GB2429841A - Selective area deposition and devices formed therefrom - Google Patents

Abstract

Patterned thin film layers (12) are applied to a substrate (10) surface by masking selective areas of a substrate surface, e.g., with a printed pattern (11) of a deposition inhibiting material such as an oil, and vapor-depositing thin film material. The masking material is subsequently removed.

Description

SELECTIVE AREA DEPOSITION AND DEVICES FORMED THEREFROM

The present invention is directed to selective area deposition of thin films, including, but not limited, to thin films useful for forming electronic circuitry.

Circuitry traces of printed electronic circuitry are typically formed by a photolithographic process. In a typical process for forming a layer of circuitry traces, a blank is prepared comprising a metal layer, typically copper, on a dielectric substrate, such as a fiberglass-epoxy composite. A layer of photoresist is applied to the metal layer, and patterned artwork laid over the photoresist layer. Then the photoresist is exposed to actinic radiation so as to affect the exposed portions of the photoresist. Then the photoresist is developed with an appropriate developing solution that washes away exposed portions of the photoresist, in the case of negative-acting photoresists, and non- exposed portions, in the case of positive-acting photoresists. Next, the metal layer is etched to remove the metal from the portions of the metal layer from which resist has been removed. Finally, the remaining photoresist is stripped.

Advances in resists and photolithography have yielded finer and finer resolution in electronic printed circuitry, and photolithography has many advantages and is well studied as a method of producing a printed circuit. Nevertheless, it can be seen that the printing of electronic circuitry is a multi-step process, each step requiring time, effort and a variety of materials. Specific chemicals, some of them harsh, and all requiring ultimate disposal, are required for producing the photoresist layer, developing the photoresist layer, etching the metal layer, and stripping the remaining photoresist layer.

U.S. Patent No. 6,210,592 describes thin film layers of resistive materials that can be deposited, for example, by combustion chemical vapor deposition (CCVD) and patterned by photolithography to form patches of resistive material. When used in conjunction with circuitry traces, a plurality of thin film resistors may be formed by this technique. Again, multi-step photolithography steps are used for forming the patterned resistive material layer.

U.S. Patent No. 6,207,522 describes thin film layers of dielectric materials that can be deposited, for example, by CCVD and used as the dielectric material in thin film capacitors. This patent describes patterning of the dielectric material layer by photolithographic processes. However, patterning of dielectric materials often proves more difficult than patterning metal circuitry layers or metal-based resistive material layers. Accordingly, dielectric layers in thin film electronic circuitry composites may be left as un-patterned planer layers with capacitive electrical pathways formed between conductive (and/or resistive) layers on both sides of the dielectric layers. In some cases it would be desirable to form capacitive circuitry paths horizontally across the dielectric material layer rather than or in addition to vertically through the dielectric material layer.

Accordingly, a simple method of patterning dielectric layers would be desirable.

U.S. Patent No. 6,212,078 describes multi-layer nanolaminated thin film circuitry traces. In this patent, nanolaminated structures containing conductive layers, resistive layers, and dielectric layers are used to form complex electronic circuitry containing conductive pathways, resistors, capacitors, and inductors. In the structures described in this patent, certain layers are patterned by photolithographic techniques, such as those described above. Other layers are left un-patterned with subsequently formed via holes used to interconnect layers and define electrical pathways, e.g., resistive and capacitive pathways. It would be desirable to provide simpler ways of patterning thin films so as to reduce processing steps, reduce the use of processing chemicals, and provide greater flexibility in the design of multi-layer thin film circuitry.

While the techniques described herein may not be as fully developed as photolithography techniques and may not yet provide the same resolution as the most advanced photolithographic techniques, the processes have reduced steps, and use fewer steps and reduce the use of chemicals. In many cases the techniques described herein may provide greater flexibility of circuitry design. The techniques are especially useful for forming relatively inexpensive, simple electronic circuitry that may be designed for a variety of applications. Further, it is possible to form entire circuitry composites, including conducting, resistive, capacitive, and inductive elements of thin film materials, or at least flexible materials.

