Sunggook Park

Controlled co-evaporation of silanes for nanoimprint stamps

A new chemical vapour deposition setup for the generation of anti-adhesive coatings on Si stamps used in nanoimprint lithography has been developed. This is suitable for controlled co-evaporation of more than one type of silane by directly injecting a premixed silane into an evacuated deposition reactor through a septum. This process was found to be very flexible and resulted in reproducible coatings. A surface coated with a mixture of mono- and trichlorosilanes shows a higher water contact angle than those of individual coatings, which is attributed to the interaction between the two types of silane molecules. In addition, the influence of process parameters, e.g. water content, temperature and number of imprints, on the coating quality will be discussed.

Flexible fabrication and applications of polymer nanochannels and nanoslits.

Fluidic devices that employ nanoscale structures (<100 nm in one or two dimensions, slits or channels, respectively) are generating great interest due to the unique properties afforded by this size domain compared to their micro-scale counterparts. Examples of interesting nanoscale phenomena include the ability to preconcentrate ionic species at extremely high levels due to ion selective migration, unique molecular separation modalities, confined environments to allow biopolymer stretching and elongation and solid-phase bioreactions that are not constrained by mass transport artifacts. Indeed, many examples in the literature have demonstrated these unique opportunities, although predominately using glass, fused silica or silicon as the substrate material. Polymer microfluidics has established itself as an alternative to glass, fused silica, or silicon-based fluidic devices. The primary advantages arising from the use of polymers are the diverse fabrication protocols that can be used to produce the desired structures, the extensive array of physiochemical properties associated with different polymeric materials, and the simple and robust modification strategies that can be employed to alter the substrates surface chemistry. However, while the strengths of polymer microfluidics is currently being realized, the evolution of polymer-based nanofluidics has only recently been reported. In this critical review, the opportunities afforded by polymer-based nanofluidics will be discussed using both elastomeric and thermoplastic materials. In particular, various fabrication modalities will be discussed along with the nanometre size domains that they can achieve for both elastomer and thermoplastic materials. Different polymer substrates that can be used for nanofluidics will be presented along with comparisons to inorganic nanodevices and the consequences of material differences on the fabrication and operation of nanofluidic devices (257 references).

Photon-beam lithography reaches 12.5nm half-pitch resolution

We have printed dense line/space patterns with half-pitches as small as 12.5nm in a negative-tone calixarene resist using extreme ultraviolet (EUV) interference lithography. The EUV interference setup which is based on transmission diffraction gratings is illuminated with spatially coherent radiation from a synchrotron source. The results show the extendibility of EUV lithography to printing features measuring less than 15nm in size. We discuss the potential impact of effects such as photoelectron blur and shot noise in high-resolution EUV lithography.

Fabrication of polymer photonic crystals using nanoimprint lithography

Dense two-dimensional periodic photonic bandgap structures are produced in poly(methyl methacrylate) thin films using nanoimprint lithography (NIL). The stamp original was made by electron beam lithography. Then, a high versatility of the process was achieved by fabricating copies of the stamp original via NIL, which enables varying and optimizing both fill factors and aspect ratios independently. For reliable stamp copying over the whole structural areas, the nanorheological behaviour had to be considered. Using those stamp copies, polymeric photonic band gap structures with an aspect ratio as high as 2 were successfully replicated.

Complete plastic nanofluidic devices for DNA analysis via direct imprinting with polymer stamps

Development of all polymer-based nanofluidic devices using replication technologies, which is a prerequisite for providing devices for a larger user base, is hampered by undesired substrate deformation associated with the replication of multi-scale structures. Therefore, most nanofluidic devices have been fabricated in glass-like substrates or in a polymer resist layer coated on a substrate. This letter presents a rapid, high fidelity direct imprinting process to build polymer nanofluidic devices in a single step. Undesired substrate deformation during imprinting was significantly reduced through the use of a polymer stamp made from a UV-curable resin. The integrity of the enclosed all polymer-based nanofluidic system was verified by a fluorescein filling experiment and translocation/stretching of λ-DNA molecules through the nanochannels. It was also found that the funnel-like design of the nanochannel inlet significantly improved the entrance of DNA molecules into nanochannels compared to an abrupt nanochannel/microfluidic network interface.

