Non-evaporable Getter Research Articles

Comparison of TiZrV Non-Evaporable Getter Films Deposited by DC Magnetron Sputtering or Quantitative Deposition

Ti-Zr-V non-evaporable getter (NEG) films have been widely used in vacuum chambers of various accelerators since their discovery. Recently, we have used a new method called “quantitative deposition” to deposit Ti-Zr-V NEG films on nichrome substrates. The surface morphology and surface chemical bonding information were collected by scanning electron microscopy. Although the film deposited by DC magnetron sputtering has more uniform grain growth, smoother grain boundaries and higher porosity, the two films all have porous network structure and can be used as getter films.

Analysis of a Non-Evaporable Getter of Ti-Al-Zr Thin Film with Enhanced Performance Used for Ultra-High Vacuum Generation

Non-evaporable getter (NEG) thin films of Ti-Al-Zr and Ti-Zr were prepared on stainless-steel substrate by using the cathodic arc deposition technique. Scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), and in-situ X-ray photo-electron spectroscopy (XPS) were used to analyze the getter behavior of the non-evaporable getter thin films. The SEM images displayed the porous and rough surfaces of the thin films, which is an indication of high adsorption and sticking probabilities for the residual gases present in the chamber. The volume elemental composition of the thin films was measured by using EDX. The XRD patterns of the synthesized thin films indicated that the films had an amorphous nature and high structural stability. The in-situ X-ray photoelectron spectroscopy (XPS) studies revealed that the superficial layers disappeared when the non-evaporable getter of Ti-Al-Zr thin films was heated at a temperature below 250 °C for 30 minutes, there by activating the surface. However, for Ti-Zr thin films, no reasonable activation was observed at 250 °C. This outcome reveals that thin films of Ti-Al-Zr are good for reducing the outgassing and producing an ultra-high vacuum.

Hydrogen incorporation and release from nonevaporable getter coatings based on oxygen-free Pd/Ti thin films

Oxygen-free Pd/Ti thin films have been developed as novel nonevaporable getter coatings for ultrahigh vacuum (UHV) chamber walls, which can maintain high pumping speed even after many cycles of air-vent and only require activation at a relatively low temperature of 133°C. Here, the authors clarify the hydrogen (H) incorporation and release properties of such oxygen-free Pd/Ti thin films using scanning transmission electron microscopy (STEM), energy dispersive x-ray spectroscopy (EDS), and H depth profiling by 1H(15N,αγ)12C nuclear reaction analysis (NRA). EDS mapping and STEM show that the layers of Pd (6 nm) and Ti (14 nm) prepared by sequential UHV deposition on a stainless steel 304L substrate are clearly separated and that the Ti is completely covered by Pd, which prevents oxidation of Ti. NRA reveals that H accumulates preferentially in the Ti layer, where the H-content increases almost linearly with the H2 dosage. Quantitative analysis indicates that TiH1.92 is produced in the Ti layer after an exposure of 4×104 Langmuir, where the H uptake is still not saturated. Upon heating in UHV, H starts to desorb above 170°C and the release rate increases above 200°C. Repeated cycles of H incorporation at room temperature (20°C) and release at about 240°C are possible. Although the H incorporation rate changes during the initial absorption/desorption, it stabilizes after a few (∼3) incorporation/release cycles.

Commissioning of vacuum system for SuperKEKB positron damping ring

The first stage commissioning of the new positron damping ring, which was constructed in the upgraded injector system of the SuperKEKB, was carried out from February to July 2018. Vacuum scrubbing in the arc sections, which occupy most of the ring, progressed smoothly, and a beam lifetime of 1000 s was obtained with a stored beam current of 10 mA and a beam dose of 0.7 A h. However, the beam lifetime did not increase with subsequent vacuum scrubbing because the beam lifetime was primarily determined by the Touschek effect. The residual gas composition during beam operation was typical of what would be expected for a vacuum system pumped by nonevaporable getter pumps. Rough Molflow+-based estimates of the photon-stimulated desorption yields of the beam pipe at the arc sections revealed that they decreased to 1.4 × 10−4 molec. photon−1 for H2 and to 2.3 × 10−5 molec. photon−1 for CO at a beam dose of 2 A h, which corresponds to a photon dose of 1 × 1023 photons m−1.

