Float-zone Wafers Research Articles

Impact of silicon substrate, iron contamination and perimeter on saturation current and noise in n+p diodes

In this article two different generations of silicon material from the early-eighties and the mid-nineties are compared. The impact of iron contamination and perimeter on the current voltage characteristics and low-frequency noise of n + p diodes was investigated. All diodes showed an ideality factor one over at least seven decades in current. Iron contamination reduces the minority carrier lifetime and thus increases the saturation current. The higher the oxygen content in the silicon substrate, the lower the minority carrier lifetime. At a given forward current this results in a lower number of excess minority carriers outside the depletion region and a higher 1 f noise. Czochralski-grown wafers with a high oxygen content have the highest 1 f noise. Epitaxial and floating-zone wafers did not show 1 f noise above 1 Hz for currents smaller than 0.2 mA. Above 0.2 mA 1 f noise was observed because of series resistance fluctuations, with S I ∝ I 2 f . For Czochralski-grown wafers with the lowest values for minority carrier lifetimes, the noise spectral density was found to be proportional to S I ∝ I k fA k − 1 , with k ≈ 3 2 and A the diode area. This indicates the absence of perimeter and series resistance effects. A model is proposed to explain the current and area dependence of the noise in diodes. In this model a position dependent 1 f noise parameter α is assumed.

The effect of oxygen on secondary defect formation in MeV self-implanted silicon

The effects of ion fluence and oxygen concentration on secondary defect formation in MeV self-implanted silicon has been studied for Czochralski (Cz) and float zone (FZ) wafers by means of transmission electron microscopy (TEM) and optical microscopy with bevel polishing/chemical etching. We found that the density, distribution and number of extended defects is strongly dependent upon the oxygen concentration. The dislocation density was found to be up to one order of magnitude lower in FZ wafers. At high ion fluences (∼ 10 15 cm −2), secondary defects form in a well-defined band near the ion projected range, R p. At lower ion fluences, dislocations extend from the defect band to increasingly large depths. For ion fluences approaching the threshold for secondary defect formation (∼ 10 14 cm −2), defects are observed from the surface to depths of ⋍ 10 μm, i.e., five times R p.

Oxygen-impurity interactions in crystalline silicon: The cases of aluminum and erbium

Impurities such as oxygen and carbon have a strong influence on the electrical and optical behaviour of other species introduced in Si. The influence can be either beneficial or detrimental according to the considered case. Two different examples are discussed in the present paper. The electrical activity of ion implanted Al, the fastest diffusing p-type dopant, amounts to a few percent of the fluence in Czochralski wafers while is practically unity in epitaxial or float-zone wafers, having a low content of dissolved oxygen. This behaviour is accounted for in terms of AlO inactive complexes whose formation is enhanced by defects. Good activation and deep junctions can be obtained by avoiding the simultaneous presence of O and defects. Erbium ions introduced by ion implantation in Si are optically active with an emission at 1.54 μm. The luminescence, however, strongly quenches with temperature and room temperature emission cannot be achieved. The co-introduction of Er and O, with the subsequent formation of ErO complexes in Si, strongly reduces this temperature quenching and room temperature light emission can be easily achieved. These two cases are presented and possible applications discussed.

Gettering of Fe to below 1010 cm−3 in MeV self-implanted Czochralski and float zone silicon

The effects of Si ion fluence and oxygen concentration on secondary defect formation and gettering of metallic impurities in MeV self-implanted silicon have been studied for Czochralski (Cz) and float zone (FZ) silicon by means of deep level transient spectroscopy, secondary ion mass spectroscopy, transmission electron microscopy, and optical microscopy/chemical etching. We found that the density, depth distribution, and number of extended defects is strongly dependent upon both the Si ion fluence and the oxygen concentration. Effective gettering of iron to below 1010 cm−3 can be achieved in both FZ and Cz wafers at implantation doses of 1015 cm−2.

Influence of Si Substrate Crystallinity on Device Performance

AbstractWe have demonstrated the influence of surface microroughness on the electrical characteristics of MOS devices and investigated the influence of wafer's manufacturing methods, such as Czochralski(Cz), floating-zone(FZ), and epitaxial(Epi) silicon wafers, on the susceptibility to the surface microroughness when some chemical treatment was performed. As a result, it was found that Cz and FZ wafers are very susceptible to the surface microroughness and the amount of the vacancy of Epi wafer is much smaller than that of another wafers. It was also demonstrated that the electrical characteristics of very thin gate oxide films are strongly influenced by the silicon substrate quality. Epi wafer is a strong candidate for fablication of highly-reliable devices on 300mm wafers.

