Use of Plasma Processing in Making Integrated Circuits and Flat-Panel Displays
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cessing large area substrates: 300 mm for Si and more than 550 X 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. Relative to wet processing, the plasma, or dry, process offers greater opportunity for control of the process and less environmental waste. These factors are more important in the FPD industry where wet etching is not precluded like it is in processing the submicron dimensions of Si device technology. Consider the etching of indium tin oxide (ITO), the transparent conductor used as part of the pixel in an FPD. To wet etch this material, hot solutions of HC1 and HNO3 are sprayed onto a spinning glass substrate. In plasma etching, by contrast, the glass is shuttled into a vacuum-tight processing chamber where it is exposed to an electrical discharge through low-pressure Cl2. The exhaust is easily funneled into a scrubber that extracts residual Cl2 and etch byproducts, neutralizes them, and converts them to salts. The processing pressure is also sufficiently low to ensure uniform distribution and minimum consumption. Even with these advantages however, the Cl2 required for
the etch imposes significant cost of ownership owing to its corrosiveness and toxicity and the resulting special handling required. Plasma Reactors Plasma-processing systems come in a variety of forms.2 The earliest systems were capable of processing large batches of wafers—desirable for maximum utilization of the expensive capital equipment. However to reduce the cost of making a single chip, the wafer diameter has increased from 50 to 300 mm so that the uniformity and control afforded by such batch reactors rapidly became inadequate. Today single-substrate systems are used almost exclusively to provide the process control required. Most reactors used in production are either low-density, parallel-plate (plasma densities from 109 to 1010 cm' 3 ) where power is capacitively coupled, or highdensity (10"-1012 cm"3) where power is either inductively coupled or waveinjected.2 Radio-frequency (rf) excitation is used to apply radio frequency power to the wafer in both cases because most substrate surfaces are insulating. In the low-density systems, this same rf power also generates the plasma. In high-density systems however, a separate energy source is used as the principle plasma generator. This source could operate at almost any frequency so long as it is capable of producing high-density plasmas. Today high-charge-density plasma reactors are used in semiconductor fabrication facilities (fabs) because they provide high throughput and uniformity while offering superior control. After the plasma is created, electrons rapidly diffuse to all surfaces, charging them negatively, impeding further electron lo
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