Semiconductor wafer (“wafer”) fabrication often includes exposing a wafer to a plasma to allow the reactive constituents of the plasma to modify the surface of the wafer, e.g., remove material from unprotected areas of the wafer surface. The wafer characteristics resulting from the plasma fabrication process are dependent on the process conditions, including the plasma density profile across the wafer surface. It should be appreciated that differences in plasma density profile during processing of different wafers will result in different wafer surface characteristics. Thus, a drift in process results between different wafers can be caused by variations in the plasma density profile. Additionally, because an amount of reaction between the plasma and a particular portion of the wafer surface is directly proportional to the plasma density over the particular portion of the wafer surface, variations in the plasma density profile can result in center-to-edge wafer uniformity problems. Such center-to-edge wafer uniformity problems can adversely effect a die yield per wafer.
Some objectives in wafer fabrication include optimizing a die yield per wafer and fabricating each wafer of a common type in as identical a manner as possible. To meet these objectives, it is desirable to control the uniformity of features across an individual wafer and among various wafers of a common type. Previous plasma processing techniques have attempted to control wafer uniformity in an indirect manner by compensating for an uncontrolled plasma density profile overlying the wafer surface. Such compensation has been provided through control of various process parameters, such as reactant gas flow and wafer temperature, that influence reactions between the plasma and the wafer, but do not directly control the plasma density profile overlying the wafer surface. A solution is needed to enable more direct control of the plasma density profile overlying the wafer surface such that wafer uniformity can be controlled in a more direct manner.