Synchrotron Radiation Checks Ultralow Contamination Levels

A wide variety of analytical techniques are used to monitor contamination in industrial semiconductor manufacturing processes and in the raw materials used, since contamination control is a key parameter for yield and reliability management. In MOSFET silicon technology, one of the most stringent contamination control monitoring requirements is for the insulating oxide film that separates the transistor gate contact structure from the silicon channel. As the dimensions of transistors are reduced, the thickness of this film must be reduced too, to only a few atoms in thickness. Any metallic atoms contaminating this oxide film may greatly increase the gate leakage and lead to short circuits and device failure.

Researchers at the European Synchrotron Radiation Facility (ESRF, Grenoble, France) have developed an extremely sensitive surface analysis technique using X-radiation from the synchrotron. This can verify that there is an adequate degree of cleanliness of the wafer surfaces prior to oxide formation and other processing steps. The researchers use a total reflection X-ray fluorescence (TXRF) technique. This is a non-destructive quantitative method that depends on the detection of the characteristic X-ray fluorescence emission from any contaminating atoms when they are excited by a grazing incidence primary X-ray beam. If this grazing incidence beam strikes the wafer at a sufficiently small angle, the beam will be totally reflected.

Under these conditions, the beam will not penetrate more than a few nanometers into the substrate. Thus, the characteristic fluorescence from layers under the surface layer is minimized, while the sensitivity to surface contaminants is greatly increased.

The intensity of the X-ray fluorescence radiation is measured by an energy-dispersive detector system and plotted against the incident X-ray photon energy. The energies of the peaks in this plot enable the contaminating elements present in the sample to be identified, as each element present produces a discrete and well-defined peak as its fluorescence signature.

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