Abstract:
Polymer materials make useful materials as electrode array bodies for neural stimulation. They are particularly useful for retinal stimulation to create artificial vision. Regardless of which polymer is used, the basic construction method is the same. A layer of polymer is laid down. A layer of metal is applied to the polymer and patterned by wet etch to create electrodes and leads for those electrodes. The base polymer layer is activated. A second layer of polymer is applied over the metal layer and patterned to leave openings for the electrodes, or openings are created later by means such as laser ablation. Hence the array and its supply cable are formed of a single body.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a divisional application of U.S. patent application Ser. No. 13/962,752, filed Aug. 8, 2013, for Method of Manufacturing and Flexible Circuit Electrode Array, which is a divisional application of U.S. patent application Ser. No. 11/926,459, filed Oct. 29, 2007, for Method of Manufacturing and Flexible Circuit Electrode Array, which is a divisional application of U.S. patent application Ser. No. 11/702,735, for Method of Manufacturing a Flexible Circuit Electrode Array, filed Feb. 6, 2007, which claims the benefit of provisional Application No. 60/772,099, for Flexible Circuit Electrode Array and Method of Manufacturing the Same, filed Feb. 10, 2006, the disclosures of which are incorporated herein by reference. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    This invention was made with government support under grant No. R24EY12893-01, which has been awarded by the National Institutes of Health. The government has certain rights in the invention. 
     
    
     BACKGROUND OF THE INVENTION 
       [0003]    1. Field of the Invention 
         [0004]    The present invention is generally directed to a flexible circuit electrode array especially for biomedical implants, especially implantable medical devices, such as retinal prosthesis and a method of manufacturing the flexible circuit electrode array. 
         [0005]    2. Description of the Related Art 
         [0006]    In U.S. Pat. No. 3,699,970 “Striate Cortex Stimulator” to Giles Skey Brindley et al. an implantable device is disclosed comprising a plurality of electrodes for stimulating the striate cortex. 
         [0007]    In U.S. Pat. No. 4,487,652 “Slope Etch of Polyimide” to Carl W. Amgren a semiconductor having an insulating layer overlying a metal layer is disclosed, wherein the insulator comprises an upper oxide layer, an intermediate polyimide layer, and a lower oxide layer in contact with the metal layer, a method for etching a via from an upper surface of the polyimide layer to the metal layer comprising the steps of applying photoresist; etching an opening from an upper surface of the photoresist layer to the upper oxide layer at a location for forming the via so that an upper surface of the upper oxide layer is exposed at the via location; heating the photoresist to cause a more gradual slope of the photoresist layer from the upper surface of the upper oxide layer at the via location to the upper surface of the photoresist layer; applying reactive ion etchant with a predetermined selectivity between photoresist and oxide to transfer the slope of the photoresist layer to the upper oxide layer at a predetermined ratio; and applying a reactive ion etchant with a predetermined selectivity between oxide and polyimide to transfer the slope of the upper oxide layer to the polyimide layer at a predetermined ratio, whereby the lower oxide layer is simultaneously etched to expose the metal layer at the via location. 
         [0008]    In U.S. Pat. No. 4,573,481 “Implantable Electrode Array” to Leo A. Bullara an electrode assembly for surgical implantation on a nerve of the peripheral nerve system is disclosed. 
         [0009]    In U.S. Pat. No. 4,628,933 “Method and Apparatus for Visual Prosthesis” to Robin P. Michelson a visual prosthesis for implantation in the eye in the optical pathway thereof is disclosed. 
         [0010]    In U.S. Pat. No. 4,837,049 “Method of Making an Electrode Array” to Charles L. Byers et al. a very small electrode array which penetrates nerves for sensing electrical activity therein or to provide electrical stimulation is disclosed. 
         [0011]    In U.S. Pat. No. 4,996,629 “Circuit Board with Self-Supporting Connection Between Sides” to Robert A. Christiansen et al. a copper supporting sheet is disclosed having vias for connecting semiconductor chips to surface mount components. A laminate of polyimide has vias corresponding to the supporting layer vias with copper covering those vias. 
         [0012]    In U.S. Pat. No. 5,108,819 “Thin Film Electrical Component” to James W. Heller a thin film electrical component is disclosed comprising a rigid glass carrier plate, a substrate bonded to the rigid glass carrier plate, the substrate comprising a polyimide establishing a bond with the rigid glass carrier plate that is broken upon immersion of the substrate and the rigid glass carrier plate in one of a hot water bath and a warm temperature physiologic saline bath to release the polymer from attachment to the rigid glass carrier plate, and means for providing an electrical circuit, the providing means being bonded to the substrate and undisrupted during release of the substrate from attachment to the rigid glass carrier plate. 
         [0013]    In U.S. Pat. No. 5,109,844 “Retinal Microstimulation” to Eugene de Juan Jr. et al. a method for stimulating a retinal ganglion cell in a retina without penetrating the retinal basement membrane at the surface of the retina is disclosed. 
         [0014]    In U.S. Pat. No. 5,178,957 “Noble Metal-Polymer Composites and Flexible Thin-Film Conductors Prepared Therefrom” to Vasant V. Kolpe a composite article is disclosed comprising a polymeric support selected from the group consisting of a polyimide, polyethylene terephthalate, and polyester-ether block copolymer having a noble metal deposited directly onto at least one surface, wherein the deposited metal exhibits a peel force of at least about 0.05 kg per millimeter width after 24 hour boiling saline treatment. 
         [0015]    In U.S. Pat. No. 5,215,088 “Three-Dimensional Electrode Device” to Richard A. Norman et al. a three-dimensional electrode device for placing electrodes in close proximity to cell lying at least about 1000 microns below a tissue surface is disclosed. 
         [0016]    In U.S. Pat. No. 5,935,155 “Visual Prosthesis and Method of Using Same” to Mark S. Humayun et al. a visual prosthesis is disclosed comprising a camera for receiving a visual image and generating a visual signal output, retinal tissue stimulation circuitry adapted to be operatively attached to the user&#39;s retina, and wireless communication circuitry for transmitting the visual signal output to the retinal tissue stimulation circuitry within the eye. 
         [0017]    In U.S. Pat. No. 6,071,819 “Flexible Skin Incorporating MEMS Technology” to Yu-Chong Tai a method of manufacturing a flexible microelectronic device is disclosed comprising first etching a lower side of a wafer using a first caustic agent; depositing a first layer of aluminum on an upper side of the wafer; patterning the first layer of aluminum; depositing a first layer of polyimide on the upper side of the wafer, covering the first layer of aluminum; depositing a second layer of aluminum on the upper side of the wafer, covering the first layer of polyimide; depositing a second layer of polyimide on the upper side of the wafer, covering the second layer of aluminum; depositing a third layer of aluminum on the lower side of the wafer; patterning the third layer of aluminum; second etching the lower side of the wafer using the third layer of aluminum as a mask and the first layer of aluminum as an etch stop and using a less caustic agent than the first caustic agent, such that the wafer is divided into islands with gaps surrounding each island; and depositing a third layer of polyimide on the lower side of the wafer, such that the gaps are at least partially filled. 
         [0018]    In U.S. Pat. No. 6,324,429 “Chronically Implantable Retinal Prosthesis” to Doug Shire et al. an apparatus is disclosed which is in contact with the inner surface of the retina and electrically stimulates at least a portion of the surface of the retina. 
         [0019]    In U.S. Pat. No. 6,374,143 “Modiolar Hugging Electrode Array” to Peter G. Berrang et al. a cochlear electrode array for stimulating auditory processes is disclosed. 
         [0020]    In U.S. Pat. No. 6,847,847 “Retina Implant Assembly and Methods for Manufacturing the Same” to Wilfried Nisch et al. a retina implant is disclosed comprising a chip in subretinal contact with the retina and a receiver coil for inductively coupling there into electromagnetic energy. 
         [0021]    In US Patent Application No. 20010037061 A1, “Microcontact structure for neuroprostheses for implantation on nerve tissue and method therefore” to Rolf Eckmiller et al. a four layer microcontact structure is disclosed in which the active connection between the microcontact structure and the nerve tissue is brought about by electrical stimulation. The layer adjacent to the nerve tissue to be stimulated is composed of the polymer polyimide and contains penetrating electrodes made of platinum which forms the adjoining layer. There follows a further layer of the polyimide and a layer of the polymer polyurethane. Polyurethane has the property of thermal expansion relative to polyimide. 
         [0022]    In US Patent Application No. 2003/0158588 A1 “Minimal Invasive Retinal Prosthesis” to John F. Rizzo et al. a retinal prosthesis is disclosed comprising an RF coil attached to the outside of and moving with an eye to receive power from an external power source; electronic circuitry attached to and moving with the eye and electrically connected to the RF coil; a light sensitive array electrically connected to the electronic circuitry and located within the eye for receiving incident light and for generating an electrical signal in response to the incident light; and a stimulating array abutting a retina of the eye and electrically connected to the electronic circuitry to stimulate retinal tissue in response to the electrical signal from the light sensitive array. A supporting silicone substrate has a polyimide layer spun onto its surface and cured. The copper or chrome/gold conducting layer is then added and patterned using wet chemical etching or a photoresist lift-off process. Next, a second polyimide layer is spun on, and the regions where circuit components are to be added are exposed by selective dry etching or laser ablation of the upper polyimide layer in the desired areas. Finally, the completed components are removed from their supporting substrate. 
         [0023]    Polyimide, also known as PI, has been mass-produced since 1955. It is used in bearing materials, thrust washers, and semiconductor wafer clamps. It has high impact and dielectric strength, high heat resistance and a low coefficient of thermal expansion and excellent mechanical, thermal, and electrical properties. Polyimide is typically applied in liquid form, and then thermally cured into a film or layer with the desired properties. The film can be patterned using photographic or other processes. Microelectronic applications include stress buffer, passivation layer, chip bonding, and interlayer dielectric. 
         [0024]    Eugene de Juan Jr. et al. at Duke University Eye Center inserted retinal tacks into retinas in an effort to reattach retinas that had detached from the underlying choroid, which is the source of blood supply for the outer retina and thus the photoreceptors. See for example E. de Juan Jr., et al., “Retinal tacks”, Am J Ophthalmol. 1985 Mar. 15; 99 (3):272-4. 
         [0025]    Hansjoerg Beutel et al. at the Fraunhofer Institute for Biomedical Engineering IBMT demonstrated the bonding of a gold ball by force, temperature, and ultrasound onto an aluminum metal layer. See for example Hansjoerg Beutel, Thomas Stieglitz, Joerg-Uwe Meyer: “Versatile Microflex-Based Interconnection Technique,” Proc. SPIE Conf. on Smart Electronics and MEMS, San Diego, Calif., March 1998, vol. 3328, pp 174-182. A robust bond can be achieved in this way. However, encapsulation proves difficult to effectively implement with this method. Gold, while biocompatible, is not completely stable under the conditions present in an implant device since it dissolves by electromigration when implanted in living tissue and subject to an electric current. See for example Marcel Pourbaix: “Atlas of Electrochemical Equilibria in Aqueous Solutions”, National Association of Corrosion Engineers, Houston, 1974, pp 399-405. 
         [0026]    A system for retinal stimulation comprising a polyimide-based electrodes being coated with platinum black are described by Andreas Schneider and Thomas Stieglitz. See for example Andreas Schneider, Thomas Stieglitz: “Implantable Flexible Electrodes for Functional Electrical Stimulation”, Medical Device Technology, 2004. 
         [0027]    A process for activating a base polyimide layer prior to applying a top polyimide layer is described by Balasubrahmanyan Ganesh, who suggests to clean, roughen, and oxygenate the base polyimide by using reactive ion etching (RIE) in oxygen plasma for 10 s at 50 W, and 800 mTorr pressure in an Oxford Plasmalab-80 Plus system. The top polyimide layer is then spun-on immediately after the plasma after the plasma roughening and the wafer is set aside for 45 min. See for example Balasubrahmanyan Ganesh: “A Polyimide Ribbon Cable for Neural Recording and Stimulation Systems”, a Thesis for the Degree of Master of Science, Department of Materials Science and Engineering, The University of Utah, March 1998. 
         [0028]    A process for activating a base polyimide layer prior to applying a top polyimide layer is described by Nancy Stoffel, Crystal Zhang and Edward J. Kramer. Stoffel et al. suggest using wet chemical treatments to hydrolyze the films according to a method reported by K. W. Lee et al. (1990). Stoffel et al. found solutions of both 1 M KOH and 1M tetramethyl ammonium hydroxide to be effective for polyimide films. Solutions of 0.2 M HCl and acetic acid were used and found to be effective for converting polyamate salt into a polyamic acid. See for example Nancy Stoffel, Crystal Zhang and Edward J. Kramer: “Adhesion of Polyimide Laminates”, Application of Fracture Mechanics in Electronic Packaging and Materials, ASME, EEP-Vol. 11/MD-Vol. 64, pp. 79-84, 1995. 
       BRIEF SUMMARY OF THE INVENTION 
       [0029]    The flexible circuit electrode array is electrically coupled by a flexible circuit cable, which pierces the sclera and is electrically coupled to an electronics package. The flexible circuit electrode array and the flexible circuit cable are formed of a single body. 
         [0030]    The pressure applied against the retina, or other neural tissue, by an electrode array is critical. Low pressure causes increased electrical resistance between the array and retina, along with electric field dispersion. High pressure may block blood flow within the retina causing a condition similar to glaucoma. Common flexible circuit fabrication techniques such as photolithography generally require that a flexible circuit electrode array be made flat. Since the retina is spherical, a flat array will necessarily apply more pressure near its edges, than at its center. The edges of a flexible circuit polymer array may be quite sharp and cut the delicate retinal tissue. Most polymers can be curved when heated in a mold. By applying the right amount of heat to a completed array, a curve can be induced matching the curve of the retina. With a thermoplastic polymer such as liquid crystal polymer, it may be further advantageous to repeatedly heat the flexible circuit in multiple molds, each with a decreasing radius. 
         [0031]    The present invention provides a method for manufacturing a flexible circuit electrode array, comprising: 
         [0000]    a) depositing a metal trace layer on an insulator polymer base layer;
 
