3.7 The role of technology Technology is an increasingly important, if not dominant, element in operations. It was included as one of the resource inputs in the Figure 2 transformation model developed earlier, but treating it in this way potentially underestimates its role. Technology is often an important element of t
3.4 Managing across interfaces Increasingly, operations management is seen as an interface discipline (Voss, 1995). Managing across interfaces, both internal and external to the organisation, is a particular challenge for managers and this is discussed further in this section. Information and communications technology is an important means of linking across the various interfaces. Author(s):
7.4.6 Molecular beam epitaxy (MBE) Where a suitable ALD chemistry cannot be found, or where cleanliness and high crystallinity are required, molecular beam epitaxy may be necessary. This is more akin to evaporation than to CVD, with multiple molecular beams of the separate chemical constituents each focused onto the hot wafer surface. Deposition is performed under extreme vacuum conditions (10−11 mbar) to prevent any contaminants from being incorporated, and the substrate must present a perfect cleaved crystal fac
7.4.1 Spin-on For a thick, quick, blanket coating of material, it is sometimes possible just to pour it over the surface! The technique is familiar from photolithography, where photoresist-polymer precursors are dissolved in a solvent. The wafer is spun at high speed (up to several thousand rpm) and a droplet is dripped onto its centre. This is thrown outwards, coating the surface instantly. The wafer is then baked to drive off the solvents and leave a solid film, perhaps 1 mm thick. This spin-on technique
7.2 Film properties In practice, we can hardly ever use just the fastest technique to put some material down onto the wafer. Before deciding how to deposit a particular layer, we must consider which film properties are important for the function of the device. The commonest requirements relate to uniformity, step coverage, composition, micro structure and stress. We shall consider each of these in detail.
7.1 Introduction Micro fabrication often involves multiple layers of different materials, each following a sequence of process steps, usually deposition/lithography/etching. The microelectronics industry has been a driver for many of these techniques and that momentum has carried over into the MEMS community. The treatment that follows covers examples from both industries. The appropriate deposition technique depends on the purpose of the layer – a thin insulating barrier layer in a transistor or capa
3.1 Charge-coupled device (CCD) detectors The first digital imagers to be developed were called charge-coupled devices, because of the way in which accumulated charges are passed along rows in order to read the contents of each element of an array. A CCD array has at each pixel an MOS photocapacitor. Potential on a gate electrode holds the accumulating, photo-generated charges in place during the exposure interval. The first thing to appreciate is that the photons that generate the electron–hole pairs have to penetrate the po
2 Specifications for image capture This section is about the performance specification for a captured image. Cameras do not attempt to copy the way in which light signals are detected and processed by nerve cells in the eye – the latter is based on quite different principles from those we have seen here, using changes in molecular shape within proteins to detect light and initiate a response. Photography began as a means of capturing images that the human eye would have seen. That task involves obtaining full-colour i
1.3 The capacity of an MOS structure to store charge
Figure 1 shows a schematic section through an MOS structure and sets up a colour scheme that distinguishes the different layers. In this case the M-layer is provided by heavily doped polysilicon and the semiconductor base material is p-type silicon. Learning outcomes After you have completed this unit you should be able to: describe how to use metal–oxide–semiconductor (MOS) structures for light capture, switches and latches; distinguish between CMOS and CCD strategies for image capture. 5.15 Further investigation is possible There are still many mysteries that surround the Tay Bridge disaster, largely because so little was recorded at the time of construction. For instance, questions remain about the details of reject rates for the castings, and modifications made to the first designs of the piers and their component parts. Although enlargement of the BoT set of pictures has helped clarify the various failure modes described by Henry Law and others at the enquiry, it has also revealed yet more mysteries. Wh 5.5 Evidence of Henry Law Henry Law's report is brief and to the point, and includes a substantial appendix giving detailed calculations of the effects of wind pressure on the structure (not included in Paper 1). Further information on his inspection of the remains – the two standing piers, the twelve wrecked piers the high girders and the train within – was given during his testimony before the enquiry. Law was able to examine the extant remains in considerable detail, and noticed numerous defects in the br Bridge oscillations Testimony was taken from the many workers employed during construction and painting of the structure just after completion. Their evidence was more compelling, especially from painters working at the top of the high girders piers during passage of trains, as well as during windy weather. They were painting the cast iron of the piers during the summer of 1879. In the main, they reported feeling strong sideways as well as vertical motion: 4.11 Model for pier failure Figure 39 shows a simple model to explain the failure of the piers. The lateral wind loading on the top of the pier bends to shear the pier from a rectangle into a parallelogram. In turn, this stretches the tie bars and also strains the bolted joints at the top and bottom of each column. 4.10 High girders and the train Divers found the high girders lying on their sides in the shallow water of the river bed a short distance away (Figure 22), within which the almost intact remains of the train itself was found. No bodies were recovered because they had all been washed away by the river or tide. Although bodies were recovered i 4.9 Survey results We have inspected some of the remains of the collapsed bridge using the set of photographs taken shortly after the disaster for the official inquiry. It is important to emphasise that the pictures form only a small part of the total of fifty, but those chosen were selected to give the clearest evidence of the failure modes in the cast-iron piers that supported the high girders. They are by the far the best real evidence to rely on to understand how and why the structure failed. It would 4.7 Photographs showing the detail: damage to pier 1 The final example of a partly collapsed pier is pier 1, photographed from the base of pier 28 and shown in Figure 32. Fracture damage to the flange at the top of the second tier is visible on the east-most column (top right); a large chunk of metal has broken off. The southern column (right of centre) exhibits 4.6 Photographs showing the detail: debris field The relatively clear platform of pier 3 is also visible in Figure 28. There are several pieces of shaped metal on the floor, at least one appears to be a bolt. A close-up of the floor, taken looking east, is shown in Author(s): 4.5 Photographs showing the detail: broken lugs The bases of the columns to which they were attached originally on pier 3 deserve closer inspection. Even at this scale, the two fractured lugs where the tie bars were formerly fixed are clearly visible at the right-hand and left-hand sides of Figure 28 (arrowed). The southern (left-hand) column base in 4.2 An introduction to the Board of Trade photographs It is important to bear in mind that these shots show the bridge remains in the state they were in just after the accident, and are almost exactly what the investigators would have seen when they inspected the bridge (see Input 8, linked below). Click 'View document' below to open Input 8
