This paper reviews the present knowledge on tantalum pentoxide (Ta 2 O 5 ) thin films and their applications in the field of microelectronics and integrated microtechnologies. Different methods used to produce tantalum oxide layers are described, emphazing elaboration mechanisms and key parameters for each technique. We also review recent advances in the deposition of Ta 2 O 5 in the particular field of microelectronics where high quality layers are required from the structural and electrical points of view. The physical, structural, optical, chemical and electrical properties of tantalum oxide thin films on semiconductors are then presented and essential film parameters, such as optical index, film density or dielectric permittivity, are discussed. After a reminder of the basic mechanisms that control the bulk electrical conduction in insulating films, we carefully examine the origin of leakage currents in Ta 2 O 5 and present the state-of-the-art concerning the insulating behaviour of tantalum oxide layers. Finally, applications of tantalum oxide thin films are presented in the last part of this paper. We show how Ta 2 O 5 has been employed as an antireflection coating, insulating layer, gate oxide, corrosion resistant material, and sensitive layer in a wide variety of components, circuits and sensors.

Tantalum pentoxide (Ta2O5) thin films for advanced dielectric applications

Glow discharge sputtering

This report is about the preparation and the optical transmission loss of rf‐sputtered SiO2‐Ta2O5 films that are suitable for use as transmission media in integrated optical circuits. It is shown that the films, which can be produced with a refractive‐index range from 1.46 to 2.08 at 6328 A wavelength by suitably selecting the ratio of the target constituents, exhibit a low optical transmission loss of less than 0.8 dB/cm for the TE0 mode.

Refractive‐index‐adjustable SiO2‐Ta2O5 films for integrated optical circuits

The complexes [Ta(OEt)4(β-diketonate)] (β-diketonate = acetylacetonate, 2; hexafluoroacetylacetonate, 3; 1,1,1-trifluoroacetylacetonate, 4; dipivaloylmethanate, 5; 2,2-dimethyl-6,6,7,7,8,8,8-heptafluoro-3,5-octanedionate, 6) are established as volatile liquid precursors for low-pressure chemical vapor deposition (CVD) of films of tantalum(V) oxide. They give thermal CVD at a slightly lower temperature than the commonly used precursor [{Ta(OEt)5}2], 1. In all cases, the CVD at 300−450 °C gives amorphous films of Ta2O5, which crystallize on annealing at 800 °C under oxygen. Promotion of Ta2O5 film formation is established using catalyst-enhanced CVD with a palladium precursor as “catalyst” and the minimum CVD temperature is then reduced to 200 °C.

Chemical Vapor Deposition of Tantalum Oxide from Tetraethoxo(β-diketonato)tantalum(V) Complexes

Significant reductions in the optical scattering losses of Si 3 N 4 , Nb 2 O 5 , and Ta 2 O 5 waveguides fabricated on SiO 2 /Si substrates have been measured following CO 2 laser annealing. The largest improvements were observed for Si 3 N 4 waveguides, where waveguide attenuation values of about 6.0 dB/cm before laser annealing were reduced to as low as 0.1 dB/cm afterwards. An improvement of more than an order of magnitude was obtained for a Nb 2 O 5 waveguide upon laser annealing, the attenuation coefficient decreasing from 7.4 to 0.6 dB/cm. In the case of one Nb 2 O 5 waveguide no improvement was obtained upon laser annealing. The attenuation coefficient of a reactively sputtered Ta 2 O 5 waveguide was found to decrease from 1.3 dB/cm before laser annealing to 0.4 dB/cm afterwards. In the case of a thermally oxidized Ta 2 O 5 waveguide a small initial improvement in waveguide attenuation was followed by degradation upon further laser annealing.

CO 2 laser annealing of Si 3 N 4 , Nb 2 O 5 , and Ta 2 O 5 thin-film optical waveguides to achieve scattering loss reduction

Optical waveguides with losses <1 dB/cm at 632.8 nm have been prepared by sputtering tantalum in O(2)-N(2) mixtures. As the fraction of N(2) in the sputtering atmosphere increases, the refractive index of the guides exhibit a sharp maximum and then decreases to a much lower value, and the films become birefringent. Using an oxygen discharge, the indices at 632.8 nm for both TE and TM modes are 2.083 +/- 0.003, but the values are 1.850 and 1.877, respectively, for the highest fraction of nitrogen used. The indices have been measured at wavelengths down to 457.9 nm. For low fractions of N(2), the results are consistent with a decreasing film density and a constant molecular polarizability, but a number of effects, including the birefringence, are inconsistent with this picture. Waveguide structures, such as interconnects, branching waveguides, and layered directional couplers, have been demonstrated using films with different indices. The birefringence has also been verified by observation of interference between TE and TM modes simultaneously coupled into the guide.

Variable refractive index and birefringent waveguides by sputtering tantalum in O(2)-N(2) mixtures.

A novel technique is presented of making graded-refractive-index slab waveguides for integrated optics. Using the dependence of the optical properties on sputtering conditions, any variation in refractive-index profile in a film can be produced.

Y. C. Cheng

Papers

Variable refractive index and birefringent waveguides by sputtering tantalum in O(2)-N(2) mixtures.

Novel technique of making graded-refractive-index slab waveguides