Proton Beam Lithography Research Articles

In-situ generation of Au nanoparticles in poly(methyl methacrylate) films via MeV proton irradiation

It has been shown that a radiation-sensitive polymer such as PMMA, which is widely utilised in lithography as a positive resist, can be used to create a flexible nanocomposite with metal nanoparticles by proton irradiation without any additional treatment. Three 50-μm-thick free-standing polymer films with 4, 10 and 16 wt% of gold were prepared with the spin-coating technique from a mixture of PMMA and hydrogen tetrachloroaurate trihydrate and irradiated with 2-MeV protons. Fluences in the range of 2 × 1013–2 × 1014 protons/cm2 were used to reduce gold ions to neutral metal atoms with the subsequent formation of nanoparticles. UV–Vis and FTIR spectroscopy was employed to study the Au-nanoparticle coalescence after the radiation modification. AFM and SEM microscopy showed the morphology of the nanoparticles on the polymer surface, while TEM microscopy was used to visualise the nanoparticles formed inside the polymer. It was found that the fluence of the Au-nanoparticle formation can be lower than that used in proton-beam lithography on PMMA. Although the free-standing foil allowed generated gases to diffuse from both sides, it did not withstand the high fluence applied to it on the substrate. The concentration of the generated nanoparticles depended both on the primary Au-atom concentration in the PMMA matrix and on the proton irradiation fluence used. In Au–PMMA composites containing 10 and 16 wt% of gold, the fluence of 2 × 1013 protons/cm2 was enough to produce a sufficient amount of nanoparticles to obtain the resonance absorption of light, whereas the sample with 4 wt% of gold required a higher proton fluence to exhibit Au-atom coalescence. It has been ascertained that the amount of the nanoparticles synthesized by proton irradiation is directly proportional to the concentration of gold in PMMA and to the proton fluence used. As for the size of the nanoparticles, it is directly proportional to the concentration of gold in PMMA, but inversely proportional to the fluence used.

Improvement in mixing efficiency of microfluidic passive mixers functionalized by microstructures created with proton beam lithography

In this work, different microfluidic passive mixer devices were designed, simulated and realized. The mixing unit area was functionalized by microstructures, whose shapes and layouts were designed in the simulations to improve the mixing efficiency of the chips. It is known that a micro wall array is a promising tool for mixing in a microfluidic passive mixer device. In the present work, the length and the areal density of the walls in the micro wall array were varied in order to find good mixing performance. The devices were then created with ion microlithography and UV lithography techniques. Mixing test were performed from which the mixing efficiencies were determined. It was found, that the mixing efficiency of the fabricated passive micromixers can be improved to an excellent value by more densely distributed and longer micro-walls.

Functionalization of microfluidic devices by microstructures created with proton beam lithography

Microfluidic devices have become important part of technology nowadays in numerous fields of application, such as chemical reactions, biosensors, synthesis of nanomaterials, diagnostic device for detection of biological samples, etc.In this work, as a new fabrication approach, poly(dimethyl-siloxane) (PDMS) microstructures were integrated into PDMS microdevices by combining proton beam lithography and conventional UV lithography fabrication techniques. This way, the microstructures and the microdevices can be made from the same material, which is not only useful, but in case of certain applications, it is essential. This work deals with the design of various microfluidic devices with and without integrated microstructures, for passive mixing purposes. The investigation involves the computer simulation and optimization, and then the fabrication of the chips with the above mentioned methods. Finally, the analysis of the mixing efficiency of the micromixer devices was realized with UV/vis spectroscopy at different fluid flow rates. Compared to the small size of the whole mixing chip, high mixing efficiency could be achieved.

