Metal Deposition Filament Evaporation E-beam Evaporation Sputter Deposition 1
Filament evaporation metals are raised to their melting point by resistive heating under vacuum metal pellets are placed on a filament that acts as a resistor multiple filaments can be mounted inside the system and selected with a switch 2
Evaporation Apparatus S.A.Campbell, 1996 3
Filament Evaporation the metal pellets give off a vapor and the atoms travel in a straight line away from the source until they strike the sample the start and stop of the deposition is controlled by a mechanical shutter 4
Ideal Gas Law PV = nrt n = PV/RT P = pressure V = volume n = number of moles R = gas constant T = temperature 5
Mean Free Path distance a molecule travels in a vacuum in a straight line, before its motion is randomized by a collision with another object 6
Mean Free Path average velocity time to collision υ = t coll = 8RT πm 1 2πnd 2 υ mean free path MFP = υ t = coll 1 2πnd 2 R = gas constant M = molecular weight n = number of moles d = molecular diameter 7
Mean Free Path n ~ P number of moles proportional to pressure MFP = 1 2πnd 2 MFP ~ 1/n ~ 1/P as P decreases MFP increases 8
Vapor Pressure P ~ e e T S.A.Campbell, 1996 9
Evaporation Rate r = M 2πkT evap P e r evap = evaporation rate M = atomic mass k = Boltzman s constant T = temperature P e = vapor pressure 10
Deposition Rate deposition rate depends on the location and orientation of the wafer in the chamber 11
Deposition Rate r dep = r Ωd evap 2 ρ cosθ d θ Ω r dep = deposition rate (thickness/sec) r evap = evaporation rate (mass/sec) Ω = solid angle over which source emits (unit less steradians) d = source to substrate distance ρ = material density θ = inclination of substrate away from direction to source 12
Case 1: d x x Uniformity x Case 2: d >> x d 1 d 2 d 1 d 2 d 1 d 2 d 1 < d 2 r dep (d 1 ) > r dep (d 2 ) r dep (d 1 ) r dep (d 2 ) 13
Filament evaporation can try to increase temperature to compensate for low dep rate at far distance too high of a evaporation rate can result in condensation of the material into droplets. droplets on wafer cause poor surface morphology 14
Deposition & Uniformity it s a tradeoff! better uniformity = lower deposition rate 15
Filament Evaporation multiple wafer deposition will result in wafers with different thicknesses d 2 > d 1 d 1 d 2 16
Planetary a rotating planetary can help improve uniformity d 1 d 2 d 2 = d 1 17
Quartz monitor used to monitor deposition rate quartz oscillates at a resonant frequency and is exposed to evaporated material as material deposits on quartz, the resonant frequency changes due to additional mass on the crystal 18
Quartz monitor Quartz monitor in evaporation chamber likely a relative reference (due to position dependency) and must be calibrated with profilometry evaporated materials have different atomic mass, and thus rates are material dependent when enough material has been deposited on the quartz, it must be changed because it no longer shows a clear resonant frequency 19
Step Coverage early days all metal deposition done by evaporation lateral dimensions decreased more rapidly than vertical making step coverage more critical S.A.Campbell, 1996 20
Step Coverage recall that r dep = r evap Ωd 2 ρ cosθ θ θ = 90 º, cosθ = 0 21
Step Coverage arriving material is highly collimated for two reasons d 1. evaporation sources are small and the source to wafer distance (d) is large, therefore the arrival angle (θ ) is limited θ 2. also, MFP are long for evaporation (molecule motion not randomized) 22
Step coverage planetaries can be used so that the source appears hemispherical and thereby improve step coverage heating of substrates (~60% of melting temperature) during deposition can improve step coverage by promoting movement of molecules over the surface after impact 23
Improving Step Coverage W.S.Ruska, 1987 24
One person s trash... poor step coverage can be used to advantage in lift off process film is deposited on top of patterned photoresist layer, layer on top of resist is easily lifted resist substrate 25
Filament Evaporation since the filament must be as hot as the material being evaporated, contamination from the filament itself evaporating can be a concern at high temperatures 26
E-beam evaporation typically used where high temperature is needed and filament evaporation is a concern thermal emission of electrons from a heated tungsten filament 27
E-beam evaporation magnetic field and rastering used to steer and focus the electron beam into a crucible containing material beam bends 270 to minimize Tungsten deposition from evaporation off of filament different materials can be selected by a rotating crucible selector 28
