Deposition of Thin Metal Films " (on Polymer Substrates)!

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1 Deposition of Thin Metal Films " (on Polymer Substrates)! Shefford P. Baker! Cornell University! Department of Materials Science and Engineering! Ithaca, New York, 14853! MS&E 5420 Flexible Electronics, 18 September, 2008! How to make a thin layer Suppose you want to make a thin (< 1 µm) metal film on a substrate e.g. to provide electrical connections to a device on a flexible substrate. How could you do it?! bonding! painting! atom by atom! difficult to handle nm-scale layer! trap dirt and voids at interface! easy! poor control of geometry! trap dirt and voids! can be clean! good geometry control! expensive!

2 Atom by atom deposition processes MECHANISM! physical! chemical! plasma! sputtering! reactive! sputtering! plasma assisted" chemical vapor deposition! PHASE! vapor! liquid! evaporation! hot dip coating! reactive! evaporation! chemical vapor deposition! plating, anodization! solid! segregation! Oxidation,! case hardening! Overview Introduction! Physical Vapor Deposition! Vacuum! Evaporation! Sputtering! Chemical Vapor Deposition! Deposition of Metals onto Polymers!

3 Physical Vapor Deposition (PVD) The basic idea: evaporate atoms at a source, condense them on a substrate! substrate! Source atoms: may be vaporized by heating or bombardment with ions and/or electrons! Substrate: may be heated, biased, rotating, translating etc! source! Vacuum: why and what Why vacuum? Need to remove other atoms and molecules (here generically called particles ) from the deposition chamber! # ensure that only film material deposited! # deposit at an adequate rate (! long enough)! " Have vacuum! Units of pressure! # Pascal (Pa) = N/m 2 (official SI units)! # torr (Torr) = 1 mm Hg = Pa (conventional unit in the US)! # Bar = 100,000 Pa = 750 Torr (used by some)! Properties of vacuum! # Density of particles low compared with atmosphere! # Particles travel long distances between collisions (average distance is mean free path)! # Particles bombard surfaces at a high rate! # Particles travel at about the speed of sound (in dry air, approx 330 m/s)! Levels of vacuum! # Low vacuum: to about 10-2 Torr! # High vacuum: Torr! # Ultra high vacuum (UHV): less than 10-8 Torr!

4 Vacuum: important quantities Using ideal gas approximation:! particle density:! mean free path:!! n = 9.67 "10 18 P T particles /cm3 " = kt #P = 2.33$10%20 T Pd 2 cm bombardment flux:!! " = n kt 2# = 3.51$1022 P MT particles /sec Where n = number of particles/cm 3, P = pressure in Torr, T = temperature in K,! = mean free path in cm,! " = d 2 collision cross section, G = bombardment flux of particles on surfaces inside the chamber, M = molecular weight of the particles.! Important quantities CO at 25 C! N 2 at 25 C!

5 Vacuum: numbers and flow pressure! (Torr)! particles/cm 3!! H 2 0! (cm)!! H e! (cm)! flux" ptcls/cm 2 /s! time to " form monolayer! (10 15 ptcles/cm 2 )! 1! 10 16! ! 0.011! 10 21! 1 µs! 10-3! 10 13! 3.3! 11! 10 18! 1 ms! 10-6! 10 10! 3300! 11000! 10 15! 1 s! 10-9! 10 7! 33 # 10 6! 11 # 10 7! 10 12! 1000 s! Purity of film depends on rate of impingement of film atoms compared with other particles! Behavior of particles depends Knudsen number, K n $!/l where l is dimension of vacuum chamber and! is mean free path! # K n >> 1 " laminar viscous flow: particle-particle interactions dominate, particles move in coherent manner along streamlines (turbulent flow only during initial stages of pumping)! # K n >> 1 " molecular flow: particle-particle interactions rare, flow controlled by wall collisions, particles rest on surface briefly and are reemitted (in random direction)! Vacuum: how to get one (spend $!) Vacuum level depends on! # Pumping speed! # Outgassing rate! # Leak rate! Pumping speed depends on! # Pump speed! # Conductance (flow rate/%#)! Note: in molecular flow regime, conductance depends on extent to which pump can see all internal surfaces!

