Metal Redeposition Research Articles

Multi-Metal Additive Manufacturing with Submicron Resolution by Electrohydrodynamic Redox 3D Printing

The ability to design metals with control over the 3D chemistry at a submicron level unlocks unique possibilities for tailoring and programming their properties. Small scale additive manufacturing of metals has recently shown promising results towards the direct synthesis of chemically heterogeneous 3D geometries. However, the fast and reliable deposition of a wide range of device-grade materials as well as the ability the change the chemistry on-the-fly still pose challenges to the field.Here, we will show how electrohydrodynamic redox (EHD-RP) 3D printing can serve as a toolbox to manufacture multi-metal structures [1]. This technique is based on the dissolution, transfer and re-deposition of metals in solvents using electrochemistry. The on-demand generation of metal ions enables the local modification of the chemical composition in a voxel-by-voxel fashion. The confined electroplating process on the substrates allows feature sizes down to 120 nm, while providing an as-deposited dense microstructure. We’ll focus first on the challenges involved in depositing a large range of metals with EHD-RP [2]. Second, we’ll show how the deposition of metals using different solvents can be combined to manufacture multi-metal structures with a high chemical resolution.[1] A. Reiser, M. Linden, P. Rohner, A. Marchand, H. Galinski, A. S. Sologubenko, J. M. Wheeler, R. Zenobi, D. Poulikakos, and R. Spolenak. Multi-metal electrohydrodynamic redox 3D printing at the submicron scale. Nat. Commun., 10(1):1-8, 2019.[2] M. Nydegger, A. Pruška, H. Galinski, R. Zenobi, A. Reiser and R. Spolenak. Additive Manufacturing of Zn with Submicron Resolution and its Conversion into Zn/ZnO core-shell structures. Nanoscale, 2022

Effects of metal redeposition in plasma sputtering

Ion sputtering, whereby atoms on target surfaces are bombarded by energetic ions, is a key process in the film-coating industry. However, its detailed mechanism has not yet been clarified and remains an empirical operation. This article demonstrates that the redeposition of metal on the target surface significantly enhances the sputtering rate. Although the effect differs depending on the metal species, significant enhancement in the sputtering rate occurs in glow-discharge plasma sputtering––a maximum increase of approximately 2.4 times was observed under experimental conditions. The experimental data agree well with the model equation, which superimposes two processes: sputtering by the desorbed redeposited metal and reduction of the enthalpy of sublimation by the adsorbed redeposited metal. The results of this study demonstrate that the metal redeposition effect is a key mechanism missing in the plasma sputtering process and that taking redeposition into account frees us from its empirical use. These findings are expected to address several of the existing questions related to sputtering, enabling the development of a wide variety of applications.

Demonstration of High Endurance and Retention Spin-Transfer-Torque-Assisted Field-Free Perpendicular Spin-Orbit Torque Cells by an Etch-Stop-on-MgO Process

Back-end-of-line compatible 400°C thermally robust perpendicular spin-orbit torque (p-SOT) cells with reduced MgO short fails are demonstrated by the etch-stop-on-MgO process. The stop-on-MgO cell features the SOT channel continuity and no metal redeposition at MgO sidewall after ion beam etching. To the best of our knowledge, the endurance as high as 1010 cycles using the field-free spin-transfer torque (STT) assisted SOT writing is achieved for the first time. The SOT switching current density can be reduced by increasing the STT current density to save write energy. The stop-on-MgO cell does not degrade the cell switching speed, since the switching always starts from the inner free layer and the domain propagation at the extended free layer does not affect junction resistance, as shown by micromagnetic simulation. The simulation also reveals that the thermal stability factor of stop-on-MgO cells is enhanced by the extended free layer, which suffers less from the interference of pinned layer edge stray field.

