Gettering Capacity Research Articles

Properties of boron layers deposited during boronisations in W7-X

Boronisation was first used for wall conditioning in W7-X during the OP 1.2b operational period, which was characterized by the use of the fine-grain graphite Test Divertor Unit (TDU) and inertial cooling only. After this period, deposited layers were observed on all inner surfaces. Deposited layers were analyzed on 21 inner wall tiles using ion beam analysis methods, the deposited layers consisted mostly of boron with additional carbon and oxygen. During the operational period OP2.1 with an actively water cooled divertor made of carbon fiber reinforced carbon, different materials were exposed during two individual boronisations using the multi-purpose manipulator. Deposited boronisation layers on the samples were analyzed using nuclear reaction analysis. The deposited layer thicknesses showed some variation depending on substrate material and surface roughness, but a systematic dependence on material and/or roughness was not observed. Under the typical boronisation conditions at W7-X, one A × h (Ampere times hour) of boronisation results in a boronisation layer with a thickness of about 30 ± 15 × 1015B-atoms/cm2 (about 3 ± 1.5 nm) at the position of the multi-purpose manipulator. The oxygen gettering capacity of the layers is up to 0.5 – 0.9O/B.

Evaluating H2 Infiltration via Drone-Based Thermal Imaging

This work discusses the analysis of thermal survey data from operating parabolic trough plants. A thermal survey consists of IR images of individual HCEs in a parabolic trough collector, these images are the basis of a non-intrusive methodology for evaluating the heat losses. The HCE performance is affected by issues such as H2 infiltration and lost vacuum, which are difficult to identify visually but significantly increase the heat losses. In this work the glass temperatures from survey data are compared to predictions from a reduced order model of the HCE with good agreement. The model is then used for parametric studies looking at the variation of important ambient conditions, glass envelope conditions, and optical properties. Results indicate the model is a useful and computationally efficient tool to determine the status of a given HCE; however, it can be difficult to distinguish between lost vacuum (from outside air infiltration), and certain levels of H2 infiltration (from decomposition of the HTF). The main methodology for identifying H2 infiltration in these cases involves thermal surveying at different times of the day, taking advantage of the temperature dependence of the getter capacity.

Impact of yttrium hydride formation on multi-isotopic hydrogen retention by a getter trap for the DONES lithium loop

Compliance with imposed hydrogen concentration limits in the lithium loop of the DEMO-Oriented Neutron Source (DONES) requires the installation of an yttrium-based hydrogen trap. To determine an appropriate H-trap design, it is essential to have access to a numerical tool capable of simulating hydrogen transport in the DONES lithium loop connected to an yttrium pebble-bed. In the past, a simplified model was created that allows such calculations when hydrogen concentrations in the lithium are low. However, in certain DONES operating phases, the concentration in the lithium is high and in a range where yttrium dihydride (YH2) formation is likely. Due to the anticipated great impact of YH2 formation on the H-trap performance a new model is developed that includes the mechanism of hydride formation. It is based on a mathematical reproduction of complete pressure-composition isotherms of the Li–H and Y–H systems. Thus, the conditions that trigger YH2 formation are determined and the variation of hydrogen solubility in different yttrium hydride phases is deduced. An approximate concentration-dependent relationship of hydrogen diffusivity in yttrium is derived and incorporated into the model. Simulations are performed to analyze the dynamics of the concentration decrease during purification of the lithium circuit prior to the experimental DONES phase by varying design parameters of the trap. It is found that hydride formation greatly increases the hydrogen gettering capacity of the H-trap and limits the maximum concentration in the lithium. Indeed, YH2 formation may be purposefully triggered to exploit its beneficial properties for DONES. Simulations of the hydrogen purification process during the experimental phase of DONES show that the H-trap must be replaced at least every 28 days to meet tritium limits. This work sets the conditions for the required pebble-bed mass of the H-trap at a given temperature to comply with the DONES safety requirements. Finally, the model is validated by numerical reproduction of experimental results.

