Lecture 12 MNS 102: Techniques for Materials and Nano Sciences

• Overview of dry chemistry methods – also known as thin film deposition: usually involve exposing gas onto substrate

• Chemical vapour deposition (CVD): expose reactive gases to substrate > rxn > deposition – atmosphere to high vacuum (HV)

• Physical vapour deposition (PVD) including magnetron sputtering, thermal or e-beam evaporation, pulsed laser deposition (PLD): expose to substrate vapours of constituent materials created in-situ > condense > deposition – low vacuum (LV) to HV (except for PLD – atm plus UHV)

• Epitaxy, and Molecular beam epitaxy (MBE): same as PVD, except single-crystal substrate and ultrahigh vacuum (UHV)

• Other techniques: Atomic Layer Deposition (ALD)

LV = above 10-3 mbar, HV = 10-3 to 10-6 mbar, UHV = below 10-9 mbar

1 bar = 1000 mbar = 100 kPa = 100,000 N m-2 = 0.987 atm = 750 Torr = 14.5 psi

“Thin” film ≡ 2D layer with thickness from a monolayer to a few microns; not completely dense; under stress; different defects from bulk; quasi-2D; surface and interface effects important

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Dry Chemistry Methods

Steps:

• Get precursor into vapour phase “somehow”

• particle transport to substrate in some state of vacuum

• condense/adsorb on substrate • nucleate • grow

Parameters:

• nature (and subsequent rxn) of depositing particles

• deposition rate, deposition directionality • P and T (and nature) of deposition

ambient • T of substrate, crystal phase (if any) of

substrate

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Quality:

• Physical and chemical properties • Electrical property, breakdown voltage • Mechanical properties, stress and

adhesion to substrate • Optical properties,

absorption/transparency, refractive index

• Composition, stoichiometry • Film density, defect (pinhole) density • Texture, grain size, boundary property,

and orientation • Impurity level, doping Other:

• Conformal vs non-conformal • Cost of ownership and operation • Increasingly used for nanostructure

synthesis

“Three” Film Growth 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 (c)

• Island growth (Volmer‐Weber): Film atoms more strongly bound to each other than to substrate and/or slow diffusion (a)

• Mixed growth (Stranski‐Krastanov): Initially 2D or layer-by-layer growth, then 3D island growth (b)

• Vs no film growth when “reactive intermixing” occurs – no film vs substrate distinction

Types of Films

• Epitaxial (single‐crystalline, layer-by-layer, lattice match to substrate): no grain boundaries; needs high T and slow rate for growth; e.g. III‐V semiconductor films (GaAs) and complex oxides.

• Polycrystalline (island or mixed growth): lots of grain boundaries; e.g. most elemental metals grown near room temperature.

• Amorphous (island or mixed growth): no‐crystalline structure but could have some short-range atomic ordering; no crystalline defects, e.g. common insulators such as amorphous SiO2.

Growth Mode Control

• Lattice mismatch: Structural compatibility; important to epitaxy growth

• Supersaturation: Energetics re Change of Gibbs free energy from vapour to solid ~ ∆μ = k TSubstrate ln(P/Peq)

• Surface or Interface free energy: Adhesion/wetting properties; e.g. Noble metals (Au, Ag, Cu) do not wet Si/SiO2 substrate island growth; Ti or Cr could reduce island formation b/c chemically bonding to O in SiO2 and is often used as an interface layer.

• Substrate T effect: Higher TS > higher mobility > increased agglomeration, i.e. smaller islands (or grains) coalesce with each other to reduce the surface area (and to lower the surface energy).

• Substrate T vs deposition rate: Higher T increase mobility to get to equil. But too high a deposition rate could terminate surface diffusion before adspecies find the lowest energy position and bury the adspecies by a later arrival adspecies

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Wetting conditions

• Layer-by-layer growth: θ=0, γSV > γLV + γSL

• Island growth: θ>0, γSV < γLV + γSL

γXY ≡ interface energy b/w phase X and phase Y

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Cu film grown on NaCl(111)

Source: Pratontep et al. Synth. Met. 146 (2004) 387. “Nucleation and growth of ultrathin pentacene films on silicon dioxide: effect of deposition rate and substrate temperature “

Source: Sun et al. Appl. Opt. 51 (2012)

8481. “Effects of substrate

temperatures and deposition rates on

properties of aluminum fluoride thin films in deep-

ultraviolet region “

Chemical Vapour Deposition

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Susceptor

Wafer

12

3 4 5

67

Gas stream

Reaction/growth rate depends on:

• Gas transport to/from substrate • Surface rxn rate (strongly T

dependent)

Steps 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 promoted by heat or plasma,

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.

