AR-P 8100 (Phoenix 81) thermal pattern-able positive resist

High-contrast, thermal pattern-able resist for integrated circuit and holographic structure

Patterning of PPA with e-beam Lithography

PPA layers can also be positively patterned directly by electron irradiation. Similar to the irradiation of common- ly used e-beam resists like CSAR 62 or PMMA, electron beam exposure induces fragmentation of the polymer chains. Polymer fragments resulting from PPA are how- ever unstable and decompose into the volatile ortho- phthalaldehyde.

Only very small amounts of monomeric phthalaldehyde are directly released in the device during e-beam expo- sure; only the subsequent PEB leads to an almost com- plete thermal development. But even in the range of the dose to clear (approx. 30 - 40 μC/cm2), a resist layer with a thickness of a few nanometres will remain. A residue- free substrate surface can nevertheless be obtained if a short plasma etching step is added. The gradation passes through a minimum, but with increasing dose, also the concurrent cross-linking processes become increasingly important. This undesirable side reaction is due to radicals which are generated during electron irradiation and stabilise the layer by cross-linking. These effects also occur in PMMA layers, but only at much higher exposure doses, and are here used to produce negative PMMA architectures. To determine the resolution limits of AR-P 8100, line patterns were examined in detail at the company Raith. Lines of different width were written into the PPA layer. After PEB and subsequent platinum metallisation, metal bridges of < 20 nm width were obtained. The highest resolution that could be achieved was 16 nm.

Lines written in PPA (resist AR-P 8100

Bridge with width of 16 nm, obtained after sputter coating with platinum (film thickness: 4 nm)

Adding PAGs (photo acid generator) to PPA (sample SX AR-P 8100/5) can increase the sensitivity and allow a bet- ter control of the gradation. The exposure causes a release of acid in situ which decomposes the PPA layer at 95 - 100 °C during the following PEB (positive development). The thermally induced, solvent-free development proceeds al- most completely. Despite the addition of PAGs, a very thin residual resist layer however remains.

If PAG-containing resists are used together with AR-P 617 in two-layer process, the thin remaining resist layer will not disturb the further process sequence since it is dis- solved during the subsequent development. After e-beam exposure and PEB, bottom resist AR-P 617 is selectively developed with developer AR 600-50. The undercut is ad- justed specifically by varying the duration of the develop- ment step. Reliably processable lift-off resist architectures can thus be produced. This method allows the realisation of metal bridges (platinum):

The process window is however quite narrow; already small variations of the dose affect the obtained line width considerably.