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
Microsystems Group Neuroprosthetics Glucose monitoring Biomedical Wireless Devices Gas sensors on Materials for harsh MEMS and electronics microhotplates environment electronis Harsh-environment Characterization of Parylene-C 2 L. Rieth
Introduction Characterization of Parylene-C 3 L. Rieth
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
Introduction 2.E+07 • Neuroprosthetics device research: Sputtered 1.E+07 Activated – Encapsulation: Parylene-C ; a-SiC:H; silicone 5.E+06 – Electrode metallization 0.E+00 • IrO x by reactive sputter deposition or -1 -0.5 0 0.5 1 “activation” of metallic Ir -5.E+06 • Improve charge injection -1.E+07 – Wafer level processing V vs Ag/AgCl (V) • 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 Characterization of Parylene-C 5 L. Rieth
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
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
Experimental Methods • Parylene-C Deposition: – 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 ���������� ���������� ����������� ����������� ����������� ����������� ��������� ��������� ��������� ��������� ������� ������� ������� ������� 120–170 ˚ C 550–690 ˚ C ~ 25 ˚ C < -70 ˚ C < 10mTorr Characterization of Parylene-C 8 L. Rieth
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 3B 2B 1B 0B – Heat treatments • 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 Characterization of Parylene-C 9 L. Rieth
Results • Adhesion testing – All films have 5B adhesion before heat treatments – Procedures: A - cut before heat treatment; B - cut after HT Thickness Film Condition Test 75 ° C 85 ° C 120 ° C 150 ° C 120 ° C 150 ° C 20 min 20 minutes 20 minutes 20 minutes 100% RH 100% RH 2 hours 2 hours 3 Procedure A B A B A B A B A B A B ± 0.3 µm c-Si 5B 5B 5B 4-5B 5B 0-3B 2-4B 0B 5B 0B 3B 0B BSG -- -- -- -- 5B 5B 5B 4-5B -- -- -- -- – Soldering compatibility: • 350 ° C soldering iron • 10 second contact to multiple bond pads Characterization of Parylene-C 10 L. Rieth
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: 2.6 × 10 -6 /K (2.6 ppm/K) • Parylene-C: 35 × 10 -6 /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
X-ray diffraction 85 � C + 120 � C 120 � C 20 min 85 � C 85% RH as deposited 10 12 14 16 18 20 2 θ θ θ θ Bragg-Brentano geometry with Cu K α radiation Characterization of Parylene-C 12 L. Rieth
X-ray diffraction • Peak parameters – 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 d -spacing Peak Position FWHM Condition Heat Treatment ( ° 2 θ ) ( ° 2 θ ) (Å) As deposited -- 14.27 6.200 1.94 85 ° C / 85% RH Accelerated 14.41 6.141 1.03 days 150 ° C Annealed 14.55 6.083 0.64 20 min 85 ° C / 85% RH Annealed / days 14.54 6.083 0.64 accelerated 150 ° C 20 min Characterization of Parylene-C 13 L. Rieth
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
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 Failure Acceptable 1.E-08 1.E-10 1.E-12 1.E-14 1.E-16 0 20 40 60 80 100 120 140 160 Time in Days Characterization of Parylene-C 15 L. Rieth
Electrical Characterization • Impedance Spectroscopy: – Sensitive to changes in encapsulation thickness – Desire pure capacitive behavior – Most action potentials ~ 1 kHz 1.E+09 0 Day 1-Z -10 1.E+08 Day 16-Z -20 Day 185-Z 1.E+07 Impedance (ohm) Day 1-P -30 Day 16-P 1.E+06 -40 Day 185-P Phase 1.E+05 -50 -60 1.E+04 -70 1.E+03 -80 1.E+02 -90 1.E+01 -100 1.0E+00 1.0E+01 1.0E+02 1.0E+03 1.0E+04 1.0E+05 Frequency (Hz) Characterization of Parylene-C 16 L. Rieth
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
Patterning • Use oxygen plasma to etch Parylene-C – Compared capacitively coupled and inductively coupled plasma reactive ion etch systems • Capacitive system - Oxford 80 plus - 200 W - 0.28 µm/min • Inductive system - March Plasmod - 150 W - 0.35 to 0.58 µm/min Characterization of Parylene-C 18 L. Rieth
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