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].

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Figure 100 Linear PPA (endgroups not shown)

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Figure 101 Cyclic PPA (here: 5-ring topology)

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.

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Figure 102 Standard solutions of PPA in Anisole

Safety

GHS07-pictogram

Figure 103 GHS07

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