New installation of in-vessel Non Evaporable Getter (NEG) pumps for the divertor pump in the LHD

A case of the application of Non Evaporable Getter (NEG) pumps to fusion devices is discussed. Compared with other pumping technologies used in fusion devices, the NEG pumps have advantages in terms of operation temperature at 100–200 °C (in which there is a high tolerance of the temperature increase due to a plasma radiation and/or an unexpected heat load from heating devices), low power consumption and compactness. Moreover, in the case of a power outage getters continue pumping and do not release hydrogen and its isotopes. The NEG pumps have been installed as an in-vessel pump into the divertor region in the Large Helical Device (LHD) for the first time. The pumping performance test shows that the effective pumping speed is 10 m3/s in hydrogen, which is close to the target value. In addition, the exposure of the NEG pumps to boronization and glow discharge treatments have been tested. While the boronization and glow discharge under noble gases do not show effect on the pumping performances of the NEG pumps, the establishment of an operational scenario including hydrogen and deuterium glow discharges consistent with other pumping facilities is still an aspect to be refined. However, the use of NEG pumps in LHD divertor is a milestone for the possibility of using this technology in future fusion devices.

Optimisation of the RGA location in the evaluation of NEG coating pumping properties

Non-evaporable getter (NEG) coatings are widely used in vacuum chambers of particle accelerators. Vacuum solutions group at STFC Daresbury Laboratory is continuously developing and optimising the NEG coatings are for various applications and vacuum chamber geometries. The pumping properties of the NEG coatings are characterised by sticking probability and pumping capacity. Initial sticking probability of the NEG coated tube can be measured by gas injection at one end of the tube and partial pressure measurements at either sides, P1 and P2 respectively. This requires the NEG coating facility to be modelled with the Test Particle Monte-Carlo (TPMC) method to obtain the pressure ratio R = P1/P2 as a function of sticking probability, R(α), for each type and geometry of the with each of the NEG coated tubes. The purpose of TPMC modelling reported in this paper was to find R(α) for the NEG coating evaluation facility with a sample in a form of the 0.5-m long NEG coated tubes with inner diameters of 5, 7, 9 and 38 mm. Furthermore, the effect of RGA2 location was also studied. It has been demonstrated that the RGA2 location could significantly (from a few percent up to a factor 3.1) affect the pressure ratio R(α). The effect of pressure measurement errors on calculated sticking probability was studied for ΔR/R = 10, 20 and 50%. The sticking probability accuracy varies: Δα/α = 0.04–0.5 for ΔR/R = 10%, Δα/α = 0.07–0.8 for ΔR/R = 20% and Δα/α = 0.23–2.0 for ΔR/R = 50%.

Characterization of ZAO® sintered getter material for use in fusion applications

The use of non-evaporable getter (NEG) pumps is common in many UHV applications, and the properties of the new sintered getters based on the ZAO®alloy make them appealing for applications dealing with large fluxes of hydrogen and its isotopes, as typically occurs in fusion research. In particular, the use of NEG pumps is interesting for the vacuum system of negative ion-based neutral beam injectors, which require huge deuterium gas throughputs for long durations. The key advantages of a NEG pumping solution are the high specific pumping speed and capacity, the ability to withstand a large number of absorption/desorption cycles, the ease of integration. In addition, NEG do not release hydrogen unless power is supplied to heat them, addressing an important safety issue related to uncontrolled release in case of power outage or subsystem failures.This paper reports the experimental characterization of ZAO®sintered getters, in pressure regimes and sorption amounts relevant for the use in the next generation NBI. Single discs and complete pumps were characterized in different conditions of H2/D2 pressure between 10−4 and 10-1Pa and operating temperatures up to 150°C. In particular, the feasibility of getter regeneration within time windows compatible with current NBI operation scenarios was demonstrated.

A new high precision, broad range gas micro-flow calibration apparatus

In this paper, a new calibration apparatus has been developed for gas micro-flow calibration. The apparatus employs both the constant volume method and the constant conductance method. The calibration range from 10−1 to 10−13 Pa·m3/s, and the combined standard uncertainty is 1.2%∼1.8%. When calibrating extremely low gas flow rate, the non-evaporable getter pump is used to reduce the effect of chamber wall outgassing on the measurement results, so that the lower limits of the constant volume method and constant conductance method are extended to 10−10 Pa·m3/s and 10−13 Pa·m3/s, respectively. Then a method for measurement of orifice conductance is proposed, the on-line precise measurement of the orifice conductance is realized; the consistency of flow measurement results by constant volume and constant conductance methods is better than 1.3%, then compared with the same standard leak was calibrated in both constant conductance method of the calibration apparatus and the constant pressure gas micro-flow calibration apparatus of the laboratory, and the deviation of those results was less than 1%.

