Parylene Encapsulated Sub-micron Structures for Implantable Biomems

We present a method for sub-micron machining of flexible, thin-film structures fully encapsulated in biocompatible polymer poly(chloro-p-xylylene) (Parylene C) that improves feature size and resolution by an order of magnitude compared to prior work. A low temperature electron beam lithography (EBL) process compatible with Parylene-coated silicon substrates was developed, and characterized using patterned Ti structures with critical dimensions down to 250 nm, including conducting traces, serpentine resistors, and electrodes with a nano-patterned texture. Using this newly developed technique, the first flexible, free-film Parylene-Ti-Parylene devices with nanoscale components were fabricated and characterized. One application of these sub-micron structures encapsulated in Parylene is next generation minimally invasive implants. Thus, we also demonstrated a prototype high density Parylene-based microelectrode neural probe using our nanopatterning approach. INTRODUCTION Polymer MEMS on flexible substrates enable novel microdevices ideal for minimally invasive biomedical implants. The typical approach is to employ surface micromachined structures encapsulated in biocompatible polymers such as Parylene C, polyimide, or PDMS. The selection of polymeric substrates over conventional MEMS materials like silicon or glass conveys several advantages: low elastic moduli, high electrical resistivity, chemical resistivity, optical transparency, and mechanical flexibility. In particular, a major motivation for the use of polymers are their mechanical properties, especially for applications involving contact with soft tissue, such as retinal or neural microprobes, where rigid substrates can induce tissue damage due to mechanical mismatch, and subsequently trigger immune responses that impede performance and device lifetime. In recent years several successful examples of implantable polymer bioMEMS have been demonstrated, including implantable neural probes [1, 2], flow sensors, and pressure sensors [3, 4]. Despite the many advantages of polymeric micromachined devices, a major limitation remains the poor feature resolution (> 5μm) achievable with typical micromachining processes that are compatible with polymer substrates. While sub-micron fabrication is routinely employed on conventional MEMS materials, there has been limited work transitioning this capability to soft polymer substrates [5]. Nanoscale patterning has been applied to polymers to create flexible stamps and shadowmasks for fabrication on other substrates [6, 7], but direct sub-micron fabrication on polymer surfaces, or for polymeric biomedical implants remains a technology gap [8]. Existing nanolithography protocols involve exposure of substrate materials to high temperatures and energies that exceed recommended operating conditions for most commonly used MEMS polymers. Methods such as nano-imprint lithography and self-assembly benefit from their highlyparallelized approach to cover large surface areas and have been used to pattern repeated arrays of simple structures on polymers [9, 10]. However, these techniques place restrictions on material choice and are pose difficulties in aligning multi-layer patterns, such as those required for many implantable microdevices. As such, existing polymer MEMS cannot compete with silicon devices on feature density or feature size, placing limits on structures that can be easily produced and the resulting device capabilities. For example, polymer neural microprobes typically contain less than a dozen electrodes for stimulating or recording, whereas the state-of-the-art comprises silicon devices with more than a hundred electrodes per probe [11]. This limitation precludes polymer MEMS with the capabilities, complexity or minimal footprint achievable with conventional materials. We developed a modified electron beam lithography (EBL) protocol that enables fabrication of biocompatible polymer MEMS with sub-micron feature resolution; our design consists of thin, free-film Parylene devices encapsulating titanium structures with feature size as small as 250 nm. This resolution was achieved with minimal optimization and represents a starting point for achievable nanoscale resolution. We describe our fabrication process and present initial electrical and mechanical characterization, showing our devices as functional, flexible and robust. Finally, we present a demonstrative application of this technology: a prototype Parylenebased cortical neural microprobe, with a greater number and density of recording sites than any prior polymer probe. Figure 1: Process flow for sub-micron fabrication on Parylene C: (a) Ti contact pads and traces were fabricated on a vapor deposited Parylene film using conventional contact microlithography, (b) PMMA bilayer (PMMA 495, 950) was coated with Cr (15 nm) then patterned with e-beam, (c) Ti features were deposited with lift-off, (d) features were encapsulated in Parylene contact pads were exposed with plasma etch, and device was released.