Masking Material Research Articles

Novel microelectromechanical systems image reversal fabrication process based on robust SU-8 masking layers

This paper discusses a novel fabrication process that uses a combination of negative and positive photoresists with positive tone photomasks, resulting in masking layers suitable for bulk micromachining high-aspect ratio microelectromechanical systems (MEMS) devices. Mi- croChem's negative photoresist Nano TM SU-8 and Clariant's image rever- sal photoresist AZ 5214E are utilized, along with a barrier layer, to effec- tively convert a positive photomask into a negative image. This technique utilizes standard photolithography chemicals, equipment, and processes, and opens the door for creating complementary MEMS structures without added fabrication delay and cost. Furthermore, the SU-8 masking layer is robust enough to withstand aggressive etch chemistries needed for fabrication research and development, bulk micromachining high-aspect ratio MEMS structures in silicon substrates, etc. This processing tech- nique was successfully demonstrated by translating a positive photomask to an SU-8 layer that was then utilized as an etching mask for a series of trenches that were micromachined into a silicon substrate. In addition, whereas the SU-8 mask would normally be left in place after process- ing, a technique utilizing Rohm and Haas Microposit TM S1818 as a re- lease layer has been developed so that the SU-8 masking material can be removed post-etching. C 2011 Society of Photo-Optical Instrumentation Engineers

Fabrication of Micro-Cantilevers Using RF Magnetron Sputtered Silicon Carbide Films

In the present work, silicon carbide (SiC) films were deposited by RF magnetron sputtering process on Si (100) substrates for micro-cantilever fabrication. The films were deposited without external substrate heating using a ceramic SiC target at 10 mTorr sputtering pressure, 200 W power and 50 mm target-to-substrate spacing. X-ray diffraction pattern shows that the films were amorphous in nature. In order to investigate the chemical inertness, the SiC coated Si substrates were dipped in buffered HF (BHF) at room temperature and in 40% KOH solution at 80 °C for varying length of time. Atomic force microscope was used to investigate surface roughness and morphology of the films before and after chemical processing. Micro-cantilever beams of the SiC film were fabricated by a single mask process. The SiC film was patterned using reactive ion etching (RIE) in SF6-O2 plasma. Thermally evaporated Al film was used as a mask during RIE process. This process also resulted in the formation of convex corners which were exploited for anisotropic etching of Si under the SiC film. The SiC cantilever beams were released by anisotropic etching of Si in KOH at 80 °C without using additional masking material. Scanning electron microscopy was used to observe the fabricated SiC micro-cantilever beams. The morphology of the SiC film after prolonged exposure to KOH was observed to be similar to that of the as-deposited film. The RF magnetron sputtered SiC films were found to be highly inert in KOH and BHF solutions. Due to difficulty in micromachining of bulk SiC material and its high cost, the RF sputtered SiC films on Si can provide a low cost structural material in MEMS.

Influence of the Masking Material and Geometry on the 4H-SiC RIE Etched Surface State

The roughness of etched SiC surfaces must be minimized to obtain surfaces with a smooth aspect, avoiding micromasking artifacts originating from re-deposited particles during the etching process. Four varieties of masks, Al, Ni, Si and C, were deposited on the SiC surface by photolithographic process. The C structures were formed by annealing conversion of patterned thick photoresist. On these surfaces, dry etching was performed with an SF6/O2 plasma produced in a Reactive-Ion-Etching (RIE) reactor. Although a better aspect of the surface is obtained with Ni in comparison with Al mask, micromasking could also occur even with Ni if the mask design was not enough spaced out. With C and Si masks, which produce fluorides species with negative boiling temperature, smooth etched surface was obtained without micromasking, even for tight masks covering up to 90% of the SiC surface.

