Thin film electrical component with polymer substrate

Abstract: A biocompatible thin film electrical component is configured for use in a human body or other ionic liquid environment. A polyimide substrate (11) is bonded to a glass carrier plate (10) sized for handling by automatic equipment and a multiple-layer metal conductor (18) is deposited on the substrate and patterned to define an electrical circuit or biosensor. The polyimide and the glass establish a bond therebetween that withstands handling yet is broken using biocompatible releasing agents and techniques. The polyimide substrate and glass carrier plate preferably have similar thermal expansion properties to reduce the likelihood of fracture and delamination problems during release of the substrate from the carrier plate. An insulation layer (28) covers the metal conductor and, in one embodiment, is made of a polyimide having a cure temperature lower than the temperature at which interdiffusion occurs in the metal layers in the conductor.

Description: Background and Summary of the Invention The present invention relates to thin film electrical components, and particularly, to a flexible, micro-miniature, waterproof, biocompatible, thin film circuit package that can be patterned to function as a connector, sensor, or other electrical component. More particularly, the present invention relates to improvements in handling and configuring thin film circuit packages during manufacture. A thin film is typically a film of 0.01 µm to 0.5 µm deposited on a glass, ceramic, or semiconductor substrate to form, for example, a capacitor, resistor, coil, or other circuits component. Thin film integrated circuits consist entirely of thin films deposited in a patterned relationship on a substrate. In Research Disclosure 283 November 1987 No. 28310 there is described a polyimide top coat applied to the circuit side of a polyimide film chip carrier to provide electrical insulation of the circuitry. One aim of the present invention is to develop an improved biocompatible thin film sensor or circuit suitable for use in the human body. It has been observed that satisfactory biocompatible thin film sensors or circuits cannot generally be produced by relying exclusively on the materials and techniques used to make conventional thin film structures. Testing and monitoring of many biological conditions in the human body or other ionic liquid environments would be enhanced by the development of a waterproof thin film biosensor of miniature size and flexible character. However, conventional thin film structures are not generally designed to be biocompatible with the human body and are made using processes which are not suitable for use with biomaterials. Also, conventional thin film structures are not flexible. A silicon wafer typically provides the foundation, platform, or carrier plate on which a conventional thin film structure is built. Automatic handling equipment is used to move the silicon wafer between manufacturing stations as a thin film structure is developed layer-by-layer on the silicon wafer. In this invention, the first thin film layer bonded to the silicon wafer carrier plate is used as a substrate of the thin film structure. Once the thin film structure is constructed, the silicon wafer carrier plate is released from attachment to the substrate of the thin film structure and discarded A thin film structure can be damaged during the release of the thin film structure from its silicon wafer carrier plate if the structure is made of any material which is incompatible with the acid etching solution or other release agent used to break the bonds coupling the substrate of the thin film structure to its silicon wafer carrier plate. It has been observed that many biomaterials of the type that could be used in the construction of a biologically compatible thin film structure are damaged or lose function upon exposure to conventional silicon wafer-releasing agents. The etching solutions normally used to dissolve a silicon wafer carrier plate are not compatible with biomaterials contained in a biocompatible thin film structure. It has also been observed that thin film metal conductors deposited on a polymer substrate coupled to a carrier plate tend to fracture and delaminate upon release of the polymer substrate from its carrier plate foundation. It is thought that any difference in the coefficients of thermal expansion of the polymer substrate and its carrier plate foundation leads to the development of internal stresses in the polymer substrate as it is heat-cured, because its coefficient of thermal expansion is different from the carrier plate. Thin film structure fracture and delamination problems can result unless careful attention is given to the thermal expansion coefficients of the carrier plate foundation and substrate materials used to construct the thin film structure. Conventional thin film structures are known to contain an adhesion metal film to enhance bonding of a noble metal film to its underlying polymer substrate. Such an adhesion metal film layer is a useful bonding tool because noble metals used to form electrical circuits in