Glassy Carbon Mold Research Articles

CMOS-compatible electrochemical nanoimprint: High throughput fabrication of ordered microstructures on semiconductor wafer by using a glassy carbon mold

Metal assisted chemical etching (MACE), as an emerging wet chemical etching method for fabricating semiconductor micro/nano-structures (MNS), has attracted wide attentions because of the advantages of simple processes, high accuracy and no damages. However, the conventionally used noble metal catalysts (such as Au, Pt,) trapped in the micro/nano-structures are difficult to be removed or reused, not only harming the electronic performances of the CMOS devices but also increasing the fabrication cost. Here, we propose an electrochemical nanoimprint strategy by using a glassy carbon (GC) imprint mold. To overcome the poor electrocatalytic activity of GC, the GC imprint mold was connected to a Pt sheet in the cathodic chamber. To avoid the contaminations of metal and metallic cations in the fabricated MNS, the cathodic chamber was separated from the anodic one by a proton exchange membrane. By optimizing the technical parameters, including the contact pressure between GC imprint mold and GaAs wafer, the concentration of electron acceptor in the cathodic chamber, the imprinting time, etc., a remarkable corrosion rate of about 130 nm/min was achieved, comparable with that of platinum electrocatalyst. Furthermore, by using a GC imprint mold in the spatially separated corrosion cell, the contaminations caused by both the metal catalysts and metallic ions can be eliminated. Thus, this work provides a potential approach to the high throughput fabrication of CMOS-compatible semiconductor microdevices.

Transfer durability of high aspect ratio moth-eye structure on glassy carbon mold using release agent

The moth-eye structure is an anti-reflection structure that is expected to effective over a wide range of wavelengths and angles of incident light. In addition, since the moth-eye structure is a single layer, it can be easily replicated using ultraviolet nanoimprint lithography (UV-NIL). However the moth-eye structure is fragile, because it has a lot of small sharp needles, and it has been reported that the durability decreases as the aspect ratio increases. The moth-eye structure exhibits anti-reflection function even with a low aspect ratio, but in this study, a fragile moth-eye structure with a high aspect ratio was used to facilitate evaluation of the releasability. In this experiment, a glassy carbon mold with high aspect ratio was release treated using a newly developed release agent. Furthermore, using the “partial-filling method” led to a dramatic improvement in the durability when compared to the conventional method using repeated cycles of the UV-NIL machine.

Direct Metal Forming of a Microdome Structure with a Glassy Carbon Mold for Enhanced Boiling Heat Transfer.

The application of microtechnology to traditional mechanical industries is limited owing to the lack of suitable micropatterning technology for durable materials including metal. In this research, a glassy carbon (GC) micromold was applied for the direct metal forming (DMF) of a microstructure on an aluminum (Al) substrate. The GC mold with microdome cavities was prepared by carbonization of a furan precursor, which was replicated from the thermal reflow photoresist master pattern. A microdome array with a diameter of 8.4 μm, a height of ~0.74 μm, and a pitch of 9.9 μm was successfully fabricated on an Al substrate by using DMF at a forming temperature of 645 °C and an applied pressure of 2 MPa. As a practical application of the proposed DMF process, the enhanced boiling heat transfer characteristics of the DMF microdome Al substrate were analyzed. The DMF microdome Al substrate showed 20.4 ± 2.6% higher critical heat flux and 34.1 ± 5.3% higher heat transfer coefficient than those of a bare Al substrate.

Micro-structuring of glassy carbon for precision glass molding of binary diffractive optical elements

Precision glass molding is a more cost efficient process for the large volume manufacturing of highly complex optical surfaces than direct manufacturing. Glassy carbon (GC) molds are used for precision glass molding, because they can be operated at temperatures up to 2000°C. Used today mainly for manufacturing aspheric lenses, we consider here material technology for diffractive optical element (DOE). For diffractive optics the surface structuring is in the micrometer range and a surface roughness Ra lower than 20 nm is required. We introduce a reactive ion etching process with a titanium hard mask. Fused silica (FS) molds with identical optical functionality were fabricated for comparison. All molds were used for precision glass molding of a low Tg glass L-BAL42. We will compare GC and FS as mold materials in terms of quality and robustness. Optical performance measurements of the molded glass DOEs are shown and are in good agreement with the theoretical predictions. The results confirm that precision glass molding based on GC molds is a very promising technology to economically fabricate small structures in glass for DOEs.

