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Chapter 9 Thin film deposition

1. Introduction to thin film deposition.
2. Introduction to chemical vapor deposition (CVD).
3. Atmospheric Pressure Chemical Vapor Deposition (APCVD).
4. Other types of CVD (LPCVD, PECVD, HDPCVD…).
5. Introduction to evaporation.
6. Evaporation tools and issues, shadow evaporation.
7. Introduction to sputtering and DC plasma.
8. Sputtering yield, step coverage, film morphology.
9. Sputter deposition: reactive, RF, bias, magnetron, collimated, and ion beam.
10. Deposition methods for thin films in IC fabrication.
11. Atomic layer deposition (ALD).
12. Pulsed laser deposition (PLD).
13. Epitaxy (CVD or vapor phase epitaxy , molecular beam epitaxy).

NE 343: Microfabrication and thin film technology

Instructor: Bo Cui, ECE, University of Waterloo; http://ece.uwaterloo.ca/~bcui/

Textbook: Silicon VLSI Technology by Plummer, Deal and Griffin

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Thin film: thickness typically <1000nm.

Special properties of thin films: different from bulk materials, it may be –

• Not fully dense
• Under stress
• Different defect structures from bulk
• Quasi ‐ two dimensional (very thin films)
• Strongly influenced by surface and interface effects

Typical steps in making thin films:

1. Emission of particles from source (heat, high voltage . . .)
2. Transport of particles to substrate
3. Condensation of particles on substrate

Thin film

Lithography, thin film deposition and its etching are the three most important processes for micro-nano fabrication.

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Two main deposition methods are used today:

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Chemical Vapor Deposition (CVD)

Reactant gases introduced in the chamber, chemical reactions occur on wafer surface leading to the deposition of a solid film.

E.g. APCVD, LPCVD, PECVD, most commonly used for dielectrics and Si.

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Physical Vapor Deposition (PVD) (no chemical reaction involved)

Vapors of constituent materials created inside the chamber, and condensation occurs on wafer surface leading to the deposition of a solid film.

E.g. evaporation, sputter deposition, most commonly used for metals.

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Other methods that are increasingly gaining importance in ULSI fabrication:

1. Coating with a liquid that becomes solid upon heating, e.g. spin-on-glass used for planarization.
2. Electro-deposition: coating from a solution that contains ions of the species to be coated. E.g. Cu electroplating for global interconnects.
3. Thermal oxidation.

Thin film deposition methods

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• Deposition rate
• Film uniformity:
• Across wafer uniformity.
• Run-to-run uniformity.
• Materials that can be deposited: metal, dielectric, polymer.
• Quality of film:
• Physical and chemical properties
• Electrical property, breakdown voltage
• Mechanical properties, stress and adhesion to substrate
• Optical properties, transparency, refractive index
• Composition, stoichiometry
• Film density, defect (pinhole…) density
• Texture, grain size, boundary property, and orientation
• Impurity level, doping
• Deposition directionality:
• Directional - good for lift-off, trench filling
• Non-directional - good for step coverage
• Cost of ownership and operation.

General characteristics of thin film deposition

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Step coverage

Figure 9-1 Step coverage of metal over non-planar topography.

1. Conformal step coverage, with constant thickness on horizontal and vertical surfaces.
2. Poor step coverage, here thinner for vertical surfaces.

Poor (non-conformal) step coverage is good for liftoff.

Conformal film is good for electrical connection…

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Thin film filling of holes/trenches

Figure 9-2 Thin film filling issues.

1. Good metal filling of a via or contact in a dielectric layer.
2. Silicon dioxide filling of the space between metal lines, with poor filling leading to void formation.
3. Poor filling of the bottom of a via hole with a barrier or contact meal.

Aspect ratio (AR):

AR = feature height/width AR = h/w

Voids in a chemical vapor deposition (CVD) oxide layer for narrow spaces between metal lines.

More difficult to fill without void for higher aspect ratio.

