Unlike charging damage, edge damage does not anneal, even at high temperatures. Energetic ions from the plasma impinge on the gate oxide edge, causing damage to the oxide bulk as well as at the Si–SiO 2 interface at the device edge. Such “edge damage” mainly arises from plasma-etch definition of polysilicon gates. This charging damage is usually hidden by the back-end processing and annealing steps.ĭevice edges directly exposed to plasma are also subject to damage ( Brozek et al. Charging of the conductor stops when the current through the insulator is equal to the local current imbalance from the plasma ( Cheung and Chang 1994). As such charging proceeds, an increasing voltage builds up across the insulator, opposing the current imbalance. If the wafer surface is nonconducting, or covered with isolated conductors (e.g., polysilicon gates) over insulators (gate dielectrics), surface currents between two separate polysilicon regions cannot flow, and significant surface charging will result. When the wafer surface is conducting, surface currents flow to balance the local nonuniformity between the electron and ion currents. These effects combine for “inverted boat” sheath shapes to deliver more net electrons to the wafer periphery and more net positive ions to the wafer center. In addition, the magnitude of positive ion current, which depends on plasma density and electron temperature, is generally more uniform than the electron current. Wafer portions underlying thinner sheaths experience significantly higher ratios of average electron/positive ion current flows. Typically, nonuniform plasma sheaths exhibit a cross-sectional “inverted boat” shape-the central section has a uniform sheath, but the end points show reduced sheath thickness. The “sheath thickness,” defined as the distance between the plasma and the wafer, is not uniform across a wafer exposed to a nonuniform plasma. For the case of nonuniform plasmas, the ion and electron currents do not balance spatially over the RF cycle, e.g., regions of the wafer can experience net charging. Plasma nonuniformity can be caused by a number of factors, including nonuniform fields, hollow cathode effects (holes in the electrodes), gas composition and gas flow conditions inside the reactors, shape of the etch chamber, etc. In most practical systems, plasmas are not uniform. For a uniform plasma in steady state, the flux of electrons during one half of an RF cycle is equal to the flux of positive ions during the second half, and net wafer charging is, therefore, not a problem ( Fang and McVittie 1992). Thus, electrons and positive ions impinge upon the wafer mounted on top of the substrate.
During alternate RF cycles, negative voltage is applied to the substrate, which then attracts positive ions from the plasma. When a positive voltage is applied to the substrate, electrons from the plasma are attracted to the substrate. The wafer to be etched is mounted onto a substrate usually, the wafer is not electrically connected to the substrate. The plasma is suspended above the wafer surface with the help of electromagnetic fields within the chamber. The reactor electrodes are driven by an alternating polarity voltage. Plasma reactors used for etching generate and sustain the plasma with a radio-frequency (RF) power supply. Kerns, in Encyclopedia of Materials: Science and Technology, 2001 1 Cause of Damage
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