An examination of tantalum pentoxide thin dielectric films using grazing incidence x-ray reflectivity and powder diffraction

 

Christine H. Russell, Scott M. Owens, and Richard D. Deslattes
Quantum Metrology Group
Atomic Physics Division, NIST,
Alain Diebold
SeMaTeCH, 2706 Montopolis Drive, Austin, Texas, 78741

New materials with high dielectric constants (high-K materials) are needed for the further development of dynamic random access memory (DRAM) chips. The problem arises because the continued scaling of such chips toward smaller dimensions using traditional low-K dielectric materials is limited by the electron tunneling through these very thin layers. To make smaller and higher density chips while keeping storage capacitance at the required level, new materials with higher dielectric constants need to be found.

Tantalum pentoxide, Ta2O5, is one member of this new class of materials with high dielectric constants. It has been shown that amorphous Ta2O5 thin films have dielectric constants in the 29.2 — 29.5 range, while crystalline Ta2O5 thin films have dielectric constants in the 45.6 — 51.7 range. One difficulty with Ta2O5 thin films is that they are difficult to grow reliably and precise measurements of film properties are needed for process qualification.

Grazing incidence x-ray reflectivity (GIXR) is a non-destructive analytical technique that gives accurate measurements of layer thickness, layer interface widths, and densities for single and multilayer samples. Powder diffraction of patterns such as shown in Figure 1 allow detection of crystallinity, identification of specific phases or verification of amorphous structure. The combination of these two methods allows detailed characterization of polycrystalline thin films.

In this work, we use GIXR and x-ray powder diffraction to a small group of tantalum pentoxide films (thickness range 2 nm to 8 nm) fabricated under different process conditions. The densities vary over a considerable range from low values (6.31 and 5.08 g/cm3 respectively) to near the tabulated bulk value of ~8.7 g/cm3. For the hexagonal form (d phase) expected from CVD process, the theoretical density is 8.316 g/cm3. The alternative orthorhombic form (b phase) of Ta2O5, produced when Ta is subjected to oxygen radical oxidation during growth, has a closely equal theoretical density. Crystal databases give structures for other oxides and an additional tetragonal form of Ta2O5.

While this work has demonstrated useful structural characterization, useful conclusions await study of a well-designed sample matrix.


Figure 1