182 PHILlPS TECHNICAL REVIEW VOLUME 27

Integrated circuits with evaporated thin films

E. C. Munk and A. Rademakers 539.234 :621.3.049.7

In this article we shall discuss a form of integrated itance or inductance value and the Q that can be circuit in which the conductors, the resistors and the achieved with the thin film components may be too capacitors (and in some cases the inductors as well) are limited. deposited as thin films in a pattern of strips and rec- An example of a thin film circuit is shown in jig. J. tangles on an insulating substrate. The advantages General considerations usually considered in the integration of circuits have been made sufficiently clear in the previous article [11, The - unlike a circuit built up and need not therefore be reiterated here. from individual components - is not flexible, and sub- The thin film technique we shall deal with in this sequent correction is acceptable only in exceptional article may be regarded as an extension of the tech- cases. This means that it is necessary to control the niques in use for making conventional resistors and production process so thoroughly as to make later

Fig. I. Example of a pattern of conductors and resistors in an integrated circuit. The conductor layers (dark patches) are tinned. capacitors, in which thin layers of carbon or metal are correction unnecessary; in other words, it should be applied to an insulating body or dielectric foil. There cheaper to scrap a product that fails to meet the speci- are various methods of applying the layers, but we shall fications of the design than to correct it. be concerned only with those where the material is This lack of flexibility becomes particularly evident if evaporated on to the substrate in a high vacuum. one compares a resistor in an integrated circuit with a The su bstrates are small flat plates, or wafers. This resistor as a separate component. In both the resistive is important both in the manufacturing process, as it material is applied in the form of a thin film to an facilitates work with masks and photographic processes, insulating substrate (which is cylindrical in the separate and in the assembly of the circuits in larger systems, resistor). The val ue of the resistance is given by: since the plates can be stacked and interconnections e I R=- - made at the edges. tb' It should be emphasized that the circuits treated in this article are not completely integrated but of a where e is the resistivity, and I, band t are the length, hybrid nature. In a completely integrated circuit the width and thickness ofthe resistive film. The factor elt, active elements as well (diodes and ) would the sheet resistivity (also referred to as the resistance have to be made by techniques. This is per square, RO), is a property of the film itself, and the possible in principle, as explained elsewhere in this length-to-width ratio lib is called the aspect ratio. The number [21, but such elements are still at the research relative error in R is the sum of the relative errors in. the stage. In practice, the active elements are therefore sheet resistivity and the aspect ratio. A conventional added as separate components to the thin film circuit. resistor is made by first applying the resistive material Moreover, it is quite often necessary to fit separate to the body. The accuracy of the sheet resistivity is not capacitors (and possibly inductors), since the capac- critical, since the resistor is given its value with the required precision by grinding a spiral groove into the Drs. E. C. Munk is witli Philips Research Laboratories, Eindhoven, surface . This corrects the aspect ratio after the and Dr. A. Rademakers is with the Nijmegen Plant, Philips Elcoma Division. evaporation process. Precisely the opposite applies to a 1966, No. 7 TH1N-FILM INTEGRATED CIRCUITS 183

