Thin film circuits—Part II. The development of thin film resistors and capacitors for microcircuits

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Peqpmma Prim 1964. Vol. 3, pp 13-23.

Printed in Gre~ Britain

T H I N FILM C I R C U I T S Part II. THE DEVELOPMENT OF THIN FILM RESISTORS AND CAPACITORS FOR MICROCIRCUITS Royal Radar F.~d~lishment ~Miczmniniatuee clrcuim ma be made from thin fiitm of haulatin8, nm~ve or conductive m t t e r i ~ which may be depmited on to suitable m ~ r a t e t by nmam of eValmration. Thin article de~rib~ the making of m~h fxlnmmd of compon~m mw.hu remism~ ~ ~ ~ them. Nickel chromlum is used for r e ~ m n snd silicon monoxide for c a p ~ m n ; ~ ~ ~ rmtde from Ip~d or ~ alloy. The mirah., been to produce depmieed circuit elements into which uemiconductora will later be in~; the ~ stt~ are then to be n~de into frillyselded modttlat contalnle~lool~olete functimml ~culm. Before incorporating into ~ded modula the individual componenm must be md:)la and eesi~nt to ordinary climatic conditlom likely to be met during manufacture, storase and mbly. MODERN electronic equipmenm are so complex that traditional componenta and methods of construction are no longer satidactory if the requirements of high reliability are to be met. At present, electronic components are made for a wide range of equipments and by a large number of different manufacturers; this results in lack of standardization which is a serious matter if the maximum component density is required. Recently there has been a trend towards the manufacture of components to a "professional" instead of to a "domestic" standard. This is because the makers of computers, communication and navigational apparatus have insisted on improved quality components and have been in a position to place orders with those manufacturers who can meet their SlX'Cificatiom; the Service m e n have not been so favourably placed becatme their orders are usually small and sporadic, thus they have been forced to use the best available components tested against their own rigid q)ecifications and not always meeting them. The microc~-uit concept a' is more than a method of packing a large number of components into a small space; it is a method of making all lmmive components (and perhape later, active ones as well) under one set of controls and to rigid specifications, including climatic, vibration and

shock testing. As described, the methods are part of a complete system for making circuits which are later assembled with semiconductors to make functional subu~mblles. Thin films can be nmle by nm~y methods, but the ones to be described use the vacuum deposition of metals, oxidea and alloys becanse it is possible to obtain very good control of the processes under production conditions. Modern plant has been developed in recent years and it is possible to buy excellent eqnipment (l~ to carry out the processes described in this article. Before any thin films can be made, the material for the sutntrate must be selected. Becanse heat is used in many of the processes, organic nutterish are unsuitable; many have high vapour pressures and are unsatisfactory when a vacuum of the order of 0-1-04)1 I~ (10"4-10-s tort) is emential. Of the many possible materials only ceramics and glasses are satisfactory, and even the former cannot be made with a sufficiently good surface finish. Soda glass, used in common m i c r m c x ~ slides, has a very good surface but suffers from high conduct/vity at high temperature and is unstable in the presence of high h-midity by nature of its absorbed water layer. Borosilicate glass fit not usmdly available in the form of smooth, flat pieces but am be optically worked to the 13

14

H. G. M A N F I E L D

required order of smoothness, but this is an expensive p ~ro,:ess__.Where it is available it makes the ideal substrate when the requirements of longterm stability in unfavourable environments must be met. Thin films for electrical purposes are in the range 50A to a few microns in thickness, cs, If it is n e e e ~ tO reprodu¢~ similar properties over the entire area of a reasonably sized substrate, say 3 cra × 2 cm, the surface smoothness should be of the order of about 10-50A. Large irregularities such as ripples may not be important, but the absence of scratches is vital because a small scratch across a thin resistive line will result in a localized voltage stress and will lead to subsequent failure. A microphotograph of the surface is the only way of ensuring that the substrates are'of the necessary quality. Resistive films will he comidered first. Although the properties of thin films are not the same as the bulk materials, it can be seen from a survey of the latter c'j that the pure metals are not likely to yield a high r'-~dstance within a small unit area. Copper, gold, silver, aluminum and nickel all have resistivities less than 10-eft cm; a strip of any of these metals 1001~thick and 0.04 cm wide has a resistance of about l t~/cm length and this is completely inadequate because the resistors made from them would be too large to insert into a micmcirouit. Nickel-chromium alloys are an order higher in resistivity and have a history of use in talking resistors, but alloys with at least an order higher resistivity, if available, would solve many problems; in the absence of such materials the nickelchromium alloys are attractive because of their inherent stability and low temperature coefficient of about 100 parts per 10s/°C. While the actual temperature coefficient of resistance must be chosen in terms of the circuit function and the environment of the equipment, the stability of the resistors is essential, and it is desirable to state this in quantitative terms. For resistive films under discussion the maximum working temperature could be 150°C. If 1000 hr is taken as a reasonable time for purposes of measurement and test, then a change of 0.1 per cent is considered utiafactory; this is to be measured after any short-term changes have occurred during ageing cycles, etc. As a comparison, the standard method of endurance t~ting metal oxide film

