Albert Sacco Jr., Daniel D. Burkey
Katherine S. Ziemer
Date of Award
Master of Science
Department or Academic Unit
College of Engineering. Department of Chemical Engineering.
Chemical engineering, Initiated CVD, Plasma CVD, Organosilicon thin films
Electric insulators and insulation - Organosilicon compounds, Semiconductors, Dielectric films
In contemporary semiconductor devices speed and functionality depends on signal propagation and, subsequently, on the geometrical design of the device and its material properties. Power dissipation, current leakage, and cross-talk noise are the issues plaguing signal propagation. These issues can be crucial at the dimensions of ultra-large scale integrated devices, as they increase exponentially with decreasing dielectric layer thickness. Thus, having a dielectric with excellent insulating properties is critical, since it can abate these issues. Organosilicon thin films are a natural choice for dielectric layers, since they are similar to the previously used silicon dioxide, can be deposited using the same technology, but have significantly lower dielectric constants. Organosilicon films, in addition to being good electrical insulators, have been shown to be biocompatible, which makes them promising candidates for passivating coatings in bioimplantable devices, such as neuroprosthetics. The solvent-free chemical vapor deposition technique insures that the biocompatibility of the passivation coating will not be compromised by solvent leaching. Additionally, it allows the deposition of the passivation coating in conjunction with a reactive polymer coating, which can then be functionalized with biomolecules. The goal of this investigation was to obtain an organosilicon thin film with lowest possible dielectric constant via CVD: co-polymerizing a cyclic precursor with intrinsic porosity and a linear precursor acting as a spacer for additional porosity. Trivinyltrimethylcyclotrisiloxane (V3D3) was the cyclic precursor used for the depositions. Vinylmethyldieothoxysilane (VDEMS), divinyldimethylsilane (S1), divinyltetramethyldisiloxane (S3), and divinylhexamethyltrisiloxane (S5) were the linear spacer precursors used. This study also investigated depositions of V3D3 film with a top layer of poly(acrylic acid) (pAA) that could be functionalized with biomolecules. Plasma enhanced CVD and initiated CVD techniques were used for deposition of V3D3 and V3D3-spacer films. PECVD was shown to be too aggressive as a deposition technique, and resulted in precursor structure loss. The lowest as-deposit refractive index obtained for a V3D3 film deposited at a low substrate temperature (25 ºC) and low power (10 W) was 1.482 ± 0.001. By contrast the mildest iCVD conditions yielded a V3D3 film with significant retention of cyclic structure, as evidenced by FTIR spectra and peak deconvolution, and with a refractive index of 1.454 ± 0.002. This showed that iCVD was a more suitable milder alternative for depositing films from precursors with delicate functionality. Both techniques yielded V3D3-spacer films with refractive indices either higher or statistically the same than those of V3D3-only films deposited at the same conditions. This suggested that the linear siloxane molecules were not acting as spacer molecules as had been hypothesized. It was likely that the spacers polymerized through the vinyl bonds only, similar to acrylates, resulting in densification rather than formation of additional porosity. Additionally, in PECVD the abundance of energy could have resulted in precursor fragmentation leading to film densification, as opposed to additional pore formation. The ratio of the monomer partial pressure to its saturation pressure (PM/Psat) was proven to be a crucial variable in iCVD by Lau and Gleason [52, 53], particularly for co-polymerizations, since it quantified the amount of monomer adsorbed onto the substrate, which, in turn, controlled the deposition rate. It was shown that a V3D3-S5 film (film that added divinylhexamethyltrisiloxane to V3D3) that was deposited with equal PM/Psat ratios for both monomers had a refractive index of 1.453 ± 0.010, whereas the film deposited with different PM/Psat values had a refractive index of 1.461 ± 0.002. V3D3 films were investigated as base layers for biofunctionalized films. Biofunctionalization would be achieved by protein attachment to carboxylic acid groups in the top pAA layer, which had to be stable in aqueous environment. It was shown that increasing plasma power and increasing initiator flowrate improved the film stability in water, but resulted in the decrease of the carbonyl peak in their FTIR spectra, suggesting that cross-linking took place at the expense of the carboxyl acid functionality. This was hypothesized to be due to the fact that the abundance of tBPO radicals in the plasma not only initialized the linear polymerization of AA through the vinyl chemistry but also reacted with other available bonds in the growing pAA linear chains and cross-linked these, thus, consuming carboxylic acid functional groups. Co-polymerization with ethylene glycol diacrylate (EGDA) showed to have the desirable effect on film stability without depleting the films of their important functionality. A V3D3/pAA-co-EGDA film was shown to have good thickness retention, ~95 %, in DI water for an hour, due to cross-linking of AA with EGDA and the likely grafting between the V3D3 film and the AA/EGDA top layer. Protein tethering on the same combination V3D3/pAA-co-EGDA film using fluorescein isothiocyanate anti-mouse immunoglobulin G was shown to be successful by fluorescent microscopy proving that was is possible to combine electrically insulating and biocompatible and biofunctional surfaces into one film using CVD techniques.
Nariné Razmik Malkhasyan
Malkhasyan, Nariné Razmik, "Chemical vapor deposition of low-dielectric constant organosilicon-based thin films" (2009). Chemical Engineering Master's Theses. Paper 2. http://hdl.handle.net/2047/d10019224
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