resistor in a thin film circuit: the dimensions of the nesses are still extremely small, i.e. of the order of resistor are fixed before the film is deposited on the 10 nm. substrate. If there are to be no subsequent corrections, Various other considerations besides those already the aspect ratio and the sheet resistivity are therefore mentioned play a role in the fabrication of evaporated established independently of one another, which means resistive films. In particular we should mention the that this must be done with tolerances which together efforts to achieve films of low temperature coefficient do not exceed the rated tolerance of the resistor. The and high stability: this will be discussed later. requirements for the process of applying ~he resistive If capacitors are to be built up from evaporated thin layer are therefore much stricter than in the manufac- films (a dielectric layer between two metal layers) ture of separate resistors. These requirements become similar considerations indicate the requirements to be even harder to meet if the films for the resistors in a met in the control of the evaporation process, in large number of circuits are to be evaporated simul- particular for the deposition of the dielectric layer. For taneously. The resistive layer must then not only be the conductors, the process is of course less critical. extremely homogeneous over the entire surface of each For inductances the thin-film technique is by its substrate, but must also be identical from one substrate nature not particularly suitable. Without going into to another. These requirements can be met by using the detail, two points may be mentioned in passing: the evaporation technique. three-dimensional character is of more significanee with the inductor than with other elements, and it is The problem can be solved in an entirely different way if difficult to concentrate magnetic in a small subsequent individual corrections to the sheet resistivity are allowed. The methods developed by IBM and the Bell Laborator- volume. For simple requirements, inductors may be ies are examples of this approach [3][4]. In both methods evaporated in the form of spirals. In practice this relatively thick films are used, and the thickness is reduced after method cannot be used for inductances greater than a deposition in the required pattern. In the IBM method the few tens of (.LHat the most, and to achieve a fairly resistive material, in the form of a paste, is screen-printed in the reasonable Q the films have to be thickened, e.g. with required pattern upon ceramic wafers. After heat-treatment, the resistance is corrected by sand-blasting. In the second method the a layer of gold. resistive material (tantalum with tantalum nitride TaN) is applied Thin-film effects to substrates by in a gas discharge. After de. position, part of the film is converted by electrolytic oxidation Before going further into the production of thin-film into non-conducting Ta205. The latter process can now be so circuits, we shall first discuss some physical peculiarities well controlled that a very high accuracy can be achieved. During the process, however, each resistor has to be individually checked. of thin metal films. Moreover, optimum properties can only be obtained provided the The resistance of thin metal films is usually greater sheet resistivity is given a relatively Iow value (e.g. 40 Q). This than the value calculated on the basis of the film thick- means that resistors with a high nominal value can only be ness and the resistivity of the metal; in other words the produced if the aspect ratio has a high value, and this makes it "effective resistivity" of the film is greater than that of difficult to meet the requirement for small dimensions. the bulk metal. An effect of this nature is to be expected in principle By way of introduetion to the subject, let us make an from the theory of electrical conduction in metals. Here estimate of the thicknesses of the films used in these if the thickness of the films becomes smaller than the techniques. Suppose that we wish to form a combi- of the conduction electrons, the scatter- nation of resistors with a tolerance of 5%, the highest ing of the electrons at the surface will make a percep- nominal value among them being 10 000 0. We tible extra contribution to the resistance. It has in fact choose for this a strip 1 cm long and 100 (.Lmwide. proved possible to determine fairly accurately the mean The aspect ratio is then 100 and so the sheet resistivity free path from resistance measurements on thin films, at is 100 0. If we now unsuspectingly put in the bulk least for very pure metals in virtually ideal thin films. resistivity value for say aluminium (2.5 X 10-8 Om), we Ideal films, that is to say films that are continuous, arrive at a film thickness of 0.25 nm! The tolerance of uniform and homogeneous, are however very rarely 5 % now has to be shared between the aspect ratio and encountered. Usually resistance anomaly in thin films the sheet resistivity. This implies that we have to reproduce not only the width of 100 (.Lmbut also the [1] P. w. Haaijman, Integration of electronic circuits, Philips tech. Rev. 27, 180-181, 1966. thickness of 0.25 nm with a tolerance certainly below [2] H. C. de Graaff and H. Koelmans, The thin-film , 5%. This is scarcely practicable, and it follows that Philips tech. Rev. 27, 200-206, 1966. . [3] E. M. Davis, W. E. Harding, R. S. Schwartz and J. J. Coming, materials of higher resistivity must be used. These are Solid logic technology: versatile, high-performance micro- to be found in alloys having resistivities of the order of electronics, IBM J. Res. DeveI. 8, 102-114, 1964. [4] D. A. McLean, N. Schwartz and E. D. Tidd, Tantalum-film 8 100 X 10- Om. Even with such alloys, the film thick- technology, Proc. IEEE 52, 1450-1462, 1964. ,

184 PHILlPS TECHNICAL REVIEW VOLUME 27

a b

c d

e Fig. 2. Some stages in the formation of an evaporated thin film of tin on glass, photographed with the electron microscope. a) After 5 seconds: the beginning of nucleation. b) and c) After 7 and 10 seconds, respectively: the crystallization nuclei grow and join up to form islands with channels in between. d) After 20 seconds: channels are still present. e) After 30 seconds: a practi- cally continuous film has formed. These photomicrographs were made available by J. van de Waterbeemd [5].