resistors,s, (up to a nominal value of 100 kl~) requires that they should be fully loaded and then submitted to 125°C for 2000 hr, after which they must not change by more than 0"3 per cent. As has already been stated, the films are of a different character from the starting material and they behave d/fferently.~S' The assessment of the composition of film a' deposited on to a glass substrate is difficult. Ordinary analysis is not possible because of the microscopic quantity of material deposited, but even ff it were it would be of little value. Spectrophotometers have given some idea of the composition, but the electron microscope has been used to show how the film is actually deposited; from a study of pictures published by Pashley c*' it is concluded that the particles of an alloy of nickel-chromium behave very much in the manner of a liquid when joining to each other. It is also thought that most of the resistance occurs at the interface between the particles; if these become large, as is the case when the films are very thin, the resistance is anomalous and extremely temperature-sensitive" besides being very noisy. When deposited by vacuum methods, fractioaation occurs. The original nickel-chromium aUoy contains a small quantity of silicon; the vapour pressures of the three constituent elements are shown in Table I. The chromium is deposited first and supplies the good adhesion to the glass substrate which is essential ff the films are to be stable and to make it possible to solder, or otherwise connect to electrodes. The silicon is the next most volatile element, to be followed by the nickel. Analysis shows that the most likdy proportion of chromium in the resulting film is about 42 per cent, but the relative proportions of nickel and silicon is unknown. There are various ways in which the alloy can be evaporated on to the surface of a substrate. A sintered alumina crucible has been used with su _cces_s, but there is some contamination from the alumina; a more reliable and simpler method is to sublime the alloy from a heated wire maintained at such a temperature that it is held just below its mehing point, 1400°C. If the wire has a --;form cross-section which is ac~ar~_~_!y controlled, and this is the case with n i c k e l ~ u m wire, its temperature can be controlled by the simple expedient of m s i n t a i n ~ a constant current

T H I N FILM CIRCUITS~PART II

piked in am oven E :~O°C, dse vatuz a/the radmm incremm =pin due to mrfEe osidafion. To emure stability it is ementisl to q e in this way. From these remdm it might be concluded that d z mmt m b l e filnm are ~ dter tbe third sublimmio~ as in this cme the fmsl value is clmmt to dse r~luired and monimmd value, but dsen= are two disadvantq~ in this. Fmmtthat the ~ are

T~I ~ t

Vspour Pmmue (un.r) at 1200~

Chromium Silicon Nicl~

9.12 x 10 -4 8.91 x 10 -4 2-82 × 10"*

IS

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fibsment, and secondly that the ~ent of these 6s,,, is nesmive; mmurements and long-term ~ests have proved conclusively that this is undesirable in the ~ of stability and that a film with a positive unzqpaztu~ coeffscient of about I00 parts/lO'/°C is the most stable. It is obvious from , h ~ mndm that the ageing process is vital because the final value of" the resistors is dependent on it as well as their longterm stability. In practice the filmment is used only once and the ageing process is continued for a controlled time---lO rain at ~O°C is muisfacu~-but from curve No. 1 of Fig. 1 it cml b¢ seen thgt it is necessary to dose the shutter over the filmmmt

throughout the sublinmtion time. Because the evaporation occurs from the surface of the wire, the alloy content changes with time; in order to produce ilium of controlled and uniform quality it is b a t m use each filament only once altlmush, from the history of previous use, it is pmsible to make films with known but different properties by using ~ szme filmnent a number of times. F'~ 1 slmws that five successive sublimations from o n e filament differ from each other when aged in sir after removal from the bell-jar. When the monitor value of 300 fl/sq was reached the sublinmtion was discontinued, but the resistance value does not remain constant. When removed and

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16

H.G.

