Advanced Materials Research Vols. 60-61

Abstract: Membrane electrode assembly (MEA) is the key component of direct methanol fuel cell (DMFC), the structure and its preparation methods may bring great effects on the cell performances. Due to the requirement of the high performance of the MEA for the micro direct methanol fuel cell (DMFC), we provide a novel double-catalyst layer MEA using CCM-GDE (Catalyst Coated Membrane，CCM；Gas Diffusion Electrode，GDE) fabrication method. The double-catalyst layer is formed with an inner catalyst layer (in anode side: PtRu black as catalyst, in cathode side: Pt black as catalyst) and an outer catalyst layer (in anode side: PtRu/C as catalyst, in cathode side: Pt/C as catalyst). The fabrication procedures are important to the new structured MEA, thus three kinds of fabrication methods are studied, including CCM-GDE, GDE-Membrane and CCM-GDL methods. It was found that the CCM-GDE technology may enhance the contact properties between the catalyst and PEM, and increase the electrode reaction areas, resulted in increasing the performance of the DMFC.

Abstract: Microlenses are important components in optical data interconnects and MEMS-based microscopy imaging. Several microfabrication processes have been developed for the production of microlens arrays. However, assembly of microlens arrays is usually required. In this paper, we demonstrate the application of PDMS membrane patterning based on parylene C lift-off to the construction of PDMS microlens arrays on a glass substrate without an assembly step.

Abstract: Planar Microtoroid cavities have important application in many researching and application domains due to their ultrahigh quality factor (Q). But the special toroid structure can not be fabricated through traditional technology. In this paper, a novel processing method for fabricating microtoroid cavities using high-power CO2 laser after a series of traditional processing steps is introduced. The processing details are discussed including light path, focusing system and other important parameters. Test using scanning electronic microscope and laser-Raman spectrum shows that fabrication of planar microtoroid cavity using CO2 laser is feasible. The final microtoroid cavity is fabricated with slight impact to the silicon strut-beam, ensuring excellent optical performance in Planar Microtoroid cavity.

Abstract: This paper proposes a rapid dynamic analysis method for microgyroscope using system vibration modes to solve the problems concerning to the computing time in the performance analysis of microgyroscope. The results of eigenvalue solution are employed to construct the state space model. The response of the microgyros cope can be reconstructed as a response superposition of the vibration modes, and then the system equation is decoupled into an uncoupled equation. The dynamic response of the microgyroscope can be calculated by a simple superposition.

Abstract: The stress gradient of a deposited thin-film is a mechanical parameter that affects the performance of MEMS devices, so in-situ measuring stress gradient of a thin-film is great significant. A new in-situ measuring method based on a center-anchored circular plate is presented. The Mirau interferometer has been used to measure the out-of-plane height at the edge of circular plate, then the curvature radius of the plate and the stress gradient of the film can be calculated. The measuring method has been verified by CoventorWare. The accuracy of the presented measuring method is ideal. The advantages of the method also have been discussed.

Abstract: The previous research showed that highly doped polysilicon nanofilms (PSNFs) have a high gauge factor and stable temperature characteristics, promoting their applications in piezoresistive sensors. For correcting the resistance deviation and improving the resistance matching of sensing elements, the electrical trimming (ET) characteristics of PSNFs deposited by LPCVD at different temperatures were studied. By the interstitial-vacancy (IV) model, the resistance falloff in trimming phenomenon is considered as a result of the decrease of scattering centers caused by the recombination of IV pairs. By modeling grain boundaries (GBs) as the accumulation of IV pairs (basic defects), it is believed that IV pairs can recombine layer-by-layer under the energy excitation of joule heat generated by high current conduction. The experiment results show that the threshold current density for ET of PSNFs is an order of magnitude lower than that of polysilicon common films, while the trimming accuracy and stability of directly deposited PSNFs are superior to the recrystallized ones. Finally, it is gained that reducing amorphous phases at GBs by optimizing deposition temperature can improve ET characteristics of PSNFs.

