Ultra‐thin SiO2 on Si VIII. Accuracy of method,linearity and attenuation lengths for XPS

In the study of ultra‐thin films (<10‐nm thick), there is a range of methods that can provide accurate measurements of differences in thickness. However, in a pilot study under the auspices of the Consultative Committee for Amount of Substance (CCQM), results for the archetypal system of SiO₂ on Si show that the methods have different offsets such that, at all thicknesses, positive or negative amounts in the range up to 1 nm may be observed between methods. All the methods studied give thicknesses that are greater than those measured by X‐ray photoelectron spectroscopy (XPS) by amounts between 0.2 nm and approximately 1 nm. Significant parts of these offsets, of other methods with respect to XPS, may be attributed to contaminations which increase the apparent thickness but that do not affect XPS. However, not all of these offsets can yet be explained at the 0.2 nm level. The remaining part of the offsets could be thought to have arisen either from the XPS or from the other methods. In this study, by measuring SiO₂ deposited in situ on amorphous Si by XPS it is shown that the XPS linearity is consistent with the previous estimate of ± 0.025 nm, down to fractions of a monolayer, with no significant offset and that, therefore, it is the offsets seen using other methods that need further study. Recent calculations of the film thickness dependence of the attenuation lengths (ALs) for this system, using NIST SESSA software, are not consistent with these data although earlier calculations are. This work shows that XPS, with the AL calibrated by one or more other methods that are valid for differences in thickness, can provide a traceable measurement of thickness in all laboratories. © Crown Copyright 2007. Reproduced with the permission of Her Majesty's Stationery Office. Published by John Wiley & Sons, Ltd.     

Ultrathin SiO2 on Si IV. Intensity measurement in XPS and deduced thickness linearity

The procedures for measuring the intensities and for subsequent calculation of the thickness of thermal SiO₂ layers on Si in the range 0.3–8 nm have been evaluated to determine the best measurement protocols. This work is based on earlier work where the measurements for (100) and (111) Si surfaces indicate the need to work at a reference geometry. In the spectra, the Si 2p peaks may be separated clearly into the substrate Si and the overlayer SiO₂ but it is recommended here that, for accuracies better than 1%, the interface oxides are also analysed. The analysis here is for thermal oxides. Oxides grown by other routes may require a modification of this analysis. It is shown that, in evaluating the data to determine the layer thickness, the failure to remove x‐ray satellites or the use of a straight‐line background will both lead to unacceptable errors that may exceed 5%. On the other hand, if a Shirley background is used consistently for both the peak area analysis and for evaluation of the ratio of intensities for bulk SiO₂ and Si, R_o, the results should be linear over the above range to within ±0.025 nm. This excellent result includes the non‐linearities arising from elastic scattering effects and the data reduction method. Equations are provided, together with a value of R_o for Mg and Al x‐rays, to calculate the oxide thicknesses with the above linearity. In order to determine the oxide thickness accurately, the relevant inelastic mean free paths also must be known. Theoretical evaluations are only accurate to 17.4% and so better values need to be obtained by calibration. This paper provides the infrastructure to do this. Crown Copyright © 2003 Published by John Wiley & Sons, Ltd.     

Considerations of the intermediate oxides via XPS elemental quantitative analysis for the thickness measurements of ultrathin SiO2 on Si

The thicknesses of intermediate oxides at the interface between ultrathin SiO₂ and Si substrates have been measured via XPS elemental quantitative analysis for some SiO₂/Si(100) and SiO₂/Si(111) samples with the silicon oxide thickness less than 2 nm. The measurements involve XPS determination of the Si relative atomic ratio, calculation of Si atomic densities for the intermediate oxide, etc. and then the intermediate oxide thicknesses and the number of monolayers are obtained by referencing the thickness data from two international comparisons for these samples. The results show that the thickness of the intermediate oxides is in the range 0.14–0.16 nm with an average value of 0.15 nm. The number of monolayers for the intermediate oxides at the interface is less than one monolayer with an average value of 0.60. In the present work, there are a series of approximations. By making these approximations many parameters, including L and R₀, used in the conventional calculation method are removed to give a simpler equation, which is valid when the thicknesses of SiO₂ overlayer and the intermediate oxides are very small. This, therefore, appears to be a simple and quick method to obtain approximate oxide thicknesses of modest accuracy. The present work does not in any way replace or improve on Eqns ( 2 –6) cited in the text. Copyright © 2010 John Wiley & Sons, Ltd.     

