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 (WSi 2, TaSi 2, CoSi, MoSi 2 )

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 Si 3 N 4 - Implantation mask 3 SiH 2 Cl 2 + 4 NH 3  Si 3 N 4 + 6 HCl + 6 H 2 –780C, vacuum –Carrier gas with NH 3 / SiH 2 Cl 2 >>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 SiH 4  Si + 2 H 2 –620C, vacuum –N 2 Carrier gas with SiH 4 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 SiO 2 – Dielectric Si0C 2 H 5 +O 2  SiO 2 + 2 H 2 –400C, vacuum –He carrier gas with vaporized(or atomized) Si0C 2 H 5 and O 2 and B(CH 3 ) 3 and/or P(CH 3 ) 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 3H 2 +WF 6  W + 6HF –T>800C, vacuum –He carrier gas with WF 6 –Side Reactions at lower temperatures Oxide etching reactions 2H 2 +2WF 6 +3SiO 2  3SiF 4 + 2WO 2 + 2H 2 O SiO 2 + 4HF  2H 2 O +SiF 4 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 –Plug flow reactor Microscopic Analysis –Surface Reaction Film Growth Rate

12. Macroscopic Analysis Plug Flow Reactor (PFR) –Like a Catalytic PFR Reactor –F Ao = Reactant Molar Flow Rate –X = conversion –r A =Reaction rate = f(C A )=kC A –C i =Concentration of Species, i. –Θ i = Initial molar ratio for species i to reactant, A. –ν i = stoichiometeric coefficient –ε = change in number of moles

13. Combined Effects Contours = Concentration

14. Reactor Length Effects SiH 2 Cl 2 (g) + 2 N 2 O(g)  SiO 2 (s)+ 2 N 2 (g)+2 HCl(g) How to solve? Higher T at exit!

15. Deposition Rate over the Radius r C As Thiele Modulus Φ 1 =(2kR w /D AB x) 1/2

16. Radial Effects This is bad!!!

17. Combined Length and Radial Effects Wafer 20 Wafer 10

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

19. Microscopic Analysis -Reaction Steps Adsorption –A(g)+S  A*S –r AD =k AD (P A C v -C A*S /K AD ) Surface Reaction-1 –A*S+S  S*S + C*S –r S =k S (C v C A*S - C v C C*S /K S ) Surface Reaction-2 – A*S+B*S  S*S+C*S+P(g) –r S =k S (C A*S C B*S - C v C C*S P P /K S ) Desorption: C*S C(g) +S –r D =k D (C C*S -P C C v /K D ) Any can be rate determining! Others in Equilib. Write in terms of gas pressures, total site conc.

20. Rate Limiting Steps Adsorption –r A =r AD = k AD C t (P A - P C /K e )/(1+K A P A +P C /K D +K I P I ) Surface Reaction –(see next slide) Desorption –r A =r D =k D C t (P A - P C /K e )/(1+K A P A +P C /K D +K I P I )

21. Surface Reactions

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

23. Silicon Deposition Overall Reaction –SiH 4  Si(s) + 2H 2 Two Step Reaction Mechanism –SiH 4  SiH 2 (ads) + H 2 –SiH 2 (ads)  Si(s) + H 2 Rate=k ads C t P SiH4 /(1+K s P SiH4 ) –K ads C t = 2.7 x 10-12 mol/(cm 2 s Pa) –K s =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 P SiH4 –Miss Orientation angle

25. Surface Diffusion

26. Monocrystal vs. Polycrystalline P SiH4 =? 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 –S=α(E 1/2 -E th 1/2 ) Deposition Rate  –Ion current into Target *Sputtering Yield – Fundamental Charge

35. RF Plasma Electrons dominate in the Plasma –Plasma Potential, V p =0.5(V a +V dc ) –V a = applied voltage amplitude (rf) Ions Dominate in the Sheath –Sheath Potential, V sp =V p -V dc Reference Voltage is ground such that V dc is negative Plasma rf Sheath

36. Floating Potential Sheath surrounds object Floating potential, V f k B T e =eV –due to the accelerating Voltage

37. Plasma Chemistry Dissociation leading to reactive neutrals –e + H 2  H + H + e –e + SiH 4  SiH 2 + H 2 + e –e + CF 4  CF 3 + F + e –Reaction rate depends upon electron density –Most Probable reaction depends on lowest dissociation energy.

38. Plasma Chemistry Ionization leading to ion –e + CF 4  CF 3 - + F –e + SiH 4  SiH 3 + + H + 2e Reaction depend upon electron density

39. Plasma Chemistry Electrons have more energy Concentration of electrons is ~10 8 to 10 12 1/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 –O 2 +e  O 2 + + 2e –O 2 +e  2O + e –O + e  O - –O 2 + + e  2O

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