Jari Koskinen, Sami Franssila jari.koskinen@aalto.fi 19.9.2016 MT-0.7076P Thin Film Technology Doctoral course (8 credits) 1. Source of material for thin film process Jari Koskinen, Sami Franssila jari.koskinen@aalto.fi 19.9.2016

Contents Material from solid and/or gas Vapourizing ion sputtering laser processing Arc evaporation Sputtering Gas ambient neutral gas reactive gas Gas kinetic theory vacuum basics molecular mean free path, accumulation of a molecular film thermalization Impurities

SOURCE solid liquid vapor gas SUBSTRATE component tool

Extraction of materials from solid Evaporation Sputtering Arc (evaporation) Laser heating Thermal Kinetic

Evaporation Thomas M. Christensen UCCS 2000

electron beam evaporator view of sample with beam shutter

Evaporation temperature for p = 10-2 torr =1.33 10-2 mbar [Mahan, p149]

Vaporization Evaporation or sublimation of atoms from surface due to thermal energy If the surface of source is in equilibrium with the vapor, the rate of molecules flowing from the source is:

Vapor pressure

Vapor pressure

Vapor pressure from tabulations

Vapor pressure of alloys

Vapor pressure of alloys

Thomas M. Christensen UCCS 2000

Methods to heat target Resistive electron beam easy to upscale clean process cooled crusible may be used

Sputtering of target Erosion of target due to collision of ions to the surface

Sputtering Source of energetic ions plasma ion source ( plasma inside)

Features of sputter prosses Acceleration of an ion across the cathode sheath Penetration of the target, resulting in a series of atomic collisions Backward ejection of one or more recoiling target atoms The maximum possible projectile energy – difference between the plasma potential and that of the cathode Ions,photons, electrons and neutral cause sputtering The target may emit neutral, ions and clusters

Sputtering yield of elements Thomas M. Christensen UCCS 2000

About binding energy

Sputtering yield of elements

Thomas M. Christensen UCCS 2000

Filtering of particles from vacuum arc plasma plasma follows magnetic field lines plasma bent around corner particles go straight Andre Anders LBL

Laser ablation

Plasma plume Copper target

Laser plasma plume Carbon plasma from graphite target

Particularly suitable for alloyed materials Organic Spintronics

SOURCE solid liquid vapor gas TRANSPORT gas phase vacuum liquid SUBSTRATE component tool

Residual gas Pumping of: Residual gas: Adsorption – Desorption Diffusion of dissolver or trapped gas Permeation trough materials Leaks diffusion of dissolved molecules desorption pump permeation leak

Total pressure of residual gasses Average mean free path (distance between collission) in nitrogen residual gas <λ> Ultra Good High High Inter-mediate Rough Total pressure of residual gasses

Total pressure of residual gasses Average mean free path (distance between collission) in nitrogen residual gas <λ> Ultra Good High High Inter-mediate Rough Total pressure of residual gasses

Vapour and liquid in vacuum - residual gas is a major source of impurity pump balance: condensation = evaporation balance: pumping = evaporation pressure constant until all liquid is pumped

Total pressure of residual gasses Time to form one molecular layer on surface Ultra Good High High Inter-mediate Rough Total pressure of residual gasses

Critical temperatures and pressures for some residual gasses Gas or vapor pump Helium Hydrogen Nitrogen Carbon monoxide Argon Oxygen Methane Carbon dioxide Chlorine Ether Ethanol Carbon tetraclor. Water above Tc no liquid

Gas sources in reactive sputtering or reactive evaporation controlled vapor pressure by using mass flow controllers oxides, nitrides, carbides, hydrides oxygen containing gas or vapor are main sources of contamination in thin films e.g. Al, Ti, -> AlOx and TiOx