A simple thermal oxidation technique and KOH wet etching process for fuel cell flow field fabrication

The relatively inexpensive SiO 2 layer was studied as the silicon etch mask in a KOH etching process for the fabrication of a flow field pattern that consists of channels as well as through-holes on the substrate of a fuel cell. The SiO 2 layer was grown in a wet thermal oxidation process to obtain a thickness of up to 6.5 μm and the growth was predicted using the Deal–Grove model. The flow fields on the cathode and anode sides were done using a wet etch process with KOH as etchant. Etching time was utilized to control the depth of the required pattern and the pattern shape was found to have an influence over the etch rate. With constant monitoring of the conditions and parameters used throughout the micro-fabrication process, the thick SiO 2 layer was found to be a reliable masking material for a long KOH etching.

X-ray characterization of Ge epitaxially grown on nanostructured Si(001) wafers

Selective growth of Ge on nanostructured Si(001) wafers is studied to evaluate the applicability of nanoheteroepitaxy approaches on the Ge–Si system for photonics applications under particular consideration of possible growth mask materials. A gate spacer technology established in advanced silicon microelectronics is used to generate a periodic array of nanoscaled Si pillars. The spacing of these Si pillars is 360 nm; diameter and height are about 100 nm, which is still above the expected geometrical parameters to obtain a compliant behavior of the Si lattice in the pillars. Ge dots are deposited by reduced pressure chemical-vapor deposition on top of the Si pillars. The characterization is done by different x-ray diffraction methods and transmission electron microscopy. It is found that even 30 nm thick Ge dots are fully relaxed and they exhibit a clear network of misfit dislocations. Significantly thicker Ge dots generate additional structural defects, mainly microtwins. A strain partitioning between Si pillar and Ge dot was not observed. The main reason for this is not the geometry of pillars but the compressive in-plane strain inside the uncovered Si pillar caused by the surrounding SiO2 layer required as masking material to guaranty the selective growth of Ge on top of the pillars only. High temperature diffraction experiments show that a partial annealing of this strain is possible.

Laser Processing of 2-D and 3-D Structures for Tunable Embedded Capacitors

Material formulation, structuring, and modification are key to increasing the unit volume complexity and density of next generation electronic packaging products. Laser processing is finding an increasing number of applications in the fabrication of these advanced microelectronic devices. This is due, in part, to the ability to achieve highly localized treatment of materials with a spatial resolution of tens of microns. In addition, the process is data-driven, that is, patterns can be generated without the need for masking materials. In the present investigation, a variety of structures have been generated in polymer nanocomposites, nanoparticle films, and sol-gel thin films using a computer-controlled laser system. Specifically, micromachining technology has been used to produce both variable-thickness and discrete capacitors from a single sheet (layer) of capacitor material, such that both types of structures can be integrated into the same layer. In addition, the laser micromachining technology has been extended to design and develop new vertical multilayer embedded capacitors for high speed applications. High speed packages require thick dielectrics. Calculations show that multilayer vertical capacitors can be better than thick capacitors formed from a single layer. In general, multilayer embedded capacitors are fabricated by repeated lamination of resin-coated copper, or pre-preg with a capacitor core. This is a time consuming, lengthy process. As an alternative, we have deposited a single, thick capacitance layer, and subsequent laser micromachining has been used to form multiple parallel channels of a controlled depth. Metal deposition in the channels results in a multilayer embedded capacitor structure. Lasers micromaching can also provide various complex patterns such as 3-D spiral channels within a dielectric or magnetically active nanocomposite, subsequently filled with conducting materials to form inductors. This technique can be used to prepare inductors and capacitors in the same layer of nanocomposite material. Hence, the technique can be used to generate multi-functional structures for tunable device applications.

Nanofabrication of surface-enhanced Raman scattering device by an integrated block-copolymer and nanoimprint lithography method

The integration of block-copolymers (BCPs) and nanoimprint lithography (NIL) presents a novel and cost-effective approach to achieving nanoscale patterning capabilities. The authors demonstrate the fabrication of a surface-enhanced Raman scattering device using templates created by the BCP-NIL integrated method. The method utilizes a poly(styrene-block-methyl methacrylate) cylindrical-forming diblock-copolymer as a masking material to create a Si template, which is then used to perform a thermal imprint of a poly(methyl methacrylate) (PMMA) layer on a Si substrate. Au with a Cr adhesion layer was evaporated onto the patterned PMMA and the subsequent lift-off resulted in an array of nanodots. Raman spectra collected for samples of R6G on Si substrates with and without patterned nanodots showed enhancement of peak intensities due to the presence of the nanodot array. The demonstrated BCP-NIL fabrication method shows promise for cost-effective nanoscale fabrication of plasmonic and nanoelectronic devices.

