High-performance ultra flexible silicon device and circuits towards bio-medical application

Abstract

This dissertation focuses on the development of high-performance ultra-flexible silicon device and circuits for utilizing it for bio-medical applications. In order to accomplish this research, we have developed a high performance transistor based on FDSOI operation using SOI wafer and developed and optimized the transfer process of high performance single crystal silicon device. The purpose of this study is to develop and characterize high performance flexible silicon based electronic device and process that can be applied to bio-medical field, especially in application field where wearable/flexible electronic devices are applied.

In order to fabricate a high-performance flexible silicon device, the entire thickness of the device must be reduced. As a basis for accomplishing the research purpose, we observed the room temperature quantum confinement effect as the thickness of silicon decreased and analyzed its mechanism. As the thickness of the channel silicon is reduced to 7 nm or less, a room temperature quantum confinement effect is exhibited. In particular, when the channel silicon thickness is 2.5 nm, the increase in mobility is electrically confirmed to be 500%. This is due to the fact that the intervalley scattering is greatly reduced due to the sub-band gap, which is due to the two effects, quantum confinement effect as the thickness of the silicon decreases and strain effect as the repetitive low temperature oxidation process used to reduce the thickness of the silicon.

For high-performance flexible silicon devices, both high-performance device development and development of structure for flexible electronics are important. In order to fabricate a high-performance device, CMOS-friendly inorganic materials must be used, but the breakdown strain of these materials is less than 1%, and that of the electrical characteristic failure is about 0.1%. For this purpose, a method to control the neutral mechanical plane, which is a point where the strain is zero, was developed and the strain of the element was controlled to be within 0.01% even when the bending radius was 1 mm. The proposed structure exhibited high performance of the wafer state without degradation and showed no deterioration even after 24 hours continuous immersion in body fluids measured for bio-medical applications.

Recent electronic circuits stack several layers of metal and insulating films on top of devices to achieve high performance and versatility. Therefore, a guideline for the control of the neutral mechanical layer is required, and it is necessary to analyze the behavior of the device and its long-term reliability in softening the thick integrated circuit. It has been found that it is most advantageous to soften the integrated circuit by arranging the neutral mechanical layer at the middle point of the total thickness during the adjustment of the neutral mechanical layer in various ways. As the radius of curvature decreases during the flexible operation, it is revealed that increase in interface state density leads degradation of the device with integrated circuit structure. In addition, NBTI analysis has been conducted for long-term reliability analysis and it has been found that the polymer material, substrate, used for softening is vulnerable to heat.

Finally, we developed a flexible 2 stage amplification system in which temperature sensors are integrated using the developed technologies above. The flexible electronic circuit consists of three resistors, two FDSOI transistors, and a silicon resistor for one temperature sensor. It was then encapsulated with SU-8 and PDMS to be bio-compatible and to maintain its original performance even at 1 mm bending radius. The prepared circuit was tested for encapsulation performance in artificial cerebrospinal fluid and showed normal operation in body fluids. Thereafter, the mouse also operated normally in the brain, and the circuit in the brain of the rat for 5 days also showed a reliable operation. Conclusively, immunohistochemistry was processed and compared with the sham control sample, it was found to be bio-compatible.