The mechanics of polymer swelling on microcantilever sensors

This paper investigates the swelling mechanics of polymer capture layers integrated into piezoresistive cantilever biochemical sensors. A finite element model investigates mechanical deformations in a polymer layer affixed to a silicon microcantilever. The polymer swells during analyte absorption, inducing deformations in the silicon cantilever which are sensed by a piezoresistive sensor integrated into the cantilever. The highest sensitivity is predicted for short and wide cantilevers that are coated with stiff polymer whose thickness is twice that of the cantilever. While the polymer swelling induces the deformations, the silicon carries most of the load. When portions of the silicon beam are removed to introduce stress concentrations, the system sensitivity can increase by 18% compared to the cantilever without stress concentrations. This study of stress distributions in the cantilever system allows sensor optimization that considers the full 3D effects of polymer swelling mechanics.     

Design and development of a novel paddle test structure for the mechanical behavior measurement of thin films application for MEMS

A new technique was developed for studying the mechanical behavior of thin films on substrate applications for micro-electro-mechanical system (MEMS). The test structure was designed on novel “paddle” cantilever beam coated thin film specimens with dimensions of a few hundred to 50 nm. This beam has a triangle shape that provides a uniform plane strain distribution. Standard clean room processing was used to prepare the paddle sample. The experiment can be operated using the electrostatic force to deflect the “paddle” cantilever beam and measure the mechanical response of the sample with surface deposited thin film. A capacitance measurement is used to observe the deflection of the cantilever plate on the other side of the sample with respect to the electrostatic force on the one side. The measured strain was then converted through this capacitance measurement to conduct mechanical behavior studies on the coated thin film. Both system performance experiments and calculations were studied to verify the design concepts. The residual thin film stress measurements were performed and compared with the calculated results from three different forces exerted on the “paddle” cantilever beam, including the force due to the film, compliance force, and electrostatic force.     

Effect of electrolyte storage layer on performance of PPy-PVDF-PPy microactuators

Trilayer electroactive conducting polymer actuators comprising of two active polymer layers separated by an electrolyte storage layer in between are capable of working in air. Ions are shuttled between the two active layers via the middle inactive layer during electrochemical switching, which leads to a bending motion like the bending of a cantilever beam under a load. Performance of these types of cantilevered actuators greatly depends on the middle electrolyte storage layer, which holds electrolyte and eliminates the necessity of an external electrolyte reservoir, as required by their predecessors. In this paper, we used different types of poly(vinylidene fluoride) (PVDF) films as an electrolyte reservoir and compared bending displacement performance of the actuators made from such PVDF films. The results indicate that the thickness of the PVDF layer has significant effect on the tip displacement of the actuators. With a reduction in the PVDF thickness, the bending stress generated by the actuator decreases while the free tip displacement increases. Several types of PVDF thin films were also compared. As expected, porous PVDF thin films retained more liquid electrolyte and produced larger actuation than non-porous films of the same thickness. It was also found that addition of the electrolyte salt directly to the PVDF film during its spin casting increased the film conductivity and actuation tip displacement.     

Integration of Reactive Polymeric Nanofilms Into a Low-Power Electromechanical Switch for Selective Chemical Sensing

This paper presents the fabrication and demonstration of an ultrathin microelectromechanical chemical sensing device. Microcantilevers are etched from 100-nm-thick silicon nitride, and a 75-nm-thick reactive copolymer film for sensing is deposited by initiated chemical vapor deposition. Cross-linking densities of the polymer films are controlled during the deposition process; it is shown that greater cross-linking densities yield greater cantilever deflections upon the polymer's reaction with the analyte. Considering that chemical reactions are necessary for stress formation, the sensing is selective. Cantilever deflections of greater than 3 $muhbox{m}$ are easily attained, which allow a simple switch to be designed with resistance-based outputs. When exposed to a hexylamine vapor-phase concentration of 0.87 mol%, the resistance of the switch drops by over six orders of magnitude with a response time of less than 90 s. $hfill$[2008-0132]      

Selective heat-transfer dye diffusion technique using laser irradiation for polymer electroluminescent devices

The heat-transfer process described in this paper will be useful for controllably and selectively doping polymer films used to make hyperfine full-color polymer electroluminescent (EL) displays. A polymer receiver film is placed in direct contact with a dye-dispersed polymer film coated onto an efficient photo-absorbing substrate to permit heat-transfer dye diffusion. The widely used heat-mode magneto-optical storage material, TbFeCo was used as the photo-absorbing material. We irradiated it with a laser in order to generate heat. This method can be used to control the lateral and vertical dopant distributions in polymer films for EL devices. We demonstrated that coumarin 6 and Nile red can be doped into the rather thermally stable polymer poly(N-vinylcarbazole) containing the electron-transporting 2-(4-biphenylyl)-5-(4-tert-butyl-phenyl)-1,3,4-oxadiazole from a viewpoint of luminescent properties. The diffusion power and time for about 1-mm² doping area were 1.2 W and about 10 s.     

