ICMCTF2010 Session F2: In Situ Characterization

In microelectronic devices or flexible electronic components the metallic interconnects are prone to failure due to limited plasticity or debonding at the interface between the metal and the adjacent materials. In order to study the mechanical behaviour of the metallic films and the film/substrate interface quantitative approaches are required. Two examples will be provided in this talk:

(i) Lithography techniques can be used to fabricate free-standing tensile samples into the metallic film material. As a consequence of the small dimensions of several micrometers down to a few hundred nanometers electron microscopy is employed to perform controlled tensile tests. The in situ straining experiments provide quantitative load and displacement data as well as insights in the deformation mechanisms. Complementary synchrotron experiments are performed to unravel the deformation induced dislocation structures.

(ii) The “adhesion energy” of thin metallic films on compliant substrates can be probed by in situ tensile testing. Cracking of the metallic film occurs under tensile load, while the lateral contraction can impose compressive film stresses which lead to buckling and debonding. Quantification of the buckle dimensions and the debonded area is used to deduce the “adhesion energy” of the interface by a thermodynamically based energy consideration. The concept and its limits will be discussed in the talk.

The origin of stress in polycrystalline thin films has gained recently a renewed interest with the potentiality offered by real-time techniques to measure stress evolution during growth. Such in situ measurements not only yield the magnitude of the growth stress, but also provide insights into the growth mechanism itself. Stress models have been proposed in the literature to explain mainly the complex behaviour of high-mobility materials grown by evaporation or sputtering.

The present work is focused on the stress development during growth of low mobility metallic thin film. For this purpose, Mo and Mo_1-xSi_x alloy thin films, with x ranging from 0 to 0.4 and film thickness up to 200 nm, were grown by magnetron sputtering at 300K onto either amorphous Si (a-Si) or crystalline bcc Mo buffer. The stress evolution during growth was monitored in-situ with a multiple-beam optical stress sensor. Very distinct stress evolutions were observed as a function of the Si content and type of buffer layer. For films grown on a-Si, the different stages of stress evolution were correlated to change in surface stress, growth of an interfacial amorphous layer, followed by an amorphous-to-crystal phase transition occurring at a concentration-dependent critical thickness. While a significant tensile stress contribution was observed for crystalline alloys grown on a-Si, a steady-state compressive stress evolution was noticed for the equivalent films grown on bcc-Mo. The results are interpreted and discussed based on microstructural features revealed by ex-situ XRD, cross-sectional HRTEM and AFM surface topography. Finally, these results show the importance of the initial crystalline nucleation conditions on the different stress generation and relaxation mechanisms.

The tribological performance of hard, protective coatings is largely governed by their mechanical properties; great hardness and a moderate elastic modulus are most desirable and generally lead to reduced wear. In many tribological studies on protective hard coatings, the formation and retention of transfer film material within the contact interface is given only little or no attention. This may be the case because of technical limitations with respect to transfer film detection or because the influence of a transfer film on the tribological behaviour of protective hard coatings is underestimated.

Within this research, the influence of transfer film formation and –retention on friction and wear is studied in tribological experiments on a variety of nano-composite hard coatings consisting of carbides, nitrides, or carbonitrides of titanium and silicon. An in situ Raman tribometer is employed to study tribomechanical and –chemical processes determining friction and wear in interfacial sliding. Featuring a 1/4” diam. sapphire hemisphere as counterface, the tribometer allows the recording of transfer film formation and fluctuation by means of video microscopy. It is additionally possible to study chemical changes occurring within the sliding interface by means of Raman spectroscopy.

Initial tests were conducted on nano-composite carbide coatings with varying TiC/SiC ratios between 100:0 and 50:50, deposited onto Si wafers by means of Plasma-Enhanced Chemical Vapor Deposition (PE-CVD). The experiments were performed as stripe tests, implementing 600 cycles at a track length of 6 mm, half-way overlapped by an additional 1,800 cycles at a track length of 3 mm. The average Hertzian contact stress was 460 MPa; relative humidity was kept between 40 and 50 %. The coefficient of friction was recorded at high spatial resolution as well as processed as averages over cycle. To determine the variation of transfer film thickness with proceeding cycle number, video frame shots were analyzed by means of a Newton’s rings method [Wahl, Chromik, 2008]. Ex situ white-light interferometry was applied additionally to measure coating and counterface wear, as well as final transfer film thickness and morphology.

In situ tribometry generally revealed rapid build-up of transfer film material during run-in followed by variability in the transfer film thickness. In this initial test series, in situ Raman tribometry proved to be an effective tool for the study of dynamic changes within third bodies produced when sliding on hard coatings. Future tests will explore the correlations between transfer film changes and friction of further coating chemistries.

In order to grow high purity, high crystallinity film through epitaxy, the understanding of growth process and the control of surface and interface are critical. UltraHigh Vacuum Chemical Vapor Deposition (UHV-CVD) using zirconium borohydride (Zr(BH₄)₄) as a source has advantages in the purity of source material compared to solid source processes, and also in in situ surface characterization capability[1,2]. A unique UHV-CVD apparatus with Reflection High Energy Electron Diffraction (RHEED) system was constructed, and the epitaxial growth of ZrB₂ on Si(111) and sapphire(0001) were carried out successfully[3,4].

RHEED pattern evolution during the growth on both substrates was studied. On Si(111), two-dimensional nucleation of ZrB₂ with streaky ZrB₂(0001)-(2x2) pattern, and then ZrB₂(0001)-(1x1) pattern with increased thickness was observed, which is consistent with earlier report using low energy electron diffraction[2]. On sapphire substrate, transmission diffraction pattern was observed upon the nucleation of ZrB₂ indicating three-dimensional nucleation, and then under optimized conditions, ZrB₂(0001)-(√3x√3) was observed with increased thickness. The difference in surface reconstruction during the growth seems to originate from substrate atoms segregating to the surface, and clear surfactant effect of these atoms was observed as an order of magnitude difference in growth rate for films grown on different substrates, but otherwise under same conditions.

[1] J. Tolle, R. Roucka, I.S.T. Tsong, C. Ritter, P.A. Crozier, A.V.G. Chizmeshya and J. Kouvetakis, Appl. Phys. Lett. 82, 2398 (2003).

[2] C. W. Hu, A. V. G. Chizmeshya, J. Tolle, J. Kouvetakis, and I. S. T. Tsong, J. Cryst. Growth 267, 554 (2004).

[3] S. Bera and Y. Yamada-Takamura, Kyoto-Advanced Nanotechnology Network JAIST Report H19-028 (2008).

[4] S. Bera, Y. Sumiyoshi, and Y. Yamada-Takamura, J. Appl. Phys. 106, 063531 (2009).