Background

The improvement in device density and performance has significantly impacted the feature size of the wiring structure for interconnects. As the feature size in integrated circuit is scaled down, the increase in propagation, Resistance-Capacitance (RC) delay, crosstalk noise and power dissipation of the backend interconnect becomes a limiting factor [1]. The impact can be examined by evaluating the RC delay of the multilevel interconnects [2]. To minimize the increase of RC delay, the industry first introduced copper metallization (1.9 μΩ-cm) to reduce the resistance (vs. Al~3.3 μΩ-cm) as well as improve the electromigration performance of wiring [3]. As the minimum device dimensions reduce beyond 250 nm, SiO2 insulator is no longer suitable. In order to continually reduce the RC delay of interconnects, low dielectric constant (low-k)

materials have been introduced primarily to replace the interlayer dielectric (ILD), SiO2

(k ~ 4.2). Therefore, the need of lowering the k-value of bulk SiO2 can be attained by lowering the density of matrix and/or addition of lower polarizability atoms or bonds such as fluorosilicate glass (FSG, k = 3.7) [4]. Organic SiLK^TM (k = 2.65) was first introduced by IBM at 0.13 m technology node [5]. However, inorganic dielectric such as carbon-doped oxide (CDO) was adopted by the majority of semiconductor companies mainly in 90 nm and beyond. Major commercially available CDOs are Aurora^TM (k = 2.9, ASM), Coral^TM (k = 2.85, Novellus), and Black Diamond^TM (k = 2.65-3.0, Applied Materials) [6,7].

Moving further toward 45 nm node, the incorporation of porosity, in which air has kair =1, the lowest attainable value, becomes essential for producing viable low-k materials with k < 2.5 [8]. And next 22 nm technology node and beyond, it is well recognized that the incorporation of porosity is critical to the search for viable low-k dielectrics ultra low-k materials with k value < 2.3 [9]. Conventionally, porous low-k dielectric films are generally formed following the deposition of a low-k matrix with a thermally labile templating agent or pore generator (porogen)[10], which are burned out by thermal treatment at low temperatures (typically ≤ 200°C) immediately following the deposition of the film as illustrated in Figure 1.1(a). However, such porous films might encounter reliability issues, such as high leakage and low dielectric breakdown strength

at the barrier/low-k interface, because of the insufficient coverage for large pores induced by plasma damage during the etching process [11,12,13]. To circumvent such reliability issues, a novel post-integration porogen removal method is proposed, which uses a high decomposition temperature (Td > 350^oC) porogen in the material design and an integration scheme [14,15,14,15]. It uses a high temperature porogen, such as poly (styrene-b-butadiene-b-styrene), and poly(styrene-b-4-vinylpyridine) [16] to defer the formation of a porous low-k dielectric, until the completion of the copper chemical mechanical polishing (CMP) step, and then thermally removes the sacrificial porogen from the hybrid dielectric film, as illustrated in Figure 1.1(b). In the post-integration porogen removal scheme, the porogen in hybrid low-k matrix/porogen films must survive at the highest possible processing temperature (preferably 300-350^oC) cycling of back-end-of-line (BEOL) processing steps. For such hybrid films to survive the backend processing such as CMP in the post-integration porogen removal scheme, the hybrid low-k material should possess the sufficiently strong mechanical strength.

Moreover, the mechanical properties such as Young’s modulus of the hybrid films depend on the porogen morphology, size, and size distribution. Yet, the interaction between porogen and low-k matrix during curing and their impact on the porogen size of low-k/porogen hybrid films are not fully understood. Thus, it is of critical interest to understand the porogen behavior and how to control its size in the low-k matrix/porogen

hybrid films before burning out porogen to form a porous low-k film. In this study, porogen aggregation behavior was elucidated by in situ grazing incidence small-angle X-ray scattering (GISAXS) and viscosity test. In general, 2D GISAXS is a powerful instrument to measure the characteristics of pore in the porous materials. That can measure pore size in the interlayer of porous film and do not cause any damage of sample. The principle is that detect the scattering signal from the second phase such as pore in the sample. In this study, we also choose GISAXS to analysis the pore size and distribution of porous low-k film. Specifically, we use 2D GISAXS to analysis the porogen size variation in the hybrid low-k film by in-situ test. On the other hand, we use in-situ viscosity test and Fourier transform infrared analysis (FT-IR) to define the

relationship between of matrix structure variation and porogen aggregation.

Therefore, in this study, methylsilsesquioxane (MSQ) and high Td polymers, poly(styrene-block-butadiene-block-styrene) (PS-b-PB-b-PS, SBS) and were used as low-k material matrix and high temperature porogens. In order to define the aggregation and diffusion of porogen before burning out, the thesis prior to focus on the porogen activity in the hybrid film during thermal profile. The effects of curing rate (slow:

2^oC/min vs. rapid: 200^oC/min) and cure temperature on the SBS porogen size in the hybrid low-k films cured up to 200^oC were studied by in-situ GISAXS, viscosity measurement, and FT-IR analysis. The next task is to disperse/control porogen in the

hybrid low-k film. We replace the lower molecular weight polymer polystyrene (PS) as high temperature porogen. An anionic surfactant, sodium dodecylbenzenesulfonate (NaDBS) and a cationic surfactant, domiphen bromide (DB), were used to modify the PS porogen surface in the solution and the low-k hybrid films. The effect of surface modification on the porogen and pore size in the low-k MSQ/PS hybrid films at 10wt%

PS loading under a slow curing rate will be studied. And the well-disperse porogen in low-k solution and hybrid low-k film will be described. To further reduce the dielectric constant down to 2.0, we adjust the different the porogen loading (10 to 50wt%) to increase the porosity. The pore size, dielectric constant and mechanical strength of different porosity porous low-k films with and without surfactant modification will be elucidated.