Characterization of single- to multi-layer surface treatments - ToF-SIMS

A surface treatment is a coating applied to the surface of an object by mechanical, chemical, electrochemical or physical means. The aim of this operation is to modify the appearance or surface function of materials. From self-cleaning car windshields, anti-reflective, anti-scratch and anti-fog spectacle lenses... to antibacterial medical prostheses, surface treatments are used in the vast majority of everyday objects.

These coatings are so powerful that with a thickness of just a few nano- or micrometers, they radically alter a product's functionality and performance.

Plasma is a state of matter in the same way as solid, liquid or gaseous states. There is no phase transition from one of these states to plasma and vice versa. It's matter in the form of ions and electrons that is electrically neutral overall.  On Earth, it is visible in its natural state at high temperatures to promote ionization, the best-known example being lightning.

Its high chemical reactivity enables it to modify the surface of many materials.

In industrial applications, plasma is used for three types of reaction:
- Surface cleaning: plasma technologies are used to remove any contamination present on the surface of a material, whether organic or hydrocarbon in nature.
- Surface activation: plasma is used to modify the nature of a material's surface by adding atoms or molecules. The polarity of the surface is increased, and its adhesive wettability is improved, to enhance the adhesion of glue, varnish, paint, etc.
- Plasma thin-film deposition is used to cover surfaces to be functionalized with an ultra-thin coating.

Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) will enable chemical evaluation of these thin films. This highly sensitive method of elemental and molecular analysis of the extreme surface (' 1 nm) is a valuable source of information for characterizing these deposits. The profile mode gives access to in-depth distribution chemistry down to 20 µm by alternating analysis and abrasion cycles with an Argon cluster (GCIB) for organic materials, or a Cesium (Cs+) or Oxygen (O2+) gun for inorganic materials.

ToF-SIMS can detect trace elements down to ppb at the extreme surface (< 1 nm) and up to 20 µm in profile mode on vacuum-compatible samples of all types.

By alternating acquisition and abrasion sequences, a composition profile can be traced with nanometric depth resolution. The primary ion beam, reduced to a small diameter spot, scans the surface to be imaged. The secondary optics for extraction and mass analysis are fixed. The image is reconstructed by synchronizing the secondary signal with the primary beam scan. Lateral image resolution depends on the size of the micro-beam (from 100 nm to 3 µm in diameter, depending on analysis conditions).

TESCAN ANALYTICS has over 20 years' expertise in the use of ToF-SIMS on all types of materials, whether insulating or conducting... With state-of-the-art instruments, our team of experts works with all industrial sectors.

 

Figure 1: ToF-SIMS profile of a wafer coated with 40 thin films


Figure 2: 2D ToF-SIMS image of the lateral section (a) over the entire depth of the profile (b) with enlargement of the thin layers Figure 3: 3D ToF-SIMS image of the lateral section over the entire depth of the profile
 

ToF-SIMS analysis in profile mode is performed via analysis/abrasion cycles with an argon cluster source. The profile is acquired over a depth of 1.5 µm and an area of 400 x 400 µm2, with one image every 10 abrasion scans.

Figure 1 shows the chemical profile of the ions of interest on the substrate (silicon wafer) and the various coatings deposited on its surface. A first silicon top coat layer of around 100 nm is observed. A 1 µm barrier coating is then detected, this plasma deposit of 40 thin layers is of organic nitrogen composition. Carbon ions show 40 peaks spaced at around 23 nm in the barrier layer. A 200 nm adhesion layer separates this barrier from the silicon-only substrate, visible from a depth of 1.2 µm.



Figure 2 and 3 show lateral reconstructions in 2 and 3 dimensions. Figure 2 shows a 2D view of the lateral section of the substrate covered with different layers. These images show each of the 40 deposits making up the barrier layer. It is worth noting the very good z-resolution of this imagery, making it possible to distinguish each of the layers spaced 23 nm apart.

With elemental detection down to ppb and abrasion depths down to 20 µm, ToF-SIMS gives access to depth distribution profiles enabling highly informative 3D reconstructions for nanometric coating evaluation and control. This method provides information on the elemental and molecular composition of each layer, as well as its thickness.

ToF-SIMS is a high-resolution tool for the chemical characterization of nanometric deposits.

In other work, ToF-SIMS has been used to map 2D and 3D contaminants on the surface of technical parts.

For more application notes with ToF-SIMS or our other techniques, see our thematics.

Other complementary techniques can be used to characterize thin films, such as XPS, TEM or SEM.