GDOES Theory

What is GDOES?

Glow Discharge Optical Emission Spectroscopy (GDOES) is a spectroscopic method for the qualitative and quantitative analysis of metallic and non-metallic solid materials.

GDOES allows for the investigation of the elemental composition as well as the layer thickness and layer structure of a sample. Moreover, coating weights and concentration gradients can be determined.
The thickness of analyzable layers ranges from thin layers (< 50 nm) to thick layers (several hundreds µm). Possible sample materials include metals, semiconductors, glass, ceramics and polymers.

Short History of GDOES

In 1968, Werner Grimm introduced a glow discharge tube as a light source for spectroscopic analyses investigating the chemical composition of metallic materials. The so-called Grimm discharge tube is characterized by a special arrangement of the electrodes: The two electrodes of the DC current source are set up of a cylindrical hollow anode and the sample as cathode which is sealing the anode tightly. Since then, this technique and its applications have been continuously refined. Today, GDOES is one of the most precise methods of elemental analysis and layer thickness measurement.

How does GDOES work? (Simplified description)

Electroconductive materials: The sample is placed into the Glow Discharge Source in such a way that it is in direct contact with the cathode, so it is switched as cathode itself.

The Glow Discharge Source is filled with Argon gas under low pressure (0.5 – 10 hPa). A high direct voltage (DC) is applied between the anode and the sample (≙ cathode). Due to the DC voltage, electrons are released from the sample surface and accelerated towards the anode gaining kinetic energy. By inelastic collisions the electrons transfer their kinetic energy to Argon atoms, which causes them to dissociate into Argon cations and further electrons. This avalanche effect triggers an increase in the charge carrier density making the insulating Argon gas conductive. The resulting mixture of neutral Argon atoms and free charge carriers (Argon cations and electrons) is called plasma.

The Argon cations are accelerated towards the sample surface because there is a high negative potential. Striking the sample surface the Argon cations knock out some sample atoms. This process is referred to as sputtering. The sample surface is ablated in a plane-parallel manner.

The knocked out sample atoms diffuse into the plasma where they collide with high-energy electrons. During these collisions, energy is transferred to the sample atoms promoting them to excited energy states. Returning to the ground state, the atoms emit light with a characteristic wavelength spectrum.

Passing through the entrance slit, the emitted light reaches a concave grating where it is dispersed into its spectral components. These components are registered by the detection system. The intensity of the lines is proportional to the concentration of the corresponding element in the plasma.

 

Schematic layout of a Glow Discharge Spectrometer GDOES

 

Insulating materials: Using radio frequency for plasma generation also insulating materials can be analyzed.