Understanding, Reducing Copper Defects
Laura Peters, Senior Editor -- Semiconductor International, 8/1/2001
 |
Anne Miller led a research group at Intel that identified the causes and impact of galvanic corrosion, pitting corrosion and chemical etching. They found changes that could be made to the slurry chemistry and process modules to help control these defects. For instance, galvanic corrosion occurs in a Ta/Cu system, where two dissimilar metals both contact an electrolyte solution. At the Cu/Ta interface in a Ta slurry formation at pH of 4.1, Cu+2 is reduced and plated out of solution leaving copper nodules at the Cu/Ta interface, as seen with optical inspection. Raising the pH of the solution puts the copper in a realm of self-passivation (according to copper's Pourbaix diagram), eliminating galvanic corrosion.
Another common corrosion mechanism - pitting corrosion - occurs when an isolated region of the metal is exposed to an increasingly more corrosive environment than the bulk metal. Pitting corrosion, called crevice corrosion when it occurs below a precipitate on the metal surface, is often initiated by impurities in the metal or in topography changes. Propagation of the pits occurs when Cu oxidizes to Cu2O, during both anodic and cathodic reactions. Pitting agents, commonly Cl-, but also SO4-2 (from ammonium persulfate oxidizer), reduce the Cu2O, releasing Cu+2 into solution. Since the H+ is not consumed, as it would be in H2O2 oxidizer, sulfate corrosion can have an autocatalytic effect, lowering the pH and increasing pitting corrosion, eventually extending through the thickness of the copper film. Removing halides from the slurry can reduce their corrosive effect. Sulfate ion levels can be controlled by adding Ba+2 getter or adding a buffer to reduce the pH drop that accompanies the reaction.
Copper CMP rates are typically increased by formulating slurries at low pH in the absence of corrosion inhibitors. This combination can result in copper loss and surface texturing. Chemical etching of the copper can be minimized by increasing the pH or adding BTA (benzotriazole) inhibitor, but Cu+2 is still being released into solution. The Cu+2, along with complexors such as glycine, catalyzes the chemical etching. Intel researchers recommend optimized polisher design, operating conditions and consumables to minimize chemical acceleration during CMP. To limit corrosion overall, engineers can use a Ta CMP step with finite copper removal, along with increased slurry flow and platen speed.
Researchers Hsueh-Chung Chen and others from UMC and UMC's United Foundry Service (Hopewell Junction, N.Y.) used a 1P/3M logic product to study defectivity after oxide etch, after liner/seed deposition, following copper electroplating and following Cu CMP using a KLA 2138 inspection tool and an in-line SEM. Using an orbital motion CMP tool, they tested different combinations of slurries and filtration systems to reduce killer defects from corrosion, scratching and contamination.
In the early stages of development, slurry residues are common. UMC eliminated the residues by optimizing post-CMP cleaning chemicals and the conditions of the cleaning process. Corroded metal, which appears as voids in metal trenches and vias, not only increases resistance but can result in opens. The researchers added corrosion inhibitor to the slurry and also to the cleaning bath. UMC also claims that dry-in/dry-out CMP is absolutely necessary to controlling corrosion. Grain size optimization can also limit corrosion along grain boundaries.
Fine-tuning the plating, annealing and CMP processes is important to reducing pitting, which can lead to pinholes on the plating surface. Photo-assisted corrosion, which plays into galvanic corrosion, can be eliminated by isolating the wafers from light. Scratching, a common problem in copper CMP, can cause shorts in closely spaced metal lines. Careful filtration of the slurry and maintenance of clean tools and consumables reduced scratch levels to <10/wafer.
UMC found many non-CMP-related copper defects. Contamination during barrier/seed deposition can lead to pinholes in the plated copper film. The defect is often removed during CMP, but the visual pinholes can be mistaken for pitting or corrosion. Pinholes in trenches from plating or barrier/seed defects are normally killer defects that cause increased line resistance and opens.
For additional information on yield management, go to www.semiconductor.net/yield
