New-Tech Europe Magazine | Q2 2022

Figure 4. Demonstration of the write process: (fluorescent) polystyrene nanoparticles are attracted by an alternating electric field generated by electrodes addressed in a checkboard arrangement.

Figure 3. Schematic of the colloidal memory concept (also presented at IMW 2022).

in this bit sequence that information can be encoded. The nanoparticles can be selectively induced (and sensed) by electrodes positioned at the entrance of each capillary. A CMOS peripheral circuit controls the array of electrodes. One of the main challenges relates to ‘wr i t ing’ the sequence of the nanoparticles, in other words, to attract and insert particles into the capillary selectively. Imec researchers are exploring both theoretically and experimentally the feasibi l ity of using frequency- dependent dielectrophoresis as a write mechanism. Following this mechanism, an alternating electric field generated across the electrode exerts a force on the nanoparticle. Whether this force is attractive or repulsive depends, among other things, on the type of particle and the frequency of the evoked electric field. A selective writing process can be created by choosing two particles that respond differently to the applied frequency (attractive versus repulsive). The colloidal memory technology is in an exploratory stage of research and development. The first set of experiments with µm-sized electrodes in different configurations (including interdigital and checkboard arranged arrays) marked the first milestone. Using the dielectrophoresis effect, they showed the feasibility of selectively extracting polystyrene nanoparticles from a mixed solution. But the required technology still

the common counter electrode form an electrochemical cell for each capillary. The dense array of working electrodes is connected to a CMOS integrated circuit for addressing each electrode individually. By applying a certain potential at the working electrode within the capillary, thin layers of metal A can be deposited on the electrode. Metal B will behave similarly but deposits at a different onset potential – determined by its chemical nature. Information can now be encoded in the stack of alternating layers, suggestive of geolithical stone (lithos) strata – hence the name of the new memory. We can now think of several ways to encode the information. In one possible encoding scheme, 1nm of metal A can be used to encode binary 0, while 2nm thick layers of A encodes a binary 1. A layer of metal B of fixed thickness (e.g., 0.5nm) can be used to delineate subsequent layers of A. In reality, assuming a higher onset potential of B compared to A, layers

needs significant development. Further investigations are ongoing to finetune the concept and provide the first proof of principle on a nanometer scale. Electrolithic memory: exploiting electrochemistry Like the colloidal memory, the electrolithic memory also uses a fluid reservoir and an array of capillaries. But in this case, metal ions are dissolved in the liquid, and the read and write operations are achieved by the more conventional electrodeposition and -dissolution techniques. In more detail, the reservoir contains a fluid in which (at least) two metal ions (A and B) are dissolved. This reservoir connects to an array of capillaries (or wells). A working electrode (made of an inert metal such as ruthenium (Ru)) resides at the bottom of each capillary. The reservoir is also in contact with a single counter electrode. Together, the reservoir, the working electrode, and

Figure 5. Schematic of the electrolithic memory concept (also presented at IMW 2022).

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