arises from the very fact that the film is not ideal. substrate an extensive labyrinth of channels remains It has been found from electron-micrographs that a however for some time. These channels also gradually thin film forms by a process of nucelation and growth disappear, so that finally a continuous film is formed. of crystallization nuclei during the evaporation (see This process can to some extent be checked by continu- jig. 2). In the initial stages the film consists of a large ously measuring the conductivity of the film during the number of separate islands. As the deposition continues. evaporation process. At first the conductivity remains this number increases and the islands already formed close to zero for some time, and then it increases grow gradually larger. If two of them come into con- linearly with time. l n some cases, e.g. tin on glass, the tact they may join together to form a larger one. In the channel network is extremely persistent, and a continu- stage where the islands cover practically the whole ous layer does not form until the film has reached an ... <., 1966, No. 7 THIN-FILM INTEGRATED CIRCq:ITS 185

appreciable thickness, of the order of a few tenths of a on the thermal conductivity of the substrate .. micron tsi, Glass is a useful material for this purpose. It ,i~ A structure may also be formed in other ways. If, sufficiently smooth and heat-resistant. A type of glass for example, an unbroken, homogeneous thin. film is with adequate electrical quality must be chosen ~cheap heated in air, recrystallization takes place in the film. soda-lime glass, for example, has limitations in this It is possible that oxidation at the grain boundaries respect. above 100 ~c,Normal organic materials are causes some electrical separation between the various ruled out because they are not heat-resistant. Ceramics domains. are not as a rule smooth enough; even after thorough The presence of such structures obviously has con- lapping they cannot be made much smoother than siderable influence on the resistance. Quite often elec- 0.5 [.Lm.Ceramic substrates coated with a thin layer of trical conduction occurs even before the island system glass can, however, be used. If thermal conductivity is has completely closed. How this conduction comes important, one might consider using ceramics such as about is not yet quite clear. It is probable that therm- porcelain and steatite, which are a few times better than ionic emission or tunnel processes play some part in glass in this respect. Sintered AhOa or sintered-Beï), the transport of electrons from one island to another. whose thermal conductivities are about ten and a Where the grain boundaries are oxidized the con- hundred times betterthan glass respectively, could also duction must take place through the oxide films. be employed. These phenomena have a considerable effect on the temperature coefficientof resistance. In general, thermal Resistors excitation of the electrons is necessary to (or at least We shall now consider the resistivefilms in somewhat assists) the kinds of conduction we have referred to, so more detail, confining ourselves to a type made by that more electrons take part in the conduction as the Philips. The starting material isa nickel- alloy. temperature increases. This makes a negative contribu- In wire form and with the composit!.pn 80% Ni, ,10% tion to the temperature coefficient. The result is that the Cr, this has a resistivity of roughly 100 X .10-8 Qm. Due temperature coefficient, which is positive in the bulk to oxidation and the thin-film effectsmentioned above, metal (higher resistance at higher temperatures), is the effective resistivity in the vacuum-deposited form is smaller for thinner films and in extremely thin films considerably higher. say 400 X 10-8 Qm: Moreover; if may even be negative. the composition is suitably chosen" the température For practical purposes the thin-film effects are there- coefficient of NiCr is small. fore advantageous in two respects: the effective resist- NiCr has two further important properties that ivity is greater than in the bulk metal and the temper- strongly indicate the choice of this material. FirstIy, it ature coefficient is smaller. adheres well to glass, - clearly an important feature. Secondly, the material begins to evaporate quite rapidly Choice of material and evaporation technique at temperatures below the melting point. This means that a solid source can be used for evaporation. A linear Substrate source can therefore be used, and this is of considerable The first requirement of a substrate is that it must be advantage in the uniform oflarge areas. This is sufficiently smooth. In order to deposit a film having a much more difficult if the source is a molten metal. ' thickness of the order of 10 nm with adequate precision, The vacuum-evaporation equipment, which is the roughness of the substrate surface should at the completely contained in a bell-jar, is illustrated infig. 3. most be ofthe order of 10 nm. A further requirement is The substrates are fixed to the inside of a cylinder which that the material should be sufficiently heat-resistant, rotates at a uniform speed around its axis during the for during the process it has to be evaporation. Inside the cylinder, parallel with its axis, heated to 250-300 °C to obtain a stable and firmly is a linear NiCr' source in the form of a wire or strip, bonded film. Other requirements that may have to be which is heated by an electric current to a temperature met depend on the heat generation in the circuit. just below the melting point. The extent of the source Because ofthe miniaturization the dissipation per unit allows a reasonable rate of deposition. For instance, volume may be substantially greater than in conven- with an 80 cm nickel-chromium wire of 2 mm diameter tional circuits. Ifthis results in a high operating temper- as the source, a film of sheet resistivity RO = 300 Q can ature, the electrical quality must be correspondingly be deposited on an area of 0.3 m2 in 10 minutes. The high, so that for example ion transport damage in the source is partly screened, so that only a few at a time substrate does not occur. On the other hand, high operating temperatures can be avoided by measures to [5] J. van de Waterbeemd, Philips Res. Repts. 21, 27-48, 1966, ensure adequate heat removal. This sets requirements (No. I). VOLUME 27 186 PHJLIPS TECHNICAL REVIEW of the substrates on the cylinder wall receive the thick wire and pre-heating it for a sufficiently long time; sublimed material. In addition to the NiCr source there if this is done a steady state is ultimately reached [6l. are a number of radiant heaters inside the cylinder, In practice, however, it proved more attractive to use which heat the glass substrates to 300°C prior to a wire once only, for a relatively short period and after evaporation, in order to ensure well-bonded and stable an appropriate pre-heating time. This makes it possible films, and also a nickel source for depositing the con- to control both the temperature coefficient and the ductor films. stability of the film, by choice of the pre-heating time - and hence the composition. The properties of the film are also determined by the quantity of oxygen it contains; this can be controlled by ~ means of the pressure in the bell-jar, which as a rule is c ..------... ,n~, about 10-5 torr . .) L ,~ Thin films are in general unstable, in the sense that - "-TI I , I their resistance increases in the course of time, in