Dynamic vacuum simulation for the Booster Ring in the high-intensity heavy ion accelerator facility

The High-Intensity Heavy Ion Accelerator Facility (HIAF) will provide intermediate charge state ion 238U35+ with intensity up to 2 × 1011 particle per pulse (ppp) to different experiments. However, large dynamic vacuum pressure rises of orders of magnitude, caused by lost heavy ions can seriously limit the maximum ion intensity and beam lifetime. Therefore, in order to predict the dynamic vacuum effects induced by the ionization beam loss for the BRing in the HIAF project, a new simulation program (ColBeam) has been developed together with GSI's simulation code StrahlSim are both conducted. The calculation algorithm for dynamic vacuum effect is introduced in this paper. According to the simulation result, up to 3 × 1011 ppp 238U35+ particles can be extracted for the current designed BRing vacuum and collimation system, which testifies the rationality of the system design and achieves the requirements of the physics experiments. Higher beam intensity can reach to 5 × 1011 ppp if the Non-Evaporable Getter (NEG) coating technology is implemented on the dipole and quadrupole chamber to further suppress the dynamic vacuum effect. The total vacuum pressure evolution with time in the BRing is shown in three-dimensional figure to explain the dynamic vacuum effect induced by the charge exchanged beam loss. The self-developed simulation program was implemented at 320 kV high voltage platform in IMP to simulate the dynamic vacuum evolution and compared with the measured vacuum pressure. Based on the simulation and benchmark results with the experimental data, it is concluded that the software can be used to confirm that dynamic vacuum effect can be suppressed by the collimation system and high speed pump distribution in the BRing.

Improved pumping speeds of oxygen-free palladium/titanium nonevaporable getter coatings and suppression of outgassing by baking under oxygen

Oxygen-free palladium/titanium (Pd/Ti) is a new nonevaporable getter material with an activation temperature as low as 133 °C. Because pumping speeds of oxygen-free Pd/Ti for H2 and CO have been reported to be improved by baking under an O2 atmosphere, the authors investigated oxygen-free Pd/Ti samples heated in an ultrahigh vacuum (UHV) or under an O2 pressure of 1.3 × 10−4 Pa by x-ray photoelectron spectroscopy. The authors found that carbon contamination decreased to an extent on heating in UHV, but decreased considerably on heating in O2. Pressure-curve measurements in an oxygen-free Pd/Ti-coated chamber show that pumping speeds for H2 and CO were considerably improved after baking the chamber under an O2 pressure of 1.3 × 10−4 Pa (O2 baking) in comparison with baking in UHV. Furthermore, partial-pressure measurements suggested that O2 baking removes adsorbed carbon and hydrogen, and consequently suppresses the formation of H2, CO, H2O, and CH4. Catalytic chemical reactions on the Pd surface appear to be responsible for the removal of adsorbed carbon and hydrogen. The pumping speeds of the oxygen-free Pd/Ti-coated chamber for H2 and CO were measured by using the orifice method and were found to be improved after O2 baking. O2 baking is therefore useful for improving pumping speeds for H2 and CO and for reducing the partial pressures of H2, CO, H2O, and CH4.

Conceptual design of scalable vacuum pump to validate sintered getter technology for future NBI application

Non-evaporable getter (NEG) technology is a potential candidate as pumping system for future neutral beam injector applications. The strategy to validate the use of getter pump technology is based on the realization of a relatively large pump mock-up that has to be tested in fusion-relevant conditions. The objectives of this mock-up are to demonstrate that a pump of large dimensions and capacity is usable. This paper deals with the design of the mock-up, based on conceptual studies which involve at first 3D gas flow simulations considering different modular mock-up pumps based on NEG sintered disks. In addition transient thermal simulations with FE method have been performed with the aim to analyze the thermal response of the mock-up. The conceptual design has been carried out in order to define the best configuration to obtain high pumping speed with low spatial gradient of gas concentration inside the getter material. The suggested solution will exhibit a modular structure of getter disks, which on one hand simplifies the mechanical assembling, and on the other hand allows interpretative modelling at different scales. It is foreseen to test the pump mockup in the TIMO facility at KIT Karlsruhe.

