Th2E.004
FABRICATION OF POLYCRYSTALLINE DIAMOND ON A FLEXIBLE PARYLENE SUBSTRATE Bin Fan1, Robert Rechenberg2, Michael F. Becker2 and Wen Li1 1 Michigan State University, East Lansing, MI, USA 2 Fraunhofer USA-CCD, East Lansing, MI, USA ABSTRACT
which causes sensor deterioration over time. However, BDD is typically grown at high temperatures (500-900°C) on temperature insensitive materials such as metals and silicon[8]. These high temperature synthesis conditions do not allow BDD to be grown directly onto polymers and alternative fabrication methods need to be implemented to combine BDD with a flexible polymer. Hess et.al. reported on diamond-on-polymer electrode arrays[9]. Selectively grown diamond electrodes were transferred onto a spincoated polynorbornene-based polymer utilizing a chemical transfer process. Bergonzo et.al. reported on flexible diamond microelectrode arrays for eye implant applications[10]. Selectively grown diamond was coated with polyimide and released from a silicon wafer substrate. However, the method mentioned above only showed transferring small diamond microelectrodes from silicon wafer. The interconnects and contact pads were made from patterned metal separately. In this paper, an alternative fabrication method is discussed to transfer patterned diamond devices, including interconnects and contact pads, from a silicon wafer to a flexible Parylene-C substrate. Parylene-C is an biocompatible, transparent, flexible, room-temperaturedeposited polymer, which has been reported as moisture and dielectric barriers for implanted devices[11]. In addition, 6 Parylene-C has Young’s modulus (~10 Pa [12]) closer to tissue and is widely used as a structural and packaging polymer for many implantable devices. The conceptual diagram of the proposed method is shown in Figure 1. Three steps include: Pre-substrate transfer process (Pre-STP), substrate transfer process (STP) and post-substrate transfer process (Post-STP). Firstly, during the Pre-STP (1 (a)), silicon dioxide (SiO2) was first grown on a silicon wafer, followed by BDD growing and patterning on the silicon substrate. Then, SiO2 was etched and undercut to form anchors. A mesh structure was introduced to form more anchors under contact pads. Next, Parylene-C was conformally coated, which will wrap and hold those anchors during the wafer transfer step. Secondly, during the STP (1(b)), a special jig made of Teflon was used to protect the patterned surface and only expose backside of the silicon wafer through the opening of the lid. The jig along with the wafer was immersed in KOH to etch silicon completely. Finally, during Post-STP (1(c)), BDD on Parylene was then attached onto a silicon carrier wafer using photoresist, followed by metallization, patterning of the contact pads for µLED assembly, and wiring the interconnects.
This paper reports a novel fabrication process of transferring polycrystalline diamond (PCD) from a 3-inch silicon wafer onto a flexible Parylene-C substrate. Combining Parylene-C with PCD can improve the mechanical flexibility while preserving the benefits of PCD (biocompatibility, chemical inertness, high thermal conductivity and feasibility of being selectively doped to be a semiconductor or insulator). This fabrication process breaks the barrier that PCD cannot grow on flexible polymeric substrate due to high temperature processes (500900ºC), which exceeds the melting point of most flexible polymers. As a demonstration of the technique, we transferred boron-doped polycrystalline diamond (BDD) on a Parylene-C substrate and assembled µLEDs on BDD for a potential application in optogenetics. Electrical and optical characteristics of a fabricated device were investigated and discussed in the paper.
KEYWORDS Flexible, Boron Doped Polycrystalline Diamond, Parylene-C.
