Targets for Infra-Red Television Camera Tubes

270 PHILlPS TECHNICAL REVIEW. VOLUME 30

Targets for infra-red television camera tubes

M. Berth and J.-J. Brissot

Introduetion Television camera tubes employing the principle of charge storage have nowadays completely superseded opto-mechanical scanning devices in the visible part of the spectrum. Their use in the infra-red however is a more difficult matter. To clearly illustrate these difficulties, we must first recall the principles underlying the construction and R operation of such camera tubes. We shall take as our example a vidicon-type tube with a photoconductive target as shown schematically infig. 1. The target con- Fig. 1. Schematic diagram of a vidicon. T image electrode on which the optical image is projected. K cathode. W Wehnelt sists of a thin photosensitive layer whose front face, cylinder. e scanning electron beam. Acc accelerating electrode. the one which receives the light, is covered with a Al alignment coil, Deft deflection coil, Foe focusing and collimating coil. The current from the signal anode of the image electrode transparent film which is a good conductor of electri- flows through the resistance R, and the signal voltage obtained city. The back face is scanned with a low-velocity beam across R is amplified and processed further. of electrons which is deflected and focused by electric and magnetic fields. On striking this face, the electron ted bands, the mobility p, ofthe free charge carriers and beam brings it to the potential=-Ps of the cathode (the the energy gap !.lE by the equation: front face being at zero potential), and the sensitive layer, of thickness d and dielectric constant e, stores a a =qp,Nexp_ (LIE)-- , charge of 2kT where q is the electron charge, k the Boltzmann constant and Tthe absolute temperature. The parameters p, and R do not differ greatly from one material to an- per unit surface area. Between two scans by the elec- other, so that to obtain a conductivity lower than a tron beam, the charge leaks away via the transverse certain value it will be necessary to choose either a resistance of the layer whose value per unit area is: fairly high value for LIE or a fairly low temperature. Now the wavelength ,10 of the photoconductivity R = dia, threshold is related to the energy gap LIE by the expres- where a is the conductivity of the material, which in- sion: creases in proportion to the incident light level. Ao = he/LIE, For the tube to operate it is essential that in dark- ness the time constant RC of the discharge is large in where h is Planck's constant and c the velocity of light. relation to the period 0 ofthe scanning cycle. This con- A threshold far into the infra-red therefore requires a dition is written: small energy gap LIE. In addition, in order to obtain a suitable discharge time constant, a maximum target elo ~ r. temperature T is required such that: Assuming that the material has a dielectric constant which is equal to that of free space (8.85 X 10-12 Fm-I) AoT« he [In qop,R] -1. 2k c and that the scanning cycle takes 0 -:- 4 X 10-2 second (frame frequency 25 Hz), it will be seen that a must Numerical calculation using normal values of p, and R have a value less than 2.2 X 10-10 (Qm)-I. A value as (p, = 10-~ m2V-ls-l and R = 2 X 1025 m-3) yields: small as this can only be obtained with semiconductors. AoT «221 [Lm oK. In these materials the conductivity is related to the effective concentration R ofthe electrons in the permit- This equation only gives a general idea of the form the problem takes: in fact the presence of impurities always M. Berth, Ingénieur C.N.A.M., and J.-J. Brissot, Ingenieur E.N.S.C.P., L. ès Sc., are with Laboratoires d'Electronique et de leads to an increase in conductivity, so that the target Physique Appliquée, Limeil-Brêvannes ~'Val-de-Marlle), France. has to be operated at an even lower temperature, and 1969, No. 8/9/10 Tt\.RGETS FOR INFRA-RED TV CAMERA TUBES 271 on the other hand, layers of a microcrystalline or pow- detection. Using relations and data given earlier, it can dered nature result in very Iow apparent conductivities be expressed by the equation: but at the cost ofundesirable side effects (unfavourable ÀoT « 395 (LmoK. rise times, fatigue and hysteresis). An attempt to overcome the difficulties encountered It will be seen that this condition is less severe than that can be made by going back to a microscopically homo- relating to the photoconductive target since it theoreti- geneous photoconductive substance but in the form of cally permits operation at an absolute temperature a P-N junction mosaic produced on a monocrystalline which is twice as high. substrate. The charge-storage effect in such junctions Targets of two types developed at LEP and based has been studied in detail [11 [21, and we shall merely on the above considerations, will now be described .. outline its principles. Polycrystalline photoconductive target Under reverse bias a semiconductor P-N junction can be represented by the equivalent diagram shown in Point detectors with a polycrystalline photoconduc- fig. 2. The capacitance C depends on the voltage V. tive layer and operating between 77 "K and 300 "K have been in existence for some time now. The compounds used are chiefly thallium sulphide, the lead sulphides, lead selenide and lead telluride [31. The sen- CfVJ s sitive elements generally consist of very thin layers de- posited on an insulating substrate and bounded by two electrodes several mm apart; their overall impedance at operating temperature is between 104 and lOB ohms. However, the use of such compounds in the form of thin layers operating in the direction oftheir thickness to form electrical images is much -more difficult because of their relatively Iow resistivity. Pure lead sulphide, for instance, has a resistivity of the order of 104 ncm and would not make a suitable target for a

