Characterization of Parylene-C Thin Films as an Encapsulation for Neural Interfaces

Transcription

1 Characterization of Parylene-C Thin Films as an Encapsulation for Neural Interfaces Hsu, Jui-Mei 1 ; Kammer, Sascha 2 ; Jung, Erik 3 ; Rieth, Loren 4 ; Normann, Richard 5 ; Solzbacher, Florian 1,4,5 1 Materials Science and Engineering - University of Utah 2 IBMT - Fraunhofer Institute 3 IZM - Fraunhofer Institute 4 Electrical and Computer Engineering - University of Utah 5 Bioengineering - University of Utah L. Rieth

2 Microsystems Group Neuroprosthetics Glucose monitoring Wireless Biomedical Devices Harsh-environment MEMS and electronics Gas sensors on microhotplates Materials for harsh environment electronis Characterization of Parylene-C 2 L. Rieth

3 Introduction Characterization of Parylene-C 3 L. Rieth

4 Introduction Wireless Neural Interface Devices Record or stimulate neural action potentials AuSn flip-chip integration Inductive power and forward telemetry 5 mm width; electroplated Au on polyimide; 2.67 MHz INI3 custom signal processor 1000x gain; tuned to neural frequencies; < 10 mw power; FSK transmitter Utah Electrode Array (UEA) 10 x 10 array of electrodes; central and peripheral nervous system designs Encapsulation Electrical isolation; conformality; hermetic; biocompatibility Characterization of Parylene-C 4 L. Rieth

5 Introduction Neuroprosthetics device research: Encapsulation: Parylene-C; a-sic:h; silicone Electrode metallization IrO x by reactive sputter deposition or activation of metallic Ir Improve charge injection Wafer level processing Electrode etching Tip deinsulation Flip-chip integration UEA to ASIC integration Hermetic seals Lid concept Thin film pancake coils Optimized designs Maximum power transfer GLP/GMP fabrication Sputtered Activated 2.E+07 1.E+07 5.E+06 0.E E+06-1.E+07 V vs Ag/AgCl (V) Characterization of Parylene-C 5 L. Rieth

6 Introduction Revolutionizing Prosthetics Develop a bionic arm Wireless neural interface devices implanted in peripheral nerves Read and decode muscle control signals to control the robotic limb Stimulate neural signals for sensory feedback Temperature; force; slip Requires wireless stimulating neural interface Ability to code sensor output for nervous system Neural interface must be robustly packaged for practical use Characterization of Parylene-C 6 L. Rieth

7 Introduction Microfabrication lab ~ 6000 ft 2 facility with both cleanroom and lab space Photolithography Film deposition (PVD; CVD; Epitaxy) Micromachining RIE and DRIE Diffusion doping Annealing furnaces Surface analysis lab Kratos Axis UltraDLD XPS/AES Optical Profilometer AFM VASE FEI field-emission ESEM Characterization of Parylene-C 7 L. Rieth

8 Parylene-C Deposition: Experimental Methods Substrates: c-si; borosilicate glass (BSG) Adhesion promoter: Silquest A-174 Chemical Vapor Deposition - Gorham Process Constant vaporizer pressure: ~0.03 g/min Dimer pyrolysis: 670 C Room temperature substrates Film thickness: ~3 µm Cl H 2 C CH 2 n VAPORIZER C PYROLYSIS FURNACE C DEPOSITION CHAMBER ~ 25 C COLD TRAP < -70 C VACUUM PUMP < 10mTorr Characterization of Parylene-C 8 L. Rieth

9 Experimental Methods Film characterization methods Adhesion testing - Tape test ASTM D3359B 10 x 10 grid of cuts in Parylene film (1 mm pitch) Scotch tape (#810 by 3M) % Removed 0% 0-5% 5-15% 15-35% 35-65% >65% Score 5B 4B Heat treatments 3B Steam sterilization compatibility (120 C with 100% relative humidity (RH)) Accelerated aging (85 C at 85% RH) Soldering compatibility (~ 350 C soldering iron) X-ray diffraction - crystallinity Electrical and stability testing Leakage current - 5 VDC in 37 C saline Electrochemical Impedance Spectroscopy 2B 1B 0B Characterization of Parylene-C 9 L. Rieth

10 Adhesion testing Results All films have 5B adhesion before heat treatments Procedures: A - cut before heat treatment; B - cut after HT Film Thickness Condition Test 75 C 20 min 85 C 20 minutes 120 C 20 minutes 150 C 20 minutes 120 C 100% RH 2 hours 150 C 100% RH 2 hours 3 ±0.3 µm Procedure c-si BSG A 5B -- B 5B -- A 5B -- B 4-5B -- A 5B 5B B 0-3B 5B A 2-4B 5B B 0B 4-5B A 5B -- B 0B -- A 3B -- B 0B -- Soldering compatibility: 350 C soldering iron 10 second contact to multiple bond pads Characterization of Parylene-C 10 L. Rieth

