sputtering

PVD: SputteringIn sputtering, atoms are physically removed from a target.Energy is transferred from incoming ions to the target atoms to break the bonds holding the atom onto the solid.1. Using an electricfield, an Al targetis bombarded withAr+ ions.Ar+Ar+Ar +2. The Ar+ions knock Al atoms off the target. The Al lands on the wafer (andeverywhere else).San Jose State University: MatE 129Why Sputtering (vs Evaporation)?Sputtering can get better uniformity over a largesize (from larger targets).There can be tighter (and easier) control over alloy composition.Pre-surface sputter cleaning of surface anddeposition/ etching processes to control uniformityare possible.CON: Certain sputtering systems (glow discharge plasmas) require a medium level vacuum that can increase contamination over evaporation!San Jose State University: MatE 129Where is sputtering used?Sputtering is used in the Al interconnect stacksand diffusion barrier/ seed layer for Cuinterconnects.Al or Cu metal stack•Al (sputter full layer)•Cu (sputter seed for electroplating)•Ta (or Ti)»For adhesion»As a diffusion barrier for Cu•TaN(or TiN)»To prevent Al & Ta from reacting»As an anti-reflective coatingSan Jose State University: MatE 129The Basics of Sputtering: TargetA high purity target is needed.Typically the target is the size of thesubstrate (or larger) to improveuniformity.Water cooled from the back.Cluster tools are often used to sputtermultiple films in series.San Jose State University: MatE 129The Basics of Sputtering: IonsA gas (typically Ar) is converted to aplasma to act as a source of ion•Remote plasma ion source that provides anion beam•Glow dischargeEnergy of the incoming ion is fine tuned.•We want to maximize the knocking off oftarget surface atoms and minimizeimplantation of the incoming ion.San Jose State University: MatE 129Chamber Size & PressureA vacuum is required for both purity andcontrol.•Typical pressures are a few to 100 mTWe want Kn(λ/L) >1•This would give a mean free path greater than the chamber length.•This allows the beam to not get scatteredbefore reaching the target, giving goodcontrol over the energy and directionality ofthe incoming ions.San Jose State University: MatE 129Fine Tuning the Energy of Our IonsIn sputtering, the goal of the plasma is to generate ions. We will set a sustainable plasma (correct pressure of gas based on the chamber dimensions).We need to accelerate the ions toward the target and control their impingement energy.•<5eV: physisorption or reflection•5-10eV: can have surface damage and surface migration•10-30eV: sputtering •>10eV: ion implantationSan Jose State University: MatE 129What happens when an ion hits the target?A lot of stuff could happen•Bounce Back •Implant•Absorb on surface•Reflection by grazing collision sequenceSan Jose State University: MatE 129Energy TransferThe energy transfer can be approximated using a simple, head on collision (but is much more accurately modeled using statistical software such as TRIM).Compare the energy transfer difference if a He ion (m=2 g/mol) collides with Al (m=27 g/mol) versus if an Ar ion (m=40 g/mol) is used.()ion,incident m ion ,incident 2et arg t ionetarg t ion m transfer E E m mm m 4T E γ=+==San Jose State University: MatE 129What energies are needed forsputtering?There is a lot more going on when the ion hits the surface that need to be considered in sputtering including:•Angle of impact•Subsequent deflection & other impacts of the ion(statistical)•Path & energy of any freed atoms»May escape, go back to its spot, or move inwards•Binding energy (heat of sublimation ) of target atomsSan Jose State University: MatE 129Threshold EnergyThe minimum energy required for sputtering depends on the mass of the target and ions. A good fit to experimental data for m ion <0.3m targetA good fit to experimental data for m ion >0.3m target()m m so1HE γ−γΔ=5/2etarg t ions o m mH 8E ⎟⎟⎠⎞⎜⎜⎝⎛Δ=San Jose State University: MatE 129Sputtering YieldTo calculate deposition rates, we need to know how many sputtered atoms we create per incoming ion, sputtering yield.This can be calculated theoretically but is most accurate from experimentally determined numbers.San Jose State University: MatE 129Angular Dependence of theSputtering YieldThe sputtering yield depends on the angle of the incident ion.•Grazing incidence (theta values approaching 90º)»Some ions bounce back.•Low values of theta (approach head on)»Reduced due to the fact that there is a greaterlikelihood for freed atoms to be knocked deeper into the material.