In the production of photovoltaic cells, mono- or multicrystalline silicon is further processed into blocks. From these blocks, also called ingots, wafers of the desired thickness are produced by wire sawing. During this sawing process, sawing damage and residues occur on the wafer surface due to mechanical stress. These must be removed by etching and texturing before further processing, as the damaged layer contains a large accumulation of mechanically induced crystal defects and thisshortens the lifespan of the solar cell.
In addition, texturing the surface of a wafer creates a rough, structure-optimized surface that absorbs more light and thus significantly influences the efficiency of the later cell. Depending on the process and type of wafer, an acidic or alkaline etching with sodium or potassium hydroxide or nitric, sulfuric, and hydrofluoric acid is then carried out. This is followed by neutralization and removal of any residues.
LiquiSonic® Sound velocity measurement in potassium hydroxide
Application
Wafers are cleaned in alkaline or acidic etch baths, freed from saw damage, and textured. Basic etching solutions, such as KOH, NaOH, or TMAH, are used for anisotropic etching of silicon, while acidic etching solutions, such as HF and HNO3, are used for isotropic etching. The texture baths optimize the surface structure of the wafer, creating a kind of light trap and increasing the energy absorption capacity. During the process, the etching depth or removal is controlled by the bath concentration and residence time.
Due to consumption and discharge during texturing, the acidic or alkaline etching solutions and water must be constantly monitored and replenished if necessary. With the help of LiquiSonic® inline measurement technology the current etch bath concentration can be determined and replenishment controlled via a control system. This ensures continuously consistent product quality.
Installation
The LiquiSonic® immersion sensors can be easily installed in the piping of the bath circulation flow or directly in the texture bath. The LiquiSonic® Controller 30 can be connected to up to 4 sensors. This makes it possible to monitor several measuring points simultaneously.
Typical measuring range:
Concentration range KOH: 0 to 55 m% | Temperature range: 80 to 120 °C
Customer benefits
The robust sensor construction and the choice of special materials, such as Halar or PFA, ensure long process service life of the system. LiquiSonic®reduces time-consuming laboratory measurements:
- Time required: 1 hour per day
By avoiding over- and under-dosing, material costs are also saved.
LiquiSonic® ensures precise analysis of the etching bath concentration with permanent data recording. This allows the replenishment of the etching solution to be controlled to maintain the etching bath at the optimal concentration and ensure the best possible efficiency. This avoids under- and overdosing of the etching solution.