A Multi-Process Microfabrication Simulatorb on Cellular Automata

We describe a 3D simulator for several fabrication techniques utilized to create MEMS. The software is based on a cellular automata model and allows the user to simulate several processes, such as anisotropic wet etching in alkaline solutions and DRIE on (100) oriented Si substrates. The simulator allows for arbitrarily shaped masking materials and several processes can be applied in sequence to the same substrate. This enables the software to simulate the fabrication of MEMS devices which require more than one etching step. Although the automata used are not the most complex available, many of the results shown here are still acceptable from an engineering point of view.

Combining retraction edge lithography and plasma etching for arbitrary contour nanoridge fabrication

Edge lithography in combination with fluorine-based plasma etching is employed to avoid the dependence on crystal orientation in single crystal silicon to create monolithic nanoridges with arbitrary contours. This is demonstrated by using a mask with circular structures and Si etching at cryogenic temperature with SF6+O2 plasma mixtures. Initially, the explored etch recipe was used with Cr as the masking material. Although nanoridges with perfect vertical sidewalls have been achieved, Cr causes severe sidewall roughness due to line edge roughness. Therefore, an SU-8 polymer is used instead. Although the SU-8 pattern definition needs further improvement, we demonstrate the possibility of fabricating Si nanoridges of arbitrary contours providing a width below 50 nm and a height between 25 and 500 nm with smooth surface finish. Artifacts in the ridge profile are observed and are mainly caused by the bird's beak phenomenon which is characteristic for the used LOCOS process.

Cryogenic inductively coupled plasma etching for fabrication of tapered through-silicon vias

Vertical interconnects pose an interesting method for heterogeneous integration of electronic technologies allowing three-dimensional (3D) stacking of microelectromechanical systems devices and integrated circuit components. The vertical interconnects, referred to as through-silicon vias, begin with the formation of blind vias in silicon that are eventually exposed by mechanically lapping and polishing the wafer back side. Inductively coupled plasma (ICP) etching using SF6∕O2 gas chemistry at cryogenic temperatures has been investigated as a way to form vias with a tapered sidewall. The point in creating a controlled taper is so that subsequent thin films can be deposited along the sloped sidewall that line the via with insulation, barrier, and seed films. This tapering is necessary if the via lining processes do not provide adequate conformal coverage, a common problem for conventional low temperature deposition processes. In our process for lining the via sidewall, plasma enhanced chemical vapor deposited silicon dioxide is used to insulate vias from the surrounding silicon. Both Ti and Cu are sputter deposited and provide protection from copper migration and a seed film for Cu electrodeposition, respectively. After etching and lining, the vias are filled by reverse pulse plating of Cu. Vias are 20–25μm in diameter and etched using different masking materials. The effect of changing gas flow rates, chamber pressure, ICP power, and substrate temperature on etch rate, via profile, and sidewall morphology will be presented. These parameters are critical in the optimization of an etch process for vias of specific dimensions to be used in 3D integration.

Fabrication technology for single-material MEMS using polycrystalline diamond

Diamond, due to its large band gap of 5.5 eV, offers the possibility of making MEMS structures out of a single material by varying the doping level to achieve the semiconducting, metallic and insulating (undoped) properties needed in a typical MEMS structure. Polycrystalline diamond (poly-C) is inexpensive and retains many of the unique properties of single-crystal diamond. However, the development of diamond-based single-material MEMS (SMM) technology faces two major challenges; (a) producing highly-insulating and highly-conducting poly-C films in a multilayer structure and (b) developing dry-etching technology to produce multilayer structures made of poly-C. Furthermore, poly-C can be layered to perform a number of functions, whereas a complex stack of materials would otherwise be required. Consequently, due to poly-C's high selectivity as a masking material, the SMM fabrication process developed in the current work allows the reduction of the number of fabrication masks by a factor of 1.5–2 as compared to that used in a conventional MEMS process. A number of complex poly-C SMM structures were fabricated using SiO 2 as a sacrificial layer to address the SMM related issues in a single paper.

