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(Color online) Titanium oxide and Au-Ti thicknesses versus 1 h annealing temperature for Au (5 nm)/Ti (100 nm). Squares: experimental values extracted from SEM measurements; solid black line: fit with Ti diffusion model; solid red line: simulated Au-Ti thickness.
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Non-evaporable getter (NEG) thin films based on alloys of transition metals have been studied by various authors for vacuum control in wafer-level packages of micro electro mechanical systems (MEMS). These materials have typically a relatively high activation temperature (300-450 °C) which is incompatible with some temperature sensitive MEMS device...
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... images using both secondary electrons (SE) and backscattered electrons (BSE) detection show that the as-deposited gold films are smooth (Fig. 7). After an- nealing at 350 °C, the titanium oxide layer completely covers the buried gold film. These images allowed the measurement of the TiO x thickness as a function of the annealing tem- perature (Fig. 8). By fitting these values using the model presented previously, the diffusion coefficient D of Ti in Au was estimated. When thermally activated, D is express ...
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Citations
... In the former case, a separate lid wafer is bonded to the device wafer, enclosing the MEMS structures and forming a sealed cavity. To achieve a high vacuum level inside the cavity, a non-evaporable getter is often employed [7], [8], [9], [10]. The getter is a chemically reactive solid material capable of capturing gas molecules and preventing their re-desorption. ...
In this study, we introduce an innovative approach to vacuum-encapsulation of MEMS resonators using Silicon Migration Seal (SMS) technology, a novel wafer-level vacuum packaging method. SMS utilizes silicon reflow phenomena under high-temperature (
$>$
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... The thin film titanium getter's activation temperature ranges from 300 to 500 • C [22], [23], [24], with pressures in the Fig. 9. Thermal impedance of two sealed MEMS vacuum packages. A slight increase in the thermal impedance confirms the pressure decrease inside the MEMS package for the initial days before stabilization. ...
This work reports a thin-film encapsulated package with porous alumina as the capping layer and titanium as a pore-sealing getter. The titanium-gold film seals the thin film package and acts as the getter. Since the getter is not exposed to the elements, fouling of the getter is prevented. EDX measurements confirm that the getter material did not penetrate the package through the nanopores. The process is a low thermal budget process, with the getter activation (300
$^{\circ}$
C for 1 hour) being the only step where the temperature is raised. A Silicon Pirani gauge was used to monitor the pressure changes inside the sealed cavity. After the getter activation, a decrease in the pressure from 50
$\mu$
Torr to 3.9
$\mu$
Torr was seen for the first few days, and no noticeable change afterward. The hermeticity of the thin-film encapsulated package was examined, and the vacuum level inside the package remained the same for the last 510 days. In addition to providing a stable hermetic package, the getter may be activated in-circuit by resistive heating in case of pressure increases after many years of operation. 2023–0034
... With the advantages of suitable shape, excellent adsorption performance, and heat activation compatibility with wafer bonding and sealing technologies, NEG films deposited at the inner cavity wall by magnetron sputtering have significant application in vacuum sealing. [6][7][8][9] The two main types of NEG films are zirconium alloys and titanium alloys or dual-layer films, such as Pd/Ti thin films. [7][8][9][10][11][12][13][14][15] NEG films adsorb gases through different processes. ...
... [6][7][8][9] The two main types of NEG films are zirconium alloys and titanium alloys or dual-layer films, such as Pd/Ti thin films. [7][8][9][10][11][12][13][14][15] NEG films adsorb gases through different processes. As for hydrogen, the gas can quickly diffuse into an alloy and form a solid solution. ...
The vacuum level in micro-electro-mechanical system devices needs to be achieved and maintained using non-evaporable getter film technology. Zr–Co–RE getter films are deposited by direct current (DC) magnetron sputtering using krypton (Kr) as the sputtering gas. The influence of sputtering gas pressure and DC sputtering power on the microstructure and adsorption characteristics of films is investigated. Zr–Co–RE films deposited at different gas pressures all grow in a columnar shape. Films grown at low Kr pressures are relatively dense and have a structure with fewer cracks, whereas high Kr pressures result in a uniform cluster and columnar structure with more gaps and micro-cracks. In addition, it is revealed that DC sputtering power has great influence on the film structure and adsorption performance. The films deposited at low sputtering power contain more micro-cracks, which are distributed uniformly. At high sputtering power, the cluster structure is not obvious. Consequently, high Kr pressures and low sputtering power are beneficial to improve the adsorption performance. Hydrogen adsorption tests are carried out using a special vacuum system, keeping the pressure constant. The highest initial adsorption speed is 89 (ml/s)/cm2, obtained at 4.0 Pa Kr pressure and 300 W sputtering power. Meanwhile, the Zr–Co–RE films deposited under these conditions have excellent adsorption stability.
... These NEG films are deposited and patterned on the assemblies for vacuum packaging. The underlayer metal atoms diffuse into the inner surface of the package and absorb the residual gases by activating the NEG film at a high temperature after packaging [7][8][9][10] . ...