thin film structures do not adhere well directly to polymer substrates. For that reason, the adhesion metal layer is situated between the polymer substrate and the noble metal film layer. It has been observed that thin film structures suffer performance losses because of interdiffusion of abutting adjacent adhesion and noble metal film layers during any high-temperature processing of the thin film structure, for example, during heat-curing of a polymer precursor solution to provide a polymer insulation layer on the metal film layers. Performance losses include, for example, loss of flexibility of the thin film structure itself and development of metal film layer adhesion problems. In addition to these mechanical problems, metal interdiffusion can cause changes to occur in the electrical properties of the noble metal layer. The common solution to the metal interdiffusion problem in the semiconductor industry is the addition of a refractory metal layer (e.g., tungsten or tantalum) between the adhesion and noble metal film layers. This extra layer acts as an interdiffusion barrier. Processing of a thin film structure, however, is severely complicated by the deposition and patterning of such an additional refractory metal film layer. One object of the present invention is to provide a biologically compatible thin film electrical component suitable for use in the human body or any other ionic liquid environment. Another object of the present invention is to provide a thin film electrical component configured and patterned to function as a biosensor includable in a medical device implanted in a human body and a method of making this thin film electrical component to preserve its biosensor function capability. Still another object of the present invention is to provide a thin film structure which can be fabricated without the occurrence of stress damage at the release of the thin film structure substrate from its carrier plate foundation. Yet another object of the present invention is to provide a thin film structure that can be insulated without interdiffusion of metal film layers contained in the thin film structure. The above objects are solved by a thin film electrical component as set out in claim 1. Preferred embodiments of the component of the invention are set out in dependent claims 2-5. Also according to the present invention, a method is provided of making a thin film electrical component as set out in claim 6. Preferred embodiments of the method according to the invention are set out in dependent claims 7-10. Additional objects, features, and advantages of the invention will become apparent to those skilled in the art upon consideration of the following detailed description of a preferred embodiment exemplifying the best mode of carrying out the invention as presently perceived. Brief Description of the Drawings The detailed description particularly refers to the accompanying figures in which: Fig. 1 is a diagrammatic sectional view of a glass carrier plate during deposition of a polymer precursor solution onto a flat surface of the glass carrier plate; Fig. 2 is a diagrammatic elevation view of a polymer substrate on the glass carrier plate of Fig. 1; Fig. 3 is a diagrammatic view of a metal composite conductor consisting of three metal layers on the polymer substrate of Fig. 2; Fig. 4 is a top plan view of the component of Fig. 3 showing configuration of the multiple-layer metal conductor to provide a circuit element; Fig. 5 is a diagrammatic sectional view of the component of Fig. 4 showing an access opening formed in the top metal layer and an insulation coating covering portions of the metal conductor; Fig. 6 is a top plan view of the component of Fig. 5 showing a pair of uninsulated lead-attachment sites; Fig. 7 is a diagrammatic sectional view of the component of Figs. 5 and 6 immersed in a liquid bath to release the glass carrier plate from attachment to the polymer substrate; Fig. 8 is a diagrammatic sectional view of a component ; and Fig. 9 is a top plan view of the component of Fig. 8 showing a pair of uninsulated lead-attachment sites. Detailed Description of the Drawings A process for making a thin film electrical component is illustrated in Figs. 1-7. The finished component is illustrated in Fig. 7. As shown in Fig. 1, a clean glass plate 10 is used at the beginning of the manufacturing process to provide a rigid carrier plate or foundation for the thin film electrical component. A "photomask" blank is preferred because of its precise flatness and smoothness and thermal stability. In the preferred embodiment, the plate 10 is a low-expansion glass consisting essentially of 60% SiO₂, 18% RO, 15% Al₂O₃, 5% B₂O₃, 1% Na₂O, and 1% K₂O. It will be understood that RO stands for "unknown metal oxide impurities." Square or circular glass plates may be used to provide plate 10 with outside dimensions ranging from 3 inches (7.62 cm) to 6 inches (15.24 cm) and thicknesses of about of 0.030 inches (0.076 cm) to 0.090 inches (0.228 cm). A standard "silicon wafer or photomask" size can be selected for plate 10 to accommodate automatic handling and processing equipment. Glass plate 10 can be