Siegfried Pahlke

Determination of ultra trace contaminants on silicon wafer surfaces using total-reflection X-ray fluorescence TXRF ‘state-of-the-art’☆

Abstract In a well balanced system of highly motivated, well trained personnel and automated equipment, pure reagents and bulk media, cleanrooms and integrated data management, total-reflection X-ray fluorescence (TXRF) can and must contribute to quality assurance and process stability, support and canalize creative engineering by continuous learning about materials and processes. TXRF has the advantage of controlled one-point calibration, a linear dynamic range of three orders of magnitude, high grade of automation in operation and data management, high up-time, and a simple control of data plausibility.

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.

Optimization of the Electrokinetic Sample Introduction in Capillary Electrophoresis for the Ultra Trace Determination of Anions on Silicon Wafer Surfaces

SummaryA capillary electrophoretic method for the ultra trace determination of anions on silicon wafer surfaces is presented. In several sets of experiments designed according to the methodology of Taguchi, electrokinetic sample introduction with transient isotachophoretic preconcentration was simultaneously optimized for peak height, peak area, peak asymmetry, efficiency, peak resolution, and reproducibility of migration time and peak area. The blank reading of the method showed no cross contamination. Thus, a detection limit of 10 nmol L−1 and a linear range from 50 to 500 nmol L−1 were obtained and verified by two independent instruments. Furthermore, the method was applied to the determination of anions on wafers from a regular production line wetting the whole wafer surface with ultrapure water. The capillary electrophoretic results agree with those obtained by ion chromatography.

Quo Vadis total reflection X-ray fluorescence?

Abstract The multielement trace analytical method ‘total reflection X-ray fluorescence’ (TXRF) has become a successfully established method in the semiconductor industry, particularly, in the ultra trace element analysis of silicon wafer surfaces. TXRF applications can fulfill general industrial requirements on daily routine of monitoring wafer cleanliness up to 300 mm diameter under cleanroom conditions. Nowadays, TXRF and hyphenated TXRF methods such as ‘vapor phase decomposition (VPD)-TXRF’, i.e. TXRF with a preceding surface and acid digestion and preconcentration procedure, are automated routine techniques (‘wafer surface preparation system’, WSPS). A linear range from 10 8 to 10 14 [atoms/cm 2 ] for some elements is regularly controlled. Instrument uptime is higher than 90%. The method is not tedious and can automatically be operated for 24 h/7 days. Elements such as S, Cl, K, Ca, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, As, Br, Sn, Sb, Ba and Pb are included in the software for standard peak search. The detection limits of recovered elements are between 1×10 11 and 1×10 7 [atoms/cm 2 ] depending upon X-ray excitation energy and the element of interest. For the determination of low Z elements, i.e. Na, Al and Mg, TXRF has also been extended but its implementation for routine analysis needs further research. At present, VPD-TXRF determination of light elements is viable in a range of 10 9 [atoms/cm 2 ]. Novel detectors such as silicon drift detectors (SDD) with an active area of 5 mm 2 , 10 mm 2 or 20 mm 2 , respectively, and multi-array detectors forming up to 70 mm 2 are commercially available. The first SDD with 100 mm 2 (!) area and integrated backside FET is working under laboratory conditions. Applications of and comparison with ICP-MS, HR-ICP-MS and SR-TXRF, an extension of TXRF capabilities with an extremely powerful energy source, are also reported.

Analysis of organic contaminants on Si wafers with TXRF- NEXAFS

Abstract Organic contamination is starting to play an important role in the production and quality control of Si wafers. For the traceability of the source of contamination, information on the chemical binding conditions is very valuable. A near edge X-ray absorption fine structure (NEXAFS) investigation is the natural development of total reflection X-ray fluorescence (TXRF) analysis of the wafer surfaces able to solve the problem of speciation. The plane grating monochromator beamline for undulator radiation of the Physikalisch-Technische Bundesanstalt at the electron storage ring BESSY II, which provides photon energies between 30 eV and 1.9 keV for the specimen excitation, is an ideal excitation source for TXRF-NEXAFS experiments that require a high resolving power and a sufficient photon flux for trace analysis of low Z elements. The contaminants have been diluted and deposited as droplets on wafer pieces thoroughly cleaned after the cutting. The K edges of C, N, O have been examined. Some discrepancies have been found in the analysis of the same compounds in two different beamtimes; molecular orientation is pointed to as the cause for the difference in magnitude of the resonances. The unintentional contamination has been identified as mainly composed of aliphatic chains.

Improvement of total reflection X-ray fluorescence analysis of low Z elements on silicon wafer surfaces at the PTB monochromator beamline for undulator radiation at the electron storage ring BESSY II☆

Abstract Several different total reflection X-ray fluorescence (TXRF) experiments were conducted at the plane grating monochromator beamline for undulator radiation of the Physikalisch-Technische Bundesanstalt (PTB) at the electron storage ring BESSY II, which provides photon energies between 0.1 and 1.9 keV for specimen excitation. The lower limits of detection of TXRF analysis were investigated for some low Z elements such as C, N, O, Al, Mg and Na in two different detection geometries for various excitation modes. Compared to ordinary XRF geometries involving large incident angles, the background contributions in TXRF are drastically reduced by the total reflection of the incident beam at the polished surface of a flat specimen carrier such as a silicon wafer. For the sake of an application-oriented TXRF approach, droplet samples on Si wafer surfaces were prepared by Wacker Siltronic and investigated in the TXRF irradiation chamber of the Atominstitut and the ultra-high vacuum TXRF irradiation chamber of the PTB. In the latter, thin C layer depositions on Si wafers were also studied.

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.

Analysis of low Z elements on Si wafer surfaces with synchrotron radiation induced total reflection X-ray fluorescence at SSRL, Beamline 3-3: comparison of droplets with spin coated wafers☆

Abstract The unique properties of synchrotron radiation, such as high incident flux combined with low divergence, its linear polarization and energy tunability, make it an ideal excitation source for total reflection X-ray fluorescence (TXRF) spectroscopy in order to non-destructively detect trace impurities of transition metals on Si wafer surfaces. When used with a detector suitable for the determination of low energy radiation this technique can be extended to the detection of low-Z elements, such as Al, Na and Mg. Experiments have been performed at SSRL Beamline 3-3, a bending magnet beamline using monochromatic radiation from a double multilayer monochromator. The wafer was mounted vertically in front of the detector, which was aligned along the linear polarization vector of the incoming synchrotron radiation. This configuration allows the detector to accept a large solid angle as well as to take advantage of the reduced scattered X-ray intensity emitted in the direction of the linear polarization vector. A comparison between droplet samples and spin coated samples was done, in order to compare the capabilities of vapor phase decomposition (VPD-TXRF) with conventional SR-straight-TXRF. Detection limits in the range of 50 fg corresponding to 2E10 atoms/cm2 have been obtained for Na. The spin coated samples, prepared from solutions containing an equal amount of Na, Mg and Al showed an unexpected result when performing a scan of the angle of incidence of the incoming X-rays suggesting a different adsorption behavior of the elements in a multielement solution on the wafer surface. The observation of this behavior is important because the spin coating technique is the standard method for the preparation of surface standards in semiconductor quality control. This effect could be characteristic of the Na, Mg, Al solution used, but the angle dependence of the fluorescence signal of a standard should always be investigated before using the standard for calibration of the apparatus and quantification.