Routes to the Formation of Air Gap Structures Using PECVD

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0914-F10-02

Routes to the Formation of Air Gap Structures Using PECVD Raymond N. Vrtis, Dingjun Wu, Mark L O'Neill, Mary K. Haas, Scott J. Weigel, and Eugene J. Karwacki Electronics, Air Products and Chemicals Inc, 7201 Hamilton Blvd, R4203, Allentown, PA, 18195 ABSTRACT Fabrication of air gap features have been achieved by three processes utilizing the diffusion of materials through a porous OSG layer. The first process involves the decomposition of a PECVD deposited organic material, either thermally or via UV anneal, to create a void with the decomposition by-products diffusing through the porous OSG layer. The second process uses the etch selectivity of XeF2 or BrF3 towards silicon versus OSG to diffuse through the porous OSG layer to etch the underlying silicon. Finally the water solubility of films such as GeO2 or B2O3, which can be easily deposited by PECVD, can be utilized for void formation via dissolution of the sacrificial inorganic layer through the porous OSG. INTRODUCTION As device dimensions continue to shrink there is a need for ever lower dielectric constant materials. Over the past 12 years there has been a progression from SiO2 to florosilicate glasses, to organosilicate glasses (OSGs), and now to porous OSGs with dielectric constants as low as 2.0.1 In order to achieve dielectric constants of ~ 2.0 it is necessary to incorporate 25% or more porosity into the OSG films. As the amount of porosity incorporated into the OSG films increases it not only lowers the dielectric constant but results in an even greater decrease in mechanical properties. For example, a dense OSG film with a dielectric constant of 2.9 has a mechanical hardness of ~3.0 GPa while a PDEMSTM Interlayer Dielectric (ILD) material1 with ~25% porosity exhibits a dielectric constant of 2.2 has a mechanical hardness of only 0.8 GPa. Extrapolating this trend to even lower dielectric constants, it is likely that a material with a dielectric constant below 1.9 may have a mechanical hardness of less than 0.3 GPa. Another trend which is seen is that as the porosity increases there is a corresponding increase in the interconnect path length as measured by Positron Annihilation Lifetime Spectroscopy (PALS). This interconnect path length can be viewed as a measure of the longest string of connected pores and interconnectedness may to be detrimental to ALD processes due to diffusion of reactive species, as well as for interactions with wet processes such as resist developers, resist strippers and CMP slurries. At some point, the very low mechanical strength of highly porous structures having completely interconnected pores may not offer many advantages over a void space having a dielectric constant of 1.0. To date the fabrication of air gaps has focused on three main avenues: (i) the use of extremely non-conformal SiO2 depositions that result in large key-hole structures as the air gaps,2 (ii) the use of thermally labile polymeric materials deposited by either spinon processes or hot filament CVD3 (iii) isotropic etching of air gaps either by

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