1. Thin films

2. Thin films: different from bulk?
Thickness: nm to µm
Properties thickness dependent
electrical (resistivity)
optical (transmission)
mechanical (Young’s modulus)
thermal (conductivity)
Structure: amorphous, polycrystalline or single crystalline
Structure depends on deposition method
Structure changes in high temperature steps
Often severe stresses (tensile or compressive)

3. Thickness dependent resistivity

4. Thickness dependent dielectric constant
Atomic Layer Deposited SrTiO Vehkamäki

5. Thickness dependent structure
High resolution TEM pictures
ALD deposited ZrO2: the 4 nm thick film Is amorphous.
ALD deposited ZrO2: 12 nm thick film is polycrystalline.
from ref. Kukli 2007.

6. PVD: Physical Vapor Deposition
The whole wafer is covered by the deposited film.

7. Evaporation
Simple:
Heat metal until vapor pressure high enough  metal vapor will be transported in vacuum to the wafer.
Metal vapor condensation results in film growth.
Very few parameters to change 
Cannot optimize film quality.
electron
beam gun
wafer
crucible

8. Sputtering Electric field excites argon plasma.
wafer
target
Electric field excites argon plasma.
Accelerated argon ions hit metal atoms from target.
Target atoms are transported in vacuum to the wafer.
Many parameters:
power, pressure, temperature, gas specie (Ar usually)

9. Metallic films usually by PVD
conductors (Al, Au, Cu)
resistors (Ta, W, Pt)
capacitor electrodes (poly-Si, Al, Mo)
mechanical materials (Al-movable mirrors)
magnetic materials (Ni coils)
protective coatings (Cr, Ni etch masks)
optical materials (mirrors, reflectors, IR filters)
catalysts (Pt, Pd in chemical sensors)

10. Rs  /T Sheet resistance
Rs is in units of Ohm, but it is usually denoted by Ohm/square to emphasize the concept of sheet resistance. Resistance of a conductor line can now be easily calculated by breaking down the conductor into n squares: R = nRs
Aluminum film 1 µm thick, sheet resistrance ?
Tungsten film, 100 nm thick, sheet resistance ?

11. Resistor sheet resistance
Figure 2.8: Conceptualizing metal line resistance: four squares with sheet resistance Rs in series gives resistance as R = 4Rs.

12. Resistor design How to increase resistor resistance ?L W
How to increase resistor resistance ?
Change L: make it longer
Change W: make it narrower
Change T: make it thinner
Change ρ: choose material with higher resistivity

13. PVD films for solar cells
50 nm
2 µm
1 µm
Poortmans: Thin film solar cells

14. CVD: Chemical Vapor Deposition
gas phase convection
diffusion through boundary layer
surface processes
(adsorption, film deposition, desorption)
The whole wafer is covered by the deposited film.

15. Common CVD processes SiH4 (g) ==> Si (s) + 2 H2 (g)
SiCl4 (g) + 2 H2 (g) + O2 (g) ==>
SiO2 (s) + 4 HCl (g)
3 SiH2Cl2 (g) + 4 NH3 (g) ==>
Si3N4 (s) + 6 H2 (g) + 6 HCl (g)

16. Thermal vs. CVD oxide thermal, 1000oC Applicable only on silicon.
High temperature.
silicon
Deposition also on metals.
Low temperature.
silicon
(PE)CVD oC

17. Plasma Enhanced CVD Deposition can be done at 300oC.
Thermal CVD is usually oC
(Thermal oxidation at 1000oC)
Oxide:
SiH4 (g) + N2O (g) ==> SiO2 (s) + N2 (g) + 2H2 (g)
Nitride:
3 SiH2Cl2 (g) + 4 NH3 (g) ==> Si3N4 (s) + 6 H2 (g) + 6 HCl (g)

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

19. Dielectric films SiO2 gate oxide in CMOS 1-50 nm
SiO2 isolation oxide in CMOS nm
SiO2 diffusion mask nm
SiO2 etch mask in MEMS nm
Si3N4 oxidation mask nm
Si3N4 membrane in MEMS nm
Si3N4 capacitor dielectric nm
Al2O3 capacitor dielectric nm
HfO2 capacitor dielectric nm
SiNx passivation coating nm

20. Sample wafers !

21. Thin film patterns The whole wafer is covered by the deposited film.
If you want patterns of films, you have to do lithography and etching.
Lithography
Etching
Remove photoresist

22. Etching two-layer films
<Si>
<Si>
<Si>
Two separate layers: make upper pattern larger than bottom pattern!
If two layers are perfectly aligned, they were made in the same litho & etch steps.
Otherwise alignment error would be visible.

