Endless demand for faster computing and computer memory devices is pushing the development of technological advances aimed at miniaturization and novel materials. Improvements in speed and power efficiency of computer memory elements are also sought at Nanocenter, the umbrella organization of several Slovenian research institutions and technology companies which provides facilities and state of the art equipment in the field of nanotechnology. An important piece of their equipment is the miDALIX’s DaLI. A maskless laser direct writing device that was used in the process, which led to an impressive discovery, enabling new concepts in non-volatile memory devices.
The client faced a challenge in an incorporated experimental setup. Its core part was a device consisting of thin (50–100 nm) single crystal flakes of 1T–TaS2 on a sapphire substrate, with electrical contacts deposited over the crystal. Four gold electrodes needed to be deposited on top of the selected crystal flakes. The key technology for that is lithography combined with sputtering or evaporation of gold layer.
The standard lithography approach combines masks and homogeneous illumination to achieve selective activation of photoresists. While masks allow for fast and reproducible production cycles, design changes required in this project are not possible and precise alignment with existing features requires advanced equipment and recognizable positioning markers. Alignment of a mask with an individual micron-sized crystal flakes would not even be possible.
This lithography project required precise alignment on individual crystal flakes and frequent design changes as geometry and size varies with each crystal. Therefore, use of an affordable direct laser maskless lithography system, as miDALIX’s DaLI is, was an optimal solution.
The 50 to 100 nm thick crystal flakes were deposited on sapphire substrate and covered with a photoresist by spin-coating. The tiny (< 5×5 mm2) substrate was then fixed on a DaLI’s substrate holder and inserted into the device. The appropriate crystal candidates with few to several tens of micrometers in size were selected with the use of an integrated microscope. In the next step electrodes’ geometry was designed based on experiment’s requirement and single crystal shape using the integrated software, followed by the assignment of laser illumination parameters.
The complete direct laser illumination process was then started by a single click and controlled by software. After photoresist development the electrodes on 1T–TaS2 crystals were generated by gold deposition.
The electrodes in gold were manufactured below 1 µm in width with positioning accuracy lower than 1 µm. The maskless lithography enabled by DaLI delivered a small but important piece of the whole research and characterization of the phenomena that was also published in NATURE COMMUNICATIONS under title “Fast electronic resistance switching involving hidden charge density wave states”.