Structural Evolution of Au Electrodes during Cathodic Corrosion: Initial Stages of Octahedral-Nanocrystal Growth

Cathodic corrosion of Au wires in highly concentrated NaOH (5–20 M) is systemically investigated to monitor the structural transformation as a function of applied electrode potential, polarization time, and electrolyte concentration. Current–potential curves for the restructured Au electrodes in 0.1 M H2SO4 demonstrate preferential (111)-orientation of nanofacets and an increase in electrochemically active surface area as a function of the applied parameters. SEM shows that polarizing the Au surfaces at sufficiently negative potentials leads to the growth of nanosized octahedral and triangular crystallites that can be tailored by varying NaOH concentrations. The nanocrystal growth occurs through dissolution and metal redeposition. The formation of etching pits indicates that Au dissolution is dominating during the initial stages of cathodic corrosion. The number of nanocrystals increases at more negative potentials due to the greater nucleation density induced by the higher concentration of dissolved Au species. Polarization times above 5.0 min result in fractal growth indicated by the appearance of multi-armed nanostructures. This work not only advances our understanding of the initial stages of nanocrystal formation but also offers a powerful means for controlling the shape and size of metal nanocrystals simply by the rational design of the cathodic corrosion.

The effect of reactive ion etch (RIE) process conditions on ReRAM device performance

The recent surge of research on resistive random access memory (ReRAM) devices has resulted in a wealth of different materials and fabrication approaches. In this work, we describe the performance implications of utilizing a reactive ion etch (RIE) based process to fabricate HfO2 based ReRAM devices, versus a more unconventional shadow mask fabrication approach. The work is the result of an effort to increase device yield and reduce individual device size. Our results show that choice of RIE etch gas (SF6 versus CF4) is critical for defining the post-etch device profile (cross-section), and for tuning the removal of metal layers used as bottom electrodes in the ReRAM device stack. We have shown that etch conditions leading to a tapered profile for the device stack cause poor electrical performance, likely due to metal re-deposition during etching, and damage to the switching layer. These devices exhibit nonlinear I–V during the low resistive state, but this could be improved to linear behavior once a near-vertical etch profile was achieved. Device stacks with vertical etch profiles also showed an increase in forming voltage, reduced switching variability and increased endurance.

Grinding of Ti-6Al-4V Under Small Quantity Cooling Lubrication Environment Using Alumina and MWCNT Nanofluids

ABSTRACTTi-6Al-4V is a difficult-to-grind material as chips tend to adhere to the grit materials of an abrasive wheel due to its chemical affinity. In the present work, it has been attempted to improve the grindability by application of small quantity cooling lubrication (SQCL) technology using nanofluids, namely, multiwalled carbon nanotube (MWCNT) and alumina nanofluid. The suitability of nanofluids was experimentally evaluated in reciprocating surface grinding using a vitrified SiC wheel. Substantial improvement in grindability under the influence of MWCNT nanofluid (SQCL) could be achieved compared to soluble oil (flood). Reduction of specific grinding forces and specific energy was observed due to the combined effect of superior heat dissipation and lubrication abilities; when the latter one was realized through on-site rolling of MWCNT strands, inter-tubular slip and solid lubrication of the film adhered onto the wheel surface. These outperforming characteristics of MWCNT nanofluid helped in retaining grit sharpness superiorly, thus resulting in better surface finish and less re-deposition of metal on the ground workpiece. On the contrary, Al2O3 nanofluid (SQCL) underperformed even soluble oil (flood). For an ageing effect, Al2O3 nanoparticles resulted in abrasive agglomerates, which led to its failure, despite its good heat dissipation capability.

SU8 etch mask for patterning PDMS and its application to flexible fluidic microactuators.

Over the past few decades, polydimethylsiloxane (PDMS) has become the material of choice for a variety of microsystem applications, including microfluidics, imprint lithography, and soft microrobotics. For most of these applications, PDMS is processed by replication molding; however, new applications would greatly benefit from the ability to pattern PDMS films using lithography and etching. Metal hardmasks, in conjunction with reactive ion etching (RIE), have been reported as a method for patterning PDMS; however, this approach suffers from a high surface roughness because of metal redeposition and limited etch thickness due to poor etch selectivity. We found that a combination of LOR and SU8 photoresists enables the patterning of thick PDMS layers by RIE without redeposition problems. We demonstrate the ability to etch 1.5-μm pillars in PDMS with a selectivity of 3.4. Furthermore, we use this process to lithographically process flexible fluidic microactuators without any manual transfer or cutting step. The actuator achieves a bidirectional rotation of 50° at a pressure of 200 kPa. This process provides a unique opportunity to scale down these actuators as well as other PDMS-based devices.