Thin Film Ag Masking for Deep Glass Micromachining

We report a single thin film and low cost masking material for deep, wet isotropic etching of glass in HF, which has applications in microfluidic devices and systems. With a 100 nm thin silver (Ag) mask, microcavities with an etch depth exceeding 200 μm were achieved and, by further thickening the silver film to 300 nm, etch depths up to 340 μm were observed. The thin film was deposited by evaporation and patterned in a mixture of nitric acid and deionized water at a ratio of 1:3. Silver had good adhesion to glass.

Study on cellular adhesion of human osteoblasts on nano‐structured diamond films

AbstractThe need of new biocompatible materials with superior properties and flexibility in their surface termination and functionalization for medical applications is constantly increasing. Due to the combination of intrinsic properties, chemically deposited diamond thin films are proposed as one of the most promising material in this field. They are chemically/mechanically/physically stable and their surface can be modified by a variety of chemical or plasmatic techniques. In the present work, different diamond nano‐structures, i.e. nano‐rods, ultra nano‐cones and nano‐cones, were prepared by plasma etching process by applying various masking materials (Ni, Au and diamond powder). The impact of these diamond nano‐structures on the growth of human osteoblasts was investigated by fluorescence microscopy. We observed that adhesion of osteoblasts varied, depending upon the diamond nano‐structure as visualized, 1 or 48 h after seeding. Many but very fine focal adhesions are formed on nano‐rods; however, few but large ones are formed on ultra nano‐cones and nano‐cones. The cell adhesion was controlled by the nano‐structuring, thereupon their usage in bio‐medical applications is considered.

Fabrication of nano‐structured diamond films for SAOS‐2 cell cultivation

AbstractThe present work reports on application of gold, nickel, and diamond nano‐particles as a masking material for realization of diamond 3D nano‐structures after a low power (100 W) plasma etching process in a gas mixture of CF4/O2. Obtained 3D structures strongly depend on the masking material. Application of the Ni mask results in diamond nano‐rods, whereas the Au mask leads to homogenous “ultra” nano‐cone structures. The diamond powder application results in irregular nano‐cone structures. The present procedure uses a strategy where no lithographic steps are implemented. Prepared samples are tested as the substrates for human osteoblast‐like cells cultivation. Obtained results exhibit a distinctive behavior in cell adhesion on used surface morphologies, as demonstrated by immunofluorescent staining of vinculin (a structural protein of focal adhesion).

Photoelectrochemical Undercut Etching of m-Plane GaN for Microdisk Applications

Undercut etching is a necessary technique for a variety of device applications, including microdisk lasers. We have explored bandgap-selective photoelectrochemical etching of nonpolar -plane GaN for undercut etching applications, including microdisks. These nonpolar optical devices are not limited by the quantum-confined Stark effect that hampers the performance of polar -plane GaN devices. We discuss the dependence of undercut quality on etchant concentration, illumination intensity, masking material, and epitaxial structure and use this technique to fabricate -plane microdisks. In these nonpolar microdisks, the in-plane polarization fields have a dramatic effect on the symmetry of the etching in both the undercut etching and in the unwanted etching of the GaN disk layer. With a careful balance of etchant concentration and illumination intensity and a well-designed epitaxial structure, we have achieved smoother optical cavities than were possible in -plane GaN.

Electric Characterization and Selective Etching of Aluminum Oxide

AbstractThis work presents results of investigations of Al2O3 thin films, deposited on Si substrates by means of impulse plasma deposition (IPD). Round, aluminum (Al) electrodes were evaporated on the top of deposited layers. Thus, metal‐insulator‐semiconductor (MIS) structures were created with aluminum oxide thin films playing the role of the insulator. It enabled subsequent electrical of studied material. Al2O3 layers were selective etched in a buffer of hydrofluoric acid. The photoresist was used as a masking material. The influence of etching time on the photoresist mask, etching progress, and the state of exposed Si surface was subsequently studied and is discussed. Films' microstructure were additionally studied by scanning electron microscopy (SEM).

