CONFORMALLY ENCAPSULATED MULTI-ELECTRODE ARRAYS WITH SEAMLESS INSULATION
Thin-film multi-electrode arrays (MEA) having one or more electrically conductive beams conformally encapsulated in a seamless block of electrically insulating material, and methods of fabricating such MEAs using reproducible, microfabrication processes. One or more electrically conductive traces are formed on scaffold material that is subsequently removed to suspend the traces over a substrate by support portions of the trace beam in contact with the substrate. By encapsulating the suspended traces, either individually or together, with a single continuous layer of an electrically insulating material, a seamless block of electrically insulating material is formed that conforms to the shape of the trace beam structure, including any trace backings which provide suspension support. Electrical contacts, electrodes, or leads of the traces are exposed from the encapsulated trace beam structure by removing the substrate.
This application is a divisional of U.S. application Ser. No. 13/792,708, filed Mar. 11, 2013, which claims the benefit and priority of U.S. Provisional Application No. 61/661,751, filed on Jun. 19, 2012, both of which are hereby incorporated by reference.
FEDERALLY SPONSORED RESEARCH OR DEVELOPMENTThe United States Government has rights in this invention pursuant to Contract No. DE-AC52-07NA27344 between the United States Department of Energy and Lawrence Livermore National Security, LLC for the operation of Lawrence Livermore National Laboratory.
FIELD OF THE INVENTIONThe present invention relates to multi-electrode arrays (MEA) and fabrication methods thereof, and more particularly to a conformally encapsulated MEA and method of fabricating the MEA while in a suspended state over a substrate to conformally encapsulate the MEA in a seamless block of insulating material.
BACKGROUND OF THE INVENTIONNeural probes and interfaces are an essential tool in neuroscience. They typically comprise a multi-electrode array (MEA) configuration with exposed metal pads or electrodes located on rigid silicon shanks and connected, via interconnection traces, to output leads or to signal processing circuitry on a monolithic substrate. The exposed metal pads/electrodes provide a direct electrical interface with the neurons of a biological entity's nervous system to stimulate and/or record neural activity. Such neural probes enable researchers and clinicians to better explore and understand neurological diseases, neural coding, neural modulations, and neural topologies, as well as treat debilitating conditions of the nervous system. Moreover, implantable neural probes and interfaces in particular enable extended interaction with neural tissue. However, such probes and devices typically require invasive surgeries for implantation, and also often require additional surgeries to remove and replace devices that have failed or otherwise require maintenance or servicing. The medical industry continues to search for methods to improve patient comfort and device reliability, including developing methods to reduce the size and extend the lifetime of chronically implanted devices.
Standard polymer-based MEAs are made using multiple layers of polymers coated layer by layer (typically by spin-coating) after each metal film deposition and patterning, to insulate the patterned conductive wiring and lines. However, the multiple polymer-polymer and polymer-metal interfaces provide opportunities for water and solution infiltration leading to delamination and separation of the polymer layers. Penetration of bodily fluids at these compromised areas can result in trace metal corrosion, electrical shorts between interconnects, and ultimately device failure. Therefore, maintaining adequate adhesion between adjacent layers of polymer and polymer-metal layers is a consistent concern using this approach, and one of the reasons thin-film microelectrode arrays have been unsuccessful for long term implantation.
Previous attempts to solve the delamination problem for such polymer-based MEAs have included various polymer treatment methods to alter the physical or chemical properties of the polymer layers to improve adhesion therebetween. However, these methods have largely failed by either not providing sufficient improvements in adhesion, or the polymer treatment parameters required for effective adhesion is detrimental to the metal thin films. What is needed therefore is a durably insulated polymer-based MEA, and a method of fabricating such MEAs using reproducible microfabrication processes which extends operational lifetime by reducing the number and area of interfaces of the MEA that can be infiltrated and thereby reducing the opportunities for failure.
SUMMARY OF THE INVENTIONOne aspect of the present invention includes a method of fabricating an electrode array, comprising: suspending at least one thin-film electrically conductive trace beam(s) over a substrate, wherein the trace beam(s) is suspended by at least one support portion(s) thereof in contact with the substrate; encapsulating the trace beam(s) with an electrically insulating material; and removing the substrate to expose an electrically conductive surface of the support portion(s) of the encapsulated trace beam(s).
Another aspect of the present invention includes an electrode array comprising: at least one thin-film electrically conductive trace beam(s) encapsulated in a seamless block of electrically insulating material except an electrically conductive surface of at least one support portion(s) of the encapsulated trace beam(s) is exposed through the electrically insulating material.
