Tape Casting of Composite Ceramic Substrates
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DEVELOPMENT OF Al2O3/AlN COMPOSITE SUBSTRATES FOR MICROELECTRONICs In the microelectronics industry, the development of high speed interconnects for semiconductor devices demand new materials which have low dielectric constant, low propagation delay and minimized capacitive line loading. Other crucial properties required include a high thermal conductivity, as well as a coefficient of thermal expansion (CTE) similar to that of silicon. Currently, alumina (Al2O3), glass and glass ceramics are used to manufacture substrates. These materials, however, do not possess the increasing property demands like high thermal conductivity and compatible CTE. Aluminum Nitride (AlN) whilst having excellent properties like high thermal conductivity, low dielectric constant and compatible CTE, its materials and processing cost are prohibitively high. Figures 1, 2 & 3 compares properties among various ceramic materials.
Figure 1 Thermal Conductivity
Figure 2 CTE
Figure 3 Dielectric Strength
Therefore, with the inability of a single material to meet such demands, a composite substrate is developed, where the properties of each material are optimized. Al2O3/AlN substrates were produced using the tape casting process via three different starting routes:
In the tape casting process, the starting powders are mixed with 5% Yittria (Y2O3) and ball milled. The powders are then added to a deflocculant and solvents to form a suspension and ultrasonically agitated. Sequential addition of plasticizer and a binder follows, and the mixture is mechanically mixed to achieve a homogeneous slurry. This slurry is de-aired in a vacuum chamber and sieved into the tape casting chamber.
Figure 4 Batch-process type caster Tape casting was performed on a batch-process type caster (Figure 4), where the casting head and doctor blade traversed over a stationary floating glass slab, depositing slurry onto the surface. The freshly cast tape is subsequently dried, stripped and laminated into a multilayered structure. This multilayered structure is then pyrolized and sintered into a hard multi-layered ceramic package. Previous work done indicated the formation of ALON during the sintering of such composite substrates. ALON is an acronym for aluminum oxynitride - a pseudo-binary solid solution of AlN in Al2O3, which may be regarded as a nitrogen-stabilized cubic g -Al2O3 phase (spinel). The presence of ALON is undesirable, as its thermal conductivity is very poor. Figures 5 &6 show the microstructure and phase composition of a sintered Al2O3/AlN multilayered substrate.
Figure 5 Microstructure (SEM)
Figure 6 XRD pattern of substrate
As shown in the micrograph, the microstructure exhibits good sinterability with a very low porosity. Liquid phase sintering occurs, where YAlO3 is present at the grain boundaries, as confirmed by the x-ray diffraction (XRD) patterns. The XRD pattern also confirms the absence of ALON, which could be detrimental to its properties. XRD pattern indicates good retention of Al2O3 and AlN. A composite Al2O3/AlN substrate was successfully developed, and work is currently in progress to characterize its properties. Boey, F., Khor, K.A., Cao, L.H., Tok, A.I.Y.** Graduate Student Tape Casting of Composite Ceramic Substrates using Microspheres A novel approach is presently taken in producing a composite ceramic substrate with controlled porosity using micro-spherical powders via the tape casting method. Existing substrates employ thin ceramic tapes, which are cast using solid starting powders. Microspheres are spherical particles (< 200m m) which differ from non-spherical particles (platelets, fibres, whiskers, granules etc.) in several ways:
Figure 1 shows a photo of the microspheres.
Figure 1 Hollow microspheres
With the current device of miniaturization and higher device counts in IC (integrated circuit) packages, a significant increase in the usage of both multilayer ceramic packages (MLCP) and multilayer capacitors (MLC) has been apparent. The demand for the development of interconnection and packaging techniques which accommodate faster system signal propagation, low dielectric loss and medium range permittivity is also increasing.
Delay time (t ) of an electrical signal on a substrate can be reduced by decreasing the dielectric constant (Î ) of the substrate material by the following relationship: t = Î 1/2/c where c is the velocity of light. Since air has a dielectric constant of about 1.0, the encapsulated air within the hollow microsphere enables its dielectric constant to be significantly reduced, thus decreasing propogation delay. A linear relationship between the porosity created by the microspheres and the dielectric constant has been suggested. The dielectric constant (Î ) was related to the volume fraction (Vf) of a substrate by the following equation: Î = (-7.54 x Vf ) + 8.48 Two routes were taken in this work.
In the first case, the solid microspheres were mixed with the alumina powder and tape cast using a batch process type caster, where the casting head and doctor blade traversed over a stationary floating glass slab, depositing slurry onto the surface. The tape is dried, stripped and laminated into a multi-layered structure. The laminate is then pyrolized in flowing air to burn off the polymeric microspheres, and subsequently sintered at 1600° C.
Figure 2 shows a cross-section of this substrate using 10% weight ratio of microspheres. Work is currently in progress to refine the process, and to characterize the properties.
Figure 2 Cross-section of substrate In the second case, work is in progress to produce the hollow alumina microspheres via the sol-gel method. Work was also done to test the feasibility of tape casting the hollow microspheres. One problem faced was the positive buoyancy of the microspheres. This problem was overcome by increasing the slurry viscosity. The tapes cast using hollow microspheres had a density gradient across its thickness due to the buoyancy, but this can be overcome by laminating the tapes into a multi-layered structure. Another method used to overcome the buoyancy was to coat the microsphere surface with alumina to increase its density. This was successfully done via a powder method, as shown in Figure 3.
Figure 3 Al2O3 coated microspheres
Further works include the development of composite Al2O3/AlN ceramic hollow microsphere powders and substrates. Boey, F., Khor, K.A., Tok, A.I.Y.** Graduate Student (Information provided by Mr Alfred Tok) |
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