EE-527: MicroFabrication

Physical Vapor Deposition

R. B. Darling / EE-527 / Winter 2013 Physical Vapor Deposition (PVD)

Gas Phase Gas Phase

Transport

Evaporation Condensation

Condensed Phase Condensed Phase (solid or liquid) (usually solid)

R. B. Darling / EE-527 / Winter 2013 Equilibrium Vapor Pressure

• P* is the partial pressure of a gas in equilibrium with its condensed phase at a given temperature T. – There is no net transfer of material from one state to the other. • For a given material, P* is only a function of T. – But the dependence of P* on T is rather complicated.

R. B. Darling / EE-527 / Winter 2013 Evaporation Rates - 1

• P* is the equilibrium vapor pressure of the evaporant at T. • P is the ambient hydrostatic pressure acting upon the evaporant in the condensed phase. • Heinrich Hertz found experimentally that the evaporation rate was proportional to (P* - P). – This is consistent with kinetic theory in which the impingement rates are proportional to pressure. – Hertz also found that the evaporation rate could not be increased by supplying more heat unless the equilibrium vapor pressure was also increased by this action. – Thus, there is a maximum evaporation rate set by P*, and this is only achieved in a vacuum, where P = 0.

R. B. Darling / EE-527 / Winter 2013 Evaporation Rates - 2

• This can be viewed as two opposing fluxes:

The net evaporation flux is the difference between the impingement rates for the evaporant vapor two fluxes:

rate goes as P dNe 1/ 2 *  2mkBT P  P Aedt rate goes as P*

condensed evaporant

R. B. Darling / EE-527 / Winter 2013 Evaporation Rates - 3

• Hertz only measured rates of about 1/10 of the above using Hg vapor. • Knudsen postulated that the evaporant vapor molecules impinging upon the condensed phase surface may be reflected back.

– v = sticking coefficient for vapor molecules onto the surface.

– Then a (1 - v) fraction of the vapor molecules contribute to the evaporant pressure, but not to the evaporant flux.

– Therefore, the vapor pressure must be higher by a factor of 1/v to obtain the same evaporation rate. • This gives the general Hertz-Knudsen equation:

dNe  1/ 2 *  v 2mkBT P  P Aedt R. B. Darling / EE-527 / Winter 2013 Mass Evaporation Rates

•  = mass evaporation rate in g/cm2-sec:

1/ 2 dN   m  e   *   m  v   P  P Aedt  2kBT  • For most elements,  ~ 10-4 g/cm2-sec at P* = 10-2 torr. • The mass of the evaporated material is

t Ae M   dA dt e  e 00

R. B. Darling / EE-527 / Winter 2013 Free Evaporation Versus Effusion

• Evaporation from a free surface is termed Langmuir evaporation.

• Because v is often much less than unity, the general Hertz-Knudsen expression must be used. • Effusion refers to evaporation through an orifice by which the area of the orifice appears as an evaporation source of the same area. • Free evaporation is isotropic. • Effusion is somewhat directional. – Ideally, it is a Lambertian angular distribution.

R. B. Darling / EE-527 / Winter 2013 Knudsen Cell

• In 1909 Knudsen invented a technique to force v to 1. • This is called a Knudsen cell, effusion cell, or “K-” cell.

• The orifice acts like an evaporating surface of area Ae at P*, but it cannot reflect incident vapor molecules, so v = 1.

Isothermal enclosure with a small orifice of area Ae. Surface area of the evaporant inside the enclosure is large compared to the area of the orifice.

An equilibrium vapor pressure P* is maintained inside the enclosure.

Effusion from a Knudsen cell is therefore: dN e  A 2mk T 1/ 2 P*  P dt e B

R. B. Darling / EE-527 / Winter 2013 Directional Dependence of a Knudsen Cell - 1

• First find the evaporant flux into a solid angle of d from a

source area of dAe: – In a time dt, molecules that are within vdt of the orifice will exit. – If molecules are uniformly distributed within the volume V, then the fraction of molecules which are within striking distance in dt of

the orifice with an exit angle of  is vdt cos  dAe/V. – The fraction of molecules entering the solid angle d is d/4.

