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

micromachines

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

El-Mostafa Bourim 1,* , Hee Yeoun Kim 1 and Nak-Kwan Chung 2

1 National Nanofab Center, Department of Nanostructure Technology, Daejeon 34141, Korea; [email protected] 2 Korea Research Institute of Standard and science, Center of Vacuum and Technology, Daejeon 305-340, Korea; [email protected] * Correspondence: [email protected] or [email protected]; Tel.: +82-42-366-1524

Received: 12 June 2018; Accepted: 13 September 2018; Published: 25 September 2018

Abstract: 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.

Keywords: non-evaporable getter; non-evaporable getter (NEG); activation temperature; sorption properties; sorption speed; sorption capacity

1. Introduction Non-evaporable getters (NEGs), based on the thin ﬁlm deposition technique, were until now still the practical technical solution for integrating a scaled-down getter into microelectronic devices, that need special residual gas control within their cavities for keeping their functioning performances optimal after packaging under a controlled atmosphere. Among these devices, micro-electro-mechanical systems (MEMS) are of high utility for scalable NEGs. Depending on their global functions, MEMS can be exploited to work either under vacuum or some controlled residual gas pressure of an inert gas inside their cavities [1,2]. Unfortunately, degradation of the desired pressure atmosphere inside the MEMS devices is commonly primed by effects such as surface degassing, micro-leaks as well as permeation. Thus, a very slight leak would cause signiﬁcant variations in the

Micromachines 2018, 9, 490; doi:10.3390/mi9100490 www.mdpi.com/journal/micromachines Micromachines 2018, 9, 490 2 of 20 adjusted internal atmosphere pressure into the MEMS cavities and hence alter their assigned operation. Therefore, the ability of the getter to trap these residual and/or additional gases can be exploited to efﬁciently master the vacuum or atmospheric pressure conditions into the MEMS cavity that assures their optimal performance and reliability throughout the system lifetime [2,3]. As known, getter materials are chemical pumps able to absorb active gases (H2, CO, CO2,O2,N2,H2O, etc.) on their surfaces and the most common basic metals for NEGs are Ti and Zr either in their single phase states or as alloys composed of other metallic elements in intermetallic compounds [4–7]. Ti and Zr belong to the group V elements in periodic table possessing a high oxygen solubility limit and high oxygen diffusivity physical properties, which are behind the high gas sorption properties (sorption speed and capacity) manifested by NEGs. In fact, the NEGs gettering performances are tightly linked to the NEGs fabricated microstructure, which is required to be essentially formed of a small columnar grain structure that promotes the gas sorption process by diffusion through a NEG grain boundary microstructure. NEGs are operational only after their activation under vacuum at a high temperature. However, the getter should be developed for low activation temperatures compatible with the sealing process temperature of MEMS devices. Usually, the most well-known bonding processes for MEMS cavity hermetic sealing their processing temperature are performed at temperatures ranging from 300 ◦C to 450 ◦C[8]. Therefore, a getter with an activation temperature less than 450 ◦C would be considered as a solution to the issue that MEMS device can withstand during the sealing process. To optimize sorption performances and reduce thermal activation temperature of NEGs systems, in the literature, many investigations have been undertaken to improve the NEG materials diffusivity by fabricating them from alloys of several mixed metals [5,9–12] or by controlling the NEG microstructure and surface roughness using pure multi-metallic-layer thin ﬁlms forming NEGs in a stacked structure [13–15]. The investigation of getter formed of metal alloys needs a minute analysis of each metallic element involved in the alloy composition, which makes it certainly not easy to elucidate precisely the mechanism controlling the gettering process. In this work, we have chosen to investigate the getters formed of pure metallic thin films stacked in single and multi-layered structures such as Ti/Si, Ti/Ru/Si, and Zr/Ti/Ru/Si. The insertion of the ruthenium as a seed sub-layer, on which the elementary getter film layers are deposited, is for growing the getter microstructure in small columnar grains with upper surface morphology manifesting a high roughness. The all metallic films composing the getter are deposited under the same temperature and pressure conditions, and the finer crystalline microstructure growth depends on the highest melting point. Therefore, as the ruthenium metal possesses a higher melting point, its film layer growth will therefore form in a thinner columnar microstructure defined by a higher grain boundary density [16,17]. Thus, the growth of the subsequent getter film layer is partially controlled by a limited mobility of its metal atoms at the Ru sub-layer surface thereby leading to a finer microstructure in the getter film, consisting of a columnar grain structure and a high getter surface roughness. Such a microstructure promotes the getter to be activated at a low temperature due to the diffusion process mostly taken along the grain boundaries, as well as provides the getter with a large sorption capacity, sufficient for pumping throughout the MEMS device lifetime. Therefore, to confirm the Ru seed sub-layer effect on getter microstructure control, activation temperature and sorption characteristics, in this study, we compared the microstructure and morphology of the getters fabricated with and without a Ru seed sub-layer. In addition, we analyzed the in situ temperature XPS getter surface chemical state modifications after thermal activation and we measured the N2 sorption behavior in the pressure–time dependence and sorption performance properties after the activation process at different temperatures. A correlation relating the XPS getter surface chemical state changes to the getter sorption behavior was evidenced, and hence the getter activation and the corresponding gas sorption mechanisms were elucidated.

2. Experimental Section NEG thin ﬁlms were fabricated by deposition on 4” (100) Si substrates using an e-beam and thermal evaporation system (KVE & T-C500200, Korea Vacuum Tech, Ltd., Gimpo-si, Korea). Micromachines 2018, 9, 490 3 of 20

The substrates were prepared in accordance with the standard RCA cleaning method. Depositions were performed at room temperature and at a base pressure of 5 × 10−8 Torr, the vacuum environment avoids getter contamination by the carrier gas as used in the deposition by the sputtering technique. Generally, such deposition conditions promote nano-grain formation in a columnar-like structure basically controlled by the reduced atomic mobility of the deposited species on the substrate and self-shadowing during the film growth [16,17]. The evaporated materials used were titanium (99.9995% stated purity), zirconium (99.9995% stated purity) and ruthenium (99.95% stated purity). The deposition rate was 1 nm/s for the Ti and Zr layers and 0.5 nm/s for the Ru one. The average thicknesses of the deposited films were 30 nm for the Ti adhesive layer, 600 nm for the Ti layer in the getter structure with a single active film, 300 nm for each of the Ti and Zr layers in the getter structure with double active films, and 60 nm for the Ru seed sub-layer. The surface morphology and the fracture cross-sectional structure of the fabricated NEGs were analyzed by field emission scanning electron microscopy (FE-SEM, S-4800, Hitachi, Tokyo, Japan) system. An atomic force microscope (AFM, SPI4000, Seiko instruments, Chiba, Japan) was used to determine the surface roughness. In situ temperature X-ray photoelectron spectroscopy (XPS) analyses were recorded using an ESCALAB 250 (Thermo Fisher Scientific, Loughborough, UK) XPS spectrometer equipped with a monochromatic Al Kα X-ray source and a ceramic heater stage. The NEG sample was heated by direct contact with the ceramic heater and the temperature was controlled with a temperature controller that covered a temperature range from RT to 600 ◦C. Before each temperature XPS measurement, the NEG sample was placed in an ultra-high vacuum (UHV) chamber and maintained for one hour at the targeted temperature before the XPS data were recorded. For consistency, the chamber pressure was maintained below 1 × 10−7 Torr during the measurements, regardless of the NEG sample temperature. As the film surface of the fabricated getter was exposed to the air during its transfer, it was easy to oxidize and to be contaminated with others gases. Thus, a passive layer was formed on the getter surface. This passivation layer was a protective barrier against more gas absorption and diffusion, resulting in gettering deterioration. It was necessary, before testing the gettering property performance, to regain the getter active surface by annealing the getter sample under a high vacuum and a high temperature, this dissolves and diffuses the protective oxide surface into the getter film bulk. Such operation, leading to a fresh getter surface, is called an activation process. For an effective activation, the getter material must also have the ability to allow diffusion of the contaminating gases once they have been absorbed. For characterizing the activation effect on the getter gas sorption behavior and for evaluating the getter sorption property performances, a getter test system was used (Figure1). This experimental system was designed based on the standard ASTM-F798 method [18] for the evaluation of gas sorption characteristics. The system consisted of two communicating chambers (getter chamber and gas inlet chamber), through a conductance orifice. These chambers could be simultaneously evacuated or isolated by actuating manually the vacuum valves (AV; EV-150, Huntington Mechanical Labs, Inc., Grass Valley, CA, USA and GV; E-GV-2500M, MDC Vacuum Products, LLC, Hayward, CA, USA) located on the bypass passages connected to the pumping units (Turbo-molecular pump (TMP) and Scroll pump). The getter chamber was made in a cylindrical tube form (diameter = 35 mm and length = 150 mm) from stainless steel, as the rest of all vacuum system parts, and the measured getter sample installed inside was of a rectangular shape with gettering surface area dimensions of 2 cm × 6 cm. The pressure in each chamber was measured using a pressure gauge (Pg, Pm) Bayard Alpert type hot cathode ion gauges (PBR 260, Pfeiffer Vacuum, Asslar, Germany). Nitrogen (N2) gas was admitted in the inlet chamber via a UHV high-precision variable leak valve (VLV; ULV–150, MDC Vacuum Products, LLC). The total system was evacuated by a 60 L/s TMP (TMU 071P, Pfeiffer Vacuum) in series with an oil-free dry scroll vacuum pump (ISP–90 Scroll Meister, Anest Iwata, Inc., Cincinnati, OH, USA). The ceramic heater was made from an alumina plate with a smooth surface finish with a resistance heating element of 50 Ω at RT; it could reach a maximum temperature of 500 ◦C (CMTECH; Daegu, Korea). A K–type thermocouple (TC) inside the getter chamber and in contact with the heater was used for measuring the temperature of the getter. Regarding the residual gas analyzer Micromachines 2018, 9, 490 4 of 20

(RGA) element, ﬁguring on the getter test system schematic diagram, it is worth to mention that this Micromachines 2018, 9, x FOR PEER REVIEW 4 of 20 device was not installed on the system in the experiments for measuring the effective total pressure in thepressure chambers. in the Such chambers. action Such was consideredaction was consider in ordered toin avoidorder to any avoid parasitic any parasitic effect oneffect the on measured the pressures,measured leading pressures, to ambiguous leading to results ambiguous and difﬁcultiesresults and difficulties in interpretations. in interpretations.