Dielectric flow observation at inter-electrode gap in micro-electro-discharge-milling process

The material removal phenomenon of sparking and melting in micro-electro-discharge-milling process occurs at inter-electrode gap of dimension less than 50 µm. The behavior of fluid flow properties at inter-electrode gap is not well discussed in the literature and its information will be useful to understand the material flow behavior and tool wear in micro-electro-discharge-milling process. Based on our previous findings, it was well recognized that tool rotation is an inherent part of micro-electro-discharge-milling and directly influences debris flushing and redeposition. Also for a stable machining performance, flow of dielectric will play an important role in flushing away debris from the gap. The objective of this work is to investigate the fluid flow along the inter-electrode gap and to study its effect on debris movement and molten metal redeposition. The fluid flow along the narrow gap of micro-electro-discharge-milling process for different machining conditions is analyzed by computational fluid dynamics simulation. By particle simulation, the effect of different sized particles formed at various positions and their subsequent movement was also analyzed. The computational fluid dynamics analysis results were validated with scanning electron micrographs obtained for various machining conditions of the experiments. The effect of inlet nozzle velocity, tool rotation and size of electrode gap on the dielectric fluid flow was primarily reported and the findings were later superimposed on particle injection velocity to determine the movement of debris along inter-electrode gap. The effect of debris movement on redeposition at workpiece/tool surface and its ejection from inter-electrode gap is recounted.

(Invited) Metallic Contamination Control in Advanced ULSI Processing

As semiconductor devices continue to shrink, metallic contamination on silicon surfaces become to have a detrimental impact on ULSI device performance and yield. Surface metal impurities degrade gate oxide integrity [1] and cause the whisker growth in film deposition [2] and the crystal defect growth on a photomask for 193nm exposure [3], while metal impurities dissolved in silicon cause recombination centers and result in junction leakage [4] and cause dark current in image sensors [5]. In this article, we have extensively reviewed metallic contamination control including the analysis, prevention, removal, and gettering in advanced ULSI manufacturing. Trace metal contamination on the silicon surfaces can be detected with VPD- or LPD TXRF, AAS, and ICP-MS and these methods have been widely used. These analyses provide average concentration on the whole wafer surface and even direct TXRF provide average concentration on the 10 mm diameter area. However, metal impurities accidentally deposited in wafer processing such as reactive ion etching are localized [6]. Therefore, the methods to easily detect localized metal atoms are required. When III-V compound semiconductor materials are introduced for high mobility channels, the detection sensitivity on the III-V semiconductor surface by TXRF and the metal collection efficiency become critical issues [7]. Changing chemistry for the cleaning of the III-V surface will be inevitable [8]. As wafer diameter becomes larger and gate dielectric thickness become smaller, single-wafer cleaning have become popular. The single-wafer cleaning can prevent re-deposition of metals and use ozone-based liquid or dilute chemistries. In order to control metal impurities in silicon, understanding of the penetration and diffusion behaviors and efficient gettering are necessary. Surface metals penetrate silicon even through SiO2 or Si3N4 films by the collision of the surface metals and dopant atoms during ion implantation and subsequent annealing [9]. The amount of metals penetrating the silicon substrate depends on metal species and the oxide thickness. W is more difficult to penetrate the SiO2 film than Fe, Cr, Ni and Cu. The penetration of metals with a low diffusion coefficient in SiO2 such as W can be controlled by varying the SiO2 thickness. On the other hand, fast diffusers such as Fe and Cu cannot be always reduced by an increase in the SiO2 thickness. It is remarkable that Cr and W metals penetrate the silicon substrate through the Si3N4 film more easily than through the thermally-grown SiO2 [9]. This phenomenon can be explained by the ab-initio calculations of the diffusion barriers of metals in high temperature cristobalite SiO2 and β-Si3N4. These metals on Si3N4 films must be strictly controlled before ion implantation. Metals penetrating silicon generate deep levels in silicon. The depth profiles of the metal-induced deep level depend on metal species. We have demonstrated the deep level, capture cross-section, and the depth profile of thermally-diffused W in silicon [10]. In-wafer distribution of recombination centers can be detected with high sensitivity and visualized with μ-PCD and photoluminescence. The distributions of surface metal concentration measured with TXRF before ion implantation are in good agreement with that of recombination lifetime after ion implantation followed by annealing. The low level surface Fe and Ni contamination as little as 2 × 1011 atoms/cm2 can generate recombination centers even when dopants were implanted through even 20-nm-thick SiO2 [9]. Gettering is effective to mitigate the influence of metal impurities on device characteristics. Transition metals are trapped at the high-dose boron or phosphorous implanted region and the dislocation loops in low-energy ion implanted silicon [11]. The oxygen-containing dislocation loops generated by phosphorous ion implantation can be gettering sites for Fe and Cr, while high concentration of phosphorous and/or vacancies will chemically bond with Cu and Ni. Gettering design based on physical properties of metal species must be necessary. Reference [1] P. S. D. Lin, J. Electrochem. Soc., 131, 1878 (1983). [2] G. M. Choi, ECS transactions, vol. 11, no. 2, p.133. [3] K. Saga, et.al, Proc. of SPIE, 6730-44 (2007). [4] T. Kuroi, et. al, Tech. Digest of SSDM ’92 (1992). [5] W. C. McColgin, et.al, Mat. Res. Soc. Symp., Proc. Vol. 262, 1992 Material Research Society. [6] T. Tomita, et. al, ECS proceeding volume, PV 98-13. [7] H .Fontaine, T. Lardin, ECS transactions,58 (6) 327-335 (2013). [8] D. H. van Dorp, et. al, Solid State Phenomena Vol. 219 (2015) pp 56-58 [9] K. Saga, et.al, ECS Journal of Solid State Science and Technology, 4 (5) P131-P136 (2015). [10] E. Simoen, K. Saga, J. Lauwaerta, and H. Vrielincka, ECS Transactions, 64 (11) 219-228 (2014). [11] K. Saga, Solid State Phenomena Vol. 187 (2012) 283-286

