Etching in Two Easy Steps! - Diego Alba

As we finish making the last batch of nanogrippers, a novel type of  self-folding tetrodes capable of holding single cells, I went ahead and prepared an older wafer in order to start imaging and measuring. On a single wafer, there can be up to 18 dies, and each can hold between 20 and 48 grippers, with their respective gold lines. This dies will be the actual chips, so they must be separated from the wafer in a process called “wafer dicing”. Then, each die must be “etched”, which means to remove a thin layer of material, in this case copper. This is a critical step, since copper is toxic for the cells, but also holds the grippers open: if too much of the copper is etched, the grippers will close without having a cell inside; on the other hand, if there’s too much copper left, the cells won’t survive.

Etching is carried out in two steps. Firstly, only the part that connects the die to the printed circuit board (PCB) is etched. This also facilitates the next step, which is wire bonding the die to the PCB, a process carried out by a completely different department and can take from two days to two weeks. Once we receive the chips back, we cover the connecting wires with epoxy, since the chips will be submerged in a conductive solution and would short circuit, and put a ring around the die in order to hold the cell medium. Next, for the second etching step, we add the etching solution and put the chip under a microscope, allowing us to visually follow the reaction. When most of the copper has dissolved, the chip is rinsed with distilled water and then 70% ethanol, for sterilization.  

Now the chips are ready for cell seeding: cardiac cells from rats will be placed on top of the die and allowed to grow for a few days. The cell solution will dissolve the minimal amount of copper left, making the grippers close without damaging the cells. The next step will be to image the grippers with cells and record action potentials.