WO2006008736A1

WO2006008736A1 - Fabrication of electrical components and circuits by selective electrophoretic deposition (s-epd) and transfer

Info

Publication number: WO2006008736A1
Application number: PCT/IL2005/000763
Authority: WO
Inventors: Assaf Thon; Martin Zarbov; Liat Shemesh
Original assignee: Cerel (Ceramic Technologies) Ltd.
Priority date: 2004-07-22
Filing date: 2005-07-18
Publication date: 2006-01-26

Abstract

The present invention is a method of manufacturing high density interconnects, electrical passive components (such as resistors, capacitors, transmission lines and inductors) and circuits on dielectric carriers. The method comprises performing electrophoretic deposition (EPD) on a conductive substrate that has been previously prepared in such a way that electrochemical deposition is possible only on selected areas of said conductive substrate. The present invention is a method of forming a green film in the form of fine lines and passive electrical elements and circuits on a dielectric tape, and in particular, on a ceramic tape such as the LTCC. This method has the capability to produce features of smaller dimensions (and therefore improved resolution) cost effectively and with improved uniformity in the properties of said green film. The invention is also electrical passive components and circuits manufactured totally or partially by EPD on dielectric carriers and to devices using such components and circuits.

Description

FABRICATION OF ELECTRICAL COMPONENTS AND CIRCUITS BY SELECTIVE ELECTROPHORETIC DEPOSITION (S-EPD) AND

TRANSFER

Field of the invention

The present invention relates to methods of manufacturing high density interconnects, electrical passive components (such as resistors, capacitors, transmission lines and inductors) and circuits on dielectric carriers. More particularly, the present invention relates to the manufacture of electrical components and circuits by electrophoretic deposition (EPD). Moreover, the invention relates to devices using such electrical passive components manufactured totally or partially by EPD, and to methods of making such electrical passive components and circuits using EPD with dielectric carriers.

BACKGROUND OF THE INVENTION

Functional systems or subsystems based on electronics (such as, for example, personal computers, cellular phones, audio-receivers, and television sets) usually include several semiconductor Integrated Circuits (ICs, or "chips"). In addition to the ICs, electronic systems include electrical connections between the ICs, transmission lines, antennae, and additional passive elements, such as resistors, capacitors, and inductors. Some or all of these components, have to be mounted on an insulating board, or integrated on a dielectric substrate. The mounting or integrating of the individual ICs and/or passive elements is presently accomplished by several dedicated technologies, for example as described in the book "Hybrid Microelectronics Handbook" (Second Edition), by Jerry E. Sergent and Charles A. Harper (Editors), published by McGraw Hill, New York, 1995.

A common technology for mounting ICs and the additional elements to form a functional electronic system is the "Printed Wiring Board Technology". The printed wiring boards are constructed from reinforced organic dielectric sheets, with copper metallization. The copper metal film is patterned by photolithographic methods. Copper-dielectric layers are then stacked and laminated under heat and pressure. Holes are then drilled in the laminated composite and plated to produce a thin copper film in the hole walls. The copper plated into the holes serve as electrical contacts to copper traces on the inner layers of the laminate, and also provide for an access to various levels of metal interconnects. Additional treatments make the whole composite assembly suitable for soldering the pins of ICs, and the contacting wires of individual passive electrical components. Hybrid electronic packages using printed wiring boards are often referred to as chip-on-board (COB) assemblies or laminated multi-chip modules (MCM-L). In an effort to produce functional systems with increased complexity operating at radio frequencies, newer implementations of electronic packages use ceramic substrates or laminates instead of organic dielectric substrates. One example of such ceramic substrate is the "Low Temperature Co-fired Ceramic" substrate (LTCC). Using LTCC substrates, an LTCC based technology allows for the fabrication of high circuit density by the efficient production of multiple layers and buried components. One review of LTCC based technology has been presented by CQ. Scranton and J. C. Lawson "LTCC technology: Where we are and we are going?" in IEEE Symposium on Technologies for Wireless Applications, New York, 1999. pp.193-200.

In LTCC based technology, individual layers of metals or insulators are deposited on a green LTCC tape by screen printing. These layers will be referred to as functional layers, being the constituents of the electrical components. The material to be printed to form the functional layers is referred to as an ink or paste. The ink contains three components: a functional phase which determines the electrical properties of the fired film, a binder which provides adhesion between the fired film and the substrate, and the solvent or dispersion medium that serves as the vehicle which establishes the printing characteristics. For conductors, the functional phase may be gold, silver, copper, palladium- silver, platinum-silver, or other suitable metals. Dielectric pastes may be applied by screen printing, but screen printed parallel capacitors are not widely used.

In the process of screen printing, the pattern definition is achieved by pressing a thin metal screen against the tape. The screen has a pattern of open and filled holes which correspond to the pattern of ink that is to be printed on the tape. The screen is brought into close proximity with the substrate, and the paste is applied on the screen. A rubber squeegee then forces the paste to flow into the open holes in the screen. The paste sticks to the surface, fills the openings in the screen, and passes through the mesh to cover the substrate only in the regions of open holes. The screen is then separated from the substrate leaving the paste adhering to the substrate in a pattern pre-defined by the screen. Current screen printing technology is normally limited to lines of not less than 100 micrometer width, and 100 micrometer spacing, although some advanced methods for screen printing have been reported with a resolution for lines and line spacing in the range of 50 micrometer. A review of screen printing can be found in the book "Hybrid Microcircuit Technology Handbook" by J. Licari, and L. Enlow, Noyes Publications, N. Y. 1998.

