By P. Markondeya Raj, Devarajan Balaraman, Isaac Robin Abothu, and Rao Tummala, Packaging Research Center, Georgia Institute of Technology
Ceramics’ unique properties such as low loss factor, high thermal stability, and hermeticity have secured their role for applications in RF, high-temperature, and high-reliability devices for decades. The discovery that polymers, such as BCB and PTFE, have low-loss factors comparable to ceramics, and that liquid crystalline polymers are nearly hermetic, is changing that role. Integrating functional components in organic packages is difficult because of the inherent process incompatibilities forcing the industry to use ad-hoc solutions.
Emerging technologies can integrate thin film capacitors directly in organic packages to take advantage of low inductance, leading to low impedance over broad frequency band, miniaturization, improved reliability, and potentially low cost.
Ferroelectric ceramics are traditionally formed by one of two processes: sintering of powders at temperatures greater than 1000°C or by vacuum thin-film technologies such as sputtering, which results in amorphous film with low dielectric constant. However, sputtered films can be crystallized into ferroelectrics at high temperatures.
Various alterations of adapting these high-temperature processes for use in organic substrates are being explored. Sol-gel technology is being developed for depositing ceramic films that typically require a foil lamination technique to integrate then into organic substrates; imposing possible design, process, and performance constraints. Rather than relying on these processes, researchers are developing ceramic films from direct solution processing at organic-compatible temperatures (Figure 1).
 Figure 1. 200-nm film hydrothermally deposited at 95°C and baked at 160°C, directly on organic substrates. |
With hydrothermal technique, crystalline ferro or paraelectric films can be directly deposited on large-area organic panels at temperatures less than 85°C. Because of the water-based chemistry and wet processing, no significant cost increments or environmental hazards can be expected by scaling up this technology.
In a typical hydrothermal process, a film of titanium or its derivative organic compound is reacted with highly alkaline barium, or strontium solution, to heterogeneously precipitate films of crystalline titanates on organic substrates. Such ceramic films with ultra-high capacitance density have been in-situ-deposited on organic packages without high-temperature or high-energy processes like sputtering, sintering, or annealing. Both titanium and titanium alkoxide coatings are shown to yield dense, crystalline, and pore-free barium titanate films within a reaction time of 20 minutes. The reaction is self-limiting, so the film thickness can be controlled accurately. All the processes can be engineered to be just solution-based, with the highest baking temperature at less than 200°C. The grain size of these films is typically within 100 nm, making the film super-paraelectric with dielectric constant of 200 to 300, and film thickness of about 200 nm. Direct deposition of crystalline barium titanate and strontium titanate films on FR4 substrates showed a capacitance density above 1 µF/cm2, and loss of 0.04 – the highest capacitance density that could be embedded in large organic substrates today. Post-hydrothermal baking at 200°C has been shown to lower the leakage of barium titanate to <1 µA/cm2 at 5 V, and improve the breakdown voltages to values similar to those from sputtered films on copper electrodes (60 V/µm.). Hydrothermal films satisfied both the DC and high-frequency performance for decoupling applications. Low-impedance power supply requirements dictate capacitance densities of the order of µF/cm2, which has not been achieved with any other polymer-compatible technology. Integration of high-k thin-film capacitors is simpler with the hydrothermal method, because these films can be directly deposited on large organic panels in plastic containers, similar to electroless and electroplating wet metallization processes.
Solution-based processing technology for integrating capacitor films at organic compatible temperatures is not new. Other researchers have applied this to other functional ceramic materials, such as inductor core and embedded batteries. If completely explored, these wet chemistry-based in-situ syntheses of functional ceramic films can have predominant effect on the new packaging industry that is evolving.
P. MARKONDEYA RAJ, assistant research director; DEVARAJAN BALARAMAN, graduate research assistant; ISAAC ROBIN ABOTHU, research scientist; and RAO TUMMALA, Ph.D., professor and director, may be contacted at the Packaging Research Center, Georgia Institute of Technology, 813 First Drive NW, Atlanta, GA 30332-0560; 404/894-2652; E-mail: [email protected], [email protected], [email protected], and [email protected].