Epitaxial silicon

Epitaxy is a term that denotes the growth of a thin crystalline film on a crystalline substrate. When the epitaxial film is of the same material as the substrate (for instance silicon on silicon), the process is known as homoepitaxy. When film and substrate are of different 

The CVD reactions used to produce epitaxial silicon are described in Ch. 8. Originally, atmospheric-pressure CVD was used, but it is gradually being replaced by low-pressure CVD in spite of higher equipment cost and complexity. Low-pressure CVD appears to yield a better film with less autodoping and pattern shift. 

In the previous section, we discussed the CVD of silicon thin films. For the pressures and temperatures at which those depositions were carried out, the films were polycrystalline. If the depositions had been carried out at higher temperatures, single-crystal (epitaxial) films would have been possible. In this section, we will discuss some of the factors that govern the growth of epi silicon films. 

Some of the specifics of the epi silicon CVD process will be covered in the balance of this chapter. 

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Silicon Epitaxy. Silicon epitaxial films have superior properties. The applications are, however, limited by the high temperature of deposition, which is generally above 1000°C. These reactions use chlorinated compounds of silicon (tetrachloride, trichlorosilane, or dichlorosilane) as precursors as follows ... 

This reaction is irreversible. Epitaxial silicon is deposited. Polycrystalline silicon is obtained in the range of 610-630°C, which is close to the crystalline-amorphous transition temperature. 

This reaction was also used to deposit epitaxial silicon at the temperature range of 1000-1040°C, but the deposit was generally unsatisfactory and the reaction is no longer used for that purpose. However, if the reaction is enhanced with a plasma using electron cyclotron resonance (ECR), fluxes may be independently controlled and high-quality epitaxial silicon deposits are obtained at temperatures below 500°C.P 1... 

Polysilicon is a contraction of polycrystalline silicon, (in contrast with the single-crystal epitaxial silicon). Like epitaxial silicon, polysilicon is also used extensively in the fabrication of IC s and is deposited by CVD.f l it is doped in the same manner as epitaxial silicon. Some applications of poly silicon films are ... 

Epitaxial Silicon Wafers using Plasma-Enhanced, Chemical-V apor-Deposition... 

When classifying chemical products, Seider et al. [3] identify three categories (1) basic chemicals (commodity and specialty chemicals, bio-materials, and polymeric materials) (2) industrial chemicals (films, fibers, paper,. ..) and (3) configured consumer products (dialysis devices, post-it notes, transparencies, drug delivery patches,. ..). In the manufacture of epitaxial silicon wafers, a thin film of crystalline silicon is often deposited on a polished crystalline silicon... 

Epitaxial Silicon Wafers using Plasma-enhanced Chemical-Vapor-Deposition 291... 

Using a 260/wafer sales price for epitaxial silicon wafers and the United States MACRS tax-basis depreciation schedule, the investor s rate of return (IRR) is 18.3%. In addition, the return on investment (ROI) is 25.3%. These measures increase significantly with small changes in sales price for example, at 273/wafer, the IRR is 29.9%. Note that the economic analysis is somewhat shielded from variations in the price of epitaxial wafers because it is strongly linked to the price of the incoming polished wafers. In other words, the key metric of interest is the value added to the wafer by the epitaxial film deposition. 

For epitaxial silicon wafers, product design focuses on optimizing the geometry of the plasma-enhanced, chemical-vapor-deposition (PECVD) reactor. To increase productivity, and maintain acceptable thickness uniformity, on the order of 5%, a simple optimization strategy locates a design that completes the deposition in 62 s. Then, for a standard manufacturing process, the economics are driven by the wafer costs, which are provided by a vendor at 206/wafer. At a sales price of 260/epitaxial wafer, the investor s rate of return is 18.3% and the return on investment is 25.3%. 

D. A. Brass and A. G. Lee, The Production of Epitaxial Silicon Wafers via Plasma Enhanced Chemical Vaposition, Univ. Pennsylvania, Towne Library, 2003. 

In the present chapter, we will turn our attention to films deposited by thermal CVD that are either dielectrics or semiconductors. There are, as one would expect, many films that can be deposited by this technique. In addition, there are many gaseous reactants that one can use to create each film, the choice depending on the film characteristics desired. Rather then attempt to catalogue all of the possible films and reactants, we will choose instead to focus on silicon dioxide, silicon nitride, polysilicon, and epitaxial silicon as the films of interest. At the same time, we will only look at those reactant gases that have been used for integrated circuit manufacture. An excellent survey of the film types that can be deposited by CVD and the many reactants that have been used to obtain them has been given by Kern.1... 

When we consider silicon films, on the other hand, the nature of the solid deposit is crucial to the behavior of the film. Depending on deposition conditions, we can deposit amorphous, polycrystalline, or single crystal films. As was noted in Chapter 1, the morphology of polycrystalline films can be complex. In the present section, we will review some aspects of polysilicon (poly) thin films deposited by CVD. The final section of this chapter will be devoted to epitaxial silicon thin films. 

As noted earlier in Chapter 3, epitaxial silicon films deposited by CVD can be affected by autodoping. If diffusion of the doping species is excessive, the film is not a useful one. Therefore, quite a lot of effort has been spent to accurately measure the distribution of dopant through the film thickness. 

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