Development and optimization of a laser carbonized polyimide film as a sensor substrate for an all-polymer humidity sensor - Polyimides and Other High Temperature Polymers - page 435

Development and optimization of a laser carbonized
polyimide film as a sensor substrate for an all-polymer
humidity sensor
JOHN M. INGRAM, JAMES A. NICHOLSON and
AUGUSTUS W. FOUNTAIN III ∗
Photonics Research Center and Department of Chemistry, United States Military Academy,
West Point, NY 10996
Abstract—Kapton polyimide film was studied as a viable sensor substrate using a capacitive type
humidity sensor design. An argon ion laser was used to pyrolyze the Kapton film to form raised,
carbonized filaments. Using a graphics program and a set of computer controlled servo mirrors, an
interdigitated circuit was formed on the surface of the polymer. This circuit was coated with a hy-
grosensitive polymer, HMPTAC (2-hydroxy-3-methacryloxypropyltrimethylammonium chloride),
to form an all-polymer humidity sensor. The sensor substrate was optimized for sensitivity by vary-
ing pyrolysis wavelength and energy density. The results showed that the polyimide based sensor
performed as well as the most commonly used silica/gold and alumina/gold sensor substrates.
Keywords: Polyimide; polymeric sensors; humidity sensor.
1. INTRODUCTION
The problem of developing small, real time point detection systems for chemical
agents is being addressed all over the world [1-17]. All proposed approaches use
various detection schemes, but virtually all use the same substrate to build their
sensing device: a silicon or alumina platform with deposited gold or platinum wir-
ing. This substrate introduces special problems to the sensor design. Silicon sub-
strates with gold wiring are expensive and sometimes need to be post-processed at
the manufacturing facility [1-4]. Both silica and alumina substrates have shown
adhesion problems [1]. Aluminum is a relatively good thermal conductor, which
can lead to sensor film breakdown. Silicon is brittle and alumina is rigid which
could lead to sensor damage under torsion or bending stress. While cost issues
and fabrication problems are not that critical in the laboratory environment, future