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International conference on thin and thick film technology, Augsburg, F.R.G., 28–30 September, 1977
Thin Solid Films, 48 (1978) 257-260 '~ ElsevierSequoia S.A., Lausanne Printed in the Netherlands
257
Conference Report International Conference on Thin and Thick Film Technology, Augsburg, F.R.G., 28-30 September, 1977
Professor Ernst Liider from the University of Stuttgart opened the conference with a reminder to the audience that hybrid technology has to be ready for everything. It will be involved with components as diverse as optical devices and displays, sophisticated microwave assemblies and thermal printers where accurate resistors without trimming at a size of only 100 x 200 Jam are required. Dr. von Stein from Siemens referred to the development of nickel chromium films on plastic supports (including both polyimide and Teflon) and the possible use of nickel chromium-aluminium alloys. He stated that the temperature coefficient of resistance could be brought down to 10 ppm ° C - 1 by the utilization of a 55 0/o chromium ratio, with stabilities of better than 0 . 1 "O/ i n 4000 h at 125 °C. He used aluminium as a conductor in his system and the plastic foil could itself be utilized for low value capacitors which could be trimmed by the action of an yttrium aluminium garnet laser on the conductor film, He pointed out that by using a thin flexible substrate one of the continuing disadvantages of thin film technology could be partially overcome in that an effective "crossover" facility could be achieved by using a through-hole interconnection technique. The thin film contributions represented a very coherent account of the state of the art and in the main related to resistive films involving tantalum technology. These sessions were off to a good start by the high quality papers on magnetron sputtering which showed that, if one wishes to utilize tantalum film circuits, there is production deposition technology readily available to produce the film. A reminder came from Dr. Pfahnl of Bell Laboratories, which must represent the greatest source of information on tantalum techniques, that the demands of their own high volume manufacturing system put considerable emphasis on the need to control, for example, the photoresist materials which control an essential stage in the process. Following closely on this emphasis on photoresist control was another paper in a different session from AEG-Telefunken relating to mask and pattern inspection systems, clearly of great importance in maximizing yields in a large-scale production state. An automatic scanning technique is used with double beam comparators. The recommended process involves syntactical imaging and classification but requires data handling at 1 megabit per second which does require several minutes per 3 in mask at a resolution of 2 lain for point defects. In an overall historical review Dr. Berry from Bell Laboratories referred to the use in about 1975 of highly nitrogen-doped tantalum for the production of combined R C networks and then referred to the interesting progression of buffer or barrier layers which have for a long time been regarded as essential between the tantalum layer and the overlying gold. Originally the use of a film of titanium under
258
(ON|-I~REN( |! REPOR I
the gold was replaced in 1971 by a double buffer layer of titaniunl and palladium. In 1975 this was further superseded by the use of titanium, palladium, copper, nickel and then gold for the overlying conductor with a recent indication of a possible elimination of the palladium layer. He indicated that regular use of Auger analysis had been particularly valuable in an understanding of the interracial effects between the films involved. He referred to a particular advantage of laser trinmmlg, compared with the original anodization technique, that it permits an increase in the sheet resistance of the tantalum to about 300 ~2/~. He pointed out that the m,e of~tantalum with 15". nitrogen allows the capacitors to be completed bel\)rc the associated resistors are stabilized (at 350 C),thereby reducing the numberofsteps in the rnanufacturing process. Using a negative working fihn resist (Rislon-T193) good adhesion is obtained and etching down to 50 Hin lincwidth is possible. This process would then use additive gold (plated through Rislon windows) and thereby avoid gold etching and the problems associated with the need for gold recover>,, with in addition a considerable reduction in gold inventory. Following this theme Dr. Berr~ wondered whether in due course gold would be replaced b? copper. In this competent and upto-date review of thin film tantalum technology it was interesting to note that the author was prepared to consider making full use of neighbouring technologies, particularly with regard to overcoming the difficult~ of establishing satisfactory crossover or mullilayer techniques in thin films. In this case he proposed the use of thick fihn technology for effecling the crossover point of the circuil with the subsequent addition of the overlying thin fihn conductors in a process which he called "'beam crossover". His final reference to the possible use of a plastic layer with a conductor superimposed over the top to