Image reversal revisited

Image reversal methods can be useful in those cases, where a resist of a specific tone is lacking particular desirable properties, e.g. etch resistance. Reversal of the image, e-beam written in Hydrogen SilsesQuioxane (HSQ), by a proper ‘work around’ method may result in a positive tone high resolution process with a high etch resistance, which is lacking in PMMA. HSQ patterns have already been reversed in tone by electroplating in between HSQ lines, and by chemical mechanical polishing (CMP) of silicon nitride or silicon oxide deposited on top of the HSQ patterns. The present paper shows the latest results in our continuous quest for such processes: a bi-layer to bi-layer image reversal method based on spin coating, CMP and Reactive Ion Etching (RIE). The achievable resolution depends on the original HSQ resolution, the anisotropy of the RIE process and the ability of the HSQ spin deposition or the CVD oxide process to fill the spaces in between the original bi-layer features. In this particular set-up, the resolution seems to be limited to 50nm. A general flowchart schedule is presented which clarifies the relation between various image reversal techniques, e.g. electroplating, CMP and lift-off.