Development of an uncompensated diagnostic for the measurement of phase-resolved characteristics in radio-frequency plasmas
Linnane, Shane (2007) Development of an uncompensated diagnostic for the measurement of phase-resolved characteristics in radio-frequency plasmas. PhD thesis, Dublin City University.
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In recent years, plasma technology has found its way into a wide and diveme array of manufacturing techniques and facilities, ranging from biomedical applications to microprocessor fabrication. In all cases, greater overall efficiency and usefulness can be improved by understanding the fundamentals of how and why plasmas behave the way they do. In order to achieve this understanding, we use a wide variety of plasma diagnostics to perform measurements of all the Important plasma parameters. The work presented in this thesis is focused on electrostatic probes, in particular, the Langmuir probe, which is arguably the most common diagnostic in the world of experimental plasma physics.
Eventhough the Langmuir probe was invented over 80 years ago and was the first diagnostic tool used for studying plasmas in detail, it is still widely used today. The reason it has survived, even in the presence of more accurate and advanced methods, is because no other diagnostic can obtain so many of the important parameters in such a relatively simple way. However, in many cases the Langmuir probe can only be used as an estimator of plasma parameters, as its accuracy is questionable, particularly in radio frequency(rf) discharges. The inaccuracies are caused by the non-linear dynamics of the plasma sheath, which result in an extra dc current component being measured. This extra component, AI, results in a difference between the time average I-V characteristic and the effective dc I-V characteristic. In 'normal' operation, the Langmuir probe only measures time averaged values, and so, errors result, such as, over-estimation of electron temperature, Te.
Compensated Langmuir probes have been developed to try and eliminate the distortion effect of the plasma potential oscillation on the probe measurements RF compensation usually involves high impedance inductors placed near the tip and a large compensation electrode, both des~gnedt o make the probe tip follow the plasma voltage oscillation, thus reducing it impact on the measurements.
Many semiconductor processing tools use capacitively coupled plasma discharges (CCP's) for etching, deposition and sputtering. CCP's generally have a large voltage oscillation so the ability to accurately measure the plasma parameters under these conditions is highly desirable. Unfortunately, even highly compensated probes have difficulty in providing accurate measurements under most CCP conditions. This 1s because, in practice, it is very difficult to achieve sufficient probe impedance relative to the sheath impedance and also probe construction can be difficult, especially for commercial discharges. For these reasons, and others, even compensated probes do not give reliable results in plasmas with large voltage oscillations.
This was the main motivation for the work presented in this thesis - to design a probe capable of accurately measuring the I-V characteristics and plasma potential oscillations without the need for complex compensation techniques. To achieve this goal, a simple uncompensated Langmuir probe was mvestigated. A computer model was constructed in MatLab to aid in the understanding. A technique was developed in which the dzstortzon-free I-V characteristic is obtained from the time-averaged (distorted) I-V trace by utilizing the rf data measured through a capacitively-coupled, synchronized rf current sensor. This allows phase dependant characteristics to be obtained and also allows reconstruction of the plasma potential oscillation, complex sheath Impedance and other important properties.
The probe deslgn and circu~try are relatively simple, thus providing a useful diagnostic for probing radio-frequency plasmas.
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