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Self-scanned photo-diode arrays — characteristics and applications
Self-scanned photo-diode arrays characteristics and applications M. A. VANN A general review of the properties of self-scanned photo-diode arrays is given. The mechanisms of charge storage, scanning, and signal processing are described and this is followed by a discussion of the electrical, optical, and noise characteristics of the devices. The final section presents a variety of current industrial applications.
Solid state image-forming devices have been newsworthy for some years. The possibility that solid state technology could yield a compact, light-weight, fast, low voltage, low power, stable, long-life image sensor has spurred considerable research and development effort, largely because the above characteristics are not those generally found in traditional electronic imaging devices. In the process, several new technologies have been developed or adapted; and at this interim point, practical devices are on the market whilst the quest continues for a solid state sensor that will deliver full broadcast-television image quality. The development of the new breeds of image sensor has been made possible by continuing progress in silicon integrated circuit technology. The high packing densities and geometric fidelity exhibited by integrated circuit devices led inevitably to consideration of how photo-sensitive array structures might be fabricated with these same techniques: a one or two-dimensional light intensity distribution might then be sampled on a point-by-point basis and the information extracted in a usable form. The virtues of silicon in this respect are that it allows the construction of p-n junction photo-diodes and photo-transistors which are compatible with a variety of active circuit devices necessary for signal processing, the whole assembly being fabricated on a single silicon chip. Scanning methods Around the middle 1960s several techniques fundamental to the successful operation of solid state image sensors emerged. With a very small number of sampling points it is feasible to consider a sensor in which every element is available for interrogation on a 100% time basis. Such ‘parallel output’ arrays have been made but present impossible accessing problems long before they reach a practically useful size. The obvious solution is to adopt some form of sequential scan-
The author is with integrated Photomatrix Ltd. The Grove Trading Estate, Dorchester, Dorset, UK. Received 16 May 1974.
OPTICS AND LASER TECHNOLOGY.
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1974
ning of the individual array elements so that a spatially varying light intensity distribution is converted to a time-varying electrical signal. The source and analbgue for this approach is the traditional image tube such as the vidicon, in which an optical image is converted to a pattern of charge on a photoconductive layer and sequentially sampled by a swept electron beam. The scanning beam restores the layer potential at each point and the resulting beam current modulation constitutes the time-varying video signal. The equivalent procedure in a solid state image sensor is to connect the output of each photo-sensitive element in turn to a common line by means of suitable seque!ltially operated switches. Chargestorage The majority of scanning techniques have been devised for arrays operating in what is known as the ‘integrating’ or ‘charge storage’ mode. This is a method of overcoming the inherent low sensitivity of an array in which each element is sampled only for its instantaneous photo-current. Parallel output arrays operate in the instantaneous mode, as do most other photo-detectors, but this leads to great inefficiency in scanned arrays where the time spent sampling one element is very small compared with the time taken to scan a complete image. Analogy with the vidicon tube again provides the clue. The charge level at any point on the tube’s photo-conductive layer builds up continuously from the time it is sampled until the scanning electron beam next returns to the same point, when it is almost instantaneously discharged. The output signal is therefore proportional to the time integral of intensity at that point. A similar procedure is followed in the operation of scanned arrays and the principles of charge storage and signal multiplexing will be examined with reference to arrays fabricated using silicon planar photo-diodes and metal oxide semi-conductor (MOS) technology. MOS transistors possess very favourable characteristics as switching elements when integrated with planar photo-diodes, and the commercial photo-diode arrays currently available all use this approach.
209
MOS photo-diode
x register
arrays
m+l
m
The method of implementing the charge-storage technique in these arrays is to utilize the self-capacitance of each p-n junction diode element. When the diode is reversebiased and then left open circuit, charge is stored on the depletion layer capacitance. This charge decays only slowly in the dark because of recombination with thermally generated electron/hole pairs. When light falls on the junction, electron/hole pairs are created at a rate proportional to the incident illuminance and the junction capacitance discharges at a corresponding rate. In a fixed interval of time, therefore, the amount of stored charge removed is proportional to the total incident flux during that interval. The point-to-point signal across the array consists of the amount of charge lost by each individual diode. It thus can be sampled either by restoring the initial condition and monitoring the quantity of charge required to do so (recharge sampling) or by looking at the actual level to which the voltage across each diode has decayed (voltage sampling). The time between samples, called the integration period, is the linescan time for linear arrays or the framescan time for area arrays. The sequential switching process which is necessary for interrogating the diodes is achieved universally by the use of shift registers. A pulse appearing at one end of a shift register can be clocked through at high speed, opening and closing diode sampling switches as it goes. As an illustration, one form of array structure will be described to demonstrate the application of the general principles. Fig. 1 shows three elements of a linear array. Each element consists of a photo-diode D and three MOS transistors. In operation a pulse is propagated through an MOS shift register on the chip to appear sequentially at lines n, n+l, nt2, which are connected to the gates of the MOS switches Tr . When these gates are driven negative the switches turn on and the photo-diode capacitances are charged to a negative potential. After the pulse has passed, the switches turn off again and the diodes begin to discharge at a rate dependent on the incident illuminance. The pulse continues to propagate through the array and eventually, after one integration period, reappears at line II. At this point it turns on MOS switch Ta and allows current to flow through T2, Ta, and the external load resistor. The magnitude of this current is controlled by the voltage on the gate of the amplifJing transistor T2, this voltage being dependent on the degree
Substrate --__
-
+ve ---_-Load resistor
Video eput Substrate ____
----
Q+
%
-----a--
Fig.1
210
n+l
Three elements of a typical
/-I+2
linear self-scanned
array
I / I I
I I
Fig.2 array
Four elements of a simple two-dimensional
self-scanned
of discharge of the photo-diode. Having sampled the voltage on the diode, the pulse moves on to n+ 1 where it recharges diode n as before and at the same time samples the voltage on diode tit 1. The above outlines the voltage-sampling mode of scanning photo-diode arrays. In recharge sampling, transistors Tz and Ts are not used and the video signal is derived from the magnitude of the current pulses required to recharge the photo-diodes when switches Tr are closed. The relative merits of the two methods are discussed later. Two-dimensional
arrays
This description of the operation of a linear array is basically valid for a two-dimensional array. The extra complication of x-y scanning is dealt with by having two shift registers on the chip, one for rows (y) and the other for columns (x). Fig.2 shows the simplest possible element structure for an area array. In the recharge sampling mode the x-shift register switching lines are connected to all the MOS switch gates in their respective columns. Similarly the y-shift register lines are connected to all the MOS switch drains in their respective rows. A pulse propagated through the ‘slow’ x register will switch whole columns in sequence but the only diode connected to the video output at any instant will be in the row activated by the ‘fast’ y register. When a column has been scanned out iny, the x register moves along one line and the next column is scanned, giving rise to a video output in the form of the familiar television raster. Alternative
n
t
\
technologies
The structures described above are typical of those which have proved most successful in the development of commercially feasible devices. Other monolithic structures have been devised - and some realized experimentally - using various combinations of diodes and transistors. (Reference 1 gives a general survey.) Somewhat aside from the mainstream, thin film and hybrid thin film/silicon techniques have been used * in the fabrication of arrays. More recently, charge-coupled and charge-injection devices ahave appeared on the scene as contenders for large area arrays. 3
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It is not intended to make critical comparison between the various types of self-scanned arrays which are, or are becoming, available for general use. This has been attempted by others 4 but in this author’s opinion the technologies have not sufficiently settled, nor is there yet an adequate body of application experience, for long term judgements to be attempted. In what follows, the characteristics and operating parameters of photo-diode arrays are reviewed. The final section provides a general survey of current applications to a variety of sensing and measurement problems.
Performance characteristics Geometry
Photo-lithography, as developed for integrated circuit technology, allows the fabrication of photo-diode arrays that have element size and pitch small enough to be comparable with electron beam scanned tubes. Diode pitches are in the range 0.025 mm (0.001 in) to 0.125 mm (0.005 in). Diode shapes are generally nearly square but this can vary depending on the application or on chip layout constraints. For example, in a linear array intended for sampling a onedimensional intensity distribution it is feasible to make each diode one or two orders of magnitude wider than the basic array pitch without affecting the inherent resolution; a proportionately larger light collecting area is thereby achieved. With linear arrays it is possible to make the diode length a large fraction of the element pitch (up to 90%) since the scanning circuitry can be placed on either side of the row of diodes. This is not possible with two-dimensional arrays,
Fig.4 0.075
Enlarged portion mm pitch
of a two-dimensional
array with diodes on
where each resolution element must contain the active components associated with the diode, plus the interconnection pattern. Here the effective sensing area falls to a region 20-50% of the element size. Figs 3 and 4 show sections of typical one and two-dimensional arrays. Linear arrays are available in a wide range of lengths, vary ing from 16 to 1 024 elements depending on the manufacturer. Area arrays currently vary between 32 x 32 and 64 x 64 elements. The smaller linear arrays can be obtained in TO5 cans but the customary mounting is a multipin DIL package with glass or quartz window. Figs 5 and 6 show the packaged versions of the arrays illustrated in Figs 3 and 4. Peripheral drive and output circuitry
Self-scanned arrays are not completely self-contained entities. Clock pulses and scan-start pulses must be generated externally with the correct phases to drive the shift registers. The array output may be accessed directly, but can usefully be subjected to additional signal processing to improve performance. Two methods of obtaining a video output have been mentioned previously: voltage sampling and recharge sampling. Some arrays offer only one of the two possibilities, others can be operated in either mode.