•Sputtering peaks at around 60º.San Jose State University: MatE 129Step CoverageStep coverage is a measure of how uniform thickness is over the topography of the surface (such as in filling a via hole in integrated circuits).In sputtering, deposition is usually thicker on corners because of the wider angle gas atoms approach it from.San Jose State University: MatE 129Deposition RateThe deposition rate can be quantified from the sputtering yield and the incident ion rate.ei s s q I Y s atoms Q =⎥⎦⎤⎢⎣⎡I i measured in a ion beam sourceVPower I i =I i calculated from measured power and applied voltage in a diffuse gas plasma.San Jose State University: MatE 129Deposition Rate vs PressureAs you increase the gas pressure, more collisions result in the formation of more ions. Increased ion current to the target results in higher deposition rate.However, at very high pressures, sputtered target atoms get scattered before reaching the target. This reduces the deposition rate.There is a peak in the deposition versus pressure curve. (For Ar ions this is around 110 mT .)San Jose State University: MatE 129Deposition Rate vs PressureThese dependences of the deposition rate on pressure can be expressed as:d: cathode anode spacing P D : Power Density E: sputtering energy<x th >: distance where the sputtered atom’s motion becomes random due to collisions rather than directed with a velocity vector()E1d x P G e density thD γ+ρ=•San Jose State University: MatE 129Limitations of Basic SputteringAs seen above, in order to get a reasonabledeposition rate, a sputtering chamber must operate at relatively high pressures.These high pressures mean that there are a fairly high concentration of impurities in the gas (air)•Also, at high pressures there is a lot of interaction between gas molecules and chamber walls resulting in increased contamination from the walls.Several modifications to sputtering exist to minimize these problems:•Bias sputtering: small negative bias to substrate reduces contamination to substrate•Magnetron sputtering: use of magnetic field to trap electrons around target gives a higher deposition rate at lower pressuresSan Jose State University: MatE 129Small Bias SputteringA small negative bias is applied to the substrate•-1000 to –3000 eV on target•-50 to –300V on substrateThis small bias is enough to remove physisorbedcontaminants (such as O2molecules)The bias also increases energy of surface atomsresulting in enhanced surface diffusion•This promotes better adhesion, nucleation, and crystalstructureSan Jose State University: MatE 129Large Bias SputteringA lrage bias can be applied to the substrate to etch the substrate (sputter the substrate)The non-uniformities can be reduced by sputter etching the surface. In other words, reverse the bias so that ions go towards the substrate rather than the target.The thick points will etch faster (for the same reasons they deposited faster).This technique is also used to clean the surface prior to deposition.San Jose State University: MatE 129Magnetron SputteringThe fraction of ions in a plasma is significantly lessthan the total concentration of gas atoms. In general,this leads to reduced deposition rates of sputtering ascompared with evaporation.•Typical ion densities in a plasma are 0.0001%!Magnetron sputtering is used to increase thedeposition rate.Lorenz forces force electrons in the plasma into ahelical path. The longer path results in morecollisions with gas atoms (and thus more ions).San Jose State University: MatE 129RF SputteringIf insulating targets (such as oxides or nitrides) are sputtered, the target negative charge applied to the target is neutralized by the Ar ions. Eventually, the ions won’t be attracted to the target anymore.To overcome this, an alternating current in the radio frequency (rf) is used rather than DC.•13.5MHzIons can not follow this frequency (too heavy and slow). Electrons do, however, building up a negative self bias on the target.San Jose State University: MatE 129Reactive SputteringCompounds are also commonly sputtered using a combination of chemical & physical techniques. Sputtering in the presence of a reactive gas is used to deposit out compounds such as:•Oxides from O2gas: Al2O3, Ta2O5, SiO2•Nitirdes from N2or NH3: TaN, TiN, Si3N4•Carbides from CH3or CH4: TiC, WC, SiC•Also combinations such as oxycarbides, oxynitridesSan Jose State University: MatE 129。