1. CVD & ALD

2. CVD & ALD Chemical Vapor Deposition, CVD Atomic Layer Deposition, ALD
Both chemical reactions and fluid dynamics must be considered.

3. Common CVD reactions

4. Thermal CVD reactions
Gaseous precursor + surface reaction  solid film + gaseous byproducts

5. Thermodynamics of CVD ΔG < 0 for reaction to take place
Pierson: Handbook of CVD, p. 22

6. Silicon CVD from SiH4 Unwanted side reaction
Wanted deposition reaction
Smith: Thin-film deposition

7. Boundary layer = stagnant gas layer
Pierson: Handbook of CVD, p. 36

8. Boundary layer thickness
Re = Reynolds number
density
viscosity

9. Surface limited vs. mass transport limited reactions
surface reaction
slope = Ea1
slope = Ea2
high T low T (1/T)
log rate
When temperature is low, surface reaction rate is slow, and overabundance of reactants is available. Reaction is then surface reaction limited.
Above a certain temperature all source gas molecules react immediately. The reaction is then in mass-transport limited regime (also known as diffusion limited and supply limited regime).

10. Si epitaxy:
surface controlled vs.
mass flow controlled

11. Surface limited vs. mass transport limited reactions (2)
A batch reactor operating in surface reaction limited mode:
-slow reaction
-many wafers
A mass transport limited reactor:
-single wafer
-high deposition rate

12. Thermal CVD reactor: gaseous precursors, resistive heating
Franssila: Introduction to Microfabrication

13. Ramped furnace

15. Thermal CVD reactor: liquid precursors, lamp heating
Changsup Ryu, PhD thesis, Stanford University, 1998

16. LRP= Limited Reaction Processing
Very fast and powerful lamp heating
Introduce gases
Flash the lamp  T up  reaction
Pump away the gases
Introduce new gases
...

17. PECVD: Plasma Enhanced CVD
Plasma aids in chemical reactions
Can be done at low temperatures
Wide deposition parameter range
High rates (1-10 nm/s) (thermal 10% of this)

18. PECVD @ 300oC: can be deposited on many materials
Oxide:
SiH4 (g) + N2O (g) ==> SiO2 + N2 + 2H2
Nitride:
3 SiH2Cl2 (g) + 4 NH3 (g) ==> Si3N4 (s) + 6 H2 (g) + 6 HCl (g)

19. SiNx:H: thermal vs. plasma
Thermal CVD at 900oC
PECVD at 300oC
Smith: J.Electrochem.Soc. 137 (1990), p. 614

20. Nitride thermal vs. PECVD (1)

21. Nitride thermal vs. PECVD (2)

22. Grain size & roughness
Vallat-Sauvain

23. Hydrogen in PECVD films
3SiH2Cl2 (g) + 4 NH3 (g) ==> Si3N4 (s) + 6 H2 (g) + 6 HCl (g)
30 at% hydrogen is usual; % is as small as it gets

24. CVD films lose thickness upon anneal (H2 escapes)
Polysilicon

25. Film quality: etch rate
Low pressure equals more bombardment, and thus denser film, and compressive stress
With SiO2, BHF etch rate is a film quality measure (should be no more than 2X thermal oxide reference
Hey et al: Solid State Technology April 1990, p. 139

26. Stress affects by:
pressure
frequency
temperature

27. ALD: Atomic Layer Deposition
Precursors introduced in pulses, with purging in-between

28. Materials deposited by ALD
Important missing materials:
silicon
silicon nitride

29. ALD applications 1 nm thick catalysts (Pt, Pd)
2 nm thick TiN barrier layers underneath copper
6 nm thick CMOS gate oxides like HfO2
10 nm thick etch masks for plasma etching (Al2O3)
30 nm thick antireflection coatings in solar cells (Al2O3)
200 nm thick barrier layers in flat panel displays (Al2O3)

30. ALD applications

31. ALD process based on:
conformal film

32. Surface saturation Irreversible saturation ALD reactions:
Surface saturates with a monolayer of precursor, strong chemisorption (=chemical bonds formed)
Reversible saturation:
Physisorption only (weak bonds like van der Waals): once precursor flux is stopped, surface specie will desorb.
Irreversible non-saturating.
CVD regime:
more reactants in, more film is deposited (continuosly)

33. ALD window

34. Pulse-by-pulse mass monitoring

35. Growth rate

36. Layer thickness control

37. Layer thickness (2)

38. Layer thickness: nanolaminates
Al2O3 and Ta2O5
Adriana Szeghalmi,Stephan Senz, Mario Bretschneider, Ulrich Gösele, and Mato Knez, APL 2009

41. Impurities in ALD

42. ALD uniformity (=thickness across the wafer)
HfO2 dielectric marathon test: 2000 wafers

43. Uniformity (2)
ALD SiO2
Rate 700 nm/hr Pt

44. ALD conformality (=step coverage in microstructures)
Excellent conformality: all surfaces coated by diffusing gaseous precursors in the surface reaction limited mode.
Al2O3/TiO2 nanolaminate
TiN barrier

45. Step coverage (2)
Step coverage good also in high aspect ratio grooves,
BUT pulse lenghts have to to be increased
(in coating porous materials, pulses last for minutes !!).

46. Structure:
amorphous vs. polycrystalline ?

47. Structure (2) amorphous aluminum oxide
Polycrystalline strontium titanate
Vehkamäki et al. (2001)

48. Plasma-enhanced ALD

49. PEALD

52. Plasma ALD benefits
Plasma can break down precursors at lower temperature
New precursors become available because plasma can break down precursors that could not be used in thermal ALD
Ions can kick of loosely bound specie from surface, densifying the film

53. ALD reactors: single wafer

54. ALD reactors: batch

55. Gas pulsing

56. Spatial ALD
jes.ecsdl.org

57. Spatial ALD reactors

58. Spatial ALD since 1977 !

59. ALD process development

60. PVD CVD & ALD Atoms as source material Molecules as source materials
Solid source materials Solid, liquid, gas precursors
Vacuum/high vacuum Fluid dynamics important
Elemental films mostly Molecular/compound films mostly, Chemical bonds broken & formed
Room temperature Needs elevated temperatures
(or plasma activation)
Alloy films easily (W:N) Elements and compounds OK, alloys more difficult
One process, many materials Each process materials specific
Al, Au, Cu, Pt, … SiO2 SiO2, Si3N4, Al2O3, HfO2, … Si, W

61. ALD CVD

62. CVD vs. ALD difference

63. Summary Thermal CVD: excellent film quality
PECVD: reasonable film quality at low T
ALD: excellent film quality at low T
Thermal CVD: high temperature needed
PECVD: very high rate possible
ALD: best for very thin films