Thin Films and Vapor Deposition

Abstract

In the previous chapter we described how thick films of ceramics are produced. The difference between thick films and thin films is not really the thickness of the layer; it is how the layer is formed. In general, thin films are ≤500nm in thickness, whereas thick films may be several tens of micrometers in thickness or even thicker depending on the particular application. Thin films are generally prepared from the vapor phase, whereas for thick films we use a solution or slurry. Furthermore, thin films are often crystallographically oriented in a particular way with respect to the underlying substrate. This orientation relationship, known as epitaxy, is determined by the crystal structures and lattice parameters of the film and the substrate. In general, thick films and coatings have no specific orientation and contain a large number of randomly oriented crystalline grains.

1 People and history

Pascal, Blaise (1623–1662) was a French mathematician and philosopher who invented the first digital calculator to help his father, a tax collector. Among his other most notable contributions to mathematics and science was laying the foundation of the theory of probability. The SI unit of pressure, the pascal Pa, is named after him.

Torricelli, Evangelista (1608–1647) was born in Faenza (the home of faence pottery). He invented the barometer and worked in geometry. He died in Florence. The unit of pressure, torr, is named after him.

2 Exercises

• 28.1 What reactive gases would be suitable for forming the following ceramic thin films by reactive sputtering? (a) Al₂O₃; (b) TaN; (c) TiC; (d) CdS.

• 28.2 Name two of the ways that you might use to make sputtering targets.

• 28.3 Thin films can grow by three distinct mechanisms. Name the three mechanisms and explain how they differ.

• 28.4 Why is it often desirable to form thin films at the lowest possible substrate temperature?

• 28.5 What advantages are there, if any, of working at high substrate temperature?

• 28.6 What reactant gases might you use for making the following films by CVD? (1) ZrC; (2) TaN; (3) TiB₂.

• 28.7 Consider the data given below: The values of A, B, and C are given for ΔG ⁰ = A + BTlogT + CT (ΔG ⁰ in cal.). From these data, determine whether it would be thermodynamically feasible to form the following ceramic films by CVD at a temperature of 850°C. (a) TiN from the nitridation of TiCl₄; (b) SiC from the reaction between SiCl₄ and methane; (c) SiO₂ from the oxidation of SiCl₄.

  Table 10

• 28.8 What technique would you use to produce a 100-nm thin film of AlN on silicon? Explain why you chose your technique and its pros and cons.

• 28.9 What technique would you use to produce a 5-nm thin film of BaBiO₃ on MgO? Explain why you chose your technique and its pros and cons.

• 28.10 Which of the techniques described in this chapter is most suitable for producing thin films on large substrates? What is the largest substrate that can be coated?

• 28.11 We suggest that molecular beam epitaxy is often a misnomer. Discuss this viewpoint with examples. (Always reference your sources.)

• 28.12 If ${G}$ for the formation of TiC were actually −159 kJ, what would the implications be for the growth of TiC using the method discussed here? Is it particularly important that TiCl₄ is used?

• 28.13 Look up the ${G}$ values for the reaction in equations 28.10 and 28.11 and explain the reason they are different.

• 28.14 Using the library or other sources, give examples of oxide deposition, using each of the forms of CVD listed in Section 28.7.

• 28.15 Sputtering can be used to grow films of yttrium barium copper oxide (YBCO). Discuss the difficulties and how you would overcome them.

• 28.16 Molecular beam epitaxy can be used to grow oxides. Discuss the state of the art with examples.

• 28.17 Pulsed laser deposition is our favorite technique for growing oxide films. What is the largest substrate currently being used? What are the thickness limitations? Why do we use different lasers for different materials?

• 28.18 Ion-beam-assisted deposition has been referred to as (for example) “MBE with a hammer.” Discuss this statement.

• 28.19 Figure 28.1C shows silicon wafers used as a substrate for deposition in a quartz tube. Do we need to control the ${p}$O₂ in this situation?

• 28.20 Figure 28.1C shows a three-zone heater. Give examples of film growth where such a system might be used.