Vacuum-Sealed MEMS Resonators Based on Silicon Migration Sealing and Hydrogen Diffusion

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 (>1000°C) hydrogen environments to seal release holes effectively. We successfully demonstrated this technique on a MEMS resonator made on a standard SOI wafer, commonly used in inertial sensors and timing devices. After the encapsulation, hydrogen diffusion from the sealed cavity was performed through annealing at 430°C for 27 hours in a nitrogen environment. Further analysis using focused ion beam (FIB) penetration outside the resonating element confirmed an impressive vacuum level improvement in the sealed cavity, estimated at ~60 Pa. Notably, after additional air-baking at 145°C, the maintained high Q factor suggests a potential vacuum level below 10 Pa. These findings not only illustrate the efficiency of SMS in wafer-level vacuum packaging but also open up possibilities for optimizing sealing pressure in MEMS packaging. [2024-0014]

Vacuum-Sealed MEMS Resonators Based on Silicon Migration Sealing and Hydrogen Diffusion Tianjiao Gong, Muhammad Jehanzeb Khan, Yukio Suzuki , Member, IEEE, Takashiro Tsukamoto, Member, IEEE, and Shuji Tanaka, Fellow, IEEE Abstract-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 (>1000 • C) hydrogen environments to seal release holes effectively.We successfully demonstrated this technique on a MEMS resonator made on a standard SOI wafer, commonly used in inertial sensors and timing devices.After the encapsulation, hydrogen diffusion from the sealed cavity was performed through annealing at 430 • C for 27 hours in a nitrogen environment.Further analysis using focused ion beam (FIB) penetration outside the resonating element confirmed an impressive vacuum level improvement in the sealed cavity, estimated at ∼60 Pa.Notably, after additional air-baking at 145 • C, the maintained high Q factor suggests a potential vacuum level below 10 Pa.These findings not only illustrate the efficiency of SMS in wafer-level vacuum packaging but also open up possibilities for optimizing sealing pressure in MEMS packaging.

