Total reflection X-ray fluorescence analysis with chemical microchip

A chemical microchip, which has a flat region on the surface, was recently designed for total reflection X-ray fluorescence (TXRF) analysis. A sample solution was introduced from an inlet by a microsyringe and flowed into a microchannel. Finally it overflowed from the well-type microchannel on the flat region. The sample solution on this region was dried, and then measured by TXRF. The TXRF spectra could be measured with a low background level. This preliminary result indicated that the edge of the well-type channel would not cause a serious problem for TXRF analysis. In addition, a good linear relationship was obtained for Zn Kα in Zn standard solution. This suggests that quantitative analysis by TXRF is feasible in combination with a chemical microchip.     

Applications of total reflection X-ray fluorescence spectrometry in trace element and surface analysis

Total reflection X-ray fluorescence spectrometry (TXRF) is presented as a genuine surface analytical technique. Its low information depth is shown to be the characteristic feature differentiating it from other energy dispersive X-ray fluorescence methods used for layer and surface analysis. The surface sensitivity of TXRF and its analytical capability together with the limitations of the technique are discussed here using typical applications including the contamination control of silicon wafers, thin layer analysis and trace element determination. For buried interfaces and implantation depth profiles in silicon a combination of TXRF and other techniques has been applied successfully. The TXRF method has the particular advantage of being calibrated without the need for standards. This feature is demonstrated for the example of the element arsenic.     

The use of Si carriers for aerosol particle collection and subsequent elemental analysis by total-reflection X-ray fluorescence spectrometry

The adoption of polished Si carriers was studied for the sensitive elemental analysis of aerosol particles using total-reflection X-ray fluorescence (TXRF) spectrometry. The surface roughness of the Si carrier measured by atomic force microscopy was found to be smaller than those of glassy carbon and quartz glass carriers, which are commonly used for TXRF analysis. The detection limits of elements for the Si carrier were superior to those for the glassy carbon and the quartz glass carriers, presumably due to its smaller surface roughness. For example, the detection limit of Sr for the Si carrier was 9 pg, which was 100 times and 3 times lower than those for the glassy carbon and the quartz glass carriers, respectively. The Si carriers could be successfully applied to the direct aerosol particle collection by impaction and the subsequent elemental analysis by TXRF. From the results of the elemental analysis of aerosol particles, the variations in the concentrations of K, Ca, Ti, V, Cr, Mn, Fe, Ni, Cu, Zn Sr and Pb with time could be clarified.     

Trace analysis for 300 mm wafers and processes with total reflection X-ray fluorescence

Total reflection X-ray fluorescence (TXRF) is essential for 300-mm silicon wafer production and fabrication of semiconductor devices. The 300-mm TXRF enables non-destructive contamination analysis on wafers for process development and process control. The TXRF system shows a very stable continuous operation, which allows accurate trace and ultra trace analysis on the silicon surface. It is equipped with two excitation sources covering the requirements of very sensitive measurement and wide element range. The TXRF is a key technology for contamination control during wafer reclaim. For this purpose we show that the system is able to examine the wafers during different processing states of reclaim. The system sensitivity is influenced by the surface of the wafer. For important processing steps, e.g. double side polishing, the sensitivity is as good as for measurements on hazefree polished wafers. We show with TXRF that cross-contamination with copper during double side polishing is suppressed.     

