Development of laser direct writing for fabrication of micro/nano-scale magnetic structures

• Main Author: Alasadi, Alaa
• Other Authors: Allwood, Dan ; Frederik, Claeyssens
• Published: University of Sheffield 2018
• Subjects: 620
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.766543

Traditional lithographic techniques used to fabricate a magnetic structure are often complex, time consuming, dependent on other techniques and expensive. Laser direct writing (LDW) can potentially overcome many of these drawbacks and may be a cheaper, faster and easier route to fabricating technique micro-/nano-magnetic structures. The main aim of this project is to fabricate magnetic structures through LDW. Two types of LDW were used to fabricate magnetic structures: subtractive LDW (LDW-) and laser-induced forward transfer (LIFT). LIFT was used to transfer permalloy (Ni81Fe19) using three laser systems. Numerous parameters were varied, including thin film thickness, scanning speed, pulse energy, distance between donor/acceptor and acceptor material. These attempts did not succeed in transferring the magnetic materials as a uniform shape. The differences of heat conductivity between the permalloy and acceptor substrate (glass and silicon), shock wave effects and the landing speed of material on the acceptor are the most possible reasons that the uniform structures and the magnetic properties were lost. LDW- was used to successfully pattern 90nm thick Permalloy into 1-D and 2-D microstructures. Magnetic wires with a range of widths, arrays of squares, rectangles with a range of aspect ratios and rhombic elements were patterned. These structures were fabricated using an 800-picosecond pulse laser and a 0.75 NA lens to give a 1.85µm diameter spot. Scan speeds were controlled to give 30% overlap between successive laser pulses and reduce the extent of width modulation in the final structures compared with lower levels of pulse overlap. Continuous magnetic wires that adjoined the rest of the film were fabricated with widths from 150 nm - 6.7µm and showed coercivity reducing across this range from 47 Oe to 10 Oe. Squares, rectangles and diamonds These elements demonstrated shape-sensitive magnetic behaviour with increasing the shape aspect ratio. Wires of different width were also fabricated by LDW- and their anisotropic magnetoresistance (AMR) determined to show a simple width-dependent magnetic field response, making them interesting as magnetic field sensors. This approach is extremely rapid and does not requires masks or chemical processing as part of the patterning procedure. The time required to patterned 1-D area of 4 x 0.18 mm was 85 s and the average fabrication time per element of 2-D structures was 4.7x10 4 s. The microstructures may be of use for AMR sensors or for biological applications, such as cell trapping.