Plasma polymerization is a favorable technique for coating surfaces that have thin films of certain chemical functionality, for example, alcohols, carboxylic acids, aldehydes, etc.
Such coatings make it possible to use materials in various applications, such as nanoparticle attachment, polymer grafting, and biomaterials. Previously, inorganic phosphate coatings have been used for rheological modification, mineral dispersion, and corrosion prevention.
Three methods are currently used for depositing phosphates. These include (1) sol-gel preparation, (2) direct deposition from solution, and (3) graft copolymerization. However, these existing techniques have limitations when it comes to depositing solvents, with consequences concerning the substrate’s chemistry, where polyphosphates adsorb to specific groups of chemicals like hydroxides and transition metals, but not silica.
Therefore, the plasma polymerization method is believed to be beneficial and offers a scope to overcome these problems. This is because plasma polymerization is a dry process, and the energy needed to chemically attach the precursor to the substrate may be offered by kinetic energy, and not by chemical energy intrinsic to the interaction between the precursor and the substrate.
Organic groups are incorporated in the precursor (phosphate) to promote adhesion to certain substrate molecules. For plasma polymerization, the precursor needs to be a volatile chemical. Organophosphates are also selected because their volatility is relevant for plasma sensing, and they exist as liquids at room temperature. By contrast, inorganic phosphates exist as solids at room temperature, and hence they are not considered.
In this study, the authors studied the deposition of organophosphate thin films from low-pressure plasma. Triethyl phosphate (TEP) was used as the precursor to study the impact of power on the chemistry of the plasma phase, and that of the deposited thin film at steady pressure in the absence of atmospheric nitrogen.
This was done to prevent the unnecessary incorporation of nitrogen onto the ensuing thin films. The chemistry of the plasma phase and the surface chemistry of the films created were subsequently compared.
After placing the samples on the grounded stage, RF power was applied to the chamber through a 0.28-m internal RF electrode powered at 13.56 MHz. The Octiv VI Probe from Impedans was used to quantify the ion flux to the powered electrode, connected in series between the RF electrode and the matching network. Figure 1 shows a schematic of the plasma reactor.
Figure 1. Schematic of the plasma system. Image Credit: Impedans Ltd.
Ion Flux and Deposition Rate
Figure 2 illustrates the ion flux for 1 Pa TEP plasma as a function of RF Power. It can be seen that there is a constant increase in the ion flux from 5 to 40 W. The result is consistent with the ion flux quantified for this plasma system utilizing other precursors. Also shown is the deposition rate for the selected plasma conditions. At first, the deposition rate increases quickly with RF power, and subsequently plateaus at a power greater than 10 W, with a slight reduction seen above 25 W.
Figure 2. Ion flux and deposition rate as a function of applied RF Power for 1 Pa TEP plasma. Image Credit: Impedans Ltd.
Figure 3 shows the ion energy distributions (IEDs). At an RF power of 2 W, the highest ion energy is 18 eV, with the highest intensity at 15 eV. When the RF power is increased to 10 W and 25 W, the sheath voltage also increases to 22 eV and 26 eV, respectively.
Figure 3. Ion energy distributions for 1 Pa TEP plasma. Image Credit: Impedans Ltd.
Low power causes the ions to collide with the surface with adequate energy to deposit, but it is not sufficient to cause atomic rearrangements and extensive etching. Increased power results in an increased proportion of ions with energies above 15 eV; at 25 W, most of the ions have energies above 15 eV. Hence, the impact of ions on etching, their contribution to the deposition, atomic scrambling, and cross-linking are likely to increase with the applied RF power.
Plasma Phase Mass Spectra
Figures 5 and 6 show the appearances of the different peaks, indicating that the increased fragmentation of the precursor with the increase in RF power correlates with the reaction mechanism illustrated in Figure 4.
Figure 4. Reaction pathways for some of the important species observed in the plasma mass spectra. Image Credit: Impedans Ltd.
Surface analysis by the XPS technique demonstrated that all the TEP plasma polymers contained phosphorus, oxygen, and carbon. In contrast to earlier research, the presence of nitrogen was not observed. The surfaces of the plasma polymer were a combination of hydrocarbon species, retained P=O groups, with just a minimal loss of P-O groups.
The AFM images demonstrate that, although the preliminary roughness of every bare substrate was different, the roughness slightly decreased with the addition of plasma polymer. Hence, the TEP plasma polymer coatings were mostly conformal and also smoothed out the substrate’s rougher parts. Since plasma polymers remained stable in water on different substrates, the applied RF power had only a slight impact on the solubility of films. Therefore, the nature of the substrate-plasma species interaction was crucial in enabling the formation of insoluble structures.
Figure 5. Neutral mass spectra of 1 Pa TEP plasmas. Image Credit: Impedans Ltd.
Figure 6. Positive ion mass spectra of 1 Pa TEP plasmas. Image Credit: Impedans Ltd.
TEP plasma polymers have been created under low-pressure conditions. Impedans’ Octiv VI probe was used to quantify the ion flux when depositing the organophosphate thin films. Analysis has demonstrated that when the applied RF power input is increased, the contribution of ions to the deposition is also increased.
The plasma phase ion mass spectra also revealed that the phosphate ion is highly stable in the plasma. This has been confirmed through surface analysis of the plasma polymers that displayed excellent retention of the phosphate structure, joined together by hydrocarbon species.
Finally, substrate interaction is essential in the formation of insoluble plasma polymers. Organic substrates enable adhesion through the aliphatic carbon species and form cross-linked structures that are highly insoluble in water.
Raphael V. F. P., Solmax S., Andrew M. “The chemistry of organophosphate thin film coatings from low-pressure plasma and the effect of the substrate on adhesion.” Plasma Processes and Polymers. doi:10.1002/ppap.20170003
This information has been sourced, reviewed and adapted from materials provided by Impedans Ltd.
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