like individual particles that need to be moved individually It is like the flow ofgravel from ground to the truck shown below Collect individual Bulk mover moves the stones into a pile gravel to the truck 1e IC vh ory baees vacatsm Sysrems Prof Milo Koretsky Chemical Engineering Dept Cleaning 75 of Yield loss due to particles 3pronged appraoch 1 Clean Factories 2 Wafer Cleaning 3 Gettering From Intel Prof Milo Koretsky Chemical Engineering Dept 1 Clean Factories HEPA lm m z a ME Class of the clean room 1 Laminar G3 3 Flow fl Elawer Air V l VVVVVV a lulu i1 t m a Mum mil L 39 A 39 mg I 7 FIGURE 3 Schema t Iiif a 131mm From Mlddleman in ll Pm Cubic l 7 ll 1 I ll Illll mm Prof Milo Koretsky Chemical Engineering Dept 2 Wafer Cleaning It is important to have clean wafers at all stages of fabrication Cleaning is especially important before any high temperature process Improper cleaning can result in yield loss and process variability Cleaning comprises roughly 14 of the IC process steps Wafe rs i l I w 9 Cleaning I I solution Prof Milo Koretsky Chemical Engineering Dept Typical Chemicals Involved Chemical Name H20 DI DeIonized water H2804 Sulfuric Acid H202 Hydrogen Peroxide H20 Water HF Hydrofluoric Acid NH4OH Ammonium Hydroxide HCI Hydrochloric Acid IPA IsopropyI aIcohoI Prof Milo Koretsky Chemical Engineering Dept Si Surfaces The chemical nature of the Si surface affects reactivity with contaminants O O O O H HH H HO HHO H O O O O H H H H P Si Sislisli SrO SI SrO SI SrO SI ISI O ISI O O Si Si Si Si I I I I I I I I I I I I SiOSiOSiOSi SiOSiOSiOSi S SFSi I I I I I I I I 39 39 39 39 SiO Si O SiO Si SiOSi O SiO Si Bare Si Silicon oxide Silanol on silicon oxide Prof Milo Koretsky Chemical Engineering Dept Generic Contaminated Si Surface o 39 0 Air o O O 0 O 0 Absorbed gas 0 O Q Q 39 W Particles Non oar or anics 391 Ac 039 39ODOoOA 06 p 9 Polar Organics Water Water Silicon Oxide Oxide Silicon Wafer Prof Milo Koretsky Chemical Engineering Dept Deionized DI Water Ions are reactive They can make liquid products from solids They can also bond to the solid Normal tap water has many different types of ions If we want to clean wafers without causing any reactions we can not use tap water We need special water which is free of ions Tap water Deionized water lon free water is called Deionized water or DI water Prof Milo Koretsky Chemical Engineering Dept Relative Strength of Binding Forces Important for Aqueous Cleaning Adsorption Type Energy eV Chemical Bonds Ionic 611 Covalent 067 Metallic 135 Physical Forces H bonds ltO5 dipoledipole ltO2 Despersion ltO4 Prof Milo Koretsky Chemical Engineering Dept General Scheme RCA Clean H2804 H20 Rinse H202 gt Room Temp 125 C Sulfuric Clean Organics H20HF 101 Room Temp 10 to 1 Oxide Etch H20 Rinse H20 NH4OH H202 H20 Rinse H20 HCI H202 511 Room Temp 811 80 C 80 C SC 1 Particles SC 2 Surface Metals H20 Rinse H20 HF H20 Rinse Alcohol IPA gt Room Temp gt39 100 1 gt Room Temp gt Dry Room Temp 100 to 1 Oxide Etch W Kern and DA Puotinen Cleaning solutions based on hydrogen peroxide in silicon semiconductor technology RCA Rev 31 187 206 Room Temp Wafer dry Prof Milo Koretsky Chemical Engineering Dept of these steps Removes Organics sulfuric clean Lets see the purpose of each Sulfuric CleanPiranha Recipe HZSO4 H202 125 C H202 has two oxygens H20 Rinse H20 HF H20 Rinse Room Temp gt 10 1 gt Room Temp H20 NH OH H202 H20 Rinse H20 HCI H202 gt 51 1 Room Temp 61 1 80 C 80 C H20 Rinse H20 HF H20 Rinse Alcohol IPA gt Room Temp 100 1 gt Room Temp gt Dry Room Temp Room Temp Hydrogen peroxide H202 provides the chemical action for the Hydrogen peroxide H202 Prof Milo Koretsky Chemical Engineering Dept Sulfuric Clean Piranha Action Organics are mainly carbon C and hydrogen H Hydrogen peroxide H202 wants to give up one of its oxygen atoms lt readily produces C02g and HZOI on reacting with organics Hydrogen peroxide H202 Organic Impurity Prof Milo Koretsky Chemical Engineering Dept DI Water Rinse Recipe H20 Room Temperature H202 125 C H20 HF 10 1 Room Temp Removes Cleaning chemicals HzoNHOHHzoz gt 511 and reaction products from an 80 C