Photolithographic Structuring Research Articles

Photolithographic Structuring of Ordered Silicon Micropillar Electrodes for Lithium-Ion Batteries and Electrochemical Performance

Ordered and vertically oriented silicon (Si) micropillar structures, when optimally designed, can accommodate the lithiation-induced volumetric expansion that is limiting the implementation of Si as negative electrodes in Li-ion batteries. As there is not much work in this area, this study is focused on evaluating systematically the potential of Si micropillar electrodes prepared by photolithographic structuring of Si. The specific objective is to determine the effect of micropillar length and the free space around the pillars on the electrochemical performance of the electrode. The Si micropillar electrode with the pillar length of 14 μm showed a high specific capacity (2600 mAh/g) and a high areal capacity (3 mAh/cm2), which were stable for over 100 cycles. However, there was significant capacity degradation in 32 and 48 μm-long Si pillar electrodes due to cracking and electrode damage during cycling. In the 32 μm Si micropillar electrode, a small change in porosity from 69 to 86% caused a substantial increase in specific capacity from 1400 to 2000 mAh/g over 100 cycles, with the areal capacity decreasing from 3 to 2 mAh/cm2. The findings also show the directions for further work for the optimization of Si micropillar electrodes.

Novel structuring technique for diamond coatings

Here we report a novel structuring technique for diamond coatings using a copper masking on a previously deposited diamond layer to epitaxially grow a subsequent diamond layer only at the exposed areas of the mask. An 8 μm thick copper mask is applied onto the substrate using physical vapor deposition and structured by standard photolithographic methods. Hot filament chemical vapor deposition is used to grow the 14 μm thick structured diamond layer on the diamond coated silicon wafer. Micro and nano crystallinity of the diamond layers are varied, as well as the doping with boron to verify the applicability of the new structuring method for different kinds of diamond qualities. It is shown that four-inch wafers can be fully structured with this high temperature masking technique and the resolution was mostly affected by the quality of the printed mask used for the photolithographic structuring of the copper mask. This diamond structuring method could pave the way for diamond being used in electronics or adding additional functionalities to diamond coated tools.

Femtosecond laser-contoured micro-strain gages

A new concept for foil based micro strain gages (μSG) with sensor elements smaller than 1 mm2 and made possible by a laser contouring process will be presented. In contrast to photolithographic structuring, strain gages structured by laser ablation can be precisely tailored at their measuring destination after the strain sensor blank has been applied. Sensors for measuring very spatially confined elongations in particular, require a precisely positioned and small grid. Accurate mechanical positioning of the measuring grid at the micrometer scale can be challenging due to the much larger, bulky, foil substrate. In cases where a blank is positioned prior to grid patterning by laser ablation, it is even possible to write the measuring grid on smoothly curved, three-dimensional surfaces. A “contour cutting” method is used to minimize grid dimensions and reduce laser processing time, which enables the realization of micrometer-sized grids in a fast, flexible and maskless process. This will extend the range of possible applications for this sensor type (e.g. to include use in very confined spaces). This paper will present the development of a suitable process for the gentle, selective ablation of nickel‑chromium on polyimide substrates by means of ultrashort-pulse lasers. The best results were achieved at a laser fluence of ϕ= 0,43 J/cm2 and a scan speed of 2000 mm/s. Furthermore, the thermal stability of the sensor layer will be investigated and sensors will be characterized in bending tests.

Photolithographic structuring of soft, extremely foldable and autoclavable hydrophobic barriers in paper

Method for photolithographic structuring of foldable hydrophobic barriers in paper for the creation of microfluidic paper-based analytical devices (μPADs).

GaN/Cu2O Heterojunctions for Photovoltaic Applications

Several growth methods were employed to investigate the photovoltaic behavior of GaN/Cu2O heterojunctions by depositing cuprous oxide thin films on top of gallium nitride templates. The templates consist of a thin layer of GaN:Si grown on a sapphire substrate by metal organic vapor deposition. The deposition procedure was followed up by photolithographic structuring and thermal evaporation of metal contacts. For device characterization, J-V characteristics and external quantum efficiency were measured, pointing to a possible energy barrier in the conduction band. To gain further insight X-ray photoelectron spectroscopy was applied.

