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Lecture 12.0. Deposition. Materials Deposited. Dielectrics SiO2, BSG Metals W, Cu, Al Semiconductors Poly silicon (doped) Barrier Layers Nitrides (TaN, TiN), Silicides (WSi 2 , TaSi 2 , CoSi, MoSi 2 ). Deposition Methods. Growth of an oxidation layer Spin on Layer

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Lecture 12 0

Lecture 12.0

Deposition


Materials deposited
Materials Deposited

  • Dielectrics

    • SiO2, BSG

  • Metals

    • W, Cu, Al

  • Semiconductors

    • Poly silicon (doped)

  • Barrier Layers

    • Nitrides (TaN, TiN), Silicides (WSi2, TaSi2, CoSi, MoSi2)


Deposition methods
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


Critical issues
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


Cvd of si 3 n 4 implantation mask
CVD of Si3N4 - Implantation mask

  • 3 SiH2Cl2 + 4 NH3Si3N4 + 6 HCl + 6 H2

    • 780C, vacuum

    • Carrier gas with NH3 / SiH2Cl2 >>1

  • Stack of wafer into furnace

    • Higher temperature at exit to compensate for gas conversion losses

  • Add gases

  • Stop after layer is thick enough


Cvd of poly si gate conductor
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


Cvd of sio 2 dielectric
CVD of SiO2 – Dielectric

  • Si0C2H5 +O2SiO2 + 2 H2

    • 400C, vacuum

    • He carrier gas with vaporized(or atomized) Si0C2H5and 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


Cvd of w metal plugs
CVD of W – Metal plugs

  • 3H2+WF6 W + 6HF

    • T>800C, vacuum

    • He carrier gas with WF6

    • Side Reactions at lower temperatures

      • Oxide etching reactions

      • 2H2+2WF6+3SiO2 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



Cvd reactor
CVD Reactor

  • Wafers in Carriage (Quartz)

  • Gasses enter

  • Pumped out via vacuum system

  • Plug Flow Reactor

Vacuum


Cvd reactor1
CVD Reactor

  • Macroscopic Analysis

    • Plug flow reactor

  • Microscopic Analysis

    • Surface Reaction

      • Film Growth Rate


Macroscopic analysis
Macroscopic Analysis

  • Plug Flow Reactor (PFR)

    • Like a Catalytic PFR Reactor

    • FAo= Reactant Molar Flow Rate

    • X = conversion

    • rA=Reaction rate = f(CA)=kCA

    • Ci=Concentration of Species, i.

    • Θi= Initial molar ratio for species i to reactant, A.

    • νi= stoichiometeric coefficient

    • ε = change in number of moles


Combined effects
Combined Effects

Contours = Concentration


Reactor length effects
Reactor Length Effects

SiH2Cl2(g) + 2 N2O(g) SiO2(s)+ 2 N2(g)+2 HCl(g)

How to solve? Higher T at exit!


Deposition rate over the radius
Deposition Rate over the Radius

CAs

r

Thiele Modulus Φ1=(2kRw/DABx)1/2


Radial effects
Radial Effects

This is bad!!!



Cvd reactor2
CVD Reactor

  • External Convective Diffusion

    • Either reactants or products

  • Internal Diffusion in Wafer Stack

    • Either reactants or products

  • Adsorption

  • Surface Reaction

  • Desorption


Microscopic analysis reaction steps
Microscopic Analysis -Reaction Steps

  • Adsorption

    • A(g)+SA*S

    • rAD=kAD (PACv-CA*S/KAD)

  • Surface Reaction-1

    • A*S+SS*S + C*S

    • rS=kS(CvCA*S - Cv CC*S/KS)

  • Surface Reaction-2

    • A*S+B*SS*S+C*S+P(g)

    • rS=kS(CA*SCB*S - Cv CC*SPP/KS)

  • Desorption: C*S<----> C(g) +S

    • rD=kD(CC*S-PCCv/KD)

  • Any can be rate determining! Others in Equilib.

  • Write in terms of gas pressures, total site conc.


Rate limiting steps
Rate Limiting Steps

  • Adsorption

    • rA=rAD= kADCt (PA- PC /Ke)/(1+KAPA+PC/KD+KIPI)

  • Surface Reaction

    • (see next slide)

  • Desorption

    • rA=rD=kDCt(PA - PC/Ke)/(1+KAPA+PC/KD+KIPI)



Deposition of ge
Deposition of Ge

Ishii, H. and Takahashik Y., J. Electrochem. Soc. 135,1539(1988).


Silicon deposition
Silicon Deposition

  • Overall Reaction

    • SiH4 Si(s) + 2H2

  • Two Step Reaction Mechanism

    • SiH4 SiH2(ads)+ H2

    • SiH2 (ads)  Si(s) + H2

  • Rate=kadsCt PSiH4/(1+Ks PSiH4)

    • Kads Ct = 2.7 x 10-12 mol/(cm2 s Pa)

    • Ks=0.73 Pa-1


Silicon epitaxy vs poly si
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




Dislocation density
Dislocation Density

  • Epitaxial Film

    • Activation Energy of Dislocation

      • 3.5 eV


Physical vapor deposition
Physical Vapor Deposition

  • Evaporation from Crystal

  • Deposition of Wall


Physical deposition sputtering
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


Dc plasma
DC Plasma

  • Glow Discharge



Sputtering chemistries

Target

Al

Cu

TiW

TiN

Gas

Argon

Deposited Layer

Al

Cu

TiW

TiN

Poly Crystalline Columnar Structure

Sputtering Chemistries


Deposition rate
Deposition Rate

  • Sputtering Yield, S

    • S=α(E1/2-Eth1/2)

  • Deposition Rate 

    • Ion current into Target *Sputtering Yield

    • Fundamental Charge


Rf plasma

Sheath

RF Plasma

Plasma

rf

Sheath

  • Electrons dominate in the Plasma

    • Plasma Potential, Vp=0.5(Va+Vdc)

    • Va = applied voltage amplitude (rf)

  • Ions Dominate in the Sheath

    • Sheath Potential, Vsp=Vp-Vdc

  • Reference Voltage is ground such that Vdc is negative


Floating potential
Floating Potential

  • Sheath surrounds object

  • Floating potential, Vf

  • kBTe=eV

    • due to the accelerating Voltage


Plasma chemistry
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.


Plasma chemistry1
Plasma Chemistry

  • Ionization leading to ion

    • e + CF4  CF3- + F

    • e + SiH4  SiH3+ + H + 2e

  • Reaction depend upon electron density


Plasma chemistry2
Plasma Chemistry

  • Electrons have more energy

  • Concentration of electrons is ~108 to1012 1/cc

  • Ions and neutrals have 1/100 lower energy than electrons

  • Concentration of neutrals is 1000x the concentration of ions


Oxygen plasma
Oxygen Plasma

  • Reactive Species

    • O2+eO2+ + 2e

    • O2+e2O + e

    • O + e  O-

    • O2+ + e  2O


Plasma chemistry3
Plasma Chemistry

  • Reactions occur at the Chip Surface

    • Catalytic Reaction Mechanisms

      • Adsorption

      • Surface Reaction

      • Desorption

    • e.g. Langmuir-Hinshelwood Mechanism



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