Resistivity of CVD

The reported bulk resistivity (p) of tungsten films deposited with the H2/WF6 chemistry varies between 7 and 12 n(lcm. Compared with the bulk resistivity of sputtered Al(Cu), (ca. 4 /iflcm)this represents an increase of a factor 2 or 3. Since the line resistance can always be considered a parasitic resistance, it is important to keep the bulk resistivity as low as possible. Only a few reports have dealt specifically with the resistivity of blanket tungsten (mostly H2/WF6 chemistry). Before we review these reports, let us first briefly summarize some concepts of conductivity in metals [Maissel178, Eckertova179]. 

For bulk material, simple theory leads to the following equation  

The mean free path for the electrons in single crystalline tungsten is about 40 nm. Equations 5.10 and 5.11 hold for diffuse reflections of the electrons at the surfaces of the film. In the case that a certain fraction, p, of the electrons reflect specularly at the surface boundaries, equation 5.10 will become  

In the case that all reflections are specular, i.e. p=l, there will be no thin film effects on the resistivity. The theory shows further that for ratios t/X0 5, hardly any thin film effect is predicted and the value of p should approach that of bulk tungsten. This is of course only true for structurally perfect (single crystalline) films. 

The resistivity of CVD-W for both the H2/WF6 and the SiH4/WF6 chemistries has been the subject of several papers. 

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Figure 9.12. Resistivity of CVD-WSiz film as a function of the anneal temperature. As-deposited Si/W ratio is 2.3. [Data from ref. 232]. 

J.T. Harding, V. Fry, R.H. Tuffias, R.B. Kaplan, Oxidation Resistance of CVD Coatings, AFRPL-TR-86-099, Air Force Rocket Propulsion Laboratory, Edwards AFB, CA, February, 1987. 

It is worthwhile noting that there are several sources of error in the determination of the specific wear rate which give approximately an order of magnitude uncertainty in the quoted values, in particular, the mass loss through wear of the pins is minuscule - a consequence of the excellent abrasion resistance of CVD diamond - and such a low loss is easily affected by any residual lubricant. 

The corrosion resistance of CVD layers and solids that are made out of (nominally) the same products fabricated with standard ceramic techniques is quite different. Glasses also are often more resistant against chemical attack than the polycrystalline modification of the same compound. Why ... 

Wear resistance of CVD diamond is generally superior to that of the single-crystal material since the wear of diamond occurs by chipping. Since CVD diamond is a poiycrystalline material, chipping stops at the grain boundary whiie, in a single-crystal, the entire crystal is sheared off. 

The third part identifies and describes the present and potential applications of CVD in semiconductors and electronics, in optics and optoelectronics, in the coating of tools, bearings and other wear- and corrosion-resistant products, and in the automobile, aerospace, and other major industries. 

The compact structure of diamond accounts for its outstanding properties. It is the hardest of all materials with the highest thermal conductivity. It is the most perfectly transparent material and has one of the highest electrical resistivities and, when suitably doped, is an outstanding semiconductor material. The properties of CVD and single-crystal diamonds are summarized in Table 7 2.[1][18]-[20]... 

Like graphite, h-BN is soft and lubricious and it has a low density. It is a refractory material which sublimes at 3000°C and has exceptional chemical resistance. The characteristics and properties of CVD h-BN are shown in Table 10.2. 

The number of CVD coating materials for wear and corrosion resistance is relatively small. The most important are as fol-... 

The main characteristic of refractory oxides is their excellent resistance to oxidation. Their brittleness however makes them prone to thermal shock with the exception of silica which has a compensating low coefficient of thermal expansion. The chemical resistance of the major oxides deposited by CVD is rated in Table 17.7. 

Iridium Coating for Spacecraft Rocket Nozzles. The coating of rocket nozzles with iridium is a good example of the ability of CVD to provide a complete composite material, in this case a structural refractory shell substrate coated with a corrosion- and oxidation-resistant component. The device is a thruster rocket nozzle for a satellite. The rocket uses a liquid propellant which is a mixture of nitrogen tetroxide and monomethyl hydrazine. 

Another drawback of the immersion cell concept is that the active area is badly defined, because of the meniscus formed at the electrolyte-air interface. The form of the meniscus greatly depends on whether the sample is hydrophilic or hydro-phobic, which again is a function of applied potential. This problem can be circumvented, if the active area of the sample is defined by a window in an inert layer, for example resist or CVD nitride, which is fully immersed into the electrolyte, as shown in Fig. 1.6 a. 

A 1 ym thick polycrystalline silicon (polysilicon) layer was then deposited by chemical vapor deposition (CVD). Phosphorus doping of polysilicon was done by ion implantation with a dosage of 1Cr° cm-2 and a voltage of 200 keV. The polysilicon sheet resistance of 50 SI/ was obtained after post-implant activation (Figure 1a). 

A large class of coordination compounds, metal chelates, is represented in relation to microwave treatment by a relatively small number of reported data, mainly p-diketonates. Thus, volatile copper) II) acetylacetonate was used for the preparation of copper thin films in Ar — H2 atmosphere at ambient temperature by microwave plasma-enhanced chemical vapor deposition (CVD) [735a]. The formed pure copper films with a resistance of 2 3 pS2 cm were deposited on Si substrates. It is noted that oxygen atoms were never detected in the deposited material since Cu — O intramolecular bonds are totally broken by microwave plasma-assisted decomposition of the copper complex. Another acetylacetonate, Zr(acac)4, was prepared from its hydrate Zr(acac)4 10H2O by microwave dehydration of the latter [726]. It is shown [704] that microwave treatment is an effective dehydration technique for various compounds and materials. Use of microwave irradiation in the synthesis of some transition metal phthalocyanines is reported in Sec. 5.1.1. Their relatives - porphyrins - were also obtained in this way [735b]. 

Concerning the use of CVD ZnO films as buffer layers between the absorber part of the cell and the front TCO (see Sect. 6.3.1.1), highly resistive... 

Chemical vapor deposition is not restricted to the microelectronics industry. It is the key process in the fabrication of optical fibers where it enables grading of the refractive index as a function of the radial position in the fiber (JO. In the manufacturing industry the technique provides coatings with special properties such as high hardness, low friction, and high corrosion resistance. Examples of CVD reactions and processes are given in Table 1. 

In the semiconductor industries, a number of materials are deposited on silicon wafers using CVD technologies such as plasma-CVD and thermal-CVD as described in the previous section. As the CVD process is repeated, a thick film is inevitably deposited on the various parts of the chamber and the internal walls of the exhaust tubes. This thick film generates contamination particles that affect the electrical resistance of the devices. Accordingly, cleaning of the apparatus is periodically necessary to eliminate the deposits and to improve the reliability of the device. 

As chlorine trifluoride is a corrosive gas, suitable conditions must be selected in each cleaning process from a sufficient study on the corrosion-resistances of materials used in the CVD system, the characteristics of the semiconductor materials etched by chlorine trifluoride and so on. 

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