Application of ETV-ICPMS in semiconductor process control

Submicron semiconductor manufacturing requires ultra-clean processes and materials to achieve high product yields. It is demonstrated that electrothermal evaporation (ETV) in a graphite furnace coupled with ICPMS offers a new possibility for a fast simultaneous analysis of eight elements with detection limits below 0.2 ng/g in conc. hydrofluoric acid and buffered oxide etch (ammonium fluoride/hydrogen fluoride mixture). ETV-ICPMS also comprises significant improvements in the analysis of metal contamination on silicon wafer surfaces with respect to currently used methods. The contaminants on the surface are usually analyzed by total reflexion X-ray fluorescence spectrometry (TXRF) or dissolved by HF vapour (vapour phase decomposition; VPD) or a mixture of hydrofluoric acid and hydrogen peroxide (droplet surface etching, DSE) and analyzed by GFAA or TXRF. ETV-ICPMS combines the advantages of both analytical methods: the multielemental advantage of TXRF and the possibility to analyze light elements like Al, Mg, Na which may not be analyzed by TXRF. With VPD/DSE-ETV-ICPMS detection limits between 0.2 and 2×10⁹ atoms cm^?2 on a 6″ wafer have been achieved in a simultaneous analysis of eight elements. The main advantage of ETV-ICPMS versus conventional ICPMS in both applications — chemical and surface analysis — is its capability to analyze Fe in the sub-ng/g range. As Fe is one of the most important impurities in semiconductor manufacturing ETV-ICPMS is much more useful for semiconductor applications than low-resolution ICPMS. For the present application potassium iodide was used as a modifier. It enhances the sensitivity by a factor of 3–4 and improves the reproducibility significantly.     

Nanoliter deposition unit for pipetting droplets of small volumes for Total Reflection X-ray Fluorescence applications

A nanoliter droplet deposition unit was developed and characterized for application of sample preparation in TXRF. The droplets produced on quartz reflectors as well as on wafers show a good reproducibility, also the accuracy of the pipetted volume could be proved by a quantitative TXRF analysis using an external standard. The samples were found to be independent of rotation of the sample carrier. Angle scans showed droplet residue behavior, and the fluorescence signal is relatively invariant of the angle of incidence below the critical angle, which is useful for producing standards for external calibration for semiconductor surface contamination measurements by TXRF. Further it could be demonstrated that the nanoliter deposition unit is perfectly able to produce patterns of samples for applications like the quantification of aerosols collected by impactors.     

Vapor phase decomposition–droplet collection–total reflection X-ray fluorescence spectrometry for metallic contamination analysis on Ge wafers

Ge substrates are recently being reconsidered as a candidate material for the replacement of Si substrates in advanced semiconductor devices. The reintroduction of this material requires reengineering of the standard IC processing steps. In this paper, we present the extension of the methodology of vapor phase decomposition–droplet collection–total reflection X-ray fluorescence spectrometry (VPD–DC–TXRF) for metallic contamination analysis towards Ge substrates. A first step that asked for adaptation was the collection chemistry as the Ge wafers surface is not hydrophobic after the VPD treatment. The contact angle could be significantly increased using a concentrated HCl solution. This chemistry has been proved to perform well in the collection of metals from intentionally contaminated Ge wafers. A second step that needed optimization was the matrix removal method as a sample preparation step prior to the TXRF analysis. First, the upper limits of TXRF on Ge containing solutions have been characterized. The accuracy of TXRF is found to be acceptable for Ge contents lower than 1×10¹⁴ atoms (250 ppb in 50 μL) but decreases systematically with higher Ge contents. Fortunately, Ge can be volatilized at low temperatures as GeCl₄ by the addition of HCl. The parameters within this method have been investigated with respect to the removal of Ge and the recovery of metal traces. Finally, the full VPD–DC–TXRF method has been applied on intentionally contaminated Ge wafers and proved to be very accurate.     

Determination of metallic contaminants on Ge wafers using direct- and droplet sandwich etch-total reflection X-ray fluorescence spectrometry

An analysis methodology for the metallic contamination control of Ge wafer substrates has been developed and evaluated for six elements (K, Ca, Cr, Fe, Ni and Zn). Detection limits (DL) of Direct-total reflection X-ray fluorescence spectrometry (D-TXRF) analysis on Ge wafers have been determined and found to be at the E10 at/cm² level. The values have been found to be a factor between 1 and 3 higher than on Si wafers, exclusively caused by differences in the background intensity. Additionally, a preconcentration procedure based on the Droplet sandwich etch (DSE) method has been developed. This method relies on the transfer of the surface and subsurface contaminants from the wafer to the liquid phase by wet chemical etching. Application of the DSE method on reference Ge wafers followed by analysis of the etch liquid by TXRF resulted in recovery rates (RR) of 40%. In an optimization study, it was found that the main DSE method parameters had limited influence on the RR. However, a detection efficiency study clearly demonstrated an underestimation by the TXRF analysis. An independent analysis for Ca, Cr, Fe and Zn by GF-AAS resulted in RR varying at approximately 100%. By internal standardization with the element La for the TXRF analysis, recovery rates could be increased to the 60% level. This underestimation by TXRF may find an origin in a matrix effect caused by the Ge etch products. By application of the developed DSE-TXRF method, DL at the E9 at/cm² level could be realized, with values, which are at least one order of magnitude lower compared to the DL of D-TXRF on Ge wafers.     