Fig. 2. Equivalent circuit diagram of a P-N junction which is picture-scanning tube since its charge storage times are charged by an electron beam acting as a switch S. incompatible with normal scanning frequencies. This drawback will become less and less significant, however, if the resistivity of the microcrystalline photo- conductor can be increased to values higher than When the scanning beam passes over the P-N junction, 1011 ncm. it gives to the capacitor a charge Q which, between successive sweeps of the beam, drains off via the two Production of the target current generators iph and ip. These represent respec- One possible way of obtaining photosensitive layers tively the photoelectric effectand the parasitic currents that meet the condition just mentioned is to intro- (dark current, surface leakage). If, as a first approxi- duce oxygen into the lead sulphide lattice. This can mation, we ignore the effect of the current generator ip, be done either by oxidizing lead sulphide or by then the current flowing in R during the next scan and sulphurating lead oxide [41. The latter process, which corresponding to recharging the capacitor will produce has been developed by LEP, is by far the more a voltage pulse whose area is proportional to the inte- flexible of the two and enables the resistivity and gral of the illumination received by the junction be- spectral range of the photoconductive layer to be tween the two scans. controlled over a sufficiently wide' range. It should The capacitance of the junction per unit surface area however be noted that any increase in the resistivity, is almost independent of the temperature and is deter- which varies with the quantity of oxygen present.vis mined by the properties of the' space charge layer separating the Nand P regions .. On the other hand, [1) F. Desvignes and M. Robert, Phénomène d'accumulation dans des jonctions NP au germanium refroidies, C.R. Acad. ip changes rapidly with the junction temperature and Sci. Paris 252, 2693-2695, 1961. . with the energy gap LlE of the semiconductor. [2] M. Robert, Les phénomènes d'accumulation dans les photodiodes. Cas particulier du germanium, Acta Electronica 6, Since the maximum time during which the stored 341-407, 1962. charge IS retained is in fact equal to Q/i, there is, as [3) J.-J. Brissot and A. Dauguet, Les détecteurs actuels de rayon- nement infrarouge, Acta Electronica 5, 459-516, 1961. - in the case of the photoconductive layer, a condition [4) Similar principles have been applied in the "Plumbicon" linking the operating température to the threshold of camera tube with improved red sensitivity .": 272 PHILIPS TECHNICAL REVIEW VOLUME 30 obtained at the cost of response at the higher wavelengths. A satisfactory compromise consists in cooling the sensitive layer to the temperature ofliquid nitrogen. A lightly doped silicon disc forms a suitable substrate for the layer; in fact, being an electrical and thermal conductor, it serves as a signal electrode for the tube, while at the same time it transmits the cold required to cool the layer; moreover, it acts as a filter for the short wavelengths which are generally undesirable for observation in the infra-red. Preparation of the sensitive layer [5] [6] consists in raising the silicon disc to a temperature of 120 oe and depositing on it a layer of pure lead oxide with a thickness of 12 to 15 microns in a vacuum of approximately Fig. 3. Relative spectral sensitivity of a camera tube with a lead- oxysulphide image electrode ("SAPHIR" tube), at 77 "K, The 10-2 torf. The disc with the deposit is then transferred bracket indicates the visible part of the spectrum.