earlier wet clean H20 HCI H202 6 1 1 80 C Alcohol IPA Dry Room Temp H20 HF 100 1 Room Temp Action Dissolves chemicals and reaction products A DI water rinse may also be used to stop the action of cleaning chemicals Prof Milo Koretsky Chemical Engineering Dept Oxide Etch I HZSO H20 Rinse H20 Rinse ReCIpe H20 HF 1222 R miemp Removes Oxide SiOZ L H20 NH OH H202 H20 Rinse H20 HCI H202 39 ACtIOnI 30101 e RoomTemp 30101 HF reacts with SiC2 to i make liquid products L 53323 C i 39 quotPAgt oom emp Si02 s 6HFI gt HZSiF6l 2H20I The etch rate or reaction rate of HF with oxide can be slowed by adding more water The lowers the concentration ofHF A DI water rinse is used to stop the action of acid after this cleaning step Prof Milo Koretsky Chemical Engineering Dept 101 Oxide Etch The wafer goes through many processing steps before gate oxide can be deposited A thick layer of poor quality oxide is grown in these steps to help ion implant e Now we need to remove this poor quality oxide before we can grow high quality gate oxide This removal is done in the 101 Oxide Etch A high concentration of HF is used because we have a lot of oxide Prof Milo Koretsky Chemical Engineering Dept 1001 Oxide Etch Importance The silicon at the bottom of contact holes reacts with oxygen in the air It forms a thin layer of oxide This oxide is called native oxide We want to remove the native oxide This allows the metal to make a good contact with the source and drain The trick here is To remove the oxide at the bottom native oxide Not to remove much of the oxide from the sides of the contact holes This removal is done in the 1001 Oxide Etch A low concentration of HF is used because we don t want to remove oxide from the sides Prof Milo Koretsky Chemical Engineering Dept v i i SC1 Clean Recipe H20 NH4OH H202 80 C 511 Removes Particulates Action This works in two ways 1 By repulsion of like charges Both the particle and the wafer get negatively charged when dipped in the solution The particle is repelled from the surface Negatively charged particle moves away Negatively charged surface Prof Milo Koretsky Chemical Engineering Dept SC1 Clean Electrochemistry Reduction Half reaction E0 08 A O32H 26 02H2 207 3 g H202 2H 2e g 2H2 177 l a 0 LL Cu2 26 D Cu 034 Fe3 36 I Fe O17 Ni2 2e D Ni o25 Cr3 36 I Cr O71 E E 3102 4H 26 D Si 2H20 O84 adj 559V Mn226 Mn 105 A13 36 g A1 166 Prof Milo Koretsky Chemical Engineering Dept SC1 Clean 2 By chemical action i Hydrogen peroxide gives its extra oxygen to the silicon substrate and makes a thin oxide film Sis 2 H202I gt Si02s 2 H20I The oxide film lifts the particles ii Ammonium Hydroxide etches the oxide and loosens the particles 2NH4OHI Si02s gt NH4ZSiO3I H20I iii The loose particles are removed by shaking Particle Substrate Prof Milo Koretsky Chemical Engineering Dept Surface Metal Removal SC2 HZSO H20 Rinse H20 HF HZORinse Surface metal contaminants 123336 Roomtemp Rogngmp are first oxidized They are then removed by c Zo TEJET HZOZ 55323 reaction with hydrochloric H20 Rinse H20 HF H20 Rinse Alcohol IPA V Room Temp 100 1 gt Room Temp gt Dry Room Temp Room Temp The name of the solution used in this clean is Standard Clean 2 or SC2 solution Also called HPM or RCA2 The cleaning solution is a mixture of six parts deionized waterH20 one part hydrochloric acid HCI and one part hydrogen peroxideH2C2 Prof Milo Koretsky Chemical Engineering Dept SCZ Clean Electrochemistry Reduction Half reaction E0 8 A O32H 2 I 02H2 D g H202 2H 2e D 2H2 177 l 1 52 Le Cu2 26 D Cu 034 Fe3 36 I Fe O17 N12 26 D Ni 025 Cr3 36 I Cr O71 g 3N3 3102 4H 26 D Si 2H20 O84 u 1 g adj 53V Mn226 Mn 105 A13 36 m A1 166 Prof Milo Koretsky Chemical Engineering Dept SC2 Clean Recipe H20 HCI H202 80 C 611 Removes Surface metal ions Action hydrochloric acid reacts with the ions and removes them Chloride ions from HCI react with aluminum ion and carry it away Prof Milo Koretsky Chemical Engineering Dept Vapor Dry HZSO H20 Rinse H20 HF HZORinse H202 gt Room Temp gt 10 1 gt Recipe Isopropyl Alcohol IPA 125C R mTemp I Removes water from wafers L OH A 511 Room Temp 39 C Ion 80 C 39 AICOhOI vapors