Another method to deposit thick films onto a ceramic substrate is by Ink-jet printing. In one version of ink-jet printing, denominated the "drop-on- demand mode", the fluid containing the material to be deposited is confined in a dispenser, and a volumetric change in the fluid is produced either by a piezoelectric displacement, by heating, or by other methods. The dispenser has a small orifice. The volumetric change produces a pressure transient causing a drop of ink to be ejected through the orifice. The drop is created only when desired and has a lateral dimension comparable to the diameter of the orifice. Available ink-jet printing technology can produce drops as small as 50 micrometers in diameter, and by using certain fluids in specific cases, down to 20 micrometer. The drops are dispensed serially, but an array of dispensers can be used to increase the printing speed.

After several layers of ceramic green tape have been covered by functional layers, they may be stacked together, pressed, and fired at elevated temperatures to form a multi-layer laminate.

If internal interconnections between layers in a laminate are needed in the final device, via-holes must first be added. Via-holes are formed by mechanically punching holes in the tape which are then filled with a particular paste. This paste contains a powder that forms an electrical connection when sintered, or an embedded capacitor between two layers of the laminate.

Cost reduction and increases in functionality of electronic packages are strong motives driving current trends towards reduction in size of the assembly. A further motivation for size reduction is the desire to produce electrical circuits operating at high radio frequencies, in the range of 2 to 5 GHz, and above 5 GHz. This reduction in the size of the electronic package is achieved by reducing the minimal size of the patterns, primarily the thickness and lateral width of the conducting lines, and the spacing between vicinal parallel lines. In the above described technologies, the narrowest lines and minimal spacing between the lines are in the range of 100 micrometers, or more, although in certain cases, 50 micrometer wide lines have been demonstrated. Some alternative technologies such as laser direct write of the functional layers, or photolithographic patterning on the green tape have been proposed and demonstrated, these methods can provide a better resolution but their use may compromise the cost-effectiveness of fabrication.

As a result there is a recognized need for provision of single and multiple functional layers with line patterns that can be produced with one or more of the following characteristics: reduced line width and inter-line spacing, reduced line thickness, improved homogeneity of particle packing, and improved reproducibility of the geometrical dimensions, such as line width and thickness. An application based description of the same requirements would be the recognized need to provide electrical components on the dielectric tape with improved resolution and uniformity, in order to support further miniaturization and to increase the functionality of the final product.

The present invention makes possible an alternative method to form a green film in the form of fine lines and passive electrical elements and circuits on a dielectric tape, and in particular, on a ceramic tape such as the LTCC. This alternative method, according to the present invention, has the capability to produce features of smaller dimensions (and therefore improved resolution) cost effectively and with improved uniformity in the properties of said green film. This said alternative method is based on electrophoretic deposition (EPD).

EPD is used to form thin or thick films on the surface of a conducting substrate, by depositing material from a suspension of particles in the size range between 4 micrometer and a few nanometer (the EPD suspension or dispersion) in a specially prepared "liquid medium" (the EPD medium). The particles in the suspension are electrically charged (positively or negatively), and the suspension placed between a pair of electrical plates (the electrodes), to which an electrical voltage is applied by an external electrical circuit. The potential difference between the electrodes causes charged particles in the medium to migrate towards the electrode of opposite polarity. As an illustration, certain embodiments of EPD will be described hereinafter, in which positively charged particles migrate towards the electrode of negative potential (the cathode), where they aggregate and bond to the surface of the cathode and to each other to form a deposited film. It is, however understood, that the positive charge of the particles, and the resulting deposition on the cathode, is only for illustrative purpose in describing this invention, since opposite polarity (negatively charged particles depositing on a positive electrode) is also possible, and it will be also contemplated here.

The deposited film can be dense, or porous, with the option to control the packing density by adjusting the EPD parameters. This specific processing technique relates to the teaching of, for example, U.S. Patents Nos. 5,919,347, 6,059,949, 6,127,283, 6,410,086, and 6,479,406, the entire disclosures of which are hereby incorporated by reference.

Additional information on EPD can be found in the publication "Electrophoretic Deposition - A Review", by M.S.J. Gani published in "Industrial Ceramics" Vol. 14, pages 163-174 (1994), and references therein. An additional article describing the mechanisms involved in EPD is "Electrophoretic Deposition, Mechanics, Kinetics, and Applications to Ceramics", by P. Sarkar and P.S. Nicholson, published in "Journal of American Ceramics Society" Vol. 79, pages 1987-2002 (1996). One feature of the EPD process, important for the realization of the present invention is that in the process of packing the charged particles in the deposited film at the deposition electrode, they neutralize their charge at the surface of the electrode, and an electrical current is established. The electrical current flows locally at the surface of the electrode, so if a certain portion of the surface is blocked by an insulating film, then current can not be established in this blocked portion, and EPD will not occur in this region.

In electronic printed board applications the EPD films have to be provided on an insulating substrate, such as a ceramic or plastic tape where they constitute passive electronic components, such as conductive wires, or capacitors (metal-dielectric-metal composites), or inductors (conducting coils). As a result of the above said, the product considered is an EPD film having a certain geometrical shape (lines, squares, circles, spirals, etc), in which the minimum lateral dimension is in the range of 5 to 100 micrometers, and more specifically between 10 and 50 micrometers, with minimum thickness in the range of about 1 micrometer but typical thickness in the range of 5 to 25 micrometers. Since patterning of the film on the ceramic board by prior methods may not result in the best possible resolution, the present invention is related to methods in which the pattern definition is performed simultaneously with the EPD process. This process, will be referred to hereinafter as "Selective EPD". Subsequent to the deposition of a selective EPD film on a conductive substrate, the green patterned film might be transferred to a dielectric substrate.

SUMMARY OF THE INVENTION

The present invention contemplates, in accordance with at least one presently preferred embodiment, a method to perform electrophoretic deposition on a conductive substrate that has been previously prepared in such a way that electrochemical deposition is possible only on selected areas of said conductive substrate. The method is herein defined as selective- electrophoretic deposition.

