Wafer Cleaning: A Technology Assessment
It is vital to reduce foreign particles, foreign metals, organic chemicals and other contaminants that occur during the chip making process. These contaminants degrade the quality and reliability of semiconductor devices.
As a result, silicon wafer/substrate cleaning to remove these contaminants has become a critical step that influences device yield. There are several cleaning methods available to semiconductor makers.
Wet cleaning is still the most widely used process in mainstream chip manufacturing because it is robust and risk-free. However, at the 90nm node and below, new non-etching and damage-free cleaning methods will be required.
One promising new cleaning technique makes use of supercritical fluids. Supercritical fluids enjoy enhanced diffusion properties and can therefore be used to etch or remove photoresist residues regardless of device sizes and pattern density.
One such supercritical fluid is supercritical CO2 (SCCO2). Boasting negligible surface tension, high diffusivity, low viscosity and high density. SCCO2 is considered to be the best candidate for the treatment of porous ultra-low-k (ULK) materials.
SCCO2 technology is likely to become a viable wafer cleaning method when ULK materials enter high-volume production. Companies such as SC Fluids and BOC Edwards have already shipped supercritical systems to their selected customers for use in development lines. However, fullyfledged production systems are still probably 18 months away.
Just as wet cleaning is playing an increasingly important role in semiconductor fabrication, so too is wafer drying.
In order to meet the surface preparation requirements for 100nm and beyond, novel wafer drying technologies will have to be developed or existing methods will have to be honed.
One technology that has been refined in recent years - and as a result shows promise for sub- 100nm drying - is the popular marangoni-style drying technique.
This drying methods has been rengineered, eliminating previous issues with watermarks on wafer surfaces and maintaining the techniques benefits, These include the use of only a small amount of alcohol, minimising the risk of combustion. In addition, the whole drying process can be undertaken in ambient conditions within an inert atmosphere.
Further SCCO2 technology is likely to become a mainstream technology of choice for semiconductor manufacturing due to its significant diffusion properties. Because of its cost and an established knowledge base, SCCO2 technology is slowly garnering increased attention in the manufacturing industry. Continuing R&D efforts and investment in this technology, integrated infrastructure of CO2, and materials/equipment research, is strong evidence of the belief in the technology's substantial wafer processing potential. However, the semiconductor industry, is reluctant to adopt a drastically novel wafer process technology unless it substantially delivers process results and proves over other conventional approaches.
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