Submicron-scale surface acoustic wave resonators fabricated by high aspect ratio X-ray lithography and aluminum lift-off

A submicron-scale surface acoustic wave (SAW) resonator fabricated by high-aspect-ratio X-ray lithography (XRL) and metal lift-off that operates at microwave frequencies is presented. We demonstrate that XRL is especially well suited for SAW device templating, as long submicron-scale interdigitated transducer structures can be batch patterned with excellent structure quality. 0.4–2.0 μm thick PMMA layers were structured by X-ray lithography shadow projection using silicon nitride-based X-ray masks. Structures with a critical lateral feature size of down to 200–700 nm were processed. The polymer structures served as templates in a subsequent aluminum lift-off process. The metal electrodes were successfully tested as SAW resonators for high frequency applications, e.g. around 1.3 GHz, using calibrated 1-port RF wafer probing measurements. Compared with standard fabrication techniques, the high structure quality of submicron-scale polymer templates made of unusually thick PMMA layers offers additional possibilities to fabricate thicker metal transducers.     

A new sacrificial layer method of LIGA technology to fabricate movable part of a gripper

 Movable microstructures are required for many applications in accelerator sensors, microvalves, micromotors, grippers and so on. With the LIGA technique, movable components can be fabricated directly by using sacrificial layer technology. This is considerably extended by the application of the LIGA technique. Normally thin metallic layers are sputtered onto an insulating substrate (e.g. silicon wafer or ceramic) and patterned by conventional photolithography and wet etching. They are used as a metalized layer and a sacrificial layer. The movable parts of the microstructures are positioned on the surface of the sacrificial layer, whereas the fixed parts are placed on the metalized area of the substrate. After stripping the resist and the sacrificial layer, the movable parts on the sacrificial layer are finished and the fixed parts remain firmly on the metalized layer. This process is rather complicated. A new technology to produce the movable parts is developed in our Lab. Firstly the normal LIGA process is used to make the sample with both metal and resist structures. The sacrificial layer pattern will be placed on the sample surface with UV lithography. A metal layer is sputtered on the sample and sacrificial layer surface as a metalized layer. By electroplating, the metalized layer will grow up to the milimeter thickness and be used as the fixed substrate. Finally removing the nonmetal substrate, resist and the sacrificial layer, the movable parts could be completed. As an example, a magnetic gripper structure is designed and fabricated by this method.     

Releasing high aspect ratio SU-8 microstructures using AZ photoresist as a sacrificial layer on metallized Si substrates

This paper presents a successful method for releasing high aspect ratio SU-8 micro-structures by the use of positive photoresist (AZ 4620) as sacrificial layer. The AZ 4620 photoresist sacrificial layer was dissolved by the SU-8 developer (propylene glycol monomethyl ether acetate). Thus, this process reduces the need for complex microfabrication steps and equipments which are otherwise required in traditional methods using metal sacrificial layers. The current method is both cost-effective and time-effective because no additional releasing method or material is needed to remove the fabricated SU-8 structures. Further, the influence of surface energy on the adhesion between Si and SU-8 was demonstrated and metallic thin layer coating on Si was employed to further reduce the lift-off duration. The results obtained showed that the duration for lift-off of SU-8 structures from metal (Al) coated Si substrate is much lower (approximately 90 % time saving) and the surface morphology of the released structures has lesser micropore concentration compared to the process employing bare Si as the substrate. In both processes AZ 4620 was the sacrificial layer whereas the metalized Si substrate could be re-used.     

A new process for fabricating tip-shaped polymer microstructure array with patterned metallic coatings

In this paper, we present a new process for fabricating tip-shaped polymer microstructure array coated by patterned metal layer. This new process involves three techniques including: micro-molding, patterned metal layer transfer, and electrochemical-based sacrificial layer. As we know, it is very difficult to remove the extra photoresist (PR) in the concave areas in traditional micro-fabrication technology, which hinders patterning metal layers on three-dimensional (3D) microstructures. The electrochemical-based sacrificial layer technique can effectively resolve this problem, which is verified by scanning electron microscopy (SEM) characterization. Comparative study between the 3D metal-coated polyimide microstructures fabricated with and without the electrochemical-based sacrificial layer step is also performed and SEM images proved the effect of the presented process. The applicability of the developed process is also demonstrated with the successful fabrication of a pyramid-shaped polyimide microelectrodes array for neural stimulation.     