The number of molecules emitted from an area dAe  d of the source in a time dt into a solid angle of d about an exit angle of  with velocities in the range

vdt of v to v + dv is: dA e 4 v dt cos dAe d 2 d Ne  N (v ) dv volume V V 4

R. B. Darling / EE-527 / Winter 2013 Directional Dependence of a Knudsen Cell - 2

• Integrate over all velocities: – A Maxwellian distribution:

3/ 2  m   mv2  (v2 )  4   v2 exp       2 kBT   2kBT   1/ 2 1/ 2 2 4  2k T  4  8k T  v (v ) dv  B  v   B     m   0  m    m  • Obtain:  1/ 2  N  8k T  d 3  B  d Ne    cos dt dAe V  m  4

R. B. Darling / EE-527 / Winter 2013 Directional Dependence of a Knudsen Cell - 3   • Since PV = NkBT, the impingement rate out through the orifice into a solid angle of d about an exit angle of  is:

1/ 2 d 3 N N  8k T  d d e  B  1/ 2     cos  2 mkBT P cos dAe dt V  m  4  • The total impingement rate out through the orifice into all forward solid angles is: 2 d Ne 1/ 2   2mkBT P dAe dt   2 / 2   Note that: cos d  cos sin d d   0 0 R. B. Darling / EE-527 / Winter 2013 Directional Dependence of a Knudsen Cell - 4

• The mass evaporation rate through the orifice is

2 1/ 2 d Ne  m    m    P dAedt  2 kBT  • The mass flux into a solid angle of d about an exit angle of  is thus d d 3M  m d 3 N   cos dA dt e e  e t Ae M   dA dt • The total mass of evaporated material is e  e 00 • Cosine law of emission: (Effusion; Lambertian in optics)  d  dM e  M e cos    • Normalized angular distribution  function: 1  f ( , )d d  cos sin d d

R. B. Darling / EE-527 / Winter 2013 Directional Dependence of Condensation

• The area upon which the evaporant is condensed is r 2d dA  c cos • The deposited evaporant mass per unit area of condensation surface is thus:

dM M  dA c e c  2 cos cos  r dAc r



K-cell

R. B. Darling / EE-527 / Winter 2013 Application of the Cosine Law - 1

• The cosine law was verified by Knudsen by depositing a perfectly uniform coating inside a spherical glass jar:  r cos  cos  2ro 

r r o dM M  c e  2  uniform coating! dAc 4 ro

K-cell This geometry is commercially used for coating the inside surfaces of spherical vessels, e.g. light bulbs, as well as for planetary wafer tooling in vacuum coating equipment.

R. B. Darling / EE-527 / Winter 2013 Application of the Cosine Law - 2

• Uniformity of an evaporated film across a wafer:

substrate wafer  dM c M e   2 cos cos r w dAc r  reduction factor for edge thickness: 2 ro re 2 2 dedge ro   ro  1  2 2 cos cos   2 2   2 d r  r r  r 2 center o w  o w    r    1  w    r    o  

Example: For a 3-inch diameter wafer suspended 18 inches above a Knudsen cell: K-cell dedge/dcenter = 0.986, or a non-uniformity of 1.4 %

R. B. Darling / EE-527 / Winter 2013 Application of the Cosine Law – 3 • Computing the required source mass to create a given thickness on the substrate:

– df = thickness of the film [cm]

– ρf = density of the film [g/cm3]

dMce M d ff 2 cos cos dAc r • For normal incidence, φ = ψ = 0,

2 Mrdeff  

3 • Example: To deposit df = 100 nm of Au (ρf = 19.32 g/cm ) at normal incidence r = 40 cm away from the source, a

source charge of Me = 0.971 grams is needed.

R. B. Darling / EE-527 / Winter 2013 Directional Dependence of a Free Point Source

• The evaporated mass from a point source is isotropic: d d d 3M   dA dt dM  M e e 4 e e 4 2 • For a condensation surface of dAc = r d/cos, obtain:

dM c M e  2 cos dAc 4 r • To put a uniform coating inside a spherical surface, a point source must be located at the center of the sphere.

R. B. Darling / EE-527 / Winter 2013 Thermodynamic Potentials - 1

• U = U(S,V,N1,N2,…,Nr) is the internal energy of a system of particles. – S = entropy – V = volume

– N1, N2, … ,Nr are the number of particles in each of r components. • For a monoatomic gas, – Equation of state: PV = NRT – Internal energy is U = 3/2 NRT • U is a measure of the potential for work by the system.

