GB2117796A - Forming ceramic layers; dielectric structures - Google Patents

Abstract

A multilayer dielectric structure, e.g. for use as a substrate for an electronic circuit, is formed by electrodeposition of a ceramic material from a liquid medium. The ceramic, in the form of a powder, is coated with a resin whereby the powder is dispersed in the liquid. The ceramic and resin are co-deposited, the resin being burned off during the subsequent firing process to form the ceramic dielectric. The ceramic laser 11 may be deposited onto a nickel foil substrate 12 which has been treated with a release agent. After removal of the foil, the ceramic sheet is printed with a conductive ink to provide conductive electrodes 22, eg of nickel or nickel alloy, heated at 300 to 350 DEG to remove organic material, then at 900 to 1500 DEG C to fire the ceramic. <IMAGE>

Description

SPECIFICATION Forming ceramic layers This invention relates to layered and multilayer dielectric structures, and in particular to processes for forming such structures by electrodeposition of a finely divided dielectric material from a liquid.

Layered dielectric structures, for example circuit boards or substrates for film circuits, are made conventionally from pressed sheets of dielectric material on which metal conductor patterns are deposited.

With the advent of very large scale integrated (VLSI) circuits a need has arisen for circuit boards or substrates having high density complex circuit patterns and which are difficult, if not impossible, to make by conventional fabrication techniques.

In the manufacture of circuit boards or circuit substrates it would be a considerable advantage to employ very thin dielectric layers so that a multilayer structure of acceptably small dimensions would be formed.

However, attempts to produce thin dielectric layers by conventional process have not been successful owing to the occurrence of pinholes in the layers. As the layer becomes thinner the occurrence of pinholes, with consequent electrical breakdown, increases.

Various methods are used to make dielectric layers as thin 'leaves', usually formed from a mix of a finely powdered ceramic material and an organic binder solvent system. For example, in a typical conventional process, a ceramic/binder/solvent mixture is coated on to polyethylene strip, by a tape-drawing process.

After drying, the ceramic/binder film is peeled off and then silk screen printed with electrodes using an ink formed from precious metal powders in an organic binder. A number of such 'leaves' are stacked and pressed together, heated to remove the binder, then fired at a high temperature. In the manufacture of such components, considerable advantage may be gained by having good control over the thickness dimensions of the ceramic layer, its porosity, and the number of faults or discontinuities appearing in it. The term pinholes is used to describe such faults. Following the present industry trend to decrease dielectric thickness these factors of thickness variation and film integrity assume greater importance.

From the intrinsic voltage breakdown point of view much thinner ceramic dielectric films are theoretically possible, but the limitations of all the methods so far described do not allow this. In such mechanical processes the control of layer thickness and integrity are decided by such factors as the concentration and rheology of the medium, the type of substrate surface, and the coating speed.

In the past, attempts have been made to use electrochemical deposition techniques to obtain greater control over the deposition of thin dielectric layers. This is in contrast to the mechanical methods outlined above. For example H.F. Bell and J.M. Drake of IBM have suggested an electro-chemical technique based on the flocculation of acid modified polyethylene/epoxy ester for the fabrication of multiple polymer/metal layers. Pinhole free dielectric layers (13 #m) under good control are claimed. However, these dielectric films are plastics materials of low permitivity and are not formed from fired ceramic materials. Other workers have attempted to exploit the excellent control of film thickness and integrity by the use of another physical/electrical process, that of electrophoresis.For example, Lamb & Salmon (National Bureau of Standards, Washington DC USA) have attempted the deposition of barium dimethyl ether. A voltage of 600 was used and deposits of about 40 lim in thickness were obtained prior to firing at 1350 to 1400 C. These attempts were not entirely successful.

The object of the present invention is to overcome the disadvantages present in the methods hereinbefore described. A further object of the invention is to provide a process for the deposition of multilayer structures under controlled conditions with high layer integrity.

It is well known that excellent paint films can be formed by electropainting techniques. The technology is well described in the Handbook of Electropainting Technology by Willibald Machu (Electrochemical Publications 1978). Water soluble resin systems are available which may form the basis of an electrocoating bath.

These film forming resins contain, for example, carboxyl groups in sufficient numbers that, when neutralised by a base, typically an amine, stable aqueous dispersions are obtained. If such a dispersion is electrolysed, a deposit of resin forms at the anode (or cathode). The presence of pigment particles stabilised in the aqueous resin system does not affect the course of the deposition; the particles migrate with the resin miscelles (and generally form part of these miscelles if the resin has been used as the pigment dispersant).

After deposition the pigment remains on the anode as part of the de-stabilised resin film.

We have found that similar techniques can be employed for the deposition of ceramic/binder films which can subsequently be stacked with electrodes and fired to form a multi-layer structure. Surprisingly, we have found that resin systems can be formulated which accept a high loading of a finely comminuted ceramic. We have also found that such high loaded resin systems provide controlled and satisfactory burn-out during the later stages of baking, firing and sintering of the body.