cleaned using a conventional cleaning solution prior to use and may be reused indefinitely. Referring to Figs. 1 and 2, a polymer substrate 11 is formed by spin-coating liquid polyamic acid 12 onto a top flat surface 14 of the rigid quartz-glass plate 10 as the plate 10 is spun about an axis 16 that is orthogonal to the flat surface 14. Illustratively, the glass carrier plate 10 is spun at a speed between 1,000 and 6,000 revolutions per minute. This spin-coating technique is desirable because it results in a smooth and uniform coating. The polyamic acid precursor solution 12 on glass plate 10 is imidized using a two-stage heat cure process to provide a polymer substrate 11 comprising a flat, uniform sheet of polyimide. Substrate 11 is made of a polymer that has desirable electrical properties, a surface that metal can be deposited onto, and desirable release properties. PMDA-ODA (pyro-mellitic dianhydride oxydianaline) type polyimide without adhesion-promotion additives is the preferred polymer. This material bonds very weakly to glass and such bonds may be broken with boiling water or warm saline exposure. However, the bond between the polyimide and the glass is sufficient to withstand thin film circuit fabrication processes. For example, the bonding of this type of polyimide to glass is sufficient to withstand typical photolithographic processes such as baking, acid etching, photoresist developing, and sputtering. Further, the coefficient of thermal expansion of the polyimide is about the same as the coefficient of thermal expansion of the preferred glass plate 10 so that fracture and delamination problems will not adversely affect the thin film electrical component upon release of the polyimide substrate 11 from the glass carrier plate 10. The thickness of polymer substrate 11 can be in the range of 10 to 100 µm. This is controlled by the viscosity of the polyamic acid precursor solution 12 and the spin speed of the glass carrier plate 10. Thick coatings (greater than approximately 25 µm) of polyimide are achieved by multiple coat-cure cycles. In another embodiment, a pre-made KAPTON® sheet is used to provide polymer substrate 11 instead of a spin-coated liquid polyamic acid 12 subjected to a two-stage heat cure process. The metal layers are deposited and patterned as shown in Figs. 3 and 4. A flexible conductor 18 is formed on the top surface 20 of polymer substrate 11 and includes a first adhesive metal layer 22 bonded to top surface 20, a noble metal layer 24 bonded to first adhesive metal layer 22, and a second adhesive metal layer 26 bonded to the noble metal layer 24 to trap the noble metal layer 24 between the first and second adhesive metal layers 22, 26. The three metal layers 22, 24, and 26 are deposited successively using standard thin film vacuum deposition methods. D.C. magnetron sputtering is the preferred method because of its relatively high deposition rate and ability to form highly adhesive films. Gold is the preferred noble metal because of its high ductility, although platinum and palladium are suitable alternatives. Chromium is the preferred adhesive metal, although titanium, tungsten, and tantalum are suitable alternatives. The adhesive metal layers are about 0.01 µm to 0.02 µm thick and the noble metal layer is about 0.1 µm to 0.2 µm thick. It will be appreciated that it is not necessary to include a refractory metal layer at the interface between each adhesive and noble metal layer because of the selection of a polymer insulator coating having a low heat-cure temperature as described below. To provide an electrical component on the substrate 11, the metal layers 22, 24, and 26 can be patterned using a variety of standard photolithography processes (not shown). A representative circuit pattern 27 (e.g., a resistor) is shown in Fig. 4. It will be appreciated that circuit pattern 27 could be any electrical element, integrated circuit, sensor, or other component depending upon the intended function of the thin film structure. A suitable photolithography process generally involves the application of a photoresist or masking material by spin coating. This material is baked until dry. The material is then exposed to ultraviolet light through a mask which contains the circuitry patterns. The photoresist is developed, leaving only selected areas masked. Exposed metal may be removed by wet chemical or dry plasma etch. The photoresist can then be stripped by solvent dip or plasma etching. In a preferred embodiment, positive photoresist, wet etching, and solvent stripping is used. Line widths and spaces as small as 5 µm have been made and dimensions as small as 1 µm are possible. An insulator cover 28 is needed to insulate the conductor 20 electrically and prevent exposure of the potentially corrosive adhesive metal to liquids. The same patterning process that was used to define the electrical circuit 27 in conductor 20 is now usable to remove the top adhesive metal layer 26 in areas of the conductor 20 that will be uninsulated. As shown in Fig. 5, the second adhesive metal layer 26 is formed to include a pair of access openings 30 therein to expose a portion of the underlying