FigureFigure 1. Schematic1. Schematic diagramdiagram of of the the getter getter test system. test system. Pm, Pg: PressurePm, Pg :gauge, Pressure AV: Angle gauge, valve, AV: GV: Angle valve,Gate GV: valve, Gate valve,VLV: Variable VLV: Variable leak valve, leak TMP: valve, Turbo TMP: molecular Turbo molecular pump, FV: pump, Fore-line FV: valve, Fore-line RGA: valve, Residual gas analyzer (device dismantled for total pressure measurements). RGA: Residual gas analyzer (device dismantled for total pressure measurements). To ensure comparability of the sorption tests between the samples, once the getter chamber was To ensure comparability of the sorption tests between the samples, once the getter chamber was opened to the atmosphere, a conditioning assuring the same initial getter chamber pressure before opened to the atmosphere, a conditioning assuring the same initial getter chamber pressure before starting the pressure measurements was performed. The getter sample was attached directly to the startingheater, the pressureand both measurementswere mounted on was a holder performed. isolated The from getter the samplechamber was walls; attached such set-up directly focused to the the heater, and bothheat werelocally mounted on the sample on a holder only. Once isolated the sample from the was chamber placed in walls; the getter such set-upchamber, focused all the thegetter heat test locally on thesystem sample was only. backed Once out the at a sample temperature wasplaced of 130 °C in thefor 48 getter h in chamber,order to clean all the the getterchambers test internal system was backedwalls out that at a minimized temperature the of vacuum 130 ◦C forchamber 48 h in outgassing order to clean and also the chambersallowed the internal getter chamber walls that to minimizedreach the vacuuman UHV chamber of 10−8 Torr. outgassing and also allowed the getter chamber to reach an UHV of 10−8 Torr. RegardingRegarding the the investigations investigations ofof the the activation activation effect effect on the on pressure–time the pressure–time dependence, dependence, the the measurementsmeasurements were were performedperformed by means means of of a astatic static method. method. In this In this method, method, after after activating activating the getter the getter sample at a specific temperature for one hour, and subsequent cooling back down to room sample at a specific temperature for one hour, and subsequent cooling back down to room temperature, temperature, the all valves (AV and GV) of the getter test system were closed to isolate the getter the all valves (AV and GV) of the getter test system were closed to isolate the getter chamber from the chamber from the pumping line (TMP and scroll pump), and then immediately after a moment of pumpingtime lineof about (TMP 3 min, and scrollwhen pump),the getter and chamber then immediately pressure had afterstabilized a moment around of 10 time−7 Torr, ofabout a flow3 of min, pure when −7 the getternitrogen chamber (N2) gas pressure was induced had stabilized through aroundthe high-precision 10 Torr, avariable flow of leak pure valve nitrogen (VLV) (N to2 )rapidly gas was reach induced −5 througha pressure the high-precision of 10−5 Torr inside variable the getter leak valvechamber. (VLV) The to gas rapidly was immediately reach a pressure stopped, of by 10 closingTorr the inside the getterleak valve, chamber. once The the gasset pressure was immediately has been reached. stopped, Pressure–time by closing the dependence leak valve, was once recorded the set for pressure a has beenperiod reached. of about Pressure–time 3 h, starting from dependence the moment was of gas-inlet recorded valve for aclosing period until of aboutthe chamber 3 h, starting outgassing from the momentpressure of gas-inlet became valve largely closing dominant. until the chamber outgassing pressure became largely dominant. RegardingRegarding the measurementsthe measurements of the of sorptionthe sorption properties properties (sorption (sorption speed speed and capacity),and capacity), a standard a standard called the dynamic method was used. It is described by the standard conductance technique called the dynamic method was used. It is described by the standard conductance technique in in ASTM–F798 [18], and its basic principle is based on the measurement of the pressure difference ASTM–F798 [18], and its basic principle is based on the measurement of the pressure difference between two chambers; the gas inlet chamber (Pm) and the sample chamber (Pg) (see schematic betweendrawing two of chambers; the setup in the Figure gas 1). inlet These chamber chambers (P arem) connected and the sampleto each other chamber through (P ag )small (see orifice. schematic drawingThe ofsorption the setup speed in of Figure a getter1). was These calculated chambers as follows: are connected to each other through a small oriﬁce. The sorption speed of a getter was calculated as follows:

Micromachines 2018, 9, 490 5 of 20

Micromachines 2018, 9, x FOR PEER REVIEW 5 of 20 Q S = and Q = C0 Pm − Pg . (1) 𝑆=Pg and 𝑄 = 𝐶 𝑃 −𝑃. (1) 2 2 Here, S is the sorption speed of the getter (L/s·cm ), Q is the sorption capacity (mbar·L/cm ), C0 is the Here, S is the sorption speed of the getter (L/s·cm2), Q is the sorption capacity (mbar·L/cm2), C0 is the conductanceconductance oriﬁce orifice with with diameter diameter == 2 mm and and C C = = 0.4 0.4 L/s. L/s. −8 InIn this this method, method, after after the the getter getter was was activated activated and and the the pressure pressure reached reached the the UHV, UHV, around around 10 10−8Torr (inTorr both (in gas both inlet gas and inlet getter and chambers) getter chambers) at room at temperature, room temperature, all the valvesall the (AVvalves and (AV GV) and were GV) closed were and −7 afterclosed a few and minutes after a (aboutfew minutes 3 to 5 (about min) of 3 pressureto 5 min) stabilizationof pressure stabilization around 10 aroundTorr (in 10− both7 Torr gas (in inletboth and gettergas inlet chambers), and getter the chambers), nitrogen gas the was nitrogen admitted gas was in the admitted gas inlet in chamberthe gas inlet through chamber the through high-precision the variablehigh-precision leak valve variable (VLV) leak manually valve (VLV) tuned manually to let atuned weak to ﬂow,let a weak maintaining flow, maintaining a constant a constant pressure of 10−pressure5 Torr, intoof 10 the−5 Torr, gas into inlet the chamber. gas inlet The chamber. pressures The pressures into the two into chambersthe two chambers were recorded were recorded with time untilwith they time equalized until they when equalized the getter when surfacethe getter became surface saturated. became saturated.

3.3. Results Results and and Discussion Discussion

3.1.3.1. Microstructure Microstructure and and Morphology Morphology AnalysesAnalyses

MicrostructureMicrostructure analysis analysis by by SEMSEM and AFM observations observations for for the the three three fabricated fabricated NEGs NEGs are areshown shown inin Figure Figure2. 2. Generally, Generally, the the images images show show partiallypartially entangled leaflets leaflets (small (small leaves) leaves) grown grown vertically vertically onon each each getter getter sample sample surface. surface. TheseThese leaflets leaflets were were stacked stacked almost almost vertically vertically with with their their normal normal plane plane orientatedorientated in in different different directions, directions, which which allow allow an an inter-granular inter-granular arrangement arrangement with with cavities cavities lying lying down fromdown the from surface the surface to the filmto the bulk. film Furthermore,bulk. Furthermor multilayerede, multilayered NEGs NEGs structures structures with with a Rua Ru sub-layer sub- showlayer a show dense a leafletdense leaflet surface surface morphology morphology compared compar toed the to singlethe single layer layer Ti/Si Ti/Si NEG, NEG, whose whose surface surface was a mixturewas a mixture of an inhomogeneousof an inhomogeneous distribution distribution of embedded of embedded leaflets leaflets and and flat flat grain grain clusters. clusters. GivenGiven the complexthe complex morphology morphology showed showed by all theby NEGall the surfaces, NEG surfaces, it was quite it was difficult quite to difficult give a representativeto give a grainrepresentative size value fromgrain thesize NEGvalueupper from the surface. NEG upper surface.

FigureFigure 2. 2. SEMSEM surfacesurface micrographs ( (aa),), 3–D 3–D AFM AFM images images (b (),b ),and and SEM SEM fracture fracture cross-section cross-section micrographsmicrographs ( c()c) of of single single Ti/Si Ti/Si NEGNEG (left(left column);column); multilayer multilayer Ti/Ru/Si Ti/Ru/Si NEG NEG (middle (middle column) column) and and multilayermultilayer Zr/Ti/Ru/Si Zr/Ti/Ru/Si NEG NEG (right (right column). column).

Micromachines 2018, 9, 490 6 of 20 Micromachines 2018, 9, x FOR PEER REVIEW 6 of 20

Therefore,Therefore, from from a acombination combination of of the the differen differentt information information gained gained by by SEM SEM observations observations and and AFMAFM roughness roughness measurements measurements (Figure (Figure 2a,b),2a,b), an an average average estimation estimation of of the the leaflet leaflet width width and and surface surface roughnessroughness is isgiven given in in Table Table 1.1. In In addition, addition, an an important important insight insight into into microstructure microstructure growth growth was was obtainedobtained from from the the cross-sectional cross-sectional SEM observationobservation ofof the the fracture fracture surfaces surfaces (see (see Figure Figure2c). 2c). For For the NEGthe NEG samples without a Ru sub-layer, it was observed that at the Sisubstrate/Ti interface there was a samples without a Ru sub-layer, it was observed that at the Sisubstrate/Ti interface there was a dense densecompact compact Ti thin Ti layerthin layer with with a variable a variable thickness thickn ofess a few of a tens-of-nmfew tens-of-nm all along all along the sample the sample cross-section. cross- section.This was This just was an just indication an indication that at that the beginningat the begi ofnning Ti film of Ti deposition, film deposition, the early the growth early growth of the film of the had filmdeveloped, had developed, on the Sionsubstrate, the Si subs atrate, nano-undulated-patterned a nano-undulated-patterned thin layer thin composedlayer composed of hills of and hills craters and craterswith awith wide a distancewide distance between between the hill the summits. hill summits From. this From type this of type hills andof hills craters and pattern,craters pattern, it looked it as lookedthough as thethough nucleation the nucleation and growth and of growth large columnar of large columnar grains were grains taking were place taking from place the cratersfrom the and cratersthin leaflets and thin were leaflets taking were place taking from theplace hill from summits. the hill Thus, summits. the Ti microstructureThus, the Ti microstructure had been raised had up. beenRegarding raised up. the Regarding Ti/Ru/Si andthe Zr/Ti/Ru/SiTi/Ru/Si and Zr/Ti/Ru/Si NEG samples, NEG the samples, deposited the Ru deposited sub-layer Ru (~60 sub-layer nm) had (~60shown nm) ahad very shown narrow a very and longnarrow columnar and long structure, columnar which structure, had a which significant had influencea significant on influence the surface oninterface the surface for controllinginterface for the control subsequentling the compact subsequent layer compact of the early layer deposited of the early Ti film deposited to be manifested Ti film to in bea manifested microstructure in a with microstructure a high nano-undulated with a high density nano-undulated pattern. Hence, density from pa thettern. resulting Hence, small from craters, the resultingwith very small close craters, hill summits with very at close the beginning hill summits of Tiat layerthe beginning growth, aof very Ti layer well-defined growth, a Tivery columnar well- definedstructure Ti columnar formed only structure in a leaflet formed shape only hadin a beenleaflet nucleated shape had and been grown nucleated up. For and the grown Zr/Ti/Ru/Si up. For theNEG Zr/Ti/Ru/Si sample, theNEG already sample, deposited the already Ti film, deposited conditioned Ti film, in conditioned leafleted microstructures in leafleted microstructures by Ru sub-layer, byhas Ru served sub-layer, as a seedhas served layer for as Zra seed film growth,layer for and Zr film thus growth, the Zr film and microstructure thus the Zr film has microstructure manifested once hasagain manifested only in aonce well-defined again only leaflet in a well-defined columnar structure, leaflet columnar but with very structure, small sizebut with and randomvery small direction size andorientation random direction of leaflet faces.orientation of leaflet faces. FromFrom the the above above analysis, analysis, it itturned turned out out that that the the mu multilayerltilayer thin thin film film based based getters getters using using Ru Ru as as a a sub-layersub-layer had had a film a film microstructure microstructure composed composed of of inclined inclined columnar columnar leaflets leaflets that that were were finely finely packed packed asas nano-crystal nano-crystal columns columns with with different different levels levels emer emergingging from from the the getter getter surfac surface.e. Such Such microstructure microstructure givesgives rise rise to toa high a high density density of ofgrain grain boundaries, boundaries, a high a high porosity porosity and and a high a high surface surface roughness roughness with with a higha high specific specific area. area. High High roughness roughness and and porosity porosity promote promote a surface a surface with, with, both, both, a high a high gas gas capacity capacity andand a high a high gas gas sticking sticking probabilit probability,y, while while the the high high grain grain boundary boundary density density promotes promotes a high a high gas gas bulk bulk diffusion.diffusion. All All of ofthese these features features are are requested requested for for a practical a practical NEG NEG to to present present high high sorption sorption properties properties andand performance. performance.