Generally, the present invention is directed to multi-electrode arrays (MEA) (e.g. microelectrode arrays with micron-sized features) with one or more electrically conductive beams conformally encapsulated in a seamless block of electrically insulating material, and methods of fabricating such MEAs using reproducible, high throughput, thin-film micro-fabrication processes. The fabrication method may be utilized to make MEA devices in a variety of structures, e.g., device shapes, number of metal layers, number of electrodes. For example, the encapsulated MEA device may be used as neural interfaces for a variety of applications in peripheral and cortical nerve stimulation and recording, including as a chronic, fully-implanted neural interface capable of extended operational lifetime due to reduced modes of failure through interface delamination.
In particular, the fabrication method generally includes first suspending one or more electrically conductive trace beams over a substrate by support portions of the trace beam in contact with the substrate. Suspension of the trace beam may be performed in various ways. In one example embodiment, the trace beam or beams may be suspended by selectively removing the underlying material, i.e. scaffold material upon which the trace beam is formed, from underneath the trace beams while the support portion or portions of the trace beams remain in contact with the substrate. In particular, the scaffold material may be a portion, e.g. a sacrificial layer, of the substrate upon which a trace beam is formed. In this manner, selective removal of the underlying material upon which the trace beam is formed, while keeping the support portion or portions of the trace beam in contact with the substrate, leaves behind the trace beam as a free-standing suspended beam that is suspended over the substrate.
To aid in suspension, a trace backing or backings may be formed on a single side or opposite sides of a trace beam, so as to provide single or double-sided suspension support. Such trace backings may be used to suspend single trace beams individually or a stack of trace beams in a multi-tier trace beam stack. Furthermore, the trace backings may be patterned to facilitate encapsulation. The trace backings may be made of, for example, polymers; other insulating materials such as for example ceramics, oxides, nitrides, glasses; or semiconductors such as for example silicon carbide and silicon. And various types of polymers may be used, including for example: polyimides, parylene, polyurethanes, polycarbonates, polymers that are vapor-deposited or spin-on liquids, and shape memory polymers.
It is appreciated that traces are the conductive lines or pathways of electronic circuits which extend and communicate between contact terminals, pads, or electrodes. In the present invention, the trace beam is characterized as a trace having a beam structure that is suitably rigid (either by itself or with reinforcement) to be laterally suspended as a suspension beam, either by a single cantilevered support structure, or multiple support structures. When the trace beam or beams is suspended over the substrate, it may be characterized as having a suspension span portion and a support portion or portions supporting the beam on the substrate upon which the trace beam is formed, similar to a bridge. It is notable that the support portion also forms the electrodes or electrical contacts when exposed in a subsequent step. And the trace beam may be formed and patterned from an electrically conductive layer or film, preferably a thin film metal layer. It is appreciated that various types of materials may be used for the trace beam, including for example: gold, titanium, platinum, iridium, Pt—Ir, Pt-black, iridium-oxide, Ti—Pt alloys, titanium nitride, chrome and aluminum, as well as other metals, conducting polymers, and conductive doped dielectrics. And various deposition methods are possible, including evaporation and or sputtering.
While in the suspended state, the trace beams are encapsulated, either individually or together, with a single continuous layer, film, or coating of an electrically insulating material, such as for example ceramics, oxidized metals, dielectrics, polymers. In particular materials that may be vapor-deposited may be used, such as for example polyimides, parylene, polyurethanes, polycarbonates. Because the trace beam is encapsulated while suspended, the single insulating layer forms a seamless block of electrically insulating material that conforms to the shape of the trace beam structure, including any trace backings which provide suspension support. By encapsulating the conductive traces and electrodes of the array with a single coating of an insulator, multiple-layered polymers are eliminated, along with the associated delamination concerns at the polymer-polymer interfaces/seams and polymer-metal interfaces/seams when exposed to external fluids, thereby reducing opportunities for failure.
One advantage of this method of directly encapsulating the trace structure with a polymer or other insulating material is that the entire surface of the suspended structure can be treated with an adhesion promoting material to enhance adhesion between itself and the encapsulating material. This may involve either treating the surface of bare suspended metal traces, or treating a composite of trace metal and backing material that make up the suspended structure. Adhesion promoters may also be applied in between the trace metal and backing material that make up the suspended structure. For example, an adhesion promoter can be applied to the suspended traces that promotes covalent bonds with a polymer that is cured directly onto the metal. Or, an intermediate metallic layer can be deposited on the traces onto which the encapsulating material will adhere better. Adhesion-promoting materials include metals (e.g. titanium, titanium-nitride, chrome), dielectrics (e.g. silicon dioxide, silicon nitride) or organic and non-organic (e.g. organic and non-organic chemicals with the appropriate chemical groups to adhere the trace and electrode material to the trace backing and the encapsulation polymer) adhesion promoters. The adhesion-promoting layer may be deposited using vapor deposition, spin coating, atomic layer deposition, or oxidation (but not limited to these techniques).