Fig. 4. This reproduetion of a photograph viewed in infra-red light by a tube equipped with the lead oxysulphide target illustrates the definition obtainable.

to a reaction chamber which is first evacuated and then The peak in the sensitivity at about 0.9 micron is connected to a source of very pure gaseous hydrogen probably due to the transfer of carriers from the silicon sulphide. By controlling the temperature, the pressure to the layer of lead oxysulphide. of the H2S and the moment at which the reaction occurs, A target of this kind when cooled to 77 "K and the characteristics of the lead oxysulphide formed are scanned in accordance with the conventional 625 line adjusted in such a way that the operation of the standard is able to detect objects at a minimum tem- target when placed in a vidicon-type tube is optimal. perature of 130 oe (403 OK) with a sensitivity of 2 oe if the camera tube is equipped with a fluorite optical Characteristics system with an aperture off/4. The spectral sensitivity curve of a photoconductive The minimum detectable power found by progressive- layer obtained in the above manner is shown in fig. 3. ly attenuating by means of neutral density filters the 1969, No. 8/9/10 TARGETS FOR INFRA-RED TV CAMERA TUBES 273

radiation from a tungsten filament at 2830 "K, is about 11"""__ 100V 50 20. 10 5 2 0.5 0.2 6 X 10-6 W per cm", Fig.4 gives an indication of the 2 definition of the infra-red picture obtained when aiming 19JOK 5 the camera tube at a photograph. I 10-12 Owing to the microcrystalline composition of the -- 1/ 2 photoconductive layer, there is a time constant, though ! I1 Isat not a very large one, that affects the response when 5 21 OK I--- the light signal decreases, and a slight after-image is 10-11

observed with high illumination. This after-effect corre- ....J..- 2 sponds to a value much lower than 5 % of the residual 1/ 5 signal after a 200 ms blocking pulse. 228°K ,_. 10-10 [/1--" P-N junction mosaic 2 We now give a description of a target using the 5 photovoltaic effect and based on the considerations _. 10-9 outlined in the introduction. 258°K 2 I Production of the target 5 The target consists of a mosaic of P-type regions 10-8

formed upon a thin substrate of N-type germanium. 2 The successive stages in manufacture can be summar- 294°K - 5 ized as follows: ,t,- . 10-7 a) cutting out, grinding, polishing and cleaning of the " germanium substrate (diameter 25 mm, final thick- 2 ness 250 [Lm); . f 5 b) deposition of a mosaic of indium dots on one