(a) epi-Si (b) poly-Si, a-Si

CVD Reactions

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• Thermal decomposition: AB(g) → A(s) + B(g)

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

Ni(CO)4(g) → Ni(s) + 4CO(g) (180oC)

• Reduction (using H2): AX(g) + H2(g) → A(s) + HX(g)

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

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

• Oxidation (using O2): AX(g) + O2(g) → AO(s) + [O]X(g)

SiO2 deposition from silane and oxygen at 450oC (lower T than thermal oxidation): SiH4(g) + O2(g) → SiO2(s) + 2H2(g)

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

(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)

Deposition of wear-resistant film (BN) at 1100oC: BF3(g) + NH3(g) → BN(s) + 3HF(g)

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

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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 rxn, uniform film thickness. • Gas P ~ 1-1000 mTorr (lower P > higher diffusivity of gas to substrate). • Better film uniformity and step coverage, with fewer defects. • Process T 500°C.

PECVD (Plasma Enhanced CVD): • Plasma helps to break up gas molecules: high reactivity, able to process at lower T and

lower P (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 T ~ 100-400°C.

MOCVD (Metal-organic CVD) or OMVPE (Organo metallic Vapour Phase Epitaxy): Epitaxial growth for many optoelectronic devices with III-V compounds for solar cells, lasers, LEDs, photo-cathodes and quantum wells.

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LPCVD vs PECVD

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RF power input

Electrode

Electrode

Wafers

Plasma

Gas outlet, pump

Heater

Gas inlet( SiH4, O2)

1

2

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PECVD Applications

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Physical Vapour Deposition

• Material vapour is deposited physically onto substrate without any chemical rxn.

• Evaporation or Vacuum Deposition

– Material source is heated to high temperature in high vacuum (P<10-5 Torr) either by thermal (resistive heating) or e-beam methods.

– Material vapour is transported to and then condenses onto the substrate in high vacuum. HV is required to minimize collisions of source atoms with the background gas to achieve line-of-sight deposition.

– Deposition rate depends on flux, geometry of source and substrate; it could be fast (>1μm/min); for good quality film 0.1-1 nm/sec is more reasonable.

• Sputter Deposition

– Neutral atoms are knocked off by gaseous ions (usually Ar+) striking the target, which are generated by glow discharge. This is usually done in low pressure (5-50 mTorr) and not in vacuum.

– Sputtered atoms move in line-of-sight fashion and condense onto the substrate.

• Film thickness can be monitored precisely using a quartz micro-balance.

• Film quality obtained by vacuum deposition is often not as good as that obtained by sputter deposition (that gives a denser film due to energetic bombardment of ions to the as-deposited film).

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Mean Free Path vs Pressure (Vacuum)

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• For a particle of diameter d travelling at speed v for time ∆t, the number of collisions N is:

N = ρ (d2 v∆t), where ρ is the number density (n/V), d2 is the

collision cross-section and v∆t is the distance travelled by the particle over time ∆t.

• Average mean free path between collisions is:

= v∆t / N = (ρ d2)-1

• Using kinetic theory and ideal gas law (PV=nkT),

• For d = 3 Å, T = 300 K P (mbar) × = 6.8×10-3

1 mbar = 0.75 Torr

Pd2

kT)2d

V

n(

2

12

P (mbar) ML / sec

1 68 m 330,000

1x10-3 68 mm 330

1x10-6 68 m 0.33

1x10-9 68 km 0.00033

P (mbar) Time to form 1 ML

10-4 27 ms

10-5 0.27 s

10-6 0.27 s

10-7 27 s

10-8 4.4 m

10-9 44.3 m

10-10 7.4 h

10-11 3 days

Other Considerations

• Source design: point vs flat head vs other geometry.

• Source-to-substrate separation: larger > more uniformity across substrate.

• Deposition orientation > GLAD.

• Deposition rate depends on the vapour pressure of source materials and the evaporation temperature. For Knudsen cell or K cell, i.e. a box of gas, with molecular mass m, at equilibrium pressure Pe and temperature T emitting from a small opening with area As, the evaporation rate is:

• Vapour pressure of 1-10 mTorr or more is needed to obtain reasonable deposition rates of 0.1-1 μm/min (or 1-20 nm/s).

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e

21

s2

evp PT

mA1083.5R

Vapour Pressure Curves of Elements

12- 15 Source: http://www.mcallister.com/vacuum.html

Mo, Ta, W good materials for crucibles

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Source: “Materials Science of Thin Films” 2nd ed, M. Ohring , Academic , New York (2002)

Vacuum Deposition

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Wafer holder

Wafers

Vacuum Source material

Vacuum system Exhaust

Heater (resistanceor E-beam)

Atomicflux

Vacuum Deposition Thermal vs E-beam plus Pro & Con

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Thermal evaporation • Simple, robust, and in widespread use, low cost. • Good for metals (Au, Ag, Al, Cr, Sn, Sb, Ge, In, Mg, Ga)

or low MP materials (CdS, PbS, CdSe, NaCl, KCl, AgCl, MgF2, CaF2, PbCl2).