Application of Matrix-Assisted Laser Desorption/Ionization Mass Spectrometric Imaging for Photolithographic Structuring

The aim of this contribution is the application of matrix-assisted laser desorption/ionization mass spectrometric imaging (MALDI-MSI) in the area of photolithographic structuring. As proof of concept, this method was used to image an UV exposed negative photoresist layer, which is generally used to manufacture printed circuit boards (PCB) for electronic components. The negative photoresist layer consisting of the main component novolac, benzophenone as the active component, and the solvent tetrahydrofuran was mixed with the matrix dithranol and the salt additive LiTFA and spin-coated onto an ITO-conductive glass slide. To imprint an image on the created surface, a transparency with a printed wiring diagram was placed on top of it and irradiated by UV light for 15 min. The inspection of the efficient imprinting of the microstructure onto the photoresist layer was performed by MALDI-MSI. This unique application represents a further step toward the surface analysis of polymer films by this emerging life science imaging technique.

Fabrication of patterned silane based self-assembled monolayers by photolithography and surface reactions on silicon-oxide substrates

Self-assembled monolayers (SAMs) have received increasing attention since their introduction 30 years ago. Soon it was discovered that they can be used as alternative resist materials and are compatible with different established lithographic techniques commonly used in silicon semiconductor technology. Besides these possibilities to structure SAMs, other attractive properties emerged from the use of SAMs. E.g., the introduction of addressability into the patterns by selective functionalization with reactive precursor molecules and/or by applying suitable surface reactions was established. In this feature we highlight developments of photolithographic techniques that have been used in combination with SAMs serving either as resists for the patterning process or as precursor molecules for surface reactions, which can be performed on non-structured and mainly photochemically structured surfaces to obtain multifunctional surfaces with tunable surface properties. The aim is to provide an overview about the versatile possibilities to use silane based SAM systems to structure silicon-oxide substrates by introducing topographical as well as chemically heterogeneous surface structures. In particular the chemical activation of SAMs includes a large number of functionalization concepts which are intended to be summarized in this review. They will be introduced here according to the class of chemical reaction that has been used. Therefore, an introduction into the plethora of possible structures, which have been created by the combination of photolithographic structuring approaches, and the integration of tailor made surface functionalities into these systems will be highlighted. Additionally effective strategies to implement a diversity of chemical functionalities onto one substrate are summarized.

Structured electrodes on optical fibers

We have investigated three different techniques for electrode production directly on surfaces of D-shaped optical fibers. All three techniques were capable of producing suitable structural shapes and structural sizes for different metallic electrodes. The produced electrodes differ, however, in their electrical insulation properties and in the reproducibility of the electrode structures. Best results were achieved by a photolithographic structuring process.

Photolithographic structuring of a thin metal film coil on a Zerodur cylinder

Photolithographic structuring is a common method for generating micro structures (<approximately 20 μm) on flat substrates, especially on silicon in the mass production of integrated circuits. After applying a thin, homogeneous, UV-sensitive photoresist film to the substrate, a mask (generally a transparent film) with the desired structure is positioned in the beam path. After a photochemical reaction has taken place in the photoresist, a chemical base is used to remove the exposed (photo positive process) or unexposed (photo negative process) layer. Subsequently, a PVD (Physical Vapour Deposition) process based on cathode sputtering is used to apply the metal layer. In the last step, the metal layer on the photoresist structure as well as the photoresist structure itself are removed by a chemical process. Applying a metal layer (50–200 μm wide and 800 nm thick) to a Zerodur cylinder, the conventional photolithographic process leads to an unacceptably high reject rate in the fabrication process on cylindrical structures. The weak point of this process is the area in which the two sides of the transparent film overlap, as these have to be adjusted very precisely to the Zerodur substrate. An alternative method is to write the structure directly on the photoresist using a UV-beam, or to ablate the photoresist and to proceed then with the same fabrication steps as in the previously described process. A direct-writing photolithographic process in combination with a cathode-sputtering PVD method has made it possible to fabricate an accurate pair of thin niobium film coils on a Zerodur cylinder. Superconductive properties of the niobium film were obtained by the use of a low-power PVD process. Final tests confirmed the superconductive behaviour of the niobium thin film structure.

