Mastering Precision: End Point Detection in Semiconductor Manufacturing 

As the size of highly integrated semiconductor devices moves toward nano-scale, the need for precise control in semiconductor manufacturing is also increasing. The production of semiconductor devices relies heavily on plasma etching processes. When patterns are etched on a wafer, the in-line control of etching progress is vital to avoid over and under-etch, ensure optimized process parameters, high yield, and productivity. Precise endpoint detection (EPD) during the plasma etch process is a crucial element.  

Understanding End Point Detection: End point detection refers to the process of identifying the precise moment when a specific manufacturing step reaches its completion. This is particularly crucial in semiconductor fabrication processes, where even the slightest deviation from the desired endpoint can have significant implications on the final product’s performance and reliability. 

Challenges and Solutions : Achieving reliable end point detection poses several challenges. Variations in material properties, process conditions, and equipment performance can all introduce uncertainties that complicate the detection process. To address these challenges, a variety of sophisticated techniques and technologies are in practice and are discussed below 

Optical emission spectroscopy (OES): OES is the most common technique used in plasma etching processes for etch endpoint detection. The method is based on measuring and tracking intensity changes in optical emission signals from the plasma during the etch process. As the process nears completion, the concentration of certain elements or compounds changes, indicating that the desired thickness or depth has been achieved. The endpoint detection consists of detecting slope changes (upward or downward) in the intensity of the relevant wavelengths to stop processes at layer interfaces. With accurate endpoint detection algorithms, etch processes can be stopped precisely. 

Figure 1. The Optical Emission Spectrometer, or OES setup installed in a plasma chamber. It measures optical light, from 200 to 800 nm, emitted by the energized atoms and molecules participating in the etch reaction.

The OES technique is very popular and has its own merits such as simple setup, direct measurement, real time monitoring and wider application range suitable to all kinds of plasma sources. However, the amount of OES sensor data is enormous and also noises are embedded in the data, it is not easy to process the collected data in real-time. It has been observed that the sensitivity of OES is not adequate to detect endpoints in some challenging processes, such as those with less than 1% open area.  

To overcome these difficulties one technique that is gaining wider use is that of end point detection using an RF spectrometer.  

 

Radio Frequency Emission Spectroscopy: The RF Spectrometer is an RF detector that directly monitors the electrical state of a plasma from outside the plasma chamber. An RF spectrometer collects the radio frequency emission of harmonics from the plasma during the etching process. The harmonic emissions come from the electron motion in the plasma. As the etching progresses and the endpoint is approached, the composition of the plasma changes, resulting in characteristic shifts in the RF emissions. By monitoring these changes, the endpoint can be detected.

Figure 2.  The RF Spectrometer operates in a similar way to an Optical Emission Spectrometer, or OES. The difference is in the frequency range measured. Instead of optical light, radio frequency electromagnetic waves are measured, from 400 kHz to 500 MHz. The RF spectrometer captures the motion of electrons in plasma.

The RF harmonic spectrum can be measured noninvasively by putting a radio antenna outside the plasma chamber to detect the RF signals. The antenna can also be placed anywhere outside the plasma source where small RF leakage is present – at a window port, for example. The radio antenna collects the electric and magnetic signal from the chamber and sends them to the Acquisition Unit, which extracts the RF harmonics. The RF spectroscopy technique has been found to exhibit superior sensitivity than OES enabling it to be used in complex processing scenarios such as those with less than 1% open area. 

 

 Current status and future perspectives 

Looking at both techniques it can be shown that each has its own particular advantages. The OES is a very well stablished technique, but it is still far from ideal due to the run to run etch variation, the lack of etch depth control and limited visibility on low open area processes. RF emission spectroscopy generally offers higher sensitivity to changes in plasma composition compared to OES and has shown promising results for low open area cases too. The performance of precise detection of endpoint by OES can be enhanced by adding additional sensors like RF spectrometers. 

As semiconductor technology continues to advance, the demand for even greater precision and control in manufacturing processes will only intensify. This necessitates ongoing research and development efforts to enhance end point detection capabilities further. Emerging technology such as Impedans Moduli RF spectrometer hold promise for pushing the boundaries of endpoint detection possible for less than 1% open area situations too.  Up to 5 fundamental frequencies and 31 harmonics can be monitored simultaneously using Moduli for both continuous wave and pulsed RF plasmas. The RF frequency range is from 300 kHz to 500 MHz, and pulse range 10 Hz to 100 kHz. Integrating the Moduli device with other metrology like in-situ monitoring, and advanced process control will enable fabrication of next-generation semiconductor devices with unprecedented performance and functionality. 

Conclusion:

End point detection stands as a cornerstone of precision in semiconductor manufacturing. By accurately identifying the optimal moment to conclude each process step, it ensures the quality, reliability, and performance of semiconductor devices that power our modern world. As technology advances and demands grow, continuous innovation in end point detection will remain essential to drive the semiconductor industry forward into a future defined by excellence and precision.  

Impedans is continually enhancing the capabilities of its sensors to enable improved control and efficiency in plasma-based applications, thereby supporting the advancement of next-generation semiconductor technologies. The Moduli RF Spectrometer is a new and effective solution for mass production monitoring, due to being as non-invasive as an optical system and offering real-time data output. The RF Spectrometer can detect endpoint in scenarios where OES cannot, such as in deposition chambers and for small open area etch processes.  

To know more about Impedans RF Moduli Spectrometer and how we can help you better understand your plasma contact us at info@impedans.com