Crack Formation during Electrodeposition and Post-deposition Aging of Thin Film Coatings - 5th Quarterly Report
This NASF-AESF Foundation research project report covers the fifth quarter of project work (January-March) on this AESF Foundation Research project at the University of Houston. The objective of the work is to study fundamental and practical aspects of crack formation in electrodeposited thin films.
Prof. Stanko R. Brankovic*
University of Houston
Houston, Texas, USA
Editor’s Note: This NASF-AESF Foundation research project report covers the fifth quarter of project work (January-March) on this AESF Foundation Research project at the University of Houston. Access information to past project reports referred to in this paper is listed at the end of this report. A printable PDF version of this report is available by clicking HERE.
- Stanko R. Brankovic, PI, Electrical and Computer Engineering and Chemical and Biomolecular Engineering, University of Houston,
- Kamyar Ahmadi, PhD Student, Material Science Program, University of Houston,
- Wenli Yang, PhD Student, Material Science Program, University of Houston.
The objective of the proposed work is to study fundamental and practical aspects of crack formation in electrodeposited thin films. The aim is to identify and quantify the key parameters of the electrodeposition process affecting the crack formation in thin films. This study should enable development of an effective strategy generally applicable in practice whenever electrodeposition process for crack-free films is demanded.
The activities in this period were focused on structural and compositional studies of chromium thin films, ~ 8 micron in thickness. The chromium films were deposited from Cr+3-containing electrolytes (EXDBA 1411 Bath with pH=5). In addition to these efforts, the work on developing an effective pulse deposition strategy to produce chromium films with minimum amount of hydride phase has started.
XRD Measurements for chromium films electrodeposited on 100 nm copper and ruthenium seed.
The compressive stress developed in chromium films during electrodeposition process from Cr(III) solutions indicates qualitatively different stress evolution as compared to traditionally seen ones in chromium films produced from Cr(VI) electrolytes (see previous reports). Our in situ stress and impedance studies during annealing of chromium films and during their aging in air (see previous reports) show that the fracturing occurs during postdeposition treatment. The tensile stresses developed during post-deposition treatment are relatively low (<100 MPa), which also indicated that chromium films produced from Cr(III) solutions are characterized with low fracture toughness. These results prompted study of the structure of chromium films using XRD method. The seeds on which chromium films were deposited were copper and ruthenium, 100 nm thick films. These two seeds were chosen to allow complementary investigation of chromium structure avoiding the interference from the peaks related to the substrates. The data are shown in Fig. 1 (copper seed) and Fig. 2 (ruthenium seed). The XRD pattern of the seed layer and chromium film are over layered for each sample so that misreading of the peaks related to the underlying seed is avoided. The chromium films were electrodeposited using current densities in the range from 200 to 400 mA/cm2. The common and somewhat surprising result from these studies (Figs. 1 and 2) is that the metallic part/phase of the chromium films shows no crystallinity. A possible presence of a chromium (110) peak is shown in Fig. 2.
However, a more careful analysis of this peak and comparison to the ruthenium substrate pattern yields inconclusive evidence about any crystallinity of chromium metal phase. Therefore, the XRD results demonstrate that metallic phase of the films does not show any peak pattern in the 2θ scan. The lack of characteristic peaks for the chromium lattice indicates that the coherence length in the metallic phase of chromium films is smaller than ~3 nm. Another surprising result is that the XRD patterns on the chromium films on the copper seed show presence of crystalline Cr-oxide (Fig. 1). The Cr2O3 (012), (104) and (110) peaks are indicated in Fig. 1. The oxide presence in the chromium films could be related to two different processes which have been discussed in our previous reports. The first one is the precipitation of Cr(OH)3 in the chromium film during the electrodeposition process. This type of oxide is expected to have an amorphous structure as seen in oxides in FeCo alloys originating from Fe(OH)3 precipitation. However, the crystalline nature of Cr-oxide in this case indicates most likely that the source of this oxide is different. It is likely that crystalline Cr-oxide is produced upon oxidation of native chromium liberated after Cr-hydride decomposition. Therefore, one expects that structure of metallic chromium and Cr-oxide to be intermixed yielding an overall amorphous appearance of the metallic chromium films where significant portion of the chromium thin film body is comprised of the Cr-oxide.