RSS Feed for the unit Microelectronic solutions for digital photography

Introduction

The Open University — Thu, 30 Jun 2011 09:02:22 GMT

This unit demonstrates how matter can be manipulated at the atomic and molecular scale to serve the engineering needs of society for ever-smaller systems acting as intelligent monitors, controllers and micro-environments. The unit will teach you about the directions in which micro and nano technologies are being advanced, particularly in the field of electronic/optical devices.

This unit is an adapted extract from the Open University course Engineering small worlds: micro and nano technologies (T356).

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Learning outcomes

The Open University — Thu, 30 Jun 2011 09:02:22 GMT

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.

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1.1 Conductor–insulator–semiconductor structures

The Open University — Thu, 30 Jun 2011 09:02:22 GMT

A forensic examination of the inside of any silicon chip would reveal a miniature network of metal tracks criss-crossing on several levels, separated by insulating layers of silicon dioxide and periodically stitched down to the underlying tracks and the underlying silicon. Down in the silicon proper there is an intricate pattern of islands of p-type material in pockets of n-type material and vice versa. The precision and regularity of the patterns of different materials tells of a highly sophisticated design comprising the deployment of standard components that are, apparently, configured to bring about specific electronic functions.

We will now look at the triplet formed by a conductor, an insulator and a semiconductor. This can be arranged to provide an electronic switch, which is just the sort of thing that is needed to connect efficiently to individual pixels in an imaging sensor or a display and to address individual locations in an electronic memory. Furthermore, the same structure is as sensitive to light as a p–n junction and it provides the basis of a semiconductor memory. Thus, conductor–insulator–semiconductor structures are central to the story of the digital camera. We begin, therefore, with a closer look at this three-layer sandwich.

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1.2 MOS structures

The Open University — Thu, 30 Jun 2011 09:02:22 GMT

Carefully designed metal–oxide–semiconductor (MOS) structures are a common building block in digital electronics, primarily intended to form part of a transistor-based switch. However, throughout the active regions of a microelectronic chip there will be secondary MOS structures that arise because metal tracks are insulated from the semiconductor substrate by a layer of oxide; equally careful design is necessary to ensure that these do not form part of a switch. The acronym is a mixture of materials classification and materials, but it is so well established that it's too late to argue that conductor–insulator–semiconductor is more generic, so MOS it is.

This text is written presuming that MOS will be pronounced ‘em-oh-ess’. The alternative pronunciation, ‘moss’, is reserved for use within the longer acronym CMOS: ‘see-moss’.

SAQ 1

Note down three important functions that can be performed by MOS structures.

Answer

MOS structures are central to the following functions:

electronic switches;
the detection of light;
electronic memory.

In practice, especially for mass-market consumer products, the semiconductor is silicon; the insulator is then silicon dioxide (see Box 1: Silicon and silicon dioxide). There are various options for the conductor: for several years it was aluminium; then it became expedient to use something more refractory such as heavily doped polycrystalline silicon, deposited from silane; the interconnecting tracks typically involve an exotic intermetallic compound layer followed by copper.

Box 1: Silicon and silicon dioxide

Solid-state electronics has made much use of silicon dioxide and silicon as archetypal insulator and semiconductor. They are excellent partners for the following reasons:

Silicon can be manufactured as wafers of extremely pure, single-crystal material.
Silicon of high purity can also be grown out of the vapour phase, for instance silicon tetrahydride (silane, SiH₄), at elevated temperature.
The conductivity of silicon is readily manipulated locally by the addition of controlled quantities of dopant in combination with photolithography.
The oxide of silicon – namely silicon dioxide, SiO₂ – can be readily grown onto a silicon surface on exposure to oxygen or steam at elevated temperature.
Silicon dioxide can also be grown out of the vapour phase at elevated temperature, so it can be incorporated over surfaces other than silicon.

Table 1 shows some bulk physical properties for three silicon-based materials and, as a comparator, aluminium. Exercise 1 will help you to appreciate the data.

Table 1 Comparisons of silicon, silicon dioxide and aluminium at 300 K

	Si (intrinsic)	Si (heavily doped polycrystalline)	SiO₂	Al
Structure	diamond	diamond	amorphous	FCC
Density / 10³ kg m⁻³	2.3	2.3	2.2	2.7
Dielectric constant	12	12	4	—
Resistivity / Ω m	2.3 × 10³	1 × 10⁻⁵	1 × 10¹⁵	2.7 × 10⁻⁸
Breakdown strength / V m⁻¹	3 × 10⁷	—	1 × 10⁹	—
Melting temperature / °C	1415	1415	1600	660
Energy gap / eV	1.1	1.1	9.0	—
Atom density / 10²⁸ atoms m⁻³	5	5	2.2	6

Exercise 1

Using data in Table 1, say:

(a) how much less conductive than aluminium is heavily doped polycrystalline silicon
(b) how much less conductive than aluminium is pure silicon
(c) how much less resistive than silicon dioxide is pure silicon.

