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TimeDomain CVD, Inc. |
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ChargingIons are constantly impinging on surfaces exposed to a plasma, giving rise to a net positive charge to the surface. In an RF-excited capacitive plasma, the sheath grows and shrinks during each RF cycle. During the brief time when the sheath voltage becomes very small, large electron currents can also flow. Typically, the sheath potential adjusts itself so that the integrated electron current balances the integrated ion current, giving no net charging of the surface. When differences in plasma potential exist, the situation changes. The RF voltage is roughly the same throughout the chamber, so if the DC potential varies, some regions will receive no electron current at all (the plasma potential is more positive and the sheath never gets small enough for electrons to escape). The surfaces will charge up until the change in surface potential compensates for this inhomogeneity, or until lateral current flows in the substrate to correct the disparity in electron and ion currents. 
If the lateral currents flow through the oxide insulator of an MOS transistor, the reliability of the gate may be impaired, leading to reduced circuit yields or (worse) early failures once the circuits are shipped to customers. The effect can be greatly enhanced depending on the circuit design: a single transistor may be connected to a large area of exposed metal (e.g. several long wiring runs -- an "antenna") which collect a much larger total current than the transistor electrode alone would. Charging damage is a more serious problem in etching than in deposition, since generally the CVD film being deposited is either an insulator or a conductor: if the former, no current can flow, and in the latter case the current flows harmlessly through the deposited layer. However, charging damage has been observed in PECVD of insulating films on integrated circuit wafers. Some authors have suggested that thin insulating films remain conductive, especially under the large flux of UV light created by the plasma. The effects are exacerbated if magnetic fields are present, since as we noted the fields can cause variations in plasma potential across the reactor. Charging may also result from localized variations in ion and electron flux due to topography ("electron shading"). Such phenomena are again more significant in plasma etching than in PECVD. Damage from Photon and Ion BombardmentTransistor gate oxides can also be damaged by energetic particle bombardment. The effects of ion bombardment are localized to the near-surface region, and thus generally of modest interest in PECVD, since they are ameliorated as soon as the surface is covered with a few nanometers of deposited film. [However, mechanical damage to structures can occur in high-density plasma deposition.] Photons with energies greater than about 10 eV can excite electrons into the conduction band of silicon dioxide, thus creating trapped holes. If the oxide is an MOS gate oxide, reliability may be impaired. Photon energies higher than 16-20 eV can also penetrate polysilicon or metal layers, damaging the gate oxides even when they are covered by gate metal. However, CVD oxides are also absorbers of the same photons that could damage gate oxides. Once a circuit has received a layer of CVD oxide dielectric, the gate oxide is well-protected from further radiation damage.
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