NSF ERC for Wireless Integrated MicroSystems (WIMS)NSF ERC for Wireless Integrated MicroSystems (WIMS)

A Poly-silicon Strain Gauge Position Sensing System for a Cochlear Prosthesis

Engineering Research Center for Wireless Integrated MicroSystemsThe University of Michigan, Ann Arbor, MI 48109

Jianbai Wang, Cathy Morgan, Pam Bhatti, Yan Yang, Katharine Beach, Danielle M. Merriam, Kensall D. Wise, Ph.D.

Goals

• Determine depth of implantation, distance from scala tympani wall and tip forces during insertion• Design a bulk-machining cochlear electrode with position sensing system and integrate it with an on-chip active circuitry• Create a closed–loop articulated position control system

Abstract

The position of the electrode array within the scala tympani is extremely important for high stimulation efficiency. Our project is to design a poly-silicon-based strain gauge position sensing system for guinea pig electrode array. The 2 nd generation passive system (Gen-2) based upon the design of half of the wheatstone bridge was fabricated and calibrated; in addition, a labVIEW-interfaced system was initiated to demonstrate the curvature- and tip-sensing system. Our current active design is wrapped up to integrate an on-chip digital control and analog readout circuitry with the MEMS-based sensor array.

ACKNOWLEDGEMENTSThis work is supported by the Engineering Research Centers Program of the National Science Foundation under Award Number EEC-9986866 and by a gift from Ms. Polly Anderson.

FUTURE WORK

• Further the study of integrating poly-diamond as sensors and encapsulating materials in the position-sensing system• Fabricate and calibrate the active position sensing electrode array• Set up the labVIEW-interfaced position-sensing system; in the future, it will be developed to a closed-loop feedback system for MTU position-

control devices• In-vitro and in-vivo stimulation and position-sensing guinea pig experiments with 2nd- arrays with position-sensing system

Fundamentals of Cochlear Implant

• Right implant depth – avoid distortion of sound

• Lower down the damage to cochlea

• Sensing the distance to scala tympani wall -- high stimulation efficiency

• Integration with insertion tool for self-articulated positioning during insertion and position control post-operatively

Why Position Sensing?

Fabrication: This is a 12-mask bulk-micromachining processing. Two interconnect layers of Aluminum and highly-doped poly-Si were utilized. Sputtered gold and sputtered Iridium work as the materials for bondpads and electrode individually. Deep and shallow boron diffusions are utilized to define the backend bond region and the shank individually. PECVD-poly-diamond was integrated into the processing on the top of poly-Si and below Aluminum layer.

The 2nd-gen Passive Arrays of Position Sensing

• Rs(highly-doped poly-Si)= 22.2 ohm/sq Rs(lightly-doped poly-Si)=21.3, 4.24, 30.2 Kohm/sq Rs(poly-diamond)=1.09 Mohm/sq

• High-impedance Arsenic-doped polysilicon sensors

• Eight strain gauge sensors having lengths of 930µm and widths of 18µm are distributed uniformly down the middle of the shank covering the first 8mm from the tip

• Eight wall contact sensors along the two edges of the shank

• Tip Sensor

• Test chip

• The position- and wall-sensing system was integrated with an eight-site stimulation array in one design

• Poly-diamond sensors prepared by MSU were also used to replace the polysilicon sensors after resizing.

• Soak test cables with four types of top dielectric layers: an LPCVD dielectric stack, low-temperature oxide (LTO), a composite stack of LPCVD dielectrics and LTO, and poly-diamond

— An Iridium electrode site integrated in the positioning system

— Wall contact sensor with only dielectrics as substrate

— Wall contact sensor with shallow boron diffusion layer and dielectrics as substrate

— Wall contact for tip sensing

— Reference sensors fabricated on the backend of the array

Active Arrays for Position Sensing

The sensors distributed along the shank follow the passive design with the structure of half of the Wheatstone bridge. In the analog circuit, the other half of the bridge is replaced by a 5-bit voltage digital-to-analog converter (VDAC) in order to compensate for the resistance variation induced by interconnect routing and processing variations. This strain gauge bridge is simulated to output a signal resolution of 0.0204 V/kΩ with respect to the piezoresistive variation.

The Calibration and Demonstration System for Position Sensing

•Natural State: almost flat • Bent away from the array surface. ( Radius of curvature = 1’)

Intermediate calibration results for comparison from the oscilloscope

This system is composed of the passive position-sensing array, a breadboard multiplexer and amplifier circuit, and a LabVIEW interface. The eight curvature sensors are connected to the reference sensors during continuous clock cycles to form half of a wheatstone bridge structure; the output voltage is differentiated and amplified by an instrumentation amplifier. A consistent linear relationship between the output voltage and curvature is observed. The gauge factor of the polysilicon sensors having Rs=30.2 Kohm/sq averages 27.6.

Analog Readout

Digital Control