1. Methods of Thin Films Deposition: Advance Techniques
M.Sc. Material Science and Technology (3rd Sem.)
Course: Thin Film Deposition and Technology
Course Code: PGMST3E001T
Unit-V
Methods of Thin Films Deposition: Advance Techniques
By:
Dr. Pragati Kumar

2. Outlines Molecular Beam Epitaxy Laser Molecular Beam Epitaxy
Atomic Layer deposition
Ion Sputtering
Atom Beam Sputtering

3. Molecular Beam Epitaxy
Surface processes in epitaxial growth
Growth modes and surface energies:
Island or Volmer-Weber mode: Islands of the bulk deposit are initially formed on the bare substrate. Further deposition causes these islands to grow and maybe coarsen; they may sometimes rearrange extensively before forming a continuous film, and any epitaxial orientation may be established quite late in the growth process.
Layer or Frank-van der Merwe growth: One layer is more or less complete before the next layer starts to form, and the epitaxial orientation is typically established within the first monolayer; after a relatively large thickness has been deposited, misfit dislocations and other defects may be introduced to relieve strain.
Layer plus island or Stranski-Krastanov (SK) growth: One or more (epitaxial) layers form first, but then further deposition is in the form of (usually epitaxial) islands.

4. These distinctions can be understood qualitatively in terms of relative surface energies, γA and γB, as illustrated in Figure 1.
If A is deposit over the substrate B, the condition γA+γ* < γB leads to layer growth (Fig. 1a).
The condition is therefore not satisfied if B deposited over substrate A and this leads to island growth (Fig. lb).
However, if we have layer growth initially, then it is often the case that the effective interfacial energy γ* increases with thickness of the layer, for example due to strain in this layer. In such a case, the thermodynamic conditions for layer growth are terminated after a certain layer thickness. Further growth of the layers is then in competition with growth of the more stable islands. This thermodynamic situation is very common, so that many crystal growth systems can be classified as SK growth.

6. The classification immediately tells us to expect trouble in the growth of A/B/A superlattices. Even if we assume no inter-diffusion or chemical reactions between A and B, then the interfaces A/B and B/A may well grow in a different mode, and one interface is likely to be more perfect than the other.
One of the difficulties of describing the whole epitaxial growth process quantitatively, and the final structures of films in general, is that there are many possible types of kinetic effect, each with associated length and time scales which can be very different.

7. Surface processes in crystal growth
Atomistic processes responsible for nucleation and growth of epitaxial thin films are indicated in Fig. l(c).
Atoms arrive from the vapour at a deposition rate R=p/(2πmkT)1/2. This creates adatoms (or ad-molecules) on the surface, whose areal density n1(t) increases initially as n1= Rt.
At the highest temperatures, these adatoms will only stay on the surface for a short time, the adsorption stay time τa. This time is determined by the adsorption energy Ea, and is conventionally written as
where υa is an atomic vibration frequency, of order 1-10 THz. Before evaporating, the adatom will have moved, maybe quite a long way from its point of arrival, with a diffusion constant D Ed diffusion energy, a= jump distance

8. The number of substrate sites visited by an adatom in time τais Dτa/No, where No is the areal density of such sites, of the same order as a-2. The rms displacement of the adatom from the arrival site before evaporation is
Since Ea is typically several times Ed, (xs/a) can be large at suitably low temperatures. Then, in their migration over the surface, the adatoms will encounter other atoms. Depending on the size of the binding energy between these atoms, and on their areal density n1, they will form small clusters, which may then grow to form large clusters of atoms on the surface, in the form of 2Dor 3D islands. This binding energy between a pair of atoms, Eb, and the energy of the critical cluster, Ei, are centrally important to the understanding of nucleation and growth processes. Subsequently, surface- and inter-diffusion processes may occur.

9. Thermodynamic and kinetic arguments
The growth of a thin epitaxial film is a non-equilibrium kinetic process in which one or more steps are rate-limiting. Two limit cases may be instructive. The thermodynamic limit is illustrated by the equilibrium vapour pressure, pe, of bulk material (A). In this case, by equating the chemical potential, μ, of the low pressure vapour, which is exact, and an approximate description of the free energy of the solid (primarily due to the lattice vibrations), we can find an expression for pe. Using the Einstein approximation in the high temperature limit, for vibrations of frequency υ, we obtain the standard result
First, pe is dominated by the sublimation energy L0 (L0>> kT at all T below the melting point) and is also influenced strongly by υ3.
Second, this equilibrium result is independent of the state of the surface, which acts only as an intermediary between the vapour and the solid.
When the vapour and solid are not in equilibrium, the nature of the surface does play a significant role. Burton, Cabrera and Frank (BCF) argued that the condensation coefficient, αc, in the Hertz-Knudsen eqn.

