Electrical and optical properties of thin films

sami.franssila@tkk.fi

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