1. Electrical and optical properties of thin films

2. Outline Metallic films
Thickness dependent resistivity
Limit of Ohm’s law
Metallization for flexible electronics
Semiconducting films (Silicon microtechnology 2009 slides !)
Dielectric films, electrical properties
Dielectric films, optical properties

3. Resistivity ρ = ρresidual + ρtemp
Linear TCR above Debye temperature (typically K)
Murarka: Metallization

4. Resistivity: impurity effects
Murarka

5. Resistivity: alloying effects
Murarka

6. Alloying (1)

7. Alloying (2)
Zirconium at grain boundaries acts as an extra barrier, preventing formation of high resistivity Cu3Si

8. Annealing defects away
Annealing defects at elevated temperature lowers resistance (no reaction with underlying film/substrate)
Murarka: Metallization

9. Thin film reaction: Co+Si
Murarka

10. Resistivity: substrate & thickness

11. Thickness dependent resistivity

12. Thickness dependent resistivity

13. Resistivity as a function of film thickness
γ = film thickness/mean free path
Mean free paths typically tens of nanometers at RT
Murarka

14. Resistivity in polycrystalline films
R = reflectivity at grain boundaries (0.17 for Al, 0.24 for copper)
lo = mean free path inside grain
d = spacing between reflecting planes
Grain boundaries trap impurities, and above
solubility limit, this leads to segregation
Murarka

15. Resistivity depends on patterns!
You cannot calculate thickness from resistance
R = ρL/Wt
because thin film resistivity ρ is linewidth and thickness dependent
(use e.g. X-rays to get an independent thickness value)
G.B. Alers, J. Sukamto, S. Park, G. Harm and J. Reid, Novellus Systems, San Jose -- Semiconductor International, 5/1/2006

16. Grain size affected by:
-underlying film (chemistry and texture)
-deposition process (sputtering vs. plating; & plating A vs. plating B)
-material purity
-thermal treatments
-geometry of structures on wafer
                                                                                          
G.B. Alers, J. Sukamto, S. Park, G. Harm and J. Reid, Novellus Systems, San Jose -- Semiconductor International, 5/1/2006

17. Flexible metallization: Pt on PI

18. Stretchable metallization: Au/PDMS

19. Strain-resistivity

20. Stretchable metallization (2)

22. Brute force metallization of an elastic polymer membrane:
PDMS casting
Seed metal, lithography and electroplating
Resist removal, PDMS casting
Resist removal and DRIE
DRIE
Brute force metallization of an elastic polymer membrane:
Sputtering+
electroplating
on polymer
Anchored metallization by metallization of silicon followed by polymer casting
Yin, H-L et. al.: A novel electromagnetic elastomer membrane actuator with a semi-embedded coil, Sensors and Actuators A 139 (2007), pp. 194–202.

23. Electromigration
Electromigration is metal movement due to electron momentum transfer. Electrons dislodge metal atoms from the lattice, and these atoms will consequently move and accumulate at the positive end of the conductor and leave voids at the negative end.

24. Stability of metallization
Ti and
Ti/TiN barriers
To prevent reaction between Si and Cu

25. Specific contact resistance, rc
Ti reduces any SiO2 at the interface to TiO  rc down
TiN is high resistivity material  higher rc
CuTi starts to form above 300oC
TiN is a better barrier and rc is reduced the higher the anneal temperature

26. Semiconductor films LPCVD polysilicon In-situ vs. Ex-situ
α-Si vs. true poly
α-Si (annealing, crystallization)

27. LPCVD Poly-Si

28. LPCVD-poly (2)

29. Dielectric films: electrical
Dielectric constant
Breakdown field
Structure vs. Stability vs. Leakage

30. Low-k dielectrics

31. SiOC

32. SiOC

33. Pores

34. Subtractive porosity

35. High-k dielectrics Amorphous initially,
polycrystalline as thickness increases

37. Leakage current

38. Optical thin films
The technique must allow good control and reproducibility of the complex refractive index
k (λ) < 10-4 for transparent films
Two materials with

39. Optical Amorphous Isotropic No birefrongence
Losses below 10-4 required
Waveguide losses < 1 dB/cm

40. Refractive index

41. General requirements Mechanical scratch resistance Reflection
Environmental stability
Waveguiding requires large
nhigh-nlow
Transmission, absorption

42. General requirements (2)
Depositon rate
Uniformity, thickness <3%, even <1%
Uniformity, refractive index <0.001
Stresses
Defect density

43. Smart windows Layers correspond to (1) polyester-based
laminated double foil, (2) ITO transparent electrodes, (3)
nanoporous tungsten oxide, (4) polymer serving as a conductor
of ions, (5) nanoporous nickel oxide. The application of a
voltage (denoted as V) changes the transparency

44. Diamond as optical material
pc-D (polycrystalline diamond)
High transparency 200 nm µm
High refractive index, n = 2.35
Crystal size, ~ µm, leads to scattering at visible wavelengths
>600oC deposition rules out many optical substrates
DLC-films not transparent in visible but in IR yes
nf ~
k ~ up to 0.8 (heavy absorption)

45. SiOxNy:H Truely oxynitride, Si-O-N bonds, not SiO and SiN domains
Amorphous and homogenous till 900oC
Open pores lead to H2O adsorption and lower n
Closed pores lead to density and nf reduction
Excellent material for graded index filters: n=
Reproducibility of n is ~1%

46. Optical filters (1) Multilayer (step index) design
Inhomogenous graded index design
Quasi-inhomogenous design (λ/4 layers)

47. Optical filters (2)

48. Optical filters (3) Refractive index profile On glass substrate
On polycarbonate substrate
Nitrous oxide flow rate