i particular at high temperatures; the resistance increases ~ quickly at first, then more slowly. The films are there- I' fore artificially aged by heating in air. The change of I1 resistance which then occurs - at the most a few per b- cent - is at the same time a measure of the stability. 11 I This change is taken into account in the evaporation illli process. Generally speaking, the thicker the film, the better its stability. The film finally obtained is the result of a compro- l= - cr 'Q mise between efforts to achieve high sheet resistivity, low temperature coefficient and high stability; this a I~ ,( v~ compromise is arrived at by an appropriate choice of v- 1----- v11~ pre-heating time, evaporation time and pressure. A u typical practical example is a film with a sheet resistivity c of 300 Q, a temperature coefficient smaller than IO-4;oC, and a resistance change of less than I % after Fig. 3. A sketch of the evaporation system. The cylinder a, with detachable shields b to which the substrates are fixed, rotates a thousand hours of heavy-duty operation. during the evaporation. The substrates are pre-heated by radiant heaters c. The resistive films are evaporated from the nickeI- Conductors chromium source d, the conductors from the Ni source e. The cylinder is mounted inside a bell-jar. The problems involved in depositing the conductor films are not so great as with the resistive films: the The speed of rotation of the cylinder and the evapora- main consideration is that the conductivity should be tion rate are chosen so that the required sheet resistivity sufficiently high. There is no question of close toler- is reached after a large number of revolutions, say 100. ances. The obvious choice is a material that has a low This enables a high degree ofuniformity to be obtained resistivity, i.e. a pure metallic conductor. Gold is often for the sheet resistivity of substrates all round the used for this purpose. Some care does have to be circumference ofthe cylinder. Uniformity in the vertical exercised to obtain a sufficiently high conductivity. If, direction is achieved by shielding the centre of the for example, a conductor with an aspect ratio of 10 linear source rather more than the top and bottom. (i.e. 10 times as long as it is wide) is required to have a With this method it proved possible to cover the 0.3 m2 resistance less than 0.1 Q, then if gold is used the film glass areas mentioned above with a uniformity within must be at least 2.5 [.Lmthick. In evaporation technique 5 %. During the evaporation the resistance of one ofthe this is a considerable thickness. At Philips the conductor substrates is continuously monitored. connections are made by depositing a nickel film and A point of importance in the reproducibility of the then tinning the complete nickel conductor pattern. sheet resistivity RO is that in the evaporation of nickel- The tinning is used to overcome the disadvantage ofthe chrome the constituents are not equally volatile, so that relatively poor conductivity of nickel, which in other the composition at the surface of the wire changes in respects, for various technological reasons, is a very the course of time. As a result, the composition of suitable material. The conductor pattern can be successively evaporated films is not identical. This selectively tinned because the tin does not adhere to the difficulty could be circumvented by using a sufficiently nickel-chrome resistor pattern. 1966, No. 7 THIN-FILM INTEGRATED CIRCUiTS 187

