1.  Field of the Invention
This invention relates to a plasma-enhanced process for forming integrated circuit structures.  More particularly, this invention relates to a process for controlling the concentration of the active reactant or reactants in a reaction chamber during plasma processing to form an integrated circuit structure on a substrate such as a semiconductor wafer. 
2.  Description of the Related Art
During the plasma processing of a substrate such as a semiconductor wafer to form an integrated circuit structure thereon, the reactant or reactants concentration in the reaction chamber may become depleted during a plasma etch (or plasma deposition).  For example, in a plasma etch process using chlorine as the reactive species to etch a pattern in a metal layer such as aluminum, the density of the lines for a particular integrated circuit structure being formed may vary widely from the pattern of another integrated circuit structure.  This, in turn, can lead to a depletion of the chlorine reactant during the etch of some integrated circuit structures, due to the excessive amount of metal being etched away for the particular structure.  Such a depletion of chlorine can lower the etch rate, result in an increase in etch residues, change the shape of the aluminum lines being formed, and change the selectivity of the etchant for the resist mask. 
Conventionally, the concentration of the active reactant (or reactants) for etching and/or deposition processes has been controlled by controlling the flow rate of the reactant or reactants into the chamber by a previous empirical determination of the flow rate needed for a particular etch or deposition process for an average integrated circuit structure.  Rather than attempt to empirically determine the required modifications to such a process (the change in flow rates of reactants) needed for each individual integrated circuit structure being processed, attempts have been made, instead, to modify the particular structure being processed to arrive at an average pattern density. 
If, for example, the pattern density of the metal lines in a particular desired structure is below about 30%, a fill material is sometimes added to provide dummy metal lines to that particular integrated circuit structure to make it more closely resemble the "average" structure for which the process was designed.  This addition of dummy lines will, thereby, adjust the amount of metal being etched away to more closely resemble the average amount for which the flow rates of etchant in the etch process were determined.  In this manner, the degree or amount of variation in density from one structure to another is reduced and the amount of depletion of the reactant (etchant in this case) for a particular structure will be correspondingly reduced, thus enabling one to use the predetermined reactant flow rate to maintain the concentration of reactant (e.g., etchant) in the reactor. 
However, such a remedy is less than satisfactory.  The amount of such fill material which can be added to a given integrated circuit pattern is limited, and therefore, the amount of the reduction in the variation of pattern density achievable with this method is limited.  Furthermore, the presence of the fill material can be deleterious in some instances.  For example, in the aforementioned plasma chlorine etch of a metal layer to form a pattern of metal lines, the addition of further dummy metal lines into the pattern, as fill material, adds further conductive material to the pattern.  This, in turn, can raise the chances of a short or other defect occurring. 
The addition of such fill in material also complicates mask design and manufacture, and limits the ability to use masks and patterns not specifically designed by the manufacturer (processor) of the integrated circuit structure to accommodate the particular process to be carried out. 
It would, therefore, be desirable to be able to provide a process for the formation of integrated circuit structures wherein the concentration of one or more active reactants being used in a plasma etch or a plasma deposition could be monitored and dynamically adjusted throughout the process to thereby avoid the above-discussed undesirable depletion of the reactant or reactants during the process.  This, in turn, would free the processing from dependence upon the density of the pattern being etched or deposited for satisfactory operation of the process.