Deposition

1
Lecture 12.0

• Deposition

2
Materials Deposited

• Dielectrics
• SiO2, BSG
• Metals
• W, Cu, Al
• Semiconductors
• Poly silicon (doped)
• Barrier Layers
• Nitrides (TaN, TiN), Silicides (WSi2, TaSi2,
  CoSi, MoSi2)

3
Deposition Methods

• Growth of an oxidation layer
• Spin on Layer
• Chemical Vapor Deposition (CVD)
• Heat decomposition T of gasses
• Plasma enhanced CVD (lower T process)
• Physical Deposition
• Vapor Deposition
• Sputtering

4
Critical Issues

• Adherence of the layer
• Chemical Compatibility
• Electro Migration
• Inter diffusion during subsequent processing
• Strong function of Processing
• Even Deposition at all wafer locations

5
CVD of Si3N4 - Implantation mask

• 3 SiH2Cl2 4 NH3??Si3N4 6 HCl 6 H2
• 780C, vacuum
• Carrier gas with NH3 / SiH2Cl2 gtgt1
• Stack of wafer into furnace
• Higher temperature at exit to compensate for gas
  conversion losses
• Add gases
• Stop after layer is thick enough

6
CVD of Poly Si Gate conductor

• SiH4 ??Si 2 H2
• 620C, vacuum
• N2 Carrier gas with SiH4 and dopant precursor
• Stack of wafer into furnace
• Higher temperature at exit to compensate for gas
  conversion losses
• Add gases
• Stop after layer is thick enough

7
CVD of SiO2 Dielectric

• Si0C2H5 O2??SiO2 2 H2
• 400C, vacuum
• He carrier gas with vaporized(or atomized)
  Si0C2H5 and O2 and B(CH3)3 and/or P(CH3)3
  dopants for BSG and BPSG
• Stack of wafer into furnace
• Higher temperature at exit to compensate for gas
  conversion losses
• Add gases
• Stop after layer is thick enough

8
CVD of W Metal plugs

• 3H2WF6 ?? W 6HF
• Tgt800C, vacuum
• He carrier gas with WF6
• Side Reactions at lower temperatures
• Oxide etching reactions
• 2H22WF63SiO2 ?? 3SiF4 2WO2 2H2O
• SiO2 4HF ?? 2H2O SiF4
• Stack of wafer into furnace
• Higher temperature at exit to compensate for gas
  conversion losses
• Add gases
• Stop after layer is thick enough

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Chemical Equilibrium
10
CVD Reactor

• Wafers in Carriage (Quartz)
• Gasses enter
• Pumped out via vacuum system
• Plug Flow Reactor

Vacuum
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CVD Reactor

• Macroscopic Analysis
• Plug flow reactor
• Microscopic Analysis
• Surface Reaction
• Film Growth Rate

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Macroscopic Analysis

• Plug Flow Reactor (PFR)
• Like a Catalytic PFR Reactor
• FAo Reactant Molar Flow Rate
• X conversion
• rAReaction rate f(CA)kCA
• CiConcentration of Species, i.
• Ti Initial molar ratio for species i to
  reactant, A.
• ?i stoichiometeric coefficient
• e change in number of moles

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Combined Effects
Contours Concentration
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Reactor Length Effects
SiH2Cl2(g) 2 N2O(g)?? SiO2(s) 2 N2(g)2 HCl(g)
How to solve? Higher T at exit!
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Deposition Rate over the Radius
CAs
r
Thiele Modulus F1(2kRw/DABx)1/2
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Radial Effects
This is bad!!!
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Combined Length and Radial Effects
Wafer 10
Wafer 20
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CVD Reactor

• External Convective Diffusion
• Either reactants or products
• Internal Diffusion in Wafer Stack
• Either reactants or products
• Adsorption
• Surface Reaction
• Desorption

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Microscopic Analysis -Reaction Steps

• Adsorption
• A(g)S??AS
• rADkAD (PACv-CAS/KAD)
• Surface Reaction-1
• ASS??SS CS
• rSkS(CvCAS - Cv CCS/KS)
• Surface Reaction-2
• ASBS??SSCSP(g)
• rSkS(CASCBS - Cv CCSPP/KS)
• Desorption CSlt----gt C(g) S
• rDkD(CCS-PCCv/KD)
• Any can be rate determining! Others in Equilib.
• Write in terms of gas pressures, total site conc.

