The 70th Japan Society of Applied Physics Spring Meeting 2023
Highlighted Presentations Press Release
April 18, 2023

Diagnosis of Spatial Distribution of Electron Temperature and Electron Density of Argon Inductively Coupled
Plasma by Tomographic Optical Emission Spectroscopic Measurement

Plasma process essential for large-scale integrated circuits
to measure 3D spatial distribution of electron temperatureand electron density
Tokyo Institute of Technology 1, ULVAC2
Yuya Yamashita 1, Kenta Doi2, Tetsuji Kiyota2, Keiichiro Asakawa2, Sotaro Hosoya 1, Wataru Kikuchi 1, Atsushi Nezu 1,Hiroshi Akatsuka 1Summary• We established a method to diagnose the three-dimensional spatial distributions ofelectron temperature and electron density in plasma using tomographic optical emission
spectroscopic measurement and collisional-radiative model-based analysis.
• The correlation between the changes in pressure and power in the reaction chamber andthe three-dimensional spatial distributions of electron temperature and electron density inthe plasma process has not yet been quantified.
• Anomaly detection systems for plasma processes contribute to higher accuracy inplasma simulation.

A research group led by Mr. Yuya Yamashita (Ph.D. student), Associate Professor Dr. Hiroshi Akatsuka, and colleagues
at the Department of Electrical and Electronic Engineering, School of Engineering, Tokyo Institute of Technology, andULVAC, Inc. successfully performed the world’s first quantitative diagnosis of the three-dimensional spatial distributionsof electron temperature and electron density in plasma processes using tomographic optical emission spectroscopic
measurement and collisional radiation modeling. This study is expected to promote the development of plasma
electronics in the future and process development, which will considerably contribute to the improvement ofsemiconductor processes in the industry.

Detail

Plasma process essential for semiconductor processing
Plasma (*1), also known as the fourth state of matter, is widely used in semiconductor
manufacturing and fundamentally supports modern electronics. For example, the plasma process,which is a processing technology using plasma, is indispensable for the ultrafine processing oflarge-scale integrated circuits in semiconductor chips, which are the fundamental components ofsmartphones and PCs, and for synthesizing transparent conductive films, which are typically usedin solar cells and LCD TVs.
This study contributed to the development of advanced and precise plasma processes. Plasma is
widely used for etching (*2) and other processes in integrated circuits. It is necessary to control
parameters, such as the power input and pressure in the plasma equipment, to determine theoptimal processing conditions. However, the correlation between the amount of power andpressure required and the type of processing that can be achieved has not yet been clarified
sufficiently. Therefore, optimizing the processing conditions in the semiconductor process ischallenging.
In actual production, test processing is performed by changing the power and pressure of theplasma system in several ways, and the processing conditions are optimized by further evaluating
the physical properties, which is an extremely expensive and time-consuming process.
Electrons play an important role in the progress of reactions in the plasma used in semiconductor
processing. Electron temperature, which indicates the energy of the electrons, and electron
density, which indicates the number of electrons per unit volume, are important indicators to control
the plasma. In this study, we established a method for diagnosing the three-dimensional spatial
distributions of electron temperature and electron density. We expect this method to optimize theconditions of plasma processing and contribute to yield improvements.

The world’s first diagnosis of the 3D spatial distributions of electron temperatureand electron density in plasma
In this study, argon (Ar) plasma was generated using an inductively coupled plasma system, which
is an experimental device that simulates a dry etching system. The generated plasma wasobserved simultaneously through 18 lenses installed in the device, and the spectral radiance (*3)
of the optical emission spectrum (*4) was measured (Figure 1). A “spectral tomography calculation”
was performed on the spectral radiance for each line-of-sight. From the line-of-sight dependenceof the spectral radiance obtained in the experiment, the three-dimensional spatial distribution of thespectral emission coefficient (*5) was calculated based on the lens position relative to the plasma
dimensions.
The spectral emission coefficients at each coordinate were analyzed based on the collisionalradiative model, which is a mathematical model of the atomic and molecular processes in plasma(*6). This method achieved the noncontact diagnosis of electron density and electron temperature,
which are important indices for plasma control, without disturbing the plasma. Based on thisdiagnostic method, the relationship between the power and pressure applied to the plasma device
and the electron temperature and electron density of the generated plasma (Figure 2) wasclarified. This correlation was not apparent before, and we could quantify how the changes in
power and pressure affected the electron temperature and electron density in the plasma device.