Fig.3 pitch
Enlarged portion
of a linear array with diodes on 0.1 mm
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In voltage sampling the video signal is derived from the actual voltage on each diode at the end of an integration period. The voltage decay is proportional to the total incident flux on the diode and for a stationary image is therefore proportional to the instantaneous light intensity. The resultant video waveform appears directly across an external load resistor. No interface circuity is required, except perhaps a buffer stage emitter follower to avoid loading the array. However, voltage sampling is limited in operational
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speed because the transitional rise and fall times from one diode to the next are dependent on the size of the external load resistor and can easily become a large fraction of the sampling time. In addition noise levels are greater for reasons which are discussed in the section ‘Noise considerations’.
Fig.5
Packaged 128 element
linear array: array length 13.0 mm
The preferred mode of operation is recharge sampling, although sophisticated signal processing is required to fully exploit the technique. With recharge sampling the video output consists of a train of rapid current pulses on the supply line. It is possible to amplify these and display them as a series of spikes, where the intensity distribution across the array is then given by the locus of the peak amplitudes. However, there is a tendency for the signal to be submerged in multiplexing noise and a more satisfactory visual appearance coupled with a great reduction in noise level is obtained by processing the output to obtain a ‘box-car’ waveform similar to that resulting from voltage sampling. This is done by integrating each current pulse for a one bit period, and sampling and holding the integrator level until it is reset by the next integrated pulse. Fig.7 shows a section of a recharge-sampled waveform after processing. The highest multiplexing rates and lowest noise levels are obtainable using this technique.
Dynamic range
The operation of arrays in the charge storage mode allows a wide dynamic range in the same sense as that of ordinary photography, where a compensation for a low light level is obtained by increasing the exposure time. The analogy with photographic film is quite close, in that both sensors have a saturation level, a fog level, and give a specific signal for a specific exposure where that exposure can be arrived at by varying combinations of intensity and time. Unlike film, array outputs are linear with exposure and do not suffer from reciprocity failure, but they offer a more restricted range of time/intensity combinations.
Fig.6
Packaged 64 x 64 element area array; array size 4.9 x 4.9 mm
Fig.7 Appearance of a typical recharge-sampled video output signal after processing. (a linescan across the bar target)
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The absolute limits of operation are set at high irradiance levels by the maximum multiplexing rate of the array, and at low irradiance by the rate of self-decay of diode charge. Each diode can store only a limited quantity of charge and if the incident irradiance is sufficiently high to completely discharge the diode in any given integratioir period, saturation level is reached. When this occurs for the minimum obtainable integration period (maximum scan rate) there is absolute saturation. At the other end of the scale, selfdecay of charge is apparent because of recombination of thermally generated hole/electron pairs with the stored charge. This is known as thermal leakage, or dark current. With a low-intensity image and increasing integration time, part of the observed video signal results from this leakage current until, in the limit, saturation level can be reached through dark current alone. Cooling the array below ambient can extend the dynamic range since leakage currant falls by approximately a factor of two for every 10°C drop in temperature. Between these two extremes there is a linear operating range of three to six orders of magnitude, according to array length and diode size. The dynamic range is greatest with the short arrays, since the maximum multiplexing rate is fixed irrespective of the number of diodes and therefore the minimum integration time falls with decreasing array length. For any fixed integration period the
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dynamic range is typically 100: 1 in recharge sampling, governed by the ratio of noise level to saturation. Linear arrays can have their dynamic range increased by the addition of ‘padding capacitance’. The diode area is increased in a direction normal to the array but most of it is covered with an opaque aluminium layer. This provides additional self-capacitance and charge storage capability without enlarging the photo-sensitive area or seriously increasing the leakage current. Maximum multiplexing rates for currently available arrays are in the region of 4- 10 MHz and maximum integration times range from 2-80 ms. The dynamic range of two-dimensional arrays operating in the frame storage mode is inevitably reduced in comparison with linear arrays because of the greatly increased number of photo-diodes. For example, a 64 x 64 element array contains 4 096 diodes and at a multiplex rate of 4 MHz the integration time is approximately 1 ms; this corresponds to a frame rate of 1 000 frames s-l. It should be compared with a 256 element linear array running at the same speed, where the integration time is 64 ps.
Table 1. Spectral conversion factors for silicon detectors. To obtain equivalent watts of 2 870 K tungsten, multiply source watts by column A factor or source lumens by column B factor
Source
conversion factor
B Lumens conversion factor
2 500 K tungsten
0.7
0.07
2 870 K tungsten
1.0
0.05
3 200 K tungsten
1.3
0.04
1 000°C blacK body
0.015
1.6
Sunlight (sea level)
1.9
0.016
White fluorescent tube
2.4
0.007
Low pressure sodium lamp
2.6
0.005
GaAs/P solid state lamp
3.2
0.04
GaAs solid state lamp
3.4
only emits in infra-red
He-Ne laser
3.0
0.02
A
Watts
Responsivity
Since self-scanned arrays operate in the integration mode, their response to incident radiation is expressed in terms of output signal per unit energy input (or more usually, energy density). The output of an individual diode is either a voltage, or a charge in picocoulombs. The input power is customarily given as an irradiance in /JW cm-* from a source of stated spectral distribution. Responsivities are therefore quoted in units of V s-1 PW-1 cm* or pC s-r pW-l cm*. Sometimes the latter is expressed in the equivalent form of pA E.IW-’cm*. The magnitude of the video signal from an array is very much dependent on the method of subsequent processing and the calculation of operating levels is usually performed relative to a saturation signal taken as 100%. Thus, calculation of the signal that will be obtained in any particular situation involves manipulating three parameters - multiplex rate, array length, and irradiance level. If the array contains n diodes and the scan rate is to be f (Hz)the integration time for each diode is n/f seconds. For a responsivity quoted as R (PC s-r pW-i cm*) and an irradiance of W (yW cm-*) the resultant signal charge will be: R n
w/f[PC]
Manufacturers data will give saturation charge along with R, from which the signal level can be estimated as a percentage of saturation. A similar calculation can be performed in the voltage sampling mode. In absolute terms, the video output signal at saturation is in the I- 10 V range after processing. The responsivity R is invariably quoted radiometrically for a tungsten lamp running at a colour temperature of 2 870 K. For this source, typical values of R vary from 10 pC s-l pW_’ cm* for diodes on 0.1 mm pitch to 0.7 pC s-l /JW-’ cm* for diodes on 0.025 mm pitch. To deduce the responsivity to other sources or to obtain the equivalent photometric quantities has always proved a headache with spectrally selective detectors such as silicon, since it involves numerical integration using the source and detector spectral curves.5 Table 1 presents a selection of conversion factors for various common light sources when they are used with a typical silicon detector. The numerical values
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P20 phosphor
1.6
0.006
Xenon arc lamp (1 600 W)
2.3
0.03
Xenon arc lamp (150 W)
2.4
0.10
are used as multiplying factors, applied either to the watts radiated by the source (column A) or the lumens (column B), the product in each case being the equivalent watts of 2 870 K tungsten. Noise considerations
Photo-diode arrays are subject to a number of noise sources not found in other forms of detector. These are generally grouped together under the heading ‘fixed pattern noise’ (FPN), so called because it is stationary with respect to the array, and it arises from the discrete nature of the sensing elements and the sampling methods used to extract the video signal6 Each element of the array has associated with it several parameters which exhibit small deviations from nominal value due to the variability of materials and processing. Since the functioning of integrated circuit devices is essentially geometry-dependant, photo-lithographic errors and mask misalignment are one source of variations. Additionally, device parameters are affected by any inhomogeneity of the monocrystalline silicon substrate material. The result of these small variations is twofold: 1. There is a corresponding variation in the desired output signal (apparent responsivity fluctuations from diode to diode). 2. A random contribution is added to the switching transients which appear on the video output. The second of these needs further explanation. Interrogation of the diodes requires sampling pulses whose magnitudes are considerably larger than the signal to be sampled. In-
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evitably these pulses break through on to the video output because of parasitic capacitative coupling between the multiplexing switches and the output line. For example, in Fig.1 the gate-drain capacitances of switches Tr transfer charge spikes to the negative rail at the leading and trailing edges of each clock pulse. Random variations occur in the amplitudes of these spikes, dependent on the degree of coupling at each switch. It is possible to remove most of the pulse breakthrough by low pass filtering of the video signal since the spikes appear at twice the multiplexing rate; however, the random variations occur at lower frequencies and are not susceptible to filtering. The ‘integration plus sample-and-hold’ technique will also remove the switching spikes because, being alternately of opposite sense, they integrate to zero in any given bit period. Whether or not this procedure removes the random variations as well depends on the duration of a sample pulse in one bit period. At the slower scan rates the clock pulse width may be only half the sample period, confining the spikes to one cycle of the integrator and producing complete cancellation. At high scan rates rise and fall times of the sampling pulse become significant and the negative-going spike from one bit may overlap and combine with the positive-going spike of the following bit. If these are not equal, because of random variations, complete cancellation will not occur. In voltage sampling the most significant contribution to fixed pattern noise arises from variations in the threshold voltages of the amplifying transistors (T, in Fig.1) and the general noise level tends to be higher than in recharge sampling. The effect of these various sources of FPN is to produce an output video waveform which exhibits a stationary noise structure even when the array is uniformly illuminated (or in the dark). Noise arising from switching spikes is signal independent, observable under dark conditions, and is typically f 0.5% of the saturation level in recharge sampling or + 3% in voltage sampling. Noise arising from those sources which produce apparent responsivity variations is signal dependent and has typical values off 5% of signal at 50% saturation. These two distinct forms of noise are commonly quoted separately in specifications. Other sources of noise in the video processing system are generally negligible in comparison with the noise from the array itself.
