ULTRA-TRACE MATERIALS ANALYSIS USING RIS

REQUIREMENTS FOR MATERIAL CHARACTERIZATION ARE BECOMING MORE STRINGENT AS THE ROLE OF THE TRACE ELEMENT COMPOSITION OF HIGHTECHNOLOGY DEVICES BECOMES APPARENT. IN SEMICONDUCTOR RESEARCH, FOR INSTANCE, THERE IS A NEED FOR ULTRA-TRACE ELEMENT ANALYSIS IN SOLIDS BELOW THE PPBA LEVEL WITH SPATICAL RESOLUTION IN THE FEW MICRON RANGE. RESONANCE IONIZATION SPECTROSCOPY (RIS) INVOLVES THE STEPWISE EXCITATION, IONIZATION, AND COUNTING OF ATOMS OF A PRE-SELECTED ELEMENT, GENERALLY IN A LARGE BACKGROUND OF OTHER ELEMENTS. NARROWBAND LASERS ALLOW SUFFICIENT OPTICAL POWER TO IONIZE MOST ATOMS OF THE SELECTED ELEMENT WITH NEGLIGIBLE IONIZATION OF THE OTHER ELEMENTS.IN RIS, THE SENSITIVITY DEPENDS ON THE NUMBER OF ATOMS THAT ARE IN THE LASER BEAM. LASER ABLATION CAN PROVIDE ATOM DENSITIES AS HIGH AS 10 TO THE 20TH POWER CM-3 WITH RESOLUTION OF A FEW MICRONS. AT >10 TO THE 7TH POWER W CM-2, PLASMAS ARE FORMED, WITH HIGH ENERGY IONS DEGRADING MASS DISCRIMINATION. BECAUSE OF THE HIGH ION YIELD, MOST WORKERS ATTEMPT TO OPERATE CLOSE TO THIS THRESHOLD. WITH RIS, LASER ABLATION NEED ONLY VAPORIZED, PERMITTING POWER DENSITIES OF TO THE 7TH POWER W CM-2, AT WHICH MOSTLY NEUTRALS ARE FORMED. WE PROPOSE INVESTIGATING QUANTIFICATION OF LASER ABLATION TO DETERMINE IF COMBINED WITH RIS, UNPRECEDENTED SENSITIVITY IN MATERIAL ANALYSIS BECOMES POSSIBLE.