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 saws. 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.
The texturing of the wafer surface also 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 hydroxide 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, freed from saw damage, and textured in alkaline or acidic etching baths. Basic etching solutions, such as KOH, NaOH, or TMAH are used in the anisotropic etching of silicon, while acidic etching solutions, such as HF and HNO3, are used in isotropic etching. The texturing baths optimize the surface structure of the wafer, creating a kind of light trap and increasing energy absorption capacity. During the process, the etching depth or removal is controlled via the bath concentration and dwell 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 etching bath concentration can be determined and replenishment can be controlled via a control system. This ensures continuously consistent product quality.
Installation
The LiquiSonic® immersion sensors can be easily installed in the pipes of the bath circulation flow or directly into the texturing bath. The LiquiSonic® Controller 30 can be connected to up to 4 sensors. This makes it possible to monitor multiple measurement points simultaneously.
Typical measurement range:
Concentration range KOH: 0 to 55 wt% | Temperature range: 80 to 120 °C
Customer benefits
The robust sensor design and the choice of special materials, such as Halar or PFA, ensure long process lifetimes of the system. LiquiSonic®reduces time-consuming laboratory measurements:
- Time required: 1 h per day
Avoiding over- and under-dosing also results in savings on material costs.
LiquiSonic® ensures precise analysis of the etching bath concentration with continuous 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.