A novel approach for measuring ultra‐thin,buried SiO2 film thickness

Accurate measurement of physical thickness of thin SiO₂ films is of great interest to the semiconductor industry. Existing inspection techniques are subject to large error when the oxide thickness falls below 2 nm. This work explored a new approach with improved accuracy to the measurement of thickness of thin oxide films. The new method utilizes dynamic SIMS with a primary ion beam of one isotope of oxygen (¹⁶O or ¹⁸O) at normal incidence and detecting negative secondary ions of another isotope (¹⁸O or ¹⁶O, respectively) inside SiO₂. The experiment was performed by using an ¹⁶O₂⁺ primary beam and detecting ¹⁸O^? as characteristic secondary ions for SiO₂. We substantiated that the matrix effect was eliminated during profiling through the SiO₂/Si interface in a poly Si/SiO₂/Si stack with an O₂⁺ beam at normal incidence, which is crucial for reliable quantification of oxygen amount inside SiO₂. The high ion yield of ¹⁸O^? and negligible contribution from the mass interference of¹⁶ OH₂^? ensured measurement of the total amount of oxygen inside an SiO₂ film with good sensitivity. By assuming that the silicon oxide is in the form of stoichiometric SiO₂, which is the case for those layers grown with dry oxidation, the measured amount of oxygen can be readily converted into the thickness of SiO₂. This technique provides reproducible measurement of the thickness of SiO₂ films and potentially a good accuracy if a reference material is well calibrated. Copyright © 2009 John Wiley & Sons, Ltd.     

Critical review of the current status of thickness measurements for ultrathin SiO2 on Si Part V: Results of a CCQM pilot study

Results are reported from a pilot study under the Consultative Committee for Amount of Substance (CCQM) to compare measurements of and resolve any relevant measurement issues in the amount of thermal oxide on (100) and (111) orientation silicon wafer substrates in the thickness range 1.5–8 nm. As a result of the invitation to participate in this activity, 45 sets of measurements have been made in different laboratories using 10 analytical methods: medium—energy ion scattering spectrometry (MEIS), nuclear reaction analysis (NRA), RBS, elastic backscattering spectrometry (EBS), XPS, SIMS, ellipsometry, grazing—incidence x‐ray reflectometry (GIXRR), neutron reflectometry and transmission electron microscopy (TEM). The measurements are made on separate sets of 10 carefully prepared samples, all of which have been characterized by a combination of ellipsometry and XPS using carefully established reference conditions and reference parameters. The results have been assessed against the National Physical Laboratory (NPL) data and all show excellent linearity. The data sets correlate with the NPL data with average root‐mean‐square scatters of 0.15 nm, half being better than 0.1 nm and a few at or better than 0.05 nm. Each set of data allows a relative scaling constant and a zero thickness offset to be determined. Each method has an inherent zero thickness offset between 0 nm and 1 nm and it is these offsets, measured here for the first time, that have caused many problems in the past. There are three basic classes of offset: water and carbonaceous contamination equivalent to ～ 1 nm as seen by ellipsometry; adsorbed oxygen mainly from water at an equivalent thickness of 0.5 nm as seen by MEIS, NRA, RBS and possibly GIXRR; and no offset as seen by XPS using the Si 2p peaks. Each technique has a different uncertainty for the scaling constant and consistent results have been achieved. X‐ray photoelectron spectroscopy has large uncertainties for the scaling constant but a high precision and critically, if used correctly, has zero offset. Thus, a combination of XPS and the other methods allows the XPS scaling constant to be determined with low uncertainty, traceable via the other methods. The XPS laboratories returning results early were invited to test a new reference procedure. All showed very significant improvements. The reference attenuation lengths thus need scaling by 0.986 ± 0.009 (at an expansion factor of 2), deduced from the data for the other methods. Several other methods have small offsets and, to the extent that these can be shown to be constant or measurable, these methods will also show low uncertainty. Recommendations are provided for parameters for XPS, MEIS, RBS and NRA to improve their accuracy. Copyright © 2004 John Wiley & Sons, Ltd.     

