G. Queirolo

Comparison among lifetime techniques for the detection of transition metal contamination

Abstract A systematic comparison among the most common methods (surface photovoltage (SPV), Elymat, and microwave-detected photoconductive decay (μ-PCD)) for lifetime measurements is presented. Though these techniques are very different from each other, we show that, where bulk-diffused impurities are concerned, they agree very well with each other, provided they are properly used. In order to validate these techniques for the quantitative evaluation of bulk-diffused contaminants, iron and chromium implantations were carried out. An excellent correspondence was found between Elymat and μ-PCD data. In addition, the lifetime dependence on iron dose was studied and the expected behaviour was verified over two orders of magnitude. Elymat and μ-PCD measurements have also been applied to the study of contaminants segregated at wafer surface, such as nickel and copper. Both these techniques are very sensitive to surface-segregated metals, though under these conditions the correlation between Elymat and μ-PCD data is somewhat different with respect to samples with metals dissolved in the bulk. A measurement procedure is proposed in order to discriminate bulk and surface recombination.

Quantitative evaluation of bulk-diffused metal contamination by lifetime techniques

Abstract In this paper we present a systematic comparison among the most common methods (surface photovoltage, Elymat and microwave-detected photoconductive decay) for lifetime measurements. The possibility to identify contaminants and to quantitatively evaluate their concentration by lifetime techniques is investigated. Though these techniques are very different from each other, we show that in relation to bulk-diffused impurities, they agree very well with each other, provided that the dependence of carrier lifetime on the injection level is taken into account when comparing them. Iron and cromium implanted wafers are used in order to validate these techniques for the quantitative evaluation of bulk-diffused contaminants. Iron concentrations obtained from lifetime data are compared to measurements of chemical concentration of iron at wafer surface. A very good correlation is obtained between estimates of iron concentration from lifetime data and O 2 -leak SIMS data in samples contaminated by implantation of dopant ions.

Molibdenum contamination in silicon 1. Molibdenum detection by lifetime techniques

Abstract In this work the use of lifetime techniques in the presence of a non-uniform profile of recombination centers is discussed. It is shown that measurements with different probing depths can be used in order to obtain information about the in-depth distribution of recombination centers. Molibdenum was chosen for this study, because it is probably the most common slowly diffusing contaminant. In this study, it is also shown that molibdenum inactivation does take place by segregation at the wafer surface. The evidence from lifetime techniques is confirmed by TEM analyses, revealing tiny molibdenum clusters at the wafer surface.

Initial reactions in Ti–Si bilayers: New indications from in situ measurements

The interactions occurring in a Ti–Si bilayer have been investigated by in situ resistance measurements and the reaction products have been analyzed by complementary techniques, such as x-ray diffraction, Auger electron spectroscopy, and MeV 4He+ Rutherford backscattering. The measurements were performed by heating the samples at a constant rate in the 5–425 °C/min range. The samples are 60 nm of titanium sputter deposited on 250 nm of a silicon film deposited by chemical vapor technique; the bilayers were placed on 650 nm of SiO2, thermally grown on p-type 〈100〉 silicon wafers. Samples were also prepared by reactively depositing 25 nm of TiN on the Ti film. The TiN cap, deposited without breaking the vacuum, protects the metal films from contaminants, namely oxygen, during handling and heat treatments. It has been shown that, at temperatures around 400 °C and before any massive Ti–Si interaction, the resistance increases. This effect, attributed in the literature to the silicon diffusion in the metal fil...

Quantitative TOF-SIMS analysis of metal contamination on silicon wafers

Time-of-flight secondary ion mass spectrometry (TOF-SIMS) is one of the most promising candidates for high sensitivity surface analysis, in line with the requirements reported by the 1997 Semiconductor Industry Association (SIA) roadmap. However, the data quantification is not straightforward because of strong matrix effects and must be still completely developed. This work aims at obtaining TOF-SIMS calibration for the quantification of metals on oxidized silicon wafers. Standards with different amounts of various elements are prepared by dipping and spinning methods. TOF-SIMS results are compared with TXRF, VPD-AAS and other techniques. The required uniformity and reproducibility of the standards are discussed, as well as the TOF-SIMS experimental set-up and the sampling strategy for the comparison between different techniques.