Figure 9-3

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Four equilibrium growth/deposition modes

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• Layer by layer growth (Frank ‐ van der Merwe): film atoms more strongly bound to substrate than to each other and/or fast diffusion
• Island growth (Volmer ‐ Weber): film atoms more strongly bound to each other than to substrate and/or slow diffusion.
• Mixed growth (Stranski ‐ Krastanov): initially layer by layer then forms three dimensional islands.

Thin film types based on crystallinity:

• Epitaxial (single‐crystalline, formed layer-by-layer, lattice match to substrate): no grain boundaries, requires high temperatures and slow growth rate. High quality thin films such as III‐V semiconductor films (e.g. GaAs) and complex oxides.
• Polycrystalline (island or mixed growth): lots of grain boundaries, e.g. most elemental metals grown near room temperatures.
• Amorphous (island or mixed growth): no‐crystalline structures (yet with some short range atomic ordering), no crystalline defects, e.g. common insulators such as amorphous SiO₂.

Four growth modes

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• Noble metals don’t bond (“wet”) to Si/SiO₂ substrate, so tend to have island growth.
• Ag always form island (not continuous film); Au is better than Ag.
• Adhesion layer Ti or Cr can reduce island formation, but for Ag, surface is still very rough.
• Here, higher adhesion is because Ti or Cr bond chemically to O in SiO₂.

Equilibrium growth modes

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Effect of substrate temperature on the lateral grain size

100 Å thick Au films deposited at 100, 200, and 300℃ by vacuum evaporation

The small islands start coalescing with each other in an attempt to reduce the surface area.

This tendency to form bigger islands is termed agglomeration and is enhanced by increasing the surface mobility of the adsorbed species, such as by increasing the substrate temperature.

Except under special conditions, the crystallographic orientation and the topographical details of different islands are randomly distributed.

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Dependence on substrate temperature and deposition rate

Cu films deposited on (111) NaCl substrate.

In this case, if equilibrium is achieved for all ad-atoms, film will be mono-crystal (epitaxy).

Higher temperature increases ad-atom’s surface mobility. It will stop once it finds the lowest energy position nearby.

But too fast deposition stops the movement (before the ad-atom finds the lowest energy position nearby) when that ad-atom is covered by a later arrival ad-atom.

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Chapter 9 Thin film deposition

NE 343: Microfabrication and Thin Film Technology

Instructor: Bo Cui, ECE, University of Waterloo, bcui@uwaterloo.ca

Textbook: Silicon VLSI Technology by Plummer, Deal, Griffin

1. Introduction to thin film deposition.
2. Introduction to chemical vapor deposition (CVD).
3. Atmospheric Pressure Chemical Vapor Deposition (APCVD).
4. Other types of CVD (LPCVD, PECVD, HDPCVD…).
5. Introduction to evaporation.
6. Evaporation tools and issues, shadow evaporation.
7. Introduction to sputtering and DC plasma.
8. Sputtering yield, step coverage, film morphology.
9. Sputter deposition: reactive, RF, bias, magnetron, collimated, and ion beam.

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CVD steps:

• Introduce reactive gases to the chamber.
• Activate gases (decomposition) by heat or plasma.
• Gas absorption by substrate surface .
• Reaction take place on substrate surface, film firmed.
• Transport of volatile byproducts away form substrate.
• Exhaust waste.

Chemical Vapor Deposition (CVD)

CVD : deposit film through chemical reaction and surface absorption.

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Chemical vapor deposition (CVD) systems

Atmospheric cold-wall system used for deposition of epitaxial silicon.

(SiCl₄ + 2H₂ → Si + 4HCl)

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Low pressure hot-wall system used for deposition of polycrystalline and amorphous films, such as poly-silicon and silicon dioxide.

Figure 9-4

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CVD advantages and disadvantages

(as compared to physical vapor deposition)

Advantages:

• High growth rates possible, good reproducibility.
• Can deposit materials which are hard to evaporate.
• Can grow epitaxial films. In this case also termed as “vapor phase epitaxy (VPE)”. For instance, MOCVD (metal-organic CVD) is also called OMVPE (organo-metallic VPE).
• Generally better film quality, more conformal step coverage (see image below).

Disadvantages:

• High process temperatures.
• Complex processes, toxic and corrosive gasses.
• Film may not be pure (hydrogen incorporation…).

