Chemical deposition
المؤلف:
Peter Atkins, Tina Overton, Jonathan Rourke, Mark Weller, and Fraser Armstrong
المصدر:
Shriver and Atkins Inorganic Chemistry ,5th E
الجزء والصفحة:
ص604-605
2025-10-08
259
Chemical deposition
Key point: Thermal decomposition of volatile inorganic compounds can be used to deposit films of materials, particularly those with uses in electronic devices, on substrates. The technological applications of the materials described in this chapter often require the inorganic material to be generated as a thin layer or film, for example on a silicon substrate, and it has become necessary to develop techniques to deposit films of many inorganic materials. The principal method is chemical vapour deposition (CVD), in which a volatile inorganic compound is decomposed above the substrate. When the compound is a metallo-organic complex (that is, a complex of a metal atom with organic ligands), this route is known as metallo-organic chemical vapour deposition (MOCVD). A large area of research has grown up around the design and synthesis of volatile inorganic compounds that can be used for CVD. For many electronic compounds, simple metal alkyls provide a route to depositing the metal or, through reaction with other gas molecules, its compounds. Thus, Me2 Zn, which has a vapour pressure of 0.3 bar at room temperature, can react with H2 S above a substrate to generate the Group 12/16 (II/VI) semiconductor ZnS (and methane). For others, such as In, the metal alkyl is a solid with a low vapour pressure at room temperature (for Me3In, less than 2 mbar) and it cannot easily be used. In such cases other volatile compounds must be found. This search focuses on other organometallic compounds such as carbonyls, cyclopentadienyl complexes, acetoacetonates, and thiocarbamates, although it should be noted than many of these volatile metal compounds are highly toxic. For complex oxides, such as the superconducting cuprate YBa2Cu3O7, suitable precursors are required for each of the metals; the challenge is to find volatile molecules of the electropositive elements Ba and Y, which normally form ionic compounds. Metal -diketonates have been found to be useful in this respect, as compounds such as the 2,2,6,6-tetramethyl-3,5-heptanedione complex of yttrium sublime at about 150ºC.
Another approach to molecules that can be used for CVD involves incorporating into a single-molecule precursor more than one of the atom types to be deposited. This procedure has the potential advantage of improving the control of the product stoichiometry. Thus, zinc sulfide can be deposited from a variety of zinc Thio complexes, such as Zn(S2PMe2)2. A future goal is to make complex volatile molecules containing, for example, several different metal atoms that can be deposited simultaneously to make a complex oxide. Other methods of deposition of thin films, at the nanometer scale, of inorganic materi als on substrates include sputtering and laser ablation; these techniques are dealt with in greater detail in Section 25.5.
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