Laser Induced Three Dimensional Metal Nanowriting On Nonplanar Substrate And Its Application On LoC

Recent years, metal nanowiring for interconnection has attracted much attention due to the growing requirements on highly integrated microcircuits which is of benefit to miniaturization of devices feature. To the best of our knowledge, both photolithography and TPA micronanofabrications are generally constructed on flat substrates.In this paper, we demonstrate flexibly metal nanowiring on nonplanar substrates by femtosecond laser induced electroless plating .The fundamental concept is, immersing the surface where metal interconnection is to be made with a metal salt solution, and then tightly focusing a near infrared femtosecond laser beam , to the site to wire. Because of the negligible linear absorption of the solution at the laser wavelength, the beam penetrates deeply into the solution without power loss, while it pinpoint triggers photoreductive reactions of the metal ion via multiphoton absorption at the focal spot. Scanning the laser focus along a preprogrammed trace on the top surfaces, either flat or nonplanar, desired metal nanowiring is thus achieved.In this work, transparent silver precursor prepared by silver nitrate, ammonia and trisodium citrate was used as metal source. Through the optical absorption spectrum of the silver precursor, It could be clearly identified that there is only one absorption band peaking at 302 nm, attributable to silver-ion. The absence of absorption at ~790 nm (laser wavelength) indicates that the photoreduction of silver-ion is a multiphoton absorption (MPA) process, most probably two-photon absorption, which is of benefit to both the deep transmission of laser and high precision writing.The width of the silver nanowires can be adjusted by simple control of laser power. The thinnest silver nanowire patterned by the laser of 1.2 mW was only 125 nm, suggesting its high resolution. In addition, the surfaces of these silver nanowires are smooth and no obvious bulky silver particles were formed all over the nanowires, indicating that our silver precursor is very tractable for microstructuring. We estimated the resistivity of our femtosecond laser reduced silver is estimated to be ~1.7×10-7 ?m, which is only 10 times larger than that of bulk silver (1.6×10-8 ?m), as might be due to the slight oxidation on the surface.One of the greatest advantages of femtosecond laser micronanoprocessing is its flexibility for integration. Through careful control of the laser spot, various 3D structures with nanometer spatial resolution could be fabricated on any substrates. in this work, the powerful femtosecond laser induce silver electroless plating was also applied to the substrates with 3D microstructures. A hemisphere with a radius of 20μm was firstly prepared through two-photon absorption (TPA) induced photopolymerization of SU-8 . In this case, the above silver microheater was patterned on this hemisphere. Notably, the silver microwire was well connected around this curve surface. Figure 3c gives another example, five frustum of pyramids were also fabricated previously on the substrate for silver nanowiring. A single silver wire circle which passes the frustum of pyramids was created. Clearly, when viewed form top, the patterned silver wire is well round even on the 3D structured substrate. Magnified SEM images shows that the silver wire has a good continuity even at the corners.For demonstrating the application of this novel metal nanowiring, a simple microheater was prepared on the bottom of a"Y-shaped"microchannel. Then a PDMS slab was covered on the microfluidic chip to form a sealed channel. This heating bubbles indicates that our microheater efficiently calorified the local liquids. In order to analyze the heating process in a quantificational manner, local temperature inside the channel was monitored by the photoluminescence (PL) spectra of 1-pyrensulfonic acid sodium salt (PS-Na).The accuracy of this method for temperature determination is about±1°C. This microheater which is set directly inside the microchannel would show great potential for highly localized temperature regulation. The femtosecons laser induced metal nanowiring would open a door for flexible integration of microcircuits, and therefore would be widely used in MEMS, LoC and other intelligent microsystems.