Low-Temperature Molecular Layer Deposition Using Monofunctional Aromatic Precursors and Ozone-Based Ring-Opening Reactions

Abstract

Molecular layer deposition (MLD) is an increasingly used deposition technique for producing thin coatings consisting of purely organic or hybrid inorganic-organic materials. When organic materials are prepared, low deposition temperatures are often required to avoid decomposition, thus causing problems with low vapor pressure precursors. Monofunctional compounds have higher vapor pressures than traditional bi- or trifunctional MLD precursors, but do not offer the required functional groups for continuing the MLD growth in subsequent deposition cycles. In this study, we have used high vapor pressure monofunctional aromatic precursors in combination with ozone-triggered ring-opening reactions to achieve sustained sequential growth. MLD depositions were carried out by using three different aromatic precursors in an ABC sequence, namely with TMA + phenol + O, TMA + 3-(trifluoromethyl)phenol + O, and TMA + 2-fluoro-4-(trifluoromethyl)benzaldehyde + O. Furthermore, the effect of hydrogen peroxide as a fourth step was evaluated for all studied processes resulting in a four-precursor ABCD sequence. According to the characterization results by ellipsometry, infrared spectroscopy, and X-ray reflectivity, self-limiting MLD processes could be obtained between 75 and 150 °C with each of the three aromatic precursors. In all cases, the GPC (growth per cycle) decreased with increasing temperature. In situ infrared spectroscopy indicated that ring-opening reactions occurred in each ABC sequence. Compositional analysis using time-of-flight elastic recoil detection indicated that fluorine could be incorporated into the film when 3-(trifluoromethyl)phenol and 2-fluoro-4-(trifluoromethyl)benzaldehyde were used as precursors.

About the importance of purge time in molecular layer deposition of alucone films

“…By using organic monomers as precursors, MLD has been used to deposit polymer films such as polyamides, 8 polyimides, 9 polyurea, 10 and others. 11 Recently, the field of MLD has received renewed interest as evident by the exploration of new MLD chemistries, like for example, in the form of ring opening mechanisms 12,13 and click reactions. 14 Further, hybrid organic-inorganic films can also be prepared using common metal-organic precursors and bifunctional organic co-reactants such as alcohols, 13,15,16 phenols, 17 amines 18 and acids.…”

Molecular Layer Deposition Using Ring-Opening Reactions: Molecular Modeling of the Film Growth and the Effects of Hydrogen Peroxide

“…Modeling also explains the differences between the contact angles in the previous study. 14 With the most favorable cleavage in the case of 4F, the cleavage of the C5–C6 and backbonding of the carboxylic acid to the surface structure lead to the surface, where the CF 3 group and one fluorine atom point perpendicularly to the surface, causing the lower surface energy, more hydrophobic surface, and higher contact angle compared with the 3F process. 30,31 In the case of 3F, there is the CF 3 group, but the additional fluorine atom is missing.…”

“…Therefore, the ring-opening reaction is an intriguing approach for enabling the new MLD thin film materials. 12−14…”

“…Previously, we presented novel MLD processes with monofunctional aromatic precursors, when TMA, one of the three aromatic compounds (phenol, 3-(trifluoromethyl)phenol, and 2-fluoro-4-(trifluoromethyl)benzaldehyde) and ozone were utilized in the three-step MLD process. 14 The processes were carried out at low temperatures (75–100 °C), and the ALD-type film growth was achieved. Furthermore, we demonstrated the ring-opening reaction between the aromatic ring and ozone, which introduced new functional groups on the film surface, thus, enabling the continuous film growth (Figure 1).…”

“…Suggested reactions during one cycle in the TMA + phenol + O 3 (+H 2 O 2 ) process. Figure is reconstructed from ref (14). …”