The mechanism of iron gettering in boron-doped silicon

High-energy B implantation was used to introduce gettering layers into float-zone Si wafers contaminated with 2×1014 Fe/cm3. Secondary ion mass spectrometry shows that about 5% of the Fe contamination is collected at the 4 μm deep peak of a 4×1014/cm2, 3.3 MeV B implant after annealing at 1000 °C for 1 h. Deep level transient spectroscopy demonstrates that increasing the gettering B dose from 4×1012 to 4×1014/cm2 reduces the Fe concentration from 3×1012 to below ∼1010/cm3 in the 1–3 μm deep region from the surface, indicating very efficient gettering. Measurements of the Fe depth profile imply that the depletion of Fe near the gettering layer occurs upon cooling down from 1000 °C. The gettering behavior can be qualitatively understood in terms of a Fermi-level-enhanced pairing reaction between Fe and B.

Impact of the starting interstitial oxygen concentration on the electrical characteristics of electron irradiated Si junction diodes

This paper studies the effect of the initial interstitial oxygen concentration on the electrical characteristics of high-energy electron irradiated Si n+p junction diodes. Emphasis is on the low frequency noise behaviour in forward operation, which is correlated with the static curren-voltage characteristics. It is shown that diodes processed in float-zone wafers give rise to a post-radiation reduction of the noise over a considerable forward bias range, while Cz devices tend to show an increase of the noise after electron irradiation. The observed noise behaviour, both before and after irradiation, cannot be explained by standard theories. Adequate modelling should take into account the generation-recombination centres in the substrate as a primary noise source.

Characterization of Nickel Contamination in Float Zone and Czochralsky Silicon Wafers by Using Electrolytic Metal Tracer or Microwave Photoconductivity Decay Measurement

The influence of nickel contamination on the free carrier recombination lifetime and on the electrolytic diodes reverse dark current of electrolytic metal tracer (ELYMAT) have been studied on both float zone (FZ) and Czochralsky Conductivity Decay (CZ) wafers. The reverse currents of the ELYMAT electrolytic cell's measured on moderately doped FZ wafers are clearly increasing with nickel concentration. The recombination lifetime in both FZ wafers, and CZ wafers after oxygen precipitation (Internal gettering) treatments is influenced by nickel contamination. In CZ wafers with oxygen precipitation nickel contamination causes a drop of the lifetime which is directly related to the oxygen precipitate density. The characteristic variation of recombination lifetime dominated by oxygen precipitates as a function of injection level changes significantly after intentional nickel contamination. All these results bring new insights for the fast identification of Ni contamination in device fabrication lines.

Static and low-frequency noise characteristics of n+p junction diodes fabricated in different silicon substrates

This paper describes the impact of the starting Si material and thermal pretreatment on the static current-voltage characteristics of n+p junction diodes, using a combination of large-area, large-perimeter and gated diodes. In addition, the low-frequency noise spectrum in forward operation is studied for different p-type substrates. It is shown that in Czochralski-grown (Cz) wafers, the starting oxygen concentration plays a crucial role: highly precipitated high-oxygen diodes show a large bulk and perimeter leakage current and a high ideality factor m. Near ideal behaviour (m approximately=1) is found for epitaxial and floating-zone (FZ) grown substrates. The surface generation/recombination properties, however, are marginally affected by the interstitial oxygen content. Simultaneously, a clear impact of the substrate type on the low-frequency noise is observed, especially for high forward currents, whereby the lowest noise figures are obtained for epitaxial and FZ wafers. It is demonstrated that in Cz wafers both perimeter (surface) and bulk noise sources are active, explaining the higher noise level.

Low Temperature Anneal of the Divacancy in P-Type Silicon

AbstractResults are reported of a Deep Level Transient Spectroscopy (DLTS) study of the conversion of the divacancy, with energy level at Ev+0.19eV, to a level at Ev+0.24eV after anneal at temperatures below its dissociation temperature (300°C). In literature both levels have been associated with the donor level of the divacancy.Diodes processed on p-type Float Zone (FZ) and Czochralski (Cz) silicon wafers with boron concentration between 0.2 and 3E15 cm−3 are irradiated with 2 MeV electrons. Before and after anneal (200°C and 250°C) DLTS spectra are recorded to get a full electrical characterisation of the induced defects.The observed conversion is proposed to be a gradual transformation of the divacancy to a divacancy-oxygen complex.

Structural and Electrical Characterization of Hydrophobic Direct-Bonded Silicon Interfaces

ABSTRACTDirect wafer bonding is a viable technique for fabricating high-voltage devices. An understanding of the microstructure and electrical behavior of the bonded interface is critical for device fabrication. In this paper, we investigated the microstructure of the silicon-to-silicon bonded interface using cross-sectional transmission electron microscopy and the corresponding electrical behavior using spreading resistance probing. Results indicate that oxide precipitates were present at the bonded interface when Czochralski silicon wafer were used in the process. Oxide precipitates were noticeably absent from the bonded interface when float zone wafers were bonded to each other. We find that oxide precipitates at the interface arise not due to the residual oxide at the surface prior to wafer bonding but due to gettering of oxygen from the Czochralski wafer. Spreading resistance measurements show occurrence of a high resistivity region at the bonding interface whether or not oxide precipitates are present.