MANFIELD

before the t~tuited value of ohrr~ per square is obtained and then to age until thi, is achieved. Although the m e c h a ~ m of depmition is not completely undenttood, it is known that a heated subsuite is dmixab~ If this is controlled at 300°(2 30° the rmulting films are very stable and Within the criteria of smbilit7 already laid down. HoUand~7~suggests that the heated substrate ~ids diffusion of the nickel into the chromium to produce a more uniform alloy. A baking process after removal from the evaporator is essential to produce stable films and those that have the minimum non.cyclic change of value with time. The resuha are more satisfactory if this is carried out in sir rather than in ~ agrees with the knowledge that the film must be fully oxidized to be stable. Films made on cold substrates and without the correct ageing cyde are poorly adhesive to the glass and in an extreme case may be rubbed off. The correctly-made film ¢mmot be attacked with normal reglents: typically, boiling in concentrated hydrockloric acid (36 per cent HCl) has no effect on the resistor value. In production it is very difficult to maintain a vacuum better than 10-5 torr when a large number of substrates are to be coated and when a fair proportion of the apparatus within the vacuum system must be maintained at a high temperature. This order of pressure is quite satisfactory, although it is best to maintain this within a close tolerance--say between 8 x 10-s and 2 X 10-s to.r~ this is rarely done as deposition is generally carried out as soon as a suitable pressure has been obtained, and this varies according to the state of the equipment and the degree of sealing obtained at the many joints. For making resistors two approaches are possible; the films can be deposited through stencils or masked as described previously; c'' this requires that the substrates and masks are held together in exact register and as close as possible to avoid shadowing. T o ensure uniformity of deposition the substrate holders are rotated at a speed of about 120 rev[min; for monitoring the value of the film as it is deposited a controlled square is used. The connexions from the rotating table are taken through slip rings and a vacuum-tight joint to a suitable bridge external to the evaporator. Figure 2 shows the rotating table together with

insets and retainers for the substrates. Masks are located on the dowel pins which emure reproducibility between sequential patterns. The controlled square is deposited on to a piece of glass which has an electrode at each end, making contact with springs insulated from the metal by means of mica plates. As the film is deposited it bridges the gap between the electrodes, so that its resistance can be measured throughout the time of deposition and the operation controUed by actuating a shutter between the source and the subsu'ate. Fig. 3 is a diagranmmtic view of the ew_porator, in which the relatiomhip of the mtatlng table to the source and to other pieces of the apparatus is shown. A diffusion pump is backed by a rotary pump and a b~lle valve isolates the bell-jar from the pumping system for changing sources, substrates, etc. The substntes are heated by a simple radiant heater, and a thermocouple in contact with the glass measures the temperature. The source, in this case the nickel-chromium wire, is wound as a fiat spiral and is mounted about 4 in. below the substrates. An alternative approach to making resistors is to deposit the alloy in the same way as already described but over the entire surface of the substrates and then to print and etch this as for a printed circuit, with the difference that the film is resistant to simple acids and must be etched in a mixture of suiphuric and nitric acid with acetic acid added as a modifier to slow down the reaction, tsj

When resistors are to be made by either method it is necessary to deposit a uniform film thickness and to obtain the maximum value of resistance within the smallest area. The r~istance per unit square should be as high as possible, but it has been pointed out that a very thin film is unstable and noisy. The minimum thickness on glass known to result in a stable film is about 50 A, which produces a resistance of 300 Q/sq. Using the wello known aspect ratio techniques for making resistors of many values from one uniform starting material, and allowing an area of 25 mm z per resistor (this is just about as much space as can be tolerated for each component in a circuit on a 3 × 2 cm substrate), a 10 kf2 resistor can be made with a line width of 0.1 mm and with similar spaces between. If the resistor is to comist of a straight line, and this is usually the case if the value is low, it is

