Characterization of thin film adhesion

Abstract: Semiconductors are coated with thin metallic films to enable the formation of a reliable mechanical bond that allows for electrical and thermal conductivity. The decisive factor for the reliability of the connection is the adhesion of the thin films to the semiconductor and to one another. However, the perceived adhesion that is relevant for the product is not solely the sum of the interfacial energies, but a complex interaction of the interfacial energies with the plasticity of the material and the residual stresses in the films.

Within the scope of this work, it was determined which methods can best characterize the adhesion of thin metallic films on semiconductors. Furthermore, it was investigated how the adhesion is influenced by deposition parameters or heat treatments.
The evaluation was performed on a metallization system widely used in the field of semiconductors for power electronics, and accompanied by a detailed analysis of the microstructure and residual stress state of the thin film system.

The selected thin film system consists of a Cr-NiV-Au metallization with a total thickness of 650 nm, deposited on a Si substrate. The lateral grain sizes are in the range of less than 10 nm, where especially the NiV layer is grown in a columnar grain structure and has a high density of nanotwins. In the initial state, no diffusion is visible in between the individual films. However, after a heat treatment at 300°C for 500 h, significant diffusion between the films becomes apparent. Ni diffuses locally into the Cr layer, and Au diffuses into the Ni layer. Ni, and especially V diffuse through the Au layer to the film surface. But also Cr diffuses along the NiV grain boundaries and all the way up to the surface. Solely silicon seems to be effectively prevented from diffusion. A significant change in grain size or an alteration of the critical interface between semiconductor and metallization was not observed.

With the heat treatment, the hardness of the NiV layer also changes measurably. However, the change is not a continuous process, but rather the hardness increases and decreases depending on the annealing time.
Residual stress in the thin films was investigated using three different methods: XRD, the wafer curvature method and the newly developed FIB-DIC micro-ring-core analysis at the Università degli Studi RomaTre.
X-ray diffraction did not yield usable results due to the strong texturing of the films. The other two methods consistently yielded residual stress levels in the range of 0.6 GPa to 1.5 GPa for specimens in the initial state without heat treatment. The lowest residual stress values were measured in the outer area of the wafer, the highest in the center of the wafer. This indicates a significant gradient in deposition temperature between wafer edge and center. In addition to that, an increase in residual stress of up to 400 MPa following different heat treatment steps was shown by wafer curvature measurements in an oven setup.

After an extensive literature review, three methods were identified as the most promising techniques for adhesion measurement of thin metallic films on semiconductors: The scratch test, the four point bending method and the cross sectional nanoindentation.
For scratch testing, a commercial device was already available. To perform the four point bending tests, a four point bending setup was developed based on the micro tensile setup designed by Tobias Kennerknecht. The cross sectional nanoindentation experiments were also initially conducted with an available commercial nanoindenter. Due to its limitations, however, an additional dedicated cross sectional nanoindentation setup was developed. Again, based on the micro tensile setup, it allows the indentation process to be controlled manually and the delamination of the films and the displacement of the detached island to be observed at the same time via two optical systems. In addition, the method itself was also adapted, which results in a significant improvement in the quality of the results via a more precise determination of the delaminated area. A direct comparison of the methods has shown that they are very differently influenced by the films material properties and residual stress, based on how they apply the test load to the thin films. Also, in terms of reliability in the product, the behavior of the coatings is very much dependent on the type of load they are subjected to. Instead of focusing on one method for testing thin film adhesion, the test method should be selected according to the particular load case that leads to failure.

In the practical use case of an observed adhesion gradient across the wafer radius, the gradient was successfully measured using the three selected methods. For this case, the scratch test method has proven to be the most suitable method. However, the cross sectional nanoindentation method, with the improvements described in this paper, has the potential to provide even higher quality results.
The effect of heat treatment at 300°C was investigated using both the scratch test method and the improved cross sectional nanoindentation method. Similar to the hardness measurements, a clear influence of the heat treatment time was found. The largest change in adhesion compared to the initial state was measured after 150 h. The investigation with the scratch test method implied improved adhesion after 150 h, while the investigation with the cross sectional nanoindentation method implied reduced adhesion. This clearly shows the different influence of the material properties changed by the heat treatment on the adhesion measurement methods. However, since these not only influence the measurement methods but also the behavior in the product, the answer to the question of whether a heat treatment is beneficial for the reliability of the product depends decisively on how the load is in the event of damage. In addition, it turns out that as long as the properties of the thin films are not better understood, a qualitative value representative of the load case is much more important than a quantitative value of the underlying thermodynamic adhesion

Angaben zum Objekt

• Standort: Deutsche Nationalbibliothek Frankfurt am Main
• Umfang: Online-Ressource
• Sprache: Englisch
• Anmerkungen: Universität Freiburg, Dissertation, 2021

Klassifikation und Themen

• Schlagwort: Adhesion ; Diffusion ; Gold ; Vanadium ; Nickel ; Adhäsion ; Messung ; Härteeindruck ; Metallisierungsschicht ; Halbleiter ; Leistungshalbleiter ; Röntgendiffraktometrie ; Eigenspannung ; Diffusion ; Korngröße ; Mikrostruktur ; Elastizitätsmodul ; Chrom ; Nickel ; Vanadium ; Gold ; Delamination

Beteiligte, Orts- und Zeitangaben

• Ereignis: Veröffentlichung
• (wo): Freiburg
• (wer): Universität
• (wann): 2021
• Urheber: Schiebel, Felix
• Beteiligte Personen und Organisationen: Eberl, Christoph ; Wilde, Jürgen ; Albert-Ludwigs-Universität Freiburg. Fakultät für Angewandte Wissenschaften

Weitere Informationen

• DOI: 10.6094/UNIFR/223045
• URN: urn:nbn:de:bsz:25-freidok-2230459
• Rechteinformation: Kein Open Access; Der Zugriff auf das Objekt ist unbeschränkt möglich.
• Letzte Aktualisierung: 15.08.2025, 07:23 MESZ