TableTable 1. 1.AverageAverage leaflet leaﬂet width width size size an andd NEG NEG upper upper surface surface roughness. roughness.

GetterGetter Samples Samples  Ti/SiTi/Si NEG NEG Ti/Ru/Si Ti/Ru/Si NEG NEG Zr/Ti/Ru/Si Zr/Ti/Ru/Si NEG NEG

LeafletLeaﬂet Width Width Size Size 225 225 nm nm 200 200 nm nm 170 170 nm nm Roughness (Ra) 10.089 nm 16.654 nm 8.958 nm Roughness (Ra) 10.089 nm 16.654 nm 8.958 nm 3.2. In-Situ XPS Getter Surface Analysis during Thermal Activation 3.2. In-Situ XPS Getter Surface Analysis during Thermal Activation The surfaces of the fabricated getters were investigated by in situ temperature XPS under the sameThe conditions surfaces of (annealing the fabricated under getters UHV were for one inve hour)stigated as duringby in situ their temperature activation inXPS the under getter the test samesystem. conditions Figure 3(annealing shows the under in situ UH XPSV for surface one hour) characteristics as during of their the threeactivation studied in the NEG getter samples. test system.The measured Figure 3 datashows are the related in situ to XPS the surface evolution char ofacteristics the XPS of spectra the three of Ti studied 2p, Zr 3dNEG and samples. C 1s orbitals The measuredduring the data activation are related process to the atevolution various of temperatures. the XPS spectra The of analysisTi 2p, Zr of3d thesesand C 1s XPS orbitals characteristics during theallowed activation information process at about various the gettertemperatures. upper surface The analysis chemical of statetheses modiﬁcations XPS characteristics to be obtained allowed as informationwell as the about reactions the ofgetter the surfaceupper surface contaminants chemical (O, state C, H, modifications etc.) with the to main be obtained metals (Ti as or well Zr) as of the each reactionsNEG upper of the layer. surface Indeed, contaminants the reduction (O, C, of H, the et oxidesc.) with of the these main elements metals to (Ti their or Zr) metallic of each states NEG is a upperclear layer. indication Indeed, of NEGthe reduction activation of by the demonstrating oxides of these a cleanelements reactive to their surface metallic [19– 21states]. Furthermore, is a clear indicationknowing of how NEG the contaminantsactivation by bounddemonstrating to the getter a clean metal reactive may provide surface insight [19–21]. into Furthermore, understanding knowingthe gettering how the mechanism contaminants and performance. bound to the In gette ther following, metal may in provide the XPS insight analysis, into since understanding the evolution thetrends gettering of XPS mechanism oxidation and states performance. with temperature In the fo forllowing, Ti upper-layer in the XPS NEG analysis, are almost since similar the evolution to the Zr trendsupper-layer of XPS NEG;oxidation therefore, states towith avoid temperature repetition infor the Ti explanation,upper-layer NEG when are describing almost similar the chemical to the stateZr upper-layer NEG; therefore, to avoid repetition in the explanation, when describing the chemical

Micromachines 2018, 9, 490 7 of 20 Micromachines 2018, 9, x FOR PEER REVIEW 7 of 20 changestate change undergone undergone by the titanium,by the titanium, the corresponding the corresponding zirconium zirconium state transformation state transformation is mentioned is inmentioned parentheses. in parentheses. AsAs shown shown in in Figure Figure3a–c, 3a–c, the the evolution evolution with with temperature temperature of theof the XPS XPS spectra spectra associated associated with with the Tithe 2pTi orbital2p orbital of the of Tithe upper-layer Ti upper-layer NEGs NEGs and theand one the associatedone associated with with the Zr the 3d Zr orbital 3d orbital of the of Zr the upper-layer Zr upper- NEGlayer haveNEG twohave regimes. two regimes. In the In ﬁrst the regimefirst regime from fr theom as-deposited the as-deposited NEG NEG sample sample to the to the activation activation at 275at 275◦C, °C, the the Ti 2p Ti (Zr2p 3d)(Zr spectrum3d) spectrum evolution evolution was mainlywas mainly controlled controlled by the by decrease the decrease of the of doublet the doublet peak 4+ 4+4+ 4+ Tipeak(Zr Ti ) (Zr of bending) of bending energy energy (BE): Ti(BE):2p3/2 Ti =2p 458.8/ =eV, 458.8Zr 3eV,d5/2 Zr= 3d 182.8/ = eV 182.8 related eV torelated the TiO to 2the(ZrO TiO2)2 x+<4 x+<4 oxide,(ZrO2) accompanied oxide, accompanied simultaneously simultaneously with, both, with, an increaseboth, anin increase the Ti in the(Zr Tix+<4) doublet(Zrx+<4) doublet peaks related peaks torelated TiOx<2 to(ZrO TiOx<2x<2 )(ZrO suboxidesx<2) suboxides [13,21–27 [13,21–27]] and an emergence and an emergence of a metal of hydroxide a metal Ti(OH)hydroxide2 (Zr(OH) Ti(OH)2)2 doublet(Zr(OH) peak2) doublet located peak at higherlocated bending at higher energies bending ( Tienergies2p3/2 = (Ti 459.3 2p/ eV = (459.3Zr 3d eV5/2 (=Zr 183.4 3d/ eV)) = 183.4 [28– eV))31]. ◦ ◦ In[28–31]. the second In the regime,second regime, over 275 overC 275 and °C up and to up 450 to 450C, the°C, the Ti 2pTi 2p (Zr (Zr 3d) 3d) spectrum spectrum evolution evolution was was mainlymainly controlledcontrolled byby aa simultaneoussimultaneous decreasedecrease ofof metal-oxide,metal-oxide, metal-hydroxide,metal-hydroxide, andand metal-suboxide,metal-suboxide, 0 0 accompaniedaccompanied withwith aa signiﬁcant significant increase increase of of doublet doublet peak peak related related to to the the metallic metallic state state Ti Ti0(Zr (Zr)0) with with a a 0 0 BEBE of ofTi Ti2 p2p3/2/= = 454 454± ± 0.20.2 eVeV (BE(BE ofof ZrZr 3 3dd5/2/ = =178.9 178.9eV) eV) asas wellwell asas somesome contaminant-basedcontaminant-based phases phases peakspeaks suchsuch asas carbidescarbides TiC,TiC, TiC–OTiC–O (ZrC,(ZrC, ZrC–O)ZrC–O) [13[13,21–27].,21–27]. Thus,Thus, itit seemsseems thatthat thethe TiTi 2p2p (Zr(Zr 3d)3d) spectrumspectrum has has gradually gradually moved moved towards towards the the right, right, towards towards lower lower binding binding energies energies by comparison by comparison with thewith initial the initial spectrum spectrum position position of the oxidizedof the oxidized as-deposited as-deposited NEG. Otherwise,NEG. Otherwise, this implies this implies that the that initial the oxideinitial surface oxide layersurface of layer the as-deposited of the as-deposited getter was getter gradually was gradually reduced to reduced a metallic to statea metallic withincreasing state with thermalincreasing activation thermal temperatures activation temperatures [13,21–27]. [13,21–27].

Figure 3. Cont.

Micromachines 2018, 9, 490 8 of 20 Micromachines 2018, 9, x FOR PEER REVIEW 8 of 20

Figure 3. Evolution of XPS spectra after activation at various temperatures. (a,a’) Ti 2p and C 1s orbitals Figure 3. Evolution of XPS spectra after activation at various temperatures. (a,a’) Ti 2p and C 1s for the single layer Ti/Si NEG; (b,b’) Ti 2p and C 1s orbitals for the multilayer Ti/Ru/Si NEG; (c,c’) Zr orbitals for the single layer Ti/Si NEG; (b,b’) Ti 2p and C 1s orbitals for the multilayer Ti/Ru/Si NEG; 3d and C 1s orbitals for the multilayer Zr/Ti/Ru/Si NEG. (c,c’) Zr 3d and C 1s orbitals for the multilayer Zr/Ti/Ru/Si NEG.

