Integrated circuits that have improved functionality and speed in a smaller package and that consume less power are desired by the microelectronics industry as well as by end users, to increase device performance and reduce costs. The fabrication of high-performance integrated circuits requires the availability of materials with low or ultralow dielectric constant (low-k: k
The first in-situ two-dimensional grazing incidence small-angle X-ray scattering (2D GISAXS) study on the evolution of nanopores during the thin film formation of porous dielectrics from composite films is reported. A soluble poly(methylsilsesquioxane) (PMSSQ) precursor and a four-armed poly( -caprolactone) (PCL4) were chosen as the model matrix and porogen components within the composite film. The measured 2D GISAXS data were analyzed quantitatively using a GISAXS formula derived under the distorted wave Born approximation. It is shown that in-situ GISAXS is a powerful tool for monitoring the evolution of nanopores in dielectric thin films, providing structural characteristics such as size, size distribution, shape, electron density, and porosity, all as a function of temperature and time. In addition, the mechanism for forming imprinted nanopores in the dielectric films by sacrificial thermal degradation of the porogen was determined by in-situ GISAXS analysis. Phase separation of the PCL4 porogen was induced below 200 °C by cross-linking of the PMSSQ precursor matrix during thermal curing. This process generated porogen aggregates, each individually imprinted pore in the film through thermal degradation; the shape, size, and size distribution of the porogen aggregates are directly reflected in the dimensions of the imprinted pores. Moreover, it was found that higher porogen loadings caused larger porogen aggregates with a greater size distribution. The present results thus show that the structural characteristics of nanopores imprinted within PMSSQ dielectric films are governed by the PCL4 porogen aggregates formed through curing of the PMSSQ precursor matrix.
Layered materials with nanoporous layers (e.g., , [1][2][3][4] AMH-3 [5,6] ) have structures intermediate between those of crystalline nanoporous frameworks, such as zeolites, and typical layered materials, such as clay minerals. Each layer includes a porous network, while the gallery between layers provides the ability for intercalation, pillaring, and exfoliation. [3][4][5][7][8][9][10][11][12][13] One such material, AMH-3, is the first layered silicate with internal porosity accessible from all directions through eight-membered ring (8 MR) apertures (i.e. pore openings made of eight SiO 4 tetrahedra). [5,6] It has been proposed to use exfoliated silicate layers of AMH-3 as a selectivity-enhancing additive in polymers. [14,15] A recent simulation study provides further motivation for the fabrication of nanocomposites incorporating dispersed AMH-3 layers for gas separation membranes.[16] However, fabrication of nanocomposites has not been demonstrated. Herein, we report on AMH-3 swelling using a novel procedure; the swollen material is used to prepare polymer nanocomposite membranes with improved selectivity.Typically, layered materials can be swollen by the intercalation of organic surfactants, such as quaternary alkyl ammonium ions or amine molecules, by cation exchange or hydrogen-bonding interaction with intergallery moieties. [3,4,[17][18][19][20][21][22][23][24] The swollen derivative of AMH-3 was prepared by intercalation of primary amine molecules (dodecylamine) after proton exchange in the presence of amino acid. In this procedure, an aqueous solution of dl-histidine was employed as a buffer and a source of protons to exchange the strontium and sodium cations in the original structure. The initial pH value was adjusted to be approximately 6.0 by addition of hydrochloric acid, and ion exchange was allowed to proceed at room temperature until the pH value reached approximately 6.4. At this point, the aqueous solution of dodecylamine was added. The swollen AMH-3 was obtained after twelve hours at 60 8C. As will be described in detail elsewhere, several alternative procedures failed to yield swollen AMH-3. A brief account of these attempts is shown in Figure 1.The emergence of a swollen material was monitored by various characterization techniques. ICP (inductively coupled plasma) chemical analysis shows that Na and Sr cations of the original structure (4.8 wt % Na, 20.3 wt % Sr) were exchanged, leaving almost no Na (0.7 wt %) and a smaller Figure 1. AMH-3 structure projection along the b axis showing silicon atoms (Si1, Si2, Si3, and Si4, light sticks), oxygen atoms (dark sticks), sodium cations (Na, single balls), and strontium cations (Sr, double balls). Details on the structure can be found in reference [5]. The arrows indicate attempted ion-exchange and swelling procedures. Only the last procedure, combination of ion exchange in the presence of dlhistidine and swelling using dodecylamine, resulted in swollen AMH-3. CTAB = cetyltrimethylammonium bromide, DTAB = dodecyltrimethylammonium bromide, DOA = do...
Both the microelectronics industry and end users desire the high performance and lower cost of multilevel integrated circuit devices with higher circuit densities.[1] To achieve higher circuit densities, the circuit feature size has to be reduced and the dielectric constant, k, of the interposed dielectric material must be decreased to enable closer spacing of circuit lines without increasing crosstalk and capacitive coupling.[1]Polymethylsilsesquioxane (PMSSQ: (CH 3 SiO 3/2 ) n ) has recently received much attention as an alternative to the workhorse dielectrics silicon dioxide (k = 3.9±4.3) and silicon nitride (k = 6.0±7.0) because of its relatively low dielectric constant (k = 2.7), minimal moisture uptake, and high thermal stability.[2±4] Although the k value of PMSSQ is considerably lower than those of silicon dioxide and silicon nitride, it is still much higher than that of air (or vacuum), k = 1.0, which is the lowest value attainable. Hence there has been much interest in incorporating air into dielectric materials to produce porous materials with low k values (£ 2.5).[2±10] One approach is the template-curing reaction of PMSSQ precursors in the presence of thermally labile organic polymer porogens, followed by the creation of pores in the resulting dielectric material by sacrificial thermal decomposition of the porogens.[2±10]However, during the template-curing reaction, the porogen molecules tend to be phase separated, causing aggregation; this phenomenon leads to the formation of larger-sized pores and limits the porosity of the final dielectric thin film.[2±10] In particular, star-shaped and dendritic porogens with a high number of arms show a strong tendency toward segregation and aggregation, even at porogen loadings as low as 10 wt.-%, to generate large and interconnected pores in the dielectric thin films.[8±10] Hence, in order to generate a porous dielectric material containing a uniform distribution of nanopores with pore sizes much smaller than the feature size of integrated circuits, it is necessary to minimize the aggregation of porogen molecules loaded in the dielectric matrix.In the present study, we aimed to minimize aggregation of a six-armed porogen in a dielectric matrix by chemical modification of the porogen end-groups. To test the efficacy of the proposed modification, the nanostructures and properties of porous dielectrics prepared using different amounts of the modified porogen were quantitatively characterized. A soluble PMSSQ precursor containing reactive ethoxysilyl and hydroxysilyl groups was used as the dielectric material, while a six-armed poly(e-caprolactone) with and without triethoxysilyl termination (mPCL6 and PCL6, respectively) were used as thermally labile porogens (Scheme 1). On heating, the PMSSQ precursor was found to undergo curing over the temperature range 75±340 C, which was accompanied by weight loss due to the evaporation of the water and ethanol by-products. The cured PMSSQ product starts thermal decomposition above 500 C; the curing reaction of the mPCL6 po...
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