• Use W, Ta, or Mo filaments (with typical current 200-300 Amp) to heat evaporation source (up to 1800°C).

• Low deposition rate: 0.1-2 nm/s. • Exposes substrates to visible and IR radiation. • High impurity due to contamination from heated

boat/crucible.

E-beam evaporation • More complex, but extremely versatile, high cost. • Virtually any material, both metals and dielectrics, up

to 3000°C (above materials, plus Ni, Pt, Ir, Rh, Ti, V, Zr, W, Ta, Mo, Al2O3, SiO, SiO2, SnO2, TiO2, ZrO2).

• High deposition rate: 1-10 nm/s. • Less contamination, less heating to wafer (b/c only

small source area heated to very high T). • Exposes substrates to secondary electron radiation. • X-rays can also be generated by high voltage electron

beam.

Evaporation “Pro” • Films can be deposited at high rates (e.g.,

1μm/min, though for research typically < 0.1μm/min).

• Low energy atoms (~0.1 eV) leave little surface damage.

• Little residual gas and impurity incorporation due to high vacuum conditions.

• No or very little substrate heating.

Evaporation “Con” • Accurately controlled alloys/compounds

are difficult to achieve. • No in-situ substrate cleaning. • Poor step coverage (but this is good for

liftoff). • Variation of deposit thickness for

large/multiple substrates – Need to rely on quartz crystal micro-balance for thickness monitoring.

• X-ray damage.

GLAD (GLancing Angle Deposition)

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• Simple implementation by manipulating the incident angle (usually > 80).

• “Self-assembly” due to shadowing effect.

• Multiple sources can be used to create films with different porosity.

• Great for producing a variety of directional and low-dimensional nanostructures.

Sputter Deposition

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• A plasma is generated to create ions used to strike off source/target atoms. [Plasma = less than 0.01% ions plus equal amount of electrons, the other 99.99% is neutral gas.]

• Pressure (1-100 mTorr) is higher than that used in vacuum deposition (< 10-5 Torr).

• Can be used to deposit virtually any materials (metals, insulators, alloys, composites, if an appropriate target can be made).

• Better film quality than vacuum deposition due to more energetic adatoms.

• More reproducible deposition, easy film thickness control (over time), scalable to large area. BUT: Substrate damage by ions bombardment or UV irradiation; more contamination due to higher P; slow deposition rates for some materials; heat generation.

• DC or RF mode – DC mode for conducting targets (metals); RF mode for conducting and insulating targets (oxides, ceramics).

• Parameters: Pressure (1-100 mTorr); power (a few 100s W); DC high voltage -2 to -5 kV; substrate bias voltage possible; substrate temperature (20-700C).

DC Sputtering

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• Plasma is first ignited by high voltage discharge of the sputtering gas (Ar) at pressure P between the anode (substrate) and cathode (target), separated by spacing d. Once started, the voltage will drop to 100 V or so due to the conducting plasma.

• Self-sustaining Discharge: Ar ions create secondary electrons (<50 eV) as they strike the target. The secondary electrons collide with Ar to create more ions. We need:

P ×d > 0.5 cm Torr e.g., d = 10 cm, P = 50 mTorr

RF (Radio Frequency) Sputtering

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-V (DC)

Electrode/target(cathode)

Electrode (anode)

Wafers

Argon plasma(glow discharge)

Ground

Heater

Sputtering gas inlet( Ar)

Vacuum

• Good for insulating materials b/c positive charge (Ar+) builds up on the cathode (target) in DC sputtering systems. Alternating potential (i.e. AC) can avoid charge buildup.

• When frequencies < 50 kHz, both electrons and ions can follow the switching of the anode and cathode, basically DC sputtering of both surfaces.

• When frequencies >> 50 kHz (e.g. 13.56 MHz), heavy ions cannot follow the switching, and electrons can neutralize positive charge buildup on each electrode during each positive half cycle. During the negative half cycle, bombardment continues.

• As electrons gain energy directly from RF powder (no need of secondary electrons to maintain plasma), and oscillating electrons are more efficient to ionize the gas, RF sputtering is capable of running in lower pressure (1-15 mTorr), so fewer gas collisions and more line-of-sight deposition.

Magnetron Sputtering (DC or RF)

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• The Magnetron: A series of magnets with alternating polarity is attached to the backside of the target in a circle. The magnetic field lines confine and trap the secondary electrons. The highly concentrated electron gas generates a denser plasma (by at least 2 orders of magnitude) near the target without the need for a higher pressure.

• Deposition rate is increased by 10-100 times, up to 1 μm/min.

• Unwanted substrate heating is reduced.

