Elsevier

Carbon

Volume 107, October 2016, Pages 180-189
Carbon

Brazing techniques for the fabrication of biocompatible carbon-based electronic devices

https://doi.org/10.1016/j.carbon.2016.05.047Get rights and content

Abstract

Prototype electronic devices have been critical to the discovery and demonstration of the unique properties of new materials, including composites based on carbon nanotubes (CNT) and graphene. However, these devices are not typically constructed with durability or biocompatibility in mind, relying on conductive polymeric adhesives, mechanical clamps or crimps, or solders for electrical connections. In this paper, two key metallization techniques are presented that employ commercially-available brazing alloys to fabricate electronic devices based on diamond and carbonaceous wires. Investigation of the carbon—alloy interfacial interactions was utilized to guide device fabrication. The interplay of both chemical (“adhesive”) and mechanical (“cohesive”) forces at the interface of different forms of carbon was exploited to fabricate either freestanding or substrate-fixed carbonaceous electronic devices. Elemental analysis in conjunction with scanning electron microscopy of the carbon—alloy interface revealed the chemical nature of the Ag alloy bond and the mechanical nature of the Au alloy bond. Electrical characterization revealed the non-rectifying nature of the carbon—Au alloy interconnects. Finally, electronic devices were fabricated, including a Au circuit structure embedded in a polycrystalline diamond substrate.

Introduction

Owing to their unique combination of biological, mechanical, electrochemical, electrical, thermal, and optical properties, carbon-based materials such as graphene, carbon nanotubes (CNT), carbon fiber (CF), and diamond have attracted significant scientific and industrial interest. Exploiting CNT and graphene properties in micro- and macroscale structures is of significant interest for engineering research and product development [1], [2], [3]. For example, high strength and high conductivity fabrics based on these materials could enable a variety of applications from lightweight body armor to space elevators [4], [5]. Several solutions to the problem of scale have been demonstrated for both graphene and CNT films. Spinning CVD-grown CNT forests into a continuous yarn has enabled conductive, high-strength structures with micro-scale diameters and macro-scale lengths [3]. Control of mechanical strength and electrical/thermal conductivity of CNT yarns is established by adjusting component CNT lengths, as well as the spinning angles of the fibers [6], [7]. Twisted and/or coiled structures of CNT yarns have been used to fabricate electrochemical as well as electrolyte-free CNT torsional actuators (artificial muscles) with large-stroke and high work-capacity [8], [9]. Cell culture dishes containing CNT yarns have shown that neurons grow and respond to electrical stimulation delivered by the yarn [10]. As bulk carriers of electricity and data, CNT yarns have been evaluated as replacements for standard Cu and Al wires due to their light weight (alleviating stress on joint connections), ballistic conduction (lack of scattering reduces risk of Joule heating), and capacity to handle high frequencies [11]. Expanding upon the application of long-distance electricity transfer, CNT yarns have been used to build wireless data transfer networks that were mechanically-resilient and displayed frequency-independent resistive behavior [12]. As a demonstration of their favorable electrochemical characteristics, CNT yarns have been used as an alternative to the standard CF electrochemical sensor for neurotransmitters due to an intrinsic ability to resist surface fouling [13].

All of the aforementioned research, and much that was not mentioned, was performed using prototype devices containing silver epoxy, gold paste, solder, or mechanical clamps to create electrical connections to carbonaceous wires. While these connections served their respective experimental purposes, their use in fabrication does not generally yield scalable or integrated electronic devices, nor are these connection methods resilient or biocompatible enough for consideration as materials for biomedical devices. There have been several methods developed for bonding a single CNT to metals, as well as joining a single CNT to another CNT, end-to-end [14]. However, making electrical connections to larger (micro, rather than nano) CNT composites and yarns is a different challenge. Current approaches include mechanical clamps, Ag- and Au-based epoxy adhesives, carbon solder, ultrasonic welding, and vacuum brazing [15], [16]. A recent, very exciting approach involves use of transition metal soldering alloys to join carbon wires using standard solder conditions (e.g., 350 °C in air) [17]. While it is a technique that will certainly revolutionize the utility of carbonaceous wires, the Cu- and Sn-based alloys are unlikely to display biocompatibility due to the known cytotoxicity of these metals [18], [19]. We have recently developed a technique to create a hermetic diamond capsule using biocompatible gold alloys [20]. Additionally, we have demonstrated a method for the construction of hermetic, biocompatible feedthroughs for a retinal prosthesis device based on conductive nitrogen-doped diamond electrodes [21]. In the present paper, we extend our previous work and describe the construction of diamond-based electronic circuit substrates with embedded Ag- or Au-based interconnects and soldering pads. Finally, we describe a method to metallize the aforementioned carbonaceous wires (CNT yarn, CF, and graphene fiber), with the option to incorporate them into diamond circuit substrates. The methods we describe constitute an important toolkit to allow for CNT-based and other similar carbonaceous “super materials” to be incorporated into electronic devices and integrated with traditional surface-mount electronics, with specific focus on biocompatible systems for implantation into the body.