FIG~ 2, Rotating table with insets a~d reta/ners for the substrates

THIN

FILM CIRCUITS-PART

II

17

men viral of tort of mtery t e ~

Fro. 3. The evaporator.

merely m a m a r y to divide the required value by the rmistance per square and multiply this by the line width which givm the length of line required. A simple ~ , - p i ¢ illustrates th;, technique. From a resistive Sire of 300~/sq it is required to make two r a h a b , one of I k ~ and the other of 100 kfL In the fmR came it is essential to decide on a fine width which is mmemed in terms of current carrying c q ~ i t y as its upper limit and on the practicability of making in production as its lower limit; this latter is between 04)5 Jund 0-1 ram. For digital and similar low power circuits the current carrying capacity is negligible and a width of 0-1 mm could be used as # compromise. T h e sises of the two resistotu taken as an example thin bec(ane I

l ~ l u i r e d resistance

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100,000

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Both of these are extreme ames and are equally impracticable-the f o r m ~ becmme the very short length makes an impossible demand on accunte pmcmaing if a d i n e tolertuce is requinW; the latter because it is too cumbea~xne to be incorporated into mint circuit patterns. T h e almmatives are to increase the width and in comequence the of the first example and to permmde the designer to reduce the value of the latter Lfit is at all possible. T o reduce the line width in order to red u ~ its size ia imaardmm as it m the chances of a f~ilure during ~ ; a small dust partide can produce faults in lines of this width, and although the same argummnt could be used that a dust particle csn creme a blemish to occur on a wide line, there is less chance of this latter being reduced to an ummable ctom4ecdon. A practical range of values of resistms to be deposited on a 3 × 2ona mahatrate is from 200 × to 30 kQ ~ith an accuracy of I0 per cent on any rmistons w i t ~ - • batch, although ratios between resistors on a particular aubstrate can be closer than this--pomibly 5 per ceaL Rmistom oumide those values can be ~ but, in the cme of the ~wcr

18

H.G.

MANFIELD

values, the tolerance on value may not be better In the making of capacitors the same equipment than 20 per cent; higher values than 30 kf~ can be is used as for reaistm,s, in fact this is why the obtained but they are obviously rather large and evaporative methods are so attractive, as it is may have appreciable capacitance to adjacent possible to make all the pamive components with conductors. similar equipment and methods. In the U.S.A. Current-carrying capacity has been mentioned. resistors, capacitors and conductors have all been Where this is at all critical, a l b ~ m c e must be made during one pumping cycle, but this calls for made in design. The limiting factor is when the elaborate tools and masks. The films have eventuresistors become so hot that they lift off the sub- ally to withstand adverse environmental constrate, or when they exceed the ageing temperature ditions, and there is little to be gained by processso that in effect this is extended with a comequent ing inside a vacuum chamber when exposure to change of resistance. In measurements made on air will rapidly oxidize them. suitable resistors the surface teml:erature was A capacitor consists of two electrodes with a 200°C when dissipating 4 W over an area of 1 era =. dielectric sandwiched between. Although gold or The resistors were still stable after this abnormal copper would be more convenient, the best elecloading as the temperature was well below the trodes are made from aluminium because this ageing temperature of 300°C. metal adheres well to the dielectric and so elimiWhen the resistors are to be used in a circuit or nates a small void which could be ionized to result even for test and measurement purposes, it is in low voltage breakdown. When making several essential to provide them with sound electrodes to circuits at once with the apparatus similar to that of which connexions can easily be made externally. Fig. 3, it is convenient to load one set of masks at a The nickel-chromium alloy adheres strongly to the time and to evaporate each component part in glass, but gold and copper have a very weak bond turn. and cannot be soldered satisfactorily. If the entire There are many possible choices for a dielectric circuit area is underlaid with nickel-chromium alloy material, but stability is of the utmost importance, the subsequent deposition of gold or copper ad- with power factor comparatively unimportant for heres well and may be soldered byordinarymethods, most applieatiom. Table 2 compares several using tin-lead solder and unactivated fluxes. materials. The choice of permittivity is dictated by Two methods are used to make strong, solder- much the same considerations as were discussed able connexions. First the gold is depmited over for resistors; if a material with a high permittivity the nickel-chromium to a thickness of about a is used the capacitance value per unit area is high, micron; then it is heated for 10 rain at 200°C and when a small capacitor is needed, usually with which diffuses the gold into the alloy and prevents close tolerance, it becomes very small in physical it dissolving in the solder on application of the size. A simple formula deduced from classical soldering iron. Another method which is used theory states that a dielectric thickness of when it is not required to use a nickel-chromium 1 l~/cms of electrode area yields a capacitance of underlayer is to evaporate manganese and copper 1000 pF when the permittivity (k) equals unity. simultaneously from a single source---a molybWhen the dielectric is silicon monoxide, whose denum boat is very satisfactory T h e manganese k is 6, the yield is 6000 pF/cm s or more conhas a high vapour pressure and so is fractionated veniently, 60 pF/mm 2. Thus capacitors of a few to "wet" the glass and provide adhesion before the hundred picofarads, which are most common in arrival of the copper, which is then deposited to the digital circuits, will be fitted into a maximum area required thickness and can be soldered as before. of about 25 mmL A proprietary copper-manganese alloy is available; The choice of silicon monoxide has been dicwith this it is merely necessary to cut a length of tated by the fact that it is very stable even in the wire, watch it melt with a given current and con- presence of high humidity and is fully compatible tinue evaporating until the required thickness has with the masking methods and equipment used to make resistive films. Its properties are shown in been built up. Examples of resistors with suitable electrodes Fig. 5 where it is compared with silicon dioxide '°' which has losses at least an order lower than the made as described are shown in Fig. 4.