R. B. Darling / EE-527 / Winter 2013 Thermodynamic Potentials - 2

U  T  temperature S V ,N1...Nr U   P  pressure V  S ,N1...Nr U  j  electrochemical potential N j  S ,V ,Ni N j dU  TdS  PdV  1dN1  2dN2

R. B. Darling / EE-527 / Winter 2013 Thermodynamic Potentials - 3

• Helmholtz potential:

– F = F(T,V,N1,N2,…,Nr) = U - TS – potential for work by system at constant temperature • Enthalpy:

– H = H(S,P,N1,N2,…,Nr) = U + PV – potential for work by system at constant pressure – examples: heat engines, turbines, diesel motors • Gibbs free energy:

– G = G(T,P,N1,N2,…,Nr) = U - TS + PV – potential for work by system at constant temperature and pressure – examples: chemical reactions, electrochemistry

R. B. Darling / EE-527 / Winter 2013 Maxwell Relations

V T   S P P S dU  TdS  PdV   k dNk k S V    dF  SdT  PdV   k dNk P T T P k  P S dG  SdT VdP   k dNk  k T V V T dH  TdS VdP   dN T P  k k   k V S S V

R. B. Darling / EE-527 / Winter 2013 Mnemonic Diagram

• Originally due to Max Born:

VTF

UG

S HP

R. B. Darling / EE-527 / Winter 2013 van der Waals Equation of State

NRT aN 2 P   V  bN V 2 – {a,b} are van der Waals parameters. – For large volumes and small particle numbers, van der Waals equation of state is approximated by the ideal gas law: P = NRT/V. P

isotherms

T4 T3 T2 T1 V

R. B. Darling / EE-527 / Winter 2013 Phase Transitions - 1

– A system will only remain homogeneous if certain thermodynamic stability criteria are met. If not, it will separate into two or more portions.

P 100 % condensed phase mixture of condensed and gas phases 100 % gas phase

cg

T = constant isotherm V

R. B. Darling / EE-527 / Winter 2013 Phase Transitions - 2

• Similarly: T

P = constant isobar gas

liquid

solid V

R. B. Darling / EE-527 / Winter 2013 Phase Transitions - 3

• Internal energy = U = U(S,V,N) – dU = TdS - PdV + dN, in general. – No material is created or destroyed during a phase transition: dN = 0.

• U = Ug - Uc = T(Sg - Sc) - P(Vg - Vc)

– U = Lgc - P(Vg - Vc),

–where Lgc = T(Sg - Sc) is the latent heat of the phase transition. •For water: – At 0°C the latent heat of fusion is 80 cal/g = 1440 cal/mole – At 100°C the latent heat of vaporization is 540 cal/g = 9720 cal/mole

R. B. Darling / EE-527 / Winter 2013 Equilibrium Vapor Pressure - 1

• Pressure of a gas in equilibrium with its condensed phase. • Evaporation of the condensed phase converts thermal energy into mechanical energy (expansion of the vapor). • Process is less than 100 % efficient, because some thermal energy must be used to increase the entropy of the system. – (Second Law of Thermodynamics)

R. B. Darling / EE-527 / Winter 2013 Equilibrium Vapor Pressure - 2

P 100 % condensed phase mixture of condensed and gas phases 100 % gas phase

I P* cg II

T = constant isotherm V

Vc Vg

P* is the pressure which makes areas I and II equal.

R. B. Darling / EE-527 / Winter 2013 Equilibrium Vapor Pressure - 3

• Need to integrate along the isotherm to find S = Sg - Sc. • In general, S S dS  dV  dT V T T V • Temperature is constant during the phase change, so: S P dS  dV  dV V T T V – (Using the Maxwell relation)

•(P/T)V is independent of volume during the phase transition, so P dP*  T dT • P* is a function of only T.

R. B. Darling / EE-527 / Winter 2013 Equilibrium Vapor Pressure - 4

• Integrating along the isotherm then produces dP* S S  S   g c dT V Vg Vc

• The latent heat of the phase transition is Lgc = T(Sg - Sc). • Substituting gives the Clapeyron Equation: dP* L  gc dT T (Vg Vc )

• For low vapor pressures, Vg - Vc  Vg  NRT/P, which gives the Clausius-Clapeyron Equation: dP* L dT  gc P* NRT 2

R. B. Darling / EE-527 / Winter 2013 Equilibrium Vapor Pressure - 5

• If Lgc is constant with temperature, integrating the Clausius-Clapeyron equation produces an Arrhenius relationship for the equilibrium vapor pressure: L ln P*   gc  C NRT 1  L  *  gc  P  C2 exp   NRT 

• Lgc/N is tabulated for many substances, but it does have a temperature dependence which can alter the above integration.

R. B. Darling / EE-527 / Winter 2013 Equilibrium Vapor Pressure - 6

• The heat capacity of a material at constant pressure is: H c   a  bT  cT 2  dT 2 p T –The {a,b,c,d} parameters are given in tables of thermochemical data, e.g. the CRC Handbook of Chemistry and Physics. • Integrating over temperature gives the temperature dependence of the latent heat of the phase transition: T d L  L  (c  c ) dT  L  aT  1 bT 2  1 cT 3  gc gco  pg pc gco 2 3 T T0

R. B. Darling / EE-527 / Winter 2013 Equilibrium Vapor Pressure - 7

• Integrating Lgc(T) in the Clausius-Clapeyron equation gives: L a bT cT 2 d ln P*   gco  lnT     C NRT NR 2NR 6NR 2NRT 2 3

– Both Lgco and C3 are unknown integration constants. • Standard thermochemical data is specified at a “standard” state of 1 atmosphere of pressure and a temperature of 25°C or 298.15 K, the triple point of water. – Need to rephrase the Clausius-Clapeyron equation to be able to use this data. – The standard state of temperature and pressure (STP) is usually denoted by a “0” subscript or superscript.

