Laser Secondary Neutral Mass Spectrometry for Metal Impurity Detection on Silicon Wafers

A new surface analytical technique has been developed for measuring the surface concentration of transition metals on a silicon wafer at levels below 10⁹ atoms/cm². The method is based on laser secondary neutral mass spectrometry (Laser-SNMS) in which neutral species are desorbed from the surface by one laser and ionized by a second laser before being mass analyzed.

Sensitivity tests were performed with prepared silicon wafer samples having transition metal contamination levels in the range of 5 x 10¹³ to 1 x 10¹⁰ atoms/cm². The wafers were characterized by Laser-SNMS, and independently by the standard TXRF and TOF-SIMS methods.

At present the detection sensitivities of laser-SNMS are comparable to existing state-of-the-art methods. The detection sensitivity will be improved by optimizing the desorption and postionization steps. Detection sensitivity at the level required by the SIA Road-Map for the new generation of solid-state devices will be possible with minor changes in the present instrument design. Work is also in progress to develop absolute quantification capabilities for trace metal contaminants on silicon wafers.

Advantage
Laser-SNMS provides clear advantages over the existing surface contamination characterization methods based on TXRF and TOF-SIMS. Not only can Laser-SNMS provide the highest sensitivity for detecting metals on silicon, but the relatively uniform response factors for different metals allows quantification of the contamination level.

Opportunity
SRI is seeking potential commercial users of this technology to further develop the instrument and to assist with evaluation and testing in real-world applications.