STED-Inspired Cationic Photoinhibition Lithography

Direct laser writing by two-photon lithography has been enhanced substantially during the past two decades by techniques borrowed from stimulated emission depletion (STED) microscopy. However, STED-inspired lithography was so far limited to radical polymerizations, mostly to acrylates and methacrylates. Cationic polymers did not derive benefits from this technique. Specifically, epoxide polymerization, which plays a paramount role in semiconductor clean-room technology, has not yet been reported with a second, depleting laser focus in the outer rim of the point spread function. We now found that using a thioxanthone as a sensitizer and sulfonium or iodonium salts as photoinitiators enables at least partial optical on/off switching of two-photon polymerization and, in the case of the sulfonium salt, allows for writing epoxy lines with widths shrunk by approx. two-thirds compared to lines written with two-photon polymerization alone.

show the results of similar measurements with other concentrations in Figure S1 for ITX/Ar3S:SbF6 and in Figure S2 for ITX/Ph2I:PF6 as the starter systems.In case of ITX/Ar3S:SbF6, the composition 4 wt.%ITX, 1 wt.%Ar3S:SbF6 clearly shows the fastest depletion and the lowest residuum of undepletable line height.In case of Ph2S:PF6, the composition 4 wt.%ITX, 1 wt.%Ph2S:PF6, shows the smallest residuum, as well, however, some other compositions show very similar results.

Development of pedestal with ordinarily shaped TAD PSF
In the main text, we show in Figure 3 how the width of the central line first shrinks and then widens again with increasing TAD power, and how a broad pedestal evolves as a function of increasing TAD power (Figure 3).In these measurements, a donut-shaped TAD PSF was used and the starter system was 4 wt.%ITX and 1 wt.%Ar3S:SbF6.
For the starter system 2 wt.%ITX and 1 wt.%Ph2I:PF6, we wrote a series of lines with an ordinarily shaped TAD PSF, confocalized with the excitation PSF and show the results in Figure S3.The excitation power was 3.4 mW.While the width of the central line stays at around 450 nm, the width of the pedestal increases up to 900 nm for 15 mW of TAD power.

Several batches of linewidths with donut-shaped TAD beam
Figure S4 shows the four different TAD-power series used to determine the average widths of the pedestal and the central line that lead to Figure 3 in the main text.The excitation power is given to the left of each SEM image and the applied TAD power is given above each line.- The starter was 4 wt.%ITX and 1 wt.%Ar3S:SbF6.four lines.The averages in the last column are the data for the graph presented in Figure 3b in the main manuscript.S2: Outer width of the pedestals for different TAD powers from up to 4 lines from the batches shown in Figure S4.

TAD photoinhibition lines with Ph2I:PF6 as onium starter
While we were able to repeatedly write thin lines with TAD photoinhibition lithography using Ar3S:SbF6 as onium salt, the results using Ph2I:PF6 were far less reliable.Many lines detached from the samples and those which stuck were frequently distorted.Some examples are shown in Figre S5 with excitation and TAD powers as indicated.Two examples are enlarged, one with 0 mW TAD (i.e.pure MPL) and one with 6 mW TAD. . 3We therefore assume that 0.3 f  is a reasonable average. . 3This corresponds well with results of the Fouassier-group, who published that the TT absorption of ITX peaks at 605 nm in methanol, 4 and they note that the spectra of thioxanthones bathocromically shift by approx.30 nm in nonpolar solvents. 5Therefore we assume that the triplet-triplet absorption cross section at 660 nm is 17 2 TT 3.9 10 cm  − = .

Figure S3 .
Figure S3.Lines written with confocalized ordinary point spread functions of 780 nm, 3.4 mW and 660 nm TAD.The starter comprised 2 wt.%ITX and 1 wt.%Ph2S:PF6.(a) Three examples of lines with 0, 6, and 15 mW of TAD power applied.The width of the central line stays essentially the same, while the pedestal widens.(b) Width of the central line (full squares) and the pedestal (open circles) as a function of TAD power.
papers have been published where the oscillator strengths of the transient triplettriplet absorption where calculated for ITX or very similar thioxanthones.Rai-Constapel et al. showed that for thioxanthone the 18 TT → transition is by far the dominating one with an oscillator strength of 0.33 f = , 1 while the other TT absorptions show negligible oscillator strength.Without specifying the specific TT transition, Mundt et al. found 0.28 f = . 2Harke et al. calculated the TT absorptions for ITX as a single-component type II initiator of an acrylate polymerization and found that again the 18 TT → is by far the dominating one with 0.26 f =

Figure S5 .m
Figure S5.Lines written with 3.2 to 3.5 mW of excitation power and increasing 660 nm TAD power as indicated.The starter system was 4 wt% ITX and 1 wt.%Ph2I:PF6.Two lines highlighted with black boxes are enlarged on the right hand side.

Table S1 :
Inner linewidths for different TAD powers from up to 4 lines from the batches shown in FigureS4.