Construction of X-Ray Optics Elements with a Proton-Beam Lithography

Construction of X-Ray Optics Elements with a Proton-Beam Lithography

Electrothermal application of novolac-derived carbon micropatterns prepared by proton beam lithography and carbonization

High performance electric heaters based on carbon micropatterns (CMs), which were fabricated from a novolac resin by proton beam lithography (PBL) and carbonization, have been explored in this study. For the purpose, well-defined negative-type novolac micropatterns with crosslinked network structures were fabricated by PBL at the optimized fluence of 5 × 1015 ions cm−2 and then carbonized to form CMs. The fabricated 60–nm–thick CMs were characterized to have a high electrical conductivity of 2.12 × 102 S cm−1 and a high optical transmittance of 78%. In an application as electric heaters, the 60–nm–thick CMs showed a high maximum temperature of 141.7 °C at 60 V with an excellent heating rate of 16.06 °C s−1. Therefore, the prepared transparent electric heaters can be used in window defrosting/defogging, display, gas sensing, and medical equipment.

Facile construction of electrically-conductive carbon patterns from a cheap coal-type pitch and their application to electric heating devices

Electrically-conductive carbon patterns (ECPs) from cheap pitch thin films were fabricated using a simple proton beam lithography and pyrolysis. Well-defined negative-type pitch patterns were formed at the optimized fluence of 3×1015ionscm−2, and then pyrolyzed at various temperatures to create ECPs. The precursory pitch patterns formed at the optimized conditions were successfully temperature-dependently converted to the ECPs by pyrolysis. The formed ECPs through pyrolysis at a higher temperature exhibited good electrical conductivity. Moreover, the ECPs exhibited good electric heating characteristics, demonstrating the possibility of using the ECPs as a cheap electric heating element.

Micro-patterns fabrication using focused proton beam lithography

Proton beam writing technique was recently introduced at 3MV Tandetron accelerator at Nuclear Physics Institute in Rez (Czech Republic). It has been used, to produce three-dimensional (3D) micro-structures in poly(methylmethacrylate) by 2.0MeV and 2.6MeV protons micro-beam. Micro-channels (52μm×52μm) have been realized.After chemical etching, the quality of the bottom and side walls of the produced structures in PMMA were analyzed using Scanning Transmission Ion Microscopy (STIM).

Creation of microstructures using heavy ion beam lithography

In this work, three-dimensional (3D) structures were produced in PMMA and CR-39 polymer resists using a carbon ion microbeam. To investigate possible advantages of heavy ions compared to the well-established proton beam lithography, the same resist materials were also irradiated with protons that had a range in the materials studied here similar to that of carbon ions. The microstructures produced in different resists were analysed after chemical etching. The quality of the bottom and side walls of the structures produced by protons and carbon ions were compared using scanning electron microscopy (SEM) and atomic force microscopy (AFM). The results showed that, for the resist materials tested, lithographic structures made with the 8MeV carbon beam had more rough lateral and bottom surfaces compared to those made with 0.6MeV proton beam lithography.

Aqueous base developable: easy stripping, high aspect ratio negative photoresist for optical and proton beam lithography

A variety of different photo resists are used for the fabrication of polymer and metal high aspect ratio structures. Among them SU-8, a chemically amplified negative tone photoresist is the mostly used. However, after processing the finished resist pattern (SU-8) is hardly removed from the substrate. In the present work the formulation and process optimization of a negative tone chemically amplified photoresist (TADEP) is presented. TADEP resist owns two advantages: the dissolution of the uncrosslinked areas in IC standard aqueous developers and the easy stripping in acetone by the assistance of ultrasonic bath. The TADEP resist is successfully applied for the fabrication of polymer and metal structures, after electroplating and stripping and also in the case of Proton Beam Writing.

Proton microbeam irradiation effects on PtBA polymer

Proton beam lithography has made it possible to make various types of 3D-structures in polymers. Usually PMMA, SU-8, PS polymers have been used as resist materials for lithographic purpose. Microbeam irradiation effects on poly-tert-butyl-acrylate (PtBA) polymer using 20 MeV proton microbeam are reported. Preliminary results on pattern formation on PtBA are carried out as a function of fluence. After writing the pattern, a thin layer of Ge is deposited. Distribution of Ge in pristine and ion beam patterned surface of PtBA polymer is studied using the optical and secondary electron microscopic experimental methods.

