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WIRELESS PARYLENE-BASED RETINAL
Jay Han-Chieh Chang
In Partial Fulfillment of the Requirements for the
Doctor of Philosophy
California Institute of Technology
(Defended on August 5, 2013)
Jay Han-Chieh Chang
All Rights Reserved
Foremost, I would like to express my sincere gratitude to my advisor Prof. Yu-Chong Tai for the continuous support during my Ph.D. study and research, and for his patience, motivation, enthusiasm, and immense knowledge. His guidance helped me all throughout my research and writing of this thesis. I could not have imagined having a better advisor and mentor for my Ph.D. study. It was an honor to have you as my advisor.
Besides my advisor, I would like to thank the rest of my candidacy and thesis defense committee:
Prof. Hyuck Choo, Prof. Azita Emami, Prof. Joel Burdick, and Prof. James Weiland for their encouragement, insightful comments, and hard questions.
I thank my fellow labmates in the Caltech MEMS Group, in particular Dr. Bo Lu, Dr. Ray Huang, Dr. Jeffrey Chun-Hui Lin, and Dr. Mike Liu; thank you for mentoring and training me when I was in the earlier years of my research career in the lab, and enlightening me on the first glance of research.
To the rest of the members, Dr. John Chen, Dr. Luca Giacchino, Dr. Justin Young-Hyun Kim, Dr.
Wendian Shi, Dr. Charles Deboer, Mandheerej Nandra, Yu Zhao, Zhao Liu, Dongyang Kang, Yang Liu, Shell Zhang, and Nick Scianmarello: thank you for your support, generous assistance, stimulating discussions, and brilliant ideas. The lab would not have been this organized and efficient if it were not for the collaboration and hard work from each and every one of you. Special credit must be given to Yang Liu and Shell Zhang, who helped on the device fabrication and electroplating setup.
My great appreciation also goes to Christine Garske and Tanya Owen, thank you for your help in purchasing and administrative tasks. You have made the lives of all members in the lab so much easier and smoother. My thanks to Mr. Trevor Roper, without whom we would not have any device we have made to date.
iv Many thanks go to the PI of the project, Dr. James Weiland and Dr. Mark Humayun, who have invested their full effort in guiding the team in achieving the goal. I have to appreciate the guidance given by them. I also appreciate my USC co-workers, including Dr. Yi Zhang, Dr. Artin Petrossians, and Dr. Alice Cho, who have dedicated their time and effort in advancing the research program in both USC and Caltech and have helped me tremendously in my research endeavor in BMES ERC;
my UCLA co-workers including Dr. Wen-Tai Liu, Dr. Kuan-Fu Chen, and Yi-Kai Lo, who developed many generations of retinal IC chips for the project; my Caltech co-workers, including Dr.
Azita Emami, and Manuel Monge, who also developed retinal IC chip for the project and helped on the measurement of parylene-chip integration.
Last but not the least, I would like to thank my parents, who have always been there for me and have given me unconditional support over the years in every aspect of my life. Most importantly, special thanks to my wife, Selene Chih-Wei Hu, and my daughter, Sophie Chang. Their love and
The degeneration of the outer retina usually causes blindness by affecting the photoreceptor cells.
However, the ganglion cells, which consist of optic nerves, on the middle and inner retina layers are often intact. The retinal implant, which can partially restore vision by electrical stimulation, soon becomes a focus for research. Although many groups worldwide have spent a lot of effort on building devices for retinal implant, current state-of-the-art technologies still lack a reliable packaging scheme for devices with desirable high-density multi-channel features. Wireless flexible retinal implants have always been the ultimate goal for retinal prosthesis. In this dissertation, the reliable packaging scheme for a wireless flexible parylene-based retinal implants has been well developed. It can not only provide stable electrical and mechanical connections to the high-density multi-channel (1000+ channels on 5 mm × 5 mm chip area) IC chips, but also survive for more than 10 years in the human body with corrosive fluids.
