Chapter 1 Introduction to Chemical Vapor Deposition (CVD)

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© 2001 ASM International. All Rights Reserved. Chemical Vapor Deposition (#06682G) 2 PERIODIC TABLE IA IIA IIIA IVA VA VIA VIIA VIII IB IIB IIIB IV VA VIB VIIB O Creighton and Ho 2 He 4.003 1 H 1.008 10 Ne 20.179 9 F 19.00 8 O 16.00 7 N 14.01 6 C 12.01 5 B 10.81 4 Be 9.012 3 Li 6.941 18 Ar 39.948 17 CI 35.45 16 S 32.07 15 P 30.97 14 Si 28.09 13 Al 26.98 12 Mg 24.305 11 Na 22.990 36 Kr 83.80 35 Br 79.90 34 Se 78.96 33 As 74.92 32 Ge 72.61 31 Ga 69.72 30 Zn 65.39 29 Cu 63.546 28 Ni 58.70 27 Co 58.933 26 Fe 55.847 25 Mn 54.938 24 Cr 51.996 23 V 50.942 22 Ti 47.90 21 Sc 44.956 20 Ca 40.08 19 K 39.098 54 Xe 131.30 86 Rn (222) 53 I 126.9 52 Te 127.6 51 Sb 121.8 50 Sn 118.7 49 In 114.8 48 Cd 112.4 47 Ag 107.868 46 Pd 106.4 45 Rh 102.905 44 Ru 101.07 43 Tc (99) 42 Mo 95.94 41 Nb 92.906 40 Zr 91.22 39 Y 88.906 38 Sr 87.62 37 Rb 85.468 85 At (210) 84 Po (210) 83 Bi 209.0 82 Pb 207.2 81 TI 204.37 80 Hg 200.59 79 Au 196.966 78 Pt 195.09 77 Ir 192.22 76 Os 190.2 75 Re 186.2 74 W 183.85 73 Ta 180.948 72 Hf 178.49 57 La 138.9 89 Ac 227.0 56 Ba 137.33 88 Ra (226) 55 Cs 132.905 87 Fr (223) www.asminternational.org 71 Lu 174.96 103 Lr (257) 70 Yb 173.0 69 Tm 168.9 68 Er 167.3 67 Ho 164.9 66 Dy 162.5 65 Tb 158.9 64 Gd 157.2 63 Eu 152.0 62 Sm 150.4 61 Pm (145) 60 Nd 144.2 59 Pr 140.1 58 Ce 140.115 57 La 138.9 LANTHANIDES 102 No (259) 101 Md (256) 100 Fm (257) 99 Es (252) 98 Cf (251) 97 Bk (247) 96 Cm (247) 95 Am (243) 94 Pu (244) 93 Np 237.0 92 U 238.0 91 Pa 231.0 90 Th 232.0 89 Ac 227.0 ACTINIDES Fig. 1: Periodic table, where shaded boxes indicate elements that have been deposited using CVD.
© 2001 ASM International. All Rights Reserved. Chemical Vapor Deposition (#06682G) In its simplest incarnation, CVD involves flowing a precursor gas or gases into a chamber containing one or more heated objects to be coated. Chemical reactions occur on and near the hot surfaces, resulting in the deposition of a thin film on the surface. This is accompanied by the production of chemical by-products that are exhausted out of the chamber along with unreacted precursor gases. As would be expected with the large variety of materials deposited and the wide range of applications, there are many variants of CVD. It is done in hot-wall reactors and cold-wall reactors, at sub-torr total pressures to above-atmospheric pressures, with and without carrier gases, and at temperatures typically ranging from 200-1600°C. There are also a variety of enhanced CVD processes, which involve the use of plasmas, ions, photons, lasers, hot filaments, or combustion reactions to increase deposition rates and/or lower deposition temperatures. There are also many derivatives of the CVD terminology, such as metal-organic chemical vapor deposition (MOCVD) 16,17 or, less commonly, organo-metallic chemical vapor deposition (OMCVD), which are sometimes used to note the class of molecules used in the deposition process. Some practitioners chose to differentiate epitaxial film deposition from polycrystalline or amorphous film deposition, so they introduced a variety of terms that include “epitaxy” in the acronym. Two of the more common variants are organometallic vapor phase epitaxy (OMVPE) 7 and metalorganic vapor phase epitaxy (MOVPE) 18 which are often used in the compound semiconductor epitaxy literature. CVD has a number of advantages as a method for depositing thin films. One of the primary advantages is that CVD films are generally quite conformal, i.e., that the film thickness on the sidewalls of features is comparable to the thickness on the top. This means that films can be applied to elaborately shaped pieces, including the insides and undersides of features, and that high-aspect ratio holes and other features can be completely filled. In contrast, physical vapor deposition (PVD) techniques, such as sputtering or evaporation, generally require a line-of-sight between the Introduction to Chemical Vapor Deposition 3 surface to be coated and the source. Another advantage of CVD is that, in addition to the wide variety of materials that can be deposited, they can be deposited with very high purity. This results from the relative ease with which impurities are removed from gaseous precursors using distillation techniques. Other advantages include relatively high deposition rates, and the fact that CVD often doesn’t require as high a vacuum as PVD processes. CVD also has a number of disadvantages. One of the primary disadvantages lies in the properties of the precursors. Ideally, the precursors need to be volatile at near-room temperatures. This is non-trivial for a number of elements in the periodic table, although the use of metal-organic precursors has eased this situation. CVD precursors can also be highly toxic (Ni(CO) 4 ), explosive (B 2 H 6 ), or corrosive (SiCl 4 ). The byproducts of CVD reactions can also be hazardous (CO, H 2 , or HF). Some of these precursors, especially the metal-organic precursors, can also be quite costly. The other major disadvantage is the fact that the films are usually deposited at elevated temperatures. This puts some restrictions on the kind of substrates that can be coated. More importantly, it leads to stresses in films deposited on materials with different thermal expansion coefficients, which can cause mechanical instabilities in the deposited films. CVD processes can be categorized according to the type of: 1. Application, 2. Process and reactor used, or 3. Precursor and chemical reaction used. The next three sections of this chapter are arranged around such subdivisions. These sections are followed by a discussion of the fundamental processes underlying CVD, such as mass transport, thermodynamics, and chemical kinetics. Finally, we present a brief analysis of the historical and current status of CVD R & D. CVD Applications www.asminternational.org One of the earliest examples of a large-scale CVD application was a carbonyl process for
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Chapter 1 Introduction to Chemical Vapor Deposition (CVD)

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