Recycling Electrochemical Machining for Metal Recovery and Elimination of Waste

This paper will discuss progress in development by Faraday of an integrated technology to recover and recycle metals from electrochemical machining (ECM) operations. ECM is suited for low mass removal, high value-added manufacturing steps that cannot be easily performed using conventional machining, whether due to workpiece material properties, tooling limitations, or high surface integrity requirements. Sludge byproducts formed during conventional ECM processes are difficult to recover from the electrolyte, and discarding the sludge results in the loss of potentially valuable “waste” metal as well as entrained electrolyte salts. The FARADAYIC® Recycling ECM [(R)ECM] technology, which is based upon patented FARADAYIC®cell designs and pulsed waveforms, is proposed as a novel augmentation of ECM to allow operation at conditions where both (a) sludge formation is avoided and (b) recovery of the machined metals in compact, solid form is enabled. Conventional, direct-current ECM typically operates with neutral salt electrolytes, which affords two particular advantages: the sludge formation itself is beneficial as it limits the dissolved metal concentration, in turn effectively eliminating any metal re-deposition onto the tool/counter­electrode; and empirical observation from industrial ECM practice is that neutral electrolytes generally yield superior surface finish as compared to acidic or alkaline electrolytes [[i]]. Faraday has previously demonstrated the capability of carefully tuned, pulsed FARADAYIC® waveforms to achieve extremely high quality surface finish in acidic and alkaline electrolytes, as well as neutral salt solutions. Also, separately tuned waveforms should allow on the one hand minimal re-plating on the tool in the ECM cell, and on the other an efficient electrowinning (EW) recovery process in an integrated electrowinning cell. As will be described, the FARADAYIC®(R)ECM technology successfully exploits both of these capabilities, enabling near-complete internal recycle of electrolyte and recovery of metals in compact, solid form. Figure 1 schematizes the (R)ECM process flow: blanks are machined into finished parts in the ECM cell and the dissolved metal is recovered in the EW cell, while substantially all of the electrolyte is retained within the system. Faraday initiated (R)ECM development efforts by screening candidate electrolytes for metal solubility limits and facility of metals electrodeposition using Hull cell tests. After initial electrolyte down-selection, ECM of 1″x1″ coupons was performed using various FARADAYIC® waveforms both to test the achievable surface finish and also to generate “live” ECM-derived electrolytes with which to reconfirm their suitability for the metal recovery function. The best-performing electrolyte for each material was then used in a larger “pilot-scale” (R)ECM system, in which high-rate ECM and in situ EW unit operations were tested, first separately and then in an integrated fashion. Beryllium-free nickel-copper (C18000) and Cr-Mo alloy steel (SAE4150) were separately machined in electrolyte to concentrations between 800-3500 ppm and subsequently recovered by electrowinning in solid, metallic form devoid of hydroxides/hydrated oxides, without intermediate electrolyte processing. Metal concentrations were maintained within a target operating window defined by polishing performance exhibited in the coupons studies (1500-2000 ppm) by appropriate adjustments to the electrical parameters of one of the (R)ECM unit operations. Development of FARADAYIC® (R)ECM for other alloys including 316L stainless steel and Inconel®718 is currently underway. [[i]] Wessel, “Electrochemical machining of Gun barrel Bores and Rifling” Naval Ordnance Station, Louisville, KY, September 1978; http://handle.dtic.mil/100.2/ADA072437. Figure 1

