Young-Je Kwark

Superamphiphilic Janus Fabric

Janus fabrics with superamphiphilicity were fabricated via electrospinning of polyacrylonitrile (PAN). PAN nanofibrous mats were formed on an aluminum foil substrate and then thermally treated to cause hydrolysis. An identical PAN solution was subsequently electrospun onto the hydrolyzed PAN layer, followed by peeling off of the bicomposite film from the collector substrate to produce a free-standing Janus fabric. On one side, the electrospun PAN mat exhibited superhydrophobic properties, with a water contact angle of 151.2°, whereas the initially superhydrophobic PAN sheet on the opposite side of the fabric was converted to a superhydrophilic surface (water contact angle of 0°) through hydrolysis of the surface functional groups induced by the thermal treatment. The resulting Janus fabrics exhibited both superhydrophobicity, repelling water on the one side, and superhydrophilicity, absorbing water on the other side. The organic solvent resistance of the PAN nanofibrous sheets was remarkably improved by incorporation of a tetraethyl orthosilicate. This facile and simple technique introduces a new route for the design and development of functional smart, robust fabrics from an inexpensive, commercially available polymer.

Doxorubicin-loaded alginate-g-poly(N-isopropylacrylamide) micelles for cancer imaging and therapy.

Chemotherapy is a widely adopted method for the treatment of cancer. However, its use is often limited due to side effects produced by anti-cancer drugs. Therefore, various drug carriers, including polymeric micelles, have been investigated to find a method to overcome this limitation. In this study, alginate-based, self-assembled polymeric micelles were designed and prepared using alginate-g-poly(N-isopropylacrylamide) (PNIPAAm). Amino-PNIPAAm was chemically introduced to the alginate backbone via carbodiimide chemistry. The resulting polymer was dissolved in distilled water at room temperature and formed self-assembled micelles at 37 °C. Characteristics of alginate-g-PNIPAAm micelles were dependent on the molecular weight of PNIPAAm, the degree of substitution, and the polymer concentration. Doxorubicin (DOX), a model anti-cancer drug, was efficiently encapsulated in alginate-g-PNIPAAm micelles, and sustained release of DOX from the micelles was achieved at 37 °C in vitro. These micelles accumulated at the tumor site of a tumor-bearing mouse model as a result of the enhanced permeability and retention effect. Interestingly, DOX-loaded alginate-g-PNIPAAm micelles showed excellent anti-cancer therapeutic efficacy in a mouse model without any significant side effects. This approach to designing and tailoring natural polymer-based systems to fabricate nanoparticles at human body temperature may provide a useful means for cancer imaging and therapy.

Absorbance measurement of polymers at extreme ultraviolet wavelength: Correlation between experimental and theoretical calculations

Performance requirements for extreme UV (EUV) resists will require the development of polymer platforms. A challenge in designing photoresists for EUV wavelengths is the selection of molecular structures that have minimal absorbance. For example, elements that are commonly used in photoresists at other wavelengths, such as oxygen and fluorine, are highly absorbing at ∼13nm making them problematic for EUV applications. In order to provide a tool for EUV resist design, this article presents a study of the absorbance of common photoresist structures and compares it to theoretical estimates of resist absorbance based on composition and density. On this basis, several potential structures suitable for EUV resists are assessed.

Interpenetrating polymer network dielectrics for high-performance organic field-effect transistors

We have demonstrated the preparation of interpenetrating polymer network (IPN) dielectrics for use in high-performance organic field-effect transistors by blending commercially available polymers (PMMA, PtBMA, and PS) with the crosslinkable polymeric silsesquiazane (SSQZ). This facile blending method is a powerful means of enhancing the electrical strength of polymer dielectrics due to the formation of a siloxane network structure interspersed among the polymer chains. We found that the leakage currents for the PMMA and PtBMA gate dielectrics blended with SSQZ significantly decreased, by as much as two orders of magnitude, compared with the pristine cases. These remarkable enhancements in the dielectric properties arose from decreases in the free volume and in the thermal dynamic motions of the polymer chains due to formation of the polysiloxane network. The IPN gate dielectrics provide a facile method for using commercially available polymers to fabricate polymer gate dielectrics with strong electrical strengths.

Materials for future lithography

The demands for high resolution and issues of line edge roughness require a reconsideration of current resist design strategies. In particular, EUV lithography will provide an opportunity to examine new resist concepts including new elemental compositions and low molar mass resists or molecular resists. In the former case, resist compositions incorporating elements such as silicon and boron have been explored for EUV resists and will be described. In an example of the latter case, molecular glass resists have been designed using synthetic architectures in globular and core-arm forms ranging from one to multiple arms. Moreover, our studies include a series of ring and irregularly shaped small molecules modified to give imaging performance. These materials have been explored to improve line edge roughness (LER) compared to common polymer resists. Several examples of polymeric and molecular glass resists will be described. Several compositions showed high glass transition temperatures (Tg) of ~ 120°C and possessed no crystallinity as seen from XRD studies. Negative-tone molecular glass resists with a T-shaped phenolic core structure, 4-[4-[1,1-Bis(4-hydroxyphenyl)ethyl]]-α,α-dimethylbenzylphenol, have demonstrated feature sizes as small as 50mn. Similarly, negative-tone images made using spiro-based compounds showed feature size as small as 60nm in lines/space patterns using e-beam lithography. Most recently we have demonstrated that fully and partially tert-butoxycarbonyl (t-Boc) protected calix[4]resorcinarene derivatives can be successfully studied as a positive-tone resist using EUV and E-beam lithography. Resolution as low as 35nm was obtained by EUV exposure.