The device is based on a parylene-metal-parylene sandwich structure. In which, the adhesion between the parylene layers and the metals embedded in the parylene layers have been studied.
Integration technology for high-density multi-channel IC chips has also been addressed and tested
schemes have been tried in application to IC chips and discrete components to gain the longest lifetime. The effectiveness has been confirmed by the accelerated and active lifetime soaking test in saline solution. Surgical mockups have also been designed and successfully implanted inside dog's and pig's eyes. Additionally, the electrodes used to stimulate the ganglion cells have been modified to lower the interface impedance and shaped to better fit the retina. Finally, all the developed
TABLE OF CONTENTS
LIST OF FIGURES
LIST OF TABLES
1.1 RETINAL PROSTHESIS
1.2 MEMS TECHNOLOGY
1.2.2 BULK MICROMACHINING
18.104.22.168 WET ETCHING
22.214.171.124 DRY ETCHING
1.2.3 SURFACE MICROMACHINING
1.2.4 HIGH-ASPECT-RATION MICROMACHINING
126.96.36.199 LASER MICROMACHINING
1.4 INTEGRATION TECHNOLOGY
1.4.1 TRADITIONAL CHIP INTEGRATION TECHNOLOGY
1.4.2 STATE-OF-THE-ART CHIP INTEGRATION TECHNOLOGY
1.5 LAYOUT OF THE DISSERTATION
2. HIGH-DENSITY MULTI-CHANNEL CHIP INTEGRATION
2.2 OVERVIEW OF REQUIREMENTS FOR DEVICE PACKAGING
2.3 DEVICE DESIGN
2.3.2 FIRST–GENERATION PARYLENE-C FLEX FABRICATION
2.3.3 SECOND-GENERATION PARYLENE-C FLEX FABRICATION
2.4 DEVICE PACKAGING
2.4.1 ALIGNMENT AND SQUEEGEE
2.4.2 SQUEEGEE ISSUES
2.4.3 LASER CUTTING
2.4.4 DEVICE TESTING
2.5 ADHESION-ENHANCEMENT SURFACE TREATMENT
2.5.1 DEVICE DESIGN AND FABRICATION
2.5.2 EXPERIMENTS AND RESULTS
188.8.131.52 PEELING TEST
184.108.40.206 SOAKING TEST
2.6 DRY MECHANICAL LIFTOFF TECHNOLOGY
2.6.1 DESIGN OF SOAKING SAMPLES
2.6.3 FABRICATION PROCESS
220.127.116.11 SU-8 THICKNESS
18.104.22.168 SMALLEST FEATURE
22.214.171.124 COMPARISON OF RESISTANCE
3. PHOTO-PATTERNABLE ADHESIVES AND THEIR APPLICATIONS
3.3 INVESTIGATION OF PHOTO-PATTERNABLE ADHESIVES
3.3.1 EXPERIMENTAL PROCEDURE AND SETUP
3.3.2 RESULTS AND DISCUSSION
3.4 APPLICATIONS ON HIGH-DENSITY MULTI-CHANNEL CHIP INTEGRATION...85 3.4.1 YIELD TEST
3.4.2 LIMITS OF THE PROPOSED TECHNOLOGY
3.5 REAL CHIP FUNCTIONAL TESTING
3.5.1 TESTING ON 268-CHANNEL CHIP INTEGRATION
3.5.2 TESTING ON 1024-CHANNEL CHIP INTEGRATION
4. LIFETIME STUDY OF PACKAGING AND SURGICAL MOCKUP
4.2 LIFETIME STUDY AND ANALYSIS
4.2.2 EXPERIMENTAL RESULTS
4.3 SURGICAL MOCKUP DESIGN AND IN VIVO TEST
4.3.1 512-CHANNEL CHIP
4.3.2 1024-CHANNEL CHIP
4.3.3 MAXIMUM PULLING FORCE
5. 1024-CHANNEL RETINAL PROSTHESIS
5.2 DEVICE DESIGN
5.3 MULTI-ELECTRODE ARRAY (MEA)
5.4 INTEGRATION OF IC CHIP, DISCRETE COMPONENTS, AND COILS.................164
Diagram of the human eyeball structure
System overview and comparison of locations of epiretinal and subretinal implants........3 Figure 1.3. Basic components of the retinal implant system
Complexity of MEMS device by structure layers
Comparison of the fabrication process between positive and negative photoresist............9 Figure 1.6. Comparison of the bulk micromachining process: (a) Anisotropic etching. (b) Isotropic etching. (c) RIE
Electrostatic comb-drive actuator fabricated by Deep Reactive Ion Etching (DRIE) of silicon-on-insulator (SOI) wafer
Comparison of bulk micromachining and surface micromachining in cross-sectional view.