Micromachines 2018, 9, 490 9 of 20

It is to be noted that the metal-hydroxide (Ti(OH)2, Zr(OH)2) XPS doublet peaks appeared after activating at 200 ◦C (see peak indexations on Figure3a–c). For the Ti upper-layer of Ti/Si and ◦ Ti/Ru/Si NEGs, the Ti 2p3/2 peak maximum level was reached also at 200 C, with almost a small shift of the Ti 2p spectrum to a higher bending energy (BE = 459.4 eV) [28,29]. Whereas for the Zr upper-layer of the multilayered Zr/Ti/Ru/Si NEG, the Zr 3d5/2 peak maximum level was reached at an activation temperature of 275 ◦C, with a substantial shift of the Zr 3d spectrum towards a higher BE = 183.4 eV [27,30,31]. The C 1s XPS spectrum evolution with thermal activation for all three NEGs also showed two different regimes (Figure3a’–c’) [ 13,21–27]. In the first regime, for the Ti upper-layer of the Ti/Si and Ti/Ru/Si NEGs, the main elementary surface-adsorbed C–C or C–H peak (BE = 285.3 eV) underwent a transformation to a metal-oxycarbide (TiC–O), accompanied with a metal-carbide (TiC) at an activation temperature of 300 ◦C. While for the Zr upper-layer of the multilayered Zr/Ti/Ru/Si NEG, only one direct transformation to a metal-carbide (ZrC) at 275 ◦C was observed. In the second regime, the only persisting peak of the carbide phase (BE = 281.6 eV) showed a slight intensity increase up to 450 ◦C for multilayered NEGs with a Ru sub-layer than for a single layer Ti NEG. For more additional details, the origin of the metal-hydroxide, carbide and oxycarbide bonds results probably from the dissociation of water molecules, O–H and C–H groups already adsorbed on the as-deposited NEG surface, or from vacuum chamber contaminants. It is important to note that the metal-hydroxide bond formation manifested at the first stage of heating, and its maximal concentration was accompanied by oxide reduction on the film surface as confirmed to happen around 250 ◦C in many reports in the literature [27,30,32]. As the hydroxide group elements were randomly distributed on the getter surface, M–HOx chemical-bond clusters formed through the hydrogen dissociation and migration into the getter sub-surface would generate an inhomogeneous oxygen-depleted region all around these clusters on the surface with different Tix+,y+<4 (Zrx+,y+<4) low oxidation states [27,30,32]. Regarding carbide/oxycarbide formation, the oxide reduction leading to metallic and suboxide states was a necessary condition. Thus, the temperature at which the carbide/oxycarbide starts to be formed correlates well with the metallic Ti0 (Zr0) and suboxide (Ti2+,3+, Zr1+,2+) state appearance [13,21–27]. Furthermore, the characteristic energy of these carbon-based compounds on the Ti 2p (Zr 3d) spectrum was very close to their corresponding metallic and suboxide states; such as the TiC (ZrC) bending energy superposes on that of the Ti0 (Zr0) metallic state, and the one for TiC–O x,y<4 x,y<4 (ZrC–O) superposes on that of Ti (Zr ) in TiOx,y<2 (ZrOx,y<2) suboxides [25,27]. In order to establish a comparison elucidating the performance related to each investigated getter structure, we plotted (Figure4) the intensities of the metallic and carbidic states evolution during the thermal activation. The metallic states generation on the getter surface enhanced the sorption properties, while the formation of carbide or other stable compounds reduced the getter activity by reducing the density of available metallic adsorption sites favorable to react with gas species on the surface. Hence, the distribution and evolution of the metallic and carbidic species on the getter surface gave insight into the gettering mechanism and performance evaluation after each activation process. Figure4a shows the comparison between the metallic state intensity variation and activation temperature. The metallic state (Ti0, Zr0) intensity was shown to be higher for getters with a structure using a Ru sub-layer, whereas the Zr/Ti/Ru/Si NEG presented better values compared to the Ti/Ru/Si NEG before activation at 400 ◦C. Furthermore, it was also seen that the activation of the Zr/Ti/Ru/Si NEG starts to be triggered at a temperature lower than the other getters. Meanwhile, the carbidic state intensity was found to be higher for the Zr/Ti/Ru/Si NEG compared to Ti-upper layer based NEGs without and with a Ru sub-layer (Figure4b). Also to be noticed is that the carbidic intensity level, for the getters of a structure with a Ru sub-layer, underwent an apparent continuous increase with activation temperature from its appearance in the second regime. Micromachines 2018, 9, 490 10 of 20 Micromachines 2018, 9, x FOR PEER REVIEW 10 of 20

Figure 4. Plots of XPS elemental intensities asas aa functionfunction ofof activationactivation temperature.temperature. ((aa)) ForFor TiTi00 andand ZrZr00 metallic states; (b)) forfor TiCTiC andand ZrCZrC carbidecarbide compounds.compounds.

From thethe above above analysis, analysis, two two key key conclusions conclusions can be can drawn. be drawn. Firstly, theFirstly, metal-hydroxide the metal-hydroxide (Ti(OH)2, Zr(OH)(Ti(OH)22), formation,Zr(OH)2) formation, which induces which a high induces getter a sub-surfacehigh getter reductionsub-surface at lowreduction activation at low temperatures activation ◦ ◦ locatedtemperatures between located 200 Cbetween and 275 200C, °C would and improve275 °C, would the getter improve surface the activity getter leadingsurface toactivity highsorption leading performance.to high sorption And secondly,performance. the surfaceAnd secondly, poisoning the effect surface by the poisoning adsorbed contaminanteffect by the species adsorbed (CO, COcontaminant2, C–C, H–C, species etc.) (CO, induces CO2, carbide C–C, H–C, phase etc.) (TiC, induces ZrC) carbide formation, phase which (TiC, reduces ZrC) formation, the metallic which site densityreduces and,the hence,metallic leads site todensity lower getterand, hence, surface le activityads to andlower sorption getter performance.surface activity and sorption performance. 3.3. Static Sorption Characterization of Non-Evaporable Getters 3.3. StaticSince Sorption the residual Characteri gaszation pressure of Non-Evaporable in a vacuum chamberGetters is very sensitive to the getter surface activity,Since therefore the residual to reveal gas the pressure activation in a effect vacuum on the chamber getter surface is very metallic sensitive state to evolution,the getter thesurface time dependenceactivity, therefore of the to getter reveal chamber the activation pressure effect (P–time on thecurve) getter surface was recorded metallic as state described evolution, in Section the time2. Basically,dependence after of baking-outthe getter chamber the whole pressure getter test(P–time system, curve) this was method recorded measures as described the pressure in Section change 2. inBasically, the getter after chamber baking-out after the activating whole getter the getter, test syst coolingem, this it tomethod room temperature,measures the isolatingpressure thechange getter in chamberthe getter from chamber the pumping after activating line, and the then getter, immediately cooling it admittingto room temperature, an amount ofisolating sorption the gas getter to a certainchamber pressure from the level pumping into the line, getter and chamber. then immediately However, admitting having a reference an amount test of to sorption which the gas effect to a ofcertain the getter pressure on the level vacuum into the pressure getter chamber. can be evidenced, However, the having vacuum a reference chamber test characteristics to which the withouteffect of the getter getter on were the alsovacuum measured. pressure Figure can be5 evidenced,shows pressure–time the vacuum dependency chamber characteristics curves, with without no getter the incorporatedgetter were also in the measured. chamber, Figure recorded 5 shows immediately pressure–time after admitting dependency N2 gas curves, into thewith UHV no sealedgetter chamber to different pressure levels such as 1 × 10−3, 1 × 10−4 and 1 × 10−5 Torr (corresponding, incorporated in the chamber, recorded immediately after admitting N2 gas into the UHV sealed × −3 × −4 × −5 respectively,chamber to todifferent1.33 10pressure, 1.33 levels10 suchand 1.33as 1 × 1010−3, mbar1 × 10 as−4 indicated and 1 × on10− the5 Torr plot) (corresponding, and closing the controllerrespectively, pressure to 1.33 leak × 10 valve−3, 1.33 (VLV). × 10 As−4 and can 1.33 be seen, × 10 no−5 mbar signiﬁcant as indicated change inon the the pressure plot) and was closing observed the × −3 × −4 forcontroller the chamber pressure sealed leak at valve higher (VLV). N2 gas As pressures can be seen, of 1 no 10significantTorr and chan 1 ge 10in theTorr. pressure However, was the sealed chamber with N gas admitted at 1 × 10−5 Torr showed ﬁrst a slight detectable gettering observed for the chamber sealed2 at higher N2 gas pressures of 1 × 10−3 Torr and 1 × 10−4 Torr. However, effect manifested as a small decrease in pressure lasting just for few minutes (~5 min), followed then by the sealed chamber with N2 gas admitted at 1 × 10−5 Torr showed first a slight detectable gettering aeffect gas releasemanifested effect as manifested a small decrease as a moderate in pressure pressure lasting increase just for with few time.minutes Such (~5 pressure min), followed evolution then is generallyby a gas release related effect to factors manifested affecting as thea moderate vacuum pres chambersure increase stability with like gas time. adsorption Such pressure by the evolution chamber wallsis generally that reduces related the to pressure factors ataffecting the beginning the vacuum of chamber chamber sealing, stability and wallslike gas outgassing adsorption with by other the chamber partswalls that that lead reduces later the to a pressure pressure increaseat the beginning when the of wall chamber sorption sealing, rate becomes and walls less outgassing dominant. Itwith can other be deduced chamber that parts the static that lead sorption later characteristics to a pressure performedincrease when in a vacuumthe wall chamber sorption admitted rate becomes with × −5 Nless2 gas dominant. limited toIt acan pressure be deduced of 1 10 that Torrthe wouldstatic sorption be useful characteristics and informative performed for getter in performance a vacuum characterization. In fact, such a low pressure appeases the fast gettering saturation of the getter by chamber admitted with N2 gas limited to a pressure of 1 × 10−5 Torr would be useful and informative lastingfor getter the performance sorption measurement characterization. with time, In fact, and su thisch isa practicallow pressure for the appeases analyses the of fast the getteringgettering processsaturation behavior. of the getter Therefore, by lasting any difference the sorption in the me getterasurement sorption with characteristics time, and this to is the practical reference for testthe willanalyses be systemically of the gettering related toprocess the getter behavior. effect. InTher addition,efore, itany is worthwhile difference toin note the that getter the referencesorption characteristics to the reference test will be systemically related to the getter effect. In addition, it is worthwhile to note that the reference test pressure level and pressure evolution tendency were not

Micromachines 2018, 9, 490 11 of 20 Micromachines 2018, 9, x FOR PEER REVIEW 11 of 20 testchanged pressure after level getter-chamber and pressure evolutionheating at tendency differen weret temperatures not changed up after to getter-chamber450 °C; using heatingthe same at differentannealing temperatures conditions as up in to the 450 getter◦C; usingactivation. the same annealing conditions as in the getter activation.

