Moisture uptake and dielectric property of methylsilsesquioxane/high-temperature porogen hybrids and porous low- k films
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Two high-temperature pore generators (porogens) have been used to study the effect of porogen structure on moisture uptake and k-value in methylsilsesquioxane/porogen hybrid films and their corresponding porous films in a postintegration porogen removal scheme. Poly(styrene-b-4-vinylpyridine) containing di-block structure and pyridine polar group leads to higher moisture uptake and k-value in the hybrid films as compared to poly(styrene-blockbutadiene-block-styrene) with symmetrical structure and nonpolar groups. Moreover, the moisture uptake behavior in both as-prepared hybrid films is in physical sorption mode based on their reversible adsorption–desorption curve measured by quartz crystal microbalance. After porogen removal, the k-values of porous films are favorably not influenced by porogen structures, and their moisture uptake is as low as 1.78 wt% even at 40 vol.% porosity. However, based on the simulation of the modified-Rayleigh model, the porous films are found to possess 0.4 vol.% chemisorbed moisture on the pore surface, resulting in 17–23% deviation from the ideal k-values.
I. INTRODUCTION
As device scaling has developed beyond the 0.25- to 0.18-lm node, Resistance-Capacitance (RC) delay in the backend interconnect has become an obstacle.1 To alleviate this problem, copper was first implemented. Later, lowdielectric constant (low-k) materials such as carbon-doped oxide or SiLK (k 5 2.6–3.0) were introduced.2,3 For dielectric with k , 2.5, which is required for 22-nm node and beyond,4 incorporation of porosity is a necessity to further reduce k-value. However, porous low-k thin film may encounter reliability issues such as (i) delamination and cracks during chemical–mechanical polishing (CMP) and other processes due to low mechanical strength5 and (ii) biastemperature stressing failure due to noncontinuous side-wall coverage of barriers.6 To circumvent such reliability issues, a postintegration pore generator (porogen) removal approach in a material design using a high-temperature porogen and an integration scheme have been proposed to defer the formation of porous dielectric after the completion of a metal/low-k layer, followed by the thermal removal of the sacrificial, high-temperature porogen.7–9 Besides the reliability issues, the postintegration porogen removal approach can also be applied to reduce plasma-induced damage to porous low-k dielectrics because porogen can reduce effective pore size and limit the plasma radical diffusion inside pore.10,11 In our previous study, we demonstrated porous/hybrid low-k materials (k 5 2.7–2.0) using an amphiphilic block a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2011.384 J. Mater. Res., Vol. 26, No. 23, Dec 14, 2011
copolymer, poly(styrene-b-4-vinylpyridine) (PS-b-P4VP), as a high-temperature porogen in the postintegration porogen removal scheme.12 The hybrid films below a critical porosity (;45 vol.%) possess higher modulus (.4.0 GPa) than their porous forms, meeting the minimum requirements for passing the CMP test i
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