A late porogen removal scheme was used to make low-k materials (k = 2.72 to 2.02) using methylsilsesquioxane (MSQ) and a high-temperature porogen, poly(styrene-b-4-vinylpyridine) (PS-b-P4VP), to circumvent the reliability issues related to as-deposited porous dielectric. Based on the nanoindentation and Fourier transform infrared spectroscopy (FTIR) analysis, the moduli of the hybrid films were found to be higher than their porous forms, and even better than the dense MSQ film, for porogen loading below a critical level (˜69.5 vol%). This could be attributed to their enhanced degree of cross-linking in MSQ as evidenced by the network/cage structural ratios. Besides, high-temperature porogen plays different roles during the cross-linking of MSQ depending on its loadings. In this study, with immediate loading at 16.7 vol%, PS-b-P4VP can serve as plasticizer to enhance the degree of cross-linking, but at a large loading >16.7 vol%, it becomes a steric hindrance reducing the degree of cross-linking.
A methyltrimethoxysilane (MTMS) modified silica zeolite (MSZ) film has been prepared using a high ratio of MTMS/tetraethyl orthosilicate (TEOS). This study investigated the effect of MTMS addition on the low-k matrix structure, elastic modulus, and pore geometry. High MTMS loading reduced the k-value of MSZ film down to 2.0, but yielded a lower elastic modulus, 2.7 GPa. Based on grazing-incidence small-angle X-ray scattering (GISAXS) 2D pattern analysis, the pore geometry of the MSZ film was found to be small but elliptical (R in-plane ∼3.75 nm; R out-of-plane ∼3.04 nm). The elliptical pore shape was formed by a collapse of film structure at 150-160 • C as a result of ∼32% thickness shrinkage due to the decomposition of tetra-n-propylammonium hydroxide (TPAOH), a structure directing catalyst, and due to a large degree of crosslinking reaction in the silica matrix. Combining GISAXS, 29 Si-NMR, and FT-IR results, we propose that the lower elastic modulus was caused by the incorporation of a large amount of methyl groups from the MTMS precursor and the elliptic pores.In order to alleviate the signal delay issue in the backend interconnects as the scaling of the IC devices continues, the search for low-dielectric-constant (low-k) interlayer dielectrics remains to be the key materials solution, in addition to 3D interconnects and air-gap approaches. 1 According to 2010 ITRS, upcoming 22 nm technology node of IC industry requires dielectric materials with k-value <2.3. 2 Extensive efforts have been made in the last decade to attain ultra-low-k dielectrics (k ∼ 2.3-2.0). Researchers have introduced nanometer-scale pores into spin-on organosilicate dielectric films such as silsesquioxane based materials, whose k is ∼2.8 to 3.0 and E is in ∼3 to 7 GPa range at no porosity. 3 However, the mechanical strength of their corresponding ultra-low-k materials, which typically have high porosity, ∼50-60%, degrades significantly. 4 Moreover, the pore generators (porogens) within the spin-on organosilicate matrix tend to aggregate during the curing step and result in larger pores upon thermal decomposition, which further degrades mechanical strength of the dielectric film. 5 Overall, the mechanical strength of spin-on dielectric films has been a challenge for its use in the manufacturing of advanced integrated circuit.Recently, researchers have focused on low-k films with high mechanical modulus, such as pure silica-zeolite (PSZ) low-k film. PSZ low-k film offers several advantages over amorphous silica including crystalline structure as well as intrinsically uniform and small pore size. 6, 7 Typical PSZ materials have high modulus and low dielectric constant, but have challenges such as high surface roughness, 8 which can be resolved by adding a chemical mechanical polishing step. The other major problem is the high moisture absorption of PSZ film. 9, 10 This is disadvantageous for the practicability of PSZ film due to the k-value of water is close to 80.
A high-temperature reactive porogen, triethoxy(polystyrene)silane (TEPSS) (M w =3,500 g/mole), suitable for late-porogen removal integration scheme has been synthesized in p-xylene via atom transfer radical polymerization. TEPSS was then grafted onto poly(methyl-silsesquioxane) (MSQ) matrix (k=2.9) to circumvent possible phase separation between matrix and porogen in the hybrid approach and porogen aggregation. Our results shows porous low-k MSQ films possess uniform pore size, 24 nm for porosity up to 40%, primarily due to low PDI and reactive porogen, and the dielectric constant is decreased to 2.37 at 40% porosity. In addition, less porogen aggregation was observed at porogen loading ~40 v%.
A solid-first scheme was employed for making ultra low-k materials (k < 2.5) based on methylsilsesquioxane (MSQ) and high-temperature porogens in order to circumvent the reliability issues related to as-deposited porous dielectric. Based on thermal stability data, the MSQ/poly(styrene-b-4- vinylpyridine) material system was recommended to be cured at 300 {degree sign}C for subsequent backend processes, and the porogen to be removed at 400 {degree sign}C for 2 hours after CMP step. The mechanical properties of ultra-low-k films used in different integration schemes were further investigated using nano-indentation and FT-IR for various porogen loading. The moduli of 2-phase films were higher than their porous films, and even better than dense MSQ, for porogen loading below a critical level (~50%), which could be attributed to their enhanced degree of crosslinking in MSQ. In addition, MSQ/porogen films cured at 300 {degree sign}C possessed good elastic modulus (> 4.0 GPa) to pass CMP test.
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