The present invention also contemplates, in accordance with at least one presently preferred embodiment, a method to perform selective electrophoretic deposition by using a pre-patterned electrode.

The present invention also contemplates, in accordance with at least one presently preferred embodiment, a single or multiple component dielectric tape with a single or multiple green patterned films or layers deposited on it, produced by selective electrophoretic deposition.

The present invention also contemplates, in accordance with at least one presently preferred embodiment, a method to fabricate a pre-patterned electrode for electrophoretic deposition using conventional micro-fabrication methods.

The present invention also contemplates, in accordance with at least one presently preferred embodiment, a single or multiple component dielectric tape with a single or multiple films or layers deposited on it, where said films or layers were produced by selective electrophoretic deposition on a pre-patterned electrode and transferred to a dielectric tape.

The present invention also contemplates, in accordance with at least one presently preferred embodiment, a single or multiple component dielectric tape with electronic components produced by selective electrophoretic deposition and transfer.

The present invention also contemplates, in accordance with at least one presently preferred embodiment, a laminate of several tapes in which single or multiple component dielectric tapes with a single or multiple films or layers deposited on them were fabricated by selective electrophoretic deposition and transfer.

The present invention also contemplates, in accordance with at least one presently preferred embodiment, a laminate of several tapes in which a single or multiple films or layers deposited on them constitute electrical components or circuits, and were fabricated by selective electrophoretic deposition and transfer.

In a first aspect, the invention provides a method for forming a green patterned film on the surface of a conductive or semiconducting substrate. The method comprises the steps of:

(a) treating the surface of said conductive or semiconducting substrate such as to make electrochemical coating possible only in selected areas of said surface; and

(b) electrophoretically depositing particles on said conductive or semiconducting substrate to form a green patterned film, only on said selected areas on the surface of said surface.

The green patterned film formed according to the invention is selected from the following group:

(a) a conductor, including but not limited to silver, gold, aluminum, copper, platinum, titanium, or a conducting polymer;

(b) A resistor;

(c) a dielectric, including but not limited to barium titanate; and

(d) a piezo- electric material.

The green film of the invention can be a multi-layer composed of conductors and dielectrics. The conducting or semiconducting substrate can be a silicon substrate or a glass, quartz, or sapphire carrier covered with a thin conducting film. The thin conducting film can be Indium Tin Oxide (ITO).

In another aspect, the invention provides a single or multiple -layer functional film deposited on a substrate, in which at least one layer of the functional film is in the form of a fine pattern, produced by the method of the invention.

In yet another aspect, the invention provides an electronic component formed by single or multiple films on the surface of a conductive or semiconducting substrate, produced by the method of the invention. The electronic component is selected from:

(a) Capacitors

(b) Inductors

(c) Resistors.

In still another aspect, the invention provides a radio-frequency (RF) component produced by the method of the invention and formed by single or multiple films on the surface of a conductive or semiconducting substrate. The RF component is selected from: (a) Microstrip transmission lines (b) Couplers

(c) Antennas

(d) Power dividers and combiners

(e) Eesonators.

In other aspects, the invention provides electrical interconnects between electronic components and a piezoelectric component both formed by single or multiple films on the surface of a conductive or semiconducting substrate produced by the method of the invention. The piezoelectric component, is selected from sensors or actuators.

In another aspect, the invention provides a functional electrical circuit or part of a functional electrical circuit formed by electronic components and electrical interconnects formed by single or multiple films on the surface of a conductive or semiconducting substrate produced by the method of claim 1.

In a further aspect, the invention provides a method for preparing the surface of a conductive or semiconducting substrate for the selective electrochemical coating to form a green film in selected areas on said surface. The method of the invention comprises the steps of:

(a) Forming a thin, electrically insulating film on the surface of said conductive surface; (b) Producing a photoresist mask pattern by photolithographic methods on said insulating film, thereby to protect certain regions and to expose other regions of the surface of said insulating film;

(c) Etching said exposed regions of said insulating film by a method selected from chemical etching and physical sputtering, or by a combination of chemical etching and physical sputtering; and

(d) Removing the photoresist by solvent cleaning.

According to the method of the invention, the conducting or semiconducting substrate can be a silicon substrate, the insulating film can be a Silicon- dioxide layer formed by oxidation of the silicon surface, and the etching is performed by:

(a) inmersing the silicon substrate with the silicon- dioxide layer in a buffered HF solution; or by

(b) Reactive Ion Etching.

The method of the invention for forming a green patterned film on the surface of a conductive or semiconducting substrate can further comprise the step of subsequently transferring the green patterned film to a dielectric tape. In which case, the transfer method comprises the steps of:

(a) bringing the surface of the conductive substrate with the electrophoretically deposited green patterned film in close proximity to the surface of the dielectric tape; (b) applying a uniform pressure normal to a bilayer consisting of the conductive substrate, on one side, and to the dielectric tape, on the other side;

(c) subsequent to the application of the uniform pressure, separating the conductive substrate from the dielectric tape.

Embodiments of the method of the invention containing the above additional step, can further comprise the steps of:

(a) applying to the conductive substrate, prior to the deposition of the green patterned film, a thin coating suitable to reduce the adhesion of the green patterned film to the conductive substrate; or

(b) applying to the dielectric tape a binder suitable for increasing the bonding of the green patterned film to the dielectric tape during the transfer step; or

(c) combining steps (a) and (b).

