Reaction Probability

In reactive deposition, the depositing material is continually being buried and the time available for reaction is limited. The probability of chemical reaction of the growing film surface depends on a number of factors, including:

• Temperature of the surface

• Chemical reactivity of the species

• Extent of prior reaction on the surface (e.g., whether the surface composition is TiN01 or TiN0.95)

• Relative fluxes of condensing species and incident gaseous species (i.e., the "availability" of the reactive species)

• Residence time (adsorption) of reactive species on the surface

• Radiation by electrons capable of stimulating chemical reactions on the surface

• Radiation by photons capable of stimulating photochemical reactions

• Energy of the incident reactive species

• Concurrent bombardment by energetic species not involved in the reaction (e.g., concurrent argon ion bombardment during titanium plus nitrogen deposition)

In many cases, surface reaction occurs first at active sites on a surface providing a nonhomogeneous growth mode. The extent to which this occurs in reactive deposition is not known.

Adsorption on a Surface. For an ambient pressure of 10-3 torr (0.13 Pa) and 25 °C (77 °F), gaseous particles will impinge on a surface at about 103 monolayers/s, compared to a typical atomistic deposition rate of 10 or so monolayers per second. The impinging species may be reflected with little residence time, or they may be adsorbed with an appreciable residence time (Ref 42). Adsorbed species will be available for reaction for a longer period of time than the reflected species. The adsorption probability and adsorbed film thickness will depend on a number of factors, such as the impinging species, nature of the surface, adsorption sites, and so on. For instance, it has been shown that atomic oxygen on silicon will adsorb with a higher probability and to a greater thickness than molecular oxygen and that ozone (O3) is strongly adsorbed on Al2O3, whereas O2 is not (Ref 41). It has also been shown that the surface stoichiometry affects the adsorption. For example, stoichiometric TiO2 surfaces do not adsorb oxygen but substoichiometric surfaces do, with the amount depending on the degree of substoichiometry. In plasma CVD of silicon from silane (SiH), it has been shown that the disilane species formed in a plasma has a high adsorption probability and is important in the deposition of the silicon at low temperatures (Ref 22). Oxygen molecules will react with a pure aluminum film, but nitrogen molecules will not react. The probability that the oxygen molecules will react with the aluminum decreases as the aluminum reacts with the oxygen molecules and the oxygen coverage increases.

Reaction Extent. The extent of the reaction before the surface is buried also depends on the factors listed above. For example, in the case of atomic oxygen on silicon surfaces, the reaction probability will decrease monotonically with coverage through several monolayer coverages. If the material can form a series of compounds (e.g., TiN, T2N) the probability of reaction is further decreased as the extent of reaction increases, making it more difficult to form the higher compound (i.e., TiN will be more difficult to form than the Ti2N).

Reactant Availability. The degree of reaction of codepositing species depends on the availability of the reactive species; therefore, the relative fluxes of the reactants are important. This gives rise to the "loading factor," which means that there is a relationship between the surface area for reaction (deposited film area) and the amount of reaction gas available.

Stoichiometry. Many materials form a series of stable compounds that have different crystal structures. For example, titanium and oxygen form TiO, Ti2O3, TiO2 (brookite), TiO2 (anatase), and TiO2 (rutile). By controlling the availability of the reactive gas, the stoichiometry of the resulting film material can be controlled. The stoichiometry of the material can be changed during the deposition by changing the reactant gas availability.

Reactively Graded Interface. The composition of the reactively deposited material can be controlled by controlling the availability of the reactive species. This allows the gradation of composition from an elemental phase to the compound phase. For example, in the deposition of titanium nitride TiN, the deposition may start by having no nitrogen available, so as to deposit pure titanium, and then increase the nitrogen availability so as to grade the composition to TiN. This technique is often helpful in obtaining good adhesion of compound films to surfaces. Another example is the deposition of a nitride on an oxide where the deposited material is graded from an oxide through an oxynitride composition to the nitride by controlling the availability of both oxygen and nitrogen.

Free electrons can enhance chemical reactions in the vapor phase and on a surface. Electron energies of about 50 eV are the most effective. The effect of electrons on reactive deposition is relatively unknown.

Photon radiation can enhance chemical reactions by exciting the reacting species (photoexcitation), thereby providing internal energy to aid in chemical reactions (Ref 18).

Energetic Inert Particle Bombardment Effects. The reactivity between codeposited or adsorbed species can be increased by using concurrent energetic particle bombardment by a reactive species or with an inert species that does not enter into the reaction. Concurrent energetic particle bombardment during reactive film deposition has been shown to have a substantial effect on the composition, structure, and properties of compound films (Ref 43, 44). In general, the bombardment:

• Introduces heat into the surface

• Generates defects that can act as adsorption and reaction sites

• Dissociates adsorbed molecular species

• Produces secondary electrons that can assist chemical reactions

• Selectively desorbs or sputters unreacted or weakly bound species

This process has been termed "bombardment-enhanced chemical reaction" (Ref 44). It is of interest to note that Coburn and Winters attribute the major portion of bombardment-enhanced etching of silicon with fluorine to the development of the volatile high-fluoride compound (SiF4) (i.e., more complete reaction) under bombardment conditions. Periodic bombardment of a depositing species by energetic reactive species can accomplish many of the same effects (Ref 45). For example, oxide films can be formed by depositing several monolayers of aluminum, then alternately bombarding the film with energetic oxygen ions and depositing more aluminum to form an aluminum oxide film.

Beam Deposition Using Activated Species. Ions of an activated species can be produced in a separate source, accelerated, and then used to bombard the depositing material in a vacuum environment to give "reactive ion beam" deposition. For particle energies greater than a few tens of electron volts, the energetic particle will physically penetrate into the surface, thereby increasing its "residence time." For example, it has been shown that for N+, ions having an energy of 500 eV impinging on a depositing aluminum film, all of the nitrogen will react with the aluminum up to a N:Al deposition ratio of 1:1 (Ref 46).

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