Patterning of PPA with e-beam lithography
PPA layers can also be positively patterned directly by electron irradiation. Similar to the irradiation of commonly used e-beam resists like CSAR 62 or PMMA, electron beam exposure induces fragmentation of the polymer chains. Polymer fragments resulting from PPA are however unstable and decompose into the volatile orthophthalaldehyde. Only very small amounts of monomeric phthalaldehyde are directly released in the device during e-beam exposure; only the subsequent PEB leads to an almost complete 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 residuefree 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 better 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 almost completely. Despite the addition of PAGs, a very thin residual resist layer however remains. Gradation of SX AR-P 8100/5 after PEB at 98 °C 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 dissolved 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 adjusted specifically by varying the duration of the development step. Reliably processable lift-off resist architectures can thus be produced. This method allows the realisation of metal bridges (platinum): [/av_textblock] [/av_two_fifth][av_one_half min_height='' vertical_alignment='' space='' custom_margin='' margin='0px' padding='0px' border='' border_color='' radius='0px' background_color='' src='' background_position='top left' background_repeat='no-repeat' animation='' mobile_display='' av_uid='av-4u16rqo'] [av_textblock size='' av-medium-font-size='' av-small-font-size='' av-mini-font-size='' font_color='' color='' id='' custom_class='' av_uid='av-4dicl8w' admin_preview_bg='']
Patterning of PPA with photolithography
PPA layers can also be structured directly by means of photolithography. Irradiation with UV-light of a wavelength of < 300 nm results in a cleavage of the polymer chains to form volatile components. By adding PAGs (photo acid generators), the photosensitivity can be significantly increased. The xposure releases acid in situ which then decomposes the PPA layer at 95 - 100 °C during the subsequent PEB (positive development). The thermally induced, solvent-free development step proceeds almost completely. Cross-linking processes which are also induced by UV-exposure may however cause a thin, only a few nanometres thick residual resist layer. A residue-free substrate surface is obtained after addition of a short plasma etching step. [/av_textblock] [av_textblock size='' av-medium-font-size='' av-small-font-size='' av-mini-font-size='' font_color='' color='' id='' custom_class='' av_uid='av-3www3ow' admin_preview_bg='']
Structuring with laser (pulse)
PPA layers can also be structured by laser ablation. Substrates coated with AR-P 8100 were patterned at the IOM Leipzig with pulsed laser light at different wavelengths. This enabled the realisation of architectures with very low edge roughness. In the absorption range of PPA (at 248 nm), complete ablation was achieved without damage of the silicon substrate.
0.5 J/cm2, 248 nm, 20 ns, double pulse exposure, 700 nm PPA on Si-wafer
Even though PPA shows only a very low absorption at a wavelength of 355 nm, a selective ablation with comparatively high sensitivity is nevertheless possible. The structures realised here are again characterised by very smooth edges.
0.1 J/cm2, 355 nm ps-laser, single-pulse exposure, 700 nm PPA on Si-wafer
The laser beam can also be used to generate 3D structures. Interference projection through a phase mask allows the production of lattice structures with sinusoidal shape and very low surface roughness.
Experimental setup of interference projection
SEM-image of PPA lattice with sinusoidal progression (period ~750 nm); 248 nm, 20 ns pulses, number of pulses: 10; 700 nm PPA on Si-wafer