Introduction to Thin Films

• 'Thin' has no precise definition, but films less than 500 nm thick are usually considered thin
• The substrate is typically much thicker than the thin film
• Important thin film parameters include thickness, composition, surface roughness, reactivity, optical absorption, uniformity, conformity, crystallographic orientation, and film-substrate interface properties

Why Study Thin Films?

• Thin films are used in fundamental research and industrial mass production
• Applying a thin film can completely change the surface chemistry of a solid, imparting new properties such as self-cleaning
• Reducing a material to a thin film (nanoscale) changes its properties and allows investigation of quantum size effects
• Interfaces between different thin film materials can show high electronic or ionic conductivity and magnetism, which is vital for silicon solar cells
• Thin films are used to build micro-scale electronic components and silicon chips
• Making a thin film changes the lattice parameters of a material
• Advanced thin film methods deposit single atomic layers to build up artificial chemical structures

Examples of Thin Film Applications

• Copper Indium Gallium Selenide (CIGS) solar cells: thin films are used in solar cell technology
• Self-cleaning windows: thin films impart strong oxidizing properties

Methods of Film Deposition

• A precursor is the starting material used to make the film
• Chemical Vapor Deposition (CVD) occurs if the precursor reacts to form the film
• Physical Vapor Deposition (PVD) occurs if the precursor undergoes a physical change like condensation
• A precursor can be a molecular species carried by a gas stream at atmospheric pressure or a metal evaporated at high temperature and low pressure
• Nucleation and growth of the film are governed by surface energetics

Film Growth Energetics

• If the partial pressure of the gas phase precursor, Pv, is greater than the equilibrium vapor pressure Ps, then adsorption is more energetically favorable than desorption, and a film will grow
• Surface is saturated when Pv = Ps, it will be supersaturated to grow a film
• Adsorbed atoms move around to find the lowest energy sites if they have enough thermal energy

Film Growth Modes

• If the interaction between adsorbed atoms and the surface is stronger than between two adsorbed atoms, layer-by-layer growth occurs
• If the interaction between two adsorbed atoms is stronger than with the surface, island growth occurs
• In mixed growth, the relative strength of interaction changes with film thickness; a film starts growing layer by layer and switches to island growth

Epitaxial and Polycrystalline Films

• Films are categorized based on the substrate they grow on
• Epitaxial films grow on single crystal surfaces, where the film's structure and orientation are influenced by the substrate
• Non-epitaxial films grow on non-single crystal substrates and the crystal orientation may not be completely random

Analysis of Films

• In situ analysis is performed while the film is growing, controlling growth by changing deposition conditions. This has a limited range of techniques
• Ex situ analysis is performed after the film has been deposited, and has a wider range of techniques

Techniques

• Transmission Electron Microscopy (TEM): A thin cross-section of the film is made and an electron beam is fired through it, producing an atomic resolution image
• X-ray diffraction (XRD): Crystal planes diffract X-rays; by measuring this diffraction, crystal structures present in the sample can be determined

X-ray diffraction

• In a diffraction pattern from an epitaxial film, diffraction peaks from the substrate are also seen
• Substrate peaks are the strongest because the substrate is thicker than the film
• Film peaks are lower in intensity and broader: the width of a diffraction peak is related to the size of the crystallites
• In non-epitaxial films, substrate diffraction peaks can still occur if the substrate is crystalline
• With amorphous substrates like glass, a large, broad peak is seen

Lattice Mismatch

• Occurs in epitaxial films when the sizes of the film and substrate unit cells are different
• Lattice Mismatch Formula: f = (aBulk - as)/as, often expressed as a %
• 'As' = lattice parameter of substrate material in the plane of the surface
• 'aBulk' = lattice parameter of film material in the bulk
• Positive lattice mismatch = the film has a larger lattice parameter than the substrate
• Negative lattice mismatch = the film has a smaller lattice parameter than the substrate
• Mismatch can be accommodated through lattice strain or by introduction of defects
• Positive lattice mismatch experiences compressive strain to compensate within the IP plane, a. Film expands in the OOP direction, c
• Negative lattice mismatch causes tensile strain in the IP direction, a, and film contracts OOP (out of plane), c
• Lattice mismatch can affect crystal structures

Defect Formation

• Defects consist of one or more missing bonds or atoms in the wrong place for a given crystal structure, which influences properties such as conductivity and magnetism

Critical Thickness

• At low thickness, lattice mismatch = films are fully strained
• Film increases in thickness = more energy is required to keep the film strained, and defects start to form and film becomes partially strained
• At large thicknesses, only defects are present, and film is now fully relaxed
• In large lattice mismatches, critical thickness can be very small, maybe only 1-2 unit cells thick
• The estimated critical thickness is given as hc=CBulk/2f, where CBulk is the bulk lattice of the film parameter of the film, and f is lattice mismatch

