Tone Lithography Research Articles

Annular Gray Tone Lithography for the Fabrication of Rotationally Symmetric Continuous Relief Meso- and Microscale Optical Elements

Annular lithography is a recently introduced, flexible technique that has been tailored to the fabrication of rotationally symmetric optical structures in the meso and micro range. The optical concept for the exposure tool is based on a combination of axicons with movable components that create a ring-shaped light distribution with variable diameter in the image plane. This contribution demonstrates for the first time the use of gray tone exposure in annular lithography to fabricate continuous relief structures, overcoming the previous limitation using binary structures. For the controlled exposure of the continuous relief structures, the sensitivity curve of the resist, the exposure dose decreasing with increasing ring diameter, and the exposure time have to be considered. A control and simulation tool is introduced to provide radius-dependent exposure data and, furthermore, to control and iteratively improve the fabricated structures. To demonstrate the gray tone capabilities, various diffractive elements as well as refractive spherical and aspherical elements with a maximum diameter of ~6 mm and a maximum height of 4 µm are shown as examples. Profile shape measurements of fabricated elements show good agreement with the expectations.

Focussed Helium Ion Beam Exposure of Polymethylmethacrylate: Positive or Negative Tone Images, Polyenes, and Fluorescent Carbon Layers

Understanding chemical transformations in polymethylmethacrylate (PMMA) during helium ion beam exposure will help researchers involved in nanofabrication, materials synthesis or transformation, patterning polyenes, developing phantoms for radiation therapy, and realizing carbonaceous quantum emitters. The level of conjugation in PMMA can be controlled using a helium ion beam to realize patterns that are suitable for positive tone lithography, polyenes, dye‐like fluorescent materials, and polycyclic aromatic compounds with very similar properties to carbon dots. High‐resolution dose studies employing Raman scattering and atomic force microscopy (AFM) reveal the conditions under which these very different conjugated carbonaceous materials form, their spatial distribution, and dissolution characteristics in common solvents.

Vertically tapered waveguide spot size converters fabricated via a linewidth controlled grey tone lithography for InP-based photonic integrated circuits.

We report a novel and simple fabrication process to realize vertically tapered spot size converters (SSC) on InP photonic integrated circuits. The vertical tapering was achieved via a linewidth controlled local optical dose variation, leading to a grey tone photoresist profile. The fabricated SSCs are compact, polarization insensitive and demonstrate a very high mode conversion efficiency of 95%. Integrated SSCs improved the overall loss by 5 dB giving a coupling loss as low as 1.3 dB/facet, for a lensed fibre with a mode field diameter of 3.0 µm. A good agreement was found between the fibre-to-fibre optical loss measurements and those predicted from simulations.

Lithographic tone reversal in optical exposure of polymethyl methacrylate (PMMA) resist

Tone reversal in electron beam lithography when using polymethyl methacrylate (PMMA) as a resist is well known. At low electron beam dose, chain scissioning—resulting in positive tone lithography, is observed in PMMA. However, above a certain threshold dose, electron beam exposure results in cross-linking of PMMA, enabling negative tone resist action. We describe a similar phenomenon in PMMA (containing Irgacure 379—a free radical generator)—brought about through UV radiation exposure at 365 nm wavelength. The photo-active Irgacure 379 serves two purposes here: making PMMA sensitive to wavelengths as short as 365 nm (i-line) and amplifying the free radical concentration so that the tone reversal could be observed at relatively low UV dose. Thus, an optical analogue of the well-known tone reversal seen in electron beam lithography exists where the reversal is brought about by changing the intensity of UV illumination on PMMA containing a certain amount of Irgacure 379.

Polycrystalline Au Nanomembrane as a Tool for Two-Tone Micro/Nanolithography

A Au nanomembrane is utilized as a hydroxyl ion channel to develop negative and positive-tone micro/nanolithography. Less than 50 nm thick Au films, that is, Au nanomembranes, have nanopores through which ions can pass. Micro- and nanostructures can be fabricated by using an Au nanomembrane-deposited Al layer with KOH solution loading on it. Hydroxyl ions diffuse over Au nanomembranes to react with an underneath Al layer. Depending on the pH of the fabrication process, which is governed by the volume of KOH solution loaded onto an Au nanomembrane-deposited Al layer, protruded aluminum hydroxide structures (positive tone lithography) or micro/nanotunnels (negative tone lithography) are able to be obtained. A flexible THz photonic structure and microfluidic channels have been demonstrated as possible applications of these structures.

Photomask displacement technology for continuous profile generation by mask aligner lithography

Mask aligner lithography is one of the most widely used technologies for micro-optical elements fabrication. It offers a high throughput with high-yield processing. With different resolution enhancement technologies shadow printing is a mature alternative to the more expensive projection or electron-beam lithography. We are presenting a novel mask aligner tool that allows shifting the photomask with high accuracy between sequential exposure shots. It offers possibilities such as double patterning or gray tone lithography by applying different light doses at different locations. Within this publication, we show the first results for high resolution blazed grating structures generated in photoresist by multiple exposures using a conventional binary photomask.

