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ISO/TS 25138:2025 — Surface Chemical Analysis of Metal Oxide Films by Glow Discharge Optical Emission Spectrometry
GDOES Method for Depth Profiling, Thickness Determination, and Chemical Composition of Metal Oxide Coatings (5 nm to 10,000 nm)
Precision Depth Profiling of Metal Oxide Films
Metal oxide films play a critical role in modern engineering — from corrosion protection on steel and anodized aluminium coatings to semiconductor oxide layers and decorative finishes. Their thickness (typically 5 nm to 10,000 nm) and chemical composition directly determine performance characteristics such as corrosion resistance, wear behaviour, electrical properties, and adhesion. ISO/TS 25138:2025 (third edition) provides a standardized glow discharge optical emission spectrometry (GDOES) method for measuring these critical parameters with high precision and reproducibility.
GDOES offers a unique advantage over competing techniques: it combines rapid sputtering rates (minutes instead of hours for XPS or SIMS depth profiling) with the ability to analyse both conductive and insulating layers, making it ideal for industrial quality control of coated products.
Principle of the GDOES Method
The method involves five fundamental steps: (a) sample preparation as a flat disc or plate; (b) cathodic sputtering of the metal oxide surface in a DC or RF glow discharge; (c) excitation of sputtered atoms in the plasma; (d) spectrometric measurement of characteristic emission line intensities versus sputtering time (qualitative depth profile); and (e) conversion of intensity-versus-time data into mass-fraction-versus-depth profiles using calibration functions (quantification).
A key strength of the GDOES method is its ability to analyse both conductive and insulating materials when using an RF-powered source. This is particularly important for metal oxide films, which are often electrically insulating. The RF source can sustain a stable glow discharge even with non-conductive samples by capacitively coupling the RF power through the dielectric layer, avoiding the charge build-up problems that plague DC sputtering of insulating materials. Modern instruments increasingly offer RF-only or switchable DC/RF configurations, providing maximum analytical flexibility.
The applicable metallic elements include Fe, Cr, Ni, Cu, Ti, Si, Mo, Zn, Mg, Mn, Zr, and Al. Non-metallic elements that can be determined include O, C, N, H, P, and S — giving the method broad applicability across industrial sectors from automotive manufacturing to microelectronics.
| Parameter | Specification |
|---|---|
| Film thickness range | 5 nm to 10,000 nm |
| Anode diameter options | 2 mm, 2.5 mm, 4 mm, 8 mm |
| Power source types | DC (conductive samples) / RF (conductive & insulating) |
| Detector types | PMT (photomultiplier) / CCD / CMOS / CID array |
| Data acquisition speed | ≥ 100 measurements/second per channel (recommended) |
| Analytical elements (metals) | Fe, Cr, Ni, Cu, Ti, Si, Mo, Zn, Mg, Mn, Zr, Al |
| Analytical elements (non-metals) | O, C, N, H, P, S |
| Minimum repeatability (RSD) | Specified in Clause 6.4.2 |
Critical Technical Aspects
Source Parameter Optimization
The standard provides detailed instructions for optimizing glow discharge source parameters, which is the most critical step in developing a GDOES method. Three competing objectives must be balanced: adequate sputtering rate (for reasonable analysis times), good crater shape (for depth resolution), and constant excitation conditions (for analytical accuracy). For DC sources, typical starting parameters are 700 V with current ranges of 5-10 mA (2 mm anode), 15-30 mA (4 mm), or 40-100 mA (8 mm). RF sources require additional consideration of power losses in cables and connectors, which can vary from 10% to 50% depending on the instrument model.
Calibration and Quantification
The standard specifies calibration procedures using certified reference materials covering low-alloy steel, stainless steel, nickel alloys, copper alloys, titanium alloys, silicon, aluminium alloys, and specialized high-oxygen, high-carbon, high-nitrogen, or high-hydrogen samples. Validation samples include anodized Al₂O₃, TiN-coated samples, TiO₂-coated samples, and oxidized silicon wafers. The emission yield method forms the basis for quantification — converting intensity data into mass fraction and depth information via empirically derived sputtering rates.
The quality of crater shape directly determines depth resolution. A non-flat crater bottom produces misleading depth profiles because signals from different depths are mixed. The standard provides guidance on optimizing crater shape, although this has been changed from mandatory to optional in the 2025 revision to accommodate modern instruments with better inherent plasma stability.
What's New in the 2025 Third Edition
This edition introduces several important updates: expanded anode size options (adding 2.5 mm to the previously listed 2 mm, 4 mm, and 8 mm); updated detector types to include CMOS and CID array detectors alongside traditional CCD; revised optical system check procedures for array-type detectors; greater emphasis on vacuum seal verification between the sample and glow discharge source; and revised minimum performance requirements in subclause 6.4.
The addition of 2.5 mm anode size reflects industry demand for intermediate-diameter analysis that offers better heat management than 4 mm while maintaining higher sensitivity than 2 mm — a practical engineering compromise for challenging samples.
Frequently Asked Questions
Q: What is the main advantage of GDOES over XPS or AES for metal oxide analysis?
A: GDOES offers significantly faster sputtering rates (typically 1-10 μm/min versus 0.1-1 nm/min for XPS), enabling analysis of thicker films (up to 10 μm) in reasonable time frames. It also handles both conductive and insulating samples with equal ease when using RF source, whereas XPS/AES require more careful charge compensation for insulators.
A: GDOES offers significantly faster sputtering rates (typically 1-10 μm/min versus 0.1-1 nm/min for XPS), enabling analysis of thicker films (up to 10 μm) in reasonable time frames. It also handles both conductive and insulating samples with equal ease when using RF source, whereas XPS/AES require more careful charge compensation for insulators.
Q: Can GDOES distinguish different oxide phases (e.g., FeO vs. Fe₂O₃)?
A: GDOES provides elemental composition as a function of depth, not direct phase identification. However, by measuring the O/Fe ratio through the depth profile, you can infer the oxide stoichiometry and distinguish different oxide layers based on their elemental composition.
A: GDOES provides elemental composition as a function of depth, not direct phase identification. However, by measuring the O/Fe ratio through the depth profile, you can infer the oxide stoichiometry and distinguish different oxide layers based on their elemental composition.
Q: What sample size is required for GDOES analysis?
A: Samples should be flat discs or plates with a width greater than 5 mm, typically 20 mm to 100 mm. The specific size depends on the anode diameter — a 4 mm anode requires at least a 10 mm x 10 mm flat area for reliable sealing and analysis.
A: Samples should be flat discs or plates with a width greater than 5 mm, typically 20 mm to 100 mm. The specific size depends on the anode diameter — a 4 mm anode requires at least a 10 mm x 10 mm flat area for reliable sealing and analysis.
Q: Is the method suitable for organic or polymer coatings?
A: Yes, but only with an RF-powered source since polymer coatings are electrically insulating. The method is primarily specified for metal oxide films, but the RF-GDOES technique can be extended to other coating types with appropriate calibration.
A: Yes, but only with an RF-powered source since polymer coatings are electrically insulating. The method is primarily specified for metal oxide films, but the RF-GDOES technique can be extended to other coating types with appropriate calibration.
