Liang Pan

Maskless Plasmonic Lithography at 22 nm Resolution

Optical imaging and photolithography promise broad applications in nano-electronics, metrologies, and single-molecule biology. Light diffraction however sets a fundamental limit on optical resolution, and it poses a critical challenge to the down-scaling of nano-scale manufacturing. Surface plasmons have been used to circumvent the diffraction limit as they have shorter wavelengths. However, this approach has a trade-off between resolution and energy efficiency that arises from the substantial momentum mismatch. Here we report a novel multi-stage scheme that is capable of efficiently compressing the optical energy at deep sub-wavelength scales through the progressive coupling of propagating surface plasmons (PSPs) and localized surface plasmons (LSPs). Combining this with airbearing surface technology, we demonstrate a plasmonic lithography with 22 nm half-pitch resolution at scanning speeds up to 10 m/s. This low-cost scheme has the potential of higher throughput than current photolithography, and it opens a new approach towards the next generation semiconductor manufacturing.

A Novel Approach of Carbon Embedding in Magnetic Media for Future Head/Disk Interface

A novel method of carbon embedding (≤1 nm) is used as a surface modification technique to produce overcoat free media surfaces. The filtered cathodic vacuum arc technique at ion energy of 90 eV is used to embed carbon in the top surface of a ~25 nm iron/platinum (FePt) film. Transport of ions in matter (TRIM) simulations and X-ray photoelectron spectroscopy (XPS) are used to study carbon embedding profiles and surface chemical composition. XPS results show that carbon embedding is effective in improving the oxidation resistance of FePt. Conductive atomic force microscopy (CAFM) is done on samples after exposure to a 780 nm IR laser with an effective output power of 40 mW to study the thermal stability. No change in the conductivity is observed in the case of carbon embedded FePt surface. Ball-on-disk tribological tests are conducted at a contact pressure of 0.26 GPa on bare and modified FePt surfaces. It is observed that the coefficient of friction is reduced considerably from a value of approximately 0.8 to ~0.27 after the surface modification.

A graphene-based Fabry-Pérot spectrometer in mid-infrared region

Mid-infrared spectroscopy is of great importance in many areas and its integration with thin-film technology can economically enrich the functionalities of many existing devices. In this paper we propose a graphene-based ultra-compact spectrometer (several micrometers in size) that is compatible with complementary metal-oxide-semiconductor (CMOS) processing. The proposed structure uses a monolayer graphene as a mid-infrared surface waveguide, whose optical response is spatially modulated using electric fields to form a Fabry-Pérot cavity. By varying the voltage acting on the cavity, we can control the transmitted wavelength of the spectrometer at room temperature. This design has potential applications in the graphene-silicon-based optoelectronic devices as it offers new possibilities for developing new ultra-compact spectrometers and low-cost hyperspectral imaging sensors in mid-infrared region.

High throughput optical lithography by scanning a massive array of bowtie aperture antennas at near-field.

Optical lithography, the enabling process for defining features, has been widely used in semiconductor industry and many other nanotechnology applications. Advances of nanotechnology require developments of high-throughput optical lithography capabilities to overcome the optical diffraction limit and meet the ever-decreasing device dimensions. We report our recent experimental advancements to scale up diffraction unlimited optical lithography in a massive scale using the near field nanolithography capabilities of bowtie apertures. A record number of near-field optical elements, an array of 1,024 bowtie antenna apertures, are simultaneously employed to generate a large number of patterns by carefully controlling their working distances over the entire array using an optical gap metrology system. Our experimental results reiterated the ability of using massively-parallel near-field devices to achieve high-throughput optical nanolithography, which can be promising for many important nanotechnology applications such as computation, data storage, communication, and energy.

Flying plasmonic lens at near field for high speed nanolithography

Optical lithography has been the key for continuous size reduction of semiconductor devices and circuits manufacturing. Although the industry is continually improving the resolution, optical lithography becomes more difficult and less cost effective in satisfying the ever increasing demands in nano-manufacturing. Besides manufacturing, the dramatic advancements in nanoscale science and engineering also call an urgent need for high-throughput nano-fabrication technologies that are versatile to frequent design changes. Here we experimentally demonstrated the capability of patterning with 50 nm linewidth with a high flying speed at 10 meter/second. This low-cost nano-fabrication scheme has the potential of a few orders of magnitude higher throughput than current maskless techniques, and promises a new route towards the next generation nano-manufacturing. Besides its application in nanolithography, this technique can also be used for nanoscale metrology, imaging and data storage.