Development of wide frequency range-operated micromachined piezoelectric generators based on figure-of-merit analysis

This paper presents a novel micromachined piezoelectric generator harvesting mechanical energy over a wide frequency range. According to systematic figure of merit (FOM) analysis, four optimized design devices of cantilevers with proof-masses are selected to be connected in parallel to form a generator of size 3 × 3 × 5 mm. Moreover, the devices are fabricated by an innovative process where the piezoelectric active areas on the cantilever are supported by flexible membranes to achieve a large displacement with minimum residual stress. When the generator is operated in a vibration environment, the deposited piezoelectric films on the cantilever structures induce charge for power generation. According to experimental characterization of the sputtered ZnO film, the developed generator produces from tens of picowatts to hundreds of nanowatts of power over a reasonably wide frequency range.     

Detection of ethanol and methanol vapors using polymer-coated piezoresistive Si bridge

A novel alcohol vapor sensor based on the swelling effect of polymer film is proposed for detection of alcohol such as methanol and ethanol vapor, which is composed of a Si bridge embedded with piezoresistive Wheatstone bridge and an epoxy acrylate thin polymer layer-coated Si bridge. When the polymer layer absorbs alcohol vapor molecules from ambient air, the swelling of the thin polymer layer leads to bending of the Si bridge which causes the piezoresistive Wheatstone bridge to produce an output voltage. A theoretical model of the interaction between Si bridge and polymer layer is presented based on elastic mechanics theory. An equivalent transverse gas load is presented. The output expression of sensor is obtained. The model of the sensor is verified by the experimental results. The experimental results show that the alcohol vapor sensor has good linearity, selectivity and reproducibility. The proposed alcohol vapor sensor has simply principle, mature technology, low cost, and can be integrated with signal processing circuit using the MEMS processes.     

Non-solvent induced phase separation as a method for making high-performance chemiresistors based on conductive polymer nanocomposites

The fabrication of novel porous conductive composite vapor sensors characterized by different porosities and specific surface areas is described in this study. These samples were obtained by the dry-cast non-solvent induced phase separation (NIPS) method. Porous composite structures have been studied by the SEM, BET and water evaporation methods. Testing different concentrations of several organic vapors, the porous sensors showed improved sensitivities and response times as compared to their dense counterpart. Improved characteristics of the sensor response were correlated to better sorption and diffusion properties of sensing film due to increased porosity and specific surface area obtained by this method of film fabrication. A competition theory was proposed that describes the optimum porosity and thickness of sensing films in which the highest sensitivities were observed.     

CO-sensing properties of hafnium oxide thin films prepared by electron beam evaporation

Thin films of hafnium oxide were deposited by electron beam evaporation. The effects of the film thickness and preparation conditions (films prepared on the heated substrate with or without the presence of oxygen environment during deposition) on the optical and carbon monoxide sensing properties of the films were studied. The films were characterized using X-ray diffraction and X-ray photoelectron spectroscopy and optical spectroscopy techniques. Films deposited on unheated substrates were amorphous, whereas those deposited on heated substrates showed a mixture of amorphous and polycrystalline structure. It was found that the sensitivity of the films to CO increased with the thickness and the porosity (as reflected by the refractive indices) of the films.     

MEMS薄膜平均应力梯度及等效弹性模量在线提取方法

薄膜沿厚度方向的平均应力梯度及薄膜的弹性模量对器件性能有重要影响.提出了一种利用静电作用下悬臂梁的吸合电压提取薄膜沿厚度方向的平均应力梯度及等效弹性模量的方法,该方法的关键在于实现悬臂梁吸合电压的快速精确计算.考虑了悬臂梁由应力梯度引起的沿宽度方向的弯曲及实现其固定端接近理想固支的方法,提高了吸合电压的计算精度.实际模拟表明该测量方法计算速度快、精度高,能够应用于实际工艺过程中材料参数的在线测量.     

