The Research Of Probe Diagnostic For RF Discharge Plasma

Plasma is the one of the four fundamental states of matter, and more than 99.9% of the material in the visible universe is plasma. According to the degree of ionization, plasma can be divided into cold plasma and hot plasma. For cold plasmas, the degree of ionization is low and the ion temperature is far less than the electron temperature, the ion temperature can even be near to room temperature. For cold plasmas, on the one hand electrons have high enough energy to prompt molecules excitation and ionization, on the other hand the reaction system can keep low temperature, and this feature makes cold plasmas be widely used in various fields. In laboratory, cold plasmas are usually generated throuth gas discharge. According to the working frequency of power supply, cold plasma includes DC discharge plasma, AC discharge plasma, RF discharge plasma and microwave discharge plasma. The RF discharge plasma includes RF capacitively coupled plasma, RF inductively coupled plasma, spiral wave plasma and so on. This dissertation is aimed at the RF inductively coupled plasma.The diagnositic methods of RF discharge plasma include probe diagnostic method, microwave diagnostic method and emission spectrometry method. Among them, probe diagnostic method has the advantages of simple structure, high precision and large measuring range compared with other methods. However, in RF environment, probe is susceptible to RF interference, and the measurement results are not reliable or even can’t be got. In order to use probe diagnostic for RF inductively coupled plasma, the tuned single probe method is adopted, the LC tuned circuit can greatly reduce RF interference in this method. At the same time, photoelectric isolation, grounding, shielding, filtering and other methods are adopt to furtherly reduce RF interference.The probe diagnostic electronics system comprises a shielding box with tuned circuit and filter, a photoelectric isolation board, a signal sampling board and PC software. The shielding box has LC tuned circuit and low pass filter inside and is shield with aluminum. The photoelectric isolation board isolates the probe scanning signal, the probe voltage signal and the probe current signal. The scanning signal from DAC is also amplified in this photoelectric isolation board. The signal sampling board is based on FPGA. Probe scanning signal is generated by using the DDS method. The probe voltage and current are sampled with ADC, processed by FPGA and then sent to PC through USB interface. Considering the portability, the signal sampling board also has LED module, switch arrays and ethernet interface, which are controlled by ARM micro controller.Aiming at the probe diagnostic of RF discharge plasmas, the dissertation has designed the probe diagnostic electronics system. The probe diagnostic system of RF discharge plasma is mainly monopolized by several foreign companies, and a set of probe diagnostic system is more than one million yuan. This dissertation conducts a preliminary exploration of the probe diagnostic electronics system of RF discharge plasmas and has made the following innovative work:1. The shielding, grounding, tuning and filter, common mode choke coil, photoelectric isolation, and under sampling methods are used to realize the suppression of RF interference.2. The FPGA based on direct digital synthesis (DDS) method is employed to generate sine wave and triangular wave as probe scanning signals whose amplifier, frequency and phase are adjustable.3. The embedded ethernet interface based on ARM microcontroller is designed to extend the portability of probe diagnosis electronics system.At present, the probe diagnostic electronics system has completed the hardware design and the electronics test, and the test results show the probe diagnostic electronics system almost meet the design requirements. Probe diagnostic test of RF plasma is currently underway, after the test this system will be used to carry out a series of plasma experiments, and based on the results the probe diagnostic electronics system can be further improved.