Dielectrics produce the required pattern of strips and rectangles, Dielectric films as well can be deposited as the di- as illustrated for example in fig. I. electrics for the capacitors in the circuit. The object is An obvious method, and one which is very widely to obtain the highest possible capacitance per unit area used, is that of evaporation through masks. The films while ensuring that the breakdown voltage remains at a are then formed directly in the pattern required. The safe margin above the working voltage. Low dielectric whole process takes place in a number of stages, in losses and not too high a temperature coefficient are which each successive material is evaporated through also desirable. an appropriate mask. Fig. 4 shows the steps required The film that has been most investigated and used is for making various elements. For making a circuit with a oxide film produced by evaporating silicon monoxide. The cornposition of the film again depends on what exactly happens in the vacuum during evapora- tion. It is certain that there is more oxygen present in the deposited film than indicated by the formula SiO. A safe field-strength for films of this kind is 10 VIfLm. At an operating voltage of a few volts the minimum thickness must then be 0.5-1 fi.m, which, for Sr = 4 a7 to 5, gives a capacitance of 40 to 80 pF per mmê.

Dielectric films are much more sensitive to imperfec- "----, "'---, I I f" J tions than resistive films. If there is a hole in the di- I I :I;):~'; I h r _.J L_-, r-g. '1+--' electric film, the result will be a short-circuit when the I I -l I I I I I I I I next electrode is deposited, whereas in the resistive film I I J J I I J J L J I. .I --- .•••• '(' the current would simply flow around the hole. It is L_,,,t,: -- _ ....J therefore particularly important to avoid dust when making dielectric films. Experience has shown that the b7 b2 b3 chance of short-circuiting increases sharply with the size of the surface. In practice the capacitances used are limited to a few thousand pF. Various ways of increasing the capacitance per unit area have been tried, so far without conspicuous success. One is to use much thinner dielectric layers such as those found in the electrolytic capacitor. This is done for example by evaporating tantalum, anodically oxidizing it and then, after it has been dried, evaporat- ing an upper electrode on to it r41. By this method Fig. 4. The formation of a pattern by means of masks during 2 evaporation. The masks that have to be used successively to form 1000 pF/mm can be achieved for voltages of the same a given element are shown for three different elements. order as mentioned above (5 to 10 V). As yet, however, a) Resistor with conductor contacts: } mask for the resistive film, 2 for the conductive layer. reasonably reliable capacitors have been made by this bl Capacitor: I mask for the first electrode, 2 for the dielectric, method on only a modest scale. Much less progress has 3 for the second electrode. cl Crossed connection: 1 mask for the first conductor, 2 for the been made in efforts to deposit materials that have a , 3 for the second conductor. relatively high dielectric constant, such as Ti02 and bl and c) can be carried out simultaneously. BaTi03. It has proved to be a practical proposition to use thin resistors, capacitors and crossed connections, as many ceramic wafers ofthe latter materials (e.g. 0.1 mm thick) as six steps may be needed. If the production is to be which have been provided with evaporated electrodes. reasonably efficient, the operations corresponding to If these are soldered flat in the circuit they take up little these steps, including changing of the sources and space, and the existing choice of materials and thick- masks, should be performed while maintaining the nesses makes it possible to achieve capacitances up to vacuum. Moreover, the masks must be aligned with the 200 pF/mm2. utmost precision. All this calls for machines of such complexity that this method has not yet in practice been Generating the pattern used for economic production runs.

