micromachines Review Techniques and Considerations in the Microfabrication of Parylene C Microelectromechanical Systems Jessica Ortigoza-Diaz 1, Kee Scholten 1 ID , Christopher Larson 1 ID , Angelica Cobo 1, Trevor Hudson 1, James Yoo 1 ID , Alex Baldwin 1 ID , Ahuva Weltman Hirschberg 1 and Ellis Meng 1,2,* 1 Department of Biomedical Engineering, University of Southern California, Los Angeles, CA 90089, USA; [email protected] (J.O.-D.); [email protected] (K.S.); [email protected] (C.L.); [email protected] (A.C.); [email protected] (T.H.); [email protected] (J.Y.); [email protected] (A.B.); [email protected] (A.W.H.) 2 Ming Hsieh Department of Electrical Engineering, University of Southern California, Los Angeles, CA 90089, USA * Correspondence: [email protected]; Tel.: +1-213-740-6952 Received: 31 July 2018; Accepted: 18 August 2018; Published: 22 August 2018 Abstract: Parylene C is a promising material for constructing flexible, biocompatible and corrosion- resistant microelectromechanical systems (MEMS) devices. Historically, Parylene C has been employed as an encapsulation material for medical implants, such as stents and pacemakers, due to its strong barrier properties and biocompatibility. In the past few decades, the adaptation of planar microfabrication processes to thin film Parylene C has encouraged its use as an insulator, structural and substrate material for MEMS and other microelectronic devices. However, Parylene C presents unique challenges during microfabrication and during use with liquids, especially for flexible, thin film electronic devices. In particular, the flexibility and low thermal budget of Parylene C require modification of the fabrication techniques inherited from silicon MEMS, and poor adhesion at Parylene-Parylene and Parylene-metal interfaces causes device failure under prolonged use in wet environments. Here, we discuss in detail the promises and challenges inherent to Parylene C and present our experience in developing thin-film Parylene MEMS devices. Keywords: Parylene; microfabrication; MEMS 1. Introduction 1.1. History and Types of Parylene Parylene is the trade name for poly-(para-xylylene), a class of semicrystalline, hydrophobic polymers which can be deposited as thin, conformal, pinhole-free films using chemical vapor deposition (CVD). Parylene was discovered by Michael Mojzesz Szwarc in the late 1940s but was not commercialized until 1965 following the development of the Gorham deposition process at Union Carbide [1,2]. Types of Parylene that are commonly available include Parylene N, Parylene C, Parylene D and Parylene HT (Figure1). A comparison of the properties of these films can be found in [ 3]. Parylene N is composed of an aromatic ring with attached methylene groups [4]; it has been used as a dielectric and has the slowest deposition rate of the Parylenes [5]. Parylene C features a single chlorine atom on its benzene ring and is recognized for its chemical inertness, electrical resistivity, low moisture permeability and proven biocompatibility. Parylene D has two chlorine atoms, resulting in similar properties to Parylene C with slightly higher temperature resistance. Parylene HT is a fluorinated Micromachines 2018, 9, 422; doi:10.3390/mi9090422 www.mdpi.com/journal/micromachines Micromachines 2018, 9, 422 2 of 25 Micromachines 2018, 9, x FOR PEER REVIEW 2 of 25 variant of the Parylene N polymer and is marked by high temperature stability [6]. The two most stabilitycommon [6]. Parylene The two types most in common commercial Parylene applications types in arecommercial Parylene applications N and C, whereas are Parylene Parylene N Cand is theC, whereasmost common Parylene material C is the encountered most common in MEMS material devices. encountered in MEMS devices. FigureFigure 1. 1. ChemicalChemical structure structure of of Parylene Parylene types. types. ParyleneParylene C, C, referred referred from from here here on on as as Parylene, Parylene, is is a a US US Pharmacopeia Pharmacopeia class class VI VI material material [7], [7], having having thethe highest highest biocompatibility biocompatibility certification certification for for a a plas plastictic material. material. For For several several decades, decades, thin thin Parylene Parylene coatingscoatings were were used used to to create create waterproof waterproof insula insulationtion for for electronics electronics inte intendednded for for use use in in harsh harsh environments,environments, a a category category that that increasingly increasingly includ includeses biomedical biomedical implants. implants. Parylene Parylene exhibits exhibits low low intrinsicintrinsic stress stress inin additionaddition to to optical optical transparency, transparency, mechanical mechanical flexibility flexibility and