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Types of CVD reactions

• Thermal decomposition

AB(g) ---> A(s) + B(g)

Si deposition from Silane at 650^oC: SiH₄(g) → Si(s) + 2H₂(g)

Ni(CO)₄(g) → Ni(s) + 4CO(g) (180^oC)

• Reduction (using H₂)

AX(g) + H₂(g) → A(s) + HX(g)

W deposition at 300^oC: WF₆(g) + 3H₂(g) → W(s) + 6HF(g)

SiCl₄(g) + 2H₂(g) → Si(s) + 4HCl (1200^oC)

• Oxidation (using O₂)

AX(g) + O₂(g) → AO(s) + [O]X(g)

SiO₂ deposition from silane and oxygen at 450^oC (lower temp than thermal oxidation): SiH₄(g) + O₂(g) ---> SiO₂(s) + 2H₂(g)

2AlCl₃(g) + 3H₂(g) + 3CO₂(g) → Al₂O₃ + 3CO + 6HCl (1000^oC)

(O is more electronegative than Cl)

• Compound formation (using NH₃ or H₂O)

AX(g) +NH₃(g) → AN(s) + HX(g) or AX(g) + H₂O(g ) → AO(s) + HX(g)

Deposit wear resistant film (BN) at 1100^oC: BF₃(g) + NH₃(g) → BN(s) + 3HF(g)

(CH₃)₃Ga(g) + AsH₃(g) → GaAs(s) + 3CH₄ (650 – 750^oC)

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Chemical reactions for silicon epitaxial growth

Except SiH₄ decomposition, ALL other reactions are reversible.

Which direction (etching of Si or growth of Si) to go depends on the partial pressures of the reactants and temperature.

(HCl etches Si at high T, which is used to prepare electronic grade Si)

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Thermal (not plasma-enhanced) CVD films

(Al₂O₃)

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CVD sources and substrates

• Types of sources
• Gasses (easiest)
• Volatile liquids
• Sublimable solids
• Combination
• Source materials should be
• Stable at room temperature
• Sufficiently volatile
• High enough partial pressure to get good growth rates
• Reaction temperature < melting point of substrate
• Produce desired element on substrate with easily removable by-products
• Low toxicity
• Substrates
• Need to consider adsorption and surface reactions
• For example, WF₆ deposits on Si but not on SiO₂

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Types of CVD

APCVD (Atmospheric Pressure CVD), mass transport limited growth rate, leading to non-uniform film thickness.

LPCVD (Low Pressure CVD)

• Low deposition rate limited by surface reaction, so uniform film thickness (many wafers stacked vertically facing each other; in APCVD, wafers have to be laid horizontally side by side.
• Gas pressures around 1-1000mTorr (lower P => higher diffusivity of gas to substrate)
• Better film uniformity & step coverage and fewer defects
• Process temperature ≥500°C

PECVD (Plasma Enhanced CVD)

• Plasma helps to break up gas molecules: high reactivity, able to process at lower temperature and lower pressure (good for electronics on plastics).
• Pressure higher than in sputter deposition: more collision in gas phase, less ion bombardment on substrate
• Can run in RF plasma mode: avoid charge buildup for insulators
• Film quality is poorer than LPCVD.
• Process temperature around 100 - 400°C.

MOCVD (Metal-organic CVD, also called OMVPE - organo metallic VPE), epitaxial growth for many optoelectronic devices with III-V compounds for solar cells, lasers, LEDs, photo-cathodes and quantum wells.

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Types of CVD

For R&D, PECVD is most popular, followed by LPCVD.

(can be higher)

and epitaxy Si…

(can have high deposition rate)

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Chapter 9 Thin film deposition

NE 343: Microfabrication and Thin Film Technology

Instructor: Bo Cui, ECE, University of Waterloo, bcui@uwaterloo.ca

Textbook: Silicon VLSI Technology by Plummer, Deal, Griffin

1. Introduction to thin film deposition.
2. Introduction to chemical vapor deposition (CVD).
3. Atmospheric Pressure Chemical Vapor Deposition (APCVD).
4. Other types of CVD (LPCVD, PECVD, HDPCVD…).
5. Introduction to evaporation.
6. Evaporation tools and issues, shadow evaporation.
7. Introduction to sputtering and DC plasma.
8. Sputtering yield, step coverage, film morphology.
9. Sputter deposition: reactive, RF, bias, magnetron, collimated, and ion beam.

