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The Plasma Etch ProcessThe etching process removes patterned material from the surface of the wafer. With the advent of sub-micron feature sizes, "dry" (plasma) etching has become one of the most often-used processes in semiconductor manufacturing. Gases are the primary dry etch medium, and plasma energy is used to drive the reaction.Plasma is created by applying an electrical field to a gas or combination of gases. The plasma affects the etching process both physically and chemically. First, it breaks down the molecules of the etch gases into chemically reactive species that can etch materials very rapidly. It also ionizes the chemicals, giving them an electrical charge which renders them maneuverable, given the process chamber design. Inside the chamber, the wafer to be processed sits on a negatively charged cathode. The positively charged ions are attracted and accelerated toward the cathode's negative charge, and the ions impact the wafer surface at a perpendicular angle. This allows chipmakers to achieve nearly perfect vertical etching profiles, a very important requirement for etching features on today's chip designs. The plasma etch process occurs in two ways, either separately or in combination, depending on the material to be etched and the etch chemistry used: (1) chemically reactive ions and free radicals formed in the plasma strike the wafer surface, where they react with the surface material removing it from the surface and allowing it to be pumped away; (2) the ions may be accelerated to strike the wafer surface with sufficient impact energy to break the chemical bonds in the film, physically dislodging or "sputtering" material away, molecule by molecule.
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