(Left) Illustration of surface micromachining process. (Right) Polysilicon micromotor fabricated using a surface micromachining process
LIGA process: (a) Exposure. (b) Electroplating. (c) Finishing to height. (d) Removal of the substrate
Chemical structure of Parylene-N, -C, -D, and-HT and the process temperature...........17 Figure 1.12. Illustration of the deposition system of Parylene and the process flow
Demonstration of soldering
Demonstration of wire bonding
Demonstration of the process of flip chip assembly
(Left) Overview of the Microflex technique. (Right) SEM pictures of the contact array.
Final implant with parylene C and silicone rubber encapsulation
chip are placed on the eyeball. The electrode array is placed in the subretinal space through a scleral flap
(Left) Illustration of the CL-I2 packaging concept. (Right) Concept of embedded chip integration
Parylene cracks occur on the edges of the metal lines, resulting in solution attack and causing delamination
Requirements for packaging issues for a wirelessly flexible parylene-based retinal implant device…
Figure 2.3. Lift-off process flow on parylene substrate
Comparison of patterns on sample after development without (Left) and with (Right) undercut
Fabrication process flow of the first-generation parylene-C flex
Fabrication process flow of the second-generation parylene-C flex
Alignment of the bonding pads with the metal pads on the chip. Alignment accuracy of 10µm can be achieved
Squeegee process: after alignment, the rubber squeegee is used to push the excess conductive epoxy away from the surface into the wells to make connections between parylene flex and chips
(Left) Add a layer of patternable epoxy to increase the aspect ratio. (Right) Footprint on a testing substrate with SU-8 wall after squeegee process
(Left) The surface of the bonding pads after squeegee and surface cleaning. (Right) The surface profile of the bonded conductive epoxy; average height is around 20 µm
(Left) Edge of the chip where the squeegee process may become a problem. (Right) PDMS mold fitted with a 256 channel stimulation chip
(Left) Short circuit defects on dense metal lines. (Right) Short circuit defects repair process
(Left) Shortage of neighboring metal pads underneath the parylene film due to low viscosity of the conductive epoxy. (Right) Electrically isolate the neighboring pads after UV laser cleaning
(Left) Dummy chip integration with squeegee technique. (Right) On average, less than 10 fixes for short and open circuits are needed after each chip bonding. 100% of the pads are functional after these repairs
xiii Figure 2.15.
Process steps of different treatments: (Top) treatment for interface of silicon and parylene. (Bottom) Treatment for interface of parylene and parylene
Sample layouts for peeling tests. (Left) Peeling test between parylene and silicon.
(Right) Peeling test between parylene and parylene
Sample layouts for soaking tests (a) between parylene and silicon and (b) between parylene and parylene. (c) Fabricated sample for soaking test. (Bottom right) 0.9 wt% NaCl solution
(Left) Fabricated sample with sacrificial PR for peeling test. (Right) Partially released film at initial condition for peeling test after PR releasing
(a) Setup of peeling test. (b) Time v.s. Force plot of a parylene film being pulled away from the substrate. SEM of the peeled interface of (c) parylene-silicon and (d) parylene-parylene...56 Figure 2.20. Undercut and vertical attack bubbles after soaking in saline (0.9 wt% NaCl solution) and ST-22. The trench is 200μm