Cheng Yu Chiu

Micro-/nano-lithography based on the contact transfer of thin film and mask embedded etching

This paper reports a simple but useful contact imprinting method for micro- and nano-patterning and fabrication. It utilizes an anti-adhesion layer to release a patterned metal film from a mold to a polymer layer deposited on a substrate. The transferred metal pattern then serves as an etching mask for subsequent etching on the polymer layer to complete the patterning and fabrication processes. Experimental tests have been successfully carried out on several samples with linear grating and dot-arrayed features with the feature size ranging from 65 nm to 500 nm. The strength of this method is in its simplicity, ease of implementation and flexibility, which paves the way for fabricating submicron- and nano-structures on different substrates without using sophisticated and expensive facilities.

IR laser-assisted micro/nano-imprinting

We report on the development of a new imprint method capable of transferring patterns down to nano-sized features from a silicon mold assisted by an infrared pulse laser during imprinting. We demonstrate that successful pattern transfer can be achieved easily on photoresist films of SU-8 5 from a silicon mold with a feature size of either 5 ?m or 266 nm using a 1064 nm Nd:YAG laser. It is found that the IR pulse laser energy is absorbed predominantly by the silicon mold, and the pattern transfer is completed through heat transfer to the polymer. We also find that large area imprinting over 4 mm2 and pattern transfer with concave features can be achieved by taking advantage of flexible Si mold fabrication. This method integrates strengths from hot embossing and laser-assisted direct imprinting. The rapid imprint speed and variety of three-dimensional structures available in silicon technology render this technique a strong candidate for many real-world applications.

Application of nanoimprinting technology to organic field-effect transistors

The charge carrier transport efficiency and issues of patterning in organic semiconductors limit the potential range of microelectronic and optoelectronic applications of organic devices in nanoscale. We demonstrate high-performance organic field-effect transistors (OFETs) with a mobility of approximately 2.5 cm2/V s using nanogroove gate-dielectrics formed by nanoimprinting. The preferred flow of charge carriers in OFETs parallel to the nanogrooves yields a high mobility anisotropic ratio (above 220), providing a built-in autopattern organic semiconductor function with nanoscale resolution. This nanostructure embedded device has great potential for use in the manufacture and lithography-free patterning of organic semiconductor films in integrated circuits.

Fabrication of polyimide micro/nano-structures based on contact-transfer and mask-embedded lithography

Polyimide materials are well known for their excellent mechanical and chemical stability which, as an adverse consequence, makes their fabrication processes much more difficult, especially in micro- and nano-scales. In this paper, we demonstrate an innovative and powerful method for fabricating micro/nano-structures on polyimides. The proposed method first adopts an imprinting approach to transfer a patterned metal film from a mold to a polymer layer coated on a polyimide layer. The patterned double polymer layers are then dry etched using the transferred metal pattern as an etching mask. Finally, polyimide structures are obtained by lifting off the top polymer layer and the metal film through wet etching. Experiments have been carried out and important parameters to achieve high pattern-transformation fidelity are determined. Fine structures of polyimides with a feature size of 500 nm and a total patterned area of 8 × 8 mm2 are demonstrated. Advantages of the proposed method include low-temperature, low contact pressure, small feature size, high throughput and ease of in implementation. Most importantly, it is applicable for a large number of tough polymers which are difficult to deal with by other methods in terms of micro/nano-fabrication.

Polymorphic transformation induced by nanoimprinted technology in pentacene-film early-stage growth

Polymorphism control of organic semiconductor films greatly affects optoelectronic device performance. The thin-film phase is always the dominant phase in the early growth of pentacene films; however, the performance of the bulk phase is superior to that of the thin-film phase. In the present study, a nanoimprinted polyimide layer is used to induce the growth of the bulk phase in the early stage. The bulk phase appears in the initial few layers and its dominance of the polymorphisms in the pentacene film increases with increasing thickness. The mechanism of the phase transformation is explained.

