Introduction

ION-BEAM-ASSISTED DEPOSITION (IBAD) refers to the process wherein evaporated atoms produced by physical vapor deposition (PVD) are simultaneously struck by an independently generated flux of ions. The extra energy imparted to the deposited atoms causes atomic displacements at the surface and in the bulk, as well as enhanced migration of atoms along the surface. These resulting atomic motions are responsible for improved film properties, including better adhesion and cohesion of the film, modified residual stress, and higher density, when compared with similar films prepared by PVD without ion bombardment. When the ion beam or the evaporant is a reactive species, compounds such as refractory silicon nitride (Si3 N4) can be synthesized at very low temperatures. Furthermore, adjustment of the ratio of reactive ions to atoms arriving at the substrate surface allows adjustment of the stoichiometry of solid solutions. Detailed reviews of the IBAD process can be found in Ref 1, 2, and 3.

Process Utilization. The feature that distinguishes IBAD from the other PVD processes discussed in this Section of the Volume is that the source of vapor and the source of energetic ions are separated into two distinct hardware items, as opposed to plasma-based techniques, such as direct current (DC), radio frequency (RF), and magnetron sputtering; plasma-enhanced chemical vapor deposition; and certain forms of ion plating in which both the evaporant flux and ion flux are derived by extraction from a plasma. Therefore, there is more control over the deposition parameters in the IBAD process, because the ion flux and the evaporant flux can be varied independently. The other major difference between the plasma techniques and IBAD is the higher pressure (0.13 to 13 Pa, or 10-3 to 10-1 torr) required by the operation of the plasma-based methods in order to sustain a plasma. Because IBAD techniques typically operate in the collision-free pressure regime, the evaporant and beam atoms follow straight-line paths to the substrate. This also limits IBAD to line-of-sight applications.

The two most common geometries used in IBAD processing are shown in Fig. 1. Details of the methods are described elsewhere (Ref 4, 5, 6). Normally, a broad beam from an ion source, such as a Kaufman-type ion gun, impinges on a substrate simultaneously with the deposited atoms. The PVD source is usually an electron-beam source, but it could also be a thermal source or a sputter target, in the case of dual-ion-beam sputtering. The simplest geometry for the generation of uniform films and the treatment of complex geometries is a small angle (<30°) between the vapor and ion sources.

Fig. 1 Two common processing techniques. (a) Ion-beam-assisted deposition (IBAD). (b) Dual-ion-beam sputtering (DIBS)

Typical levels of ion-beam energy are between approximately 50 and 1000 eV for Kaufman-type ion guns. The ion beam has a uniform flux over a large area, depending on the diameter of the ion extraction apertures (typically, 30 to 80 mm, or 1.2 to 3.1 in.). The energy levels for beams from an ion implanter are 20 to 100 keV. Large areas are obtained by rastering the beam over the exposed surfaces. Research indicates that while the adhesion of IBAD films at low energies (500 eV to 3 keV) is excellent (Ref 7, 8, 9), an additional improvement in adhesion can sometimes be gained at higher energies (20 to 40 keV) if the film and substrate are not normally miscible. On the other hand, other properties, such as the absorption in optical films, increase as the energy increases because of greater displacement damage (Ref 10, 11, 12). Minimal absorption is obtained at energies below 500 eV. Therefore, for films intended for environmental protection, wear resistance, decorative coatings, and similar applications, a slight advantage might be derived from utilizing higher beam energies, although it would not normally be enough to warrant the greater expense of high-energy ion sources and beam-handling optics. However, for films intended for optical and microelectronics applications, lower energies are preferred in order to minimize beam damage resulting in optical absorption and the formation of electrically active defects.

For the low-energy range (50 to 1000 eV), inelastic collisions of the incident ions with the surface atoms deposit the energy into the surface according to the expression:

where R is the ratio of beam atoms to evaporant atoms arriving at the surface and Eb is the energy per atom in the ion beam. For the arrival of four beam atoms for every ten evaporant atoms (R = 0.4) and a beam energy of 200 eV, the average energy deposited in the surface for every film atom is 80 eV. This should be contrasted to the thermal energy of evaporated film atoms without ion bombardment, which is E = 0.15 eV/atom. This parameter can be tightly controlled and independently varied over a wide range by changing the beam output of the ion source and the energy of the ion beam. In plasma-based techniques, there is a larger spread of energies of the energetic flux components, with unknown ratios of the ion to thermal atom flux. The independent control of these parameters is more limited.

