Sputter Deposition Processes
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linearly with ion energy up to about 500 eV. Because of the momentum transfer process, the yield depends on the relative masses of atom and ion, and most of the atoms are sputtered from the top two layers of atoms in the target. Because of this, sputter removal or etching is almost universally used in surface physics experiments to prepare clean surfaces for study (e.g., LEED, XPS) and to obtain depth profiles by Auger electron spectroscopy and secondary ion mass spectroscopy. In AES the altered and disordered target surface is analyzed, whereas sputtered ions are measured in SIMS. Usually, only 1 or 2% of the material is sputtered as ions but important exceptions exist. For example, higher percentages of O" are sputtered from materials like ZnO and YBa2Cu307.j, with potentially important consequences. Typical values of sputter yield are 0.1-3 atoms/ion for ion energies up to 1,000 eV. The atoms are ejected in different directions, roughly following a cosine distribution and with a range of energies giving an average value of 5-30 eV. Over 95% of the incident ion's energy remains in the target, so sputtering targets must be cooled. For deposition, sputtering provides a significantly different source than evaporation. Evaporation requires increasing the source temperature to where the vapor pressure of the metal is approximately 1 torr. The energy input, from an electron beam for example, must therefore be concentrated so that the evaporant flux emanates from a small area. Since the energy of the atoms is equivalent to the source temperature (—0.15 eV for Cu), the gas pressure between source and substrate must be low enough ( CD
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50 100 150 'Molecular" weight
200
Figure 3. Average energy deposited in the substrate per deposited "molecule." For sputtering a metal in Ar, the "molecule" is an atom. Note that a value of 100 eV for £m0, does not mean that an Ar atom with 100 eV energy strikes the substrate for each metal atom; e.g., 10 Ar atoms with 10 eV would produce this effect.
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Sputter Deposition Processes
The importance of these energetic neutral particles is not only in raising the substrate temperature; they also affect film stress11 and composition.12 These energetic atoms result from ions (e.g. Ar+) that bombard the target but are neutralized at the target and scattered back by a collision with a target atom. Of course, all the atoms leaving the target, both sputtered and reflected, have to pass through the plasma region of the magnetron before reaching the substrate. In this passage they may collide with particles in that region — electrons, gas atoms, ions. The number of collisions an atom makes is determined by the cross section. For gas atoms, the probability is determined by the mean free path, which is inversely proportional to the gas pressure. When a Cu target is sputtered in Ar at a target voltage of 500 V, Cu atoms are sputtered with an average energy of about 5 eV. Using laser induced fluorescence, Ball et al.13 showed that the atom energy decreased to 0.3 eV at a distance of 25 mm fr
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