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Micro 3D printing of a functional MEMS accelerometer

Engineering and Technology

Micro 3D printing of a functional MEMS accelerometer

S. Pagliano, D. E. Marschner, et al.

Discover how a team of researchers from KTH Royal Institute of Technology and EPFL have pioneered a revolutionary 3D-printed MEMS accelerometer using innovative techniques like two-photon polymerization. Their groundbreaking work demonstrates a path towards cost-efficient, custom MEMS devices for varied applications.

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Playback language: English
Introduction
Microelectromechanical systems (MEMS) accelerometers are widely used but their production in small to medium batches for specialized applications is hampered by high start-up costs. Conventional large-scale semiconductor manufacturing techniques are cost-prohibitive for low-volume production. This necessitates the use of suboptimal existing devices or makes some applications economically infeasible. The authors propose that micro 3D printing could bridge this gap, enabling the rapid prototyping and cost-effective manufacturing of customized MEMS devices for niche markets. While macroscale 3D printing has been used for prototyping and final component manufacturing, its limitations hinder microscale applications. Two-photon polymerization, offering sub-micrometer resolution, is identified as a suitable technique for creating MEMS devices, but integrating functional transducers at the microscale remains a challenge. This research aims to overcome this hurdle by demonstrating a functional 3D-printed MEMS accelerometer, utilizing two-photon polymerization for the structure and metal evaporation for the strain gauge transducers.
Literature Review
The paper reviews existing MEMS manufacturing techniques, highlighting the limitations of current methods for low-to-medium volume production. It then discusses macroscale 3D printing techniques and their limitations in miniaturization. The suitability of two-photon polymerization for microscale fabrication is presented, along with its successful application in other micro- and nanofabrication areas such as microfluidics and optics. However, the literature lacks examples of functional microscale inertial sensors fabricated using this method, emphasizing the novelty of this research.
Methodology
A MEMS accelerometer structure was designed to be compatible with two-photon polymerization and subsequent metal deposition. The design, using a supporting pillar, two single-sided clamped cantilevers, and a proof mass, allowed for the creation of T-shaped shadow-masking structures for the strain gauge transducers, interconnects, and probing electrodes. The structure was 3D printed using a Nanoscribe Photonic Professional GT2 3D printer and IP-S resin. A 10 nm Ti and 30 nm Au layer was then deposited using directional evaporation, creating electrically isolated metal coatings. A parametrized finite-element model in COMSOL was used to determine the geometrical parameters for a target measurement range of 1-10xg. The model's parameters were adjusted based on the measured cantilever length, which deviated from the designed length. The fabricated accelerometers were characterized using a setup including a piezoshaker, a laser Doppler vibrometer (LDV), and a lock-in amplifier. Resonance frequency, responsivity, and long-term stability were measured. The mechanical response was evaluated by sweeping the frequency of the driving voltage and measuring oscillation amplitude. Responsivity was determined by measuring the relative resistance change of the strain gauge transducers as a function of applied acceleration. Long-term stability was assessed by continuous frequency sweeps over a 10-hour period.
Key Findings
Three accelerometers were fabricated and characterized. The resonance frequency was consistently measured within the range of 1.775 kHz ± 5 Hz, with Q-factors between 31 and 36. The mechanical response showed linear behavior within the evaluated displacement range. The measured resonance frequency corresponded to a Young's modulus of 6.5 GPa, different from the datasheet value. The responsivity ranged from 322 to 420 ppm/g at the resonance frequency, yielding a responsivity of 11 ± 0.7 ppm/g at standard testing frequencies (100-160 Hz). The gauge factor of the thin film strain gauge transducers was computed as 3.5 ± 0.6. This was higher than the value obtained from the COMSOL simulation. Long-term stability tests over 10 hours showed minimal drift in resonance frequency (±4 Hz) and relative resistance change (±4 ppm for one device and approximately 20 ppm for the other two). Two theoretical models based on Euler-Bernoulli beam theory were developed and predicted resonance frequencies within 10% of measured values.
Discussion
The results demonstrate the successful fabrication and operation of a functional 3D-printed MEMS accelerometer. The achieved performance characteristics are comparable to those of commercially available devices for certain applications. The discrepancies between the simulated and measured Young's modulus and gauge factor might be attributed to the limitations of the material models used in the simulations and the complex behavior of thin-film conductors. The observed variations in long-term stability might be due to temperature fluctuations or variations in the applied acceleration. The success of this approach opens possibilities for producing customized MEMS devices for various applications previously inaccessible due to manufacturing limitations. The high degree of design freedom offered by 3D printing allows for the creation of complex structures tailored to specific needs.
Conclusion
This research successfully demonstrated the 3D printing of a functional MEMS accelerometer using two-photon polymerization and metal evaporation, showcasing the potential of this approach for cost-effective, low-to-medium volume MEMS production. Future work could focus on refining the fabrication process to improve precision and reduce variability, exploring different materials for improved performance and investigating new sensor designs enabled by 3D printing capabilities.
Limitations
The study utilized a limited number of devices (three), which might not fully represent the variability inherent in the 3D printing process. The observed variations in the long-term stability suggest potential improvements to be made in the fabrication or packaging of the devices to minimize the effects of external factors like temperature fluctuations. Further research is needed to fully understand the discrepancy between the simulated and measured gauge factor, as well as the precise impact of the probe placement on the Q-factor.
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