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Evaluation of strength distribution and toughness of LiTaO3 and LiNbO3 single crystals for smartphone applications
Keywords: Frequency filters, Single crystals, Mechanical characterization
Smartphones have become a substantial aspect of daily life. Due to our request for ever faster up- and download rates, around every 10 years a new standard for mobile communication is introduced. In this context, todays 4G LTE network is expected to be superseded by the 5G technology in 2020, providing data rates of up to 10 Gigabit/s [1]. The corresponding increase in utilized frequency bands together with need for larger bandwidths leads to a huge demand for precise and efficient frequency filters. Especially for frequencies below 2 GHz, surface acoustic wave (SAW) filters are the leading technology due to their suitability for cost-efficient mass production. Thin interleaved metal finger electrodes are thereby deposited onto a piezoelectric substrate. When an AC voltage is applied, a periodic displacement of the surface is induced and SAWs are radiated at each finger. All occurring frequencies should thereby be damped, except for the desired one within its bandwidth. Since more and more filters are to be integrated into mobile devices, a high volume density has to be achieved in order to meet required geometric dimensions. In this context, two examples for miniaturized SAW filers are Die-Sized-Surface-acoustic wave Packages (DSSPs) and Thin-Film-Acoustic-Packages (TFAPs) [2]. For the choice of the substrate material various considerations need be taken into account to ensure functionality. Among others, a high coupling between electrical and mechanical field together with a low temperature dependence of the emitted resonance frequency, are highly desired. In this regard, two materials that seem to meet an adequate compromise are special cuts of LiTaO3 and LiNbO3 single crystals. Since significant thermo-mechanical stresses may occur during the fabrication, assembly processes and qualification of SAW microelectronic devices [3], the functionality of the filters may be compromised when the strength of the substrate material is overcome (leading to cracking and failure). Furthermore, an additional aspect in the design of these devices concerns the management of the longitudinal and transversal bulk acoustic waves emitted by the finger electrodes, which may lead to undesirable distortion of phase, amplitude and delay of the filter frequency. Strategies to enhance functionality are (i) thinning the substrate and thus reduce the available volume for bulk acoustic waves and (ii) roughening the back-side of the functional wafer in order to scatter away unwanted bulk waves [4]. From the structural integrity point of view, thinning may lower the bearable loads. In addition, roughening of the wafer may significantly reduce the strength of the substrate material associated with the surface defects introduced, as widely documented for other brittle materials [5]. The aim of this work is to characterize the mechanical properties of single crystalline LiTaO3 and LiNbO3 samples, drawing special focus on the effect of crystal orientation with respect to loading and surface conditioning of corresponding wafers. Biaxial strength measurements, in-situ small scale toughness measurements and nanoindentation experiments were conducted. Anisotropy in strength, toughness and deformation behaviour could be discerned and linked to their atomistic origins. Overall, LiTaO3 showed significantly larger strength values for the same surface conditioning than LiNbO3 [6]. By changing the crystallographic orientation of LiNbO3 a harder surface could be provided and the disadvantage eliminated. Fractographic analyses showed different fracture patterns for both materials due to the different wafer orientations. Both materials tended to fail along the known family {011 ̅2} of cleavage planes [6]. The anisotropic fracture response of LiTaO3 and LiNbO3 single crystals was explained through the toughness measurements in notched micro-cantilevers along the {011 ̅2} cleavage planes, and supported by atomistic modelling calculations of cleavage fracture energies using density functional theory. It is demonstrated that differences in fracture behaviour between LiTaO3 and LiNbO3 are related to the different chemical bonding in LiTaO3 as compared to LiNbO3 within the loaded crystallographic planes. The knowledge on the alignment of tough as well as weak planes (i.e. cleavage planes) can be used to tailor the design of single crystal based functional components, aiming to exhibit enhanced mechanical reliability without compromising the functionality. Methods proposed in the framework of these investigations regarding anisotropic mechanical performance and deformation mechanisms can be extended to other brittle single crystalline materials which may be used in different microelectronic systems.
Manuel Gruber,
Institute of Structural and Functional Ceramics, Montanuniversitaet Leoben
Leoben, Styria

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