A microgripper based on electrothermal Al-SiO2 bimorphs

Abstract

Microgrippers are essential for assembly and manipulation at the micro- and nano-scales, facilitating important applications in microelectronics, MEMS, and biomedical engineering. To guarantee the safe handling of delicate materials and micro-objects, a microgripper needs to be designed to operate with exceptional precision, rapid response, user-friendly operation, strong reliability, and low power consumption. In this study, we develop an electrothermal actuated microgripper with Al-SiO₂ bimorphs as the primary structural element. The fabricated microgripper naturally adopts a closed state due to process-induced residual stresses. The thermal expansion mismatch between Al and SiO₂ allows for an easy transition of the microgripper between open and closed states by temperature control. Experimental data reveal that the microgripper can achieve impressive deformability, bending over 100 degrees at just 5 V, and responding within 10 ms. Its capability to handle micro-objects is verified using polymethyl methacrylate (PMMA) microbeads and its gripping strength is quantitatively assessed. It is demonstrated that the microgripper holding a microbead with a diameter of 400 μm and a weight of 0.1 mg can withstand an average acceleration of 35 g during vibration test and over 1600 g in impact tests, highlighting its exceptional grasping performance. Additionally, the "pick-and-place" task for handling and positioning solder beads (0.25 mg for each bead) with diameters of 400 μm on a bulk silicon inductor chip has been successfully completed. This unique microgripper is anticipated to be highly beneficial for various micro-assembly and micromanipulation applications, particularly in the field of electronic packaging.

Fig. 1. The concept of the microgripper.

a The formation of curved actuators based on the utilization of residual stresses. b The operating method of the microgripper

Fig. 2. Design of the electrothermal microgripper.

a Working principle of the electrothermal bimorph actuator. b Topology design. c Cross-sectional view of two adjacent bimorphs. d Enlarged view of an actuator showing the key structural parameters. D: diameter of the center plate. L: length of the actuator. W_b: width of the bimorph. W_g: width of the release gap

Fig. 3. Fabrication process steps.

a, b PECVD-based SiO₂ depostion and dry etch. c Sputtering Pt & lift-off. d PECVD SiO₂ deposition and dry etch. e Sputtering Al and dry etch. f Device release

Fig. 4. SEM images of fabricated microgrippers.

a, b Bimorph beams are arranged along the longitude direction. c, d Bimorph beams are arranged along a 45-degree direction. e, f Bimorph beams are arranged along the latitude direction. The scale bar in all figures are 250 μm

Fig. 5. Actuation characteristics of the microgrippers.

a(I) Profile of the bending angle versus applied DC voltage for the actuators. a(II–VII) Shapes of an actuator at different driving voltages. b Profile of the average temperature versus applied DC voltage for the actuators. c Profile of the DC power versus applied DC voltage for the actuators. d Curves of the thermal response time of the microgripper

Fig. 6. Manipulation process.

a Schematic illustration of the experiment setup. b–g Step-by-step manipulation process. h, i SEM images of the microgripper after capturing the microbead

Fig. 7. Reliability tests.

a, b Two wrapping modes of the microgripper. c, d Detailed setup of the vibration test. e Vibration test results of the two wrapping modes. f, g Detailed setup of the impact test. h Impact test results of the two wrapping modes

Fig. 8. Potential applications use of microgrippers in electronic packaging.

a–c Micrgrippers complete the handling and alignment of solder beads on the surface of inductors. d Schematic diagram of microgrippers completing solder bead handling and alignment in a small system. e–h Manipulation process of the microgripper. All scale bars in Fig. 8 are 500 μm