b) applying a layer of photoresist on the metal trace layer and patterning the metal trace layer and forming metal traces on the insulator polymer base layer; and
 
c) activating the insulator polymer base layer and depositing a top insulator polymer layer and forming one single insulating polymer layer with the base insulator polymer layer; wherein the insulator polymer layers were treated at a temperature from 80-150° C. and then at a temperature from 250-350° C.
 
         [0032]    The present invention provides a flexible circuit electrode array with excellent adhesion and insulating properties of a polymer insulator reached by a new technique of activation of a base polymer layer prior to applying a top polymer layer wherein both polymer layers result in one polymer layer. The adhesion and insulating properties are further improved by applying a top metal layer on the electrode layer as an adhesion aid to the polymer. 
         [0033]    The method of the present invention solves the long term problem of week adhesion between the insulator polymer base layer and the top insulator polymer layer. The method of the present invention provides an excellent adhesion between the polymer base layer and the polymer top layer and an excellent insulation of the trace metal. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0034]      FIG. 1  depicts a perspective view of the implanted portion of the preferred retinal prosthesis including a twist in the array to reduce the width of a scleratomy and a sleeve to promote sealing of the scleratomy. 
           [0035]      FIG. 2  depicts a perspective view of the implanted portion of the retinal prosthesis showing the fan tail in more detail. 
           [0036]      FIGS. 3   a - 3   e  depicts a perspective view of molds for forming the flexible circuit array in a curve. 
           [0037]      FIG. 4  depicts an alternative view of the invention with ribs to help maintain curvature and prevent retinal damage. 
           [0038]      FIG. 5  depicts a top view of a body comprising a flexible circuit electrode array, a flexible circuit cable and a bond pad before it is folded and attached to the implanted portion. 
           [0039]      FIG. 6  depicts a top view of a body comprising a flexible circuit electrode array, a flexible circuit cable and a bond pad after it is folded. 
           [0040]      FIG. 7  depicts a top view of a body comprising a flexible circuit electrode array, a flexible circuit cable and a bond pad after it is folded with a protective skirt. 
           [0041]      FIG. 8  depicts a cross-sectional view of a flexible circuit array with a protective skirt bonded to the back side of the flexible circuit array. 
           [0042]      FIG. 9  depicts a cross-sectional view of a flexible circuit array with a protective skirt bonded to the front side of the flexible circuit array. 
           [0043]      FIG. 10  depicts a cross-sectional view of a flexible circuit array with a protective skirt bonded to the back side of the flexible circuit array and molded around the edges of the flexible circuit array. 
           [0044]      FIG. 11  depicts a cross-sectional view of a flexible circuit array with a protective skirt bonded to the back side of the flexible circuit array and molded around the edges of the flexible circuit array and flush with the front side of the array. 
           [0045]      FIG. 12  depicts a top view on a supporting glass plate substrate which is marked with a batch and plate identification code. 
           [0046]      FIG. 13  depicts a cross-sectional view of a layer of polyimide which is applied onto the front side of the glass plate and cured. 
           [0047]      FIG. 14  depicts a cross-sectional view of layer deposition of titanium/platinum/titanium thin film stack from which conductor traces are patterned. 
           [0048]      FIG. 15  depicts a cross-sectional view of photoresist layer deposition on the titanium/platinum/titanium thin film stack after the photoresist layer is exposed to irradiation through a mask and the radiated areas of the photoresist layer are removed during photolithographic processing to selectively mask titanium/platinum/titanium thin film stack in areas where conductor traces are to remain. 
           [0049]      FIG. 16  depicts a cross-sectional view of the layer structure after the removal of the top layer from titanium/platinum/titanium trace metal stack in areas exposed during photolithography. 
           [0050]      FIG. 17  depicts a cross-sectional view of the layer structure after the removal of the middle layer from titanium/platinum/titanium trace metal stack in areas exposed during photolithography. 
           [0051]      FIG. 18  depicts a cross-sectional view of the layer structure after the removal of residual photoresist from trace metal surface. 
           [0052]      FIG. 19  depicts a cross-sectional view of the layer structure after the removal of bottom layer from titanium/platinum/titanium trace metal stack in areas exposed during photolithography and preceding etches. 
           [0053]      FIG. 20  depicts a cross-sectional view of the layer structure after activating and roughening and partial removal of base polyimide surface layer in all areas not covered by trace metal conductors. 
           [0054]      FIG. 21  depicts a cross-sectional view of the layer structure after a layer of polyimide is spun over the underlying structures and cured. 
           [0055]      FIG. 22  depicts a cross-sectional view of the layer structure after curing the polyimide and after depositing an aluminum thin film layer on top of the polyimide layer that will serve to define openings in the top polyimide insulation. 
           [0056]      FIG. 23  depicts a cross-sectional view of the layer structure after a photoresist layer is deposited on the aluminum layer and exposed to irradiation through a mask and the radiated areas of the photoresist layer are removed during photolithographic processing to selectively mask aluminum thin film layer in areas that are to remain. 
           [0057]      FIG. 24  depicts a cross-sectional view of the layer structure after the removal of aluminum layer in areas exposed during photolithography and preceding etches. 
           [0058]      FIG. 25  depicts a cross-sectional view of the layer structure after the removal of top polyimide to create vias to trace metal in areas defined by aluminum etch mask. 
           [0059]      FIG. 26  depicts a cross-sectional view of the layer structure after the removal of the remaining photoresist from surface. 
           [0060]      FIG. 27  depicts a cross-sectional view of the layer structure after the removal of remaining aluminum mask layer. 
           [0061]      FIG. 28  depicts a cross-sectional view of the layer structure after the removal of exposed top titanium layer in areas opened through the top polyimide. 
           [0062]      FIG. 29  depicts a cross-sectional view of the layer structure after the singulation of arrays to create individual parts prior to removal from the supporting glass plate substrate. 
           [0063]      FIG. 30  depicts a cross-sectional view of the layer structure after the deposition of additional material on electrode sites which depicts a part of the flexible circuit electrode array. 
           [0064]      FIG. 31  depicts a top view of the flexible circuit electrode array. 
           [0065]      FIG. 32  depicts a perspective view of a part of the flexible circuit electrode array. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0066]      FIG. 1  shows a perspective view of the implanted portion of the preferred retinal prosthesis. A flexible circuit electrode array  10  is mounted by a retinal tack or similar means to the epiretinal surface. The flexible circuit electrode array  10  is electrically coupled by a flexible circuit cable  12 , which pierces the sclera and is electrically coupled to an electronics package  14 , external to the sclera. 
         [0067]    The electronics package  14  is electrically coupled to a secondary inductive coil  16 . Preferably the secondary inductive coil  16  is made from wound wire. Alternatively, the secondary inductive coil  16  may be made from a flexible circuit polymer sandwich with wire traces deposited between layers of flexible circuit polymer. The electronics package  14  and secondary inductive coil  16  are held together by a molded body  18 . The molded body  18  may also include suture tabs  20 . The molded body  18  narrows to form a strap  22  which surrounds the sclera and holds the molded body  18 , secondary inductive coil  16 , and electronics package  14  in place. The molded body  18 , suture tabs  20  and strap  22  are preferably an integrated unit made of silicone elastomer. Silicone elastomer can be formed in a pre-curved shape to match the Curvature  40  of a typical sclera. However, silicone remains flexible enough to accommodate implantation and to adapt to variations in the Curvature  40  of an individual sclera. The secondary inductive coil  16  and molded body  18  are preferably oval shaped. A strap  22  can better support an oval shaped coil  16 . 