I. INTRODUCTION
H IGH-VACUUM wafer-level packaging is essential especially for resonant-type MEMS to improve the performance by reducing air damping and to reduce the fabrication cost by wafer batch process.For wafer-level packaging, there are typically two sealing methods [1], [2]; one is to use a lid wafer and the other is to deposit a sealing layer.In the former Wafer-level vacuum packaging is one of the most important technologies in the field of MEMS (Micro Electro Mechanical Systems), especially for resonant-type MEMS.This technique addresses one of the critical challenges, the minimization of squeeze film air damping (SFD) for high Q factor [1], [2], [3].This effect is particularly significant in MEMS based on capacitive sensing and driving, which usually lead to narrow air gaps.When MEMS include a moving plate which interacts with a trapped film, the squeeze film air damping becomes critical as it dominates the damping mechanism and significantly impacts the system's frequency response.The mechanical structures of various devices such as gyroscope sensors and timing resonators exhibit a close correlation between their performance and squeeze film air damping [4].Wafer-level vacuum packaging effectively eliminates the negative impact of air damping and significantly enhances the performance of resonant-type MEMS.
Wafer-level vacuum sealing can be typically achieved by two methods [5], [6]; one is to use a lid wafer and the other is to deposit a thin film as a sealing layer.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.It is initially inactive and requires subsequent thermal activation to be activated after sealing.The use of the getters for MEMS wafer-level packages increases cost, and more importantly, it is not always useful for high vacuum encapsulation.
In the latter case, a sealing layer is directly deposited onto the device wafer, forming a hermetic seal to enclose the MEMS structure [11], [12].In this method, the residual gas determines the pressure of the sealed cavity.To achieve a high vacuum level within the sealed cavity, the residual gas must be effectively removed.If the residual gas is mostly hydrogen, the evacuation of the residual gas is performed through annealing the sealed package under specific conditions before the sealing layer deposition.
The vacuum sealing method using epi-Si deposition was first proposed by K. Ikeda et al. from Yokogawa Electric Corporation [13].Later, another "Epi-seal" process was developed by Stanford University and Robert Bosch [14], [15], [16], [17], which is now a typical example utilizing this method.Epi-seal process has proven to be an excellent platform for the fabrication and packaging of MEMS resonators.However, it is more complex compared to other vacuum packaging methods, because a special epitaxial growth technique with severe stress control is essential [18].In this paper, we report a new wafer-level packaging technology based on silicon migration.Silicon migration refers to the phenomenon of self-organizing recrystallization, which occurs due to the physical migration of silicon atoms on the surface [19], [20].It has been observed that silicon migration takes place under deoxidizing ambient, usually in hydrogen environments, at elevated temperatures (900∼1100 • C), which is below the silicon melting point.During the migration process, silicon atoms on the surface exhibit a self-organizing behavior, redistributing themselves to minimize surface energy.Silicon migration has been extensively studied for its potential in fabricating complex structures which are challenging to achieve using traditional techniques [21], [22], [23].For instance, through-holes can deform and finally close in the top surface, leaving a sealed cavity inside.This structure was used for pressure sensor [24] and pMUT [25].The silicon migration sealing (SMS) technique utilizes this phenomenon to achieve hermetical sealing by occluding through-holes [26].
The concept of applying SMS technique to the waferlevel vacuum packaging of MEMS was presented in [27].The general packaging process is shown in Figure 1.Firstly, a cap wafer with sub-micron release holes is directly bonded with a device wafer with MEMS structures, and then the MEMS structures in the cavities are released by sacrificial layer vapor HF etching through the release holes.Following the release of the MEMS structures, the release holes are closed through silicon migration in a hydrogen atmosphere at high temperature, resulting in the hermetical sealing of the cavities.The use of sub-micron diameter release holes is preferred to ensure a shorter occlusion time, considering the deformation of the internal MEMS structures and fabrication cost.Additionally, the remaining hydrogen within the sealed cavity can thermally diffuse out, thereby achieving a high-vacuum environment.After metal electrode fabrication, the sealed device can be connected to other packages by wire bonding.The SMS-based wafer-level vacuum packaging technique requires neither getter nor sealing layer.
In our previous study, the vacuum level after SMS has been investigated using a silicon membrane [28].The vacuum level of the sealed cavity was evaluated using "Zero-balance method" in a vacuum chamber with a diaphragm pressure gauge.Although the accuracy is limited by the method, it was confirmed from the result that a vacuum level of less than 10 Pa can be achieved in the SMS-sealed cavity by 35 hours of nitrogen annealing at 650 • C after SMS.Moreover, a yield of more than 95% has been confirmed in terms of the sealing pressure below a few hundred Pa.In this study, MEMS resonators were fabricated and vacuum-sealed by SMS technology.The fabrication process is explained in detail as well as the hydrogen diffusion condition.After vacuum encapsulation, Q factors of the MEMS resonators were measured.By opening the package and measuring the Q factors in a chamber with varying pressure, the relationship between the Q factor and vacuum level was obtained, thus the vacuum level after SMS was confirmed.

A. Design of SMS-Encapsulated MEMS Resonator
A dual mass symmetric MEMS resonator with 4 anchors was purposefully designed as the Q factor of the resonator is used to assess the internal pressure of a sealed cavity.Figure 2 illustrates the design of the resonator.The tuning fork design of the resonator effectively minimizes anchor loss in anti-phase vibration mode, which is necessary for lowering the sealing pressure limit measurable by the squeezed film damping effect.Standard comb electrodes are employed for both driving and sensing, and a gap between the sensing (B) and driving (C) combs is 5 µm.The through holes in the resonator body for sacrificial SiO 2 etching are designed to have a dimension of 20 × 20 µm 2 .To accommodate wire bonding after packaging, the pad area is 350 × 350 µm 2 .The packaged device measures 3.31 ×3.31 mm 2 in the total size.
It is crucial for SMS to have small-diameter release holes with a high aspect ratio in the cap wafer, which ensures easy closure of the holes and guarantees that the release holes remain significantly smaller than any functional structures within the internal resonator.Therefore, the maximum diameter of 0.6 µ and the minimum aspect ratio of 8 for the release holes are applied.Five sets of multiple release holes are evenly distributed across the chip for effective sacrificial layer etching by vapor HF [29].