A Survey of Deposition Techniques

• General parameters apply to any deposition:
  • Temperature of the substrate is usually heated to promote crystal growth
  • The dimensionless Knudsen number describes the different pressure regimes: Kn = λ/L
  • Kn = Knudsen number -L = the characteristic length of the reactor
  • λ = the mean free path of a gas phase molecule

Molecular Beam Epitaxy

• Each element needed for the film is evaporated at high temperature at very low pressures
• Evaporated precursors travel in molecular or atomic beams
• Elements Ga and As would be evaporated simultaneously onto the substrate to grow GaAs
• High temperature evaporation cells heat the required elements up to 1000°C, causing vaporization
• Typical pressures 10-10 mbar, meaning the mean free path of the atomic beams is longer than the distance to the substrate
• Film growth is monitored by Reflection High Energy Electron Diffraction (RHEED) & high energy electron gun which controls film growth on a unit cell level

Pulsed Laser Deposition

• The precursor is a solid pellet made of the desired film material called the target
• A laser beam is fired to cause a small volume of the target to become a plasma of high energy ions that rapidly expands towards the substrate to grow a desired film
• Pressures can be up to 10-4 mbar
• An ultraviolet 'eximer' (excited dimer) laser is used to hit the target, causing rapid dissociation of chemical bonds & forming a gas phase plasma that contain substrate hits

RHEED Intensity Oscillations

• RHEED is used for in situ monitoring and control of MBE and PLD
• In layer-by-layer growth this allows deposition of a single unit cell thickness to switch materials during deposition for layered films
• Integer number of unit cells of different materials are layered on top of each other to create Superlattices
• A perovskite can be built up from individual SrO and TiO2 layers to create Artificial unit cells

Comparison of MBE and PLD

• MBE can grow higher quality films with higher purity because evaporation of precursors is easier to control
• PLD is more versatile; almost any material can be made into a target, but MBE each element needs its own Knudsen cell

Chemical Vapor Deposition (CVD)

• A film is deposited by chemical reaction of precursors in the gas phase
• Precursors are volatile inorganic or organometallic molecules transported into the reaction chamber and can be carried out at different pressures using single crystal substrates or polycrystalline/amorphous substrates
• CVD is used extensively in industry for mass production with a 5 step process: -Volatilisation of the precursor -Transport of the precursor to the reaction chamber -Chemical reaction of the precursor before or after adsorption onto the substrate -Desorption of unwanted by-products -Nucleation and growth of the film
• Two main types of CVD reactors: -Hot wall ( uniform coating but precursor is wasted) -Cold wall (thermal gradients may not be uniform but film quality improves)

CVD Kinetic Regimes

• Dominated by Rate of reaction, rate of transport of precursor to the reaction zone, and rate of competing reactions
  • Reaction rate limited regime: At low temperatures, reaction rate is much slower than precursor delivery.
  • Diffusion rate limited regime: At higher temperatures, reaction rate is fast compared to precursor diffusion to the substrate.
  • Parasitic Reaction Regime: At very high temperatures, competing decomposition reactions can reduce the growth rate.

CVD Precursors Requirements

• Volatile
• Decompose
• Low cost
• Environmentally friendly
• Designing act focuses on balancing factors by using Single & Multi source precursors

CVD Precursors:

• Assessing Precursor Volatility with Thermogravimetric Analysis (TGA) a small sample the pan up that is heated over time to reveal evaporative properties

CVD Precursors:

• Al metal films are used for conductive interconnects in some applications

Atomic Layer Deposition (ALD)

• Technique related to conventional CVD where each precursor is individually 'pulsed' onto the substrate sequentially
• This process is repeated until the required thickness is achieved
• Pulse: individual precursor is flowed into the reactor chamber of inert gas
• Cycle: combination of pulse (precorse), inert gas, second process & inert gas again
• Self Limiting: the growth process where only a single monolayer of precursor adsorbs onto the substrate which requires precursors with a limit that is removed by the inert gas pulse
• High temperatures causes metal degradation - becoming more like CVD
• This process has a set monolayer growth rate (unlike CVD)

Steps in the ALD of Al2O3 from AlMe3 and H2O

-Pulse of H2O forms OH groups on the substrate surface followed by an inert gas pulse to clear excess H2O

• AlMe3 reacts with the OH growing AlOH = gas. This process is repeated with heat and inert gas to monitor the result -H2O reacts with AlMe to grows the desired thickness. All this helps achieve conformal substrate or surface