         [0068]    The implanted portion of the retinal prosthesis may include the additional feature of a gentle twist or fold  48  in the flexible circuit cable  12 , where the flexible circuit cable  12  passes through the sclera (scleratomy). The twist  48  may be a simple sharp twist, or fold; or it may be a longer twist, forming a tube. While the tube is rounder, it reduces the flexibility of the flexible circuit cable  12 . A simple fold reduces the width of the flexible circuit cable  12  with only minimal impact on flexibility. 
         [0069]    Further, silicone or other pliable substance may be used to fill the center of the tube or fold  48  formed by the twisted flexible circuit cable  12 . Further it is advantageous to provide a sleeve or coating  50  that promotes healing of the scleratomy. Polymers such as polyimide, which may be used to form the flexible circuit cable  12  and flexible circuit electrode array  10 , are generally very smooth and do not promote a good bond between the flexible circuit cable  12  and scleral tissue. A sleeve or coating  50  of polyester, collagen, silicone, Gore-Tex® or similar material would bond with scleral tissue and promote healing. In particular, a porous material will allow scleral tissue to grow into the pores promoting a good bond. 
         [0070]    The entire implant is attached to and supported by the sclera. An eye moves constantly. The eye moves to scan a scene and also has a jitter motion to improve acuity. Even though such motion is useless in the blind, it often continues long after a person has lost their sight. By placing the device under the rectus muscles with the electronics package  14  in an area of fatty tissue between the rectus muscles, eye motion does not cause any flexing which might fatigue, and eventually damage, the device. 
         [0071]    Human vision provides a field of view that is wider than it is high. This is partially due to fact that we have two eyes, but even a single eye provides a field of view that is approximately 90° high and 140° to 160° degrees wide. It is therefore, advantageous to provide a flexible circuit electrode array  10  that is wider than it is tall. This is equally applicable to a cortical visual array. In which case, the wider dimension is not horizontal on the visual cortex, but corresponds to horizontal in the visual scene. 
         [0072]      FIG. 2  shows a side view of the implanted portion of the retinal prosthesis, in particular, emphasizing the fan tail  24 . When implanting the retinal prosthesis, it is necessary to pass the strap  22  under the eye muscles to surround the sclera. The secondary inductive coil  16  and molded body  18  must also follow the strap  22  under the lateral rectus muscle on the side of the sclera. The implanted portion of the retinal prosthesis is very delicate. It is easy to tear the molded body  18  or break wires in the secondary inductive coil  16 . In order to allow the molded body  18  to slide smoothly under the lateral rectus muscle, the molded body  18  is shaped in the form of a fan tail  24  on the end opposite the electronics package  14 . 
         [0073]    The flexible circuit electrode array  10  is a made by the following process. First, a layer of polymer is applied to a supporting substrate (not part of the array) such as glass. The polymer layer or films of the present invention can be made, for example, any one of the various polyfluorocarbons, polyethylene, polypropylene, polyimide, polyamide, silicone or other biologically inert organic polymers. Layers may be applied by spinning, meniscus coating, casting, sputtering or other physical or chemical vapor deposition, or similar process. Subsequently, a metal layer is applied to the polymer. The metal is patterned by photolithographic process. Preferably, a photoresist is applied and patterned by photolithography followed by a wet etch of the unprotected metal. Alternatively, the metal can be patterned by lift-off technique, laser ablation or direct write techniques. 
         [0074]    It is advantageous to make the metal thicker at the electrode and bond pad to improve electrical continuity. This can be accomplished through any of the above methods or electroplating. Then, the top layer of polymer is applied over the metal. Openings in the top layer for electrical contact to the electronics package  14  and the flexible circuit electrode array  10  may be accomplished by laser ablation or reactive ion etching (RIE) or photolithograph and wet etch. Making the electrode openings in the top layer smaller than the electrodes promotes adhesion by avoiding delaminating around the electrode edges. 
         [0075]    The pressure applied against the retina by the flexible circuit electrode array  10  is critical. Too little pressure causes increased electrical resistance between the array and retina. Common flexible circuit fabrication techniques such as photolithography generally require that a flexible circuit electrode array  10  be made flat. Since the retina is spherical, a flat array will necessarily apply more pressure near its edges, than at its center. With most polymers, it is possible to curve them when heated in a mold. By applying the right amount of heat to a completed array, a curve can be induced that matches the curve of the retina. To minimize warping, it is often advantageous to repeatedly heat the flexible circuit in multiple molds, each with a decreasing radius.  FIG. 3  illustrates a series of molds according to the preferred embodiment. Since the flexible circuit will maintain a constant length, the curvature  30  must be slowly increased along that length. As the curvature  30  decreases in successive molds ( FIGS. 3   a - 3   e ) the straight line length between ends  32  and  34 , must decrease to keep the length along the curvature  30  constant, where mold  3 E approximates the curvature  30  of the retina or other desired neural tissue. The molds provide a further opening  36  for the flexible circuit cable  12  of the array to exit the mold without excessive curvature. 
         [0076]    It should be noted that suitable polymers include thermoplastic materials and thermoset materials. While a thermoplastic material will provide some stretch when heated a thermoset material will not. The successive molds are, therefore, advantageous only with a thermoplastic material. A thermoset material works as well in a single mold as it will with successive smaller molds. It should be noted that, particularly with a thermoset material, excessive curvature  30  in three dimensions will cause the polymer material to wrinkle at the edges. This can cause damage to both the array and the retina. Hence, the amount of curvature  30  is a compromise between the desired curvature, array surface area, and the properties of the material. 
         [0077]    Referring to  FIG. 4 , the edges of the polymer layers are often sharp. There is a risk that the sharp edges of a flexible circuit will cut into delicate retinal tissue. It is advantageous to add a soft material, such as silicone, to the edges of a flexible circuit electrode array  10  to round the edges and protect the retina. Silicone around the entire edge may make the flexible circuit less flexible. It is advantageous to provide silicone bumpers or ribs to hold the edge of the flexible circuit electrode array  10  away from the retinal tissue. Curvature  40  fits against the retina. The leading edge  44  is most likely to cause damage and is therefore fit with molded silicone bumper. Also, edge  46 , where the array lifts off the retina can cause damage and should be fit with a bumper. Any space along the side edges of curvature  40  may cause damage and may be fit with bumpers as well. It is also possible for the flexible circuit cable  12  of the electrode array to contact the retina. It is, therefore, advantageous to add periodic bumpers along the cable  12 . 
         [0078]    It is also advantageous to create a reverse curve or service loop in the flexible circuit cable  12  of the flexible circuit electrode array  10  to gently lift the flexible circuit cable  12  off the retina and curve it away from the retina, before it pierces the sclera at a scleratomy. It is not necessary to heat curve the service loop as described above, the flexible circuit electrode array  10  can simply be bent or creased upon implantation. This service loop reduces the likelihood of any stress exerted extraocularly from being transmitted to the electrode region and retina. It also provides for accommodation of a range of eye sizes. 
         [0079]    With existing technology, it is necessary to place the implanted control electronics outside of the sclera, while a retinal flexible circuit electrode array  10  must be inside the sclera in order to contact the retina. The sclera must be cut through at the pars plana, forming a scleratomy, and the flexible circuit passed through the scleratomy. A flexible circuit is thin but wide. The more electrode wires, the wider the flexible circuit must be. It is difficult to seal a scleratomy over a flexible circuit wide enough to support enough wires for a high resolution array. 