T H I N F I L M C l R C U I T S - ~ P A R T II

!

powders. Again, a ,hatter controls the ~-,:y time of deposition and keeps the mhstrates ,way from

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Fro. 5. Dielectric properties of silicon monoxide and dioxide.

monoxide, but also has a lower permittivity (3.5) and yields, in consequence, a lower value of capacitance per unit area. Although a great deal has been written about SIC), little is known of its true composition. It is assumed to be an atomic mixture of silicon and silica and is sometimes referred to as a sub-oxide. It is deposited in a vacuum of about 10-s tort (as for resistors), but there is a strong gettering action which can reduce this still further. A proprietary mixture of silicon and silica in powder form is a sathfactory material from which to evaporate. It does not melt and is inclined to jump out of a crucible and, in consequence, the most ~ s ~ c t o r y source is a molybdenum boat with a doeely fitting lid in which a number of small holes have been perforated with a needle. A heavy current source and supply are essential, as a substantial boat rr~_de of molybdenum sheet 0-002 in. thick represents • low resistance and, in consequence, the current is typically about 150 A. Control is difficult because the mixture does not melt but is sublimed; a boat temperature of 12500C is usually correct, but this obviously depends on the thermal conditiom inside and the contact made between the boat and the

Because the cempmition of the films is unknown, the methods of c o n t e m n , the d e p ~ t i o n a t =om~hat arbitrary but, as judl~! by the dectrical quality of the films, a depmition rate of between 16 and 18 A/sec h a been shown to be burst stable. The permittivity is then 6 at low radio frequencies and the power factor about 0.015 to 04)2 which is adequate for most purposes excluding special falters and r.L circuits where low losses are essential. The apparatus for making these dielectric films and the associated electrodes looks identical exteruaUy to that used for making resistors, but it is convenient to keep the processes separate to avoid contamination. The contour of the capacitors is made by out-of-contact masks, as in this way the edges of the electrodes are diffused and the stress is less than at a clean sharp edge. When capacitors of higher value than a few hundred picofarads are required, materials with a higher dielectric constant could he used, but these are not developed in the form of thin films and it is doubtful if they would yield values of capacitance several orders higher than the present one. At pr'-~oent the only solution to this problem is to use electrolytic capacitors added separately. To sum up the properties of re3istive and capacitive films that have been developed for microminature techniques, it can be said that they are adequate for most digital electronic applications where modest values of resistance and capacitance are required and where the power factor of the capacitors is not critical. There is a need for materials with higher resistivity, but similar temperature coefficient to nickel--chromium. There is also a need for dielectric materials with lower power factor than SiO for high quality "low loss" circuits, and with much higher permittivity for circuits where a high capacitance is essential; in this connexion it must be noted that the normal "high k" materials have poor behaviour at high temperatures and their Curie point may be below 120"C. Among pmaible starting materish for higher value reshtor5 arc tantalum and tellurium, of which the first is probably the more satisfactory and

20

H.G.

