Course Tutor Dr R E Hurley

1. Vacuum evaporation of thin films Course Tutor Dr R E Hurley Northern Ireland Semiconductor Research Centre School of Electrical & Electronic Engineering The Queen’s University of Belfast

2. Course Content • Applications • Reasons for using vacuum environment • Fundamentals of evaporation • Practical requirements (types of source) • Uniformity issues • Compounds, alloys and mixtures

3. Applications of vacuum evaporation Late 19th century saw first metal films but mainly in research labs until after 2nd World War when vacuum became an industrial reality. Anti-reflection coatings, front-surface mirrors, interference filters, spectacles, decorative coatings onto many substrates including plastics, (metal + lacquer) hard-coatings for mechanical applications, corrosion resistance and protection CRT’s and electronics.

4. Evaporation in vacuum Why evaporate in vacuum? Evaporation in atmosphere means the following: 1. Chemical reactions with intervening atmosphere. 2. Nucleation in gas phase together with high bombardment rate at surface leads to porous coatings 3. Considerably reduced coating rate. i.e inefficient 4. H20, HCs, N2, O2, H2, etc. and other impurities react and combine with growing film. Hence remove air until no collisions (mfp > 10cm), and impingement rate is reduced to acceptable level (10-6 mbar, 1015/cm2s)

5. Evaporation a 3-stage process • A condensed phase( solid or liquid) transforms to a vapour phase • The vapour crosses the intervening space • The vapour condenses on the substrate • Each step is critical in determining film properties.

6. Stage 1 - evaporation • Hertz, 1882 found that a liquid cannot exceed a certain • evaporation rate for a given temperature. • At surface 2 phases exist in equilibrium, liquid (or solid) • and vapour. (Atoms are return (condensing), as much as they leave (evaporating).) • This condition is characterised by a vapour pressure, p(T). • The evaporation is given by Hertz-Knudsen equation: • R = (2πmkT)-1/2p mols/sec.cm2 • or, as given by Langmuir (equally for solids) • R = 0.0583(M/T)1/2 p g/cm2s • (about 10-4 gms/cm2sec for p = 10-2 mbar)

7. Evaporation data for elements

8. Distribution of evaporant on substrate • A Knudsen cell (box with hole) allows measurements of • mass distribution with angle without substrate effects. • Theory and experiments show that distribution is cosine • (mass of material evaporated from a small area at a given • angle x from normal is proportional to cosx) • The Cosine Law applies to most materials of interest and both solids and liquids • Of practical importance since this has major effect on uniformity of deposit. • Depends on size and shape of source which may be broad area

9. Evaporation Sources (a) hairpin source, (b) wire helix (c) wire basket, (d) dimpled foil (e) plus aluminacoating (f) canoe or boat type

10. Evaporation source for SiO The Drumheller design of large heating area and good baffling

11. Ta crucible for Cu, Au, Ag (R. Glang, IBM)

12. Ceramic crucible sources Oxide crucible with resistive heating element RF induction heated source for aluminium, BN/TiB2 crucible

13. Commercial e-beam gun

14. Evaporation onto a plane substrate For evaporation from small area dAe, apply the cosine law to give: t/t0 = [1 + (l/h)2]-2 where t0 is the thickness of deposit at centre, l = 0

15. Film thickness distribution S = small area source P = point source Solid lines are a broad disc source. Numbers are ratios r/h of radius, r, of a disc source to substrate distance,h. l is distance from centre The graph shows that even with a large source diameter, non-uniform deposition will always be obtained for a stationary substrate.

16. Film thickness uniformity • Methods to improve film uniformity • Move substrate w.r.t. source during coating • Rotate symmetrically, some improvement. (10-20%) • Rotate eccentrically, (calculation, 5-10%). • (l = 0.1h, when eccentricity, s = 0.71h) (Behrndt et al.) • 2 degrees of freedom + ecc. Substantial improvement (<1%), • this is typically sun and planets (high packing possible) • A specially shaped shutter (calculation) with rotation (5-10%) • Very large areas (mirrors), use a ring of sources • and fire off simultaneously.

17. Evaporation of Compounds • Compounds fractionate on evaporation, experiment needed • Proportions in melt will change and • composition in film will vary with thickness. • Different molecular forms will evaporate, • dissociation and association (chalcogenides) • Sticking coefficients at substrate will alter, • causing metal-rich layers as 2nd phase (optical absorption). • Solutions • Two separate sources • Reactive evaporation (oxides, nitrides) • Flash evaporation. • Control substrate temperature to achieve stoichiometry

18. Evaporation of Compounds - Oxides • T >15000C and dissociation common. • High oxides dissociate to lower oxides which do not vapourise • and film is deficient in oxygen. • Metal of source (Ta,W, Mo, combines with • released oxygen and ends up in deposit • Residual HCs in vacuum can reduce and remove oxygen • Fresh oxide deposits will getter H20, contaminating film • and affecting density, stress, optical charac. Etc. • Solutions: Fast, high temperature evap, (laser, pulsed e-beam) • sub-oxides evaporate. Localised spot, source metal thus not hot, • minimum time for impurities to react with growing film.

19. Evaporation of compounds Borides, nitrides, carbides Constituents having different v.p.’s decompose - one ends up as a second phase in film. In the melt, metal crystallites form and evaporate preferentially. TiC, AlN, ZrC are exceptions (L. Holland). Alloys Can be analysed using Raoult’s Law taking account of v.p. and the fact that the composition of the melt remains homogeneous. As more volatile, A, is depleted, B increases in film. (Used as a production technique in Se/Te alloy evap. for photocopier drums) For homogeneous films use solid evap.(?!), flash evap., or sputtering!

20. Evaporation of Compounds

21. Evaporation of Compounds (oxides)

22. Vapour pressure compounds

23. Reactive evaporation for compounds

24. Flash evaporation

25. Nucleation of a thin film