Ion implantation of tantalum thin films.

Tantalum thin films have been implanted with argon, nitrogen or oxygen to change their physical, chemical and electrical properties. The films were deposited either by electron beam evaporation or by D.C. diode sputtering in an atmosphere or argon. Physical and chemical changes of the films were examined with the aid of the transmission electron microscope (TEM). Some X-ray diffraction techniques and Electron Spectrum for Chemical Analysis (ESCA) were also undertaken in conjunction with the above. Present results show that the electrical characteristics, viz. sheet resistance, resistivity and temperature coefficient of resistance (TCR) of the thin films before and after irradiation are very dependent on the growth history of the films. In general for a film of given thickness, resistivity increases with impurity content with the TCE becoming more negative. When the films were implanted with energetic ions, resistivity and TCR change according to the type of ions used. Films irradiated with argon showed a gradual, almost linear rise in resistivity initially, after which a nonlinear region was observed, followed by a very rapid change for a small increase in dose. The TCR remained almost constant throughout. Electron diffraction analysis of the films at increasing doses showed the presence of the lower oxide of tantalum (Ta2O). When the films were implanted with oxygen, resistivity versus dose plots similar to the argon treated films were obtained but with a distinct difference in the middle dose region. The TCR for the films changed sign from negative initially, to positive and back to negative again with increasing doses. At light doses, highly crystallised b.c.c. tantalum was precipitated out from the films. This was quickly replaced by less crystalline Ta2O as the dose increased. The crystal structure of the Ta2O became more ordered with increasing dose but deteriorated at very high doses, the films becoming amorphous. When the films were implanted with nitrogen two peaks were observed in the resistivity versus dose plots with correspondingly large negative excursions in their TCR. The compound Ta4N5 has been identified in these films. It was observed that the films were very resistant to radiation damage, up to very high doses. Finally, where possible we have attempted to correlate the physical and chemical effects of implantation to the measured electrical behaviour of the films