According to the method of the invention comprising the step of subsequently transferring the green patterned film to a dielectric tape, the dielectric tape can be a green Low Temperature Cofired Ceramic tape or an organic polymer tape or a plastic sheet. The Low Temperature Cofired Ceramic can be sintered. According to the method of the invention comprising the step of subsequently transferring the green patterned film to a dielectric tape, the green patterned film can be selected from:

(a) A conductor, including but not limited to silver, gold, aluminum, copper, platinum, titanium, and a coducting polymer;

(b) A dielectric, including but not limited to barium titanate;

(c) A piezo-electric material;

(d) A multi-layer, composed of conductors and dielectrics; and

(e) A multi-layer, composed by a piezo-electric material and a conductor.

According to the method of the invention comprising the step of subsequently transferring the green patterned film to a dielectric tape, the conducting or semiconducting substrate can be a silicon wafer.

In another aspect, the invention provides a single or multiple green patterned film, produced by the method of the invention comprising the step of subsequently transferring the green patterned film to a dielectric tape and deposited on a dielectric tape, in which at least one deposited film is in the form of a fine pattern.

In another aspect, the invention provides a laminate of dielectric tapes, which includes a single or multiple green patterned film formed on one or on several of the constituent dielectric tapes, produced by the method of the invention comprising the step of subsequently transferring the green patterned film to a dielectric tape, in which at least one deposited film is in the form of a fine pattern.

In another aspect, the invention provides electronic components formed by single or multiple films on a dielectric tape produced by the method of the invention comprising the step of subsequently transferring the green patterned film to a dielectric tape. The electronic components are selected from:

(a) Capacitors;

(b) Inductors; and

(c) Resistors.

In another aspect, the invention provides Radio-frequency (RF) components formed by single or multiple films on a dielectric, produced by the method of the invention comprising the step of subsequently transferring the green patterned film to a dielectric tape. The RF components are selected from:

(a) Microstrip transmission lines;

QS) Couplers;

(c) Antennas;

(d) Power dividers and combiners; and

(e) Resonators. In another aspect, the invention provides Piezoelectric components formed by single or multiple films on a dielectric tape produced by the method of the invention comprising the step of subsequently transferring the green patterned film to a dielectric tape. The piezoelectric components can be sensors or actuators.

In another aspect, the invention provides a laminate of dielectric tapes that includes electronic components formed by a single or multiple film deposited on a single or several of the constituent dielectric tapes, in which at least one deposited film is in the form of a fine pattern, produced by the method of the invention comprising the step of subsequently transferring the green patterned film to a dielectric tape.

In another aspect, the invention provides electrical interconnects between electronic components formed by single or multiple films on a dielectric tape produced by the method of the invention comprising the step of subsequently transferring the green patterned film to a dielectric tape,.

In another aspect, the invention provides a laminate of dielectric tapes with electrical interconnects formed by a single or multiple film deposited on one or on several of the constituent dielectric tapes, in which at least one deposited film is in the form of a fine pattern, produced by the method of the invention comprising the step of subsequently transferring the green patterned film to a dielectric tape.

In another aspect, the invention provides a functional electrical circuit, or part of a functional electrical circuit formed by electronic components and electrical interconnects formed by single or multiple films on a dielectric tape produced by the method of the invention comprising the step of subsequently transferring the green patterned film to a dielectric tape,.

In another aspect, the invention provides a laminate of dielectric tapes with a functional electrical circuit or part of a functional electrical circuit formed by a single or multiple film deposited on one or on several of the constituent dielectric tapes, including at least one deposited film in the form of a fine pattern, produced by the method of the invention comprising the step of subsequently transferring the green patterned film to a dielectric tape.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention, in accordance with preferred and exemplary embodiments, together with further advantages thereof, is more particularly described in the following description, taken in conjunction with the accompanying drawings. In the drawings, like reference numbers generally refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead generally being placed upon illustrating the principles of the invention.

FIG. IA schematically illustrates a cross section of an exemplary electrophoretic deposition bath, for uniform cathodic deposition, according to the prior art.

FIG. IB illustrates a cross section of an exemplary electrophoretic deposition bath, for selective cathodic deposition, by the use of a cathode with a patterned insulating film.

FIG. 2A shows a cross section of a conducting or semiconducting electrode including regions on its surface covered with an insulating thin film, and other regions where the surface of the electrode is exposed for selective electrophoretic deposition, in accordance with the invention.

FIG. 2B is an isomeric view of an electrode for selective electrophoretic deposition with an exemplary patterned insulating film, in accordance with the invention.

FIG. 3 schematically shows an exemplary electrophoretic deposition bath designed for improved capability for control of the conditions of selective EPD.

FIG. 4 schematically shows an exemplary jig for transfer of the patterned green film from the pre-patterned electrode to a dielectric tape. FIG. 5 illustrates a cross section of a dielectric laminate with

electronic circuits comprising a generic representation of hybrid

microelectronic assembly.

FIG. 6 is a micrograph of an exemplary implementation of a Silicon-

based pre-patterned electrode.

FIGS. 7A and 7B are micrographs of an exemplary implementation

of a Silicon-based pre-patterned electrode with a green patterned film

formed by EPD.

FIG. 8 is a micrograph of an exemplary green patterned film formed

by selective EPD on silicon and transferred to a ceramic tape.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention relates to the formation of films or layers by using the

techniques and the methods of electrophoretic deposition (EPD).

Conventionally EPD is a process in which electrostatically charged particles

suspended in a fluid medium migrate under an applied electric field and

deposit onto a conductive surface having an applied potential opposite in

sign to the particle charge. As an example, positively charged particles are

considered, to be deposited on a negatively charged electrode (a Cathode).

