Nano Device Technologies

Plasma Diagnostics

Novel methods to analyze plasma parameters and process conditions in situ


Research and development topics

A lot of processes in microelectronic device fabrication take place under reduced pressure or in plasma, e.g. CVD, PVD and RIE. Complex chemical reactions occur in the used plasma reactors. In these reactions the process gases and the materials at the surfaces of the processed wafers are involved. Process parameters, plasma conditions and the current state of the plasma chamber influence these chemical reactions and therewith the results of the processes. For the development and optimization of processes for the todays microelectronics fabrication it is not enough to analyse process results. Rather it is necessary, to understand the chemical reactions, which take place during a plasma process.

The department Nano Device Technologies of the Fraunhofer ENAS has been enlarging its activities in the field of in situ analysis methods in the last years. Figure 1 shows an etch reactor, which is equipped with the in situ diagnostic methods available at the Fraunhofer ENAS. The following in situ investigations are possible:


Langmuir probe (LP)

  • A small metal dip will be introduced into the plasma using a stepper motor.
  • Measuring of IV curves along the radius of a radial plasma distribution
  • Calculation of the spatial distribution of the electron density or the electron temperature


Optical absorption spectroscopy (OAS)

  • A wavelength tuneable laser beam will be passed through the plasma repeatedly using a multipath arrangement with concave mirrors. The wavelength dependent attenuation of this laser beam is an indicator for the species, which are existent in the plasma.
  • Exact determination of the concentration of different ions, radicals and molecules in the plasma with a very low detection limit


Optical emission spectroscopy (OES)

  • Determination of wavelength and intensity of light, emitted from the plasma
  • Fast method to measure the changes in plasma conditions during the whole process
  • A typical OES spectrum for an etch process of porous SiCOH in a CF4 plasma is shown in figure 2.


Quadrupole mass spectrometry (QMS) methods

  • Measuring of single ions and complex compounds separated by their atomic weights (see fig. 3)
  • Excellent method to analyze the complete gas composition in the process chamber
  • Using a tracer substance, the determination of the exact partial pressure of ions and compounds are possible (actinometry).

The spectral data will be collected centrally (see fig. 1). The evaluation of the measured data will be performed using corresponding tabulations plants. At the present, novel mathematics based methods to process and to evaluate the measured spectral data are also tested at the Fraunhofer ENAS. These methods are:


Statistical methods

  • Principal Components Analysis (PCA)
  • Covariance analysis to find hidden correlations between the different spectral data as well as spectral data and process results


Virtual metrology

  • Mathematical estimation of process results using transiently measured spectral data
  • Reduction of expensive, destructing and time consuming analytics
  • Two-step process, which consists of a construction and an application phase (see fig. 4)