Magnetic lithography for servowriting applications using flexible magnetic masks nanofabricated on plastic substrates

Magnetic lithography (ML) is a process qualitatively analogous to contact optical lithography which transfers information from a nanopatterned magnetic mask (analog of optical photomask) to magnetic media (analog of photoresist), and is interesting for applications in instantaneous parallel magnetic recording, in particular for servowriting applications. The magnetic mask consists of nanopatterned magnetically soft material (FeNiCo, FeCo) on a thin flexible plastic substrate, typically Polyethylene teraphtalate (PET) or polyimide. When uniformly magnetized media is brought into intimate contact with the magnetic mask, an externally applied magnetic field selectively changes the magnetic orientation in the areas not covered with the soft magnetic material. Flexible substrate of the magnetic mask offers superior compliance to magnetic media which is likely to have imperfect flatness and surface particulate contamination. We discuss nanofabrication challenges of magnetic masks on plastic substrates, including electron beam lithography, electroplating and lift-off processing on the nanometer scale, adhesion of metal thin films on PET and polyimide substrates, and release of plastic films from rigid substrates used during the processing. We present results on fabricated magnetic masks, magnetic force microscopy images of the magnetic transition patterns and disk spinstand tests of servowritten patterns.     

Asymmetric focusing microlens array fabricated using off-axis lithography

An asymmetric microlens with a given inclination angle was fabricated. Two circular pattern masks with different diameters were used to form a metal pattern and photoresist column on the substrate using the photolithography process. The metal pattern on the substrate was used to control the asymmetric microlens profile using thermal reflow. A lift-off process was applied to the first lithography to precisely define the metal pattern. A second lithography used deviation counterpoint exposure to pattern the photoresist column. The photoresist column was converted into a rubbery state when its temperature was increased to its glass transition temperature (Tg) during the thermal reflow. The asymmetric microlens structure was formed by shifting the arc vertex of the microlens toward one direction taking into account the fact that the copper coating surface has superior hydrophobicity to the silicon substrate surface. A 55° asymmetric microlens array was fabricated in this research by properly controlling the copper pattern size and the offset of two centers.     

A FCOB packaged thermal wind sensor with compensation

A two dimensional wind sensor was designed, fabricated and packaged on ceramic substrate instead of silicon substrate. The Ti/Pt heater and thermistors were fabricated using single lift-off process. The gold bumps were then sputtered and patterned on the chip using lift-off process again. Correspondingly, the Pb/Sn bumps were fabricated on the FR4 substrate using stencil printing method after metallization. The sensor chip was flip-chip packaged on the FR4 substrate, and the gap was filled with epoxy-based underfill to improve the structure strength. The packaged sensor was tested in wind tunnel in constant power mode. The wind velocity and direction offsets of the sensor were compensated using software and hardware calibration. Both the simulation and test results show that the thermal wind sensor can measure wind speeds up to 8 m/s with an accuracy of 0.3 m/s, and wind direction in a full range of 360° with a resolution within 5°.     

Fabrication and flow-sensor application of flexible thermal MEMS device based on Cu on polyimide substrate

A Cu on polyimide (COP) substrate was proposed as a MEMS material, and the fabrication process for a flexible thermal MEMS sensor was developed. The COP substrate application to MEMS devices has the advantage that typical MEMS structures fabricated in a SOI wafer in the past—such as a diaphragm, a beam, a heater formed on a diaphragm—can also be easily produced in the COP substrate in the flexible fashion. These structures can be used as the sensing element in various physical sensors, such as flow, acceleration, and shear stress sensors. A flexible thermal MEMS sensor was produced by using a lift-off process and sacrificial etching of a copper layer on the COP substrate. A metal film working as a flow sensing element was formed on a thin polyimide membrane produced by the sacrificial etching. The fabricated flexible thermal MEMS sensor was used as a flow sensor, and its characteristics were evaluated. The obtained sensor output versus the flow rate curve closely matched the approximate curve derived using King’s law. The rising and falling response times obtained were 0.50 and 0.67 s, respectively.