Spotless hybrid thin-film encapsulation stack for organic light-emitting diodes on organic foils

In order to protect a flexible OLED on plastic foil against water from the ambient atmosphere a top and bottom thin-film encapsulation (TFE) is necessary. A TFE stack SiN-OCP-SiN with getter in OCP results in excellent shelf lifetime. However, the getter particles result in light scattering that is lost upon water ingress in the stack. As a result spots of reduced electroluminescence are observed during exposure to the ambient atmosphere that limit the practical lifetime of the device drastically. For the bottom emitting OLED we reduced the getter content in the bottom barrier to a level that does not exhibit visible light scattering. The reduced getter capacity with respect to the top barrier does not result in a lower performance of the bottom barrier due to the lower pinhole density in SiN-ITO with respect to Al-SiN. The projected shelf lifetime at ambient conditions is 10 years. © 2018 Elsevier B.V.

Bronze Alloy Development for Zinc Vapor Capture

After gamma-emitting 65Zinc was detected in a vacuum pumping system contained in a tritium glovebox, a series of experiments were undertaken to develop a method and material to trap zinc vapors in an area that is more suitable for preventing dose to workers. In this study, bronze alloys with 0–30% tin were prepared using a powder metallurgical process and exposed to three levels of zinc vapors. All of the alloys demonstrated acceptable zinc gettering capacity; however, low tin content bronzes are considered for further testing.

Development of 1K-class Joule–Thomson cryocooler for next-generation astronomical mission

This paper reports on the development of a 1K-class Joule–Thomson (JT) cryocooler in Japan for application to upcoming next-generation astronomy missions. In this development, engineering models (EMs) were designed and manufactured for verification tests. The survival of the models in the mechanical and thermal vacuum environment tests of the JT compressors was proven to be possible with stable compression performance. In addition, the electromagnetic noise and disturbance force associated with the JT compressors were evaluated. Gas analysis showed that the estimated total amount of CO2 gas contaminant was less than the getter capacity for the required lifetime. A nominal cooling power of 10mW at 1.7K was verified using the EM test units.

Germanium doping for improved silicon substrates and devices

During the last decade, the 300 mm silicon wafer has been optimized and one is studying the move to 450 mm crystals and wafers. The ever increasing silicon crystal diameter leads to two important trends with respect to substrate characteristics: the interstitial oxygen concentration decreases while the size of grown in voids and crystal originated particles (COPs) in vacancy-rich crystals is increasing. The first effect is due to the large melt in which movements have to be controlled and partly suppressed by the use of magnetic fields. This magnetic confinement leads to a more uniform dopant incorporation but at the same time to a more limited transport of oxygen from the quartz crucible to the melt and the growing crystal. The reduced interstitial oxygen concentration and the lower thermal budget of modern device processing leads to strongly reduced oxygen precipitation and thus internal gettering capacity. The increasing COP size (accompanied by a decreasing density) is caused by the decreasing pulling rate and thermal gradient that have to be used in order to avoid dislocation formation. The slower cooling of the crystal leads to a decreased void nucleation rate and at the same time to an increased thermal budget for void growth as well as a larger number of vacancies available per void. In the present paper the effect of germanium doping in the range between 10 16 and 10 19 cm −3 on COP formation and oxygen precipitation is discussed and illustrated. Also the beneficial effect of germanium doping with respect to wafer breakage during processing, with respect to the suppression of thermal donor formation and with respect to improving device radiation hardness is addressed.

Gettering by Ba films in color cathode ray tubes

Ba getters are applied to maintain high vacuum during the life of a cathode ray tube (CRT). In a color CRT the shadow mask is electron bombarded and electrons stimulated desorption of gas from the shadow mask (and other tube components) is the result. When the Ba film is covering the shadow mask, this desorption is largely prevented because of the effective gettering of the Ba film. We found that the consumption of Ba in the getter film during life in this case cannot be attributed only to the reaction between Ba and reactive residual gases, but also, to some extent, to a reaction between Ba and the oxide layer that covers the shadow mask. This conclusion could be drawn after analyzing the getter capacity of Ba films in 26 in. CPTs with H2 as a test gas.

