Determination of boron in the thin surface layer of a silicon wafer by instrumental charged particle activation analysis

Instrumental charged particle activation analysis (CPAA) for determining boron in a thin surface layer of silicon was developed. The nuclear reaction and incident energy were selected in order to minimize any interference from surface or bulk impurities. Thin boron film was used as a standard sample and its boron content was determined by neutron induced prompt -ray analysis. As a result, we were able to determine¹¹B and¹⁰B at 10¹⁵ atoms/cm² with an accuracy of better than 3% by 4 MeV proton and 7 MeV -bombardment, respectively. Each boron isotope could be determined down to 10¹³ atoms/cm². Our CPAA was applied to determine boron in a boron implanted silicon wafer of a SIMS standard sample.     

Charged particle activation analysis for carbon,nitrogen and oxygen in semiconductor silicon

Convenient processes are described for the charged particle activation analysis for carbon, nitrogen, and oxygen in semiconductor silicon. Suitable activation reactions and incident particle energies were selected, and the interferences examined; the activation curves for the Si+³He→¹¹C and Si+³He→¹⁸F reactions, which may seriously interfere with³He activation analysis, were measured, and the interference caused by the fission of the matrix itself is discussed. A simple technique for the separation of¹¹C present in silicon is proposed. Reliable determination of as low as several parts per billion of the three elements has thus become possible. Semiconductor silicons of various origin were analyzed for C, N and O, and the behaviour of these elements during zone-melting is reported.     

Trace determination of iron on a silicon wafer surface by silicon direct bonding and neutron activation analysis

In order to determine iron on silicon wafer surface at a level of 10¹⁴ atoms·m^–2 the efficiency of a well-type Ge detector for⁵⁹Fe -ray emissions was measured and a low temperature silicon direct bonding technique was developed. With silicon direct bonding at a temperature of 350 to 650°C iron remains near the interface of the bound silicon. The iron contamination of the interface escaped from the interface can be ignored with this technique. The value of iron obtained was (2.7 to 5.9)·10¹⁴ atoms · m^–2 in the surface on silicon wafers.     

Charged particle activation analysis for surface oxygen under various ambient pressures

Charged particle activation analysis has proved to be useful for the determination of surface oxygen under any atmospheric conditions. The¹⁶O(³He, p)¹⁸F reaction was used for activation. The sample in a form of two plates was contacted intimately with each other and bombarded with³He particles of such an energy as to give the maximum cross section at the inside surfaces of the plates. The¹⁸F thus formed in the forward plate was then measured. Oxygen in the gap between the two plates can be shown to cause no noticeable interference. This method was applied to silicon, aluminium and lead in four different ambient pressures.     

Charged particle activation analysis coupled with X-ray counting

A survey is given on the analytical use of X-ray emitting radioisotopes produced by charged particle activation. Thirty-nine proton and deuteron reactions were considered on twentysix elements (34≤Z≤82). Thick target yields and sensitivity estimates are presented. The features and limitations of this method and the scope of non-destructive and destructive determinations are discussed. The main interest of this approach is to open an avenue for trace analysis with simplified data acquisition and reduction.     

Charged particle activation analysis of phosphorus in biological materials

Charged particle activation analysis of phosphorus in biological materials using the³¹P (α,n)^34mCl reaction has been studied. Since^34mCl is also produced by the³²S (α,pn) and the³⁵Cl (α, α′ n) reactions, the thick-target yield curves on phosphorus, sulfur and chlorine were determined in order to choose the optimum irradiation conditions. As a result, it was found that the activation analysis for phosphorus without interferences from surfur and chlorine is possible by bombarding with less than 17 MeV alphas. The applicability of this method to biological samples was then examined by irradiating several standard reference materals. It was confirmed that phosphorus can readily be determined at the detection limit of 1 μg free from interferences due to the matrix elements.     

Charged particle activation analysis applied to the detection of heavy elements

Trace analysis methods have been developed for determining thallium, lead and bismuth. Proton or deuteron activation is used followed by a radiochemical separation of the reaction products:²⁰³Pb from thallium,²⁰⁶Bi from lead, and²⁰⁷Po from bismuth. Activation curves are presented for different nuclear reactions occuring on the elements studied. Determinations have been carried out on high purity samples containing varying amounts of thallium, lead, and bismuth. Based on experimental data, the detection limits are estimated at 0.01 ppm for lead, and 0.001 ppm for thallium and bismuth, respectively.     