MANFIELD

certainly the one with the better pt~ioua history. It can be anodised to c~atrol its rmistance, tlthongh this brings in a ~ process which is not desirable when mixed with evaporative procem

Aaermtiny, by proved en0aeer ,

narrower lines and spaces for higher value resistor8 could be made. There is a wide choice for dielectries including tantalum pentoxide (either sputtered or chemicatly dep~__i_ted), silicon dioxide, cerium fluoride, lanthanum fluoride, magnesium fluoride and many others. As it is ~ l t to protect thin films from the effects of moisture, it is essential that they are stable in the presence of high humidity. Table 2 compares the various materials,

resistive or capacitive, deanlinas at every stage is essential The cleaning of the glare meat be carried out thorougldy; alternative methods are given in an appendix to this article. A "'dean room" or dean areas are essential if faults are to be eliminated, and this is particularly so with the capacitors; whereas in a resistive film a foreign body could cause a small indentation or cause a piece to be missing from a line, it could cause a shortcircuit in a capacitor. Every process should be controlled so that handling is reduced to a minimum, even that by operators wearing clean clothes including gloves and caps to prevent contamination by body fluids.

Tab& 2 Material

Silicon Monoxide Silicon Dioxide Cerium Fluoride Cerium Oxide Magnesium Fluoride Tantalum Pentoxide

Permittivity 1 kc/t--I Mc/s (at 20"C)

Power factor 1 kc/~-I Mc/s (at 20"C)

Working voltage per micron (approx.)

Resistanceto moisture, and remarks

6-0 3.7 7-9 12 5 25

ff01--0-02 0-001 005 High 0-01 0.001

50 50 50 30---50 30--50 50

Very Good Very Good Poor Very Poor Variable: tends to craze Excellent

and it can be seen that only silicon dioxide, silicon monoxide and tantalum pentoxide are resistant to moisture. Which of these is selected as a dielectric is decided by the conditions of manufacture. If an aU-evaporative system is required but the capacitors are not required to possess very low losses, sih'con monoxide is the best material. For a chemical method there is little doubt that tantalum has much to commend it, especially as it is possible to make resistors and capacitors from this same material 'ze~ and to use anodizing as a method of value control. Where high value capacitors are needed, it may be possible to use a thin dielectric---say 1000 A in thickness--and m,e many layers; this requires good engineering to establish accorate reghtration between layers, but with a material such as tantalum pentoxide a capacitor with ten layers each I000 A thick and 1 cm s in area would have a capacitance of 2.5 ttF which could be very useful as a decoupler in tramistor circuits. Throughout the procesaing of thin films, either

Fluxes used in soldering should be non-activated and their residues removed by suitable solvents. Figure 6 shows a dean room especially designed for making thin film circuits. The evaporators, monitoring and glass-cleaning equipments are shown; apparatus for photomechanical processing is in a comer section of the same room, so it is possible to process and evaporate the film circuiis without removing them from dean surroundings; they are taken into another lahouratory for hole drilling and for the addition of semiconductors prior to testing and assembly into modules. T o illustrate the making of a circuit using resistive and capacitive films, as well as microminiature semiconductors, Figs. 7 and 8 show all stages in the many proceaaea, including drilling holes by means of a diamond impregnated drill. This latter stage is inserted between resistor and capacitor making so as not to damage the thin capacitor films. Both out-of-contact and incontact masks are used to r-lk~ patterns of the required size and value.

laqc. 4, Example~ of reslstor~_ fa~i~g pogc 21}

FI<;. 6. A "c!ean rtmm" dmigned for making the titm circ~lits.

T H I N FILM CIRCUITS-PART II

21

~'il

8orcmlicaR ~ J,*~ i J i ~,,,-

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- - - w O.,I micrcm thick

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Subllmte coated with a film of evaporated copper

iw,.~m~

the copper and dried

~ , ~ l i l

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A photo positive is placed in close

The unexposed lacquer is rlmove¢l during development and ~ unpcotlcted copper is etched owoy in ferric chloride

contact wire tt~ i a c q ~ . The unshieided Iocquer is exl~Imcl to ultra violet light

/lot

Sto~ ~1

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NiCkel chromo o t ~ is ~ ~ h the high ~ lin ¢mNoctecoppl¢

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resistive pattern

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22

H.G.

