Thin film technology, intro lecture sami.franssila@aalto.fi

This lecture Thin films vs. bulk Thickness dependent properties Deposition processes in general Growth modes and structure Structural and interfacial stability Interfaces and multilayer films Film quality Film characterization measurements

Thin films vs. bulk Properties different from bulk materials Density Resistivity Thermal conductivity Refractive index Dielectric constant … Properties & structure thickness dependent

Thickness dependent resistivity

Thickness dependent dielectric constant Atomic Layer Deposited SrTiO3 Vehkamäki

Thickness dependent structure ALD deposited ZrO2: the 4 nm thick film is amorphous but the 12 nm thick film is polycrystalline, from ref. Kukli 2007.

1D & 2D structures Freund & Suresh

Generic deposition process

Starting surface Starting surface extremely important Film structure depends on starting surface Film adhesion depends on starting surface Surface preparation before deposition -baking (thermal) -etching (chemical) -bombarding (physical) Cleaning either -ex-situ (just prior to loading into reactor) -in-situ (in the deposition reactor)

Starting surface (2) Chemical nature: passive or active surface groups Physical nature: smooth, porous Stability: thermal, chemical, UV Softness/hardness: film growth on surface or penetration into substrate

PVD: Physical Vapor Deposition

PVD films Poortmans: Thin film solar cells

CVD: Chemical Vapor Deposition gas phase convection diffusion through boundary layer surface processes (adsorption, film deposition, desorption)

CVD step coverage

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

ALD: surface reactions

Plasma enhancement CVD & ALD can be run at lower T in plasma enhanced mode Activation of gas phase processes Activation of surface processes UV activation as a byproduct

Temperature ranges Liquid phase processes: LB, ECD, ... RT PVD: RT but can be run at elevated T ALD: 100-400oC PECVD & PEALD: 50-300oC CVD: 400-900oC

Film viewpoint amorphous nanocrystalline microcrystalline polycrystalline epitaxial textured Allen: MEMS

Epitaxial films No lattice match Polycrystalline film Lattice match + other conditions  epitaxy possible but not guaranteed

More complex films non-stoichiometric: TiN vs. TiNx, x≈0.8 hydrogenated: a-Si vs. a-Si:H, 30 at% H doped: USG vs. PSG, 5 wt% phosphorous metals, oxides: 1-5% dopant polysilicon: 10-3...10-6 dopant porous (on purpose !) oriented crystals (piezoelectric, magnetic)

Doped oxides SiOxCyFz SiO2

Multilayer films Al SiO2 SiNx W Ti/TiN

Generic structure surface interface 2 interface 1 thin film 1 substrate thin film 1 thin film 2 surface interface 2 interface 1 thin film 1

Interfaces Stability of interface in subsequent processing and during use ? Barrier layers: extra films to stabilize interfaces

Annealing: chemical reactions Surface reaction: Titanium nitride formation 2Ti + N2  2 TiN <Si> Ti N2 heat Interface reaction: Titanium silicide formation Si + 2 Ti  TiSi2

Annealing: physical effects

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

Acoustic multilayers 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

Magnetic multilayers 1 nm CoFe 2.6 nm Cu 2.5 nm CoFe 0.8 nm Ru substrate 5 nm Ta 15 nm PtMn 5 nm NiFe

Film quality (1) High density (non-porous) Defect-free Low impurity content Low stress (or tailored stress) Stability Smoothness ? SiO2/Ti; Weileun Fang

Is smooth good ? Fluorine-doped zinc oxide (FZO) (Poortmans) Superstrate solar cell: Repmann

Film quality (2) Uniformity: Homogeneity: across-the-sample uniformity of thickness, resistivity, refractive index… Usually given as U = (max-min):(2*ave), 1-10% typical Homogeneity: film has same structure and composition all over e.g. grain size or dopant concentration is independent of position, no stress gradient, no interfacial layer, …

Inhomogenous films e.g. phosphorous-doped oxide with phosphorous gradient (and therefore refractive index gradient) Grain size gradient Messier 1986 JVST GLAD/sculpted film: rotation and inclined deposition used to create inhomogenous film on purpose (evaporated MgF2; Messier book)

Structure variants SiH4 + nH2  Si + (n+2) H2 Percentage of silane increasing

Use quality Is this material compatible with our environment (e.g. tissue) ? Is this film erosion resistant in our application ? Is this film free of pinholes ? Does the film have spikes/protrusions ? Are the impurities mobile ? Getter films (consumed during use on purpose)

Productivity measures Deposition rate Thru-put (very different from rate !) Yield of precursors (source gases/targets expensive) Deposition on various substrates (e.g. porous) Uniformity across the substrate Uniformity across the batch Repeatability run-to-run Repeatability day-to-day

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 -defects (particles, pinholes,…)

Sputtered TiN characterization