N admission to 1.33x10-3 mbar 2 10-3 N admission to 1.33x10-4 mbar 2 10-4

10-5 N admission to 1.33x10-5 mbar 2 10-6

10-7 Getter pressure (mbar) chamber

10-8 0.5 1.0 1.5 2.0 Time (hour)

FigureFigure 5.5. VariationVariation ofof vacuumvacuum chamberchamber pressurepressure withoutwithout thethe gettergetter samplesample (reference(reference test)test) forfor NN22 gasgas admittedadmitted intointo thethe chamberchamber toto differentdifferent pressurepressure levels.levels.

TheThe pressure–versus–timepressure–versus–time curvescurves afterafter eacheach activationactivation temperaturetemperature forfor thethe threethree investigatedinvestigated gettersgetters are shown shown in in Figure Figure 6a–c.6a–c. These These curves curves were were recorded recorded immediately immediately after after closing closing the leak the valve leak valve (VLV) with which N gas was admitted into the UHV sealed getter chamber to a pressure of (VLV) with which N2 gas was2 admitted into the UHV sealed getter chamber to a pressure of 1 × 10−5 1 × 10−5 Torr. It can be seen that all getters with different structures exhibit a similar N sorption Torr. It can be seen that all getters with different structures exhibit a similar N2 sorption 2trend. The trend.curves The indicate curves that indicate the getter that chamber the getter pressure chamber decreased pressure quickly decreased and quickly reached and a kinetic reached equilibrium a kinetic equilibriumcorresponding corresponding to the minimum to the pressure minimum within pressure a peri withinod of atime; period varying of time; from varying 0.5 to from1.5 h depending 0.5 to 1.5 h dependingon the thermal on the activation thermal activation temperature temperature value. value.After this After equilibrium, this equilibrium, the pressure the pressure began began to toprogressively progressively increase increase with with measurement measurement time. time. Such Such behavior behavior could could be be mainly mainly attributed to thethe vacuumvacuum chamberchamber walls walls and and the the sample sample holder holder outgassing, outgassing, whose whose ﬂow flow rate rate became became higher higher than than the rate the ofrate the of gettering the gettering process. process. It was It also was seen also that seen in thethat early in the stage early of thestage pressure–time of the pressure–time records as records the rate ofas thethe pressurerate of the decreasing pressure decreasing was higher was as the higher reached as the equilibrium reached equilibrium pressure level pressure was minimal level was (minimum minimal pressure(minimum value pressure on the value curve). on the This curve). minimum This levelminimum was reached level was for, reached both, the for, multilayered both, the multilayered Ti/Ru/Si ◦ andTi/Ru/Si the Zr/Ti/Ru/Si and the Zr/Ti/Ru/Si NEGs at theNEGs activation at the activati temperatureon temperature of 250 C, exceptof 250 for °C, the except single-layered for the single- Ti/Si ◦ NEGlayered sample Ti/Si NEG with whichsample the with minimum which the pressure minimum level pressure happened level after happened activating after at activating 200 C. Above at 200 those°C. Above activating those temperatures, activating temperatures, the equilibrium the equilib pressurerium levels pressure increased levels and, increased again after and, reaching again after an ◦ activatingreaching an temperature activating temperature range around range 375 aroundC, the pressure375 °C, the level pressure slightly level re-decreased, slightly re-decreased, but effectively but onlyeffectively for the only multilayered for the multilayered getters with getters Ru sub-layer. with Ru Furthermore, sub-layer. Furthermore, such structured such getters structured containing getters a ◦ Rucontaining seed sub-layer a Ru seed (Ti/Ru/Si sub-layer and (Ti/Ru/Si Zr/Ti/Ru/Si and Zr/Ti/Ru/Si NEGs) at higher NEGs) activation at higher temperatures activation temperatures (425 C and ◦ 450(425 °CC) manifestedand 450 °C) amanifested notable resistance a notable to resistance the outgassing to the outgassing effect by lowering effect by the lowering pressure the level pressure as is seenlevel atas the is seen right at part the ofright the part curves of the at around curves 3at h around of pressure 3 h of recording pressure(see recording Figure 6(seeb,c). Figure 6b,c).

Micromachines 2018, 9, 490 12 of 20 Micromachines 2018, 9, x FOR PEER REVIEW 12 of 20

(a) Ti/Si getter structure substrate -7 -7 o 8x10 6x10 400 C as-received o 4x10-7 200 C -7 6x10 250oC Getter chamber pressure (mbar) pressure chamber Getter 2.90 2.95 3.00 o Time (hour) 275 C 300oC -7 4x10 350oC 375oC 400oC 425oC 450oC 2x10-7 Getter chamber pressure (mbar) 0.5 1.0 1.5 2.0 2.5 3.0 Time (hour)

(b) Ti/Ru/Si getter structure substrate 6x10-6 -6 5x10 as-received -6 4x10 200oC 250oC -6 3x10 275oC 300oC o 2x10-6 325 C 350oC o -6 5x10 o 375 C 375 C o -6 400 C 4x10 o -6 425 C 10 o 3x10-6 450 C Getter chamber pressure (mbar) pressure chamber Getter

Getter chamber pressure (mbar) pressure Getter chamber 2.75 Time (hour) 0.5 1.0 1.5 2.0 2.5 3.0 Time (hour)

-6 -7 10 8x10 o as-received 375 C o 8x10-7 6x10-7 200 C 250oC o

Getterchamber pressure (mabar) 275 C 2.70 2.75 2.80 2.85 2.90 -7 o 6x10 Time (hour) 300 C 325oC 350oC 375oC -7 4x10 400oC 425oC 450oC Getter chamber pressure (mabar) 0.5 1.0 1.5 2.0 2.5 3.0 Time (hour)

FigureFigure 6. 6.Variation Variation of of the the getter getter chamber chamberpressures pressures after different activation activation temperatures. temperatures. (a) ( aSingle) Single layerlayer Ti/Si Ti/Si NEG; NEG; (b ()b multilayer) multilayer Ti/Ru/Si Ti/Ru/Si NEGNEG andand ((cc)) multilayermultilayer Zr/Ti/Ru/Si NEG. NEG. Insets Insets are are detailed detailed viewsviews of of the the magniﬁed magnified zone zone of of pressure pressure evolution evolution inin thethe right side of of the the P–time P–time curves. curves.

Micromachines 2018, 9, 490 13 of 20

From the getter effect on the pressure change during the thermal activation process and the temperature in situ XPS analysis of the getter surfaces, the equilibrium pressure minimum levels measured at an activation temperature of 250 ◦C correspond to special changes of the getter surface chemical properties as already probed by the XPS spectra at around 275 ◦C in our investigated samples and 250 ◦C in the literature [27,30,32]. For this temperature range, the change in the getter surface chemical states during the activation was demonstrated to be related to hydroxide and hydrocarbon dissociations and diffusion into the getter surface, accompanied simultaneously with oxygen bulk diffusion primed through the reduction of the passive oxide layer. Generally, this described mechanism leads to a fresh getter surface manifesting more metallic sites favorable for more N2 sorption and pressure lowering. However, in the subsequent activation temperature of 300 ◦C, the diffused carbon in the getter sub-surface and probably also the nitrogen stuck on the getter surface from the previous sorption test, react with the appeared metallic Ti0 (Zr0) elements in forming carbide and nitride compounds. Thus, leading to a lowered number of metallic sites on the getter surface that consequently weakens N2 sorption by manifesting a low rate of pressure decrease and a rise in the equilibrium pressure level. For a further activation temperature increase, it was seen that the pressure decrease rate continued to become slower and the equilibrium pressure level also continued to increase. This behavior is presumably due to an important increase to the poisoned metal sites (carbide/nitride) on the getter sub-surface, despite the metallic peak (Ti0, Zr0) signal increase as probed in the XPS profiles with an increasing of the activation temperature (Figure4a). After activating at 375 ◦C and above, the equilibrium pressure levels and the right side of the P–time curves underwent just a little decrease indicating that all investigated getters were not completely activated as was proved by the incomplete disappearance of the XPS peaks related to the metal suboxides (Tix+<4, Zrx+<4) and the C 1s orbital. To get more information about the mechanisms involved in the nitrogen sorption kinetics by our investigated getters, the nitrogen sorption reaction mechanism was proposed by comparing the sorption rate characteristics with the help of Hirooka’s kinetic theory [33]. The reaction rate in this method can be expressed by the rate of gas pressure change due to the sorption:

dP − = k ·P − P (2) dt a t eq where ka is the sorption rate constant, supposedly independent of pressure; Pt is the getter chamber pressure at an arbitrary time; and Peq is the pressure at the final equilibrium state. The solution of Equation (2) allows us to extract the ka from the slope of the straight line expressed by Equation (3): " # Pt − Peq ln = −ka· t (3) P0 − Peq where P0 is the initial pressure. The typical plots of ln[(Pt − Peq)/(P0 − Peq)] versus reaction time, from which the nitrogen sorption reaction rate constant characteristics (ka1, ka2) can be extracted, are depicted in Figure7a–c. It was observed that for the Ti upper-layer based getter without a Ru sub-layer (Ti/Si NEG), the curves can fit into two different linear segments characterized by two different slopes corresponding, respectively, to a rapid sorption stage with ka1 constant and a stable sorption one with ka2 constant. Whereas, for the Ti and Zr upper-layer based getters with a Ru sub-layer (Ti/Ru/Si and Zr/Ti/Ru/Si NEGs), the curves fit into almost only one aligned linear segment including the rapid and stable sorption stages with approximately equal slopes ka1 ≈ ka2. The reaction rate constants (ka1, ka2), calculated after each activated temperature for all studied ◦ getters, are shown in Figure8a,b. The ka1 constants demonstrate an increase until the 250 C activating ◦ temperature (except Ti/Si NEG with ka1 max at an activation temperature of 200 C) followed by a subsequent decrease with increasing activation temperatures. The origin of such behavior has already been discussed above in the sense that a higher rate constant was found to be related to getter surface cleaning (NEG sub-surface reduction by metal-hydroxide formation) and its drop in Micromachines 2018, 9, 490 14 of 20

Micromachines 2018, 9, x FOR PEER REVIEW 14 of 20 carbide/nitride sites formation. Regarding the rate constant ka2, whose effect was considerable only inwithout the getters a Ru without sub-layer, a Ru their sub-layer, values theirdid not values undergo did not any undergo substantial any change substantial with changeactivation with activationtemperature. temperature.

