Optical Emission Spectroscopy In The Fabrication Of Integrated Circuits

INTRODUCTION

Optical Emission Spectroscopy (OES) is crucial in the fabrication of integrated circuits (ICs), where precise control over each process step is essential. ICs often comprise numerous layers, with each layer requiring exact deposition and etching to create complex structures. In thin-film deposition, a process used to deposit material layers on a substrate, controlling the plasma environment is critical for achieving uniform and high-quality films. OES plays a significant role by monitoring the light emitted during plasma ionization, which provides real-time insights into the deposition process. This allows for precise adjustments to ensure optimal plasma conditions, preventing defects and ensuring that each film is deposited with the correct thickness and composition. Thus, OES is an indispensable tool in modern electronics manufacturing, helping to maintain the high standards required for effective IC fabrication.

METHOD

In thin-film deposition processes such as Plasma-Enhanced Chemical Vapor Deposition (PECVD), OES is employed to monitor and control the plasma environment. The technique involves analyzing the light emitted from excited atoms and molecules in the plasma to gain insights into its composition and behavior. By evaluating these emission spectra, OES allows for precise adjustments of parameters such as gas flow rates, pressure, and power. This control ensures that the deposited thin film meets the desired specifications for thickness, uniformity, and material properties. OES is particularly useful in the fabrication of semiconductor devices, solar cells, and other advanced materials, where the quality of the thin film directly impacts device performance. For example, in the PECVD process, OES helps analyze the emission spectra of elements in the plasma, ensuring that thin films like SiO₂ or TiN are deposited uniformly and precisely, meeting the technical requirements of the material. This is crucial in the production of semiconductor devices and solar cells.

Figure 1 from J.J. Robbins’s experiments compares two emission spectra within a specific wavelength region to identify the effects of adding SnCl₄ on the plasma composition. The spectrum from plasma with an oxygen (O₂) flowrate of 6 sccm and an argon (Ar) flowrate of 32 sccm is shown by the solid line. After 2.2 sccm of SnCl₄ were added to the plasma, the spectrum is displayed by the dotted line. The additional features in the dashed line spectrum indicate the introduction of many new spectral lines due to the addition of SnCl₄. Eight notable new signals were detected and divided into two groups: Group B lines, whose source molecules are unknown, and Group A lines, which behave similarly and are recognized as atomic chlorine. The goal of the experiment is to illustrate how the introduction of SnCl₄ alters the emission spectrum, providing insights into the plasma’s chemical environment and helping to optimize the plasma conditions.

Figure 1. Comparison of the OES spectra for ArqO plasma and Ar 2 q O2 4 qSnCl plasma.The circled peaks are a sample of the wavelengths used for analysis.

EQUIPMENT

Setting up Optical Emission Spectroscopy (OES) involves several crucial steps to ensure precise measurement and real-time monitoring, particularly in plasma-based processes like etching or deposition used in the fabrication of integrated circuits (ICs). The spectrometer is the central component of the OES system, responsible for separating emitted light into its constituent wavelengths (spectrum). To achieve accurate results, the spectrometer must provide sufficient resolution to differentiate between the emission lines of various elements and cover a broad wavelength range, typically spanning ultraviolet (UV), visible (VIS), and sometimes infrared (IR) regions. This allows the detection of emissions from elements such as oxygen, fluorine, and chlorine.

An Ocean Optics USB2000+ spectrometer along with a 200-mm bifurcated cable with a 25-mm slit was used for the collection of optical emission. The spectrometer contained two gratings for the separation of the wavelengths. One detected a wavelength range from 200 to 850 nm: the other from 530 to 1100 nm. was used. The optical resolution with this arrangement was approximately 1.5 nm

Another suitable spectrometer is the HR4000 Spectrometer which is a high-resolution spectrometer designed specifically for applications requiring fine spectral detail, such as gas analysis, laser characterization, and plasma monitoring. With a resolution of up to 1.0 nm and a broad wavelength range from 190 - 1025 nm that includes UV, VIS, and near-infrared (NIR) regions, the HR4000 is well-suited for precise analysis of plasma-based processes, like those used in IC fabrication. Its compact design and compatibility with fiber optic systems make it easy to integrate into both industrial and laboratory settings.

Figure 3. Ocean Insight’s HR400 High-Resolution Spectrometer

CONCLUSION

In conclusion, Optical Emission Spectroscopy (OES) is indispensable for the fabrication of integrated circuits (ICs), providing crucial insights into plasma-based processes such as etching and deposition. The precision of OES, facilitated by high-resolution spectrometers like the HR4, enables detailed analysis of emission spectra, allowing for the accurate monitoring of elements like oxygen, fluorine, and chlorine. By delivering real-time data on plasma composition, density, and temperature, OES enhances process control, ensures high-quality IC production, and supports ongoing advancements in semiconductor technology.