While initial use of the patterning techniques of the present invention may be first applied in formation of thin film electronic circuitry, and while the invention will be described initially in reference thereto, the patterning techniques are applicable to a wide variety of materials that may be deposited by vapor deposition processes.

Such uses include artistic uses as well as functional puiposes. CCVD, as described in U.S. Patents 5,997,956 and 5,652,021, is useful for depositing certain metals in metallic (zero valence) form. This is especially true for noble metals, such as platinum and gold. The deposited layers may be very thin, e.g., on the nanometer or micron thickness levels, and therefore decorative layers of these precious materials may be deposited with minimal use of the noble metal.

In accordance with the invention, vapor-deposited thin films are deposited on a substrate that is masked, the films depositing only on the unmasked area. The mask may be a mechanical mask. A preferred mask is a printed chemical substance that may be applied in a pattern by a variety of printing techniques, such as silk screening and/or roller printing. The vapor deposited thin film is then applied, the thin film material either adhering or depositing only in the unmasked regions. The masking substance is then removed, leaving a patterned thin film. If no functional deposition occurs on the mask.

then it could remain.

Multiple layers of patterned thin film may be applied by this method, such multi- layer patterned laminates being applied, for example, to form thin film electronic circuitry including circuitry elements, including traces and inductors, and passive components, including resistors.

The masking procedure of the present invention involves substantially fewer processing steps than photolithographic processes. In forming printed circuitry, even if resolution is sacrificed to some extent, economic benefits may be realized. Furthermore, because of the flexibility in forming devices with multiple layers of patterned circuitry components, even if resolution is lost in some cases, an entire circuitry device may be even smaller than is practically achievable by such devices produced by photolithographic processes.

The masking material may be a simple chemical, such as a common, environmentally safe oil. The chemical for removing the masking chemical may be a common, environmentally friendly solvent, such as an alcohol, typically isopropyl alcohol, or an aqueous detergent solution.

Alternative masks include photoresist on the substrate prior to deposition, ink printing techniques, e.g., ink jet, and blank masking with selective area removal via photons or electrons, e.g., laser, x-ray, etc. ablation. Such techniques may produce very fine resolution.

A further advantage of the present technique is the accuracy and precision that are inherent to devices formed that do not require post processing (i.e. etching). As the material forming the device (for example, resistive material for resistors), is deposited, the desired characteristic (i.e. resistance) can be measured either continuously, or periodically. Once the desired value is achieved, the deposition is terminated. As no etching of the material is required, this value remains the same, resulting in a method to produce resistors or other components with improved accuracy and reduced manufacturing and processing costs, when compared to other methods of forming accurate components (such as laser trimming).

Preferred embodiments of the invention will now be described with reference to the accompanying drawings, wherein: Figures Ia - If illustrate the formation of a simple electronic circuitry comprising a plurality of resistive material pads interconnected by electronic circuitry. This resistor elements formed by this method may be used for selective area micro-heating; Figure la is a cross-sectional view of a metal foil layer to which a pattern of masking material has been applied to one surface.

Figure lb is a cross-sectional view of the structure of Figure 1 a on which resistive material has been deposited on the masked surface.

F1igure Ic is a cross-sectional view of the structure of Figure lb in which the masking material has been removed leaving patterned resistive material on the surface of the foil.

Figure 1 d is a cross-sectional view of the structure of Figure ic in which the patterned resistive material is embedded in a material that provides a support, e.g., a flexible support.

Figure le is a cross-sectional view of the structure of Figure id in which the foil layer has been patterned, e.g., by a standard photolithography technique.

Figure if is a plan view of the formed circuit in which discrete thin fihn resistors are connected to electronic circuitry traces.

Figures 2a - 2d illustrate the formation of a flexible printed circuit that has a plurality of thin film layers, including a plurality of patterned thin film layers produced in accordance with the present invention.

