Molecular models and activation energies for bonding rearrangement in plasma-deposited a -SiN x :H dielectric thin films treated by rapid thermal annealing F. L. Martı ´ nez, A. del Prado, I. Ma ´ rtil, and G. Gonza ´ lez-Diaz Departamento de Fı ´sica Aplicada III, Universidad Complutense de Madrid, E-28040 Madrid, Spain W. Bohne, W. Fuhs, J. Ro ¨ hrich, B. Selle and I. Sieber Hahn-Meitner-Institut Berlin, Glienicker Strasse 100, D-14109 Berlin, Germany Received 29 September 2000; revised manuscript received 24 January 2001; published 6 June 2001 Hydrogen and nitrogen release processes in amorphous silicon nitride dielectrics have been studied by MeV ion scattering spectrometry in combination with infrared spectroscopy. The outdiffusion of those light con- stituents was activated by the thermal energy supplied to the samples by rapid thermal annealing treatments. Molecular models of how these reactions proceed have been proposed based on the information obtained from the infrared spectra, and the validity of the models has been tested by an analysis of the activation energy of the desorption processes. For this purpose, the evolution of the hydrogen concentration versus the annealing temperature was fitted to an Arrhenius-type law obtained from a second-order kinetics formulation of the reactions that are described by the proposed structural models. It was found that the low values of the activation energies can be consistently explained by the formation of hydrogen bonding interactions between Si-H or N-H groups and nearby doubly occupied nitrogen orbitals. This electrostatic interaction debilitates the Si-H or N-H bond and favors the release of hydrogen. The detailed mechanism of this process and the temperature range in which it takes place depend on the amount and the proportion of hydrogen in Si-H and N-H bonds. Samples with higher nitrogen content, in which all bonded hydrogen is in the form of N-H bonds, are more stable upon annealing than samples in which both Si-H and N-H bonds are detected. In those nitrogen-rich films only a loss of hydrogen is detected at the highest annealing temperatures. DOI: 10.1103/PhysRevB.63.245320 PACS numbers: 68.60.Dv, 61.43.Er, 81.40.Tv, 82.20.Pm I. INTRODUCTION Amorphous hydrogenated silicon nitride ( a -SiN x :H) is a high-permittivity dielectric whose properties are specially suitable to look into the physics of amorphous solids and into the important role of hydrogen in semiconductors. 1 In par- ticular, its response to thermal treatments is a convenient way of investigating details of the bonding structure and net- work topology, by means of the lattice reactions that may be started up by thermal activation. Thereby, the hydrogen con- tent in the dielectric, its amount and distribution, plays a key role in determining the special kind of lattice reaction and network rearrangement. 2 In plasma-deposited silicon nitride films, this hydrogen originates from the precursor gases used in the deposition process, and it may be bonded to nitrogen and silicon or trapped in molecular or atomic form in micro- voids within the network structure. 3,4,5 Silicon nitride is used in inversion layer solar cells, 6 thin film transistors, 7 and memory devices 8 because of its excel- lent dielectric properties. Particularly useful is its high di- electric permittiviy, which makes it a promising candidate to substitute silicon dioxide in the gate structure of field effect transistors. Current levels of integration in microelectronic devices demand a rigorous decrease of the gate dielectric thickness in order to keep pace with the downscaling of the transistor lateral dimensions. This trend has taken silicon di- oxide to its limit of performance, where direct tunneling cur- rents become comparable to off-state drain to source currents. 9,10 The higher dielectric constant of SiN x :H makes possible increasing the physical thickness of the dielectric without reducing the capacitance and transconductance of the structure. In other words, a small equivalent oxide thick- ness is retained as needed by present technological require- ments, but with a physically thicker film that avoids the limi- tations of a thin oxide layer. 11,12 However, the practical realization of the above approach finds a serious limitation due to the higher stress of silicon nitride compared to silicon dioxide. 13,14 This stress originates from its higher average coordination number and results in a defective interface with the silicon substrate, which in turn causes a large density of charge-trapping defects. 15 A thor- ough understanding of the dielectric structure is needed in order to advance in this direction. Up to now a number of technological approaches have been tried to overcome this difficulty. Essentially, these techniques keep up silicon diox- ide as the main dielectric layer, especially at the interface with the silicon wafer, and then incorporate silicon nitride to increase the thickness up to a value that eliminates tunneling currents and ion migration 16,17 or, alternatively, achieve the same effect by means of an interfacial monolayer nitrogen nitridation that avoids tunnel conduction and a top surface monomolecular layer nitridation that blocks ion migration. 18,19 This article provides some basic understand- ing of the network topology of SiN x :H, which we believe will be valuable to progress in the technological applications. The study of the response of the dielectric to thermal treat- ments not only helps to achieve this understanding, but it is also important because of the potential benefits that con- trolled thermal processes may have on the bonding structure and interfacial characteristics. Rapid thermal annealing RTA has proved to be a useful PHYSICAL REVIEW B, VOLUME 63, 245320 0163-1829/2001/6324/24532011/$20.00 ©2001 The American Physical Society 63 245320-1