1. Lecture 12.0
Deposition

2. Materials Deposited Dielectrics Metals Semiconductors Barrier Layers
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 >>1
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 3H2+WF6  W + 6HF Stack of wafer into furnace
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

9. Chemical Equilibrium

10. CVD Reactor Wafers in Carriage (Quartz) Gasses enter
Pumped out via vacuum system
Plug Flow Reactor
Vacuum

11. CVD Reactor Macroscopic Analysis Microscopic Analysis
Plug flow reactor
Microscopic Analysis
Surface Reaction
Film Growth Rate

12. 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

13. Combined Effects
Contours = Concentration

14. Reactor Length Effects
SiH2Cl2(g) + 2 N2O(g) SiO2(s)+ 2 N2(g)+2 HCl(g)
How to solve? Higher T at exit!

15. Deposition Rate over the Radius
CAs r
Thiele Modulus Φ1=(2kRw/DABx)1/2

16. Radial Effects
This is bad!!!

17. Combined Length and Radial Effects
Wafer 10
Wafer 20

18. CVD Reactor External Convective Diffusion
Either reactants or products
Internal Diffusion in Wafer Stack
Adsorption
Surface Reaction
Desorption

19. 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.

20. Rate Limiting Steps Adsorption Surface Reaction Desorption
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)

21. Surface Reactions

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

23. Silicon Deposition Overall Reaction Two Step Reaction Mechanism
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 mol/(cm2 s Pa)
Ks=0.73 Pa-1

24. 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

25. Surface Diffusion

26. Monocrystal vs. Polycrystalline
PSiH4=? torr

27. Dislocation Density Epitaxial Film Activation Energy of Dislocation
3.5 eV

28. Physical Vapor Deposition
Evaporation from Crystal
Deposition of Wall

30. 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

31. DC Plasma
Glow Discharge

32. RF Plasma Sputtering for Deposition and for Etching
RF + DC field

33. Sputtering Chemistries
Target Al Cu
TiW
TiN
Gas
Argon
Deposited Layer Al Cu
TiW
TiN
Poly Crystalline Columnar Structure

34. Deposition Rate Sputtering Yield, S Deposition Rate  S=α(E1/2-Eth1/2)
Ion current into Target *Sputtering Yield
Fundamental Charge

35. RF Plasma Electrons dominate in the Plasma Ions Dominate in the Sheath
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

36. Floating Potential Sheath surrounds object Floating potential, Vf
kBTe=eV
due to the accelerating Voltage

37. 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.

38. Plasma Chemistry Ionization leading to ion
e + CF4  CF3- + F
e + SiH4  SiH3+ + H + 2e
Reaction depend upon electron density

39. Plasma Chemistry Electrons have more energy
Concentration of electrons is ~108 to /cc
Ions and neutrals have 1/100 lower energy than electrons
Concentration of neutrals is 1000x the concentration of ions

40. Oxygen Plasma Reactive Species O2+eO2+ + 2e O2+e2O + e O + e  O-

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

42. Plasma Transport Equations
Flux, J