P.C. Russell

ON-LINE FIBRE OPTIC BASED INSPECTION USING CHROMATIC MODULATION FOR SEMICONDUCTOR MATERIALS PROCESSING

This contribution describes the use of chromatic sensing for monitoring and controlling the state of industrial processing plasmas and for providing an indirect but approximate indication of the quality of the end product of the process on-line. Control systems for a Silicon Nitride plasma deposition and a Reactive Ion Etching system have been developed utilising the chromaticity of the optical emissions from the plasma as a control feedback. It is shown that there is a correlation between the plasma chromaticity and the refractive index of the thin film of semiconductor being deposited so that the chromaticity parameters can be used as indicators of the film quality. This indicates that a chromaticity monitoring system may be used as the basis of an on-line intelligenl control system. It has been demonstrated that there exists potential advantages in combining such chromatic sensing with neural network methodologies to achieve significant improvements in reliability and efficiency of such approaches.

RESOLUTION CAPABILITIES OF CHROMATIC SENSING IN THE MONITORING OF SEMICONDUCTOR PLASMA PROCESSING SYSTEMS

An on-line monitoring system for semiconductor plasma processing systems, based on the chromatic monitoring of the emission spectra of a plasma has been developed. The system offers low cost operation, lack of interference with the processing plasma and a fast response. Previous work has shown that the chromatic signature is a good indicator of the quality of the final product. In this contribution an analysis of the signal-to-noise performance of the chromatic system is presented. It is shown that chromatic systems, because of their integrative nature, offer a higher signal-to-noise ratio than comparable optical systems. These results provide further evidence that the chromatic monitoring is suitable for on-line control of plasma processing systems.

Near-infrared techniques for LPG quality control

The analysis of the composition of liquified petroleum gases (LPG) is currently carried out by gas chromatography. This method is accurate, but is expensive and is not suitable for use in field portable instruments. In this work we show that there is sufficient information in the near infrared absorption spectra to allow the main components of LPG to be distinguished. Classical spectroscopic methods can be used for this, but currently there are no near infrared spectrometers available suitable for use in a low cost portable instrument. Nor is it clear how such detailed spectroscopic data should be used to yield an LPG quality factor. We show how a chromatic system based ont three broad band detectors may be used as a low cost robust method of quality control of LPG products.

Knowledge-based monitoring and control of plasma processes using chromaticity measurements

Chromatic monitoring techniques have been developed to provide rapid feedback on the state of electrically induced plasmas. These techniques have been used to provide diagnostic information, based on the plasma emissions, and also to give rapid feedback on the thickness of insulating films while being processed. It is also possible that the chromatic technique could be used to monitor other important aspects of plasma systems, such as the concentration of particulates in plasmas and substrate temperature. The chromatic monitors provide information to a knowledge based system, which also has access to data from the conventional gas flow, pressure and rf power meters. The knowledge base consists of a set of rules, obtained by interviewing processing experts, and a statistical model of the plasma chamber. As well as being able to diagnose coarse errors in the plasma chamber equipment, the knowledge based system can compensate for undesirable variations during process runs.

Near-infrared spectroscopy for LPG quality control

The analysis of the composition of liquefied petroleum gases is currently carried out by gas chromatography. This method is accurate, but is expensive and it is not practical for use in a low cost, field portable instrument. It has been shown that sufficient information can be obtained from the near infrared absorption spectra for the components of LPG to be distinguished. Two instruments are described: a novel device based on a number of low cost infrared light emitting diodes with a single detector, and a more conventional filter wheel based device. Experimental results indicate that an infrared system may be used to provide a robust and cost effective method of quality control of LPG products.

Vibration monitoring using 2D speckle pattern images

A high speed data acquisition system has been developed for monitoring the speckle pattern produced by a fiberdyne interferometer. The system captures data from a 256 element, linear, CCD array with sample frequencies of up to 18.5 kHz. Data from the system is in the form of a 2D image showing the variation in pixel intensity as a function of time. The system is used to address the speckle pattern produced by a fiberdyne interferometer which is used as a vibration sensor. Information taken using this system is compared with the predictions of an existing theoretical model which indicates that all speckles should be affected by a vibration signal but, that the amplitude and phase of the resulting variations will be speckle dependent. Data taken using this system agrees with these predictions. All of the pixels in the CCD array show some variation when subjected to a sinusoidal vibration signal. The amplitude and phase of the pixel signals show a considerable variation in both amplitude and phase, indicating that the model is valid. By averaging the spectra obtained by performing a Fourier transform on each pixel it is possible to substantially reduce noise effects making the system more sensitive to changes in the acoustic signal.