ENGINEERED THIN FILMS OF PARYLENE C AS ELECTRICAL INSULATORS FOR FLEXIBLE ELECTRONICS
Open Access
- Author:
- Khawaji, Ibrahim Hussain
- Graduate Program:
- Electrical Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- May 20, 2019
- Committee Members:
- Osama O Awadelkarim, Dissertation Advisor/Co-Advisor
Jerzy Ruzyllo, Committee Chair/Co-Chair
Akhlesh Lakhtakia, Committee Member
Noel Christopher Giebink, Committee Member
John B Asbury, Outside Member - Keywords:
- ENGINEERED THIN FILMS
PARYLENE C
ELECTRICAL INSULATORS
FLEXIBLE ELECTRONICS
low-k - Abstract:
- The use of a single material as a multifunctional insulator (i.e., substrate, gate dielectric, interlayer dielectric, and passivation layer) in the same device will reduce the cost and improve the sustainability of flexible devices. The major goal of this dissertation was to examine the potential use of the multifunctional insulator Parylene C as a low-k interlayer dielectric in flexible electronics. Towards this goal, columnar microfibrous thin films of Parylene C were fabricated and their electrical and mechanical properties, stability, and reliability were studied. The columnar microfibrous thin films were fabricated using a collimated flux of Parylene-C monomers directed at a range of angles from 30 deg to 90 deg with respect to the substrate plane in a modified vacuum chamber using oblique angle deposition. Also, bulk Parylene-C thin films were fabricated to explore the stability of bulk Parylene C as a gate dielectric in flexible electronics. The significant results of this research are the following: • Columnar microfibrous thin films of Parylene C can be highly porous. The porosity decreases as the deposition angle increases and lies between 0.38 and 0.56. Both the static Young’s modulus and the yield strength of the columnar microfibrous thin films of Parylene C are higher in the morphologically significant plane than in the plane normal to it. In both loading directions, static Young’s moduli and yield strengths are about two orders of magnitude lower for the columnar microfibrous thin films of Parylene C than the corresponding parameters of the bulk Parylene C, making the columnar microfibrous thin films of Parylene C softer. The lowest relative permittivity of the fabricated columnar microfibrous thin films of Parylene C in the 1–1000 kHz frequency range is about 70% of that of the bulk Parylene C. The static Young’s moduli, yield strengths, and the relative permittivity can be correlated to the porosity, crystallinity, and the deposition angle. • The d.c. leakage current in the columnar microfibrous thin films of Parylene C at temperatures not exceeding 100 C (373 K) arises from the Poole–Frenkel conduction mechanism with a barrier energy of about 0.77 eV. The a.c. conduction in the columnar microfibrous thin films of Parylene C is attributable to small-polaron-tunneling hopping conduction and depends on the frequency f as fs, with s 2 [0.82, 0.85] increasing with temperature. Also, a.c. conduction in the columnar microfibrous thin films of Parylene C is temperature-activated with an activation energy that decreases from 0.020 to 0.012 eV as f increases from 1 to 1000 kHz. • Before and after the application of a constant-voltage stress (CVS), roomtemperature leakage current in a metal-insulator-metal (MIM) structure incorporating columnar microfibrous thin film of Parylene C as the insulator is space-charge limited. The space charge comprises defects introduced during fabrication. No new defects are induced by the CVS. Kohlrausch–Williams–Watts relaxation can be exploited to understand transient leakage-current behavior, and characteristic times in the 3.5–3.9 s range and stretch factors in the 4.2–5.2 range were determined. These parameters suggest that carrier trapping at defects and their polarization orientation are related to space-charge formation. Moreover, capacitance dependence on time and frequency is a good indicator of the CVS induced degradation and stability. Charge buildup in the columnar microfibrous thin films of Parylene C is accompanied by capacitance decrease with CVS duration. Extrapolation of the capacitance-decrease dependence on CVS duration indicates that the capacitance would degrade by about 20% in 10 years. • CVS induces charges in bulk Parylene C and its interfaces with gold and Pentacene. The net induced charge is positive and negative for, respectively, negative and positive gate bias polarity during CVS. The magnitude of the charge accumulated following positive CVS is significantly higher than that following negative CVS in the range of 4 to 25 nC cm^-2. In contrast, the leakage current during the negative CVS is three orders of magnitude higher than that during the positive CVS for the same bias stress magnitude. The charge buildup and leakage current can be explained in terms of electron trapping in the bulk Parylene-C/Pentacene interface and bulk Parylene C. Before the application of the CVS, a dielectric breakdown occurs at an electric field of 1.62 MV cm^−1. After the application of the CVS, the breakdown voltage decreases and the density of the trapped charges increases as the stress voltage increases in magnitude, with the polarity of the trapped charges opposite to that of the stress voltage. Trapped-charge buildup occurs in the bulk Parylene-C layer and in the proximity of the bulk Parylene-C/Pentacene interface during CVS, the magnitude and direction of the capacitance-voltage curve-shift depending on the trapping and recombination of electrons and holes in those regions. The overall conclusion is that both mechanical and dielectric properties of the columnar microfibrous thin films of Parylene C can be controlled by selecting the deposition angle appropriately. As a result, columnar microfibrous thin films of Parylene C can be fabricated to deliver k = 2.02, which is lower than k = 3.0 of bulk Parylene C by 30%. Therefore, columnar microfibrous thin films of Parylene C are promising candidates for deployment as ultralow-k ILDs beside their electrical stability and reliability. Finally, the buildup of trapped charges in the bulk Parylene-C used as a gate dielectric and near the Parylene-C/Pentacene interface plays a major role in the degradation of Au/bulk Parylene-C/Pentacene structures. The first-level understanding of charge buildup in Au/bulk Parylene-C/Pentacene structures obtained will serve as the basis of future studies on the defect-generation process and the trapping of charge carriers within the insulator layer in OFETs.