Carbon Microelectromechanical Systems Research Articles

Two-Step Deformation Mechanism of Self-Forming Three-Dimensional Carbon Micro Structures

We have developed a novel fabrication method for three-dimensional carbon micro structures using a carbon-microelectromechanical system (C-MEMS) and a thermomechanical deformation technique. Three-dimensional carbon micro structures could be fabricated as pyrolyzed SU-8 film with sputtered Au layer. The SU-8 film with sputtered Au layer was deformed autonomously by thermal deformation. Fabrication of three dimensional structures from two dimensional films is like origami structure. In this paper, we investigated deformation behaviour, including temperature-dependent deformation, shape-dependent deformation, and size-dependent deformation to design carbon structures. We found a two-step deformation. The SU-8 film with sputtered Au layer was deformed toward the Au-inside direction by thermal expansion at temperatures under 200 °C. However, it was deformed toward the Au-outside direction by the glass transition between 200 and 300 °C. Shape- and size-dependent deformations were also confirmed. This process could allow fabrication of various micro devices such as large surface area electrodes for micro fuel cells, microreactors, and microsensors.

Carbon microelectromechanical systems as a substratum for cell growth

The study of the biocompatible properties of carbon microelectromechanical systems (carbon-MEMS) shows that this new microfabrication technique is a promising approach to create novel platforms for the study of cell physiology. Four different types of substrates were tested, namely, carbon-MEMS on silicon and quartz wafers, indium tin oxide (ITO) coated glass and oxygen-plasma-treated carbon thin films. Two cell lines, murine dermal fibroblasts and neuroblastoma spinal cord hybrid cells (NSC-34) were plated onto the substrates. Both cell lines showed preferential adhesion to the selectively plasma-treated regions in carbon films. Atomic force microscopy and Fourier transform infrared spectroscopy analyses demonstrated that the oxygen-plasma treatment modifies the physical and chemical properties of carbon, thereby enhancing the adsorption of extracellular matrix-forming proteins on its surface. This accounts for the differential adhesion of cells on the plasma-treated areas. As compared to the methods reported to date, this technique achieves alignment of the cells on the carbon electrodes without relying on direct patterning of surface molecules. The results will be used in the future design of novel biochemical sensors, drug screening systems and basic cell physiology research devices.

Carbon post-microarrays for glucose sensors

A novel design and fabrication method of glucose sensors based on high aspect ratio carbon post-microarrays is reported in this paper. Apart from the fact that carbon has a wide electrochemical stability window, a major advantage of using carbon post-microarrays as working electrodes for an amperometric glucose sensor is the large reactive surface per unit footprint substrate area, improving sensitivity of the glucose sensor. The carbon post-microarrays were fabricated by carbon-microelectromechanical systems (C-MEMS) technology. Immobilization of enzyme onto the carbon post-electrodes was carried out through co-deposition of glucose oxidase (GOx) and electrochemically polymerized polypyrrole (PPy). Sensing performance of the glucose sensors with different post-heights and various post-densities was tested and compared. The carbon post-glucose sensors show a linear range from 0.5 mM to 20 mM and a response time of about 20 s, which are comparable to the simulation result. Sensitivity per unit footprint substrate area as large as 2.02 mA/(mM cm 2) is achieved with the 140 μm high (aspect ratio around 5:1) carbon post-samples, which is two times the sensitivity per unit footprint substrate area of the flat carbon films. This result is consistent with the hypothesis that the number of reaction sites scales with the reactive surface area of the sensor. Numerical simulation based on enzymatic reaction and glucose diffusion kinetics gives the optimum geometric design rules for the carbon post-glucose sensor. Glucose sensors with even higher sensitivity can be achieved utilizing higher carbon post-microarrays when technology evolution will permit it.

Evaluation of bacteria compatible microporous surfaces for biofuel cells

Prototypes of photosynthetic-bacterium micro-biofuel cells have been developed in our laboratory, in which electrons are extracted from bacterial cells while they are in contact with an electrode. The material and shape of the electrode should be electroconductive and biocompatible with a low electrical resistance and a high-specific surface area, therefore, pyrolyzed carbon was fabricated either to a flat or porous surface electrode using carbon microelectromechanical systems (C-MEMS). The developed electrodes were applied in micro-biofuel cells, and the porous electrode showed a fourfold higher performance than the flat one.