     

Fabrication of air-channel structures for microfluidic,microelectromechanical, and microelectronic applications

A method is presented for fabricating micro-air-channel structures encapsulated by a dielectric material using a sacrificial polymer based on polynorbornene (PNB) chemistry. A spin-coated film of PNB was patterned to define the exact geometry of the air-channels using conventional lithographic and etching techniques. The sacrificial polymer was encapsulated with a permanent dielectric material. The composite was then raised to elevated temperatures to produce gaseous products which permeate through the encapsulating material (SiO₂ , SiN_x or other polymer) leaving behind minimal solid residue. Air-channels integrated with metal interconnections can be formed via a Damascene, or in-lay process. After patterning the sacrificial polymer, copper was electroplated, followed by encapsulation with the dielectric. Various issues pertaining to the processing steps have been investigated and are discussed, such as type of encapsulants, feasible air-channel sizes, and processing conditions. Such air-channel structures are believed to have potential applications in microelectronics, displays, printers, multilevel wiring boards, microscale chemical reactors on a chip, and microelectromechanical devices     

基于声表面波技术的微流体混合及仿真

报道了在微流系统中使用基于叉指聚焦换能器的声表面波的一种有效方法.利用lift-off和软光刻技术在铌酸锂( LiNbO3)压电单晶片基底上制作器件.使用有限元分析软件COMSOL Multiphysics分别模拟聚焦叉指电极和平行叉指电极两种声表面波的形成和分布.通过与相同条件下的平行叉指电极比较,说明聚焦叉指电极可...     

Fabrication of electrostatic MEMS microactuator based on X-ray lithography with Pb-based X-ray mask and dry-film-transfer-to-PCB process

Lithographie Galvanoformung Abformung (LIGA) is a promising approach for fabrication of high aspect ratio 3D microactuator for dual-stage slider in hard disk drive. However, this approach involves practically challenging X-ray lithography and structural transfer processes. In this work, electrostatic MEMS actuator is developed based on a LIGA approach with cost-effective X-ray lithography and dry-film-transfer-to-PCB process. X-ray lithography is performed with X-ray mask based on lift-off sputtered Pb film on mylar substrate and photoresist application using casting-polishing method. High quality and high aspect ratio SU8 microstructures with inverted microactuator pattern have been achieved with the interdigit spacing of ~5 μm, vertical sidewall and a high aspect ratio of 29 by X-ray lithography using the low-cost Pb based X-ray mask. A new dry-film-transfer-to-PCB is employed by using low-cost dry film photoresist to transfer electroplated nickel from surface-treated chromium-coated glass substrate to printed circuit board (PCB) substrate. The dry film is subsequently released everywhere except anchor contacts of the electrostatic actuator structure. The fabricated actuator exhibits good actuation performance with high displacement at moderate operating voltage and suitably high resonance frequency. Therefore, the proposed fabrication process is a promising alternative to realize low-cost MEMS microactuator for industrial applications.     

Preparation of sacrificial layer for MEMS devices by lift-off technology

To simplify the preparation of sacrificial layer in micro-electromechanical system structure, new processes have been developed. By using lift-off technology, sacrificial layer was selectively deposited into the pit that prepared for sacrificial layer releasing. Then a short time polishing process was used to remove the burrs around the pit. This method offers several advantages including reducing the difficulty of polishing and hence processing cost. Methods and apparatuses to provide a continuously smoothly sacrificial layer surface in the pit were disclosed. However, it is important to control the homogeneity of filling and avoid the shadow effect at the corner of the pit. The unpadded regions would form a steep step and cause the cracking of structure layer. This problem was discussed and solved by rotating the tilted wafer during sacrificial material deposition. The surface roughness after the preparation process of sacrificial layer was measured. The root mean square roughness of sacrificial layer was 1.247 nm and reduced to 0.861 nm after a short time polishing process. Finally, a film bulk acoustic wave resonator with stacked Mo/AlN/Mo films had been fabricated on the sacrificial layer based on this method.     