Active brazing, a variation of vacuum brazing, utilizes an alloy containing an active component that reacts chemically with a relatively inert surface, such as diamond or ceramics [22], [23]. Transition metals are of particular interest due to electron vacancies in their d-orbitals, with greater numbers of vacancies leading to greater reactivity (overlap) with carbon’s p-orbitals [24]. In the case of carbon substrates, the active component—typically Ti, V, or Cr—forms a carbide interface layer that acts as a surfactant to enable the wetting of other filler metals and produce non-rectifying electrical contacts [25], [26]. A Ag-based active brazing alloy (ABA) has been used previously to make electrical contact to a CNT bundle, and formation of the TiC interface layer was confirmed with X-ray photon scattering (XPS), though the electrical and mechanical properties were not fully characterized [16]. The formation of interfacial TiC particles has also been shown to significantly increase the tensile strength of carbon composites brazed with Ti-based alloys containing CNT reinforcements [27]. In the present work, commercially-available ABAs were investigated to determine their suitability to make electronic connections to several carbonaceous materials, including polycrystalline diamond (PCD), CNT yarns, graphene oxide (GO) fibers, and PAN-based carbon fiber (CF) to enable various types of electronic devices (Fig. 1).

In the first technique described here, ABAs were used to create embedded circuit boards in PCD, wherein the solidified ABA forms the interconnects and contact pads following mechanical polishing. The second metallization system involves making free-standing Au contacts to carbonaceous fibers using a graphite “lift-off” method which exploits the balance of cohesive and adhesive forces of the liquid metal/graphite interface (Equation (1)).cosθ=WAσL1

Where θ is the contact angle between the substrate and liquid metal, WA is the work of adhesion (adhesion force) between the substrate and the liquid metal, and σL is the cohesion force (liquid-vapor surface tension) within the liquid metal droplet [28]. Equation (1) describes the balance of forces at the liquid metal/solid substrate interface, wherein liquid metals (intrinsically very high cohesion force materials) tend to create large contact angles on most substrates unless σL < WA, where contact angles <90° are considered “wetting”. In one facet of this work, we use active brazing to modify WA and create robust electrical connections to carbonaceous materials such as PCD and CF. In another technique, we exploit the lack of adhesion and wetting between graphitic carbon and Au, as well as the cohesion forces within the liquid metal droplet, to create a freestanding carbonaceous wire with metallic connections. Broadly speaking, the techniques described here are applicable to the development of devices in which a strong, biocompatible, and non-rectifying connection to carbon-based materials is required.

Section snippets

Sample preparation and wettability studies

Thermal grade polycrystalline diamond (PCD) substrates (TM-250, Element Six) with a thickness of 250 μm were laser-cut into 5 × 5 mm squares with a 532 nm Nd:YAG laser (Oxford Lasers). Approximately 6.5 mg of either TiCuNi, Ag ABA, or Au ABA paste were placed ontop of either the PCD substrate, the PCD substrate with a carbonaceous wire, or a graphite substrate with a carbonaceous wire. The elemental composition and melting points of all braze alloys tested is listed in Table 1.

Samples were

Investigating PCD-braze interactions

Three different commercially-available active brazing alloy (ABA) pastes were assessed for their ability to wet and bond to polycrystalline diamond (PCD) substrates. The elemental composition of these brazes is included in Table 1 of the Experimental section. Ag ABA was found to spread and adhere strongly to PCD, which was likely facilitated by the formation of an interfacial TiC layer as has been observed previously for Ti films on diamond [22] (Fig. 6a). SEM and EDX elemental analysis of the

Conclusion

Two key methods for fabricating electronic microdevices with PCD and carbonaceous wires (CNT yarns, CF bundles, and rLCGO fibers) have been described, with biocompatibility of all components paramount. For PCD electronics, Ag ABA is used due to its wetting and covalent bonding to diamond via reactive formation of TiC, though Ag is susceptible to corrosion in physiological saline [46]. Au ABA can be used following creation of an Ag ABA interfacial layer, as it does not wet PCD. Our group has

Acknowledgements

Many thanks to David Thomas for assistance with Micro-CT imaging and Owen Burns for insightful technical conversations. NVA is supported by a MMI-CSIRO Material Science PhD Scholarship. DJG is supported by ARC DECRA grant DE130100922.This research was supported by the Australian Research Council (ARC) through its Special Research Initiative (SRI) in Bionic Vision Science and Technology grant to Bionic Vision Australia (BVA). JF is supported by the Australian Research Council under Discovery

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