Spin-on, low-k dielectrics remain viable

Maex: The real question is how to design a material that has both maximum porosity and a pore structure that can be sealed.

Historically, spin-on materials have an important advantage over CVD materials: the ability to introduce a high degree of porosity in the films, reaching k values as low as 2.0. But problems with integrating ultra-low-k materials have caused a delay of two device generations in bringing low-k dielectrics into production. Hence, CVD materials with "moderate" low-k values (2.8 to 2.4) have regained considerable attention.

Spin-on and CVD low-k approaches present numerous integration challenges compared with conventional SiO₂, which is mechanically tough, relatively inert, thermally stable and easy to deposit. A series of critical material properties for low-k materials determines their integratability in damascene processes.

Mechanical strength and thermal stability decide whether a material can withstand chemical-mechanical polishing or survive wire-bonding and packaging processes. Due to their porosity, low-k materials have lower mechanical strength compared with SiO₂. As a rule, spin-on-deposited dielectric materials show a lower degree of cross-linking and thus lower mechanical strength than CVD-deposited films. CVD films also show better thermal stability.

Porosity determines the k value of the dielectric, but the pore structure and distribution also influence other basic material properties and the integratability of the dielectric films. Process interactions like interaction with plasmas, swelling of low-k dielectric films in liquid media and thin-film deposition on porous dielectrics especially pose problems for all films with a high degree of subtractive porosity. This is because the introduction of a high porous volume in the dielectric films goes together with the presence of a high degree of meso-connectivity among the pores. Any exposure of the dielectric will lead to contamination.

As chipmakers put low-k dielectrics into production, manufacturability, tool productivity and cost of ownership become increasingly important. Those issues have increased interest in CVD materials. First, CVD allows a simpler process sequence and many companies have CVD tools at their disposal. Spin-on integration still requires CVD-based dielectrics for hard-mask and barrier applications, for example. A spin-on dielectric flow therefore demands CVD tools in addition to the required spin-on coaters and furnace. Also, the chemicals needed for spin-on materials cost far more than the CVD precursors.

The real question is how to design a material that has both maximum porosity and a pore structure that can be sealed. Also, the material must have the right properties for mechanical and electrical reliability. Hence, both CVD and spin-on-deposited low-k materials are being explored, with a special focus on materials with small pores and high porosity.

Spin-on materials with mechanical properties as good as CVD films are becoming available, as are CVD films with k values approaching 2.0. Differences are becoming less pronounced.

- Karen Maex

Strategic Research Coordinator

Silicon Process and Device Technology Division, IMEC