face of the substrate by evaporating through a metal . ''!I grid in a high vacuum; Fig. 5. Static current-voltage characteristicsjof the germanium photodiodes in the reverse-bias region, for various temperatures. c) alloying the indium to the germanium in a hydrogen The saturation current Is':t'is plotted as afunction of the reverse atmosphere; voltage Vl. .' '.;.: d) electrolytic etching of the junctions thus forrnedj. e) filling up the grooves separating the junctions with an insulating material so that the electrons of the are given below were obtained under the following con- scanning beam do not have direct access to the ditions: N-type zone. Target temperature: 77 "K' (boiling point of liquid The resultant structure is a mosaic of photodiodes at nitrogen). 50 [Lm between centres (side of cell 35 [Lm, space be- Frame frequency: 50 Hz.· -. tween cells 15 urn). With à picture size in the propor- Objective: made of fluorite with' a maximum relative tion of 4:3 this gives a definition of 300 lines with aperture equal to f/4. 400 dots per line. It should be mentioned that the operating tempera- Fig. 5 shows an example of the electrical reverse ture was chosen for reasons of convenience; in fact, . characteristics of the junctions as a function of tem- the tube operates satisfactorily at any target tempera- perature. It is seen that the lower the temperature, the ture below 170 OK. less clearly the breakdown voltage is defined and the smaller the useful voltage range becomes. At very low 1) Spectral.sensitivity temperatures, this effect limits the permissible potential Fig. 6 shows that with such a mosaic it is only pos- difference between the target and the cathode of the sible to obtain pictures-in a fairly narrow spectral zone tube to a few volts. centred on 1.5 [Lm. The shape of the spectral sensitivity .curve and the position of the detection limits in Characteristics The detection characteristics of a photodiode operat- [5] French Patent No. 1460817, granted to LEP, Oct. 1965: ing in a charge-storage mode obviously vary with the Procédé de fabrication de couches photosensibles. [0] G.-A. Boutry, J.-J. Brissot, R. Legoux, J. Périlhou and operating conditions'{i.e, temperatures, frequency and . G. Piétri, Un tube convertisseur d'image pour l'infrarouge associated optical system). The measured values which moyen, "le Serval", Philips Res. Repts. 20, 684-706, 1965. 274 PHILlPS TECHNICAL REVIEW VOLUME 30

3 '; achromatic objectives of large aperture, and another is ., that since carriers created at a large distance from the , r-, junction are not collected, there is no danger that ( lateral diffusion of these carriers will introduce any haziness into the picture. 2 1\ \ .The selective nature of the spectral sensitivity curve is illustrated byfig. 7. The first exposure (a) was made , I in visible light with a normal emulsion; the second (b) shows the picture obtained on the screen ofthe receiver \ when the subject is "illuminated" by an infra-red source. " \ '" At·I.S [Lm,water in liquid form gives an intense ab-

0. sorption band; it is the quantity of moisture in the skin, the hair or the clothing that determines the contrasts.

o 'J \ 2) Minimum detectable energy 1.4 T,S T.6 1.71lm _À r: The minimum detectable energy was measured by Fig. 6. Spectral sensitivity (in arbitrary units) of the mosaic of germanium photodiodes. progressively reducing by means of neutral filters the radiation of a tungsten filament heated to 2830 "K. With this light source the target illumination corre- ...... ".' sponding to a picture which is not distinguishable from the spectrum are easily explained. At ordinary tem- its background is about 5 X 10-8 W per cm''. peratu~es the photoelectric threshold of germanium is 3) Minimum detectable temperature 1.9. ~in; since, the energy gap of the material increases when the temperature decreases, the detection thresh- The minimum detectable temperature depends on old drops" to 1.7 [Lmat 77 "K, The shape of the curve the objective used. With a relative aperture of 1/4.5, on-the.lowerside of 1.5 [Lmis determined on the one it is possible to obtain a picture of a black body hand by the rapid increase of the absorption coefficient raised to a temperature of 180°C (453 OK). This limit ofgermanium with photon energy and on the other is lowered to 150 oe (423 OK) if an objective with an by' the short life-time of the carriers at low tempera- aperture of 1/1.5 is 'Used, this being the maximum tures. The diffusion length of the holes is much smaller value that the aperture can have in view of the geom- than the thickness of the germanium layer so that when etry of the tube. they are released near the front face by radiation of short 'wavelength (i.e. by radiation which is totally 4) Resolving power absorbed in a thin layer of the material), they recom- To measure the resolution the image of a test pattern bine before reaching the junctions. Consequently, the consisting of alternate black and white parallel bands only' carriers' which can contribute to the signal are is projected on to the target. The resolution limit is a thóse created near the junctions by photons penetrating function of the relative orientation of the scan, the sufficiently deeply into the germanium . bands in the test pattern and the grooves in the mosaic. .When the observed scene is illuminated by a source Generally speaking, interference between these regular with'à high colour -temperature, this absence of sensitiv- structures produces moiré effects which prevent the ity ~t short wavelengths is a-drawback sirice the tube resolution from being isotropic. The curve in fig. 8 only utilize's a 'fraction of the energy radiated by the was obtained by orientating the diagonal formed by source. On the other hand, if the objects concerned are thë square islands on the mosaic parallel to the line- warm objects detectable by their own radiation, the scan direction and arranging the lines of the test pat- loss of overall sensitivity is insignificant as long as the tern perpendicular to this same direction. If allowance wavelength of the emission maximum of the radiating is made for the imperfections of the pattern projector body remains much higher than 1.7 urn (T« 1400 °C used, i.e. for the contrast transfer function of this or 1673 "K for a black body). It should be noted that instrument, the curve is in good agreement with the in any case this behaviour of the target is conducive to predicted result. obtaining a clear image whose sharpness depends only on tlie distance between the. centres of the mosaic Structure of the tubes used dots and the diameter of the scanning spot. One Itis useful to give a brief description here ofthe tube reason for this is, that observation in a narrow spectral developed at this Laberatory for use with the targets band' makes it easier to design and manufacture which we havejust described. 1969, No. 8/9/10 TARGETS FOR INFRA-RED TV CAMERA TUBES 275

