Nowadays, understanding the in-depth structure and composition of thin films and more generally nanomaterials has become crucial for process engineering, making depth profiling a key method for quality control and product development. In this article, depth profiling techniques providing the elemental composition of samples as a function of depth are discussed. Several techniques can be used for quantitative or semi-quantitative depth profile analysis, and it is important to know their differences to select the proper method that provides all the necessary information.
Factors to consider for method selection
The following parameters should be considered when choosing a depth profiling method:
Elements of interest and their amount – Techniques present different sensitivity or detection limits. Some elements, like H, cannot be measured by all techniques, for example.
Probing depth – It is particularly important to consider for thin films, surface, or interface measurements. If very thin layers have to be differentiated, a small probing depth is to be considered to obtain information that is localized in-depth.
Area to be analyzed – If a small pattern or a complete wafer is to be measured then the area of analysis matters for the technique selection.
Profile thickness – The maximum depth that the method can profile is of importance if the profile has to be measured for more than 1 µm.
Table 1: Most common depth profiling techniques and their parameters
Method | Detection limit | Probing depth | Analysis area diameter | Max. profiling depth | Quantification | Obtained information |
RBS | 0.1 at.% ppm for heavy elements | 5 - 15 nm | > 1 mm | 1 µm | Quantitative | Elemental composition (All elements except H and He) |
ToF-ERDA | 0.1 - 0.5 at.% (ppm for H) | 2 - 10 nm | > 1 mm | 500 nm - 1 µm | Quantitative | Elemental composition (all elements + different isotopes of hydrogen) |
XPS | 0.1 - 1 at.% | 3 - 10 nm | 10 µm - 600 µm | 1 µm | Semi-quantitative | Elemental composition and chemical bondings (elements Li - U) |
AES | 0.1 - 1 at.% | 3 - 10 nm | 10 nm - 1 µm | 1 µm | Semi-quantitative | Elemental composition |
SIMS | ppm - ppb | 3 - 10 nm | 0.5 µm - 1 mm | 10 µm | Quantitative with standards | Elemental composition (all elements) |
ToF-SIMS | ppm - ppb | sub-nm | 50 nm - 1 mm | 500 nm | Qualitative | Elemental composition (all elements) Molecular composition |
GD-OES | 1-100 ppm | 3 nm | 2-10 mm | 150 µm | Quantitative | Elemental composition |
LA-ICP-MS | ppm - 100 ppb | > 10 nm | 10 - 100 µm | 2 µm | Quantitative | Elemental composition (Except for H, He, C, N, O, and F) |
Depth profiling with RBS and ToF-ERDA
Rutherford Backscattering Spectrometry (RBS) and Time-of-Flight Elastic Recoil Detection Analysis (ToF-ERDA) are cousin techniques often performed with the same tool together with PIXE. During the analysis, ions are targeted to the sample. RBS will measure the energy and the number of the backscattered ions after their collision with near-surface atoms. ToF-ERDA analysis is done with heavier ions at grazing angle and the atomic nuclei are recoiled in the forward direction and then separated by energy and mass to be detected.
While RBS will offer a better sensitivity for heavy elements, it is difficult to differentiate elements with similar mass. This is why PIXE is often associated with RBS measurements. ToF-ERDA offers the additional possibility to measure all elements, including He and H, as well as H isotopes. It has to be noted that ToF-ERDA is only suitable for flat samples with negligible roughness. Both techniques offer in-depth and quantitative profiling of trace elements.
Depth profiling with XPS and AES
X-ray Photoelectron Spectroscopy (XPS) and Auger Electron Spectroscopy (AES) are similar techniques for semi-quantitative depth profiles. Both techniques measure the energy of electrons that escape the sample after it has been targeted by either electrons (AES) or X-rays (XPS). The emitted electron energy is characteristic of the sample atoms from which they escaped.
XPS and AES provide a semi-quantitative elemental composition of the sample as a function of depth. XPS's strength is that it can also provide the atom’s chemical bonding information as a function of depth, allowing it to differentiate different oxides or alloys of the same element, for example. AES can be very useful for analyzing small patterns, but the sample has to be conductive.
Depth profiling with SIMS and ToF-SIMS
Dynamic Secondary Ion Mass Spectrometry (SIMS or D-SIMS) and Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) are highly sensitive thin film depth profiling methods that are both based on the mass analysis of secondary emitted ion beams. SIMS uses a continuous ion beam to bombard the sample, allowing analysis and etching simultaneously. ToF-SIMS uses a dual beam system with a pulsed ion beam to analyze the sub-nm surface and a second beam for depth profiling. The time-of-flight analyzer allows for molecular information to be gathered in addition to the elemental composition.
SIMS is particularly used for dopant and impurity analysis. It is a quantitative measurement if calibrated with standards. ToF-SIMS provides qualitative measurements with molecular information. Both techniques are very sensitive for trace element analysis.
Depth profiling with GD-OES or LA ICP-MS
Glow Discharge Optical Electron Spectroscopy (GD-OES) and Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) are both quantitative methods for elemental composition analysis. LA-ICP-MS uses a pulsed laser to release a heterogeneous mixture of particles, clusters, and aggregates from the sample. The resulting mixture is carried to ICP-MS with a gas flow where it will be ionized by the plasma and then measured by mass spectroscopy. LA-ICP-MS is a good solution for quantitative and sensitive elemental analysis.
For the GD-OES technique, a discharge is applied between the sample and an anode on which it is positioned. Plasma ions are accelerated toward the surface of the sample inducing the sputtering. Atoms from the sample are consequently ejected and excited by the plasma; hence emitting photons with characteristic wavelengths allowing elemental composition analysis. GD-OES is very fast and does not require a vacuum atmosphere. Recent tools also contain a differential interferometry profiling feature providing a very precise thickness measurement during the sputtering with nanometer-scale resolution which is a great advantage for thin films.
Still not sure which method to choose?
In addition to the methods described above, other analysis techniques, such as SEM-EDX, can be used for depth profiling. Measurlabs offers analysis services using all these techniques and more. You can ask for more information and request a quote using the form below. Alternatively, you can read more about the following measurements and proceed to order them through the links:
Our experts review all orders before processing them to ensure that the selected method is suitable for the sample and conducive to the goal of the analysis.