Electron cloud buildup and impedance effects on beam dynamics in the Future Circular e+e− Collider and experimental characterization of thin TiZrV vacuum chamber coatings

The Future Circular Collider FCC-ee is a study toward a high luminosity electron-positron collider with a centre-of-mass energy from 91 GeV to 365 GeV. Due to the beam parameters and pipe dimensions, collective effects and electron cloud can be very critical aspects for the machine and can represent the main limitations to its performance. An estimation of the electron cloud build up in the main machine components and an impedance model are required to analyze the induced instabilities and to find solutions for their mitigation. Special attention has been given to the resistive wall impedance associated with a layer of nonevaporable getter (NEG) coating on the vacuum chamber required for electron cloud mitigation. The studies presented in this paper will show that minimizing the thickness of this coating layer is mandatory to increase the single bunch instability thresholds in the proposed lepton collider at 45.6 GeV. For this reason, NEG thin films with thicknesses below 250 nm have been investigated by means of numerical simulations to minimize the resistive wall impedance. In parallel, an extensive measurement campaign was performed at CERN to characterize these thin films, with the purpose of finding the minimum effective thickness satisfying vacuum and electron cloud requirements.

Erratum: “Comparative study of Ar-implanted Ti-Zr-V non-evaporable getter films on the Al-alloy substrate” [AIP Advances 8, 075025 (2018)

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Novel measurement technique for the electromagnetic characterization of coating materials in the sub-THz frequency range

This paper describes a reliable, handy, and inexpensive measurement system to characterize, in the sub-THz frequency range, the coating materials used in the beam pipes of accelerators. The method is based on time domain measurements of an electromagnetic wave passing through a waveguide having a thin central slab, where the coating material is deposited on both sides. Two horn antennas are integrated on both sides of the device to optimize the signal collection and detection. This novel technique is tested on slabs coated with a nonevaporable getter (NEG) and allows to evaluate the surface impedance in the frequency range from 0.1 to 0.3 THz.

Impact of a cryogenic baffle system on the suppression of radon-induced background in the KATRIN Pre-Spectrometer

The KATRIN experiment will determine the effective electron anti-neutrino mass with a sensitivity of 200 meV/c2 at 90% CL. The energy analysis of tritium β-decay electrons will be performed by a tandem setup of electrostatic retarding spectrometers which have to be operated at very low background levels of <10−2 counts per second. This benchmark rate can be exceeded by background processes resulting from the emanation of single 219,220Rn atoms from the inner spectrometer surface and an array of non-evaporable getter strips used as main vacuum pump. Here we report on the impact of a cryogenic technique to reduce this radon-induced background in electrostatic spectrometers. It is based on installing a liquid nitrogen cooled copper baffle in the spectrometer pump port to block the direct line of sight between the getter pump, which is the main source of 219Rn, and the sensitive flux tube volume. This cold surface traps a large fraction of emanated radon atoms in a region outside of the active flux tube, preventing background there. We outline important baffle design criteria to maximize the efficiency for the adsorption of radon atoms, describe the baffle implemented at the KATIRN Pre-Spectrometer test set-up, and report on its initial performance in suppressing radon-induced background.

Development and Characterization of Non-Evaporable Getter Thin Films with Ru Seeding Layer for MEMS Applications.

Mastering non-evaporable getter (NEG) thin films by elucidating their activation mechanisms and predicting their sorption performances will contribute to facilitating their integration into micro-electro-mechanical systems (MEMS). For this aim, thin film based getters structured in single and multi-metallic layered configurations deposited on silicon substrates such as Ti/Si, Ti/Ru/Si, and Zr/Ti/Ru/Si were investigated. Multilayered NEGs with an inserted Ru seed sub-layer exhibited a lower temperature in priming the activation process and a higher sorption performance compared to the unseeded single Ti/Si NEG. To reveal the gettering processes and mechanisms in the investigated getter structures, thermal activation effect on the getter surface chemical state change was analyzed with in-situ temperature XPS measurements, getter sorption behavior was measured by static pressure method, and getter dynamic sorption performance characteristics was measured by standard conductance (ASTM F798–97) method. The correlation between these measurements allowed elucidating residual gas trapping mechanism and prediction of sorption efficiency based on the getter surface poisoning. The gettering properties were found to be directly dependent on the different changes of the getter surface chemical state generated by the activation process. Thus, it was demonstrated that the improved sorption properties, obtained with Ru sub-layer based multi-layered NEGs, were related to a gettering process mechanism controlled simultaneously by gas adsorption and diffusion effects, contrarily to the single layer Ti/Si NEG structure in which the gettering behavior was controlled sequentially by surface gas adsorption until reaching saturation followed then by bulk diffusion controlled gas sorption process.

