electrical and optical properties of thin films [email protected]
TRANSCRIPT
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
Resistivity
ρ = ρresidual + ρtemp
Linear TCR above Debye temperature (typically 200-400K)
Murarka: Metallization
Resistivity: impurity effects
Murarka
Resistivity: alloying effects
Murarka
Alloying (1)
Alloying (2)
Zirconium at grain boundaries acts as an extra barrier, preventing formation of high resistivity Cu3Si
Annealing defects away
Annealing defects at elevated temperature lowers resistance (no reaction with underlying film/substrate)
Murarka: Metallization
Thin film reaction: Co+Si
Murarka
Resistivity: substrate & thickness
Thickness dependent resistivity
Thickness dependent resistivity
Resistivity as a function of film thickness
γ = film thickness/mean free path
Mean free paths typically tens of nanometers at RTMurarka
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
Resistivity depends on patterns!
G.B. Alers, J. Sukamto, S. Park, G. Harm and J. Reid, Novellus Systems, San Jose -- Semiconductor International, 5/1/2006
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)
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
Flexible metallization: Pt on PI
Stretchable metallization: Au/PDMS
Strain-resistivity
Stretchable metallization (2)
PDMS casting
Seed metal, lithography and electroplating
Seed metal, lithography and electroplating
Resist removal, PDMS casting
Resist removal and DRIE DRIE
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.
Brute force metallization of an elastic polymer membrane:
Sputtering+electroplatingon polymer
Anchored metallization by metallization of silicon followed by polymer casting
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.
Stability of metallization
Ti andTi/TiN barriersTo prevent reaction between Si and Cu
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
Semiconductor films
• LPCVD polysilicon
• In-situ vs. Ex-situ• α-Si vs. true poly• α-Si (annealing, crystallization)
LPCVD Poly-Si
LPCVD-poly (2)
Dielectric films: electrical
• Dielectric constant
• Breakdown field
• Structure vs. Stability vs. Leakage
Low-k dielectrics
SiOC
SiOC
Pores
Subtractive porosity
High-k dielectrics
Amorphous initially,
polycrystalline as thickness increases
22
SiOkhighkhigh
SiO ttEOT
Leakage current
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
Optical
• Amorphous
• Isotropic
• No birefrongence
• Losses below 10-4 required
• Waveguide losses < 1 dB/cm
Refractive index
General requirements
Transmission, absorption
Waveguiding requires large nhigh-nlow
ReflectionMechanical scratch resistanceEnvironmental
stability
General requirements (2)
• Depositon rate
• Uniformity, thickness <3%, even <1%
• Uniformity, refractive index <0.001
• Stresses
• Defect density
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
Diamond as optical material
pc-D (polycrystalline diamond)
High transparency 200 nm ... 20 µmHigh 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 yesnf ~ 1.6-2.2k ~ up to 0.8 (heavy absorption)
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 nClosed pores lead to density and nf reduction
Excellent material for graded index filters: n=1.48-2.0
Reproducibility of n is ~1%
Optical filters (1)
1) Multilayer (step index) design
2) Inhomogenous graded index design
3) Quasi-inhomogenous design (λ/4 layers)
Optical filters (2)
Optical filters (3)
Nitrous oxide flow rate
Refractive index profile On glass substrate
On polycarbonate substrate