Summary of ISO/TC 201 Standard: ISO 14701:2011 – Surface chemical analysis – X‐ray photoelectron spectroscopy—measurement of silicon oxide thickness

This International Standard specifies several methods for measuring the oxide thickness at the surfaces of (100) and (111) silicon wafers as an equivalent thickness of silicon dioxide when measured using X‐ray photoelectron spectroscopy. It is only applicable to flat, polished samples and for instruments that incorporate an Al or Mg X‐ray source, a sample stage that permits defined photoelectron emission angles and a spectrometer with an input lens that may be restricted to less than a 6° cone semiangle. For thermal oxides in the range 1‐ to 8‐nm thickness, using the best method described in this International Standard, uncertainties at a 95% confidence level around 2% may be typical and around 1% at optimum. A simpler method is also given with slightly poorer, but often adequate, uncertainties. Copyright © 2012 Crown copyright.     

Ultrathin SiO2 on Si. VII. Angular accuracy in XPS and an accurate attenuation length

Following an earlier study of the uncertainties for defining thicknesses by angle‐resolved XPS, one of the major instrumental uncertainties has been evaluated that limits both precision and accuracy. For analysis of the thicknesses of SiO₂ on Si, certain angles of emission have been recommended, but an error of 1° in these angles leads directly to an error in the thickness of ～1% from this contribution alone. This is significant since the total uncertainty required in the (International technology roadmap for semiconductors) ITRS is only 1%. In many instruments, to reduce sample‐stage vibration and for other reasons, the angle and other setting adjustments are engineered with backlash. This, combined with the manufacturer's tolerances, can lead to angular errors above 1°. We report here a device using a laser and reflectors, fixed to the sample mount, that allow the angle of emission to be set to a precision better than 0.1° and, furthermore, a method to set the zero angle of emission to 0.1°. Using this geometrical device as well as by measurements of intensities in XPS, it is deduced that the data in our earlier report for the CCQM (Consultative Committee for Amount of Substance) intercomparison were for angles 1.89° ± 0.15° too high. Consequently, by a re‐analysis of all of that data, we find that our recommended attenuation length data for the Si 2p photoelectrons in thermal SiO₂ using Mg or Al Kα X‐rays should be increased to 2.996 nm and 3.485 nm, respectively, an increase of 1.2% on the originally calculated values. These values now have standard uncertainties of 0.54% instead of the 20% of the TPP‐2M calculations. This leads to an improved accuracy in the measurement of ultrathin thermal oxides on silicon by XPS, of better than 1% for thicknesses greater than 1.5 nm but less than 8 nm. © Crown copyright 2005. Reproduced with the permission of Her Majesty's Stationery Office. Published by John Wiley & Sons, Ltd.     

Reversible CO2 Addition to a Si=O Bond and Synthesis of a Persistent SiO2–CO2 Cycloadduct Stabilized by a Lewis Donor–Acceptor Ligand

The donor‐stabilized sila‐β‐lactone 1 reacts with CO₂ via a remarkable reversible [2+2]‐cycloaddition reaction to form the spiro‐cyclic silicon carbonate derivative 2 . Furthermore, photolysis of 2 under pressure of CO₂ affords the first persistent SiO₂–CO₂ cycloadduct 3 , presenting a Si₂O₄‐like structure, which is stabilized by a Lewis donor–acceptor type ligand. As predicted by theoretical calculations, in marked contrast to the thermodynamically stable SiO₂ dimer, the SiO₂–CO₂ mixed cycloadduct 3 is labile and readily releases CO₂.     