which arrays can operate. It is not fundamentally a noise mechanism since the primary effect is to progressively decrease the available signal swing - or video window - with increasing integration time. However, noise is associated with it: this arises from random variations in leakage curren from diode to diode. Leakage current is a function of diode geometry, bias voltage, the bulk properties of silicon, and the parameters of the processing technology, in ways which are not all well under-stood. The dependence on diode perimeter length is ~nuch stronger than the dependence on diode areas. Because of this and since photo-current is proportional to area, the signal current falls faster than the leakage current as the diode size is reduced. As a result, the ratio of signal to dark current deteriorates with decreasing diode area and arrays with very small diodes have a correspondingly reduced low light level capability. Fig.8 shows the effect of average leakage current on video window as a function of integration time for diodes measuring 0.1 x 0.086 mm. The minimum irradiance depends on the signal to dark current ratio that can be tolerated. The signal charge has previously been given as: Rn
W/f
[PC1
The dark current equivalent Idn/f
charge is:
[PC1
The signal to leakage ratio is therefore: R ‘/Id which is independent of integration time. Taking typical values for the diodes represented by Fig.8: id = 35 pA R= 10 pApW_t cm* For a signal to leakage ratio of one: W=ld/R
= 3.5 /.lW Ctli2
Since FPN is of constant form it is possible in principle to remove it by suitably processing the video output. Both additive and multiplicative variations may be cancelled using two digital store read-only memories programmed with me-determined correction factors and clocked in synchronism with the array. The memories would store, respectivcly, an additive term for switching noise and a multiplicative factor for responsivity variations, determined manually from the dark and uniform light level array response. After conversion to analogue form these Factors would be applied, diode by diode, to the video output. This noise cancellation technique can also bc implemented in purely analoguc form. At the expense of intr-educing considerable extra complexity the memories may be programmed automatically from dark and light scans. integration period
[msl
Low light level operation Thermal leakage current has already been mentioned as the limiting factor in determining the minimum irradiance at
214
Fig.8 Variation of signal and dark current with integration time for various levels of irradiance from a 2 870 K tungsten source; diode size 0.1 x 0.086 mm
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concepts are not strictly applicable since arrays do not possess the required property of spatial invariance.
1.2
~ I.Oc” g OS-
Sampling theory indicates that the maximum spatial frequency unambiguously detectable by an array is one half the frequency corresponding to the reciprocal diode pitch, eg 5 cycles mm-r for a 0.1 mm pitch. In a sampled data system, the frequency spectrum of the input signal is repetitive in the frequency domain, at intervals equal to the reciprocal of the sampling pulse separation. This characteristic arises from the amplitude modulation of the sampling pulses by the continuous input waveform, which is equivalent to convoluting the signal frequency spectrum with the spectrum of the sampling pulses.
e
b 06.$ cl 0.4 z
0.2 -
Wavelength
I pm1
Fig.9 Typical smoothed spectral response of silicon phatodiode arrays to an equi-energy source
Fig. 10 illustrates the situation as it applies to arrays. The intensity distribution along the array (1 Oa) is sampled at intervals p by the diodes (lob) to give the resultant sampled signal (10~). Figs. 10d to 10f show the equivalent frequency domain process in which d and e are convoluted to give f. The overlap of adjacent spectra in f indicates that spurious modulation will appear in the output sampled intensity distribution: a phenomenon known as ‘aliasing’. To avoid aliasing it is necessary to restrict the bandwidth of the input waveform to a maximum spatial frequency of 1/2p.
Cooling the array from ambient to -10°C would reduce this figure to about 0.35 pW cme2. If the array is being operated well clear of thermal leakage effects, signal-to-noise ratios are governed by the various forms of FPN previously discussed. Spectral response
Photo-diode arrays have a spectral response broadly similar to other silicon photo-detectors (Fig.9), covering a useful range in the visible and near infra-red.
In practice, the sampling pulses are not the infinitely narrow delta functions shown. The sampling ‘aperture’ is the length of the diode, and this can be a large fraction of the diode pitch in some arrays. Finite aperture width does not effect the spacing of the repeated spectra, but exerts a filtering action on the spatial frequencies to which the array will respond. If, for example, the diode length is equal to p/2 for an array pitch p, the filtering characteristic would be of the form (sinx)/x falling to a first zero at a spatial frequency 2/p. As the diode length approaches p, the (sinx)/x cut-off falls towards l/p but the filtering action is mild and does
I rnage quality
The video output obtained from a self-scanned array is the result of a spatial sampling process in which, for a stationary optical image, each diode delivers a signal proportional to the integrated irradiance over the diode area. It would be convenient to express the imaging capabilities of arrays in terms of a modulation transfer function, but although array transfer functions are occasionally found in the literature,’ mtf
Iiw 1 1
X
=
--I
P
Distance
a
7
Distance
b
Distance
C
*
l/P Spatial
frequency
Spatial
d Fig.1 0
Spatial
e Effect of sampling on the spatial frequency
OPTICS AND LASER TECHNOLOGY.
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l/P
ZIP
frequency
frequency
f of the video signal (explained
1974
in the text)
215
reasonable approximatioii to the input intensity distribution requires additional (electrical) filtering of the video, to cut off at the same ‘frequency as the input. Aliasing is relevant mainly in applications of arrays to OCR, facsimile, and two-dimensional imaging generally. It is not significant in dimensional gauging of the type described in a later section. Applications General The application of self-scanned arrays to a variety of technological problems is briefly described below. Their role has often been to displace traditional spatially resolved photo-sensitive systems such as flying-spot scanners or vidicon tubes. The photo-diode array does not offer an improvement in every operational parameter, but its ‘plus factors’ of compactness, geometric accuracy and stability, large dynamic range, low power, and digital scanning frequently outweigh its relatively limited spatial resolution and low light level performance. Fig.1 1 Spatial frequency filtering action of the diode length and ‘recording aperture’ on the video output: A - (sin x)/x spectrum of diode of lengthp; B - first replication at l/p; A is also the spectrum of ‘recording aperture’ of the box-car output waveform which multiplies A and B to give C and 0
little to reduce the spectrum overlap. The box-car type of waveform produced in recharge processing exerts a much stronger filtering action on the higher harmonics since its (sinx)/x frequency characteristic multiplies the entire replicated spectrum resulting from the basic diode sampling. Fig. I 1 illustrates these various filtering operations.