In this study, we developed a metal multilayer that can provide hermetic sealing after degassing the assemblies and absorbing the residual gases in the package. A package without a leak path was obtained by the direct bonding of the Au/Pt/Ti layers. After packaging, annealing at 450 °C caused thermal diffusion of the Ti underlayer atoms to the inner surface, which led to absorption of the residual gas molecules. These results indicated that a wafer coated with a Au/Pt/Ti layer can provide hermetic sealing and absorb residual gases, which can simplify vacuum packaging processes in the electronics industry.
... Additionally, it is known that the Au/Ti layer can function as a non-evaporative getter (NEG), which absorbs residual gas molecules in the package [12]. When the Au/Ti layer was annealed, the thermally diffused Ti atoms reached the surface and reacted with the residual gas. ...
... However, such thin Au/Ti layers cannot absorb large amounts of gaseous molecules. To study the gettering process [12], thicker Ti layers were employed; for instance, Ming Wu employed Au and Ti layers with thicknesses in the range of 5-20 and 100 nm, respectively [10]. To address this dilemma, the present study developed fabrication techniques for an Au/Tibased getter layer with a smooth Au surface and a thick Ti layer. ...
... In this study, the thickness of the Ti layer was adjusted to 100 nm, which is the same as that in a previous study on the gettering process [12]. However, in the industry, thicker Ti layers are sometimes used to getter large amounts of gas molecules. ...
This study demonstrates room-temperature bonding using a getter layer for the vacuum packaging of microsystems. A thick Ti layer covered with an Au layer is utilized as a getter layer because it can absorb gas molecules in the package. Additionally, smooth Au surfaces can form direct bonds for hermetic sealing at room temperature. Direct bonding using a getter layer can simplify the vacuum packaging process; however, typical getter layers are rough in bonding formation. This study demonstrates two fabrication techniques for smooth getter layers. In the first approach, the Au/Ti layer is bonded to an Au layer on a smooth SiO2 template, and the Au/SiO2 interface is mechanically exfoliated. Although the root-mean-square roughness was reduced from 2.00 to 0.98 nm, the surface was still extremely rough for direct bonding. In the second approach, an Au/Ti/Au multilayer on a smooth SiO2 template is bonded with a packaging substrate, and the Au/SiO2 interface is exfoliated. The transferred Au/Ti/Au getter layer has a smooth surface with the root-mean-square roughness of 0.54 nm and could form wafer-scale direct bonding at room temperature. We believe that the second approach would allow a simple packaging process using direct bonding of the getter layer.
... A number of researchers have developed a non-evaporative getter (NEG) to remove the residual gas in the package. [8][9][10][11][12][13][14][15][16] NEG films installed in the package, which serve as chemical pumps, can be activated by thermal treatment. This method has been generally used in MEMS device fabrications. ...
A Au/Ta/Ti metal multilayer was developed to improve the high vacuum packaging process for microdevice fabrication. This study revealed that the wafer coated with the Au/Ta/Ti layer could form direct bonding and absorb residual gas. We investigated the effect of Ta layer thickness on the diffusion of Ti atoms. The Au/Ta/Ti metal multilayers were successfully bonded after a degassing process when the Ta barrier layer is thicker than 1.5 nm. Moreover, the Au/Ta/Ti metal film effectively absorbed the residual gas molecules by annealing at 350 °C. As the annealing temperature for the gas gettering is lower than the previous reports, the Au/Ta/Ti metal multilayer could be useful for the future vacuum packaging process.
... To prevent noble gas release by getter films, evaporation instead of sputtering was used in our previous works for the deposition of various getter multilayers and alloys. (63)(64)(65)(66)(67)(68)(69) In the ASTM 798-97 standard, (70) the getter film sorption capacity is defined as the quantity of sorbed gas beyond which the gettering rate decreases to 5% of its value 3 min after the start of the test. Since the getter pumping rate requirement during the few hours of the outgassing, wafer bonding, and getter film thermal activation steps is expected to be ≥10 3 higher than that after few days at room temperature, using this getter capacity definition is not very appropriate in the case of getter films for wafer-level packaging. ...
... Getter activations by out-diffusion and dewetting are respectively illustrated in Figs. 6(a) and Fig. 6(b) by SEM cross sections after the thermal annealing and air exposure of an evaporated Au/Ti multilayer (63,65) and of an evaporated Au/Zr film. (65) Solid-state dewetting is a less common case and will be first briefly discussed. ...