displace L H20 Rinse H20 HF H20 Rinse Room Temp 100 1 gt Room Temp water from surface Roomtemp The alcohol evaporates more easily than water It leaves a dry surface I lK I 39i IPA displaces water IPA evaporates g Drop of water Prof Milo Koretsky Chemical Engineering Dept 3 Gettering PM Layer Devices in new quot Emma mg39m surtare re inn 1 1 g7 511 rt 10 20 pm Metals Bounded Zone ur L Ira Traps on back quotr 1quot 33 9quot or in bulk Difiitlsium Alkali ions Ilnirin ic 39 attering quot Dielectric layer Regi n Troop iii 39 011 11 m on topside Backside Y attering tum I g 399mg Damage induced Prof Milo Koretsky Chemical Engineering Dept Predeposition Diffusion do Ci ii grim Historically diffusion was the process by by by by by which ntype P As and ptype B dopants were introduced into Si Sir 540 Doping was accomplished by exposing y I the Si wafer to a P As or B containing Si source and then driving it into the wafer at high temperature by diffusion Ion Implantation has replaced diffusion i DriVe39in doping Si Many fabs still have Diffusion groups I g T which can include a range of activities 53950 55 SM or from high T processes thermal CVD m Si oxidation and anneal to ion implant Prof Milo Koretsky Chemical Engineering Dept Doping Si ln doping the concentration of the dopant in the lattice is an important parameter It is defined as the number of dopant atoms per volume Concentration can change with position and time Cxt Dopant atoms l w l fv o o i l 17 j 7 l t 23 a re f a aixuiu QK QCDU a 0 D w y Low concentration Prof Milo Koretsky Chemical Engineering Dept Diffusion If an ink drop is added to one side of a glass full of Ink drop t still water the ink spreads in the water It spreads from high concentration of ink to low concentration This process is known as diffusion Mathematically in 1 D Ink spreads with time Flux of ink 9Cxt 9x ltdrIVIng force F D Proportionality constant Diffusion coefficient Prof Milo Koretsky Chemical Engineering Dept Mass Shell Balance Balance on of diffusing species in differential volume element JX E p JXW Accum in out C E iii Ax en 3 CH MAM CtAAx JXIAAt JXAXAAt J E 2 I 1 I 39 cm S CHM Ct JxAxJx J At Ax t x Prof Milo Koretsky Chemical Engineering Dept Mathematics of 1D unsteady Diffusion 8Com Mm J Bacon at x x If the diffusion coefficient D is independent of position 0quotCxt 072Cxt 19 2 at 07x Need one initial condition in t and 2 boundary conditions in X Prof Milo Koretsky Chemical Engineering Dept Solid state diffusion EA Solid state diffusion is an activated process Interstitial Vacancy Diffusion of B in Si DifoSiOH DifoSiOH 0 Cu Au Fe Ni P B As Al Ge In T D 800 C 103916 cm2s 1200 C 103912 cm2s Diffusion of 02 in H20 I z 10395 cm2s Prof Milo Koretsky Chemical Engineering Dept Solid state diffusion in Si TEIEIFITIWIquot FIE Timmr tu it m3 Id 1311 IEIEI lion mm I39m mum mmnm EIJEI inn 39 I I I I I w I I i I 1 i 140ImI I 1y5 or m g E I ill H i 139 a i E E l H39 E g E Lill H p E E 5 a E IIIT 1121 dI i u 1M ll39 iI L If I I 113 I I II g5 355 37 njr LE 1135 H151 0 113 339 110 Ll Emmaan mom EHquot Temwrniurep l i 39 iTiE39W Prof Milo Koretsky Chemical Engineering Dept Solid solubitliy in Si 022 E 1m Solubility atoms cm393 39 I I I ii Him I MM mm 1 311i Temperature I 9m Prof Milo Koretsky Chemical Engineering Dept Case I Predeposition aCOCat 32Cxt D 2 at 07x T c8 T IC CxO I Dopant source 839 COI KSentration 09 constantly pro les C OOJ O replenished Solution T Cx t Cserfclx ZJDt I I complimentary error function D39ffus39on length Prof Milo Koretsky Chemical Engineering Dept acltxatgtprzclt rgt Case ll Drive in at 6x Dirac delta func on concentration IC CXO QOCSOC profiles BCS CO t 0 time 07x Thin surface layer with total impurity present C OOJ O of QC atomscm2 Solution 2 x J39L39Dt Prof Milo Koretsky Chemical Engineering Dept Annealing Doping of Si 00 o O o Si Wafer Silicon is doped with boron 0 o phosphorous and arsenic by ion implantation lons from the ion beam damage the lattice Prof Milo Koretsky Chemical Engineering Dept Annealing E Si Wafer 39 Annealing is a process where the wafer is heated to repair the a damage to the