Plasma etching is commonly used in semiconductor fabrication for removing material from substrates.  FIG.  1 illustrates a plasma reactor 10 in accordance with the prior art.  As shown in FIG.  1, the plasma reactor 10 includes a chamber 11, a support 12 inside the chamber 11, and a power source 14 electrically coupled to the support 12.  The chamber 11 includes a vessel 16 coupled to an electrically grounded lid 18 to form a sealed environment inside the chamber 11.  The chamber 11 also includes a gas inlet 20 proximate to an upper portion of the vessel 16 and a gas outlet 22 proximate to a bottom portion of the vessel 16.  The plasma reactor 10 can also include a vacuum pump (not shown) coupled to the gas outlet 22 for evacuating etching gases from the chamber 11. 
In operation, an etching gas enters the chamber 11 via the gas inlet 20.  The power source 14 creates a bias voltage between the support 12 and the lid 18 to establish and/or to maintain a plasma 24 between the lid 18 and a wafer 28 held on the support 12.  The plasma 24 then removes material from a surface of the wafer 28 via chemical, physical, and/or a combination of chemical/physical processes. 
One drawback of the foregoing plasma reactor 10 is that the etching uniformity over the surface of the wafer 28 may be difficult to maintain.  For example, the plasma 24 may have a non-uniform charge density that can cause the plasma 24 to etch a central portion more rapidly than a peripheral portion of the wafer 28.  As a result, feature dimensions in the central portion may be larger than those of the peripheral portion of the wafer 28.  Such etching non-uniformity can have a significant impact on subsequent wafer processing and/or final wafer yields.  Accordingly, there is a need for a plasma reactor with improved etching uniformity over a wafer.