6 Vacuum pumps Vane pump: fast, only good at low vacuum, oil can backstream! Diffusion pump: works by entrainment of gas by supersonic jet of pumping fluid vapor, only good at high vacuum, oil can backstream, needs forepump (very common, nothing to do with diffusion)! Sorption pump: condensation/" adsorption (depending on M) onto cold zeolite, fast, good to high vacuum, no backstreaming, M dependence, limited capacity! Cryo pump: condensation/" adsorption (depending on M) onto stage cooled by liquid He, fast, good to UHV except for H and He, no back-streaming, capacity better than sorption pump but still limited! Vacuum pumps Sublimation pump: evaporates Ti on walls, traps by chemisorption all gasses except inert gasses and other certain molecules, good to UHV, capacity limited, need forepump! Turbomolecular pump: a high speed (up to 100,000 rpm) turbine that works by capturing atoms on leading edges of blades, where they are re-emitted further into the pump (blade speed must be faster than gas particle speed!), only works in molecular flow regime, good to UHV! Sputter ion pump: ionizes gas, implants it in Ti layer and buries in with more Ti. Traps essentially all molecules. The best for UHV, capacity limited, need forepump! Typical strategy for UHV is turbopumps, backed by vane pumps (vane pump gets system to molecular flow regime, turbopump brings it to UHV), with ion pumps to get the light fast species that escape the turbopumps. Every separate chamber must be pumped. All components must be baked to >100 C regularly. Pumps are typically more than half the cost of any evaporation system, and between a quarter and half the cost of sputter deposition systems.!

7 Vacuum: composition changes during pumping Inside the vacuum chamber - start pumping, see change in composition of residual gasses! Initially pumping gas from volume of chamber " fast!! At about 10-4 Torr, outgassing from chamber walls dominates (Note H 2 0 can be eliminated by heating chamber walls to 100 C + during pumping!)! At lower pressure, diffusion of gas through seals or walls dominates.! Vacuum system key features Base pressure! Composition of base atmosphere! Pumping speed (cycle time)! Depends on! # Pumps! # Seals! # Bakeout capability! Better vacuum always means! # More cost! # Lower throughput!

8 Evaporation Oldest, best established PVD method (Edison patent 1884, earliest use Al reflective coatings for telescope mirrors)! Simple:! # Raise T at source to evaporate material! # If vacuum good, particles travel in straight line! # Stick to any surface that$s cool enough, incl substrate! Complicated: film nucleation and growth! Need pressure less than 10-4 Torr to avoid interaction/reaction with background gas! Evaporation rate depends on vapor pressure of evaporant! Evaporation characteristics of some metals 2. Below this temperature, more heat radiated than used for evaporation!

9 Evaporation: vapor pressure curves Evaporation sources Resistive heaters! Filament heaters:" cheap, good area coverage, need rate control! Oven source:" fast, powerful, with aperture can control deposition geometry ( effusion cell )! Boat, mesh, and coated wire:" faster than filaments!

10 Evaporation sources RF induction heaters! E-beam source! Electrically conductive material in non-conducting crucible:! # easy rate control! # less contamination from heated container! Electrons generated by thermionic emission from hot W filament, steered to source material by electromagnet:! # easy rate control! # can evaporate high-t m materials! # no contamination from heated container! # complicated!! Evaporation geometry To first order line of sight with source profile (cosine law)! To ensure uniform thickness! # Rotate substrates through flux gradients! # Change geometry of source!

11 Evaporation and flexible electronics Evaporation good for roll to roll processing! roll coater! Considerations for evaporation! Sputtering In sputtering, a glow discharge plasma is used to remove atoms from source.! Plasma = mix of gas ions and molecules (Langmuir 1928), the 4th state of matter! For deposition and etching, plasmas are cold and dilute compared with, say the sun or a plasma torch. Think of as a nearly equal number of electrons and positively charged ions in a sea off gas atoms and molecules. Glow discharge plasmas:! # Are electrically neutral! # Conduct electricity! # Exist at low pressures (typically 1 mtorr to 2 Torr)! # Electrons are hot % 2 ev (20,000 C)! # Ions typically 0.02 to 0.1 ev (avg T between RT and 300 C)! # Dilute 1:10,000 mixture of charged particles!