In situ redeposition of trace metals mobilized by CO2‐charged brines

Mobilization of contaminants by CO2‐charged brines is one concern relating to injection of CO2 as part of carbon capture and storage projects. This study monitors the mobility of trace metals in an exhumed CO2‐charged aquifer near the town of Green River, Utah (USA), where CO2‐charged brines have bleached red sandstones, and concentrated trace metals at the bleaching reaction front. Mass balance calculations on the trace metal enrichments are used to calculate time‐integrated fluid fluxes and show that a significant fraction of the metals mobilized by the CO2‐rich brines are redeposited locally. A sequential extraction procedure on metal‐enriched samples shows that these metals are incorporated into secondary carbonate and oxide phases which have been shown to grow at the CO2‐promoted bleaching reaction front. We argue that while CO2‐charged brines are capable of mobilizing trace metals, local metal redeposition implies that the potential for contamination of overlying freshwater aquifers is low.

Lithography-free fabrication of single crystalline silicon tubular nanostructures on large area

Graphical abstractDisplay Omitted Highlights? We develop facile fabrication for single crystalline Si tubular nanostructures. ? Thin metal film is thermally dewetted to form nanodots for etch mask in Si RIE. ? Redeposition of metal from nanodot during RIE produces metal rim around nanodot. ? Etch rate difference between Si and nanodot/rim results in tubular nanostructure. ? Lithography-free and large-area fabrication of tubular nanostructure is proposed. A novel method to fabricate single-crystalline silicon tubular nanostructures on large area was developed. Utilizing the thermal dewetting of a thin metal film, redeposition of dewetted metal nanodots, and etch selectivity between silicon substrate and metal masks, the tubular nanostructures were formed from single crystalline silicon substrate on large area without using any nano-patterning process. This lithography-free fabrication method composed only of sputtering, rapid thermal process and reactive ion etch (RIE) is simple and cost effective batch-process. The transmission electron microscopic inspection revealed that the silicon tubular nanostructures are in the range of 1µm in length, 250nm in diameter, 75nm in wall-thickness and 380nm in hollow-depth.

Percolation effects in metal-cation-exchange membrane nanocomposites

121 Modification of ion exchangers (granules, mem branes, fibers) by an inorganic component (metal, salt, oxide) is used for imparting new properties to the initial matrix or improving the existing ones [1–3]. If a dis perse material in introduced into an ion exchanger, the forming metal–ion exchanger composites have as many as two sorts of spatially separated active sites: metal particles and ionogenic groups of the matrix [3– 5]. The bifunctionality of such materials allows one to use them as catalysts of chemical and electrochemical reactions, electrode sensors, and highly reactive mate rials. The electronic conduction of nanocomposites, which determines their electrochemical activity, is a function of the metal content [6, 7]. In this work, by chemical deposition of a metal (copper, silver) into heterogeneous sulfo cation exchange membrane MK 40, nanocomposites with bulk distribution of metal particles were obtained. Multiple metal redeposition gives rise to a percolation threshold of specific electronic conduction of the composite. The number of deposition cycles necessary for the emergence of the percolation threshold is determined by the nature of the metal. It was shown for the first time that, in the course of the formation of a percolation cluster of metal particles in the ion exchange membrane, the boundary potential of the membrane, which characterizes its ion exchange properties, undergoes a step change. The purpose of this work was to study the depen dence of the electronic conduction and the equilib rium boundary potential of nanocomposites on the content of the metal component. Nanocomposites with different metal (Cu0, Ag0) contents were synthesized by cyclic alternation of ion exchange saturation and chemical reduction of metal ions according to a published procedure [5]. The matrix was heterogeneous styrene–divinylbenzene sulfo cation exchange membrane MK 40. Figure 1 presents scanning electron microscopy images of nanocomposites Cu0 · MK 40 (panel (a)) and Ag0 · MK 40 (panel (b)) after three cycles of metal deposi PHYSICAL CHEMISTRY

