Characterization of Parylene-C Thin Films as an Encapsulation for - - PowerPoint PPT Presentation

• L. Rieth

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

Hsu, Jui-Mei1; Kammer, Sascha2; Jung, Erik3; Rieth, Loren4; Normann, Richard5; Solzbacher, Florian1,4,5

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

Characterization of Parylene-C 2

• L. Rieth

Microsystems Group

Wireless Biomedical Devices Gas sensors on microhotplates Materials for harsh environment electronis Neuroprosthetics Glucose monitoring Harsh-environment MEMS and electronics

Characterization of Parylene-C 3

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Introduction

Characterization of Parylene-C 4

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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 5

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Introduction

• Neuroprosthetics device research:

– Encapsulation: Parylene-C; a-SiC:H; silicone – Electrode metallization

• IrOx 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

• 1.E+07
• 5.E+06

0.E+00 5.E+06 1.E+07 2.E+07

• 1
• 0.5

0.5 1 V vs Ag/AgCl (V)

Sputtered Activated

Characterization of Parylene-C 6

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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 7

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Introduction

• Microfabrication lab

– ~ 6000 ft2 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 8

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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

• < -70˚C

~ 25˚C 550–690˚C 120–170˚C < 10mTorr

CH2 H2C

n

Cl

Characterization of Parylene-C 9

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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)

– 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

0B 1B 2B 3B 4B 5B Score >65% 35-65% 15-35% 5-15% 0-5% 0% % Removed

Characterization of Parylene-C 10

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Results

• Adhesion testing

– All films have 5B adhesion before heat treatments – Procedures: A - cut before heat treatment; B - cut after HT – Soldering compatibility:

• 350 °C soldering iron
• 10 second contact to multiple bond pads
• 4-5B

5B 5B 5B

• BSG

0B 3B 0B 5B 0B 2-4B 0-3B 5B 4-5B 5B 5B 5B c-Si B A B A B A B A B A B A Procedure

3 ± 0.3 µm

150 °C 100% RH 2 hours 120 °C 100% RH 2 hours 150 °C 20 minutes 120 °C 20 minutes 85 °C 20 minutes 75 °C 20 min Test Condition Film Thickness

Characterization of Parylene-C 11

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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 12

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X-ray diffraction

10 12 14 16 18 20

2 θ θ θ θ

85 C + 120 C 120 C 20 min 85 C 85% RH as deposited

Bragg-Brentano geometry with Cu Kα radiation

Characterization of Parylene-C 13

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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

0.64 6.083 14.54 85 °C / 85% RH days 150 °C 20 min Annealed / accelerated 0.64 6.083 14.55 150 °C 20 min Annealed 1.03 6.141 14.41 85 °C / 85% RH days Accelerated 1.94 6.200 14.27

• As deposited

FWHM (° 2θ) d-spacing (Å) Peak Position (° 2θ) Heat Treatment Condition

Characterization of Parylene-C 14

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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 15

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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-16 1.E-14 1.E-12 1.E-10 1.E-08 1.E-06

20 40 60 80 100 120 140 160 Time in Days

Failure Acceptable

Characterization of Parylene-C 16

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Electrical Characterization

• Impedance Spectroscopy:

– Sensitive to changes in encapsulation thickness – Desire pure capacitive behavior – Most action potentials ~ 1 kHz

1.E+01 1.E+02 1.E+03 1.E+04 1.E+05 1.E+06 1.E+07 1.E+08 1.E+09 1.0E+00 1.0E+01 1.0E+02 1.0E+03 1.0E+04 1.0E+05

Frequency (Hz) Impedance (ohm)

• 100
• 90
• 80
• 70
• 60
• 50
• 40
• 30
• 20
• 10

Phase Day 1-Z Day 16-Z Day 185-Z Day 1-P Day 16-P Day 185-P

Characterization of Parylene-C 17

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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 18

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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 19

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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

• f 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 20

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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

• HHSN265200423621C

– DARPA Revolutionizing Prosthetics

• 908164