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 using wire saws. During this sawing process, saw damage and residues occur on the wafer surface due to mechanical stress. These must be removed before further processing by etching and texturing, 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, 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, beim isotropen Ätzen genutzt werden. Durch die Texturbäder wird die Oberflächenstruktur des Wafers optimiert, sodass eine Art Lichtfalle erzeugt und die Energieaufnahmekapazität erhöht wird. Während des Prozess wird die Ätztiefe bzw. der Abtrag über die Badkonzentration sowie die Verweilzeit gesteuert.
Due to consumption and discharge during texturing, the acidic or alkaline etching solutions and water must be constantly monitored and, if necessary, replenished. With the help of LiquiSonic® inline measurement technology the current etching bath concentration can be determined and the 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 into the texture bath. The LiquiSonic® Controller 30 can be connected with up to 4 sensors. This makes it possible to monitor several measuring points simultaneously.
Typical measuring range:
Konzentrationsbereich KOH: 0 bis 55 m% | Temperaturbereich: 80 to 120 °C
Customer benefits
The robust sensor construction and the choice of special materials, such as Halar or PFA, ensure long process life of the system. LiquiSonic®reduces time-consuming laboratory measurements:
- Time required: 1 h 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 helps avoid under- and overdosing of the etching solution.