A study on the micro‐injection molding of multi‐cavity ultra‐thin parts

In this study, we report the micro‐injection molding of ultra‐thin parts (100, 250, and 500 µm). The results show that the flow resistance increases as the cavity becomes thinner. The melt front is not symmetric when filling a four‐cavity ultra‐thin part and filling the eight‐cavity mold under a low temperature. If we increase the mold temperature or cavity thickness, the melt front becomes symmetric. Finally, we construct the operation windows of molding for three kinds of plastics (PS, PMMA, PC) and provide a molding range based on mold temperature and injections speed. Meanwhile, the relationship between the thickness and the operation windows are also investigated. The thinner the cavity is, the smaller the operation window is. We need to increase the injection speed significantly for molding the ultra‐thin parts with micro‐features on both surfaces which are 60 µm in thickness. Furthermore, we succeed in molding 30 µm ultra‐thin parts in this experiment. Copyright © 2009 John Wiley & Sons, Ltd.     

Synthesis of Hybrid Hollow Sub‐microspheres Assisted by Pre‐added Colloidal SiO2

A novel method was developed to synthesize organic–inorganic hybrid hollow sub‐microspheres (HHSs) through the addition of colloidal SiO₂. The hydrolysis rate of 3‐(methacryloyloxy)propyltrimethoxysilane (MPS) was accelerated by SiO₂ particles; meanwhile, the condensation rate of the hydrolytic species was decelerated. Thus, the hydrolytic monomers and oligomers of MPS were preserved as emulsifiers. These emulsifiers can then emulsify the isopentyl acetate (PEA) to form a steady O/W emulsion. The HHSs were produced by subsequent free radical polymerization and removal of the oil core. The hydrolytic MPS acted as emulsifiers and polymerizable monomers at the emulsification and polymerization stage, respectively. Thus, extra emulsifiers, co‐emulsifiers, and organic monomers were omitted, which simplified the synthesis process. The good dispersion of HHSs in water and oil, as well as the EDX results, indicated the organic–inorganic hybrid structure of HHSs.     

Review on the thickness measurement of ultrathin oxide films by mutual calibration method

Mutual calibration was suggested as a method to determine the absolute thickness of ultrathin oxide films. It was motivated from the large offset values in the reported thicknesses in the Consultative Committee for Amount of Substance (CCQM) pilot study P-38 for the thickness measurement of SiO₂ films on Si(100) and Si(111) substrates in 2004. Large offset values from 0.5 to 1.0 nm were reported in the thicknesses by ellipsometry, X-ray reflectometry (XRR), medium-energy ion scattering spectrometry (MEIS), Rutherford backscattering spectroscopy (RBS), nuclear reaction analysis (NRA), and transmission electron microscopy (TEM). However, the offset value for the thicknesses by X-ray photoelectron spectroscopy (XPS) was close to zero (−0.013 nm). From these results, the mutual calibration method was reported for the thickness measurement of SiO₂ films on Si(100) by combination of TEM and XPS. The mutual calibration method has been applied for the thickness measurements of hetero oxide films such as Al₂O₃ and HfO₂. Recently, the effect of surface contamination was reported to be critical to the thickness measurement of HfO₂ films by XPS. On the other hand, MEIS was proved to be a powerful zero offset method which is not affected by the surface contamination. As a result, the reference thicknesses in the CCQM pilot study P-190 for the thickness measurement of HfO₂ films on Si(100) substrate were determined by mutual calibration method from the average XRR data and MEIS analysis. Conclusively, the thicknesses of ultrathin oxide films can be traceably certified by mutual calibration method and most thickness measurement methods can be calibrated from the certified thicknesses.     

Glow discharge deposition of silicon dioxide and aluminum oxide films: A kinetic model of the surface processes

Silicon oxide films have been deposited in a RF discharge plasma using hexamethyl-disiloxane (HMDSO) and Ar/O₂ or N₂O. Also, aluminum oxide layers have been deposited using trimethylaluminum (TMA) and N₂O or CO₂. The influence of the nature and the proportion q( the oxidizing gases and file suhstrate temperature effect have been studied. As far as the films obtained with HMDSO and TMA tire concerned, the most important experimental finding is the decrease of the deposition rate with increasing substrate temperature. FTIR, ESCA, and refractive index measurements show that the decrease of the deposition rate correlates with a nearly stoichiometric film. Low negative apparent activation energies are deduced from Arrhenius plots and are representative of deposition rates controlled hr fire adsorption of radical and surface di fusivities. A kinetic rnodel shows that the rate-limiting step is the adsorption of HMDSO or TMA radicals at low temperatures while for temperatures above 250°C the oxidation reactions control the deposition rate.     