compatibilityand compatibility with several with severalstandard standard micromachining micromachining processes processes [8–11]. [8–11]. While While Parylene Parylene can be can used be used in combination in combination with with rigid rigidsubstrates substrates such such as silicon as silicon and glass, and glass, it has beenit has increasingly been increasingly utilized utilized as a flexible as a structuralflexible structural material materialin the growing in the field growing of polymer-based field of polymer-based biomedical microelectromechanical biomedical microelectromechanical systems (bioMEMS). systems As a (bioMEMS).dynamic polymer As a material,dynamic Parylenepolymer presents material, unique Parylene challenges presents during unique microfabrication challenges andduring use microfabricationwhich are further and explored use which here. are further explored here. 1.2.1.2. Thin-Film Thin-Film Parylene Parylene Device Device Microfabrication Microfabrication Parylene-basedParylene-based bioMEMS bioMEMS are preparedprepared throughthrough a a combination combination of of microfabrication microfabrication processes processes and andcommonly commonly consist consist of a simpleof a simple sandwich sandwich design, design, with a with base layera base of layer Parylene, of Parylene, a thin layer a thin of patternedlayer of patternedmetal defining metal tracesdefining and traces other and components, other componen and a topts, and layer a top of Parylene. layer of Parylene. Figure2 depicts Figure 2 a depicts flexible adevice flexible in device which Parylenein which servesParylene as bothserves the as structural both the materialstructural and material insulator. and However, insulator. devices However, may devicesalso be may supported also be on supported standard rigidon standard substrates rigid such substrates as glass andsuch silicon as glass when and flexibilitysilicon when is not flexibility required. isRegardless not required. of the Regardless final format, of the microfabrication final format, microfabrication requires that Parylene requires be supportedthat Parylene by rigidbe supported substrate by rigid substrate during fabrication, with optional release of a free film device towards the end of duringMicromachines fabrication, 2018, 9, x withFOR PEER optional REVIEW release of a free film device towards the end of the process. 3 of 25 the process. Devices are realized using a combination of three process categories: additive processes (deposition), subtractive processes (etching) and patterning (e.g., photolithography). Parylene is deposited exclusively through a highly conformal CVD process at room temperature, with typical deposition rates on the order of ~2 μm/h. Other deposition methods are critical for polymer MEMS processing; spin coating is used to deposit photo-patternable polymer layers (photoresist) for use as an etch mask or shadow mask, or as a sacrificial layer, while physical vapor deposition (PVD) methods including evaporation and sputtering are used for metal deposition. Etching processes are limited to dry methods due to Parylene’s high chemical inertness. Reactive ion etching (RIE) and deep reactive ion etching (DRIE) are oxygen plasma-based techniques commonly used to etch Parylene.Figure Parylene 2. Cross devices section areof a predominantlytypical device showing patterned insulated using and standard exposed UV metal lithography features, suchprocesses, as Figure 2. Cross section of a typical device showing insulated and exposed metal features, such as traces and, moretraces andrecently, electrodes, nanoscale respectively. features can be patterned using electron beam lithography with appropriateand electrodes, protective respectively. layers [12] to prevent beam damage. 1.3. State-of-the-Art Parylene-Based Devices Devices are realized using a combination of three process categories: additive processes (deposition),Parylene subtractiveis used extensively processes to coat (etching) printed and circuit patterning boards, (e.g.,wires, photolithography). MEMS devices, and Parylenebiomedical is depositedimplants, and exclusively less commonly through as a a highly structural conformal or substrate CVD processmaterial at for room electronic temperature, and MEMS with devices. typical depositionStill, over the rates last on several the order years, of many ~2 µm/h. devices Other for depositiona large variety methods of applications are critical have for polymer been described MEMS processing;in the literature spin coatingfeaturing is usedParylene to deposit as either photo-patternable the structural or polymer substrate layers material (photoresist) (Figure for3). useParylene as an has been used as a flexible coiled cable to connect medical implants to external circuitry [13–15].