Semiconductor applications of nanoliter droplet methodology with total reflection x-ray fluorescence analysis

In this study, the nanoliter dried spot method was applied to semiconductor contamination analysis to enhance vapor phase decomposition processes with total reflection X-ray fluorescence detection. Nanoliter-sized droplets (10 and 50 nl) were deposited onto native silicon oxide wafer surfaces in a clean room environment from both single and multielemental standards containing various concentrations of iron in different matrices. Direct comparisons were made to droplets formed by conventional VPD with similar iron standards. Nanoliter dried spots could be reproducibly deposited and dried in air with typical drying times ranging from 20 s to 2 min depending on the nanoliter volume deposited, compared to VPD spots which have drying times ranging from tens of minutes to several hours. Both types of residues showed a linear relationship between Fe intensity and mass deposited. Variable angle experiments showed that both nanoliter and VPD deposits of single element standards were film-like in character, while residues formed from much more complex matrices and higher mass loadings were particulate in character. For the experimental conditions used in this study (30 kV, 100 mA), typical TXRF spectral Fe limits of detection were calculated to be on the order of picograms or ∼1×10¹⁰ atoms/cm² for a 0.8 cm² X-ray excitation beam area for both nanoliter dried spots and VPD spots prepared from single elemental standards. Calculated Fe detection limits for 200 mm diameter silicon wafers used in this study were in the ∼1×10⁸ atoms/cm² range. By using nanoliter sized droplets, the required sample volume is greatly reduced resulting in higher sample throughput than with conventional VPD methods.     

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.     

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.     

Application of vapor phase decomposition/total reflection X-ray fluorescence in the silicon semiconductor manufacturing environment

Total reflection X-ray fluorescence (TXRF), in combination with vapor phase decomposition (VPD), provides an efficient method for analyzing trace metal contaminants on silicon wafer surfaces. The progress made in applying these techniques to the analysis of silicon wafers in a wafer fabrication cleanroom environment is reported. Methods of standardization are presented, including the preparation and characterization of VPD standards. While the VPD wafer preparation process increases the sensitivity of the TXRF measurement by at least one order of magnitude, inherent uncertainties associated with the VPD technique itself are apparent. Correlation studies between VPD/TXRF and VPD/inductively coupled plasma mass spectrometry (ICP-MS) are presented.     

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.     

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.     

Accurate calibration of TXRF using microdroplet samples

TXRF has been applied in combination with VPD to the analysis of trace impurities in the native oxide layer of Si wafer surfaces down to the range of 10⁸ atoms · cm^–2. Proper quantification of VPD/TXRF data requires calibration with microdroplet standard reference wafers. The precision of calibration function has been evaluated and found to allow quantification at a high level of 3 confidence with microdroplet standard reference.     

Potential of Total Reflection and Grazing Incidence XRF for Contamination and Process Control in Semiconductor Fabrication

 The actual detection limits of total reflection X-ray fluorescence (TXRF) are determined and compared to those of destructive physical analytical methods like secondary ion mass spectrometry (SIMS) and chemical methods like vapour phase decomposition in combination with inductively coupled plasma-mass spectrometry (VPD-ICP-MS). The elements Ca, Ti, Cr, Fe, Cu were analyzed on a Si wafer with 10 nm thermal oxide using TXRF and VPD-ICP-MS. The deviation of the TXRF and the VPD-ICP-MS results is less than 30%. The thickness, composition and density of a Co/Ti two-layer stack were determined using angle dependent total reflection and grazing incidence X-ray fluorescence (A-TXRF). The obtained data were compared with X-ray reflectometry (XRR) and energy filtered transmission electron microscopy (EFTEM). The agreement between TEM and A-TXRF is excellent for the determination of the thickness of the metal layers. From these results we conclude, that A-TXRF permits the accurate determination of composition, thickness and density of thin metallic layers. The results are discussed regarding detection efficiency, acquisition time, accuracy and reproducibility.     

Trends in total reflection X-ray fluorescence spectrometry for metallic contamination control in semiconductor nanotechnology

The development of transistor manufacturing into the nanoscale regime is accompanied by a continuous awareness concern for contamination control. The ever-increasing demands for analysis sensitivity (in the sub-10⁹ at/cm² regime) combined with the introduction of new materials (i.e. non-silicon based) put severe challenges on the application of analytical techniques for atomic level contamination monitoring. Since many years, total reflection X-ray fluorescence spectrometry (TXRF) has developed as a preferred technique, ideally suitable due to the excellent reflectivity and flatness of the starting Si substrates. Driven by performance enhancement requirements, many new materials are being introduced at the substrate level (Ge, III/V compounds for higher mobility), gate stack (alternative dielectric materials and gate electrodes for capacitance scaling) and interconnect level (low-k and copper for faster switching). This paper reviews some recent developments in the state-of-the-art TXRF developments for semiconductor applications. Among the focus areas are the expansion of the elemental range (through multi-excitation line selection or multi-excitation source to excite low Z as well as high Z elements in one analysis sweep) and dynamic range (by pre-concentration techniques, synchrotron radiation analysis and detector developments). Further, emphasis is also focused towards quantification issues—whereby the three methodologies (micro-droplet, film and bulk type standards) are critically reviewed. Also, a recent development of sweeping TXRF, suitable for fast screening of large surface areas is being discussed. The applications of TXRF in a semiconductor environment are being reviewed. Finally, the performance of TXRF for the various semiconductor applications is assessed with respected to competitive techniques.     

