Micropatterning of sulfonated polyaniline using a soft lithography based lift-off process

A combination of soft lithography and lift-off processing is presented for the fabrication of sulfonated polyaniline (SPAN) microstructures. A soft lithography based micromolding process was used to pattern sacrificial layers using a thermoplastic polymer. SPAN was then polymerized in situ to coat the patterned substrate. The sacrificial layer was removed by lift-off in an organic solvent, leaving the patterned SPAN on the substrate. This process was performed on several rigid and flexible substrates including glass, silicon, and polyimide. The film thickness and roughness were measured as a function of reaction time using atomic force microscopy. Patterns were also imaged using scanning electron microscopy. This process provides a cost effective and versatile method of patterning SPAN and has potential applications in a number of conducting polymer devices.     

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.     

微F-P腔可调谐滤波器关键工艺研究

采用表面加工工艺,AZ5214E光刻胶进行光刻并反转,磁控溅射NiCr合金,剥离出高度为2.3μm的金属桥墩,填充聚酰亚胺作为牺牲层,再在牺牲层上光刻、沉积金属形成金属桥面,在金属桥面的中心嵌入第二布拉格反射镜。采用O2等离子体刻蚀去除聚酰亚胺膜,制作成微法布里—珀罗（ F-P）腔,不需要硅片键合,克服了传统F-P腔高度不够高、调谐范围有限、腔平整度不好以及对设备要求高的缺点,并且可以做出大阵列结构,易于探测器集成。着重对腔体关键工艺,即金属桥墩的NiCr剥离工艺进行研究,针对现有技术缺陷,提出解决办法。     

Polyimide micro-channel arrays for peripheral nerve regenerative implants

Mechanical guidance is one way in which regenerating axons can be directed towards an appropriate target. In this paper, we present the design and fabrication process of a three-dimensional (3D) device comprising a bundle of parallel micro-channels, which can be used as a 3D regenerative implant for peripheral nerve repair. The skeleton of the device is entirely made of flexible polyimide films. Gold micro-electrodes and micro-channels of photosensitive polyimide are patterned directly on polyimide substrates. After fabrication, the 2D electrode channel array is rolled into a 3D channel bundle fitting the peripheral nerve.The efficiency with which axons enter the 2D channel array was evaluated in vitro as a function of channel width, spacing and pitch. Axon outgrowth is maximised when micro-channels are wide (>30 μm), and when the array transparency (the channel width to pitch ratio) is at least 50%. To ensure the metallic electrodes remain functional in the rolled device, substrate thickness and micro-channel height must also be optimized to position the metal film in the neutral plane of the rolled structure. Electrodes embedded in the implant polyimide structure are robust to rolling. Their impedance at 1 kHz in Ringer solution is of the order of 1 MΩ on flat samples, and changes little when the same samples are rolled and inserted into 1.5 mm inner diameter tube. Such 3D, electrode channel devices on polymer not only provides a novel technological approach to physical guidance of regenerating neurons in vivo but also enables the fabrication of an electrode implant with direct electrical communication with multiple groups of nerve fibres in a regenerating peripheral nerve.     

Sacrificial layer process with laser-driven release for batchassembly operations

A dry sacrificial layer process is presented in which microstructures fabricated on UV-transparent substrates are released by excimer laser ablation of a polymer sacrificial material using laser light incident from the reverse side of the substrate. We investigate the application of this technique to the batch assembly of hybrid microelectromechanical systems (MEMS) built from parts fabricated on different substrates. Preliminary measurements of initial velocity are presented for nickel test structures released from polyimide sacrificial layers using a KrF excimer laser. At fluences in the range 50-250 mJ/cm ² (i.e., close to the ablation threshold), structures with heights of 100 μm are shown to exhibit initial velocities in the range 15 ms^-1, allowing controlled transfer of parts between substrates. Application of the new assembly method to a MEMS device is demonstrated by assembling arrays of electrostatic wobble motors from component parts fabricated on separate substrates by UV-LIGA processing     

Different methods for the fabrication of UV-LIGA molds using SU-8 with tapered de-molding angles

De-molding is one of the most crucial steps for successful mass production of high aspect ratio microstructures in microreplication technologies of LIGA process. With a proper taper angle in mold structure, normal contact pressure on the structure will be abated; this will facilitate the de-molding procedure and at the same time prevent the microstructures of mold from being damaged. However, in the case of UV lithography, the top area of the patterned SU-8 resist is observed to be larger than the bottom area especially in cases of thick layer and high aspect ratio structures. In order to obtain an applicable metal mold for hot-embossing process, we purpose here several novel methods with backside exposure which can fabricate different taper angles with proper direction on the mold structures easily. In this paper, we described the technology concept, process details and related experimental results both in mold structures and molded PMMA replicas. In addition, various interesting 3D microstructures can be produced by combining these exposure methods. On-chip microneedle arrays were selected to demonstrate this ability.     