Thin film evaporation in microchannels with interfacial slip

We investigate the role of interfacial slip on evaporation of a thin liquid film in a microfluidic channel. The effective slip mechanism is attributed to the formation of a depleted layer adhering to the substrate–fluid interface, either in a continuum or in a rarefied gas regime, as a consequence of intricate hydrophobic interactions in the narrow confinement. We appeal to the fundamental principles of conservation in relating the evaporation mechanisms with fluid flow and heat transfer over interfacial scales. We obtain semi-analytical solutions of the pertinent governing equations, with coupled heat and mass transfer boundary conditions at the liquid–vapor interface. We observe that a general consequence of interfacial slip is to elongate the liquid film, thereby leading to a film thickening effect. Thicker liquid films, in turn, result in lower heat transfer rates from the wall to liquid film, and consequently lower mass transfer rates from the liquid film to the vapor phase. Nevertheless, the total mass of evaporation (or equivalently, the net heat transfer) turns out to be higher in case of interfacial slip due to the longer film length. We also develop significant physical insights on the implications of the relative thickness of the depleted layer with reference to characteristic length scales of the microfluidic channel on the evaporation process, under combined influences of the capillary pressure, disjoining pressure, and the driving temperature differential for the interfacial transport.     

悬臂梁大变形的向量式有限元分析

为分析悬臂梁的几何非线性行为,用向量式有限元法将结构离散成质点系以及质点间的连接单元.根据牛顿第二定律得到每个质点在内力和外载荷作用下的运动方程以及悬臂梁在每个时刻的变形用该时刻质点系的运动表示.结合刚架元的节点内力和等效质量得出质点位移的迭代计算公式,采用FORTRAN编制计算程序,对悬臂梁分别承受集中载荷和弯矩下的大变形进行算例分析.计算结果与理论解吻合较好,表明该方法能很好地模拟分析悬臂梁的大变形.     

Micro-tribological interface model for friction-induced cold start-up running dynamics

The robustness and noise warranty costs of many automotive friction materials like transmission belts or brake pad are directly affected by the frictional properties in cold start-up running. This paper presents a friction model for start-up running under cold conditions. The absorbed water, phase changes and variable lubrication regimes in cold start-up running are taken into account. The model includes the breakage of ice adhesion, ice–ice friction between the ice films on friction pairs, melting water-mediated mixed lubrication and boundary lubrication, friction elevation due to capillary adhesive effect, as well as dry contact in the end of cold start-up running. Thermal analysis is applied in conjunction with the friction model to estimate the parameters in phase transitions. A meniscus model is also integrated into the friction model to address the friction elevation due to the variation of water film thickness. It is illustrated that if the thickness of surface ice film is larger than a critical value, the static friction coefficient could be close to 1; if the thickness of melting water film is higher than the average roughness of the surface, static friction could increase due to capillary effect, and kinetic friction could decrease due to mixed lubrication resulting in wide modulation of friction during intermittent start-up transitions. This paper also presents the application of the model to elucidate the friction mechanisms in cold brake noise where the cold wet coefficient of friction (cof) could be substantially higher than the dry cof. The effects of temperature, roughness and load on cof are also characterized.     

Study of residual stress-induced deformation of multilayer cantilever for maskless microplasma etching

In the scanning probe microscopy-based microplasma etching system proposed by our group, the microcantilever probe integrated with microplasma device is a multilayered structure. However, the thin film residual stress generated by microfabrication process may cause undesirable bending deformation of the cantilever. In order to predict and minimize the stress-induced deformation in the cantilever design, we experimentally measure and calculate each thin film stress of the cantilever based on Stoney equation. Then the stress-induced bending deformation of the cantilever is simulated by finite element simulation. By adjusting the thickness of reserved silicon layer of the cantilever, the deflection can be minimized to <5 μm for a 750 μm-length cantilever. Finally the microcantilever probes with different thickness of reserved silicon layer are successfully fabricated by MEMS process. The bending deformation of actual fabricated cantilevers agree well with simulation results, which verifies the feasibility of the cantilever structural design. The results of this paper may lay a foundation for further scanning plasma maskless etching.     

Zirconium tungstate (ZrW/sub 2/O/sub 8/)-based micromachined negative thermal-expansion thin films

Negative expansion materials are relatively rare but promise to be particularly useful in designing thermally sensitive mechanical devices. Although negative thermal expansion (NTE) in bulk materials such as ZrW/sub 2/O/sub 8/ has been extensively studied, this paper reports the first deposition of a NTE material thin film. ZrW/sub x/O/sub y/ films were deposited by electron beam evaporation and reactive cosputtering. The films were processed and patterned for various microstructures. The coefficients of thermal expansion of the deposited thin films were determined by measuring the change in curvature with temperature. It was found that evaporated films but not sputtered films, which were denser than the evaporated films, exhibited NTE. It was also found that NTE behavior occurred across a variety of stoichiometries. Since crystalline ZrW/sub 2/O/sub 8/ and thin film ZrW/sub x/O/sub y/ both have low densities and show negative expansion, it is speculated that similar physical mechanisms, as discussed in the text, are at work. Further, since the deposition conditions of a thin film can often be changed to control density, it is speculated that a wider variety of thin films than bulk crystals might be made to have NTE.     