         [0080]      FIG. 5  shows a body  1  containing the flexible circuit electrode array  10 , the flexible circuit cable  12  and the interconnection pad  52  prior to folding and attaching the array to the electronics package  14 . At one end of the flexible circuit cable  12  is an interconnection pad  52  for connection to the electronics package  14 . At the other end of the flexible circuit cable  12  is the flexible circuit electrode array  10 . Further, an attachment point  54  is provided near the flexible circuit electrode array  10 . A retina tack (not shown) is placed through the attachment point  54  to hold the flexible circuit electrode array  10  to the retina. A stress relief  55  is provided surrounding the attachment point  54 . The stress relief  55  may be made of a softer polymer than the flexible circuit, or it may include cutouts or thinning of the polymer to reduce the stress transmitted from the retina tack to the flexible circuit electrode array  10 . The flexible circuit cable  12  is formed in a dog leg pattern so than when it is folded at fold  48  it effectively forms a straight flexible circuit cable  12  with a narrower portion at the fold  48  for passing through the scleratomy. 
         [0081]      FIG. 6  shows the flexible circuit electrode array  10  after the flexible circuit cable  12  is folded at the fold  48  to form a narrowed section. The flexible circuit cable  12  may include a twist or tube shape as well. With a retinal prosthesis as shown in  FIG. 1 , the interconnection pad  52  for connection to the electronics package  14  and the flexible circuit electrode array  10  are on opposite side of the flexible circuit. This requires patterning, in some manner, both the base polymer layer and the top polymer layer. By folding the flexible circuit cable  12  of the flexible circuit electrode array  10 , the openings for the bond pad  52  and the electrodes are on the top polymer layer and only the top polymer layer needs to be patterned. 
         [0082]    Also, since the narrowed portion of the flexible circuit cable  12  pierces the sclera, shoulders formed by opposite ends of the narrowed portion help prevent the flexible circuit cable  12  from moving through the sclera. It may be further advantageous to add ribs or bumps of silicone or similar material to the shoulders to further prevent the flexible circuit cable  12  from moving through the sclera. 
         [0083]    Further it is advantageous to provide a suture tab  56  in the flexible circuit body near the electronics package  14  to prevent any movement in the electronics package  14  from being transmitted to the flexible circuit electrode array  10 . Alternatively, a segment of the flexible circuit cable  12  can be reinforced to permit it to be secured directly with a suture. 
         [0084]    An alternative to the bumpers described in  FIG. 4 , is a skirt of silicone or other pliable material as shown in  FIGS. 7 to 10 . A skirt  60  covers the flexible circuit electrode array  10 , and extends beyond its edges. It is further advantageous to include windows  62  adjacent to the attachment point  54  to spread any stress of attachment over a larger area of the retina. There are several ways of forming and bonding the skirt  60 . The skirt  60  may be directly bonded through surface activation or indirectly bonded using an adhesive. 
         [0085]    Alternatively, a flexible circuit electrode array  10  may be layered using different polymers for each layer. Using too soft of a polymer may allow too much stretch and break the metal traces. Too hard of a polymer may cause damage to delicate neural tissue. Hence a relatively hard polymer, such a polyimide may be used for the bottom layer and a relatively softer polymer such a silicone may be used for the top layer including an integral skirt to protect delicate neural tissue. 
         [0086]    The simplest solution is to bond the skirt  60  to the back side away from the retina of the flexible circuit electrode array  10  as shown in  FIG. 8 . While this is the simplest mechanical solution, sharp edges of the flexible circuit electrode array  10  may contact the delicate retina tissue. Bonding the skirt to the front side toward the retina of the flexible circuit electrode array  10 , as shown in  FIG. 9 , will protect the retina from sharp edges of the flexible circuit electrode array  10 . However, a window  62  must be cut in the skirt  60  around the electrodes. Further, it is more difficult to reliably bond the skirt  60  to the flexible circuit electrode array  10  with such a small contact area. This method also creates a space between the electrodes and the retina which will reduce efficiency and broaden the electrical field distribution of each electrode. Broadening the electric field distribution will limit the possible resolution of the flexible circuit electrode array  10 . 
         [0087]      FIG. 10  shows another structure where the skirt  60  is bonded to the back side of the flexible circuit electrode array  10 , but curves around any sharp edges of the flexible circuit electrode array  10  to protect the retina. This gives a strong bond and protects the flexible circuit electrode array  10  edges. Because it is bonded to the back side and molded around the edges, rather than bonded to the front side, of the flexible circuit electrode array  10 , the portion extending beyond the front side of the flexible circuit electrode array  10  can be much smaller. This limits any additional spacing between the electrodes and the retinal tissue. 
         [0088]      FIG. 11 , shows a flexible circuit electrode array  10  similar to  FIG. 10 , with the skirt  60 , flush with the front side of the flexible circuit electrode array  10  rather than extending beyond the front side. While this is more difficult to manufacture, it does not lift the electrodes off the retinal surface as with the array in  FIG. 10 . It should be noted that  FIGS. 8 ,  10 , and  11  show skirt  60  material along the back of the flexible circuit electrode array  10  that is not necessary other than for bonding purposes. If there is sufficient bond with the flexible circuit electrode array  10 , it may be advantageous to thin or remove portions of the skirt  60  material for weight reduction. 
         [0089]    The electrode of the present invention preferably contains platinum. Platinum can be present in any form in the electrode. The electrode has preferably increased surface area for greater ability to transfer charge and also having sufficient physical and structural strength to withstand physical stress encountered in its intended use. The electrode contains platinum having a fractal configuration so called platinum gray with an increase in surface area of at least 5 times when compared to shiny platinum of the same geometry and also having improved resistance to physical stress when compared to platinum black. Platinum gray is described in US 2003/0192784 “Platinum Electrode and Method for Manufacturing the Same” to David Zhou, the disclosure of which is incorporated herein by reference. The electrodes of the preferred embodiment are too small to display a color without significant magnification. The process of electroplating the surface coating of platinum gray comprising plating at a moderate rate, i.e., at a rate that is faster than the rate necessary to produce shiny platinum and that is less than the rate necessary to produce platinum black. 
         [0090]    The flexible circuit electrode array  10  is manufactured in layers. A base layer of polymer is laid down, commonly by some form of chemical vapor deposition, spinning, meniscus coating or casting on a supporting rigid substrate like glass. A layer of metal (preferably platinum), preferably sandwich by layers of another metal for example titanium, is applied to the polymer base layer and patterned to create electrodes and traces for those electrodes. Patterning is commonly done by photolithographic methods. The electrodes may be built up by electroplating or similar method to increase the surface area of the electrode and to allow for some reduction in the electrode over time. Similar plating may also be applied to the bond pads. See  FIGS. 5 to 7 . A top polymer layer is applied over the metal layer and patterned to leave openings for the electrodes, or openings are created later by means such as laser ablation. It is advantageous to allow an overlap of the top polymer layer over the electrodes to promote better adhesion between the layers, and to avoid increased electrode reduction along their edges. Alternatively, multiple alternating layers of metal and polymer may be applied to obtain more metal traces within a given width. 
         [0091]      FIG. 12  depicts a supporting rigid substrate  70  which may be marked with a batch and plate identification code by mechanical engraving. The supporting substrate  70  is a rigid material like glass. 
         [0092]      FIG. 13  shows a polymer layer  71   a  which is applied onto the front side of the supporting substrate  70 . The polymer can contain polyfluorocarbons, polyethylene, polypropylene, silicone, polyamide, polyimide, liquid crystal polymer (LCP), poly-para-xylylene (GALXYL® Parylene), polyaryletherketone (Peek®) or other similar polymers. The preferred polymer according to the present invention is polyimide. Polyimide is biocompatible and shows excellent properties as insulator for the trace metal and electrodes in the electrode assay. Polyimide is preferably obtained by imidization of polyamic acid. Polyamic acid is a precursor of the polyimide. By heating the polyamic acid to a temperature of preferably 100° C. to 400° C. the imidization of the polyamic acid is conducted. The following formula shows an example of imidization of polyamic acid to polyimide. 
         [0000]    
       