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Rate Limiting Steps

• Adsorption
• rArAD kADCt (PA- PC /Ke)/(1KAPAPC/KDKIPI)
• Surface Reaction
• (see next slide)
• Desorption
• rArDkDCt(PA - PC/Ke)/(1KAPAPC/KDKIPI)

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Surface Reactions
22
Deposition of Ge
Ishii, H. and Takahashik Y., J. Electrochem. Soc.
135,1539(1988).
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Silicon Deposition

• Overall Reaction
• SiH4 ?? Si(s) 2H2
• Two Step Reaction Mechanism
• SiH4 ?? SiH2(ads) H2
• SiH2 (ads) ?? Si(s) H2
• RatekadsCt PSiH4/(1Ks PSiH4)
• Kads Ct 2.7 x 10-12 mol/(cm2 s Pa)
• Ks0.73 Pa-1

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Silicon Epitaxy vs. Poly Si

• Substrate has Similar Crystal Structure and
  lattice spacing
• Homo epitaxy Si on Si
• Hetero epitaxy GaAs on Si
• Must have latice match
• Substrate cut as specific angle to assure latice
  match
• Probability of adatoms getting together to form
  stable nuclei or islands is lower that the
  probability of adatoms migrating to a step for
  incorporation into crystal lattice.
• Decrease temp.
• Low PSiH4
• Miss Orientation angle

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Surface Diffusion
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Monocrystal vs. Polycrystalline
PSiH4? torr
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Dislocation Density

• Epitaxial Film
• Activation Energy of Dislocation
• 3.5 eV

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Physical Vapor Deposition

• Evaporation from Crystal
• Deposition of Wall

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Physical Deposition - Sputtering

• Plasma is used
• Ion (Ar) accelerated into a target material
• Target material is vaporized
• Target Flux ? Ion Flux Sputtering Yield
• Diffuses from target to wafer
• Deposits on cold surface of wafer

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DC Plasma

• Glow Discharge

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RF Plasma Sputtering for Deposition and for
Etching
RF DC field
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Sputtering Chemistries

• Target
• Al
• Cu
• TiW
• TiN
• Gas
• Argon

• Deposited Layer
• Al
• Cu
• TiW
• TiN
• Poly Crystalline Columnar Structure

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Deposition Rate

• Sputtering Yield, S
• Sa(E1/2-Eth1/2)
• Deposition Rate ?
• Ion current into Target Sputtering Yield
• Fundamental Charge

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RF Plasma
Sheath
Plasma
rf
Sheath

• Electrons dominate in the Plasma
• Plasma Potential, Vp0.5(VaVdc)
• Va applied voltage amplitude (rf)
• Ions Dominate in the Sheath
• Sheath Potential, VspVp-Vdc
• Reference Voltage is ground such that Vdc is
  negative

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Floating Potential

• Sheath surrounds object
• Floating potential, Vf
• kBTeeV
• due to the accelerating Voltage

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Plasma Chemistry

• Dissociation leading to reactive neutrals
• e H2 ? H H e
• e SiH4 ? SiH2 H2 e
• e CF4 ? CF3 F e
• Reaction rate depends upon electron density
• Most Probable reaction depends on lowest
  dissociation energy.

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Plasma Chemistry

• Ionization leading to ion
• e CF4 ? CF3- F
• e SiH4 ? SiH3 H 2e
• Reaction depend upon electron density

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Plasma Chemistry

• Electrons have more energy
• Concentration of electrons is 108 to 1012 1/cc
• Ions and neutrals have 1/100 lower energy than
  electrons
• Concentration of neutrals is 1000x the
  concentration of ions

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Oxygen Plasma

• Reactive Species
• O2e?O2 2e
• O2e?2O e
• O e ? O-
• O2 e ? 2O

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Plasma Chemistry

• Reactions occur at the Chip Surface
• Catalytic Reaction Mechanisms
• Adsorption
• Surface Reaction
• Desorption
• e.g. Langmuir-Hinshelwood Mechanism

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Plasma Transport Equations

• Flux, J