Single component photoacid/photobase generators: potential applications in double patterning photolithography

193 nm light as an excitation source for resist patterning is limited due to the inability to achieve pitch division much below limits of λ/2. Current techniques are examining the use of a variety of photochemical manipulations as a means to extend lithographic patterning to small, more defined images. Double patterning, or dual tone lithography, has recently garnered considerable interest due to the potential of patterning two features within one excitation exposure. In this contribution, single component carbamate photoacid/photobase (PAG/PBG) generators are studied as potential substrates for implementing dual tone lithography. At lower exposure powers, only the acid is generated and complete film dissolution was observed, while at higher 193 nm laser powers, photobase activation resulted in little to no film dissolution. Ideally, at intermediate laser doses, both the photoacid and photobase are activated giving rise to the desired double patterning. The energy required to initiate dual tone characteristics was found to be easily adjusted using the additional of amine quenchers or via manipulation of the PAG/PBG concentration. Film thickness measurements were used to determine the energies required for both photoacid and photobase activation, while laser flash photolysis and NMR spectroscopy studies were used in an attempt to understand the PAG/PBG activation mechanism.

819 グレイトーンリソグラフィによる立体微細形状板バネの製作プロセスの開発(機素潤滑設計III)

819 グレイトーンリソグラフィによる立体微細形状板バネの製作プロセスの開発(機素潤滑設計III)

Layer thickness-induced lens distortions during thermal nanoimprint

Arrays of convex spherical lenses with heights in the micrometer range and diameters of 100 and 200μm⁠, respectively, were imprinted into thin layers of polystyrene by thermal imprint. The masters were prepared by gray tone lithography and dry etching. They feature three distinct lens heights, so that during imprint into medium layer thickness three different situations prevail: imprint into thick layers, imprint into thin/adequate layers, and imprint into highly thinned layers. Only during imprint in such highly thinned layers slight recovery was detected combined with stretching-induced polymer layer breakup. For all other cases the replication quality was good and did not show recovery effects. This is in contrast to the imprint of flat stamps into layer thickness of similar sizes. Differences detected in height of the replicated lenses compared to the master are due to thermal expansion in the thermal nanoimprint lithography process. Negligible residual layers were obtained at only slightly changed curvature radius.

Evaluation of poly(hydroxyethyl methacrylate) imaging chemistries for micropatterning applications

Chemical reactions leading to imaging of poly(2-hydroxyethyl methacrylate) (PHEMA) based materials are investigated. Positive imaging is obtained upon exposure to deep UV and electron beam radiation following a chain scission mechanism. The presence of the OH group adds the possibility of development in alkaline aqueous solutions with considerably higher sensitivities than in the case of PMMA. On the other hand, negative tone lithography can be achieved with the addition of photo-acid generators that induce crosslinking, arising mainly from transesterification reactions. Reaction mechanisms have been investigated mostly by MS studies supported by FTIR and UV spectroscopies. Micropatterning characteristics for deep UV and electron beam radiation have also been evaluated. Submicron patterning capabilities are demonstrated for both positive and negative imaging although higher resolution is hindered by strong swelling phenomena during development. Nevertheless, specific patterning applications including novel schemes for use in biomaterials and microsystems are anticipated.

Experiments for 3-D structuring of thick resists by gray tone lithography

With the conventional micromachining technologies: isotropic and anisotropic dry and wet etching, a few shapes can be done. To overcome this limitation binary multi-masking technique, laser micro-stereolithography, or direct electron-beam-writing were used, but an inexpensive one-step UV-lithographic method, using the so-called “gray-tone lithography”, seems to be the best choice to produce local intensity modulation during exposure process. The paper reports on the study of arbitrary three-dimensional (3-D) shaping of negative and positive thick resists, using this method, and common technologies in standard ICs fabrication. Particular emphasis is placed on the design, manufacturing and use of half-tone transmission masks, required for UV-lithographic step in the fabrication process of mechanical, optical or electronics components. The original design and fabrication method, for the gray-tone reticles, were supported by experiments showing the main advantage of this new technology: the 3-D structuring of thick resists in a single exposure step, and also a high aspect ratio obtained over 9:1. Experimental results are presented in SEM micrographs, only for positive thick resists, showing different 3-D shapes in positive and negative polarity, and also the results obtained by using the wall-type test structure for aspect ratio evaluation. Finally, by optimization of the lithographic process, interesting applications are shown.

Protein patterning via radiation-assisted surface functionalization of conventional microlithographic materials

Several technological options available for protein high resolution patterning on conventional microlithographic materials were investigated: (i) two mechanisms for radiation-assisted selective attachment on polymer surface, i.e. chemical linkage and hydrophobicity-controlled patterning; (ii) two classes of microlithographic materials, i.e. diazo-naphto-quinone (DNQ)/novolak and poly( tert-butyl-methacrylate-co-methyl-methacrylate) [P(tBuMA-co-MMA)] resists; (iii) two pattern exposure techniques, i.e. contact printing using near-UV light on DNQ/novolak resist, and projection printing using e-beam lithography on P(tBuMA-co-MMA) resist; and (iv) two patterning tones, i.e. positive and negative tone lithography. The mechanism based on crosslinking protein amino groups to carboxylic groups generated by photolysis can be used to print (i) high contrast, positive or negative tone, protein features on DNQ/novolak resists; and (ii) good contrast, positive tone protein features on P(tBuMA-co-MMA) resist. The hydrophobicity-controlled mechanism based on the hydrophobic interaction between the protein and hydrophobic unexposed polymer surface can be used to print high contrast, negative tone, protein features on P(tBuMA-co-MMA) resist with 125 nm resolution. It was found that optical and e-beam lithography materials and techniques can be easily transferred in bio-microlithography, with impact on biodevices fabrication and combinatorial chemistry.