10. Where n is the total areal density of atoms condensed, is a function of the step structure of the surface in the absence of island nucleation. At low supersaturation S, defined by p/pe or R/Re, such that Δμ = kT ln S, they showed that
Where d is the step separation and Xs=(Dτa)1/2 as given in previous topic. When Xs >> d, all atoms are captured, and αc →unity.
At low Δμ, growth did not occur at a measurable rate on a flat surface, but was mediated by (screw) dislocations which produce spiral arrays of steps at the surface.
At higher supersaturations, adatoms will come together before they reach the steps, and if they are bound strongly enough, will form nuclei which can develop into islands. The rate limiting step is the formation of ‘critical nuclei’of size i, which is defined as the size which is more likely to grow than decay.
The use of atomistic expressions was prompted by the realisation that, in
many deposition experiments the driving force Δμ can be so large that the critical nucleus is only a single atom.

11. i=1 represents the extreme kinetic limit on a perfect surface.
i=0 can arise on a defective surface; i.e. adatoms diffuse to defect sites, and that clusters nucleate from such filled sites.
Only for the lowest substrate temperatures surface difussion of adatom is suppressed, and in this limit the film would grow simply by accreting atoms which stick where they fall. This does not happen at temperatures involved in the growth of less reactive thin films for practical purposes.

12. MBE System
Involves highly controlled evaporation in an ultrahigh-vacuum (10-10 torr) system.

13. The image on the left shows effusion, where the image on the right shows diffusion.
Effusion cells consisting of an isothermal cavity with a hole through which the evaporant exits are used for compound semiconductor elements and their dopants. Effusion cells behave like small-area sources and exhibit a cosφ emission.
number of evaporant species striking the substrate is
Sketch showing the main components and rough layout and concept of the main chamber in a MBE system

14. Irrespective of whether homo-or heteroepitaxy is involved, it is essential to grow atomically smooth and abrupt epitaxial layers.
In solid source MBE, elements like Ga and As in ultra-pure form, are heated in separate quasi-Knudsen effusion cells or electron beam evaporators until they begin to slowly sublime (solid phase directly transferred into gasses phase). The gaseous elements then condense on the wafer, where they may react with each other and form single crystal.
When evaporation sources such as copper or gold are used, the gaseous elements impinging on the surface may be adsorbed (after a time window where the impinging atoms will hop around the surface) or reflected. Atoms on the surface may also desorb.
Controlling the temperature of the source will control the rate of material impinging on the substrate surface and the temperature of the substrate will affect the rate of hopping or desorption.
The term "beam" means that evaporated atoms do not interact with each other or vacuum chamber gases in between the path of

15. evaporator to substrate.
Reflection high energy electron diffraction (RHEED) is often used for monitoring the growth of the crystal layers during operation.
A computer controls shutters in front of each furnace, allowing precise control of the thickness of each layer, down to a single layer of atoms. Intricate structures like (QWs,QDs) of layers of different materials may be fabricated this way.
In systems where the substrate needs to be cooled, the ultra-high vacuum environment within the growth chamber is maintained by a system of cryopumps, and cryopanels.
Molecular beam epitaxy is also used for the deposition of some types of organic semiconductors. In this case, molecules, rather than atoms, are evaporated and deposited onto the wafer. Other variations include gas-source MBE, which resembles chemical vapor deposition.
MBE systems can also be modified accordingly to the needs.

16. Real space representation of the formation of a single complete monolayer;θ is the fractional layer coverage; corresponding RHEED oscillation signal is shown.

17. Laser MBE
Laser MBE uses the both merits of PLD and MBE especially for high melting point ceramics and multi-component solids.

18. Atomic Layer Deposition (ALD)
ALD is a more recent variation on the older technology referred to as CVD.
In CVD a mixture of gases flows over a heated substrate causing a thin solid film to grow on the surface. This heated surface has to be hot enough to allow the surface reaction to proceed rapidly.
In the ideal case, there will be no reaction between the reactant gases in the gas phase, in other words no homogeneous reactions, which would cause the formation of particulates. The gases approaching the heated surface will be heated by gas phase conduction and should not react until they impinge on the surface where they form a solid film of deposited material by a heterogenoeous reaction.
If a homogenoeous reaction takes place, in the worst case one can have particles forming in the gas phase and ending up embedded in the growing thin film, clearly an unacceptable result.
The optimum choice of reactants for a CVD process is generally a mixture of the most reactive gases available. This allows film deposition at the highest rates, and at the lowest substrate temperatures.

19. Unfortunately, this choice leads to a high probability of gas phase reactions, which as noted can compromise the deposited film quality.
In ALD, the presentation of the two reactants to the heated surface is separated into two steps.
In step one, the substrate is exposed to the first reactant after which this reactant is pumped away. During this exposure a monolayer of the first reactant adsorbs to the substrate, and remains after the chamber is evacuated.
In step two second reactant is introduced into the chamber, and it reacts with the monolayer of the first reactant. This then forms one layer (generally less than one complete monolayer) of the solid film being sought. After this, the remaining second reactant and any gas phase reaction products are removed from the chamber.
This process is repeated as many times as necessary to grow a fi lm of the desired thickness.
The conformality of the film is also excellent, since film growth depends only on the formation of monolayers on the surface and not on the arrival rate of reactants. However, the one disadvantage is that the film deposition rate may be slow.

