Aspect Ratio Microstructures Research Articles

Fabrication of a polymeric tapered HARMs array utilizing a low-cost nickel electroplated mold insert

A simple low-cost technique has been developed to fabricate a mold insert for replicating polymeric tapered high aspect ratio microstructures. A backside exposure technique is used to first obtain a tapered sidewall structure as an electroplating mold in SU-8 photoresist on a glass wafer. Nickel electroplating is utilized to form the mold insert. The lowest average surface roughness of the nickel mold insert on the side that interfaces with the glass wafer during electroplating is measured to be 7.02 nm. A novel technique involving use of titanium putty is introduced here to reduce cost and effort required to fabricate the mold insert. Replication of tapered microstructures in polymeric materials utilizing the fabricated mold insert is demonstrated here in polydimethylsiloxane by a direct molding process and in polymethyl methacrylate by hot embossing. The fabrication details for the mold insert are described. Advantages and disadvantages of the use of titanium putty for achieving superior metal surface finish are given.

High-aspect-ratio microstructural posts electroforming modeling and fabrication in LIGA process

This paper describes a theoretical model that predicts the metal ion concentration distribution during electroforming high aspect ratio microstructures (HARM). The applied current density and microstructure aspect ratio were found as two important factors that affect the electroforming outcome. The analytical results are verified using experiments that electroforming microstructural posts with an aspect ratio of 10. Good agreement was obtained between the experimental and analytical solutions. Based on the ion concentration analytical prediction on the cathode surface, one can estimate the electroforming time required for fabricating a microstructure for a given aspect ratio.

Direct writing of microtunnels using proton beam micromachining

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. During ion exposure of positive photoresist like PMMA, scission of molecular chains occurs. These degraded polymer chains are removed by the developer. The aim of this paper is to investigate the capabilities of proton micromachining as a lithographic technique. We show the realization of sub-surface channels, or microtunnels, which have been fabricated in only one exposure and without cutting or resurfacing the material. Using our Van-de-Graaff accelerator, the resist (PMMA) has been exposed with high-energy protons (2.5 MeV). The range of charged particles in matter is well-defined and depends on the energy. Therefore, it is possible to obtain a dose which is sufficient to develop the bottom part of the ion paths but not the top part. Thus, by selecting the energy and the exposure time, a big variety of microtunnels can be realized.

Tolerance analysis for comb-drive actuator using DRIE fabrication

Deep reactive ion etching (DRIE) process is specially invented for bulk micromachining fabrication with the objective of realizing high aspect ratio microstructures. However, various tolerances, such as slanted etched profile, uneven deep beams and undercut, cannot be avoided during the fabrication process. In this paper, the origins of various fabrication tolerances together with its effects on the performances of lateral comb-drive actuator, in terms of electrostatic force, mechanical stiffness, stability and displacement, are discussed. It shows that comb finger with positive slope generates larger electrostatic force. The mechanical stiffness along lateral direction increases when the folded beam slants negatively. The displacement is 4.832 times larger if the comb finger and folded beam are tapered to +1° and −1°, respectively. The uneven deep fingers generate an abrupt force and displacement when the motion distance reaches the initial overlap length. The undercut reduces both the driving force and the mechanical stiffness of the lateral comb-drive actuator. The fabricated comb-drive actuator, with comb finger of +1° profile and 0.025 μm undercut, and folded beam of −1° slope and 0.075 μm undercut, is measured and compared with the models where both show consistent results. These analytical results can be used to compensate the fabrication tolerances at design stage and allow the actuators to provide more predictable performance.

High energy focused ion beam lithography using P-beam writing

The term “P-beam writing” is used to describe the technique of using focused high energy proton microbeams for micro or nanofabrication applications. The P-beam technique can be used to rapidly fabricate three-dimensional, high aspect ratio microstructures in a variety of materials without the use of masks and it is proving to be a versatile lithographic method. Recent developments in the application P-beam writing of microstructures at the Louisiana Accelerator Center are presented.