Method and mechanism of vapor phase treatment–total reflection X-ray fluorescence for trace element analysis on silicon wafer surface

Vapor phase treatment (VPT) is a pretreatment with hydrofluoric acid vapor to raise the sensitivity of total reflection X-ray fluorescence spectroscopy (TXRF) for trace metal analysis on silicon wafers. The International Organization for Standardization/Technical Committee 201/Working Group 2 (ISO/TC201/WG2) has been investigating the method to analyze 10⁹ atoms/cm² level of metallic contamination on the silicon wafer surface. Though VPT can enhance the TXRF signal intensity from the metallic contamination, it has turned out that the magnitude of the enhancement varies with the type of methods and the process conditions. In this study, approaches to increase TXRF intensity by VPT are investigated using a fuming chamber in an automated VPD instrument. Higher signal intensity can be obtained when condensation is formed on the sample surface in a humidifying atmosphere and with a decreasing stage temperature. Surface observations with SEM and AFM show that particles with ~ 4 μm in diameter are formed and unexpectedly they are dented from the top surface level.     

Investigation of element distribution and homogeneity of TXRF samples using SR-micro-XRF to validate the use of an internal standard and improve external standard quantification

Total reflection X-ray fluorescence analysis (TXRF) offers a nondestructive qualitative and quantitative analysis of trace elements. Due to its outstanding properties TXRF is widely used in the semiconductor industry for the analysis of silicon wafer surfaces and in the chemical analysis of liquid samples. Two problems occur in quantification: the large statistical uncertainty in wafer surface analysis and the validity of using an internal standard in chemical analysis. In general TXRF is known to allow for linear calibration. For small sample amounts (low nanogram (ng) region) the thin film approximation is valid neglecting absorption effects of the exciting and the detected radiation. For higher total amounts of samples deviations from the linear relation between fluorescence intensity and sample amount can be observed. This could be caused by the sample itself because inhomogeneities and different sample shapes can lead to differences of the emitted fluorescence intensities and high statistical errors. The aim of the study was to investigate the elemental distribution inside a sample. Single and multi-element samples were investigated with Synchrotron-radiation-induced micro X-ray Fluorescence Analysis (SR-μ-XRF) and with an optical microscope. It could be proven that the microscope images are all based on the investigated elements. This allows the determination of the sample shape and potential inhomogeneities using only light microscope images. For the multi-element samples, it was furthermore shown that the elemental distribution inside the samples is homogeneous. This justifies internal standard quantification.     

Electro-deposition as a sample preparation technique for total-reflection X-ray fluorescence analysis

A one-step sample preparation by electro-deposition for total-reflection X-ray fluorescence (TXRF) analysis has been developed using a common three-electrode arrangement with a rotating disc as the working electrode. Several elements such as Cr, Mn, Fe, Co, Ni, Cu, Zn, Ag, Cd, Pb, As and U have been determined simultaneously in saline matrix. A special electrode tip has been constructed as a holder for the TXRF sample carrier, which consists of polished glassy carbon. The influence of parameters such as time, pH value, and trace element concentration on the deposition yield has been examined for 14 elements. From repeatability studies, the uncertainty in deposition yields at the 95% confidence level has been found to be less than 20% for most of these elements. Typical detection limits range from 5 to 20 ng/l under the experimental conditions applied here. By an appropriate choice of the reference element and by calculation of yield factors, reliable quantification can be achieved directly by internal standardization. First results obtained for the standard reference material CRM 505 are presented.     

