Selective etching of PDMS: etching as positive resist

TL;DR

This paper introduces a novel method for selectively etching PDMS using ion irradiation and chemical development, enabling its use as a positive resist in microfabrication.

Contribution

It demonstrates for the first time how to chemically modify and selectively etch additive-free, cured PDMS as a positive resist using proton microbeam irradiation and alkaline solutions.

Findings

Maximum etching rate with KOH is approximately 3.5 μm/min at 7.5×10^15 ion/cm².

NaOH etches at about 1.75 μm/min at 8.75×10^15 ion/cm².

Selective etching enables PDMS to be used as a positive resist material.

Abstract

Although, poly(dimethylsiloxane) (PDMS) is a widely used material in numerous applications, such as micro- or nanofabrication, the method of its selective etching has not been known up to now. In this work authors present two methods of etching the pure, additive-free and cured PDMS as a positive resist material. To achieve the chemical modification of the polymer necessary for selective etching, energetic ions were used. We created 7 um and 45 um thick PDMS layers and patterned them by a focused proton microbeam with various, relatively large fluences. In this paper authors demonstrate that 30 wt% Potassium Hydroxide (KOH) or 30 wt% sodium hydroxide (NaOH) at 70 oC temperature etch proton irradiated PDMS selectively, and remove the chemically sufficiently modified areas. In case of KOH development, the maximum etching rate was approximately 3.5 um/minute and it occurs at about 7.5*10^15 ion*cm-2. In case of NaOH etching the maximum etching rate is slightly lower, 1.75 um/minute and can be found at the slightly higher fluence of 8.75*10^15 ion*cm-2. These results are of high importance since up to this time it has not been known how to develop the additive-free, cross-linked poly(dimethylsiloxane) in lithography as a positive tone resist material.