Tai-Chang Chen

Micromachined Faraday cup array using deep reactive ion etching

A micromachined Faraday cup array (MFCA) for position sensitive ion detection has been developed using a deep reactive ion etching (DRIE) process. Linear, closely spaced arrays of 64, 128, and 256 cups have been produced with pitches of 250 /spl mu/m and 150 /spl mu/m. Low leakage MOS capacitors formed into DRIE trenches form effective ion collection traps with stable and electrostatically isolated capacitances. These closely spaced arrays of Faraday cups enable a new generation of compact mass spectrometers with true multi-channel detection capability. Since all of the incident ion flux is continuously intercepted by the array, no ion flux is lost as in scanning systems, and the overall sensitivity of the mass spectrometer is drastically improved by a factor approximately equal to the number of cups in the array. The MFCA is thus an ideal component for miniaturized mass spectrometers, ion beam profiling, and chemical analyzers which must work with very small sample volumes or high throughputs.

An electron energy loss spectroscopy (EELS) study of thermal effects in LiF :Mg, Cu, P phosphors

LiF:Mg,Cu,P is a promising new high sensitivity material for use as a solid state radiation dosimeter. However, it suffers from a considerable loss in sensitivity upon heat treatment above 265 °C, for reasons not yet understood. This electron energy loss spectrosopy study of this phosphor indicates the presence of previously undetected inclusion particles in this material. As-received pellets contain small concentrations of inclusions analysed to contain Li, F and O. After heat treatment at 320 °C, the inclusions are found to contain principally Li, P and O. Evidence is also found that the Li-F-O particles may be the precursor to the formation of the Li-P-O particles. Since this heat treatment also causes a 95% loss in thermoluminescent sensitivity in this material, this observation may be important in understanding the cause of this loss in sensitivity, and may also indicate an important role for oxygen in the process. Along with other reactions that occur in this temperature range, these results indicate the importance of defect interactions, which appear to be complex in this system, in determining the mechanisms of luminescence in halide systems.

Microfabricated components for miniaturized chemical analysis systems

The roles of microfabricated components for miniaturized chemical analysis systems are reviewed and the fundamental advantages of these components are illustrated in typical analysis and separation systems, including mass spectrometry, electroanalytical chemistry, capillary electrophoresis, and chromatography. In each instance the scaling laws that affect the resolution of the miniaturized instrument are supported by key enabling micromachined components. Micromachined components are also enabling elements for system integration and for coupling multiple techniques together in parallel or cascade.

Fundamentals of Laser Ablation of the Materials Used in Microfluiducs

Microfluidics falls into an intermediate range within the spectrum of applications for microfabrication techniques. The width and depth of most microfluidic channels fall in the range of 10-1000 μm, and this feature size is thus small for conventional machine tool microfabrication, but quite large for photolithographically defined etching processes of the type used within the microelectronics industry. In addition, most microfluidic channels occupy only ~10% or less of the surface area of a microfluidic device. Wet chemical or plasma etching processes to produce microfluidic devices therefore take considerable time to complete, based upon the comparatively deep depths that are required for the channels. A comparatively fast wet or dry etching rate of 1 μm/min would still require up to several hours per wafer to achieve these depths. The small surface areas that are etched within this time make conventional batch processing of wafers less attractive economically. In many cases, photolithographically defined microfluidic features with micron scale accuracy are more precise than what is required for these applications. At high volumes, other microfabrication processes become more applicable for the manufacture of microfluidics. Roll-to-roll stamping, lamination, hot embossing, and injection molding of plastic components offer excellent accuracy, repeatability, and cost effectiveness once the non-recoverable engineering (NRE) costs of molds, dies, and master templates have been paid for. However, the cost of these NRE items is comparatively high, and in most circumstances, production volumes of >1 million parts are required to recover this cost. For part volumes from 1 to 1 million, laser microfabrication offers an excellent balance between speed, cost, and accuracy for microfluidics. Laser micromachining is also unmatched in the breadth of different of materials that it can process. A single laser system can micromachine materials all the way from lightweight plastics and elastomers up through hard, durable metals and ceramics. This versatility makes laser micromaching extremely attractive for prototyping and development, as well as for small to medium run manufacturing. The most common criticism of laser micromachining is that it is a serial, rather than batch process, and it is therefore too slow to be economical for high volume manufacturing. While certainly true in some instances, as a generalization, this is not always the case. The processing time per part is the sum of the beam exposure time plus the beam positioning time. For parts which require only minimal volumes of material to be removed, serial