Mechanism of Reactive Ion Etching of Polymeric Films in Oxygen-Based Plasmas
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MECHANISM OF REACTIVE ION ETCHING OF POLYMERIC FILMS IN OXYGEN-BASED PLASMAS
Sandra W. Graham and Christoph Steinbrtichel Materials Engineering Dept. and Center for Integrated Electronics Rensselaer Polytechnic Institute, Troy, NY 12180
ABSTRACT
The etching of polymeric films in 02 plasmas containing small amounts of CF 4 has been studied at low pressures (10-60 mtorr) in an RIE reactor. The relationship between etch rate, ion flux, and 0 atom concentration is investigated for photoresist, polyimide, and amorphous carbon. The applicability of a surface-chemical model proposed by Joubert, et al. (Q. Appl. Phys. 65(12) 1989, 5096) is explored. Modifications to the model are made to include physical sputtering and direct reactive ion etching. The modified model provides an improved description of the process, showing the various contributions of the ions and 0 atoms to the overall etching.
INTRODUCTION
Dry etching of polymers in 02 and 0 2/CF 4 plasmas has been an essential part of semiconductor fabrication for many years, yet an understanding of the basic mechanisms by which etching occurs is still incomplete [1-3]. Four basic etch mechanisms have been proposed [4,5], however, a knowledge of which of these are the dominant ones and under what conditions they dominate is still being investigated. The possible mechanisms for ionenhanced etching are (1) surface damage promoted etching, (2) chemical sputtering, (3) chemically enhanced physical sputtering, and (4) direct reactive ion etching. Joubert, et al. [6] have proposed a surface-chemical model for plasma etching of polymers based upon ionenhanced etching. Their model does not specify which mechanisms are active, but it implies that mechanisms (1)-(3) could all be involved. In this article we discuss the relationship between etch rate, etch yield, ion flux, and 0 atom concentration for photoresist, polyimide, and amorphous carbon. The model of Joubert, et al. is modified to include the contributions of direct reactive ion etching and sputtering. This new model provides a better description of plasma etching for these materials.
EXPERIMENTAL
Etching experiments were performed in an Applied Materials 8130 hexode reactor. The reactor is equipped with a laser interferometer for etch rate measurements. A stainless steel flange containing two small ports is located over two of the wafer positions. A
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Langmuir probe is mounted on one of these ports; a monochromator is mounted to a quartz window on the other port. The Langmuir probe is a 1 cm long cylindrical probe made from 5 mil tungsten wire. The probe is used to determine the ion density and electron temperature of the plasma as a function of pressure, gas composition, and reactor loading configuration. From these parameters, an estimate of the ion flux is obtained. A tuning circuit [7] on the output of the probe is used to minimize the rf impedance to ground thereby minimizing interference with the probe signal. Optical emission spectro
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