Research Areas

Organic electronics promises to allow the expansion of electronics into new applications beyond the reach of conventional silicon-based technologies. Examples include large-area lighting and robust color displays that can be rolled up. Before such devices are possible there is much development work to be done to facilitate their fabrication and study the fundamental science upon which they are founded. Orthogonal processing is a step along that path.

Orthogonal processing leverages creative chemistries to allow photolithographic patterning of materials for organic electronics. Solvents with a high fluorine content are typically immiscible with organic solvents and water. Working together in the Ober group, synthetic chemists and materials scientists have developed both highly fluorinated active materials and highly fluorinated photoresists. These materials can, respectively, be patterned with conventional photoresists and be used to pattern standard organic electronic materials such as pentacene and PEDOT:PSS. Small molecule and polymer resists have been synthesized that are sensitive to either exposure to UV light or e-beam conditions allowing sub-100 nm patterning of materials for organic electronics.

There are two key advantages to orthogonal processing. First, it allows patterning to much smaller dimensions than techniques commonly used for organic materials such as shadow-masking. Second, the fluorinated materials’ orthogonality to organic materials allows the fabrication of otherwise unattainable device architectures. We believe that these advantages will both advance the commercialization of organic electronics and permit further study of the fundamental science behind organic electronics.

In January 2010 a start-up company, Orthogonal Inc. was set up to commercialize these promising fluorinated materials.

Organic electronic materials (e.g., conducting and semi-conducting polymers) are uniquely suited for the interface with biology, due to their intrinsic (and tunable) mechanical, chemical, and electronic properties. These properties enable organic electronics to not only support cell culture, but also actively communicate and interact with living tissues in ways that are inaccessible to traditional inorganic electronics. Developing an understanding of the bioelectronic interface, as well as learning how to control the interaction between cells and conducting polymers, is important from both a scientific as well as a technological viewpoint; the ultimate goal is to design and implement better, cheaper, and more effective biomedical devices, ranging from sensors to neural interfaces.

Our work focuses on designing and fabricating organic electronic devices, and studying their electrical and chemical interactions with a variety of cell types and biomolecules. The electrochemically-active devices range in size from centimetres to microns, enabling studies of the bioelectronic interface from the macroscopic scale down to the scale of single cells.

We have been studying the conducting polymer poly(3,4-ethylenedioxythiophene) (PEDOT) doped with p-toluenesulfonate (TOS) or polystyrenesulfonate (PSS), and its interactions with fibroblasts, neurons, tumour cells, and stem cells. We have shown that by applying a voltage to a PEDOT surface, we can locally control the molecular conformation of important adhesion proteins such as fibronectin. This in turn enables control over a range of cell behaviours, including adhesion, motility, and paracrine signalling.

Part of our ongoing effort is to extend these studies to three-dimensional cell-culture systems that remain electrically active. These efforts aim to study the behaviour and response of cells to their local microenvironment in increasingly physiologically-relevant systems.

Polymer brushes have recently attracted considerable interest for generating molecularly defined surfaces for applications in nanotechnology, molecular biology, and biomedical sciences. Two main advantages of using polymer brush systems are the ability to mitigate non-specific adsorption and the creation of tailor-made surfaces to control the immobilization of bioanalytes through specific receptor recognition interactions. The use of polymer brushes allows the formation of uniform surfaces with controlled chemical architecture that exhibit good chemical and thermal stability.

Currently we are in collaboration with other groups to develop a microfluidic electrochemical biosensor that utilizes polymer brushes for the detection of antibodies. The ability to detect selective antibodies is essential for diagnosing infectious diseases and advancing medical applications. Our device is tailored to eliminate non-specific absorption and other limitations associated with current assays, but can be modified for the identification of antibodies specific for any infectious agent.

Another area of polymer brushes being explored is that of patterned brushes, which can be used to create surfaces with tailored surface properties. Various patterning methods have been used to fabricate patterned polymer brushes, the most conventional methods involve patterning of surface immobilized initiator followed by surface initiated polymerization. To simplify the process, our group patterns features in the nanometer regime by direct e-beam lithography. Currently, we are investigating the responsive nature in different solvents of binary brush systems at selected feature sizes. Polymer brushes are chosen by their ability to swell in a good solvent and cover the neighboring brush which is in a collapsed state. Other patterning projects include exploring the morphology of e-beam patterned block copolymer brushes. Our results show that pattern size and solvent treatment influence the morphology of phase separated block copolymer brushes.

The fouling of submerged materials by marine algae is an age-old and costly economic problem; biofouling on the surface of ship hulls is well known for its undesirable consequences such as decreasing fuel efficiency and maneuverability. While antifouling paints currently exist, many incorporate biocides that are now considered to be environmentally undesirable.

Our research focuses on the synthesis of environmentally-friendly, self-assembled surface active block copolymers (SABC) as anti-fouling coatings. These SABCs contain mixtures of hydrophobic/hydrophilic side chains that alter surface properties and improve foul-release efficiency. The side chains include perfluoro groups as hydrophobic chains and PEG chains as hydrophilic chains and have been found to show excellent release of both Ulva sporelings and Navicula diatoms from the surface. The possible reason for the better performance is due to the amphiphilic surface which undergoes an environment-dependent transformation when in contact with water.