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Steps involved in a CVD process

1. Transport of reactants to the deposition region.
2. Transport of reactants from the main gas stream through the boundary layer to the wafer surface.
3. Adsorption of reactants on the wafer surface.
4. Surface reactions, including: chemical decomposition or reaction, surface migration to attachment sites (kinks and ledges); site incorporation; and other surface reactions (emission and redeposition for example).
5. Desorption of byproducts.
6. Transport of byproducts through boundary layer.
7. Transport of byproducts away from the deposition region.

Steps 2-5 are most important for growth rate.

Steps 3-5 are closely related and can be grouped together as “surface reaction” processes.

Reaction rate may be limited by:

• Gas transport to/from surface.
• Surface chemical reaction rate that depends strongly on temperature.

Figure 9-5

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F₁ = diffusion flux of reactant species to the wafer through the boundary layer (step 2) = mass transfer flux

(1)

F₂ = flux of reactant consumed by the surface reaction (steps 3-5) = surface reaction flux,

where h_G is the mass transfer coefficient (in cm/sec).

(2)

where k_S is the surface reaction rate (in cm/sec).

In steady state: F = F₁ = F₂

(3)

Equating Equations (1) and (2) leads to

(4)

The growth rate of the film is now given by

(5)

where N is the number of atoms per unit volume in the film and Y is the mole fraction (partial pressure/total pressure) of the incorporating species, C_T is total concentration of all molecules in the gas phase .

Derivation of film growth rate

(similar to/simpler than Deal-Grove model for thermal oxidation)

Figure 9-6

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(a). If k_S << h_G, then we have the

surface reaction controlled case:

(6)

(b) If h_G << k_S, then we have the mass transfer,

or gas phase diffusion, controlled case:

(7)

• k_s increases with temperature.

(Arrhenius with E_A depending on the particular reaction, e.g. 1.6 eV for single crystal silicon deposition).

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• h_G ≈ constant

(diffusion through boundary layer is insensitive to temperature)

(5)

Derivation of film growth rate (continued)

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CVD film growth rate

Actually h_G is not constant (depends on T)

Figure 9-8 Growth or deposition rate for silicon by APCVD. The partial pressure of the reactant gas is 0.8Torr (1atm=760Torr!!).

H₂ is used as the carrier or diluent gas for the solid curves.

For SiH₄, using N₂ carrier gas increases the growth rate, because the carrier gas H₂ is a reaction product of SiH₄ decomposition, thus slowing down the reaction.

Deposition rate vs. gas glow rate

Figure 9-8

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Chemical Vapor Deposition (CVD) growth rate

• k_S limited deposition is VERY temperature sensitive.
• h_G limited deposition is VERY geometry (boundary layer) sensitive.
• Si epitaxial deposition is often done at high T to get high quality single crystal growth. It is then h_G controlled, and horizontal reactor configuration is needed for uniform film thickness across the wafer.
• When a high film quality is less critical (e.g. SiO₂ for inter-connect dielectric), deposition is done in reaction rate controlled regime (lower temperature). Then one can greatly increase the throughput by stacking wafers vertically (for research, usually 25 wafers per run; 100-200 for industry).

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Gas moves with the constant velocity U.

Boundary layer (caused by friction ) increases along the susceptor, so mass transfer coefficient h_G decreases.

Source gas also depletes (consumed by chemical reaction) along the reactor.

Both decrease growth rate along the chamber.

To compensate for this, one can:

• Use tilted susceptor.
• Use temperature gradient 5-25°C.
• Gas injectors along the tube.
• Use moving belt.

Other factors affecting growth rate: thickness of boundary layer and source gas depletion

D_G: diffusivity

μ: gas viscosity

ρ: gas density

U: flow velocity

X: gas flow direction

I don’t understand why not operates in the zone where δ→tube radius (independent of x).

