Facile and Controlled Synthesis of Ultra‐Thin Low Dielectric Constant Meso/Microporous Silica Films

Abstract

Sustained and aggressive scaling within the microelectronics industry has necessitated reducing the permittivity of the back end dielectric, namely silicon dioxide, to help reduce the resistance–capacitance delay. One approach currently under consideration involves the incorporation of pores within the dielectric itself. This is achieved predominantly via the self-assembly of block copolymers and a silicon source, using sol–gel techniques—a scheme which may potentially minimise the RC delay. Self-assembled mesoporous films with defined nanostructures have received considerable attention over the last decade owing to their excellent insulating properties, well-defined porosity, reliable processing and ability to form very thin and continuous films. In recent times, focus has also expanded towards investigating spin-on zeolite (microporous) thin films as they offer increased thermal stability (i.e. no pore collapse or unidirectional shrinkage), defined micropore size and porous (intraparticle) architecture, low dielectric constants of 1.8 and significant interparticle mechanical strengths. Unfortunately zeolite films suffer from a variety of problems such as poor cohesive strengths and unwanted large diameter, non-ordered mesoporosity caused by irregular nanoparticle shapes. Therefore, these films present an enormous integration challenge for high-volume manufacturing. Furthermore the zeolite films are essentially agglomerations of nanoparticles deposited across a silicon substrate and, as a result, have high surface roughness that potentially can cause two major concerns: 1) poor gap fill and 2) poor patterning (etching) properties. Non-uniformities within films results in poor etch rates/ variations, difficulties at chemical mechanical planarization (CMP) steps and interconnect misalignment problems. The addition of a dense tetraethoxyorthosilane (TEOS) binder material enhances the adhesion of the film to the silicon substrate and removes the large mesostructural porosity, but concurrently results in an increase of the dielectric constant. To offset this relationship, work has shifted towards investigating binder materials with low permittivities in an attempt to negate the increasing dielectric constant. Addition of polymer binder material was previously demonstrated by Li and co-workers, whereby 5–15 wt.% of c-cyclodextrin was added to promote adhesion between the nanoparticles. The authors reported little loss in mechanical properties whilst maintaining a low dielectric constant. Larlus and co-workers have also reported the use of silicalite-1 zeolite films covered in an acryl latex layer to enhance mechanical properties and reduce surface roughness. A dynamic dielectric constant ranging from 2.1 to 2.4 was reported using spectroscopic ellipsometry. To reduce the dielectric constant further the binder substance may be exchanged for a mesoporous material, fabricated via evaporation-induced self-assembly (EISA) routes. Previously, zeolite-beta and silicalite-1 nanoparticles were mixed with mesoporous films and demonstrated high degrees of bimodal ordering and reasonably good film-forming properties. Unfortunately the mesoporous structure-directing agent was an ammonium-based cationic surfactant, which generates pore sizes close to 2 nm and consequently reduces the overall porosity of the films. Herein we examine the insulating properties of zeolite/mesoporous films, whereby the binder material is templated by an anionic block copolymer with special emphasis on utilizing them for advanced semiconductor dielectrics. The mesophase and the orientational alignment of the mesoporous system and silicalite nanoparticles is confirmed by grazing-incident small-angle X-ray scattering (GISAXS) and wide-angle X-ray diffraction. The mesophase of a thin film deposited from a coating solution containing 50–50 volume percentage of silicalite-1 nanoparticles and silica/P123 sol–gel is presented in Figure 1a. The mesophase is compressed hexagonally, with channels oriented parallel to the substrate surface. The silicalite-1 nanocrystals within the films show preferred orientation with their a or b axis parallel to the substrate surface, evidenced by the appearance of the (h00) and (0k0) reflections only (Supporting Information, Figure 1). The silicalite-1 nanoparticles within the films exhibit orientations with either sinusoidal or straight channels perpendicular to the substrate surface. This orientation is possible because the well-developed crystalline shapes have coffin-like morphologies (Figure 1b), that allow for orientational alignment when in contact with a substrate during deposition of very thin films. Similar results are obtained for pure silicalite-1 thin films deposited by spin coating (Supporting Information, Figure 2). Further details on [a] Dr. R. A. Farrell, Dr. N. Petkov, Dr. J. D. Holmes, Prof. M. A. Morris Department of Chemistry and Tyndall National Institute University College Cork, Cork (Ireland) Fax: (+353)2144274197 E-mail : m.morris@ucc.ie [b] Dr. J. D. Holmes, Prof. M. A. Morris Centre for Research on Adaptive Nanostructures and Nanodevices (CRANN) Trinity College Dublin, Dublin 2 (Ireland) [c] Dr. K. Cherkaoui, Dr. P. K. Hurley Tyndall National Institute, University College, Cork, Cork (Ireland) [d] Dr. H. Amenitsch Institute of Biophysics and X-ray S Steyrergasse 17/VI, 8010 Graz (Austria.) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cphc.200800158.