Process Description

In a thermally driven CVD process, component elements of the film to be deposited are introduced into the reaction chamber via gaseous precursor reactants. The reactants are mass-transported to the surface of the hot substrate, where they are adsorbed. If the substrate is heated to an appropriate temperature for the desired chemical reaction to take place, films of the material to be deposited form on the substrate surface while the reaction byproducts are pumped out of the system. Thus the quality and the rate of film deposition are a function of, among other things, reactor geometry, partial pressures of the reactants, and the substrate temperature. Although the thermally driven CVD process is thermodynamically quite well understood and is widely used for the deposition of some films (e.g., undoped polycrystalline silicon), high deposition temperatures could be quite prohibitive for other applications (e.g., deposition of intermetallic dielectrics).

In the PECVD process, the gaseous precursors are most commonly subjected to time-varying electric fields of frequencies in the range of 50 kHz to 13.5 MHz. In some designs, microwave frequencies are also used. The electric field initially reacts primarily with the free electrons present in the gas. Although the electric field also reacts with the ions, these species remain initially unaffected because of their higher mass. The electrons undergo elastic and inelastic collisions with the gas molecules, but the electrons do not lose much energy during the elastic collisions because they are much lighter than the gas molecules. Loss of electronic energy during inelastic collisions with the gas molecules occurs only if the electrons, accelerated by the electric field, acquire energy that is higher than the threshold energies for excitation and ionization for a particular gas species (e.g., 11.56 eV for excitation and 15.8 eV for ionization of argon) (Ref 10). The inelastic collisions between these energetic electrons and gas molecules generate highly reactive species such as excited neutrals, free radicals, and ions, as well as more electrons. By this mechanism, the energy of the electrons is used to create reactive and charged species, while the gas temperature does not increase substantially.

In the PECVD environment, only a fraction of the precursor species in the gas phase undergo electron impact ionization and excitation, thereby generating the reactive species. The latter have a lower energy barrier to physical and chemical reactions than the parent species and consequently react at lower temperatures. Thus in the PECVD process, these reactive species lead to lower deposition temperatures and higher deposition rates than are possible with only thermally driven CVD. In film growth with the PECVD process, in addition to the unchanged parent species, these highly reactive species also diffuse to the surface of the substrate, where they are adsorbed and undergo a sequence of processes similar to that of thermal CVD. But these reactive species follow an alternate deposition pathway that proceeds parallel to the thermal pathway, because:

• The reactive species created during the inelastic collisions of gas molecules with energetic electrons have a sticking coefficient closer to unity (Ref 10); that is, once they reach the surface of the substrate, they tend not to escape.

• The activation energy for chemical dissociation is typically lower for these plasma-enhanced reactions (Fig. 1).

Fig. 1 Activation energy diagram for thermally driven (solid line) and plasma-enhanced (dashed line) chemical vapor deposition reactions. A and B, initial and final energy states, respectively, for the thermally driven reaction; D E, activation energy; A*, B*, D E*, corresponding parameters for the plasma-enhanced reaction. Source: Ref 11

Fig. 1 Activation energy diagram for thermally driven (solid line) and plasma-enhanced (dashed line) chemical vapor deposition reactions. A and B, initial and final energy states, respectively, for the thermally driven reaction; D E, activation energy; A*, B*, D E*, corresponding parameters for the plasma-enhanced reaction. Source: Ref 11

Some heat is needed to drive the reaction over E*, but as shown in Fig. 1, this energy is typically lower than the energy needed for a purely thermally driven reaction to proceed at a reasonable rate. If the substrate is kept at a temperature such that the deposition proceeds at a very slow rate without the plasma but at a reasonable rate once the plasma is switched on, a plasma can be used as a "switch" to turn the deposition reaction on and off (Ref 12). Switching the plasma on and off can be used to start and stop the deposition process. Extremely abrupt layers can be grown by this technique, because the time required for the plasma to switch is equivalent to that between gas molecule collisions (e.g., 1 ms at 1 torr) (Ref 13). The thermal energy in a PECVD process, in addition to driving the surface reaction, is also needed for desorption of the reaction byproducts, minimizing adsorption and the inclusion of undesired gases in the deposited film, thereby lowering film contamination.

Typically, radio-frequency (rf) glow discharges used for the deposition of thin films operate at frequencies between 50 kHz to 13.56 MHz and at pressures of 0.1 to 2.0 torr. The plasma density (i.e., the density of ions and free electrons) is in the range of 108 to 1012 cm-3. The degree of ionization is 10-4. Typical average electron energies are in the range of 1 to 3 eV, but the fastest electrons may reach energies as high as 10 to 30 eV (Ref 14).

In a PECVD deposition system, many factors can affect the growth, composition, and the properties of deposited films, in addition to conditions such as the substrate temperature, reactor geometry, and reactant partial pressures, which are important in thermally driven CVD film growth. Some of these factors are plasma power, frequency, electrode spacing, and substrate positioning. In these plasma glow discharges, the PECVD environment is not in thermal equilibrium because the average electron energies are much higher than the ion energies. Consequently, thermodynamic calculations cannot reliably predict the product of a PECVD reaction (Ref 13).

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