Semiconductor device manufacturing: process – Formation of semiconductive active region on any substrate – Polycrystalline semiconductor
Reexamination Certificate
1999-06-15
2001-08-28
Bowers, Charles (Department: 2813)
Semiconductor device manufacturing: process
Formation of semiconductive active region on any substrate
Polycrystalline semiconductor
C438S479000, C136S243000, C117S088000, C117S099000, C118S669000, C118S726000, C423S349000
Reexamination Certificate
active
06281098
ABSTRACT:
BACKGROUND OF THE INVENTION
CHEMICAL VAPOR TRANSPORT—general principles
Chemical Vapor Transport (generally known as CVT) is a crystal growth process in which the materials to be grown are transported to the growth zone as volatile compounds that chemically react or decompose to give the material and a by-product. The CVT technique could be used in growth of either bulk crystals or thin films on a substrate. CVT may be categorized as either an open system or a closed system depending on the usage of the gas components. In closed systems, the source material and the growth zone are both contained in a sealed reaction chamber, material transport is accomplished by gas phase diffusion and convection due to temperature difference without the aid of a carrier gas, and the by-product is recycled. The advantage of a closed system is that only a small amount of transport medium is needed, but it is unsuitable for production-scale operation because unloading the grown crystals and replenishing the source material and substrates is inconvenient. In open systems, there is a gas flow through the growth region, the by-products are normally exhausted to waste, and the volatile compounds could be formed by passing an active gas ingredient through a nutrient zone or in a completely separate system. The disadvantage of an open system is the use of a large quantity of the transport medium and handling of effluent, but it allows for the easy removal of growth product and re-introduction of new substrates. In the case of a separate source generation using open systems, the technique is commonly called Chemical Vapor Deposition (CVD). In reality, CVD is essentially one half of the CVT process, thus, claims made regarding the deposition phase of CVT are also germane to CVD processes. The current semiconductor industry almost exclusively uses only CVD processes. In general, commercial CVD silicon processes employ silicon chloride, mono- and poly chlorosilanes, or silane.
CVT is generally carried out in a chamber in which the surface to be coated is isolated from the ambient environment. Before carrying out typical CVT processes, the moisture and oxygen must be purged from the chamber and the surface to be coated. In a typical apparatus for commercial production of coated objects, a load lock (see lexicon) is employed to facilitate removal of coated objects and re-introduction of new substrates to be coated without shutting the system down.
The steps carried out in CVT or CVD growth comprise:
1) Production of a volatile precursor reagent, generally by reaction between a source of the material to be coated onto or grown on a substrate.
2) Transport of the precursor species to a reaction site.
3) Causing a reaction of the precursor species on the surface of a substrate such that it forms a film (coating) or grows bulk crystals on the surface.
Step 1 may be carried out in the CVT apparatus itself, or in a site removed from the coating process when CVD processing is employed. In the semiconductor fabrication industry, nearly all coating precursors are prepared industrially by one company, and shipped to the fabricating facility of a second company which actually manufactures the devices. Step 2 is usually accomplished by a pressure differential (in an open system) or a temperature differential (in a closed system). In most applications, the precursor stream is diluted with a carrier gas. Carrier gas aids in controlling the coating process and in moving materials into and out of the reaction zone (see lexicon). Step 3 is usually carried out by supplying energy to the system either in the form of heat, photon, or radio frequency irradiation. Radio frequency irradiation is used both in conjunction with a susceptor (which converts it to heat energy) or to form plasma. Generation of a plasma aids decomposition of the precursor species leading to film formation on the substrate. In some cases the substrate is heated, and decomposition of the precursor species occurs on the surface of the substrate. In other cases the precursor is decomposed in the gas phase above the substrate, and the building blocks of the desired coating “rain down” on the substrate. In some cases both mechanisms are employed at the same time. Each option influences coating rate, morphology, defect occurrence, quality of coverage, and in some cases coating purity.
It is not unusual, especially in manufacturing electronic devices, that a coated substrate must be further processed after coating. Frequently, the substrate is annealed by heating, or subjected to some other post-coating process such as etching or polishing to reduce defects in the coating or improve the coating quality.
CVT may be used to form films having many different morphologies and characteristics. Epitaxial coatings (see lexicon), most often used in electronic device manufacturing, may be formed through rigorous reaction rate and process control with the same lattice structure in the film as that in the substrate. Polycrystalline coatings (see lexicon) are readily available from chlorosilane-based CVD processes, and both crystal size and size deviation may be controlled through a combination of processing and post processing treatment conditions. Amorphous film (see lexicon) coatings are readily formed when CVD is employed at low temperatures.
In CVT processes, as with other film growth processes, film growth is thought to occur through “nucleation”. Nucleation occurs when clusters of atoms that will comprise the film being deposited adhere to the substrate surface on widely scattered sites. These sites promote further growth upon the nucleation sites. Further growth is energetically preferred at nucleation sites that are larger than a critical size than it is at smaller nucleation sites. The critical size depends on the free energy driving force of the growth process, which in turn affects nucleation density on a substrate. In polycrystalline film growth, it is thought that widely scattered nucleation sites (sparse nucleation) leads to the development of large grain sizes. Suppression of secondary nucleation (the formation of new nucleation sites after the initial nucleation sites have developed into crystals) is thought to lead to films with a narrow distribution of crystallite sizes. The prior art in CVT growth of silicon crystals with iodine was not concerned about nucleation on foreign substrates, which is critical in achieving adequate grain sizes for solar cell applications.
The prior art in CVT growth of silicon crystals with iodine as a transport vapor has shown that such a system gives higher growth rates when compared to the silicon-chloride-based CVD processes at the same growth temperatures, but the conditions required (growth had to be carried out in either a sealed chamber, or if carried out in an open system resulted in high iodine consumption), were not suitable for industrial production.
SOLAR CELLS—current art and problems
Solar cells are candidates for use as a source of electricity in an attempt to reduce the dependence of the world on fossil fuel for the production of electricity. Solar cells based upon silicon have long been known, and used very successfully in spacecraft and satellite power systems.
The silicon solar cells employed in high efficiency applications have by-and-large been produced by semiconductor fabrication technology. This is to say that such solar cells have been manufactured from semiconductor grade materials, using techniques developed for fabrication of semiconductor devices on high purity and single crystal wafers. The use of semiconductor materials and technology drives the cost of the electricity produced by such cells to levels exceeding $50/peak watt, even if economies of scale are factored into the cost calculation. Many practitioners in direct solar power conversion believe that for terrestrial solar conversion applications to compete with current commercial technologies, power must be produced at costs of less than $0.50/watt.
High-efficiency solar cells are primarily single crystal silicon devices. They may be obtaine
Ciszek Theodore F.
Wang Tihu
Bowers Charles
Midwest Research Institute
Sarkar Asok Kumar
White Paul J.
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