Spectroscopic reflectometry as in‐operando method for thickness determination of anodic oxide films on titanium

The present work focuses on the development of an in‐operando technique based on the visible spectroscopic reflectometry (VSR) for simultaneous determination of the oxide film formation during anodizing. The establishment of the VSR as in‐operando technique requires an extensive validation by comparative in‐situ but non‐operando thickness measurements under aqueous conditions. The investigations were carried out on anodic oxide films on pure titanium. The authors demonstrate the VSR as a simple and robust method for measurement under electrolyte covering. Additionally, an empirical correction algorithm extends the limitation of the visible reflectometry in thin film thickness. Reliable film thickness values can be measured down to ≥5 nm. The in‐operando mode yields additional information about the film growth time resolved. Copyright © 2013 John Wiley & Sons, Ltd.     

Time‐of‐flight secondary ion mass spectroscopic characterization of the nitrogen profile of shallow gate oxynitride thermally grown on a silicon substrate

Silicon oxynitride has been used as a shallow gate oxide material for microelectronics and its thickness has been reduced over the years to only a few tens of angstroms due to device size scaling. The nitride distribution and density characteristic in the gate oxide thus becomes imperative for the devices. The shallow depth profiling capability using time‐of‐flight secondary ion mass spectrometry (TOF‐SIMS) has huge potential for the nitrogen characterization of the shallow gate oxide film. In this article, both positive and negative spectra of TOF‐SIMS on silicon oxynitride have been extensively studied and it was found that the silicon nitride clusters Si_xN^? (x = 1–4) are able to represent the nitrogen profiles because their ion yields are high enough, especially for the low‐level nitride doping in the oxide, which is formed by the annealing of nitric oxide on SiO₂/Si. The gate oxide thickness measured by the TOF‐SIMS profiling method using ¹⁸O or CsO profile calibration was found to correlate very well with transmission electron microscope measurement. The nitrogen concentration in the gate oxide measured using the TOF‐SIMS method was consistent with the results obtained using the dynamic SIMS method, which is currently applied to relatively thicker oxynitride films. Copyright © 2012 John Wiley & Sons, Ltd.     

Micro/macrotribological properties of several nano‐scale ionic liquid films on modified silicon wafers

Three kinds of alkylimidazolium base room temperature ionic liquids (RTILs) were synthesized and their nano‐scale lubricant films were prepared on modified silicon wafers by dip‐coating method. The thicknesses of these films were measured and their relationship between thickness and solution concentration was obtained. Their surface morphologies were observed and contact angles of water on these films were determined. The adhesions and friction coefficients of these films were detected by contact mode AFM. As comparison, their macrotribological properties were evaluated on a UMT‐2MT tribo‐tester. It was found that, in microscale, 1‐hexyl‐3‐methyl‐imidazolium hexafluorophosphate performed the best tribological properties, while in macroscale, its tribological properties were similar with that of 1‐hexyl‐3‐methyl‐imidazolium tetrafluoroborate and better than that of 1‐hexyl‐3‐methyl‐imidazolium adipate. Copyright © 2009 John Wiley & Sons, Ltd.     

Formation of PdNiZn thin film at oil‐water interface: XPS study and application as Suzuki‐Miyaura catalyst

Nanosheet of PdNiZn and nanosphere of PdNiZn/reduced‐graphene oxide (RGO) with sub‐3 nm spheres have been successfully synthesized through a facile oil‐water interfacial strategy. The morphology and composition of the films were determined by X‐ray diffraction (XRD), X‐ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), scanning electron microscopy (SEM), energy dispersive analysis of X‐ray (EDAX) and elemental mapping. In the present study, we have developed a method to minimize the usage of precious Pd element. Due to the special structure and intermetallic synergies, the PdNiZn and PdNiZn/RGO nanoalloys exhibited enhanced catalytic activity and durability relative to Pd nanoparticles in Suzuki‐Miyaura C‐C cross‐coupling reaction. Compared to classical cross‐coupling reactions, this method has the advantages of a green solvent, short reaction times, low catalyst loading, high yields and reusability of the catalysts.     

