Nanofabrication of patterned surface structures by controlling surface-initiated and crosslinked ultra-thin polymeric films

Abstract

This thesis reports the successful development and use of surface-initiated polymer films via ring-opening metathesis polymerization (ROMP) and solid-state continuous assembly of polymers reactions as capable bottom-up processes for nanofabrication of organic and inorganic materials. Chapter 1 presents an overview of existing bottom-up nanofabrication methods. Chapter 1 also highlights the drawbacks of current top-down fabrication methods and the opportunities to expand bottom-up methods to nanofabrication, of which are the focus of this thesis.

In Chapter 2, the development of a protocol for solid-state continuous assembly of polymers via ring-opening metathesis polymerization (ssCAPROMP) from silanized substrates is presented. This method expands upon previously reported protocols by utilizing an olefinic silane to form highly crosslinked and tailorable polymer thin films which are covalently attached to surfaces. Polymer films constructed using the new anchoring layer showed similar growth characteristics and film thicknesses when compared with previous studies on CAP film growth, but overcome the high degree of steric hindrance and typical minimal macro-cross linker attachment encountered when previously working with silanized silicon surfaces and CAP processes. Films were also successfully reinitiated multiple times, a key property of the CAP process. Optimization of the protocol was carried out via thorough testing of reaction conditions, offering insight into the reactivity of the SI-ROMP process at play. The protocol was further developed to work on the organic material SU-8, a negative photoresist commonly used in microfluidic devices and bio-microelectromechanical systems (bioMEMS) and could be patterned using two methods: masking of the silanization step and subsequent lift-off prior to initiation and film growth, or through selective surface functionalization via the use of polymeric stamps and micro-contact printing procedures. The study presented in Chapter 2 translates the significant advantages of the solid-state CAPROMP protocol when forming polymer films into a method that can be integrated into the fabrication of robust sensors, bioMEMS or microfluidic devices which require tailorable yet stable polymer thin films.

In Chapter 3, the ssCAPROMP protocol developed in Chapter 2 was used as a reactive ink system for a micro/nano 3D printing platform to access spatially defined, crosslinked polymer features. The printing platform used was a modified atomic force microscope whereby a hollow cantilever and an aperture at the tip was used to deliver material from a reservoir through a nanofluidics channel to the substrate. Material delivery was controlled by applying pressure to the reservoir. Utilizing the control over polymer properties offered by CAP, a reactive polymer crosslinker was created that encompassed a number of critical parameters in order to be printed successfully; the ink must undergo rapid crosslinking, it must have a low glass-transition temperature to be printed, and the crosslinking must be of a living nature to enable the printing of multiple layers continuously. The reactive ink could be used without solvent and was delivered through the aperture of the tip onto an initiated surface where it was found to crosslink almost immediately, even in ambient conditions and without requiring an inert atmosphere. This rapid crosslinking enabled the delivery of several layers of which each crosslinked, allowing the build-up of various line heights. A comparison of line heights after washing and drying found that when printing with both the 4 micrometer aperture and the 300 nm aperture, repeated depositions in the same location resulted in an almost quantitative addition of material based on the number of depositions. Overlapping lines were also printed and showed that the height at the overlapped location was the sum of the heights of both lines, highlighting the ability to print on top of existing lines. Experiments showed that by waiting a short period of time between overlapping lines, line heights could be increased as the feature could “cure” before the next deposition. Print directionality was shown to affect line widths based on the contact between the deflected tip and the direction it moves. Structures with three distinct layers were also created, showing that this method of delivering reactive ink and crosslinking in-situ could create three-dimensional patterns. By combining this printing platform with the versatility in ink and polymer chemistry offered by CAP, a robust platform for creating three dimensional structures with a layer resolution of down to 2 nm and a minimum line width down to 450 nm was demonstrated.

In Chapter 4, the surface-initiated ring-opening metathesis polymerization of norbornene was used to direct the bottom-up construction of TiO2 and ZnO via atomic layer deposition (ALD). Norbornene monomers were used in the vapor phase to avoid excessive polymerization and cross-metathesis side reactions that occur when using solution-based SI-ROMP. This process afforded surface-bound polymer films in an extremely rapid fashion, with 100 nm films achievable in less than one minute of vapor exposure. The polymer films were exposed to TiO2 and ZnO thermal ALD processes and then analyzed via XPS to observe any inorganic material growth. It was found that 100 nm of surface-initiated polynorbornene could resist ALD of at least 1200 cycles, challenging a current paradigm of using small molecules to prevent ALD deposition. Several norbornene-based monomers were synthesized as surface-binding initiators that were selective for copper and copper oxide over silicon oxide. These initiators were used in conjunction with coplanar and topographical copper features on silicon oxide wafers in order to selectively attach initiator to the patterned features. Once coated, the functionalized features were used to grow polymer in an area-selective fashion and these substrates were subjected to ALD to test the ability of the polymers to perform area selective deposition (ASD). ASD of ZnO was achieved using a hydroxamic acid-based initiator, enabling the deposition equivalent of 38 nm of ZnO before any nucleation was observed on the polymer surface. This was also tested on large areas, where a large Sierpinski’s triangle 300 micrometers across was created on a substrate using e-beam lithography to demonstrate that large scale ASD could be performed. Polymers grown on the copper surfaces showed inhibition of ALD for up to 675 cycles of ZnO. These results show that SI-ROMP is not only an excellent tool for the bottom-up construction of organic materials as is shown in Chapters 2, 3 and 4, but that long macromolecules can be used to drive the patterned bottom-up construction of inorganic films relevant to semiconductor and device fabrication to great effect.

Finally, in Chapter 5 we propose several pathways where expansion upon the work contained in this thesis may lead to further advances in the bottom-up construction of organic and inorganic materials for nanofabrication.