Particle adhesion in nano-structured microelectronics systems

Kathryn Maureen Smith, Purdue University

Abstract

In the microelectronics industry, as expectations for integrated circuit (IC) dimensions continue to shrink and restrictions on material loss become more stringent, many devices are approaching operating conditions at which one or more of their spatial dimensions are on the same length scale as van der Waals (vdW) forces, the primary forces governing particle adhesion near contact. The vast majority of studies to-date on vdW force phenomena treat systems consisting only of bulk materials having uniform macroscopic properties. In this work, we investigate the effect of varying nanoscale surface film thickness on particle adhesion of the vdW type to layered substrates. First, we examine a model system consisting of a silicon nitride atomic force microscope (AFM) probe, representing a contaminant particle, and a silicon substrate onto which alumina films have been deposited via atomic layer deposition. Force measurements using the AFM show an increase in the probe-composite substrate adhesion with alumina film thickness (represented by δ), changing from 0.207 ±0.020 N/m for δ = 0 nm to 0.449 ±0.019 N/m for δ = 39.6 nm. We apply theory developed by Vold to describe the effect of the thickness of the surface film material on the vdW interaction force between the particle and substrate. This theory is based on the assumption that each component of the layered substrate has a vdW interaction with the particle, but that the presence of each component does not affect the vdW interaction of the other. Our experimental data reveal that this assumption is appropriate only at the limits of behavior, i.e. for very thin (< 5 nm) and very thick (> 35 nm) coatings. To address the deficiencies of the Vold theory, we turn to the Lifshitz Formulation in the form of a simulation module written in the MATLAB language. Per the simulation results for the model system, we conclude that representing the nanoscale surface film as a bulk material is incorrect, and that the properties contributing to its vdW interaction—specifically, its complex dielectric permittivity function—must depend on the thickness of the film itself. This suggests that films of nanoscale thickness present unique material (optical) properties meriting further investigation. Finally, we examine an industrial-scale system involving the removal—via a non-optimized cryogenic aerosol process—of exemplar contaminant `challenge' particles of silicon dioxide from the surfaces of 300 mm-diameter silicon wafers with titanium nitride surface films of varying nanoscale thicknesses. The challenge particles have size distributions centered around 55- and 110-nm diameters. Results from particle removal efficiency (PRE)—a macroscopic metric of nanoscale adhesion—experiments show an increase in the PRE of more than 30% for the 55-nm and 10% for the 110-nm challenge particles. These results demonstrate the significant effect that nanoscale surface film thickness can have on macroscale particle adhesion.

Degree

Ph.D.

Advisors

Beaudoin, Purdue University.

Subject Area

Chemical engineering|Electromagnetics|Nanotechnology

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