Figure 2a is a cross-sectional view of a support sheet, e.g., a flexible support sheet, on which has been applied a patterned layer of electrically conductive, e.g., copper, circuitry traces.

Figure 2b is a cross-sectiomi view of the structure of Figure 2a on which has been deposited a continuous layer of dielectric material.

Figure 2c is a cross-sectional view of the structure of Figure 2b on which a patterned layer of resistive material has been deposited.

Figure 2d is a cross-sectional view of the structure of Figure 2c on which a patterned layer of electrically conductive material has been deposited.

Figure 3A shows a cross-section of a two-layer laminate comprising an electrically conductive metal foil layer and a thin film layer of resistive material..

Figure 3B shows a cross-section of the two-layer laminate of Figure IA in which the layer of resistive material has been patterned to produce discrete patches of resistive material.

Figure 3C shows a cross-section of the patterned laminate of Figure lB adhered to a support sheet by an adhesive layer.

Figure 3D shows the laminate of Figure IC after the electrically conductive layer has been patterned.

Figure 4A shows a cross-section of a patterned laminate as shown in Figure lB adhered to a relatively thick embedding and supporting layer and to a removable backing or cover sheet.

Figure 4B shows the laminate of Figure 2A with the backing sheet removed.

Figure 4C shows the laminate of Figure 2B after the electrically conductive layer has been patterned.

Figure 5 is a plan view of electronic circuitry in which discrete patches of resistive material are connect-able to an outside power source through printed electronic circuitry, as per Figures ID or 2C.

Figure 6 is a cross-section of a laminate, similar to that of 1C, except that the resistive material layer has not been patterned.

Figure 7 is a cross-section of a laminate similar to Figure 2A except that the resistive material layer has not been patterned.

illustrated in Figure Ia is a cross-sectional view of a self-supporting, electrically conductive metal foil 10, such as copper foil, is coated with a patterned layer of masking material 11. Alternatively the substrate may be all or part insulating material (such as polyimide). The masking material of layer!! could be a mechanical mask, but is preferably an adhering substance, such as a printed layer of oil. To this structure, as shown in Figure lb is vapor-deposited, e.g., by CCVD, a layer 12 of electrically resistive material. An example of an electrically resistive material that can be deposited as a thin film by CCVD is a platinum/silica mixture, as described in U.S. Patent no. 6,212,078.

The resistive material 12 adheres to the foil 10 in the unmasked areas, but does not deposit on the masking material 11. Accordingly, the masking material 11 may be removed by an appropriate solvent, as seen in Figure Ic, leaving patches of resistive material 12.

During the deposition of resistive material 12, probes 16 can be used to measure the resistance of one or more of the resistive patches that are grown on portions of the substrate that are insulating (or an insulating film can be between the substrate and the resistive patches). This is done by passing an electrical current through the patch 12 via the probes 16. When the desired resistance is achieved, deposition of the resistive material 121s terminated, resulting in a highly accurate resistance value. Probes 16 may continuously monitor the resistance of the patch(es), or may move away from the deposition area, and periodically moved back into the monitoring position (as shown), to avoid interfering with the deposition process. For some materials, it may be desired to measure other characteristics (such as transparency), and to control the deposition based on these in-situ measurements.

After the masking material ills removed, a backing material 13 is applied to embed the resistive material patches 12 as seen in Figure Id. This backing material 13 may be a curable epoxy resin, such as that known as pre-preg or FR-4. The backing material 13 is applied to a sufficient thickness above the resistive material patches 13 such that when cured, the backing material 13 has sufficient structural integrity to support the structure as it is formed. Alternatively, the backing material 13 may be applied to a thickness just sufficient to cover the resistive material patches 12 and act as an adhesive to a supporting sheet of polymeric material (not shown). The backing material 13 (or backing material plus support sheet) may be flexible or inflexible, depending upon thickness. Some applications may call for a flexible circuit, others an inflexible circuit.

To complete the circuit, the metal foil layer 10 is now patterned into circuitry traces 10*, as seen in Figure le. A pair of such circuitry traces contacting spaced-apart locations on a resistive material patch 12 define an electrically resistive pathway.