Answer

(a) We want the inverse ratio of resistivities: heavily doped polycrystalline silicon is about 400 times less conductive than aluminium.
(b) We want the inverse ratio of resistivities: pure silicon is about 10¹¹ times less conductive than aluminium.
(c) We want the ratio of resistivities: silicon is 4 × 10¹¹ times less resistive than silicon dioxide.

There are two other things you should know about silicon and aluminium. First, they form a eutectic alloy at 11.7% Si which melts at 577 °C, so don't go thinking that process temperatures can approach 660 °C (see Table 1) once something has been made involving these two elements in contact. In fact, 450 °C is generally recognised as the safe upper limit. Second, aluminium – like gallium, boron and indium – is a well-known dopant capable of rendering silicon a p-type semiconductor.

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1.3 The capacity of an MOS structure to store charge

The Open University — Thu, 30 Jun 2011 09:02:22 GMT

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.

Figure 1 A schematic of an MOS structure (the p-type silicon is shown in pink)

Long description

In Figure 2 the schematic is modified to show what happens in response to positive bias between the M-layer and the S-layer. Initially, the holes in the p-type silicon are repelled by the arrival of positive charge on the M-layer. This is similar to the depletion region that exists at a p-n junction. The repulsion of charge from the depletion region is accomplished by an electric field that is caused to build up under the biased M-layer. Further bias extends the zone from which holes are pushed out.

View larger image

Figure 2 An MOS structure with bias of (a) 1 V; (b) 2 V; (c) 3 V

Long description

To appreciate what happens when yet more bias is applied, it is important to remember that charge carriers in semiconductor material are continually being generated by thermal energy. That is, there is a continual background fizzing of activity as pairs of electrons and holes are generated throughout the silicon. In regions where there are lots of holes, electrons don't survive for long. However, in the region under the positively biased M-layer holes have been pushed out by an electric field, so here electrons live much longer. Not only that but the field sweeps electrons in the opposite direction to the holes, and so they accumulate up against the insulating oxide layer.

The creation and annihilation of electrons and holes goes on all the time in semiconductors. In uniformly doped material, and in the absence of external bias, the local rate of generation equals the rate of loss. It is important to see how these microscopic life cycles of electrons and holes are affected near a biased MOS structure, where an electric field disturbs the local equilibrium. Given time, some kind of steady state is always achieved, with generation and loss in balance overall, but the question is, how long does it take?

Because MOS structures, like p–n junctions, involve depletion regions, they are sometimes said to constitute MOS diodes.

The quantity of the electrons that accumulate at the interface between oxide and semiconductor will establish a steady level as follows. There is a constant trickle of electrons drifting in from the sites across the depletion region where electron-hole pairs are thermally generated. This builds up negative charge that tends to offset the effect of positive charge on the M-layer; that is, it weakens the electric field that maintains the depletion region. In effect, there is a thermally generated current that charges the capacitor formed by the MOS structure. (Because the MOS structure accumulates charge it can also be described as an MOS capacitor.) The more the electrons accumulate, the less effective the field is at keeping back holes. Thus, a few more holes are able to diffuse back in, recombining with, and therefore removing, some of the accumulating electrons. Here, therefore, is a structure that can store charge, much like a capacitor. In fact it is reasonably described as an MOS capacitor, but the MOS structure is effectively a leaky capacitor; see Box 2: Filling a leaking bucket.

Box 2: Filling a leaking bucket

Imagine a bucket that has a hole in the bottom collecting water from a steadily dripping tap (Figure 3). Let's suppose that the leak rate through the hole depends on the depth of water in the bucket. What happens is that the depth of water in the bucket will increase until the level causes the leak rate to equal the filling rate from the dripping tap.

The MOS capacitor behaves in a similar fashion. The bucket is the equivalent of the capacitance. The dripping tap represents the thermally generated electrons. The quantity of water in the bucket then mirrors the stored charge.

Figure 3 Filling a leaking bucket: (a) leak rate less than supply rate; (b) leak rate equal to supply rate

Long description

A balance is struck when holes diffuse back from the p-type silicon outside the depletion region at the same rate that electrons drift in from the depletion region. The time to fill the capacitor can be several seconds, depending on the rate of thermal generation. Figure 4 illustrates the steady-state distribution of charges in the biased MOS structure.

