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Figure 1. Calibration Verification after 15 months. |
James McAndrew, Ronald Inman and Dmitry Znamnensky
Air Liquide
5230 S. East Ave.
Countryside, IL 60525
An industrial system, implementing near-infrared diode laser absorption spectroscopy, has been applied to the measurement of moisture in semiconductor processing, both in situ (on process chambers) and in high-purity reactive gases.
High purity of process atmospheres is usually considered critical to high yields in semiconductor processing. As device dimensions become smaller, purity demands increase, as manifested in the ppt (part-per-trillion) contamination levels now routinely required of carrier gases. In recent years, however, it has become clear that other sources of contamination, such as reactive gases, the process chamber and semiconductor wafers themselves, are more important. Tunable diode laser absorption spectroscopy (TDLAS) is extremely useful for such environments: As a spectroscopic technique that exposes only optical elements to the analyte, it is compatible with most matrices. In this work, we review some recent applications of this technique, both in the laboratory and in the wafer fab.
TDLAS has been applied to trace contamination measurement in high purity reactive gases - improving, for example, the achievable detection limit for H2O in NH3 from about 2 ppm (by FTIR) to 50 ppb - and to in situ monitoring in semiconductor process tools. In situ moisture monitoring enables improvements in overall equipment effectiveness (OEE) by reducing time spent on purging procedures. Moisture is an excellent indicator of atmospheric contamination, being perhaps the most difficult atmospheric impurity to remove. By measuring true contamination levels, rather than simply relying on pre-set timing, significant savings are achievable. In addition, non-product wafers for qualification and/or trouble-shooting can be reduced if the true system status is known.
Air Liquide’s TDLAS technology for moisture measurement in semiconductor gases and process atmospheres is now available commercially from SOPRA (Bois-Colombes, France), a manufacturer of optical instrumentation for semiconductor manufacturing and related industries.
TDLAS was selected because it is a relatively well-established and robust technology already widely applied, not only in the laboratory, but also in numerous demanding field applications. It has been shown to be capable of delivering the ppb (part-per-billion) sensitivity levels required for semiconductor process gas monitoring. TDLAS does not demand extremely high mirror reflectivities as do intracavity laser and cavity ring-down spectroscopies, and is therefore less likely to display performance loss over time. To date we have focussed on the application of TDLAS to H2O monitoring, but its application to other species is straightforward once a diode emitting at the appropriate wavelength is available. As TDLAS becomes widely adopted, diodes that emit at wavelengths characteristic of most molecules of interest are expected to become available.
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Figure 1. Calibration Verification after 15 months. |
Because TDLAS is based upon absorption spectroscopy, the measurement is absolute and can be directly related to fundamental properties of the molecule of interest. In practice, it is often more convenient to quantify the response of the signal-processing electronics by direct comparison with a known moisture standard. The reliability of this procedure is illustrated in Figure 1. The sensor reading was verified after 15 months on a semiconductor process tool and found to be still in calibration within the accuracy of the moisture generator used. This chart is based upon a test such as that illustrated in Figure 2, in which various moisture levels are generated by varying the flow of pure gas over a permeation device. In this figure, the rapid response of the hygrometer is clearly illustrated.
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Figure 2. Laboratory Determination of Electronic Response Factor. |
The application of TDLAS to a variety of semiconductor process environments, including RTP, CVD and etch, will be reviewed [1,2]. TDLAS has been demonstrated to be compatible with aggressive gases such as HCl, NH3, SiH2Cl2, etc. and with strongly depositing atmospheres as in Si3N4 LPCVD[3]. In addition [4], it has been shown to be capable of trace moisture measurements to 50 ppb in pure NH3 and to 10 ppb in a variety of corrosive gases including HCl. These applications will be discussed as will the key differences in approach required for in situ and for high-purity gas measurement.
This paper draws upon the work of Air Liquide researchers Melanie Bartolomey, Patrick Mauvais and Jean-Marc Girard, and owes much to the collaboration of customers including Texas Instruments, ST Microelectronics and IBM.
1. Progress in In Situ Contamination Control J. McAndrew, Semiconductor International 21(5) pp.71-78 (1998)
2. Increasing Equipment Up-time through In Situ Moisture Monitoring J.J.F. McAndrew, R.S. Inman, D. Znamensky, A. Haider, J. Brookshire, P. Gillespie, Solid State Technology 41(8) (1998)
3. In situ Measurement of Moisture Contamination Using Tunable Diode Laser Absorption Spectroscopy, J. McAndrew, R. Inman, D. Znamensky, J.-M. Girard, G. Goltz and J.M. Flan, AEC/APC Symposium X, Vail Colorado Oct. 11- 16. P. 213. SEMATECH 1998.
4. In-situ and On-line Moisture Monitoring in Reactive Gas Environments, J. McAndrew, M. Bartolomey, J.-M. Girard, G. Goltz,and J.-M. Flan MICRO 18(2) Feb. 2000.