3.1 Thin Film Deposition and Processing
3.1.3 Processing Parameters and Post-processing techniques
The success of growth of complex oxides by sputtering is attributed to the growth process and nucleation in thin film which is related to the thermodynamics and kinetics of 2-dimensional crystal growth. Numerous parameters are involved in the entire process of growth. Substrate temperature, process gas pressure and composition and sputtering power are among the most critical in the growth of thin films.
3.1.3.1
Room Temperature Growth
Due to the sputtering process being a physical bombardment of ions to the surface, room temperature growth of thin films leads to an amorphous layer of atoms at the surface in nearly the composition of the target. Due to the ability of ejecting atoms from the surface of the target, the composition of the material deposited on the substrate will vary slightly from the composition of the target. This however, is mostly limited to the size of the atoms deposited, whereas larger atoms that are ejected have a longer mean free path and a higher probability of ending up on the substrate, smaller atoms such as oxygen have a greater probability of interacting with other particles and not ending up on the substrate.
In room temperature grown films, high temperature post processing annealing is required to reach proper crystal phase such as seen in chapter 4.1, 4.2 and 5.1 . This post processing annealing step often leads to polycrystalline thin films where the crystallite size is often defined by the thickness of the film as well as the annealing temperature. Growth of polycrystalline films generally beings with the thermally activated nucleation of islands of the film material on the surface. Initial crystallographic orientation of the film that minimize surface and interface energies are favored over other mechanism and the nucleation rates for clusters with lower
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energy orientations. This can lead to textured growth.56 When sputtering as a function of temperature, difference structural evolution is seen as a function of temperature as first discussed by Thornton in 1977.57 A schematic of the work down by Thornton is shown in Figure 16.
Figure 16: Structural evolution in polycrystalline films as a function of substrate temperature and argon gas sputtering pressure. Reprinted with permission from Ref. [57]. Copyright 1977, Annual Reviews.
In this same light, the flow of process gas is required to improve stoichiometry as well as limit the formation of certain phases and crystallography. When post deposition processing of oxide films is performed, an understanding of the oxidation-reduction reactions of constituent materials must me understood.
Using the simplest oxidation reaction of a metal M, 2M(s) +O2(g) ≡2MO(s). The standard Gibbs free energy change for a reaction is given by ∆Go =−RTlnKwhere
37 2 2 2 pO a a K M MO
= , R is the gas constant, T is the temperature, K is the equilibrium constant and a is the activity.58 Assuming the activities of a pure solid to be unity, the free energy change can be written as 2 1 ln pO RT Go =− ∆
In an Ellingham diagram, an oxide is stable above the line and the metal is stable below the line. This allows one to vary the partial pressure during post processing in order to avoid oxidation of material (as will be seen with the selective oxidation in chapter 4.2) or promote the oxidation of others. The partial pressure as a function of temperature for the oxidation is derived and plotted in Figure 17. This allows for regions to be accessed where thermodynamic stability can be obtained for different compounds.
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Figure 17: pO2 vs temperature diagram showing processing window in which oxides of Ba,
Sr and Ti are stable with metallic Ni as derived from elligham diagrams. Reprinted with permission from Ref.[59]. Copyright 2008, American Institute of Physics.
Since thermodynamics often leads to limitations in processing, kinetically driving oxidation is often necessary as will be seen in chapter 4.1and 4.2.
With post-growth annealing, in order to control the partial pressure of oxygen, a water- hydrogen reaction is utilized. By varying the flow rate of forming gas (5% H2) with Argon and
by using a Yttrium stabilized tube in the process area (coated on both sides by platinum to form a half-cell reaction), a voltage is obtained from the difference in oxygen content across the half cell. The voltage (emf) is given by the equation
39 = '' ' 2 2 ln 4 O O P P F RT emf
Where F is the Faraday constant, R is the universal gas constant, T is the temperature ''
2
O
P is the
air (~0.21) reference and ''
2
O
P is the concentration of the gas in the cell.
3.1.3.2
Epitaxial Growth of oxide thin films
When considering the growth of epitaxial films, it is critical to determine the proper substrate, temperature, processing gas conditions and gun power. One of the key components for the growth of epitaxial films lay in the choice of substrate as shown in chapter 5.3. Near lattice matching must be chosen to ensure oriented growth. The proper amount of thermal energy (substrate heating) is needed to allow for the redistribution of atoms at the surface into the lowest energetic (epitaxial) formation. Process gas pressure is another substantial parameter in sputter deposition, impacting composition and growth. As stated earlier, oxygen must be included in order to promote stoichiometry in the films. Gun power also leads to a substantial parameter when considering epitaxial growth. The higher the rate, the less time the adatoms at the surface have to find the minimal energy state, and mosaicity in the film will begin to develop, sometimes referred to as island growth.
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