R. B. Darling / EE-527 / Winter 2013 Equilibrium Vapor Pressure - 8

• Standard state chemical potentials for the gas and condensed phases are  and  .  go co • Equilibrium at a given temperature occurs by letting the chemical potentials change until they equal each other at the equilibrium vapor pressure of P*: P* P*   g     (T )    dP  (T )  c dP go    co    1 atm  P T 1 atm  P T • The molar volume is given as shown, and for an ideal gas,

  g  Vg RT      P T N P

• Since Vc is small, the integral for the condensed phase can be ignored. R. B. Darling / EE-527 / Winter 2013 Equilibrium Vapor Pressure - 9

• Integrating gives the standard free energy of evaporation: * Geo (T )  go (T )  co (T )  RT ln P – P* is expressed in atmospheres. – This now gives an expression for P* which eliminates the

unknown integration constants, but Geo is only listed for 298 K. • Use the facts that:

 Geo   Seo  c p G  H TS,    Seo (T),    .  T P  T P T • Integrate twice to obtain: T T c G (T )  H (298K) TS (298K)  p dTdT eo eo eo  298K 298K T

R. B. Darling / EE-527 / Winter 2013 Equilibrium Vapor Pressure - 10

• A working formulation of the equilibrium vapor pressure from standard thermochemical data is therefore: H (298K) S (298K) 1 T T c ln P*   eo  eo   p dTdT RT R RT 298K 298K T – P* is in units of atmospheres.

– Heo(298K), Seo(298K), and cp(T) may be found in standard tables of thermochemical data. – Note: 1 kcal = 1 Cal = 1000 cal = 4186.8 J = 3.97 BTU – See Section D of the CRC Handbook of Chemistry and Physics. – Example: Aluminum:

• Heo(298K) = 70 kcal/mole, Seo(298K) = 30 kcal/mole-K, at the boiling point of P = 1 atm and T = 2327°C = 2600 K.

• Hmo(298K) = 2.57 kcal/mole, at the melting point of P = 1 atm and T = 658.5°C = 931.7 K. R. B. Darling / EE-527 / Winter 2013 Equilibrium Vapor Pressure - 11

1.00E+03 1.00E+02 1.00E+01 1.00E+00 Aluminum 1.00E-01 0 1000 2000 3000 4000 5000 Gallium 1.00E-02 Indium 1.00E-03 Silicon 1.00E-04 Gold 1.00E-05 Platinum 1.00E-06 Chromium

Vapor Pressure, Torr 1.00E-07 Titanium 1.00E-08 1.00E-09 1.00E-10 1.00E-11 Temperature, K

R. B. Darling / EE-527 / Winter 2013 Equilibrium Vapor Pressure - 12

1.00E+03 1.00E+02 1.00E+01 1.00E+00 Aluminum 1.00E-01 0 5 10 15 20 Gallium 1.00E-02 Indium 1.00E-03 Silicon 1.00E-04 Gold 1.00E-05 Platinum 1.00E-06 Chromium

Vapor Pressure, Torr Pressure, Vapor 1.00E-07 Titanium 1.00E-08 1.00E-09 1.00E-10 1.00E-11 Reciprocal Temperature, 10000/T(K)

R. B. Darling / EE-527 / Winter 2013 Evaporation System Requirements

• Vacuum: – Need 10-6 to 10-8 torr for medium quality films. – Ultra-high quality films can be prepared in UHV at 10-9 to 10-11 torr. • Cooling water: – Hearth – Thickness monitor – Bell jar • Mechanical shutter: – Evaporation rate is set by temperature of source, but this cannot be turned on and off rapidly. A mechanical shutter allows evaporant flux to be rapidly modulated. • Electrical power: – Either high current or high voltage, typically 1-10 kW. R. B. Darling / EE-527 / Winter 2013 Evaporation Support Materials

