Enhanced Point of Use Filtration for Cleaning without Small Particle Addition

Strict semiconductor cleaning demands for shrinking feature size and increased contamination sensitivity have led to projects that improve performance. The sources of contamination are generally the process chemicals themselves, where wafers are exposed to chemicals for etching or cleaning. Concerns over contamination are compounded in wet benches where chemical baths are re-circulated for periods up to 24 hours. Particle and metal impurities can arise from insufficiently pure chemicals/water, or liquid carry contaminants to the tools through plumbing components such as valves and pumps, and chemical containers/lines. Strict chemical, deionized water (DIW) and material specifications have benefited the industry. Currently, typical purity levels are below detection limits. That being said, point of use filtration offers enhanced reduction of contamination for critical cleans.

In this paper we investigate particle adsorption on an immersed wafer surface in dilute HF and rinse applications. Particle addition for various immersion times in the tank can probe the contamination concentration in a solution. In this study, point of use filtration with tighter retention rating and cleaner devices is demonstrated to be critical for small particle control.

300mm wafers were processed by immersing a batch of up to 50 at a time in dilute HF and oxidizing agents for several minutes, rinsing and then drying. Inspection was conducted on a Surfscan SP3 system by KLA-Tencor.

As shown below, the surface count of defects increases linearly with immersion time in HF for wafers processed normally. This demonstrates that particle contamination is present in the tank. Due to its acidic and etching nature (impact on particle zeta potential and silica concentration), HF processing is difficult in semiconductor processing.  There is a need for enhanced particle retention filters with adequate flow rate to remove undesirable particles.

The team evaluated tighter and cleaner Torrento® filters.  Torrento® devices are polytetrafluoroethylene membrane filters that deliver contaminant protection performance for critical wet etch and cleaning technology.

In a wafer manufacturing facility, recirculation filters were upgraded in a dilute HF tank. Tighter and cleaner Torrento® devices replaced the previous filters.  Greater than one month of sampling was conducted, before and after the exchange. SP3 data for 32nm particles show that the average decreases >35%, and the standard deviation is reduced as well. Surface metals by VPD-ICPMS confirm no adverse effects.

Separately, deionized water point of use purifier + filters were upgraded. Tighter retention Protego devices replaced the previously used purifiers.  Protego® Plus ion-exchange purifier + filters are designed to reduce particles and metallic contamination during rinse. The Protego Plus technology is capable of processing DI water applications up to 80°C. Greater than one month of sampling was conducted, before and after the exchange. SP3 data for 26nm particles demonstrate that the average is reduced by >30%, and the standard deviation is lowered in turn. Surface metals by VPD-ICPMS show improvement (e.g. reduction in Na).

Particle performance of acidic baths and DIW rinses has been shown to depend strongly on point of use filtration. Systematic studies of immersion time and large quantities of wafer production data are required to isolate the contribution and measure improvements.