Our initial experimental setup was to simply use the PLOMP 1.0 resist (described in Chapter 5) in the Nanoscribe. The results of these experiments were highly in- consistent; the response of the resist to exposure varied from exothermic explosions to boiling to completely unreactive. Figure 7.5 exemplifies some of these inconsisten- cies, one can clearly see bubble formation around the written line as well as regions of variable linewidth.
Figure 7.5: An early attempt at 2-photon PLOMP, depicting the issues of boiling and non-linear writing kinetics.
As mentioned in Chapter 5, while the parent resist was stable under ambient conditions for many months as a viscous solution, cast films were not stable to air in- definitely. Practically, it was necessary to begin irradiation of the films a few minutes
after casting to achieve consistent results. Because of the time required to prepare, load and align samples into the Nanoscribe, we observed dramatically varied out- comes depending on the delay between initial air exposure of the resist during sample preparation and writing. Within the first hour, writing was generally consistent, but rapidly declined afterwards. Much of the inspiration to find a more stable photocat- alyst came from these initial attempts on the Nanoscribe.
Figure 7.6: (a) The intensity-dependent behavior of DLW resists typically exhibits two thresholds: polymerization and damage. (b) Damage due to overirradiation appears as a solidified explosion as reagents vaporize, expand, and crosslink. Image from Reference [3].
Baldacinni describes the writing resolution and kinetics as a function of inten- sity, with two key thresholds: polymerization and damage. Figure 7.6 depicts the chemical response as a function of optical intensity, and shows an example of a struc- ture where the damage threshold was clearly exceeded. Most of these heuristics have been developed in the context of radical polymerization. One contributing factor to the polymerization threshold phenomenon is the inhibition of the radical polymer- ization by oxygen from the environment. In principle, the polymerization threshold for PLOMP could be much lower, as ROMP is exothermic and the resin could be designed to rapidly initiate after activation. The downside of this scenario is that the resolution and feature shape will be impacted by diffusion. For the purposes of writing high-fidelity structures, it is therefore more desirable to control the kinetics of ROMP, either through ligand coordination or re-quenching via excess vinyl ether addition.
7.4
Control Experiments
There are a number of key differences between the physical and chemical environ- ments of the resist in the Nanoscribe, compared to a UV-cured thin-film. First, the optical intensity is dramatically higher and focused only at the writing point. The optical flux in the Nanoscribe is approximately 5 × 108 times higher than the UV
lamp employed in our thin-film studies in the previous chapters. Second, the ex- tremely short lengthscales of the written structures demand very high mechanical contrast between the written and unwritten domains, in order to successfully isolate the free-standing nanostructures. Thus, it is desirable to achieve a system with a low viscosity before curing, that will rapidly crosslink to a high-modulus solid. At the same time, the high optical power of the laser combined with the exothermic nature of ROMP can cause the low molecular weight components to vaporize during the writing process, which results in bubbling and changes the local composition of the resist. For these reasons, we typically avoided including excess solvent in developing resist compositions for 2-photon lithography. Finally, the Nanoscribe features the ability to monitor the writing process in real time through the same 100x objective lens that the laser is focused through. The sample is illuminated by an infrared LED (835 nm), enabling the structures to be imaged due to refractive index contrast after crosslinking. While this has the advantage of providing a rapid means for prototyping the write conditions in real-time, it provides limited information about the chemical activity within the resist.
We identified 5-norbornene-2-methanol as a relatively high-boiling ROMP monomer, which is also a liquid at room temperature. During our attempts to incorporate this monomer into the PLOMP resist, we discovered that this monomer was photoac- tive, even in the absence of any ruthenium catalyst. Alcohols are well-known to undergo photo-dissociation into radical fragments.[5] The addition of the common radical inhibitor BHT (2,6-Di-tert-butyl-4-methylphenol) to the monomer dramati- cally changed the writing behavior. While BHT prevented the writing of 3D struc- tures, it is interesting to note that the base of the structure is still visible. It is unclear
whether this is the result of chemical interaction with the silica substrate, or simply due to the fact that the local concentration of BHT is too low at the interface to effectively mitigate the radical crosslinking. Nevertheless, this unique behavior at the interface provides some potential challenges to utilizing a single, well-defined writing mechanism.
Figure 7.7: (Left) 5-norbornene-2-methanol is able to undergo visible writing on its own, without any added catalyst. (Right) The addition of the radical inhibitor BHT to this molecule shuts down writing in the bulk, while some surface activity remains.
These control experiments clearly indicate the presence of competing radical pro- cesses in this highly intense optical environment. Radical formation within the re- sist presents a number of challenges to PLOMP. First, the radical polymerization of olefins will consume the necessary reactants, reducing the concentration of ROMPable monomers. Secondly, the radical species could alter the structure of the ruthenium catalysts, there is precedent for these complexes to also catalyze radical polymeriza- tions.[6–8] After these experiments were conducted, we began to add BHT to all of our resists to mitigate competing radical processes.
As mentioned above, another challenge to developing a robust multiphoton PLOMP process was the evaporation of volatile components during the writing process. De- pending on the resist composition, ∼10-50 fold reductions in the volume of resist were observed after the writing process. For PLOMP 1.0 resists, the ethylidene norbornene is the most volatile component, with a boiling point of 146 °C. This is significantly lower than the commercially-available (photoradically cured) pentaery-
thritol triacrylate, with a boiling point of 205 °C. The total resist droplet size is typically only ∼100-300 µL. The combination of the high surface area and heating from the laser and illumination sources likely accelerate the evaporation rate. To our surprise, the PLOMP 2.0 resists also exhibited significant shrinkage after the writing process. These resists are prepared by the partial polymerization of DCPD, which is quenched when the reaction mixture reaches the desired viscosity. We assumed that these highly viscous resists consisted largely of oligomeric poly(DCPD), which is inconsistent with the observed evaporation. After concentrating a typical PLOMP 2.0 resist at 50 millitorr overnight, the remaining solids only accounted for ∼15% of the mass of the resist. This suggests that there is a significant amount of unreacted DCPD remaining in the PLOMP 2.0 resists. We attempted to engineer a number of solutions to mitigate the effects of evaporation during writing.