Vacuum Deposition

In the vacuum deposition process, elements, alloys, or compounds are vaporized and deposited in a vacuum. The process is carried out at pressures of less than 0.1 Pa (1 mtorr) and usually in vacuum levels of 10 to 0.1 mPa (100 to 1 ¿'torr). The substrate temperature typically ranges from ambient to 500 °C (930 °F). Figure 1 shows a typical batch vacuum deposition system. Vacuum deposition is commonly used to deposit pure metals (for example, aluminum, silver, gold, nickel, chromium, titanium, molybdenum, and tungsten), some alloys (for example, stainless steel, nickel-chromium, lead-tin, and M-Cr-Al-Y), and selected compounds (for example, Al2O3, TiC, and TiB2).

Fig. 1 Schematic diagram of a typical vacuum deposition chamber Fundamentals of Thermal Vaporization

Equilibrium Vapor Pressure. The saturation or equilibrium vapor pressure of a material is defined as the vapor pressure of the material in equilibrium with the solid or liquid surface. At equilibrium, as many atoms return to the surface as leave the surface. The vapor pressure is measured by the use of a Knudsen cell, which consists of a closed volume with a small orifice of known conductance. When the container is held at a constant temperature the material that escapes through the hole depends on the pressure differential. In a vacuum environment and knowing the rate of material escaping, the equilibrium vapor pressure in the container can be calculated. The vapor pressures of the elements have been presented in tabular and graphical form (Ref 3). The Knudsen cell is often used as a source for molecular beam epitaxy, where the deposition rate can be carefully controlled by controlling the temperature of the source (Ref 4) or by mechanically interrupting the beam (Ref 5).

Figure 2(a) and 2(b) shows the vapor pressure of selected materials as a function of temperature. Note that the slopes of the vapor pressure curves are strongly temperature dependent (about 13 mPa/100 °C for cadmium and 13 mPa/250 °C for tungsten). The vapor pressures of different materials at a given temperature can differ by many orders of magnitude. For vacuum deposition, a reasonable deposition rate can be obtained only if the vaporization rate is fairly high. A vapor pressure of 1.3 Pa (10-2 torr) is typically considered the value necessary to give a useful deposition rate. Materials with that vapor pressure above the solid are described as subliming materials, and materials with that vapor pressure above the liquid are described as evaporating materials. Figure 3 shows the equilibrium vapor pressure curves of lithium and silver in detail and shows that at 800 K (527 °C) the vapor pressures differ by a factor of 107.

Fig. 2(a) Vapor pressure curves of the elements
Fig. 2(b) Vapor pressure curves of the elements

cd Cl

Li v

/^Ag

al 000 K

No.1: Ay-Li

-i

1 í

/

Li only 1 rl

500 1000 1500 2000 2500 Temperature, K

500 1000 1500 2000 2500 Temperature, K

Fig. 3 Plot of equilibrium vapor pressure vs. temperature for lithium and silver

Vaporization Rate. A material vaporizes freely from a surface when the vaporized material leaves the surface with no collisions above the surface. The free surface vaporization rate, dN/dt (in s-1) is proportional to the vapor pressure and is given by the Hertz-Knudsen vaporization equation (Ref 2, 6):

where dN/dt is the number of evaporating atoms per cm2 of surface area per second, C is a constant that depends on the rotational degrees of freedom in the liquid and the vapor, p* is the vapor pressure of the material at temperature T, p is the hydrostatic pressure of the vapor above the surface, k is Boltzmann's constant, T is the absolute temperature, and m is the mass of the vaporized species. The maximum vaporization rate is when p = 0 and C = 1. The actual vaporization rate will be one-third to one-tenth of this maximum rate because of collisions in the vapor above the surface (that is, p > 0 and C ■ 1), surface contamination, and other effects (Ref 7). Figure 4 shows some calculated maximum vaporization rates.

3500

3000

2500

2000

1SOO

1000

2500

2000

w

Mo

Pt

Cu

Ti

AT

Mg^

Zn

4532

2732 g.

1832

Free surface vaporisation rate in vacuum (calculated), g/cm2 * s

Free surface vaporisation rate in vacuum (calculated), g/cm2 * s

6332

5432

4532

2732 g.

1832

Fig. 4 Plot of temperature vs. free surface vaporization rate in a vacuum for selected elements. The symbol • indicates the melting point.

Vapor Flux Distribution on Vaporization. For low vaporization rates, the flux distribution can be described by a cosine distribution (Ref 2, 6). With no collisions in the gas phase, the material travels in a straight line between the source and the substrate (that is, line-of-sight deposition). The material from a point deposits on a surface with a distance and substrate orientation dependence given by the cosine deposition distribution equation:

where dm/dA is the mass per unit area, E is the total mass evaporated, r is the distance from the source to the substrate, 9is the angle from the normal to the vaporizing surface, and j is the angle from the source-substrate line.

Figure 5 shows the distribution of atoms vaporized from a point source and the thickness distribution of the film formed on a planar surface above the source based on Eq 2.

60" 45" 30° 15" 0" 15" 30" 45" 60"

60" 45" 30° 15" 0" 15" 30" 45" 60"

Point source

Distribution of deposited material

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