Plasma etching
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Plasma etching is a form of plasma processing used to fabricate integrated circuits. It involves a high-speed stream of glow discharge (plasma) of an appropriate gas mixture being shot (in pulses) at a sample. The plasma source, known as etch species, can be either charged (ions) or neutral (atoms and radicals). During the process, the plasma will generate volatile etch products at room temperature from the chemical reactions between the elements of the material etched and the reactive species generated by the plasma. Eventually the atoms of the shot element embed themselves at or just below the surface of the target, thus modifying the physical properties of the target.
Plasma systems ionize a variety of source gases in a vacuum system by using RF excitations. The frequency of operation of the RF power source is frequently of 13.56 MHz, chosen by the Federal Communications Commission (FCC) for industrial and scientific use. Nevertheless, it can be used lower frequencies (kilohertz) or higher (microwave).
The mode of operation of the plasma system will change if the operating pressure change. Also, it is different for different structures of the reaction chamber. Standard plasma etching work with very high pressures. In the simple case, the electrode structure is symmetrical, and the sample is placed upon the grounded electrode. Free radicals such as fluorine or chlorine are created in the plasma and react at the sample surface.
Without the assistance of the plasma, much higher temperature would be required. The low processing temperature is possible because the plasma generates atoms, molecular radicals and positive ions that are more chemically reactive than the normal molecular gases from which the species are created.
The key to develop successfully complex etching processes is to find the appropriate gas etch chemistry that will form volatile products with the material to be etched. For some difficult materials (magnetic materials for example), the volatility can only be obtained when the wafer temperature is increased.
