PPA handling¶
Polyphthalaldehyde (PPA) belongs to a family of polymers known as polyaldehydes, which are based on a poly-(acetal) [1] backbone, consisting of repeating C-O bonds. Each monomer unit of PPA consists of an ortho-phthalaldehyde (OPA) molecule with the molecular formula C8H6O2. Thermodynamically PPA is unstable at RT, since its ceiling temperature [2] is Tc= -43 °C [3]. This property gives rise to a high sensitivity and fast decomposition (unzipping) of PPA into its volatile monomer OPA upon various trigger signals (e.g. heat, UV, acid, e-beam or X-ray).
Chemical topology¶
PPA could be polymerized by anionic and cationic polymerization, which results in different topologies: linear PPA (lPPA) (cf. Figure 100) and cyclic PPA (cPPA) (cf. Figure 101) [4].
Stability and storage¶
PPA powder: Since PPA is unstable at RT, it should be stored in a freezer in dark (UV sensitive) and dry (moisture sensitive) environment. It is recommended to handle PPA powder and seal it in an appropriate container under inert gas (preferentially: Ar).
PPA solution: In order to get a specifically thick PPA thin film, different solutions with varying PPA concentrations need to be used (cf. Figure 102). To prepare a specific PPA solution (eg. 5 w-%) weigh the appropriate amount of PPA (eg. ~150 mg) and Anisole (eg. ~3 g) and mix them. Put the mixed solution to a shaker for around 1 hr. Filter the solution with a filter (pore size < 0.2 µm). Because the PPA might start to decompose, PPA solutions should be stored in a cool environment and used within 2 months.
PPA film: Thin films of PPA are stable for ~6 months, if kept in dark and dry place and in nitrogen atmosphere. In any case, it is recommended to use as freshly prepared thin films as possible.
Safety¶
Hazard statements
H315 | Causes skin irritation. |
H319 | Causes serious eye irritation. |
H335 | May cause respiratory irritation. |
Precautionary statements
P305+P351+P338 | IF IN EYES: Rinse cautiously with water for several minutes. Remove contact lenses, if present and easy to do. Continue rinsing. |
Specifications¶
Property | Value |
---|---|
Film preparation | Spin coating |
Solvents | Anisole (standard), Cyclohexanone |
Soft bake | 90 °C for 3 min |
Decomposition temperature | ~ 140 °C |
Glass transition temperature | > 120 °C |
Film thickness range | 5 - 200 nm |
Roughness Rq | < 0.3 nm |
Lateral resolution (half-pitch demonstrated) | 8 nm |
Vertical resolution (step size, demonstrated) | 1 nm |
Minimum heating time per pixel (demonstrated) | 1 µs |
Etch rate (RIE with CHF3) | 5 nm/min (15 nm/min SiO2) |
[1] | Köstler (Polym Int 2012), Polyaldehydes: homopolymers, block copolymers and promising applications) |
[2] | Snow and Frey (J. Am. Chem. Soc. 1943), The Reaction of Sulfur Dioxide with Olefins: the Ceiling Temperature Phenomenon |
[3] | Aso (J o Polym Sc A 1969), Polymerization of Aromatic Aldehydes Cationic Cyclopolymerisation of Phthalaldehyde |
[4] | Kaitz et al (J. Am. Chem. Soc. 2013), End Group Characterization of Poly(phthalaldehyde): Surprising Discovery of a Reversible, Cationic Macrocyclization Mechanism |