20. Vapour Adsorption
The forces holding the molecular species on the surface can be weak (physical adsorption or physisorption), or they can be quite strong (chemisorption).
In the latter case, in particular, the adsorbed molecules tend to readily form the self-limiting monolayers of interest for ALD, and such layers can be fairly stable at moderate temperatures.
A physisorbed monolayer, on the other hand may be readily desorbed if the substrate surface runs too hot, because of the weak bonding forces.

21. Physisorption
Two aspects of adsorption, essential to the under standing of ALD.
1. The bond strength that binds the gaseous species to the surface varies depending on both the gaseous species and the nature of the surface.
2. The rate at which adsorption occurs will be finite, and in some cases may be quite slow.
Physical adsorption is characterized by weak bonds to the solid surface (e.g. heats of adsorption < 20kJ/mole), on the order of van der Waals forces, and rates of adsorption that are fast.
Generally, this process is domination in “thermal” ALD”. In thermal ALD temperatures, although lower than in CVD, are high enough to cause weakly bonded monolayers to desorb from the surface.
All gases will physisorb a self-limiting monolayer under the correct temperature and pressure conditions, even if they do not chemisorb. The preferred conditions are low pressure and moderate temperatures. For a given pressure, if the temperature is too low more than one monolayer will adsorb, and at low enough temperatures a liquid or solid

22. film will be formed. As the temperature is raised a single monolayer will be left behind on the surface. Finally, if the temperature is raised enough the single monolayer will be desorbed.
therm, illustrates this effect [9].
Langmuir equated the rate of adsorption to the rate of desorption of gas atoms or molecules on a smooth surface, where there is no interaction between adsorbed particles.
Also, when a particle strikes a bare surface it adsorbs. When it strikes
an occupied site on the surface it is reflected back into the gas phase.
The rate of adsorption is ka[A](1 − θ) and the rate of desorption is kdθ, where θ is the fraction of surface covered. Equating these and defining K=ka/kd as an equilibrium constant for the adsorption process, we derive:
[A] is the gas phase concentration of the molecular species A. For low values of [A], surface coverage varies linearly with the concentration [A] or the gas pressure. For higher pressures, the coverage approaches unity. This relationship is illustrated in Figure.

23. Adsorption kinetics of O2 on Rh(111).

24. Chemisorption
In chemisorption the reactant being adsorbed forms a chemical bond with atoms on the surface. Here, the first reactant will have to adsorb and remain bound to the surface until the reaction is complete. As a reactant bonds to the surface it will, in general, form just a single monolayer. Additional reactants arriving at the monolayer coated surface would have to bond to it by van der Waals forces, and the surface temperature would discourage this. Therefore, chemisorption bonding invariably leads to the desired single monolayer of reactant. Chemisorption is often associated with an activation energy, so that unless the temperature is high enough the chemisorption bonding could proceed slowly. This is why chemisorption is often referred to as activated adsorption.
In general, the halogen and organometallic compounds seem to chemisorb as needed.

25. Atomic Layer Deposition (ALD)
ALD was developed as a new technique for the growth of polycrystalline dielectric thin films with unique characteristics. The essence of the process is that two distinct reactants are separately exposed to the growth surface in turn, and the reaction occurs between a first monolayer of reactant and a second reactant to form a solid thin film product. Since now a chemical reaction was required to form the solid film, surface temperature became an issue, and the concept of a “temperature window” was developed.
Thermal ALD Processes
ALD can be used for elemental species as well as chemical compounds. For thermal ALD processes there are temperature constraints on the feasibility of successfully carrying out a particular deposition. A schematic representation of this situation is shown in Figure.

26. At very high temperature, first chemical reactant may decompose on the surface before having time to react with the second reactant, which results higher growth rate than one would expect from an ALD process.
Alternately, if the first precursor is stable it may
Temperature window for ALD.
still desorb from the surface before having a chance to react with the second reactant, which turns in slower growth rate than expected. This would be more likely with a physisorbed first reactant than chemisorbed.
On the other hand, At very low temperature, adsorbance of more than one monolayer per cycle (or even condense a liquid or solid on the surface) is possible which results higher deposition rate than expected.
Alternately, the reaction rate may be so slow that there may not be enough time for a complete monolayer to be reacted.

27. One way to remove the restriction of the temperature window would be to use a highly reactive radical as one of the reactants.

30. Ion Beam Sputtering

31. If laser pulse is long (ns) or repetition rate is high, laser may continue interactions

32. Atom Beam Sputtering
ABS provides a fast neutral atom beam induced co-sputtering to prepare composite materials.
This is a type of DC sputtering but applicable to wide variety of
materials.
Due to its slow deposition rate, it can provide better uniformity homogeneity and control of nanoparticle formation.

33. Thanks
for your attention…!