Post Clean Treatment for Metal Layers

Maria Lester, Associate Editor -- Semiconductor International, 9/1/1998

N ew aqueous-based chemistries for post clean treatment (PCT) have been developed at EKC Technology Inc. (Hayward, Calif.) for the neutralization and removal of metal ion contamination and surface-absorbed cationic and anionic contaminants and the reduction of metal corrosion on VLSI structures. These chemistries specifically target amine residues. Typically, amine-based chemistries are used for post etch residue removal from vias, contacts and metal lines. Although they are effective, they can cause metal corrosion particularly in aluminum, copper, titanium and other metals if the amine agents are not removed thoroughly. The challenge is to prevent metal corrosion, so the metal oxide layer remains intact and unaffected. The researchers found that adjusting the rinse solution pH will prevent damage to the oxide layer.

The researchers developed a processing method to tackle metal contamination, especially for etching and photoresist ashing (Fig. 1). The challenge was to design chemistries that would remove metal ions significantly from the wafer surface. A pH of ~11 is typical for a DI water rinse system contaminated with amine chemistries. However, the amine layer cannot diffuse instantaneously from the wafer surface. It was therefore, an important design consideration to neutralize or control metal corrosion before the DI water rinse. The EKC team adjusted the pH of the aqueous solution to prevent corrosion and developed a buffer agent to neutralize the residual amines. The buffering capacity was developed to neutralize up to 10% of NMP/DGA or 8% of the alkanolamine-HAD chemistries without affecting metal lines.

Fig. 1. Post clean treatment procedural flow chart for removal of amine based chemistries. (Source: EKC Technologies Inc.)

Fig. 2. Results indicate the effectiveness of post clean treatment rinses when compared to DI water and IPA rinses. (Source: EKC Technologies Inc.)

The EKC team used TXRF and SIMS to determine the effectiveness of its PCT chemistries. Various solutions for the removal of contaminant metal ions were tested on blanket TEOS and BPSG wafers. The wafers were immersed into a solution of 100 ppb metal ions at room temperature, then dried and analyzed. The effectiveness of the PCT solution was compared to IPA and DI water rinses. The test results indicated that the PCT solution was more effective than the other two methods (Fig. 2). Sodium levels on both ashed and unashed wafers were tested as well under the same conditions, and the solution was shown to be effective in removing sodium ions from the wafers. Following a post etch residue removal step, the PCT rinse provided the lowest sodium levels. Potassium, calcium, iron, copper, chloride and sulfate ions were shown to be cleaned more effectively with PCT than DI water or IPA rinses also.

The absence of corrosion becomes more critical as device geometries shrink. This is driving the need for post cleaning. The PCT solution is an option that may be more effective than traditional IPA or DI water rinses at neutralizing residues of amine chemistries. In particular, the combination of its buffering capacity and the decreased pH provides a solution in preventing redeposition of metal contaminants and removal of metal ions.