Abstract: We use two-dimensional FDTD (Finite difference time domain) method to simulate light transmitting in Si nano-pillar arrays and get some results. The Si nano-pillar array, with diameters, heights, intervals respectively 50-80nm, less than 480nm, 26-44nm, is used as the testifying structure. The reflectivities of both experiment and simulation match well either for bulk silicon or for structure silicon even though small deviations are caused by the uneven size of the pillar’s diameters, heights, and intervals. What’s more, we find reflectivity (ref) increases along with diameter’s (d) increasing for a single pillar with diameters of 20-100nm under a constant light of 600nm, and reaches 10.48% at d=100nm. And with a constant d equal to 20nm, an infinite aspect ratio (r) and a light 600nm, ref is decreasing when the intervals (i) of pillars are increasing. Finally, under the condition of different i, the relations between ref and r are investigated. The undulation of these curves shows that the reflective waves play an important role in sub-wave length optics. Analysis also reveals that there exists a best i for achieving the lowest ref, too large or too small i will cause high ref, even if r is great; moreover, the lager i is, the higher pillar is needed to achieve the lowest ref.

Abstract: a new structure of the uncooled amorphous silicon (α-Si) infrared detector has been fabricated and characterized. The structure with thermal isolation and infrared absorption based on polyimide (PI) and bottom metal reflective layer is put forward. The fabrication process of the IR detectors is described. The temperature coefficient of resistance (TCR) of α-Si resistance has been investigated. The measurements show that the TCR is up to -2.8%. The detectivity of 1.7×108 cmHz1/2W-1 is achieved with chopping frequency of 30Hz at a bias voltage of 5V. Measurement results indicate that the polyimide layer exhibits excellent thermal isolating characteristics and the unique sandwich IR absorption structure is beneficial to the enhancement of detectivity. Compared with other techniques, the IR detectors using PI as thermal isolation layer are not only with simpler process, lower cost and higher yield, but also suitable for the application of large-scale uncooled infrared focal plane arrays (IRFPA).

Abstract: A novel electrode of supercapacitor was developed with a three dimensional (3D) structure which results in a significant increase of the electrode area per unit capacitor volume. An area of 2.58 times the area of corresponding planar design was obtained. The process flow for fabricating the 3D electrode was developed compatible with microelectromechanical system technology. The key processes were high-aspect-ratio Deep Reactive Ion Etching, electroless plating and electropolymerization. Ni as the current collector was deposited on Si/SiO2 substrate by using electroless plating, Polypyrrole was galvanostatically polymerized as electroactive film. The capacitance properties of the 3D electrode were investigated. Cyclic voltammetry tests show that the specific capacitance of the 3D electrode at 100 mV/s was 0.014 F/cm2 which is comparable with the 0.013 F/cm2 for the planar design. Electrochemical impedance spectroscopy plot of the 3D electrode is well fitted by equivalent circuit, we found the specific capacitance is 0.011 F/cm2 which is slightly larger than 0.0094 F/cm2 for similarly planar design. From the gavanostatic charge/discharge tests, the specific capacitance of the 3D electrode at 2 mA/cm2 is 0.011 F/cm2 while for planar design the corresponding value is 0.010 F/cm2. The results indicate the continuous and homogeneous Polypyrrole film with 3D structure was obtained as we designed.

Abstract: In micro electrochemical machining (micro ECM), side-insulation of tool electrode can significantly improve the machining accuracy, especially for high aspect ratio structures. This paper presented a new side-insulation approach for not only micro single electrodes but also micro array electrodes, in which spin-coating and room curing are used to coat the micro tool electrodes with a very thin film of polymer, followed by an end-rubbing process to remove the insulation material from the electrode end surfaces. Processing parameters of spin-coating were optimized experimentally, and side-insulated micro single electrodes with a double-layer epoxy coating of 3m thickness were prepared. Experiments were made to test the performance of the side-insulation film under simulated micro ECM conditions. And the results demonstrated good insulativity and high adherence strength in a 6-hour durability test. Also side-insulation for micro array electrodes was performed using this approach.