Negative hybrid sol-gel resist as hard etching mask for pattern transfer with dry etching

Hybrid organic-inorganic (HOI) sol-gel systems are emerging as a flexible class of materials in micro-fabrication, since they can be directly structured by different lithographic techniques (UV, EBL, hard and soft X-ray). In this work, we explored a new sol-gel system to be used as a sacrificial layer, highly resistant as an etch mask in fluorine containing plasmas. Commonly, the etch selectivity for hard materials (e.g. Si, W, SiC among others) with respect to most common resists is less or much less than 10:1, depending also on the plasma chemistry and details of the process and of the reactor. A previous contribution demonstrated how the selectivity of a plasma etching process for silicon masked by a hybrid sol-gel system containing a high concentration of boehmite nano-particles (Al 2O 3/AlO(OH)) could reach values as high as 60, in a continuous process (i.e. non-switched) in ICP reactor with SF 6, C 4F 8, Ar gas mixture. In order to improve the performance as a resist of such a HOI, we present a novel system which avoids loading the material with a solid fraction. The target of the synthesis is to condense in situ an alumina based network, starting from an aluminum-containing precursor. The alumina content in the final material is up to 80%. By means of soft X-ray lithography (XRL), high resolution pattern were produced in a film that after development resulted having a thickness of 30 nm. The new synthesized system shows extremely high etch selectivity even after mild thermal treatments (<100 °C). On the contrary, common hybrid systems generally require thermal treatments at relatively high temperature to convert the structures in dense ceramic layers (silica, usually), thus improving their selectivity in dry etching processes. Plasma etching in an ICP reactor was then used for pattern transfer. A process characterized by a silicon etching rate of about 170 nm/min was applied for different times, resulting in >3 ?m tall structures without appreciable loss in the critical dimension (CD) and almost vertical sidewalls. The resist showed thus a selectivity exceeding 60:1, on structures with details at the 200 nm scale. © 2012 Elsevier B.V. All rights reserved.