Surface-emitting mid-infrared quantum cascade lasers with high-contrast photonic crystal resonators

We have developed surface-emitting single-mode quantum cascade lasers which employ high-contrast photonic-crystal resonators. The devices operate on band-edge states of the photonic band-structure. The mode profile and polarization characteristics of the band-edge modes are calculated by three-dimensional finite-difference time-domain simulation. Experimentally, the spectral properties, the far-field patterns, and the polarization characteristics of the lasers are determined and compared with simulations. The good agreement between the simulations and the experiments confirms that the hexapolar mode at the Gamma-point band-edge gives rise to lasing. By using a novel and advanced fabrication method, deep and vertical PhC holes are fabricated with no metal redeposition on the sidewalls, which improves the laser performance with respect to the current status. The angular of the output beam is approximately 15 masculine, and the side mode suppression ratio of the single mode emission is about 25 dB. The threshold current density at 78 K and the maximum operation temperature are 7.6 kA/cm2 and 220 K, respectively. The performance is mainly limited by the loss induced by surface plasmon waveguide, which can be overcome by using an optimized dielectric waveguide structure.

A Geologic Framework for Mineralization in the Western Brooks Range, Alaska

The Brooks Range is a 950-km-long north-vergent fold and thrust belt, which was formed during Mesozoic convergence of the continental Arctic Alaska terrane and the oceanic Angayucham terrane and was further shortened and uplifted in Tertiary time. The Arctic Alaska terrane consists of parautochthonous rocks and the Endicott Mountains and De Long Mountains subterranes. The Endicott Mountains allochthon of the western Brooks Range is the setting for many sulfide and barite occurrences, such as the supergiant Red Dog zinc-lead mine. Mineralization is sediment hosted and most commonly is present in black shale and carbonate turbidites of the Mississippian Kuna Formation. The reconstructed Kuna basin is a 200 by +600 km feature that represents the culmination of a remarkable chain of events that includes three fluvial-deltaic and two or more orogenic cycles, Middle Devonian to Mississippian episodes of extension and igneous activity, and the emergence of a seaward Lower Proterozoic landmass that may have constituted a barrier to marine circulation. Mississippian extension and related horst-and-graben architecture in the western Brooks Range is manifested in part by strong facies variability between coeval units of allochthons and structural plates. Shallow marine to possibly nonmarine arkose, platform to shelf carbonate, slope-to-basin shale, chert and carbonate turbidites, and submarine volcanic rocks are all represented in Mississippian time. The structural setting of Mississippian sedimentation, volcanism, and mineralization in the Kuna basin may be comparable to documented Devono-Mississippian extensional sags or half-grabens in the subsurface north of the Brooks Range. Climate, terrestrial ecosystems, multiple fluvial-deltaic aquifers, and structural architecture affected the liberation, movement, and redeposition of metals in ways that are incompletely understood.

Effect of carbon divertor plates on impurities, Zeff and density limit in large helical device

LHD (Large Helical Device) has been operated during 2 years after initiating the first plasma on March of 1998. During the first year (1st and 2nd experimental campaigns) the discharges were mainly carried out with a low magnetic field of Bt = 1.5 T. Before the 3rd experimental campaign begin in 1999, carbon divertor plates were installed in the vacuum chamber at the end of the divertor legs. These were connected to the ergodic layer for the purpose of heat removal and separation from the vacuum wall (SS316L). After the installation metal impurities decreased and high influxes of carbon and oxygen were measured. During accumulated number of discharges the metal impurities increased and returned finally to the former level. The re-deposition of metals on the carbon plates was also observed. However, discharges were successful and the impurity level did not lower critically the plasma performance. The behaviour of impurities, Zeff (effective charge) and density limit is discussed and results from spectroscopic measurements are presented.