E-beam evaporation scan plates W.S.Ruska, 1987 29
X-rays x-rays can be generated by an e-beam system due to highly excited electrons in the material being evaporated decaying back to core levels radiation can damage CMOS and Silicon based devices 30
Alloys many modern silicon technologies require alloys to form reliable contacts and metal lines (e.g. Al w/ 1% Si, or Al w/ 0.5% Cu). difficult to produce well controlled alloys by evaporation, due to differences in vapor pressure between the two materials 31
Alloys in both filament and e-beam evaporation, multi layer films of different materials can easily be created by sequential processing two ways to attempt alloys 1) single alloy evaporation, 2) simultaneous evaporation of two different materials 32
Alloys S.A.Campbell, 1996 33
Sputtering better step coverage less radiation than e-beam better at producing alloys easier to deposit metals with high melting points 34
Sputter deposition parallel plate system similar to etcher gas is ionized and accelerated towards target material material is sputtered off the target if bombardment energy roughly 4 times bond energy of solid, atoms will be knocked lose (relatively easy, most bond energies several ev) 35
Sputter Deposition plasma is generated using inert gas which does not react with source material (want a pure deposition!) wafer must be placed close to target to collect ejected material (short MFP due to high pressure) 36
DC Plasma to serve as an electrode, material must be conductive, so DC sputtering is not possible with insulating materials insulating materials must use RF plasma 37
Sputter Apparatus S.A.Campbell, 1996 38
Plasma a plasma is initiated by applying large voltage across a gap containing a low pressure gas governed by Paschen s law P L V bd ~ log + ( P L) b V bd = voltage drop P = pressure L = gap between electrodes b = constant 39
Plasma dark space plasma glow W.S.Ruska, 1987 40
Plasma sputter rate depends on ion flux to the target J ion ~ 1 m ion 3/ 2 bd 2 V d J ion = ion flux m ion = mass of ion V bd = voltage drop across dark space d = dark space thickness 41
Ion bombardment S.A.Campbell, 1996 < 10 ev Ion bounce off or adsorbs to surface 10 ev to ~1000eV Ions sputter atoms off target material > ~1000eV Ions implanted into target 42
Sputter Yield Ions begin to implant into target Ions bounce off surface or adsorb onto surface S.A.Campbell, 1996 43
DC Plasma V V 1 2 A = A 2 1 4 voltage drop can be concentrated on one electrode by making it relatively small the large electrode is the entire chamber (A 2 ), while the small electrode is the target (A 1 ) 44
Step Coverage plasma deposition has short mean free path due to higher pressure (more randomization), therefore material approaches from more angles providing better step coverage 45
Step coverage as aspect ratios increase > 0.5, it is difficult for even sputtering to achieve good step coverage for high aspect ratios, CVD process is used for better fill, such as W contacts in a modern IC fab 46
Magnetic Field sputter yield can be increased by applying a magnetic field giving electrons a spiral path increasing probability of creating an ion ion density increases from 0.0001% to 0.03% to avoid excessive heating the target is cooled 47
Magnetron W.S.Ruska, 1987 Hall Effect 48
Circular Magnetron S. M. Rossnagel, 1999 49
Uniformity deposition rate under circular magnetron has a Gaussian profile as a function of lateral distance from the target to improve uniformity, the wafer can be rotated, as well as moved in an orbital motion target substrate planetary 50
Heart shaped magnetron moving parts now outside of chamber, less particles more uniform wear on target means less waste S. M. Rossnagel, 1999 51
Film morphology at low temperature and ion energy, films can be amorphous and highly porous. films of this type can oxidize readily and be resistive desire small grains in moderate temperature and ion energy range high temperature and energy can cause large grain size. films can be rough and appear hazy 52
Stress S.A.Campbell, 1996 stress dependent on substrate temperature deposition rate film thickness background chamber ambient σ = δ E T t 1 υ 3R σ = stress δ = wafer bow t = thickness of film E = Young s modulus υ = Poisson ratio T = thickness of wafer R = radius of wafer 2 2 53
Pro/Con Summary Method Pro Con Filament evaporation E-beam evaporation Sputter Deposition 1. simple apparatus 2. good for liftoff 1. good for liftoff 2. high temp materials 1. better step coverage 2. alloys 3. high temp materials 4. sputter pre-clean 1. limited source material 2. alloys difficult 3. high temp materials difficult 4. poor step coverage 1. radiation 2. alloys difficult 3. poor step coverage 54