Fabrication of microstructures with different aspect ratios in a single layer

Magnetic actuation has been very successful in both macro and micro scales and magnetic microactuators with different microstructures have also been extensively studied. Almost all of these microactuators are based on the electromagnetic effect and the force is generated by the electromagnetic coil, between the soft magnetic core and the (magnetic) cantilever. To integrate all of these magnetic components on the same chip, microstructures with different aspect ratios were fabricated in a single layer by using a photolithography process using the negative photoresist SU-8 2000. A copper microcoil, a permalloy core and a nickel metal post for suspending a cantilever beam were electroplated on the same seed layer with aspect ratios varying from 1:1, 5:1 to 10:1, and the dimensions of the features ranging from 25 µm to 500 µm. The three different features were fabricated separately in three layers. Different viscosity of SU-8 2000 was used in three layers. 50 µm, 80 µm and 200 µm thick SU-8 are used to pattern the molds for electroplating three different features. 25 µm thick copper microcoils with an aspect ratio 1:1 was first electroplated on the Ti/Cu/Ti seed layer. This is followed by electroplating of 50 µm thick permalloy layer with an aspect ratio 10:1 on the same seed layer. A 100 µm thick copper post with an aspect ratio 5:1 was electroplated on to the same seed layer in the next step. SU-8 resist molds were removed between each of these layers. Finally, the residual SU-8 was cleaned by CF4 plasma etching. The coils have 19 turns with a footprint area of 10 mm2. The microcoils were tested to ascertain their maximum current densities before burnout. Similarly, the magnetic properties of the Permalloy core were also tested. The profiles of different layers and the coil and permalloy core test results were discussed in detail in this paper