The Pennsylvania State University The Graduate School College of Engineering AMORPHOUS FILM MICROSTRUCTURE AND ITS DEVICE APPLICATIONS: STRAIN SENSOR, MICROBOLOMETER, THIN FILM TRANSISTOR AND SOLAR CELL A Dissertation in Electrical Engineering by Hang-Beum Shin 2012 Hang-Beum Shin Submitted in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy August 2012 The dissertation of Hang-Beum Shin was reviewed and approved* by the following: Thomas N. Jackson Robert E. Kirby Chair Professor Dissertation Advisor Chair of Committee Joan M. Redwing Professor of Materials Science and Engineering and Professor of Electrical Engineering Srinivas Tadigadapa Professor of Electrical Engineering Mark W. Horn Professor of Engineering Science and Mechanics Kultegin Aydin Professor of Electrical Engineering Head of the Department of Electrical Engineering *Signatures are on file in the Graduate School ii ABSTRACT This thesis reports materials and device results for several types of thin-film devices: (1) n+ µc-Si:H strain sensors and a-Si:H thin film transistor (TFT) arrays on a flexible polyimide substrates, (2) high temperature coefficient of resistance (TCR) a-Si(C):H and sputtered a-Ge films as sensor materials for uncooled microbolometers, and (3) a-Si:H solar cell integration with plasma enhanced atomic layer deposited (PEALD) ZnO TFTs and circuits. The strain sensor array was fabricated on a flexible polyimide substrate. Six photolithography layers were processed on 4-by-4 inch flexible samples, forming a-Si:H TFTs and n+ µc-Si:H strain bridges. Discrete test structures and arrays with up to 32-by-32 sensors were fabricated. By using Wheatstone bridge sensors with varying orientation for different sensors in an array it was possible to determine both strain magnitude and direction. Strain sensor array operation was demonstrated with a custom interface circuit board and a LabView program from real-time data display. In resistive microbolometer devices, the temperature coefficient of resistance (TCR) is a key property for high sensitivity. Plasma enhanced chemical vapor deposition (PECVD) silicon and sputtered germanium deposited with various deposition parameters were investigated to achieve high TCR. Real-time spectroscopic ellipsometry (RTSE) was installed onto a load-locked PECVD system, and the growth evolution of films was monitored successfully. Depending on the deposition conditions, film phases were correlated with variations in film electrical properties. One major factor that limits microbolometer performance is 1/f noise, and the normalized Hooge parameters of PECVD silicon and sputtered germanium films were evaluated. In a more theoretical approach to understanding amorphous silicon, an a-Si:H bandstructure was modeled using a commercial software, Synopsis Sentaurus, and the temperature dependence of resistivity was simulated using a variety of band structure models. An iii a-Si:H model was constructed to examine the TCR dependence on several model parameters: band-tail slope / concentration, gap-state slope / concentration, doping concentration, and mobility. Comparison of experimental and modeled conductivity over a wide temperature range can help determine the slope of the band-tail traps. Autonomously powered circuits are of interest for a range of applications. Thin film solar cells currently occupy a smaller portion of the market than solar cells made from bulk materials, but still have large potential for use in mobile and portable electronics. Here, the heterogeneous integration of a-Si:H solar cells and ZnO TFT circuits was demonstrated. The process requires only two additional photolithography steps compared to a standard ZnO TFT process. As a simple integration example, 15 series connected n-i-p solar cells were fabricated to provide the supply voltage for 7-stage ZnO ring oscillator. The resulting a-Si:H/ZnO integration requires only illumination (no other external power) and oscillates at about 28 kHz. This integration of PV cells into functional TFT circuits to create an autonomously powered devices may be groundbreaking for the development of cost-effective low-power sensors. iv TABLE OF CONTENTS List of Figures ……………………………………………………………………….…viii List of Tables ………………………………………………………………….………xvii Acknowledgements …………………………………………………………………...xviii Chapter 1 Introduction ............................................................................................................. 1 Hydrogenated amorphous silicon technology .................................................................. 1 Electronic structure of a-Si:H ........................................................................................... 2 Research Objectives and Outline ..................................................................................... 4 Chapter 2 PECVD Development and Spectroscopic Ellipsometry .......................................... 6 The principle of PECVD .................................................................................................. 6 Plasma Process: Sheath ............................................................................................ 7 Chemical kinetics and surface process in a-Si:H deposition .................................... 9 Optical Emission Spectroscopy (OES) ............................................................................ 11 Spectroscopic Ellipsometry: RC2 / M88 / IR-Vase ......................................................... 13 Light and Matter interaction: complex dielectric functions ..................................... 15 Lorentz oscillator model: Semiconductor (a-Si:H), Insulator .................................. 16 Optical components in spectroscopic ellipsometry .................................................. 20 Infrared (IR) Spectroscopic Ellipsometry ................................................................ 22 Real-time Spectroscopic Ellipsometry (RTSE)................................................................ 24 Computer-controlled PECVD gas/power control ............................................................ 26 Chapter 3 Strain sensor and a-Si:H TFT Array on a Flexible Substrate .................................. 30 Overview .......................................................................................................................... 30 Sensor Part: Strain and Whitestone Bridge ...................................................................... 32 Flexible substrates: Polyimide Kapton ............................................................................. 34 Mask design ..................................................................................................................... 35 Sample Fabrication .......................................................................................................... 36 Kapton Lamination and Surface Preparation ........................................................... 37 Bottom gate (Chromium, Metal-1) Deposition and Patterning ................................ 39 Tri-Layer Deposition and Patterning ........................................................................ 40 Via Patterning ........................................................................................................... 41 Thin Molybdenum / n+ Microcrystalline Silicon Deposition and Patterning .......... 42 Thick Molybdenum .................................................................................................. 44 Room temperature nitride or Parylene Passivation .................................................. 45 ACF Bonding ........................................................................................................... 47 Sensor and array characterization .................................................................................... 48 Layer characterization .............................................................................................. 49 PECVD silicon nitride (SiNx) ................................................................................... 50 v Amorphous silicon thin film transistors (a-Si:H TFTs) ........................................... 53 Readout circuitry for sensor array ............................................................................ 54 Discrete Sensor Characterization ............................................................................. 55 Strain Array Data Collection and Analysis .............................................................. 57 Temperature susceptibility ....................................................................................... 60 Several array results ................................................................................................. 61 Troubleshooting in the arrays ................................................................................... 62 Integration with UIUC amplifiers .................................................................................... 64 Conclusions ...................................................................................................................... 65 Chapter 4 Uncooled IR Microbolometers ................................................................................ 66 Introduction to microbolometer devices .......................................................................... 66 Parameters in microbolometer material ........................................................................... 67 Temperature coefficient of resistance (TCR) ..........................................................