XPS study of the process of oxygen gettering by thin films of PACVD boron

Numerous collector samples have been exposed in the TdeV tokamak, either to plasma assisted chemical vapor boronization only, or to boronization plus tokamak power discharges. They have been analyzed by X-ray photoelectron spectroscopy (XPS) in order to characterize and better understand the process by which the boron-rich film getter ambient oxygen. It was found that the state of oxidation of the samples after boronization is the most reliable predictor of subsequent machine performance. The gettering capacity is high, on the order of 10 21 O/m 2, and affects a surprisingly thick layer (∼100 nm), but the oxide always remains substoichiometric (with a formula BO x , x < 1). The oxidation is clearly activated by the plasma, during both glow discharge deposition and power discharges, but in this respect the latter are much more effective than the former.

Phosphorus and Aluminum Gettering of Gold in Silicon: Simulation and Optimization Considerations

AbstractUsing the diffusion-segregation equation, modeling and simulations of gettering Au (a substi-tutional-interstitial species in Si) away from the Si bulk have been performed. Three external gettering schemes have been considered: wafer frontside P indiffusion gettering, wafer backside Al deposition gettering, and a combination of the two processes. Under the same processing conditions, it has been shown that P indiffusion gettering is faster than Al gettering, but P gettering has a lower gettering capacity and is less stable than Al gettering for longer gettering times. The combined P and Al gettering process is as fast as P gettering in reaching an optimum gettered state, and possesses the capacity and stability of the Al gettering process.

Modeling of time-dependent process changes and hysteresis in Ti-O2 reactive sputtering

Understanding the mechanisms of reactive sputtering is thought to be crucial to obtain a better process control and widen applications of reactive sputtering. A model simulating reactive gas mass balance changes in reactive sputtering is described. The model is based on physical mechanisms involved in target and wall gettering behavior. This enables calculation of time-dependent condition changes that occur until the process reaches a steady state, without assuming how the process reaches a steady state. Another important feature of the model is that hysteresis curves are obtained as a result of the calculation of time-dependent target condition changes. This approach to obtain hysteresis curves is more similar to an actual reactive sputtering process. The proposed model clearly displays changes in target coverage, sputtered flux, and reactive gas partial pressure during compound-layer formation and sputter etching processes as a function of elapsed time. Effects of reactive gas flow rate, pumping speed, and sputtering current on compound-layer formation and sputter etching are also discussed. Hysteresis curves are yielded as a result of compound-layer formation and sputter etching calculations. The obtained curves are in accord with experimental results. Further, the dependence of hysteresis on pumping speed, sputtering current, and the difference of sputtering yields between metallic and compound-covered targets was investigated, resulting in showing a good agreement to that obtained experimentally. Mechanisms of hysteresis formation are also discussed on the basis of obtained results. It is concluded that hysteresis is formed because of the difference of gettering capacity before and after the target surface condition transition.

An investigation of hysteresis effects as a function of pumping speed, sputtering current, and O2/Ar ratio, in Ti-O2 reactive sputtering processes

Understanding of mode transition and hysteresis behavior in reactive sputtering is necessary to widen its applications. The effects of physical pumping speed, sputtering current, and reactive/inert gas ratio on mode transitions and hysteresis in Ti-O2 dc reactive sputtering are examined. As O2 gas flow rate is increased or decreased, mode transition takes place as a consequence of the target poisoning or cleaning. The results obtained explain the important roles of pumping speed and sputtering current on hysteresis. Also, it is found that as pumping speed increases hysteresis width decreases. In addition, for smaller sputtering currents and constant pumping speed, hysteresis width shrinks. The change in transition points as a function of pumping speed and sputtering current are successfully explained by considering getter pumping capacity or getter pumping speed. Elimination of hysteresis is not investigated in this experiment. From the discussion it is suggested that hysteresis is a result of difference in sputtering yields from a poisoned target and from a metallic target, which provides a difference in gettering capacity.