MANFIELD

s~e]

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Aluminium is evaporated thmu~ o metal foil mask

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FIG. 8. Further smiles in ruskin8 complete c/rc-_/u,.

THIN FILM CIRCUITS-PART T h e final circuit may consist of a complete dig{tal counter or other circuit, and the design and layout of such circuits are the subject of another article. T h e s e circuits are not designed to be used

in the form required into circuits and enclosed in a

shown, but they are mounted as a module consisting of several such then encapsulated in a resin or metal container with glass-to-metal

seals for the terminations. Assemblies of this kind are strong and able to withstand climatic, shock and vibration tests as imposed by military

specifications. Admo~/edgmerttt--Acknowledlprtent is made to MuUard E.C. Division and to the author's colleagues who have participated in the work described in this article. APPENDIX The cleaning j i p used are open b n m frameworks with grooves to hold the glass.

Ca) Deu~ent ,~tkod/or ~

s/as,

(1) Three dishes are filled with demlneral~ed water and 2~t per cent by volume of detergent mludon added to the first. ']'he dishes are preferably of plmtic to avoid scratching the gimm.Photographic developing dishes are suitable. (2) The operations are all carried out using rubber glov~.

(3) The slides are scrubbed with detergent in the first dish using a square piece of synthetic chamoh leather and tranderred to the second dhh. (4) The process is repeated in the second dish using clean material and the slides placed in the third dish. (5) The slides are loaded under water into jigs and transferred to the rearculating tank for S rain. (6) The jilts are rinsed in each of the twin ultrasonic beakers in deminend/zed water for 3 mln. (7) The jigs are rinsed once more for $ rain in the recirculating tank. (8) The jigs are agitated in isopmpyl alcohol to r ~ n ~ v e t h e water.

II

23

(9) The jigs are left in the degreaser in isopropyl alcuhol v~xmr until dry, taually 3-5 rain. (b) Hydvognm pmxx~/e mahod

(I) The slidesare scrubbed with • Selv~ cloth soaked in alcohol and tramferred to the jigs. (2) The jigs are heated in 30 per cent Analar hydrogen peroxide solution for 10 mln at between 60 and 750C. The upper Limitshould not be exceeded. (3) The jigs are transferred to the rec~--ulating tank for 5 win. (4) The process is then the same as in (a) sections (6) to (9) inclusive.

~ C E S I. G. W. A. DUMMY, A Review of British Work on Microminismrfimtion Techniques. E/Ktron. Rt//ab. Mk'rmm~. 1, 39 (Jsn.-Msr. 1962). 2. L. H o ~ , Vacuum Depmition of Thin Films. (Chapman & Hall 19S8). 3. R. H. At~mrroN and F. ~ w r t t . Vacuum Deposited Films of Nickel/Chromium Alloy, Bn't..7. AppI. Phys. p. 205 (May 1957). 4. G. W. C. KA~rland T. H. LAaY, Tables of Physical and Chemical Consumta. (Lon4pmtns 1952.) 5. Defence Specification DEF-5114, "Resistors, Fixed, High Stability, Film (Metal and Oxide)." 6. G. P,~m.~, Nucleation, Growth and Microetructure of Thin Film*. Conference on Evaporation.

(Proceedings pubi. by Wiley 8: Sons, 1959.) 7. L. HotJ.A~m, Lo¢. c/t., p. I94. 8. H. G. MA.,~lx.u., The Uses of Phommechamcal Processes in the Manufacture of Micro-minlature Circuits. ~ o n . E~q~g,. 35, 520 (1963). 9. B. W. ~ c m m and S. S. M r r ~ , Measuring Thic.imm m-~ Cocnpcaidon of Thin Surface Films by Means of an Electric Probe. Prec. 1962 Electronic Component, Conference, p. 153 (Washington). 10. T. V. Stra~^, High Density Tantalum Film Microc/rcuita, Proc. 1962 Electronic Components Conference, p. 24 (Washington).