The advantages given by the broad spectral response of silicon are evident in the wide range of radiation sources with which the array may be coupled. The infra-red response of silicon arrays will often permit imaging using the self-radiation of hot bodies such as steel or glass in situations where the provision of external illumination is an inconvenience. The lowest temperatures that will produce useful signals are in the region of 6OO”C--700°C. Care must be exercised, however, when refracting optical systems with large apertures are used for image formation,
A similar situation is found in applications where a twodimensional field is being scanned using a linear array for one dimension and a mechanical scan for the other. In these systems it is common for the line spacing along the mechanical sweep to be equal to the instantaneous field of view of the diodes, ie the diode width. Aliasing is less severe in this direction because the integration time of the array effectively increases the width of the diodes so that the scanned lines overlap. Apart from the limitations imposed by sampling, there is one additional source of image blur contributed by the array which is known as ‘crosstalk’. This is an effect resulting from residual photo-sensitivity outside the nominal diode area and manifests itself as an apparent sideways signal ‘leakage’. If, for example, an intensity step function is imaged on the array to fall between two diodes, there will be a small decrease in signal amplitude on the first illuminated diode and a corresponding increase on the adjacent obscured diode. The effect is negligible (at about 5%) with diodes on 0.1 mm centres but increases in magnitude with decreasing pitch. In summary: self-scanned arrays as imaging devices are prone to the same aliasing problems as other sampling systems. In those situations where aliasing must be avoided it is necessary to filter the optical input so that cut off is at a spatial frequency not greater than half the diode sampling rate. An approximation to the required filtering effect may be obtained by a slight defocusing of the image, but the result is not optimum. To obtain an output waveform which is a
216
Laboratory set-up illustrating diameter gauging. A Fig.12 collimated light source illuminates a rod which is then imaged on a linear array. The video signal is displayed on the oscilloscope
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since their aberration correction may not be adequately maintained in the near infra-red. Another problem sometimes arises when silicon arrays are used to view printed or written material. It is caused by the infra-red transparency of most organic dyes of the type used in coloured printing inks, ball point and felt-tip pens, and colour photographs. Unless the material is carbon based, it yields low contrast images with an unfiltered silicon detector, although these images are of good visual contrast. The above comments on dye transparency apply equally to dyed gelatine filters which require an infrared blocking filter when used with silicon devices. Most of the infra-red response of silicon may be removed by a visually transparent, low pass filter such as a Chance HA3 or Schott KG3; this is at the expense of 50% of the signal level if the light source is tungsten. Filtering to give a closer match to a photopic response reduces signal levels to around 10%. Fig.1 3
Optical
head of hole area measuring equipment
Dimensional gauging
The monitoring of mechanical dimensions perhaps constitutes the most diverse range of applications of linear arrays in industry. Provided that an optical image of the required dimension can be obtained, suitably scaled in size to the array, a fast non-contacting measuring system is available. The method of measurement relies on obtaining a good contrast between object and background and simply involves a digital count of the number of diodes illuminated (or obscured) by the image. A diode is classed as illuminated when its output has reached a pre-set threshold level. The field of view is quantized into the number of elements in the array, and the uncertainty of measurement is plus or minus one diode for a single scan. Since most dimensional gauging situations yield high contrast images the simpler voltage-sampling mode of operation is usually adequate. Fig. 12 shows a laboratory demonstration of diameter gauging. Where the dimension is too large to be encompassed within a single array at the required resolution, it is often possible to use two arrays - each with its own optical system - to look at opposite sides of the object. The unscanned dead space between the two systems is accounted for by electronically inserting an appropriate number of pulses in the count. An example of this type of application would be monitoring the width of steel sheet from a rolling mill, where the required resolution might be 1 mm in 1.5 m but the overall width variation is not great. The technique is ideally suited to continuous process monitoring, where the digital nature of the scanning makes it a simple matter to set upper and lower limits on a dimension, transgression of these limits producing an alarm signal. The high array speeds, and consequent large amount of redundant information in most applications, offer the possibility of reducing the inherent quantization error in those situations where relative movement exists between the array and the measured object. Normally a movement of the image along the array does not change the count by more than the basic plus or minus one diode since the measurement is differential. For a large number of scans the frequency of occurrence of different counts might be expected to yield an interpolation factor between diodes. Simple averaging over n scans produces an improvement in measurement accuracy but the degree of improvement is strongly dependent on the nature of the object movement. A high perturbation rate is necessary since several thousand scans
OPTICS AND LASER TECHNOLOGY.
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1974
might be processed in one second. In an application of this technique to diameter gauging on a high speed rod-drawing machine, an accuracy of l-2 pm was obtained with a basic resolution of 100 pm. Gauging in two dimensions may be undertaken with area arrays, a pair of linear arrays at right angles, or with one linear array and a mechanical scan. An example of the last system is illustrated in Fig. 13 which shows the optical head of an instrument for measuring hole area. Holes in an annular metal ring pass through the measuring station at a fixed increment per array scan. Areas were derived by summing the total number of illuminated diodes for all scans across each hole and multiplying by the quantization element this being defined by the array pitch at the object and the mechanical scanning increment. Inspection and flaw detection
A recurrent problem in the production of materials on a continuous basis is the on-line detection of flaws and defects. These could be, for example, bubbles in sheet glass, pin-holes in magnetic tape, dents in sheet metal, or holes in woven material. In these situations linear arrays are useful for their spatial resolution even though the position of the defect may not be important. As the material passes through the scanned line, flaws show up as pulses on the video output and these trigger a defect signal if they rise above a pre-set threshold level. The method of illumination can be critical if the defect is to be given maximum contrast against the background, because the detectability of defects, as a function of size, depends on their inherent contrast. The most favourable case is the search for holes in an otherwise opaque object. Here the geometric image of the hole may be smaller than a diode element and still produce an acceptable signal if the light source is sufficiently bright. Some degree of defocus is useful in this case to avoid losing a small hole in the dead space between diodes. It is possible to use ac-driven light sources in flaw detection, and in other situations where the video profile is not critical. High-pass electrical filtering of the output signal will then remove gross ac ripple and also slowly varying illumination non-uniformities, with little effect on small detail. In normal
217
applications dc driven sources are required to avoid beating between the supply and scan frequencies. Optical
character
recognition
and facsimile
OCR is a field where the highest scan rates are required and it provided one of the earliest spurs to the development of selfscanned arrays. In principle it is a straightforward application of linear arrays to the reading of printed characters moving at high velocity. A slightly unorthodox scanning technique is coming into use to obtain maximum multiplexing rates: the array is accessed in blocks.which are scanned in parallel. For example, a single 256 element array is divided electrically into four separate segments so that it operates as four 64 element arrays placed end to end but driven in parallel. The scrambled video signal is correctly sequenced in the subsequent processing so that the output of the four sections scanned at 5 MHz is equivalent to a single 256 element line scanned at 20 MHz.
hology, satellite solar aspect sensors, read-out of images on film, dynamic strain measurement in tensile testing, and spatially resolved optical pyrometry. In these diverse fields the three most significant attributes of arrays have been the geometrical accuracy, speed of operation, and digital scanning. The range of applications is expected to broaden as scanned arrays take their place alongside the other electronic image sensors and familiarity with their particular advantages becomes more widespread. Acknowledgements I would like to thank my colleagues at IPL, in particular P. W. Fry and R. J. Rycroft, for helpful technical discussions. My thanks are also due to the Directors of the Company for permission to publish this paper.
References
Apart from the broad categories discussed above, self-scanned arrays have been applied to a variety of problems on an experimental basis. These include measurement of onedimensional diffraction patterns, line spread functions, and spectral line profiles 8 ; and applications in blood cell morp-
Special issue on solid state imaging IEEE Tram Elecf Deu ED15 (1968) Weimer, P. K. et al. A self-scanned solid-state image sensor ProcI&YI?SS (1967) 1591-1602 Tompsett, M. 6. et ai. Charge coupling improves its image, challenging video camera tubes Electronics 18 Jan (1973) 162-169 Melen, R. The trade-offs in monolithic image sensors: MOS v CCDEIectronics May 24 (1973) 106-111 Eberhardt, E. H. Source-detector spectral matching factors App Opt 7 (1968) 2037-2047 Fry, P. W. et al. Fixed pattern noise in photomatrices IEEE J Solid-state Circuits SC-S (1970) 250-254 Nill, N. B. Imaging characteristics of photodetectors operating in the photon integration mode Appl Opt 10 (1971) 686-687 Horlick, G., Codding, E. G. Applications of self-scanning linear silicon photodiode arrays as detectors of spectral information Anal Chern 45 (1973) 1490-1494
218
OPTICS
Facsimile is an application requiring many more picture lines than OCR but a much lower operating rate, limited by the data transmission capacity of telephone lines. In general, only two grey levels need to be transmitted, but these may be close together if the original document is faint. The use of arrays in facsimile scanning is in an early stage and has only become practical with the advent of 1 024 element sensors. Miscellaneous
applications
AND
LASER
TECHNOLOGY.