Structuring of LTCC Substrates by a Combination of Pressure-Assisted Sintering and Hot-Embossing

A novel technology for the structuring of LTCC surfaces is introduced. The material is shaped in a zero-shrinkage process by embossing a glassy carbon mold into the softened LTCC directly after termination of the shrinkage. Three commercially available LTCC compositions (Ceramtape GC, Heratape CT707, and DP951) were tested. Diverse raised and lowered structures including rings, grids, and characters were fabricated. Different material behavior was observed for the tested compositions. Promising results were achieved with Ceramtape GC. Embossing of precise, 40 μm deep circular cavities and 50 μm high raised characters is demonstrated. Processing of 100 × 100 mm2 substrates is possible. DP951 showed very good moldability, but also unwanted material displacement due to evaporating lead. A high displacement capacity but uneven heights of embossed structures were observed on CT707 samples. SEM investigations proved the precise transfer of surface contours from the mold to the LTCC. Thereby, the high potential of the hot-embossing process for micro-patterning of LTCC is illustrated.

Design and fabrication of an achromatic infrared wave plate with Sb–Ge–Sn–S system chalcogenide glass

We designed and fabricated an achromatic infrared wave plate. To examine its phase retardation characteristics, the birefringence was calculated using the effective medium theory. A wave plate with a subwavelength grating was fabricated by direct imprint lithography on a low toxic chalcogenide glass (Sb-Ge-Sn-S system) based on calculated results. As a result of imprinting onto chalcogenide glass by a glassy carbon mold, a grating with 1.63 μm depth, a fill factor of 0.7, and a 3 μm period was obtained. The phase retardation of the elements reached around 30° in the 8.5-10.5 μm wavelength range. The fabrication of the infrared wave plate is less costly compared with conventional crystalline wave plates.

An electrochemomechanical polishing process using magnetorheological fluid

A new electrochemomechanical polishing process using a magnetorheological (MR) fluid is proposed in this study. The process uses the electrochemical anodic reactions on an electrically conductive workpiece under an electric field and an electrolytic solution soaked in the same carbonyl iron (CI) particles as those used in the conventional MR polishing process, where the yield stress is controllable through the external magnetic field. Through experiments, we show that the new process is suitable for polishing three-dimensionally configured workpieces made of very hard materials, such as glassy carbon (GC), that are very difficult to polish using conventional processes. To examine the benefits and limitations of the proposed polishing process, a preliminary electrochemical experiment was performed on GC specimens, and the characteristics of the resultant electrochemical reactions in an aqueous electrolytic solution were investigated. The effectiveness of the proposed process was then demonstrated by polishing GC molds designed to replicate a glass channel-mixer structure used for biochemical inspection applications.

Deliberation of Effect to Glass Imprinting Analysis by Williams-Landel-Ferry Equation

The Mems-ONE is well known software which simulates thermo-viscoelastic properties in the conduct of nanoimprinting. Assuming the glass materials to be viscoelastic body, the relaxation shear modulus was measured by the creep test, Williams-Landel-Ferry (WLF) equation is applied for expressing the temperature dependence of liquid viscosity. We compared experimental with analytic results used by Mems-ONE with the condition of fixed pressure and time. Thermo-viscoelastic properties of the glass materials were estimated using unidirectional compression creep test based on traditional thermo viscoelastic theory. Glass was Borosilicate Glass (D263, Schott). Glass imprinting was carried out on Glassy Carbon (GC) mold with line & space10 μm patterns fabricated by dicing saw. The machining accuracy is most important thing as the evaluation mold. The glass imprinting temperature consulted thermo-viscoelastic properties of the glass materials. The numerical simulation was carried out on the small portion of mold and glass. The constant value of WLF equation fitting in high temperature translates the master curve of D263 with a high degree of accuracy. It caused the accuracy improvement of analysis result. In addition, we confirm that WLF equation intended to resin can use to the glass imprinting.