Self-Assembly Based on Chromium/Copper Bilayers

In this paper, we detail a strategy to self-assemble microstructures using chromium/copper (Cr/Cu) bilayers. Self-assembly was primarily driven by the intrinsic residual stresses of Cr within these films; in addition, the degree of bending could be controlled by changing the Cu film thickness and by introducing a third layer with either a flexible polymer or a rigid metal. We correlate the observed curvature of patterned self-assembled microstructures with those predicted by a published multilayer model. In the model, measured stress values (measured on the unpatterned films using a substrate curvature method) were utilized. We also investigated the role of two different sacrificial layers: 1) silicon and 2) water-soluble polyvinyl alcohol. Finally, a Taguchi design of experiments was performed to investigate the importance of the different layers in contributing to the stress–thickness product (the critical parameter that controls the curvature of the self-assembled microstructures) of the multilayers. This paper facilitates a deeper understanding of multilayer thin-film-based self-assembly and provides a framework to assemble complex microstructures, including tetherless self-actuating devices. $hfill$[2008-0308]      

RF MEMS电容式开关结构层释放技术

研究了RF MEMS开关的制造工艺流程和聚酰亚胺牺牲层的去除工艺。在开关的设计和加工中采用在信号线两侧的地线上生长一层绝缘介质层,直流偏置线生成在绝缘介质层之上,与桥的锚点相连接,实现了交直流隔离。讨论了干法刻蚀和湿法刻蚀牺牲层技术。干法刻蚀容易造成绝缘介质层的刻蚀和损伤。采用湿法刻蚀结合临界点干燥技术,可以获得理想的微梁结构。通过测试,开关样品的下拉电压为34 V～40 V,下拉距离为(1.7±0.2)μm,满足设计要求。     

微陀螺仪敏感芯片的选择性电铸技术研究

介绍了一种新的MEMS器件敏感芯片的制备技术———选择性电铸技术。以金为检测电极、铜为牺牲层、正胶作为电铸胎膜,在牺牲层上经过数次电铸形成MEMS器件敏感芯片的各组成部分,腐蚀掉牺牲层后便得到了所需的敏感芯片。以微机械陀螺仪敏感芯片的制备为例,介绍了选择性电铸技术的工艺流程,进行了工艺流片,所制备的微机械陀螺仪敏感芯片结构完整、侧壁陡直、表面平整。该技术在电容式微加速度计及微机械陀螺仪等多种MEMS器件敏感芯片的制作中有广泛的应用前景。     

A new process for releasing micromechanical structures in surface micromachining with polysilicon support and LPCVD Si3N4 embedded mask

This paper presents a new process for releasing micromechanical structures in surface micromachining with polysilicon support and LPCVD (low pressure chemical vapor deposition) Si₃N₄ embedded mask for one polysilicon layer process, which can be adjusted to be suitable for the structure stiffness by changing the distance between two supports. The results of test structures show that this process may be a good technology to eliminate the sticking of microstructures to the substrate during the wafer drying after the sacrificial etching process.     

聚酰亚胺（PI）树脂在电容式RF MEMS开关制作中的应用

聚酰亚胺树脂(PI)因其良好的平面化特性、在氧气中易灰化、不完全固化易溶解于碱性显影液、在CHF3等离子气氛中有较强的抗蚀性等性质,在电容RF MEMS开关的制作过程中,应用它作为刻蚀保护层和牺牲层,不但可以使工艺过程得到简化,而且可以对开关的介质层尺寸、牺牲层厚度等图形参数起到很好的控制作用.     

Electrostatic aluminum micromirrors using double-pass metallization

The fabrication of aluminum spatial light modulators has so far required costly process engineering efforts. In this paper, a low-cost process approach is presented, suitable for the manufacture of electrostatic micromirror arrays. The mirrors are made from the second metallization of complementary metal oxide semiconductor (CMOS) or bipolar processes deposited in two passes. This metal2 is protected by a photoresist layer that can be patterned using the top passivation mask of the process. No additional layer deposition and layer structuring is necessary during postprocessing. The actuators are released in a simple surface micromachining postprocessing sequence based on a sacrificial aluminum and silicon dioxide etch. Our approach allows one metallization to be used for both the circuitry and the electrooptomechanicaI devices. Deformable mirror arrays of up to 16×16 pixels were fabricated. Static self-consistent electromechanical simulations using the finite-element method (FEM) toolbox SOLIDIS were performed for a theoretical analysis and optimization of the actuator devices     

Fabrication of microfluidic channels based on melt-electrospinning direct writing