Analytical approaches for Hg determination in wastewater samples by means of total reflection X-ray fluorescence spectrometry

At present, there is a considerable interest in Hg monitoring in wastewater samples due to its widespread occurrence and the high toxicity of most of its compounds. Hg determination in water samples by means of total reflection X-ray fluorescence spectrometry (TXRF) entails some difficulties due to the high vapor pressure and low boiling point of this element that produce evaporation and loss of Hg from the surface of the reflector during the drying process, commonly used for sample preparation in TXRF analysis.The main goal of the present research was to develop a fast and simple chemical strategy to avoid Hg volatilization during the analysis of wastewater samples by TXRF spectrometry. Three different analytical procedures were tested for this purpose: (i) increasing the viscosity of the wastewater sample by adding a non-ionic surfactant (Triton^® X-114), (ii) Hg immobilization on the quartz reflectors using the extractant tri-isobutylphosphine (Cyanex 471X) and (iii) formation of a stable and non-volatile Hg complex into the wastewater sample. The best analytical strategy was found to be the formation of a Hg complex with thiourea (pH = 10) before the deposition of 10 μL of sample on the reflector for following TXRF analysis. Analytical figures of merit such as linearity, limits of detection, accuracy and precision were carefully evaluated. Finally, the developed methodology was applied for the determination of Hg in different types of wastewater samples (industrial effluents, municipal effluents from conventional systems and municipal effluents from constructed wetlands).     

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

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.     

Future in-fab applications of total reflection X-ray fluorescence spectrometry for the semiconductor industry

In-fab analytical methods are of increasing interest for wafer monitoring in advanced semiconductor device manufacturing. In particular, an analytical method which allows non-destructive measurements of implant dose and surface roughness would be very beneficial. We investigated the capabilities of total reflection X-ray fluorescence spectrometry (TXRF) to determine implant dose and surface roughness. These advanced applications of TXRF can be used to monitor processes like implantation, rapid thermal annealing, and chemical mechanical polish. As implants in Si at implant energies of 2 keV, 10 keV and 50 keV were studied. Angle resolved TXRF measurements were performed with a commercial Rigaku 3750 system. The TXRF results were compared to secondary ion mass spectrometry (SIMS) measurements.     

Application of total reflection X-ray fluorescence in angle scan mode to establish the location of arsenic contamination in silicon and silicon oxide,respectively

Experiments have been carried out using total reflection X-ray fluorescence (TXRF) to determine the location of arsenic cross-contamination on or in silicon and silicon oxide, respectively, caused during argon-implantation. TXRF has been applied at varying angles of incidence — the so-called angle scan mode. By comparing the angle scan curves of implanted samples with those of a wafer, spin-coated with arsenic, at which arsenic is certainly located on top of the silicon surface, clear differences are observed. This indicates the presence of arsenic embedded in the subsurface. These observations are confirmed by Rutherford backscattering measurements, by modeling As-implantation profiles for low implantation energies as well as by step-by-step oxide etching followed by standard TXRF analysis. This fast and non-destructive application of TXRF angle scan appears a useful method for qualitative depth profiling.     

A new non-destructive method for chemical analysis of particulate matter filters: the case of manganese air pollution in Vallecamonica (Italy)

Total Reflection X-ray Fluorescence (TXRF) is a well-established technique for chemical analysis, but it is mainly employed for quality control in the electronics semiconductor industry. The capability to analyze liquid and uniformly thin solid samples makes this technique suitable for other applications, and especially in the very critical field of environmental analysis. Comparison with standard methods like inductively coupled plasma (ICP) and atomic absorption spectroscopy (AAS) shows that TXRF is a practical, accurate, and reliable technique in occupational settings. Due to the greater sensitivity necessary in trace heavy metal detection, TXRF is also suitable for environmental chemical analysis. In this paper we show that based on appropriate standards, TXRF can be considered for non-destructive routine quantitative analysis of environmental matrices such as air filters. This work has been developed in the frame of the EU-FP6 PHIME (Public Health Impact of long-term, low-level Mixed element Exposure in susceptible population strata) Integrated Project (www.phime.org). The aim of this work was to investigate Mn air pollution in the area of Vallecamonica (Italy).     