However, it is understood that this polarity is considered as an example,

and that EPD can be implemented with opposite polarity as well. The film

deposited by conventional EPD is continuous, and covers the cathode

uniformly. FIG. IA shows schematically a conventional EPD cell 10 where a container 11 contains the EPD suspension 20. An electrical power supply 30

is connected to a negative electrode (the cathode) 31 and a positive electrode

(the anode) 32. The two electrodes 31 and 32 are immersed in the EPD

suspension 20. As an example, FIG. IA depicts cathodic deposition, in

which positive particles are neutralized by electrons at the surface of the

cathode.

In certain cases, the cathode for EPD consists of a patterned conductive

surface attached to a non-conducting carrier. If the patterned conductive

surface is subjected to a negative potential, the charged particles will

deposit only on the patterned conductive surface. A major disadvantage of

this method of patterned deposition is that in order to be effective, all the

areas of the pattern need to be subjected to the electrical potential and

therefore they have to form a connected conducting pattern. Disconnected

areas, such as, for example, an array of isolated squares or lines, can not be

fabricated by EPD by using this method. As described above, conventional

EPD can be used to produce a continuous film or a patterned film where all

areas of the pattern are fully interconnected to each other in order to be

subjected to the appropriate electrical potential.

Another property of the EPD process is that in the complete electrical circuit

an electrical current is established between the cathode 31 and the anode

32. This electrical current is carried by the charged particles 40, and the arrival of each charged particle to the electrode requires charge

neutralization between the charged particle 40 and the local charge at the

surface of said cathode 31. This teaching leads to the conclusion that EPD is

based on the local exchange of charge at the electrode in order to establish a

current. Areas of the electrode covered by a current-blocking insulating

coating are excluded from the EDP process.

In at least one of the preferred embodiments of the present invention, a

method to perform a selective EPD process is contemplated, as depicted in

FIG IB by the use of a conducting electrode 31 on which an electrically

insulating film 50 was deposited to prevent the flow of current from the

electrode. An enlarged version of said pre-patterned electrode is shown in

FIG. 2A and FIG. 2B. The insulating film 50 is patterned in such a way

that parts of the film are removed in regions where the electrode is to be

exposed to the electrical current. The conducting electrode 31 can be a sheet

of metal, a metal covered carrier (such as, for example, but not limited to, a

glass plate covered with aluminum), or a doped semiconductor. The

insulating film 50 can be any dielectric applied by conventional coating

methods, and suitable to be etched by conventional methods. As used

herein, the conducting electrode 31 covered by an insulating film 50, in

which the film was patterned to produce electrical current in certain,

selected areas, and with a certain pattern, is defined as a pre-patterned

electrode. The advantage of the use of a pre-patterned electrode for EDP is that such electrode can be produced with high perfection and high spatial resolution without compromising the cost-effectiveness of the EPD process. An additional advantage is that the use of a pre-patterned electrode for EPD induces an electric current only in the exposed regions and avoids material waste by preventing deposition in undesired areas. The electrical potential of all the exposed areas is nearly the same, regardless of whether these areas spatially connected or disconnected.

In at least one of the preferred embodiments of the present invention, a pre- patterned electrode fabricated by a microelectronic fabrication methodology is contemplated. One possible implementation of said pre-patterned electrode fabricated by the microelectronic fabrication methodology is the following: a conducting or semiconducting substrate, or a non-conducting substrate covered by a thin conducting film, is covered with an insulating film such as silicon dioxide, silicon nitride, or silicon oxy-nitride. The desired pattern for EPD is made on a photolithographic mask. The substrate is coated uniformly with photo-resist by spin-coating. The photo-resist coated substrate is pre-baked on a hot plate at a temperature of 90⁰C in order to evaporate the photo-resist solvent. The photolithographic mask is then attached to the surface of the photo-resist covered substrate, and exposed to UV light for the period of time necessary to expose the complete thickness of the photo-resist. After exposure, the substrate is post-baked in a hot oven at a temperature of about 120⁰C, and developed in a developer. After the photo-resist has been developed, the desired pattern on the photo-resist serves to protect the regions of the insulating film that will not be etched. Subsequently, the substrate is subjected to an etching agent that etches away the insulating film only in the regions previously exposed in the photo¬ resist. In the last stage, the photo-resist is removed by a suitable solvent. The fabrication steps described in this paragraph are known to persons skilled in the art, as recognized steps of microelectronics fabrication technology that are described in many books. One such book is "Silicon VLSI Technology" by J.D. Plummer, M.D. Deal, and P.B. Griffin, edited in year 2000 by Prentice-Hall, Upper Saddle Eiver, New Jersey 07458.

Microelectronic fabrication technology is relatively expensive. If such an electrode were to be consumed for each patterned film to be made, the process would not be cost effective. However a single pre-patterned electrode can be used repetitively for the formation of many selective EPD films, as explained below, significantly reducing the impact of the cost of fabrication on the cost of the final product.

In at least one of the preferred embodiments of the present invention, a method for selective EPD process is contemplated. The method consists of immersing a pre-patterned electrode, and a counter electrode, in a bath filled with a suspension, better understood in conjunction with FIG IB. The pre-patterned electrode 33 and the counter electrode 32 are parallel to each other. The two electrodes are electrically connected to an electrical current supply 30 by electrical cables 29. The suspension or dispersion of charged particles 40 is contained in the bath 20 and the charged particles can migrate by electrophoresis towards the electrode of opposite polarity. In FIG. IB the movement of the charged particles 40 is represented by arrows. When the charged particles reach the near proximity of the pre-patterned electrode 33, they aggregate to become part of the deposited film 42. In this embodiment, the bath 12 is herein defined as the EPD cell, the suspension 20 is defined as the EPD suspension, the material or composite to be deposited is defined as the EPD material, and the deposited film 42 is defined as the EPD film. The EPD material is usually a powder, whose size has to be at least five times smaller than the smallest feature to be created. For example, if the minimal feature has a lateral dimension of 10 micrometers, the EPD material has to be composed of particles not larger than 2 micrometers across. In order to reduce the surface roughness of the film, and to minimize thickness fluctuations, the EPD material to be used might be composed of sub-micron size particles, commonly called nano- powders.