Alkaline cleaning of silicon wafers: additives for the prevention of metal contamination

The paper presents a new strategy for cleaning of silicon wafers. A novel class of chelating agents added to alkaline cleaning mixtures provides promising performance without negative effects such as metal redeposition due to residual metal contamination of the cleaning solution. The superior capability of the new cleaning process is confirmed by the results obtained from wafer surface metal analysis as well as from minority carrier lifetime and diffusion length measurements and gate oxide integrity tests. Particle densities and surface roughness are not influenced by the presence of the chelating agents in the cleaning solution. TOF–SIMS measurements do not indicate any deposition of chelating agent on the wafer surface. With this type of modified SC-1 cleaning procedure the acid SC-2 step used in conventional RCA cleans to remove the metals deposited in the preceding SC-1 step is unnecessary resulting in substantial cost savings with respect to chemicals, waste, equipment and space.

Pseudotachylitische Brekzien aus St�rungszonen des Witwatersrand-Beckens: Hinweise auf Autometasomatose und Alteration nach der Brekzienbildung

Pseudotachylitic breccias, associated with either bedding-parallel or normal faults, are abundant in the northern and northwestern parts of the gold- and uranium-rich Witwatersrand Basin in South Africa. They are particularly abundant in a zone tangential to the Vredefort Dome, a structure which is now widely accepted to be the eroded remnant of the central uplift of the originally 250 to 300 km wide Vredefort impact structure. Several of these fault zones were sampled along vertical traverses that included both fault breccias and host rocks. Geochemical analyses were undertaken to investigate the processes involved in fault rock formation. In addition, the temporal relationships of fault activity to regional geological events was studied. Detailed petrographic analysis shows evidence for strong hydrothermal overprints of the faults related to breccia formation (thus termed ‘autometasomatism’), as well as post-formational alteration. Mixing between two or more precursor lithologies and fluid alteration affected both the fault zones and their wall-rocks. A wide spectrum of trace elements, including Au and U, has been locally mobilised and redeposited by these processes. These fault zones, some of which are of basin-wide significance, were important channels for fluids that also modified the distribution of base metals in the Witwatersrand Basin. The regionally separated locations of our study areas suggest that these processes affected at least a major part of the basin. However, as the chemical characteristics determined for the fault zones differ from site to site, it is likely that the metals were only locally remobilised in the vicinity of fault zones or to previously hydrothermally altered country rock. Subsequent redeposition of metals was limited to the regimes in and around such fault zones.

Sulphur isotopes from the gruvåsen sulphide skarn deposit, Bergslagen, Sweden

The Gruvasen sulphide skarns in Western Bergslagen are hosted by a sequence of marbles and metacherts, intruded by a number of thin mafic sills and dykes of midProterozoic age. The sulphide skarns show a zonal distribution with Cu-Zn-Fesulphide-rich pyroxene skarns (± scheelite, molybdenite, cassiterite, bismuth) in the central, and Zn-Fe-Pb-As-sulphide-rich amphibole skarns in the peripheral zones of mineralization. The sulphide skarns in the central zone are closely related to a phase of local potassic alteration that affected the marbles, cherts and mafic intrusions. σ34S values of sulphides range between + 0.5 and + 1.3% for the central zone, and between-1.4 and + 1.5% for the peripheral zone. Sulphide-pair sulphur isotope geothermometry (Ohmoto and Rye, 1979) yields temperatures around 555°C corresponding with the temperature of 550–600°C estimated from silicate and sulphide parageneses. Although the Gruvâsen deposit is situated close to the Filipstad-type granitic intrusion, this granite is unlikely to have a genetic relationship with the sulphide mineralization, because the sulphide skarns were deformed at about 1,860 Ma, whereas the postkinematic Filipstad-type granite is dated at +- 1,640Ma (Oen, 1982). Hydrothermal fluids at estimated temperatures around 550°C, presumably heated by the coeval solidifying mafic sills and dykes, are considered responsible for the dissolution and transport of sulphur from the mafic rocks, and of metals from both mafic and felsic rocks underlying the Gruvasen marbles, and for the redeposition of metals and sulphur in the Gruvasen sulphide skarn deposit.

Distribution and dispersion of lead and zinc in anomalous soils and stream sediments, Mount Hermon area, Israel

Geochemical field and laboratory tests of soils and stream sediments in an area of weak surficial lead-zinc mineralization resulted in the delineation of two anomalous zones. Lead-zinc values are exceptionally high, but downslope and downstream mobility is low. The metals occur in base-exchangeable positions (∼5%), with iron oxides (∼45–60%), in clay lattice positions (∼15–30%), as resistant minerals (∼2–5%), and in organic matter (up to 20%). Intense dissolution and redeposition of metals occurred probably close to their point of origin in residual soils.