Picoliter solution deposition for total reflection X-ray fluorescence analysis of semiconductor samples

A deposition system capable of delivering picoliter quantities of solution in programmable arrays was investigated as a method for sample preparation for total reflection X-ray fluorescence (TXRF) spectroscopy. Arrays of trace metals in solution were deposited on Si wafers. The array deposits provide a capability of depositing closely spaced (100 μm or less), typically 5–20 μm diameter droplets in an area that can be matched to the analysis spot of the TXRF detector. The dried depositions were physically characterized and the effect of deposition type and matrix on the TXRF signal was investigated.     

Calibration of reference materials for total-reflection X-ray fluorescence analysis by heavy ion backscattering spectrometry

Total-reflection X-ray fluorescence (TXRF) is widely used for the control of metallic contamination caused by surface preparation processes and silicon materials. At least three companies supply a variety of TXRF systems to the silicon integrated circuit (IC) community, and local calibration of these systems is required for their day to day operation. Differences in local calibration methods have become an issue in the exchange of information between IC manufacturers' different FABs (Fabrication Facility) and also between silicon suppliers and IC FABs. The question arises whether a universal set of fluorescence yield curves can be used by these different systems to scale system sensitivity from a single element calibration for calculation of elemental concentrations. This is emphasized by the variety of experimental conditions that are reported for TXRF data (e.g. different angles of incidence for the same X-ray source, different X-ray sources, etc.). It appears that an instrumental factor is required. We believe that heavy ion backscattering spectrometry (HIBS) provides a fundamental method of calibrating TXRF reference materials, and can be used in calculating this instrumental factor. In this paper we briefly describe the HIBS system at the Sandia National Laboratories HIBS User Facility and its application to the calibration of TXRF reference materials. We will compare HIBS and TXRF mapping capabilities and discuss the issues associated with the restrictions of some older TXRF sample stages. We will also discuss Motorola's cross-calibration of several TXRF systems using different elements as references.     

Quo Vadis total reflection X-ray fluorescence?

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 10⁸ to 10¹⁴ [atoms/cm²] 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 1×10¹¹ and 1×10⁷ [atoms/cm²] 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 10⁹ [atoms/cm²]. Novel detectors such as silicon drift detectors (SDD) with an active area of 5 mm², 10 mm² or 20 mm², respectively, and multi-array detectors forming up to 70 mm² are commercially available. The first SDD with 100 mm² (!) 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.     

Optimization of the quantitative direct solid total-reflection X-ray fluorescence analysis of glass microspheres functionalized with Zr organometallic compounds

Quantitative determination of Zr in the system constituted by quartz microspheres functionalized with two kinds of organometallic compounds has been studied due to the importance of the correct quantization of the Zr from a catalytic point of view. Two parallel approximations were done, i.e. acid leaching and direct solid quantization. To validate the acid leaching TXRF measures, ICP-MS analysis were carried out. The results obtained by means of the optimization of the quantitative direct solid procedure show that, with a previous particle size distribution modification, TXRF obtain the same analytical results as ICP-MS and TXRF by acid leaching way but without previous chemical acid manipulation. This fact implies an important improvement for the analysis time, reagents costs and analysis facility and it proves again the versatility of TXRF to solve analytical problems in an easy, quick and accurate way. Additionally and for the direct solid TXRF measurements, a deeper study was done to evaluate the intrinsic analytical parameters of the Zr TXRF analysis of this material. So, the influence of the particle size distributions (modified by means of a high power ultrasound probe) with respect to uncertainty and detection limits for Zr were developed. The main analytical conclusion was the strong correlation between the average particle sizes and the TXRF analytical parameters of Zr measurements, i.e. concentration, accuracy, uncertainty and detection limits.