Characterization of deep wet etching of fused silica glass for single cell and optical sensor deposition

The development of a high-throughput single-cell metabolic rate monitoring system relies on the use of transparent substrate material for a single cell-trapping platform. The high optical transparency, high chemical resistance, improved surface quality and compatibility with the silicon micromachining process of fused silica make it very attractive and desirable for this application. In this paper, we report the results from the development and characterization of a hydrofluoric acid (HF) based deep wet-etch process on fused silica. The pin holes and notching defects of various single-coated masking layers during the etching are characterized and the most suitable masking materials are identified for different etch depths. The dependence of the average etch rate and surface roughness on the etch depth, impurity concentration and HF composition are also examined. The resulting undercut from the deep HF etch using various masking materials is also investigated. The developed and characterized process techniques have been successfully implemented in the fabrication of micro-well arrays for single cell trapping and sensor deposition. Up to 60 µm deep micro-wells have been etched in a fused silica substrate with over 90% process yield and repeatability. To our knowledge, such etch depth has never been achieved in a fused silica substrate by using a non-diluted HF etchant and a single-coated masking layer at room temperature.

Nanostructuring of diamond films using self-assembled nanoparticles

Abstract We report the use of gold, nickel and diamond nanoparticles as a masking material for realization of diamond nano-structures by applying the dry plasma etching process. Applying low power plasma (100 W) in a gas mixture of CF4/O2 for 5 minutes results in a formation of three different types of diamond nanostructures, depending on the mask type material and particle size. Using of the Ni mask results in realization of diamond nano-rods, applying of the Au mask brings cauliflower-like structures, and using the diamond powder allows the production of irregular nano-structures. The main advance of the presented etching procedure is use of a self-assembly strategy where no lithographic steps are implemented.

A precision hot embossing mold fabricated by high-resolution powder blasting with polydimethylsiloxane and SU-8 masking technology

In this paper, we applied a three-dimensional micromold by using micropowder and micropattern masking technology. High-resolution powder blasting with polydimethylsiloxane (PDMS) and SU-8 masking is developed for the fabrication of a precision microfluidic mold for hot embossing fabrication. The PDMS is a suitable masking material for powder blasting due to its high erosion resistance. First, a 100 µm SU-8 negative pattern was developed on a stainless steel substrate by standard photolithography. A PDMS solution was then applied to the substrate, mainly filling recessed regions. Excess of PDMS was then physically removed by a blade. The PDMS/SU-8 structure was then cured. Next, silicon carbine (SiC) powder with an average diameter of 50 µm was blasted on a PDMS/SU-8 coated stainless sheet at a constant pressure between 4 and 6 bar and a PDMS mask was removed by ultrasonic cleaning in isopropanol. The three-dimensional structure was examined by an optical microscope, optical white light interferometer and scanning electron microscope. The PDMS pattern is found to be 50 µm wide, and the maximum etched depth at this thickness is around 150 µm at a blasting pressure of 6.2 bar; thus, an aspect ratio of 3 is easily obtained. In principle, the low-cost micromachining hot embossing mold developed can be improved to yield submicrometer- and nanometer-scale resolution.

Isotropic Etching of SiC

Isotropic etching of silicon carbide was achieved using a capacitive coupled parallel plate reactor in plasma etching mode and SF6 at elevated substrate temperatures. It was observed to be remarkable at substrate temperatures above 350°C. The influence of chamber pressure, masking materials, rf-power and substrate temperature were analyzed. Thereby, 8.5° off-axis oriented 4HSiC wafers exhibit a larger vertical and lateral etching rate compared to on-axis oriented SiC wafers. Additionally, the erosion of nitrogen containing masking material results in a reduction of the etching rates.