Supercritical CO₂ Finds Niche With Ultralow-k Materials

It can clean copper residue, remove photoresist, replace thermal annealing steps and can even restore low-k values to films. So, is there anything that supercritical CO₂ cannot do? So far, not much. However, chip manufacturers are reluctant to adopt a drastically different process technology unless it delivers process results that simply cannot be delivered with existing approaches. Is the cleaning of ultralow-k porous dielectrics one of those cases?

It is the unique diffusion and surface tension properties that make supercritical CO₂ (SCCO₂) the best candidate for the treatment of porous ultralow-k (ULK) materials, according to a recent study by researchers from CEA-DRT/LETI-DTS (Grenoble, France), Altis Semiconductor (Corbeil Essonnes, France), Recif (Aussonne, France), EKC Technology/DuPont (East Kilbride, Scotland) and STMicroelectronics (Grenoble, France). SCCO₂ may become a necessary method of cleaning when ultralow-k materials finally make it into production. One of the properties that make SCCO₂ so flexible is the ability to use additives to target specific applications.

The main focus of SCCO₂ development has been on post-etch and -ash cleans. In particular, ULK materials can be damaged following etch stop layer breakthrough when copper and its residues are sputtered onto the ULK sidewalls. When a chelating agent is used, copper-rich residues can be removed in a lift-off process. For instance, hexafluoroacetylacetone (HFAC) dissolved in SCCO₂ was used to remove purposely contaminated silicon wafers with JSR 5109 porous SiOCH (k=2.2) with metallic copper (Cu(0)) and oxidized copper (CuO and Cu₂O). The clean was performed in SCCO₂ + ethanol + HFAC at 61°C and 180 bar. Residual contamination levels were measured using vapor phase decomposition-atomic absorption spectroscopy. After 1 minute, a cleaning efficiency of 98% was reached, and 99% shortly thereafter.

Another interesting application for SCCO₂ is film curing. The exposure of ULK films to plasma processes can damage the film by converting Si-CH₃ bonds to Si-OH bonds and elevating the k value. Unlike thermal annealing processes, which simply dehydrate these films, SCCO₂ has the effect of permanently drying a film due to the solubility of water in SCD CO₂ (4.5 mg/g at 75°C and 180 bar). In fact, by adding a small amount of HMDS (hexamethyldisilazane) at high temperature and moderate pressure (100°C and 80 bar), a SiOCH-based dielectric (JSR LKD 5109) that has been damaged by plasma treatments can be restored, as evidenced by the low k value and change of the film's contact angle from 3 to 110°.

The researchers have also shown that organic contaminants can be removed in a similar fashion to that of water from ULK films.

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