The surface reaction model of SiO2 etching with fluorocarbon plasma has been discussed in many cases focused on the amorphous fluorinated carbon (a-C:F) film, that is deposited on the etched surface during etching. Challenges have remained to understand how the a-C:F film is deposited during the etching process and how the underlying SiO2 film was etched by simultaneous formation of the a-C:F film. Thus it needs to clarify how the underlying SiO2 is etched while the a-C:F film was being deposited.
The Langmuir adsorption model has been introduced in which the adsorption of adsorbing particles is limited to the number of adsorption sites. However, considering microscopic details of the adsorption phenomena, it has been unclear the details with the Langmuir adsorption model. During the fluorocarbon plasma etching, a-C:F film is deposited and its steady state thicknesses are more than a few nm. The thickness obtained from experiments was more than a single molecular layer. To assume any saturation factor in adsorption of more than two molecular layers is hardly to understand it based on physical aspects. Namely, it is not clear whether the mechanisms for reaching this steady state are site formation or saturation adsorption, where a-C:F films are deposited with this steady state thickness on the surface during etching. This has been a challenge to understand the process of the surface transition to the steady state by dynamic analysis including time variation.
Next, it has been explained that the etching rate is inversely proportional to the a-C:F film thickness because of the diffusion of F in the film. However, the modeling that the diffusion of the F etchant decreases as the a-C:F film becomes thicker does not seem to adequately describe the phenomenon. This is because the a-C:F film is only a few nm thick and it is unlikely that the diffusion transport in the very thin film deposited on the surface of the substrate will rate-constrain the reaction while it is being etched.
With an effective bias applied to the substrate, the system is irradiated with ions. At the same time, it is unclear how the surface reactions are affected by radicals and other elements incident from the gas phase. Some of the detailed points remain unclear, such as how this surface denaturation effect manifests itself when the surface is denatured by radical adhesion and irradiated with ions.
From above, the following questions has been required to be solved for understanding correctly the surface reactions during fluorocarbon plasma etching of SiO2;
1. Is the Langmuir model appropriate for adsorption of adsorbing particles?
2. The deposition mechanism of the a-C;F films
3. What the rate limiting step for determination of the etching rates is either the rate of diffusion of F etchant,
4. Is the effects of the impact ion energy,
It has been hoped that these points would be resolved first.
Outline of this study
The development of semiconductor device manufacturing has been greatly driven by microfabrication techniques. Today, device dimensions have been reached right down to the molecular and atomic levels, making use of the properties of the materials used in a variety of ways. In order to realize this, both in the development and manufacturing stages, it is increasingly important to control the processing of structural dimensions, electrical, mechanical and various physical properties of materials at the molecular and atomic levels. With this background, the author has been studying the control of processing and properties of atomic and molecular scale in the manufacturing process. In this study, we attempted to elucidate surface reactions in plasma processes by using spectroscopy.
The current status of the manufacturing process and its problems are discussed and the background of this study is described. The microfabrication technique of silicon oxide film as an insulator is the key, and fluorocarbon plasma is used to etch this silicon oxide film. A complex chemical reaction occurs during plasma etching, in which a fluorocarbon film is deposited on the surface while the etching proceeds. The films deposited on this surface work to inhibit etching of non-etching regions, but on the silicon oxide film of the workpiece, they are removed as soon as it is etched. Even though it was found to be feasible, obtaining the desired etching characteristics (e.g., shape and processing speed) required a great deal of trial and error. Therefore, it is necessary to understand the reactions from the perspective of efficient reaction control. In order to advance our understanding of this reaction, the author has been working with physicochemical structural analysis using spectroscopy.
Due to technical difficulties, in-situ observation of the surface during the etching process has not been a major part of our understanding of the etching reaction of silicon oxide films by fluorocarbon plasma. The author believes that in-situ observation of the surface in the etching process is the first priority, and for this purpose, the most suitable observation method for in-situ observation has been developed by using infrared spectroscopy and electron spin resonance. Throughout this development, the surface in fluorocarbon plasma etching has been investigated using a variety of measurement techniques, including Fourier-transformed infrared spectroscopy (FT-IR), laser-induced fluorescence (LIF), X-ray photoelectron spectroscopy (XPS), and in vacuo electron spin resonance (ESR) to determine surface dangling bonds.
The analyzed results of etching reactions of silicon oxide films during the fluorocarbon plasma are described. The author has focused on the interaction between the surface and the active species, radicals and ions, in the gas phase of the fluorocarbon plasma, with a focus on the near-surface behavior of the CFx radicals and the process of polymer deposition, and while the gas phase and the surface have been observed separately in the past. In this study, the author has used two-dimensional laser-induced fluorescence and the deposition process of fluorocarbon polymers on the surface was successfully measured simultaneously by infrared spectroscopy. It was found that near-surface radical fluxes alone do not determine polymer deposition, especially in high-density plasma (ne ≤ 1011 cm-3). This clearly demonstrates the limited applicability of previously proposed polymer deposition models.
Next, using a device capable of delivering a mass-separated fluorocarbon ion beam, we will clarify the surface denaturation of SiO2 when it is irradiated with ions only, apart from the plasma. In-situ X-ray photoelectron spectroscopy of the ion irradiated surfaces showed that the surface denaturation was strongly dose-dependent when irradiated with low F ions such as CF+ ions. In the initial stage of irradiation, the etching of SiO2 proceeds with the accumulation of C on the surface. It was found that the amount of C on the surface increased as the cumulative amount of irradiation (dosing) increased, and after approximately 1017 cm-2 was dosed, the amount of C on the surface reached a certain critical value, and the etching stopped and rapidly changed to continuous fluorocarbon film deposition. This result shows that the surface reaction changes depending on the incident species and surface denaturation.
The author describes the results of in situ observations of the surface during etching. Here, in-situ observation of the surface of silicon oxide films during etching was successfully performed by infrared spectroscopy. The dynamics of fluorocarbon polymer film deposition on the surface of the silicon oxide film during the etching process was analyzed to elucidate the dynamics of the deposition process. In addition, a method was developed to study the dangling bonds formed on the inner side (subsurface) near the surface during etching using electron spin resonance. Because the dangling bonds are reactive and active, they disappear when exposed to air, we successfully observed the surface dangling bonds formed during etching by transporting the samples in a vacuum and observing them by electron spin resonance.
Finally, I will summarize and discuss the future prospects.