OCTOBER
1974
Solid state image-forming devices have been newsworthy for some years. The possibility that solid state technology could yield a compact, light-weight, fast, low voltage, low power, stable, long-life image sensor has spurred considerable research and development effort, largely because the above characteristics are not those generally found in traditional electronic imaging devices. In the process, several new technologies have been developed or adapted; and at this interim point, practical devices are on the market whilst the quest continues for a solid state sensor that will deliver full broadcast-television image quality. The development of the new breeds of image sensor has been made possible by continuing progress in silicon integrated circuit technology. The high packing densities and geometric fidelity exhibited by integrated circuit devices led inevitably to consideration of how photo-sensitive array structures might be fabricated with these same techniques: a one or two-dimensional light intensity distribution might then be sampled on a point-by-point basis and the information extracted in a usable form. The virtues of silicon in this respect are that it allows the construction of p-n junction photo-diodes and photo-transistors which are compatible with a variety of active circuit devices necessary for signal processing, the whole assembly being fabricated on a single silicon chip. Scanning methods Around the middle 1960s several techniques fundamental to the successful operation of solid state image sensors emerged. With a very small number of sampling points it is feasible to consider a sensor in which every element is available for interrogation on a 100% time basis. Such ‘parallel output’ arrays have been made but present impossible accessing problems long before they reach a practically useful size. The obvious solution is to adopt some form of sequential scan-
The author is with integrated Photomatrix Ltd. The Grove Trading Estate, Dorchester, Dorset, UK. Received 16 May 1974.
OPTICS AND LASER TECHNOLOGY.
OCTOBER
1974
ning of the individual array elements so that a spatially varying light intensity distribution is converted to a time-varying electrical signal. The source and analbgue for this approach is the traditional image tube such as the vidicon, in which an optical image is converted to a pattern of charge on a photoconductive layer and sequentially sampled by a swept electron beam. The scanning beam restores the layer potential at each point and the resulting beam current modulation constitutes the time-varying video signal. The equivalent procedure in a solid state image sensor is to connect the output of each photo-sensitive element in turn to a common line by means of suitable seque!ltially operated switches. Chargestorage The majority of scanning techniques have been devised for arrays operating in what is known as the ‘integrating’ or ‘charge storage’ mode. This is a method of overcoming the inherent low sensitivity of an array in which each element is sampled only for its instantaneous photo-current. Parallel output arrays operate in the instantaneous mode, as do most other photo-detectors, but this leads to great inefficiency in scanned arrays where the time spent sampling one element is very small compared with the time taken to scan a complete image. Analogy with the vidicon tube again provides the clue. The charge level at any point on the tube’s photo-conductive layer builds up continuously from the time it is sampled until the scanning electron beam next returns to the same point, when it is almost instantaneously discharged. The output signal is therefore proportional to the time integral of intensity at that point. A similar procedure is followed in the operation of scanned arrays and the principles of charge storage and signal multiplexing will be examined with reference to arrays fabricated using silicon planar photo-diodes and metal oxide semi-conductor (MOS) technology. MOS transistors possess very favourable characteristics as switching elements when integrated with planar photo-diodes, and the commercial photo-diode arrays currently available all use this approach.
209
MOS photo-diode
x register
arrays
m+l
m
The method of implementing the charge-storage technique in these arrays is to utilize the self-capacitance of each p-n junction diode element. When the diode is reversebiased and then left open circuit, charge is stored on the depletion layer capacitance. This charge decays only slowly in the dark because of recombination with thermally generated electron/hole pairs. When light falls on the junction, electron/hole pairs are created at a rate proportional to the incident illuminance and the junction capacitance discharges at a corresponding rate. In a fixed interval of time, therefore, the amount of stored charge removed is proportional to the total incident flux during that interval. The point-to-point signal across the array consists of the amount of charge lost by each individual diode. It thus can be sampled either by restoring the initial condition and monitoring the quantity of charge required to do so (recharge sampling) or by looking at the actual level to which the voltage across each diode has decayed (voltage sampling). The time between samples, called the integration period, is the linescan time for linear arrays or the framescan time for area arrays. The sequential switching process which is necessary for interrogating the diodes is achieved universally by the use of shift registers. A pulse appearing at one end of a shift register can be clocked through at high speed, opening and closing diode sampling switches as it goes. As an illustration, one form of array structure will be described to demonstrate the application of the general principles. Fig. 1 shows three elements of a linear array. Each element consists of a photo-diode D and three MOS transistors. In operation a pulse is propagated through an MOS shift register on the chip to appear sequentially at lines n, n+l, nt2, which are connected to the gates of the MOS switches Tr . When these gates are driven negative the switches turn on and the photo-diode capacitances are charged to a negative potential. After the pulse has passed, the switches turn off again and the diodes begin to discharge at a rate dependent on the incident illuminance. The pulse continues to propagate through the array and eventually, after one integration period, reappears at line II. At this point it turns on MOS switch Ta and allows current to flow through T2, Ta, and the external load resistor. The magnitude of this current is controlled by the voltage on the gate of the amplifJing transistor T2, this voltage being dependent on the degree
Substrate --__
-
+ve ---_-Load resistor
Video eput Substrate ____
----
Q+
%
-----a--
Fig.1
210
n+l
Three elements of a typical
/-I+2
linear self-scanned
array
I / I I
I I
Fig.2 array
Four elements of a simple two-dimensional
self-scanned
of discharge of the photo-diode. Having sampled the voltage on the diode, the pulse moves on to n+ 1 where it recharges diode n as before and at the same time samples the voltage on diode tit 1. The above outlines the voltage-sampling mode of scanning photo-diode arrays. In recharge sampling, transistors Tz and Ts are not used and the video signal is derived from the magnitude of the current pulses required to recharge the photo-diodes when switches Tr are closed. The relative merits of the two methods are discussed later. Two-dimensional
arrays
This description of the operation of a linear array is basically valid for a two-dimensional array. The extra complication of x-y scanning is dealt with by having two shift registers on the chip, one for rows (y) and the other for columns (x). Fig.2 shows the simplest possible element structure for an area array. In the recharge sampling mode the x-shift register switching lines are connected to all the MOS switch gates in their respective columns. Similarly the y-shift register lines are connected to all the MOS switch drains in their respective rows. A pulse propagated through the ‘slow’ x register will switch whole columns in sequence but the only diode connected to the video output at any instant will be in the row activated by the ‘fast’ y register. When a column has been scanned out iny, the x register moves along one line and the next column is scanned, giving rise to a video output in the form of the familiar television raster. Alternative
n
t
\
technologies
The structures described above are typical of those which have proved most successful in the development of commercially feasible devices. Other monolithic structures have been devised - and some realized experimentally - using various combinations of diodes and transistors. (Reference 1 gives a general survey.) Somewhat aside from the mainstream, thin film and hybrid thin film/silicon techniques have been used * in the fabrication of arrays. More recently, charge-coupled and charge-injection devices ahave appeared on the scene as contenders for large area arrays. 3
OPTICS
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LASER
TECHNOLOGY.
OCTOBER
1974
It is not intended to make critical comparison between the various types of self-scanned arrays which are, or are becoming, available for general use. This has been attempted by others 4 but in this author’s opinion the technologies have not sufficiently settled, nor is there yet an adequate body of application experience, for long term judgements to be attempted. In what follows, the characteristics and operating parameters of photo-diode arrays are reviewed. The final section provides a general survey of current applications to a variety of sensing and measurement problems.
Performance characteristics Geometry
Photo-lithography, as developed for integrated circuit technology, allows the fabrication of photo-diode arrays that have element size and pitch small enough to be comparable with electron beam scanned tubes. Diode pitches are in the range 0.025 mm (0.001 in) to 0.125 mm (0.005 in). Diode shapes are generally nearly square but this can vary depending on the application or on chip layout constraints. For example, in a linear array intended for sampling a onedimensional intensity distribution it is feasible to make each diode one or two orders of magnitude wider than the basic array pitch without affecting the inherent resolution; a proportionately larger light collecting area is thereby achieved. With linear arrays it is possible to make the diode length a large fraction of the element pitch (up to 90%) since the scanning circuitry can be placed on either side of the row of diodes. This is not possible with two-dimensional arrays,
Fig.4 0.075
Enlarged portion mm pitch
of a two-dimensional
array with diodes on
where each resolution element must contain the active components associated with the diode, plus the interconnection pattern. Here the effective sensing area falls to a region 20-50% of the element size. Figs 3 and 4 show sections of typical one and two-dimensional arrays. Linear arrays are available in a wide range of lengths, vary ing from 16 to 1 024 elements depending on the manufacturer. Area arrays currently vary between 32 x 32 and 64 x 64 elements. The smaller linear arrays can be obtained in TO5 cans but the customary mounting is a multipin DIL package with glass or quartz window. Figs 5 and 6 show the packaged versions of the arrays illustrated in Figs 3 and 4. Peripheral drive and output circuitry
Self-scanned arrays are not completely self-contained entities. Clock pulses and scan-start pulses must be generated externally with the correct phases to drive the shift registers. The array output may be accessed directly, but can usefully be subjected to additional signal processing to improve performance. Two methods of obtaining a video output have been mentioned previously: voltage sampling and recharge sampling. Some arrays offer only one of the two possibilities, others can be operated in either mode.
