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 material = film material Heteroepitaxy: Substrate material film material

2 Repetition: Heteroepitaxy I Epitactic relation: Substrate material Film material Van-der-Waals epitaxy: The interaction between substrate material and film material is so weak, that film atoms can arrange themselves 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 i1 R q - or RMS-value: mean quadratic deviation R q R RMS RMS 1 N N h h i i1 2

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

10 Repetition: Correlation Length Surface profile Autocovariance function Within the profile exhibits similar heigth values. Periodicities are present, if R() exhibits maxima at 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: MECH T I Mechanical stress: MECH Thermal stress: T E S ( S U )(T B E S... Elastic modulus film S... CTE film 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: 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 of a thin film: Esd 6(1 2 s s )d F 1 R s1 1 R s2 a) Substrate b) Film c) Reference platelet E S... Elastic modulus substrate 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: tan(2) H Substrate Film Compressive stress Tensile stress l l R S 1 a tan 2 H l sin R S R S l 2 a tan H 2lH H Neglections and prerequisites: a) lateral displacement of the cantilever b) vertical displacement of the cantilever () 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 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 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 Cu = 16 ppm/k C = 2 ppm/k