Micro energy and chemical systems (MECS) and multiscale fabrication

Abstract

Over the past 40 years, there has been a large and growing emphasis on the fabrication of devices with ever-decreasing dimensions spread over multiple length scales. These so-called multiscale systems are here defined as integrated systems with sub-millimeter features spanning more than three orders of magnitude. One example of a multiscale system is the integrated circuit (IC) where feature sizes may range four or more orders of magnitude from 100 nanometer gates to millimeter-scale bonding pads. The drive toward multiscale systems is obvious. On one end, smaller gate sizes permit higher component densities improving both speed and cost. At the other end, macro-scale pads are required for electrical interconnection with the macro-world. As shown in Table 14.1, other examples of multiscale systems involve mechanical miniaturisation including microelectromechanical systems (MEMS), micro total analysis systems (μTAS) and micro energy and chemical systems (MECS). Over three decades ago, mechanical miniaturisation was introduced through pioneering biomedical research at Stanford (Samaun et al. 1973; Middlehoek et al. 1980). Since then, many applications have been developed such as automotive accelerometers, inkjet printers, microelectronics cooling, point-of-use chemical synthesis, and man-portable power generation to name a few. MEMS were the first of these devices to develop. MEMS are highly miniaturised electromechanical devices used for microscale energy conversion (transduction). Typical applications are as miniature sensors and actuators such as accelerometers for automotive air bags, thermal inkjet printheads, micromirror arrays for computer projection and read/write magnetic memory heads. Typical feature sizes are on the order of one to ten μm. MEMS technology enables highly complex assemblies without the need for mechanical assembly. The basis of MEMS fabrication is integrated circuit (IC) processing and silicon micromachining techniques. The traditional manufacturing engineering community did not contribute much to the process development necessary to enable ICs and MEMS. This was in part due to the fact that the science base needed to manipulate matter for the processing of electrons is fundamentally different than the manufacturing science for producing structural systems. Then to, the precision in ICs and MEMS were at levels previously unattainable in traditional manufacturing. Feature sizes in these systems were on or below the order of traditional manufacturing tolerances. Whole new fields of dimensional and compositional analysis were required. Finally, in traditional manufacturing, systems integration across multiple length scales was provided mainly through mechanical assembly, which could be managed by unskilled laborers. Consider the mechanical assembly of airplanes or automobiles, which could involve hundreds or thousands of workers in contrast with automated IC integration. Consequently, manufacturing engineers did not perceive their skill set to be applicable at the small scale. However, new microfluidic technologies are beginning to emerge providing the manufacturing engineer an opportunity to participate in the multiscale revolution. μTAS (also known as BioMEMS) are microfluidic systems used for chemical, biological or biochemical manipulation and analysis. These microfluidic devices may incorporate many of the transduction concepts from ICs and MEMS into microchannels or microwells to collect data and information based on assays performed on nanoliters or picoliters of fluid. A popular application of μTAS recently has been DNA analysis-on-a-chip which has contributed significantly to the decoding of the human genome (Weigl et al. 1999). Due to their ability to replace long, arduous chemical assaying procedures, μTAS systems are sometimes referred to as "lab-on-a-chip" technology. In constrast, MECS are microfluidic devices, which rely on highly-paralleled, embedded microchannels for the bulk processing of mass and energy. There are numerous advantages of microfluidic systems that are common to both μTAS and MECS. First of all, microfluidic devices all have high surface area to volume ratios, which shortens the diffusion distances and provides high rates of heat and mass transfer. Secondly, certain conditions such as high pressures can generally be sustained in these systems, which are hard to sustain in macro-scale systems since, in the case of high pressures, the surface areas are reasonably small and the resultant forces are also small. Lastly, the functional parts of the systems all operate with small volumes of fluid. Consequently, these systems permit excellent temperature control with the ability to rapidly mix and quench materials. The major difference in μTAS and MECS technology is the volume of fluid processed. MECS devices are typically based on arrays of microchannels in order to handle much larger volumes of fluid and also typically have more demanding chemical and thermal property requirements. Consequently, the overall size of MECS devices ranges between a few millimeters for portable power systems to over one meter for distributed fuel reforming. Fig. 14.1 shows a small slice of the current and future microfluidic applications that will benefit from nano and microtechnology integration. Already much of the biological revolution has benefited from the accelerated heat and mass transfer available within microchannels. Continued advances in lab-on-a-chip technology are fueling radical innovations in medicine. Similar microtechnology is beginning to show promise as a means for economically producing nanomaterials. Other multiscale trends are toward the decentralised processing of mass and energy. In the future, residential air conditioning will be made "ductless" through the application of many distributed micro heat pumps resulting in large energy savings. Microchannel reactors with nanostructured catalysts will make on-site waste cleanup a reality lending to the realisation of "green" manufacturing. Portable kidney dialysis based on multiscale technology will make life more manageable (and cost effective) for a whole new generation of kidney patients. New fabrication approaches are being developed to advance these technologies. While material processing requirements for MEMS revolve around electromechanical integration leading to IC processing and silicon micromachining, material processing for μTAS generally involves polymer processing due to near ambient device operating conditions. However, many of the devices shown in Fig. 14.1 are MECS devices. Unlike MEMS and μTAS devices, MECS devices require highly-paralleled arrays of microchannels made from more traditional engineering materials. MECS devices are produced by a fabrication approach known as microlamination (Paul et al. 1999). Microlamination consists of the patterning and bonding of thin layers of material, called laminae, to generate monolithic devices with embedded microchannel features. Within this context, a new role for the manufacturing engineer emerges as new methods for economical multiscale fabrication are investigated. Future challenges lie in the breadth of material and dimensional integration required. Compare the mechanical assembly of an airplane with wingspans on the order of tens of meters and electrical subassemblies on the order of millimeters (four orders of magnitude) with the integration of thermal electric generation (TEG) superlattices with resolution on the sub-nanometer scale into a portable power generator with overall system sizes approaching one meter (over nine orders of magnitude). New multiscale fabrication methods emphasise fabrication, assembly and characterisation of more traditional engineering materials including metals, ceramics and selected polymers due to a need for different thermal and chemical properties. Advances in material science are needed to refine grain sizes and provide better compositional homogeneity at the small scale. Further, many of these monolithic, integrated systems are produced by shaping and joining discrete pieces of material-the domain of the manufacturing engineer. This chapter includes the findings from seven year's of study of microlamination processes conducted at the Oregon State University (OSU) Nano/Micro Fabrication (NMF) Facility. In sum, significant progress has been made toward understanding the source and effect of shape variation within microlaminated structures. Shape variation is defined as any change from the specification of the product's design such as rough surfaces, improper alignment or warpage. Other studies have been conducted to understand the economic drivers within microlamination processes. Also, new microlamination methods have been developed based on new material and geometry requirements. Future efforts are needed to overcome the economic and technological challenges of microlamination processes in order for MECS technology to become of substantial industrial and societal benefit. This chapter is organised into three sections. The first section provides a justification and foundation for microlamination and the current research being conducted in multiscale fabrication. The second section consists of findings from specific investigations into technological issues within specific microlamination architectures. The final section explores the economics of MECS device fabrication via microlamination and provides implications for future research and development. © Springer-Verlag Berlin Heidelberg 2006.