Technique for transfer of high-density, high-aspect-ratio nanoscale patterns in UV nanoimprint lithography and measurement of the release force

Ultraviolet nanoimprint lithography (UV-NIL) is a powerful tool for nanoscale fabrication. However, the replication of high-density, high-aspect-ratio mold patterns by UV-NIL is very difficult because of the strong forces required to release the replicate from the mold. We used a glassy carbon (GC) mold with an antireflective structure, fabricated by irradiation with an oxygen-ion beam, to produce a high-density, high-aspect-ratio pattern, and we evaluated its release properties. The fabricated GC surface contained high-aspect-ratio conical structures with pitch of less than 100 nm. After fabrication of the antireflective structure, the mold surface was coated with chromium and a fluorinated silane coupling agent. By using this treatment and a peel motion during mold release, faithful replication of the mold structure in photocurable resin was possible. The release force increased with increasing mold surface area; the surface area effect is therefore the main factor in the mold-release step.

A process of glassy carbon etching without the micro masking effect for the fabrication of a mold with a high-quality surface

The surface quality of a mold is one of the major factors in imprint lithography as it determines the final surface quality and the minimum size of the replicated patterns. This paper describes a process for O2-based reactive ion etching (RIE) of glassy carbon (GC) without encountering any micro masking effect. Glassy carbon, because of its attractive properties such as its surface inertness, thermal stability and extraordinary hardness, has drawn much interest as a mold material for high-temperature imprinting on glasses and metals. Etch profiles with highly smooth surfaces free of micro masking effects were achieved by adding SF6 to the etching gas. The fraction of SF6 in the gas mixture (ranging from 0.1 to 0.6) showed little change in the quality of the etched surface, but it did lead to a proportional decrease in the etch rate of GC. A reasonable GC etch rate (115–120 nm min−1) and a smooth etch surface were obtained using SF6 at a fraction 0.2 or below. Using electron beam lithography (EBL), and processing under the established SF6/O2 RIE conditions, GC molds were fabricated and successfully applied to thermal imprinting onto glass and metals.

Molecular orbital study on a mechanism of adhesion between carbides and optical glass with high refractive index

AbstractA mechanism of adhesion between carbide molds and optical glass species with high refractive index in precision molding was elucidated on the basis of DV‐Xα molecular orbital calculations. An interfacial cluster model comprising NbPNaO glass and glassy carbon (GC) was applied in order to examine the changes in the chemical bonds between atoms at the interface and the effect of the substitution of niobium with other transition metals (denoted with; MdI) at the interface. The bond overlap population (BOP) between oxygen atoms at the surface of the glass and carbon atoms at the surface of the GC increases when the surface of the glass approaches the GC, while that between MeIO correspondingly decreases. When introducing oxygen deficiency at the surface of the glass, the BOP of the MeIcarbon bonds against the MeIoxygen increased in the order of Nb<Ti<W, which mostly coincided with the experimentally obtained order of increased probability of occurrence of adhesion between the glass and the GC mold. This supports the validity of the model where adhesion between the glass and the carbide occurs following the reduction of the transition metal in the glass. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2009

A parameter study on the micro hot-embossing process of glassy polymer for pattern replication

As a promising processing method for glassy polymer, hot-embossing process has attracted more attention in the micro/nano manufacturing field. However, the relationship between process condition and final quality has not been thoroughly and systematically investigated. Since the replication quality of the process is sensitive to the control parameter, it is necessary to study on the optimal process condition. For the four main factors influencing on the final replication fidelity of the hot-embossing process, we carried out a series of experiments with different sets of parameters, in which micro pattern arrays were directly replicated from a glassy carbon (GC) mold to a commercially available glassy polymer substrate (polycarbonate, PC) with self-made micro hot-embossing system. sWe adopt Taguchi method to minimize the sets of experiments and made an analysis of the result using ANOM and ANOVA. As the result shows, the most significant factor is the embossing temperature and the optimal process parameters are the embossing temperature at 170 °C, heating time at 120 s, embossing force at 70 kgf and force holding time at 90 s. The experiment with these parameters shows a good result.

