Porous low-k materials

The above mentioned low-k materials were all classified into dense low-k materials. In order to reach k<2, fully densified materials has seemingly reached theirs lowest capability. Hence, the research has to move on with introduction of porosity onto the dense materials. There are two type pore formation inside the dense low-k matrix. The primary one is pores that are inherently formed inside the matrix through sol-gel process, ex: aerogel and xerogel low-k film. The secondary one is formed though the template type which accommodate the present of sacrificial materials which are also called “porogen” or pore generator that are decomposed upon the thermal process. Following we will discuss the fact and issue of each porous low-k materials.

1. Silica aerogel and xerogel low-k film

SiO2 aerogel thin film has attracted significant attention because of their unique properties such as ultralow dielectric constant, high porosity, and high thermal stability. SiO2 aerogel thin films usually take the advantage of aging processes.

However, SiO2 aerogel thin films usually synthesis above the supercritical pressure (>60 bar) and high temperature of drying solvent process, which is very expensive and hazardous. Thus, it will become constraint for the production in industrial application. [59] Due to their high porosity, SiO2 aerogel thin film have not display superior mechanical properties. [60] SiO2 xerogel thin films also employ the same aging technique as SiO2 aerogel thin film. The differences between aerogel and xerogel is SiO2 xerogel prepared by the ambient drying process which involve pre-drying step that called surface modification process known as silylation. The intention of silylation process is to change the surface hydroxyl (-OH) groups into inert methyl (-CH3) groups. This procedure ensure the film absorb minimal moisture

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from the environment. [61] Both SiO2 xerogel and aerogel has been reported to reach k-value <2 at porosity level of 70-90%. [ , 62, 63, 64, 65]

2. Template-type (porogen) low-k films

Furthermore, the incorporation of thermally-labile pore generator also can produce porous SiO2 thin film. The removal of porogen upon high temperature heating will be replaced by pores inside the matrix of SiO2. First method, incorporation of porogen into the SiO2 matrix can be accomplished by simply dispersing or mixing the porogen into the solution of SiO2 precursor. [66, 67, 68] Porogen size determines the final pore size that exists inside the matrix. Percentage of porogen added or also called porogen loading determines degree of porosity inside the final cured matrix. [69, 70]

However, during thermal heating process, the random distribution of pores tends to agglomerate and coalescence which cause a burden to the mechanical strength of final SiO2 film especially when the porogen loading is increased as shown in Figure 2.12.

[71, 72] The mechanical property of porogen templated low-k film results on lower elastic modulus, the present of porogen in the surrounding of silanol matrix will disturb the condensation of silanol groups (2SiOHÎSi-O-Si+H2O) to form Si-O-Si skeleton. Since Si-O-Si skeleton cross-linking determines the modulus of film, therefore the incorporation of higher porogen loading to obtain lower κ will cause the deterioration of film modulus. [73]. Second method, the porogen is chemically linked or grafted to the SiO2 polymer. [74, 75, 76] This method can achieve better control of porogen distribution in the SiO2 dielectric film. Porogen selection must compatible with SiO2 matrix precursor in order to avoid phase separation. Closely to this theory, phase separation method can be utilized when choosing the suitable porogen.

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Figure 2.12 The conventional formation of porous low-k by using template-type porogen method.

Recently studies have shown that various organic or inorganic polymer could be applied to form more ordered pore size and pore shape with narrower pore size distribution. Regarding to their ability to self assembly and form micelle when the thermal curing process takes place, block copolymers have become one of the promising candidates for low-k dielectric. Amphibilic di-(or tri-) block copolymer such as PEO-b-PPO-b-PEO [77], PS-b-PEO [78], PS-b-P2VP [79], PS-b-P4VP [42], etc have been studied widely. Various nanoporous low dielectric films have been realized using poly(methyl silsesquioxane) (PMSSQ) as a matrix material with a wide variety of pore generating materials (porogens) such as star-shaped polymers (Polycaprolactone (PCL)) [80, 81, 82], block copolymers [83, 84], cyclodextrines [85, 86, 87], norbornenes [88, 89], dendrimers [90, 91], and hybrid type porogens [92, 93, 94, 95].

Another way to overcome the aggregation issue of porogen is by adopting fast curing and slow curing process [96]. Slow ramp-curing allows porogen diffusion and slow matrix cross-linking. During this curing, porogen percolation leads to form surface agglomerates. Using faster ramp curing, the matrix is cross-linked around porogen nanoparticles faster, and afterward the porogen is degraded at high temperature to leave a network of nanopores in a rigid ULK dielectric matrix.

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Controlling Curing temperature and ramping rate are important factor for nanoporous ULK dielectrics quality. [96] Moreover, alternative curing process, such as UV, or e-beam assisted thermal cures, would be helpful in providing good film performance.

[97, 98] For example, UV radiation could be used to break porogen bonds, leading more easily to their extraction and it also promotes cross-linking that will improve mechanical modulus. [99] The problem that UV-cure meets in the packaging process is the increase in film stress that will cause cracking of film stack and delamination.

[100] Therefore, the entire list of the UV cure performance specifications that ultimately determine the quality of the UV cure process must be considered. [101]

E-beam assisted thermal cure can improve crosslinking, but it also causes charge damage. [102]