B. Fabrication
Figure 3 illustrates the fabrication process of the MEMS resonator applying SMS.Each of two SOI wafers is used as a device wafer or cap wafer.The device wafer consists of a 50 µm thick device layer, a 2 µm thick buried oxide (BOX) layer, and a 600 µm thick handle layer.The cap wafer consists of a 5 µm thick device layer, a 1 µm thick BOX layer, and a 400 µm thick handle layer.
The fabrication process begins with the growth of a 1 µm thick bonding SiO 2 layer on the device wafer by thermal oxidation at 1100 • C for 140 minutes.This thermal SiO 2 layer will be used for direct bonding in the later process.Alignment marks on the handle layer are then patterned using photolithography and RIE (reactive ion etching) (a).Subsequently, the thermal SiO 2 layer is patterned by photolithography and etched by RIE (b).This SiO 2 layer will be the future bonding area with the cap wafer, and its thickness will determine the small space between the resonator and the cap wafer.The resonator pattern is then fabricated on the device layer by using DRIE to etch until reaching the BOX layer (c).Simultaneously, submicron release holes are patterned on the cap wafer using an i-line stepper, and the device layer is penetrated using DRIE (deep reactive ion etching).
The next step is direct wafer bonding between the device and cap wafers (d).Prior to bonding, the wafer surfaces undergo thorough cleaning processes, including RCA-1 and RCA-2 to remove organic and metallic contamination.Oxygen plasma surface activation and mega-sonic cleaning are also employed for both wafers.The bonded wafer is then processed by thermal oxidation at 1100 • C for 140 minutes, which further enhances the overall bonding quality.Strong chemical strength of bonding oxide is critical to facilitate controlled vapor HF etching [30].
After the thermal oxidation process, the oxide layers on both sides of the wafer are removed using a BHF solution.Subsequently, the handle layer of the device wafer is thinned down to a thickness of 50 µm by chemical mechanical polishing (CMP) (e), in preparation for the subsequent formation of pad openings.Some micro-cavities are then created using DRIE to access the release holes (f).This structure reduces the aspect ratio of the submicron through holes.The bonded wafer are diced into 2 cm square samples and cleaned by RCA-1 and RCA-2.The sacrificial oxide layer of the resonator and the bonding oxide are then etched through the submicron release holes using vapor HF etching (g).
Subsequently, the release holes are closed by SMS in a pure hydrogen environment at 1100 • C and 10 kPa for 20 minutes (h). Figure 4 shows the scanning electron microscope (SEM) images of an array of the release holes before and after the closure.As a result, the cavities are sealed and filled with hydrogen gas.High temperature hydrogen annealing can effectively clean the silicon surface and eliminates any degas sources, such as native silicon dioxide and organic contaminants, especially inside the sealed cavity.
After that, DRIE is carried out to create pad openings on the thinned handle layer.This pad openings are etch-stopped by the BOX layer of the resonator SOI wafer, and the area around the opening determines the minimum sealing width of 50 µ (i).Next, medium temperature annealing at 430 • C for 27 hours is performed to facilitate the diffusion of trapped hydrogen inside cavities through the SiO 2 layer.Figure 5 shows the schematic principle of hydrogen diffusion during the annealing.As a result, the vacuum level within the sealed cavity is improved.The exposed BOX layer within the pad openings is then removed using vapor HF etching.Finally, the samples are covered by the stencil mask and a deposition   of Cr/Au by sputtering is applied to the opened pad areas in order to create wire-bonding pads (j).

