Wet chemical cleaning is essential in solar cell fabrication to ensure silicon quality is maintained and to prevent contamination of equipment, which could contaminate following samples. Surface contamination arising from “dirty wafers” can have disastrous effects on the bulk minority carrier lifetime of silicon wafer solar cells, especially if impurities are allowed to diffuse into the wafers through high-temperature thermal processes such as diffusion. These impurities can also out-diffuse and contaminate production line components such as diffusion furnaces, firing furnaces and plasma enhanced chemical vapour deposition (PECVD) tools. Silicon wafers industry are cleaned through a series of wet chemical steps. In some cases, a Radio Corporation of America (RCA) cleaning process is used . RCA cleaning involves 3 primary steps:
- RCA 1: Organic Particle Removal
The first step is used to remove organic residuals from the wafer surface. Aqueous ammonium hydroxide (NH4OH) is mixed in with hydrogen peroxide (H2O2) and de-ionized (DI) water in a 1:1:5 ratio to form the RCA 1 mixture. Wafers are cleaned for 6-10 minutes at elevated temperatures (~80 °C) before rinsing in DI water. In this process, a thin oxide layer may form on the surface which would be removed later.
- HF – oxide removal
A hydrofluoric (HF) acid dip is typically performed to remove any native oxides that may have grown on the silicon surface. Wafers are immersed in an aqueous HF solution until a hydrophobic surface condition is observed. This process is usually performed with high purity materials as the bare silicon surface is highly reactive and may easily become re-contaminated.
- RCA 2: Ionic Clean
This treatment is used to remove metallic contaminants. The solution consists of a mixture of aqueous hydrochloric acid (HCl), H2O2 and DI water in a 1:1:5 ratio heated to ~80 °. Once again, wafers are immersed in the solution for 6-10 minutes. The resulting wafers have a thin, chemically grown oxide, which according to the next processing steps, is either removed in a HF acid dip, or left on the surface to act as a passivating oxide.
In the video below we show the RCA cleaning procedure performed at UNSW Sydney.
To reduce processing time, industrial cleaning may only involve 1 or 2 of these steps.
Another popular way to remove organic contaminants from a silicon wafer is the so-called ‘Piranha clean’ using sulphuric acid (H2SO4) and H2O2 which is shown in the animation below.
Drying and Storage
Wafers in cassettes are commonly dried using a spin rinse dryer or a centrifuge prior to the next process. If wafers are not immediately processed, they need to be stored in a clean environment and inert environment, such within a dedicated nitrogen (N2) storage cabinet, which has a constant nitrogen gas purge.