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PLASMA ASSISTED-ETCHINGDENNA PAUL1Contents DCGlowDischargeandRFPlasma RFPlasmaEquivalentCircuit PlasmaChemistryandSurfaceReactions EtchAnisotropyandFeatureSizeControl EdgeProfiles ReactionIonEtchers FutureTrends2

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788Dry Etch Plasma etch Reactive ion etchDifference between Plasma and Reactive Ion EtchingIn RIE, the rf-driven electrode instead of the grounded electrode holds the wafer.RIE is also called RSE Reactive Sputter Etching.99Parallel plate Etcher

Equipment descriptionA 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 enters through small inlets in the top of the chamber, and exits to the vacuum 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 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.10Method of operationPlasma is initiated in the system by applying a strong RF electromagnetic field to the wafer platter. The field is typically set to a frequency of 13.56 MHz, applied at a few hundred watts. The oscillating electric field ionizes the gas molecules by stripping them of electrons, creating a plasma.11Contd.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 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.12Contd.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 knock off (sputter) some material by transferring some of their kinetic energy. Due to the mostly vertical delivery of reactive ions, reactive ion etching can produce very anisotropic etch profiles.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 is deep reactive-ion etching, used to excavate deep features.13Equipment

An RIE consists of two electrodes (1 and 4) that create an electric field (3) meant to accelerate ions (2) toward the surface of the samples (5).1415

1516Plasma Etch Processes Dielectric etch Single-crystal silicon etch Polysilicon etch Metal etchPhotoresist stripBlanket dry etch processes1617Etch Mechanisms Damaging mechanism Blocking mechanismBoth are related to the ion bombardment Dielectric etch processes use a damaging mechanism Single-crystal silicon, poly and metal etch processes use blocking mechanism1718Damaging mechanism Closer to the physical etch side To improve anisotropic profile increase ion bombardment and RF power and decrease pressure Disadvantages Increased ion bombardment on the inner surfaces of the chamber reduces their life.Can cause plasma-induced device damage This also increases particle contamination1819

192020Blocking mechanismIn this process, Chemical deposition during the etch process protects the sidewall and blocks etching in the horizontal direction.If the hard photoresist is left on the wafer, ion bombardment will sputter some of the PR into the gap.PR coating on the side wall will block the etching in that direction.PR deposited on the bottom will be constantly removed by the bombardment from the plasma and expose the surface of the wafer to the etchants. Closure to the chemical etch side Requires less ion bombardment Disadvantage The sidewall deposition needs to be cleaned by either dry or wet clean process or both2121h334a

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PR atomic oxygenSilicon fluorine containing gas CF4Designed for isotropic etchBarrel-etcher2223Remote Plasma SystemDesigned for isotropic etch

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2425Batch RIE System h337a

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282929Comparison of etch processesChemical EtchRIEPhysical EtchExamplesWet etch, Strip, RP etchPlasma patterned etchesArgon sputteringEtch RateHigh to lowHigh, ControllableLowSelectivityVery goodReasonable, ControllableVery poorEtch ProfileIsotropicAnisotropic, ControllableAnisotropicEnd PointTimed or visualOpticTimed30h335a

3031Etch Endpoint For Wet Etch BY time pre-measured etch rate & etch thickness Visual inspection sensitive to etchant concentration and temperature For plasma etch - optical method3132h341a

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