Fabrication of high aspect ratio microstructure arrays by micro reverse wire-EDM

In this paper, a machining technique to fabricate high aspect ratio microstructure arrays of a total volume less than 1 mm3 is developed. A method for determining the appropriate tension of the micro brass wire of the micro wire-EDM mechanism designed in our previous study is proposed, and a design for suppressing the vibration of the wire is implemented. In addition, a machining approach coined ‘reverse wire-EDM’ is developed. The micro wire-EDM mechanism is mounted on the worktable rather than on the machine head while the micro workpiece is clamped on the spindle instead of the worktable by a micro chuck. Machining is carried out by a horizontal moving micro brass wire of 20 µm diameter located beneath the micro workpiece to accelerate the removal of debris and to eliminate the heat accumulated in the micro gap during machining. The possible occurrence of short circuit discharge and thermal deformation of the machined part are therefore minimized. Experiments are conducted to machine various high aspect ratio miniature structures including a microstructure array of ten 10 µm sharp-edge lamellae at the tip, a microstructure array of ten 10 µm uniform thickness lamellae and a microstructure array of ten by ten 21 µm squared pillars. It is found that a microstructure array of an aspect ratio more than 33 is satisfactorily and precisely fabricated. The dimensional accuracy and geometric accuracy are less than 0.6 and 1.0 µm, respectively, while the surface roughness Rmax is kept within 0.44 µm.

A quantitative study on the adhesion property of cured SU-8 on various metallic surfaces

SU-8 has received wide attention in recent years because of its application in the fabrication of high aspect ratio microstructures and devices. This negative resist is known for its excellent lithography properties using ultraviolet light source. As the microfabrication technology based on UV-lithography of SU-8 finds wide applications, a good understanding and characterization of cured SU-8 polymer on various substrate materials are therefore very important. A good adhesion on various substrate materials is essential to both the fabrication process and to the functionality of any final products that have cured SU-8 as part of the structural material. There are very limited studies reported in this important area in the literature. This paper presents a theoretical and experimental work to quantitatively study the adhesion properties of cured SU-8 on some of the most commonly used metallic surface materials. The adhesion strengths of cured SU-8 samples on Au, Ti, Cu, Cr, and Ni coated glass substrates were measured following ASTM-C633 standard. A detailed analysis of the experimental results was also provided based on the atomic structures and electron configurations of the respective metals.

Advanced etching of silicon based on deep reactive ion etching for silicon high aspect ratio microstructures and three-dimensional micro- and nanostructures

Different processes involving an inductively coupled plasma reactor are presented either for deep reactive ion etching or for isotropic etching of silicon. On one hand, high aspect ratio microstructures with aspect ratio up to 107 were obtained on sub-micron trenches. Application to photonic MEMS is presented. Isotropic etching is also used either alone or in combination with anisotropic etching to realize various 3D shapes.

MODULIGA: The LIGA process as a modular production method-current standardization status in Germany

The LIGA process combines X-ray lithography with electroplating and molding. It is a technique used worldwide for the fabrication of high aspect ratio microstructures. In Germany, this technique will be established as a production line to manufacture micro system devices. A project named (modular production method using the LIGA process) MODULIGA has the goal to link together the activities of the German LIGA centers Forschungszentrum Karlsruhe / Institut fur Mikrostrukturtechnik, Karlsruhe (FZK/IMT), Institut fur Mikrotechnik, Mainz (IMM) and Berliner Elektronenspeicherring-Gesellschaft fur Synchrotronstrahlung, Berlin (BESSY). LIGA process steps will be standardized and interface specifications will be defined to establish a modular production method. Interface specifications are defined for masks, substrate and resist, exposure technology, development and finally electroplating. The standards are defined by testing criteria. The challenges and the current status of standardization will be described.