Hydrogen determination in Si‐rich oxide thin films

Quantitative in‐depth distribution of the elements contained in silicon‐rich oxide thin films deposited on single‐crystal silicon by low temperature plasma‐assisted deposition has been performed by a combination of various MeV ion beam techniques. The quantity of oxygen and nitrogen has been measured by nuclear reactions, the silicon content has been determined by Rutherford backscattering, and elastic recoil detection was used for hydrogen. All the samples contain not only Si and O, but also N and H, which are residuals from the reactions involved in the deposition process. We did find that the MeV beam used in the nuclear techniques can induce a process of hydrogen desorption, which causes the measured H content to be a function of the He dose received by the sample. This phenomenon, not previously reported, must be taken into account to give the correct H content. The study of the kinetics of the He‐induced hydrogen desorption has been used to correct the experimental data and to determine the original...

Study of SiO2Si interfaces by photocurrent measurements

Abstract An experimental study is presented about surface recombination velocity obtained from photocurrent measurements by the Elymat technique. Surface recombination velocity has been studied as a function of process conditions (oxidation cycle, hydrogen annealing, cleaning procedures) with the aim to investigate how this parameter can be used for determining some properties of oxide-silicon interfaces. The data of surface recombination obtained from photocurrent measurements have the expected dependence on the oxidation cycle and on a H2/N2 post-oxidation annealing, so we concluded that these data actually reflect the status of the oxide-silicon interface. In addition, these data are shown to be affected by the cleanliness of the oxidation equipment, and they are unaffected by bulk properties of the oxide. By using these measurements, we show that N2O treated interfaces have a better stability under baking treatments than N2 annealed interfaces. The dependence of surface recombination velocity on injection level can be modeled by analogy with bulk recombination by Shockley—Read—Hall recombination statistics.

Radiation enhanced transport of hydrogen in SiO2

Abstract 2.2 MeV 4He+ and 7 MeV 15N2+ ion beams have been used to investigate, by in situ measurements, the hydrogen desorption processes in SiO2 thin films deposited by plasma enhanced chemical vapor deposition (PECVD). An effective cross-section of 3 × 10−16 cm2 for He and 17 × 10−16 cm2 for N has been measured for the ion-thin-film interaction phenomena. The structure crystalline silicon/thermally grown SiO2/PECVD film has also been investigated for the hydrogen radiation-enhanced diffusion in thermally grown SiO2. The data are consistent with a diffusion process with a diffusion coefficient of (5.0 ± 0.5) × 10−26 cm2/ion from a constant source at a concentration C0 = (2.5 ± 0.2) × 1021 at/cm3.

AFM measurement of the grain size in polycrystalline titanium silicides

Abstract In titanium silicide, the transition from the high (C49) to the low resistivity phase (C54) is difficult in small areas. This has been attributed in the literature to the lack of the nucleation sites for phase transition in small areas [1] . The mean grain size is directly related to the nucleation point density and its evaluation is important in obtaining data on nucleation mechanism of a new crystalline phase. TEM and SEM cross sections show that a silicon ridge is formed on the silicon substrate at the silicide grain boundaries, due to the equilibrium between the forces related to the grain–grain and to the grain–silicon interface energies [2] , [3] . AFM measurements have been performed on the interface between silicon and silicide after the removal of the silicide film. The grain boundaries of the silicide were imaged using the silicon ridges, allowing us to obtain unambiguous and statistically relevant data on the grain size.

Impact of plasma treatment time on MOCVD-TiN properties and on the electrical performance of deep contacts

MOCVD-TiN deposition from TDMAT precursor consists of two steps: a deposition and a plasma treatment in hydrogen. The effects of different plasma treatment times are tested on high aspect ratio contacts, finding an increase in the number of high resistance contacts for short plasma treatments. Moreover, the W nucleation layer thickness on MOCVD-TiN is impacted by the plasma time: for a short treatment a thin nucleation layer is found. Various investigations are performed on different MOCVD-TiN layers, comparing samples exposed to the air and samples in-situ capped by Ti to prevent oxygen contamination. It is shown that the plasma treatment influences the film morphology.