According to one aspect of the invention there is provided a process for fabricating a dielectric structure comprising a fired ceramic dielectric layer having a conductor pattern on at least one surface, wherein said ceramic layer is formed by joint electrodeposition of a ceramic powder and a resin from a liquid medium.

According to another aspect of the invention there is provided a dielectric structure comprising a fired body of electrodeposited ceramic powder, and a conductor pattern disposed on at least one surface of said body.

We have found that high quality pinhole-free ceramic-loaded resin films are obtained by this electrodeposition process which favours the prevention of pin-holes or discontinuities in the film. For example, at the site of a bursting bubble, where the film is thinner, the region of lower resistivity attracts a higher current, to aid deposition in that area. The process is also self limiting in thickness. Once a particular film thickness, dependent on the applied voltage, has been deposited the deposition rate drops very rapidly.

This ensures that highly uniform and reproducible films are obtained over relatively large areas.

The ceramic is dispersed to a finely comminuted powder in a liquid medium. To effect this dispersion each ceramic particle is coated with a resin. The ceramic and resin are codeposited in the electrodeposition process, the resin being removed during the subsequent firing process to form the ceramic dielectric.

Typically the resin forms an emulsion or a colloidal solution in the liquid.

The use of such techniques in the fabrication of ceramic capacitor structures is described in our co-pending application No.8210135 (H.F. Sterling-E.L. Bush-J.H. Alexander 84-24-18).

Embodiments of the invention will now be described with reference to the accompanying drawings in which: Figures 1 and 2 illustrate in cross-section successive stages in the fabrication of a circuit board or substrate, and Figures 3 and 4 illustrate the fabrication of a multilayer (multilevel) board or substrate.

Referring to Figures 1 and 2, a laminar dielectric structure, e.g. a printed circuit board or substrate, is made from a ceramic dielectric sheet or film 11 (Figure 1) formed by electrodeposition from a liquid containing the finely divided ceramic material carried in a resin on to a conductive substrate 12. Typically we employ nickel foil, e.g. 4 to 6 microns in thickness for this purpose but other suitable substrate materials can of course be employed. The ceramic loaded film 11 may be deposited on the substrate 12 and then separated therefrom to provide a self supporting ceramic sheet which can be subsequently processed.In order to effect this separation a suitable release agent is employed which has sufficient electrical conduction to allow deposition of the ceramic/binder electrocoating medium, but on the other hand prevents good film adherence and so allows for stripping. Such materials are known in the art and include suspensions of either colloidal graphite (AQUADAG RTM), materials such as oxygen-deficient (black) titanium dioxide or metal (e.g. Aluminium) depositions on to a plastic substrate. In the subsequent firing of the ceramic sheets any release agent remaining will either burn away completely in the case of graphite or produce a fully oxidised form of a metal which is compatible with the ceramic dielectric and benign to its electrical characteristics.

The self supporting ceramic/binder film 11 is next dried in air at a temperature of 100 to 150 C. This heat treatment serves to evaporate any volatile species which remain.

The sheet or film 11 may next be printed on one or both faces with a conductive ink so as to provide a conductor pattern of conductive electrodes 22 (Figure 2) and may, in some applications, be punched to provide openings 23 therethrough. The assembly is then heated, typically 300 to 350 C, to remove organic materials and then at a temperature of 900 to 1 500"C to form the fired ceramic. The firing temperature and conditions will of course depend on the particular ceramic dielectric employed, and these temperatures and conditions will be well known to those skilled in the art. Typically the conductor pattern 22 comprises nickel, or a nickel alloy, but other metals compatible with the ceramic and the firing conditions can of course be employed.In a further application deposition of the conductor pattern may be effected after firing has been completed.

In an alternative embodiment (Figures 3 and 4) a multilayer (multilevel) structure may be prepared by a multiple deposition process. In this technique, a layer 41 of ceramic material is electrodeposited on to a conductive substrate foil 42. Typically the substrate may comprise a nickel or nickel alloy foil 4 to 6 microns in thickness, but other suitable substrates, e.g. metalised plastic foil, can be used. The ceramic layer is then coated in selected regions, e.g. by electroless plating or screen printing, with a metal 43 that will form the permanent conductor pattern of the finished structure. Those regions left uncoated by the metal are coated with a conductive layer 44 of a second temporary or fugitive electrode material, e.g. graphite, which will disappear or become insulating during the firing process.The purpose of this temporary electrode is to allow electro-deposition of the ceramic/resin medium. This secondary electrode may be termed an evanescent or fugitive electrode. Such electrodes are formed from similar materials to those already described as release agents. On top of the conductor pattern/secondary electrode system a second ceramic/binder layer is electrodeposited and the process is continued until a sufficient number of layers has been built up after which the assembly is punched to provide contact openings 45 to the various conductor levels. When the assembly is fired to form the ceramic dielectric, the evanescent electrode material is lost or rendered inactive, thus leaving only the permanent conductor patterns 43. Contact to the various conductor levels may then be effected e.g. by through-hole plating 46.