23. poly = CVD polysilicon =polycrystalline silicon
SiH4 (g) ==> Si (s) + 2 H2 (g)
Deposited by CVD at 625oC
Usually deposited undoped
Doping after deposition by diffusion/implantation
Annealing typically 950oC, 1 h to active dopants
Heavy doping ca. 500 µΩ-cm
Grain size ca nm
Annealing changes film stress (and grain size)
Typical thickness 100 nm-2 µm

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

25. ALD: surface reactions

26. ALD films Al2O3 diffusion barrier 1-20 nm
Al2O3 hard mask in etching nm
HfO2 capacitor dielectric nm
TiN electrode nm
TiN protective coating nm
TaN barrier layer nm
Pt catalyst nm

27. Growth modes
layer-by-layer island growth columnar growth

28. Step coverage in depositionH B
Ratio of film thickness on sidewall to horizontal surfaces (100% = conformal coverage)
Cote, D.R. et al: Low-temperature CVD processes and dielectrics, IBM J.Res.Dev. 39 (1995), p. 437

29. ALD step coverage
Excellent conformality: deposition is a surface controlled reaction.
Al2O3/TiO2 nanolaminate
TiN barrier
Franssila: Microfabrication

30. Electroplating Typical plated metals: -nickel (Ni) -copper (Cu)
-gold (Au)
Not applicable to:
-aluminum (Al)
-most refractory metals (W, Ti, ...)

31. Electroplated structures
a) Seed layer sputtering and lithography
b) Electroplating metal
c) Resist stripping
d) Seed layer removal
Why use electroplating ?
Metals like copper and gold do not have anisotropic plasma etch processes available
 If you want vertical walls, electroplating is a solution.

32. Released plated metals

33. Stresses in thin films
The substrate is in opposite stress state !

34. Origin of stress Extrinsic stresses: thermal expansion mismatch
Intrinsic stresses: deposition process dependent
low energy deposition
 no energy for relaxation process
high energy deposition
 non-equilibrium, forced positions
impurities, voids, grain boundaries

35. Cantilever bending
Fang, W. & C.-Y. Lo, On the thermal expansion coefficients of thin films, Sensors &Actuators 84 (2000), p. 310

36. Stresses in bimetal cantilever

37. Generic thin film structure
substrate
thin film 1
thin film 2
surface
interface 2
interface 1
Various interfacial processes take place during following process steps and during device operation !
thin film 1

38. Reactions in thin films
Surface reaction:
Titanium nitride formation
2Ti + N2  2 TiN
<Si> Ti N2
heat
Interface reaction:
Titanium silicide formation
Si + 2 Ti  TiSi2

39. Interfaces
Stability of interface in subsequent processing and during use ?

40. Barriers and adhesion layers
In order to stabilize the interfaces, additional films are introduced: -to improve adhesion, e.g. Ti/Pt, Cr/Au -to prevent interdiffusion, Si/TiW/Al, SiO2/TaNx/Cu; SiO2/SiNx/Cu -to prevent ion movement: glass/Al2O3/poly-Si -to protect from ambient: SiO2/SiNx

41. Multilevel metallization with Ti/TiN barriers
SiO2
SiNx W
Ti/TiN

42. Copper for IC metallization
General:
low variation
low particle generation
large process window
Barrier:
t < 10 nm thick
ρ < 500 µΩ-cm
Cl conc. < 2%
unif. < 2%
step coverage >90%
rate > 3 nm/min
Low-k:
CMP compatible
Tdepo < 400oC
adhesion on etch stopper
Copper seed
t > 2 nm
step coverage ~100%
rate > 10 nm/min
growth and adhesion on etch stopper
Etch stopper:
growth and adhesion
on dielectric
on barrier

43. Acoustic multilayers Glass wafer Al (300 nm) Mo (50 nm) ZnO (2300 nm)
Au (200 nm)
Ni (50 nm)
SiO2 (1580 nm)
W (1350 nm)
TiW (30 nm)
TiW (30 nm)
Glass wafer

44. Film characterization needs
-spatial resolution (image spot size)
-depth resolution (surface vs. bulk properties)
-elemental detection (constituents, impurities)
-structural information (grain structure)
-dimensional characterization (thickness)
-mechanical properties (curvature, stress,…)
-surface properties (roughness, reflectivity,…)
-top view vs. cross sectional imaging -…

45. Sputtered TiN characterization

46. Thin films
On this course, we are interested in applications of thin films in microfabrication
Prof. Jari Koskinen is teaching Thin film technology (period IV):
“Principles of vacuum technology, surface physics and surface-ion interactions and low pressure plasma. Thin film methods: Physical vapor deposition, chemical vapor deposition, and other plasma. Characterization methods for thin films to determine, structure, composition, and mechanical and optical properties.”