A Miniaturized Electrochemical Angular Accelerometer with the Integration of Microelectrodes and Channel

This paper reports a miniaturized electrochemical angular accelerometer with the integration of planar microelectrodes and toroid channel. Specifically, the proposed sensor adopted a three-layer bonding structure of glass-silicon-glass in preference to manual mechanical compression structure, which reduced the device size and simplified the assembly processes. The anodic bonding technology was used as a packaging method, which realized the chip-level package of MEMS electrochemical angular accelerometer for the first time and solved the problem of cathode outlet. From the experimental results, it has the characteristics of larger measurement range (up to 3.5rad/s2) and lower noise level (-144dB@1Hz), which is a significant improvement compared with the previously reported counterparts. A novel approach for the production of electrochemical angular accelerometers is presented in this study, which could be utilized for rotational seismic measurement with large angle variations.


Introduction
Angular motion is a fundamental type of motion that is widely present in nature and in the objective world [1].The popularity of angular motion seismic sensors has increased due to the precise measurements of rotational seismic signal, that can help acquiring additional valuable characters of seismic signal, testing of mechanical structures in civil engineering and realizing the azimuth positioning [2][3].
Nowadays, angular accelerometers according to inertial mass dissimilarity can be classified into two categories: solid [4][5][6][7] and liquid inertial mass [8][9][10].Different from inertial mass sensors which use solids, the electrochemical angular accelerometers have high performance in low-frequency domains due to electrochemical principles [10] and strong shock resistance because of the lack of mechanical structure [11].
Conventional electrochemical angular accelerometers were based on gauze electrodes, which were manufactured through the ceramic sintering technology, leading to the complex fabrication process and poor consistency [8][9].Moreover, the epoxy resin adhesive was used to fixed the sensitive unit to the ceramic shell.When epoxy resin was solidified, it would absorb massive electrolyte, leading to sensor failure.Over the years, the latest breakthrough in MEMS technology has led to an innovative proposal for an integrated plane structure of electrodes.However, this kind of sensor has defects of poor longterm consistency and complex assembly due to the use of rubber rings for physical fastening or UV adhesive for sealing [12].
In this paper, a novel miniaturized electrochemical angular accelerometer that integrates microelectrodes and channel was proposed to simplify manufacturing and reduce device size.Also, a prototype of the device was characterized.This work built on the plane electrode reported elsewhere [12][13].By comparison with the previously reported alternatives, the anodic bonding technology was used as a packaging method, which realized the chip-level package of MEMS electrochemical angular accelerometer for the first time and solved the problem of cathode outlet.Based on the characterization results, the proposed device is featured with larger measurement range and lower noise level.

Principles
Figure 1(a) demonstrates how the electrochemical angular accelerometer works, consisting of an electrolyte-filled toroidal channel and sensitive microelectrodes.The electrolyte, which acts as a liquid inertia mass sensitive to external angular accelerations, is composed of a mixed solution of I2 and KI.The microelectrodes, as the sensitive component, are arranged in an Anode-Cathode-Anode-Cathode-Cathode-Anode-Cathode-Anode configuration [12][13].The working principle can be observed in Figure 1(b).When a positive voltage (preferred 0.3V) is applied on anode, the electrochemical redox reactions occur on the surface of electrode as follows: Anodes: Cathodes: More specifically, upon powering on, the electrodes initiate an electrochemical reaction which rapidly reaches an equilibrium state.When there is no external angular acceleration applied, the ion concentration gradient between the anodes and cathodes remains stable and unchanged.In addition, since the active ions (I 3-) concentration around the two cathodes is nearly identical, the output currents of two pairs of cathodes equally (I1=I2), resulting in a differential output voltage of zero (U0=0).In the presence of an angular acceleration, electrolyte starts moving relative to the electrodes due to inertia, leading to convective transport of ions among electrodes, which generates imbalanced currents on the two cathodes(I1>I2).Consequently, an output voltage is produced (U0>0).
Hence, the differential currents between two cathodes can be served as the output signal of an electrochemical angular accelerometer, displaying linearity across a wide range of fluid velocities and incoming angular accelerations.The first anodic bonding process involves bonding glass 1 with holes to the silicon substrate, creating a substrate structure where the sensitive electrode and its leads are located.The second anodic bonding process involves bonding glass 2 with a narrow groove to the silicon substrate, forming a closed  annular channel.Among them, low-resistance silicon is selected as the substrate material, and BF33 glass is used for bonding glass.The anodes and cathodes are separated by insulating slots on the silicon substrate.Finally, the device is completed by injecting electrolyte.

Fabrication and Characterization
The miniaturized electrochemical angular accelerometer designed in this paper can simplify the manufacturing process while achieving a large measurement range and anodic bonding technology is an appropriate method for producing the integrated sensors without requiring manual mechanical compression.
Figure 3(a) illustrates the MEMS based manufacturing processes, including key steps such as anodic bonding, deposition and etching.Figure 3(b)-(d) show the prototypes of the proposed angular accelerometer with the integration of microelectrodes and channel.
In addition, the measurement results were carried out in laboratory through a national standard rotational turntable with the frequency and amplitude of the angular acceleration under control.Figure 4 presents the characterizations of the fabricated angular accelerometer with the integration of microelectrodes and channel.and the device was featured with excellent linearity with a correlation coefficient R 2 of 0.999.Figure 4(b) is the amplitude-frequency curve of this fabricated device and the peak sensitivity is 51.5 V/rad/s 2 at the low frequency domain and decreases at the high frequency domain.As shown in Figure 4(c), the noise level of the proposed device was quantified as -144 dB at 1Hz.The noise level of this work is lower than other sensors listed in Table 1, while its measurement range is larger than previously reported counterparts for the conventional electrochemical angular sensor, which is essential for those measurement with large angular accelerations.

Conclusions
In this study, the design and fabrication of a miniaturized electrochemical angular accelerometer with the integration of microelectrodes and channel was conducted.To shrink the device, the angular accelerometer was made 40mm*40mm*1.5mmwith anodic bonding technology, offering high consistency without manual alignment and a larger measurement range compared to other counterparts, which creates an opportunity to monitor angular accelerations on a wide-range.

Figure 1 .
Figure 1.The typical structure of electrochemical angular accelerometer consists of an electrolytefilled toroidal channel and sensitive microelectrodes.The electrolyte serves as the inertial mass and the sensitive microelectrodes are immersed in the electrolyte.

Figure 2 .Figure 3 .
Figure 2. the structure diagram of this sensor with the integration of microelectrodes and channel, including a silicon-based substrate with integrated anodes and cathodes, and two layers of glass.

Figure 4 .
Figure 4. Characterization results of the electrochemical angular accelerometer with the integration of planar microelectrodes and toroid channel by using anodic bonding technology with key parameters including linearity, sensitivity and noise level demonstrated in (a), (b), and (c), respectively.

Table 1 .
Performance comparison among this work and other counterparts.