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 4.1 Condition of the bridge: an overview An investigation was put into motion by the Board of Trade (BoT) as soon as news of the catastrophe reached London. Three commissioners were appointed to consider the evidence. They proceeded at their task with haste, knowing the country looked to them for an explanation of the accident. Fifty photographs were taken of the remains of the bridge about a week after the collapse at the request of the BoT enquiry team. The photographs are vital evidence of the way the bridge piers fai 3.5 Sunday 28 December 1879 The morning of Sunday 28 December 1879 was quiet. When Captain Wright took his ferry boat, the Dundee, across the firth at 1.15 pm, he noted that the weather was good and the water was calm. The 4.15 pm crossing was just as uneventful, but the captain noted that the wind had freshened. By 5.15 pm a gale was moving in from the west and the river, in the words of the captain ‘was getting up very fast’. The local shuttle train left Newport at 5.50 pm and arrived at Dundee statio Bridge girders Figures 11 and 12, below, are photographs of the bridge taken from the south and north banks of the firth. The girders of the bridge were supported on a total of 85 piers. The first 14 piers were made from brick and masonry, built up as a solid structure. The rest were fabricated from iron on masonry platforms, and by comparison, appeared rather insubstantial (Author(s): 3.1 Overview The Tay Bridge disaster came towards the end of a period of intense development of the railway system in the UK. The bridge had materials that were well known. Cast iron was used for the columns and wrought iron for the trussed girders. The construction of the bridge was, at the time, the largest single engineering project in Britain, the Tay estuary being about two miles wide near Dundee, and the bridge was the longest in the world. In the shallower approaches in the estuary, con 2.4 Early disasters Many of the earliest bridges were simply a wooden trestle type of construction, an efficient and easy-to-build structure, yet providing a secure and safe passage for heavy metal trains. Although we tend to associate such structures with the United States, they were in fact widely used in Britain in the early days of steam locomotion. However, they had a limited lifetime owing to rot, so were gradually replaced by wrought iron girder bridges, often laid on brick or masonry piers. Designe