Microfluidics is a flourishing field, enabling a wide range of applications. However, the current fabrication methods for creating the microchannel structures of microfluidic devices, such as photolithography and 3D printing, mostly have the problems of time-consuming, high cost or low resolution. In this work, we developed a simple and flexible method to fabricate PDMS microfluidic channels, based on poly(ε-caprolactone) (PCL) master mold additive manufactured by a technique termed melt-electrospinning direct writing (MEDW). It relies on the following steps: (1) direct writing of micrometric PCL 2D or 3D pattern by MEDW. (2) Casting PDMS on the printed PCL pattern. (3) Peeling off of patterned PDMS from the embedded sacrificial PCL layer. (4) Bonding the PDMS with microchannel to another PDMS layer by hot pressing. The process parameters during MEDW such as collector speed, nozzle dimension and temperature were studied and optimized for the quality and dimension of the printed micropatterns. Multilayer fiber deposition was developed and applied to achieve microscale architectures with high aspect ratio. Thus, the microchannels fabricated by the proposed approach could possess tunable width and depth. Finally, T-shape and cross-channel devices were fabricated to create either laminar flow or microdroplets to illustrate the applicability and potential of this method for microfluidic device manufacture.     

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.

     

Characterization and optimization to improve uneven surface on MEMS bridge fabrication

This paper presents an optimized fabrication method for developing a freestanding bridge for RF MEMS switches. In this method, the sacrificial layer is patterned and hard baked a 220 °C for 3 min, after filling the gap between the slots of the coplanar waveguide. Measurement results by AFM and SEM demonstrate that this technique significantly improves the planarity of the sacrificial layer, reducing the uneven surface to less than 20 nm, and the homogeneity of the Aluminum thickness across the bridge. Moreover, a mixture of O₂, Ar and CF₄ was used and optimized for dry releasing of the bridge. A large membrane (200 × 100 μm²) was released without any surface bending. Therefore, this method not only simplifies the fabrication process, but also improves the surface flatness and edge smoothness of the bridge. This fabrication method is fully compatible with standard silicon IC technology.     

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.     

Microfabricated small metal cantilevers with silicon tip for atomicforce microscopy

Atomic force microscopy with small cantilevers is faster due to higher resonant frequencies and has a lower noise level. We report a new process to microfabricate small metal cantilevers with integrated silicon tips. This process is used to fabricate gold cantilevers that are 13-40-μm long, 5-10-μm wide, and 100-160-nm thick. The tip is first formed at the free end of a sacrificial oxide cantilever. The cantilever layer of the desired metal is then deposited on the nontip side of the sacrificial oxide cantilever. The oxide layer is removed to form the cantilevers with tips on them in a batch process. The highly stressed cantilevers are rapid thermal annealed for 60 s at 300°C to relieve the stress. The gold cantilevers have been characterized through their thermal spectra and used to image in tapping mode. The process can be used, not only for gold, but also for any metal or compound that can withstand removal of sacrificial oxide cantilevers     

LIGA技术及其在制造可动微结构件中的应用

介绍了将LIGA技术及其与牺牲层技术相结合 ,制造可动微结构的工艺方法。并给出了可实现摆动、转动的可动微结构———微执行器、微传感器和三维加速度传感器的制造过程等应用实例     

Partial release and detachment of microfabricated metal and polymer structures by anodic metal dissolution

The large majority of microelectromechanical systems (MEMS) are fabricated on silicon, glass or Pyrex substrates by manufacturing techniques, which originated from the semiconductors industry. However, their final application often requires removal of the fabrication substrate or at least a partial release of some section of the device. This paper describes a technique based on anodic dissolution of sacrificial metal layers for the complete or partial detachment of microstructures. As an example, a thin-film of sacrificial aluminum is selectively removed in a neutral sodium chloride solution by applying a small positive potential to the aluminum. The method is evaluated theoretically and experimentally in a defined geometry and compared to diffusion-limited, chemical etching. It is shown experimentally that the process is significantly faster than conventional wet chemical etching and the method has been used to release planar and nonplanar thin-film devices made from polymers and metals. The method is applicable for a wide range of metals as sacrificial materials and is very versatile with respect to electrolyte composition and applied voltages. Ease of sacrificial material deposition (sputtering or evaporation) and structuring and the possibility of high process temperature and the nondestructive chemical environment (also environmentally friendly) during detachment make the process technology an interesting alternative to conventional chemical etching.     

Flexible, dry-released process for aluminum electrostatic actuators

We present an all-aluminum MEMS process (Al-MEMS) for the fabrication of large-gap electrostatic actuators with process steps that are compatible with the future use of underlying, pre-fabricated CMOS control circuitry. The process is purely additive above the substrate as opposed to processes that depend on etching pits into the silicon, and thereby permits a high degree of design freedom. Multilayer aluminum metallization is used with organic sacrificial layers to build up the actuator structures. Oxygen-based dry etching is used to remove the sacrificial layers. While this approach has been previously used by other investigators to fabricate optical modulators and displays, the specific process presented herein has been optimized for driving mechanical actuators with relatively large travels. The process is also intended to provide flexibility for design and future enhancements. For example, the gap height between the actuator and the underlying electrode(s) can be set using an adjustable polyimide sacrificial layer and aluminum “post” deposition step. Several Al-MEMS electrostatic structures designed for use as mechanical actuators are presented as well as some measured actuation characteristics