Lift-off of thick metal layers using multilayer resist

Abstract

We have developed a modified multilayer resist technique which allows lift-off of very thick (≳2 μm) metal layers while retaining fine resolution. The trilayer structure consists of a thick (3 μm) lower layer of AZ 2430, the top of which is plasma-hardened in a CF4 discharge in order to render it impervious to solvents, an intermediate layer of SiO (∠1000 Å), and a thinner (1 μm) upper resist layer, typically AZ 1350 J. After standard patterning of the upper resist, the pattern is transferred to the SiO and lower resist layers by reactive ion etching in CF4 and O2 respectively. By varying the pressure of the O2 plasma, either vertical walls, or walls with controlled undercut were produced. Because the lower resist layer remains soluble in acetone, it is easily removed, making this method appropriate for lift-off. Using this technique, we have made 2 μm thick microwave electrodes of aluminum and of gold coated copper. This procedure is potentially useful for fabricating x-ray masks or other structures requiring thick high resolution metal patterns, as well as for these specialized electrodes.