noble metal layer 24 to provide a lead-attachment site 32 on the noble metal layer 24. The metal in layer 26 is removed about 5-10 µm beyond the intended location of the insulation opening edge 34. This distance must be sufficient to ensure coverage of adhesive metal edge 30 but minimal so that most of the insulation layer 28 is in contact with the adhesive metal. Polyimide substrate and insulation layers 11, 28 do not adhere well to the noble metal 24 directly. It is this precise alignment step that makes the use of spun-on substrates preferable in the present invention. It is difficult to hold free-standing films sufficiently flat to ensure alignment accuracies of 5 to 10 microns across a 3 inch (7.62 cm) to 6 inch (15.24 cm) sheet. As shown in Fig. 5, the polyimide coating defining insulator cover 28 includes a wall 31 that extends into the access opening 30 formed in the second adhesive metal layer 26 to contact the noble metal layer 22. The insulation layer 28 is deposited and patterned. In one preferred embodiment, the insulation layer 28 is comprised of a BTDA-ODA type polyimide which can be cured at a temperature as low as 250°C to minimize the opportunity for metal interdiffusion problems in conductor 20 to develop. Coating thicknesses of 1-25 µm can be used to provide insulation layer 28. Alternatively, a PMDA-ODA type polyimide can be used to provide insulation layer 28. Photoimageable polyimides are preferred because non-photoimageable polyimides require more processing steps. The insulation layer 28 is fabricated by spin-coating a polyamic acid precursor solution on conductor 20 as mounted on polymer substrate 11, curing the precursor solution to form the chosen polyimide at a temperature that is less than the minimum temperature at which interdiffusion of the noble metal layer 24 and the adhesive metal layers 22, 26 occurs, applying photoresist, etching, and stripping. Advantageously, substantial interdiffusion of the noble metal layer 24 and adhesive metal layers is avoided by curing the polyamic acid precursor solution-forming insulator cover 28 at a low temperature. It will be appreciated that the cure temperature of the polymer precursor used to form the polymer substrate 11 is not limited to this low temperature because it is heat-cured before any of the metal layers 22, 24, and 26 are deposited thereon. It has been discovered during work on the present invention that if gold and chromium are employed as the noble and adhesion metals, respectively, interdiffusion (which leads to sensor cracking) between the noble and adhesion metal layers does not occur even when a polyimide having a high cure temperature is used to provide insulation layer 28. If a gold/chromium metal combination is used, high and low temperature polyimides can be used to provide insulation layer 28. It has been observed that a metal layer interdiffusion problem exists if any of the other noble or adhesion metals listed above are substituted for gold or chromium, e.g., platinum for gold or titanium for chromium. Accordingly, the gold/chromium combination is the best mode at present. A component not forming part of the present invention illustrated in Figs. 8 and 9, wherein those elements referenced by numbers identical to those in Figs. 1-7 perform the same or similar function. The primary features distinguishing this component from the last embodiment is the use of a room temperature vulcanizing polymer to form the insulation layer and the absence of an adhesive metal layer between the noble metal layer and the insulation layer. A conformal coating is used to form insulation layer 128 as shown in Figs. 8 and 9. Preferrably , this coating is a synthetic polymer elastomer consisting essentially of a silicone resin such as dimethyl siloxane and an adhesion promoter such as methoxy silane. A suitable coating material is MS-460™ resin available from Miller-Stephenson Chemical Company, Inc. of Danbury, Connecticut. This resin is available in solution for spin-on application or aerosol for spray-on application. Use of the spray-on technique is preferred because it is more amenable to mass production. The silicone resin used to provide insulation layer 128 is a one-part system that cures in place at room temperature. It is an alternative to heat-cure polymers of the type curable only at the high temperatures that can cause metal interdiffusion problems. As noted above, there are some thin film applications where the metal conductors cannot be exposed to the high temperatures required to cure the polymer insulation layer film. Another advantage of conformal coating materials, particularly silicone resins, is that they are more hydrophobic than other materials such as polyimides. The hydrophobic nature of conformal coatings, particularly the silicones, results in a lower amount of moisture pickup. Also, the preferred conformal coating adheres well to gold, which eliminates the need for a layer of adhesive metal between the noble metal layer 24 and the insulation layer 128. This decreases the number of processing steps required for fabrication of the thin film electrical component and, in turn, decreases the cost