Reactive ion etching
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Reactive ion etching (RIE) is a technology using plasma to etch material deposited on wafers. It is mainly used in the microelectronics industry.
A typical (parallel plate) RIE system consists of a cylindrical vacuum chamber with a wafer platter situated in the bottom portion of the chamber. The wafer platter is electrically isolated from the rest of the chamber, which is usually grounded. Gas flow is introduced through small inlets in the top of the chamber and is evacuated out to the vacuum pump system through the bottom of the chamber. The types and amount of gas used are determined by the etch process; for instance, sulfur hexafluoride is commonly used for etching silicon. Gas pressure is typically maintained in a range between a few millitorr and a few hundred millitorr by adjusting gas flow rates and/or adjusting an exhaust orifice.
Plasma is initiated in the system through the use of RF (radio frequency) power applied to the wafer platter. This power is typically at a frequency of 13.56 megahertz, applied at a few hundred watts. The RF power creates an oscillating electric field that ionizes the gas molecules by stripping them of electrons, creating a plasma. In an RF cycle, the electrons are electrically accelerated up and down in the chamber, sometimes striking both the upper wall of the chamber and the wafer platter. At the same time, the much more massive ions move relatively little in response to the RF electric field. When electrons are absorbed into the chamber walls they are simply fed out to ground and do not alter the electronic state of the system. However, electrons absorbed into the wafer platter cause the platter to build up charge due to its DC isolation. This charge build up develops a large negative voltage on the platter, typically around a few hundred volts. The plasma itself develops a slightly positive charge due to the higher concentration of positive ions compared to free electrons. Because of the large voltage difference, positive ions tend to drift toward the wafer platter where they collide with the samples to be etched. The ions react chemically with the materials on the surface of the samples, but can also physically etch some material due to their high kinetic engery. Due to the mostly vertical delivery of reactive ions, reactive ion etching can produce very anisotropic etch profiles, which contrast with the typically isotropic profiles of wet chemical etching.
Etch conditions in an RIE system are very much dependent on the many process parameters, such as pressure, gas flows, and RF power.
Other types of RIE systems exist, including inductively coupled plasma (ICP) RIE. In this type of system, the plasma is generated with an RF powered magnetic field. Very high plasma densities can be achieved, though etch profiles tend to be more isotropic.
A combination of parallel plate and inductively coupled plasma RIE is possible. In this system, the ICP is employed as a high density source of ions which increases the etch rate, whereas a separate RF bias is applied to the substrate (silicon wafer) to create directional electric fields near the substrate to achieve more anisotropic etch profiles.