导电聚合物驱动器悬臂梁模型建立及柔性抓取装置设计

基于导电聚合物具有柔韧性好、驱动电压低、能耗小等特性,采用自制的多层弯曲型导电聚合物驱动器搭建实验系统,依据等效悬臂梁理论建立驱动器力学模型。通过测量驱动器的弯曲变形量建立偏转位移与电压、长度的函数关系式,并且计算出等效均布载荷值。实验结果表明,驱动器偏转位移与电压、长度成线性关系;当驱动电压达到1.0 V时,驱动器偏转速度趋于稳定,且偏转效果最佳。为改善普通微操作装置结构复杂、能耗大的缺点,采用导电聚合物智能材料设计并制作出微型手爪制动器,最后验证了手爪可稳定抓起0.0111 g左右的重物。     

薄膜结构的复合氟离子敏感电极的研究􀀁

复合氟离子敏感电极是用热蒸发的方法在硅基片上制备的。参比电极用热蒸发的银－氟化银薄膜电极,为了提高氯化银薄膜的坚固性和可靠性,又在氯化银表面涂复一层１０００Ａ的含氯离子的有机聚合物薄膜。采用溶胶－凝胶法＾「１,２」制备二氧化硅对电极引线的焊点和器件的侧、背面进行封装,因此这种电极不仅体积小、可靠性高,而且,由于这些薄膜可以在蒸发台内依次完成,减少了污染,适于大规模生产。     

Nanodroplets on a solid plane: wetting and spreading in a Monte Carlo simulation

The wetting behavior and spreading dynamics of small polymer melt droplets in the course of transition from partial to complete wetting conditions on a flat structureless solid substrate have been studied by dynamic Monte Carlo simulation. From the density profiles of the drops we determine the contact angles at varying strength of the van der Waals surface forces in the whole interval of partial wetting. The validity of Young's equation is then tested whereby the surface tension of the melt/vapor interface is derived independently from interfacial fluctuation analysis, and the surface free energy of the melt at the substrate—from the anisotropy of the local pressure at the wall. The bending rigidity of the melt/vapor interface turns out negative, as recently predicted for short-range interactions.We carry out computer experiments which show that Tanner's law for the kinetics of drop spreading holds also on nanoscopic scales. The observed density profiles of spreading droplets confirm earlier predictions that the central cap-shaped region of the droplets shrinks at the expense of a transition region (“foot”) surrounded by a precursor film which is roughly one monolayer thick. At later times the precursor film breaks into individual polymer chains and advances in typically diffusive manner as found in laboratory experiments.Eventually we investigate the impact of line tension on nanodroplets behavior at varying strength of adhesion and demonstrate that the Gretz equation which incorporates line tension into Young's rule holds even on nanoscale and predicts important properties of the drops subject to droplet size.     

阵列化锰铜高压传感器的研制

通过传感器的结构设计、敏感材料和封装材料的研制以及采用新的传感器制备工艺,制作了一种新型的薄膜式锰铜传感器。采用熔融石英材料作为绝缘基板。在绝缘基板上沉积锰铜敏感薄膜。并在敏感薄膜的上面沉积SiO2封装层薄膜。根据"后置"式传感器由阻抗匹配原则,计算出铝靶板中的最高压力为51.68GPa,SiO2封装材料中的压力为35.396GPa。     

Silver (Ag) as a novel masking material in glass etching for microfluidics applications

In this paper, we report the use of a single masking film for deep glass etching in hydrofluoric acid (HF). Thin film silver (Ag) is the key masking material in this work enabling a simple and low cost fabrication of microfluidic structures. The Ag film was deposited by evaporation and etched in a diluted nitric acid and de-ionized water solution at a ratio of 1:3. Surface morphology for different thicknesses of Ag film and its correlation to the maximum achievable etch depth is analyzed. AFM results shows low roughness values (<5 nm), indicating the Ag films are of smooth surface. With a 100 nm Ag film, a 220 μm etch depth in borosilicate glass substrates were produced and by further thickening the Ag to 300 nm, etch depths exceeding 300 μm were successfully achieved. SEM images show that thinner Ag films are of finer grains, potentially a source for pinholes formation where rapid penetration of HF along the grain boundaries peels off the Ag film from the glass surface. However, the Ag film was found not to react with HF. The process was demonstrated in the fabrication of cavities for integration with other microfluidic devices.