A particular attribute of selective EPD is that the electrical current inside the EPD cell, and the associated electrical field are locally related to the concentration of the EPD material (usually called the "solid load" in the EPD suspension), and to the distance between the electrodes. The distance between the electrodes will vary if the electrodes are not strictly parallel, so the electrodes must be kept parallel. The concentration of the EPD material may also vary, as a result of slow sedimentation of the solid particles in the EPD medium due to gravitation. One efficient way to control the concentration of the EPD material is to perform the EPD in a cell configuration with the electrodes horizontal and parallel to each other, the pre-patterned electrode being the upper one. In this configuration, the EPD charged particles migrate up, balancing the force of gravity and any tendency to sedimentation. FIG. 3 schematically illustrates an exemplary EPD cell designed by the inventors to implement selective EPD using a pre- patterned cathode in accordance with at least one embodiment of the present invention. In a container 120 containing the EPD suspension, a special electrode holder 125 made of a non-conducting material is inserted at the bottom of the cell 120 to hold the anode 132 horizontal and concentric within the cell 120. The pre-patterned cathode 150 is held by a cathode holder 129 supported by supporting rods 128. The cathode holder 129 and the supporting rods 128 are made of a non-conducting material. The supporting rods 128 are secured to a cell cover 127 in such a way that the height of the pre-patterned cathode 150 can be adjusted at 3 points until the best conditions for selective EPD are found. Said best conditions are determined, among other factors, by the voltage supplied by the power supply 130 and by the separation distance between the anode 132 and the pre-patterned cathode 150.

In the above described selective EPD process, a patterned film of some functional material, such as a green film that is conducting after firing, can be produced on the pre-patterned electrode 150. In electronic printed board applications the EPD films have to be produced on a ceramic substrate where they constitute passive electronic components, such as conductive wires, or capacitors (metal-dielectric-metal composites). In view of that, a need has been recognized in connection with selective EPD for applications in electronics, to transfer the EPD patterned film onto a dielectric tape.

The present invention also contemplates, in accordance with at least one presently preferred embodiment, a single or multiple component dielectric tape with a single or multiple film or layer deposited on it, where the film was produced by selective electrophoretic deposition on a pre-patterned electrode and transferred to a dielectric tape. Jn order to accomplish the process of transfer, it is recognized that the patterned film has to be detached from the pre-patterned electrode, and therefore, the adhesion of the film to the electrode has to be weak. To provide this requirement, the present invention contemplates pre-conditioning of the pre-patterned electrode with a thin sacrificial film to reduce the Van-der-Vaals binding forces between the electrode surface and the EPD film. The film has to be thin enough to permit the EPD current to flow by electrical conduction, and to be resistant to the EPD medium. This embodiment can be better understood in conjunction with FIG. 2A showing the thin film 56. One such thin film is, for example, Pyralin PD PI-2721 or PI-2722, or PI-2723 produced and supplied by DuPont Electronics, Bartley Mill Plaza, POBox 80019, Wilmington, DE 19880-0019. Pyralin can be applied to the surface of the pre-patterned electrode by spin-coating, to produce a film of several micrometers thickness. When a thinner film is needed, the Pyralin can be diluted with Pyraline Thinner Pl-2555 of the same supplier. In order to produce a very thin film, the inventors of the present invention produced a diluted mixture of Pyralin with Thinner with a ratio of 1 to 5, 1 to 10 and 1 to 30 in the process of implementing this invention. It was observed that the diluted Pyralin in the ratio 1 to 30 produced a film that does not inhibit formation of the EPD film, but that this film only adheres weakly to the pre- patterned cathode.

In the contemplated process, the pre-patterned cathode with the EPD film (the green tape) is brought into close proximity with a ceramic tape. The surface coated with the green film has to be face to face with the surface of the tape onto which the green film has to be transferred. The inventors of the present invention observed that the application of a uniform pressure on the composite, in the range of 500N/cm² to 10,000N/cm² caused the green film to remain attached to the surface of the ceramic tape, and completely detach from the electrode. In the process used to transfer the patterned film to the ceramic tape, there was no need to apply any heating to the tape or to the electrode. However, it is contemplated that in certain cases of other substrates, other ceramic tapes, other patterned film material, or a modified selective EPD process, the application of pressure at elevated temperature may be beneficial to the transfer process. The above said transfer process can be better understood in conjunction with FIG. 4, in which a schematic representation of an exemplary transfer jig 200 is depicted. The transfer jig 200 is comprised of a lower cylinder 205 of which one surface 210 was previously machined to accommodate accurately the pre-patterned cathode 240 on which was the green patterned film 250. The second part of the transfer jig 200 is a hollow cylinder 260 with internal cylindrical face 261 snuggly fitting the external cylindrical face 207 of the lower cylinder 205. This provision is one possible implementation capable of ensuring strict parallelism between the internal circular base 262 of the hollow cylinder 260, and the upper circular base 210 of the lower cylinder 205. A green dielectric tape 270, such as LTCC, Mylar, a flexible polymer, or any other dielectric tape, is inserted into the internal circular base 262, the lower cylinder 205 inserted into the hollow cylinder 260, until the dielectric tape 270 and the green patterned EPD film 250 approach each other. Then a force 290 is applied to produce a pressure on the internal base 262 and the upper base 210. The magnitude of this pressure needed to cause the green patterned EPD film 250 be released from the cathode 240, and become attached to the dielectric tape 270 is a variable depending on the EPD film properties such as the EPD material, thickness, density, and minimal lateral dimensions of the pattern. The magnitude of said pressure also depends on the dielectric tape onto which the green patterned EPD film is to be transferred. An optimal value of such pressure for each particular set of conditions can be found by experiment.

