Yang-Chun Cheng

Talbot lithography: Self-imaging of complex structures

The authors present a self-imaging lithographic technique, capable of patterning large area periodic structures of arbitrary content with nanoscale resolution. They start from the original concept of Talbot imaging of binary gratings—and introduce the generalized Talbot imaging (GTI) where periodic structures of arbitrary shape and content form high-definition self-images. This effect can be used to create the complex, periodic patterns needed in the many lithographic fabrication steps of modern semiconductor devices. Since the process is diffraction limited, the achievable resolution depends only on the wavelength, mask patterning, and degree of coherence of the source. Their approach removes all the complex extreme ultraviolet (EUV) reflective masks and optics, replacing them with nanopatterned transmission masks and makes the whole process simple and cost effective. They have successfully verified the GTI concept using first a He–Ne laser, and then demonstrated its potential as a nanolithography method using a compact table-top soft x-ray (EUV) 46.9nm laser source. These sources provide the high degree of coherence needed by diffraction-based imaging and are extendable to shorter wavelengths. They have recorded EUV GTI images up to the sixth Talbot plane, with consistent high quality good results, clearly demonstrating the ability of the GTI method to record high-resolution patterns at large distances.The authors present a self-imaging lithographic technique, capable of patterning large area periodic structures of arbitrary content with nanoscale resolution. They start from the original concept of Talbot imaging of binary gratings—and introduce the generalized Talbot imaging (GTI) where periodic structures of arbitrary shape and content form high-definition self-images. This effect can be used to create the complex, periodic patterns needed in the many lithographic fabrication steps of modern semiconductor devices. Since the process is diffraction limited, the achievable resolution depends only on the wavelength, mask patterning, and degree of coherence of the source. Their approach removes all the complex extreme ultraviolet (EUV) reflective masks and optics, replacing them with nanopatterned transmission masks and makes the whole process simple and cost effective. They have successfully verified the GTI concept using first a He–Ne laser, and then demonstrated its potential as a nanolithography method...

Extreme ultraviolet holographic lithography: Initial results

The authors report the initial results from a holographic lithography technique using extreme ultraviolet (EUV) radiation. This approach removes the need for complex EUV reflective masks and optics, replacing them with a binary, nanopatterned transmission mask. Computer generated holograms were fabricated on 100nm thick silicon nitride membranes with a 100nm thick chromium absorber layer. Reconstructed images have been recorded in an 80nm thick polymethylmetacrylate photoresist using 13nm wavelength EUV radiation from an undulator source. The pattern was characterized by optical and atomic force microscopies, and compared with simulation results from the TOOLSET diffraction simulation program, yielding excellent agreement.

Progress in extreme ultraviolet interferometric and holographic lithography

The Center for Nanotechnology has developed an advanced beamline dedicated to nanopatterning using the radiation from a new undulator on the Aladdin storage ring at the Synchrotron Radiation Center of the University of Wisconsin-Madison. Computer generated holograms and transmission interferometric gratings were fabricated and tested on the new extreme ultraviolet (EUV) exposure system. The authors have developed an accurate model, based on Fresnel-Kirchhoff integral diffraction theory, to analyze performance of real EUV interferometric and holographic lithography systems.

Engineering study of extreme ultraviolet interferometric lithography

Extreme ultraviolet interferometric lithography (EUV-IL) is a powerful nanopatterning technique, exploiting the interference of two beams of short-wavelength radiation (13 nm) to form high-accuracy fringe patterns. Transmission diffraction gratings of appropriate period (40-100 nm) are used to form the beams; the substrate is located in the region of overlap to expose the photoresist material, recording 20-50 nm interference fringe patterns. Although the physics of EUV-IL is simple, its actual implementation is not and requires attention to detail in order to fully exploit the power of the technique. In order to understand the impact of realistic physical conditions on the performance of EUV-IL, we have developed a set of accurate numerical models based on the Rayleigh-Sommerfeld diffraction theory. These modeling tools are then applied to generate a complete and accurate analysis of EUV-IL, taking into account all the relevant physical processes, from finite extent of the gratings to the partial coherence of the source, and including detailed physical structure of the mask. The results are used to guide the design and implementation of EUV-IL exposure systems, down to the sub-11-nm range.