Should be ∂C/∂y, since diffuse along y-direction

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Mass transport in gas

• Two flow Regimes
• Molecular flow (diffusion in gas, particle transfer).
• Viscous flow (laminar & turbulent flow, moment transfer).
• Laminar flow is desired.
• In CVD growth rate model, it was assumed that mass transport across the stagnant layer proceeds by diffusion.

Mass transport depends on:

δ→ tube radius when x is large

Transport of reactants:

• Flow along x-direction.
• Diffusion along y-direction. (anyway, no flow along y-direction)

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Doping in CVD films

• Doping is usually done for epitaxial (thus single crystal) film during film growth.
• Dopant will be grown directly onto crystalline site (no need of dopant activation).
• Doping is realized by adding gas containing the dopant. Such as PH₃, B₂H₆, AsH₃ (all gas phase at room temperature); or PCl₃, BCl₃, AsCl₃ (all liquid at RT).
• They will go through: dissociation, lattice site incorporation, and burying of dopants by other atoms in the film.
• The dopant concentration C: (P is partial pressure of he dopant species, and v growth rate)

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• However, there is also unintentional doping process:
• Out-diffusion of dopant from heavily doped substrate into the epi-layer.
• Auto-doping – dopant from substrate diffuses into gas stream first, then back into epi-layer.

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Auto-doping and out-diffusion in CVD film growth

Figure 9-11 Auto-doping processes in an epitaxial reactor. Illustrated are evaporation from 1) the wafer front side; 2) the wafer backside or edges; 3) other wafers; and 4) the susceptor.

Out-diffusion:

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Auto-doping:

That is, the growth is faster than diffusion after certain time t.

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Auto-doping and out-diffusion in CVD film growth

Figure 9-12 Dopant profile in a Si epi-layer due to out-diffusion and auto-doping.

Here substrate is more heavily doped than the epitaxial layer.

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Chapter 9 Thin film deposition

NE 343: Microfabrication and Thin Film Technology

Instructor: Bo Cui, ECE, University of Waterloo, bcui@uwaterloo.ca

Textbook: Silicon VLSI Technology by Plummer, Deal, Griffin

1. Introduction to thin film deposition.
2. Introduction to chemical vapor deposition (CVD).
3. Atmospheric Pressure Chemical Vapor Deposition (APCVD).
4. Other types of CVD (LPCVD, PECVD, HDPCVD…).
5. Introduction to evaporation.
6. Evaporation tools and issues, shadow evaporation.
7. Introduction to sputtering and DC plasma.
8. Sputtering yield, step coverage, film morphology.
9. Sputter deposition: reactive, RF, bias, magnetron, collimated, and ion beam.

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Low Pressure Chemical Vapor Deposition (LPCVD)

Atmospheric pressure systems (APCVD) have major drawbacks:

• At high T, a horizontal configuration must be used (few wafers at a time).

• At low T, the deposition rate goes down and throughput is again low.

The fundamental reason (I think) for the low throughput of APCVD is that only a small percentage of the gas is reactant gases, with the rest carrier/diluent gas.

Obviously, the solution is to operate at low pressure – LPCVD.

But

(8)

In the mass transfer limited regime,

This is not one expects: lower pressure means less reactants, so lower rate. But for APCVD, the reactant gas is only a small portion of the total gas.

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• LPCVD reactors use: P = 0.25 – 2.0Torr, T = 500 – 900°C.
• Transport of reactants from gas phase to surface through boundary layer is still not rate limiting (despite the high T), so wafers can be stacked vertically for high throughput (100-200 wafers per run).
• Because LPCVD operates in reaction limited regime, it is VERY sensitive to temperature and so temperature needs to be controlled closely (within +/- 1^oC), so use hot walled reactor for this precise control.
• Again, a 5-25^oC temperature gradient is often created to offset source gas depletion effects (or one can use distributed feeding).
• Requires no carrier gas, and low gas pressure reduces gas-phase reaction which causes particle cluster that contaminants the wafer and system.
• Less auto-doping (at lower P), as out-diffused dopant gas pumped away quickly.

