Research on Electrothermally Driven Al-SiO₂ Bimorph Microgripper

Academic Background

Microgrippers play a crucial role in assembly and manipulation at the micro and nano scales, with wide applications in microelectronics, MEMS (Micro-Electro-Mechanical Systems), and biomedical engineering. To ensure the safe handling of delicate materials and micro-objects, microgrippers need to exhibit high precision, rapid response, ease of operation, strong reliability, and low power consumption. Although various microgrippers with different actuation mechanisms, such as electrostatic, electromagnetic, and optical actuation, have been developed, these technologies still have limitations. For example, optically driven microgrippers require specific light sources and optical paths, electrostatic microgrippers require high voltages, and electromagnetic microgrippers involve complex magnetic field generation systems. Therefore, developing a high-performance, compact, easy-to-operate, and widely applicable microgripper remains of great significance.

Electrothermally driven microgrippers have attracted attention due to their simple structure, low driving voltage, and ability to achieve significant structural deformation. The research team developed an electrothermally driven microgripper based on Al-SiO₂ bimorphs, featuring large deformation, fast response, ease of operation, strong gripping force, and high stability, making it suitable for various microassembly and micromanipulation applications, particularly in the field of electronic packaging.

Paper Source

This paper was co-authored by Hengzhang Yang, Yao Lu, Yingtao Ding, and others, affiliated with the School of Integrated Circuits and Electronics, Beijing Institute of Technology, and the Engineering Research Center of Integrated Acousto-Opto-Electronic Microsystems, Ministry of Education of China, among other institutions. The paper was published in 2024 in the journal Microsystems & Nanoengineering, titled “A microgripper based on electrothermal Al–SiO₂ bimorphs”.

Research Process and Results

1. Design and Working Principle of the Microgripper

The core structure of the microgripper is an electrothermal bimorph actuator, composed of two materials (Al and SiO₂) with a large difference in thermal expansion coefficients (CTE). When a voltage is applied, the bimorph generates stress due to thermal expansion mismatch, causing structural bending and enabling the opening and closing of the microgripper. The microgripper naturally adopts a closed state due to residual stresses induced during fabrication, and the state can be easily switched by temperature control.

Design Details:

• Bimorph Actuator: The large CTE difference between Al and SiO₂ enables significant deformation.
• Resistive Heater: A Pt resistor is embedded in the bimorph to convert electrical energy into thermal energy via Joule heating, driving structural deformation.
• Independent Control: Each actuator is equipped with an independent resistive heater, allowing precise individual control.

2. Fabrication Process

The microgripper was fabricated using a unique microfabrication process, which included the following steps: 1. SiO₂ Deposition and Etching: A 400 nm-thick SiO₂ layer was deposited on a silicon wafer and patterned via reactive ion etching (RIE). 2. Pt Deposition and Lift-off: A 100 nm-thick Pt layer was sputtered and patterned using a lift-off process. 3. Insulation Layer Deposition: A 100 nm-thick SiO₂ layer was deposited to ensure electrical insulation between Pt and Al. 4. Al Deposition and Etching: A 500 nm-thick Al layer was deposited and patterned via RIE to define the actuators and circuits. 5. Release Step: SF₆ gas was used to etch the silicon beneath the actuators, rapidly completing the release process.

3. Experimental Results

3.1 Deformation Characteristics of the Microgripper

Experiments showed that the microgripper could achieve over 100 degrees of bending deformation at 5 V, with a response time of less than 10 ms. By adjusting the driving voltage, the deformation angle of the microgripper could be precisely controlled. Additionally, the four actuators could be controlled independently, enabling multiple operation modes.

3.2 Grasping Experiment

The microgripper successfully grasped a 500 μm diameter PMMA microbead, and its gripping strength was verified through vibration and impact tests. In the vibration test, the microgripper could withstand an average acceleration of 35 g; in the impact test, it could withstand over 1600 g of impact acceleration, demonstrating excellent gripping performance.

3.3 Application Test

The microgripper successfully completed a “pick-and-place” operation on 400 μm diameter solder balls, showcasing its potential in electronic packaging applications.

4. Discussion and Improvements

Although the microgripper exhibited excellent performance, there is still room for improvement. For example, the large gaps between adjacent actuators may affect the grasping of smaller objects. The introduction of through-silicon via (TSV) technology could further enhance device integration. Additionally, the uneven thermal distribution of the actuators resulted in lower temperatures at the roots, affecting deformation. Future work could consider using thermal isolation materials to address this issue.

Conclusion

This study developed an electrothermally driven microgripper based on Al-SiO₂ bimorphs, featuring low driving voltage, large deformation, and fast response, making it suitable for various micromanipulation applications. Vibration and impact tests verified its excellent gripping performance and reliability. The microgripper has broad application prospects in fields such as electronic packaging.

Research Highlights

1. Large Deformation and Fast Response: The microgripper achieved over 100 degrees of bending deformation at 5 V, with a response time of less than 10 ms.
2. Strong Gripping Force: The microgripper demonstrated excellent gripping performance in vibration and impact tests, withstanding impact accelerations of up to 1600 g.
3. Independent Control: The four actuators could be controlled independently, enabling flexible operation modes.
4. Wide Applications: The successful “pick-and-place” operation on solder balls showcased its potential in practical applications, particularly in electronic packaging.

Research Significance

This study provides a high-performance microgripper for the field of micromanipulation, featuring low driving voltage, large deformation, and fast response, making it suitable for various microassembly and micromanipulation applications. Particularly in the field of electronic packaging, the microgripper enables high-precision solder ball manipulation, offering significant application value.