Figure 4 Steady-state distribution of charges in a biased MOS structure (note that details of the thermal generation and back diffusion of holes are not shown)

Long description

While charge is building up at the oxide–silicon interface, electrons are displaced from the metal into the external circuit; that is, current flows in the external circuit. Extra positive charge is thereby left behind on the M-layer. This charging of the M-layer is driven by thermal generation of electron–hole pairs, swept in opposite directions by the field. However, once a steady state is reached the thermal current is shorted internally by the back diffusion of holes, so there is no longer any current in the external circuit. The charge added to the M-layer cannot pass through the insulating O-layer. So, when the M-layer is disconnected from the bias supply, positive charge remains there, locked in place by the combined effects of the depletion region and the accumulation of electrons at the oxide interface.

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1.4 Sensitivity

The Open University — Thu, 30 Jun 2011 09:02:22 GMT

Since a biased MOS structure builds up a depletion region with an electric field similar to that in a p–n junction, it's worth thinking about what would happen if light were to be absorbed in an MOS capacitor, generating electron–hole pairs.

Exercise 2

Suggest what would happen to electron–hole pairs generated by photon absorption within the depletion region of a biased MOS structure that incorporates a p-type semiconductor.

Answer

The electrons will be swept towards the semiconductor–oxide interface and the holes will be swept into the p-type material.

The generation rate of charge carriers follows the standard in terms of the incident optical power, the wavelength and the efficiency of conversion.

Whereas photoconductors and photodiodes generate current in proportion to incident light, the MOS structure automatically integrates photocurrent into accumulated charge. The total accumulated charge after a shutter time is:

The sensitivity in this case, measured in coulombs (C) per watt, is .

Absorption of photons generates a current in a biased MOS structure; see Figure 5. Photocurrent builds up charge in the electron accumulation region in proportion to the number of absorbed photons. Here, then, is another way of translating light intensity into electronic information. Provided it is not ‘over-exposed’, an array of MOS photocapacitors can be charged in proportion to the light intensity to which it is exposed. However, the thermal generation that was discussed in section 1.3 remains active, so the two mechanisms proceed in parallel. To be an effective pixel, the photocurrent must significantly dominate the thermally generated dark current. (To distinguish them from p–n photodiodes, I have chosen to refer to MOS-based light detectors as photocapacitors.)

Figure 5 Generation of photocurrent in a biased MOS structure (note that details of the thermal generation and back diffusion of holes are not shown)

Long description

SAQ 2

The most sensitive light-detecting systems based on MOS capacitors are cooled to the temperature of liquid nitrogen (77 K). Explain why this will improve the sensitivity.

Answer

The dark current introduces a limit to sensitivity, as photo-generation must outstrip thermal generation if significant changes are to be registered. Cooling the detector reduces the thermally generated dark current, so that lower rates of photo-generation can be observed.

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1.5 Photocapacitors as pixels

The Open University — Thu, 30 Jun 2011 09:02:22 GMT

Perhaps you can also imagine an image-capturing array made up of MOS photocapacitor pixels. It's well within the scope of microelectronics to fabricate such an array of virtually identical, isolated MOS structures that would translate the pattern of light intensity falling on it into a pattern of charge. There's more to it than that, though, because:

each capacitor must be simultaneously connected to a power supply that provides the reverse bias
to transfer the information into a stored image requires the read-out of the charge at each pixel.

Working out how to meet these challenges requires a clear target in terms of performance specification.

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2 Specifications for image capture

The Open University — Thu, 30 Jun 2011 09:02:22 GMT

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 imitation of a scene. Because of the way the eye works with colour, the techniques that have been developed reduce the scene to a combination of three primary colours, red, green and blue, by which the eye reconstructs the sensation of full colour. Furthermore, photographic images have always used dots of information that blend into smooth pictures when viewed from the appropriate distance. In film photography the dots are somewhat irregular grains of chemical photoreceptor; in digital photography the dots are arranged in a regular array of pixels. In practice, therefore, photography is all about deception. The trickery begins with a means to capture the pattern of light intensity in a focused image.

SAQ 3

Look back over the story so far and suggest three ways in which patterns of light intensity can be translated by arrays of pixels into electronic information. In each case, identify the basic device and the nature of the primary electronic signal that it registers.