A large set of micro- and nanodevices requires hybrid vacuum packaging at the wafer level
to optimize their performance and lifetime. When the temperature of the packaging fabrication
process is limited, getter films must be integrated into the packaging cavity to obtain a low
and sustainable pressure. In this paper, gas generation and residual gases inside a wafer-level
package, as well as the main getter alloy films and multilayers investigated previously, are
reviewed. An emphasis is put on the effect of a gaseous environment on the physicochemical
mechanisms involved in getter film activation and gas sorption. A large outgassing occurs
during the packaging fabrication process. It produces a gas load larger than or similar to that
generated during the entire lifetime of the device at room temperature. The resulting partial
pressures of gases in a micropackage are quite different from those found in ultrahigh vacuum
during characterization by surface analysis techniques and when getter films are used in
accelerators. Consequently, in micropackages, activation and sorption steps can no longer
be analyzed separately and surface oxidation and/or oxygen bulk diffusion during activation
should be taken into account. It is thus recommended to use and develop complementary
characterization techniques that enable the investigation of surface and bulk phenomena at
various temperatures, pressures, and gas mixtures for the assessment of getter films for vacuum
packaging.
... measurements under vacuum as a function of the temperature. Previous works have shown that this technique provides useful and reliable information on getter films activation[3]. ...
Transition metals alloys are the most studied getter films for wafer-level vacuum packaging of MEMS. In this work we investigated yttrium and Y x Ti y alloys films that could possibly overcome the limitations of usual getter materials, i.e. reversible sorption of hydrogen and low sorption ability for hydrocarbon gases. As a preliminary step towards this objective, properties of (co-)evaporated yttrium and Y x Ti y films were analyzed by SEM, EDS, XPS and in-situ sheet resistance measurements before, after and/or during annealing in vacuum. As deposited and annealed films have a small grain size and a columnar structure. It is shown that yttrium and Y x Ti y films can be activated after 1 hour of annealing in vacuum at 250 °C if the Y-content is larger than ∼9 %. These results are promising for the use of Y-based films as low temperature getters for vacuum packaging.
... 13 The activation of such composite getter films is obtained by out-diffusion of getter material atoms through the protective layer and their diffusion on the surface or/and by degradation of the protective layer, both of which lead to exposure of the underlying reactive material. In our previous works, [16][17][18] the interdiffusion phenomenon in the Au/Ti dense film has been studied and modelled. It was shown that an annealing temperature as low as 200 C for 1 h can cause the out-diffusion of titanium atoms required to activate the getter film. ...
... For such a bilayer getter, thermal activation is achieved by Ti out-diffusion through the gold film as demonstrated in previous works. [16][17][18] Ideally, the gold film must be as thin as possible but must completely cover the surface of the porous getter film. For a conformal deposition near the surface, the minimum Au film thickness would be the normal distance c between two tilted columns [ Fig. 4(a)], which can be calculated from the measured width of the columns and from a (or b). ...
... This TiO 2 thickness is also larger than the one measured on a dense Ti film with a 5 nm Au layer. 16 This shows that Ti porosity increases the effective gettering ability of the titanium getter. It is attributed to a larger interdiffusion of titanium and gold and a larger oxidation. ...
Interdiffusion in gold/titanium films with titanium evaporated on silicon at different oblique angles (0°, 30°, 50°, and 70°) and gold evaporated under normal incidence is investigated using a scanning electron microscope and in-situ sheet resistance measurements for the optimization of the films' effective gettering ability during and after a low-temperature thermal activation. Films have a tilted columnar structure and a porosity increasing with the deposition angle in agreement with calculated values. The oxygen effective gettering ability, characterized by energy-dispersive X-ray spectroscopy, increases with the deposition angle. It is shown that the gold/titanium film with titanium deposited with an oblique angle of 50° is an optimized condition to obtain the largest sorption capacity after 1 h thermal activation at temperatures lower or equal to 300 °C. This film has a low initial residual stress that becomes highly tensile if thermal activation is performed above 300 °C.
At the wafer-level package stage, the interior of the sensor can be easily converted into a vacuum environment. However, after packaging, degassing occurs due to the accumulation of fumes with additional processing, resulting in a significant reduction in sensor reliability. To counteract this, non-evaporable getter (NEG) film is commonly packaged together with the sensor to absorb the outgassing gas molecules and maintain a vacuum environment within the sensor. Most NEG films require an activation process to migrate the adsorbed gas molecules from the surface to the bulk by thermal annealing. Recently, NEG films have been considered to reduce the activation temperature and time to avoid heat damage. Depositing an anti-oxidant layer on NEG film or alloying the NEG film with metallic materials through co-sputtering to create a distinct valence state during activation was found to prevent further oxidation of NEG film. However, these methods require expensive materials and fabrication equipment. In this study, we demonstrate that a much lower activation temperature (T = 350 °C) and time (t = 10 min) for Ti NEG film can be achieved by controlling the surface morphology depending on the deposition method and condition, without requiring further treatment such as the deposition of a capping layer or co-sputtering. Increasing the grain size of the Ti NEG film results in a larger surface area, which enables more efficient adsorption of gas molecules. Additionally, higher porosity in the film increases the diffusion of gas molecules, thus enhancing the overall gas adsorption capacity. Our experiments show that the Ti NEG film, which was deposited at 7.8 Å/s using a sputtering method, exhibited a grain size of 411 nm and a surface roughness of 59.185 nm. Furthermore, after an activation process at 350 °C for 10 min, the atomic ratio of the adsorbed gas molecules was 23.14%.
