Single-crystal – oriented-crystal – and epitaxy growth processes; – Forming from vapor or gaseous state – With decomposition of a precursor
Reexamination Certificate
2002-08-16
2004-11-02
Kunemund, Robert (Department: 1765)
Single-crystal, oriented-crystal, and epitaxy growth processes;
Forming from vapor or gaseous state
With decomposition of a precursor
C117S093000, C117S103000, C117S108000
Reexamination Certificate
active
06811611
ABSTRACT:
BACKGROUND OF THE INVENTION
1 Field of the Invention
The present invention is directed to a method and system for growing highly regular crystalline structures using an electrically shielded radio frequency (ESRF) plasma source.
2 Discussion of the Background
Monocrystalline layers of silicon have been grown on silicon substrates using several techniques, including liquid phase epitaxy, vapor phase epitaxy, molecular beam epitaxy, and ion plating. The goal of epitaxial growth is to produce a grown layer that replicates the crystalline structure of the substrate on which it has grown. In addition, with regard to certain kinds of defects, an epitaxially grown layer is typically more nearly defect-free than the substrate on which it grew. Chapter 7 of
Semiconductor integrated circuit processing technology
, Addison-Wesley Publishing Company, Inc., Reading, Mass., 1990, by W. R. Runyan and K. E. Bean, describes epitaxy.
High resistivity layers grown epitaxially on low resistivity substrates are used to reduce the base-collector resistance and thereby increase the performance of bipolar transistors. Although transistors fabricated in MOS integrated circuits are self-isolating, higher performance and/or smaller circuit size is often possible if epitaxial techniques are incorporated in the circuit design. Consequently, epitaxial growth technology remains an important technique among the tools available to the designer of integrated circuits.
An important application of vapor phase epitaxy is the growth of high resistivity layers on low resistivity substrates. Whenever doping is achieved using either solid-state diffusion or ion implantation, the dopant is introduced from the surface; so the dopant concentration near the surface is generally greater than the dopant concentration deep inside the wafer. During growth from a melt, after a dopant has been added to the melt, it cannot, in practice, be removed; so during growth from a melt the impurity concentration may be abruptly increased, but not decreased. On the other hand, in vapor phase epitaxial growth, the dopant is deposited simultaneously with the host semiconductor (e.g., silicon), and its concentration in the epitaxially growing layer may be readily decreased or increased. Furthermore, the species of dopant can also be readily changed. Therefore, using vapor phase epitaxy, one can, in principle, sequentially grow on the same substrate uncompensated layers of p-type and n-type material having widely different resistivities.
The epitaxial layer can grow defect-free only if no solid material is formed in the gas adjacent to surface of the growing layer. That is, if a solid forming chemical reaction is involved, that reaction must be surface catalyzed. If no solid forming chemical reaction is involved, steps must be taken to insure that no groups of atoms or molecules form prior to their impinging on the growing layer.
It is known from nucleation theory that nucleation is favored at steps or ledges on a growing surface. Indeed, in many cases the observed growth rates can be explained only if a continuous supply of ledges or steps is assumed. Nevertheless, the atoms that strike the surface of the growing layer do not all suffer the same fate. Five possibilities may be identified:
1. After striking the surface, the atom may be desorbed before it becomes incorporated in the growing layer.
2. If the surface mobility of the atom is very small or if the deposition rate is very high, the atom may be surrounded by other atoms and effectively be trapped at a location that is not coincident with an appropriate crystallographic site. In such a case, a polycrystalline or amorphous layer will result.
3. If the surface mobility of the atom is great enough to permit it to move appreciably across the surface but the ledges are widely separated, it may, before reaching a ledge, join with other atoms on a terrace (i.e., a flat surface between ledges or steps) to form a stable and perhaps even properly oriented cluster that can serve as a nucleating ledge for subsequently arriving atoms.
4. If the surface mobility of the atom is great enough to permit it to move appreciably across the surface, it may diffuse to a ledge, bond properly at an appropriate crystallographic site, and thereby become a proper component of the epitaxially growing layer. For silicon and other crystalline materials characterized by the diamond lattice, a sufficient number of atomic bonds are available at a ledge or step on any crystalline surface to orient properly and bond stably any atom that diffuses to it. Ledges are most important when the depositing species is chemically hindered from forming a solid. These species have two surfaces at the ledge to remove the partially bonded atoms from the crystal forming species.
5. The atom may bond properly at an appropriate crystallographic site without having diffused to a ledge or having joined with other atoms.
In possibilities 2 through 5, the atom becomes incorporated in the growing layer and is therefore referred to as an adatom. This brief description of the several ways in which an adatom may be incorporated into the growing layer suggests that the mechanism by which such incorporation occurs will depend in a significant way upon the energy of the impinging atom. To facilitate the incorporation of adatoms at proper lattice sites, temperatures in the range from 950° C. to 1150° C. are usually used for the epitaxial growth of silicon on silicon. For temperatures in this range, growth rates of the order of a micrometer per minute are achieved. As the growth temperature is increased, the maximum growth rate at which a certain quality of epitaxial growth (as determined, perhaps, by defect density) can be achieved is also increased.
Interest in lower temperature procedures continues because lower processing temperatures minimize both slip defects and impurity and dopant redistribution by diffusion. Conventional epitaxial reactors have been used at temperatures in the range from 800° C. to 1200° C. Process temperatures in the range from 600° C. to 800° C. are possible using molecular beam epitaxy (MBE).
Another factor that greatly influences the quality of an epitaxially grown layer is the condition of the substrate surface. To prevent defects, such as undesired polycrystalline growth and stacking faults, from originating at the interface between the substrate and the growing layer, the substrate surface must be both damage-free and clean. Residual damage from mechanical polishing operations must be removed. One procedure for removing the damaged layer uses high-temperature HCI vapor phase etching. Standard surface preparation techniques such as the so-called “RCA Cleanup,” satisfactorily remove most contaminants that adversely affect subsequent epitaxial growth. However, the thin layer of oxide that remains after chemical etching, and residual carbon from adsorbed organic solvents (if epitaxial growth occurs at relatively low substrate temperatures), and any heavy metal contaminants must also be removed. The major problem, however, is the removal of the residual oxide.
Surface cleaning may also be affected with low-energy ions of an inert gas produced in a plasma. Argon is most commonly used, because it is the least expensive inert gas and the ions are heavy enough to have high sputtering yield cross-sections on most materials. For this purpose, an inductively-coupled plasma generator is especially well-suited, because it permits independent control of both plasma density and, using the independently controlled substrate bias, the energy of the impinging ions.
Finally, in cases where thermal decomposition of a silicon containing compound (e.g., SiHCI
3
) at the growing surface is used, other products of the reaction (e.g., chlorine) may be incorporated in the growing film and adversely affect its quality.
Various techniques for achieving single-crystal epitaxial growth of silicon on silicon have been described in the non-patent and patent archival literature. U.S. Pat. No. 3,379,584, to Bean and Runyan, entitled “Semiconductor w
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