AlN陶瓷微热板MEMS传感器阵列设计与工艺实现

针对陶瓷基微热板MEMS器件难以微加工,器件表面加热Pt膜使用普通正性光刻胶难以实现光刻剥离的工艺难点问题,提出了激光微加工和柔性机械剥离相结合的微加工方法。以AlN陶瓷为衬底基片,采用激光微加工技术实现热隔离刻蚀体加工,刻蚀梁宽可达0.2 mm。采用柔性机械剥离工艺制备方法解决普通正性光刻胶形成倒梯形凹槽Pt膜难实现图形化问题,可在复杂表面特性的陶瓷基衬底上实现Pt膜剥离线宽10μm。同时利用有限元法进行传感器阵列设计和热结构仿真,验证设计工艺的可行性。     

E-beam lithography of nano-interdigital transducers on insulating and semiconducting substrates

This work describes the micro-fabrication process developed to manufacture nano-interdigital transducers (nano-IDTs) to be used in surface acoustic wave applications. The combination of electron-beam (e-beam) lithography and lift-off process is shown to be effective in fabricating IDT finger patterns with a line width below 100 nm and good yield. It is also shown how a very thin organic anti-static layer can be used to avoid charge accumulation on the resist layer during e-beam lithography, which is easy to occur on insulating piezoelectric substrates and results in e-beam deflection. However, it is also shown how the use of this anti-static layer is not required with the insulating piezoelectric layer resting on a semiconducting substrate such as highly doped silicon. The effect of the e-beam dose on insulating and semiconducting layers is also discussed.     

Surface modification with self-assembled monolayers for nanoscalereplication of photoplastic MEMS

A release technique that enables to lift microfabricated structures mechanically off the surface without using wet chemistry is presented. A self-assembled monolayer of dodecyl-trichlorosilane forms a very uniform ~1.5-nm-thick anti-adhesion coating on the silicon dioxide surface, on full wafer scale. The structural layers are formed directly onto the organic layer. They consist here of a 100-nm-thick aluminum film and a high-aspect ratio photoplastic SU-8 structure. After the microfabrication the structure can be lifted off the surface together with the aluminum layer. This generic technique was used to make a variety of novel structures. First, aluminum electrodes that are embedded in plastic are made using lithography, etching and surface transfer techniques. Second, using a patterned monolayer as defined by microcontact printing, resulted in a spatial variation of the surface adhesion forces. This was used to directly transfer the stamped pattern into a metal structure without using additional transfer etching steps. Third, the monolayer's ability to cover surface features down to nanometer scale was exploited to replicate sharp surface molds into metal coated photoplastic tips with ~30-nm radii for use in scanning probe instruments such as near-field optical techniques. The advantage compared to standard sacrificial layer techniques is the ability of replication at the nanoscale and the absence of etchants or solvents in the final process steps     

An intermediate-layer lithography method for generating multiple microstructures made of different conducting polymers

An intermediate-layer lithography (ILL) method has been developed in this work to generate multiple microstructures of different conducting polymers on the same substrate. Previous and current efforts in developing conducting polymer microsystems mainly focus on generating a device of a single function. When multiple micropatterns of different conducting polymers are produced on the same substrate, many microsystems of multiple functions can be envisioned. However, existing techniques present significant technical challenges of degradation, low throughput, low resolution, depth of field, and/or residual layer in producing conducting polymer microstructures. To circumvent these challenges, the ILL method has been explored to generate multiple micropatterns of different conducting polymers in a parallel manner. In this method, conducting polymer materials and a non-conducting polymer intermediate layer are first coated on a substrate, and are then patterned through a mold insertion at a raised temperature. In this work, the ILL has been used to successfully pattern three types of commonly used conducting polymers on the same substrate under a single mold insertion, and simulation has been conducted to gain a good understanding of the molding process. Due to distinctive advantages of simplicity, low cost and high throughput, the ILL has promising applications in fabricating micropatterns for polymer-based microsystems.     

