Thin silicon nitride films are used as passivation barriers, interlayer dielectrics, and structural materials in semiconductors, MEMS, sensors, medical devices, and packaging. Thin film, low stress, silicon nitride mechanical properties are strongly dependent on the deposition method and process parameters and can also vary from their bulk material counterparts.
It is essential to measure the thin film post-processing to assure the mechanical properties are within the process target. It is also important to use the actual properties when simulating structures that include these films.
Veryst measured elastic modulus on a low stress silicon nitride thin film using nanoindentation. The 400 nm thick silicon nitride film was deposited using a low temperature PECVD (plasma-enhanced chemical vapor deposition) tool.
We used a diamond Berkovich indenter tip with a 1 mN maximum load and performed eight indents.
Load versus displacement curves show (in Figure 1) that the residual indent depths are repeatable at 15 nm while maximum load depths repeat at approximately 48 nm. The indenter tip at the maximum load depth gives the contact area; this area is related to the hardness of the material. The unloading curve slope, S, is related to the stiffness of the material. The contact area and stiffness are used to calculate the modulus of the thin film.
We used atomic force microscopy (AFM) line profiling to assure that the nanoindents were performed properly. A line profile of the indentation mark agrees with the load-displacement curve residual depth of 15 nm, as circled in the line scan shown in Figure 2. This indentation width at this location is approximately 300nm.
Nanoindentation penetration depths of a thin film on a substrate should be ~10% of the overall thin film thickness to eliminate substrate effects (see Figure 3).
This low stress silicon nitride 400 nm thin film is on a silicon wafer, and a maximum load depth of 48 nm meets this criteria at 12%.
Veryst calculated the elastic modulus per ASTM E2546 for this low stress silicon nitride film. This matches the literature for films deposited at similar deposition settings. Table 1 summarizes the results.
|Elastic Modulus Mean
Veryst is experienced in the measurement and characterization of thin film properties, whether silicon nitride or other materials, including polymeric. This characterization includes elastic, inelastic, and rate-and-temperature-dependent properties.
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