Ion Implantation of Polymers
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ION IMPLANTATION OF POLYMERS
M.S. DRESSELHAUS,#*+ B. WASSERMAN,#+ AND G.E. WNEK#@ Department of Electrical Engineering and Computer Science; #Center for Materials Science and Engineering; +Department of Physics; @Department of Materials Science and Engineering; Massachusetts Institute of Technology, Cambridge, MA 02139, USA
ABSTRACT Ion implantation provides a mechanism for radically modifying the electronic and transport properties of a variety of polymers that are normally insulating. By using masks and tailoring the implanted species and ion energies, conducting paths in an insulating medium can be fabricated between specific reference points, an application of obvious relevance to the microelectronics industry. Specific results are reported for modification of the structure, electrical conductivity, thermoelectric-power, optical transmission and electron spin resonance for several polymers under a variety of implantation conditions. The temperature and frequency dependence of the conductivity suggest a onedimensional variable range hopping mechanism for conduction along the polymer chains. Comparison is made between implantation in the 200 keV and 2 MeV energy ranges.
INTRODUCTION Most polymeric materials can be regarded as good insulators and many have electrical
resistivities
1
of > 10 '
acm.
The
electronic
structure of
these insulating polymers consists of saturated bonds and tangled and interrupted chains which result in few if any conduction mechanisms. The low cost and easy processibility of some polymers make them attractive as electronic materials. The general instability of chemically doped conducting polymers presents problems with regard to their practical utilization. In contrast, the alternate doping scheme by ion implantation produces stable conducting materials. However, the conducting mechanism for ion implanted polymers is more difficult to determine because of the physical damage associated with the implantation process. Extensive studies of ion implantation in graphite have been reported [1] and it is found that the extensively damaged graphite material bears many similarities to highly damaged ion-implanted polymeric materials with regard to their electrical properties and lattice modes [2]. Recent developments in microelectronics technology make extensive use of energetic beams of radiation (electron, x-ray, and ion) for patterning. Of significance is the higher sensitivity of the polymer (by -2 orders of magnitude) to ion beams relative to electron beams, so that ion beam lithography is feasible at fluences of less than 10 cm[3]. For positive resists (e.g., PMMA), ion implantation results in scission of the molecular chains, while for negative resists (e.g., polystyrene), implantation results in crosslinking of polymer chains. In order to understand this technology in a more fundamental way, the investigation of radiationinduced defects in polymers has become an area of interest. The use of ion beams to define conducting paths (for example as interconnects) through an insulating
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