Mechanisms of particle removal from silicon wafer surface in wet chemical cleaning process

A quantitative mechanism of particle removal from silicon wafer surfaces by a wet chemical cleaning process is proposed. The particles are removed from the surface due to the combined effects of chemical etching and a net repulsive interaction between the particle and surface. The mechanism suggests that a critical etching depth, which has been determined theoretically, and an optimal etching rate, which can be determined from etching profile calculation, are required for particle removal. The study will help in the optimization of cleaning processes and formulation of superior cleaning solutions.     

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     

Charged particle activation analysis of light elements at sub-ppb level

We have developed charged particle activation analysis to determine light elements at a sub-ppb level. This analytical method is characterized by sample bombardment with charged particles at a few tens of mA and substoichiometric separation for¹³N,¹¹C and¹⁸F within two half-life times and with decontamination factors of more than 10⁸. Nuclear reactor interference is also estimated with this method. This analytical method is confirmed to be useful for characterizing highly purified materials from analytical results for boron, carbon, nitrogen and oxygen in Nb refined by the floating zone melting method.     

Determination of boron, carbon, nitrogen and oxygen in nickel by charged particle activation analysis

Summary The determination of boron, carbon, nitrogen and oxygen in nickel by charged particle activation using the reactions ¹⁰B(d, n)¹¹C, ¹²C(d, n)¹³N, ¹⁴N(p, )¹¹C and ¹⁶O(³He, p)¹⁸F is studied. The interference of ¹¹B(p, n)¹¹C with the ¹⁴N(p, )¹¹C reaction is taken into account. ¹¹C, ¹³N and ¹⁸F are separated by oxygen combustion followed by trapping of ¹¹CO₂ in NaOH and by steam distillation of ¹³NH₃ and of H₂Si¹⁸F₆, respectively. The results obtained were 0.129±0.035 g g^–1 for boron, 86.4±8.3 g g^–1 for carbon, 1.077±0.037 g g^–1 for nitrogen and 8.91±1.00 g g^–1 for oxygen. The results for nitrogen and oxygen agreed satisfactorily with those of other analytical methods.Grateful acknowledgement is made to Prof. J. Hoste for his interest shown in this work, to J. Pauwels (B.C.M.N., Geel) for providing the samples, to B. F. Schmitt (Bundesanstalt für Materialprüfung, Berlin) for helpful information concerning the radiochemical separation of ¹¹C and to the NFWO and the IIKW for financial support.     

Charged particle activation of medium Z elements

A survey has been made on the application of charged particle activation analysis for the detection of traces of medium Z elements (40≤Z≤58, 72, 74) using protons and deuterons of 20 MeV,³He and⁴He ions of 40 MeV. The product nuclides considered were γ-ray emitting radioisotopes with half lives ranging from 10 min to 3 days. Based on the thick target yields obtained, proton activation was found to provide an optimum compromise between sensitivity and selectivity.     

Charged particle activation analysis at RIKEN: Past,present and future

To study the effect of rhizosphere pH condition on the cadmium uptake movement, ¹⁰⁹Cd, was applied as a radioisotope tracer to a soybean plant grown in a water culture at pH 4.5 or pH 6.5. The distribution of ¹⁰⁹Cd in the soybean plant was observed radiographically with an imaging plate (IP). The amount of Cd transported from the root to the upper part of the plant at pH 4.5 was approximately two times higher than that at pH 6.5. However, the movement of Cd in the upper part of the plant was similar under both pH conditions. The distribution of Cd inside the internodes at pH 4.5 also showed similar pattern to that at pH 6.5, suggesting that once Cd reached to the vessel of the root, the movement of Cd was not dependent on rhizosphere pH conditions.     