Quantification issues of trace metals analysis on silicon oxide and nitride films by using VPD‐ICP‐MS and VPD‐GF‐AAS

Vapor phase decomposition (VPD) is a pretreatment technique for collecting trace metal contaminants on the surface of a Si wafer. Such trace metals can be identified and quantified by inductively coupled plasma mass spectrometry (ICP‐MS) or graphite furnace atomic absorption spectroscopy (GF‐AAS). However, the analytical results can be influenced by the Si‐matrix in the VPD samples. This article discusses the approaches to eliminate the interference caused by Si‐matrix. When the thickness of oxide film on wafer surface is less than 100 Å, the quantification results of ICP‐MS analysis will not be affected by Si‐matrix in the VPD samples. Except this, the Si‐matrix must be removed before analysis. An improved heating pretreatment approach has been adopted successfully to eliminate the Si‐matrix. For GF‐AAS analysis, the Si‐matrix will influence the sodium and aluminum analyses. Adding HNO₃ to the graphite furnace tubing after sample injection could also eliminate the interference caused by the Si‐matrix. The method detection limits (MDLs) of VPD‐GF‐AAS and VPD‐ICP‐MS range from 0.04 to 0.55 × 10¹⁰ atoms cm^?2 and 0.05 to 1.73 × 10⁹ atoms cm^?2, respectively. Copyright © 2008 John Wiley & Sons, Ltd.     

Nanostructural properties of zinc oxide thin films grown on non‐planar substrates

The crystallographic structure of zinc oxide thin films grown on optical fibres using single source chemical vapour deposition (SSCVD) was analysed using near edge X‐ray absorption fine structure (NEXAFS). Zinc diethyl carbamate was used as a precursor for the growth of highly conformal films in a one‐step deposition process without substrate rotation and at substrate temperatures of 400–575 °C. It was found that the growth temperatures greatly affected the crystallographic structure of the film with no preferred crystallographic orientation and negligible crystallinity at low temperatures and very high crystallinity with pure c‐axis orientation at high temperatures. Cross‐sectional analysis of the films by scanning electron microscopy (SEM) showed the presence of a film at all points around the fibre. These films generally consisted of densely packed columns that bore a strong resemblance to c‐axis‐oriented films grown on planar substrates. Copyright © 2005 John Wiley & Sons, Ltd.     

Brushless and controlled microphase separation of lamellar polystyrene‐b‐polyethylene oxide thin films for block copolymer nanolithography

Creating perpendicular alignment in lamellar block copolymer (BCP) systems has considerable industrial and commercial significance, most importantly for generating nanowire structures in electronic devices. In general, these lamellar systems require careful interface engineering to obtain vertical orientation of the blocks. To avoid the strong preferential adsorption of one block to either the substrate or the polymer/air interface, the surface must be “neutralized” by chemical brushes or external forces, for example, solvent fields. Reported here is a stepwise thermo/solvent annealing process allowing the formation of perpendicular domains of polystyrene‐b‐polyethylene oxide lamellar structures while avoiding brush or other surface modifications. This BCP has a relatively small minimum feature size and can be used to generate substrate patterns for use in fabrication of nanowire electronic device structures. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012     

用XPS法精确测量硅片上超薄氧化硅的厚度

简单介绍了硅片表面超薄(0.3～8nm)氧化硅厚度的XPS测量方法。方法根据XPS测得的元素硅和氧化硅的Si2p谱线强度,使用较简单的厚度分析公式计算。本文还介绍了厚度分析公式中两个关键参数(光电子衰减长度LSiO2和氧化硅纯硅体材料的Si2p电子强度比R0)的理论计算和实验测量方法。此外,文中还介绍了用XPS测量厚度的实验步骤,包括样品方位角和光电子发射角的选择以及XPS数据处理方法。使用上述XPS测量方法,目前对于超薄SiO2的厚度测量不确定度已可达到1%。