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Passivation mechanisms of Si trenches involved in SF 6 /O 2 cryogenic plasma etching were investigated in order to better control the process and avoid defects. Trench sidewalls and profiles were ex situ characterized geometrically by SEM and chemically by spatially resolved XPS experiments. These measurements reveal that the passivating layer is removed during the increase of the wafer chuck temperature leading to a very clean surface of the sidewalls after processing. Nearly no SiO 2 formation on the sidewalls was observed after the very low temperature etching (−100 • C). A two-step process was defined to rebuild the passivating layer after its destruction and continue the trench etching. The necessary conditions for properly rebuilding the passivating layer give precious information about its chemical composition. These experiments clearly show that sulfur is not a necessary element to form an efficient passivating layer.

The SiOxFy passivation layer created on structure sidewalls during silicon cryoetching is investigated. This SiOxFy passivation layer formation strongly depends on O2 content, temperature and bias. It is a fragile layer, which mostly disappears when the wafer is warmed up to ambient temperature. A mass spectrometer was used to analyze the desorbed species during the warm-up and using this instrument allowed us to find a large signal increase in SiF3+ between −80°C and −50°C. SiF4 etching products can participate in the formation of the passivation layer as it is shown by a series of test experiments. SiF4∕O2 plasmas are used to form a thin SiOxFy layer on a cooled silicon wafer. Thickness and optical index of this thin film can be determined by in situ spectroscopic ellipsometry. It is shown that the passivation layer spontaneously desorbs when the silicon wafer temperature increases in good agreement with the mass spectrometry analysis. Two physical mechanisms are proposed to explain the SiOxFy passivation layer buildup involving either the etching products or the SiFx sites created during etching. In both cases, oxygen radicals react at the surface to form the SiOxFy layer.

Articles you may be interested inProfile control of high aspect ratio trenches of silicon. II. Study of the mechanisms responsible for local bowing formation and elimination of this effect J.A cryogenic etching method with SF 6 /O 2 chemistry plasma in an Alcatel inductively coupled plasma reactor is used to achieve deep trenches with high aspect ratio ͑depth/width Ͼ10͒ and high anisotropy. The mean etch rate in 2 m wide and 100 m deep trenches is about 3.5 m/min. The slope of the trenches can be adjusted from 88°to 90°by selecting appropriate process conditions and selectivity ͑silicon/SiO 2 mask͒ is higher than 400:1. However, profiles still present a defect: they need to be improved, mainly by reducing the local bowing effect. Local bowing is lateral etching located on the sidewalls and resulting in profiles destruction. This article deals with the study of the local bowing and profile improvement by changing process parameters. We investigated the effect of the process parameters ͑pressure, bias voltage, temperature, gas flow rates, etc.͒, and the mechanisms responsible for local bowing are discussed and evaluated. The final aim is to find the cause of the appearance of local bowing and thus the means to reduce or eliminate it. We especially showed that local bowing depends on the efficiency of the trench passivation mechanism and on ion energy and density: ions seem to be responsible of the destruction of the passivation layer.

3 pagesInternational audienceThe oxyfluorinated silicon passivation layer created during various cryoetching processes is of interest in order to improve high aspect ratio profiles. In this work, the desorption of a SiOxFy layer obtained in an overpassivating SF6 /O2 regime was investigated during the wafer warm-up from the cryogenic temperature to room temperature. An in situ x-ray photoelectron spectroscopy XPS device is used in order to probe the top-surface layer and understand the desorption mechanism. A new mechanism can be proposed using the evolution of fluorine, oxygen, silicon, and carbon contributions evidenced by XPS