Pmma Gate Dielectric - Ossila FACT1 User Manual

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Appendix II - PMMA gate dielectric
For top-gate devices the choice of dielectric material is a critical consideration as it must have low
leakage and also have an orthogonal solvent to the semiconductor layers below.
Solution processed gate dielectrics are therefore an area of active research with many new polymers
being developed with the desired properties. However, of the commercially available polymers we
have found that PMMA gives the best combination of performance, simplicity and compatibility.
However, the molecular weight and solvent used has a large impact on performance.
We have found that PMMA with M
Molecular weights significantly higher than this can cause solubility issues and uneven films while
significantly lower can cause pin-holes and solvent incompatibility with solution process gates. While
there are many non-polar solvents that will dissolve PMMA very well, these often also dissolve
polymeric semiconductor layers. We have therefore found that butanone gives the right
combination of solubility for the PMMA with a moderate boiling point ideal for film forming
properties and with little effect on most polymeric semiconductors such as P3HT, PBTTT and PCDTBT
(it will however still dissolve small molecule materials such as TIPS-pentacene and PCBM).
Process routine for PMMA gate dielectric

PMMA with Mw=120,000 bought from Sigma Aldrich

Dissolved in butanone at 100 mg/ml using stirbar and hotplate set to 80 C

Cooled for 10 mins before filtering with 0.45 um PVDF syringe filter

Spin coat using a static dispense of 100 ul and fully coat the substrate before spinning

Spin speed of 1000 RPM for 30 seconds (fastest acceleration).

Bake at 105 C for 5 mins to fully evaporate solvent and take the PMMA to the glass
transition temperature to reduce leakage.

Solutions stable for at least several weeks.
The thickness of the resulting layer should be around 1200 nm and we generally obtain a film
uniformity of better than 10%.
Ossila Ltd
=120K gives good performance when dissolved in butanone.
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