The ever-shrinking dimensions of microelectronic devices has mandated the use of plasma processing in integrated circuit (IC) factories worldwide. Today the plasma-processing industry has grown to over $3 billion in revenues per year, well in excess of predictions made only a few years ago. Plasma etching and deposition systems are also found throughout flat-panel-display (FPD) factories despite the much larger dimensions of thin-film transistors (TFTs) that are used to switch picture elements (pixels) on and off. Besides the use of plasma in etching and depositing thin films, other processes include the following: removal of photoresist remnants after development (descumming), stripping developed photoresist after pattern transfer (ashing), and passivating defects in polycrystalline material. Why are plasma processes so prevalent?
In etching, plasmas are used for high-fidelity transfer of the photolithographically defined pattern that defines the device or circuit. More generally, plasma provides the means to taper sidewalls. In Si processing, the sidewalls must be nearly vertical to obtain high density integration and faster performance. However in making FPDs, sidewalls are tapered to obtain uniform step coverage and reduce shorting. In deposition, plasmas are used to enable processing at low temperature. For both etching and deposition, only plasma processing provides an economically viable means for processing large area substrates: 300 mm for Si and more than 550 × 650 mm for FPDs. It is the ability to scale uniform reactant generation to larger areas that sets plasma apart from beam-based processes that might otherwise offer the desired materials modifications. The nonequilibrium characteristics of plasma further distinguish this processing method. Energetic electrons break apart reactant precursors while ions bombard the surface anisotropically.