In addition to the problem of producing films with [6.] P. Huijer, W. T. Langendam and J. A. Lely, Vacuum de- specific properties there is also the problem of how to position of resistors, Philips tech. Rev. 24, 144-149, 1962/63. 188 PHILlPS TECHNICAL REVIEW VOLUME 27

An entirely different solution of the problem consists given fabrication process, however, the relative accuracy in evaporating a film that completely covers the sub- in the width, and hence in the resistance, then decreases. strate and then, having coated the film with a resistant The strip width is therefore generally chosen to give a lacquer in the pattern required, etching away the super- compromise between a high resistance per unit area fluous parts. and a high resistance accuracy. With the methods de- A particularly suitable etching method is the photo- scribed above it is possible to reproduce strip widths etching process (see fig. 5), which has long been used with tolerances as small as 0.01 mm. Strips 0.3 mm wide for making printing-blocks and printed wiring, and which is also employed in the fabrication of solid circuits, as described else- where in this number [71. The lacquer used becomes solu bie after illumination, in certain solvents (or in- soluble; both types are available). The lacquer is /1'/ /h /H .....<>::: ~ •. applied to the whole surface e of the wafer, illuminated Fig. 5. The photo-etching method. a) Substrate with metal layer (yellow); b) the metal layer through a photographic covered with a lacquer layer (green); c) the lacquer layer is exposed through a photographic mask and "developed", i.e. mask; d) the lacquer is photographically developed; e) the uncovered metal is etched away; the exposed parts (or the f) the residual lacquer is removed. unexposed parts, as appro- priate) are dissolved. Finally, the parts of the deposit- can then easily be made with a tolerance of ± 5 %. ed material not protected by lacquer are etched away. With Ro = 300 D the expression quoted shows that If the film is not readily susceptible to chemical a value of 1600 Dfmm2 can be obtained. attack, use can be made of an underlayer of an easily etchable material, e.g. copper. The copper film is first The complete circuit and the encapsulation evaporated on to the substrate, the pattern is etched out, and then the desired material is evaporated. Next, To complete the circuit the active elements, diodes the copper is removed by etching, the material de- and transistors, have to be added to the network of posited on it being removed with it at the same time. resistors and capacitors. The active elements should An advantage of this variant is that the details of the preferably be adapted to the thin-film technique; they pattern can be checked before evaporating the resistive should be small and easy to mount in the flat circuit. film. The semiconductor devices are generally small enough, In practice the techniques described can be combined but the method of attaching them calls for some atten- in various ways for producing a circuit. One example is tion. the following method of making a combination of In practice conventional transistors are often used. resistors and conductors [81. In a single vacuum cycle Their connections are wires or tags, which can be a double layer is deposited on the substrate, first the connected appropriately, for example by pressing the resistive layer and then the conductive layer on top. connectors into the tinned conductors of the circuit Next, the double layer is etched away so as to leave the with a heated pin. In a refinement of this procedure the required pattern, and finally the conductive material is connectors can be soldered either ultrasonically or by selectively etched away at the places where the resistors means of localized current pulses. Better prospects for are to be located. This method obviously sets special fast production in long runs are offered by the use of requirements on the materials; the resistive film must transistors mounted on a ceramic wafer, which carries be etchable, but it must be resistant to the etch that the connectors in the form of solid, pre-tinried metal removes the conductive layer. contacts (see fig. 8). If the pattern of the contacts If the element is a "meander" of strips (see fig. 1) of matches that of the circuit, these transistors can be width b separated by insulating strips ofthe same width, directly mounted and soldered on the circuits. This can the resistance that can be produced per unit area is be done automatically, and for a large number of equal to tRofb2, where RO is the sheet resistivity. This circuits at a time, by using alignment jigs, and heating value thus increases as the strips get narrower. In a in a furnace. This method is often used with planar 1966, No. 7 THIN-FILM INTEGRATED CIRCUITS 189