                 
         
             
             
         
       
     
         [0093]    R is a bivalent organic group, like 4,4′ oxydianiline or p-phenylendiamine and n is an integer &gt;1. Two polyimides are preferably used within the present invention. PMDA/ODA (derived from polymellitic dianhydride (PMDA) and 4,4′ oxydianiline (ODA)) and BPDA/PDA (derived from 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA) and p-phenylendiamine (PDA)). The polyimide layer  71   a  is preferably obtained according to the present invention with a thickness of the liquid precursor of 1.0 μm to 10 μm, preferably 4.0 μm to 7.0 μm, and most preferably 5.0 μm to 6.0 μm. The plate is placed in a plasma cleaner. An adhesion promoter is applied on the plate and dried by letting the plate sit for at least 5 min. Spin speed of a spinner and ramp up time, spin time, and ramp down time are adjusted and polyamic acid is applied. The plate is soft baked on a hotplate at 90° C. to 110° C. whereby the solvents in the polyimide are evaporated. Then the polyimide is cured in a high temperature nitrogen purged oven. 
         [0094]      FIG. 14  shows that a first thin metal layer  72   a  containing titanium, chromium, tantalum or an alloy thereof or a combination of two or more alloys or metal layers thereof, having a thickness of 0.01 μm to 0.1 μm, preferably 0.03 μm to 0.07 μm, and most preferably 0.04 μm to 0.06 μm is applied on the base polymer layer  71   a  preferably by magnetron sputtering. A layer of electrode  73 , containing platinum, tantalum, iridium, palladium, rhodium, rhenium, or alloys thereof or a combination of two or more alloys or metal layers thereof, having a thickness of 0.1 μm to 1.0 μm, preferably 0.3 μm to 0.7 μm, and most preferably 0.4 μm to 0.6 μm is applied on the thin metal layer  72   a  preferably by magnetron sputtering. A top thin layer of metal  72   b , having a thickness of 0.05 μm to 0.15 μm, preferably 0.08 μm to 0.13 μm, and most preferably 0.08 μm to 0.12 μm, which preferably contains titanium, chromium, tantalum or an alloy thereof or a combination of two or more alloys or metal layers thereof, and particularly the same as the metal layer  72   a , is applied onto the layer of platinum  73  preferably by magnetron sputtering yielding a thin film stack. 
         [0095]    The trace metal of the present invention most preferably contains a lower layer of 0.04 μm to 0.06 μm thick titanium film  72   a , a 0.4 μm to 0.6 μm thick platinum layer  73  and a 0.08 μm 0.12 μm thin top titanium film  72   b . The present invention provides trace metal with a thin top titanium film  72   b . The top titanium film  72   b  performs an expected strong adhesion with the upper polyimide layer  71   b , which is applied as a precursor and is subsequently cured to polyimide. The state of the art teaches to omit a top layer of titanium on top of the platinum layer, because partial oxidation of the titanium surface would weaken the adhesion to polyimide. The present invention has shown a strong and unexpected adhesion a top titanium layer  72   b  to polyimide  71  despite the contrary prior art teaching. 
         [0096]      FIG. 15  shows a positive photoresist  74  which is applied on the metal titanium layer  72   b . The photoresist layer  74  is irradiated by UV light through a mask whereby a pattern is obtained. The irradiated areas of the photoresist layer  74  are removed. The removed areas of the photoresist layer  74  masked selectively the areas where vias and traces are obtained. In this process the plate front side is first dehydrated. A mask aligner is turned on prior to use. The spin speed of a spinner is adjusted to 1700 rpm to 1900 rpm. The ramp up time, spin time, and ramp down time are adjusted to 3 s to 7 s, 15 s to 25 s, and 3 s to 7 s. Positive photoresist is poured onto the center of the plate to form a puddle that is 5.8 cm to 6.6 cm in diameter. The plate is soft baked on a hot plate at 100° C. to 110° C. for 25 s to 35 s. The required exposure dose is 80 mJ/cm 2 ±5 mJ/cm 2  at 436 nm. The plate is aligned under a mask pattern in the mask aligner. The plate is exposed in the mask aligner. The plate is developed in developer for 40 s to 50 s (seconds) with manual agitation of a carrier boat. The photoresist is dissolved during development. The exposed photoresist is dissolved during the development. Then the plate is first rinsed in a lower cascade rinse bath for 50 s to 70 s then in a middle cascade rinse bath for 50 s to 70 s and finally in a bubbler cascade rinse bath for 50 s to 70 s. The plate is dried with filtered nitrogen. 
         [0097]      FIG. 16  shows how the thin top layer  72   b  is removed in the exposed areas. Typically the preferred titanium is removed at 20° C. to 30° C. for 50 s to 70 s with dilute hydrofluoric acid in a container. The plate is loaded into a carrier boat before etching the titanium. The plate is first rinsed in a lower cascade rinse bath for 50 s to 70 s then in a middle cascade rinse bath for 50 s to 70 s and finally in a bubbler cascade rinse bath for 50 s to 70 s. The plate is dried with filtered nitrogen, front side only. 
         [0098]      FIG. 17  shows platinum being removed from the layer  73  by wet etch in the exposed areas. The plate is loaded into a carrier boat for removing platinum. Typically the preferred platinum is removed with Aqua Regia, which is a mixture of hydrochloric acid and nitric acid and water, at 60° C. to 70° C. for 7 min to 8 min to 10 min with continuous automatic agitation by 5 cm stir bar rotating at 400 rpm to 600 rpm. The plate is first rinsed in a lower cascade rinse bath for 50 s to 70 s then in a middle cascade rinse bath for 50 s to 70 s and finally in a bubbler cascade rinse bath for 50 s to 70 s. The plate is dried with filtered nitrogen, front side only. 
         [0099]      FIG. 18  shows the residual photoresist layer  74  being removed with a liquid immersion solvent. The plate is soaked in acetone for 1 min to 3 min with manual agitation of the carrier boat at least every 15 s to 25 s. Then the plate is soaked in isopropanol for 1 min to 3 min with manual agitation of the carrier boat at least every 15 s to 25 s. 
         [0100]      FIG. 19  shows that metal is removed from the thin titanium layer  72   a  by wet etch in the masked areas. Typically the preferred titanium is removed at 20° C. to 30° C. for 30 s to 40 s with dilute hydrofluoric acid. The plate is first rinsed in a lower cascade rinse bath for 50 s to 70 s then in a middle cascade rinse bath for 50 s to 70 s and finally in a bubbler cascade rinse bath for 50 s to 70 s. The plate is dried with filtered nitrogen, front side only. The plate is then dehydrated in preheated oven at 110° C. to 130° C. for 8 min to 10 min. 
         [0101]      FIG. 20  shows that the areas where titanium  72   a  and  72   b  and platinum  73  were removed are not covered by trace metal conductors but are open and base polyimide  71   a  is the surface layer. Base polyimide surface layer  71   a  is then activated by RIE in all areas not covered by trace metal conductors. The plate is loaded in this process into a plasma cleaner. The plate is cleaned in 400 mTorr to 600 mTorr O 2  for 4 min to 6 min at 180 W to 220 W (W stands for Watt). Then the plate is treated in KOH-deimidization bath at 20° C. to 30° C. for 4 min to 6 min with manual agitation of the carrier boat at least every 50 s to 70 s. The plate is first rinsed in a lower cascade rinse bath for 50 s to 70 s then in a middle cascade rinse bath for 50 s to 70 s and finally in a bubbler cascade rinse bath for 50 s to 70 s. The plate is dried with filtered nitrogen, front side only. The plate is treated in an HCl-deimidization bath at 20° C. to 30° C. for 4 min to 6 min with manual agitation of the carrier boat at least every 50 s to 70 s. The plate is rinsed in the lower cascade rinse bath for 50 s to 70 s. The plate is rinsed in the middle cascade rinse bath for 50 s to 70 s. The plate is rinsed in the bubbler cascade rinse bath for 50 s to 70 s. The plate is dried with filtered nitrogen, front side only. The plate is soaked in isopropanol at 20° C. to 30° C. for 1 min to 2 min with manual agitation of the carrier boat at least every 15 s to 25 s. The plate is dried with filtered nitrogen, front side only. The plate is dehydrated under vacuum pressure of less than 100 mTorr for at least 2 hr. 
         [0102]    The activation process according to the present invention provides a surface of the base layer  71   a  which is deimidized and therefore the surface is similar to the uncured state. The imidization process obtained during curing is now reversed. 
         [0103]    The activated surface layer of the base polyimide layer is chemically very similar to the precursor of the top polyimide layer  71   b  which is applied to yield a top layer. When the precursor of the top layer  71   b  is applied on the activated and deimidized base layer  71   a  then there is no boundary between the two layers in the contact area. 
         [0104]      FIG. 21  shows how a top layer of polymer  71   b  is applied onto the top of base polymer layer  71   a . The polymer contains polyfluorocarbons, polyethylene, polypropylene, silicone, polyamide, polyimide, liquid crystal polymer (LCP), poly-para-xylylene (GALXYL® Parylene), polyaryletherketone (Peek®) or other similar polymers. Polyimide shows excellent properties as an insulator for the trace metal and electrodes in the electrode array. Polyimide is preferably obtained by imidization of polyamic acid. Polyamic acid is a precursor of the polyimide. The imidization of the polyamic acid to polyimide is conducted by heating the polyamic acid at a temperature of preferably 100° C. to 400° C. The polyimide layer  71   b  is preferably obtained according to the present invention with a thickness of 1.0 μm to 10 μm, preferably 4.0 μm to 7.0 μm, and most preferably 5.0 μm to 6.0 μm. The plate is placed in this process in a plasma cleaner. An adhesion promoter is applied on the plate and dried off by letting the plate sit for at least 5 min. Spin speed of a spinner and ramp up time, spin time, and ramp down time are adjusted and polyamic acid is applied. The plate is soft baked on a hotplate at 100° C. to 110° C. whereby the solvents in the polyimide are evaporated. The polyimide is cured in a high temperature nitrogen purged oven. 
         [0105]      FIG. 22  shows that base polyimide layer  71   a  and top polyimide layer  71   b  become one polyimide layer  71  performing a high adhesion to the trace metal. An insulation mask of aluminum thin film  75  is applied preferably by magnetron sputtering on top of the polyimide layer  71  having a thickness of 0.5 μm to 1.5 μm, preferably 0.7 μm to 1.3 μm, and most preferably 0.9 μm to 1.1 μm. 
