Issues in PAT are clearly diverse, and their inclusion in the system design can add substantial complexity to the product development process. Conversely, the exclusion of PAT issues in the development process can render an otherwise good R&D project useless. The definitions in Table 6.3 summarize the diversity of issues that should be considered in the following sections.
The following examples provide a quick overview of a series of progressively more complicated examples of fabrication, packaging (open or closed), and assembly (integrated or manual).
Summary of Diversity of Issues
Figure 6.9 shows a micromachined accelerometer developed by Analog Devices Corporation. The system includes microelectronics, micromechanical flexure-supported masses, sensors, and actuators for self-testing the sensor prior to each use. The microsensor is assembled via integrated techniques, and then packaged in a closed-rigid package. An integrated approach for packaging such systems is also shown in Figure 6.10.
Figure 6.9. Integrated accelerometer - Analog Devices (USA).
Figure 6.10 shows an integrated pressure sensor developed by NOVA Corporation. The sensor includes a flexing diaphragm, with deflection sensed by strain sensors integrated into the chip. Pressure must act directly on the chip or be isolated by a package that itself includes a sealed, flexible member. Assembly techniques involve very sensitive manual techniques with demands on placement and electrical connection. Equivalent systems are produced in Japan, Europe, and the United States using similar approaches.
Figure 6.10. Integrated pressure sensor - NOVA (USA).
Figure 6.11 shows a novel approach to the development of electrostatic microactuation being developed at Tohoku University (Japan). The system uses deforming films in series-parallel combinations in order to achieve desired strength-displacement characteristics. PAT methods for rigidly enclosed systems (for example, optical or null-sensor applications) are available. However, flexible seals for small actuation deflections must be developed that permit low force movement, easy assembly, and low cost. Methods for large excursions, with seals, are nonexistent and will involve significant difficulty in development and manufacture. Figure 6.12 shows a very interesting integrated micropackage for a micromachined resonant element used to detect the deflection of a pressure-measuring diaphragm. The system, developed by Yokogawa (Japan), involves the formation of a beam-type oscillator driven at resonance by integrated surface actuators. The beam system is then sealed in on the pressure diaphragm, and shifts in resonance indicate diaphragm strain. The package is ideal in that important parts are totally isolated and output is easily converted to digital form. Assembly and testing procedures are similar to the system shown in Figure 6.10.
Figure 6.11. Deforming film actuators - Tohoku University (Japan).
Figure 6.12. Micromachined resonant pressure sensor - Yokogawa (Japan).
Figure 6.13 shows a set of microforceps developed at UC Berkeley (United States). The forceps is a system of components that includes a base for mounting and electrical interconnection. The forceps, an integrated extension of a base chip, are actuated electrostatically. The system, which is intended to be a demonstration of concept, will now require a not insignificant focus on procedures for packaging and assembly. In short, the value of such a system and the beauty of the concept are unquestionable -- its practical realization will require additional work and perhaps a totally different fundamental approach due to PAT issues.
Figure 6.13. Microforceps - Berkeley Sensor and Actuator Center (USA).
Figure 6.14 shows the internal and package elements for a rotary displacement transducer (RDT) being developed at the University of Utah (United States). The RDT is physically assembled at open-sealed component level. The housing, with seals and bearings, contains a base chip that contains rotation-sensing arrays of detectors. The shaft holds a circular array of emitter features that drive the detectors. Assembly procedures are direct and require substantial attention to shaft runout and alignment. The package is sealed, but not hermetically, due to the rotary motion required between the input shaft and the sensor housing. Testing procedures involve conventional access methods to the chip, but are unique and difficult (high rotational precision) when evaluating total system performance.
Figure 6.14. Rotary displacement transducer (RDT) - University of Utah (USA).
Figure 6.15 shows the emitter-detector chips for the RDT. The base chip includes arrays (grey-code and vernier) of capacitive detectors and associated circuitry for sensor management and multiplexing information onto a digital bus in coordination with 127 other sensors. The base chip is CMOS with after-processing etching, to produce different planar levels and surface treatments appropriate to form long-life bearings. The base chip interacts with emitter elements on a close-proximity sapphire disk, which is suspended over the base chip by a central rotor shaft, and the integrated bearings on the chip and disk surfaces.
Figure 6.16 shows an electromagnetically driven microvalve being developed at NTT (Japan). The system includes a base over which a magnetizable valving element is suspended by silicon micromachined flexures. Magnetic field interactions actuate the valving element, which modulates a 30-micron orifice. The field is produced externally (sealed), but internal components are exposed to the flow field (open). Packaging requires new types of structures, and assembly procedures are mixed in that they involve both integrated and manual steps. With low gas flows and high band width (up to 100 Hz), testing also requires unique approaches for flow measurement. Fouling, liquid effects, corrosion, particulate intrusion, and other problems will all present unique PAT and use problems for the system.
Figure 6.15. RDT emittor-detector chips - University of Utah (USA).
Figure 6.16. Electromagnetically-driven microvalve - NTT (Japan).
Figure 6.17 shows a very interesting odor sensor being developed by Yokogawa (Japan). The sensor uses a quartz crystal microbalance, driven at resonance, to detect the presence of odorants such as amyl acetate, ethanol, acetic acid, and others. Fabrication includes the semi-integrated assembly of the crystal resonator and a PVC-blended lipid membrane. During use, the membrane is exposed to the medium to be sensed, so that selective uptake-outgo processes cause mass variations that alter the resonance frequency of the system. The packaging approach will be unique, since it requires the isolation of electronically active elements and open exposure to fluids to be sensed. Testing requires conventional chemistry and the pattern-like recognition of the output of five sensor elements. Details of the PVC sensing element were not disclosed.
Figure 6.17. Odor sensor - Yokogawa (Japan).
Figure 6.18 shows another interesting concept for an integrated sensor system that falls in the open-exposed category (under development at the University of Michigan). The system proposes the use of heaters (actuators), temperature sensors, pressure sensors, and thin film gas sensors, all integrated onto a single chip. Due to its integration, packaging and assembly of the system will be straightforward. Its successful, chronic, and direct exposure to fluids-being-sensed will present problems.
Figure 6.19 shows a microciliary motion system being developed at the University of Tokyo (Japan). The system is composed of a large array of individual, thermally actuated microbeams that can act in groups to produce mechanical effects at the surface of the array. Assembly of the system at the chip level is integrated, but if multiple systems are to be used, manual techniques are required. Packaging represents substantial problems if protection of the apparently fragile elements is to be achieved. Testing procedures so far involve optical examination of individual beam performance and the ability of the coordinated arrays to interact with objects in direct contact.
Figure 6.18. Silicon micromachined gas detector - University of Michigan (USA).
Figure 6.19. Microciliary motion system - University of Tokyo (Japan).