INTRODUCTION Boron doped polycrystalline diamond (BDD) exhibits a unique combination of properties including making it a promising material for implantable neural stimulation and recording devices. Among the favorable properties are high thermal conductivity, low background current response, large electrochemical potential window in aqueous solutions, chemical inertness, high resistance to surface fouling and biocompatibility[1][2]. So far, BDD is predominantly used for chemical sensing. For example, highly boron doped diamond deposited on tungsten and platinum wires has been used for in vivo detection of dopamine (DA) in the mouse brain [3] and in vitro for the detection of serotonin release from the guinea pig mucosa [4], respectively. A BDD probe has been reported to be implanted in the auditory cortex area of a guinea pig brain for in vivo neural recording [5]. Since BDD is also a hard and rigid material, for specific application such as implanted biomedical devices, the mechanical property mismatch between the rigid PCD (with 9 Young’s modulus of ~10 Pa [6]) and brain tissue (with 3 5 Young’s modulus ranging from ~10 to 10 Pa [7]) may cause tissue irritation and damage for chronic applications. The same requirement applies to wearable sensors since a rigid material cannot have a conformal contact to skin,
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Figure 1: Concept diagram of the proposed method for transferring polycrystalline diamond from a silicon substrate onto a mechanical flexible substrate. first evaporated (Auto 306, Edward, Inc) and patterned
FABRICATION METHOD Figure 2 shows a detailed process flow. (a1) one micron SiO2 was coated on a 3 inch Si wafer using plasma enhanced chemical vapor deposition (PECVD) (PlasmaLab 80plus®, Oxford Instruments). (a2) A ~2.7µm microcrystalline BDD film was grown using a customdesigned microwave plasma assisted chemical vapor deposition reactor (MWPACVD) (Lambda Technologies, Inc.) with a gas mixture of hydrogen-diborane and methane. (a3) A 1.3 µm thick aluminum hard mask for diamond etching was deposited (Denton Desk Top Pro Sputtering System, Denton Vacuum, Inc) and (a4) patterned via photolighograhpy, with small holes on the contact pads for undercutting SiO2 in Step-a6. (a5) BDD was plasma etched in an electron cyclotron resonance reactive ion etcher (Lambda Technologies, Inc.) using SF6/Ar/O2 as processing gases. (a6) SiO2 was over-etched in buffered oxide etchant (BOE) to create undercuts for forming Parylene anchors in Step-b1. (b1) The wafer was treated with the Silane A174 adhesion promoter (Sigma Aldrich, Inc), followed by conformal coating of ~15µm Parylene-C (PDS 2010, Specialty Coating System, Inc) partially wrapping around the BDD structures through the SiO2 undercuts. (b2) The backside of the Si wafer was etched in 35% potassium hydroxide (KOH) at 70ºC for ~9hrs using the custom-made etching jig. After the BDD structures were successfully transferred onto a Parylene-C substrate, the BDD/ParyleneC film was attached onto a carrier wafer coated with photoresist for subsequent steps. A BDD-based µLED probe was designed and fabricated to be used for a potential application in optogenetic neuromodulation. In this case, (c1) Ti/Cu was
Figure 2: Fabrication process of making a BDD device on a flexible Parylene-C substrate. (ABM, Inc) to form contact pads onto the nucleation side of the BDD film. (c2) After applying low melting point (LMP) solder (62 ºC, 144 ALLOY Field’s Metal) onto the contact pads in an acidic solution, (c3) µLEDs (Samsung, Inc) were self-assembled on the pads[13]. (c4) The probe was patterned, released from the carrier wafer with acetone, and rinsed with isopropanol and deionized water. Flexible wires
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Figure 3: Etching jig before (a) and after (b) assembled. (c) BDD on a flexible Parylene-C thin film after removing the Si substrate via KOH etching. (d) A close-up view of pads after KOH etching, the dimension of the contact pads is 1.2mm×0.9mm (without area 2) and 1.8mm×0.9mm(with area 2). (e) A close-up view of pads after metal deposition and patterning. (f) A fabricated LED probe showing its mechanical flexibility. (g) The LED probe is powered up, demonstrating the integrity of the conductive BDD leads and contacts. Hitachi, Inc), which indicates Si and O2 elements on the surface of BDD, as shown in Figure 5 (a). This causes the high contact resistance between BDD pads and µLEDs,
were soldered onto the pads using LMP. Epoxy was applied to strengthen the bonding between the wires and the pads. Finally, another layer of 5µm Parylene-C was deposited to encapsulate the device.