         [0106]      FIG. 23  shows that a positive photoresist  76  is applied on the aluminum layer  75 . The photoresist layer  76  is irradiated by UV light through a mask whereby a pattern is obtained. The irradiated areas of the photoresist layer  76  are removed by a solvent. The removed areas of the photoresist layer  76  expose selectively the areas where vias are to be obtained in the further process. The plate front side is dehydrated at 110° C. to 130° C. for 15 min to 25 min. A mask aligner is turned on prior to use. The spin speed of a spinner is adjusted to 2800 rpm to 3200 rpm. The ramp up time, spin time, and ramp down time are adjusted to 15 s to 25 s, 1 s to 2 s, and 2 s to 4 s respectively. Positive photoresist  76  is poured onto the center of the plate to form a puddle that is 3.6 cm to 4.0 cm in diameter. The plate is soft baked on a hot plate at 110° C. to 120° C. for 4 min to 5 min. The required exposure dose is 470 mJ/cm 2  to 490 mJ/cm 2  at 436 nm. The Insulation Vias photomask is used in the aligner. The plate is aligned under mask pattern in the mask aligner. The plate is exposed in the mask aligner. The plate is developed in a developer for 40 s to 50 s with manual agitation of the carrier boat. The plate is developed in heated developer mixture at 20° C. to 30° C. for 4 min to 6 min with manual agitation every 30 s. The plate is first rinsed in a lower cascade rinse bath for 50 s to 70 s then in a middle cascade rinse bath for 50 s to 70 s and finally in a bubbler cascade rinse bath for 50 s to 70 s. The plate is dried with filtered nitrogen, front side only. The plate is hard baked in oven for 25 min 35 min at 80° C. to 100° C. 
         [0107]      FIG. 24  shows that the aluminum layer  75  is removed in exposed areas by wet etch. The plate is plasma cleaned in this process in 500 mTorr to 600 mTorr O 2  for 4 min to 6 min at 180 W to 220 W. The plate is treated in heated Al Etch, containing 71% to 73% phosphoric acid, 9% to 11% acetic acid, and 1% to 3% nitric acid, at 30° C. to 40° C. for 9 min to 12 min with manual agitation every 50 s to 70 s. The plate is first rinsed in a lower cascade rinse bath for 1 min to 3 min then in a middle cascade rinse bath for 1 min to 3 min and finally in a bubbler cascade rinse bath for 1 min to 3 min. The plate is dried with filtered nitrogen, front side only. The plate is dehydrated for 25 min to 35 min at 90° C. to 110° C. 
         [0108]      FIG. 25  shows that polyimide surface layer  71  is removed by RIE (Reactive Ion Etching) in all areas not covered by the aluminum layer  75 . 
         [0109]      FIG. 26  shows that the residual photoresist layer  76  was removed from the aluminum layer  75  with a liquid immersion solvent. The plate is soaked in acetone for 1 min to 3 min with manual agitation of the carrier boat at least every 15 s to 25 s. Then the plate is soaked in isopropanol for 1 min to 3 min with manual agitation of the carrier boat at least every 15 s to 25 s. The plate is first rinsed in a lower cascade rinse bath for 50 s to 70 s then in a middle cascade rinse bath for 50 s to 70 s and finally in a bubbler cascade rinse bath for 50 s to 70 s. The plate is dried with filtered nitrogen, front side only. 
         [0110]      FIG. 27  shows that remaining insulation mask aluminum layer  75  is removed from the polyimide  71  layer by wet etch. The plate is plasma cleaned in 550 mTorr O 2  for 4 min to 6 min at 180 W to 220 W. The plate is treated in heated Al Etch at 30° C. to 40° C. for 9 min to 11 min with manual agitation every 50 s to 70 s. The plate is first rinsed in a lower cascade rinse bath for 1 min to 3 min then in a middle cascade rinse bath for 1 min to 3 min and finally in a bubbler cascade rinse bath for 1 min to 3 min. The plate is dried with filtered nitrogen, front side only. The plate is dehydrated for 25 min to 30 min at 100° C. to 110° C. 
         [0111]      FIG. 28  shows that the titanium  72   b  top surface layer is removed in the vias by wet etch to leave platinum  73  as the top surface layer in the vias. Typically the preferred titanium is removed at 20° C. to 30° C. for 50 s to 70 s with dilute hydrofluoric acid. The plate is first rinsed in a lower cascade rinse bath for 50 s to 70 s then in a middle cascade rinse bath for 50 s to 70 s and finally in a bubbler cascade rinse bath for 50 s to 70 s. The plate is dried with filtered nitrogen, front side only. 
         [0112]      FIG. 29  shows that the arrays are singulated by cutting through the polyimide  71  layer by laser  77  whereby individual parts are created. The single circuit electrode array is then removed from the supporting glass plate substrate  70  by a solvent. 
         [0113]      FIG. 30  shows that the supporting glass substrate  70  is removed. The vias are electroplated with preferably platinum, most preferably platinum gray  78 . The vias are preferably filled with platinum gray  78  are electroplated on the platinum trace  73 . Platinum gray is described in US 2003/0192784 “Platinum Electrode and Method for Manufacturing the Same” to David Zhou, the disclosure of which is incorporated herein by reference. The method to produce platinum gray according to the present invention comprises connecting a platinum electrode, the anode, and a conductive substrate to be plated, the cathode, to a power source with a means of controlling and monitoring either the current or voltage of the power source. The anode, cathode, a reference electrode for use as a reference in controlling the power source and an electroplating solution are placed in a electroplating cell having a means for mixing or agitating the electroplating solution. Power is supplied to the electrodes with constant voltage, constant current, pulsed voltage, scanned voltage or pulsed current to drive the electroplating process. The power source is modified such that the rate of deposition will cause the platinum to deposit as platinum gray, the rate being greater than the deposition rate necessary to form shiny platinum and less than the deposition rate necessary to form platinum black. 
         [0114]    The electrode region of the array  10  is immersed in this process into sulfuric acid. The electrode region of the array  10  is rinsed under a flow of distilled water for at least 30 s. The plating solution is kept at 21° C. to 23° C. The plating solution is being stirred at 180 rpm to 220 rpm. The electrical contact fixture is loaded into the rotary holder. The electrode end of the array  10  is immersed into plating bath and the apparatus is aligned. The system is allowed to stabilize for 4 min to 6 min. After starting a potentiostat program a pneumatic control timer is started immediately. The array is rotated by 90° every 4 min to 6 min for the duration of plating. After the plating cycle is completed, the electrode region of the array  10  is rinsed under a flow of distilled water for at least 30 s. The array is then removed from the electroplating electrical contact fixture. The entire array is rinsed in a vial with fresh distilled water for up to about 24 hours. The array  10  is then dried in nitrogen. 
         [0115]    It becomes apparent especially from  FIG. 30  that the electrode  78 , preferably platinum gray has a direct contact to the conducting layer  73 , preferably platinum. The electrode  78  is slightly slimmer than the trace  72   a / 73 / 72   b , preferably titanium/platinum/titanium. Polymer  71 , preferably polyimide, covers the part of the trace, which does not have a contact with the electrode  73 . This part of the trace is covered by titanium  72   b . There is a surprisingly strong adhesion between titanium  72   b  and polyimide  71 . The trace  72   a / 73 / 72   b  is referred to in  FIG. 30  and subsequently in  FIGS. 31-32  simplified as number  79 . 
         [0116]      FIG. 31  shows an enlarged top view of the flexible circuit electrode array  10  which is a part of the body  1  as shown for example in  FIG. 5 . The preferred positions of the electrodes  78  and the preferred wiring by the trace metal  79  both embedded in the polymer  71  are shown in the  FIG. 31 . 
         [0117]      FIG. 32  shows a three dimensional view of a part of the flexible circuit electrode array  10 . It shows one electrode  78  which has a contact with the trace metal  79 . It also shows that the trace metal  79  overlaps the electrode  78  and the electrode  78  overlaps the via in the polymer  71 . The  FIG. 32  further shows the adhesion of the polymer  71  with the trace metal  79  and the electrode  78  which results in a very high effective insulation of the trace metal  79  and the electrode  78 . The figure shows also that the trace metal  79  is preferably composed of platinum conducting trace  73  covered on the lower and upper side preferably with a thin titanium layer  72   a  and  72   b . The figure finally shows that the first applied base polymer  71   a  and the subsequently applied top polymer layer  71   b  form a single polymer layer  71 . 
         [0118]    The present invention will be further illustrated by the following examples, but it is to be understood that the invention is not meant to be limited to the details described herein. 
       Example 1 
       [0119]    A 10.2 cm×10.2 cm×0.15 cm supporting glass plate substrate  70  was marked with a batch and plate identification code by mechanical engraving. Then a 5.5 μm thick layer of polyimide BPDA/PDA (derived from 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA) and p-phenylendiamine (PDA)) layer  71   a  was applied onto the front side of the glass plate  70  as a liquid precursor by spin coating and cured to Polyimide, PI2611. 
         [0120]    Then a 0.05 μm layer of titanium  72   a  was applied on the polyimide layer  71   a  preferably by magnetron sputtering, a 0.5 μm layer of platinum  73  was applied on the titanium layer  72   a  preferably by magnetron sputtering, and a 0.10 μm layer of titanium  72   b  was applied onto the layer of platinum  73  preferably by magnetron sputtering yielding a titanium/platinum/titanium thin film stack. 
         [0121]    Subsequently a 2 μm layer  74  of positive photoresist AZ 1512 (Microchemicals GmbH, Germany) was applied on the titanium layer  72   b . The photoresist layer  74  was irradiated by UV light through a mask whereby a pattern was created. The irradiated areas of the photoresist layer  74  where removed by a developer solution AZ 300-MIF (Microchemicals GmbH, Germany). The remaining areas of the photoresist layer  74  masked selectively the areas where metal traces were obtained in the further process. 
         [0122]    Titanium from the layer  72   b  was removed by wet etch, 50:1 Dilute HF (Ashland Chemical), in the exposed areas. Platinum from the layer  73  was removed by wet etch, DI water:HCl:HNO 3  in volume ratio 1:3:1, in the exposed areas. Then the residual photoresist layer  74  was removed with a liquid immersion 20 s in acetone and 20 s in isopropanol. Titanium from the layer  72   a  was removed by wet etch, 50:1 Dilute HF (Ashland Chemical), in the exposed areas. The areas where titanium  72   a  and  72   b  and platinum  73  were removed expose polyimide  71  as the surface layer. 
         [0123]    The base polyimide surface layer  71   a  was activated and partially removed by RIE in all areas not covered by trace metal conductors. The surface was treated in 100 mTorr, 85% O 2 , 15% CF 4  for 2 min at 200 W and 20° C. as shown in the following table 1. 
         [0000]    
       
