Automated mass spectrometry to detect impurities in harsh acid chemistries
06/01/2006
An automated, in-line mass-spectrometry (ILMS) system using basic time-of-flight principles has been developed that can detect part-per-trillion contamination levels of metals in harsh acid chemistries. Special hardware and software are needed to handle concentrated hydrofluoric, hydrochloric, sulfuric, and phosphoric acids. Prior control strategies relied on slow and imprecise manual sampling using off-line instrumentation. The in-line system can automatically test samples from up to five different locations in the chemical management/distribution system, providing the information needed to respond to both routine variations and extreme contamination events.
Leading-edge semiconductor fabrication plants use large volumes of high-purity aggressive acid chemistries in automated wafer processing tools. These harsh chemistries are delivered to the process tools by the fab’s bulk chemical distribution system (BCDS), and include concentrated hydrofluoric, hydrochloric, sulfuric, and phosphoric acids. They are used at full strength or can be mixed with other process chemicals for various etch and wafer cleaning steps. While the chemical suppliers make every effort to deliver high-purity chemicals that meet required specifications, it is the fab’s responsibility to ensure that they arrive uncontaminated at the point of use (POU).
Harsh chemistry management
Surprisingly, the purity of harsh chemicals is largely taken for granted except when limited “grab samples” are dispatched to analytical labs, and the supplier’s Certificate of Acceptance is secured. As a result, problems attributed to contamination continue to occur, often causing fab managers to halt specific processes until the source of the contamination has been identified. Larger 300mm wafers and ongoing process scaling add to the susceptibility and cost of excursions.
The current manual grab-sample approach to analyzing purity is reaching its practical limit, especially in modern fabs that essentially function as sophisticated chemical processing facilities. This non-automated approach is labor-intensive and prone to costly human error (see table). Obstacles include double-walled piping and automated tool interlocks that limit operator access to certain points in the process line, as well as safety hazards associated with the manual handling of extremely harsh chemicals.
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In one recent incident, the grab-sampling process itself precipitated a 48-hour fab line shutdown, when the vial used to collect the sample was contaminated. The fab line was returned to operation only after the error was discovered and a new sampling demonstrated that the affected wet stations were not contaminated. In another case involving a high-volume fab, a contamination problem caused by the wear-out of a valve in the BCDS resulted in contamination that caused the loss of several weeks’ worth of production material. The severity of this issue was reflected in the company’s bottom line, which stated lowered quarterly financial results.
Automated metrology
Recently introduced process chemistry metrology tools designed for in-fab trace contamination measurement, including those for harsh chemistries, have altered this situation. These automated metrology tools use ILMS. Mass spectrometry, conventionally considered a lab method used by lab experts, requires a number of adaptations for the basic technology to function in the fab line.
For one, the hardware has to be “hardened” to confront the aggressive chemistries and 24/7 operation required in semiconductor manufacturing. Moreover, automated calibration techniques are required for accurate species identification and quantitative analyses. The latter problem is solved by the use of in situ calibration standards, or “spikes,” in every sample. This real-time calibration method corrects for instrument drift and matrix changes. The alternative is frequent manual recalibration by experts, which must be repeated for each process solution that is to be analyzed, and must occur at least daily to account for instrument drift.
To further equip ILMS technology for fab operation, new ionization techniques and mass spectrometer designs developed for the life sciences have been adopted to enable not only elemental analysis (normally the limit of existing lab tools), but also simultaneous molecular identification of contamination species as well. The incorporation of a time-of-flight (TOF) mass spectrometer provides high spectral resolution in a small form factor (approx. 3m × 3m × 3m). Depending on the process solution to be analyzed, sample preparation modules perform precise dilution of samples and add spikes at concentration levels that provide the best possible quantitative accuracy.
Harsh chemistry analysis
Analysis of harsh chemistry presents unique challenges for any analytical method. Beyond the obvious hardware corrosion problems are the issues of preferential ionization of the hydrogen species versus other species in solution and the generation of multiple spectral interferences. These problems were solved by the development of sample treatment methods that remove the harsh acid matrix while keeping metallic contamination species in solution.
In these methods, the sample is passed through a porous ion exchange resin. The resin exchanges H+ and the acidic anion from the acid matrix (Cl- in the case of HCl) for species that form soluble and easily ionized complexes with metallic cations. The resin is used multiple times with regeneration treatments. For harsh chemistry analysis, these systems have been automated and integrated into the sample preparation capability of the ILMS instrument.
This new in-line chemical metrology capability (Fig. 1) offers an alternative for the management of complex harsh acids, as well as for more benign process chemistries. Up to five different chemistries can be routinely sampled at any point selected by the fab-from the point of delivery to the POU. Results are available in minutes rather than the hours or days typical of the older grab-sample approach (see table).
 Figure 1. Diagram of the ILMS metrology tool including the sampling system. |
ILMS allows for the measurement of metallic species in concentrated hydrofluoric and sulfuric acids after matrix treatment. Figure 2 depicts the responsiveness and repeatability of the ILMS measurements to changing contamination levels in concentrated hydrochloric acid.
Figure 3 illustrates the detection of aluminum (Al) contamination excursions in an NH4OH/H2O2/H2O Standard Clean 1 (SC-1) bath deployed in production. These data graphically demonstrate Al excursions that occurred as a result of routine wet station maintenance, in this case up to ~500ppt. The much more serious excursion, up to ~3500ppt Al, was correlated to the processing of a large volume of wafers with Al contamination through the bath. These results demonstrate how difficult it would have been to detect and to understand the factors creating these excursions based on low frequency grab-sample measurements.
 Figure 3. Aluminum excursions in SC-1 (NH4OH/ H2O2/H2O) show routine contamination up to 500ppt and the rapid detection of 3500ppt from an extremely contaminated bath. |
These new chemical metrology techniques provide a routine, proactive in-line process measurement capability that enables rapid detection of process contamination at the earliest possible time. More important, this detection can occur before large volumes of production material are affected. These new capabilities offer a novel and improved alternative to conventional approaches for the general management of complex process chemistries as well as for excursion response.
Acknowledgments
ILMS is a trademark of Metara Inc.
Reference
1. R. McDonald, M. J. West, June Wang, Jason Wang, Y. Han, F. Liu, R. Mui, “Real-Time, Unattended, Trace Contamination and Chemical Species Analysis of Semiconductor Cleaning and Processing Solutions,” SEMI Technology Symposium: Innovations in Semiconductor Manufacturing (STS: ISM), 361, SEMICON West, July 2003.
Robert McDonald received his PhD in materials science from the U. of California, Los Angeles, and is VP and fellow at Metara Inc., 1225 East Arques Ave., Sunnyvale, CA 94085; ph 408/331-5221, fax 408/523-0945, e-mail [email protected].