Silicon wafer plasma cleaning

Among silicon-based materials, silicon (Si), as the first generation semiconductor, has the advantages of large reserves and good mechanical properties. In addition, the application and development of ion implantation, diffusion, oxidation and other processes have further optimized the development and manufacturing of silicon devices. At the same time, silicon is the most important structural material in optoelectronic devices. Due to the single SiO2 component structure, silicon wafers have good light transmittance in a wide spectral range and excellent mechanical and electrical properties. They have unparalleled advantages over other materials in optoelectronic device packaging and the manufacturing of various sensors and micro-nanofluidic chips.

Plasma cleaning of silicon wafers improves hydrophilicity

Plasma is a special state of matter. It is the fourth state of matter after solid, liquid and gas after gas is ionized under the action of high-frequency alternating electric field. Plasma is composed of excited atoms, molecules, free radicals, electrons, etc. Plasma cleaning technology is a process that uses high-energy particles in plasma to remove pollutants from workpieces through physical and chemical methods such as bombardment and activation reaction.

The horizontally placed silicon wafer was cleaned with oxygen plasma for different lengths of time, and the wetting conditions of the water droplets on the surface of the silicon wafer with surface cleaning times of 0s, 5s, 10s, 15s, and 20s were obtained as shown in Figure 1.

Figure 1. Spread of droplets on the silicon wafer surface at different plasma cleaning times

As shown in Figure 1, the droplets on the untreated silicon wafer surface maintain a spherical crown with a small radius. As the oxygen plasma surface cleaning time increases, the droplets on the silicon wafer surface spread more and more. When the treatment time reaches 20s, the droplets are completely spread on the silicon surface.

The contact angles of the droplets on the silicon wafer surface at each cleaning time were measured separately, and the image of the contact angle of the silicon wafer surface changing with the treatment time was obtained as shown in Figure 2.

Figure 2. Changes of contact angle of silicon wafer surface with plasma cleaning time

The contact angle of the silicon wafer surface before plasma cleaning is 71.8°, and the wettability is poor. When the cleaning time is increased to 20s, the contact angle of the silicon wafer surface drops to 0°. After continuing to increase the surface cleaning time to 25s and 30s, the contact angle does not change and stabilizes at 0°. The final plasma cleaning effect of the silicon wafer is shown in Figure 3.

Figure 3 Comparison of water drop angles before and after plasma cleaning of silicon wafers

Principle analysis

The reason why the surface wettability of silicon wafers that have not been plasma cleaned is poor and the contact angle of the droplet is large is that pollutants such as hydrocarbons and carbon oxides exist on the surface of the silicon wafer before treatment.

As the plasma surface cleaning process proceeds, the pollutants on the surface are desorbed by high-energy particle bombardment, oxidized to generate volatile gases and then released from the surface. The reactive oxygen reacts with the organic pollutants on the surface as follows: CxHy+O*→CO+CO2+H2O

At the same time, oxygen plasma directly acts on some areas of the silicon wafer surface, generating more Si- and Si-O-dangling bonds, increasing surface energy and surface activity, and generating some Si-OH with free -OH in the surrounding environment, as shown in Figure 4, which improves the surface wettability and reduces the contact angle. As the surface cleaning time increases, more surface contaminants are cleaned, and more clean surfaces are exposed to direct plasma action, the number of Si-OH generated increases, the surface wettability is further improved, and the wetting angle continues to decrease. Until the surface treatment time exceeds 20s, the hydrophilic group Si-OH that improves the surface wettability increases to the extent that the droplet can spread completely on the silicon wafer surface, and the wetting angle is 0°.

Figure 4 Plasma cleaning activates the silicon wafer surface and introduces hydroxyl groups

Plasma cleaning activates the surface of silicon wafers through chemical or physical effects, removes contaminants at the molecular level to increase the activity of the surface, increases the number of dangling bonds on the surface of the silicon wafer, and greatly improves the hydrophilicity of the silicon wafer surface.