silicon transistors. In a promising variant of these distortion to a level at which signals do not interfere transistors the ceramic wafer is completely dispensed with one another. Apart from the transit times in the with, and the silicon is soldered directly into the transistors, capacitive and inductive elements also circuit. The , which are about 0.6 X 0.6 mm, are contribute to the parasitic phase shift in the feedback pre-coated with an extremely thin layer of glass. The loop. To prevent oscillation tendencies the parasitic contacts are minute tinned beads mounted in small phase shift should be kept a great deal smaller than 1800 holes in the glass film [9l. up to that frequency at which the loop gain has dropped As a proteetion against atmospheric effects, in par- to unity. This frequency is much higher than 230 Mc/s, ticular against moisture, the whole circuit is encapsu- the highest frequency to be transmitted. The phase shift lated. The requirements to be met by the encapsulation is kept small by making the feedback loop very short. depend on the conditions in which the circuit is to be This means that the circuit must be small. used. The semiconductor devices are always the most A second point is that the pattern should be sharply sensitive elements in the circuit. They can be fitted defined dimensionally and be reproducibly fabricated. hermetically sealed into the circuit, so that less exacting This is important because unwanted couplings - in requirements have to be made on the encapsulation of spite of the screening effect of a "ground-plate" (see the circuit as a whole. There is, however, a tendency to fig. 7) - cannot entirely be avoided; these unwanted use semiconductor devices that are covered merely with couplings can be taken into account in the design, but plastic or lacquer and to enclose the entire circuit in a since the tolerances are small they must be sharply herrnetically sealed can. An example can be seen in reproduced. jig.6. The can is fitted with pins, so that it can be In these respects the thin-film technique is superior to connected to a printed wiring panel. other manufacturing methods. Although printed-wiring assembly is excellently reproducible, it is not so small (the capacitance of two neighbouring soldered joints for example, is nearly I pF). Miniature circuits can indeed be made by the direct interconnection of separ- ate components, but this method is not adequate where reproduci bility is concerned. Finally, the resistors and capacitors must be purely linear in view of the linearity requirements which the amplifier has to meet. In this respect the thin-film circuit is preferable to the solid type, in which the resistors tend to be non-linear owing to their semiconductor nature. The amplifier contains a few small isolating induct- ances, which need only have a low Q (about 20). The possibility of producing these in the form of evaporated

Fig. 6. Example of a hermetically sealed thin-film circuit. spirals has been considered but rejected because the spirals take up too much space (20 mm diameter for 1.2 fI.H). Moreover, even though only a low Q is needed, special measures would have to be taken to improve the Some applications conduction sufficiently. We shall now briefly describe two examples of cir- The second example (jig.8), a digital (double cuits in which the foregoing technique has been success- NAND) circuit, will be dealt with briefly. Circuits of fully employed. Both are hybrid types of circuit in this kind are used in large numbers in electronic com- which the conductors and resistors have been deposited puters. Here again, the principal virtue of the thin-film as thin films and the other elements - capacitors, technique is that it makes miniature circuits possible, so transistors, and, in the first example, inductors - are that compact computers can be built. Moreover added later. The first example is a stage of a broad-band amplifier for the frequency range from 40 to 230 Mc/s (jig. 7). [7] A. Schmitz, Solid circuits, Philips tech. Rev. 27, 192-199, This amplifier is used for the simultaneous distribution 1966. [8] See C. W. Skaggs, Photo-etching thin-film circuits, Electronics of a large number of signals from television and FM 37, No. 18, 94-98, 1964. broadcast aerials. To make this possible the amplifier is [9] E. M. Davis, W. E. Harding and R. S. Schwartz, An approach to low cost, high performance , 1963 provided with negative feedback which reduces the WESCON tech. Papers, Part 2, publ. No. 13.1. 190 PHILIPS TECHNICAL REVIEW VOLUME 27

c d

b

Fig, 7. The second stage of a linear broad-band amplifier for 40-230 Mc/s. a) Circuit diagram, b) design for a thin film version of the circuit, c) and d) the circuit before and after soldering separate components. The negative feedback loop, which must be short, is shown dashed in (a) and marked with a thick dashed line in (b). Unwanted couplings are pre- vented to some extent by a ground- plate, indicated by the wide cross- hatching in (b).

