Plasma frequency generators

Plasmas can be used in CVD reactors to activate and partially decompose the precursor species and perhaps form new chemical species. This allows deposition at a temperature lower than thermal CVD. The process is called plasma-enhanced CVD (PECVD) (12). The plasmas are generated by direct-current, radio-frequency (r-f), or electron-cyclotron-resonance (ECR) techniques. Eigure 15 shows a parallel-plate CVD reactor that uses r-f power to generate the plasma. This type of PECVD reactor is in common use in the semiconductor industry to deposit siUcon nitride, Si N and glass (PSG) encapsulating layers a few micrometers-thick at deposition rates of 5—100 nm /min. 

A glow-discharge (non-isothermal) plasma is generated in a gas by a high-frequency electric field, such as microwave (2.45 GHz), at relatively low pressure.P9] In such a plasma, the following events occur ... 

An RF plasma is generated at a frequency of 13.56 MHz. A typical equipment consists ofparallel electrodes as shown inFig. 5.20. It is a cold-wall design which is used extensively forthe deposition of silicon nitride and silicon dioxide for semiconductor applications. 

Here, results are shown from experiments performed in ASTER, reported by Biebericher et al. [512. 519], A SiH4-H2 (50 50 flow ratio, total flow 60 seem) plasma was generated at an RF excitation frequency of 50 MHz. The substrate temperature was 250°C. The RF signal was ampitude modulated (AM) by a square wave. The modulation frequency has been varied in a range of 1-400 kHz. The modulation depth was always 90%. The duty cycle was fixed at 50%. The pressure amounted to 0.2 mbar, and the average power was kept at 10 W. With a duty cycle of 50%, this leads to a power of 20 W during the plasma-on period. 

Indeed, most of the applications of laser-plasmas rely on the efficient production of energetic electrons driven by the interaction of ultraintense laser pulses with plasmas created from solids or gases. In fact, in these interaction conditions, laser energy is efficiently transferred to electrons generating a population of so-called fast or hot electrons. The process of fast electron generation often takes place near the critical density (the density at which the laser frequency iv0 equals the local plasma frequency wpe) surface [8, 9]... 

Emission spectroscopy utilizes the characteristic line emission from atoms as their electrons drop from the excited to the ground state. The earliest version of emission spectroscopy as applied to chemistry was the flame test, where samples of elements placed in a Bunsen burner will change the flame to different colors (sodium turns the flame yellow calcium turns it red, copper turns it green). The modem version of emission spectroscopy for the chemistry laboratory is ICP-AES. In this technique rocks are dissolved in acid or vaporized with a laser, and the sample liquid or gas is mixed with argon gas and turned into a plasma (ionized gas) by a radio frequency generator. The excited atoms in the plasma emit characteristic energies that are measured either sequentially with a monochromator and photomultiplier tube, or simultaneously with a polychrometer. The technique can analyze 60 elements in minutes. 

The atomic emission detector is a tunable, element-specific detector that uses microwave-induced helium plasma to generate temperatures high enough to break molecular bonds. The generated free atomic species undergo electron excitation to higher energy states, followed by relaxation and photon emission at characteristic frequencies... 

The chamber is a quartz tube that perpendicularly penetrates the waveguide, and the plasma is generated at the cross section in the quartz chamber. The microwave frequency is 2.45 GHz, and usually = 300-500 W is used. 

For glow discharges and inductively coupled high-frequency plasmas ion generation takes place in the plasmas. In the first case mass spectrometry can be performed directly on solids and in the second case on liquids or solids after sample dissolution. In the various atomic spectrometric methods, real samples have to be delivered in the appropriate form to the plasma source. Therefore, in the treatment of the respective methods extensive attention will be given to the techniques for sample introduction. 

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