         
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Reactive Iron Etch (RIE) 
               
             
          
           
               
                   
                   
                 Pressure 
                 Power 
                 Time 
                 Temperature 
               
               
                 Step 
                 Gases 
                 [mTorr] 
                 [W] 
                 [m:s] 
                 [° C.] 
               
               
                   
               
             
          
           
               
                 Pump 
                 — 
                 1 
                 0 
                 00:01 
                 20 
               
               
                 Down 
               
               
                 Etch 
                 85% O 2 , 15% CF 4   
                 100 
                 200 
                 02:00 
                 20 
               
               
                 Pump 
                 — 
                 1 
                 0 
                 00:01 
                 20 
               
               
                 Down 
               
               
                   
               
             
          
         
       
     
         [0124]    The surface was subsequently treated in KOH-deimidization bath at 25° C. for 5 min with manual agitation of the carrier boat at least every 60 s. The surface was first rinsed in a lower cascade rinse bath for 60 s, in a middle cascade rinse bath for 60 s, and finally in a bubbler cascade rinse bath for 60 s. The surface was dried with filtered nitrogen. The surface was then treated in an HCl-deimidization bath at 25° C. for 5 min with manual agitation of the carrier boat at least every 60 s. The surface was first rinsed in a lower cascade rinse bath for 60 s, in a middle cascade rinse bath for 60 s, and finally in a bubbler cascade rinse bath for 60 s. The deimidization process is shown in the following table 2. 
         [0000]    
       
         
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Deimidization 
               
             
          
           
               
                   
                   
                   
                 Pressure 
                 Time 
                 Temperature 
               
               
                   
                 Step 
                 Concentration 
                 [Torr] 
                 [m:s] 
                 [° C.] 
               