Physical and Chemical Aspects. The physical processes that occur at the surface during the film formation of IBAD are shown in Fig. 2. The lower right portion of Fig. 2 shows a very dilute neutral plasma consisting of background gas ("g") at a thermal energy of 0.03 eV, which is present throughout the chamber, as well as vapor atoms ("v") with an energy of either 0.15 eV (produced by the electron-beam evaporator shown in Fig. 1a) or 1 to 10 eV (produced by the sputter source shown in Fig. 1b). The vapor atoms have a velocity vector directed toward the substrate and are confined to the region between the vapor source and the substrate. The gas and the vapor impinge on the film surface at individual rates that are determined by the chamber pressure and the evaporation rate, respectively. The lower left portion of Fig. 2 shows a 200 eV N+ beam directed toward the substrate. Some of this charged plasma is charge-exchange neutralized in near collisions with the ambient gas atoms, resulting in an energetic 200 eV neutral N0, which continues in the direction of the substrate, and a 0.03 eV N+ charged atom that gets expelled from the positive column of charge represented by the ion beam. As a result of charge exchange, the neutralized (high-velocity) atoms will not be counted by the charge-collection system.

Fig. 2 Physical and chemical processes at the film-vacuum interface during ion-beam-assisted deposition and dual-ion-beam sputtering

In the central portion of Fig. 2, the ions, which consist of N+ in this example, are either implanted into the first few atomic layers or reflected from the surface with a reflection coefficient, r. As a result of ion bombardment, some of the deposited and/or implanted atoms are sputtered from the surface with a sputtering coefficient, S. The sputtering process is shown as resulting from a three-step process, in which energy is transferred in a collision sequence, or cascade, from the ion to atom A, and then to atom B, which causes atom C to be sputtered. Additional surface processes shown are:

The ion-stimulated thermal desorption of a physisorbed gas atom (binding energy ~0.05 eV) caused by a thermal spike event in the vicinity of the ion impact • The ion-stimulated chemisorption of the gas impurity to form chemical bonds with the deposited atoms (binding energy ~2 eV per bond)

The upper portion of Fig. 2 shows the processes that occur within the film. For this example, imagine that the vapor atoms are titanium and the ions are nitrogen. The large, open circles are titanium atoms condensed out of the vapor phase that form the film. They are more numerous near the surface because the nitrogen ions represented by the small, shaded circles have a finite range in the lattice of five atomic layers. In the deepest three layers shown, there are equal numbers of titanium and nitrogen atoms, giving stoichiometric titanium nitride. This also means that the rate of arrival of titanium atoms has been adjusted to be nearly equal to the rate of arrival of the nitrogen ions. Also shown in the near-surface region is a void that is formed in most metallic depositions at thermal energies. The random nature of the positions along the surface, where the vapor atoms land, coupled with the low mobility of the atoms after impact, leads to void formation.

The upper left portion of Fig. 2 represents the process of film densification. As the ion enters the lattice, it slows down upon inelastic collisions with other atoms in the lattice. The dashed line enclosing atoms A' and 2' indicates a void, identical to the one shown on the right, which was present prior to the arrival of the ion. In this schematic, the unprimed symbols are the atom positions before the ion collision, the primed symbols are their positions after the ion collision, and the faint dashed circles represent missing atoms after the collision is over. Upon entering the lattice, the ion strikes atom A, knocking it into one of the void positions (A'), but on the way, atom A also strikes atom B, whereupon its motion causes atom C to be sputtered, as already noted. Next, the ion knocks atom 2 into void position 2', leaving position 2 empty, and the thermal excitation of the ion impact causes atom 1 to jump into position 1', the position formerly held by atom A. Thus, before the collision, there were two atoms missing in the bulk, whereas after the collision, only one atom is missing, and the film has a net increase in density. Finally, the N ion comes to rest in an interstitial position, which represents a defect in the crystalline structure. This defect can be eliminated by a subsequent collision or it can remain in the film.

The atomic displacement mechanisms in the bulk of the film depicted in Fig. 2 are generic to any energetic deposition process. The plasma and surface processes depicted in that figure are conceptually simple, compared with the physical description of processes that occur in plasma-based deposition systems. For that reason, the production of reproducible films and graded composition films by IBAD is straightforward.

0 0

Post a comment