• Refractory metals: – Tungsten (W); MP = 3380°C, P* = 10-2 torr at 3230°C – Tantalum (Ta); MP = 3000°C, P* = 10-2 torr at 3060°C – Molybdenum (Mo); MP = 2620°C, P* = 10-2 torr at 2530°C • Refractory ceramics: – Graphitic Carbon (C); MP = 3700°C, P* = 10-2 torr at 2600°C -2 –Alumina (Al2O3); MP = 2030°C, P* = 10 torr at 1900°C – Boron nitride (BN); MP = 2500°C, P* = 10-2 torr at 1600°C • Engineering considerations: – Thermal conductivity – Thermal expansion – Electrical conductivity – Wettability and reactivity R. B. Darling / EE-527 / Winter 2013 Resistance Heated Evaporation

• Simple, robust, and in widespread use. • Can only achieve temperatures of about 1800°C. • Use W, Ta, or Mo filaments to heat evaporation source. • Typical filament currents are 200-300 Amperes. • Exposes substrates to visible and IR radiation. • Typical deposition rates are 1-20 Angstroms/second. • Common evaporant materials: – Au, Ag, Al, Sn, Cr, Cu, Sb, Ge, In, Mg, Ga

– CdS, PbS, CdSe, NaCl, KCl, AgCl, MgF2, CaF2, PbCl2

R. B. Darling / EE-527 / Winter 2013 Resistance Heated Evaporation Sources

foil dimple boat

wire hairpin

alumina coated foil dimple boat

wire helix foil trough

chromium coated tungsten rod wire basket

alumina crucible with wire basket alumina crucible in tantalum box

R. B. Darling / EE-527 / Winter 2013 Characteristics of Specific Resistance Evaporator Sources • Principal considerations: -2 – Required temperature to achieve P* ~ 10 torr, TP*-2. • About 1800C is normally maximum for most resistance heated systems. – Wetting of support material – Alloying with support material – Sublimation versus evaporation from a liquid state

• Melting point temperature, Tmp versus TP*-2. – Decomposition of non-elemental sources – Expansion or contraction upon freezing – Tendency to splatter – Reactivity with oxygen – Toxicity – Effects of chamber wall deposits R. B. Darling / EE-527 / Winter 2013 Aluminum (Al)

• Most commonly used for silicon IC interconnects.

•Tmp = 660C; TP*-2 = 1220C. • Wets most support materials and crawls out of its containers. – Much of the initial charge can go only towards wetting the container. – Wire filaments can become shorted out by Al crawling along their length. • BN crucibles with a W wire heater on the outside work well. • e-Beam evaporation is very effective, but can cause splattering at high rates.

R. B. Darling / EE-527 / Winter 2013 Gold (Au)

• Most commonly used for compound semiconductor interconnects.

•Tmp = 1063C; TP*-2 = 1400C. • Wets W and Mo supports. • Alloys with Ta boats, usually resulting in their breakage. • Usually a well-behaved evaporant. • Capable of creating very thin films with high uniformity. • Substrate heating often used to create single crystal gold films on mica substrates for AFM use.

R. B. Darling / EE-527 / Winter 2013 Silver (Ag)

• Mostly used for chemical sensor electrodes; too reactive for most interconnect purposes.

•Tmp = 961C; TP*-2 = 1030C. • Wets Ta and Mo supports, but not W. • Generally a well-behaved evaporant.

R. B. Darling / EE-527 / Winter 2013 Tin (Sn)

• Assorted uses in chemical sensors; less commonly used as an interconnect metal because of its low melting point.

•Tmp = 232C; TP*-2 = 1250C. • Alloys with and destroys Mo supports. • Works well with Ta and W supports.

• Works well in Al2O3 lined boats and crucibles. • Generally a well-behaved evaporant.

R. B. Darling / EE-527 / Winter 2013 Chromium (Cr)

• Most commonly used as an adhesion film; too resistive for most interconnects.

•Tmp = 1900C; TP*-2 = 1400C; sublimes. • Evaporation from Cr-coated W rods is usually the easiest, but the amount that can be deposited from one rod is limited. • Can be used in Ta or W boats, but thermal contact to boat from Cr shot or pellets may be a problem because the charge will not melt.

R. B. Darling / EE-527 / Winter 2013 Copper (Cu)

• Creates high conductivity films; useful for interconnects where it will not interfere with the semiconductor.

•Tmp = 1084C; TP*-2 = 1260C. • Works well with Ta, W, and Mo support materials.

• Works well in graphite, Mo, and Al2O3 crucibles. • Generally, a well-behaved evaporant.

