טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentJakobson Claudio Gabrie
SubjectIon Sensitive Field Effect Transistors in Standard CMOS for
Brain Monitoring
DepartmentDepartment of Biomedical Engineering
Supervisors Professor Emeritus Yael Nemirovsky
Professor Emeritus Uri Dinnar (Deceased)


Abstract

The Ion Sensitive Field Effect Transistor (ISFET) is a potentiometric pH sensor that is easily adapted to a wide range of chemical, biochemical and biomedical measurements. The operation of this sensor is based on the surface adsorption of charges from the solution under test in the solid-electrolyte interface that is part of the gate of the ISFET. As a result of this process, the threshold voltage of the ISFET is modulated. Today’s challenge is the integration of ISFETs in microsystems for applications such as in-vivo analysis, lab-on-a-chip and electronic tongues.

This study presents new ISFETs fabricated by post-processing of a standard silicon chip. These sensors can perform as the building blocks of new microsystems incorporating the ISFETs, electronics, as well as other sensors and actuators. This goal is achieved due to the compatibility of the ISFET studied with the Metal Oxide Semiconductor Field Effect Transistor, enabling the integration of both devices monolithically without giving up the basic advantages of the standard process. Beyond their advantage in integration, the new ISFETs are technology independent. Hence, these devices can directly incorporate future advances in integrated circuit technology.

In addition, the study focuses on fundamental ISFET issues: noise and drift. In spite of the considerable progress in the ISFET field, drift and noise sources in this device are still a matter of research. This study presents the first extensive measurements of noise in ISFETs, showing for the first time that the dominant noise is the 1/f noise originated in the trapping-detrapping of carriers at the interface between the silicon and the silicon dioxide. 1/f noise spectrum is observed down to very low frequencies.

The ISFET studied is aimed at brain local pH monitoring. In this research, ISFETs are introduced in a catheter used nowadays in standard neurosurgical procedures. The ISFET catheter can measure in-vivo pH parameters of the cerebrospinal fluid, entering in contact with this fluid through a tiny hole in the skull. The main objective is the detection of ischemic deficiently oxygenated tissue, preserving it from secondary damage that may result from the therapeutic treatment. It is expected that the new information provided by the catheter will lead to the improvement of the life expectancy and the quality of life of patients undergoing neurosurgery.

Preliminary clinical measurements of the ISFET catheter at cerebrospinal fluid are presented. The catheter is tested in continuos operation at intraventricular cerebrospinal fluid flowing from the head of the patient. The pH measurements are compared with those obtained by gas analysis of the same fluid as well as venous blood samples. The measurements validate the operation of the catheter, and probe the feasibility of in-vivo measurements. This research enables future work on pH as well as multi-modal brain local monitoring.