Macrochannelling: Characterisation of nano-structures by ion beam analysis

High aspect ratio microstructures produced by novel fabrication methods have important applications in micro-electro mechanical machines (MEMS) and photonics. Investigation of the internal structure of sub-micron scale devices is challenging although scanning electron microscopy (SEM) and scanning tunnelling microscopy (STM) are two commonly used techniques for this purpose. These techniques have the disadvantage that important information about sub-surface structure can be difficult to obtain. Especially difficult to analyse are high aspect ratio structures because of the limited reach of the scanning probe into deep wells, or the shallow penetration of the electron beam. Ion beam channelling, with Rutherford backscattering spectrometry (RBS) is already well-established as a technique for providing information about sub-microscopic structure at the atomic level. Based on this method, we are investigating the use of macroscopic ion beam channelling into micro and nano-scale high aspect ratio textured surfaces. We have developed a numerical simulation program that can calculate the RBS spectrum of textured surfaces, as a function of ion beam tilt angle, which can be used to interpret the experimental spectra from a real specimen. In this paper we compare our simulations with control samples.

A numerical and experimental study on gap compensation and wavelength selection in UV-lithography of ultra-high aspect ratio SU-8 microstructures

This paper presents a study on UV-lithography of thick SU-8 resist using air gap compensation and optimal wavelength selection for ultra-high aspect ratio microstructures. Both numerical simulations and experiments were conducted to study effects of different lithography conditions: broadband light source with and without air gap compensation, filtered light source with glycerol liquid, and filtered light source with Cargille refractive index matching liquid. A thick PMMA sheet was used as an optical filter to eliminate most of the i-line components of a broadband light source. Using the filtered light source and gap compensation with the Cargille refractive index liquid perfectly matching that of SU-8, patterns with feature sizes of 6 μm thick, 1150 μm tall (aspect ratio of more than 190:1) and high quality sidewalls were obtained. Microstructures with height up to 2 mm and good sidewall quality were also obtained and presented. The study also proved that Cargille refractive index matching liquid is compatible with UV-lithography of SU-8 and may be used as an effective air gap compensation solution.

Micropatterning of Foturan photosensitive glass following exposure to MeV proton beams

A new proton lithography mechanism has been identified and investigated: the micropatterning of photosensitive etchable glass based on the crystallization of the glass after irradiation with MeV protons and heat treatment. The use of MeV protons results in a significantly reduced minimum feature size compared with reported results using ultraviolet (UV) irradiation and the threshold dose for etching is very low (4000 protons µm−2), offering the potential for creating complex microstructures by direct writing using very short exposures. The depth of the structures is determined only by the range of the protons in the glass, which allows structures with different depths to be fabricated. This technique appears to be very valuable for the rapid fabrication of high aspect ratio microstructures such as fluid networks and micro-optical devices in a material with useful optical and mechanical properties.

Multi-Layer Photopolymer Micromachining

AbstractThis paper presents a novel method to create and integrate micro-machined devices and high aspect-ratio (height-to-width ratio) microstructures in which the microstructures are built up using multiple layers of photopolymer film and/or viscous solution. Very high aspect-ratio 2-and 3-dimensional (2-D and 3-D) microstructures were constructed by stacking photo-imageable polymer films. Such films may be dry films applied by lamination or solution layers applied by bar coating, or doctor blade coating. Photolithography is used in both cases to define the microstructure. This additive process of thin-film micromachining facilitates high aspect-ratio microstructure fabrication. We have demonstrated structures of up to 12-layers comprising 2-D arrays of deep trenches (180 μm deep and 25 μm wide) and a 2-layer SU-8 micro-trench array with an aspect ratio up to 36 on glass substrates. Miniaturized structures of interconnected reservoirs as small as 50 μm × 50 μm × 15 μm (∼38 pico liter storage capacity) are also being fabricated, along with a novel 5-layer microfluidic channel array and a vacuum-infiltration process for fluid manipulation. This method has the potential to create functional large-area micro-devices at low-cost and with increased device flexibility, durability, prototyping speed, and reduced process complexity for applications in optoelectronics, integrated detectors, and bio-devices. The novel multi-layer photopolymer dry film and solution process also allows microstructures in micro-electro-mechanical systems (MEMS) to be built with ease and provides the functionality of MEMS integration with electronic devices and integrated circuits (ICs).