III. RESULTS AND DISCUSSION
The samples are diced into 3.31 × 3.31 mm 2 chips, and each chip is mounted on a ceramic package by conductive epoxy paste and wire-bonded with the package, as shown in Figure 6.In the measurement, the anti-phase mode was observed at 35.527 kHz, with a measured Q factor of 6000.The frequency characteristic is shown in Figure 8.Additional samples were also tested, showing Q factors of 13000, 18400, and 19000.Following the successful measurement of the Q factor without a vacuum chamber, a through-hole with a diameter in the of several tenths of a micron was drilled in the cap of one resonator sample using the focused ion beam (FIB).The place of through-hole was carefully selected between two electrodes in order not to damage the inside MEMS structure, and a low   ion beam current was applied during FIB.This is because the degree of charge-up was found to have some relation with the ion beam current; Higher ion current facilitated faster FIB penetration but have a higher possibility of stiction failure due to charge-up, while lower ion current led to slower penetration but showed less stiction failure.
The FIB-penetrated sample was subsequently measured in a vacuum chamber.By adjusting the pressure level inside the chamber, the relationship between the Q factor and vacuum level was obtained, as illustrated in Figure 9.This result revealed that the sample with a Q factor of 6000 was encapsulated at about 60 Pa, a pressure significantly lower than that of hydrogen annealing (10 kPa), which serves as the sealing environment.The other samples' Q factors roughly correspond to 20 Pa, 29 Pa, and 30 Pa.Although some deviation can be seen from this result, indicating an insufficient hydrogen outgassing, this successful outcome confirmed the effectiveness of the SMS technology for a typical MEMS resonator on the SOI wafer.It is also observed that the resonator begins to exhibit significant SFD effects at vacuum levels below approximately 300 Pa, as evidenced by the notable change in the Q factor from 10 to 300 Pa.At pressures below 10 Pa, the Q factor is less affected by pressure, suggesting that other factors such as anchor point loss and TED begin to dominate the losses.After the initial Q factor measurement, an encapsulated resonator sample with an initial measured Q factor of 18400 was put inside an oven for additional long-term air-baking at 145 • C, and its Q factor was continuously measured.The baking test is based on the quality standard of high temperature storage life (JESD22-A103).The choice of a lower temperature compared to the initial annealing temperature aimed to minimize the risk of damaging the electrode pad and bond wires.As depicted in Figure 10, the Q factor exhibited improvement throughout the subsequent baking process, which is attributed to continuous hydrogen diffusion.Notably, the Q factor showed a linear correlation with the duration of baking and exhibited no signs of saturation, suggesting the presence of a considerable amount of residual hydrogen within the sealed cavity.Again, this indicates that the initial hydrogen diffusion baking is not sufficient.After 50 days of air-baking, the Q factor corresponds to a vacuum level lower than 10 Pa, showing potential to enhance the initial hydrogen diffusion process for achieving even lower sealing pressures.Moreover, the frequency of this resonator sample is stable during the additional baking.The variations in resonance frequency remained within a 0.02% range throughout the 50-day longterm heating test, as shown in Figure 11.The above findings showed that the additional air baking can also serve as a method to further increase the sealed vacuum level in post-fabrication.

IV. CONCLUSION
In this study, the SMS technique was applied to the waferlevel packaging of MEMS resonators.The encapsulation of dual mass resonators was accomplished successfully, and subsequent measurements of Q factors were conducted.The sealing pressure was estimated by assessing the Q factor of the same sample within a vacuum chamber after deliberately inducing a leak by FIB drilling.Using this method, the sealing pressure of a sample with a Q factor of 6000 was evaluated to be about 60 Pa.One sample after the initial measurement was subjected to additional long-term air baking at 145•C and continuously measured.The results showed a continuous increase in its Q factor as the baking process continued, displaying a linear relationship with the baking duration.This result suggests that the initial hydrogen diffusion annealing may be insufficient, and optimizing process conditions could potentially result in a higher vacuum level within the sealed cavity.In summary, this study confirmed the success of the SMS wafer-level high vacuum packaging technique for MEMS resonators, demonstrating its applicability to practical fabrication processes.Furthermore, it highlights the potential of achieving high vacuum levels of single Pa in wafer-level encapsulation using SMS technique.

Figure 7
Figure 7 illustrates the measurement diagram.To measure the Q factors of the resonators, a front-end circuit and a lockin amplifier are employed.The input drive voltage is set at 0.3 Vpp, applied over an offset voltage of 1.2 V.The frontend circuit includes a pair of CV converters and demodulators.A modulation signal of 1 MHz with an amplitude of 1 Vpp is generated by a functional generator and applied to the common electrode for capacitive sensing.The measurements are conducted at room temperature, maintained at 25 • C.In the measurement, the anti-phase mode was observed at 35.527 kHz, with a measured Q factor of 6000.The frequency characteristic is shown in Figure8.Additional samples were also tested, showing Q factors of 13000, 18400, and 19000.Following the successful measurement of the Q factor without a vacuum chamber, a through-hole with a diameter in the of several tenths of a micron was drilled in the cap of one resonator sample using the focused ion beam (FIB).The place of through-hole was carefully selected between two electrodes in order not to damage the inside MEMS structure, and a low

Fig.
Fig. Measurement layout for packaged resonator.

Fig. 9 .
Fig. 9. Relationship of Q factor and chamber pressure after FIB penetrating of a packaged resonator sample processed with condition.

Fig. 10 .
Fig. 10.Q factor improvement by additional long term hydrogen diffusion at 145 • C.