Basic Concepts

What is GDOES?

Quantitative Depth Profile analysis of a CIGS thin film solar cell.

Fig 1: Quantitative Depth Profile analysis of a CIGS thin film solar cell.

As the acronym suggests, GDOES combines a glow discharge (GD) with an optical emission spectrometer (OES). This is an analytical technique that provides both the surface/depth profile and the bulk elemental composition of solid materials and layers quickly, and with high sensitivity to all elements.

Qualitative Depth Profile analysis of a PVD multilayered coating –about 10 μm thick on steel.

Fig 2: Qualitative Depth Profile analysis of a PVD multilayered coating –about 10 μm thick on steel.

This is an analytical technique that provides both the surface/depth profile and the bulk elemental composition of solid materials and layers quickly,  and with high sensitivity to all elements.

3D view of a GD crater (diameter 4 mm).

Fig 3: 3D view of a GD crater (diameter 4 mm).

Operation involves the controlled sputtering of a representative area of the sample to be analyzed by the GD plasma and the simultaneous OES observation of the sputtered species. The GD technique is destructive. A crater is made in the sample after analysis.

The technique is extremely fast and depth resolution can be excellent (nm level) if sample is flat over the sputtered area.

What is the history of GDOES?

“Glow Discharges” are low pressure type plasma sources found in many domains, from TV plasma displays to light sources.

Most coatings used in the microelectronics industry are deposited using these plasmas. Plasmas are also applied for decontamination of surfaces. Adjustment of trajectories of satellites is done through thrusters powered by plasmas.

Surprisingly, our GDOES comes from a very different background, namely the domain of analytical chemistry in the metal industry. When W. Grimm proposed the first GD source in 1968, his idea was to connect it to an Optical Emission Spectrometer (OES) and to have a superior light source, compared to his standard spark one, for elemental bulk analysis of Cu alloys.

Soon a group of researchers working with R. Berneron at IRSID in France realized the potential of this new combination for the exploration of galvanized steel plates and on passive films on steel  howing elemental depth profile analysis of thick and thin films by GDOES. For many years, the technique developed mainly in the steel industry, though the first published depth profiles with GDOES (1970) were of GaAs thin films.

What is Pulsed RF GDOES

By Pulsed RF GDOES, we mean the most recent and comprehensive GD instrument. In this instrument, the GD plasma is still combined with high resolution optics but the powering of the plasma is done by Pulsed RF.

With pulsed RF GDOES not only has the range of applications of GD been drastically increased but also the task for obtaining quantitative depth profiles has been simplified. As an example a single calibration can be done using a multilayered material containing both conductive and isolating layers.

What is the information provided by Pulsed RF GDOES?

Typical Pulsed RF GDOES result.

Fig 4: Typical Pulsed RF GDOES result: composition as a function of the depth. Sample is an electronic component made of brass coated with NiP and featuring a gold top coat layer. A 4 mm crater has been eroded by the GD process. Total analysis time: 2 minutes.

Pulsed RF GDOES provides fast elemental chemical analysis of solid materials as a function of depth.

All elements can be measured, including C, H, N, Cl, O, Na, Li, etc.

Pulsed RF GDOES does not measure isotopes except possibly Deuterium (D, isotope of H). Pulsed RF GDOES does not measure molecules (as RAMAN does) though some “molecular bands” can sometimes be seen in the plasma and provide indirect information on the material studied.

Pulsed RF GDOES relies on the controlled erosion of a representative part of the material (few mm crater diameter) by the plasma and gives the elemental distribution as a function of the penetration depth.

Multi spots on a large wafer sample.

Fig 5: Multi spots on a large wafer sample.

Pulsed RF GDOES will answer in minutes multiple questions such as:

  • Which elements are present in the sample?
  • What are the concentration levels?
  • Is the sample homogeneous in depth?
  • Were any coatings or surface treatments applied to the sample?
  • How thick are the coatings?
  • Is there any contamination at an interface?
  • Is there any oxidation of the sample?
  • Is there any diffusion in the layers?
Overlay of 5 measurements on an anodized Al with Cr delta layer.

Fig 6: Overlay of 5 measurements on an anodized Al with Cr delta layer.

    The speed of analysis allows performance of multiple measurements on different locations of a sample to assess repeatability.

    What problems can be solved by pulsed RF GDOES?

    Pulsed RF GD is the most advanced and versatile analysis method that allows all types of materials made of thin and thick films (conductive and isolating) to be measured.

    Whether you compound materials or control a production, Pulsed RF GDOES is a fast, reliable and sensitive tool to obtain elemental depth profiling and gives you results in minutes that will let you:

    • See if expected elements are present in each layer and at interfaces, and detect unexpected ones
    • Assess if the concentration levels are correct or not
    • Check the repeatability of a depth distribution and be informed of possible deviations
    • Quickly optimize a process and assure that it remains under control
    • Identify what the competitors are doing
    • Track sources of corrosion
    • And more!

    Is Pulsed RF GDOES a destructive analysis method or a non-destructive one?

    GDOES crater.

    Fig 7: GDOES crater.

    Pulsed RF GDOES is a destructive analysis method. The result of the sputtering of the material is a usually circular and flat hole having the diameter of the anode. We call this hole the crater.