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Reactive-ion etching (RIE) is anetching technology used inmicrofabrication. RIE is a type ofdry etching which has different characteristics thanwet etching. RIE useschemically reactiveplasma to remove material deposited onwafers. The plasma is generated under lowpressure (vacuum) by anelectromagnetic field. High-energyions from the plasma attack the wafer surface and react with it.
A typical (parallel plate) RIE system consists of a cylindrical vacuum chamber, with awafer platter situated in the bottom portion of the chamber. The wafer platter is electrically isolated from the rest of the chamber. Gas enters through small inlets in the top of the chamber, and exits to thevacuum pump system through the bottom. The types and amount of gas used vary depending upon the etch process; for instance,sulfur hexafluoride is commonly used for etchingsilicon. 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.
Other types of RIE systems exist, includinginductively coupled plasma (ICP) RIE. In this type of system, the plasma is generated with aradio frequency (RF) poweredmagnetic field. Very high plasma densities can be achieved, though etch profiles tend to be moreisotropic.
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 directionalelectric fields near the substrate to achieve more anisotropic etch profiles.[1] The RF power given to the substrate is often a tunable parameter called 'platen power', and controls the energy of ions bombarding the surface.[2]
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Plasma is initiated in the system by applying a strong RF (radio frequency) electromagnetic field to the wafer platter. The field is typically set to a frequency of13.56Megahertz, applied at a few hundredwatts. The oscillating electric field ionizes the gas molecules by stripping them of electrons, creating aplasma.
In each cycle of the field, 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 deposited on 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, the 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 knock off (sputter) some material by transferring some of theirkinetic energy. Due to the mostly vertical delivery of reactive ions, reactive-ion etching can produce veryanisotropic etch profiles, which contrast with the typicallyisotropic profiles ofwet chemical etching.
Etch conditions in an RIE system depend strongly on the many process parameters, such as pressure, gas flows, and RF power. A modified version of RIE isdeep reactive-ion etching, used to excavate deep features.