Fabrication and properties of a carbon/polypyrrole three-dimensional microbattery

The carbon-microelectromechanical systems (C-MEMS) microfabrication process offers a promising method for fabricating three-dimensional (3D) battery architectures. In the current study, this approach was extended to the fabrication of positive electrode arrays and their assembly in a 3D lithium-ion microbattery. The positive electrode array was fabricated by electrochemical deposition of dodecylbenzenesulfonate-doped polypyrrole (PPYDBS) on an array of carbon rods. Electrochemical measurements show that the electrodeposited PPYDBS electrode array reversibly intercalates lithium with better gravimetric capacity than that of 2D electrodeposited films. The prototype carbon/PPYDBS 3D battery was based on an interdigitated electrode array configuration. It functioned as a secondary battery but the performance was limited because of electrical shorting. These initial results with the 3D interdigitated battery help to identify the needs associated with different electrode fabrication approaches and specific 3D designs.

Diamond and diamond-like carbon MEMS

Diamond and diamond-like carbon (DLC) thin films possess a number of unique and attractive material properties that are unattainable from Si and other materials. These include high values of Young's modulus, hardness, tensile strength and high thermal conductivity, low thermal expansion coefficient combined with low coefficients of friction and good wear resistance. As a consequence, they are finding increasing applications in micro-electro-mechanical systems (MEMS). This paper reviews these distinctive material properties from an engineering design point of view and highlights the applications of diamond and DLC materials in various MEMS devices. Applications of diamond and DLC films in MEMS are in two categories: surface coatings and structural materials. Thin diamond and DLC layers have been used as coatings mainly to improve the wear and friction of micro-components and to reduce stiction between microstructures and their substrates. The high values of the elastic modulus of diamond and DLC have been exploited in the design of high frequency resonators and comb-drives for communication and sensing applications. Chemically modified surfaces and structures of diamond and DLC films have both been utilized as sensor materials for sensing traces of gases, to detect bio-molecules for biological research and disease diagnosis.

A Case for Fractal Electrodes in Electrochemical Applications

Fractal electrode designs are proposed for electrochemical devices. A zeroth-order approximation fractal model is analyzed and basic insights into the scaling laws of the electrical properties and the surface-to-area ratio are derived. Fractal electrodes can minimize internal resistance while maximizing surface-to-volume ratios. Recent carbon microelectromechanical systems (C-MEMS) techniques show great promise for use in the fabrication of fractal electrodes for electrochemical applications. Preliminary results showing carbon fractal electrodes fabricated using C-MEMS techniques are presented.

Investigation on the solid electrolyte interface formed on pyrolyzed photoresist carbon anodes for C-MEMS lithium-ion batteries

Carbon-microelectromechanical systems (C-MEMS) obtained from the pyrolysis of patterned photoresists are a powerful solution for the miniaturization of energy storage/conversion devices such as fuel-cells and microbatteries. The 3D shapes and the high aspect ratio of the resulting carbon structures are critical factors in applications where specific surface area plays a key role. Furthermore, lithographic techniques used in the C-MEMS technology may solve the downscaling problems of conventional carbon manufacturing techniques. The application of C-MEMS as a lithium-ion battery anode has already been demonstrated in our previous work. It is well known that a protective film, referred to as the solid electrolyte interface (SEI), forms on carbonaceous materials used as negative electrodes in commercial lithium-ion batteries. The passivating film forms during the first battery charging cycle. Detailed SEI investigations of this film, made up of electrolyte decomposition products, have not been explored yet. In this paper the evolution of the pyrolyzed carbon/electrolyte interface functioning as a lithium-ion battery anode is studied and discussed in detail.

Fabrication of suspended carbon microstructures by e-beam writer and pyrolysis

A fabrication process has been developed to create suspended carbon microelectromechanical system (C-MEMS) structures. SU-8, a negative photoresist, was used as the starting material and was converted to the desired carbon microstructures using pyrolysis in an inert atmosphere. Suspended carbon-micro and nano electromechanical system (C-MEMS/NEMS) structures with feature sizes down to 500 nm were fabricated by ultra violet/electron beam (EB) lithography and pyrolysis. The problem of charging of the non-conductive SU-8 surface was solved by partially masking a thin metal layer to prevent the repulsion of negative charged electrons before EB writing. Complex suspended C-MEMS structures, such as bridges and networks have been formed. This fabrication method can accurately and reproducibly produce various suspended C-MEMS structures which have applications in microelectronics and biosensing.