Answer

Three approaches have been identified to translate patterns of light intensity into electronic information using arrays of pixels:

photoconductors: photocurrent
photodiodes: photocurrent
photocapacitors: stored charge.

To specify the scale at which information is to be encoded we need to consider four basic quantities: resolution, sensitivity, contrast and dynamic range.

Resolution relates to the size of dot from which the picture is to be reconstructed. At normal close viewing distance, the eye can discern a little less than two-tenths of a millimetre. So a conventional print photograph (100 mm × 150 mm) should be made up of around 500 pixels × 750 pixels (or dots). Table 2 records some typical digital image formats: you can see that the smallest in the list is close to this. Such a format is not suitable for making larger images, unless the viewing distance is increased. Therefore denser formats are specified to build in some room for enlargement (this amounts to a ‘digital zoom’).

Table 2 Image sizes: uncompressed bitmaps

Image height/ pixels	Image width/ pixels	Total pixels	Colours	Grey scale/ bits	Image size/ megabytes
480	600	288 000	3	8	0.864
600	800	480 000	3	8	1.44
768	1024	786 432	3	8	2.359296
1200	1600	1 920 000	3	8	5.76

Let's work out the pixel size that is required to encode an image at the highest resolution in Table 2 (1600 × 1200). Electronic image sensors are a little smaller than the 35 mm × 24 mm film format, measuring about 24 mm × 16 mm. The ratios do not match exactly and the harder task is to get 1600 pixels into 24 mm: that amounts to 15 μm per pixel. Such a scale represents no real challenge to the semiconductor industry, even when you take into account that really each pixel needs to have three separate sensors sitting behind separate colour filters and a matrix of connections. Of more concern is whether there are enough photons striking such a small area to register an image.

Sensitivity concerns the amount of light that is required to register as a change in intensity. We have already carried out a few calculations about this in this section. There is some interaction here with the optical design team because larger-diameter lenses capture more light.

Contrast is about the tonal range between the brightest and the darkest features. Cameras must have means to adjust the rate at which light enters the system so that image detection is effective: too much and the image bleaches into saturation with no discernible contrast; too little and image contrast is washed out by the speckle of background noise. There are two controls available for a camera. First, there is the size of the aperture, which need not always be as large as the lens system; second, there is the shutter which sets the amount of time for which the image sensor is active.

Dynamic range defines the ratio of the extremes of brightness that are encoded. In digital data it is conventional to base this on powers of two; 256 levels corresponds with eight ‘bits’ or one ‘byte’ of information.

Figure 6 shows an array of pixels in schematic form. We now have an idea of the physical size of the array and the manner in which a pixel might capture its part of an image. This figure also includes some indication of how the various elements might be individually connected to the next stage in the process of recording a digital image, which is the storage of the data.

Figure 6 An addressed array of pixels: each of the twelve dashed lines indicates an as-yet-unspecified electronic control of a switch

Long description

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3.1 Charge-coupled device (CCD) detectors

The Open University — Thu, 30 Jun 2011 09:02:22 GMT

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 polysilicon gate and its underlying silicon dioxide. For visible wavelengths, the absorption coefficient of silicon changes from a little over 10⁵ m⁻¹ for red light to a little under 10⁷ m⁻¹ for blue light. That means for red light passing through 1 µm of silicon the intensity would fall to about 90% [= exp(−10⁵ × 10⁻⁶)] of its initial value. The same path would extinguish blue light altogether. The specification for gate thickness is therefore critical and is considerably less than 1 µm; some designs also have small holes etched in the gates of the blue sensors. Across the entire array, the thickness of gate material and the distribution of holes in the gates of blue sensors must be highly uniform, otherwise different parts of the array will see similar things somewhat differently.

The next point to note is that not all the silicon surface of an imaging chip is available for image capture. This is due to two more important strategies. The first is the electronic read-out from the pixels. The second is an electronic shutter that effectively freezes an image before reading out.

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3.1.1 Read-out

The Open University — Thu, 30 Jun 2011 09:02:22 GMT

The read-out step from a CCD is beautifully choreographed. The basic idea is illustrated in Figure 7. Charge accumulated in an MOS structure can be displaced sideways by using a synchronised sequence of gate voltages, as shown in the figure – rather like passing buckets of water down a line of poorly resourced firefighters.

