Advancements in Copper Interconnect Technology: The Effect of Sulfuric Acid on the Adsorption & Desorption of an Advanced MLI Suppressor

In the last five years the size of the copper plated interconnect features for the logic and memory market has dropped below 30nm.  This decrease in size has forced plating chemistries to operate on copper seed that is often times either non continuous or has morphology that requires special tuning of the additives to create the bottom up phenomenon.  In the past, the traditional path of development led to the continued optimization of the organics additives (accelerators, suppressors, levelers) to help generate the void free fill in the plated interconnects while mostly fixing the inorganic concentrations.  A new approach to additive development is to optimize the organic additives in different electrolyte concentrations. 

Traditionally, the copper chemical vendors were focused on the traditional 40g/L Cu²⁺, 10g/L sulfuric acid, 50ppm electrolyte formulation.  However, optimization of electrolytes can allow for a decrease in defectivity across the wafer.  This change in electrolyte formulation has sparked a new set of chemistry development at MLI to tune the organic chemistry to the electrolyte concentrations and also expand the understanding of the effect of electrolyte on the organic additives.

 The current set of work aims at showing how acid affects the adsorption and desorption of a new advanced MLI suppressor.  This work starts with a basic understanding of how acid affects the plating bath with no suppressor present and then expands to show how this new suppressor is affected by the acid changes.  A surface adsorption and desorption model is then hypothesized based on the electrochemical signal. 

The figure below outlines a typical adsorption result where an applied current of 10mA/cm² is run while solution over the electrode is transitioned from one containing the electrolyte only to one containing the electrolyte and the MLI suppressor. 

The two plots in the graph show the difference in the adsorption rate as a function of acid concentration.  One will note that the higher acid concentration shows a stronger steady state suppression and also a slower adsorption rate.  Experiments similar to these will be disclosed and the adsorption / desorption model discussed.