1. C-MEMS Significance:
Almost 45 years after the initial development of the fabrication processes which enabled MEMS technology to be the current state-of-the-art in miniaturization, the entire community is still sitting on a silicon-based material palette. Although the number of new applications is growing at a tremendous rate, researchers and industry are facing a challenging problem of material selection because of the low availability of alternatives. In more recent years, new materials such as polymers, photo curable resists, metal alloys and others have appeared to be used in conjunction with integrated circuit (IC) technology. Nevertheless, some of the more envisioning and promising applications (such as biosensors for life science, micro/nano electrochemical and chemical sensing devices, etc.) have not yet found suitable materials for their optimal fabrication.
In nature carbon is a very special element as the essential building block of most organic life. But also in human enterprise carbon in its various forms has acquired an enviable position. To just name a few: diamonds are made of carbon and also graphite, coke, and glassy carbon are all forms of carbon. So are the more recently discovered buckyballs, carbon fibers and carbon nanotubes (CNTs) . The utilization of carboneous materials has already begun in the above mentioned application fields as the widely different crystalline structures and morphologies of these materials enable different physical, chemical, mechanical, thermal and electrical uses -, specially in life sciences applications because of its high biocompatibility.
Recent developments in C-MEMS (Carbon-MicroElectroMechanical Systems)and C-NEMS (Carbon-NanoElectroMechanical Systems) microfabrication techniques, based on the pyrolysis of patterned photoresist at different temperatures and at different ambient atmospheres, have been made at the Dr. Madou's C-MEMS group at the University of California, Irvine.
2. C-MEMS Process:
Carbon MEMS, or C-MEMS [5-9], describes a manufacturing technique in which carbon devices are made by treating a pre-patterned organic structure to high temperatures in an inert or reducing environment.It has been shown recently that 3D high-aspect-ratio carbon structures can be made from patterned thick SU-8 negative photoresist (Microchem Newton, MA) layers (See figure 1) .SU-8 negative photoresist is a high transparency UV photoresist that enables creation of “LIGA  -like” structures using traditional UV photolithography.
In detail, and as seen in figure 1, carbon structures are fabricated by the pyrolysis of photopatterned positive resists (such as: AZ-4620) and negative photoresists (such as: SU-8) on silicon and fused silica wafers. The pyrolysis process is carried out in a closed ceramic tube furnace in vacuum or a forming gas (95% N2 and 5% H2) atmosphere at about 1000 °C. Carbon produced by pyrolysis of photoresist has been extensively characterized by our team [6,11].
Figure 1. Conventional C-MEMS process
The structure of carbon was studied with SEM (Scanning electron microscopy) and TEM (Transmission electron microscopy), see the Figure 2. The electrical resistance of the film was evaluated by four-point-probe resistance measurement and cyclic voltammetry carried out to characterize its electrochemical behavior. In addition, the pyrolysis process has been studied through extensive thermal analysis including TGA (Thermo-gravimetric analysis), DSC (Differential Scanning Calorimetry) and DTA ( Differential thermal Analysis).
Figure 2. TEM photos of pyrolyzed photoresist
Many different carbon microstructures have been fabricated using the C-MEMS described process. Current efforts of the team are being focused in high-aspect ratio structures for area maximization.
3. Recent results:
We are currently working in different aspect of the C-MEMS like mechancal properties of the resulting structures, microstructure of the material, but mainly we are focused in the application of the current C-MEMS technology on a 3D microbattery. Additional unpublished results include work on C-MEMS porous structures, bridging C-MEMS to C-NEMS, suspended structures and substrateless C-MEMS.
A micropower battery may be simply defined as a battery providing power in the microwatt range. Power requirements may be intermittent over short or long periods or continuous, and in some cases there may be the need to accommodate high power pulses superimposed on a low microwatt background power drain. Batteries may be single cycle, low rate primary cells or multi-cycle secondary cells capable of being recharged many times. The range of power requirements is illustrated by the variety of relatively new miniature portable electronic devices such as cardiac pacemakers, hearing aids, smart cards, personal gas monitors, microelectromechanical systems (MEMS), embedded monitors, and remote sensors with RF capability.
A conventional 2D microbattery is typically a parallel arrangement of a planar cathode and anode separated by a solid or liquid electrolyte. Current processing technologies can easily produce thin-layered structures (<100µm), and hence very thin batteries. Thin film Li batteries have yielded rather impressive performances with respect to lithium capacity, cycle life and energy density. Despite these excellent characteristics 2D battery systems face some severe limitations since thin film batteries are unable to provide meaningful power levels for reasonable periods of time .