A start-up company, Nanosurfaces Inc., was set up to commercialize these polymer formulations for foul release coatings.

Molecular glasses (MGs) have emerged as an alternative to traditional polymeric resists. As target feature sizes continue to decrease, polymers are reaching their fundamental limit as these feature sizes are approaching the size of the molecules. MGs are low molecular weight amorphous molecules that are small in size but still retain the beneficial properties of polymers, including high T_g. When first investigated, it was believed that MGs could outperform polymers in terms of higher resolution and lower line edge and width roughness because of their smaller, more uniform size. While MGs have produced patterns in the 30 nm regime, they have not yet been proven to show consistently superior resolution or reduced LER compared to polymer resists. New design strategies and processing conditions are being explored to further take full advantage of MG properties.

With the use of small molecules, Physical Vapor Deposition (PVD) has emerged as a viable method of film deposition. MGs are defined by their small, rigid structure, and hold a strong advantage over polymers as PVD candidates. Traditionally, photoresists are dissolved in a casting solvent and spin-coated onto a substrate. Impurities such as traces of residual solvent are left in the film by this method. Evaporation of resist materials creates a film free from these impurities and of a uniform, controlled thickness. PVD can eliminate solvent waste generated by spin-coating, making it an environmentally-friendly and cost-effective deposition method.

This project combines synthesis, physical characterization and state of the art processing of MG photoresists, while focusing on furthering the development of MG resists for the current workhorse 193nm lithography and the NGL platform of EUV lithography. Molecular glasses with various architectures have been synthesized and evaluated in our group. Branched, bulky architectures can be designed to produce high T_g, amorphous resist materials through a range of discrete synthetic steps. Likewise, ring structures with suitable properties for resists can also be prepared with outstanding performance characteristics. Due to our understanding of the synthesis of these materials, we have the ability to introduce additional functionality to their design through modifications to the synthetic scheme.

We are interested in composite inorganic/organic photoresist systems that may lead to performance enhancements that are difficult to achieve with traditional polymer architectures. The incorporation of metal oxide nanoparticles into traditional photoresists has been one focus of this project, which may lead to improvements in etch resistance, refractive index, dielectric constant, and mechanical properties. The nanoparticles can be synthesized using hydrothermal techniques and then can be incorporated into polymers using blending, grafting-to, or grafting-from approaches.

In order to realize the full potential of higher numerical apertures enabled by high index lens materials and immersion fluids for 193 nm immersion photolithography, a resist with a correspondingly high refractive index is required. In one example of a nanocomposite photoresist, Bae and coworkers incorporated hafnium oxide (HfO₂) nanoparticles covalently attached to an organic ligand (TDHT), that enabled their miscibility with a traditional 193 nm photoresist, poly(methyladamantane methacrylate-co-alpha-methacryloxy-?-butyrolactone) (PMAdMA-co-GBLMA). Dense 100 nm lines were achieved by e-beam lithography, demonstrating the high resolution capability of the nanocomposite photoresist.

In order to enhance the etch resistance of nanocomposite photoresists, HfO₂ nanoparticles are also in use as a core building block for an imageable photoresist, similar to the molecular glass concept developed in our group. Covalent binding of organic ligands to the surface of the nanoparticle affords high solubility, along with the ability to form amorphous, smooth films composed of greater than 60% hafnium. The different ligands on the surface of the nanoparticles give the nanoparticles the ability to act as positive or negative tone photoresists for 193 nm or e-beam lithography. Recent results have suggested that these materials have etch resistance over ten times greater than the industry standard poly(hydroxystyrene) under silicon etching conditions.

Photoresists such as chemically amplified resists (CARs) are playing a crucial role in photolithography to achieve sub-micron features sizes by providing high sensitivity, contrast and resolution. Using CARs and 193 nm deep ultraviolet platforms, semiconductor industries have been able to double the number of transistors per unit area in integrated circuits every two years according to the Moore's law. As next-generation lithography lean towards 13.5 nm extreme ultraviolet platforms or direct write e-beam techniques in order to acheive sub-32 nm features, understanding deproection and diffusion kinetics in resist polymers during heating processes become critical.

In this project, we use lasers (at various wavelengths) to heat substrates at very high temperatures in millisecond time frames. By using lasers to heat the substrate (hence the resist polymer), we can probe the resist response and model their deprotection and diffusino kinetics. The main goal of this project is to understand the resist deprotection/diffusion kinetics under various heating duration and temperatures, and ultimately create/modify resist polymers suitable for next-generation lithography.