Fig.3 pitch
Enlarged portion
of a linear array with diodes on 0.1 mm
OPTICS AND LASER TECHNOLOGY.
OCTOBER
1974
In voltage sampling the video signal is derived from the actual voltage on each diode at the end of an integration period. The voltage decay is proportional to the total incident flux on the diode and for a stationary image is therefore proportional to the instantaneous light intensity. The resultant video waveform appears directly across an external load resistor. No interface circuity is required, except perhaps a buffer stage emitter follower to avoid loading the array. However, voltage sampling is limited in operational
211
speed because the transitional rise and fall times from one diode to the next are dependent on the size of the external load resistor and can easily become a large fraction of the sampling time. In addition noise levels are greater for reasons which are discussed in the section ‘Noise considerations’.
Fig.5
Packaged 128 element
linear array: array length 13.0 mm
The preferred mode of operation is recharge sampling, although sophisticated signal processing is required to fully exploit the technique. With recharge sampling the video output consists of a train of rapid current pulses on the supply line. It is possible to amplify these and display them as a series of spikes, where the intensity distribution across the array is then given by the locus of the peak amplitudes. However, there is a tendency for the signal to be submerged in multiplexing noise and a more satisfactory visual appearance coupled with a great reduction in noise level is obtained by processing the output to obtain a ‘box-car’ waveform similar to that resulting from voltage sampling. This is done by integrating each current pulse for a one bit period, and sampling and holding the integrator level until it is reset by the next integrated pulse. Fig.7 shows a section of a recharge-sampled waveform after processing. The highest multiplexing rates and lowest noise levels are obtainable using this technique.
Dynamic range
The operation of arrays in the charge storage mode allows a wide dynamic range in the same sense as that of ordinary photography, where a compensation for a low light level is obtained by increasing the exposure time. The analogy with photographic film is quite close, in that both sensors have a saturation level, a fog level, and give a specific signal for a specific exposure where that exposure can be arrived at by varying combinations of intensity and time. Unlike film, array outputs are linear with exposure and do not suffer from reciprocity failure, but they offer a more restricted range of time/intensity combinations.
Fig.6
Packaged 64 x 64 element area array; array size 4.9 x 4.9 mm
Fig.7 Appearance of a typical recharge-sampled video output signal after processing. (a linescan across the bar target)
212
The absolute limits of operation are set at high irradiance levels by the maximum multiplexing rate of the array, and at low irradiance by the rate of self-decay of diode charge. Each diode can store only a limited quantity of charge and if the incident irradiance is sufficiently high to completely discharge the diode in any given integratioir period, saturation level is reached. When this occurs for the minimum obtainable integration period (maximum scan rate) there is absolute saturation. At the other end of the scale, selfdecay of charge is apparent because of recombination of thermally generated hole/electron pairs with the stored charge. This is known as thermal leakage, or dark current. With a low-intensity image and increasing integration time, part of the observed video signal results from this leakage current until, in the limit, saturation level can be reached through dark current alone. Cooling the array below ambient can extend the dynamic range since leakage currant falls by approximately a factor of two for every 10°C drop in temperature. Between these two extremes there is a linear operating range of three to six orders of magnitude, according to array length and diode size. The dynamic range is greatest with the short arrays, since the maximum multiplexing rate is fixed irrespective of the number of diodes and therefore the minimum integration time falls with decreasing array length. For any fixed integration period the
OPTICS AND LASER TECHNOLOGY.
OCTOBER
1974
dynamic range is typically 100: 1 in recharge sampling, governed by the ratio of noise level to saturation. Linear arrays can have their dynamic range increased by the addition of ‘padding capacitance’. The diode area is increased in a direction normal to the array but most of it is covered with an opaque aluminium layer. This provides additional self-capacitance and charge storage capability without enlarging the photo-sensitive area or seriously increasing the leakage current. Maximum multiplexing rates for currently available arrays are in the region of 4- 10 MHz and maximum integration times range from 2-80 ms. The dynamic range of two-dimensional arrays operating in the frame storage mode is inevitably reduced in comparison with linear arrays because of the greatly increased number of photo-diodes. For example, a 64 x 64 element array contains 4 096 diodes and at a multiplex rate of 4 MHz the integration time is approximately 1 ms; this corresponds to a frame rate of 1 000 frames s-l. It should be compared with a 256 element linear array running at the same speed, where the integration time is 64 ps.
Table 1. Spectral conversion factors for silicon detectors. To obtain equivalent watts of 2 870 K tungsten, multiply source watts by column A factor or source lumens by column B factor
Source
conversion factor
B Lumens conversion factor
2 500 K tungsten
0.7
0.07
2 870 K tungsten
1.0
0.05
3 200 K tungsten
1.3
0.04
1 000°C blacK body
0.015
1.6
Sunlight (sea level)
1.9
0.016
White fluorescent tube
2.4
0.007
Low pressure sodium lamp
2.6
0.005
GaAs/P solid state lamp
3.2
0.04
GaAs solid state lamp
3.4
only emits in infra-red
He-Ne laser
3.0
0.02
A
Watts
Responsivity
Since self-scanned arrays operate in the integration mode, their response to incident radiation is expressed in terms of output signal per unit energy input (or more usually, energy density). The output of an individual diode is either a voltage, or a charge in picocoulombs. The input power is customarily given as an irradiance in /JW cm-* from a source of stated spectral distribution. Responsivities are therefore quoted in units of V s-1 PW-1 cm* or pC s-r pW-l cm*. Sometimes the latter is expressed in the equivalent form of pA E.IW-’cm*. The magnitude of the video signal from an array is very much dependent on the method of subsequent processing and the calculation of operating levels is usually performed relative to a saturation signal taken as 100%. Thus, calculation of the signal that will be obtained in any particular situation involves manipulating three parameters - multiplex rate, array length, and irradiance level. If the array contains n diodes and the scan rate is to be f (Hz)the integration time for each diode is n/f seconds. For a responsivity quoted as R (PC s-r pW-i cm*) and an irradiance of W (yW cm-*) the resultant signal charge will be: R n
w/f[PC]
Manufacturers data will give saturation charge along with R, from which the signal level can be estimated as a percentage of saturation. A similar calculation can be performed in the voltage sampling mode. In absolute terms, the video output signal at saturation is in the I- 10 V range after processing. The responsivity R is invariably quoted radiometrically for a tungsten lamp running at a colour temperature of 2 870 K. For this source, typical values of R vary from 10 pC s-l pW_’ cm* for diodes on 0.1 mm pitch to 0.7 pC s-l /JW-’ cm* for diodes on 0.025 mm pitch. To deduce the responsivity to other sources or to obtain the equivalent photometric quantities has always proved a headache with spectrally selective detectors such as silicon, since it involves numerical integration using the source and detector spectral curves.5 Table 1 presents a selection of conversion factors for various common light sources when they are used with a typical silicon detector. The numerical values
OPTICS AND LASER TECHNOLOGY.
