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Plasma diagnostics


Working in progress

1. Introduction  

It is important to understand the behavior of plasmas in order to gain a better understanding of the relationship between the deposition process and the film structure and properties. In general, plasma diagnostics refers to the techniques used to gather information about the nature (properties) of the plasma used in a deposition process. Diagnostics help us obtain information about plasma properties in the sputter deposition glow discharge, such as the plasma chemical compositions and species, temperature, plasma density, ion/electron energy distributions, and neutral species. The most commonly used plasma diagnostic techniques can be categorized as optical and electrostatic spectrometry.

Optical spectrometry involves focusing radiation emitted from excited neutral and charged species in the plasma through an optical window on to the entrance slit of an optical spectrometer. A photomultiplier is used to detect radiation of a particular wavelength at the spectrometer exit slit. Alternatively, a photodiode array is mounted at the exit port of the spectrometer and a broad spectral region is detected simultaneously []. Optical spectrometry uses a spatial and temporal resolution instrument which allows measurement of the bulk plasma properties.  It is difficult for optical spectrometry to detect a specific position inside the plasma, for example, to measure the plasma properties near the cathodes or close the substrate. However, the relatively low cost and easy operation of an optical spectrometer make it a useful tool, and is used in wide plasma diagnostics applications.

Langmuir probes are used for monitoring discharge or plasma parameters including spatial distribution of potential, electron density and electron temperature. The Langmuir probe consists of molybdenum or tungsten electrodes inserted into the plasma. The plasma parameters are estimated by the current-voltage curve of these electrodes. More detailed information on determining the electron density and temperature can be found in references [] and [].

Mass Spectrometry is another powerful technique for identifying unknown species, studying ionic species, and probing the fundamental principles of chemical reactions in the plasma. Mass spectrometry is based upon the motion of a charged particle, i.e. ion, in an electric or magnetic field. The mass to charge ratio (m/z) of the ion affects this motion. Since the charge of an electron is known, the mass to charge ratio will indirectly measure an ion mass. Mass spectrometry is operated under high vacuum condition. A sample (preferably a gas) is introduced and broken down into charged fragments by electron impact or chemical ionization. The fragments, accelerated by applying a voltage, pass through a mass selector which separates them by their ratio of mass to charge (m/z). The separate fragments are detected and measured as ion current. Under constant conditions, a molecule will break up in the same number of ways, giving a reproducible mass spectrum. Compared to optical spectrometers, mass spectrometers can be placed at any position inside a plasma, thereby providing flexible and specific measurement of the plasma parameters as a function of location in the plasma.

2. Diagnostic equipments in ACSEL

We are using a Hiden electrostatic quadrupole plasma mass spectrometer (EQP) and a Hiden Espon Langmuir probe for the plasma property characterizations in ACSEL. The EQP can be operated in two modes: (i) the PI (plasma Ions) mode, in which the ions are directly extracted from the plasma and focused into the energy filter; and (ii) the EI Mode, in which neutrals and radicals are sampled from the plasma and ionized inside an electron impact (EI) ion source to be focused into the energy filter. Both the ions formed in the plasma (in the PI mode) and the artificial ions (in the EI mode) represent the instantaneous status of the plasma during thin film deposition. The ion energy distribution (IED) was obtained by varying the potential in the energy analyzer and fixing the mass filter to allow only ions with a certain mass to charge ratio to reach the detector. The mass distributions were obtained by fixing the energy filter to allow only ions with certain energy to reach the mass detector. The routine partial pressure and residual gas analysis (RGA) was obtain by ionization of a sample gas from the vacuum chamber with or without plasma by an external source.

Fig 1. Schematic drawing of the HIDEN EQP 300 [Hiden Analytical LLC, 2005]

3. Technical examples

ACSEL has carried out extensive plasma diagnostics work in recent years for various ion mass

Figure 1. Photo showing the plasma diagnostics of the MPP plasma in a unbalanced magnetron sputtering system using the EQP.






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