Carbon dioxide is relatively nontoxic, nonflammable, inert under most conditions and is a potential candidate as an environmentally benign replacement for organic solvents in many microelectronic applications. Its easily accessible critical point (31.1 °C and 1170 psi) has made it the most widely used supercritical compound. Supercritical carbon dioxide (scCO₂) exhibits the unique properties of combining liquid-like densities with gas-like diffusivities and zero surface tension. These unique properties have enabled scCO₂ to be used as a developing solvent for photoresist patterning. However, most conventional polymeric photoresists are generally not scCO₂-soluble, which limits its practical use. Therefore, certain fluorinated or silicon containing polymers have been used for pure scCO2 processing.

Our group has previously shown negative-tone patterning of fluoropolyers in scCO₂. Copolymers of tetrahydropyranyl methacrylate(THPMA) and fluorinated methacrylates were successfully patterned in scCO₂ with features as small as 0.2 µm. We have also shown positive-tone imaging by silylating poly(tetrahydropyranyl methacrylate-co-1H, 1H perfluorooctyl methacrylate) (THPMA-F7MA). However, the large amount of fluorination in photoresists can result in poor plasma-etch resistance. To eliminate the use of fluorine in photoresists, we have designed and studied a lot of molecular glass photoresists. It is known that scCO₂ is generally a good solvent for small molecules and the small sizes of molecular glass photoresists can enable high-resolution patterning. A group of phenolic molecular glass photoresists were synthesized and characterized by their dissolution properties in scCO₂ and we have shown features as small as 50 nm with hexa(hydroxyphenyl) benzene (HHPB) molecular glass developed in scCO₂.

Calix[4]resorcinarene derivatives were also studied as molecular glass photoresists and patterns as small as 70 nm were developed in scCO₂. Besides designing new photoresist structures, our group is also interested in developing conventional photoresists in scCO₂. Because most conventional photoresists are insoluble in scCO₂, it often requires a cosolvent or an additive. We have shown the development of a group of conventional photoresists with the use of quaternary ammonium salts (QAS) and sub-100 nm features were obtained. Currently, we are working on further structures of molecular glass resists and investigating additives for developing conventional photoresists in scCO₂.

Population growth and industrialization have increased the need for new fresh water sources and access to fresh drinking water is now an issue of global importance. Reverse osmosis has became the leading technology to produce potable water from non-traditional water sources such as brackish groundwater and seawater. Reverse osmosis is a low cost and low power consumption process in comparison to other seawater purification treatments. Membranes used for reverse osmosis are highly efficient in terms of salt rejection and water flux. However, the lifespan and efficiency of reverse osmosis membranes is severely affected by, among others things, organic and biological fouling.

Biofouling, the accumulation of microorganisms or biomolecules on the membrane, leads to a decrease of water flux and reduced overall performance necessitating frequent cleaning or replacement of the membrane. In order to prevent and control biofouling, we developed antifouling polymer brush coatings (Figure 1). The membrane was functionalized with an ATRP initiator from which poly(methacrylic acid) brushes were grown and then modified by esterification with different antifouling side-chains. The polymer brushes modify the surface properties of the reverse osmosis membrane by reducing the affinity between the surface and microorganisms these coatings either efficiently inhibit the settlement of the treated surface by algae and bacteria (antifouling coating) or provide weak foulant/surface adhesion so that the foulant can be easily washed off (fouling-release coating).

Preliminary results (see Figure) have shown that the polymer brush coating reduces the adhesion of protein and bacteria. While traditional antifouling coatings show a decrease in the bacterial adhesion, the fouling-release coatings are more efficient. The presence of a tangential flow at the surface of a reverse osmosis membrane in operation provides an opportunity to create a self-cleaning membrane; a 35-fold decrease in microbial deposition rate was achieved with the use of perfluorinated side-chains.

The central nervous system (CNS) has limited capacity (or no capacity in some sites) to replace neurons lost through physical injury and chronic neurodegenerative diseases (i.e., Parkinson's Disease and Alzheimer's disease). Because the injured environment provides physical and chemical inhibitory barriers for neuronal regeneration, the positive cues that elicit and guide repair are absent. Biomaterials have been widely used to help improve functional recovery in the CNS, and hydrogels are particularly attractive scaffold biomaterials for this application. Compared to other biomaterials, hydrogels have many advantages such as high oxygen and nutrient permeabilities and low interfacial tensions. Hydrogels can also be easily conformed to any defect shape, their elasticity can be tuned by controlling the crosslink density, and they can be functionalized for the control of neuronal cell adhesion, proliferation and neurite outgrowth.

We are attempting to develop new types of synthetic hydrogel scaffolds that can artificially generate favourable cellular microenvironments to support neuronal regeneration. One of the strategies is to use incorporate biologically active molecules into hydrogel scaffolds. For example, in one of our studies, by incorporating a monomer derived from the neurotransmitter acetylcholine into a biocompatible hydrogel structure (e.g., Polyethylene glycol (PEG)), we prepared a series of hydrogels which induced different neuronal cell responses. Some of those hydrogels encourage mouse hippocampus neuronal cell attachment and neurite outgrowth (Fig. 1 and 2). These types of hydrogels are also being explored for alternative applications such as scaffolds for CNS repair, drug delivery, biomedical device coating, and neural tissue engineering.