Lithographic procedures for fabrication of highly precise micron-scale conductive tracks on a variety of substrates has already become an industrial standard. We demonstrate a tabletop solution for prototyping and small series production, the miDALIX DaLI, suitable for delicate lift-off micropatterning using direct laser maskless lithography. Single-micron sized features and spacing is achieved using a two-layer photoresist on silicon wafer, aimed at characterizing novel materials’ electronic properties.
Endless demand for faster computing and computer memory devices is pushing the development of technological advances aimed at miniaturization and novel materials, as further improvements in speed and power efficiency are sought. On the other hand, scientific interest in nano- and submicron-scale science is growing year-by-year. Lithographic structures combined with thin (metal) layer deposition techniques have become a standard in both scientific1,2 and industrial3 environments, e.g. for characterization of new materials, lab-on-a-chip devices, nanowire analysis, and industrial semiconductor wafer fabrication. The standard approach combines masks and homogeneous illumination to achieve selective activation of photoresists.
While masks allow for fast and reproducible production cycles, design changes 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 feature, as shown in Figure 1, would not even be possible.
It uses precise alignment on individual features and/or frequent design changes to profit from the use of direct laser maskless lithography. Here we demonstrate delicate lift-off micropatterning using the MiDalix DaLI device for novel material characterization applications. We used a standard double layer positive photoresist for lift-off technique, achieving single-micron sized features and spacing in the final golden electrodes on silicon substrate.
Maskless lift-off lithography
Wafers up to 4 inches in diameter fit into the MiDalix DaLI’s material holder, with silicon being the typical material of choice due to either its semiconductivity, excellent thermal mechanical properties, or surface uniformity and adhesiveness. The latter is important for bonding the resist layers to the substrate and consequently holding (sub-)micron structures in place during both the illumination and further developing and rinsing steps.
The silicon substrate was covered first with a layer of lift-off promoting resist, insensitive to the light. We have chosen the Allresist AR-P 5480 and spin-coated it at 4000 RPM for 1 minute, obtaining an approximately 400 nm thick bottom layer. After a 6-minute soft bake at 150°C and cooldown to room temperature, the light sensitive photoresist was added as a top layer. We have chosen the Allresist AR-P 3510 as a compatible and manufacturer’s recommended counterpart for lift-off lithography. The top layer photoresist was again spin-coated at 4000 RPM for 1 minute and then introduced to a 2-minute soft bake at 100°C. The resulting top layer thickness was around 2.0 µm (as seen in Figure 2).
Direct laser illumination of the photoresist layer is controlled by proprietary software, enabling tuning of structure raster density, laser spot size, illumination dose, and various other parameters. The laser spot moves from spot to spot instantaneously and in an extremely precise fashion due to beam steering with acousto-optical deflectors (AODs). Contrary to other popular beam steering devices, e.g. galvo scanners, AODs feature no moving parts and an entirely digital driver, ensuring single-nanometer beam positioning precision and resolution when coupled to other optics and mechanics in the fashion of DaLI. The maskless lithography is then realized by the laser beam selectively illuminating the desired areas of the photoresist.
While creation of fine features requires a highly focused laser beam to achieve the spatial resolution, those features need connections to the outside world. miDALIX DaLI features an automatically exchangeable laser spot size to speed up the illumination process – switching from Fine tool to Coarse tool in a matter of seconds, increasing the spot size threefold. The increase in spot size switches to a high-speed gear, bringing a 10-fold decrease in the illumination time compared to the Fine tool.
After illumination, further processes are used to develop the photoresist to create overhanging structures, add the gold layer, and finally perform the lift-off step. The combination of two materials and different layer thicknesses allowed us to create overhanging structures through a single development step, using diluted Allresist AR 300-47 developer with deionized water in a ratio of 2:3, respectively, for 60 seconds. The resulting overhangs provide cover for sputtering or evaporation processes (Figure 2) and ensure smooth edges of the final, structured gold layer after the lift-off process. The chemical development step was followed by deposition of a gold layer approx. 100 nm thick. The final lift-off process is propagated using an acetone bath for two hours, which fully dissolves both resist layers. In combination with the overhanging structures, the overall process is delicate enough not to damage the gold structures and ensures sharp edges. The structures in gold were manufactured below 1 µm in size (as shown in Figure 3), while the same can be set for gaps in-between (spacing).
The miDALIX DaLI was used for delicate lift-off micropatterning of gold electrodes on a silicon substrate, using near-UV direct laser maskless lithography. We demonstrated high precision and sub-micrometer sized final structures in the gold layer. The device design ensures repeatability of the process across whole 4” wafers as well as among multiple subsequent batches, using high performance AODs, a stabilized laser source, and integrated self-calibration capabilities.
This application is but one of the machine’s possible applications, also excelling at manufacturing of microfluidic circuits, photomasks, micro-optics, etc.
- Stojchevska, L. et al. Ultrafast Switching to a Stable Hidden Quantum State in an Electronic Crystal. Science 344, 177–180 (2014).
- Vaskivskyi, I. et al. Fast electronic resistance switching involving hidden charge density wave states. Nature Communications 7, 11442 (2016).
3. Wolf, S. & Tauber, R. N. Silicon Processing for the VLSI Era: Process technology. (Lattice Press, 2000).