I I • 'u•

y 0 _,1 2~. 3 .4 5cm

Fig. 8. A double NAND circuit made by thin-film technique. 1966, No. 7 THIN-FILM INTEGRATED CIRCUITS 191

the manner indicated, as a thin film. With this method it is also possible to intro- duce high resistances (of the order of 1 MD), which cannot be done in a purely monolithic circuit. Another problem arises if, owing to the semiconductor nature of all the components in the purely monolithic circuit, the sensitivity to tempera- ture variations is too great. If the resistors are the critical elements, the ob- vious answer is to deposit the resistors in the form of thin films, which have a low temperature coeffici- Fig. 9. A solid circuit with evaporated films. A number of these Circuits are made at the same ent. Furthermore, since the time on a single crystal wafer. Each circuit covers an area of dimensions 1 X I mm. sheet resistivity for thin films is readily reproducible, - partly because the circuits are so small - large specified tolerances may be more readily maintained numbers of them can be produced simultaneously in a in production runs of these circuits. few stages, but to take advantage of this it must also The main problem in this combination of a solid be readily possible to solder large numbers of transis- circuit with thin films arises from the tolerances in the tors and capacitors into the circuits simultaneously. strip-widths: the strips have to be much narrower than This is why the special transistors with pre-tinned in the thin-film circuits discussed above, because the contact surfaces are used here (see previous section); crystal on which the pattern is to be deposited is always they are mounted by means of jigs, many at a time, at extremely small. The strip widths for such circuits are the appropriate places, and soldered simultaneously. between 10 and 25 (J.m.In order to reproduce these with tolerances within a few per cent, the etching techniq ue Thin films on solid circuits has been perfected to give ten times greater precision. To conclude we shall mention briefly a development Fig. 9 shows a laboratory example of a circuit pro- in which the solid circuit discussed in the following duced by this technique. article [7] is combined with a thin-film technique. An insulating oxide film is made on a silicon crystal, in which various functions have been incorporated by the Summary. In thin-film circuits the circuit elements are deposited diffusion method; the circuit is then completed with in the form of narrow strips and rectangles of vacuum-evaporated materials. In practice the deposited elements are generally resistive and conductor films evaporated on top of the confined to a number of passive elements; transistors and diodes, insulating film. This mayalso be considered a hybrid and frequently capacitors and inductors as well, are added sepa- rately. This article devotes particular attention to evaporated type of circuit, but compared with the hybrid type of resistors. For reproducible quantity production, the sheet resistiv- circuit referred to in the introduetion the situation has ity and the aspect ratio, which are independently chosen, must both be accurately reproducible. The characteristics of thin films been more or less reversed. While the thin film circuit as such, in particular the relatively high bulk resistivity and the described above had separate semiconductor elements low temperature coefficient, are attributed to their island struc- ture. A suitable resistive material is NiCr, which can be evapor- added to it, the solid circuit here is supplemented with ated from a wire source. The properties ofthe film are determined thin evaporated films. by the pre-heating time of the source, the evaporation time and the pr essu re in the bell-jar. Dielectric films for capacitors are often This system makes it possible to use the thin-film made from SiO. Conductors are produced by evaporation of a technique for circumventing problems arising in the highly conductive material (e.g. gold) or by tinning a moderately conductive nickel film after evaporation. The photo-etching production of solid circuits. A few such problems may method is particularly suitable for forming tbe pattern in a layer; be mentioned here. Exacting requirements for the this method can be applied selectively to different films one on top of the other. The technique is illustrated with two applica- insulation between two components in a circuit are tions: a broad-band amplifier for 40-230 Mc/s and a dou ble difficult to meet in a single crystal. The difficulty is NAND circuit. The advantages of thin-film and monolithic tech- niques can be combined by directly evaporating thin films on to esolved if one of those components can be applied, in a solid circuit coated with an insulating oxide layer.