High Aspect Ratio Microstructures in LiNbO3 Produced by Ion Beam Enhanced Etching

AbstractThis work presents data on damage evolution, volume expansion and etching behavior of LiNbO3 irradiated with Ar+-ions as a function of irradiation and etching conditions. Single crystals of x- and z-cut LiNbO3 were irradiated at room temperature and 15 K using Ar-ions with energies between 60 and 600 keV and ion fluences between 5 × 1012 and 1 × 1015 cm-2. The damage formation investigated with RBS channeling analysis depends on the crystal cut as well as on the irradiation temperature. Irradiation of z-cut material at 300 K causes complete amorphization at 0.4 dpa (displacements per target atom). In contrast 0.27 dpa are sufficient to amorphize the x-cut LiNbO3. Irradiation at 15 K reduces the number of displacements per atom necessary for amorphization to 0.18 dpa. To study the etching behavior ∼500 nm thick amorphous layers were generated via multiple irradiations with Ar+-ions. Etching was performed in HF-solution of different concentration and at different temperatures. The influence of the irradiation and etching conditions on damage formation and etching of LiNbO3 is discussed. In conclusion, negligible etching of the perfect crystal, high etching rates and high contrast of Ion Beam Enhanced Etching (IBEE) allow the realization of high aspect ratio microstructures in LiNbO3.

Using megasonic development of SU-8 to yield ultra-high aspect ratio microstructures with UV lithography

SU-8 resist is commonly used to produce high aspect ratio microstructures in MEMS field. The resist is patterned using either ultraviolet (UV) or X-ray lithography and developed to produce structures ranging from 20 microns to 1.5 millimeters in height. Three processes are currently used to develop SU-8 resist: dip development with or without stirring and dip development with ultrasonic agitation. One difficulty associated with producing high aspect ratio microstructures lies in the development of open and closed fields evenly on the same substrate. To overcome this obstacle, we have applied megasonic agitation to the developer bath, which resulted in faster development rates, uniformed development, and the ability to produce structures with higher aspect ratios. To date, this process has been used to achieve 100:1 aspect ratio open field features and 45:1 intact cylinder arrays using a broadband optical contact aligner.

Microfabrication of an electromagnetic power relay using SU-8 based UV-LIGA technology

Most of the MEMS relays reported in the field now are based on silicon fabrication and cannot be used for power applications. In this paper, we report a research effort to microfabricate an electromagnetic relay for power applications using a multilayer UV-LIGA process. A mechanically wrapped coil was used and very simple design for the magnetic circuit was adopted to increase the design flexibility and performances. The broad material selection and the capability of making high aspect ratio microstructures of the UV-LIGA make the technology best suited for fabricating microelectromechanical relays for power applications. The prototype relay has a truly three-dimensional structure and very suitable for large power capacity applications.

A comprehensive study of ion track enabled high aspect ratio microstructures in flexible circuit boards

A process to form deep, vertical and high aspect ratio microstructures of solid as well as porous nature is presented. The process is capable of producing regions with perpendicular sub-micron metal wire connections, with a regulated effective metal density at numerous, arbitrarily specified locations. The structures are created in a two-metallic-layer polyimide laminate, i.e. a flexible printed circuit board. The high aspect ratio of the process is indebted to ion track technology. The laminate is irradiated with heavy ions creating a vertical damage anisotropy (individual ion tracks) in the polymer layer. Apertures in the front metallic layer define the geometry and the positions of the vertically projected structures. The tracks are selectively developed forming nanometer wide pores, which after prolonged etching grow in diameter and eventually merge creating fully opened cavities. Metallic structures have been replicated in these pores/cavities by electrodeposition of nickel and copper. We have fabricated open and dense clusters of separated micron or sub-micron sized wires as well as solid structures. Highly vertical, through hole microvias in average 39 μm wide, with a pitch of 100 μm have been fabricated. The smallest structures obtained were 25 × 25 μm square columns. The process appears promising for ultra-high density via batch production and has a strong potential of further miniaturising via dimensions.