Determination of boron in magnesium oxide by charged particle activation analysis

The determination of boron in magnesium oxide using the¹⁰B/p, /⁷Be,¹⁰B/d, n/⁷Be, and¹⁰B/d, n/¹¹C reactions is described. Lithium interferes the nuclear reactions leading to beryllium-7. Combination of a proton and deuteron irradiation, each followed by measurement of the induced beryllium-7 activity, allows a simultaneous determination of boron and lithium. The¹⁰B/d, n/¹¹C reaction is free from nuclear interferences. The boron concentration ranges from 1.5 to 850 g g^–1. The results obtained by the two methods are in good agreement.Senior Research Associate of the Belgian National Fund for Scientific Research.     

Charged particle activation analysis of oxygen in fluoride and chalcogenide glasses used for fiber amplifiers

The proton activation analysis of oxygen was studied in fluoride and chalcogenide glasses used for fiber amplifiers. First, we studied the interfering nuclear reactions from glass matrices to determine the oxygen concentration in these glasses. By using substoichiometric separation for ¹⁸F after irradiation, we found that the oxygen concentration was 12 to 204 ppm in InF₃-based fluoride glass and 0.04% to 0.7% in chalcogenide glass containing sodium. We also discuss the relation between oxygen concentration and optical properties such as the infrared absorption spectrum and fluorescence lifetime.     

Effect of boron on the surface tension of molten silicon and its temperature coefficient

The influence of boron concentration (C(B)/mass%) on the surface tension of molten silicon has been investigated with the sessile drop method under oxygen partial pressure P(O(2))=1.62x10(-25)-2.63x10(-22) MPa, and the results can be summarized as follows. The surface tension increases with C(B) in the range below 2.09 mass%, and the maximum increase rate of the surface tension is about 30 mN m(-1)(mass% C(B))(-1). The temperature coefficient of the surface tension, ( partial differential sigma/ partial differential T)C(B), was found to increase with the boron concentration in molten silicon. At the interface between molten silicon and the BN substrate, a discontinuous Si(3)N(4) layer was reckoned to form and the layer might prevent BN from dissolving into the molten silicon. Since dissolved boron from the BN substrate into the molten silicon is below 0.054 mass% and the associated increase in surface tension is below 1.5 mN m(-1), the contamination from the BN substrate on the surface tension can be ignored. The relation between the surface tension and C(B) indicates negative adsorption of boron and can be well described by combining the Gibbs adsorption isotherm with the Langmuir isotherm.     

Application of total reflection X-ray photoelectron spectroscopy to boron and phosphorus on Si wafer surface measurement

A study is made on the chemical bonding state analysis of B and P injected into a Si wafer by means of the total reflection XPS (TRXPS) method. Inelastic scattering electrons forming a strong background of photoelectron spectrum can remarkably be reduced in TRXPS. The measurement of a small amount of B and P (<10¹² atom cm⁻²) on the Si wafer by TRXPS shows that SiB₄ is formed on the Si wafer. Organic phosphorus compounds originated from the contamination in a clean room and P₂O₅ are additionally found. It is demonstrated that the TRXPS is a very effective analysis method for semiconductor surface analysis.     

Charged-particle activation analysis studies on carbon,nitrogen and oxygen mainly in semiconductor silicon

High utility value of charged-particle activation analysis is exemplified by the study on carbon, nitrogen and oxygen in semiconductor silicon. A summary of the present authors' works is shown, and the equilibrium of carbon and oxygen in a silicon melt with ambient carbon monoxide is discussed. Also, a note is given about the chemical separation of¹⁸F for the³He activation analysis of various matrices.     

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.     

Effect of rate retardation in RAFT grafting polymerization from silicon wafer surface

Polystyrene and poly(butyl acrylate) were grafted from silicon wafer surface by reversible addition‐fragmentation chain transfer (RAFT) polymerization. Three RAFT agents were immobilized onto silicon wafer through their leaving/initiating groups (R group). Grafting polymerization of butyl acrylate (BA) and styrene (St) was then carried out from the immobilized RAFT agents. The immobilization of the RAFT agents and the subsequent grafting polymerization of St and BA were evaluated by ellipsometry and X‐ray photoelectron spectroscopy. It was found that type of monomer, structure of RAFT agent, and local RAFT concentration on the surface have dramatic influences on the thickness of grafted polymer layer. The grafting polymerization with more severe rate retardation effect yielded thinner polymer films on the silicon wafer. Selection of a RAFT agent with little rate retardation was critical in the grafting polymerization to achieve thick films. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 970–978, 2008