Patterning of SU-8 resist with Digital Micromirror Device (DMD) maskless lithography

Digital micromirror device (DMD) based maskless lithography has a number of advantages including process flexibility, no physical photomask requirement, fast turnaround time, cost effectiveness. It can be particularly useful in the development stage of microfluidic and bioMEMS applications. In this report, we describe the initial results of thick resist SU-8 patterning, soft lithography with polydimethylsiloxane (PDMS) and lift-off of Cr features using a modified DMD maskless system. Exposures of various patterns and microfluidic channels reveal that the system is well capable of printing 60 μm thick resist at a resolution as small as a single pixel (less than 13 μm) with an aspect ratio about 5:1. Both negatively and positively tapered sidewalls are achieved by projecting the UV light from front side of the SU-8 coated Si wafer and from the back side of the coated glass, respectively. The positive sidewall has an angle 88o which is ideal to serve as a mold for subsequent PDMS soft lithography. Both SU-8 and PDMS microfluidic devices for biomolecular synthesis were fabricated with this maskless system. In addition, a lift-off process was also developed with the intention to create built-in metal features such as electrodes and heaters.

Progress in extreme ultraviolet interferometric lithography at the University of Wisconsin

Extreme Ultraviolet Interferometric lithography (EUV-IL) can generate periodic patterns useful to characterize photoresist materials and to create templates for self-assembled geometries. The Center for NanoTechnology has developed a novel EUV-IL beamline dedicated to nanopatterning using radiation from an undulator on the Aladdin storage ring at the University of Wisconsin-Madison. The beamline and the EUV-IL system were commissioned in 2006; we have completed several characterization studies, and modified several key components to improve resolution and usability. The EUV-IL system can expose different pitches at the same time producing patterns with a range of halfpitch from 55nm down to 20nm and less on the wafer. We can also introduce a variable image modulation by performing double exposures, overlapping the interference pattern with the transmitted zero order. Recently we have demonstrated down to 20nm half-pitch printed IL image in PMMA resist.

Local line edge roughness in microphotonic devices: An electron-beam lithography study

Microphotonic resonators are very sensitive to sidewall roughness. Several structures with sub-100-nm gap between bus and ring have been patterned on silicon-on-insulator wafer with 50keV electron-beam lithography at different doses. The authors’ direct Monte Carlo (DMC) model has been used to model the electron scattering and energy deposition to predict proximity effects and image modulation. By comparing the point spread function from DMC with independent experimental results, the DMC model has been validated. Experimentally, the line edge roughness (LER) study has been carried out to include both the first order (standard deviation) and the second order of statistical analysis (correlation length). The authors found that the LER is not uniform and varies with the position along the pattern—this nonlocal LER may have profound implications for the performance of the devices. The highest local LER is obtained at the position when the bus is closest to the ring. This phenomenon will be exacerbated when th...

Extreme UltraViolet Holographic Lithography with a Table-top Laser

We report the demonstration of Extreme Ultraviolet Holographic Lithography - EUV-HL - using a compact table top extreme ultraviolet laser. The image of the computer-generated hologram (CGH) of a test pattern was projected on the surface of a sample coated with a high resolution photoresist. Features with a 140 nm pixel size were printed using for the reconstruction a highly coherent table top 46.9 nm extreme ultraviolet laser. We have demonstrated that the combination of a coherent EUV source with a nanofabricated CGH template allows for the extension of nanolithography in an extremely simple set up that requires no optics. The reconstructed image of CGH was digitized with an atomic force microscope, yielding to reconstructions that are in excellent agreement with the numerical predictions.

Coherent imaging nano-patterning with extreme ultraviolet laser illumination

We present a high resolution extreme ultraviolet patterning approach based on Talbot self imaging and holographic projection imaging using for illumination a table top extreme ultraviolet laser.

Extreme Ultraviolet Interferometric Lithography: A Path to Nanopatterning

The semiconductor industry continues in its relentless march to miniaturization [1]. Every four years or so, the dimensions of the features on an integrated circuit are halved, yielding an increase in density and functionality of the electronic “chip.” The economic advantages of more devices per unit area outweigh increases in fabrication costs and performance limitations, pushing the industry to seek ever-smaller patterns. At the time of writing (April 2008) advanced devices are patterned with the smallest features hovering around 45 nm, and the next generation of ∼32 nm devices is on the horizon. What is perhaps most remarkable is that this level of nanopatterning is achieved with optical imaging tools and processes that use an actinic wavelength of 193 nm, the ArF laser emission line. As taught in any elementary physics textbook, the wavelength of light ultimately limits the achievable optical resolution [2]. So how can we pattern 32 nm features using 193 nm radiation?