effect a crossover facilii> seemed somewhat surprising to sornc members of the audience but this was referred to Iatcr and in more detail by Gunter Kicssling of Siemens who described a production process based on standard tantalum technology on glass or alumina substrates which includes the screen printing of "an insulating patch" followed b~ the evaporation of a further nichroine gold layer o r b ~ an electroplating process. The proposed lacquer was based on a phenolic resin and was produced by Wiedoprinl and intended prilnarily as a solder resist lacquer. In this paper the restllts of quite severe environmental test conditions showed a remarkably satisfitctorv level of performance. The author of the paper regarded his screen-printed plastic film as a utilization of thick film technology and he claimed economic advantages for this combination of thin and thick fihns. One of those surprising but satisf,cing contributions to a conference of this nature, which seem to state the obvious and yet to remind one thai it has not been done before, came from the University of Stuttgart. A paper by Dr. Baeger described his development of a single tantalum film based on reactive sputtering in a mixture of argon, nitrogen and oxygen with the objective of producing a fihn from which both resistors and dielectric layers could be produced. The temperature coefficients of the resistors and capacitors are adapted to balance each other to within a very few parts per million. This particular method o f p r o d u c m g a single RC fihn seerns particularly obvious and desirable in the present jungle of alloys in tantalum technology. Dr. Robert Berry of Bell Laboratories had already stated thai the first RC network was not really in production until 1970 and it will be interesting
CONFERENCE REPORT
259
to see whether the Stuttgart development is adopted by any of the industrial organizations operating in the tantalum field. As an adjunct to the work by Baeger from Stuttgart University on a single tantalum film for the production of temperature-coefficient-compensated RC networks, a further paper from one of his colleagues illustrated an interesting geometrical arrangement for a distributed RC network. On a glass substrate an initial tantalum layer of about 2000 A is deposited and then anodized to provide the required dielectric. An overlayer of NiCr gold is then applied to cover the major portion of the meandered resistive tantalum layer but the ends of each of the meandered loops are allowed to protrude outside of the top conductive layer. Laser cutting is used to extend the inside gap in these loops, hence increasing the resistance value of each limb for tuning purposes. The final contribution from the team at Stuttgart related to the particular aspect of developing multilayer structures in thin film RC circuits. It must be noted, however, that this "multilayer" concept relates specifically to RC film circuits in which, after depositing an initial resistive film (Ta + N) followed by an AI layer, the capacitor film (TaOxNy) is sputtered and anodized and finally the top electrode (NiCr-Au) is applied. The technique is not intended to provide general purpose multilayer technology with low capacity crossover facilities etc. An interesting contribution to the improvement in adhesion of tantalum layers on glass or ceramic without the use of an adhesion-promoting layer of Cr or NiCr or Ti was based on the sputter etching of the surfaces prior to subsequent deposition inside the same vacuum system. Roger Duckworth from Ultra Electronics reported interesting work in association with Dr. A. G. Stevens at Surrey University and pointed out that it was now appreciated that only few materials are stable under ion bombardment. Therefore during the sputtering process which is used to deposit certain elements, the ions have sufficient energy to remove low mass atoms which may thereby result in film property variation. In addition his work programme examined variations in films caused by the implantation of inert gas atoms; if effects like this could be produced by low energy bombardment during film growth, a new parameter is available in the optimization of thin film properties. In some cases an appreciable drop in resistivity and an increase towards positive values of the temperature coefficient of resistance was shown to occur when the implanted ions probably lay within the thin film. With higher bombarding energies larger effects were observed as a result of the penetration into the substrate and consequential reactions with substrate elements. A session of considerable significance was devoted entirely to high frequency applications and amply demonstrated the virtues of the precise layout and accurate control of geometrical parameters implicit in film technology in achieving the necessary performance characteristics. Several of the papers concentrated on techniques for accurate measurement of attenuation and of properties such as dielectric constant and unloaded Q-factors of microstrip lines (and of the equivalent reactances for impedance "steps"). Other papers ranged from thin film transistors to transparent films for display devices and among the latter several papers referred specifically to the important ITO (indium-tin-oxide film).