Local chemical vapor deposition of carbon nanofibers from photoresist

The addition of nanofeatures to carbon microelectromechanical system (C-MEMS) structures would greatly increase surface area and enhance their performance in miniature batteries, super-capacitors, electrochemical and biological sensors. Negative photoresist posts were patterned on a Au/Ti contact layer by photolithography. After pyrolyzing the photoresist patterns to carbon patterns, graphitic nanofibers were observed near the contact layer. The incorporation of carbon nanofibers in C-MEMS structures via a simple pyrolysis of modified photoresist was investigated. Both experimental results considered to consist of a local chemical vapor deposition mechanism. The method represents a novel, elegant and inexpensive way to equip carbon microfeatures with nanostructures, in a process that could possibly be scaled up to the mass production of many electronic and biological devices.

Electrical Properties and Shrinkage of Carbonized Photoresist Films and the Implications for Carbon Microelectromechanical Systems Devices in Conductive Media

Recent advances in fabricating 3D micro- and nanostructures using carbon microelectromechanical systems, or C-MEMS, has opened up a wide variety of new and exciting applications. The development of 3D C-MEMS has been catapulted forward by the use of transparent, high-viscosity resists such as SU-8. The electrical characteristics and shrinkage of various thickness carbon films derived from SU-8 and AZ P4620 are quantified and discussed in the context of the decomposition and carbonization mechanisms of epoxy and phenolic resins. Measurements obtained reveal a thickness dependence of the resistivity at lower carbonization temperatures but not much dependence at . Possible explanations for this low-temperature thickness dependence are given. The electrical characteristics of carbon films obtained from both types of photoresists carbonized at are very similar to that of glassy carbon. Simulations have been carried out to demonstrate the importance of the carbon resistivity for C-MEMS devices when used in conductive media. A method for simple optimization and verification of C-MEMS device designs for use in conductive media is introduced.

3-D electrode designs for flow-through dielectrophoretic systems

Traditional methods of dielectrophoretic separation using planar microelectrodes have a common problem: the dielectrophoretic force, which is proportional to nabla|E|2, rapidly decays as the distance from the electrodes increases. Recent advances in carbon microelectromechanical systems have allowed researchers to create carbon 3-D structures with relative ease. These developments have opened up new possibilities in the fabrication of complex 3-D shapes. In this paper, the use of 3-D electrode designs for high-throughput dielectrophoretic separation/concentration/filtration systems is investigated. 3-D electrode designs are beneficial because (i) they provide a method of extending the electric field within the fluid. (ii) The 3-D electrodes can be designed so that the velocity field coincides with the electric field distribution. (iii) Novel electrode designs, not based on planar electrodes designs, can be developed and used. The electric field distribution and velocity fields of 3-D electrode designs that are simple extensions of 2-D designs are presented, and two novel electrode designs that are not based on 2-D electrode designs are introduced. Finally, a proof-of-concept experimental device for extraction of nanofibrous carbon from canola oil is demonstrated.

From MEMS to NEMS with carbon

Our work in carbon-microelectromechanical systems (C-MEMS) suggests that C-MEMS might provide a very interesting material and microfabrication approach to battery miniaturization, active DNA arrays and a wide variety of chemical and biological sensors. In C-MEMS, photoresist is patterned by photolithography and subsequently pyrolyzed at high-temperatures in an oxygen-free environment. We established that it is possible to use C-MEMS to create very high-aspect ratio carbon structures (e.g. posts with an aspect ratio >10), suspended carbon plates and suspended carbon nanowires (C-NEMS). By changing the lithography conditions, soft and hard baking times and temperatures, additives to the resist, pyrolysis time, temperature and environment, C-MEMS permits a wide variety of interesting new MEMS and NEMS applications that employ structures having a wide variety of shapes, resistivities and mechanical properties. We also demonstrate that arrays of high-aspect ratio carbon posts can be charged/discharged with Li and this enables the fabrication of a smart switchable array of batteries.

C-MEMS for the Manufacture of 3D Microbatteries

We have demonstrated that carbon-microelectromechanical systems (C-MEMS), in which patterned photoresist is pyrolyzed in inert environment at high temperature, constitutes a powerful approach to building 3D carbon microelectrode arrays for 3D microbattery applications. High aspect ratio carbon posts (>10:1) are achieved by pyrolyzing SU-8 negative photoresist in a simple one step process. Lithium can be reversibly charged and discharged into these C-MEMS electrodes. Because of the additional volume of the posts, higher capacities are achieved with the 3D array electrodes as compared to unpatterned carbon films with the same projected electrode area. These novel electrode arrays represent one of the critical components for 3D batteries, which may be interconnected with C-MEMS leads to enable smart power management schemes.