Observation of enhanced hydrogen diffusion in solar cell silicon

We report the observation of higher diffusivity of hydrogen in some solar cell silicon compared to that in Czochralski and float zone wafers. SIMS profiles of hydrogen/deuterium, implanted at low energies and in a temperature range of 100–300 °C, are compared for a variety of different types of silicon substrates. In addition, a new technique that utilizes hydrogen decoration of dislocations was applied to directly verify long diffusion depths in some solar cell silicon. Higher diffusivity of hydrogen permits backside hydrogenation of solar cells to be carried out in less than 30 min with a significant improvement in the cell performance.

Surface copper contamination of as-received float-zone silicon wafers

As-received float-zone Si wafers from two major suppliers are shown to have surface Cu contamination at a level of ∼5×1011 atoms cm−2, detectable by both low-temperature photoluminescence and room-temperature x-ray fluorescence. The effects of selective chemical removal of Cu from, or Cu adsorption onto, the front or rear surfaces of wafers have demonstrated where the majority of the contamination resides, thereby revealing its possible origin.

The Effect of Nitrogen on the Mechanical Properties of Float Zone Silicon and on CCD Device Performance

The effect of nitrogen on the mechanical properties of float zone wafers was investigated. It has been established that resistance to wafer breakage/warpage and dislocation generation can be affected by the presence of nitrogen. The improvement of mechanical properties is reflected in the reduction in the amount of process‐induced defects, and in the yield improvement of CCD imagers made in these wafers. The relationship between the mechanical properties of silicon and the nitrogen concentration and state is complex. The highest mechanical strength was achieved for nitrogen concentrations approaching the solubility limit, but only for nitrogen in a certain state. This observation indicates that the behavior of nitrogen and oxygen in silicon are very similar. The nitrogen state and concentration must be carefully controlled to achieve beneficial effects.

Impact of silicon substrates on leakage currents

Leakage currents of n <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> -p-diodes, made on four different groups of p-type silicon substrates, are investigated at temperatures between 50 and 120°C. At these temperatures, diffusion of thermally generated minority carriers from the bulk is the dominant leakage current mechanism and determines the holding time of dynamic memories. Measurements at these temperatures show that for Czochral-sky-grown wafers (CZ) with a high interstitial oxygen concentration as is used for intrinsic gettering, the leakage current densities are about 1O× higher than for CZ wafers with a low oxygen concentration or floating-zone wafers (FZ), and are about 100× higher than for p-p <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> -epitaxial substrates. Simple analytical formulas explaining these large differences will be presented. Finally a short discussion about the optimum substrate for future high-density memories will be given.

Excess solubility of oxygen in silicon during steam oxidation

1 8O was introduced into float-zone Si wafers during oxidation in H2 18O at temperatures from 600–1240 °C, and the 18O concentration profiles were measured by secondary ion mass spectrometry. The oxygen solubility, as determined from the surface 18O concentrations, was found to be significantly higher in as-oxidized wafers than in post-oxidation annealed wafers. The temperature dependence of the as-oxidized solubility, Sox, can be described by Sox(T)=1.2×1020 cm−3 exp(−0.67 eV/kT), with a higher activation energy of 1.02 eV for the equilibrium solubility. These results are compared with other published oxygen solubility determinations.

Defect Studies in Solar Cell Silicon

IntroductionOne of the requirements for low-cost silicon solar cells is that the silicon substrates be relatively inexpensive (compared to standard Czochralski and float-zone wafers). This requirement has led to the development of a number of techniques for growing silicon ‘ribbons’, e.g. edge defined film-fed growth (EFG), silicon-on-ceramic (SOC), ribbon-to-ribbon (RTR) and dendritic web. Details of these and other growth techniques can be found in ref. Most of the growth methods produce silicon ribbons which contain relatively high densities of structural defects, such as grain boundaries, twin boundaries and dislocations. In addition, small amounts of chemical impurities are introduced into the ribbons during growth from sources such as shaping dies (EFG), substrates (SOC, RTR), heat shields, etc.

Diffusivity of oxygen in silicon during steam oxidation

The diffusivity of oxygen in silicon during steam oxidation at 700–1240 °C was measured on as-oxidized Si wafers. To provide greater analytical sensitivity 18O was introduced into the float-zone Si wafers from 1-atm H2 18O steam, and the 18O depth profiles were measured by Cs+ secondary ion mass spectrometry. Over the entire temperature range studied, the diffusivity is governed by a single activation energy, 2.44 eV, and the prefactor is 7×10−2 cm2 s−1. Our results are compared with previous literature values measured only at high temperature.

Sources of oxidation-induced stacking faults in Czochralski silicon wafers

Using optical microscopy/etch pit techniques for the delineation of defects in {100} Czochralski silicon wafers we have made a one-to-one correlation between bulk stacking faults in oxidized wafers and etch hillocks identified at the same sites before oxidation. Transmission electron microscopy of the hillock defects shows them to be clusters of precipitates ranging in size from 0.01 to 0.3 μm. A discussion of these stacking-fault nucleation sites in light of previous work on ’’swirl’’ defects in float-zone wafers and attempts at preoxidation gettering are also presented along with a model for the formation of the extrinsic stacking faults.