Effect of Cavity Vacuum Pressure Diminution on Thermal Performance of Triple Vacuum Glazing

Long-term durability of the vacuum edge seal plays a significant part in retrofitting triple vacuum glazing (TVG) to existing buildings in achieving progress towards a zero-energy building (ZEB) target. Vacuum pressure decrement with respect to time between panes affects the thermal efficiency of TVG. This study reports a 3D finite element model, with validated mathematical methods and comparison, for the assessment of the influence of vacuum pressure diminution on the thermal transmittance (U value) of TVG. The centre-of-pane and total U values of TVG are calculated to be 0.28 Wm−2 K−1 and 0.94 Wm−2 K−1 at the cavity vacuum pressure of 0.001 Pa. The results suggest that a rise in cavity pressure from 0.001 Pa to 100 kPa increases the centre-of-pane and total U values from 0.28 Wm−2 K−1 and 0.94 Wm−2 K−1 to 2.4 Wm−2 K−1 and 2.58 Wm−2 K−1, respectively. The temperature descent on the surfaces of TVG between hot and cold sides increases by decreasing the cavity vacuum pressure from 50 kPa to 0.001 Pa. Nonevaporable getters will maintain the cavity vacuum pressure of 0.001 Pa for over 20 years of life span in the cavity of 10-mm wide edge-sealed triple vacuum glazing, and enable the long-term durability of TVG.

Application of compact Non-Evaporable Getter and Sputter Ion Pump combination in UHV system of HIAF

HIAF-BRing, a new multi-purpose accelerator facility of the High Intensity Heavy-Ion Accelerator Facility project, requires an extremely high vacuum lower than 10−9 Pa to fulfill the requirements of radioactive beam physics and high energy density physics. Use of large speed Titanium sublimation pump (TSP) in combination with Sputter ion pumps (SIP) is not always feasible due to space limitations. A novel combination pump, based on Non Evaporable getter (NEG) and a 10 l/s SIP (NEXTorr® D 2000-10) has been evaluated as it provides large speed in a more compact design. The two combination pumps (TSP + SIP and NEXTorr D 2000-10) have been tested by dynamic flow method to measure the ultimate pressure, the pumping speed and the suitability for HIAF. Test results show that both combination pumps can be used in the pressure range of 10−10 Pa. The ultimate pressure of TSP + SIP combination pump is slightly lower than NEXTorr D 2000-10, but the latter one has much larger sorption capacities (H2: 210200 Pa l vs 265 Pa. l; CO: 430 Pa. l vs 24 Pa. l), which means it is applicable to a wider pressure range. At the same time, after twenty activation cycles, the initial pumping speed of NEXTorr D 2000-10 for H2 and CO decreased less than 10% as compared with the initial pumping speed of the first activation, which meets the actual vacuum system requirement of HIAF. A dedicated vacuum chamber with collimator and NEXTorr D 2000-10 has been designed and manufactured to stabilize the dynamic vacuum by decreasing the heavy ion induced gas desorption and increasing the gas pumping speed. The static and dynamic vacuum pressure evolution has been simulated to verify the feasibility of the NEXTorr D 2000-10 used in the HIAF-BRing.

Oxygen-free palladium/titanium coating, a novel nonevaporable getter coating with an activation temperature of 133 °C

A novel nonevaporable getter (NEG) coating was developed with an activation temperature as low as 133 °C, that is, a palladium/titanium coating with extremely low oxygen concentration (oxygen-free Pd/Ti coating). The substrate was coated with Ti and then overcoated with Pd using sublimation under ultrahigh vacuum conditions. The morphology and surface elemental composition of the Pd/Ti thin film were investigated with electron microscopes and x-ray photoelectron spectroscopy, respectively. The thicknesses of Ti and Pd films were approximately 1.3 μm and 50 nm, and the Ti film was completely overcoated by the Pd film. Ti and oxygen were found to be negligible on the oxygen-free Pd/Ti surface. The oxygen-free Pd/Ti coating was applied to the formed bellows. The bellows was successively baked at 133 °C for 12 h, 176 °C for 3.5 h, and 200 °C for 3.5 h. After sealing off a turbomolecular pump from the vacuum system containing the bellows, the pressure reached values of 4.6 × 10−6, 1.7 × 10−7, and 6.1 × 10−8 Pa, respectively. The pumping speeds of the bellows were estimated to be 0.028, 0.23, and 0.23 l s−1, respectively. These results demonstrate that oxygen-free Pd/Ti deposition can be used as a new NEG coating for vacuum systems with a baking temperature of 133 °C or higher.