A simple device 15 formed using the method of the present invention is illustrated in Figure if. In this device, a plurality of resistive material patches 12 on a support material 13 are each interconnected at circuitry traces 10* of opposite electrical polarity.

The electrical current through the electrically resistive patches 12 generate heat. Thus device 15 may be a selective, micro-area heating pad as seen in Fig. if. A large number of miniature selective area heating pads may be formed starting with a sheet of foil and subsequently divided into multiple discrete devices.

Formation of a complete electrical circuit by vapor deposition layers is illustrated with respect to Figures 2a - 2d.

In Figure 2*, on a sheet 20 of polymeric material, e.g., flexible polymeric material, electrically conductive circuitry traces 21 aze produced by masking a surface of the polymeric material and then vapordepositing an electrically conductive metal. The polymeric support sheet 20 for electrical applications may be polyimide or polyimide/amjde, these materials exhibiting heat stability, but other polymers may be used, depending on the amount of heat expected to be generated during the deposition and subsequently by the circuitry. Noble metals, such as platinum or gold may easily be deposited by a number of processes including CCVD. More oxidation-susceptible, electrically conductive metals, such as copper or nickel, may be deposited by CCVD under reduced atmosphere, as described, for example, in U.S. Patent No. 5,652,021 and European patent document EP-0976847.

Next, as seen in Figure 2b, a layer 22 of dielectric material is deposited so as to cover the circuitry traces 21. The dielectric material may be an inorganic material, such as an oxide, e.g., silica, as described in U.S. Patent no. 6,207,522, or a polymeric material, such as polyimide. Such polymer deposition is described in U.S. Patent No. 6,939, 576.

As seen in Figure 2c, by patterned deposition in accordance with the invention, patches 23 of resistive material are formed on the dielectric material, and then, as seen in Figure 2d, by patterned deposition in accordance with the invention is formed a further patterned layer of circuitry traces 24. In this circuitry embodiment, the dielectric layer 22 is continuous; however, in other circuitry designs, the dielectric material layer may be similarly patterned.

Resistive electrical pathways are formed between circuitry traces 24 connecting resistive patches 23 at spaced apart locations, and capacitive electrical pathways are formed between resistive patches 23 and circuitry traces 21 through dielectric layer 22.

The circuitry trace patterns of 21 and/or 24 may also be patterned to form inductive elenients.

The illustrated circuitry of Figure 2d is only one of any number of circuitry arrangements that may be simply fabricated using the method of the present invention.

Again, the method of depositing patterned thin film layers is not limited to electronic circuitry, but may be used to apply patterned thin film layers of any number of thin film materials.

Illustrated in Figure 3A is a cross-section of a two-layer laminate comprising an electrically conductive metal foil layer 30 and a thin film electrically resistive layer 32 formed thereon. The foil layer is selected so as to be electrically conducting and also to be etchable by standard techniques for printing electronic circuitry, copper foil or nickel foil being examples of such foils. The foil layer 30 for the fabrication method of Figures 3A-3D need only be sufficiently thick to be self-supporting and is preferably relatively thin so that a high resolution of printed circuitry can be patterned from this layer in a later step. Typically, the metal foil layer 30 is between about 10 and about 100 microns thick, generally between about 17.5 and about 70 microns thick.

Layer 32 is formed of a material that is electrically conductive, but has a relatively high electrical resistance relative to that of the foil layer 30. Preferably the layer 32 is formed of such material and such thickness as to have a resistivity of between about I and about 15 ohms per square. U.S. Patent No.6,210,592 describes formation of thin films of resistive material comprising a metal, e.g., platinum, and a dielectric, e.g., silica, and methods of fabricating discrete patches of resistive material from such resistive material layer patches. In the 6,210,592 patent, the resistive material layer is deposited by combustion chemical vapor deposition (CCVD), but the layer 32 of resistive material for use in the present invention may be fabricated by any of known thin film tecimiques, such as sputtering and evaporation. The resistive material is a thin film, typically ranging from about 0.1 to about 10 microns in thickness.