(a) Ti/Si getter structure 1 substrate as-received 350 oC 0 200 oC 375 oC o o )] -1 250 C 400 C eq 275 oC 425 oC o o

-P -2 300 C 450 C

o k a1

P -3 ( / ) -4 eq -5 -P t

P -6 [( k -7 a2 Ln -8 -9 10000 1200200 1400400 1600600 1800800 20001000 22001200 Time (sec)

(b) Ti/Ru/Si getter structure substrate as-received o 200 oC 350 C )] 0 o 250 oC 375 C eq k o a1 275 oC 400 C -P o 425 oC o -2 300 C o o

P 325 C 450 C ( / )

eq -4 -P t k a2 P

[( -6 Ln

-8

0 200 400 600 800 1000 1200 Time (sec)

-P 300 C 425 C

o -2 k o o a1 325 C 450 C P ( / )

eq -4 -P t k P a2 [( -6 Ln

-8 0 300 600 900 1200 Time (sec)

FigureFigure 7. Kinetic7. Kinetic curves curves for for nitrogen nitrogen sorption sorption afterafter different activation temperatures; temperatures; (a ()a single) single layer layer Ti/SiTi/Si NEG; NEG; (b ()b multilayer) multilayer Ti/Ru/Si Ti/Ru/Si NEG NEG and and (c (c) )multilayer multilayer Zr/Ti/Ru/Si Zr/Ti/Ru/Si NEG. NEG.

Micromachines 2018, 9, 490 15 of 20

Micromachines 2018, 9, x FOR PEER REVIEW 15 of 20 As the variation in the slope of the Equation (3) plot, during the nitrogen sorption, is related to As the variation in the slope of the Equation (3) plot, during the nitrogen sorption, is related to the rate controlled sorption mechanism change. Consequently, for the Ti upper-layer based getter the rate controlled sorption mechanism change. Consequently, for the Ti upper-layer based getter without a Ru sub-layer (Ti/Si NEG), the high reaction rate (ka1) could be associated with the N2 without a Ru sub-layer (Ti/Si NEG), the high reaction rate (ka1) could be associated with the N2 adsorption on the getter surface and the lower reaction rate (ka2) could be associated to the absorption adsorption on the getter surface and the lower reaction rate (ka2) could be associated to the absorption by diffusion of N2 into the getter bulk, which took over to control the sorption process when the by diffusion of N2 into the getter bulk, which took over to control the sorption process when the surface was saturated [34–36]. Whereas for the Ti and Zr upper-layer based getters with a Ru surface was saturated [34–36]. Whereas for the Ti and Zr upper-layer based getters with a Ru sub- sub-layer (Ti/Ru/Si and Zr/Ti/Ru/Si NEGs), the manifestation of only one slope (ka1 ≈ ka2) in layer (Ti/Ru/Si and Zr/Ti/Ru/Si NEGs), the manifestation of only one slope (ka1 ≈ ka2) in the nitrogen the nitrogen sorption curve predicts that the sorption process was controlled simultaneously by N2 sorption curve predicts that the sorption process was controlled simultaneously by N2 adsorption adsorptionand diffusion. and diffusion.From the From above the analysis above analysis and predicted and predicted hypotheses, hypotheses, the thedifference difference in in sorption sorption mechanismsmechanisms demonstrateddemonstrated byby thethe investigatedinvestigated getters can be be assumed assumed to to be be specifically specifically due due to to the the gettergetter microstructure, microstructure, which which was was engineered engineered in in a highlya highly leafleted leafleted columnar columnar and and porous porous structure structure with awith highly a highly rough rough surface surface morphology morphology when when the getter the getter structure structure comprised comprised a Ru a metalRu metal as a as thin a thin film sub-layer.film sub-layer. Therefore, Therefore, it would it would be expected be expected for the for getters the getters presenting presenting only only one sorptionone sorption stage stage in their in sorptiontheir sorption kinetic kinetic curves curves to show to show better better sorption sorption performance performance properties. properties.

FigureFigure 8. 8.Nitrogen Nitrogen sorptionsorption raterate constants,constants, kkaa, atat differentdifferent activationactivation temperatures. ( (aa)) Reaction Reaction rate rate

constant,constant,k ka1a1,, inin thethe rapidrapid sorptionsorption stage; ( (b) reaction reaction rate rate constant, constant, kka2a2, ,in in the the stable stable sorption sorption stage. stage. 3.4. Dynamic Sorption Characterization of Non-Evaporable Getters 3.4. Dynamic Sorption Characterization of Non-Evaporable Getters InIn the the following, following, the the sorption sorption performance performance characterizations characterizations of theof the three three investigated investigated NEGs NEGs were measuredwere measured and compared. and compared. Figure 9Figurea shows 9a theshows sorption the sorp speedtion as speed a function as a function of sorption of sorption capacity capacity at room ◦ temperatureat room temperature for Ti/Si, for Ti/Ru/Si Ti/Si, Ti/Ru/Si and Zr/Ti/Ru/Si and Zr/Ti/ gettersRu/Si getters activated activated at a temperature at a temperature of 300 ofC 300 for °C 1 h. Itfor can 1 beh. seenIt can that be seen the sorption that the speedsorption for speed the single for the layer single Ti/Si layer NEG Ti/Si decreases NEG decreases rapidly and rapidly ends and with −6 2 aends low sorptionwith a low capacity sorption at around capacity 1 × at10 aroundmbar 1·L/cm × 10−6. mbar·L/cm While for the2. While Ti/Ru/Si for the and Ti/Ru/Si Zr/Ti/Ru/Si and multilayeredZr/Ti/Ru/Si multilayered NEGs, the sorption NEGs, speedsthe sorption were higherspeeds andwere manifest higher and as a manifest slightly inclined as a slightly plateau inclined shape −5 2 andplateau diminish shape at and a sorption diminish capacity at a sorption on the capacity order of on 1 the× 10 ordermbar of 1· ×L/cm 10−5 mbar·L/cm. For a higher2. For activation a higher ◦ temperatureactivation temperature of 425 C (Figure of 4259 b),°C the(Figure three 9b), getters the exhibitthree getters together exhibit a plateau together having a plateau approximately having theapproximately same sorption the speed same evolution. sorption However, speed evolutio the multilayeredn. However, Ti/Ru/Si the andmultilayered Zr/Ti/Ru/Si Ti/Ru/Si NEGs haveand demonstratedZr/Ti/Ru/Si NEGs a sorption have demonstrated capacity roughly a sorption four times capacity higher roughly than the four one times measured higher with than the the single one layermeasured Ti/Si NEG.with Thethe lowsingle measured layer Ti/Si sorption NEG. properties, The low previouslymeasured exhibitedsorption byproperties, the three getterspreviously after ◦ theexhibited activation by temperaturethe three getters of 300 afterC the (see activation Figure9a), te weremperature directly of corroborated 300 °C (see Figure by the 9a), diminution were directly of the reactioncorroborated rate constant by the diminution (Figure8a) of measured the reaction from rate the constant nitrogen (Figure pressure–time 8a) measured evolution. from Thethe nitrogen sorption weakening,pressure–time in this evolution. activation The temperature sorption weakening, range, can beinaccounted this activa fortion by temperature the getter surface range, poisoning can be effectaccounted that results, for by duringthe getter the surface activation poisoning process, effect in reacting that results, contaminating during the gaseous activation impurities process, (CO, in COreacting2, C–C, contaminating H–C, N2, etc.) gaseous adsorbed impurities on the getter (CO, surfaceCO2, C–C, with H–C, getter N2, metal. etc.) adsorbed Thus, leading on the to getter metal carbidesurface (probably with getter also metal. nitride) Thus, species leading formation to metal oncarbide the getter (probably surface also as nitride) determined species by formation XPS analyses. on Thisthe poisoninggetter surface process as determined can spread by out XPS in a analyses. thin sorption This blockingpoisoning layer process leading can tospread a sorption out in decrease a thin andsorption even theblocking degradation layer leading of the gettering to a sorption ability. decrease and even the degradation of the gettering ability.

Micromachines 2018, 9, 490 16 of 20 Micromachines 2018, 9, x FOR PEER REVIEW 16 of 20

FigureFigure 9.9.Sorption Sorption property property comparisons comparisons of single-layered of single-layered and multilayered and multilayered NEGs. (aNEGs.) NEGs ( activateda) NEGs ◦ ◦ atactivated 300 C; at (b )300 NEGs °C; ( activatedb) NEGs atactivated 425 C. at 425 °C.

