The technical basis of the metal and surface analysis methods is a gas discharge lamp as a sputtering and excitation source, in which the sample is sputtered off almost plane-parallel by argon ions. The sputtering process and the excitation take place as shown in Figure 1:
Source: SPEKTRUMA Analytik GmbH
Excitation mechanism in plasma
Argon particles are ionized by applying a DC voltage (1) and are accelerated (2,3) due to the potential difference onto the sample, which is connected as a cathode. As a result, sample particles are sputtered off and v. a. excited (6) by electron impact (5). The emitted light quanta are analyzed in an optical emission spectrometer (7).
Source: SPEKTRUMA Analytik GmbH
Structure of the optical emission spectrometer
The area of the ablation depends on the selected anode diameter, which is usually 4 or 2.5 mm.
The light quanta are imaged through a lens onto a holographic grid (Figure 2). The wavelengths are separated using an optical arrangement according to Paschen/Runge on a Rowland circle. It is thus possible to measure the characteristic spectral lines of all elements at the same time. If the measurement is also carried out in a time-resolved manner using a computer system, the changes in concentration of all elements present along the depth axis can be recorded through a layer [3,4].
Based on the development work of A. Bengtson and his co-workers, it is now possible to transform time-resolved GDO(E)S intensity curves into concentration-depth profiles using a multi-matrix calibration. Such a calibration takes into account the fact that different materials have different sputtering rates. The concentrations of the elements in an unknown sample are obtained by normalizing the element content to 100%. The depth is determined by comparing the current sputtering rate with the sputtering rate of pure iron [2,4].
Advantages of quantitative GDOS depth profile analysis:
- Depth profile analysis of round and curved samples is possible!
- Determination of all elements (incl. H) possible
- Low detection limits (0.1 – 50 ppm; exception Cl and F)
- High sputter rateà depth profile analysis down to 100 µm
- Inexpensive purchase, low analysis costs compared to other surface analysis methods
- Chemical analysis of the base material possible + reproducibility < 1% rsd + depth resolution: approx. 10% of the removed depth (depending on the surface roughness)
Disadvantages of quantitative GDOS depth profile analysis:
- Lateral resolution >2 mm
- Availability of calibration standards
- Binding states only detectable to a limited extent
In principle, the detection limits of optical emission spectrometers or glow discharge spectrometers can be determined using the following two options:
- Determination of the repeatability of the spectral background
- Determination of BEC/30
On 1. By determining the spectral background, the detection limit of the individual elements can be determined relatively quickly and easily. In practice, a sample containing the analyte in a significantly lower concentration than the detection limit for this element is z. B. measured 10 times. The absolute standard deviation * 3 is by definition the detection limit of this element. This only works if the
Standard deviation is mainly determined by the spectral background and the analyte is not present or is only present in a very low concentration.
Since the modern, high-resolution spark and glow discharge spectrometers sometimes have detection limits in the lower ppm range, such ultrapure samples are either not available or only available to a very limited extent.
To 2. Assuming a linear calibration curve in the trace range, the BEC value can be determined by a 2-point calibration in the trace range. Assuming a relative repeatability of 1%, the detection limit can be determined with BEC/30. The determination limit and detection limit, which are then important for the analysis, can also be derived from the detection limit.
The general rule:
- Only above the limit of detection (LOD) statements can be made as to whether an element is present in a sample with a certain probability or not. It is not possible to give information about the concentration of the element in a sample.
- The limit of detection is equal to twice the limit of detection. The detection limit indicates the minimum content of a corresponding sample that can be detected with sufficient certainty.
- The limit of quantification or limit of quantification (LOQ) is the smallest concentration of an analyte that can be determined quantitatively with a specified precision. Quantitative analysis results are only given above the limit of quantification.
Caution: For daily practice, both in spark spectrometry OES and in glow discharge spectrometry GDOES, it is important to be able to make quantitative statements about element concentrations only above the limit of quantification.
ISO: “Surface chemical analysis – determination of thickness and chemical composition of zinc-based metallic coatings by glow discharge optical emission spectromerty” ;04/1999 » read «
(pdf file, Size 247 KB)
A. Bengtson: “Quantitative depth profile analysis by glow discharge”;GB 1994 » read «
(pdf file, Size 476 KB)
T. Asam: Schnelle Tiefenprofilanalytik mit der Glimmentladungsspektroskopie ; VDI-Seminar “Angewandte Oberflächen-,Grenzflächen- und Dünnschichtanalytik”; May 2000 » read «
(pdf file, Size 1,37 Mb)
Herr Dr. W. Verscharen: U,I-Abhängigkeit der Sputtergeschwindgkeit ; November 2002 » read «
Prüfung von Randschichten – Übersicht
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The depth resolution is a decisive criterion for all sputtering analysis methods. It is often claimed that the GDOS can no longer properly resolve “thin” layers. The example below shows the analysis of 10 alternating Cu and Cr/Ni layers with a total thickness of 1.6 microns.
All layers can be resolved comparatively well. Notable is V. a. that the underlying layers cannot be resolved any less poorly than layers on the surface of the sample.
60 nm mit 3 nm / Schicht
Quelle: Spectruma Analytik GmbH
The depth resolution of very thin layers such as the TiN/TiAlN/…TiAlN/TiNH/Si layers shown here with a layer thickness of approx. 3 nm per layer clearly shows the individual layers with the impurities between the layers (elements H and O).
Quelle: Spectruma Analytik GmbH
Conversion of the intensity-time profiles into concentration-depth profiles as follows (information without formulas):
1. Performing a calibration with certified samples with a known sputter rate (sputter rate correction)
2. Calculation of the concentrations of all essential elements of each volume segment
3. Normalization to 100%
4. Calculation of density
5. Calculation of the thickness of each volume segment
7. Adding all volume segments