Original ArticleLi2O-ZrO2-SiO2/Al2O3 nanostructured composites for microelectronics applications
Introduction
In recent years, with the rapid development of commercial wireless technologies and satellite communication, the evolution of electronic components moves toward miniaturization, integrated electronic components and high frequency, all of which require the substrates to meet the high propagation speed, high wiring density and considerable chip packaging demands [1,2]. These circumstances have increased the interest in producing novel materials with attractive dielectric properties and low sintering temperatures.
Among the new technologies for the processing of such materials, the low temperature co-fired ceramic (LTCC) technology is of particular interest. This technology is widely used in the preparation of ceramic substrates with low dielectric constants (<10), sufficient mechanical strength (≥150 MPa), low sintering temperatures (<1000 °C) and low coefficient of thermal expansion (CTE) for a successful matching with the thermal properties of the silicon devices utilized in such systems [3].
Moreover, materials that display low CTE are especially suited for applications in which rapid changes in temperature take place, such as heat exchangers, ceramics for use in domestic furnaces and burner nozzles. Furthermore, in applications involving the joining of materials, a close compatibility of thermal expansion is required. Under such conditions, the CTE values of the coupled materials must be close enough in order to minimize thermal stresses, thus rendering the joined materials strongly bound to one another. Such would be the case of glass-ceramic/metal systems, which include hermetic seals in laser tubes, vacuum tubes and solid oxide fuel cell sealants [[4], [5], [6], [7]].
In microelectronics, a great variety of materials is used to construct a network of conductive paths that integrate the electronic components, sensors, actuators, microsystems, cooling and heating systems in a single module. Using the LTCC technology, it is also possible to produce MEMS (Micro Electro Mechanical Systems) packages [8]. Therefore, advanced electronics applications urgently need novel materials with not only excellent mechanical properties, but also tunable CTE.
The LiAlSi2O6, β-spodumene phase, which has a low CTE value of 0.9 × 10−6 ºC−1 and a low dielectric constant of ∼7, is one of the main crystalline phases of lithium aluminosilicate (LAS, Li2O-Al2O3-SiO2, LZSA, Li2O-ZrO2-SiO2-Al2O3, LZS, Li2O-ZrO2-SiO2/Al2O3 composites) glass-ceramics [[9], [10], [11], [12], [13]] and is of great interest for use in microelectronic devices. Due to their excellent thermal and dielectric properties, LAS, LZSA and LZS + Al2O3 composites containing this crystalline phase can be considered as potential candidates for applications both in the LTCC technology and in other applications that require rapid temperature variations. In the case of LZS + Al2O3 composites, appropriate amounts of Al2O3 may result in materials with low and controlled CTE suitable for specific applications, such as advanced multilayer materials [14].
The conventional processing methods utilized in the production of these materials are both time and energy consuming, mostly due to the high melting (1550 °C) and sintering/crystallization (∼900 °C) temperatures and the long milling process required to obtain the fine powders from the parent glass frits. LZS and LZS + Al2O3 were already prepared through a colloidal approach from mixtures of Al2O3, SiO2 and ZrO2 nanopowders and a Li precursor, eliminating the melting process [11]. Glass-free materials with appropriate thermal and dielectric properties are strongly desired for the multilayer structure applications because they simplify the chemical interaction with the metallic layers and reduce the possibility of cracking caused by the mismatch of the CTE between the ceramic and glassy phases [15].
In the present paper, we optimized different variables of the processing of ceramic tapes of glass-free LZS + Al2O3 nanocomposites for the effective production of substrates for microelectronics. We successfully eliminated the usual melting stage associated to these systems by using a colloidal approach. The addition of alumina also allowed the tuning of the thermal expansion coefficients of the produced tapes, which in turn enabled the co-firing of the materials with a silicon wafer. The interfacial region of such joining is pristine and crack-free, thus demonstrating the potential of such materials for application in advanced microelectronics.
Section snippets
Materials and methods
Commercially available nanosized ZrO2 powder (40N-0801, Inframat, USA, dv.50 53 nm and specific surface area of 53.2 m2/g), a colloidal suspension of amorphous SiO2 nanoparticles (Levasil 200 A, Bayer, Germany, solids content of 40 wt%, dv.50 15 nm and specific surface area of 205 m2/g), lithium carbonate (Synth, Brazil) and glacial acetic acid (Quimex, Peru) were used as starting materials in the production of LZS (19.58Li2O·11.10ZrO2·69.32SiO2) nanostructured composites. Al2O3 nanoparticles
Results and discussion
The first step towards the production of ceramic tapes is the preparation of a stable suspension of the precursors. A well-dispersed suspension, with low viscosity and high solids content, is ideal for minimizing the microstructural defects of materials subjected to firing. Nevertheless, such requisites are hard to attain when ceramic powders with nanometric particles are utilized. Their high specific surface area promotes aggregation and agglomeration, hindering the stability of suspensions.
Conclusions
Nanostructured composites of the LZS (19.58Li2O.11.10ZrO2.69.32SiO2) and LZS + (1, 2.5 and 5 vol%) Al2O3 systems were obtained successfully through a colloidal approach from nanoparticulate oxides and Li2CO3 precursors.
The addition of Al2O3 allowed the tuning of the thermal expansion coefficients of the produced tapes, which reduces up to values of 2.2 × 10−6 ºC−1 which is even lower than the obtained for similar compositions obtained by conventional glass-ceramic processing. The tapes fired at
Acknowledgments
This work has been supported by CAPES in the frame of the International Cooperation Program Science without Borders for Special Visiting Researcher PVE (MEC/MCTI/CAPES/CNPq/FAPs/Nº71/2013), Project Nº A011/2013 (Brazil), CAPES and CNPq (National Council for Scientific and Technological Development, Brazil). Authors greatly acknowledge the financial Support of the Spanish Ministry of Economy and Competitiveness (MINECO, grant MAT2015-67586-C3-2-R).
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