Anal. Chem. 1997, 69, 2434-2437
Trace Level Organics in Hydrofluoric Acid Determined by Attenuated Total Internal Reflection Infrared Spectroscopy Oliver M. R. Chyan,*,† Jin-Jian Chen,‡ Fei Xu, and JunJun Wu
Department of Chemistry, University of North Texas, Denton, Texas 76203
Trace levels of organic impurities in the hydrofluoric acid solutions were measured by a multiple internal reflection infrared spectroscopic (MIRIS) technique. The MIRIS utilizes a clean attenuated total reflection (ATR) silicon crystal to extract organic impurities from the HF solutions. An open beam single-channel background spectrum combined with statistical analysis was used to ensure the reproducibility of absorbed organics measurement. The hydrofluoric acid samples were analyzed under an ultrapure nitrogen blanket to avoid airborne organics interferences. The adsorbed organic contaminants were found to randomly orient on the silicon ATR probe surface. A higher level of organic impurities was found in the more concentrated hydrofluoric acid solutions. As the miniaturization trend continues in the microelectronics industry, the effective control and detection of trace metal and organic contaminants has emerged as the critical issue in fabricating a submicronmeter field effect transistor.1,2 Previously, we reported the nucleation and growth of nanometer-size metal particles on a Si(100) surface from hydrofluoric acid solution using atomic force microscopy.3 Recently, a novel silicon-based potentiometric sensor which is capable of detecting parts-per-billion to parts-per-trillion levels of metal ion impurities in HF and other microelectronic processing chemicals was also reported.4 In the present work, we focused our efforts on characterizing the trace levels of organic impurities in HF solutions using a silicon-based infrared organics probe. HF etching is widely used to remove the surface oxide layer before the critical thermal gate oxide growth step. Adsorption of organic compounds on the silicon wafer surface can drastically alter the surface wettability, which can yield uneven etching. In addition, the adsorbed organic compounds can lead to silicon carbide film formation on the silicon surface under certain high-temperature processing conditions.5,6 Therefore, it is highly desirable to measure and quantify the †
E-mail address:
[email protected]. Currently with Bruker Optics, Billerica, MA 01821. (1) Sze, S. M., Ed. VLSI Technology; McGraw Hill: New York, 1988. (2) Kern, W., Ed. Handbook of Semiconductor Wafer Cleaning Technology; Noyes Publications: Park Ridge, NJ, 1993. (3) (a) Chyan, O. M. R.; Chen, J. J.; Chien, H. Y.; Sees, J.; Hall, L. Proceedings of the Second International Symposium on Ultra-clean Silicon Surfaces, Brugge, Belgium, 1994, pp 213-216. (b) Chyan, O. M. R.; Chen, J. J.; Chien, H. Y.; Sees, J.; Hall, L. J. Electrochem Soc. 1996, 143, 92-96. (4) (a) Chyan, O. M. R.; Chen, J. J.; Chien, H. Y.; Wu, J. J.; Liu, M.; Sees, J.; Hall, L. J. Electrochem Soc. 1996, 143, L235-L237. (b) Chyan, O. M. R.; Chen, J. J.; Chen, L.; Xu, F. J. Electrochem Soc. 1997, 143, L17-L19. (5) Murrell, M.; Sofield, C.; Sugden, S.; Verhaverbeke, S.; Heyns, M. M.; Welland, M. ; Golen, B. Proc. Silicon Ultra-Clean Processing Workshop, Oxford, U.K., September 1991. (6) Kasi, S. R.; Liehr, M. J. Vac. Sci. Technol. A 1992, 10, 795-801. ‡
2434 Analytical Chemistry, Vol. 69, No. 13, July 1, 1997
Scheme 1. Schematic Diagram of Detecting Trace Level Organics in HF Acids by a Silicon Wafer-Based ATR Probe
organic contaminants in the HF solutions. Currently, determination of the total oxidizable carbon (TOC) levels in solution involves oxidizing all carbon-bearing compounds into carbon dioxide by a strong oxidant (such as sodium persulfate).7 Then, the change of solution conductivity caused by the dissolved carbon dioxide is used to represent the TOC values in the solution. However, it is difficult to adapt the same monitoring methods to the corrosive HF acids. Furthermore, current TOC detection methods measure the sum of both water-soluble ionic organics and nonsoluble organics. From the stand point of the contamination control, only the strongly adsorbed and nonsoluble organics will cause the aforementioned detrimental effects to the silicon surface. For the above reasons, it is advantageous to determine the level of organic contaminants directly adsorbed on the silicon wafer surface after it is immersed in the HF solution. As illustrated in Scheme 1, we utilize a clean attenuated total reflection (ATR) silicon crystal to extract organic impurities from the HF solutions by physical adsorption. The organic impurities adsorbed on the silicon ATR probe surface were characterized by multiple internal reflection infrared spectroscopy (MIRIS).8 Each total reflection can setup a surface-confined standing wave pattern (evanescent wave) which effectively activates the adsorbed organic species. Up to 86 internal reflections within the ATR silicon crystal were employed to enhance the detection sensitivity.9 In addition to the high detection sensitivity, the silicon-based ATR probe offers another important advantage as a specific organics probe which selectively measures the organics adsorbed on the (7) Moulin, J.; Melanson, P.; Retzik, M. Am. Lab. News 1996, 2, RP257. (8) Harrick, N. J. Internal Reflection Spectroscopy; Interscience Publishers: New York, 1967. S0003-2700(97)00001-2 CCC: $14.00
© 1997 American Chemical Society
silicon wafer surfaces. Therefore, the organics adsorption data acquired via a silicon-based organics probe can be directly utilized in the organics contamination control for the integrated circuit fabrication. EXPERIMENTAL SECTION Materials. All the solutions were prepared from prefiltered, high-purity electronic grade chemicals. The dilution was made directly in a Teflon beaker using ultrapure water (R >18.2 MΩ, total organic content
