Epitaxy is the deposition of layers, which are monocrystalline in large regions

Transcription

1 Repetition: Epitaxy Epitaxy is the deposition of layers, which are monocrystalline in large regions Homoepitaxy: Substrate meterial = film material Heteroepitaxy: Substrate material film material

2 Repetition: Heteroepitaxy I Epitactic relation: Substrate material Film material Van-der-Waals epitaxy: The interaxction between substrate material and film material is so weak, that film atoms can arrange themselkves in a crystallographically favorable manner.

3 Repetition: Heteroepitaxy II High temperature epitaxy: The crystallographically favorable arrangement of the atoms is reached by a high substrate temperature. Low temperature epitaxy : The crystallographically favorable arrangement of the atoms is reached by local defect structures Vicinal surfaces Dendritic Islands

4 Repetition: Growth Modes Growth modes: A: Substrate material B: Film material Frank-Van der Merwe: layer by layer W AB>WBB Volmer-Weber: islands, W AB<WBB Stranski-Krastanov: layer/island, W AB>WBB stress relief by 3d islands While the Frank-van der Merwe and Volmer-Weber Growth modes lead to mostly stress free films, in the Stranski-Krastanov-mode significant stresses are induced in the first growth phases.

5 Repetition: Stress/Film Growth Detailed mechanism: Lattice Mismatch : = a a b 100[%] b Film, lattice constant b Pseudomorphic transition zone Substrate, lattice constant a a

6 Repetition: Roughness Types h(x) h max h min x Stochastic roughness h(x) a R R' R'' h Self similarity h(x) L L' R=f(L), R''>R'>R L'' x Ballistisc aggregate x

7 Repetition: Shadowing Peaks grow faster than valleys + Formation of columnar structures (a) + Pore formation in combination with surface diffusion (b) (a) (b)

8 Repetition: Roughness Values R a -value: mean absolute deviation R = 1 N h a N h i i= 1 R q - or RMS-value: mean quadratic deviation R q = R RMS = RMS = 1 N N ( h h ) i i= 1 2

9 Repetition: Correlation Functions Non normalized quantities Autocovariance function Normalized quantities Autocorrelation function R( τ) = h(x) h(x + τ) dx ρ( τ) = R( τ) / R(0) Structure function S( τ) = [h(x) h(x + τ)] 2 S( τ) = 2R 2 q [1 ρ( τ)] Note: All heigth values are measured from the mean heigth h.

10 Repetition: Correlation Length ξ Surface profile Autocovariance function Within ξ the profile exhibits similar heigth values. Periodicities are present, if R(τ) exhibits maxima at τ 0.

11 Film Structure and Film Properties The film structure influences: + Density + Mechanical properties + Electric properties + Magnetic properties + Electronic properties

12 Application Profiles Depending on the application a certain film structure can be advantageous or disadvantageous: + Tool dense, fine grained + Biolog. material porous, soft + Elektronics dense, monocrystalline + Therm. barriers porous, hard

13 Structure Zone Models Growth phases of a coating Nucleation on substrate Partial coalescence and interface formation Total coalescence and polycrystal formation Growth of polycrystal grains Structure zone models yield a qualitative image of film growth, morphology and crystallography in dependence on the coating parameters.

14 Movchan-Demchishin: Evaporation

15 Thornton: Sputtering

16 Ion Plating

17 Zones and Growth Mechanisms Zone Mechanism Char. feature 1: T/T M <0,2 T: T/T M <0,4 2: T/T M <0,8 3: T/T M >0,8 Shadowing Particle energy Surface diffusion Volume diffusion Fibers, pores Nano grains Columnar crystlites 3d - Grains

18 Stresses Kinds of stresses: σ = σ + σ + σ MECH T I Mechanical stress: σ MECH Thermal stress: σ T = E S ( α S α U )(T B E S... Elastic modulus film α S... CTE film α U... CTE substrate T B... Deposition temperature T M... Measurement temperature T M ) Generated by clamping of the substrate and subsequent unclamping Generated by the different Coefficients of Thermal Expansion (CTE) of substrate and coating

19 Stresses and Film Structure Intrinsic stress: σ I Intrinsic stresses are a direct consequence of the film structure and of the deposition conditions. Tensile stress Compressive stress Variable Compressive stress Tensile stress

20 Intrinsic Stresses: Sputtering

21 Stress Measurement: Fundamentals Curved substrate: Tensile stress Compressive stress Total stress σ of a thin film: σ = Esd 6(1 ν 2 s s )d F 1 R s1 1 R s2 a) Substrate b) Film c) Reference platelet E S... Elastic modulus substrate ν S... Poisson-number substrate d S... Thickness of substrate d F... Film thickness R S1, R S2... Radius of curvature before and after coating, respectively

22 Stress Measurement: Cantilever Geometry: Principle: α tan(2α) H Substrate Film Compressive stress Tensile stress l l R S δ α 1 2 a tan H l sin α α = R S R S l 2 a tan H 2lH H 2α Neglections and prerequisites: a) lateral displacement of the cantilever b) vertical displacement of the cantilever (δ) c) low ratios /H

23 Stresses and Film Growth I In-Situ-measurements by the cantilever method: Tensile stress t COATING Type I: T large, mobility small M Compressive stress 0 Coalescence Type II: T small, mobility large M t Volume reduction by recrystallization after coating leads to tensile stresses Influence of the film thickness on σ I

24 Stresses and Film Growth II In-Situ-measurements by the cantilever method : Evaporation of Al -6 >10 mbar Type I Tensile stress 0 Compressive stress -11 <10 mbar mbar mbar t Type II Transition: segregation Influence of impurities in the residual gas During the evaporation process on σ I

25 Lattice Mismatch and Self Organization I Detailed mechanism: Lattice Mismatch : = a a b 100[%] b Film, lattice constant b Pseudomorphic transition zone Substrate, lattice constant a a

26 Lattice Mismatch and Self Organization II Example: self organization of island positions in InAs/GaAs Multilayers: Quantum dot Interlayer Stranski-Krastanov wetting layer The lattice strain in the interlayer generates a preferred nucleation position directly above an island.

27 Stress Measurement by X-Rays Principle: Measurement of the global strain of the elementary cell generated by: + Interstitial atoms + Impurities Advantages: + Non-destructive + In Situ possible Disadvantages: Several other influences: + Lattice defects + Dislocations + Impurities + Foreign phases

28 Example: Temperature Variation Roentgenographic stress determination at variable temperature: Stress [MPa] Tensile stress Compressive stress as deposited Delamination 1. cycle: heating 1. cycle: cooling 2. cycle: heating 2. cycle: cooling Temperature [ C] Carbon substrate coated with 4 µm Cu α Cu = 16 ppm/k α C = 2 ppm/k