Aluminum as an interconnect material in combination with silicate glasses as insulators have been exclusively used for IC metallization during the last few decades. Technology development has been driven by a continuous reduction of feature sizes. At the shrinking point of devices beyond the 0.35 μm generation, the interconnect RC delay (dominated by the metal resistance and the insulator capacitance) significantly limits, for example the microprocessor clock speed. Furthermore, beyond the 0.25 μm device generation, current densities can induce electromigration failures especially in lower interconnect levels. In order to resolve this issue, a "revolution of materials" used in semiconductor fabrication is in progress around the world. Copper as an alternative interconnect material in combination with special low permittivity insulating materials (also-called "low-k dielectrics") offers the new quality required.
The Center of Microtechnologies has focussed particularly on ultra low-k dielectric materials with k < 2.2. Fluorinated polymer thin films with k-values as low as 2.0 were formed by plasma enhanced chemical vapor deposition. This material has one of the lowest dielectric constant for nonporous materials with sufficient thermal stability. The study of its integration feasibility with copper interconnect technology is underway. On the other hand, making dielectric films porous is the way to achieve k-values lower than 2.0. The development of a xerogel process for microelectronic applications was a high-risk topic because the requirements for these films are very demanding (thermal stability, adhesion to adjacent films, moisture adsorption, leakage current, mechanical stability ...) and a new method had to be to developed, now with its own patent pending. A joint development program with the semiconductor equipment manufacturers Applied Materials GmbH and Suss MicroTec AG funded by the BMBF is currently pursued to prove the process potential for production using dedicated equipment.
The high international level of the research and development in this field has become apparent in a collaboration with leading European institutes and IC manufacturers within the European ESPRIT and IST projects Damascene, CLOK and ULISSE since 1997.
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Prof. Dr. Stefan E. Schulz