lattice o o o The dopant ions become part of the crystal lattice Activation 1 The ions also spread out during anneal Diffusion Prof Milo Koretsky Chemical Engineering Dept Annealing Tool Wafer Heating lamps Cross wise Inert atmosphere Heating lamps Length wise Prof Milo Koretsky Robot arm Chemical Engineering Dept Ion Implant Beam of ions Beam of ions Dix i Si Wafer An ion implanter is used to dope silicon Doping is done by implanting ions of Phosphorus Boron or Arsenic on the wafers Ions implanted from the ion beam damage the lattice Prof Milo Koretsky Chemical Engineering Dept Anneal Activation amp Diffusion Heating Lamp 6 7V ix x x Si Wafer Annealing is a process where the wafer is heated to repair the damage to the lattice The dopant ions become part of the crystal lattice Activation The ions also spread out during anneal Diffusion Prof Milo Koretsky Chemical Engineering Dept Ion Implanter The major subunits of an ion implanter are shown below ION IMPLANTER 3 Analyzer 4 Acceleration 5 Resolving Column Unit 2 Extractor 6 PFOCGSS Wafers Chamber 1 Ion Source All the parts of an implanter shown here are under high vacuum ll l l l l ll ll iii Prof Milo Koretsky Chemical Engineering Dept Gas sou rce 1 Ion Source 2 ExtraCtOr 3 Analyzer 4 Acceleration Column 5 Resolving Unit Outside view Ion Beam 5 6 Process Chamber Load Lock User Interface Top View 00mputer Operator ll l l l Prof Milo Koretsky Chemical Engineering Dept lon Source Operation The filament is heated 1 1 RepellerPlate Electrons are boiled off the heated Gas Slotfor Inlet extracting ions filament 2 The electrons are 1 attracted to the positive 3 39 wall 3 H On their way to the wall 2 H t d they hit neutrals and Fuejmeent ionize them 4 4 g U Electron from N heated lament eutral atom Posmve Ion Q B 8 9 l 7 l l Prof Milo Koretsky Chemical Engineering Dept on Source Operation Many different ions are made when the gas is ionized Some possible ions from BF3 are shown here B Electron from BF filament Q BF2 0 Neutral Molecule BF3 Now we want to move the ions out of the source and towards the wafer 5 l l 2 il 5 l Ovs39 Prof Milo Koretsky Chemical Engineering Dept Extractor Extraction A semb39y The extractor extracts 90 KV ions from the ion I Extracted beam source by providing them a lower voltage 4 H H I Extractor 39 OHS down the KV energy hill Ions fall down the energYhi Now we have a beam of i ions that is moving to wards the wafer We want to make sure that we get the ions we need H quot 1 quot l 3 7 m Prof Milo Koretsky Chemical Engineering Dept Analyzer The analyzer makes sure that only the ions we need reach the wafer It magnetically filters out the specified ions from the other ions in the ion beam Separation is based on the difference in mass Lin l Rad i us of the arc Analyzer I Beam Gunde T I Magnetic Analyzmg Field Potential Magnet 397 l O D 1 Selected ions Prof Milo Koretsky Chemical Engineering Dept Accelerating Column Positive ions gain energy as they fall down the energy hill Fl n Accelerating column Ion beam H H from on analyzer E source 90 KV Extractor 60 o K 39 Analyzer Accelerator KEqV 6O KV 58 KV T 9 Kinetic Energy OKV O Prof Milo Koretsky Chemical Engineering Dept Resolving Unit Magnets in the resolving unit focus the beam onto the wafer Ion beam from accelerator Magnets I I39 39 I 3quot 39 l 39 5 Now the ion beam is l l a focused I y Ion beam gets i H i We are ready to send focused it onto the wafer g Hag III IEiiMEI4I Prof Milo Koretsky Chemical Engineering Dept Process Chamber The wafers are in the process chamber They are held on a large disk It is called the implant disk The implant disk can be tilted to allow implant at different angles Implant diSk Ion beam from resolving unit Tl Wafers ll tiff n l m ll llquot l l l n Prof Milo Koretsky Chemical Engineering Dept Scanning The ion beam must be scanned over the wafer surface to cover the entire area There are three methods of scanning 1 Mechanical 2 Electrostatic Ydirection 3 Magnetic Ion beam Wafer xdirection l n l nn Prof Milo Koretsky Chemical Engineering Dept
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