Photolithographic structuring of surface-attached polymer monolayers

In this paper, we present the combination of various photolithographic techniques with the concept of “grafting-from” polymerizations by means of immobilized initiators to create patterned monolayers of covalently attached polymers. Depending on the nature of the coupling chemistry of the initiator, either gold or silicon oxide surfaces can be modified. The creation of positive and negative images of the mask on the various surfaces by choosing the appropriate photolithographic technique is described. If the pattern are to be created by photopolymerization, in situ adjustments in the structure of the initiator allow for a higher efficiency of the process. Step and repeat procedures, which take advantage of the covalent linking of the polymers to the surfaces, permit the preparation of multifunctional polymer patterns.

Studies of Structure and Local Wetting Properties on Heterogeneous, Micropatterned Solid Surfaces by Microinterferometry

The microstructure and the local wettability of functionalized patterns is reported. The patterns are formed by the photolithographic structuring of multilayers of hairy rod polymers on glass/MgF2/SiO2substrates. By application of quantitative reflection interference contrast microscopy (RICM), the RICM image analysis was improved by accounting for (i) multiple reflections and the finite illumination aperture in stratified systems and (ii) geometrical effects in wedge-shaped systems and by (iii) optimizing the contrast enhancement through adjustment of the MgF2coating. The surface free energy of hairy rod films were determined by analysing the contact angle of partially wetting liquid droplets as a function of the surface tension of the liquid. Periodic lattices of hydrophobic domains separated by hydrophilic domains were applied to check the theory of Joanny and de Gennes [J. Phys. Chem.81,552–562 (1984)] of contact angle hysteresis at periodic arrays of pinning centers. Measurements of the shape of the triple line and the contact angle hysteresis by RICM confirmed the theoretical predictions.

Encapsulation of an integrated thin-film multi-electrode sensor using photocurable poly(styrene)

The direct photolithographic structuring of poly(styrene) by bi-functional azides has been elaborated and applied to the encapsulation of a disposable thin-film three-electrode device. Effective dielectric encapsulation has been shown with a polystyrene layer only 10 μm thick when chemically bonded to the sensor surface. This process may be generally amenable to the encapsulation of a wide variety of other types of sensors and electronic devices.

Patterning molecularly thin films of polymers—new methods for photolithographic structuring of surfaces

Three novel photolithographic processes for patterning of molecularly thin polymer films are described. The polymer monolayers are prepared by immobilization of initiator molecules to surfaces of solid substrates followed by thermally or photochemically activated radical chain polymerization. Thus polymer chains which are covalently linked to the surfaces of the substrates are obtained. The films can be patterned using appropriate masks and deep or near UV irradiation before, during or after polymer formation. The procedures described in this paper allow the chemical tailoring of surfaces with high spatial resolution. Step-and-repeat procedures, which take advantage of the covalent linking of the polymers to the surfaces, permit the preparation of multifunctional polymer patterns.

PcI2: New piezoelectric composites for ultrasonic transducers

Abstract The combination of photolithographic processes and green ceramic body structuring methods allow the fabrication of 1–3 or 2–2 composites with very fine structures and a free geometric design. Two suitable methods – named lost mould resp. jet machining – are presented with their potential to make new transducer geometries in the frequency range ≥ 5 MHz.

Encapsulation of chemically sensitive field-effect transistors with photocurable epoxy resins

A method for the direct photolithographic structuring of epoxy resins has been elaborated and applied to the encapsulation of CHEMFETs. The experimental results promise the possibility of a high degree of automation in CHEMFET production. This process may also be used for the encapsulation of other sensors (e.g., pressure or gas sensors) which need a window in the encapsulation material between the sensor chip and the measuring medium.

Additive thin film technology for hybrid circuit fabrication

Thin film hybrid circuits usually are fabricated by photolithographic structuring of an evaporated or sputtered metal layer sequence (substractive technology). In contrast to this, we discuss the abilities of ‘‘evaporation masking,’’ i.e., evaporation through metal masks (additive technology). Its main advantage is the local deposition of several different materials (conducting, resistive, semiconducting, insulating, electroluminescent, etc.) enabling the production of multilayer thin film devices. In this work we emphasize the possibility of producing complex hybrid circuits in one vacuum cycle including local insulations for capacitors, crossovers, and passivating oxide layers. Moreover, we discuss the equipment used, the electrical properties of some standard materials (NiCr, Al, Al2O3, etc.) and the resulting design rules for the hybrid layout. As a typical example, a thin film hybrid circuit on 2×2 in. glazed ceramics is presented which includes 800 crossovers, 56 thin film resistors, and 3 evaporated capacitors.