R. B. Darling / EE-527 / Winter 2013 Electron Beam Heated Evaporation - 1

• More complex, but extremely versatile. • Can achieve temperatures in excess of 3000°C. • Uses evaporation cones or crucibles in a copper hearth. • Typical acceleration voltage is 8-10 kV. • Exposes substrates to secondary electron radiation. – X-rays can also be generated by high voltage electron beam. • Typical deposition rates are 10-100 Angstroms/second. • Common evaporant materials: – Everything a resistance heated evaporator will accommodate, plus: – Ni, Pt, Ir, Rh, Ti, V, Zr, W, Ta, Mo (all noble and refractory metals)

–Al2O3, SiO, SiO2, SnO2, TiO2, ZrO2 (most metal oxides)

R. B. Darling / EE-527 / Winter 2013 Electron Beam Heated Evaporation Source

270 degree bent electron beam

magnetic field evaporation cones pyrolytic graphite of material hearth liner

4-pocket rotary copper hearth (0 V) beam forming cathode aperture filament recirculating (-10,000 V) cooling water

R. B. Darling / EE-527 / Winter 2013 Electron Beam Heated Evaporation - 2

• 270° bent beam electron gun is most preferred: – Filament is out of direct exposure from evaporant flux. – Magnetic field can be used for beam focusing. – Magnetic field can be used for beam positioning. – Additional lateral magnetic field can be used produce X-Y sweep. • Sweeping or rastering of the evaporant source is useful for: – Allows a larger evaporant surface area for higher deposition rates. – Allows initial charge to be “soaked” or preheated. – Allows evaporant source to be more fully utilized. • Multiple pocket rotary hearth is also preferred: – Allows sequential deposition of layers with a single pump-down. – Allows larger evaporation sources to be used.

R. B. Darling / EE-527 / Winter 2013 Characteristics of Specific e-Beam Evaporator Sources • Principal considerations: – Thermal conductivity – Sublimation versus evaporation from a liquid state – Decomposition of non-elemental sources – Expansion or contraction upon freezing – Soft x-ray emission threshold – Tendency to splatter – Reactivity with oxygen – Reactivity with copper, i.e. the hearth – Toxicity – Effects of chamber wall deposits – Need for a hearth liner, e.g. C or BN – Magnetic elements (Co, Ni, Mn) can create some problems.

R. B. Darling / EE-527 / Winter 2013 Titanium (Ti)

• Useful as an interconnect metal and as an adhesion film.

•Tmp = 1700C; TP*-2 = 1750C; usually sublimes. • Evaporation cones that directly drop into the hearth pockets are most common. • Ti coated surfaces are extremely reactive! – Surfaces become a getter pump for the chamber – chamber pressure drops during evaporation due to this! – Surfaces are extremely reactive with oxygen – chamber must be

backfilled with dry N2 to avoid combustion until fresh surfaces are cool.

R. B. Darling / EE-527 / Winter 2013 Tungsten (W)

• Useful as an interconnect metal; has the useful property of precisely matched thermal expansion to that of silicon.

•Tmp = 3380C; TP*-2 = 3230C; usually sublimes. • Usually requires a lot of heat, but easily accomplished in an e-beam evaporator.

R. B. Darling / EE-527 / Winter 2013 Silicon (Si)

• Amorphous silicon has several applications for both devices and sensors. Polycrystalline silicon can be produced with additional substrate heating and annealing, dependent upon the substrate.

•Tmp = 1420C; TP*-2 = 1350C; usually sublimes, but puddling is also common. • Most commonly done using a graphite crucible. • Si expands upon freezing and can crack crucibles. • Si sources tend to spatter unless evaporation rate is kept low.

R. B. Darling / EE-527 / Winter 2013 Silicon Dioxide (SiO2)

• One simple method to deposit glass insulating layers, but not to any great thickness.

•Tmp = 1730C; TP*-2 = 1250C. • Reacts with and destroys Ta, W, and Mo supports. • Usually used in a graphite hearth liner for e-beam evaporation.

• Usually decomposes into SiO and O2 unless evaporated at a significant rate.

R. B. Darling / EE-527 / Winter 2013 High Throughput Evaporation Techniques

• Box coaters are used for evaporating large substrate materials, sometimes up to several meters in size. • Large amounts of source material are required, but cannot be all heated at once because of realistic power limitations. • Two popular techniques: – Powder trickler source – Wire feed source • Both can be adapted for either resistance heated or electron beam heated evaporation systems.

R. B. Darling / EE-527 / Winter 2013 Problematic Source Materials

• Arsenic (As) – highly toxic, spoils vacuum chamber. • Cadmium (Cd) – highly toxic, spoils vacuum chamber. • Selenium (Se) – highly toxic, spoils vacuum chamber. • Tellurium (Te) – highly toxic, spoils vacuum chamber. • Zinc (Zn) – wall deposits spoil vacuum chamber. • Bismuth (Bi) – toxic. • Lead (Pb) – toxic. • Magnesium (Mg) – can be done, but extremely reactive and prone to combustion upon exposure to air.