DEVICE CHARACTERISTICS Figure 3(a) and (b) show the custom-designed etching jig. A KOH resistant O-ring was used to achieve a good seal between etching jig and wafer with extra force from chromium coated c-clamp compressors. Figure 3(c) shows fabricated BDD structures transferred on a transparent, flexible Parylene-C substrate. Figure 3(d) and (e) show a close-up view of pads after KOH etching and subsequent metal deposition and patterning. The mesh structures (area 1 in 3(d)) are used to form Parylene-C anchors, which will help to keep BDD on Parylene-C during the wafer transfer step. Compared to their adjacent areas where no mesh structures present(area 2 in 3(d)), the area with mesh structures have a better adhesion to the Parylene-C substrate. The reason of using metals on the contact pads for interconnects and µLED assembly is that the LMP solder does not stick to BDD. The metal on the contact pads can be replaced with silver epoxy for prototyping in the future but still need metal for µLED connection, where it is used for µLED self-assembling process as discussed in[14]. Figure 3 (f) shows the flexibility of BDD on Parylene-C substrate, where LMP was applied at both the contact pads for wiring and for µLED. Figure 3(g) shows a prototype of a flexible BDD-based µLED probe.
Figure 4: (a) The I-V curve of a fabricated BDD probe. (b) The light intensity of the assembled µLED probe driven with different input voltages.
The current-voltage property and light intensity of the probe were characterized in Figure 4, using a semiconductor parameter analyzer (Hewlett Packard, Inc), and a digital power meter (Model 815 Series, Newport, Inc) through a RHA 2000 evaluation board (Intan Technologies, Inc). The conductivity of the BDD was 1.69×10-3Ω·cm. Compared with the data reported previously in[13], the threshold of the µLED turn-on voltage is much higher and the light intensity of the µLED is much lower. Further investigation using a microscope shows the surface color of the BDD is pinkish, which is a typical color for SiO2 residues. This is confirmed by energy dispersive X-ray spectrometry (EDS) (Hitachi S4700II Field Emission Scanning Electron Microscope,
Figure 5: (a) Energy dispersive X-ray spectrum suggests SiO2 residues left at the BDD/LED interface. (b) An image taken at the backside of the BDD trace of the probe, showing the surface morphology of the BDD.
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and leads to the low light emitting efficiency. In addition, the contact resistance between BDD to the metal layer and conductivity of BDD can also be the reasons, which needs further investigation. Figure 5(b) shows an image taken from the backside of the BDD trace of the probe, showing the surface morphology of the polycrystalline diamond. The average surface roughness was measured to be around 23.66nm. The Parylene anchors coated in the undercut SiO2 was also observed along on the edges of the BDD patterns.
[4] H. Zhao, X. Bian, J. J. Galligan, and G. M. Swain, “Electrochemical measurements of serotonin (5-HT) release from the guinea pig mucosa using continuous amperometry with a boron-doped diamond microelectrode,” Diam. Relat. Mater., vol. 19, no. 2–3, pp. 182–185, Feb. 2010. [5] H.-Y. Chan, D. M. Aslam, J. A. Wiler, and B. Casey, “A novel diamond microprobe for neuro-hemical and electrical Recording in neural prosthesis,” J. Microelectromechanical Syst., vol. 18, no. 3, pp. 511– 521, Jun. 2009. [6] Advanced Diamond Technology, “The CVD diamond booklet.” [Online]. Available: http://www.diamondmaterials.com/downloads/cvd_diamond_booklet.pdf. [7] P. Schiavone, F. Chassat, T. Boudou, E. Promayon, F. Valdivia, and Y. Payan, “In vivo measurement of human brain elasticity using a light aspiration device,” Med. Image Anal., vol. 13, no. 4, pp. 673–678, Aug. 