APPLICATION OF SYNCHROTRON RADIATION TO TXRF ANALYSIS OF METAL CONTAMINATION ON SILICON WAFER SURFACES

P. Pianetta, S. Brennan, and Katharina Baur

Stanford Synchrotron Radiation Laboratory, Stanford, CA 94309

As the dimensions of integrated circuits become smaller and smaller, the thickness of the gate oxide is being reduced to a level where it has become necessary to control the process to virtually atomic levels. With oxide thicknesses less than 100 Angstroms, surface metal impurities can have deleterious affects on the oxide properties. Metals, such as Fe, Ni, and Cu, can be deposited on the surface at any number of the processing steps ranging from wet chemical etching to ion implantation. Much of the processing steps involved in fabricating an integrated circuit are actually cleaning steps designed to remove metal contamination. The levels of allowable metal contamination are being driven down along with the feature sizes by improved processes, which in turn are made possible by improved measurement techniques. Therefore, trace impurity analysis has become essential for the development of competitive silicon circuit technologies. Current best methods for chemically identifying and quantifying surface and near surface impurities include grazing incidence x-ray fluorescence techniques using rotating anode x-ray sources. To date, this method falls short of what is needed for future process generations. However, the work described here demonstrates that with the use of synchrotron radiation (SR), Total Reflection X-ray Fluorescence (TXRF) methods can be extended to meet projected needs of the silicon circuit industry into the next century. To date, SR-TXRF has achieved a sensitivity for transition metals of 1 X 108 atoms/cm2, as determined from Fe, Ni and Zn standards.1 This represents a detection limit of 1 femtogram over the detected area on the wafer surface of 8 mm2 as compared to 500 femtograms over a 100 mm2 area in the conventional TXRF systems. These sensitivities are achieved by exploiting the high photon flux using a high-power wiggler x-ray source at the Stanford Synchrotron Radiation Laboratory (SSRL) and efficiently coupling the radiation to the silicon wafer using a high throughput multilayer monochromator. To make this technique useful for industrial applications, it has also been necessary to add cleanroom facilities and a wafer mapping capabilities to the system. This program as well as future developments to increase the sensitivity will be discussed.

1. P. Pianetta, S. Brennan, N. Takaura, H. Tompkins, A. Fischer-Colbrie, S. Laderman, D. Wherry, and M. Madden, Review of Scientific Instruments 66(1995)1293.

  1. Part of this work was performed at SSRL, which is supported by the Department of Energy, Office of Basic Energy Sciences under contract No. DE-AC03-76SF00515.