of producing the component. Access openings 134 are easily formed in the insulation layer 128 after it cures. These openings can be created, for example, using either a dry plasma etch or by masking these areas prior to application of the conformal coating to the noble metal layer 24. The conformal coating 128 is a flexible material well-suited for use in a thin film electrical component that must be able to be flexed in use. The final step is to release the polymer substrate 11 from the rigid glass carrier plate 10 as shown in Fig. 7. Two methods are suitable: 3 hour immersion in a 100°C water bath or 24-hour immersion in a 37°C physiologic saline bath. These times and temperatures may be varied slightly while still achieving the same result. The boiling water method is the most expeditious process for production. The body temperature saline process is useful where temperature sensitive materials such as enzymes, antibodies, or other biomaterials have been attached to the circuitry. An additional step can be added before the substrate 11 is released from glass carrier plate 10. The outline of the thin film electrical component may be cut out with high accuracy using an excimer laser. The ultraviolet wavelength of this laser cuts polyimide rapidly but does not damage glass used to provide the carrier plate 10. In addition, this cutting mechanism generates no mechanical or thermal disruption which could damage biomaterials. Very thin, flexible substrates are required for biosensors. A biosensor must be small enough to fit into a hypodermic needle or a venous or arterial catheter and withstand some flexing. This type of substrate is not easily handled or held flat, two necessities for cost-effective photolithographic production. Biosensor substrates must also be free of contaminants and be biocompatible. All of these requirements are met by a thin film electrical component in accordance with the present invention. High purity liquid polymer precursors are available and relatively inert biologically when cured. Glass plates are available with very good flatness and can be loaded into various cassette mechanisms for automatic handling and transfers. Also, a means is provided for releasing the thin film structure without damage to biomaterials. Flexibility is a main advantage of the circuitry used to construct the thin film electrical component of the present invention. Curved and three-dimensional thin film structures can be built as a result of the flexible character of the conductor 20. The flexibility of the thin film electrical component makes it easily adaptable to provide a sensor compliable with a surface to be measured or a connector. By contrast, typical thin film or semiconductor substrates such as silicon, alumina, and silicon dioxide are very brittle in such thin sections. The small conductor and insulator feature size results in very small overall device size. This is particularly advantageous for portable or implanted devices. It is also useful for high-density connectors such as those that interface directly with integrated circuitry. There is at least a ten-fold reduction in size compared to standard flex circuitry. The waterproof nature of this circuit gives it the advantage of being able to operate in many adverse environments not suitable for traditional circuitry. These include: fresh water, salt water, physiologic saline (body fluid), boiling water, and steam. This opens many medical, industrial, and marine applications. Polyimide is relative inexpensive compared to silicon and other typical microcircuit foundation materials. This means that one can afford to make the connecting leads on the same substrate as the active circuitry, thereby eliminating wire bonds and connectors. Polyimide has been shown to be biologically inert. Liquid polyamic acid is available in higher purity levels than sheet polyimide. Many electrical components can be made simply by patterning the metal and insulation layers 22, 24, 26, and 28. These include resistors, capacitors, connectors, antennae, contact pads, electrodes, heaters, inductors, fuses, and thermistors. In addition, piezoelectric, photoresistive, or magnetic films may be applied yielding sensing elements for force, light, and magnetic fields. The noble metal surface 24 can be electroplated to form electrochemically useful detection electrodes (i.e., platinum, silver/silver-chloride). Active devices (optoelectronic components, transistors, integrated circuits) may be attached to the circuit to achieve amplification, regulation, or other functions. An area of wide application is implanted medical devices. These could be electrochemical sensors, electrodes, or other physical sensors for strain, temperature, or fluid conductivity. Stimulating electrodes for the cochlea, heart, brain, or spinal column could also be made. The geometries allow such devices to be built to fit inside small size catheters and hypodermic needles. Although the invention has been described in detail with reference to certain preferred embodiments, variations, and modifications exist within the scope of the invention as described and defined in the following claims.