Proton beam lithography at the University of Surrey’s Ion Beam Centre

Investigations into the lithography applications using MeV protons and α-particles are being performed at the Ion Beam Centre of the University of Surrey, UK. This is a new application of the microbeam in this laboratory. Proton beam lithography has been carried out on several materials: The first microstructures were made on bulk polymers, to test the performance of our beam. Photosensitive glasses have also been micropatterned with successful results and more recently, proton lithography of semiconductor materials has been introduced. In this paper we present a few structures made in GaAs and PMMA.

Fabrication of phosphor micro-grids using proton beam lithography

A new nuclear microscopy technique called ion photon emission microscopy or IPEM was recently invented. IPEM allows analysis involving single ions, such as ion beam induced charge (IBIC) or single event upset (SEU) imaging using a slightly modified optical microscope. The spatial resolution of IPEM is currently limited to more than 10μm by the scattering and reflection of ion-induced photons, i.e. light blooming or spreading, in the ionoluminescent phosphor layer. We are developing a “Microscopic Gridded Phosphor” (also called Black Matrix) where the phosphor nanocrystals are confined within the gaps of a micrometer scale opaque grid, which limits the amount of detrimental light blooming. MeV-energy proton beam lithography is ideally suited to lithographically form masks for the grid because of high aspect ratio, pattern density and sub-micron resolution of this technique. In brief, the fabrication of the grids was made in the following manner: (1) a MeV proton beam focused to 1.5–2μm directly fabricated a matrix of pillars in a 15μm thick SU-8 lithographic resist; (2) 7:1 aspect ratio pillars were then formed by developing the proton exposed area; (3) Ni (Au) was electrochemically deposited onto Cu-coated Si from a sulfamate bath (or buffered CN bath); (4) the SU-8 pillars were removed by chemical etching; finally (5) the metal micro-grid was freed from its substrate by etching the underlying Cu layer. Our proposed metal micro-grids promise an order-of-magnitude improvement in the resolution of IPEM.

The beam blanking system for microlithography at Lund Nuclear Microprobe

A new beam blanking system was installed at the Lund Nuclear Microprobe and employed in proton beam lithography (PBL) for polymer microstructures fabrication. The blanker consists of two parallel plates connected to a high voltage generator. Measurement of the beam blanking time on a sample was performed by means of the standard PIXE system. The beam is blanked and returns to a sample within 200 ns. The blanking system is designed for the new sub-micrometer beamline under installation in the accelerator laboratory. A number of pilot MeV ion beam lithography experiments were performed to illustrate the possibility to use the blanking system in combination with the existing data acquisition and scanning system. A 2.5 MeV proton beam was used to irradiate 50 μm SU-8 negative resist. The blanker was shown to be a necessary part of the lithography system. It enables blanking between each pixel and hence fabrication of various patterns down to a single pixel. The blanker has significantly simplified beam control and enhanced process time and spatial resolution. Three-dimensional microstructures with 20:1 aspect ratio were fabricated.

Low Energy Proton Beam Lithography with a Thin Oxygen‐Etch Barrier Layer

Low energy protons in the several keV energy range have very limited penetration depths into polymer films. This limitation necessitates the use of very thin oxygen‐etch barrier layers and of dry image development by oxygen reactive ion etching. For negative tone images, the etch‐barrier layers were deposited patternwise by proton‐beam‐induced polymerizations of organo‐metallic compounds, followed with dry image development by oxygen reactive ion etching. For positive tone images, the etch‐barrier layers were deposited (prior to patternwise exposure to proton beams) on top of polymer films by plasma polymerizations of organo‐metallic compounds, or by evaporation or sputtering of certain metals. Hydrogen atoms and/or protons react with metal atoms in the etch barrier layers to yield volatile metal hydrides, making the exposed areas more vulnerable to oxygen reactive ion etching, and providing positive tone images after image development. Sub‐keV electrons can replace low energy protons for fabrications of positive‐tone polymer images. In these processes, almost any kind of carbonaceous polymer film can be used as the imaged material. With a bright ion gun available, the exposure times could be less than a second, yielding high aspect ratio and high resolution polymer patterns.