These results are expected to contribute to the elucidation of the mechanism of plasma
processing.
Several previous studies have measured the optical emission spectra of plasma from a single
perspective. However, in the current semiconductor process, silicon wafers (*7) are larger indiameter to increase the yield. The reactions occurring at the center and edge of the silicon wafer
are different; therefore, an accurate diagnosis cannot be achieved by measurement from a single
viewpoint. In addition, although previous studies have used tomography, a technique to calculate
the three-dimensional spatial distribution of optical emission spectra and complex atomic andmolecular processes has not been studied. Thus, no previous studies have considered atomic andmolecular processes in detail or have realized the diagnosis of the 3D spatial distribution ofplasma. Therefore, it was not possible to determine the electron temperature and density
accurately, which are problems in the plasma process. “We have achieved more reliablediagnostics of electron temperature and electron density by analyzing the results of tomographic
spectroscopy measurements based on our long-standing research results on plasma atomic andmolecular processes,” said Associate Professor Akatsuka of Tokyo Institute of Technology.

Applications including anomaly detection systems and more accurate plasma
simulations“If this study can be applied as a measurement tool, we can consider an anomaly detection systemfor plasma processes: in other words, a tool that can detect abnormal reactions by observing
during actual processing and eliminating the abnormalities before final processing," Yamashita
said.
Furthermore, by conducting “plasma simulation” introducing electron temperature and electrondensity based on the power and pressure dependence obtained through this study, the company
says that it can concretely contribute to cost reduction in the optimization of process conditions
depending on conventional repetitive experiments. Dr. Kenta Doi of ULVAC, a joint research
partner in this study, said, “The knowledge of electron temperature and electron density, which aredirect factors in the optimization of process conditions and process results, will enable processdevelopment with more evidence.”
Dr. Doi added, “With regard to abnormality detection, it may become possible to detect signs ofabnormality if the system is installed in actual equipment. If we can detect abnormalities, we will beable to operate the equipment more rationally.”Annotation*1 Plasma: An electrical discharge phenomenon that occurs in gas. When an electric discharge is generated in a gas,
electrons are separated from atoms and molecules owing to the electrical energy. Consequently, three types of particles(electrons, positive ions, and atoms/molecules) coexist in the plasma. In an ordinary gas that is not plasma, only atomsand molecules exist.
*2 Etching: A semiconductor manufacturing process. In semiconductors, circuits are formed by transferring a circuit
pattern onto a thin film of a substrate. Etching is used to remove thin films from unnecessary parts. Plasma has beenused for dry etching.
*3 Spectral radiance: The dependence (spectrum) of the intensity and wavelength of the light emitted from a window isexpressed as a physical quantity per window area and light spread angle. This value was obtained by adding thespectral emission coefficients of the plasma in the line of sight of the lens.
*4 Optical emission spectrum: The wavelength and intensity of light emitted when an atom or molecule transitions froma higher-energy state to a lower-energy state: The energy state of an atom or molecule can be quantitatively determinedby analyzing its emission spectrum.
*5 Spectral emission coefficient: The intensity and wavelength dependence (spectrum) of light emitted by the plasma
are expressed as a physical quantity per volume of plasma and light spread angle. As plasma has a three-dimensional
spread, the spectral emission coefficient is position-dependent.
*6 Atomic and molecular processes: Reactions in which electrons, positive ions, atoms, and molecules collide. In theplasma, energy is exchanged through these collisions, leading to the reactions used in processing.
*7 Silicon wafers: Circular plate cut from high-purity silicon. It is also used as a substrate for semiconductors. Circuit
patterns were formed on silicon wafers during the manufacturing process.

FiguresFigure 1 Plasma generated by an inductively coupled plasma system (left) observed using a “multichannel spectrograph” through 18
lenses (right) installed on the sides and top of the system. The line-of-sight dependence of the spectral radiance for each line of sightwas measured.

Figure 2 Diagnostic results of the power dependence of the spatial distribution of electron temperature. The electron temperature
increased with increasing power. This may be due to the increase in drift velocity caused by the increase in induced electromagnetic
field strength. In addition, the electron temperature showed an asymmetric distribution along the y-axis. This may be due to the antenna
geometry.