A Coupled Electromagnetic and Thermal Model for Picosecond and Nanometer Scale Plasmonic Lithography Process

Plasmonic lithography may become a mainstream nanofabrication technique in the future. Experimental results show that feature size with 22 nm resolution can be achieved by plasmonic lithography. In the experiment, a plasmonic lens (PL) is used to focus the laser energy with resolution much higher than the diffraction limit and features are created in the thermally sensitive phase-change material (PCM) layer. The energy transport mechanisms are still not fully understood in the lithography process. In order to predict the lithography resolution and explore the energy transport mechanisms involved in the process, customized electromagnetic wave (EMW) and heat transfer (HT) models were developed in comsol. Parametric studies on both operating parameters and material properties were performed to optimize the lithography process. The parametric studies show that the lithography process can be improved by either reducing the thickness of the phase-change material layer or using a material with smaller real refractive index for that layer.

Temperature mapping using molecular diffusion based fluorescence thermometry via simultaneous imaging of two numerical apertures.

We report a new optical technique to map two-dimensional temperature distributions in liquid solutions based on the thermal motion of fluorescent molecules. We simultaneously capture the fluorescence images of different numerical apertures (NAs) to resolve the temperature-dependent orientations of emission dipoles. In this work, we use two numerical apertures (2NA) prove the concept. This 2NA technique is robust against the intensity variations caused by photobleaching, unsteady illumination and nonuniform molecule distribution. Moreover, as the measured intensity of directional emission is insensitive to polarization changes, this method can be applied to polarizing materials, such as metal surfaces. Under this configuration, the 2NA technique offers another advantage of naturally filtering out the emission background that falls out of collection cones. We foresee the 2NA technique to open a new detection scheme of fluorescence thermometry.

Fabrication of silver nanostructures using femtosecond laser-induced photoreduction

Silver nanostructures were fabricated by femtosecond laser-induced reduction of silver ions and the impact of solution chemistry on the fabricated structures was evaluated. By investigating the exact photochemistry of the nanofabrication solutions, which contained varying amounts of diamine silver ions, trisodium citrate, and n-lauroylsarcosine sodium, and optimizing the laser processing parameters, we fabricated two-dimensional silver pads with surface roughness values of 7 nm and stable 2.5-dimensional shell structures with heights up to 10 μm and aspect ratios of 20 in a ready manner. Moreover, thermal annealing of these structures afforded materials where the average resistivity value was only a factor of 4 greater than that of bulk silver. In this way, the work presented here provides for a methodology that can be used for laser direct fabrication of metal nanostructures for applications in plasmonics and micro- and nano-electronics.

Extending the diffusion approximation to the boundary using an integrated diffusion model

The widely used diffusion approximation is inaccurate to describe the transport behaviors near surfaces and interfaces. To solve such stochastic processes, an integro-differential equation, such as the Boltzmann transport equation (BTE), is typically required. In this work, we show that it is possible to keep the simplicity of the diffusion approximation by introducing a nonlocal source term and a spatially varying diffusion coefficient. We apply the proposed integrated diffusion model (IDM) to a benchmark problem of heat conduction across a thin film to demonstrate its feasibility. We also validate the model when boundary reflections and uniform internal heat generation are present.

A coupled electromagnetic and thermal model for picosecond and nanometer scale plasmonic lithography process

Plasmonic lithography may become a mainstream nano-fabrication technique in the future. Experimental results show that feature size with 22 nm resolution can be achieved by plasmonic lithography. In the experiment, a plasmonic lens is used to focus the laser energy with resolution much higher than the diffraction limit and features are created in the thermally sensitive phase change material layer. The energy transport mechanisms are still not fully understood in the lithography process. In order to predict the lithography resolution and explore the energy transport mechanisms involved in the process, customized electromagnetic wave and heat transfer models are developed in COMSOL. Parametric studies on both operating parameters and material properties are performed for optimizing the lithography process. Parametric studies show that the lithography process can be improved by either reducing the thickness of the phase change material layer or using a material with smaller real refractive index for that layer.Copyright