2009. [8] R. Ramamurti, M. Becker, T. Schuelke, T. Grotjohn, D. Reinhard, G. Swain, and J. Asmussen, “Boron doped diamond deposited by microwave plasma-assisted CVD at low and high pressures,” Diam. Relat. Mater., vol. 17, no. 4–5, pp. 481–485, Apr. 2008. [9] A. E. Hess, D. M. Sabens, H. B. Martin, and C. A. Zorman, “Diamond-on-polymer microelectrode arrays fabricated using a chemical release transfer process,” J. Microelectromechanical Syst., vol. 20, no. 4, pp. 867– 875, Aug. 2011. [10] P. Bergonzo, A. Bongrain, E. Scorsone, A. Bendali, L. Rousseau, G. Lissorgues, P. Mailley, Y. Li, T. Kauffmann, F. Goy, B. Yvert, J. A. Sahel, and S. Picaud, “3D shaped mechanically flexible diamond microelectrode arrays for eye implant applications: The MEDINAS project,” IRBM, vol. 32, no. 2, pp. 91–94, Apr. 2011. [11] E. M. Schmidt, J. S. Mcintosh, and M. J. Bak, “Longterm implants of Parylene-C coated microelectrodes,” Med. Biol. Eng. Comput., vol. 26, no. 1, pp. 96–101, Jan. 1988. [12] “Parylene specifications.” [Online]. Available: http://vsiparylene.com/pdf/ParyleneProperties2013.pdf. [13] B. Fan, K. Y. Kwon, A. J. Weber, and W. Li, “An implantable, miniaturized SU-8 optical probe for optogenetics-based deep brain stimulation,” in 2014 36th Annual International Conference of the IEEE Engineering in Medicine and Biology Society (EMBC), 2014, pp. 450–453. [14] B. Fan, K.-Y. Kwon, R. Rechenberg, A. Khomenko, M. Haq, M. F. Becker, A. J. Weber, and W. Li, “A polycrystalline diamond-based, hybrid neural interfacing probe for optogenetics,” in 2015 28th IEEE International Conference on Micro Electro Mechanical Systems (MEMS), 2015, pp. 616–619.
CONCLUSION In summary, we reported a novel method for fabricating BDD on flexible Parylene-C thin films by transferring BDD from a silicon wafer to a Parylene-C substrate. The flexible Parylene-C, with a Young’s modulus matches to that of soft tissues, can significantly reduce the tissue damage and irritation for chronic implantation. In addition, the superior thermal property of BDD could be used as a heat spreader to dissipate localized heat rapidly to surrounding tissues during optical stimulation where µLEDs are used as light sources, as reported previously in[14]. Furthermore, our proposed BDD has the capability of being used as recording and electrical stimulation electrodes due to its chemical stability and biocompatibility. In addition to neural interfacing applications, the flexibility of the Parylene-C substrate and the low background signal level, wide potential window, chemical stability of the BDD make it a good candidate for wearable electrochemical sensors. After the fabrication process was demonstrated, a prototype of µLED assembled optical neural probes for optogenetics was designed, constructed and characterized based on the proposed wafer transfer technology.
ACKNOWLEDGEMENTS This work was supported by the National Science Foundation under the Award Numbers CBET-1264772 and ECCS-1407880. The authors would like to thank Brain Wright for Aluminum deposition, Dr. Baokang Bi and Karl Dersch for help on microfabrication.
REFERENCES [1] M. Hupert, A. Muck, J. Wang, J. Stotter, Z. Cvackova, S. Haymond, Y. Show, and G. M. Swain, “Conductive diamond thin-films in electrochemistry,” Diam. Relat. Mater., vol. 12, no. 10–11, pp. 1940–1949, Oct. 2003. [2] A. Kraft, “Doped diamond: a compact review on a new, versatile electrode material,” Int J Electrochem Sci, vol. 2, pp. 355–385, 2007. [3] A. Suzuki, T. A. Ivandini, K. Yoshimi, A. Fujishima, G. Oyama, T. Nakazato, N. Hattori, S. Kitazawa, and Y. Einaga, “Fabrication, characterization, and application of boron-doped diamond microelectrodes for in Vivo dopamine detection,” Anal. Chem., vol. 79, no. 22, pp. 8608–8615, Nov. 2007.
CONTACT: Bin Fan, tel:+1-517-355-3299;
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