Fig. 7.Thesamesubject,a) photo- graphed with visible light on a standard emulsion, b) as shown on the screen when illuminated by an infra-red projector and viewed with a television tube equipped with the mosaic target. The information conveyed by method (b) is rather different from (a).

c i

Fig. 8. Resolving power of the mosaic of photodiodes, for reproduetion of a test picture with black and white stripes spaced at a distance zl between centres, the relative orientation of mosaic, test picture and scan being as shown in the inset. The measured °O~~~~~------O~.2~------O~.3~------~a4mmcontrast C between the light and dark stripes is plotted as a -LI function of zl , Moiré effects occur in the shaded region. 276 PHILIPS TECHNICAL REVIEW VOLUME 30

The electronic optical system In general outline, the structure (jig. 9) is that of a vidicon, i.e. a thermionic cathode combined with The electronic optical systems at present used in vidi- an electronic optical system capable of focusing and cons are unsuitable when the spectral sensitivity range collimating low-velocity electrons and a target receiving of the tube extends into the infra-red, since these sys- the infra-red picture on theface away from that exposed tems allow the target to "see" the cathode; the energy to electronic scanning. A set of external coils, which radiated by the latter would produce a haze on the are not shown, supply the fields for focusing and de- image which is denser the greater the sensitivity. In the flection. The differences between the structure of this case of a mosaic consisting of germanium photodiodes, tube and that of the conventional vidicon are those the sensitivity is in fact so high that the level of illu- made necessary by the need to cool the sensitive sur- mination corresponding to saturation would be greatly face and to shield it from the thermal radiation of the exceeded. cathode. There are various devices which can be used to over-

Fig.9. Construction of the camera tube. K cathode. W Wehnelt cylinder. G decelerating grid. T image electrode. In order to prevent infra-red radiation ph from the hot cathode striking the image electrode, an intermediate image of the first "crossover" of the electron beam e is formed at the diaphragm D, and the electron gun is mounted skew. The signal is obtained at S. T is cooled via the metal tube M by the ring-shaped jacket Refr , which is connected to the outside by the tube P. A cooled gas may be supplied through P or the cold required can be produced by JouJe-Thomson expansion in the cylinder JK. E image window through which the infra-red age to be recorded is projected on to T.

