Advanced Microfabrication Research Articles

Microfabrication Technology for Vascularized Tissue Engineering

This work describes the application of advanced microfabrication technologies including silicon micromachining and polymer replica molding towards the field of tissue engineering of complex tissues and organs. As a general approach, tissue engineering of skin, bone and cartilage using cell transplantation on biodegradable matrices has achieved great success. However, such techniques encounter difficulties when applied to complex tissues and vital organs. The principal limitation for such applications is the lack of an intrinsic blood supply for the tissue engineered organ, which experiences significant cell death when the tissue thickness is increased above the 1–2 mm range. In this work, the concept of microfabricated scaffolds is introduced, with the goal of producing organ templates with feature resolution of 1 micron, well in excess of that necessary to fashion the capillaries which comprise the microcirculation of the organ. Initial efforts have resulted in high resolution biocompatible polymer scaffolds produced by replica molding from silicon micromachined template wafers. These scaffolds have been successfully seeded with endothelial cells in channels with dimensions as small as the capillaries.

Fabrication and characterization of a nanogap edge emitter with a silicon-on-insulator wafer

Si thin-film edge emitters with various gaps ranging from 15 nm to 55 nm were fabricated and characterized. Smooth, straight nanometer-scale gaps were made on a silicon-on-insulator wafer by using advanced microfabrication technology based on electron beam lithography, dry etching, and thermal oxidation. Electron emission occurred from voltage around 60 V and reached 100 nA at 70 V. The emission characteristics show no clear dependence on the gap distance. Measurements were also done in air to evaluate current leakage through nanogaps. Fabrication, structure, and emission characteristics are described in detail.

Applications of microfabrication technology to x-ray laser cavities

We have fabricated new, efficient x-ray optical components (x-ray mirrors, beamsplitters, and highly dispersive multilayer mirrors) and have used them to build an x-ray laser cavity. With our laser cavity, we have demonstrated multipass amplification of soft x rays with significant enhancement (>20×) of the x-ray laser emission. This is an important first step in the ultimate conversion of x-ray amplifier media into true x-ray lasers with optimized coherence and divergence properties. We used advanced microfabrication techniques to fabricate the key laser cavity components which require precise layering of alternating materials, thin-film membrane technology, and precise patterning. The x-ray laser requirements, fabrication details, and characterization results for these cavity components are discussed.

Theory of coupling of electronic systems: Experimental structures using advanced lithographic techniques

This paper focuses on the physics of disorder-induced localization and the important role of advanced material microfabrication technology for realizing novel microstructures as physical models. Present and future microfabrication capabilities for tailoring materials in the submicron and ultrasubmicron range offer the opportunity to fabricate controllable physical models, approaching atomic scale, for the fundamental study of coupling of electronic systems and the effect of dimensionality on transport properties. These physical models will require the ingenious exploitation of advanced lithographic/patterning techniques for controlling lateral dimensions, particularly if this is combined with advanced vapor or molecular beam deposition technique for controlling vertical dimensions. The type of studies discussed here have great significance in understanding and designing highly-dense very large-scale/ultra large-scale integrated (VLSI/ULSI) circuits and interconnecting-lines-dominated neural network IC systems, as well as in the studies of the scaling theories of localization. It is expected that these studies will lead to the design of novel device concepts.

Quantum transport in microstructures

Advanced microfabrication techniques allow production of experimental systems as narrow as 20 nm for study of electron transport at low temperatures, ~1K. Fine wires in metals, Si-MOS inversion layers, and GaAs modulation-doped heterostructure layers have been studied. Metal rings of diameter ~1 μm have also been studied. New electron interference effects have been discovered in these systems, due to electron partial waves which scatter elastically from impurities. The length scale over which these interference effects occur is the phase coherence length, ¢ φ, of order 1 μm or greater at 1K. In this paper the physical mechanisms and manifestations of these electron interference effects are reviewed. The microfabrication methods used to produce these ultrasmall structures are also reviewed. Other quantum transport issues, including inelastic scattering mechanisms, detection of single trap states, and wavefunction spatial quantization, are also discussed.