1. Introduction
For decades, reducing the size of the devices has been an essential issue in terms of device functionality and cost in the semiconductor manufacturing industry. Recently, the size of semiconductor devices has been reduced to nano-scale, and accordingly, the semiconductor process has become more detailed. A photoresist is widely used in nano-scale processes because it can be simply used as a mask using light and is also easy to remove [1,2,3,4]. However, photoresist (PR) is only used as a mask for patterning, so it should be removed after use. Typical methods for removing photoresists are wet and dry [5,6,7,8,9]. Wet etching is a method using a chemical solution, and since photoresist is made of polymer, it is removed using an acidic solution. Wet etching is advantageous in cost because it is simple to set up and has a relatively short time compared to dry etching, but it is limited in that it is difficult to analyze sample corrosion and trace elements due to strong acidity [10,11,12]. Therefore, in the nano-scale process, dry etching using plasma is used [13,14,15,16].
The semiconductor process, as if by Moore’s Law, has become refiner and more integrated over a few decades. Accordingly, the number of semiconductor processes has increased significantly, and it has become very important to reduce the processing time. It is necessary to increase the etching rate because dry etching, called plasma ashing, takes a longer process time than wet etching. Since the 1990s, there have been many studies to increase the ashing rate. For example, there are methods of controlling the substrate temperature, changing the gas flow rate, or inserting additional gas into oxygen plasma [17,18,19]. Among them, FC gas, nitrogen, hydrogen, etc., are used to insert additional gas into the oxygen plasma. Additional gases play a role in increasing the concentration of oxygen radical species in oxygen plasma, and nitrogen gas was used in this experiment [20,21]. In the previous study, it was reported that when nitrogen gas is added to oxygen plasma, nitrogen gas combines with oxygen radical species to form NO [22], thereby preventing the oxygen radical species recombination and increasing concentration. In addition, nitrogen plasma has relatively more vibrational and metastable states of nitrogen molecules than oxygen plasma, resulting in more dissociation of oxygen molecules [23]. Nitrogen, as well as other additional gases, similarly exhibit the effect of increasing the concentration of oxygen radical species by dissociating more oxygen molecules. In the ashing process, eventually, the most important factor is the density of oxygen radical species, so it is essential to set the ratio of additional gases to the maximum density of radical species. Then, the optimum point of oxygen radical species density can be found through plasma diagnostics.
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There are many methods to diagnose plasma parameters; electron density, electron temperature, plasma potential, radical density, etc. One of them is the Langmuir probe, which measures the current entering the metal tip driving tip voltage by penetrating the plasma [24,25,26]. We can obtain the electron energy probability function (EEPF) with the current by the Langmuir probe and determine electron density, electron temperature, and plasma potential through the EEPF. Moreover, among the methods for measuring the number of radicals in plasma, there is optical emission spectroscopy (OES) using light emitted from the excitation-relaxation process of gas. OES is a method of qualitatively determining the number of gas species by analyzing the spectrum of light emitted from the process. However, the amount of light obtained through OES depends on several variables such as electron density, rate coefficient, etc. Therefore, accurate radical density cannot be obtained by light intensity of OES only. In previous studies, many groups studied methods for converting the intensity obtained through OES into quantitative radical density [27,28,29,30]. One of the most commonly used methods is actinometry. Actinometry is a method of injecting and comparing an additional gas other than process gases, where the ratio of the rate coefficient between the gas and processing gas used should be constant depending on the electron temperature. Although the radical density can be inferred quite accurately using actinometry, there is a disadvantage in that a gas other than the process gas used should be injected [31].
In this study, compared with the previous study, we performed a more comprehensive experiment of the O2 plasma ashing process in various N2/O2 mixing ratios and RF powers; our investigation revealed that the tendency of the density measured using only OES did not exactly match the ashing rate [32]. This problematic issue can be solved by considering the plasma parameter, such as electron density. The detailed experiment procedure, result, and discussion are described in this paper [33,34].
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