Release Properties and Durability of Release Layer on UV Nanoimprint Lithography

Two photo-curable resins (PAK-01 and PAK-02) have been evaluated by a primitive test and a precise test. The primitive test is simple manual transfer process and the precise test is using UV nanoimprint lithography (UV-NIL) machine. At first, nano order high aspect ratio glassy carbon (GC) molds are prepared and then manual transfers are carried out. Both PAK-01 and PAK-02 have good filling and release properties. Next, a silane coupling agent C8F17C2H4SiCl3 is used for the release treatment of the quartz mold. To investigate of affinity between photo-curable resin and release layer, repetition manual transfers are carried out. In this release layer, durability is not sufficient for PAK-02. Therefore, more precise durability tests are carried out using PAK-01. The release treatment samples with various contact angles are prepared by changing the treatment of the silane coupling agent. The contact angle of less than 10 degrees is obtained by UV ozonation cleaning. The contact angle of 40 degrees is obtained by acetone cleaning. The contact angles of more than 80 degrees are obtained by silane coupling agent at 5 minutes treatment. The contact angles of more than 100 degrees are obtained by silane coupling agent at 2 hours treatment. After the preparation of the mold treatment, repeatedly imprints are carried out using UV-NIL machine. In the case of contact angles with less than 40 degrees, PAK-01 sticks to the surface of the quartz within several imprint times. On the other hand, in the case of contact angles with more than 80 degrees, PAK-01 does not stick on quartz surface more than 1000 times. Therefore, the combination of this release treatment and PAK-01 is suitable for mass-production.

Fabrication of Glassy Carbon Molds Using Hydrogen Silsequioxane Patterned by Electron Beam Lithography as O2 Dry Etching Mask

Glass is a good candidate material for optical devices because of its enhanced optical properties, the technique of die machining has not been established for the hot embossing of glass. In this study, we used the glassy carbon (GC) mold for the hot embossing of glass. An inductively coupled plasma reactive ion etching (ICP–RIE) using oxygen plasma was employed for the submicron structuring of the GC mold. Hydrogen silsesquioxane (HSQ) is a negative-type electron beam (EB) resist used to be resistant to oxygen plasma. HSQ patterns drawn by electron beam lithography (EBL) were used as the O2 dry etching mask. The etching selectivity between HSQ and GC was 35. The average of the extent of side etching was 40 nm at a depth of 300 nm. The side etching functioning as the draft angle was caused mainly by oxygen radicals, because HSQ patterns remained even after GC patterns were side-etched. We confirmed that the GC mold fabricated by O2 dry etching can be used for glass hot embossing. Since the mold lubricant was not rubbed on the mold surface, GC is the appropriate mold material for Pyrex glass.

Glass Imprinting Process for Fabrication of Sub-Wavelength Periodic Structures

Using direct glass imprinting with glassy carbon molds, one-dimensional surface-relief gratings with 500 nm pitch were fabricated on phosphate glasses. The maximum grating height attained in this study was 730 nm, which was formed by pressing at the softening temperature of the glass at a constant pressure of 0.4 kN/cm2. A large area pattern 6 ×6 mm2 with a 350-nm groove depth was also fabricated. Phase retardation of 0.1 λ was attained between transverse electric (TE)- and transverse magnetic (TM)-polarized light in the visible wavelength region. Calculated retardation using a rigorous coupled wave analysis agreed well with the experimental results.

A reflow process for glass microlens array fabrication by use of precision compression molding

A novel low-cost, high-volume fabrication method for glass microlens arrays was developed by combining compression molding and thermal reflow processes. This fabrication process includes three major steps—i.e., fabrication of glassy carbon molds with arrays of micro size holes, glass compression molding to create micro cylinders on a glass substrate, and reheating to form microlens arrays. As compared to traditional polymer microlens arrays, glass microlens arrays are more reliable and therefore may be used in more critical applications. In this research, microlens arrays with different surface geometries were successfully fabricated on a P-SK57 (Tg = 493 °C) glass substrate using a combination of the compression molding and thermal reflow processes. The major parameters that influence the final lens shape, including reheating temperature and holding time, were studied to establish a suitable fabrication process. A numerical simulation method was developed to evaluate the fabrication process. Finally, both surface geometry and optical performance of the fabricated glass microlens arrays were analyzed.

Surface-relief gratings with high spatial frequency fabricated using direct glass imprinting process

Surface-relief gratings with high spatial frequencies were first fabricated using a direct imprinting process with a glassy carbon mold at the softening temperature of phosphate glass. A grating with maximum height of 730 nm and 500 nm period was formed on the glass surface by the pressing at the softening temperature of glass under constant pressure of 0.4 kN/cm(2). Phase retardation of 0.1 lambda was observed between TE-polarized and TM-polarized light at 600 nm wavelength.

