Laszlo Fabry

Low Z total reflection X-ray fluorescence analysis — challenges and answers ☆

Low Z elements, like C, O, ... Al are difficult to measure, due to the lack of suitable low-energy photons for efficient excitation using standard X-ray tubes, as well as difficult to detect with an energy dispersive detector, if the entrance window is not thin enough. Special excitation sources and special energy dispersive detectors are required to increase the sensitivity and to increase the detected fluorescence signal and so to improve the detection limits. Synchrotron radiation, due to its features like high intensity and wide spectral range covering also the low-energy region, is the ideal source for TXRF, especially of low-Z elements. Experiments at a specific beamline (BL 3-4) at SSRL, Stanford, designed for the exclusive use of low-energy photons has been used as an excitation source. Detection limits <100 fg for Al, Mg and Na have been achieved using quasimonochromatic radiation of 1.7 keV. A Ge(HP) detector with an ultra-thin NORWAR entrance window is used. One application is the determination of low-Z surface contamination on Si-wafers. Sodium, as well as Al, are elements of interest for the semiconductor industry, both influencing the yield of ICs negatively. A detection capacity of 1010 atoms/cm2 is required which can be reached using synchrotron radiation as excitation source. Another promising application is the determination of low-Z atoms implanted in Si wafers. Sodium, Mg and Al were implanted in Si-wafers at various depths. From measuring the dependence of the fluorescence signal on the glancing angle, characteristic shapes corresponding to the depth profile and the relevant implantation depth are found. Calculations are compared with measurements. Finally, aerosols sampled on polycarbonate plates in a Battelle impactor were analyzed with LZ-TXRF using multilayer monochromatized Cr-Kα radiation from a 1300-W fine-focus tube for excitation. Results are presented.

Application of capillary zone electrophoresis with an isotachophoretic initial state to determine anionic impurities on as-polished silicon wafer surfaces

Abstract Mobile inorganic anions such as chloride, sulfate and nitrate were determined up to an analyte-to-matrix-ratio (ATMR) of 1:104 using capillary zone electrophoresis. On adjusting the mobility and the concentration of the co-ion of the electrolyte to an isotachophoretic initial state, the detection limit of the analytes was improved by a factor of 2 by increasing the plate numbers through the isotachophoretic state. The ATMR could be increased to 1:6 · 104. These optimized conditions were applied to the determination of anionogenic impurities on experimental silicon wafer surfaces (d = 150 mm) after dissolution of the native oxide of silicon in isothermally distilled hydrofluoric acid vapour down to the range of 109 anions/cm2. The mobile organic anion oxalate was identified on silicon wafer surfaces for the first time.

Total reflection X-ray fluorescence analysis of light elements with synchrotron radiation and special X-ray tubes

Abstract Total reflection X-ray fluorescence analysis (TXRF) of light elements, such as C, O and Al (atomic numbers 5–13) generally has poor sensitivity and detection limits due to poor excitation and low fluorescent yields. Special excitation sources are necessary to compensate for these physical limitations. Synchrotron radiation is the ideal source for TXRF due to its high intensity and wide spectral range extending into the low energy region required for light elements. For more routine use, special X-ray tubes can be constructed. Experiments have been performed at the Stanford Synchrotron Radiation Laboratory (SSRL) using beamline III-4, which is specially designed for the use of low energy photons. Light elements on Si wafers have been analyzed, leading to detection limits below 100 fg for Na, Mg and Al, which corresponds to about 10 9 atoms. A new vacuum chamber is introduced meeting the requirements of wafer handling without the risk of contamination and offering the possibility of scanning a certain area of the wafer. Boron was detected on a wafer with 10 14 atomsxa0cm −2 implanted in the surface layer. A special windowless X-ray tube with Mo, Al and Si as anode materials was also tested. With the optimization of anode geometry, beam path and excitation conditions, a detection limit of 5 pg (corresponds to 10 11 atoms) for Al was achieved.

Trace-analytical methods for monitoring contaminations in semiconductor-grade Si manufacturing

Generating systematic data on incoming materials, processes, production environments and products by contamination monitoring and analyis is the key element of quality assurance in semiconductor fabrication. To be able to match the analytical capabilities to the requirements of improving materials and processes, the level of sophistication of contamination monitoring and analysis systems must be higher than the expected demands in the fabrication line. The accuracy of each analytical method has to be cross-checked by different independent techniques. Accuracy, precision, power of detection, analysis time and expenses should always be tailored to the particular case. All monitoring methods must run under statistical process control. The methods described meet the analytical requirements of the near future in semiconductor grade silicon manufacturing.

