Katharina Baur

Application of synchrotron radiation to TXRF analysis of metal contamination on silicon wafer surfaces

Synchrotron Radiation based Total Reflection X-ray Fluorescence (TXRF) has been shown to meet the critical needs of the semiconductor industry for the analysis of transition metal impurities on silicon wafer surfaces. The current best detection limit achieved at the Stanford Synchrotron Radiation Laboratory (SSRL) for Ni is 8 x 10 7 atoms/cm 2 which is a factor of 50 better than what can be achieved using laboratory based sources. SSRL has established a TXRF facility which meets the cleanliness and stability requirements of the semiconductor industry. This has enabled both industrial and academic researchers to address industrially relevant problems. In addition research is being carried out for the analysis of light elements such as Al and Na.

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.

Aluminum impurities in silicon: Investigation of x-ray Raman scattering in total reflection x-ray fluorescence spectroscopy

Total reflection x-ray fluorescence using synchrotron radiation from the Stanford Synchrotron Radiation Laboratory has been used to study Al impurities on Si wafer surfaces. For primary excitation energies below the Si K absorption edge an inelastic resonance scattering due to resonant x-ray Raman scattering is observed. This scattering dominates the background behavior of the Al K fluorescence line, and consequently limits the achievable sensitivity for detection of Al surface contaminants. The energy and angle dependence of the resonant x-ray Raman scattering has been investigated to determine the experimental conditions for which the highest sensitivity for Al can be achieved. We find that for a precise determination of the achievable sensitivity, the specific shape of the continuous Raman background has to be taken into account. Our calculations demonstrate a minimum detection limit for Al of 6×109 atoms/cm2 for a 10 000 s count time.

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

Abstract 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.

Recent advances and perspectives in synchrotron radiation TXRF

Abstract Total reflection X-ray fluorescence (TXRF) using Synchrotron Radiation is likely to be the most powerful non-destructive technique for the analysis of trace metal impurities on silicon wafer surfaces. Of fundamental importance in TXRF is the achievable sensitivity as characterized by the minimum detection limit. This work describes the progress we achieved recently at the Stanford Synchrotron Radiation Laboratory (SSRL) in minimum detection limits for transition metals and will give an estimate of what can be achieved using a third generation synchrotron radiation source such as SPEAR3.

Detection and characterization of trace element contamination on silicon wafers

Increasing the speed and complexity of semiconductor integrated circuits requires advanced processes that put extreme constraints on the level of metal contamination allowed on the surfaces of silicon wafers. Such contamination degrades the performance of the ultrathin SiO2 gate dielectrics that form the heart of the individual transistors. Ultimately, reliability and yield are reduced to levels that must be improved before new processes can be put into production. It should be noted that much of this metal contamination occurs during the wet chemical etching and rinsing steps required for the manufacture of integrated circuits and industry is actively developing new processes that have already brought the metal contamination to levels beyond the measurement capabilities of conventional analytical techniques. The measurement of these extremely low contamination levels has required the use of synchrotron radiation total reflection x‐ray fluorescence (SR‐TXRF) where sensitivities 100 times better than conve...

X-ray Absorption Spectroscopy on Copper Trace Impurities on Silicon Wafers

Trace metal contamination during wet cleaning processes on silicon wafer surfaces is a detrimental effect that impairs device performance and yield. Determining the chemical state of deposited impurities helps in understanding how silicon surfaces interact with chemical species in cleaning solutions. However, since impurity concentrations of interest to the semiconductor industry are so low, conventional techniques such as x-ray photoelectron spectroscopy cannot be applied. Nonetheless, chemical information on trace levels of contaminants can be determined with x-ray absorption near edge spectroscopy (XANES) in a grazing incidence geometry. In this study, silicon samples were dipped in ultra pure water (UPW) and 2% hydrofluoric (HF) solutions with copper concentrations of 5 and 1000 ppb, respectively. These samples were then analyzed using XANES in fluorescence yield mode to determine the oxidation state of deposited copper contaminants. It was found that copper impurities on the silicon surface from HF solution were metal in character while copper impurities deposited from the spiked UPW solution were deposited as an oxide. These results show that XANES can provide information on the chemical state of trace impurities even at surface concentrations below a few thousandths of a monolayer.

Investigation of Trace Metals Analyses of Dry Residue on Silicon Wafer Surfaces by TXRF and ICP-MS

TXRF, ICP-MS and SR-TXRF have been used for the quantification of trace metallic contaminants such as copper (Cu) and nickel (Ni) in dry spots of a NIST solution on a silicon wafer. It is found that the element combination influences the TXRF results in a dry residue. The results of Ni in a dry spot by TXRF can be well-quantified regardless of the surface concentration if the dry spot contains Ni, Cu, Ti and Ca. However, the Ni results can be suppressed by 50% if the dry spot contains 27 elements with 1 ng each. On the other hand, the Cu results can be suppressed by 15% to >50% depending on the elemental combination and concentration of the dry spots. Noticeably however, at lower concentration, e.g.0.05 ng Cu, the “lower-than-expected” phenomenon no longer exists. A further investigation of the analysis of dry spot by SR-TXRF has shown that the Cu result at level of E9 atoms/cm can be well quantified.

Investigation of Na impurities on Si wafer surfaces using TXRF

Synchrotron Radiation from the Stanford Synchrotron Radiation Laboratory (SSRL) has been used as an excitation source for Total Reflection X-ray Fluorescence Analysis (TXRF) of Na impurities on Si wafer surfaces. A wafer intentionally contaminated by a droplet containing 1.4×1014 atoms/cm2 of sodium and a wafer uniformly contaminated with 4.4×1012 atoms/cm2 of Na were investigated. The minimum detection limit for this element has been found to be 1.1×1011 atoms/cm2 for the blanket sample and 3×1011 atoms/cm2 for the droplet sample. Theoretical considerations show that the detection limit for Na can be further improved by at least a factor of 2 by exploiting the tunability of synchrotron radiation to even lower excitation energies.