Claims: A thin film electrical component comprising: a polymer substrate (11), a first adhesive metal layer (22) bonded to said polymer substrate (11), a noble metal layer (24) bonded to said first adhesive metal layer (22), and a second adhesive metal layer (26) bonded to said noble metal layer (24), said noble metal layer (24) being sandwiched between the first (22) and second (26) adhesive metal layers, and a polymer insulator (28) bonded at least to the surfaces of said first (22) and second (26) adhesive metal layers to cover all exposed surfaces thereof unbonded to one of the polymer substrate (11) and the noble metal layer (24), characterised in that said component further comprises a rigid glass carrier plate (10), said polymer substrate being bonded to said rigid glass carrier plate (10), said substrate (11) having a coefficient of thermal expansion equivalent to that of said rigid glass carrier plate (10), said bond being breakable upon immersion of said substrate (11) and rigid glass carrier plate (10) in one of a hot water bath and a warm temperature physiologic saline bath. The component of claim 1 wherein said adhesive metal layers (22) and (26) are a material selected from the group consisting of titanium and chromium, and said noble metal layer (24) is a material selected from the group consisting of gold, platinum and palladium. The component of either claim 1 or 2, wherein said polymer substrate (11) is a PMDA-ODA type polyimide or a BPDA-PPD type polyimide. The component of any of claims 1 to 3, wherein said polymer insulator (28) is a BTDA-ODA type polyimide. The component of any of claims 1 to 3, wherein said polymer insulator (28) is a synthetic polymer insulator. A method of making a thin film electrical component, the method comprising the steps of: (a) providing a rigid glass carrier plate (10) having a flat surface (14); (b) coating the flat surface (14) with a first polyamic acid precursor solution (12); (c) curing the first polyamic precursor solution (12) using heat to provide a first layer of polyimide film (11) bonded to the flat surface (14), the first layer of polyimide film (11) having a coefficient of thermal expansion equivalent to the coefficient of thermal expansion of the rigid glass carrier plate (10) so that creation of internal stresses in the first layer of polyimide film (11) during curing is avoided; (d) providing a first adhesive metal layer (22) bonded to the first layer of polyimide film; (e) providing a noble metal layer (24) bonded to the first adhesive metal layer (22); (f) providing a second adhesive metal layer (26) bonded to the noble metal layer (24); (g) forming means for providing an electrical circuit (27) by selectively removing certain areas of the first (22) and second (26) adhesive metal layers and the noble metal layer (24); (h) coating the exposed surfaces of the means for providing an electrical circuit (27) and the first layer of polyimide film (11) with a second polyamic precursor solution; (i) curing the second polyamic precursor solution using heat to provide a second layer of polyimide film (28) bonded to all exposed surfaces of the first (22) and second (26) adhesive metal layers and the first polyimide layer (11), the second polyamic acid precursor solution being cured at a heat-cure temperature that is less than a characteristic minimum temperature at which interdiffusion of the noble metal layer (24) and the first (22) and second (25) adhesive metal layers occurs; and (j) exposing the rigid glass carrier plate (10) and the thin film electrical component comprising the first (11) and second (28) layers of polyimide film, the first (22) and second (26) adhesive metal layers and the noble metal layer (24,) to one of a hot water bath and a room temperature physiologic saline bath for a period of time sufficient to break the bond connecting the first layer of polyimide film (11) to the rigid glass carrier plate (10 so that the first layer of polyimide film (11) is released from the rigid glass carrier plate (10) without separating the first adhesive metal layer (22) from the first layer of polyimide film (11). The method of claim 6 wherein the adhesive metal layers (22) and (26) are a material selected from the group consisting of titanium and chromium, and the noble metal layer (24) is a material selected from the group consisting of gold, platinum and palladium. The method of either of claims 6 or 7, wherein the first layer of polyimide film (11) is PMDA-ODA type polyimide or a BPDA-PPD type polyimide. The method of any of claims 6 to 8, wherein the second layer of polyimide film (28) is BTDA-ODA type polyimide or a PMDA ODA type polyimide. The method of any of claims 6 to 8, wherein the second layer of polyimide film (28) is a synthetic polymer elastomer.

Inventors: HELLER JAMES WALTER, JOHNSON KIRK WILLIS, LIPSON DAVID, HELLER, JAMES WALTER, JOHNSON, KIRK WILLIS, LIPSON, DAVID
Applicants: LILLY CO ELI, ELI LILLY AND COMPANY
Application year: 1991
Main classification: H01L23
IPC codes: H01L21, H05K3, A61B18, H05K1, G01N27, H01L23, H01L49, A61B5, H01B5, H01L27