© 2002 by CRC Press LLC The shear layer separating from the edge of the delta wing is thin (order of 1 mm for the UCLA/Caltech work) and very sensitive to minute changes in the geometry. Therefore, as discussed earlier in this chapter, the use of microactuators to alter the shear-layer, and ultimately the vortical-structure, characteristics has good potential for success. Furthermore, when the edge of the wing is rounded, rather than sharp, the specific separation point location will vary with the distance from the wing apex, the flow velocity and the position of the wing relative to the flow. Therefore, a distributed sensor/actuator array is needed to cover the area around the edge of the delta wing for detection of the separation line and actuation in the immediate vicinity of it. 35.4.1 Sensing To detect the location of the separation line around the edge of the delta wing, the UCLA/Caltech group utilized an array of MEMS hot-wire shear sensors. The sensors, which are described in detail by Liu et al. (1994), consisted of 2- µ m wide × 80- µ m long polysilicon resistors that were micromachined on top of an evacuated cavity (an SEM view of one of the sensors is provided in Figure 35.3). The vacuum cavity provided thermal isolation against heat conduction to the substrate in order to maximize sensor cooling by the flow. The resulting sensitivity was about 15 mV/Pa, and the frequency response of the sensors was 10 kHz. For more comprehensive coverage of this and other MEMS hot-wire sensors, the reader is referred to Chapter 26. Because of directional ambiguity of hot-wire measurements and the three-dimensionality of the sep- aration line, it was not possible to identify the location of separation from the instantaneous shear-stress values measured by the MEMS sensors. Instead, Lee et al. (1996) defined the location of the separation line as that separating the pressure- and suction-side flows in the vicinity of the edge of the wing. The distinction between the pressure and suction sides was based on the rms level of the wall-shear signal. This was possible, as the unsteady separating flow on the suction side produced a highly fluctuating wall- shear signature in comparison to the more steady attached flow on the pressure side. A typical variation in the rms value of the wall-shear sensor is shown as a function of the position around the leading edge of the wing in Figure 35.4. Note that the position around the edge is expressed in terms of the angle from the bottom side of the edge, as demonstrated by the insert in Figure 35.4. It should also be pointed out that, because the rms is a time-integrated quantity, the detection criterion was primarily useful in identifying the average location of separation. In a more dynamic situation, where, FIGURE 35.3 SEM image of the UCLA/Caltech shear stress sensor. © 2002 by CRC Press LLC The shear layer separating from the edge of the delta wing is thin (order of 1 mm for the UCLA/Caltech work) and very sensitive to minute changes in the geometry. Therefore, as discussed earlier in this chapter, the use of microactuators to alter the shear-layer, and ultimately the vortical-structure, characteristics has good potential for success. Furthermore, when the edge of the wing is rounded, rather than sharp, the specific separation point location will vary with the distance from the wing apex, the flow velocity and the position of the wing relative to the flow. Therefore, a distributed sensor/actuator array is needed to cover the area around the edge of the delta wing for detection of the separation line and actuation in the immediate vicinity of it. 35.4.1 Sensing To detect the location of the separation line around the edge of the delta wing, the UCLA/Caltech group utilized an array of MEMS hot-wire shear sensors. The sensors, which are described in detail by Liu et al. (1994), consisted of 2- µ m wide × 80- µ m long polysilicon resistors that were micromachined on top of an evacuated cavity (an SEM view of one of the sensors is provided in Figure 35.3). The vacuum cavity provided thermal isolation against heat conduction to the substrate in order to maximize sensor cooling by the flow. The resulting sensitivity was about 15 mV/Pa, and the frequency response of the sensors was 10 kHz. For more comprehensive coverage of this and other MEMS hot-wire sensors, the reader is referred to Chapter 26. Because of directional ambiguity of hot-wire measurements and the three-dimensionality of the sep- aration line, it was not possible to identify the location of separation from the instantaneous shear-stress values measured by the MEMS sensors. Instead, Lee et al. (1996) defined the location of the separation line as that separating the pressure- and suction-side flows in the vicinity of the edge of the wing. The distinction between the pressure and suction sides was based on the rms level of the wall-shear signal. This was possible, as the unsteady separating flow on the suction side produced a highly fluctuating wall- shear signature in comparison to the more steady attached flow on the pressure side. A typical variation in the rms value of the wall-shear sensor is shown as a function of the position around the leading edge of the wing in Figure 35.4. Note that the position around the edge is expressed in terms of the angle from the bottom side of the edge, as demonstrated by the insert in Figure 35.4. It should also be pointed out that, because the rms is a time-integrated quantity, the detection criterion was primarily useful in identifying the average location of separation. In a more dynamic situation, where, FIGURE 35.3 SEM image of the UCLA/Caltech shear stress sensor. © 2002 by CRC Press LLC 36 Fabrication Technologies for Nanoelectromechanical Systems 36.1 Introduction 36.2 NEMS-Compatible Processing Techniques Electron Beam Lithography • X-Ray Lithography • Other Parallel Nanoprinting Techniques • Achieving Atomic Resolution 36.3 Fabrication of Nanomachines: The Interface with Biology Inspiration from Biology • Practical Fabrication of Biological Nanotechnology 36.4 Summary Acknowledgments 36.1 Introduction As discussed in previous chapters of this volume, microelectromechanical systems (MEMS) are typically constructed on the micrometer scale, with some thin layers being perhaps in the nanometer range. As has already been demonstrated by microelectronic circuits, the lateral dimensions of MEMS are being pushed into the nanometer range as well. This advance has been dubbed “nanoelectromechanical sys- tems,” or NEMS. The ultimate utility of nanomachining (that is to say, the application of capable robots on the molecular scale to solving a range of problems) is limitless. Such a regime will likely be attainable only by the “bottom-up” approach in which atoms are individually manipulated to construct macromol- ecules or molecular machines. Properties of pure molecules, such as heat conduction, electrical conduc- tion (low power dissipation), speed of performance and strength, without the limits of boundaries to other molecules and resulting materials defects, vastly exceed those of bulk materials. Drexler wrote about molecular machinery that could be modeled after the ultimate existing nanoelec- tromechanical system, the biological cell [Drexler, 1981]. He discussed analogs within the cell for such mechanical devices as cables, solenoids, drive shafts, bearings, etc. Proteins exhibit a remarkable range of functionality, and compared with current MEMS technology, are extremely small. Reasoning that proteins are ideal models upon which to design nanomachines, Drexler envisioned the development of machinery that would allow us to artificially produce such nanoscale mechanical components as those listed above. Imagining complex machinery operating at the molecular scale gives rise to images of Gary H. Bernstein University of Notre Dame Holly V. Goodson University of Notre Dame Gregory L. Snider University of Notre Dame . volume, microelectromechanical systems (MEMS) are typically constructed on the micrometer scale, with some thin layers being perhaps in the nanometer range. As has already been demonstrated by microelectronic. microelectronic circuits, the lateral dimensions of MEMS are being pushed into the nanometer range as well. This advance has been dubbed “nanoelectromechanical sys- tems,” or NEMS. The ultimate. coverage of this and other MEMS hot-wire sensors, the reader is referred to Chapter 26. Because of directional ambiguity of hot-wire measurements and the three-dimensionality of the sep- aration line,