A Study on the Microphysical Mechanisms of Adsorption in Polyimide Layers for Microelectronic Applications

Abstract

Polyimide films are now finding application in microelectronics and microelectromechanical systems technology. One problem with polyimide films is their tendency to adsorb water from ambient humid air, which can cause significant changes in the mechanical and electrical behavior of these films and even lead to their failure. To find ways to minimize water adsorption in polyimide films, we have studied different types of polyimides by both analytical and phenomenological means. It appears that the process of water uptake is by sorption of water molecules in micropores inside the polymer. The sorption of water in thin polymer films typically results in volume expansion. Past a certain saturation level, each sorbed water molecule causes an expansion of about 15 A 3 , which is about half of the effective volume of the water molecule. If the expansion of the polymer film is restrained by adjacent inorganic layers, the result is humidity-induced compressive stress in the polymer film. We have found that the amount of water uptake and the resulting anisotropic swelling depend strongly on the particular type of polyimide. In this paper, new experimental results are presented and discussed in detail.

A molecular dynamics simulation study of three polysulfones in dry and hydrated states

“…The influence of absorbed water on structural,1–21 physical,1, 11, 18, 21–37 mechanical,3, 14, 18, 21, 32, 38–43 and electrical properties37, 44–48 of polymers has been extensively studied and the consequent effects this can have during plastic processing has been highlighted 4. Hydrolysis of polymers by water is also a well‐known phenomenon 3, 18, 43, 49–61.…”

“…Hydrolysis of polymers by water is also a well‐known phenomenon 3, 18, 43, 49–61. For all these reasons, water sorption has been studied for many years mainly using techniques such as sorption isotherms,24, 50, 62–76 density or swelling measurements,1, 11, 22–35, 49, 77 differential scanning calorimetry (DSC),12, 78–81 infrared experiments,6, 28, 81–100 dielectric measurements24, 98, 101, 102 or nuclear magnetic resonance (NMR) 12, 78, 79, 102–106. Other specific experimental techniques have also been applied.…”

Parametrically Amplified Low-Power MEMS Capacitive Humidity Sensor

“…When the device is exposed to humidity, the sensing polymer swells after absorbing the water vapor. Since the polymer is asymmetrically patterned and constrained to the silicon cantilever beam, the swelling generates surface stress [40], which results in the bending of the cantilever beam towards one side.…”

Fabrication and characterization of a piezoresistive humidity sensor with a stress-free package

“…The swelling can be converted into an electrical signal by making use of the piezoresistive effect. The first research work on piezoresistive humidity sensors was published by a group from the Technical University of Dresden (TUD) at the beginning of the 1990s Gerlach and Sager, 1994;Buchhold et al, 1998a). A polymer layer on the top of a silicon-bending plate forms a bimorph that bends under the influence of humidity.…”

“…The moisture loading capacity was characterized by a relation of the mass of the absorbed water to the mass of the dry polymer using the value of a saturation concentration c s given by (Buchhold et al, 1998a) …”