Shown in Figure 3B is the laminate of Figure 3A in which the resistive material layer 32 of Figure 3A has been patterned so as to form discrete resistor patches 32a.

Such patterning may be done by standard photolithographic techniques, such as are described in U.S. Patent No. 6,210,592.

To the laminate of Figure 38, and as illustrated in Figure 3C, is applied a thin layer 34 of adhesive, and to this is laminated a flexible support sheet 36, preferably a polymeric material, such as polyimide. The adhesive 34 in this embodiment need only be thick enough to cover the patches 32a of resistive material, but, the adhesive, naturally, fills the space between the patches 32a. One material that may be used for the adhesive 34 is a curable epoxy, such as is sold as "pre-preg" or "FR-4". The support sheet 36 is laminated to the structure using heat and pressure, the heating acting to cure the adhesive layer 34. Alternatively, for certain embodimente of the invention, as will be described hereinafter, both the adhesive for layer 34 and the support sheet 36 may be formed of thermoplastic material.

With the support sheet 36 laminated to the structure, the foil layer 30 is no longer needed for structural support, and the foil layer is patterned, e.g., by photolithographic techniques, to form circuitry traces 30a, a pair of circuitry traces 30a contacting spaced- apart locations on each resistive mater al patch so as to form a resistive pathway that produces tissue-perforating heat when cunent is supplied to the circuitry.

illustrated with respect to Figures 4A, 4B, and 4C is an alternative method of fabricating the heating device of the present invention. In this case, after the resistive material has been patterned into patches 32a, a relatively thick layer 38 of adhesive material is applied, followed by lamination to a backing sheet 40 of relatively low

S

adhesion to the adhesive layer 38 as seen in Figure 4A. In this case, the adhesive layer 38 is intended to provide the support to the structure, and the backing sheet 40 is intended to be removed subsequent to lamination and curing of the adhesive layer 38. The structure with the backing sheet 40 removed is shown in Figure 4B. Subsequently the foil layer 30 is patterned to form discrete resistive patches 30a as seen in Figure 4C. In this case, the adhesive layer 38 is applied to a thickness of between about 1 and about 500 microns above the resistive material patches 32b. The removable backing sheet 40 may be formed of wax paper, Tedlar (polyvinyifluoride), or the like, having low adhesion to the adhesive layer.

Either by the process of Figures 3A-3D or 3A, 4A-4C, a resistive device 42 is formed, as illustrated in FigureS. In this device 42 a plurality of resistive material patches 32a are each connected to a pair of circuitry traces 30a of opposite electrical polarity. These can be connected to an external power supply, such as a battery or transformer, so as to cause the resistive patches to produce the desired heat. For micro- perforating human skin, it is desired that the resistor patches produce localized temperatures of about 400 C to about 800 C for short periods of time. For other living membranes or for other purposes, lower temperatures may sufficc. For re-usable devices, such as for bubble jet inkers, the device 42, including the resistive patches 32a and circuitry traces 30a, should be robust. For some one-time use fluid delivery devices, such as may be used to apply a drug through a living membrane, it may be acceptable or even desirable that the circuitry burn out during the heating step. If the circuitry burns out, there is no possible second triggering of the device.

The device 42 is formed of thin flexible materials and, as such, conforms to the surface, e.g., skin, to which the device is applied. The degree of flexibility required of the device is dependent upon the surface contours to which the device or pad 42 is applied.

In the illustrated circuitry of device 42, all the patches 32a are connected in parallel so as to activate all of the resistive patches at once. However, the circuitry traces could be designed such that selective resistive patches could be activated individually.

