Advanced Role of Ozone in Wet Processing
Maria A. Lester, Associate Editor -- Semiconductor International, 12/1/2002
Ozone is an environmentally friendly and highly reactive form of oxygen that has been used for several years in semiconductor fabrication in the deposition of doped and undoped oxide films.
Ozone is generated at the point of use and can be easily converted to oxygen. IMEC (Leuven, Belgium) developed a wafer cleaning process that, compared with the widely used RCA clean, shows better or comparable performance. This new cleaning process, made possible when ozone is dissolved in deionized or ultrapure water, eliminates the need for sulfuric or hydrochloric acids and significantly reduces the number of steps in the RCA clean (Fig. 1). In applications where sulfuric acid and hydrogen peroxide mixtures are used, ozone can be added to restore the peroxide and greatly extend the effectiveness of the bath. In both cases, significant savings in chemical disposal costs and a very desirable positive environmental impact are achieved.
Researchers at MKS Instruments-ASTeX Products (Berlin) developed LIQUOZON, a fully integrated system that delivers ozonated water for use in such critical cleaning applications. Using LIQUOZON, ozone cleanings led to better gate oxide quality (transconductance), subthreshold slope, charge-to-breakdown (QBD), time-to-breakdown (TBD), and mobility with the least amount of roughness and interface scattering. In addition, the susceptibility of the surface to particle and organic contamination is significantly reduced.
A key differentiator of this technology is the high ozone concentrations achieved by using a proprietary efficient contactor specifically developed for LIQUOZON. The residual O3 gas is converted back to O2 in an integrated ozone destruct unit. Also different is the SEMOZON ozone generation technology that creates the highest-purity ozone gas through the partial conversion of oxygen into ozone in a silent plasma discharge. The ozone generator incorporates an inlet for O2 and an outlet for the O2/O3 mixture.
LIQUOZON provides high-purity, dissolved ozone in concentrations of 5-90 ppm at water flows of 5-60 L/min. Typical dissolved ozone applications include wet wafer cleaning, photoresist strip, contaminant removal, surface conditioning, oxide growth and pre-gate clean/pre-diffusion. The systems are available in two models: the LIQUOZON 100 for flows up to 30 L/min and the LIQUOZON XF for extended flow operation of up to 60 L/min. They include closed-loop concentration control and an intelligent electrical interface designed to handle variable flow rates at the desired ozone concentration. Because of their high water-flow capability, they can feed up to four wet processing tools simultaneously. An optional accessory, the FLOW CONTROL BOX, allows the system to feed multiple points of use at different concentrations and flows simultaneously from a single system.
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| 2. LIQUOZON can handle the variation of flow changes and liquid ozone concentrations. (Source: MKS-ASTeX) |
For additional information on clean processing, go to www.semiconductor.net/clean.