Highly transparent resist platforms for 157-nm microlithography: an update

Hexafluoroisopropyl alcohol-functionalized acrylate monomers and their (co)polymers were prepared as photoresist platforms for 157 nm imaging. In order to balance transparency with other desirable traits such as etch resistance, we developed several copolymer systems. One is using 2-methyl adamantyl methacrylate as a comonomer, and the copolymer system showed better dissolution contrast compared to the copolymer with tetrahydropyranyl methacrylate without sacrificing transparency. To further improve the absorption properties at 157 nm, monomers having (alpha) -trifluoromethyl group were prepared and polymerized in anionic mechanism. The product polymer was unexpectedly transparent at 157 nm (A = 1.6 micrometers -1) in spite that all the monomers contain carbonyl group. The second system is the copolymer with p-t-butoxy-tetrafluorostyrene. p-Hydroxy-tetrafluorostyrene and p-t-butoxy-tetrafluorostyrene were polymerized radically using AIBN in good yield, and the two resulting polymers showed distinctive solubility differences in aqueous base solution. Finally, this paper describes the synthesis of new monomers having fluorine (e.g CF3- group) in the vicinity of the double bond to improve transparency at 157 nm. Due to the lower electron density of the double bond, these monomers can be copolymerized with electron-rich vinyl monomers using radical initiators.

Simultaneous control over the molecular weight and tacticity of poly(vinyl acetate) using a low-temperature photoinitiated RAFT process in fluoroalcohols

A photoinitiated RAFT process in fluoroalcohols (FAs) was successfully applied to the preparation of poly(vinyl acetate) and its hydrolyzed polymer, poly(vinyl alcohol), permitting the simultaneous control over the molecular weight (MW) and tacticity. The polymerization reaction displayed living polymerization characteristics, evidenced with linear relationships between ln([M]0/[M]) and time, and between the MW and the conversion. Polymers of higher syndiotacticity could be prepared by using the FA solvent, achieving a 62.4% racemic diad at −20 °C. The syndiotacticity of the polymer increased with increasing concentrations of FAs or with decreasing polymerization temperatures, and the dependence was stronger than the one observed in conventional radical polymerization reactions due to the slower RAFT process. Furthermore, by performing a sequential polymerization reaction involving the high-temperature reaction in the bulk state and the low-temperature reaction in the FAs, stereoblock copolymers of poly(vinyl alcohol) could be obtained.

Novel silicon-containing polymers as photoresist materials for EUV lithography

Performance requirements for EUV resists may require the development of entirely new polymer platforms. In the first approach, we have synthesized norbornene-based copolymers using ring-opening metathesis polymerization (ROMP). Silicon containing norbornenes were synthesized and copolymerized with a series of monomers having acid sensitive and polar groups, including nitrile, carboxylic acid, hydroxyl, and anhydride functions to achieve random copolymers with suitable properties to be applied as resist materials. Using well-characterized metal alkylidene complexes, we could prepared polymers having controlled molecular weights and low polydispersities. From initial exposure studies using an EUV interferometer, we were able to pattern 150 nm pitchs without additional optimization. In the second approach, polysilane has been copolymerized with acid sensitive monomers (acrylate and styrene derivatives) to produced chemically amplified polysilane-copolymers.

Silicon backbone polymers as EUV resists

To fulfill industry requirements for EUV resists, the development of entirely new polymer platforms is needed. In order to address transparency issues, we have been studying low absorbance materials, specifically silicon based resist platforms. In this approach, we have synthesized and studied resist materials based on polysilanes, polycarbosilane, and polysilsesquiazanes. Poly(methylphenylsilane) was chemically modified to incorporate polar groups to enhance solubility in polar solvents and developer solution. Copolymerization of the modified polysilane with an acid sensitive monomer has been used to produce chemically amplified copolymers. Preliminary studies have shown promising behavior. Polysilsesquiazanes-based resist were synthesized and tested using a 248 nm stepper. They showed excellent lithographic performance but some issues, including long term stability, are presently unknown. Our strategy to produce silicon-based resist together with outgassing and lithography issues will be discussed.

Finely Formed, Kinetically Modulated Wrinkle Structures in UV-Crosslinkable Liquid Prepolymers.

Special characteristics of wrinkles such as a scattering source and a high surface area are finding use in high-tech applications. UV-crosslinkable prepolymers are occasionally used for fabricating wrinkled films. Wavelength of the wrinkles formed from the prepolymers is several tens and hundreds of micrometers. Here, a UV-crosslinkable liquid prepolymer is synthesized to spontaneously form wrinkle structures in the order of several micrometers. Double layers with a very thin hard skin and a soft and contractible foundation are formed at the same time, by ensuring that all the absorbance wavelengths of the photoinitiator are shorter than the minimum wavelength at which the prepolymer is transparent. The rate of photo-crosslinking reaction, R(p), is also found to affect the thickness of the skin and foundation layers at the early UV-curing stage. The first-order apparent rate constant, k(app), is between ≈0.20 and ≈0.69 s(-1) for the wrinkle formation. This wrinkle structures can be simply modulated by changing R(p).