OCTOBER
1974
P20 phosphor
1.6
0.006
Xenon arc lamp (1 600 W)
2.3
0.03
Xenon arc lamp (150 W)
2.4
0.10
are used as multiplying factors, applied either to the watts radiated by the source (column A) or the lumens (column B), the product in each case being the equivalent watts of 2 870 K tungsten. Noise considerations
Photo-diode arrays are subject to a number of noise sources not found in other forms of detector. These are generally grouped together under the heading ‘fixed pattern noise’ (FPN), so called because it is stationary with respect to the array, and it arises from the discrete nature of the sensing elements and the sampling methods used to extract the video signal6 Each element of the array has associated with it several parameters which exhibit small deviations from nominal value due to the variability of materials and processing. Since the functioning of integrated circuit devices is essentially geometry-dependant, photo-lithographic errors and mask misalignment are one source of variations. Additionally, device parameters are affected by any inhomogeneity of the monocrystalline silicon substrate material. The result of these small variations is twofold: 1. There is a corresponding variation in the desired output signal (apparent responsivity fluctuations from diode to diode). 2. A random contribution is added to the switching transients which appear on the video output. The second of these needs further explanation. Interrogation of the diodes requires sampling pulses whose magnitudes are considerably larger than the signal to be sampled. In-
213
evitably these pulses break through on to the video output because of parasitic capacitative coupling between the multiplexing switches and the output line. For example, in Fig.1 the gate-drain capacitances of switches Tr transfer charge spikes to the negative rail at the leading and trailing edges of each clock pulse. Random variations occur in the amplitudes of these spikes, dependent on the degree of coupling at each switch. It is possible to remove most of the pulse breakthrough by low pass filtering of the video signal since the spikes appear at twice the multiplexing rate; however, the random variations occur at lower frequencies and are not susceptible to filtering. The ‘integration plus sample-and-hold’ technique will also remove the switching spikes because, being alternately of opposite sense, they integrate to zero in any given bit period. Whether or not this procedure removes the random variations as well depends on the duration of a sample pulse in one bit period. At the slower scan rates the clock pulse width may be only half the sample period, confining the spikes to one cycle of the integrator and producing complete cancellation. At high scan rates rise and fall times of the sampling pulse become significant and the negative-going spike from one bit may overlap and combine with the positive-going spike of the following bit. If these are not equal, because of random variations, complete cancellation will not occur. In voltage sampling the most significant contribution to fixed pattern noise arises from variations in the threshold voltages of the amplifying transistors (T, in Fig.1) and the general noise level tends to be higher than in recharge sampling. The effect of these various sources of FPN is to produce an output video waveform which exhibits a stationary noise structure even when the array is uniformly illuminated (or in the dark). Noise arising from switching spikes is signal independent, observable under dark conditions, and is typically f 0.5% of the saturation level in recharge sampling or + 3% in voltage sampling. Noise arising from those sources which produce apparent responsivity variations is signal dependent and has typical values off 5% of signal at 50% saturation. These two distinct forms of noise are commonly quoted separately in specifications. Other sources of noise in the video processing system are generally negligible in comparison with the noise from the array itself.
which arrays can operate. It is not fundamentally a noise mechanism since the primary effect is to progressively decrease the available signal swing - or video window - with increasing integration time. However, noise is associated with it: this arises from random variations in leakage curren from diode to diode. Leakage current is a function of diode geometry, bias voltage, the bulk properties of silicon, and the parameters of the processing technology, in ways which are not all well under-stood. The dependence on diode perimeter length is ~nuch stronger than the dependence on diode areas. Because of this and since photo-current is proportional to area, the signal current falls faster than the leakage current as the diode size is reduced. As a result, the ratio of signal to dark current deteriorates with decreasing diode area and arrays with very small diodes have a correspondingly reduced low light level capability. Fig.8 shows the effect of average leakage current on video window as a function of integration time for diodes measuring 0.1 x 0.086 mm. The minimum irradiance depends on the signal to dark current ratio that can be tolerated. The signal charge has previously been given as: Rn
W/f
[PC1
The dark current equivalent Idn/f
charge is:
[PC1
The signal to leakage ratio is therefore: R ‘/Id which is independent of integration time. Taking typical values for the diodes represented by Fig.8: id = 35 pA R= 10 pApW_t cm* For a signal to leakage ratio of one: W=ld/R
= 3.5 /.lW Ctli2
Since FPN is of constant form it is possible in principle to remove it by suitably processing the video output. Both additive and multiplicative variations may be cancelled using two digital store read-only memories programmed with me-determined correction factors and clocked in synchronism with the array. The memories would store, respectivcly, an additive term for switching noise and a multiplicative factor for responsivity variations, determined manually from the dark and uniform light level array response. After conversion to analogue form these Factors would be applied, diode by diode, to the video output. This noise cancellation technique can also bc implemented in purely analoguc form. At the expense of intr-educing considerable extra complexity the memories may be programmed automatically from dark and light scans. integration period
[msl
Low light level operation Thermal leakage current has already been mentioned as the limiting factor in determining the minimum irradiance at
214
Fig.8 Variation of signal and dark current with integration time for various levels of irradiance from a 2 870 K tungsten source; diode size 0.1 x 0.086 mm
OPTICS
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TECHNOLOGY.
OCTOBER
1974
concepts are not strictly applicable since arrays do not possess the required property of spatial invariance.
1.2
~ I.Oc” g OS-
Sampling theory indicates that the maximum spatial frequency unambiguously detectable by an array is one half the frequency corresponding to the reciprocal diode pitch, eg 5 cycles mm-r for a 0.1 mm pitch. In a sampled data system, the frequency spectrum of the input signal is repetitive in the frequency domain, at intervals equal to the reciprocal of the sampling pulse separation. This characteristic arises from the amplitude modulation of the sampling pulses by the continuous input waveform, which is equivalent to convoluting the signal frequency spectrum with the spectrum of the sampling pulses.
e
b 06.$ cl 0.4 z
0.2 -
Wavelength
I pm1
Fig.9 Typical smoothed spectral response of silicon phatodiode arrays to an equi-energy source
Fig. 10 illustrates the situation as it applies to arrays. The intensity distribution along the array (1 Oa) is sampled at intervals p by the diodes (lob) to give the resultant sampled signal (10~). Figs. 10d to 10f show the equivalent frequency domain process in which d and e are convoluted to give f. The overlap of adjacent spectra in f indicates that spurious modulation will appear in the output sampled intensity distribution: a phenomenon known as ‘aliasing’. To avoid aliasing it is necessary to restrict the bandwidth of the input waveform to a maximum spatial frequency of 1/2p.
Cooling the array from ambient to -10°C would reduce this figure to about 0.35 pW cme2. If the array is being operated well clear of thermal leakage effects, signal-to-noise ratios are governed by the various forms of FPN previously discussed. Spectral response
Photo-diode arrays have a spectral response broadly similar to other silicon photo-detectors (Fig.9), covering a useful range in the visible and near infra-red.
In practice, the sampling pulses are not the infinitely narrow delta functions shown. The sampling ‘aperture’ is the length of the diode, and this can be a large fraction of the diode pitch in some arrays. Finite aperture width does not effect the spacing of the repeated spectra, but exerts a filtering action on the spatial frequencies to which the array will respond. If, for example, the diode length is equal to p/2 for an array pitch p, the filtering characteristic would be of the form (sinx)/x falling to a first zero at a spatial frequency 2/p. As the diode length approaches p, the (sinx)/x cut-off falls towards l/p but the filtering action is mild and does
I rnage quality
The video output obtained from a self-scanned array is the result of a spatial sampling process in which, for a stationary optical image, each diode delivers a signal proportional to the integrated irradiance over the diode area. It would be convenient to express the imaging capabilities of arrays in terms of a modulation transfer function, but although array transfer functions are occasionally found in the literature,’ mtf
Iiw 1 1
X
=
--I
P
Distance
a
7
Distance
b
Distance
C
*
l/P Spatial
frequency
Spatial
d Fig.1 0
Spatial
e Effect of sampling on the spatial frequency
OPTICS AND LASER TECHNOLOGY.
content
OCTOBER
w
l/P
ZIP
frequency
frequency
f of the video signal (explained
1974
in the text)
215
reasonable approximatioii to the input intensity distribution requires additional (electrical) filtering of the video, to cut off at the same ‘frequency as the input. Aliasing is relevant mainly in applications of arrays to OCR, facsimile, and two-dimensional imaging generally. It is not significant in dimensional gauging of the type described in a later section. Applications General The application of self-scanned arrays to a variety of technological problems is briefly described below. Their role has often been to displace traditional spatially resolved photo-sensitive systems such as flying-spot scanners or vidicon tubes. The photo-diode array does not offer an improvement in every operational parameter, but its ‘plus factors’ of compactness, geometric accuracy and stability, large dynamic range, low power, and digital scanning frequently outweigh its relatively limited spatial resolution and low light level performance. Fig.1 1 Spatial frequency filtering action of the diode length and ‘recording aperture’ on the video output: A - (sin x)/x spectrum of diode of lengthp; B - first replication at l/p; A is also the spectrum of ‘recording aperture’ of the box-car output waveform which multiplies A and B to give C and 0
little to reduce the spectrum overlap. The box-car type of waveform produced in recharge processing exerts a much stronger filtering action on the higher harmonics since its (sinx)/x frequency characteristic multiplies the entire replicated spectrum resulting from the basic diode sampling. Fig. I 1 illustrates these various filtering operations.
