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Physical Vapour Deposition

Physical vapor deposition (PVD) involves evaporating or sputtering material from a source to deposit thin films on a substrate in a vacuum chamber. In evaporation, a thermal source heats material which travels in straight lines to the substrate. Sputtering uses plasma to bombard a target, ejecting atoms which deposit with better step coverage. Both techniques can deposit a wide range of materials but sputtering provides better step coverage and evaporation risks contamination.

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 Physical methods produce the atoms that deposit on the substrate › Evaporation › Sputtering  Sometimes called vacuum deposition because the process is usually done in an evacuated chamber  PVD is used for metals.  Dielectrics can be deposited using specialized equipment
 Rely on thermal energy supplied to the crucible or boat to evaporate atoms  Evaporated atoms travel through the evacuated space between the source and the sample and stick to the sample › Few, if any, chemical reactions occur due to low pressure › Can force a reaction by flowing a gas near the crucible  Surface reactions usually occur very rapidly and there is very little rearrangement of the surface atoms after sticking › Thickness uniformity and shadowing by surface topography, and step coverage are issues
http://www.ee.byu.edu/cleanroom/metal.parts/vaporpressure.jpg
)in torr( 05.0 Law)Gas(Ideal 2 1 2 P nRTPV ndo        ~ 63% of molecules undergo a collision in a distance less than  and ~0.6% travel more than 5 . › where do is the diameter of the evaporatant and n is the concentration of gas molecules in the chamber
 The vacuum is usually < 10-5 torr › 4x10-6 torr,  = 18 inches  The source heater can be › Resistance (W, Mo, Ta filament)  Contaminants in filament systems are Na or K because they are used in the production of W › E-beam (graphite or W crucible)  E-beam is often cleaner although S is a common contaminant in graphite  Top surface of metal is melted during evaporation so there is little contamination from the crucible  More materials can be evaporated (high melting-point materials)  A downside of e-beam is that X-rays are produced when the electron beam hits the Al melt  These X-rays can create trapped charges in the gate oxide  This damage must be removed by annealing
http://www.lesker.com/newweb /Deposition_Sources/ThermalEv aporationSources_Resistive.cfm
http://www.fen.bilkent.edu.tr/~aykutlu/msn551/evaporation.pdf
 At sufficiently low pressure and reasonable distances between source and wafer, evaporant travel in straight line to the wafer › Step coverage is close to zero › If the source is small, we can treat it as a point source › If the source emission is isotropic, it is easy to compute the distribution of atoms at the surface of the wafer
 For a source that emits only upwards,  = 2 › The number of atoms that hit the area Ak of the surface is › The deposition velocity is the above expression divided by the density (N) of the material kk R F cos r2 evap   k N R v cos r2 evap  
› Pe is the equilibrium vapor pressure of the melt (torr) › m is the gram-molecular mass › T is the temperature (K) › As is area of source  The vapor pressure depends strongly on the temperature (Claussius-Clapeyron equation) › In order to have a reasonable evaporation rate (0.1-1 m/min), the vapor pressure must be about 1-10 mtorr eS P T m AR 2/1 2 evap 1083.5        
 The velocity can be normalized to the velocity at the center of the wafer
 Corrections can be applied if the source is a small, finite area › If we now move the center of the wafer from the perpendicular position, but tile it with respect to the source, an extra term must be added ik N R v  coscos r2 evap  
 Wafer holders that rotate wafer position during deposition to increase film thickness uniformity across wafer and from one wafer to another. › Wobbling wafer holders increase step coverage
 Nonuniformity of evaporatant can occur when angular emission of evaporant is narrower than the ideal source › Crucible geometry › Melt depth to melt area ratio › Density of gas atoms over the surface of the melt
 Evaporating alloys is difficult Because of the differing vapor pressures. › Composition of the deposited material may very different from that of the target material  The problem can be overcome by › Using multiple e-beams on multiple sources  This technique causes difficulties in sample uniformity because of the spacing of the sources › Evaporating source to completion (until no material is left)  Dangerous to do in e-beam system
 Compounds are also hard to evaporate because the molecular species may be different from the compound composition › Energy provided may be used to dissociate compound. › When evaporating SiO2, SiO is deposited. Evaporation in a reactive environment (flowing O2 gas near crucible during deposition) helps reconstitute oxide.
 Advantages › Little damage to the wafer › Deposited films are usually very pure › Limited step coverage  Disadvantages › Materials with low vapor pressures ae very difficult to evaporated  Refractory metals  High temperature dielectrics › No in situ precleaning › Limited step coverage › Film adhesion can be problematic
 Evaporation technique is very directional due to the large mean free paths of gas molecules at low pressure.  Shadowing of patterns and poor step coverage can occur when depositing thin films.  Rotation of the planetary substrate holder can minimize these effects.  Heating substrate can promote atom mobility, improve step coverage and adhesion.  Shadow masking and lift-off are processes where poor step coverage is desirable.
 Other deposition techniques include › Sputter deposition (DC, RF, and reactive) › Bias sputtering › Magnetron sputtering › Collimated and ionized sputter deposition › Hot sputter deposition
 Sputter deposition is done in a vacuum chamber (~10mTorr) as follows: – Plasma is generated by applying an RF signal producing energetic ions. – Target is bombarded by these ions (usually Ar+). – Ions knock the atoms from the target. – Sputtered atoms are transported to the substrate where deposition occurs.
 Wide variety of materials can be deposited because material is put into the vapor phase by a mechanical rather than a chemical or thermal process (including alloys and insulators).  Excellent step coverage of the sharp topologies because of a higher chamber pressure, causing large number of scattering events as target material travels towards wafers.  Film stress can be controlled to some degree by the chamber pressure and RF power. http://www.knovel.com
 Temperature: Room to higher  Pressure: 100mtorr › compromise between increasing number of Ar ions and increasing scattering of Ar ions with neutral Ar atoms  Power › Heating of target material  Low temperature metals can melt from temperature rise caused by energy transfer from Ar ions
 Magnetron › Magnetic field traps freed electron near target › Move in helical pattern, causing large number of scattering events with Ar gas – creating high density of ionized Ar  Ion beam › Plasma of ions generated away from target and then accelerated toward start by electric field  Reactive sputtering › Gas used in plasma reacts with target material to form compond that is deposited on wafer  Ion-assisted deposition › Wafer is biased so that some Ar ion impact its surface, density the deposited film. May sputter material off of wafer prior to deposition for in-situ cleaning.
 Advantages › Large-size targets, simplifying the deposition of thins with uniform thickness over large wafers › Film thickness is easily controlled by fixing the operating parameters and simply adjusting the deposition time › Control of the alloy composition, step coverage, grain structure is easier obtained through evaporation › Sputter-cleaning of the substrate in vacuum prior to film deposition › Device damage from X-rays generated by electron beam evaporation is avoided.  Disadvantages › High capital expenses are required › Rates of deposition of some materials (such as SiO2) are relatively low › Some materials such as organic solids are easily degraded by ionic bombardment › Greater probability to introduce impurities in the substrate because the former operates under a higher pressure
http://www.research.ibm.com/journal/rd/444/jordansweet.html