Improvement of Mechanical/Electrical Properties of Nano-Metal Film for Neural Electrode with Curing Rate Adjustment of Photosensitive Polyimide

Abstract

Recently, research on Electroceuticals has been actively conducted as a new treatment technology for intractable chronic diseases, and one of the main technologies that make up Electroceuticals is the neural interfaces. Basically, neural interfaces require high safety and reliability because body implantation is inevitable. In terms of safety, neural interfaces are fabricated based on flexible polymer-based materials to minimize mechanical mismatch with nerves or tissues, and polyimide is used as a representative material (Fig. 1(a)) [1]. It is sufficient to secure safety in the body compared to rigid or brittle materials just by configuring the main components of the neural interfaces with flexible materials. Concerning the reliability, it is demanded at first to improve the adhesion and electrical characteristics of the metal thin film deposited on the polyimide. For these purposes, it is common to approach by increasing surface roughness by attaching additional materials such as nanoparticles on deposited metal thin film or surface treatment method of fully cured polyimide [2, 3]. However, as a result, there is a limit to characteristic retention period after the improvement of the adhesion with the metal thin film and the electrical properties. Accordingly, this paper presents a way to improve the mechanical/electrical properties of metal thin films by adjusting process steps and the curing conditions of PSPI (Photosensitive Polyimide), which is frequently used to implement flexible neural interfaces. For experimental verification, samples composed of PSPI and Au thin films have been fabricated using the MEMS process as follow; a 5 μm thick PSPI pattern was formed on a piranha cleaned silicon wafer, deposited Au 3000？, and then a PR mask was formed for a dry etching mask, and the same shapes of Au layer as PSPI by dry etching with ICP-RIE were formed. The 1st PSPI sample was fully cured at 300°C for 1 hour before Au deposition, and the 2nd and 3rd PSPI samples were partially cured at 200°C for 20 minutes and then completed Au patterning. Thereafter, the 2nd sample has undergone a surface treatment with asher after partial curing and the 3rd sample has been subjected to additional PSPI full curing. The fabrication result is shown in Fig. 1 (b). As shown in Figure 2, SEM photography and EDS measurements were conducted to evaluate the surface characteristics depending on PSPI curing conditions, and it was clearly confirmed that the roughness of the Au surface changed whether the PSPI was fully cured before Au deposition. In addition, after Au patterning, it was found that grain boundaries were formed on the surface of Au in a sample without PSPI full curing process. Also, through the EDS measurement, it was confirmed that the deformation of the Au surface or the redeposition of other materials did not occur. Peel-off experiments using Scotch tape were conducted to evaluate the adhesion force of the Au thin film to PSPI. The Au thin film adhesion of the sample that underwent PSPI full curing process after Au patterning increased significantly as shown in Fig. 3(a). Note that the gold electrode was not debonded from the PSPI substrate but the tape is detatched from the gold electrode and thus the measured force is the maximum value of the scotch tape test. In addition, through the sheet resistance measurement of each sample, it was confirmed that it had a lower resistance and a smaller standard deviation under full curing after partial curing, as shown in Figure 3(b). As a result, we have improved the mechanical/electrical properties of nano-thick gold thin film by relativizing partial curing and full curing process with the time of depositing Au on PSPI. Based on these results, we plan to conduct further research on securing safety and stability through preclinical verification experiments.