Effects of mechanical stresses, thickness and atmosphere on aging of polyimide thin films at high temperature

doi:10.1016/j.polymdegradstab.2012.09.005

Polymer Degradation and Stability

Volume 98, Issue 1, January 2013, Pages 361-367

https://doi.org/10.1016/j.polymdegradstab.2012.09.005 Get rights and content

Abstract

This work deals with the isothermal aging mechanisms and their impact on the behaviour of polyimide thin films, usually used as dielectric material in high temperature electronic applications. The objectives were to achieve a better knowledge of the atmosphere and mechanical stress effects on the degradation mechanisms of deposited thin films during high temperature isothermal aging. Hence, BPDA/PDA aromatic polyimide (PI) thin films deposited on two different substrates (stainless steel and silicon) were aged slightly below their glass transition temperature (0.8 T_g) in air and in nitrogen atmospheres. During the aging, the dielectric strength, morphological characteristics (thickness, surface profile) and chemical variation were monitored for different initial film thicknesses (<10 μm). Results show that films aged under nitrogen and those deposited on silicon substrates aged in air do not show any remarkable degradation during the aging period. In contrast, the films deposited on stainless steel substrates present significant degradation during aging in air and the material lifetime is thickness dependent. In the latter conditions, the measured mean thickness losses for long aging periods are about 0.5 nm/h for all initial thicknesses, indicating that the degradation is not related to the bulk variation of the material but to a surface degradation mechanism. In addition, the breakdown field presents stabilization after an initial aging period, leading us to conclude that the bulk of the material is not affected by the aging. The coupled results indicate that the BPDA/PDA thermal degradation mechanism at temperature below its glass transition one is due to the attack on the outer film layer, related to the presence of oxygen in the atmosphere and strongly dependent on thermo-mechanical substrate properties. Hence, studying of the thermo-oxidative stability of freestanding polyimide films can lead to misestimated reliability.

Introduction

Aromatic polyimides (PIs), with stiff molecular backbones containing aromatic rings, offer high glass transition temperatures and high thermal stability. For high temperature electronic applications, some unique properties of PIs like the chemical stability and the excellent mechanical and electrical properties are of great interest [1], [2]. Polyimides used at high temperature can be subjected to thermal and thermo-oxidative degradation which will cause the final breakdown of the insulating film under applied voltages. Consequently, it is of major interest to understand and predict the structural changes in those materials when submitted to specific conditions for long periods of time, as well as the effect of these changes on the dielectric properties.

In the aging conditions under concern here, aging-related degradation of high temperature polymers may be a result of complicated interaction of both physical and chemical aging processes, such as physical creep and relaxation during the aging period and the chemical degradation of polymer such as chain scission, hydrolysis, and oxidation induced degradation [3]. It is especially important to consider such chemical and physical aging for aeronautical applications, where hot and oxidative environments are commonly encountered [4].

The effect of long time aging of polyimide at high temperature (>200 °C) and in oxidative environment on the mechanical properties [5], weight loss [6], [7], and chemical properties [8], [9], has been widely investigated for thick polyimide matrix composites (1 mm thick) used in aerospace applications. It has been found that while thermal degradation occurred throughout the material, the oxidative degradation occurs mainly within a thin surface layer where oxygen diffuses into the material. The developed oxidized surface layer during thermal aging has shown a variation in the mechanical properties compared to the unoxidised interior material [10]. Also, a lot of papers discussed the effect of thermal aging on the electrical properties of polymer dielectric materials but for thick and freestanding films [11], [12], [13], [14]. Consequently, an overall understanding of the mechanisms of oxidative degradation of thin deposited polyimide films (thickness < 20 μm) and their effects on the electrical properties are still lacking. After having investigated the electrical properties of thin unaged polyimide BPDA/PDA (biphenyltetracarboxylic dianhydride acid p–phenylene diamine) films in a wide temperature range [15], [16], it seems important to study the effect of its aging in various conditions as well, in view of a possible use for high temperature applications and a better comprehension of the occurring degradation mechanisms at temperatures close to the glass transition one. In this work, the effect of isothermal aging at 300 °C in different conditions on the degradation kinetics of BPDA/PDA PI films, was investigated by monitoring electrical, physical and chemical properties. Based on the different obtained results, a degradation mechanism for deposited thin films aging is proposed.

Section snippets

Sample preparation

The studied polyimide is a commercial electronic grade of BPDA-PDA with a high decomposition temperature (T_d = 620 °C), a high glass transition temperature (T_g = 360 °C), a low dissipation factor (2 × 10⁻³ at 1 kHz and 25 °C), a high breakdown field (>2 MV/cm) and a low coefficient of thermal expansion CTE (3–5 ppm/°C). PI films were obtained from a polyamic acid (PAA) precursor dissolved in N-methyl-2-pyrrolidone (NMP) solvent. PI coatings were prepared by dispensing successively an adhesion

Thickness variation

The thickness losses for different PI film thicknesses, deposited on two different substrates versus the aging time in air and N₂, are represented in Fig. 2. The films that were aged under conditions C2 and C3, do not show any significant variation during the aging time. For the films deposited on stainless steel and aged in air (C1), one can observe for all thicknesses the same reduction of approximately 0.5 nm/h until 5000 h of aging. Indeed, for the C1 aging, the rate of losses does not show

Surface thermo-oxidative aging process

The presented thickness losses in Fig. 2 are thickness independent and do not occur when films are aged under N₂. So they can be related neither to the thermolysis process nor to the densification and chain ordering of the material. Whereas, it seems that this variation can be related to a surface degradation of the material, due to the oxygen presence in the air, which can induce at such high temperatures the thermo-oxidative degradation. As shown in Fig. 6, a water droplet placed on the

Conclusions

The thermal aging effects, at temperature of 300 °C slightly below the glass transition one (0.8 T_g), on polyimide thin films were investigated for different initial thicknesses deposited on two different substrates and aged in two different atmospheres. A good thermal stability is observed when films were aged in N₂ and in air on silicon substrate and degradation occurred during the aging in air for films deposited on stainless steel (S.S.) substrate. Whatever the substrate, the purely thermal

Acknowledgements

The authors warmly thank the financial support of the DGA/DGCIS brought to this work, and M. Benoît Schlegel for the sample preparation.

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Effects of mechanical stresses, thickness and atmosphere on aging of polyimide thin films at high temperature

Abstract

Introduction

Section snippets

Sample preparation

Thickness variation

Surface thermo-oxidative aging process

Conclusions

Acknowledgements

Polym Degrad Stab

Compos Part A Appl S

Mat Sci Eng A Struct

Polymer

Thin Solid Films

Polym Degrad Stab

Comp Sci Tech

Spectrochim Acta A

Polym Degrad Stab

Polyimides: synthesis, characterization and applications, I and II

Predicting thermo-oxidative degradation and performance of high temperature polymer matrix composites

High temperature degradation of 5250-4 polymer resin

Effects of prior aging at 2888C in air and in argon environments on creep response of PMR-15 neat resin

J Appl Polym Sci

Oxidative degradation of nadic-end-capped polyimides. 2. Evidence for reactions occurring at high temperatures

Macromolecules

On the oxidative degradation of nadic-end-capped polyimides. 3. Synthesis and characterization of model compounds for end-cap degradation products

Macromolecules