With the more stable PLOMP 2.0 resist in hand, we sought to explore the com- patibility scope of molecular additives to demonstrate the utility of our patterning methodology to a variety of functional materials. Our initial efforts in this area were limited to adding small molecule, olefin-containing additives directly to the PLOMP 2.0 resist, and subsequently testing the compatibility of these added functionalities with our thin-film, UV-cured photolithography process. This versatile approach en- ables rapid prototyping, as a single batch of the parent resist can yield dozens of functional variants. A wide variety of additives, shown in Figure 6.6, were success- fully employed using this technique, albeit at relatively low loadings.
These compounds are added by dissolving them in either chloroform or acetoni- trile, then adding a small amount of this solution to a portion of the resist. The compounds shown in Figure 6.6 have all been successfully incorporated into resists at 0.1% by weight without significantly effecting the photolithographic process. A few have been incorporated successfully at 1% by weight, but at this high level of load- ing, solubility is a limiting factor. Typically, the functional monomers were added as chloroform or acetonitrile solutions. Often, the addition of a small amount of water is sufficient to help polar compounds to dissolve in acetonitrile. Somewhat surprisingly, the addition of water into the resist does not lead to any apparent negative effects on the basic functionality of the resist.
The improved stability of complex 6 also necessitates different processing con-
Figure 6.6: A variety of compatible molecular additives. All of these functional monomers could be incorporated into the resist at 0.1 % by weight into the PLOMP 2.0 parent resist, without impacting the lithographic process.
ditions for the PLOMP 2.0 resists. Specifically, while the PLOMP 1.0 resists were not stable at elevated temperatures, the PLOMP 2.0 resists actually benefit from both pre- and post-exposure baking steps during thin-film patterning. The benefits of pre-exposure baking for this system can be rationalized in two ways. First, there is excess vinyl ether in the parent resist that could quench the crosslinking process after photoactivation, but can be easily removed with heating. Second, the glass transition temperature of the partially polymerized poly(DCPD) in the PLOMP 2.0 resist is likely much higher than the poly(COD)-based PLOMP 1.0 resist. Pre-heating is a common strategy to relieve residual stress, rendering higher-density thin-films before patterning. The benefits of post-exposure baking can be rationalized by the fact that the phenanthroline ligand can likely re-coordinate to the ruthenium complex during crosslinking, slowing or halting the process. By heating the film after exposure, the
rate of polymerization can be increased to achieve higher crosslinking densities. The details of the patterning process are described in Section 8.3.
The results of this substrate scope study suggest that the PLOMP 2.0 photoresist can be modified to yield a versatile toolbox. The only functionalities that we specifi- cally had trouble incorporating in any amount were alkynes. Is is possible that some form of alkyne metathesis is responsible for this observation, although we could not rule out other impurities. We were delighted to find that amines did not interfere with the patterning process, as these can be problematic for other homogeneous metathesis reactions because of their tendency to coordinate to the ruthenium center. Given that our proposed photoactivation mechanism for complex6 is through photodissociation of an amine ligand, it is possible that these amines can also be induced to dissociate under irradiation. Post-exposure heating may also mitigate the deleterious effects of the amines on reactivity.
One of the disadvantages of the molecular additive approach to diversification is the difficultly of maintaining homogeneity throughout the process. While it was possible to incorporate low loadings (0.1%) of a variety of small molecule additives, many of these underwent phase separation at even 1% loading. Even in cases where the initial resist solution was homogeneous, some additives were found to precipitate during the spin-coating process, when a large fraction of the volatiles are removed from the material. Figure 6.7 shows an example of crystallite formation within the thin film, as seen through a microscope.
Another ongoing challenge within this project is the characterization of the final composition of the patterned film. While we have shown here the compatibility of many molecular additives with the PLOMP 2.0 resist system, we have not yet fully elucidated the extent to which the added functionalities are incorporated. Ongoing work includes the development of a method of probing the degree of functional group incorporation through Fourier Transform Infrared (FTIR) spectroscopy. Additionally, we hope to elucidate the availability of the the molecular additives to serve as active surface-functional handles for further derivatization of the material. With our current process, this has been challenging to validate for many reasons, including limited
Figure 6.7: This photograph reveals that at higher loadings, the monomer addi- tives can cause phase segregation within the PLOMP films. This particular image shows crystallization of the alcohol-functional monomer N-(hydroxyethanyl)-cis-5- norbornene-exo-2,3-di-carboximide at 1 wt% loading.
surface-area, low loadings, and the difficulty of preparing samples compatible with many of the highly surface-sensitive (and non-destructive) spectroscopic techniques, such as as attenuated total reflectance (ATR) methods of FTIR.