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Dielectric Investigation of Parylene D Thin Films: Relaxation and Conduction Mechanisms Marwa Mokni, Abdelkader Kahouli, Fethi Jomni, Jean-Luc Garden, Emmanuel Andre, and Alain Sylvestre J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.5b07459 • Publication Date (Web): 14 Aug 2015 Downloaded from http://pubs.acs.org on August 20, 2015
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The Journal of Physical Chemistry
Dielectric Investigation of Parylene D Thin Films: Relaxation and Conduction Mechanisms
M. Mokni1,2, A. Kahouli1,2*, F. Jomni2, J.-L. Garden3, E. André3 , A. Sylvestre1 1
Univ. Grenoble Alpes, G2Elab, F-38000 Grenoble, France
2
Laboratoire Matériaux Organisation et Propriétés (LMOP), Université de Tunis El Manar,
2092 Tunis, Tunisia 3
CNRS, I. Neel, F-38000 Grenoble, France
ABSTRACT
Parylene is a generic name indicating a family of polymers with chemical basic structure of poly-para-xylylene. Parylene N and Parylene C are the most popular for applications. Curiously, Parylene D (Poly-dichloro-para-xylylene, (C8H6Cl2)) was forgotten for applications. This report is the consequence of a later availability of a commercial dimer of Parylene D but also to the recent advent of fluorinated Parylenes allowing extending applications at higher temperatures. In our work, from a dielectric analysis, we present the potentialities of Parylene D for applications particularly interesting for integration in organic field-effect transistors. Dielectric and electrical properties, macromolecular structures and dynamics interaction with electric field as a function of frequency and temperature are studied in 5.8 µm thick Parylene D grown by chemical vapor deposition. More exactly, the dielectric permittivity, the dissipation factor, the electrical conductivity and the electric modulus of Parylene D were investigated in a wide temperature and frequency ranges from -140°C to 1 ACS Paragon Plus Environment
The Journal of Physical Chemistry
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350°C and from 0.1 Hz to 1 MHz, respectively. According to the temperature dependence of the dielectric permittivity, Parylene D behaves two different dielectric responses. It is retained as a non-polar material at very low temperature (like Parylene N) and as a polar material at high temperature (like parylene C). The dissipation factor shows the manifestation of two relaxations mechanisms: γ and β at very low and high temperature, respectively. The γ relaxation is assigned to the local motions of C-H end of chains when approaching the cryogenic temperature range. A broad peak in tanδ is assigned to the β relaxation. It corresponds to rotational motion of some polar C–Cl groups. For temperature above 260°C a mechanism of Maxwell-Wagner-Sillars polarization at the amorphous/crystalline interfaces was identified with two activation energies of Ea1 = 2.12 eV and Ea2 = 3.8 eV. Moreover, the conductivity and the dielectric permittivity relaxation processes have been discussed in terms of nearly constant loss (NCL) and universal dynamic regime (UDR). Finally, ionic conduction and electrode polarization effects are identified at very high temperature and their physical origins are discussed.
Keywords: Parylene, relaxation phenomena, electrical conductivity, electrode effect, dielectric constant, OFET
*Corresponding Author:
[email protected] 2 ACS Paragon Plus Environment
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The Journal of Physical Chemistry
1. Introduction There is great demand for thin-film polymer coatings for advanced applications from integrated optoelectronic devices1, circuit boards2, biomaterials3 and optical or microelectromechanical systems.4,5 Traditionally polymers of poly(para-xylylene) (commercially trademarked as Parylene or PPX) are used as protective insulating thermoplastic coatings for electronic devices. This is due to the uniform conformal coating properties of Parylene resulting from the specific nature of its polymerization, called chemical vapor deposition (CVD). The high mobility of the gaseous monomer (para-xylylene) (PX) in this process enables them to polymerize in the plane of the surface and to create uniform surfaces with very low sticking coefficient (