260
('ONFEREN('E
REPOR3
When one does have to soft solder onto gold layers and to utilize materials such as indium-based solders (i.e. indium tin or indium-lead), one paper from AEGTelefunken examined carefully the processes of diffusion and leaching at various storage temperatures and suggested that, with careful control of time and temperature of soldering, high reliability joints can be assured. A "'special film" section included a wide variety of topics ranging from solar cells, implantable pH electrodes, thin film cathodes for electron emission and thin films in a galvanic cell, as well as an account of some interesting possibilities for devices based on junctions between tin oxide and antimony where ohmic contacts can be made to both materials. l'. [,. K I R B Y
Welwyn Electric Ltd., Bedlington, Northumberland NB22 7A A
257
Conference Report International Conference on Thin and Thick Film Technology, Augsburg, F.R.G., 28-30 September, 1977
Professor Ernst Liider from the University of Stuttgart opened the conference with a reminder to the audience that hybrid technology has to be ready for everything. It will be involved with components as diverse as optical devices and displays, sophisticated microwave assemblies and thermal printers where accurate resistors without trimming at a size of only 100 x 200 Jam are required. Dr. von Stein from Siemens referred to the development of nickel chromium films on plastic supports (including both polyimide and Teflon) and the possible use of nickel chromium-aluminium alloys. He stated that the temperature coefficient of resistance could be brought down to 10 ppm ° C - 1 by the utilization of a 55 0/o chromium ratio, with stabilities of better than 0 . 1 "O/ i n 4000 h at 125 °C. He used aluminium as a conductor in his system and the plastic foil could itself be utilized for low value capacitors which could be trimmed by the action of an yttrium aluminium garnet laser on the conductor film, He pointed out that by using a thin flexible substrate one of the continuing disadvantages of thin film technology could be partially overcome in that an effective "crossover" facility could be achieved by using a through-hole interconnection technique. The thin film contributions represented a very coherent account of the state of the art and in the main related to resistive films involving tantalum technology. These sessions were off to a good start by the high quality papers on magnetron sputtering which showed that, if one wishes to utilize tantalum film circuits, there is production deposition technology readily available to produce the film. A reminder came from Dr. Pfahnl of Bell Laboratories, which must represent the greatest source of information on tantalum techniques, that the demands of their own high volume manufacturing system put considerable emphasis on the need to control, for example, the photoresist materials which control an essential stage in the process. Following closely on this emphasis on photoresist control was another paper in a different session from AEG-Telefunken relating to mask and pattern inspection systems, clearly of great importance in maximizing yields in a large-scale production state. An automatic scanning technique is used with double beam comparators. The recommended process involves syntactical imaging and classification but requires data handling at 1 megabit per second which does require several minutes per 3 in mask at a resolution of 2 lain for point defects. In an overall historical review Dr. Berry from Bell Laboratories referred to the use in about 1975 of highly nitrogen-doped tantalum for the production of combined R C networks and then referred to the interesting progression of buffer or barrier layers which have for a long time been regarded as essential between the tantalum layer and the overlying gold. Originally the use of a film of titanium under
258
(ON|-I~REN( |! REPOR I
the gold was replaced in 1971 by a double buffer layer of titaniunl and palladium. In 1975 this was further superseded by the use of titanium, palladium, copper, nickel and then gold for the overlying conductor with a recent indication of a possible elimination of the palladium layer. He indicated that regular use of Auger analysis had been particularly valuable in an understanding of the interracial effects between the films involved. He referred to a particular advantage of laser trinmmlg, compared with the original anodization technique, that it permits an increase in the sheet resistance of the tantalum to about 300 ~2/~. He pointed out that the m,e of~tantalum with 15". nitrogen allows the capacitors to be completed bel\)rc the associated resistors are stabilized (at 350 C),thereby reducing the numberofsteps in the rnanufacturing process. Using a negative working fihn resist (Rislon-T193) good adhesion is obtained and etching down to 50 Hin lincwidth is possible. This process would then use additive gold (plated through Rislon windows) and thereby avoid gold etching and the problems associated with the need for gold recover>,, with in addition a considerable reduction in gold inventory. Following this theme Dr. Berr~ wondered whether in due course gold would be replaced b? copper. In this competent and upto-date review of thin film tantalum technology it was interesting to note that the author was prepared to consider making full use of neighbouring technologies, particularly with regard to overcoming the difficult~ of establishing satisfactory crossover or mullilayer techniques in thin films. In this case he proposed the use of thick fihn technology for effecling the crossover point of the circuil with the subsequent addition of the overlying thin fihn conductors in a process which he called "'beam crossover". His final reference to the possible use of a plastic layer with a conductor superimposed over the top to effect a crossover facilii> seemed somewhat surprising to sornc members of the audience but this was referred to Iatcr and in