• Lower Ar pressure (0.5 mTorr)

• The magnet ring can be made rotating to prevent too much target erosion at some spots and to improve film uniformity.

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WATLab Mantis

PVD System with Multicluster Source

E.g. Endura PVD System (Intel)

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1 mbar = 0.75 Torr 1 Bar = 1,000 mbar = 100 kPa = 750.06 Torr = 0.987 atm = 14.5 psi 1 Torr = 1.33 mBar 1 kPa = 1,000 N/m2

Pulsed Laser Deposition

• Steps: Laser pulses impinging on target > material ablation producing laser plume > material transport to substrate (in bunches) usually in high vacuum or in set environment

• Deposit reproduces composition of target

• Could deposit virtually any material including high temperature ceramics such as Tc superconductors

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PLD Key Features & Materials Deposits

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• Metal target absorption depth ~ 10 nm (wavelength dependent).

• Deposition is inherently non-equilibrium > access new phases under normal conditions.

• Laser pulse fs to ns width

• Laser pulse (MW/cm2) could produce target temperature > 10,000 K (or > 40 eV).

• High electric field (> 105

V/cm) process.

• Plasma temperature: 3000-5000 K.

• Energy of ablated species: 1-100 eV

• C60

Epitaxy

• Epitaxy: Deposition of a monocrystalline film onto a monocrystalline substrate.

• Epi – Greek for “above” + taxis – Greek for “in an ordered manner”.

• Auto or Homo-epitaxy: Extension of substrate lattice by overgrowth of layers of identical material (e.g. Si on Si, GaAs on GaAs) – no problem of compatibility or lattice mismatch.

• Hetero-epitaxy: Extension of material of different crystalline structure and different orientation from those of the substrate (e.g. GaAs(100) on Si(100) – consideration of lattice mismatch.

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Molecular Beam Epitaxy

• Vapour-Phase Epitaxy (VPE, a form of CVD): Transport of the epilayer constituents (Si, Ga, As, dopants,…) in the form of one or more volatile compounds to the substrate where they react to form the epilayer.

• Molecular Beam Epitaxy: Physical transport of individual constituents to a heated substrate in vacuum or ultrahigh vacuum condition – “Beam” means no interaction between constituents until they reach the substrate, i.e. long mean free path.

• Liquid-Phase Epitaxy (LPE): Growth of epitaxial layer on crystalline substrate by direct precipitation from the liquid phase.

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MBE

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• MBE is an epitaxial process that involves reactions of one or more thermal beams of atoms or molecules on a crystalline surface under UHV conditions.

• Precise control in both chemical composition and doping profiles.

• Very low growth rate (e.g. for GaAs, 1μm/hr is typical.)

• Single-crystal multilayer structures with dimensions on the order of atomic layers can be made.

• Effusion cell or Knudsen cell: Source put inside a crucible (made of pyrolytic BN, quartz, W or graphite), heated by radiation from a heated filament or by e-beam bombardment. Equipped with a thermocouple to measure the source temperature and to allow closed-loop feedback control. Must also have adequate water cooling, heat shield, and shutter.

• Reflection high-energy electron diffraction (RHEED): Electron diffraction technique used to monitor the surface crystallinity in-situ during material growth.

http://www.mbe-components.com/products/effusion/ntez.html

N HSi

Opposite Dimer

DiagonalDimer

Side DimerLong Dimer

Self-alignment of Adenine Dimer Nanowires

3 nm

C

Atomic Layer Deposition

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• The CVD reaction is broken into two half-reactions, keeping the precursor materials separate throughout the coating process.

• Steps: 1. Saturated chemisorption: The precursor gas is introduced into the process chamber to generate a monolayer of gas on the substrate. 2. Sequential surface reaction: A second precursor gas is let into the chamber to facilitate reaction with the first precursor to produce a monolayer of the product film. 3. Repeat cycle.

• Film growth is self-limited (monolayer adsorption + reaction = 1 layer per cycle) > precise atomic layer thickness control of film growth by controlling the number cycle.

• Outstanding film quality for dielectric layers (e.g. high-K dielectrics for CMOS, DRAM, Cu interconnect barrier)

ALD cycle of Al2O3 http://www.cambridgenanotech.com/

Nanomaterial Synthesis

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Homework 3A: 1. Do an internet search for two research papers, published in the last 5 years (2013-

2008), on using different “dry” synthesis methods (discussed above) for producing nanostructured materials.

2. Email the pdf files of these papers to tong@uwaterloo.ca. Append your last name and student number to the filenames of these papers (e.g. Leung12345678_ref1).

3. Read these papers. 4. In no more than 2 pages total, indicate the full references of these two papers, and

briefly summarize the objectives of these studies and the synthetic methods used. If applicable, indicate the “tricks” (e.g., catalysts, templates, seed layers, surfactants) that these methods may have employed.