R. B. Darling / EE-527 / Winter 2013 Adsorption

• Adsorption is the sticking of a particle to a surface. – Absorption (note the spelling difference!) is the collection of a external substance within the interior of a media. • Physisorption: – The impinging molecule loses kinetic (thermal) energy within some residence time, and the lower energy of the molecule does not allow it to overcome the threshold that is needed to escape. • Chemisorption: – The impinging molecule loses its kinetic energy to a chemical reaction which forms a chemical bond between it and other substrate atoms.

R. B. Darling / EE-527 / Winter 2013 Condensation of Evaporant - 1

• Condensation of a vapor to a solid or liquid occurs when the partial pressure of the vapor exceeds the equilibrium vapor pressure of the condensed phase at this temperature.

–Pvapor > P*(Tsubstrate) • The vapor is “supersaturated” under these conditions. • This is only true if condensation takes place onto a material which is of the same composition as the vapor. • When a material is first deposited onto a substrate of a different composition, a third adsorbed phase must be included to describe the process.

R. B. Darling / EE-527 / Winter 2013 Condensation of Evaporant - 2

• Molecules impinging upon a surface may: – Adsorb and permanently stick where they land (rare!). – Adsorb and permanently stick after diffusing around on the surface to find an appropriate site. • This can lead to physisorption or chemisorption.

– Adsorb and then desorb after some residence time a. – Immediately reflect off of the surface. • Incident vapor molecules normally have a kinetic energy

much higher than kBT of the substrate surface. • Whether an atom or molecule will stick depends upon how well it can equilibrate with the substrate surface, decreasing its energy to the point where it will not subsequently desorb. R. B. Darling / EE-527 / Winter 2013 Condensation of Evaporant - 3

• Thermal accommodation coefficient:

Ev  Er Tv Tr T   Ev  Es Tv Ts

– Ev, Tv = energy, temperature of impinging vapor molecules.

– Er, Tr = energy, temperature of resident vapor molecules; • (Those which have adsorbed, but have not permanently found a site.)

– Es, Ts = energy, temperature of substrate surface.

• If T < 1, (Er > Es), then some fraction of the impinging molecules will desorb from the surface.

R. B. Darling / EE-527 / Winter 2013 Condensation of Evaporant - 4

• Mean residence time for an adsorbed molecule:  1  G   des  a  exp  0  k BT  14 – 0 = kBT/h = vibrational frequency of adsorbed molecule (~10 Hz) • This is the frequency at which the molecule “attempts” to desorb.

– Gdes = free activation energy for desorption. • Under a constant impinging vapor flux of R, the surface density of the deposit is then:  R  G   des  ns  R a  exp  0  k BT  – R = deposition rate in molecules/cm2-sec. -2 – ns = surface density of deposited molecules in cm . R. B. Darling / EE-527 / Winter 2013 Condensation of Evaporant - 5

• If the impingement rate stops, then the adsorbed molecules will all eventually desorb. • Condensation of a permanent deposit will not occur, even for low substrate temperatures, unless the molecules interact. • Within the mean residence time, surface migration occurs and clusters form. • Clusters have smaller surface-to-volume ratios, and therefore desorb at a reduced rate. • Nucleation of a permanent deposit is therefore dependent upon clustering of the adsorbed molecules.

R. B. Darling / EE-527 / Winter 2013 Observed Growth Features of a Deposited Film

• Adsorbed monomers • Subcritical embryos of various sizes • Formation of critically sized nuclei • Growth of nuclei to supercritical size and depletion of monomers within their capture zones • Nucleation of critical clusters within non-depleted areas • Clusters touch and coalesce into new islands, exposing fresh substrate areas • Adsorption of monomers onto fresh areas • Larger islands grow together leaving holes and channels • Channels and holes fill to form a continuous film

R. B. Darling / EE-527 / Winter 2013 Stages of Thin Film Growth

• Island Stage • Coalescence Stage • Channel Stage • Continuous Film Stage

R. B. Darling / EE-527 / Winter 2013 Modes of Thin Film Growth

(1) Volmer-Weber: (island growth):

M. Volmer and A. Weber, Z. Phys. Chem. 119, p. 277 (1926).

(2) Frank-Van der Merwe: (layer growth; ideal epitaxy):

F. C. Frank and J. H. Van der Merwe, Proc. R. Soc. London, Ser. A 198, p. 205 (1949).

(3) Stranski-Krastanov: (layers + islands):