Data are taken out of a CCD array in an orderly fashion, rippling through each column in turn. To do this requires two more electrodes per pixel, one on each side of the thin-gate photocapacitor, and an additional column of electrodes that forms an output register. These new electrodes are all parts of MOS structures that are ‘blind’. Their role is to shift sideways the charge stored in a line of pixels, so that all rows can be simultaneously displaced, say to the right, by one column. At the right-hand edge the output register column can be simultaneously loaded with data from all rows in a parallel action. Before overwriting with the input from the next column, the output line is marched downwards, one pixel at a time, passing serially through a single amplifier that converts stored charge to a proportional voltage signal that is subsequently digitised and sent on for storing in memory.

One of the performance advantages of a CCD is that data from every pixel are passed through the same amplifier. Two disadvantages are that transfer into memory is a serial process, and that during read-out it is necessary to freeze the image frame to avoid overwriting during the read-out cycle. This is a major factor influencing how quickly successive photos can be taken.

Figure 7 Passing buckets of charge down the line

Long description

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3.1.2 Shutter

The Open University — Thu, 30 Jun 2011 09:02:22 GMT

The electronic shutter that freezes the frame for read-out adds more complexity, but can be based on the standard MOS fabrication steps. In effect, at the end of image capture, the charge at each pixel is first switched into another ‘blind’ MOS capacitor that sits in the read-out line for each row, as the middle of the three buckets per pixel. You should have already guessed that the switch is yet another MOS device. Once switched into the read-out line, the row data are isolated from the photocapacitors and read-out march can begin. A further disadvantage of CCDs is that a large area of silicon surface is given over to data transfer functions at the expense of data capture: for every photocapacitor, one blind switch and three blind charge-transfer MOS structures have to be incorporated.

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3.1.3 Micro lenses

The Open University — Thu, 30 Jun 2011 09:02:22 GMT

A smart way to compensate for surrendering area to data transfer is to build in microscopic lenses at each pixel: the processing sequence that is used to manufacture the MOS devices already involves transparent polymeric material and, calling again on the ingenuity of the designers of microelectronics, significant enhancement to optical efficiency can be won.

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3.1.4 The CCD in practice

The Open University — Thu, 30 Jun 2011 09:02:22 GMT

Figure 8 shows a block diagram of the essential features of a CCD camera's image capture ‘from light to bits’. The full camera includes memory, power supplies, lens and subsystems control, digital and human interfaces, etc.

Figure 8 CCD schematic

Long description

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3.1.5 Digital media for medical X-rays

The Open University — Thu, 30 Jun 2011 09:02:22 GMT

Digital cameras have revolutionised the family album. However, the initial driver for CCD imagers in particular was specialised applications such as the capture of high-quality astronomical images, especially on remote satellites and space probes, from where it is unfeasible to collect film and from where continuous, real-time, high-bandwidth data streams are impractical. The CCD camera system, with its combination of high-sensitivity image capture, high-fidelity read-out and long-term storage, provides an excellent solution for remote astronomy.

Similar criteria emerge when you consider other situations where it would be advantageous to make images that can be stored and relayed electronically. Medical imaging with X-rays, for example, fits this scenario.

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3.2 CMOS detectors

The Open University — Thu, 30 Jun 2011 09:02:22 GMT

The rival technology for CCDs is loosely called CMOS. That's not, at first sight, descriptive of the way the devices function, but it does identify the kind of production line that can be used to make them. You should recall that CMOS is the basis of mass memory. The commercial convenience of being able to produce imaging chips on a memory line has played a part in opening up the market for digital imaging, in cameras, mobile phones, remote monitoring systems, etc.

The image capture array is essentially the same as in CCDs, but instead of marching the data out through a single amplifier, CMOS amplifiers are built into each pixel, completing the conversion from charge to voltage locally. A CMOS amplifier is made from MOS transistors and so their inclusion is entirely compatible with the processing technology required for the light-capture components. However, a significant fraction of array space is given over to this local data handling, so again microlenses are incorporated to compensate for the loss of capture area. Figure 9 shows a diagram of a CMOS imager.

Figure 9 CMOS imager schematic

Long description

A disadvantage for the CMOS array is that each pixel uses a different charge-to-voltage amplifier, so there is additional statistical variation between pixels, as compared with CCDs. On the other hand, data can be drawn out of the array in parallel: a CMOS array can be interrogated row by row, as discussed in connection with CMOS memory. So, there is a trade-off between quality and speed.

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Acknowledgements

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Grateful acknowledgement is made to the following sources:

Figures

Figure 7 Adapted from In Smith, S.W. (1997), The Scientist and Engineer's Guide to Digital Signal Processing, (2nd edn), California Technical Publishing.

Course team acknowledgements

Block 2 Optoelectronic devices for digital pictures was prepared for the course team by Professor Nicholas Braithwaite.

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