11 Discussion Parylene adhesion: Wireless neural interface device and other current and future applications require robust adhesion to heterogenous materials (Si, glass, noble and other metals, etc) Heat treatments including steam sterilization and soldering can degrade film adhesion depending on substrate material Geometry/morphology of coatings affects their adhesion Smaller domains are more able to tolerate heat treatments Thermal Coefficients of Expansion: Si: /K (2.6 ppm/k) Parylene-C: /K (35 ppm/k) Profilometry: film stress 1 MPa for as deposited films 10 MPa after annealing at 120 C for 20 minutes All residual film stresses are very small Interfacial bonds broken to relieve thermal mismatch stress Characterization of Parylene-C 11 L. Rieth

12 X-ray diffraction 85 C C 120 C 20 min 85 C 85% RH as deposited θ Bragg-Brentano geometry with Cu Kα radiation Characterization of Parylene-C 12 L. Rieth

13 Peak parameters X-ray diffraction Changes in peak position from out-of-plane stress Full-width at half-maximum (FWHM) for crystallinity Peak intensity for approx. volume fraction of crystalline material Condition Heat Treatment Peak Position ( 2θ) d-spacing (Å) FWHM ( 2θ) As deposited Accelerated 85 C / 85% RH days Annealed 150 C 20 min Annealed / accelerated 85 C / 85% RH days 150 C 20 min Characterization of Parylene-C 13 L. Rieth

14 X-ray diffraction: Discussion Affect of heat-treatments on Parylene microstructure: Processes < 85 C XRD indicates small changes in microstructure over relatively long periods Microstructure changes would need to be considered for demanding applications Glass transition temperature typically 30 to 85 C Processes 120 C Crystallization occurs d-spacings decrease Consistent with out-of-plane compressive stress or film densification In-plane tensile stress measured by wafer-bow Overall stress state is consistent with densification induced stress Characterization of Parylene-C 14 L. Rieth

15 Electrical Characterization Leakage current testing: 5 VDC bias on samples in test vials 3 days of testing dry - establish surface leakage 3 months of testing in 37 C saline solution - in vitro testing 1.E-06 1.E-08 Failure Acceptable 1.E-10 1.E-12 1.E-14 1.E Time in Days Characterization of Parylene-C 15 L. Rieth

16 Electrical Characterization Impedance Spectroscopy: Sensitive to changes in encapsulation thickness Desire pure capacitive behavior Most action potentials ~ 1 khz 1.E+09 0 Impedance (ohm) 1.E+08 1.E+07 1.E+06 1.E+05 1.E+04 1.E+03 Day 1-Z Day 16-Z Day 185-Z Day 1-P Day 16-P Day 185-P Phase 1.E E E E E E E E+05 Frequency (Hz) Characterization of Parylene-C 16 L. Rieth

17 Discussion Electrical properties of Parylene-C Stable and large resistance for dry testing Suggests no surface conduction/leakage modes activated Resistance decreases with immersion in saline Lower resistance due to small film thickness Failure due to pinholes typically causes leakage currents > 10-7 A Results indicate films are continuous over the test structures Resistance and impedance large and stable Suggests no degradation via loss of material (etching) Minimal ion or water infiltration is indicated No failure modes such as electrochemical corrosion, dissolution, etc are detected Characterization of Parylene-C 17 L. Rieth

18 Patterning Use oxygen plasma to etch Parylene-C Compared capacitively coupled and inductively coupled plasma reactive ion etch systems Capacitive system - Oxford 80 plus W µm/min Inductive system - March Plasmod W to 0.58 µm/min Characterization of Parylene-C 18 L. Rieth

19 Conclusions Parylene-C is a useful material for microsystems Established track record of biocompatibility; conformal coating; few pinholes; Adhesion is critical and must be controlled for systems made of heterogenous materials Thermal treatments (e.g. soldering, steam sterilization, etc) can degrade film adhesion; process flows and alternatives need to be considered The thermal degradation mechanism is hypothesized to involve breaking of interfacial bond to relieve strain Parylene crystallization is rate is significant at > 120 C Long-term stable in saline solution under 5 V bias; no electrochemical reactions, etc Can be effectively patterned using RIE with some control on the etch anisotropy Characterization of Parylene-C 19 L. Rieth

20 Future Work and Acknowledgements Investigate adhesion to more materials to investigate encapsulation of heterogenous devices Surface treatments and adhesion promoters Better understanding of the chemistry on both interfaces Functionalization of Parylene Integrating Parylene into the packaging chain Effects on adhesion of other materials (e.g. silicone potting, acrylic resin, etc) Adhesion and stability on other materials (e.g. a-sic:h, metals and metal oxides, etc Acknowledgements NIH/NINDS Chronic Microelectrode Recording Array HHSN C DARPA Revolutionizing Prosthetics Characterization of Parylene-C 20 L. Rieth