more detail by Gunter Kicssling of Siemens who described a production process based on standard tantalum technology on glass or alumina substrates which includes the screen printing of "an insulating patch" followed b~ the evaporation of a further nichroine gold layer o r b ~ an electroplating process. The proposed lacquer was based on a phenolic resin and was produced by Wiedoprinl and intended prilnarily as a solder resist lacquer. In this paper the restllts of quite severe environmental test conditions showed a remarkably satisfitctorv level of performance. The author of the paper regarded his screen-printed plastic film as a utilization of thick film technology and he claimed economic advantages for this combination of thin and thick fihns. One of those surprising but satisf,cing contributions to a conference of this nature, which seem to state the obvious and yet to remind one thai it has not been done before, came from the University of Stuttgart. A paper by Dr. Baeger described his development of a single tantalum film based on reactive sputtering in a mixture of argon, nitrogen and oxygen with the objective of producing a fihn from which both resistors and dielectric layers could be produced. The temperature coefficients of the resistors and capacitors are adapted to balance each other to within a very few parts per million. This particular method o f p r o d u c m g a single RC fihn seerns particularly obvious and desirable in the present jungle of alloys in tantalum technology. Dr. Robert Berry of Bell Laboratories had already stated thai the first RC network was not really in production until 1970 and it will be interesting
CONFERENCE REPORT
259
to see whether the Stuttgart development is adopted by any of the industrial organizations operating in the tantalum field. As an adjunct to the work by Baeger from Stuttgart University on a single tantalum film for the production of temperature-coefficient-compensated RC networks, a further paper from one of his colleagues illustrated an interesting geometrical arrangement for a distributed RC network. On a glass substrate an initial tantalum layer of about 2000 A is deposited and then anodized to provide the required dielectric. An overlayer of NiCr gold is then applied to cover the major portion of the meandered resistive tantalum layer but the ends of each of the meandered loops are allowed to protrude outside of the top conductive layer. Laser cutting is used to extend the inside gap in these loops, hence increasing the resistance value of each limb for tuning purposes. The final contribution from the team at Stuttgart related to the particular aspect of developing multilayer structures in thin film RC circuits. It must be noted, however, that this "multilayer" concept relates specifically to RC film circuits in which, after depositing an initial resistive film (Ta + N) followed by an AI layer, the capacitor film (TaOxNy) is sputtered and anodized and finally the top electrode (NiCr-Au) is applied. The technique is not intended to provide general purpose multilayer technology with low capacity crossover facilities etc. An interesting contribution to the improvement in adhesion of tantalum layers on glass or ceramic without the use of an adhesion-promoting layer of Cr or NiCr or Ti was based on the sputter etching of the surfaces prior to subsequent deposition inside the same vacuum system. Roger Duckworth from Ultra Electronics reported interesting work in association with Dr. A. G. Stevens at Surrey University and pointed out that it was now appreciated that only few materials are stable under ion bombardment. Therefore during the sputtering process which is used to deposit certain elements, the ions have sufficient energy to remove low mass atoms which may thereby result in film property variation. In addition his work programme examined variations in films caused by the implantation of inert gas atoms; if effects like this could be produced by low energy bombardment during film growth, a new parameter is available in the optimization of thin film properties. In some cases an appreciable drop in resistivity and an increase towards positive values of the temperature coefficient of resistance was shown to occur when the implanted ions probably lay within the thin film. With higher bombarding energies larger effects were observed as a result of the penetration into the substrate and consequential reactions with substrate elements. A session of considerable significance was devoted entirely to high frequency applications and amply demonstrated the virtues of the precise layout and accurate control of geometrical parameters implicit in film technology in achieving the necessary performance characteristics. Several of the papers concentrated on techniques for accurate measurement of attenuation and of properties such as dielectric constant and unloaded Q-factors of microstrip lines (and of the equivalent reactances for impedance "steps"). Other papers ranged from thin film transistors to transparent films for display devices and among the latter several papers referred specifically to the important ITO (indium-tin-oxide film).
260
('ONFEREN('E
REPOR3
When one does have to soft solder onto gold layers and to utilize materials such as indium-based solders (i.e. indium tin or indium-lead), one paper from AEGTelefunken examined carefully the processes of diffusion and leaching at various storage temperatures and suggested that, with careful control of time and temperature of soldering, high reliability joints can be assured. A "'special film" section included a wide variety of topics ranging from solar cells, implantable pH electrodes, thin film cathodes for electron emission and thin films in a galvanic cell, as well as an account of some interesting possibilities for devices based on junctions between tin oxide and antimony where ohmic contacts can be made to both materials. l'. [,. K I R B Y
Welwyn Electric Ltd., Bedlington, Northumberland NB22 7A A