In a further application an electrocoating resin/ceramic system may be formulated containing radiation sensitive materials, e.g. ultraviolet sensitive photoresists. A resin/ceramic film which is deposited on a substrate by the techniques described herein may be delineated by light, ultraviolet light, X-radiation or an electron beam, e.g. through a mask, to cross-link the photoresist in selected areas. Complete curing of the photoresist is not effected, but sufficient cross-linking takes place to enable the unexposed regions to be washed away prior to firing the remaining ceramic.

Awide range of ceramic materials may be employed in the process. If, for example, the process is used to fabricate a circuit substrate, then a low dielectric constant material will be employed. Typical ceramics include, but are in no way limited to, alumina, alumina based ceramics, porcelains and glass ceramics. Also there are numerous resin vehicle systems that are suitable for forming the ceramic dispersion.

It will be clearly understood that where a metal conductor pattern is fired with the ceramic the metal should be stable and compatible with the ceramic at the firing temperature. For example such metals as tungsten, palladium, nickel, platinum or alloys thereof may be used having regard to the nature of the ceramic and the nature of the firing atmosphere. The selection of a suitable metal for use with a particular ceramic will be apparent to those skilled in the art.

The following examples of compositions from which a ceramic material can be electrodeposited are quoted purely as examples and are in no way to be considered as limiting.

Example I A mix was prepared of the following materials: 500 gms ME 1420/0* 100 ml Benzyl Butyl Phthalate 21 gms Serfanol (wetting agent) 20 ml n-Butanol 750 gms Barium Titanate 100 ml Water distilled * An acrylic based resin medium made by Ault and Wiborg Ltd.

The mix was sand milled for 1 hour and was then added to a further 500 grams of ME 1420/0 and 650 ml distilled water.

Electrodeposition of the suspension on to a nickel substrate was effected at 10 C and an applied voltage of +200 V. A total of 20 microns thickness was deposited in 2 minutes. The deposited material was highly uniform and free from pinholes.

Example II A mix was prepared by ball milling together for 17 hours: 5 grams Barium titanate based ceramic 9 grams Resydrol P411E (Cray Valley Products Ltd) 10 my Water The mixture was removed from the mill and 75 ml deionised water were added.

The film anodically deposited from this composition was found to have a thickness of 20 microns after 2 minutes deposition at a voltage of 60 volts.

The film was then electroless nickel plated to a depth of 2 microns. Recoating with the ceramic medium was effected as before and the process repeated. A total of 20 layers were deposited. It was found that the deposited layers showed a high degree of uniformity and reproducibility and were free from pinholes.

These examples demonstrate the feasibility of fabricating multilayer ceramic structures by the techniques described herein.

Claims (13)

1. A process for fabricating a dielectric structure comprising a fired ceramic dielectric layer having a conductor pattern on at least one surface, wherein said ceramic layer is formed by joint electrodeposition of a ceramic powdertogetherwith a resin from a liquid medium.

2. A process as claimed in claim 1, wherein said resin forms a colloidal solution in the liquid medium.

3. A process as claimed in claim 1, wherein said ceramic is alumina, an alumina based ceramic, a porcelain or a glass ceramic.

4. A process as claimed in claim 1,2 or 3 wherein a plurality of dielectric layers are formed by consecutive deposition of ceramic material and a conductive material or materials.

5. A process as claimed in any one of claims 1 to 4 wherein the structure is heated to a temperature of 300 to 3500C prior to firing the ceramic.

6. A process as claimed in any one of claims 1 to 5, wherein said resin includes or comprises a radiation sensitive material.

7. A process as claimed in any one of claims 1 to 6, wherein said electrode pattern is formed by electroless deposition of a metal or alloy.

8. A process as claimed in any one of claims 1 to 7, wherein the electrode metal is nickel, tungsten, platinum, palladium or alloys thereof.

9. A process for fabricating a dielectric structure substantially as described herein with reference to Figures 1 and 2 or Figures 3 and 4 of the accompanying drawings.

10. A dielectric structure made by a process as claimed in any one of claims 1 to 9.

11. A dielectric structure comprising a fired body of electrodeposited ceramic powder, and a conductor pattern disposed on at least one surface of said body.

12. A dielectric structure substantially as described herein with reference to Figures 1 and 2 or Figures 3 and 4 of the accompanying drawings.

13. An electrical circuit assembly incorporating a dielectric structure as claimed in any one of claims 1 to 12.