J. N. Stranski and L. Krastanov, Ber. Akad. Wiss. Wien 146, p. 797 (1938).

R. B. Darling / EE-527 / Winter 2013 Condensation Control • Condensation control of the evaporant is achieved through

control of the substrate temperature Ts. • Higher substrate temperatures: – Increase thermal energy of adsorbed molecules. • (Shortens the residence time.) – Increase surface diffusivity of adsorbed molecules. – Performs annealing of deposited film. • Substrate heaters: – Quartz IR lamps from frontside or backside – Ta, W, or Mo foil heaters from backside – Graphite impregnated cloth heaters from backside • Too much heat will desorb the deposited film, evaporating it away! (But this can be used for cleaning…)

R. B. Darling / EE-527 / Winter 2013 Deposition End Point Control

• Evaporate the source material to completion – Compute the required mass of source material to provide the desired coating thickness using Lambertian source characteristics. – Only works for sources that do not wet their supporting materials. • Quartz crystal thickness monitors (QCMs) – Based on a quartz crystal oscillator where the quartz crystal is exposed to the evaporant flux. – An increase in mass on the surface of the crystal will shift the oscillation frequency in a predictable manner, from which the deposited thickness can be determined from the change in the oscillation frequency. • Optical interference, absorption, or reflectivity – A more sensitive technique better suited for optical coatings.

R. B. Darling / EE-527 / Winter 2013 Quartz Crystal Microbalances (QCMs) - 1

• Quartz is piezoelectric; application of a voltage will create an elastic deformation of the crystal; a sinusoidal voltage will create a transverse acoustic wave. • The acoustic wave will set up a resonance when the thickness of the crystal is half of the acoustic wavelength. • This will create an electrical series resonant equivalent circuit which can be used to pull an oscillator circuit onto this frequency. • Adding mass to the surface of the quartz crystal is equivalent to increasing the effective thickness of the crystal, which will change the resonant frequency.

R. B. Darling / EE-527 / Winter 2013 Quartz Crystal Microbalances (QCMs) - 2

• Resonance condition: Gq 2 f0tq  va  q

– fo = resonant frequency, usually 5 or 6 MHz.

– tq = thickness of quartz crystal.

– va = acoustic velocity for a shear wave = 3336 m/s. 11 2 – Gq = shear modulus of quartz = 2.947 x 10 g/cm-sec . 2 – q = density of quartz = 2.648 g/cm .

–For fo = 6 MHz, tq = 275 m. 2 f 2 m • Sauerbrey equation: f   0 Zq A 1/2 5 2 – Zq = (qGq) = acoustic impedance of quartz = 8.834 x 10 g/cm -sec. – m = deposited mass. – A = piezoelectric active area of quartz (electrode area). – Valid in the limit of small added mass (thin deposits). R. B. Darling / EE-527 / Winter 2013 Quartz Crystal Microbalances (QCMs) - 3 • The deposited film thickness can be determined from the added mass: m t f  Af  f – Af = area of deposited film. • For thicker films, acoustic reflections between the quartz and the deposited film must be taken into account:    m Zquartz 1  fU  f L   tan Z tan  A 2 Zf L   fU  – “Z-match method”; reduces to the Sauerbrey eqn. for Z = 1.

– fU = unloaded (upper) frequency; fL = loaded (lower) frequency. – “Z-ratio” = acoustic impedance ratio of quartz to that of the film: Z  G Z  quartz  q q Z  G film f f R. B. Darling / EE-527 / Winter 2013 Quartz Crystal Microbalances (QCMs) - 4 • Temperature effects: – Quartz crystal oscillators are also sensitive to temperature. – Temperature dependence is minimized by using AT-cut quartz. – Water cooling of the QCM oscillator head is also used to stabilize T. • Chamber geometry effects: – Deposited thickness on QCM is usually not the same as that deposited onto the substrates. – A multiplicative tooling factor (TF) is used to correct for this. – The TF depends purely upon the relative location of the QCM to that of the wafers; once calibrated, it does not change. • Material data:

– The density (f) and Z-ratio (Z) are both needed to accurately calibrate the QCM for a given deposited film. – Usually tabulated for most common films. R. B. Darling / EE-527 / Winter 2013 Quartz Crystal Microbalances (QCMs) - 5

Material Density (g/cm3)Z-Ratio Aluminum (Al) 2.70 1.080 Chromium (Cr) 7.20 0.305 Copper (Cu) 8.93 0.437 Germanium (Ge) 5.35 0.516 Gold (Au) 19.30 0.381 Nickel (Ni) 8.91 0.331 Platinum (Pt) 21.40 0.245 Silicon (Si) 2.32 0.712 Silver (Ag) 10.50 0.529 Tin (Sn) 7.30 0.724 Titanium (Ti) 4.50 0.628

R. B. Darling / EE-527 / Winter 2013