High contrast 3D proximity correction for electron-beam lithography: An enabling technique for the fabrication of suspended masks for complete device fabrication within an UHV environment

Many devices, such as lateral spin valves, depend critically on the quality of interfaces formed between different materials, and hence require the entire device to be fabricated within an ultra-high vacuum environment. This is possible using angled deposition with a suspended mask such that, by depositing from specific angles, different patterns form on the substrate beneath. We use a bi-layer of MMA(8.5)-MAA copolymer and PMMA patterned by electron-beam lithography (EBL) to form such a mask. It is necessary, though, to perform proximity effect correction (PEC) in EBL to achieve the correct spatial distribution of electrons, and hence produce the desired pattern in the developed resist. For bi-layer processes this is a three-dimensional (3D) correction since we must optimise for two different critical doses (one for the copolymer, the other for the PMMA) at defined 3D positions within the resist stack. We perform this 3D correction using the commercial software BEAMER produced by GenISys GmbH. We show that by applying manual shape segregation and modulation to the exposure pattern, prior to the "3DPEC" algorithm, it is possible to achieve much higher contrasts in the spatial distribution of absorbed energy and hence significantly increase the processing window, and yield in the fabrication of suspended masks.