12 Sputtering: the glow discharge plasma DC glow discharge with grounded anode! Bring cathode at to V near grounded anode in appropriate gas at mtorr get plasma glow and cathode dark space! In plasma glow! # No field(!)! # No charge! # All species in random motion! In dark space! # All of the field! # Electrons and ions moving fast! # Neutrals in random motion! Sputtering: the glow discharge plasma DC glow discharge in 100 mtorr Ar, grounded anode on right at -1000V, discharge current density 0.22 ma/cm 2!

13 Sputtering: starting and maintaining a plasma A single e - -molecule collision that forms an ion can initiate plasma! e - from cathode knock e - off of neutral molecules if energy & ionization potential! Ionization potential (ev)! Each collision generates an e -, but for plasma to be maintained more e - must be generated than are lost from the plasma secondary e- from cathode (efficiencies typically ) do this! Sputtering: what the ions and neutrals do About 5-10% of high-energy ions that hit cathode are neutralized and bounce off as high energy particles, these bombard all surfaces! Some neutrals are excited by e - -molecule and ion-molecule collisions.the plasma glow is the radiation emitted by these excited species as they relax to lower energy levels! Lifetimes of some energetic species!

14 Sputtering: the glow discharge plasma DC glow discharge with electrode bias! Suppose bias applied to anode! # If positive bias, plasma potential will track applied bias! # Negative bias adds to plasma potential! # A slight positive bias can prevent ion bombardment! # A negative bias increases energy of ion bombardment! Sputtering: the glow discharge plasma RF glow discharge! If use rf potential, can use insulator for cathode (aka target) as surface charges and discharges! Sputter, Deposit, or Etch?! All are possible, depending on energetics and species in chamber! # Accelerate ions towards target! # If energy sufficient, knock atoms from target surface into vacuum, this is sputtering! # If target atoms condense on substrate, deposit film! # Can adjust fields to use ions to clean substrate or etch film (same processes)!

15 Sputtering Atomic billiards, incoming species knock atoms from surface! Incoming ion with energy and angle determined by! # Fields (electric, magnetic)! # Geometry! # Pressure! # Species! If energy sufficient, angle high enough! # Atoms sputtered from surface (mostly neutral)! # (ion may also be neutralized and reflected)! Subsurface damage can be extensive, limits use of plasma etch! Sputtering Processes during sputtering! For sputter deposition, atoms knocked from cathode (called target ) condense on substrate! Compared with evaporation! # Faster! # Not limited by vapor pressure! # Can sputter any material! # Very wide range of process control (includes etching/cleaning as well as deposition)! # Sputter gas ions can be included in film!

16 Sputtering rate and yield Sputter rate is rate at which materials leaves target! # Ion flux bombarding target! Sputter yield is the number of atoms sputtered per incoming ion! # Heat of vaporization of target! # Ion mass to target atom mass ratio! # Ion energy (voltage on the target)! # Angle of incidence! Magnetron sputtering Use magnets to control charged species, contain plasma near target! # Electrons follow helical paths about magnetic field lines, stay in plasma, undergo many ionizing collisions with neutrals in plasma! # Ions accelerated into target, sputter target atoms! Planar magnetron can make uniform deposition over large area if move substrate across target!

17 Magnetron sputtering Schematic of planar magnetron sputtering! Plasma at surface of large planar magnetron source (can be meters long)! Magnetron sputtering Cosputtering from two circular planar magnetrons (useful for making alloys)!

18 Sputter deposition and flexible electronics Anything evaporation can do, sputtering can do better! Biggest issue is adhesion of metal to polymer! # Can use plasma to etch, oxidize surface, implant metal into surface to improve adhesion! # Also commonly include metal adhesion layer (Cr, Ti) which is easy to sputter, but difficult to evaporate (especially Cr)! Sputtering much more complicated and expensive than evaporation! Resources For more on PVD processes (a huge field!)! Physical Vapor Deposition of Thin Films John E. Mahan, John Wiley and Sons (2000)! Introduction to Surface and Thin Film Processes John A. Venables, Cambridge University Press 2001! Thin Film Deposition and Patterning Robert K. Waits American Vacuum Society 1998! Thin Film Deposition Principles and Practice Donald L. Smith, McGraw-Hill Inc., 1995!