Proton beam writing

SU-8 has become the favourite photoresist for high-aspect-ratio (HAR) and three-dimensional (3D) lithographic patterning due to its excellent coating, planarization and processing properties as well as its mechanical and chemical stability. However, as feature sizes get smaller and pattern complexity increases, particular difficulties and a number of material-related issues arise and need to be carefully considered. This review presents a detailed description of these effects and describes reported strategies and achieved SU-8 HAR and 3D structures up to August 2006.

https://iopscience.iop.org/article/10.1088/0960-1317/17/6/R01/pdf

SU-8: a photoresist for high-aspect-ratio and 3D submicron lithography

Ionizing radiation has been found to be widely applicable in modifying the structure and properties of polymers, and can be used to tailor the performance of either bulk materials or surfaces. Fifty years of research in polymer radiation chemistry has led to numerous applications of commercial and economic importance, and work remains active in the application of radiation to practical uses involving polymeric materials. This paper provides a survey of radiation-processing methods of industrial interest, ranging from technologies already commercially well established, through innovations in the active R&D stage which show exceptional promise for future commercial use. Radiation-processing technologies are discussed under the following categories: cross-linking of plastics and rubbers, curing of coatings and inks, heat-shrink products, fiber–matrix composites, chain-scission for processing control, surface modification, grafting, hydrogels, sterilization, natural product enhancement, plastics recycling, ceramic precursors, electronic property materials, ion-track membranes and lithography for microdevice production. In addition to new technological innovations utilizing conventional gamma and e-beam sources, a number of promising new applications make use of novel radiation types which include ion beams (heavy ions, light ions, highly focused microscopic beams and high-intensity pulses), soft X-rays which are focused, coherent X-rays (from a synchrotron) and e-beams which undergo scattering to generate patterns.

High-energy radiation and polymers: A review of commercial processes and emerging applications

Shape memory polymer nanocomposites

3.2. Two-Photon Lithography (TPL) 919 4. Serial Writing with Charged Particles 920 4.1. Electron Beam Lithography 920 4.2. Ion Beam Lithography 920 4.3. Scanning Probe Resist Lithography 921 5. Microand Nanomachining 921 5.1. Focused Ion Beam 921 5.2. Scanning Probe Nanomachining 921 6. Direct Writing and Material Deposition 921 7. Moulding 922 7.1. Mould Fabrication 922 7.2. Demoulding: Mould Treatment 924 7.3. Embossing Thermoplastic Materials: Nanoimprint Lithography (NIL) 924

Fabrication approaches for generating complex micro- and nanopatterns on polymeric surfaces.

In recent years the process combining deep X-ray lithography with electroforming and micromoulding (i.e. LIGA), has become an important technique for the production of high aspect-ratio microstructures for the fabrication of micro-electromechanical systems (MEMS). The aim of this paper is to investigate the potential of high energy ion microbeams for carrying out similar micromachining, and in particular for overcoming the geometrical restrictions which are inherent in deep x-ray lithography. Using a scanned 2.0 MeV proton beam of approximately 1 micron diameter, we produced latent microstructures in high molecular weight PMMA resist. These resist microstructures were subsequently developed using a multi-component developer which is highly specific in the removal of exposed resist, while leaving unexposed or marginally exposed material unaffected. A suitable range of exposures has been established, and factors affecting the geometrical fidelity of the produced microstructure have been investigated. The relative advantages and limitations of this technique vis a vis deep X-ray lithography are discussed.

Micromachining using deep ion beam lithography

The production of high aspect ratio microstructures is a potential growth area. The combination of deep X-ray lithography with electroforming and micromolding (i.e. LIGA) is one of the main techniques used to produce 3D microstructures. The new technique of proton micromachining employs focused MeV protons in a direct write process which is complementary to LIGA, e.g. micromachining with 2 MeV protons results in microstructures with a height of 63 μm and lateral sub-micrometer resolution in PMMA resist. The aim of this paper is to investigate the capabilities of proton micromachining as a lithographic technique. This involves the study of different types of resists. The dose distribution of high molecular weight PMMA is compared with three other types of resist: First the positive photo resist AZ P4620 will be discussed and then PMGI SF 23, which can be used as a deep UV, e-beam or X-ray resist. Finally SU-8, a new deep UV negative type of chemically amplified resist will be discussed. All these polymers are applied using the spin coating technique at thicknesses of between 1 and 36 μm

Resist materials for proton micromachining

The combination of deep X-ray lithography with electroforming and micromoulding (i.e., LIGA) has been shown to offer high potential for the production of high aspect-ratio microstructures. The LIGA technique, employing synchrotron light and a suitable X-ray mask, allows production of 3D microstructures in PMMA with aspect ratios around 100. Here we demonstrate that the novel technique of Deep Ion Beam Lithography (DIBL), a direct process utilizing a focused beam of MeV ions scanned in a predetermined pattern over a suitable resist material, can produce three dimensional microstructures with sub-micrometer feature sizes. Microstructures extending up to 100 μm from the substrate with aspect ratios approaching 100 can be produced. Multiple exposures at different ion energies allow production of multilayer structures in single resist layers of SU-8, a newly developed, chemically accelerated, negative tone, near UV, photoresist.

Micromachining using focused high energy ion beams: Deep Ion Beam Lithography

The recently developed process of high-energy ion beam micromachining (proton micromachining) is discussed. Proton micromachining is a novel process for the production of high aspect-ratio 3D microstructures. The sub-micron lateral resolution and the well-defined range of an MeV proton microbeam are utilized to make lithographic structures in suitable polymers (e.g., SU-8, PMMA). Sub-micron structures with a depth of tens of microns and aspect-ratios approaching 100 have been achieved. The use of different energies for multiple exposures allows the production of intricate 3D multi-layer structures in a single polymer layer, and because no mask is needed the process offers a wide range of possible geometries for the production of non-prismatic or even rounded features. The throughput of the technique does not compare favourably with conventional (masked) processes for high volume batch production of microcomponents. On the other hand, significant applications of high-energy ion beam micromachining may be developed, e.g. for the rapid production of prototypes, the research into the characteristics of microstructures, and the manufacture of molds, stamps and X-ray masks.

The use of proton microbeams for the production of microcomponents

The technique of Deep Ion Beam Lithography (DIBL) allows the production of high aspect-ratio microstructures in suitable polymer resists (eg PMMA) with complex three-dimensional geometries in a fast, direct write process In conjunction with micromoulding and electroforming, the DIBL-technique may prove extremely useful for the production of microcomponents, micromachines and microelectromechanical systems (MEMS) The present scanning system (OM-DAQ) in use at the Singapore Nuclear Microscope facility is limited to a 256 × 256 pixels raster scan This system, while adequate for analytical applications, has limitations when machining high resolution structures using DIBL A new scanning system has been developed in order to overcome this limitation The new scanning system is based on a DAC PC-card that allows flexible scanning with a resolution of up to 4096 × 4096 pixels The new system allows the beam to be scanned in specific patterns, which are designed to achieve optimal resolutions The use of the new scanning system provides a mechanism for translating high resolution digital images into high resolution three dimensional microstructures Using this system we have produced submicron (300 nm) walls with an aspect ratio approaching 100, three dimensional complex microstructures with smooth walls and corners, and multiple microstructures exposed by repetitive scanning

J.L. Sanchez

Papers

Micromachining using deep ion beam lithography

Resist materials for proton micromachining

Micromachining using focused high energy ion beams: Deep Ion Beam Lithography

The use of proton microbeams for the production of microcomponents

A high resolution beam scanning system for deep ion beam lithography