Xudi Wang

Nano metamaterials and photonic gratings by nanoimprint and hot embossing

This paper reviews our recent progress in nanoimprint lithography and hot embossing for the fabrications of planar photonic meta-materials and photonic gratings. With these technologies, dielectric, metallic chiral structures and dense gratings in sizes from sub micrometres down to sub-100 nm were successfully fabricated. Characterisations of these meta-materials and photonic structures indicate these materials and structures are functional. It can be concluded that nanoimporint lithography and hot embossing are capable of achieving large area planar optic meta-materials and photonic structures such as dense gratings. This progress suggests the great prospect of these techniques for fast speed patterning of important meta materials and photonic nanostructures with high throughput at low cost in mass production in the future.

Poly (ethylene terephthalate) (PET) micro/nanostructures fabricated by nanoimprint and its applications

In this paper, we present a novel method for the fabrication of high density pattern in PET foils employing nanoimprint lithorgraphy. The temperature and pressure dependence on the imprinted pattern were investigated. Well resolved PET nanopatterns (sub-100nm resolution) were transferred successfully. It is observed that the grating with different trench depths will demonstrates corresponding changes of diffraction intensity distribution. The grating imprinted in PET foils could broader applications in the manufacture of nanophotonic structures owing to its excellent flexibility and transparency. Based on the optimized imprint process, a novel method for the fabrication of phase gratings and nanochannels is presented. This developed process can find broader applications in the manufacture of nanofluidic channels and other nanophotonic structures.

Soft substrate as a sacrificial layer for fabrication free-standing SU-8-based nanofluidic system

In this paper, we describe a new fabrication process utilizing polydimethylesiloxane (PDMS) and polyester (PET) as a sacrificial substrate for fabricating free-standing SU-8-based nanofluidic system. The soft substrate permits SU-8 UV cured patterning and layer-to-layer bonding, and allows the SU-8 structures to be easily peeled off from the substrate after complete fabrication. In the process, PDMS-on glass is used as a handling wafer, on which SU-8 based trenches is imprinted by a flexible film mold using low-pressure nanoimprint lithography. The reservoir pattern of SU-8 is fabricated on the bonding layer, in which PET serves as substrate. The nanochannel is sealed by optimized bonding process, which is flexible and easily controllable with the use of soft substrate as a sacrificial layer. After bonding process, PDMS and PET could be easily peeled off from nanaofluidic system. The SEM results shows that the height of the fully enclosed nanochannels will be about hundreds nanometer. Large area of free standing SU-8 structure layers are successfully fabricated and peeled off from the soft substrate layer as single continuous sheets.

Fabrication of self-enclosed nanochannels based on capillary-pressure balance mechanism

Polymer-based micro/nano fluidic devices are becoming increasingly important to biological applications and fluidic control. In this paper, we propose a self-enclosure method for the fabrication of large-area nanochannels without external force by using a capillary-pressure balance mechanism. The melt polymer coated on the nanogrooves fills into the trenches inevitably and the air in the trenches is not excluded but compressed, which leads to an equilibrium state between pressure of the trapped air and capillary force of melt polymer eventually, resulting in the channels’ formation. A pressure balance model was proposed to elucidate the unique self-sealing phenomenon and the criteria for the design and construction of sealed channels was discussed. According to the bonding mechanism investigated using the volume of fluid (VOF) simulation and experiments, we can control the dimension of sealed channels by varying the baking condition. This fabrication technique has great potential for low-cost and mass production of polymeric-based micro/nano fluidic devices.

Polymeric nanostructures fabricated by dynamic nanoinscribing technique and its applications

Nanoscale grating structure can be utilized in many practical applications in optics, flat-panel displays, and biosensors. Dynamic nanoinscribing (DNI) technique was newly developed to create large-area and truly continuous nanograting patterns in a variety of metal or polymer materials with feature size down to sub-50 nm and at very high speed. In this paper we investigate the nanopatterning of PC and SU-8 by DNI process and then take advantage of its superior optical and thermal properties to explore its applications in nanooptics and nanofluidics. To carry out nanoinscribing, silicon grating templates with different periods were first fabricated. The inscribing property of PC and SU-8 under various pressures and temperatures was systematically studied, in which the experimental results were compared with the simulation results described by a modified equation of Squeezed flow. Inscribed polymeric gratings with period of 700nm were achieved and excellent uniformity can now be routinely replicated using this optimized process. Using this technique, free-standing subwavelenth gratings based on SU-8 are successfully fabricated and their performance are characterized. The inscribed polymeric gratings could also be sealed with another bare layer thermally to serve as enclosed channels after oxygen plasma treatment. The fabricated nanofluidic channels were characterized using spontaneous capillary filling with dyed water, demonstrating good quality of sealing.