The advantages of this transfer process reside in the ability to perform the process while preserving the integrity of the pre-patterned electrode 240. The pre-patterned electrode can thus be re-used as a template for additional selective EPD processes. As a result of this capability, the patterned electrode can be fabricated with high resolution microfabrication methods without compromising the cost of the contemplated product.

The present invention also contemplates, in accordance with at least one presently preferred embodiment, a laminate of several tapes in which a single or multiple film deposited on the tapes constitute electrical components or circuits, and were fabricated by selective electrophoretic deposition and transfer. This embodiment can be more readily understood in conjunction with the drawing depicted in FIG. 5 of a typical hybrid microelectronic assembly. It consists of a laminate of dielectric tapes 300, on which a packaged semiconductor integrated circuit (a chip) 310 has been mounted by surface mount soldering 311. Shown also schematically is an individual electrical passive component 320, mounted by surface mount soldering. Additionally, conducting features 330 formed on the surface of each layer of the dielectric tapes 300 constitute capacitors 350, inductors 360, and resistors 370, some of which are embedded in the laminate, others being on the surface. By combining and connecting the conducting features 330, the capacitors 350, the inductors 360, and the resistors 370, electrical circuits are formed in conjunction with semiconductor integrated chips 310, and discrete surface mounted elements 320. An advantage of this embodiment of the invention is that the electrical circuits or part of them can be formed with high resolution, high uniformity across the tape, controllable density and cost effectively by selective EPD.

Example 1

A pre-patterned cathode was prepared in the following way: A 2" n-type Silicon wafer was cleaned by immersing in a mixture of HFrEbO with a 1:50 ratio for 30seconds. Subsequently, the wafer was immersed in de-ionized water for 10 minutes and dried in a spinner. The clean wafer was oxidized in a dry oxidation furnace at 1050⁰C for two hours, to form a thin layer of thermal SiO₂ of about 0.7 micrometers thick. A photolithographic mask containing patterns corresponding to long lines, inter-digitated electrodes, and coils, with line width/spacing of 20/20, 16/16, and 12/12 micrometer was designed. Photo-resist (Kodak Z-2070) was spun on the wafers at a spinning rate of 5000 r.p.m. for 1 minute, and then pre-baked on a hot plate at 90⁰C for 3 minutes. The wafer and the mask were inserted in a Mask Aligner

(Karl Zuss MJB-3) and the wafer exposed to UV illumination during 4 seconds. Post-baking was performed on a hot plate at HO⁰C for 1 minute, and the development of the exposed photo-resist in a mixture of developer and water (AZ-312:H2θ at 1:1 ratio) for 1 minute. The developed wafer was subjected to Descum in an Oxygen plasma system using a power of 100 Watts, and an Oxygen pressure of 200 mTorr for 3 minutes. After Descum the wafer was again post-baked on a hot plate at 120⁰C for 40 minutes and immersed in buffered HF for 15 minutes for etching of Siθ2. Resist removal was effected by immersing the wafer in a bath with Acetone and subjected to Ultrasonic shaking. A micrograph of parts of the Silicon wafer with the patterned Siθ2 on it, constituting the exemplary implementation of a pre- patterned cathode is shown in FIG. 6.

Example 2

The pre-patterned cathode of Example 1 was used for selective EPD of silver by the following procedure. Silver-palladium powder 7102FG (purchased from Cermet Materials, Inc. 6 Meco Drive, Wilmington, DE 19804, USA) was used as the EPD material. The powder was dispersed in ethyl alcohol. The suspension was stabilized by the electrosteric additive poly-ethylene- imine. A sonication treatment was carried out by dipping an ultrasonic head in the suspension bath. The ultrasonic head delivered 8000 Joule, and the

sonication time was 5 minutes. The dispersion was poured in the exemplary

EPD bath with horizontal electrodes. A plain Silicon wafer was used as the

anode, and the pre-pattemed electrode as the cathode. The distance between

the electrodes was about 1 cm. A voltage of 30 V was supplied by the power

-supply, during'30 seconds. Surplus deposition on top of the Siθ2 areas was

removed by brief immersion in ethyl alcohol. Photographs of the pre-

patterned cathode with the green patterned EPD film is shown in FIGS. 7A

and 7B.

Example 3

The Silicon wafer with the green patterned film of Example 2 was placed in

a transfer jig similar to the one schematically described in FIG. 4, with a

green LTCC tape purchased from Nippon Electric Glass Co, Ltd, 1-14 Miyahara 4-chome, Yodogawa-ku, Osaka 532-0003, Japan, the two parts of

the jig were pressed against each other with a force of 500 N/cm² for 7

minutes. When separated it was observed that the green patterned EPD

film was transferred to the LTCC tape preserving its integrity. The Silicon

wafer remained clean and ready for re-use in a subsequent selective EPD

run. Surface profiling of the Silicon wafer was performed by α-step

profilometry, confirming that no traces of the green patterned film remain

on the pre-patterned cathode. A photograph of the LTCC tape with the

transferred green patterned EPD film is shown in FIG. 8. The LTCC tape with the transferred green patterned EPD film was subsequently sintered at

875⁰C for 10 hours. Visual inspection confirmed the integrity of the

patterns, and electrical testing confirmed continuity of the silver lines.

Claims

What we claim is:

1. A method for forming a green patterned film on the surface of a conductive or semiconducting substrate, comprising the steps of:

2. The method of claim 1, in which the green patterned film is selected from:

(b) A resistor;

(c) a dielectric, including but not limited to barium titanate; and

(d) a piezo-electric material.