TheThe poisoningpoisoning phenomenon that hinders the gettering activity was revealed by the twotwo aboveabove static sorption (P–time) and XPS analyses, to be primed just after the thermal activation over 250 ◦C static sorption (P–time) and XPS analyses, to be primed just after the thermal activation◦ over 250 °C andand was diminished by by raising raising the the activation activation to to hi highgh temperatures temperatures over over 350 350 °C. FigureC. Figure 10 shows10 shows the thenitrogen nitrogen sorption sorption property property evolution evolution at room at room temperature temperature for the for Zr/Ti/Ru/Si the Zr/Ti/Ru/Si multilayered multilayered getter getterafter being after activated being activated at different at different temperatures temperatures for 1 h. for It can 1 h. be It canseen be that seen for that the foractivation the activation at 250 °C, at ◦ 250the resultantC, the resultant sorption sorptioncharacteristic characteristic curve has curve shifte hasd to shifted a higher to sorption a higher capacity sorption value, capacity while value, the ◦ whileactivation the at activation a higher attemperature a higher temperatureof 300 °C made of 300the sorptionC made characteristic the sorption curve characteristic shift to a curvelower ◦ ◦ shiftrange. to Furthermore, a lower range. for activations Furthermore, at higher for activations temperatures, at higher 375 °C temperatures, and 425 °C, the 375 sorptionC and capacity 425 C, thelimit sorption seemed capacity to stabilize limit seemed in between. to stabilize The inlowering between. of The the lowering sorption of thecapacity sorption for capacity activation for ◦ activationtemperatures temperatures over 250 over°C was 250 explainedC was explained above to above be due to be to due the topoisoning the poisoning effect. effect. However, However, the ◦ thepronounced pronounced sorption sorption capacity, capacity, observed observed at atan an activation activation temperature temperature of of 250 250 °C,C, was effectivelyeffectively associatedassociated with metal-hydroxidemetal-hydroxide cluster formation on thethe gettergetter upperupper surface.surface. The gettergetter surfacesurface ◦ statestate transformation, while while activating activating at at 250 250 °C,C, wa wass elucidated. elucidated. During During the theactivation activation process, process, the thegaseous gaseous impurities impurities (O–H (O–H and andC–H C–H groups), groups), already already adsorbed adsorbed on the on getter the getter surface, surface, dissociated dissociated and andresulted resulted in hydrogen in hydrogen internal internal diffusion. diffusion. This loca Thislly locally diffused diffused hydrogen hydrogen reacts reacts in the in passive the passive layer layeroxide oxideby forming by forming a distributed a distributed metal-hydroxide metal-hydroxide cluster cluster in the in getter the getter sub-surface sub-surface [30–32], [30 –thus32], thusinducing inducing an important an important reduction reduction of the of passivation the passivation oxide oxide layer layer by establishing by establishing a high a high coexistence coexistence of ofsub-oxidized sub-oxidized and and metallic metallic states; states; a aconfiguration conﬁguration wh whichich is is very very favorable favorable for for high high gettering gettering activity. activity. ◦ TheThe reestablishmentreestablishment of of the the sorption sorption capacity, capacity, after after activating activating at at 375 375C °C and and above, above, is due,is due, in in addition addition of oxideof oxide dissolution/diffusion, dissolution/diffusion, to decomposition to decomposition and diffusion and diffus of poisoningion of poisoning compounds, compounds, which probably which concernedprobably concerned only the metal-hydroxides only the metal-hydroxides not including not carbidesincluding as carbides the C 1s peakas the did C not1s peak undergo did anynot considerableundergo any decreaseconsiderable in this decrease temperature in this rangetemperat (Figureure range4b). Thereby (Figure the4b). getter Thereby surface the getter activity surface was re-improved.activity was re-improved. However, in spiteHowever, of the in obtained spite of betterthe obtained sorption better properties sorption at a properties low temperature at a low of ◦ 250temperatureC, a question of 250 is still°C, a open question whether is still or not open to exploit whether the or getter not activationto exploit atthe a highergetter temperatureactivation at ofa ◦ 375higherC. temperature In fact, this temperatureof 375 °C. In fact, is also this advantageous; temperature is italso is relatedadvantageous; to an effective it is related dissolution to an effective of the passivationdissolution of layer the compoundspassivation intolayer the compounds seeping microstructure, into the seeping engendered microstructure, in the engendered getter-bulk byin thethe appropriategetter-bulk by additional the appropriate Ru sub-layer additional in the Ru getter sub-laye structure.r in the Furthermore,getter structure. it isFurthermore, also included it is in also the temperatureincluded in the range temperature compatible range with compat the MEMSible with sealing the process. MEMS sealing process. ItIt isis to to be be noted noted that that in the in getter the getter ﬁlm patterning film patterning processes, processes, the use of the organic use solutionof organic components solution cancomponents degrade can the getterdegrade sorption the getter performance sorption performance or even lead or to even sorption lead to failure. sorption Given failure. that Given the getter that metalthe getter surface metal is very surface sensitive is very to anysensitive external to chemicalany external compounds chemical coming compounds from photoresist coming from and solventphotoresist products and solvent used in theproducts photoresist used depositionin the phot andoresist removal, deposition the interaction and removal, of these the elementsinteraction with of thethese getter elements would with deteriorate the getter the would getter deteriorate surface morphology the getter surface stability morphology and sorption stability ability. and In addition,sorption asability. most In metals addition, are hard as most to etch metals by dry are plasma hard etching,to etch theby dry MEMS plasma patterned etching, getters the canMEMS beperformed patterned easilygetters using can be a physical performed shadow easily mask, using but a thephysical patterning shadow size mask, of this but technique the patterning is limited size only of in this the technique is limited only in the range of the micrometer-scale. However, in the case of lateral getter film patterning, using lift-off or other standard lithography methods, a post wet cleaning step

Micromachines 2018, 9, 490 17 of 20

Micromachinesrange of the 2018 micrometer-scale., 9, x FOR PEER REVIEW However, in the case of lateral getter ﬁlm patterning, using lift-off 17 of 20 or other standard lithography methods, a post wet cleaning step restoring the getter performance would restoringbe highly the recommended. getter performance To this would end, anbe investigational highly recommended. study is To needed this end, to select an investigational the appropriate studycleaning is needed solution, to select which the canappropriate interfere cleaning with the solution, used speciﬁc which metalcan interfere of the getterwith the in used order specific to both metalpre-activate of the getter the getter in order surface to both by pre-activate removing the the oxides getter andsurface to clean by removing the getter the surface oxides by and eliminating to clean thethe getter contaminants. surface by eliminating the contaminants.

0 10 o Zr/Ti/Ru/Si getter structure Activation at T= 250 C Activation at T= 300oC ) 2 Activation at T= 375oC 10-1 Activation at T= 425oC

10-2

10-3 Sorption speed (L/s.cm speed Sorption

10-4 10-7 10-6 10-5 10-4 Sorption capacity (mbar.L/cm2)

Figure 10. Sorption characteristics of the multilayered Zr/Ti/Ru/Si NEG after different activation Figure 10. Sorption characteristics of the multilayered Zr/Ti/Ru/Si NEG after different activation temperatures for one hour. temperatures for one hour. 4. Conclusions 4. Conclusions Non-evaporable getters structured in single and multilayered thin films such as Ti/Si, Ti/Ru/Si andNon-evaporable Zr/Ti/Ru/Si were getters deposited structured and analyzed in single in and terms multilayered of microstructure thin films and such morphology as Ti/Si, formation,Ti/Ru/Si andactivation Zr/Ti/Ru/Si mechanism, were deposited and sorption and analyzed performances. in terms The of insertionmicrostructure of Ru asand a seedmorphology sub-layer formation, has shown, activationin term of mechanism, microstructure and growth,sorption a performances. significant improvement The insertion to theof Ru microstructure as a seed sub-layer modification has shown, in the infine term vertical of microstructure leafleted texture growth, with a roughsignificant and porousimprovement morphology to the of microstructure the upper getter modification surfaces. It wasin thealso fine found vertical that leafleted NEGs with texture a Ru with seed a sub-layerrough and exhibited porous morphology lower triggering of the activation upper getter temperatures surfaces. Itthan was unseeded also found ones. that The NEGs temperature with a values Ru seed extracted sub-layer from theexhibited right side lower of P–time triggering curves activation at the end temperaturesof tests, just atthan the unseeded sudden jump ones. after The the temperature second downward values extracted pressure levelfrom dropthe right as seen side by of the P–time broken curvesarrows at inthe the end magnified of tests, just zone at inthe the sudden insets jump of Figure after6 a–c,the second were 400 downward◦C for a pressure single layer level Ti/Si drop NEG as seenand by ~375 the ◦brokenC for both arrows the layeredin the magnified Ti/Ru/Si zone and Zr/Ti/Ru/Siin the insets of NEGs. Figure These 6a–c, temperatureswere 400 °C for were a single a little layerhigher Ti/Si than NEG the and temperatures ~375 °C for relatedboth the to layered the start Ti of/Ru/Si the oxideand Zr/Ti/Ru/Si reduction NEGs. as probed These by temperatures the increase of were a little higher0 than0 the temperatures related to the start of the oxide reduction as probed by the metallic states (Ti , Zr ) of Ti 2p3/2 and Zr 3d5/2 XPS peak intensities (Figure4a) and a decrease of the 0 0 increaseO 1s XPS of profilemetallic intensity states (Ti (not, Zr shown) of here).Ti 2p/ Such and behavior Zr 3d/ can XPS be explainedpeak intensities by the (Figure pressure 4a) sensitivity and a decreaseto the poisoning of the O 1s effect, XPS which profile takes intensity over by(not reducing shown here). the getter Such surface behavior active can site be density.explained Moreover, by the pressureit is to be sensitivity noted that to thethe increasepoisoning of effect, the XPS which metallic takes signal over by was reducing also influenced the getter by surface the metal-carbide active site density.contribution Moreover, since it this is to one be noted manifests that the almost increase at the of same the XPS bending metallic energy signal as was the also one influenced of the metallic by thestate. metal-carbide Hence, the contribution consideration since of activationthis one manife temperaturests almost from at the pressure same bending evolution energy measurements as the one is ofmore the metallic credible state. than fromHence, the the XPS consideration data analyses of that activation need high temperature skills and efficientfrom pressure software evolution tools for measurementsspectral deconvolution. is more credible than from the XPS data analyses that need high skills and efficient softwareFor tools the for analyses spectral in deconvolution. terms of activation mechanism, from the correlation between the XPS analysesFor the and analyses the sorption in terms pressure of activation evolution, mechanism, the getter activation from the passes correlation through between three stages;the XPS at a analysesmoderate and temperature the sorption until pressure 250 ◦ evolution,C, a formation the ge oftter metal-hydroxide activation passes clusters through just three under stages; the upper at a moderategetter sub-surface temperature allows until higher250 °C, sub-oxide a formation and of metallic metal-hydroxide state sites clusters creation, just which under results the upper in the getter sub-surface allows higher sub-oxide and metallic state sites creation, which results in the higher sorption rate observed in the pressure–time curves. Then at around 275 °C and up to 375 °C, the formation of a carbide species takes over, which reduces considerably the metallic sites on the getter surface. This results in lower sorption rates and an upward shift of the pressure–time curves.

Micromachines 2018, 9, 490 18 of 20 higher sorption rate observed in the pressure–time curves. Then at around 275 ◦C and up to 375 ◦C, the formation of a carbide species takes over, which reduces considerably the metallic sites on the getter surface. This results in lower sorption rates and an upward shift of the pressure–time curves. Finally, after 375 ◦C and up to 450 ◦C, despite the probed oxide dissolution and diffusion into the getter bulk, as confirmed by the continuous increase of the Ti0 and Zr0 peaks intensities in the Ti 2p and Zr 3d XPS profiles, the carbide species did not undergo any significant decomposition and diffusion as indicated by the slight variation of the C 1s XPS profile. Such behavior was physically manifested by a slight downward re-shift of the pressure–time curves, indicating that the getter surface was refreshed but the activation process was not completely achieved. To avoid the carbon poisoning effect, a covering of the getter upper surface by a thin protective layer of Cr, Ni, Pd or Pd–Ag alloy deposited in situ during the getter process would be of great interest for activating the getter at a lower temperature, less than the 375 ◦C already determined for the layered getters with Ru as a sub-layer. In addition, from the analyses of the pressure–time characteristics plotted according to Hirooka’s model, it was demonstrated that with a Ru sub-layer in the multilayered NEG structure, the gettering mechanism was controlled simultaneously by gas adsorption and diffusion processes, while a single NEG system without a Ru sub-layer manifested a gettering behavior evolving into two sequentially steps; the gas adsorption process first, when the surface sorption is dominant, followed by the gas diffusion process when the bulk sorption became predominant with a diffusion rate lower than the adsorption rate. Finally, regarding the multilayered NEGs containing a Ru sub-layer, getter sorption performances were found to be directly related to the sorption rate constants determined from the static sorption-activation temperature dependence measurements. As can be concluded, the getter sorption performance can be predicted to be maximal at an activation temperature of 250 ◦C, feeble for temperatures located in the interval 250 ◦C < T < 375 ◦C, and satisfying for temperatures from 375 ◦C and above. Also, as found in this study, the modulation of the activation temperature by an appropriate sub-layer addition would allow performing in situ NEG activation while MEMS packaging with a temperature adequate and compatible for the MEMS cavity sealing process parameters.