Surface modification on a silicon carbide mirror for space application

Silicon carbide (SiC) is a promising candidate for large-scale mirrors due to its high stiffness and thermal stability. However, it is very challenging to obtain a super smooth surface for high precision optical telescopes due to the intrinsic defects of SiC. In this letter, a super smooth surface with a roughness lower than 1 nm and a surface profile of λ/50 is achieved by depositing a uniform and dense silicon surface modification cladding by plasma ion assisted deposition (PIAD) on a lightweight concave reaction bonded (RB) SiC mirror, followed by a polishing procedure. Characterization data from the high resolution optical microscope, WYKO profilometer, Zygo interferometer, and nanoindentation are further discussed. The thermal shock resistance test indicates that the surface modification cladding is very stable and shows firm adherence. A reflectance of over 98% in the visible light region is obtained on the spectrometer after being coated with the silver-enhanced coatings. OCIS codes: 310.0310, 240.0310, 240.5450. doi: 10.3788/COL201008S1.0183.

High aspect ratio grating fabrication in SU-8 resist by UV-Curing nanoimprint

UV curing nanoimprint is demonstrated for high aspect ratio gratings fabrication based on SU-8 for nanophotonics and biochemical applications. The defects, which are caused by stress and friction between mold and resist and air bubbles are key issues. To eliminate the defects, the process parameters, such as imprinting pressure, baking time and demolding temperature, are optimized. SU-8 grating with 150nm in width and 1.5µm is presented with good uniformity in large area using Si template fabricated by non-switching DRIE process. The process could find broader applications in the manufacture of biochemical devices and nanophotonic structures.

Experimental study of roof filling rate during thermal bonding of polymer microchannel sealing

Direct thermal bonding approaches are especially desirable as they allow formation of enclosed microchannels with uniform surfaces composed entirely of the same polymeric material. It is often believed that the main phenomenon involved during thermal bonding is a chain entanglement of the polymers over the boundary. If the temperature is too high and/or the application of the force is too long then the polymer will flow and refill the channels caused by capillary forces, called as roof filling phenomenon. In order to understand this process more fully, we describe an experimental method for characterizing the roof filling rate inside a microchannel by measuring the polymer marching velocity or position of a capillary meniscus during thermal bonding. 1D nanochannels was fabricated succesfully and the vertical dimension was well controlled according to the top filling mechnism.

Free-standing SU-8 gratings fabricated by sacrificial layer-assisted UV curing imprint

We present results on the nanofabrication of high density patterns in SU-8 resist, based on nanoimprinting combined with UV curing. The bilayer process using PMMA as sacrificial layer was developed to release the SU-8 layer to form three dimensional structures. The SU-8 displays excellent imprint property and well defined patterns are achieved at at low temperature, low pressure after demolding process. Using this technology, 300nm period SU-8 subwavelengh gratings and nanochannels were fabricated on flat substrate with good fidelity. This sacrificial layer-assisted UV curing imprint technology offers versatility and flexibility to stack polymer layers and sealed fluidic channels.

Rapid nanofabrication with high density pattern with UVN30 chemically amplified resist

As a chemically amplified resist, UVN30 has been evaluated for mask use in high density pattern rapid fabrication by electron beam lithography. This resist displays excellent sensitivity and reasonable resolution for dense features. At optimum conditions proximity effect is eliminated and 75 nm and 150 nm dense lines resolved in a 300 nm thick film with writing field of 1mm2. With UVN30 mask, Si nanostructures are etched by non-switch DRIE etch chemistry developed in this work, which achieves high etch rate and smooth sidewall. This method is a promising technique for fast speed fabrication of nanophotonics, nanochannels and Si master stamps for nanoimprint.