Supercritical CO₂ Cleaning Enables Sub-65 nm Processing

What new cleaning techniques are required to enable the next wave of roadmap technology advances?

While the industry tries to extend the capabilities of existing technologies, new ones develop to address shortfalls. Semiconductor processing in supercritical carbon dioxide (SCCO₂) shows considerable promise for cleaning applications below 65 nm. For example, SCCO₂-based processing enables the removal of post-etch residue and copper. In addition, SCCO₂ formulations can dry and repair low-k materials. Properties of CO₂ at supercritical conditions — high density, low viscosity and negligible surface tension — allow these SCCO₂ applications to be used for porous low-k substrates and smaller geometries.

One company committed to the development of SCCO₂ technology is BOC Edwards (Wilmington, Mass.). It recently demonstrated that cleaning high-aspect-ratio structures and cleaning/drying porous low-k dielectric processing for 40 nm could be achieved by SCCO₂ processing.

Conventional etching and ashing processes can damage porous low-k dielectric materials. The surfaces of these often hydrophobic materials become polar, leading to adsorption of moisture and changes in k value. SCCO₂ has been shown to remove this moisture from the porous materials. In addition, specific chemistry formulation can enhance and even repair the low-k surfaces through reactive coupling.

1. Post-dielectric etch, post-ash (above), SCCO2 cleaning (below). (Source: BOC Edwards)

Special SCCO₂ formulations can even remove ions such as copper in its oxidized state. Dense CO₂ is a non-polar solvent and has no net dipole. Consequently, its solubility characteristics with ionic materials are similar to hexane, and therefore inadequate. When co-solvents are added to CO₂ in relatively small quantities of <15%, the non-polar nature of dense CO₂ is not effectively changed. However, the ionic potential of the fluid medium can be changed significantly by the creation of water-in-CO₂ microemulsions. These aqueous micro-regions can be created by the addition of specially formulated surfactants to SCCO₂ .

2. Dissolved copper salts in water-in-CO2 microemulsion. (Source: BOC Edwards)

Back-end and front-end applications could benefit from SCCO₂. Continuing investment in SCCO₂ technology, CO₂ integrated infrastructure, materials and equipment is strong evidence of the belief in its substantial process potential. Supercritical fluids are not an absolute solution to all cleaning issues.

However, many possible applications include lithography, post-CMP clean/dry, post-deposition and MEMS cleaning, to name a few.

In related news, BOC Edwards recently formed an alliance with Micell Integrated Systems (MIS, Raleigh, N.C.) to further develop and commercialize technology for dense and supercritical CO₂ semiconductor wafer processing.

MIS develops processes and chemical formulations that enable CO₂ -based process solutions for the semiconductor industry. The joint development agreement focuses on delivery systems of semiconductor-grade CO₂ at supercritical pressures; and sub-fab systems for chemical blending, pressure and temperature control of CO₂ before delivery to the process tools.