Author Contributions: E.-M.B. and H.Y.K. selected the investigated materials and conceived the sample structures. E.-M.B. and N.-K.C. designed the experiments and set up the experimental tools. E.-M.B. performed the experiments and analyzed the data; E.-M.B. and H.Y.K. prepared the manuscript. Acknowledgments: Authors would like to thank the National Nanofab Center engineers in the department of nanostructure development for thin film deposition and sample preparation. This research was supported by the Nano-Material Technology Development Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (2015M3A7B7044730). Conflicts of Interest: The authors declare no conflicts of interest.

References

1. Ramesham, R.; Kullberg, R.C. Review of vacuum packaging and maintenance of MEMS and the use of getters therein. J. Micro/Nanolithogr. MEMS MOEMS 2009, 8, 031307. [CrossRef] 2. Jin, Y.F.; Wang, Z.F.; Zhao, L.; Lim, P.C.; Wei, J.; Wong, C.K. Zr/V/Fe thick film for vacuum packaging of MEMS. J. Micromech. Microeng. 2004, 14, 687–692. [CrossRef] 3. Ferrario, B. Foundations of Vacuum Science and Technology; Lafferty, J.M., Ed.; John Wiley and Sons Inc.: New York, NY, USA, 1998; Chapter 5, Part 1; ISBN 0-471-17593-5. 4. Ferrario, B. Chemical pumping in vacuum technology. Vacuum 1996, 47, 363–370. [CrossRef] 5. Benvenuti, C.; Chiggiato, P.; Pinto, P.C.; Santana, A.E.; Hedley, T.; Mongelluzzo, A.; Ruzinov, V.; Wevers, I. Vacuum properties of TiZrV non-evaporable getter films. Vacuum 2001, 60, 57–65. [CrossRef] 6. Li, C.-C.; Huang, J.-L.; Lin, R.-J.; Chang, H.-K.; Ting, J.-M. Fabrication and characterization of non-evaporable porous getter films. Surf. Coat. Technol. 2005, 200, 1351–1355. [CrossRef] 7. Vivek, C.; Xie, L.; Chen, B.T. Titanium-based getter solution for Wafer-Level MEMS vacuum packaging. J. Electron. Mater. 2013, 42, 485–491. [CrossRef] Micromachines 2018, 9, 490 19 of 20

8. Hilton, A.; Temple, D.S. Wafer-Level Vacuum Packaging of Smart Sensors. Sensors 2016, 16, 1819. [CrossRef] [PubMed] 9. Benvenuti, C.; Chiggiato, P.; Mongelluzzo, A.; Prodromides, A.; Ruzinov, V.; Scheuerlein, C.; Taborelli, M. Influence of the elemental composition and crystal structure on the vacuum properties of Ti-Zr-V nonevaporable getter films. J. Vac. Sci. Technol. A 2001, 19, 2925–2930. [CrossRef] 10. Prodromides, A.E.; Scheuerlein, C.; Taborelli, M. Lowering the activation temperature of TiZrV non-evaporable getter films. Vacuum 2001, 60, 35–41. [CrossRef] 11. Chiggiato, P.; Pinto, P.C. Ti–Zr–V non-evaporable getter films: From development to large scale production for the Large Hadron Collider. Thin Solid Films 2006, 515, 382–388. [CrossRef] 12. Li, C.-C.; Huang, J.-L.; Lin, R.-J.; Lii, D.-F.; Chen, C.-H. Activation characterization of non-evaporable Ti–Zr–V getter films by synchrotron radiation photoemission spectroscopy. Thin Solid Films 2009, 517, 5876–5880. [CrossRef] 13. Tenchine, L.; Baillin, X.; Faure, C.; Nicolas, P.; Martinez, E.; Papon, A.-M. NEG thin films for under controlled atmosphere MEMS packaging. Sens. Actuators A Phys. 2011, 172, 233–239. [CrossRef] 14. Bu, J.G.; Mao, C.H.; Zhang, Y.; Wei, X.Y.; Du, J. Preparation and sorption characteristics of Zr–Co–RE getter films. J. Alloys Compd. 2012, 529, 69–72. [CrossRef] 15. Xu, Y.; Cui, J.; Cui, H.; Zhou, H.; Yang, Z.; Du, J. Influence of deposition pressure, substrate temperature and substrate outgassing on sorption properties of Zr–Co–Ce getter films. J. Alloys Compd. 2016, 661, 396–401. [CrossRef] 16. Movchan, B.A.; Demchishin, A.V. Structure and properties of thick condensates of nickel, titanium, tungsten, and aluminum oxides, and zirconium dioxide in vacuum. Phys. Met. Metallogr. 1969, 28, 653–660. 17. Thornton, J.A. The microstructure of sputter-deposited coatings. J. Vac. Sci. Technol. A 1986, 4, 3059–3065. [CrossRef] 18. ASTM F798-97. Standard Practice for Determining Gettering Rate, Sorption Capacity, and Gas Content of Non-Evaporable Getters in the Molecular Flow Region; Reapproved 2002; ASTM: West Conshohocken, PA, USA, 2002. 19. Lozano, M.O.; Fraxedas, J. XPS analysis of the activation process in non-evaporable getter thin films. Surf. Interface Anal. 2000, 30, 623–627. [CrossRef] 20. Matolín, V.; Drbohlav, J.; Mašek, K. Mechanism of non-evaporable getter activation XPS and static SIMS

study of Zr44V56 alloy. Vacuum 2003, 71, 317–322. [CrossRef] 21. Li, C.-C.; Huang, J.-L.; Lin, R.-J.; Chen, C.-H.; Lii, D.-F. Characterization of activated non-evaporable porous Ti and Ti–Zr–V getter ﬁlms by synchrotron radiation photoemission spectroscopy. Thin Solid Films 2006, 515, 1121–1125. [CrossRef] 22. Zemek, J.; Jiricek, P. XPS and He II photoelectron yield study of the activation process in Ti–Zr NEG ﬁlms. Vacuum 2003, 71, 329–333. [CrossRef] 23. Šutara, F.; Skála, T.; Mašek, K.; Matolín, V. Surface characterization of activated Ti–Zr–V NEG coatings. Vacuum 2009, 83, 824–827. [CrossRef] 24. Sancrotti, M.; Trezzi, G.; Manini, P. An X-ray photoemission spectroscopy investigation of thermal activation

induced changes in surface composition and chemical bonds of two gettering alloys: Zr2Fe versus Zr57V36Fe7. J. Vac. Sci. Technol. A 1991, 9, 182–189. [CrossRef]

25. Kovac, J.; Sakho, O.; Manini, P.; Sancrotti, M. Near-surface chemistry in Zr2Fe and ZrVFe studied by means of X-ray photoemission spectroscopy: A temperature-dependent study. J. Vac. Sci. Technol. A 2000, 18, 2950–2956. [CrossRef] 26. Yang, X.; Li, J.; Zhang, T.; Hu, R.; Xue, X.; Wang, X.; Fu, H. In situ investigation on transformation of valence

on the surface of the Zr0.9Ti0.1V2 alloy during thermal activation. Solid State Commun. 2011, 151, 842–845. [CrossRef] 27. Petti, D.; Cantoni, M.; Leone, M.; Bertacco, R.; Rizzi, E. Activation of Zr–Co–rare earth getter ﬁlms: An XPS study. Appl. Surf. Sci. 2010, 256, 6291–6296. [CrossRef] 28. Hirokawa, K.; Danzaki, Y. Behaviour of water containing materials in the high vacuum of an X-ray photoelectron spectrometer. Surf. Interface Anal. 1982, 4, 63–67. [CrossRef] 29. Wu, L.-H.; Cheng, C.-M.; Shueh, C.; Chan, C.-K.; Chang, C.-C.; Perng, S.-Y.; Sheng, I.-C. Synthesis of Ti-Zr-V non-evaporable getter thin ﬁlms grown on Al alloy and CuCrZr alloy. Key Eng. Mater. 2017, 730, 87–94. [CrossRef] Micromachines 2018, 9, 490 20 of 20

30. Li, Y.S.; Wong, P.C.; Mitchell, K.A.R. XPS investigations of the interactions of hydrogen with thin films of zirconium oxide II. Effects of heating a 26 Å thick film after treatment with a hydrogen plasma. Appl. Surf. Sci. 1995, 89, 263–269. [CrossRef] 31. Sharma, R.K.; Mithal, N.; Bhushan, K.G.; Srivastava, D.; Prabhakara, H.R.; Gadkari, S.C.; Yakhmi, J.V.; Sahni, V.C. Ti–Zr–V thin films as non-evaporable getters (NEG) to produce extreme high vacuum. J. Phys. Conf. Ser. 2008, 114, 012050. [CrossRef] 32. Abboud, Z.; Moutanabbir, O. Temperature-dependent in Situ studies of volatile molecule trapping in low-temperature-activated Zr alloy-based getters. J. Phys. Chem. C 2017, 121, 3381–3396. [CrossRef] 33. Hirooka, Y.; Miyake, M.; Sano, T. A study of hydrogen absorption and desorption by titanium. J. Nucl. Mater. 1981, 96, 227–232. [CrossRef] 34. Pan, L.; Wang, C.-M.; Xiao, S.-F.; Chen, Y.-G. Hydrogenation behaviors and characteristics of bulk Ti–6Al–4V alloy at different isothermal temperatures. Rare Met. 2017, 1–5. [CrossRef] 35. Yang, X.W.; Zhang, T.B.; Hu, R.; Li, J.S.; Xue, X.Y.; Fu, H.Z. Microstructure and hydrogenation thermokinetics

of ZrTi0.2V1.8 alloy. Int. J. Hydrogen Energy 2010, 35, 11981–11985. [CrossRef] 36. Zhang, Y.L.; Li, J.S.; Zhang, T.B.; Hu, R.; Xue, X.Y. Microstructure and hydrogen storage properties of non-stoichiometric Zr–Ti–V Laves phase alloys. Int. J. Hydrogen Energy 2013, 38, 14675–14684. [CrossRef]

© 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).