Pierre Ranson

Cryogenic etching of deep narrow trenches in silicon

Deep and narrow anisotropic etching of silicon structures has been investigated in a low-pressure high density plasma reactor working with a cryogenic chuck. We have previously demonstrated the feasibility of this technique on such structures. Improvement of etch rate and profiles has been studied and new results show 2 μm wide trenches etched to a depth of 50 μm at an average etch rate of 5 μm/min with highly anisotropic profiles and very high selectivity (>500:1) toward the SiO2 mask. An evaluation of a commercially available reactor from Alcatel has been carried out and similar results are obtained. A phosphosilicate glass mask has been used to study the effect on profiles. It is shown that undercut is reduced while bowing is independent of the mask material. Since surface temperature strongly affects the profiles, wafer deformations in our cryogenic chuck have been measured and temperature evolution across the wafer has been estimated. A significant temperature difference of 10 °C between the chuck an...

Passivation mechanisms in cryogenic SF6/O2 etching process

Passivation mechanisms of Si trenches involved in SF6/O2 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 SiO2 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.

SiOxFy passivation layer in silicon cryoetching

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 passivati...

Silicon columnar microstructures induced by an SF6/O2 plasma

An inductively coupled SF6/O2 plasma is used to form a columnar microstructure (CMS) on silicon samples cooled at very low temperature (~ −100 °C). The formation of this CMS is studied as a function of bias voltage, temperature, RF power and gas pressure. The characteristic mean diameter and mean height of the microstructure are evaluated by image processing tools from SEM micrographs. A crystallographic effect is also observed at very low temperature, which induces a needle-shaped structure. A physical mechanism is proposed to explain the formation of this CMS.

In situ x-ray photoelectron spectroscopy analysis of SiOxFy passivation layer obtained in a SF6/O2 cryoetching process

The 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.

Deep GaN etching by inductively coupled plasma and induced surface defects

GaN etching was studied in Cl2/Ar plasmas as a function of process parameters. In addition, for a better understanding of the etching mechanisms, Langmuir probe measurements and optical emission spectroscopy were carried out. Etch rate was found to depend strongly on bias power. After optimization, an etch rate greater than 1000 nm/min was achieved. A second part of this work is dedicated to the etched surface defects. An original method to estimate GaN dislocation density and to localize nanopipes in the material is presented. Columnar defects could also appear with impurities in the etching reactor. The authors also present a possible formation mechanism of those columnar defects.

Oxidation threshold in silicon etching at cryogenic temperatures

In silicon etching in SF6∕O2 plasmas, an oxidation threshold appears when the oxygen content is large enough. A SiOxFy passivation layer is formed under such conditions. This threshold is reached at lower oxygen proportions if the substrate is cooled down to cryogenic temperatures. In this article, we present a mass spectrometry study of this oxidation threshold in different experimental conditions (temperature, source rf power, self-bias) on bare silicon wafers. The presence of the threshold is clearly evident in the signals of many ions, for example, SiF3+, F+, and SOF2+. This helps us to determine the main reactions which can occur in the SF6∕O2 plasma in our experimental conditions. This threshold appears for higher oxygen proportions when either the source power or the chuck self-bias is increased. The ion bombardment transfers energy to the surface and makes the film desorb. A model, describing the oxygen coverage as a function of the parameters mentioned above, is proposed to interpret these result...

Chemical physics of fluorine plasma-etched silicon surfaces : study of surface contaminations

F2 plasma‐Si(100) surface interaction experiments have been conducted to understand basic mechanisms of surface modifications. Surface analysis has been investigated using x‐ray photoelectron spectroscopy and nuclear reaction analysis. The experiments show deep penetration of fluorine into silicon and limitation of etching caused by oxide layers coming from contamination of the plasma by removal of oxygen from alumina walls of the reactor. Biasing of the silicon substrate enhanced carbon contamination.

Effect of limiting the cathode surface on direct current microhollow cathode discharge in helium

This paper describes how to light several microdischarges in parallel without having to individually ballast each one. The V-I curve of a microhollow cathode discharge is characterized by a constant voltage in the normal glow regime because the plasma is able to spread over the cathode surface area to provide the additional secondary electrons needed. If one limits the cathode surface area, the V-I characteristic can be forced into an abnormal glow regime in which the operating voltage must increase with the current. It is then possible to light several microdischarges mounted in parallel without ballasting them individually.

Optimization of submicron deep trench profiles with the STiGer cryoetching process: reduction of defects

The STiGer process is a time-multiplexed cryogenic etching method designed to achieve high aspect ratio structures on silicon. SF6 or SF6/O2 plasmas are used as etch cycles and SiF4/O2 plasmas are used as passivation cycles. Trenches with a critical dimension of 0.8 µm have been etched to a depth of 38 µm with an average etch rate of 1.8 µm min−1. These features exhibit both undercut and a defect which is called extended scalloping. We describe this defect specific to the STiGer process and we discuss its origin: the extended scalloping is composed of anisotropic cavities developed on the sidewalls of the feature top (typically in the first 2–3 µm below the mask). It originates from ions scattered at the feature entrance that hit the top profile and remove the passivation layer where it is weakest. Then, we propose two methods to reduce this extended scalloping. The first consists in adding a low oxygen flow in the etching cycle. It favors a low additional passivation which reduces scalloping. The second technique consists in gradually increasing the SF6 flow from a low value to the nominal value. Consequently, the process starts with a low etch rate and an efficient passivation.