Polydimethylsiloxane (PDMS) is an organic silicon polymer commonly known as silicone rubber. Due to its many advantages, PDMS has been widely used in recent years, especially in the manufacture of various micro nano devices. For example, PDMS has become the most commonly used processing material for microfluidic chips due to its low cost, good transparency, and good biocompatibility, as well as its ease of use in the production of micro nano channels through casting and other methods. For example, due to its excellent flexibility and extensibility, PDMS is often used as a substrate material for various flexible electronic devices.

In the process of manufacturing various PDMS micro nano devices, it is often necessary to first pattern the photoresist on the surface of the PDMS substrate, and then perform various microfabrication on the PDMS substrate, or use techniques such as exfoliation to fabricate metal microstructures on the PDMS surface.

However, due to the poor wettability and high thermal expansion coefficient of PDMS surface, it is difficult to use standard photolithography processes to pattern photoresist on its surface, and defects such as cracks often appear in the photoresist. Therefore, it is necessary to modify the surface of PDMS to improve its wetting performance.

Material surface modification technology is currently at the forefront of materials science research. In the past few decades, various modification techniques (such as chemical treatment and etching, ozone treatment, ultraviolet radiation, and plasma treatment) have been widely used to customize material surface properties. By comparing the processing speed, uniformity, chemical flexibility, and environmental impact of different modification techniques, it can be concluded that plasma surface modification is one of the most promising technologies in the field of materials science research. Plasma surface modification technology modifies materials by combining various functional groups on the surface of some materials with poor surface properties to improve their surface wettability, adhesion, and biological activity reactions. Plasma treatment usually only modifies the surface of materials without changing their overall properties

PDMS surface oxygen plasma treatment improves hydrophilic wetting performance

During the experiment, it was found that the photoresist could not be uniformly spin coated onto the surface of the untreated PDMS substrate, and the photoresist could not form a complete thin film, as shown in Figure 1 (a). This is mainly due to the poor surface wettability of PDMS, which prevents the photoresist from effectively spreading uniformly on the surface of PDMS. Therefore, in order to improve the surface wettability of PDMS, oxygen plasma is used to modify the surface of PDMS substrate before spin coating photoresist. As shown in Figure 1 (b), on the surface of PDMS substrate modified by peroxide plasma, photoresist can be uniformly spin coated to form a complete thin film.

Figure 1: The photoresist spin coated on the surface of PDMS substrate

The contact angle can measure the wettability of a surface. In order to study the effect of oxygen plasma treatment on the wettability of PDMS substrate surface, an optical contact angle measuring instrument was used to measure the contact angle of PDMS substrate before and after plasma modification treatment.

The outer dimensions of the PDMS substrates used for testing are 25mm × 25mm × 1mm, and 3 PDMS substrates were randomly selected. Contact angle measurements were conducted at three different positions on the surface of each PDMS substrate, and all measurement data are shown in Table 1. It can be seen that the PDMS surface before plasma modification treatment is a hydrophobic surface, with an average contact angle of 111.03 °. After a long time, water droplets still cannot spread on its surface, as shown in Figure 2 (a). However, after oxygen plasma modification treatment, the PDMS surface changed from a hydrophobic surface to a hydrophilic surface, and the average contact angle decreased to 29.50 °. Water droplets instantly spread out on the PDMS surface (Figure 2 (b)).

Comparison of droplet angle before and after oxygen plasma treatment on PDMS substrate

It can be seen that oxygen plasma modification treatment can indeed effectively improve the hydrophilic wettability of PDMS surface, so that the photoresist can be uniformly spin coated on the surface of PDMS substrate, forming a complete photoresist film.