Simple replication methods for producing nanoslits in thermoplastics and the transport dynamics of double-stranded DNA through these slits

Mixed-scale nano- and microfluidic networks were fabricated in thermoplastics using simple and robust methods that did not require the use of sophisticated equipment to produce the nanostructures. High-precision micromilling (HPMM) and photolithography were used to generate mixed-scale molding tools that were subsequently used for producing fluidic networks into thermoplastics such as poly(methyl methacrylate), PMMA, cyclic olefin copolymer, COC, and polycarbonate, PC. Nanoslit arrays were imprinted into the polymer using a nanoimprinting tool, which was composed of an optical mask with patterns that were 2-7 µm in width and a depth defined by the Cr layer (100 nm), which was deposited onto glass. The device also contained a microchannel network that was hot embossed into the polymer substrate using a metal molding tool prepared via HPMM. The mixed-scale device could also be used as a master to produce a polymer stamp, which was made from polydimethylsiloxane, PDMS, and used to generate the mixed-scale fluidic network in a single step. Thermal fusion bonding of the cover plate to the substrate at a temperature below their respective T(g) was accomplished by oxygen plasma treatment of both the substrate and cover plate, which significantly reduced thermally induced structural deformation during assembly: ∼6% for PMMA and ∼9% for COC nanoslits. The electrokinetic transport properties of double-stranded DNA (dsDNA) through the polymeric nanoslits (PMMA and COC) were carried out. In these polymer devices, the dsDNA demonstrated a field-dependent electrophoretic mobility with intermittent transport dynamics. DNA mobilities were found to be 8.2 ± 0.7 × 10(-4) cm(2) V(-1) s(-1) and 7.6 ± 0.6 × 10(-4) cm(2) V(-1) s(-1) for PMMA and COC, respectively, at a field strength of 25 V cm(-1). The extension factors for λ-DNA were 0.46 in PMMA and 0.53 in COC for the nanoslits (2-6% standard deviation).

Simulation study on stress and deformation of polymeric patterns during the demolding process in thermal imprint lithography

Thermal imprint lithography or hot embossing is a processing technique using molding to produce surface patterns in polymer resist at micro- and nanoscales. While fast molding is important to improve the yield of the process, the process step that determines the success of imprinting high aspect ratio structures is demolding, a process to separate the mold insert from the patterned resist after conformal molding. In this paper the authors studied the stress and deformation behavior in polymer resist during the cooling and demolding process of thermal imprint lithography via finite element method. A simple model structure of the Si stamp/poly(methyl methacrylate) (PMMA) resist/Si substrate was used for the simulation, assuming that PMMA is viscoelastic. As demolding proceeds, Von Mises stress in the PMMA layer is highly localized in two locations, one at the transition corner zone between the residual layer and the replicated PMMA pattern and the other close to the contact region with the moving stamp edge...

Energy level alignment driven by electron affinity difference at 3,4,9,10-perylenetetracarboxylic dianhydride/n-GaAs(100) interfaces

Ultraviolet photoemission spectroscopy (UPS) was employed to investigate the electronic structure upon deposition of 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) on differently treated n-GaAs(100) surfaces. Interface dipoles are found to form according to the electron affinities (EA) of the substrates and PTCDA films at the interfaces and, consequently, the vacuum level alignment rule does not hold. The results demonstrate that the energy offset between the conduction band minimum of n-doped inorganic semiconductors and the lowest unoccupied molecular orbital of organic molecular films at the interfaces can be obtained using UPS by systematically varying the EA of substrates with a known band gap.

Nanostructuring of anti-adhesive layers by hot embossing lithography

Sub-100 nm patterns of (tridecafluoro-1,1,2,2,-tetrahydrooctyl)trichlorosilane (TFS), widely used as antiadhesive layers, are fabricated on SiO2 substrates using hot embossing lithography (HEL) and subsequent lift-off process. The TFS patterned surfaces are analyzed using atomic/lateral force microscopy (AFM/LFM). Low friction force is revealed on the areas of TFS relative to the SiO2 areas. The width of TFS lines is mainly determined by the quality of the embossed structure and the etching time during reactive ion etching. The border width determined by LFM is 45±10 nm, which limits the lateral resolution of the chemical patterns by HEL. Smaller structures can also be fabricated by HEL, however, with sacrifice of the friction contrast between areas of TFS and SiO2.

A Simulation Study on the Effect of Cross-Linking Agent Concentration for Defect Tolerant Demolding in UV Nanoimprint Lithography

The chemistry and composition of UV-sensitive resists are key factors determining the stress in the molded resist structure in UV nanoimprint lithography (UV-NIL) and thus the success of the process. The stress in the molded structure is mainly generated due to shrinkage of the resist in the UV curing step and also adhesion and friction at the stamp/resist interface in the subsequent demolding step. Thus, understanding of the stress generated in these steps is critical to the improvement of the process as well as the development of new UV resists. In this paper the effect of resist composition on the stress generation was studied by numerical simulations of the curing and demolding steps in UV-NIL. Parameters required for the simulation, such as resist shrinkage, Youngs modulus, fracture strength, friction coefficient, crack initiation stress, and debonding energy, were determined experimentally for different resist compositions. As the cross-linking agent concentration increases the fracture strength also improves. In addition, as more cross-linking agent is added to the resist composition, both shrinkage stress due to the curing and also adhesion at the stamp/resist interface increase resulting in a larger maximum local stress experienced by the resist on demolding. By normalizing the overall maximum local stress by the fracture stress of the resist, we found that there is an optimum for the cross-linking agent concentration that leads to the most successful imprinting. Our finding is also corroborated by qualitative experimentations performed for UV-NIL with various resist compositions.