Hydrogen pumping and release by graphite under high-flux plasma bombardment

Inert gas (helium or argon) plasma bombardment has been found to increase the surface gas adsorptivity of isotropic graphite (POCO-graphite), which can then getter residual gases in a high-vacuum system. The inert gas plasma bombardment was carried out at a flux ≊1×1018 ions s−1 cm−2 to a fluence of the order of 1021 ions/cm2 and at temperatures around 800 °C. The plasma bombarding energy was varied between 100 and 200 eV. The gettering speed of the activated graphite surface is estimated to be as large as 25 l s−1 cm−2 at total pressures between 10−6 and 10−7 Torr. The gettering capacity estimated is 0.025 Torr l/cm2 at room temperature. The gettering capability of graphite can be easily recovered by repeating inert gas plasma bombardment. The activated graphite surface exhibits a smooth, spongelike morphology with significantly increased pore openings, which correlates with the observed increase in the surface gas adsorptivity. The activated graphite surface has been observed to pump hydrogen plasma particles as well. From calibrated H-alpha measurements, the dynamic hydrogen retention capacity is evaluated to be as large as 2×1018 H/cm2 at temperatures below 100 °C and at a plasma bombarding energy of 300 eV. The graphite temperature was varied between 15 and 480 °C. Due to the plasma particle pumping capability, hydrogen recycling from the activated graphite surface is significantly reduced, relative to that from a presaturated surface. A presaturated surface was also observed to reproducibly pump a hydrogen plasma to a concentration of 9.5×1017 H/cm2. The hydrogen retention capacity of graphite is found to decrease with increasing temperature. At temperatures between 450 and 480 °C, the hydrogen retention estimated for the activated surface and presaturated surface is 7.9×1017 and 2.7×1017 H/cm2, respectively. A transient pumping mechanism associated with the spongelike surface morphology is conjectured to explain the large hydrogen retention capacity. Hydrogen release behavior under helium and argon plasma bombardment was also investigated, and the result indicated the possibility of some in-pore retrapping effect. The effective plasma impact desorption cross sections are evaluated in the plasma bombarding energy range from 15 to 300 eV. A significant fraction (70%–80%) of preimplanted hydrogen was observed to release under inert gas plasma bombardment to a fluence of 6×1019 ions/cm2. The graphite surface, once saturated, can be reactivated via plasma-impact hydrogen desorption, regardless of the helium and argon bombarding energy in the range examined here.

In-Situ Evaluation of High Pressure Sodium (HPS) Lamp Capacity for Gettering Hydrogen

A number of 400-W high pressure sodium (HPS) lamps with different gettering systems were evaluated with respect to their hydrogen gettering capacity by exposing them to known quantities of hydrogen while they were operated within their normal voltage limits. Results demonstrate that activated Zirconium/Aluminum alloys proved a superior getter system. Evaporated barium films are excellent getters of hydrogen provided that the film temperature does not approach 300°C.

Barium, strontium, and calcium as getters in electron tubes

Strontium and calcium getters have possible use replacing barium as gas sorbers in color picture tubes and other electron tubes. Stable getters were made of a calcium–aluminum or strontium–aluminum alloy, which was powdered, mixed with nickel powder, and pressed into a ring‐shaped channel of stainless steel. After exhaust pumping and tip‐off of the picture tube, the getter was heated to form a gas‐sorbing film of strontium or calcium, which was deposited by evaporation onto the internal surfaces of the picture tube. Strontium and calcium getters were evaluated and compared to barium getters in color picture tubes by the oxygen sorbtion test, which determined the maximum oxygen quantity that can be absorbed by the film without damage to the tube’s oxide cathode. When flashed in 25‐in. color tubes, barium and strontium showed the same oxygen capacity, both initially and after 11 and 25 weeks of life test. When flashed in tubes of smaller size and surface area, strontium showed lower oxygen capacity than barium, a result indicating slower bulk diffusion of oxygen through thick films of strontium oxide than through barium oxide. Calcium showed satisfactory gettering speed allowing normal tube operation out to 25 weeks of life test; however its gettering capacity was lower, being about 1/4 that of barium and indicating even slower diffusion of oxygen in calcium oxide than in strontium oxide.

Method for Determining Getter Activity in Vacuum Devices

A commonly accepted method for determining the activity of a barium getter film within vacuum devices requires the measurement of pressure over the film. The getter pumping speed may be calculated from this measurement when the gas inlet rate and manifold pressure are known. In this work an attempt is made to evaluate the utility of determining getter activity as a function of emission decay of an oxide cathode. Mass spectrometer and emission decay data obtained from experiments carried out using either new or operated color picture tubes indicate that decay to an arbitrary level of 80%, induced by slow oxygen-poisioning, occurs rapidly within a narrow pressure range. This indicates that determination of oxygen getter capacity based on oxide-cathode emission decay may serve as an alternate method for comparing relative getter film activities.