| AVS 60th International Symposium and Exhibition | |
| Electronic Materials and Processing | Tuesday Sessions |
| Session EM+MI+NS+SS+TF-TuA |
| Session: | High-k Oxides for MOSFETs and Memory Devices II/Oxides and Dielectrics for Novel Devices and Ultra-dense Memory I |
| Presenter: | W. Wang, University of Illinois at Urbana Champaign |
| Authors: | W. Wang, University of Illinois at Urbana Champaign N. Chang, University of Illinois at Urbana Champaign T. Hitt, University of Illinois at Urbana Champaign G.S. Girolami, University of Illinois at Urbana Champaign J.R. Abelson, University of Illinois at Urbana Champaign |
| Correspondent: | Click to Email |
An important fabrication challenge is to fill deep trenches or gaps with a dielectric material, such as shallow trench isolation or inter-metal dielectric in microelectronics. This is typically accomplished using chemical vapor deposition, which affords nearly conformal film growth, interspersed with one or more etching steps to prevent pinch-off of the feature opening. A superior alternative would be superconformal growth, in which the film thickness is inherently thicker towards the bottom of a deep feature than at the top, to afford complete filling in a single process.
We report a method, applicable to CVD processes that use two reactive molecules, to afford superconformal growth. It takes advantage of two insights. First, growth involves competitive adsorption on the film surface, such that the growth rate is maximum for a particular ratio of reactant fluxes (pressures) and falls on either side of this peak. Thus, there exists a regime in which decreasing the pressure of one reactant will increase the film growth rate. Second, the molecular (Knudsen) diffusion coefficient controls the rate of pressure drop down the axis of the feature. The reactant with the smaller diffusion coefficient (generally, the heavier molecule) will decrease in pressure faster than the other component. Combining these insights, we identify regimes of reactant pressure that afford superconformal growth in deep features.
We demonstrate superconformal growth, to a maximum depth beyond which the reactants are depleted, for two CVD systems. MgO is deposited at 220°C using the precursor Mg(DMADB)2 with H2O as the co-reactant; the growth rate increases from 1.0 nm/min at the trench opening to 1.8 nm/min at a depth/width ratio of 18. TiO2 is deposited at 300°C using TiCl4 and H2O; the growth rate increases from 1.5 to 4.0 nm/min at depth/width ratio of 5. The TiO2 coating inside trench is found to be stoichiometric and 88% of bulk density.
Finally, we describe a general model for the superconformal growth phenomenon. It uses as input the adsorption and reaction rate coefficients derived from growth on planar substrates, and suitable values for the molecular diffusivities. A first-order solution of the diffusion-reaction equation affords an analytic relationship that predicts the degree of superconformality in terms of the aspect ratio of the feature and the starting pressures of the reactants. It can be used to identify the regimes of useful operation and the necessary growth conditions. Given input data on reaction rates, this model can be used to predict which other two-molecule CVD systems would afford superconformal growth.