Comment on “MEL‐type Pure‐Silica Zeolite Nanocrystals Prepared by an Evaporation‐Assisted Two‐Stage Synthesis Method as Ultra‐Low‐k Materials”

Abstract

A bottle-neck in the application of zeolite films as low-k dielectrics is the presence of 40-80 nm zeolite nanoparticles composing the film, causing both large mesopores and hampering the grooving of nanofeatures. Evaporation-assisted two-stage synthesis of MELtype zeolite nanosuspensions avoiding 40-80 nm zeolite nanoparticle formation in suspension was recently proposed in this journal for the preparation of zeolite films with improved properties and with possibility to groove nanofeatures at 65 nm. [1] We reproduced such zeolite films using that procedure involving well dispersed zeolite nanosuspension and characterized the porosity using ellipsometric porosimetry. We concluded that the approach of evaporation-assisted two-stage synthesis of MEL-type zeolite nanosuspensions does not lead to films with small and narrow pore size distribution where nanofeatures can be grooved. Pores of up to 50 nm width were observed in final spin-on MEL-zeolite films The preparation of films by spin coating of nanocrystal suspensions of Silicalite-1 and -2 type zeolites is being intensively investigated for their applicability as ultra-low-k materials in nanoelectronic devices. [1][2][3][4] Despite their low dielectric constant, they offer limitations related to the use of zeolite nanocrystal suspensions with particle sizes of 40-80 nm. [1,3e,3f ] The main drawbacks are the important surface roughness, the formation of large mesopores between individual nanocrystals, and the difficulty to groove nanofeatures. Although different zeolite syntheses involve the formation of primary zeolite particles smaller than 20 nm, these tend to agglomerate into larger ones at high concentrations or during the crystallization process. [5] In a recent publication in this journal Yan's group claimed to have overcome the agglomeration drawback by optimizing the synthesis conditions of MEL zeolite suspensions. [1] The authors reported that an evaporation-assisted two-stage synthesis of MELtype zeolite nanosuspensions significantly avoids the agglomeration in suspension of 14 nm MEL primary particles into larger 60-80 nm secondary particles. Briefly, the method consists of evaporating part of the solvent between two hydrothermal treatments of the synthesis mixture of the MEL-type zeolite. By evaporating 60 wt% of solvent, 98.4% of primary particles were reported to not agglomerate and be preserved in the final suspension (so called E-60). This was found in the dynamic light scattering analysis on this MEL suspension diluted with abundant water (0.05 mL E-60 in 4 mL water). The use of MEL suspensions with 98.4% of particles being only 14 nm (i.e., avoiding almost all the agglomeration) should minimize the formation of mesopores between the nanocrystals, and moreover, grooving small features of few tens of nm should be feasible. However, we encountered that the use of such MEL suspensions still leads to films with relatively wide pore size distribution.We synthesized MEL nanocrystal suspensions following carefully the described procedures in ref.[1] ...

Metal-Organic Framework ZIF-8 Films As Low-κ Dielectrics in Microelectronics

“…Pores should be as small as possible (<5 nm wide) in low-κ dielectrics to enable an effective sealing of the material with a barrier layer that avoids diffusion of copper atoms from the interconnects. − A common technique to assess the pore size on low-κ dielectrics is EP, which consists of monitoring the changes in refractive index during a toluene adsorption cycle and applying effective medium theory (Lorentz–Lorentz) . With this technique and toluene as molecular probe, pores from approximately 0.5 to 70 nm can be detected.…”

“…But similar values indicate a good grain coalescence and absence of intergranular voids (in agreement with SEM and toluene adsorption analysis) which would have otherwise affected the mechanical properties. This is very important for the application of these films in microelectronics, since relatively large voids would be detrimental for the application as was found in spin-coated zeolite films. − …”

Solution-processed ultra-low-k thin films comprising single-walled aluminosilicate nanotubes

“…This is in agreement with the previously reported value. 72 It is worth noting that ε r = 1.05 in AlSiNT thin films satisfies the criteria for ultra-low-k materials (ε r < 2.0), 14,17,20 and the value of ε r is as low as the benchmark ultra-low-k thin film. 8,11,14,23 According to the correlation of the ε r as a function of the bulk density of zeolites, 73 ε r of hexagonally packed AlSiNTs at high frequency should be around 1.4.…”

“…72 It is worth noting that ε r = 1.05 in AlSiNT thin films satisfies the criteria for ultra-low-k materials (ε r < 2.0), 14,17,20 and the value of ε r is as low as the benchmark ultra-low-k thin film. 8,11,14,23 According to the correlation of the ε r as a function of the bulk density of zeolites, 73 ε r of hexagonally packed AlSiNTs at high frequency should be around 1.4. The obtained ε r of 1.05 might be due to that the AlSiNTs are packed disorderly, as suggested by the GIWAXS characterization.…”

“…3 Specifically, low-k thin films require a Young's modulus greater than 6 GPa for micro/nanoelectronics; 12,13 however, most existing polymeric thin films have a Young's modulus of only 1 GPa. 7 Low-k thin films made of nanoporous materials, such as zeolites, [14][15][16][17] metal-organic frameworks (MOFs), [18][19][20][21] and mesoporous silica, [22][23][24][25] have higher elastic modulus than polymeric thin films. The Young's modulus values of nanoporous low-k thin films are in the range of 5-20 GPa.…”