The advantages given by the broad spectral response of silicon are evident in the wide range of radiation sources with which the array may be coupled. The infra-red response of silicon arrays will often permit imaging using the self-radiation of hot bodies such as steel or glass in situations where the provision of external illumination is an inconvenience. The lowest temperatures that will produce useful signals are in the region of 6OO”C--700°C. Care must be exercised, however, when refracting optical systems with large apertures are used for image formation,
A similar situation is found in applications where a twodimensional field is being scanned using a linear array for one dimension and a mechanical scan for the other. In these systems it is common for the line spacing along the mechanical sweep to be equal to the instantaneous field of view of the diodes, ie the diode width. Aliasing is less severe in this direction because the integration time of the array effectively increases the width of the diodes so that the scanned lines overlap. Apart from the limitations imposed by sampling, there is one additional source of image blur contributed by the array which is known as ‘crosstalk’. This is an effect resulting from residual photo-sensitivity outside the nominal diode area and manifests itself as an apparent sideways signal ‘leakage’. If, for example, an intensity step function is imaged on the array to fall between two diodes, there will be a small decrease in signal amplitude on the first illuminated diode and a corresponding increase on the adjacent obscured diode. The effect is negligible (at about 5%) with diodes on 0.1 mm centres but increases in magnitude with decreasing pitch. In summary: self-scanned arrays as imaging devices are prone to the same aliasing problems as other sampling systems. In those situations where aliasing must be avoided it is necessary to filter the optical input so that cut off is at a spatial frequency not greater than half the diode sampling rate. An approximation to the required filtering effect may be obtained by a slight defocusing of the image, but the result is not optimum. To obtain an output waveform which is a
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Laboratory set-up illustrating diameter gauging. A Fig.12 collimated light source illuminates a rod which is then imaged on a linear array. The video signal is displayed on the oscilloscope
OPTICS AND LASER TECHNOLOGY.
OCTOBER
1974
since their aberration correction may not be adequately maintained in the near infra-red. Another problem sometimes arises when silicon arrays are used to view printed or written material. It is caused by the infra-red transparency of most organic dyes of the type used in coloured printing inks, ball point and felt-tip pens, and colour photographs. Unless the material is carbon based, it yields low contrast images with an unfiltered silicon detector, although these images are of good visual contrast. The above comments on dye transparency apply equally to dyed gelatine filters which require an infrared blocking filter when used with silicon devices. Most of the infra-red response of silicon may be removed by a visually transparent, low pass filter such as a Chance HA3 or Schott KG3; this is at the expense of 50% of the signal level if the light source is tungsten. Filtering to give a closer match to a photopic response reduces signal levels to around 10%. Fig.1 3
Optical
head of hole area measuring equipment
Dimensional gauging
The monitoring of mechanical dimensions perhaps constitutes the most diverse range of applications of linear arrays in industry. Provided that an optical image of the required dimension can be obtained, suitably scaled in size to the array, a fast non-contacting measuring system is available. The method of measurement relies on obtaining a good contrast between object and background and simply involves a digital count of the number of diodes illuminated (or obscured) by the image. A diode is classed as illuminated when its output has reached a pre-set threshold level. The field of view is quantized into the number of elements in the array, and the uncertainty of measurement is plus or minus one diode for a single scan. Since most dimensional gauging situations yield high contrast images the simpler voltage-sampling mode of operation is usually adequate. Fig. 12 shows a laboratory demonstration of diameter gauging. Where the dimension is too large to be encompassed within a single array at the required resolution, it is often possible to use two arrays - each with its own optical system - to look at opposite sides of the object. The unscanned dead space between the two systems is accounted for by electronically inserting an appropriate number of pulses in the count. An example of this type of application would be monitoring the width of steel sheet from a rolling mill, where the required resolution might be 1 mm in 1.5 m but the overall width variation is not great. The technique is ideally suited to continuous process monitoring, where the digital nature of the scanning makes it a simple matter to set upper and lower limits on a dimension, transgression of these limits producing an alarm signal. The high array speeds, and consequent large amount of redundant information in most applications, offer the possibility of reducing the inherent quantization error in those situations where relative movement exists between the array and the measured object. Normally a movement of the image along the array does not change the count by more than the basic plus or minus one diode since the measurement is differential. For a large number of scans the frequency of occurrence of different counts might be expected to yield an interpolation factor between diodes. Simple averaging over n scans produces an improvement in measurement accuracy but the degree of improvement is strongly dependent on the nature of the object movement. A high perturbation rate is necessary since several thousand scans
OPTICS AND LASER TECHNOLOGY.
OCTOBER
1974
might be processed in one second. In an application of this technique to diameter gauging on a high speed rod-drawing machine, an accuracy of l-2 pm was obtained with a basic resolution of 100 pm. Gauging in two dimensions may be undertaken with area arrays, a pair of linear arrays at right angles, or with one linear array and a mechanical scan. An example of the last system is illustrated in Fig. 13 which shows the optical head of an instrument for measuring hole area. Holes in an annular metal ring pass through the measuring station at a fixed increment per array scan. Areas were derived by summing the total number of illuminated diodes for all scans across each hole and multiplying by the quantization element this being defined by the array pitch at the object and the mechanical scanning increment. Inspection and flaw detection
A recurrent problem in the production of materials on a continuous basis is the on-line detection of flaws and defects. These could be, for example, bubbles in sheet glass, pin-holes in magnetic tape, dents in sheet metal, or holes in woven material. In these situations linear arrays are useful for their spatial resolution even though the position of the defect may not be important. As the material passes through the scanned line, flaws show up as pulses on the video output and these trigger a defect signal if they rise above a pre-set threshold level. The method of illumination can be critical if the defect is to be given maximum contrast against the background, because the detectability of defects, as a function of size, depends on their inherent contrast. The most favourable case is the search for holes in an otherwise opaque object. Here the geometric image of the hole may be smaller than a diode element and still produce an acceptable signal if the light source is sufficiently bright. Some degree of defocus is useful in this case to avoid losing a small hole in the dead space between diodes. It is possible to use ac-driven light sources in flaw detection, and in other situations where the video profile is not critical. High-pass electrical filtering of the output signal will then remove gross ac ripple and also slowly varying illumination non-uniformities, with little effect on small detail. In normal
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applications dc driven sources are required to avoid beating between the supply and scan frequencies. Optical
character
recognition
and facsimile
OCR is a field where the highest scan rates are required and it provided one of the earliest spurs to the development of selfscanned arrays. In principle it is a straightforward application of linear arrays to the reading of printed characters moving at high velocity. A slightly unorthodox scanning technique is coming into use to obtain maximum multiplexing rates: the array is accessed in blocks.which are scanned in parallel. For example, a single 256 element array is divided electrically into four separate segments so that it operates as four 64 element arrays placed end to end but driven in parallel. The scrambled video signal is correctly sequenced in the subsequent processing so that the output of the four sections scanned at 5 MHz is equivalent to a single 256 element line scanned at 20 MHz.
hology, satellite solar aspect sensors, read-out of images on film, dynamic strain measurement in tensile testing, and spatially resolved optical pyrometry. In these diverse fields the three most significant attributes of arrays have been the geometrical accuracy, speed of operation, and digital scanning. The range of applications is expected to broaden as scanned arrays take their place alongside the other electronic image sensors and familiarity with their particular advantages becomes more widespread. Acknowledgements I would like to thank my colleagues at IPL, in particular P. W. Fry and R. J. Rycroft, for helpful technical discussions. My thanks are also due to the Directors of the Company for permission to publish this paper.
References
Apart from the broad categories discussed above, self-scanned arrays have been applied to a variety of problems on an experimental basis. These include measurement of onedimensional diffraction patterns, line spread functions, and spectral line profiles 8 ; and applications in blood cell morp-
Special issue on solid state imaging IEEE Tram Elecf Deu ED15 (1968) Weimer, P. K. et al. A self-scanned solid-state image sensor ProcI&YI?SS (1967) 1591-1602 Tompsett, M. 6. et ai. Charge coupling improves its image, challenging video camera tubes Electronics 18 Jan (1973) 162-169 Melen, R. The trade-offs in monolithic image sensors: MOS v CCDEIectronics May 24 (1973) 106-111 Eberhardt, E. H. Source-detector spectral matching factors App Opt 7 (1968) 2037-2047 Fry, P. W. et al. Fixed pattern noise in photomatrices IEEE J Solid-state Circuits SC-S (1970) 250-254 Nill, N. B. Imaging characteristics of photodetectors operating in the photon integration mode Appl Opt 10 (1971) 686-687 Horlick, G., Codding, E. G. Applications of self-scanning linear silicon photodiode arrays as detectors of spectral information Anal Chern 45 (1973) 1490-1494
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OPTICS
Facsimile is an application requiring many more picture lines than OCR but a much lower operating rate, limited by the data transmission capacity of telephone lines. In general, only two grey levels need to be transmitted, but these may be close together if the original document is faint. The use of arrays in facsimile scanning is in an early stage and has only become practical with the advent of 1 024 element sensors. Miscellaneous
applications
AND
LASER
TECHNOLOGY.
OCTOBER
1974