Multilayer Thin Film Getter for Sustainable Vacuum in MEMS Packaging

Some applications require MEMS devices to be packaged in a high vacuum environment, for example, high-quality factor mechanical micro-resonators or low heat loss microbolometers can only be obtained in a vacuum better than 10-3 mbar [1-2]. For this purpose, many Non-Evaporable Getter (NEG) multilayers or alloys films based on sputtered highly reactive transition metals have been investigated. For example, R.K. Sharma [3] found the Ti21Zr24V55 getter and O.B. Malyshev [4] found the Ti-Zr-Hf-V getter can be activated at 160 °C for 2 hours and 150 °C for 24 hours respectively. The getter function after its thermal activation is to absorb gases generated inside the packaging cavity by outgassing, permeation and by real and virtual leaks after packaging cap sealing by a highly hermetic bonding process. In this work we investigate Au/Zr/V/Zr getter multilayers in order to get a sustainable low pressure in a packaging cavity. All films were initially deposited by e-beam evaporation on 100 mm in diameter Si（100）wafers. The samples of Au/Zr/V/Zr were deposited in an ultra-high vacuum set-up with a background pressure of 2 10-10 mbar. To investigate their activation and oxidation by gas sorption at typical pressures found in packaging cavities, the films were annealed for 1 hour at various annealing temperatures in a rapid thermal annealing (RTA) chamber (Jipelec Jetfirst 100) at a vacuum level 10-2 mbar. Then, these films were analyzed by using four probe resistivity measurement, SEM and TEM cross section observations, EDX measurement and SIMS. All results are consistent and support that the activation temperature of zirconium under vanadium depends on vanadium layer thickness. For example, a 25nm zirconium oxide layer is found under the vanadium film after annealing at 300°C for 1h when the vanadium thickness is less than 200nm. However, for a 450nm thick vanadium layer, the 1-hour annealing temperature must reach 350°C to obtain the same thickness of zirconium oxide layer. Next, these annealed Au/Zr/V/Zr getters were exposed to air at room temperature for 7 months. After this exposure, the sheet resistance was found to increase by 6 % and 15 % for samples annealed at 250°C-1h and 300°C-1h respectively. TEM cross section observations of these samples showed that the thickness of the zirconium oxide layer under vanadium increases by about 3 nm. Finally, to evaluate its sorption ability in a real application case, the Au/Zr/V/Zr getter film was integrated in the 1.3 cmx1.3 cmx2.5 mm packaging cavity of a resonant gyroscope and activated at 300°C for 30 min before cavity sealing by Au/Sn bonding at 285°C for 30min. The time dependence of the pressure inside the packaging was evaluated from the variation of the quality factor Q of a selected mechanical resonance of the gyroscope. Quantitative values of the internal pressure were obtained owing to a previous calibration of the Q versus pressure curve that was in agreement with a squeeze film damping model. The evaluated pressure inside the cavity was 4.5 x10-3 mbar after annealing, 3x10-3 mbar after 2 weeks, and then this vacuum level still maintained after 6 months. In conclusion, this work demonstrates that Au/Zr/V/Zr getter films have a low activation temperature and suitable sorption properties to get a sustainable low pressure in a medium size vacuum package. [1] M. Elßner, Vacuum quality evaluation for uncooled micro bolometer thermal imager sensors, Microelectronics Reliability 54 (2014), 1758. [2] G.-D. Anna, Miniature and MEMS-type vacuum sensors and pumps, Vacuum 82 (2009), 1419. [3] R.K. Sharma, Jagannath, S. Bhattacharya, R.B. Tokas, K.G. Bhushan, S. Sen, S.C. Gadkari, S.K. Gupta, Journal of Alloys and Compounds 651 (2015) 375-381. [4] O.B. Malyshev, R. Valizadeh, R.M.A. Jones, A. Hannah, Vacuum 86 (2012) 2035-2039.