L08-Sputtering_b - Sputtering(cont and Other Plasma...

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Sputtering (cont.) and Other Plasma Processes
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Sputtering Summary Create an ionic plasma by applying a high voltage to a glow tube. Ions bombard the target material at the cathode. Target atoms are ejected (sputtered) from the cathode by energy and momentum transfer. Sputtered atoms from the target are deposited on to the substrate (anode).
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Keys to Proper Sputtering The right choice of ions The atomic weights of the ions and the target atoms should be close. The right pressure for a sustainable plasma 10-1000 mTorr The right cathode voltage so that the ions have the right energy for sputtering E ion > 100 eV and V cath ~ 2-5 kV The right angle for high sputter yield 60° - 70° from normal The right substrate voltage and temperature for a clean film
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Glow Discharge Sputtering As the gas molecules are ionized and energized by collisions, eventually they will have enough energy to cause ejection of atoms from the cathode surface. This is not a very efficient system (ionic MFPs are very low due to their high mass). The film quality is not very good due to the high pressures required to maintain the glow.
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DC Sputtering Initially pump system down to 10 -6 10 -7 Torr for purity. Then let in controlled amount of the gas to be ionized (generally Argon). Eventually the chamber pressure will be around 1 100 mTorr. This number is determined by the pressure required to have a sustainable plasma given the chamber dimensions. The main control is the energy of the ions. They need to be in the right range for sputtering.
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Some Limitations of DC Sputtering The high pressures required to achieve a plasma can degrade film quality. Only a small fraction of the gas is converted to ions. All of this results in deposition rates that are low (~ 100 ǖ / min).
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Substrate Biasing Applying a small negative voltage to the substrate (50 300 V) Removes physisorbed (adsorbed) contaminants such as Oxygen Also, by increasing the energy of the surface atoms enhances diffusion This improves adhesion, nucleation and crystal structure. Other modifications to DC sputtering (hollow cathode, triode) enhance ionization and enable lower gas pressures.
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The Major Limitation of DC Sputtering Suppose instead of a metal, we want to sputter an insulator like quartz. Due to the high resistivity of the target running an appreciable current through would require an impossibly large voltage (~10 12 V).
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