Such could be useful for timed, sequential delivery of living tissue, e.g. , to deliver drug doses at predetermined periodic intervals. A large array of closely spaced resistive patches could be connected to a circuitry grid such that selected patches could be activated in a pattern. e.g., to burn a pattern in wood or another substrate, or to deliver ink in a pattern for tattooing purposes. Such an array could even be connected to a computer such that the device could be used for burning a computerized image into a surface or tattooing the skin with a computer generated device. The heating device could overlie a laminate of a support sheet and a surface-facing melt-able solid material. Upon selective activation of resistive patches, an image of melted material could be applied to a surface.

The area of the resistive material patches 32a may as small as 1 mm2 and as large as required for the application. For delivery of fluid through the human skin, the area of the patches 32* typically ranges from about I to about 1000 mm2.

In Figure 6 is illustrated a laminate comprising a metal foil 30, a thin film layer 32 of electrically resistive material, an adhesive layer 34 and a support sheet 36. In Figure 7 is illustrated a laminate comprising a metal foil 30, a thin film layer 32 of electrically conductive material. In this case, the laminate is formed without intermediate patterning of the thin film resistive material layer. In either of these cases, the foil layer 30 would be first circuitized, and then, in exposed areas, the resistive material layer 32 is patterned.

This order of patterning has the disadvantage in that the resistive material layer 32 below the traces 30* of conductive material cannot be patterned, thereby reducing the flexibility of circuitry design. On the other hand, laminates as are shown in Figure 6 and Figure 7 may be more easily mass produced and supplied to a customer who may then design an appropriate circuitry pattern according to need.

While currently envisioned circuitry formation is described above, one with ordinary skill in the art will recognize that a variety of lamination and ciruitization steps performed in a number of different orders may be utilized to form micro-heating devices in accordance with the invention.

While a very important utility for the present invention is for drug delivery, the invention could be applied to any heat sensitive utility in which localized controlled heating to high levels is required. The key is where the exact spatial distribution or very fine control of the amount of surface is at a specific temperature range is needed for optimal performance and control. Such other application include but not limited to thermal writing and marking, thermal ablation or transfer of films of material, fine thermally controlled reaction surfaces, displays, and thermal controkof mechanical, electrical or optical properties.

Example I

On a copper surface, a thin film of platinum was deposited by combustion chemical vapor deposition (CCVD) according to the method taught in U.S. Patent no. 5,652,021. Inadvertently, a finger print was left on the copper surface. The platinum deposited uniformly on the surface, except where the finger print was left; no platinwn deposited on the oily surfaces of the print.

Claims (14)

1. Claims 1. A method of depositing a patterned thin film layer on a surface

   of a substrate comprising: masking selected areas of substrate, and vapor depositing a thin film on said masked substrate, whereby said thin film deposits on said substrate only on unmasked areas of said substrate.
2. 2. The method of claim 1 wherein said masking material is a chemical that is printed on said substrate surface.
3. 3. The method of claim 1 or claim 2 wherein said thin film is formed wholly or partly of electrically conductive material.
4. 4. The method of any preceding claim wherein said thin film is formed wholly or partly of an electrically resistive material.
5. 5. The method of claim 4 wherein said thin film is a dielectric material.
6. 6. The method of any preceding claim wherein said masking material is an oil.
7. 7. The method of any preceding claim further comprising the step of measuring a characteristic of said thin film during said vapor deposition, and terminating said vapor deposition when a desired value of said characteristic is reached.
8. 8. The method of claim 7 wherein the characteristic monitored is electrical resistance.
9. 9. The method of claim 7 wherein the characteristic monitored is transparency.
10. 10. The method of any preceding claim wherein said masking material is removed after said thin film is deposited.
11. 11. The method of claim 10 wherein after removal of the masking material a backing material is applied to embed the deposited thin film.
12. 12. The method of claim 11 wherein the backing material is applied to a sufficient thickness to provide structural integrity.
13. 13. The method of claim 11 wherein the backing material is applied to a thickness sufficient to cover the deposited film, and acts as an adhesive for a supporting sheet applied thereto.
14. 14. The method of claim 11 or claim 12 wherein the substrate is a metal foil and wherein, after the application of the backing material, and the support sheet if used, the metal foil layer is patterned to form circuitry traces.