Investigation of highly diffusive point defects during Si plasma etching

Abstract

The quantitative prediction and precise control of plasma-induced damage by high-energy incident ions during Si etching for detailing the regions of amorphous, dangling bonds, and point defects (interstitials, vacancies, and clusters) are essential for optimizing the performance of CMOS devices. Particularly, regions of amorphous and dangling bonds with concentrations of an order of 1018-1019 atoms/cm3 have been studied and modeled extensively [1][2][3]. Insights obtained from these studies are used for suppressing damage to these layers to realize plasma processes with low damage. To enhance the performance of advanced CMOS devices with complex structures, understanding the mechanisms of formation and distributions of points defects with less than the order of 1017 /cm3, which cannot be accurately detected through transmission electron microscopy and secondary ion mass spectroscopy, is crucial. However, limited studies have focused on the effects of point defects generated during Si plasma etching.
Therefore, we focused on point defects as damage during Si plasma etching and proposed numerical simulation modeling in which Si interstitials (I) are generated depending on not only ion energy distribution but also the gas system. These interstitials are highly diffused and interact with nearby vacancies and incident hydrogens from plasma to form clusters (Im). Considering the energy balance between an incident ion and sum of forming energy of each cluster during Si trench etching with an ion energy (Vpp) of 1200 eV, I3 or I4 cluster corresponding to m = 3 or 4 can be formed at most. In the proposed model, this transient phenomenon was included depending on the substrate temperature.
To confirm the validity of the damage model, we analyzed photoluminescence (PL) data after Si trench etching with or without hydrogen irradiation and revealed that the model assumptions are reasonable for the variations of the observed PL intensities originating from point defects. Furthermore, we performed in-situ X-ray photoelectron spectroscopy on the B1s spectrum in the highly Boron-doped Si substrate with various X-ray irradiation angles after Ar+ ion irradiation into the doped substrate as damage with a temperature variation of 25, -50, and -120 ºC. At 1.5-nm depth from the surface corresponding to the a-Si region, the intensity of the B1s spectrum did not vary, but the intensity at the 6-nm region increased with the decrease in the temperature (i.e., -50 and -120 ºC) and decreased when the substrate temperature of -120 ºC was back to 25 ºC. This experimental result revealed various phenomena with the depth on the nanometer scale and supports the proposed formulation as the diffusion phenomena of Si interstitials.
Consequently, we revealed that the diffusion of Si interstitials induced by plasma irradiation can be suppressed by extremely low temperature. Furthermore, cryo etching exhibits considerable potential for not only improving etched profile with high selectivity but also suppressing plasma-induced Si damage.

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