A saw Sensor Study of the Relative Adhesion and Surface Properties of Polyimide Films
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A SAW SENSOR STUDY OF THE RELATIVE ADHESION AND SURFACE PROPERTIES OF POLYIMIDE FILMS D. W. GALIPEAU, C. FEGER* AND J. F. VETELINO Laboratory for Surface Science and Technology and Department of Electrical Engineering, University of Maine, Orono, ME 04469 *IBM T. J. Watson Research Center, Room 39-114, P.O. Box 218, Yorktown Heights. NY 10598 ABSTRACT A surface acoustic wave (SAW) sensor is used to study the relative humidity response of polyimide films where interfacial and surface properties are varied. The relationship between these responses and the relative adhesion strength of these films is of particular interest since it could have application as a nondestructive, quantitative measure of adhesion strength. The results show clear differences in the SAW humidity response for cured and uncured films. There are also significant differences in the SAW humidity response for films applied with and without adhesion promoter. In addition the humidity response has proven to be particularly sensitive to the surface treatment of the polyimide film. These results suggest applications of the SAW sensor as a cure indicator, an in-situ sensor for the monitoring of film surface properties in process control applications and as an adhesion sensor. INTRODUCTION A considerable amount of work has been done in the area of surface acoustic wave (SAW) sensor studies of polymer film properties [ 1-9]. The sensing mechanism was attributed to small changes in the density, elasticity and viscoelasticity of the films. It is also well known that residual stress is present in cured polyimide films due to solvent loss and differences in the thermal expansion coefficients between the film and adherend [10.111. Humidity as well as temperature can affect the magnitude of these stresses [121. Silane adhesion promoters such as Dupont's Al 100 are commonly used to improve the adhesion of polyimide films [ 13,14]. However the overall effect of improved adhesion on the SAW sensor response and the relationship between improved adhesion and the sensor response to changes in density, elasticity and residual stress of the film when the adhesion promoter is present is not well understood. Preliminary work has been done at the University of Maine using SAW sensors to study the humidity response of polyimide films and its relationship to film adhesion [15,16]. The purpose of the present work is to report the most recent results of SAW sensor studies on the effects of water uptakedowntake on polyimide films applied on both delay lines of a dual delay line SAW sensor. This SAW sensor configuration will allow for careful study of the response differences between two films where the interface or surface properties are varied. PROCEDURE The SAW sensor used in the present work is the dual delay line type on a ST-Quartz substrate. It was designed and fabricated at the University of Maine and is shown in Figure 1. Quartz is being used due to it's similarity to SiO in ICs. The interdigital transducers (IDTs) consist of a patterned metal layer approximately 1500
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