Low Pressure Chemical Vapor Deposition (LPCVD)

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Possible disadvantages:

• For too low temperature, deposition rates may be too low, film quality decreases.
• Shadowing (less gas-phase collisions) due to directional diffusion to the surface, so deterioration of the step coverage and filling.

Low Pressure Chemical Vapor Deposition (LPCVD)

Seems cold wall reactors also exist: cold wall reduce deposition on walls, which leads to depletion of deposition species and particle formation that may flake off walls and fall on wafers.

Besides poorer temperature control than hot wall, gas convection is another problem.

Cold-wall

Hot-wall

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Plasma Enhanced CVD (PECVD)

• Use RF-induced plasma to transfer energy into the reactant gases, forming radicals that is very reactive. (RF: radio-frequency, typically 13.56MHz for PECVD)
• Low temperature process (<300^oC), as thermal energy is less critical when RF energy exists.
• Used for depositing film on metals (Al…) and other materials that cannot sustain high temperatures. (APCVD/LPCVD at such low temperatures gives increased porosity and poor step coverage)
• Surface reaction limited deposition, thus substrate temperature control is important to ensure uniformity.
• At low T, surface diffusion is slow, so one must supply kinetic energy for surface diffusion – plasma (ion bombardment) provides that energy and enhances step coverage.
• Disadvantages: plasma damage, not pure film (often lots of H incorporated into film).

“Good” quality films (though generally not as good as LP or APCVD films deposited at much higher T): energy supplied by plasma (i.e. ion bombardment of film) increases film density, composition, and step coverage.

Cold-wall

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PECVD process parameter

Substrate temperature (100-300^oC, up to 1000^oC PECVD available)

• Control by external heater, very little heating from PECVD process

Gas flow (10s to 100s sccm – standard cubic centimeter per minute)

• Higher flow rates can increase deposition rate and uniformity

Pressure (P ≈ 50mTorr – 5Torr )

• Changes the energy of ions reaching electrodes
• Can change deposition rate
• Increases pressure may lead to chemical reactions in the gas
• Effects also depend on gas concentration

Power (10s to 100s watts)

• Affects the number of electrons available for activation and the energy of those electrons
• Increased power may lead to chemical reactions in gas
• Increased power increases deposition rate

Frequency (mostly 13.56MHz, same for plasma etching and sputter deposition)

• Changes plasma characteristics
• Changes ion bombardment characteristics

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Examples of PECVD systems and applications

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High Density Plasma (HDP) CVD

• High density plasma CVD gives dense layers (SiO₂) at low T (150 °C) and low P (1- 10 mTorr); T increases to 400°C by bombardment.
• Separate RF (gives substrate biasing for bombardment) from plasma generation (electron cyclotron resonance ECR and inductively coupled plasma ICP).
• Simultaneous deposition and sputtering/ bombardment. Improved planarization and filling due to preferential sputtering of sloped surface.�Mostly used for SiO₂ deposition in backend processes.

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Selective deposition:

• Especially important in microelectronics, surface patterning and 3D-growth.
• Reaction rate of precursor is limited on a non-growth surface. E.g. deposition of Cu from (hfac)Cu(PMe₃) occur on Cu, Pt… but not on SiO₂.
• Growth surface acts as co-reactant, and is selectively consumed. E.g. Si reacts with WF₆ or MoF₆, while reaction at SiO₂ or Si₃N₄ is slower.
• A chemical reaction of a gaseous co-reactant occur on the growth surface. E.g. H₂ dissociation on a metal surface, but not on SiO₂ or metal oxide surfaces.

Miscellaneous: selective deposition and laser CVD

Tungsten spring grown by laser CVD.

Laser CVD

(energy provided by laser)

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CVD reactor types: quick summary

According to the LPCVD slides, APCVD growth rate should be lower, which is not true. Because: (?? I think)

• In APCVD reactive gas partial pressure could be set much higher than that in LPCVD.
• Its pressure could be much lower (by 10×) than 1atm and is still called APCVD.
• Gas transport actually increases with T as T^3/2 (APCVD is usually done at higher T than LPCVD).
• When putting wafer side-by-side facing the gas, more exposed to gas, thus faster transport.