Excimer Laser Induced Electrical Conductivity in Polymers
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EXCIMER LASER INDUCED
ELECTRICAL CONDUCTIVITY IN POLYMERS 1 2 1 13 H. M. PHILLIPS , , T. FEURER , D. L. CALLAHAN , AND R. SAUERBREY .3
1 Department 2 Department 3
of Electrical and Computer Engineering of Mechanical Engineering and Materials Science Rice Quantum Institute
Rice University, P. 0. Box 1892, Houston, TX 77251-1892 ABSTRACT The electrical conductivity of high temperature polymers (i.e. polyimide) has been changed permanently from 10-17 1-1 cm-1 to 10 Q-I cm-1 by excimer laser irradiation. The conduction mechanism is found to be phonon assisted variable range hopping between small (-10 nm) carbon clusters that form a macroscopic percolation cluster. The critical concentration of carbon clusters is found to be Pc = 0.24 ± 0.02 and the critical exponent of the insulatorconductor transition is t = 1.97 ± 0.25. INTRODUCTION
Excimer laser processing of materials offers not only the advantage of high spatial and depth resolution in generating mechanical structures, but also the possibility of modifying other physical properties of materials with similar resolution. Recent experiments have demonstrated the ability to use excimer lasers to permanently change the electrical conductivity of high temperature polymers by more than 18 orders of magnitude [1, 2]. Fig. 1 shows typical ranges of electrical conductivities for a number of insulators, semiconductors, and metals. Also indicated is the range of electrical conductivity obtainable by
excimer laser irradiation of polyimide. The conductivity of this material ranges from being a
very good insulator (10-17 j1-1 cm-1) to a moderately doped semiconductor (10 Q- 1 cm-'). If polymers, which are widely used in the microelectronics industry as insulators, can be changed into electrical conductors in situ with sufficient spatial resolution, entirely new applications can be considered. It was demonstrated recently that conducting features in polyimide can be produced with a spatial resolution better than 0.5 gm [3]. Experiments with laser ablation have demonstrated an ultimate spatial resolution of well below 100 nm [4, 5]. It is the purpose of this article to describe the basic mechanisms which lead to this vast range of electrical conductivity in excimer laser irradiated polyimide.
EXPERIMENTS The basic experimental approach was to irradiate polyimide (Kapton) foils under different atmospheres and characterize the laser modified material by measurements of its electrical conductivity, visible and infrared absorption, as well as by transmission electron microscopy (TEM). Kapton foils of 12 and 50 gim thicknesses were irradiated with an injection controlled KrF laser which emitted 30 ns (FWHM) pulses with a repetition rate between 0.5 and 8 Hz. Only the most homogeneous part of the beam was selected and the samples were rotated at a speed of 10 revolutions/minute in order to ensure uniform irradiation. The fluence within a circular area of 4 mm in diameter was varied by changing the distance between the sample and a diverging lens. The samples were contacted using
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