Trace metal analysis on hafnium silicate deposited Si wafer by Total Reflection X-ray Fluorescence

Hafnium silicate is a so-called high-k material, which is a new key material in the semiconductor field. This material is difficult to analyze by a conventional W-Lβ1TXRF source due to the high background originating from Hf-Lα lines. In this paper, the capability of Ir source TXRF analysis on hafnium silicate films is investigated with intentional contamination of Ti, Cr, Fe, Ni and Cu elements. The spectral fitting is discussed where X-ray resonant Raman scattering and escape peak of Ir-Lα overlap with Ni-Kα peak. The detection limits are estimated to 0.9 × 10¹⁰ to 2 × 10¹⁰ atoms/cm² for the transition metals.     

Novel methods of TXRF analysis for silicon wafer surface inspection

TXRF became a standard, on-line inspection tool for controlling the cleanliness of polished Si wafers for semiconductor use. Wafer makers strive for an all-over metallic cleanliness of < 10¹⁰ atoms · cm^–2. The all-over cleanliness can be analyzed using VPD/TXRF. For VPD preparation and scanning we have developed an automatic system coupled with TXRF. With synchrotron radiation TXRF we were able to detect 13 fg of Ni in a residual microdroplet, i.e.10⁵ atoms · cm^–2. Received: 8 January 1998 / Revised: 13 July 1998 / Accepted: 30 July 1998     

Accurate calibration of TXRF using microdroplet samples

TXRF has been applied in combination with VPD to the analysis of trace impurities in the native oxide layer of Si wafer surfaces down to the range of 10(8) atoms. cm(-2). Proper quantification of VPD/TXRF data requires calibration with microdroplet standard reference wafers. The precision of calibration function has been evaluated and found to allow quantification at a high level of 3 sigma confidence with microdroplet standard reference.     

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.     

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.     

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.     

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.     

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.     

Feasibility study of SR‐TXRF‐XANES analysis for iron contaminations on a silicon wafer surface

The ability to chemically characterize the contamination on silicon wafers is of critical importance to the semiconductor industry. It provides information on possible unwanted chemical processes taking place on the wafer surface and helps in determining the true source of the contamination problem. This type of information is not readily accessible with standard laboratory equipment. Synchrotron radiation‐induced total reflection X‐ray fluorescence (SR‐TXRF) was combined with X‐ray absorption near‐edge structure (XANES) to determine the chemical state of Fe contaminations on a silicon wafer surface. Main purpose of the study was to test the method for a contamination issue as it could appear in a microelectronic VLSI production fab. Copyright © 2008 John Wiley & Sons, Ltd.     

A new total reflection X-ray fluorescence vacuum chamber with sample changer analysis using a silicon drift detector for chemical analysis

There are several TXRF spectrometers commercially available for chemical analysis as well as for wafer surface analysis, but there is up to now no spectrometer for chemical analysis available that allows to measure samples under vacuum conditions. Simply a rough vacuum of 10⁻² mbar for the sample environment reduces the background due to scattering from air, thus to improve the detection limits. The absorption of low energy fluorescence radiation from low Z elements is reduced and therefore extends the elemental range to be measured down to Na. Finally evacuation of the chamber removes the Ar K-lines from the spectrum.The new vacuum chamber for TXRF named WOBISTRAX is equipped with a 12-position sample changer, a 10-mm² silicon drift detector (SDD) with an 8-μm Be entrance window and electrical cooling by Peltier effect, so no LN₂ is required. The chamber was designed to be attached to a diffraction tube housing. WOBISTRAX can be operated with a 3 kW long fine focus Mo-X-ray tube and uses a Mo/Si multilayer for monochromatization. The modified software is performing the motion control between sample changer and MCA features.The performance is expressed in terms of detection limits which are 700 fg Rb for Mo Kα excitation with 50 kV, 40 mA excitation conditions, 1000 s livetime. Using a Cr-X-ray tube for excitation of Al the achieved detection limits are 52 pg. So it could be shown that with the same measuring chamber and using an SDD with 8 μm Be window and a Cr-tube for excitation, low Z elements can be also measured with good detection limits.     

Laboratory and synchrotron radiation total-reflection X-ray fluorescence: new perspectives in detection limits and data analysis

Having established detection limits for transition elements exceeding current requirements of the semiconductor industry, our recent efforts at the Stanford Synchrotron Radiation Laboratory (SSRL) have focused on the improvement of the detection sensitivity for light elements such as Al. Data analysis is particularly challenging for Al, due to the presence of the neighboring Si signal from the substrate. Detection limits can be significantly improved by tuning the excitation energy below the Si–K absorption edge. For conventional TXRF systems this can be done by using a W–Mα fluorescence line (1.78 keV) for excitation. At a synchrotron radiation facility energy tunability is available. However, in both cases this results in a substantial increase in background due to resonant X-ray Raman scattering. This scattering dominates the background under the Al Kα fluorescence line, and consequently limits the achievable sensitivity for the detection of Al surface contaminants. In particular, we find that for a precise determination of the achievable sensitivity, the specific shape of the continuous Raman background must be taken into account in the data analysis. The data deconvolution presented here opens a new perspective for conventional TXRF systems to mitigate this background limitation. This results in a minimum detection limit of 2.4×10⁹ atoms/cm² for Al. Based on these results it will also be demonstrated that by improving the detector resolution, the minimum detection limit can be improved significantly. For a detector resolution of 15 eV as predicted for novel superconducting tunnel junction detectors, an improvement in minimum detection limit of approximately a factor of 3 can be estimated.