PLASMA-ENHANCED ATOMIC LAYER DEPOSITION ZINC OIXDE FLEXIBLE THIN FILM ELECTRONICS

Open Access
Author:
Zhao, Dalong
Graduate Program:
Electrical Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
November 11, 2010
Committee Members:
  • Thomas Nelson Jackson, Dissertation Advisor
  • Thomas Nelson Jackson, Committee Chair
  • Srinivas A Tadigadapa, Committee Member
  • Suman Datta, Committee Member
  • Mark William Horn, Committee Member
Keywords:
  • Oxide semiconductor
  • plasma-enhanced atomic layer deposition
  • thin film transistor
  • flexible electronics
Abstract:
This thesis demonstrates high performance flexible thin film electronics fabricated by low temperature process. A novel process for forming high quality stable oxide films using weak oxidant plasma-enhanced atomic layer deposition (PEALD) has been used to achieve fastest flexible oxide integrated circuits reported to date. In addition, a unique approach based on plasma-enhanced chemical vapor deposition (PECVD) silicon nitride for organic light emitting diodes (OLEDs) encapsulation at low temperature (<70 °C) is also reported. Among several low temperature deposition approaches PEALD process provides highly crystalline and dense ZnO thin films which are uniform and conformal at 200 ºC. Crossover measurement results also demonstrate the advantage of PEALD process in thin film deposition on flexible substrates. PEALD ZnO flexible TFTs have high field-effect mobility (~ 20 cm2/V&#8729;s) and excellent bias stress stability with ALD Al2O3 passivation. 15-stage ring oscillators with propagation delay of <20 nsec/stage have been successfully fabricated on flexible substrates. To the best of our knowledge, these are the fastest oxide-semiconductor circuits on flexible substrates reported to date, and they are about 20 times faster than the best previous report. This thesis also presents the investigation of ZnO device physics by modeling. Non-ideal ZnO device characteristics, including passivation, contacts, and output conductance, have been well modeled and verified with experimental results. Two different approaches were also proposed to extract device parameters for compact models and form the foundation for later circuit design and simulations. A TCAD ZnO model is established and can well describe the operation physics from single transistor to simple circuits. This model is verified by reasonable agreement with experimental data. Building on the results of ZnO TFTs and circuits, several ZnO based applications have been demonstrated. Microsensors with ZnO pyroFETs have been fabricated on flexible substrates for implantable application, and temperature sensitivity of 7 mV/°C has been obtained. Moreover, PEALD ZnO also exhibits excellent low noise characteristics. A Hooge parameter of less than 10-4 was extracted from low frequency noise measurements. Based on the radiation hardness of PEALD ZnO, TFTs with Gadolinium as the floating gate have been also demonstrated in neutron detection.