Capillary preconditioning for analysis of anions using indirect UV detection in capillary zone electrophoresis: Systematic investigation of alkaline and acid prerinsing techniques by designed experiments

It is widely accepted in capillary zone electrophoresis, that the use of alkaline prerinse procedures can improve the reproducibility of migration times and of corrected peak areas. In this study we present a systematic investigation of alkaline and acid preconditioning procedures for anion analysis using indirect UV detection by designed experiments according to the methodology of Taguchi. Four frequently used electroosmotic flow modifiers (diethylenetriamine, hexamethonium hydroxide, tetradecyltrimethylammonium hydroxide, and hexadimethrine hydroxide) were examined. The optimized procedures were evaluated with regard to the necessary preconditioning time compared to the analysis time. Furthermore, it was demonstrated that the optimized preconditioning technique could be applied to other indirect detection based CE systems. For all examined electrolyte systems, relative standard deviations below 0.5% for migration times and below 5% for corrected peak areas (n=20) were achieved using automated peak integration without further manual reprocessing.

Diagnostic and monitoring tools of large scale Si-manufacturing: trace-analytical tools and techniques in Si-wafer manufacturing

In order to facilitate fast corrective actions, all process media, crucial process steps and intermediate product stages of wafering must be monitored by suitable analytical tools. The analyzes have to provide reliable, relevant and real-time results at justifiable economy. Preferably, they should be serviceable in on-line configuration under strict SPC conditions. The following chapters outline the performance of the few analytical techniques, that optimally satisfy these requirements.

Modification and validation of the pyromellitic acid electrolyte for the capillary electrophoretic determination of anions.

For the determination of inorganic and organic anions, the pyromellitic acid (PMA) electrolyte is widely used. The pH adjustment of the self-prepared electrolyte was very challenging to satisfy the pH of specification of pH 7.8 +/- 0.1. A modification was proposed to provide a more simple electrolyte by buffering the PMA electrolyte with triethanolamine (TEA) only instead of adjusting the pH by NaOH and TEA. Thus, the proposed electrolyte consisted of 2.25 mmol l(-1) PMA, 0.75 mmol 1(-1) hexamethonium hydroxide and 12 mmol 1(-1) TEA. The performance of the PMA electrolyte buffered by TEA only was compared to a commercial available PMA and statistically validated in accordance with the methodology of Taguchi. No statistically significant difference could be found for both electrolytes assessing the performance and detection limits of hydrodynamic, stacking and electrokinetic injection with transient isotachophoretic preconcentration as well as repeatability of migration times, peak resolutions and peak symmetries.

Synchrotron radiation induced total reflection X-ray fluorescence of low Z elements on Si wafer surfaces at SSRL — comparison of excitation geometries and conditions ☆

The analysis of low Z elements, like Na and Al at ultra trace levels (<1010 atoms/cm2) on Si wafer surfaces is required by the semiconductor industry. Synchrotron radiation induced total reflection X-ray fluorescence analysis (SR-TXRF) is a promising method to fulfill this task, if a special energy dispersive detector with an ultra thin window is used. Synchrotron radiation is the ideal excitation source for TXRF of low Z elements due to its intense, naturally collimated and linearly polarized radiation with a wide spectral range down to low energies even below 1 keV. TXRF offers some advantages for wafer surface analysis such as non-destructive analysis and mapping capabilities. Experiments have been performed at the Stanford Synchrotron Radiation Lab (SSRL) using Beamline 3-4 (BL 3-4), a bending magnet beamline using white (<3 keV) and monochromatic radiation, as well as Beamline 3-3 (BL 3-3), using a crystal monochromator as well as a multilayer monochromator. A comparison of excitation–detection geometry was performed, using a side-looking detector with a vertically positioned wafer as well as a down-looking detector with a horizontally arranged wafer. The advantages and disadvantages of the various geometrical and excitation conditions are presented and the results compared. Detection limits are in the 100-fg range for Na, as determined with droplet samples on Si wafer surfaces.

Si drift detector in comparison to Si(Li) detector for total reflection X-ray fluorescence analysis applications

Abstract TXRF is routinely used and suited to inspect Si wafer surfaces for possible impurities of metallic elements at the level of pg and below. Lightweight, compact sized, high-resolution Silicon drift detectors (FWHM=148 eV at 5.9 keV) electically cooled and with high throughput are ideally as the new spectrometer and for clean room application. A KETEK 5 mm 2 Si drift detector was compared with a NORAN 80 mm 2 SiLi in a previously commercially available ATOMIKA 8010 wafer analyzer. Results are presented and show that almost the same detection limits for both detector types were achieved analyzing a droplet sample containing 1 ng Ni on a Si wafer. Also, the performance to detect low Z elements like Na, excited with monochromatic Cr Kα radiation in a vacuum chamber was tested and detection limits of 600 pg obtained.