Analysis of morphological, electrical, and optical properties of the microheater
Electrospun nanofibers are subsequently metallized via an electroless deposition process and transformed into microheaters. These microheaters operate on the principle of Joule heating, where thermal energy is produced upon the application of external electrical energy [50]. High performance in microheaters can be evaluated by their ability to generate high temperatures under low power [53]. Dense geometry in the nanofibers is beneficial for thermal stability and heat transfer, as an increase in the percentage of the metal fiber network deposited on the substrate results in a higher generation of thermal energy [52]. Figure 2 depicts an optical microscope image that shows the distribution of PVP-palladium nanofibers collected on a silicon nitride membrane as a function of electrospinning time. Since the quantity of fibers collected varied with the electrospinning time, we compared them to ascertain optimal conditions. The optical image was digitized with a threshold to compute the fiber distribution ratio. As demonstrated in Fig. 2C, a sufficient number of nanofibers were efficiently collected at 30 seconds.
Figure 3 presents the electrical and optical characteristics of fabricated microheaters. Figure 3A shows the variation in surface resistance of the microheater with respect to heat treatment time on a logarithmic scale. From 3 to 10 minutes, the surface resistance of the microheater decreased rapidly with increasing heat treatment time, eventually saturating at 5.71 Ω/sq, representing an 89.9% decrease. This trend was attributed to the elimination of intersections between nanofibers during heat treatment, and the formation of a seed layer with stably embedded palladium as the PVP component in the fibers decomposed due to heat.
Figure 3B displays the variation in surface resistance of the microheater with respect to heat treatment temperature and time. The blue and red dots represent heat treatment times fixed at 30 and 60 minutes, respectively, with the heat treatment temperature rising from 200°C to 350°C. For 30 minutes at 200–300°C, the microheater’s surface resistance decreased from 12.42 to 5.5 Ω/sq as the annealing temperature increased, before increasing to 6.38 Ω/sq at 350°C. Similarly, for 60 minutes at 200–300°C, the microheater’s surface resistance decreased from 13.39 to 8 Ω/sq as the annealing temperature increased, but then rose to 9.98 Ω/sq at 350°C. This suggests that overlaps between nanofibers and PVP components are effectively removed and decomposed as the heat treatment temperature rises, but the resistivity gradually increases beyond a certain temperature.
Figure 3C shows the variation in surface resistance of the microheater with respect to the copper electroless deposition time. From 30 to 90 seconds, the microheater’s surface resistance gradually decreased from 1.36 to 1.34 Ω/sq with increased deposition time, and then sharply fell to 0.22 Ω/sq after 120 seconds. As the electroless deposition time increases, the thickness and linewidth of copper growing on the palladium-embedded seed layer increase, leading to enhanced electrical conductivity [53].
Figure 3D presents the transmittance of the microheater as a function of copper electroless deposition time, across a wavelength range of 300–900 nm. When electrospun nanofibers of the same density were electrolessly plated for 30, 60, 90, 120, 150, or 180 seconds, the microheaters exhibited average transmittance of 98.09%, 94.44%, 77.2%, 49.8%, 38.61%, and 14.50%, respectively. As the electroless deposition time increased, the quantity of copper deposited increased, resulting in a gradual decrease in transmittance. From Figs. 3C and D, it is evident that the surface resistance of the microheater and the transmittance in the visible region can be controlled by adjusting the electroless deposition time.
Figure 4 presents the changes in nanofiber morphology during the fabrication stages of the microheater. Figure 4A, A' displays the morphology of PVP-palladium nanofibers electrospun onto a silicon nitride membrane. Figure 4B, B' presents the seed layer with a reduced linewidth due to the thermal degradation of PVP. The intersections occurring between nanofibers were also eliminated [53]. Figure 4C, C' illustrates the nanofiber network fabricated through 2 minutes of electroless deposition, with a significant increase in linewidth. Figure 4D demonstrates the variation in nanofiber linewidth according to the microheater fabrication steps. The PVP-palladium nanofibers fabricated by electrospinning averaged 505.45 nm, which decreased to 183.25 nm after heat treatment due to PVP decomposition. After 2 minutes of copper electroless deposition, the linewidth expanded to 1296.57 nm. Figure 4E displays the variation in nanofiber linewidth with copper electroless deposition time. The linewidth increases with increasing electroless deposition time.
Performance evaluation of microheater
Figure 5 presents an evaluation of the final microheater performance. Figure 5A displays the temperature of the microheater as a function of current. When a constant current was applied, the temperature rose and then remained stable within less than 5 seconds; the average temperatures recorded were 70 ℃, 130 ℃, 200 ℃, and 350 ℃ when the current of 0.2 A, 0.4 A, 0.6 A, 0.8 A was applied, respectively. The microheater reached a maximum temperature of 353 ℃ at a current of 0.8 A, after which the silicon nitride membrane melted and the microheater broke. Figure 5B displays the final temperature distribution across each section during current application, captured using an infrared camera. In Fig. 5B I, II, III, and IV, final temperatures of 70 ℃, 130 ℃, 200 ℃, and 350 ℃ were achieved in each section at a current of 0.2, 0.4, 0.6, and 0.8 A, respectively. The infrared temperature distribution from I to IV gradually shifted from purple to white, confirming the high uniformity of heating. This process enabled the fabrication of a high-efficiency microheater with a fast thermal response and high heating temperature, even at low power.