Handbook Of Crystal Growth Thin Films Au Insb Diagram

Cu

N+, Ar +

Adhesion

50,000

10"2

Ni on Fe

Ar +

Hardness

10,000-20,000

From Ref. 32

application of ion bombardment is the enhancement of the density and index of refraction of optical coatings. This subject is treated again in Chapter 11.

3.8.4. Ionized Cluster Beam (ICB) Deposition (Ref. 33)

The idea of employing energetic ionized clusters of atoms to deposit thin films is due to T. Takagi. In this novel technique, vapor-phase aggregates or clusters, thought to contain a few hundred to a few thousand atoms, are substrate substrate

Qo-10 kV

Figure 3-26. Schematic diagram of ICB system. (Courtesy of W. L. Brown, AT&T Bell Laboratories. Reprinted with permission of the publisher from Ref. 34).

ionization crucible heating source material

Qo-10 kV

Figure 3-26. Schematic diagram of ICB system. (Courtesy of W. L. Brown, AT&T Bell Laboratories. Reprinted with permission of the publisher from Ref. 34).

created, ionized, and accelerated toward the substrate as depicted schematically in Fig. 3-26. As a result of impact with the substrate, the cluster breaks apart, releasing atoms to spread across the surface. Cluster production is, of course, the critical step and begins with evaporation from a crucible containing a small aperture or nozzle. The evaporant vapor pressure is much higher (10~2-10 torr) than in conventional vacuum evaporation. For cluster formation the nozzle diameter must exceed the mean-free path of vapor atoms in the crucible. Viscous flow of atoms escaping the nozzle then results in an adiabatic supersonic expansion and the formation of stable cluster nuclei. Optimum expansion further requires that the ratio of the vapor pressure in the crucible to that in the vacuum chamber exceed 104 to 10s.

The arrival of ionized clusters with the kinetic energy of the acceleration voltage (0-10 kV), and neutral clusters with the kinetic energy of the nozzle ejection velocity, affects film nucleation and growth processes in the following ways:

1. The local temperature at the point of impact increases.

2. Surface diffusion of atoms is enhanced.

3. Activated centers for nucleation are created.

4. Coalescence of nuclei is fostered.

5. At high enough energies, the surface is sputter-cleaned, and shallow implantation of ions may occur.

6. Chemical reactions between condensing atoms and the substrate or gas-phase atoms are favored.

Moreover, the magnitude of these effects can be modified by altering the extent of electron impact ionization and the accelerating voltage.

Virtually all classes of film materials have been deposited by ICB (and variant reactive process versions), including pure metals, alloys, intermetallic compounds, semiconductors, oxides, nitrides, carbides, halides, and organic compounds. Special attributes of ICB-prepared films worth noting are strong adhesion to the substrate, smooth surfaces, elimination of columnar growth morphology, low-temperature growth, controllable crystal structures, and, importantly, very high quality single-crystal growth (epitaxial films). Large Au film mirrors for C02 lasers, ohmic metal contacts to Si and GaP, electromigra-tion- (Section 8.4) resistant A1 films, and epitaxial Si, GaAs, GaP, and InSb films deposited at low temperatures are some examples indicative of the excellent properties of ICB films. Among the advantages of ICB deposition are vacuum cleanliness (~ 10"7 torr in the chamber) of evaporation and energetic ion bombardment of the substrate, two normally mutually exclusive features. In addition, the interaction of slowly moving clusters with the substrate is confined, limiting the amount of damage to both the growing film and substrate. Despite the attractive features of ICB, the formation of clusters and their role in film formation are not well understood. Recent research (Ref. 34), however, clearly indicates that the total number of atoms agglomerated in large metal clusters is actually very small (only 1 in 104) and that only a fraction of large clusters is ionized. The total energy brought to the film surface by ionized clusters is, therefore, quite small. Rather, it appears that individual atomic ions, which are present in much greater profusion than are ionized clusters, are the dominant vehicle for transporting energy and momentum to the growing film. In this respect, ICB deposition belongs to the class of processes deriving benefits from the ion-beam-assisted film growth mechanisms previously discussed.

1. Employing Figs. 3-1 and 3-2, calculate values for the molar heat of vaporization of Si and Ga.

Exercises

2. Design a laboratory experiment to determine a working value of the heat of vaporization of a metal employing common thin-film deposition and characterization equipment.

3. Suppose Fe satisfactorily evaporates from a surface source, 1 cm2 in area, which is maintained at 1550 °C. Higher desired evaporation rates are achieved by raising the temperature 100 °C. But doing this will burn out the source. Instead, the melt area is increased without raising its temperature. By what factor should the source area be enlarged?

4. A molecular-beam epitaxy system contains separate A1 and As effusion evaporation sources of 4 cm2 area, located 10 cm from a (100) GaAs substrate. The A1 source is heated to 1000 °C, and the As source is heated to 300 °C. What is the growth rate of the AlAs film in A/sec? [ Note: AlAs basically has the same crystal structure and lattice parameter (5.661 A) as GaAs.]

5. How far from the substrate, in illustrative problem on p. 90, would a single surface source have to be located to maintain the same deposited film thickness tolerance?

6. An A1 film was deposited at a rate of 1 /¿m/min in vacuum at 25 °C, and it was estimated that the oxygen content of the film was 10 3. What was the partial pressure of oxygen in the system?

7. Alloy films of Ti-W, used as diffusion barriers in integrated circuits, are usually sputtered. The Ti-W, phase diagram resembles that of Ge-Si (Fig. 1-13) at elevated temperatures.

a. Comment on the ease or feasibility of evaporating a 15 wt% Ti-W alloy.

b. During sputtering with 0.5-keV Ar, what composition will the target surface assume in the steady state?

8. In order to deposit films of the alloy YBa2Cu3, the metals Y, Ba, and Cu are evaporated from three point sources. The latter are situated at the corners of an equilateral triangle whose side is 20 cm. Directly above the centroid of the source array, and parallel to it, lies a small substrate; the deposition system geometry is thus a tetrahedron, each side being 20 cm long.

a. If the Y source is heated to 1740 K to produce a vapor pressure of 10 3 torr, to what temperature must the Cu source be heated to maintain film stoichiometry?

b. Rather than a point source, a surface source is used to evaporate Cu. How must the Cu source temperature be changed to ensure deposit stoichiometry?

c. If the source configuration in part (a) is employed, what minimum 02 partial pressure is required to deposit stoichiometric YBa2Cu307 superconducting films by a reactive evaporation process? The atomic weights are Y = 89, Cu = 63.5, Ba = 137, and O = 16.

9. One way to deposit a thin metal film of known thickness is to heat an evaporation source to dryness (i.e., until no metal remains in the crucible).

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