MSE 550

1. MSE 550 Presentation #2

2. What is a thin film? Science background overview of film growth crystal structure and defects (dislocations, grain boundaries) diffusion properties of vacuum Film formation Thermal accommodation Sticking and surface diffusion nucleation of film growth modes (island, layer by layer, mixed) coalescence of film continued growth (zone models) other factors energetic deposition amorphous films epitaxial growth Deposition parameters and techniques (relate the knobs on equipment to what happens on the film) Evaporation Vacuum Arc Deposition Sputter Deposition DC RF Chemical Vapor Deposition Ion Beam assisted deposition plasma enhanceddeposition (PECVD, ECR . . .

4. How are thin films made? What are the basic parts of deposition systems? source transport region substrate - deposition region What parameters can we control? temperature deposition rate deposition energy Within the framework established above, examine each of the following deposition methods in detail: evaporation cathodic arc vaporization sputter deposition DC RF Molecular Beam Epitaxy Chemical Vapor Deposition Ion Beam assisted deposition / ion implantation plasma enhanced deposition (PECVD, ECR . . . ) Spin coating Electroplating

5. What is a "thin film" ? thin = less than about one micron ( 10,000 Angstroms, 1000 nm) film = layer of material on a substrate (if no substrate, it is a "foil") Applications: microelectronics - electrical conductors, electrical barriers, diffusion barriers . . . magnetic sensors - sense I, B or changes in them gas sensors, SAW devices tailored materials - layer very thin films to develop materials with new properties optics - anti-reflection coatings corrosion protection wear resistance

6. Typical steps in making thin films: emission of particles from source ( heat, high voltage . . .) transport of particles to substrate (free vs. directed) condensation of particles on substrate (how do they condense ?) Simple model:

7. Mechanisms? thermodynamics and kinetics phase transition - gas condenses to solid nucleation growth kinetics activated processes desorption diffusion allowed processes and allowed phases

8. Kinetics and Diffusion Kinetics= how fast it will happen we will concentrate on mass transport = atoms diffusing through a solid Diffusion in one dimension - Fick's 1st and Fick's 1st Law = "stuff moves from where you have lots to where you have little"

9. 2nd Fick’s law

10. There is always a potential energy barrier to diffusion (activation energy). What do we expect mathematically for the flux to the right (from position1 to 2): Similarly we can find the flux to the left: (note: if we had used the gas constant, R, instead of Boltzmann constant, k, then the energy would be the diffusion energy/mole)

11. Examine what happens when we apply a field:

12. How fast do atoms diffuse?

13. Nucleation and Growth • Connection to Phase Diagrams • Can phase diagrams help us in understanding rates ? • Consider cooling a liquid into a solid through a eutectic point: • at point A: solid is not stable so will not form • at point B: solid and liquid are both stable so no driving force to solid • at point C: liquid is unstable - will form solid • at point D: liquid is unstable - will form solid • further from equilibrium => greater driving force to form solid

14. Nucleation depends on: • liquid phase instability • driving force toward equilibrium (as above) • increases as we move to lower temperatures • diffusion of atoms into clusters • increases at higher temperatures • combine these two terms (multiplication) to determine the total nucleation rate • The maximum rate of nucleation is at some T < Te

15. Growth • growth of the phase is diffusion controlled => increases with temperature • Transformation rate: • total rate of forming solid is product of nucleation rate and growth rate

16. Nucleation details • When moving into a 2 phase region on phase diagram - how does the new phase form ? • Two issues: • Thermodynamics: Is nucleation possible ? (energy minimization) • Kinetics: How fast does it happen ? (nucleation rate) • Homogeneous Nucleation • vapor --> liquid (solid) for a pure material with NO substrate

17. Energy minimization involves two terms: volume transition surface formation volume transition: where W is the atomic volume, PS is the pressure above the liquid (solid), and PV is the pressure in the vapor. We want PV > PS so that ÆG is negative => supersaturation provides the driving force.

18. surface formation: Change in surface energy is always positive when forming surfaces. Total energy change:

19. note: • initial formation of nuclei has increase in G => metastable • if r < r* then nuclei shrink to lower G • if r > r* then nuclei grow to lower G • r* is a critical radius for nuclei

20. Nucleation rate • How fast will the critical nucleus continue to grow ? • Consider the rate at which atoms will join the critical nuclei: expect nucleation rate to be given by N* = concentration of critical nuclei (nuclei/cm3) A* = critical surface area of nuclei w = flux of atom impingement (atoms / cm2sec) Consider each of these three terms:

22. Film Formation I • Competing Processes • adding to film: • impingement (deposition) on surface • removing from film: • reflection of impinging atoms • desorption (evaporation) from surface • We can characterize the process of getting atoms onto a surface with • sticking coefficient = mass deposited / mass impinging • Steps in Film Formation • thermal accommodation • binding • surface diffusion • nucleation • island growth • coalescence • continued growth • We will examine each of these steps in turn.