Roller-based laser assisted direct imprinting for nanofabrication

This paper reports an improved nanofabrication method based on laser-assisted direct imprinting (LADI) technology. The key element in this improvement is to introduce a quartz roller into the LADI process. The quartz roller optically focuses an incident UV laser light into a line onto a silicon substrate, and mechanically compresses a quartz mold against the silicon substrate. Under the action of both laser melting and mechanical pressure, the mold can imprint into the silicon and transfer the molds surface features directly to the silicon substrate. This roller-type LADI approach has several significant advantages as compared to the original planar type LADI. It transforms the LADI process into a large-area, continuous, and high throughput nano-fabrication method. Experimental tests in this work demonstrate the direct fabrication of nano-structures of 500 nm line width with an imprinting speed of 5 times 60 mm2 per minute in a continuous configuration.

Fabrication of quartz mold with submicrometer features based on laser-assisted contact transfer method

We report a simple method for fabricating quartz molds with submicrometer- and nanoscaled surface structures. It utilizes an excimer laser as the energy source and a roller-type contact-printing setup for directly transferring a patterned metallic thin film from a silicon mold to a quartz substrate. The transferred metallic patterns are subsequently used as an etching mask for dry etching on the quartz plate. Experiments have been carried out to determine the optimal parameters for successful pattern transformation. Experimental results on fabricating quartz molds are presented, and potential applications of this innovative pattern transformation method are addressed.

Laser Assisted Roller Imprinting

This paper presents a new roller-based nano-imprinting method based on the laser assisted direct imprint (LADI) technique. Similar to LADI, it utilizes a high-energy pulse laser to melt the silicon substrate and a pre-loaded quartz mold to imprint nano-patterns into the substrate. However, a cylindrical roller made of highly transparent silica is introduced in this new method. Optically, the roller is acting like a cylindrical lens which can focus the laser beam into a line source at the mode/substrate interface. Mechanically, the roller provides a constant line-type contact pressure between the mold and substrate. With this innovative roller design, the laser-assisted imprint process can be carried out in a smooth and continuous way and therefore open the possibility for large-area nano-imprinting and faster imprinting speed. A prototype setup of this laser-assisted roller imprinting (LARI) has been constructed and the experimental results are successfully obtained.

Determination of Residual Resist Layer Thickness in E-Beam Lithography Based on Energy Dispersive X-Ray Spectrometer

In this paper, we report a method for determining the residual thickness of resist layer during e-beam lithography processes. The method is based on the energy dispersive x-ray spectroscopic (EDS) measurements on a silicon substrate deposited with a PMMA layer. The PMMA layer acts as the resist layer in e-beam lithography. From a calibration test, an empirical relationship is established between the EDS signal, electron energy, and PMMA film thickness. Form this empirical relationship the residual PMMA layer thickness after e-beam exposure and developing can be determined within an accuracy of 83%, which is very important to the subsequent etching processes. An important feature of the proposed method is that its lateral resolution depends only on the focused e-beam spot size and can be in the order of nm. With such resolution, the thickness of the resist layer under few nm line width can be measured. The proposed method to estimate the residual resist layer thickness plays a vital role in nano-fabrication or nano-patterning based on e-beam lithography technology.

High-frequency surface acoustic wave (SAW) devices fabricated by contact-transferred and mask-embedded lithography

This paper reports the application of a newly developed lithography method, the contact transfer and mask-embedded lithography (CMEL), in fabricating high-frequency (~2 GHz) surface acoustic wave (SAW) filters. In sort, CMEL utilizes a thin metal film deposited on an anti-adhesion layer and a silicon mold with pre-fabricated features. The metal film bearing the molds surface pattern can be transferred into a photo-resist layer deposited on a substrate. Subsequent etchings complete the lithography and the micro/nano-fabrication. To demonstrate CMEL and its potential, we apply CMEL for fabricating inter-digital transducers of SAW filters with a line width down to 500 nm. Using a LiNbO3 substrate, the operating frequency can reach 1.8 to 2 GHz based on this simple and easy-to-implement lithography method. Details in the CMEL, the lithography results, the high-frequency performance of the fabricated SAW devices, as well as future developments of this simple lithography method will be addressed.