               
                   
                   
               
               
                   
                 KOH 
                 1.0N KOH 
                 1 
                 5:00 
                 25 
               
               
                   
                 HCl 
                 1.0N HCl 
                 1 
                 5:00 
                 25 
               
               
                   
                   
               
             
          
         
       
     
         [0125]    Then a 5.5 μm thick top layer  71   b  of a precursor solution was applied by spin coating and cured to Polyimide, PI2611 on the top of base polyimide  71   a . Polyimide  71   a  and  71   b  combined to polyimide  71  after curing. 
         [0126]    Then an adhesion promoter, VM652, and aluminum foil, P/N X11652-1.518 (All Foils Inc) were applied by magnetron sputtering on top of the polyimide layer  71  yielding 1.0 μm mask aluminum thin film  75 . 
         [0127]    Then a 12 μm layer of positive photoresist  76 , AZ P4620 (Microchemicals GmbH, Germany) was applied on the aluminum layer  75 . The photoresist layer  76  was irradiated by UV light through a mask whereby a pattern was created. The irradiated areas of the photoresist layer  76  were removed by a developer AZ 1:1 (Microchemicals GmbH, Germany) [Developer: DI Water]. The removed areas of the photoresist layer  76  exposed selectively the areas where vias were obtained in the further process. 
         [0128]    Aluminum  75  was removed in exposed areas by wet etch, sulfuric acid, 1.0 N (0.5 M). The polyimide surface layer  71  was removed by RIE in all areas not masked by aluminum. Then the residual photoresist layer  76  was removed from the aluminum layer  75  by immersion 20 s in acetone and 20 s in isopropanol. Vias were created with titanium  72   b  as the top surface layer in the vias. Then the aluminum mask layer  75  was removed from the polyimide  71  layer by wet etch. The titanium  72   b  top surface layer was removed in the vias by wet etch, 50:1 Dilute HF (Ashland Chemical), to leave platinum  73  as the top surface layer in the vias. 
         [0129]    The arrays were singulated by cutting through the entire polyimide  71  layer by laser to create individual parts prior to removal from the supporting glass plate substrate  70 . The array  10  was removed from the supporting glass substrate  70 . 
         [0130]    The electrodes were electroplated with platinum gray  78 . The plating solution contained 18 mM (NH 4 ) 2 PtCl 6  dissolved in 0.46 M Na 2 HPO 4 . The electrode vias were filled with platinum gray  78 , which was in contact with the platinum trace  73 . 
       Example 2 
       [0131]    Example 2 was carried out according to example 1 with the difference that the base polyimide surface layer  71   a  was activated and partially removed by RIE in all areas not covered by trace metal conductors. The surface was treated in 100 mTorr, 85% O 2 , 15% CF 4  for 2 min at 200 W and 20° C. as shown in the preceding table 1 and the deimidization was omitted. The adhesion strength between the base polyimide layer  71   a  and the top polyimide layer  71   b  is shown in table 3. 
         [0000]    
       
         
               
             
               
               
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 Adhesion Strength 
               
             
          
           
               
                   
                   
                   
                 Polyimide - 
                   
               
               
                   
                   
                   
                 Polyimide 
                 Polyimide - 
               
               
                   
                   
                   
                 Adhesion 
                 Polyimide 
               
               
                   
                   
                   
                 Strength [N] 
                 Adhesion Strength [N] 
               
               
                 Ex 
                 RIE 
                 Deimidization 
                 Dry 
                 Wet 
               
               
                   
               
               
                 1 
                 85% O 2 ,  
                   
                 3.0 
                 2.4 
               
               
                   
                 15% CF 4   
               
               
                   
               
             
          
         
       
     
         [0132]    Table 3 shows the measurement of two adhered dry polyimide films and two adhered polyimide films kept in saline solution for 7 days at 87° C. 
       Example 3 
       [0133]    Example 3 was carried out according to example 1 with the difference that the base polyimide surface layer  71   a  was activated by deimidization. The surface was subsequently treated in KOH-deimidization bath at 25° C. for 5 min with manual agitation of the carrier boat at least every 60 s. The surface was first rinsed in a lower cascade rinse bath for 60 s, in a middle cascade rinse bath for 60 s, and finally in a bubbler cascade rinse bath for 60 s. The surface was dried with filtered nitrogen. The surface was then treated in an HCl-deimidization bath at 25° C. for 5 min with manual agitation of the carrier boat at least every 60 s. The surface was first rinsed in a lower cascade rinse bath for 60 s, in a middle cascade rinse bath for 60 s, and finally in a bubbler cascade rinse bath for 60 s. The deimidization process is shown in the following table 2 and the RIE was omitted. The adhesion strength between the base polyimide layer  71   a  and the top polyimide layer  71   b  is shown in table 4. Table 4 shows the measurement of two adhered dry polyimide films (DRY) and two adhered polyimide films kept in saline solution for 7 days at 87° C. (WET). 
         [0000]    
       
         
               
             
               
               
               
               
               
             
           
               
                 TABLE 4 
               
             
             
               
                   
               
               
                 Adhesion Strength 
               
             
          
           
               
                   
                   
                   
                 Polyimide - 
                   
               
               
                   
                   
                   
                 Polyimide Adhesion 
                 Polyimide - Polyimide 
               
               
                   
                   
                   
                 Strength [N] 
                 Adhesion Strength [N] 
               
               
                 Ex 
                 RIE 
                 Deimidization 
                 DRY 
                 WET 
               
               
                   
               
               
                 1 
                   
                 KOH, HCl 
                 2.1 
                 2.0 
               
               
                   
               
             
          
         
       
     
         [0134]    While the invention has been described by means of specific embodiments and applications thereof, it is understood that numerous modifications and variations could be made thereto by those skilled in the art without departing from the spirit and scope of the invention. It is therefore to be understood that within the scope of the claims, the invention may be practiced otherwise than as specifically described herein. 
       Example 4 
       [0135]    Example 4 was carried out according to example 1 with the difference that the polyimide layers were cured at lower temperature as shown in table 5 below. 
         [0136]    The polyimide layer was treated in an initial bake, a soft-bake, for 10 minutes at 100° C. on a hot plate. The polyimide layer was then purged in a nitrogen oven ramp at 4° C./min to 325° C., and held for 30 min at 325° C. and then ramped down. 
         [0137]    The polyimide layers were treated according to example 1 in a soft-bake for 10 minutes at 100° C. on a hot plate. The polyimide layer was then purged in a nitrogen oven ramp at 4° C./min to 375° C., and held for 30 min at 375° C. and then ramped down. 
         [0000]    
       
         
               
             
               
               
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 5 
               
             
             
               
                   
               
               
                 Curing of Polyimide 
               
             
          
           
               
                   
                 Example 1 
                 Example 4 
               
             
          
           
               
                   
                   
                 Time 
                 Temp. 
                 Time 
                 Temp. 
               
               
                 Step 
                 Phase 
                 [h:m] 
                 [° C.] 
                 [h:m] 
                 [° C.] 
               
               
                   
               
             
          
           
               
                 1 
                 Purge 
                 0:30 
                 30 
                 0:30 
                 30 
               
               
                 2 
                 Initial Bake 
                 0:10 
                 100 
                 0:10 
                 100 
               
               
                 3 
                 Ramp up 
                 2:00 
                 375 
                 2:00 
                 325 
               
               
                 4 
                 Dwell 
                 0:20 
                 375 
                 0:20 
                 325 
               
               
                 5 
                 Ramp down 
                 3:00 
                 30 
                 3:00 
                 30 
               
               
                 Damper 
                 ½ open 
               
               
                 N 2  Line Pressure 
                 40-45 psi 
               
               
                 N 2  Throat Flow 
                 20 SCFH 
               
               
                 N 2  Purge Flow 
                 100 SCFH 
               
               
                 N 2  un Flow 
                 30 SCFH 
               
               
                   
               
             
          
         
       
     
         [0138]    The curing parameters according to example 1 lead to a polyimide with a low permeability. The lower temperature applied in example 4 leads to a higher but sufficient permeability. Further, polyimide layer prepared according to example 4 yields higher flexibility which advantageously minimizes the possibility of damaging to the retina.