Micromachined ultrasonic transducers based on lead zirconate titanate (PZT) films

The design, fabrication and measuring of piezoelectric micromachined ultrasonic transducers (pMUTs), including the deposition and patterning of PZT films, was investigated. The (100) preferential orientation of PZT film have been deposited on Pt/Ti/SiO₂/Si (100) substrates by modified sol–gel method. PZT film and Pt/Ti electrode were patterned by novel lift-off using ZnO as a sacrificial layer avoiding shortcomings of dry and wet etching methods. pMUT elements have been fabricated by an improved silicon micromachining process and their properties were also characterized. As measured results, the pMUT tends to operate in a standard plate-mode. The receive sensitivity and transmit sensitivity of pMUT element whose active area only has 0.25 mm² are ?218 dB (ref. 1 V/μPa) and 139 dB (ref. 1 μPa/V), respectively.     

Aluminum–germanium wafer bonding of (AlGaIn)N thin-film light-emitting diodes

Eutectic aluminum–germanium wafer bonding was used to fabricate (AlGaIn)N thin-film light-emitting diodes (LEDs). Wafer bonding was carried out on 2″ wafer level at a bond temperature of 470 °C using patterned Al bond pads on the GaN-on-sapphire LED epiwafer and plain Ge substrates. The microstructure of the joint formation was characterized via cross-section analysis using scanning electron microscopy and energy dispersive X-ray spectroscopy (EDX). Scanning acoustic microscopy was used to investigate the bond interface. The shear strength was determined to be 1–2 kN/cm². The formation of a liquid Al–Ge phase is evident from cross-section analysis and optical microscopy. During solidification, Al and Ge are separated into distinct phases again, which is revealed by EDX. The obtained bond is not free of micro-voids, yet it is mechanically stable and suited for the fabrication of thin-film LEDs by removing the sapphire substrate via laser lift-off, which is also demonstrated.     

Fabrication of high-aspect-ratio hydrogel microstructures

Microfabrication of high-aspect-ratio polymeric microstructures via deep X-ray lithography traditionally involves either crosslinking or scissioning a polymer film spun-cast on a substrate. A post-exposure development procedure is usually employed to remove the unwanted polymer, leaving behind lithographically patterned structures. Instead, we use a novel synthesis technique wherein polymerization of a mixture of monomers in solvent is initiated, through a mask, with hard X-rays. The resulting polymer precipitates out of the solvent, thus limiting the spatial propagation of the reaction only to the exposed regions. Such a technique offers a unique way for the patterned synthesis of polymers from a variety of monomer-solvent systems. Here, we present the first results on the synthesis of high-aspect-ratio microstructures of a thermoreversible hydrogel, poly (N-isopropylacrylamide), and an ionic hydrogel, poly (methacrylic acid). These stand-alone, implantable microstructures are envisioned to be potentially useful in such diverse areas as biosensors, microactuators, controlled release applications, and cell and tissue engineering.This paper was first presented at the High Aspect Ratio Microstructures (HARMST) 2003 conference in Monterey California, June 2003.We would like to thank Dr. Francesco De Carlo (APS) for his discussions on beamline simulations. Use of the Advanced Photon Source was supported by the U.S. Department of Energy, Basic Energy Sciences (BES), Office of Science, under contract number W-31-109-ENG-38.     

A4‐sized flexible ferroelectric liquid‐crystal displays with micro color filters

Abstract— We demonstrated an A4‐paper‐sized flexible ferroelectric liquid‐crystal (FLC) color displays fabricated by using a new plastic‐substrate‐based process which was developed for large‐sized devices. Finely patterned color filters and ITO electrodes were formed on a plastic substrate by a transfer method to avoid surface roughness and thermal distortion of the substrate, which induce disordering of the FLC molecular alignment. The thickness of an FLC/monomer solution sandwiched by two plastic‐film substrates was well controlled over a large area by using flexographic printing and lamination techniques. Molecular‐aligned polymer walls and fibers were formed in the FLC by a two‐step photopolymerization‐induced phase‐separation method using UV‐light irradiation. A fabricated A4‐sized flexible‐sheet display for color‐segment driving was able to exhibit color images even when it was bent.