3. The method of claim 1, in which the green film is a multi-layer composed of conductors and dielectrics.

4. The method of claim 1, in which the conducting or semiconducting substrate is a silicon substrate.

5. The method of claim 1, in which the conducting or semiconducting substrate is a glass, quartz, or sapphire carrier covered with a thin conducting film.

6. The method of claim 5, in which the thin conducting film is Indium Tin Oxide (ITO).

7. A single or multiple-layer functional film deposited on a substrate, in which at least one layer of said functional film is in the form of a fine pattern, produced by the method of claim 1.

8. An electronic component formed by single or multiple films on the surface of a conductive or semiconducting substrate, produced by the method of claim 1, said electronic component being selected from:

(a) Capacitors

(b) Inductors

(c) Resistors.

9. A radio-frequency (RF) component formed by single or multiple films on the surface of a conductive or semiconducting substrate produced by the method of claim 1, said RF component being selected from:

(a) Microstrip transmission lines

(b) Couplers

(c) Antennas

(d) Power dividers and combiners

(e) Resonators.

10. A piezoelectric component formed by single or multiple films on the surface of a conductive or semiconducting substrate produced by the method of claim 1, said piezoelectric component, being selected from sensors or actuators.

11. Electrical interconnects between electronic components formed by single or multiple films on the surface of a conductive or semiconducting substrate produced by the method of claim 1.

12. A functional electrical circuit or part of a functional electrical circuit formed by electronic components and electrical interconnects formed by single or multiple films on the surface of a conductive or semiconducting substrate produced by the method of claim 1.

13. A method for preparing the surface of a conductive or semiconducting substrate for the selective electrochemical coating to form a green film in selected areas on said surface, comprising the steps of:

(a) Forming a thin, electrically insulating film on the surface of said conductive surface;

(b) Producing a photoresist mask pattern by photolithographic methods on said insulating film, thereby to protect certain regions and to expose other regions of the surface of said insulating film;

(d) Removing the photoresist by solvent cleaning.

14. The method of claim 13, in which the conducting or semiconducting substrate is a silicon substrate.

15. The method of claim 14, in which the insulating film is a Silicon- dioxide layer formed by oxidation of the silicon surface, and the etching is performed by:

(a) inmersing the silicon substrate with said silicon- dioxide layer in a buffered HF solution; or by

(b) Reactive Ion Etching.

16. The method of claim 1, further comprising the step of subsequently transferring the green patterned film to a dielectric tape, in which the transfer method comprises the steps of:

(a) bringing the surface of the conductive substrate with the electrophoretically deposited green patterned film in close proximity to the surface of the dielectric tape;

(b) applying a uniform pressure normal to a bilayer consisting of said conductive substrate, on one side, and to said dielectric tape, on the other side;

(c) subsequent to the application of the uniform pressure, separating said conductive substrate from said dielectric tape.

17. The method of claim 16, further comprising:

(a) applying to the conductive substrate, prior to the deposition of the green patterned film, a thin coating suitable to reduce the adhesion of said green patterned film to said conductive substrate; or

(b) applying to the dielectric tape a binder suitable for increasing the bonding of said green patterned film to said dielectric tape during the transfer step; or

(c) combining steps (a) and (b).

18. The method of claim 16 in which the dielectric tape is a green Low

Temperature Cofired Ceramic tape.

19. The method of claim 18 in which the Low Temperature Cofired Ceramic is sintered.

20. The method of claim 16 in which the dielectric tape is an organic polymer tape or a plastic sheet.

21. The method of claim 16, in which the green patterned film is selected from:

(b) A dielectric, including but not limited to barium titanate;

(c) A piezo-electric material;

(d) A multi-layer, composed of conductors and dielectrics; and

(e) A multi-layer, composed by a piezo-electric material and a conductor.

22. The method of claim 16, in which the conducting or semiconducting substrate is a silicon wafer.

23. A single or multiple green patterned film deposited on a dielectric tape, in which at least one deposited film is in the form of a fine pattern, produced by the method of claim 16.

24.A laminate of dielectric tapes that includes a single or multiple green patterned film formed on one or on several of the constituent dielectric tapes, in which at least one deposited film is in the form of a fine pattern, produced by the method of claim 16.

25. Electronic components formed by single or multiple films on a dielectric tape produced by the method of claim 16, said electronic components being selected from:

(a) Capacitors;

(b) Inductors; and

(c) Resistors.

26. Radio-frequency (RF) components formed by single or multiple films on a dielectric tape produced by the method of claim 16, wherein said RF components are selected from:

(a) Microstrip transmission lines;

(b) (b) Couplers;

(c) (c) Antennas;

(d) (d) Power dividers and combiners; and (e) (e) Resonators.

27. Piezoelectric components formed by single or multiple films on a dielectric tape produced by the method of claim 16, said piezoelectric components including sensors and actuators.

28. A laminate of dielectric tapes that includes electronic components formed by a single or multiple film deposited on a single or several of the constituent dielectric tapes, in which at least one deposited film is in the form of a fine pattern, produced by the method of claim 16.

29. Electrical interconnects between electronic components formed by single or multiple films on a dielectric tape produced by the method of claim 16.

30. A laminate of dielectric tapes with electrical interconnects formed by a single or multiple film deposited on one or on several of the constituent dielectric tapes, in which at least one deposited film is in the form of a fine pattern, produced by the method of claim 16.

31. A functional electrical circuit, or part of a functional electrical circuit formed by electronic components and electrical interconnects formed by single or multiple films on a dielectric tape produced by the method of claim 16.

32. A laminate of dielectric tapes with a functional electrical circuit or part of a functional electrical circuit formed by a single or multiple film deposited on one or on several of the constituent dielectric tapes, including at least one deposited film in the form of a fine pattern, produced by the method of claim 16.