Fabrication of glasses with low softening temperatures for mold-processing by ion-exchange

Glasses having a low softening temperature are desired for compression molding techniques in order to lengthen the mold life. The final goal of this research is to obtain glasses, which are suitable for the precise molding in order to fabricate micro- to submicro-structures on the glass surfaces, utilizing the ion-exchange technique, which lowers the softening temperature of only the surface regions of the glasses. We examined variations in optical and thermal properties of a soda-lime silicate glass after ion-exchange in AgNO3-NaNO3 molten salts under various conditions. The glass transition and softening temperatures were lowered after the ion-exchange by 80 K and 70 K, respectively. Molding tests were demonstrated for the ion-exchanged glasses using a glassy carbon mold having a one-dimensional periodical structure of 500 nm in frequency and 350 nm in depth within the area of 6 × 6 mm2. It was found that the structure was successfully transferred on a part of the surface of the ion-exchanged glass at a lower temperature than that for the glass before ion-exchange.

A study on focused ion beam milling of glassy carbon molds for the thermal imprinting of quartz and borosilicate glasses**This article was presented at the 1st Topical Meeting on Microfactories ‘Desktop MEMS and Nano Factories’, Tsukuba, Japan, 17–19 October 2005.

The need for a flexible, low-cost and high-throughput process for the fabrication of a nano/micro glass-based micro fluidic device is becoming increasingly acute as the bio-MEMS industry is expanding very fast. In this study, repetitive-pass milling with a focused ion beam (FIB) was used for maskless and resistless fabrication of glassy carbon (GC) molds, which will then be the primary elements in the mass production of nano/micro glass devices by mold replication processes. The aim of the study is to establish basic process conditions and to investigate possible problems that occur during both FIB milling of GC and thermal imprint of glasses. First, the milled depth–beam dwell time curve for GC was investigated experimentally, and the relationship between the areal ion dose and the milled depth was formulated for application to the design of mold parts using FIB milling. Then, GC molds were produced and used for imprinting glasses to make micro fluidic parts. Surface contamination due to the FIB milling could be eliminated by a vacuum heat treatment at 1400 °C for 10 min. Finally, a dimensional mismatch arising from a thermal expansion coefficient mismatch between the GC mold and the glass materials was discussed.

Microstructuring of glassy carbon mold for glass embossing – Comparison of focused ion beam, nano/femtosecond-pulsed laser and mechanical machining

Various methods, including focused ion beam (FIB), femto-second laser, KrF eximer laser and dicing techniques, were employed for preparing glassy carbon (GC) micro-molds, and those methods were characterized in terms of the process rate, the roughness and the shape of machined structure. FIB milling using a repetitive pass method produced nano/microstructures with flat channel bottom and nearly vertical sidewalls. Although FIB milling was slowest process, it provided the best quality of machined surface ( R a = 4–30 nm depended on milled depth). Femtosecond-pulsed laser machining also allowed fabricating flat bottom and nearly vertical sidewalls on an area of 1.2 × 1.2 mm 2 at a scanning speed of 20 mm/s, but lead to an increase of the surface roughness ( R a = 80 nm). Femto-second laser in combination with FIB milling provided a possibility for the rapid fabrication of high quality microstructures on wide surface area. The roughness of machined surface decreased to 45 nm by the subsequent FIB milling. Microstructuring with a nanosecond-pulsed KrF eximer laser at an irradiation wavelength of 248 nm with a fluence of 13.2 J/cm 2 also allowed the fast fabrication of master structure for micro-gear, resulted in slanted sidewalls and not ideally flat bottoms. The surface roughness ( R a) of the bottom and the side wall was about 45 and 70 nm, respectively. Dicing technique allowed machining micro-channels with a rectangular and pyramidal cross-section on an area of 15 × 15 mm 2 under the feed speed of 50 mm/min. By reducing the feeding speed from 100 mm/min to 50 mm/min, surface roughness ( R a) of the structure side wall decreased from 150 nm to 70 nm. Achieved glassy carbon molds were then applied to the hot-emboss process of Pyrex and quartz glasses to investigate embossing conditions (emboss temperature, pressure and hold time) needed for the replication of Pyrex and quartz glass structures with various geometries and dimensions in glass plates with thickness of 1 mm. Replication results showed good replication at the nanoscale, resulted in the almost the same dimensions and surface roughness with that of cavities. Thicker plate provided faster filling in the emboss process of glass.