Using Multi-Channel Detectors for Plasma Monitoring Applications

A typical application for multichannel detectors is plasma diagnostics, which involves analysis of contaminant presence, and relative concentration. The spectrum of the base gas of the plasma is examined, to determine the presence of spectral lines that are uncharacteristic to the gas.

End point monitoring analyzes the plasma, to determine if unwanted substances, which are being deposited onto, or etched off of a substrate like a semiconductor wafer, are present. The intensity versus time of a single wavelength is observed during the ongoing etch/deposition process. Figure 1 depicts such an application of an InstaSpec™ IV CCD Detector.

MS260i™ Imaging Spectrograph and InstaSpec™ IV CCD, used for plasma diagnostics

Figure 1. MS260i™ Imaging Spectrograph and InstaSpec™ IV CCD, used for plasma diagnostics

Plasma Monitoring Experimental Setup

Figure 1 shows the typical setup used for most plasma monitoring experiments. Here, the sighting optics collects the light from the plasma. The collected light is then integrated into the MS260i™ Imaging Spectrograph. A simple lens is enough, in lieu of the sighting optics, if the plasma can be easily accessed, i.e. positioned near the spectrograph. A lens with an F/# comparable to the entrance angle of the spectrograph needs to be selected to reduce losses. The setup involves the use of a -55°C TE cooled CCD for the best dynamic range.

Things to Consider

The following things need to be considered in plasma monitoring applications:

Signal to Noise

Integrations can be used in the presence of low level of contaminants to facilitate collecting their weak signals from the plasma background. The signal to noise ratio improvement is in approximate proportion to the square root of the number of integrations.

Dynamic Range

Subtracting the uncontaminated plasma as a background is a useful approach to detect extremely low levels of contaminants within the overall high background of the plasma.

Variation in Instruments

The behavior of plasma generating instruments varies from system to system. Variations can even be found in duplicate systems running similar processes. Hence, users must have adequate knowledge about their process, and equipment, to improve the performance of the monitoring and feedback system.

Spectral Libraries

To perform plasma monitoring, it is recommended to have libraries of reference spectra for comparison of real data. This helps in determining unknown contaminants, and distinguishing between two species showing identical spectra. Oriel Instruments does not have libraries integrated into its hardware or software.

About Oriel Instruments

Oriel Instruments, a Newport Corporation brand, was founded in 1969 and quickly gained a reputation as an innovative supplier of products for the making and measuring of light. Today, the Oriel brand represents leading instruments, such as light sources covering a broad range, from UV to IR, pulsed or continuous, and low to high power.

Oriel also offers monochromators and spectrographs, as well as flexible FT-IR spectrometers, which make it easy for users across many industries to build instruments for specific applications. Oriel is also a leader in the area of Photovoltaics with its offering of solar simulators, that allow you to simulate hours of solar radiation in minutes. Oriel continues to bring innovative products and solutions to Newport customers around the world.

This information has been sourced, reviewed and adapted from materials provided by Oriel Instruments.

For more information on this source, please visit Oriel Instruments.

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