Near‐infrared time‐resolved fluorescence lifetime determinations in poly(methylmethacrylate) microchip electrophoresis devices

High aspect-ratio microstructures were hot-embossed in polymer substrates with a molding tool fabricated using lithography/electroplating/forming (LIGA). The resulting devices were used for the electrophoretic separation of oligonucleotides labeled with near-infrared (near-IR) dyes. Near-IR time-resolved fluorescence was used as an identification method for the labeling dyes. The detection apparatus consisted of a pulsed laser diode operating at 680 nm, a single-photon avalanche diode, an integrated microscope, and a PC-board incorporating time-correlated single photon counting electronics. Investigation of the optical quality and amount of autofluorescence generated from different polymer substrates was carried out in the near-IR region for determining compatibility with time-resolved fluorescence. Our results revealed that of several poly(methylmethacrylate)(PMMA) substrates, brand Plexiglas offered minimal replication errors in the embossed features using appropriate embossing conditions with low background fluorescence contributions to the observed decay. Near-IR dye-labeled oligonucleotides were separated to determine the applicability of fluorescence lifetime discrimination between Cy5.5 (tauf = 930 ps) and IRD700 (tauf = 851 ps) labeling dyes during the microchip separation. These dyes were used to label T-fragments (thymine) of an M13mp18 ssDNA template. The DNA ladders were electrophoresed at 130 V/cm in a 4% linear polyacrylamide gel (LPA) gel matrix in a 9.5 cm long serpentine channel heated to 50 degrees C. The electropherogram revealed that the lifetimes could be accurately read well beyond 450 bases, although single-base pair resolution in the electropherogram was difficult to achieve due to potential solute-wall interactions in the polymer microdevice or the electroosmotic flow (EOF) properties of the device. The relative standard deviations secured for individual bands in the electropherogram were similar to those obtained using capillary gel electrophoresis, in spite of the lower load volume.

Study on the postbaking process and the effects on UV lithography of high aspect ratio SU-8 microstructures

In recent years, a relatively new type of negative photoresist, EPON SU-8, has received a lot of attention in the MEMS field because of its excellent lithography properties. Significant research efforts have been made to study the lithographic properties of SU-8 to obtain high aspect ratio microstructures with good sidewall quality. Currently, selection of optimal wavelengths of the UV light for lithographic and reduction of the diffraction effects are believed to be the two most important factors for achieving high-quality lithography of SU-8 as reported in the literature. Other reported efforts also include modifications of the chemical properties of SU-8 for better lithographic quality. We report a study on stress reduction during the postbaking process and the effects on lithography of ultra-thick high aspect ratio SU-8 microstructures. Our research proves that aspect ratios up to 40:1 in isolated open field structures of thicknesses between 1 and 1.5 mm can be obtained without any modifications of the resist chemistry or changes in light spectrum applied from a standard broadband UV source. The principal factor in this achievement is the reduction of internal stress during the postexposure bake process that eliminates large plastic deformations present during standard bake procedures. This process may be used for the fabrication of ultra-thick high aspect ratio microstructures that have to date only been obtainable using x-ray lithography-based LIGA processes.

Ultrathick SU-8 fabrication for microreciprocating engines

We present a high-quality UV-lithography process for making high aspect ratio microstructures for microengines using an ultrathick SU-8 photoresist layer. The microreciprocating engine project, which is ongoing at the University of Birmingham, aims to develop a compact power plant to replace batteries. The design of the microengine imposes strict requirements on the geometry of the engine components. The microfabrication research work has been concentrated on developing a SU-8 UV-lithography process to achieve good sidewall angles and high aspect ratio. Based on the study of the photoactive property of ultrathick SU-8 layers, an optimized prebake time is found for obtaining the minimum UV absorption of SU-8. The optimization process is tested and proven effective using a series of UV-lithography experiments on different prebake times. Microstructures with aspect ratios as high as 40:1 are produced in 1000-µm ultrathick SU-8 layers using standard UV-lithography equipment. The sidewall angles are controlled between 85 to 90 deg. The engine components fabricated using this process satisfy microengine design requirements.