The cooling device come this disadvantage. We haveadopted the following. The detection threshold of the germanium target is A diaphragm D of small diameter (0.4 mm) is placed not so far into the infra-red as to make it necessary to at the entrance to the collimation chamber, a few centi- cool the walls of the tu be. The device chosen to cool metres from the cathode; an electrostatic lens placed the target is a vacuum-tight enclosure, annular in between the triode assembly of the gun and this dia- shape, connected to the flange carrying the input win- phragm forms on the latter the image of the crossover dow by a double-walled tube (see fig. 9). The vacuum (the first focusing point for the electrons from the in the enclosure acts as a thermal insulator and the cathode). Thus the aperture of the light beam emerging metal used has a very low thermal conductivity; thus from the cathode is limited without affecting the beam the heat exchange between the cooling device and the current.lt is now sufficient to tilt the gun-lens assembly exterior is reduced to a minimum. Since, therefore, slightly (approximately 4°) in relation to the axis of the the walls of the tube and, more particularly, the input tube to ensure that the light beam falls outside the sen- windoware practically always at ambient temperature, sitive surface. The electron beam is brought back into there is no danger of icing or thermal shock at the the axis of the tube by a fixed transverse magnetic field glass-to-metal seals when operation of the equipment provided by the alignment coils; then it is refocused on is started. the mosaic by the conventional axial magnetic field. Cooling can be effected either by injecting refrigerating fluid (e.g.liquid nitrogen or argon) or by producing The infra-red television camera the required cold in situ by means of a Joule-Kelvin The images obtained with the infra-red television expansion valve inserted in the double-walled tube camera developed at LEP are formed on the target of and connected to a cylinder of compressed gas or to the tube by means of an objective made of fluorite a compressor. The refrigerating power required is low, provided with an anti-reflection coating. The objective is about 1 W, which corresponds approximately to the suitable for the 1.2-2.8 [Lmband, has a focal length of evaporation of 30 mi of liq uid nitrogen per hour. 10 mm and a maximum aperture equal to 1/4. The 1969, No. 8/9/10 TARGETS FOR INFRA-RED TV CAMERA TUBES 277

a b

Fig. 10. Experimental demonstration of passive viewing in the infra-red. a) Two rods, of copper (left) and stainless steel (right), are heated to 450°C on the under- side. b) Where the temperature has fallen to about 150 "C along the rod, the luminance of the television picture falls sharply. The difference in thermal conductivity of the rods can be clearly seen.

scanning standard is 625 lines per image and 50 frames ried out with this camera equipped with a lead oxy- per second. This camera can use either of the two tubes sulphide tube. The infra-red picture (fig. lab) of two described above; its electronic circuits are almost com- metal rods (fig. lOa) of the same dimensions but of pletely identical with those of a light-weight outside- different materials heated to 450 oe (723 OK) at their broadcast camera developed at LEP for the O.R.T.F. base brings out the difference of thermal conductiv- These circuits, which are completely transistorized, in- ity between these materials. The zone of decreasing clude in particular a crystal-controlled time-base gener- luminance corresponds for both rods to a surface ator and perform all the amplification, scanning and temperature of about 150 oe (423 OK). power-supply functions which are necessary for obtaining a complete vision signal satisfying the standards of conventional television. Thevideo signals can therefore Summary. The vast majority of modern camera tubes used for the visible part of the spectrum operate on the charge-storage be connected to a direct-viewing television receiver or, principle. In the infra-red, because of the low individual energy if desired, recorded on video tape for viewing at a later of the photons, application of this principle runs into difficulties of both a fundamental and a technical nature. A brief examination moment. The target is cooled with the aid of a two- of the physical aspect of this problem permits the establishment stage Joule-Kelvin expansion valve [7] supplied with of relations between the target temperature and the spectral detection threshold; these relations differ depending on whether gaseous nitrogen (or argon) of high purity (a filter is the photoconductive effect or the photovoltaic effect is considered. incorporated in the camera) and at a minimum pres- This article discusses the technical aspects and behaviour of two types of target which have been developed: one is photoconductive sure of 100 bars. and consists of a microcrystalline, macroscopically homogeneous An experiment on closed-circuit television was car- layer of lead oxysulphide (photoelectric threshold at 2.7 fJ.m); the other is photovoltaic and comprises a mosaic of P-N junc- [7J M. C. Lefranc, Le refroidissement des détecteurs de rayon ne- tions formed on a monocrystalline substrate of germanium ment infrarouge, Acta Electronica 9, 47-90, 1965. (photoelectric threshold at 1.7 fJ.m).