Microbatteries based on 3D microstructures are shown to offer significant advantages in comparison to thin film devices for powering microelectromechanical systems and miniaturized electronic devices . A first simple observation is that with an array of 3D microelectrodes, a molecule might have to diffuses over a 10mm distance only which will be 1 million times faster than diffusing over a 1 cm distance as is easily calculated basic molecular diffusion equation . Thus, 3D configurations with narrow electrode posts offer a means of keeping the diffusion distances “short” and provide enough active material such that the 3D batteries will be capable of powering MEMS devices and microelectronic circuits for extended periods of time. Other theoretical advantages and disadvantages of a micro-3D battery compared to a 2D battery were reported by R.W.Hart et al .
Recently we have demonstrated that the pyrolysis of patterned photoresists (C-MEMS) process constitutes a powerful approach to building 3D carbon microelectrode arrays for three dimensional microbattery applications. High aspect ratio carbon posts (>20:1) are achieved by pyrolyzing SU-8 negative photoresist in a simple one step process. . Figure 3 shows our most recent 3D microelectrode arrays fabricated by lithography and pyrolysis of photoresist.
Figure 3. SEM photos of high aspect ratio (a) and (b): 3D SU-8 structures;
(c) 3D C-MEMS posts.
The power of micropower battery ranges from 20µW to 40mW, and the energy ranges from 100µWh to 5Wh for the mentioned applications. For example, the power is 20-200µw and the energy is 1-5Wh for a Li/I2 battery with an in situ formed solid electrolyte used in cardiac pacemakers.
The development of advanced batteries for micropower applications is intimately linked to the availability of new materials and the development of miniature battery designs. Lithium-based batteries have demonstrated very high values of energy density. Highly ordered graphite, hard carbon and soft carbon have extensively served as a host material for Li storage in the negative electrode of commercial Li batteries. However, the values are generally based on the performance of larger cells with capacities of up to several ampere-hours. For small microbatteries the achievable power and energy densities are diminished because the packaging and internal hardware will determine the size and mass of the completed package. Recently CMEMS technology showed potential material and microfabrication solution to this battery miniaturization problem. Very encouraging electrochemical results indicating that using C-MEMS technology the cell voltage is about 3V and theoretical specific energy is possible for the Li microbattery. Furthermore, the carbon films show an electrochemical response that is comparable to that of glassy carbon for selected electrochemical reactions in aqueous and nonaqueous electrolytes.
Recently we demonstrated that the pyrolyzed SU-8 material exhibits reversible intercalation/de-intercalation of lithium .The galvanostatic measurements of the unpatterned film show a large irreversible capacity on the first discharge followed by good subsequent cycling behavior, which is also consistent with the behavior of coke. These results are best summarized by considering the surface area normalized lithium capacity, which is determined to be 0.070 mAh cm-2 for the second and subsequent cycles. For a fully dense carbon film, this corresponds to ~ 220 mAh g-1, which is within the range of reversible capacities reported for coke .The galvanostatic measurements in Fig. 4 were found to give a surface area normalized discharge capacity of 0.125 mA cm-2 for the second and subsequent cycles. Thus, the C-MEMS electrode array possesses a nearly 80% higher capacity than that of the unpatterned carbon film, for the same defined working electrode area.
Figure 4. (a): Galvanostatic charge/discharge cycle behavior of patterned carbon arrays. (b): Cyclic Voltammetry of patterned carbon arrays. The electrolyte was 1 M LiClO4 in a 1:1 volume mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC).
On the cathode side, initial tests have already been performed to characterized 3D deposited metal oxides. Candidate materials for the cathode of the 3D microbattery are LiCoO2 and LiMn2O4. The first deposition method used to attach the metal oxide particles to the C-MEMS structures is codeposition in a PPY matrix. The first experimental tests show good results, as seen in figure 5. This approach is extremelly useful for liquid electrolyte batteries, and adressable configurable electrodes as the cathode material can be deposed on selected groups of electrodes.
Figure 5. (a) SEM of LiMn2O4 particles codeposited on a 3D carbon substrate and edges.
The electrochemical characterization of the final cathode electrodes in the first experiments is very promising, as the LiMn2O4 gives a nice discharge plateau around 2.7V as can be seen in figure 6.
Figure 6. (a) SEM of LiMn2O4 particles codeposited on a 3D carbon substrate and edges.
(b): Codeposited LiMn2O4-PPY cathode charge-discharge curve up to 4V vs. Li/Li+.
More research in the areas of solid electrolytes and castable cathode materials and their application and engineering is being done at the C-MEMS group to obtain a complete solid-state 3D microbattery within 2006.
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