The following are some examples of treatments that can improve adhesion between different layers: (1) the adhesion promoter A-174 silane may be vapor deposited onto the suspended structure to promote adhesion of parylene that is subsequently deposited as the encapsulating material; (2) the trace metal may consist of a stack of metals, with the outer (top and bottom) layers being a metal such as titanium onto which the encapsulating material will adhere better; (3) the liquid adhesion promoter, HD Microsystems VM-652, may be applied on top of the trace metal film to enhance adhesion of a polyimide layer that is subsequently spin-deposited to create a backing layer; (4) in the case of a backing structure that consists of a top and bottom layer of polyimide, the bottom polyimide layer may be treated with KOH followed by HCl in order to create a surface layer of polyamic acid. Then, this surface layer will cure convert back to polyimide) together with the top layer of polyimide, improving adhesion between the two layers.
After encapsulation, electrical contacts, electrodes, or leads of the traces are then exposed from the encapsulated trace beam structure by removing a substrate upon which the support portions of the trace beams are formed and connected. As such the substrate serves as the base foundation upon and over which the electrode array may be suspended, and conformally encapsulated while in a suspended state. And the substrate also serves to isolate/protect a surface of the support portion or portions of the trace beam from being encapsulated by the insulating material, only to expose the electrically conductive surface when it is separated/released from it. The electrically conductive surfaces of the support portions of the trace may be either an electrode site, or contact pads (leads) providing electrical connection to other devices/components.
The accompanying drawings, which are incorporated into and form a part of the disclosure, are as follows:
Turning now to the drawings,
In particular,
In
In
In
And in
In
In
And
In particular,
In
In
In
And
In
As shown in
Next in
And
In particular,
In
And
In
In
While particular operational sequences, materials, temperatures, parameters, and particular embodiments have been described and or illustrated, such are not intended to be limiting. Modifications and changes may become apparent to those skilled in the art, and it is intended that the invention be limited only by the scope of the appended claims.
Claims
1. A method of fabricating an electrode array, comprising:
- suspending at least one thin-film electrically conductive trace beam(s) over a substrate, wherein the trace beam(s) is suspended by at least one support portion(s) thereof in contact with the substrate;
- encapsulating the suspended trace beam(s) in a seamless coating of an electrically insulating material; and
- removing the substrate to expose an electrically conductive surface of the support portion(s) of the encapsulated trace beam(s).
2. The method of claim 1,
- wherein the trace beam(s) is suspended by selectively removing scaffold material upon which the trace beam(s) is formed while the support portion(s) of the trace beam(s) remains in contact with the substrate.
3. The method of claim 2,
- wherein the scaffold material is a portion of the substrate.
4. The method of claim 3,
- wherein the selectively removed portion of the substrate is a sacrificial layer adjacent a release layer of the substrate.
5. The method of claim 4, further comprising:
- forming at least one via opening(s) through the sacrificial layer to the release layer; and
- forming a thin-film electrically conductive layer on the sacrificial layer and in the via opening(s) so that removal of the sacrificial layer forms at least one post(s) of the trace beam(s) which remains in contact with the release layer.
6. The method of claim 4,
- wherein the removing step includes releasing the support portion(s) from the release layer of the substrate.
7. The method of claim 3,
- wherein the portion of the substrate is selectively removed by isotropic etch to form substrate columns in contact with the support portion(s) of the trace beam(s).
8. The method of claim 1,
- further comprising providing at least one trace backing(s) to reinforce the trace beam(s) in suspension, and
- wherein the trace beam(s) and the trace backing(s) together are encapsulated with the electrically insulating material.
9. The method of claim 8,
- wherein the trace backing(s) and multiple trace beams form a multi-tiered trace beam stack that is suspended over the substrate and encapsulated with the electrically insulating material.
10. The method of claim 8,
- further comprising providing an adhesion promoting layer between the trace backing(s) and the electrically insulating material.
11. The method of claim 1,
- wherein the trace beam(s) is encapsulated by vapor deposition.
12. The method of claim 1,
- further comprising joining two or more of the encapsulated trace beams together with an electrically insulating material.
13. The method of claim 1,
- further comprising forming an adhesion promoting layer on the suspended trace beam prior to encapsulation.
14. The method of claim 13,
- wherein the adhesion promoting layer is vapor-deposited on the suspended trace beam prior to encapsulation.
Type: Application
Filed: Oct 19, 2016
Publication Date: Feb 9, 2017
Inventors: Phillipe J. Tabada (Roseville, CA), Satinderpall S. Pannu (Pleasanton, CA), Kedar G. Shah (Oakland, CA), Vanessa Tolosa (Oakland, CA), Angela Tooker (Dublin, CA), Terri Delima (Livermore, CA), Heeral Sheth (Oakland, CA), Sarah Felix (Oakland, CA)
Application Number: 15/298,140