Impact of wavelength of UV light and UV cure time on chemical and mechanical properties of PECVD deposited porous ultra low-k films S. Godavarthi a,b , Q.T. Le a , P. Verdonck a,⇑ , S. Mardani a , K. Vanstreels a , E. Van Besien a , M.R. Baklanov a a Imec, Kapeldreef 75, 3001 Leuven, Belgium b Program of Nanoscience and Nanotechnology, Cinvestav-IPN, Mexico City, Mexico article info Article history: Available online 13 October 2012 Keywords: Ultra low-k PECVD FTIR Nanoindentation Porogen UV cure abstract We report a method to improve the chemical stability and the mechanical properties of a plasma enhanced chemical vapour deposited ultra low-dielectric constant material. This enhancement in the properties was achieved through extended UV cure times using a broad band lamp, emitting light at wavelengths higher than 200 nm. These longer cures also increased significantly the Young’s modulus, but the dielectric constant did increase less than the experimental error. It is also demonstrated that this improved property depends on the chemical bonds present in the film, such as Si–CH 3 , as well as on the density of the film. Films with a dielectric constant of 2.06, Young’s modulus of 4.9 GPa and perfect resis- tance against 0.5% HF for up to 300 s, were obtained. Another type of UV cure, using a narrow band lamp with wavelength of 172 nm, induced completely different chemical resistance characteristics of the resulting films. Ó 2012 Elsevier B.V. All rights reserved. 1. Introduction Future microelectronic systems require ultra low-k (ULK) mate- rials as interlevel metal insulators, in order to reduce RC delay and power consumption, increase signal to noise ratio etc. Ultra low-k materials consist of a porous structure and are often prepared through either a ‘‘structural method’’ or a ‘‘subtractive process’’. In the structural method, pores are created by choosing special precursors containing molecular pores [1]. The PECVD based, sub- tractive process involves the co-deposition of a SiOCH network precursor and a sacrificial organic porogen [2]. In general, the poro- gen is subsequently removed by a UV assisted thermal cure in or- der to create pores. A recently reported, improved subtractive process requires the application of a remote He–H 2 plasma treat- ment to the films immediately after the co-deposition to remove the porogens [3,4], followed by a UV cure to raise the skeleton cross linking [5]. In standard subtractive processes, the optimization of UV cure time is determined by the porogen removal, trying to obtain an as good as possible compromise between dielectric constant and Young’s modulus [6]. In the improved subtractive process, most of the porogens are removed by the remote plasma; hence the sub- sequent UV cure serves mainly to improve the mechanical charac- teristics. The purpose of this research is to find also a correlation of the cure type and time with the chemical stability and to analyze the observed phenomena. 2. Experimental procedure All the films were deposited on 300 mm Si (100) wafers. Before deposition and processing of ULK films, 1 nm dry thermal oxide route was grown in order to obtain improved quality of capaci- tance – voltage curves. The precursors of the PECVD film consist of alkylsilanes and cyclic hydrocarbons where the first is supposed to produce the network and the latter the porogens. All films were deposited to obtain a thickness of approximately 110 nm. The film deposition conditions were detailed elsewhere [6–8]. The He-H 2 remote plasma treatment was applied to all films at a wafer tem- perature of 280 °C during 350 s. Other details of this process can be found in Ref. [7]. In the third process step, either a 172 nm nar- row band (NB) or a >200 nm broadband (BB) UV cure was applied. The influence of different long BB and NB cures times, up to 4 times the standard curing time (written below as 4ÂBB or 4ÂNB), were investigated. Thickness and refractive indices of the processed films are mea- sured by spectroscopic ellipsometry. The mass of the films was measured using a very precise mass balance, with an accuracy bet- ter than 100 lg. The density was then calculated directly from the mass to volume ratio; the error on the density is less than 1%. FTIR spectra were recorded with a resolution better than 1 cm À1 and averaged over 64 scans, within the range of 400–4000 cm À1 . The Young’s modulus was measured using nanoindentation with a dy- 0167-9317/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.mee.2012.09.012 ⇑ Corresponding author. E-mail address: verdonck@imec.be (P. Verdonck). Microelectronic Engineering 107 (2013) 134–137 Contents lists available at SciVerse ScienceDirect Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee