Nonlinear optical diagnosis of oxide traps formed during reactive ion etching

J Fang,G P Li,W W Heidbrink

doi:10.1063/1.1288168

J Fang, G P Li + Show 1 more

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https://doi.org/10.1063/1.1288168

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Abstract

Oxide traps generated by reactive ion etching are studied using a pulsed femtosecond laser. The second harmonic generation (SHG) signal from the Si/SiO2 interface is sensitive to charged traps in the oxide. The time evolution of the SHG signal indicates that positive traps predominate. The angular dependence of the polarized signal shows that the electric field generated by the oxide traps alters the symmetry of the sample. The damage is greatest for an oxide thickness of 13 nm (for a plasma dc bias of 300 V). Thicker oxides have smaller SHG signals, presumably because the Fowler–Nordheim tunneling currents induced by plasma charging of the oxide surface are smaller. Very thin oxides also exhibit reduced damage. The time dependent SHG signals depend on the temperature of the samples; these data provide information on the trapping and detrapping of substrate electrons by oxide holes.

Highlights

When exposed directly to a plasma, a wafer with a thin oxide is subject to two major kinds of damage
We postulate that trapped charge sites have a nonuniform distribution in the oxide so that the isotropic secondharmonic polarization changes inhomogeneouslylaterally; the electric fields from these randomly distributed charges add incoherently, increasing the isotropic SHG signal but canceling the constructive interference between the isotropic and anisotropic contributions that generates the four-fold symmetry
The initial SHG signal was independent of oxide thickness; RIE-processed samples exhibit a strong dependence on thicknessFig. 4͒

Summary

INTRODUCTION

When exposed directly to a plasma, a wafer with a thin oxide is subject to two major kinds of damage. Bombardment by electrons, ions, atoms, radicals, and ultraviolet radiation cause damage to the exposed oxide,[1] while surface charging produces indirect damage.[2] The surface charging occurs when the electron and ion currents that flow from the plasma source to the wafer do not balance locally. As the charge builds up on the wafer surface, the voltage across the dielectric layergate oxideincreases until the oxide layer starts to conduct via a Fowler–NordheimFNtunneling current. Both direct and indirect damage create trapped charges inside the oxide bulk and at the Si/SiO2 interface. We show that a nonlinear optical probe, the second harmonic generationSHGsignal from the SiO2/Si interface, provides a nonperturbative diagnostic of charge trapping caused by plasma etching.

EXPERIMENTAL TECHNIQUE AND BASIC MEASUREMENTS

DEPENDENCE OF THE OXIDE TRAP DENSITY ON OXIDE THICKNESS

TIME DEPENDENCE OF THE SHG SIGNAL

TEMPERATURE DEPENDENCE OF THE TIME DEPENDENT SHG SIGNAL

MECHANISM OF RIE-INDUCED DAMAGE

CONCLUSIONS