23. 1. Thermal accommodation • impinging atoms must lose enough energy thermally to stay on surface • assume that E = kT so we can talk about energy or temperature equivalently thermal accommodation coefficient (aT)

24. if rebound is strong enough - atom escapes • if not - atom is trapped - oscillates and loses energy to lattice • RESULTS: • atom is trapped if Ev < 25 Edesorb • Edesorb is typically 1-4 eV • trapped if Ev < 25 - 100 eV • equivalently Tv < 2500 - 10,000 K • most deposition processes have Ev < 10 eV • MOST ATOMS ARE TRAPPED • thermal accommodation is very fast • around 10-14 seconds

25. 2. Binding • two broad types of surface bonds: • physisorption (physical adsorption) • Van der Waals type • weak bonds • 0.01 eV • chemisorption (chemical adsorption) • chemical bonds • strong bonds • 1 - 10 eV • Can we keep the atoms on the surface ? • competition between impinging atoms (deposition) and desorption of atoms • deposition: determined by deposition rate (atoms/cm2sec) = desorption: determined by DGdes = free energy of desorption • TS = temperature of substrate • no = frequency of adsorbed atom attempting to desorb = lattice vibration frequency

26. Consequences: • heat up substrate => lower coverage • stop depositing => lower coverage until not film • films are not stable !!! • What is wrong with this model ? • missing surface diffusion

27. 3. Surface diffusion • allows clusters of adsorbed atoms to form • clusters are stable => film forms • How far do they diffuse ? • from random walk analysis [see F. Reif "Fundamentals of Statistical and Thermal Physics" p. 486] • diffusion distance (X) is given by Consider two cases:

28. 4. Nucleation • How do clusters form ? => nucleation • Two competing processes in cluster formation • clusters have a condensation energy per unit volume (DGV) which lowers the desorption rate (higher barrier) • clusters have a higher surface energy than individual atoms • clusters want to break up to minimize energy • Capillarity Model (= heterogeneous nucleation) • nucleation on a substrate • assume nuclei are spherical caps

29. as with homogeneous nucleation, we can plot ÆG against r and determine a critical nucleus size:

31. How do nuclei grow initially ? Substrates are NOT flat steps, kinks, etc. have higher Edes barrier => longer residence time on surface => preferred sites for nucleation

32. Nucleation Rate How quickly do nuclei form ?

33. Nucleation Rate

34. : • Nucleation Rate • What can we learn from the capillarity model about effects of deposition rate and substrate temperature on nucleation ? from before

35. To see how the lab variable (deposition rate, substrate temperature) change the basic physics examine the derivatives (and plug in some typical values):

36. Summary: high T and/or low deposition rate => large crystal grains low T and/or high depostion rate => small polycrystalline structure Problem: Can we apply macroscopic thermodynamics to nuclei of 2-100 atoms ?

37. Atomistic (Statistical) Nucleation Model Walton - Rhodin Theory treat clusters of atoms like molecules rather than solid caps consider the bonds between atoms similar to capillarity model, but now include Ei* = energy to break apart a critical cluster of i* atoms into individual atoms. other terms: Ni* = concentration of critical clusters per unit area N1 = concentration of single atoms per unit area no = total density of adsorption sites on surface

38. advantages of this model: • depends on microscopic parameters • includes crystallographic information • since bonds between atoms are included • critical size (i*) depends on substrate temperature • model shows transitions in growth modes • preferred i* increases with T

39. Film Formation II 5. Island Growth observe 3 growth modes experimentally 1. Island growth (Volmer - Weber) form three dimensional islands source: film atoms more strongly bound to each other than to substrate and/or slow diffusion

40. 2. Layer by layer growth (Frank - van der Merwe) generally highest crystalline quality source: film atoms more strongly bound to substrate than to each other and/or fast diffusion

41. 3. Mixed growth (Stranski - Krastanov) initially layer by layer then forms three dimensional islands => change in energetics

42. When would we expect to see each of these ? The layer growth condition with cosine greater than 1 looks odd. This is the case where the angle theta is undefined because for layer growth there really is no point where the substrate, vapor and film come together and therefore, no way to define the angle.

43. 6. Island Coalescence three common mechanisms: 1. Ostwald ripening atoms leave small islands more readily than large islands more convex curvature => higher activity => more atoms escape 2. Sintering reduction of surface energy

44. 3. Cluster migration small clusters (<100 Å across) move randomly some absorbed by larger clusters (increasing radius and height)

45. 7. Thick films - zone models Further growth depends on: bulk diffusion surface diffusion desorption geometry: shadowing (line of sight impingement) • Relative importance of these processes depends on • substrate temperature (T) • deposition rate these variables to find regions with similar film structure (similar properties)

47. Columnar structures very common from limited atomic mobility often oriented slightly toward source • Films are typically lower density than bulk • more porosity at macro, micro and nano scales. • Grain size dependence on deposition rate and substrate temperature • grain size typically increases with increasing film thickness, increasing substrate temperature, increasing annealing temperature, and decreasing deposition rate.

49. Other factors affecting film growth 1. Substrate not really a featureless plane atomic structure => epitaxy relationship of film crystal structure to substrate crystal structure defects nucleation sites 2. Contamination from: poor background pressure impure deposition source dirty substrate changes the energies (surface energies, desorption energy, surface diffusion energy)

50. 3. Impinging particle energy 0.5 eV -------------------> 10 - 20 eV --------> 100-1000 eV thermal evaporation ----- sputtering --------- accelerated (bias) interactions of incident particles with film/substrate produce: sputter removal of surface atoms insertion of particles into film or substrate increased local temperature defects shock (pressure) waves