Grafting of a porous polymethyl methacrylate (PMMA) film on the silicon surface with low dielectric constant

Polymer films fabricated on a silicon substrate has been widely used as an insulation film due to its low-cost processing and excellent dielectric performance. Polymethyl methacrylate (PMMA) is a widely used thin film material. The dielectric properties of PMMA film can be further improved by the porosity process. However, the porous PMMA film is prone to non-uniform pore sizes and through-hole defects, which affect insulation performance in practical applications. In this paper, we reported an approach to optimize the pore structure of PMMA film on the previously prepared porous silicon layer (PSL). We first prepared the PSL on the substrate by the electrochemical etching process. Then, the substrate was directly dipped into an acidic aqueous media to graft the PMMA film by a one-step method. The PSL thickness could be controlled by adjusting etching time. The effects of PSL on the pore structure of the PMMA film were studied. Besides, the thickness of porous PMMA film could be controlled by the proportion of grafting solution and the grafting time. The results show that the growth rate of porous PMMA film is the fastest when the electrochemical etching time is about 90 s. Compared with a flat surface, PSL provides more Si–H active sites. With the same grafting conditions, the presence of the PSL helps to accelerate the grafting reaction in the early stage. PMMA film was finally obtained with the porous network structure. More importantly, the obtained porous PMMA film has a lower dielectric constant (ε1= 1.9~1.5) than general PMMA (ε1= 2.9~2.5). The obtained porous PMMA film is potential to be used in semiconductor industry.