High Aspect Ratio 3D-Printed Copper Pillars with Integrated Piezoresistors for High Sensitivity Force Sensing

This work presents the manufacturing and testing of MEMS lateral force, $\pm \mathrm{F}_{\mathrm{x}}$ sensors that integrate 3D printed copper pillars, on top of underlying in-plane, stress-sensing microelectronics. Using 3D metal printing allows for manufacturing of freestanding, upright pillars with (height/diameter) aspect ratios > 100, which enables higher sensor sensitivity to flow and/or lateral force $\pm \mathrm{F}_{\mathrm{x}}$. In addition, forming the sensor structural element out-of-plane allows for a compact on-substrate, sensor footprint of a few $\mu \mathrm{m}^{2}$, at least two orders lower than existing MEMS lateral force sensors. The 3D printing of copper pillars involves an electrodeposition process, which requires sample submersion in an acidic electrolyte, and an applied voltage bias with respect to the electroplating solution. This paper details our method of preparation to make the microelectronic chips, with active N-type piezoresistive devices, compatible with the acidic electrodeposition process. We also discuss how we achieve $\sim\pm 0.3\ \mu \mathrm{m}$ alignment of the 3D printed pillars to the sub-micron piezoresistor widths, despite poor system optics, using 2.5D/3D alignment marks and height mapping. To demonstrate that the microelectronics work post-electrodeposition, the pillar-based sensors were tested for lateral force $\mathrm{F}_{\mathrm{x}}$ sensitivity. Preliminary testing shows $\Delta \mathrm{R}/\mathrm{R}$ sensitivity to Fx of up to 0.23% $\mu \mathrm{N}^{-1}$. The high $\mathrm{F}_{\mathrm{x}}$ sensitivity and confined in-plane footprint, allows for future expansion into dense sensor arrays for quantifying biological cell contractions with high throughput.