Saw damage etching

The sawing process to cut ingots into wafers induces surface damage and introduces residual contaminants on the wafer that are detrimental to solar cell performance. An example of “saw damage” is shown in Figure 1 for a wafer which was sawn using diamond wire sawing. Therefore, it is necessary to etch 10 µm (slurry based sawing) or 5 µm (diamond wire sawing)  of each side of the wafer before further solar cell processing and a wet alkaline etch process is commonly used for this purpose. The most common solutions utilise sodium hydroxide (NaOH), potassium hydroxide (KOH), or tetramethylammonium hydroxide (TMAH) diluted in de-ionised water as the etch solution. The reaction process is essentially similar for all solutions, where OH– and water (H2O) plays a key role in the reaction:

Si + 2OH– + 2H2O → SiO2(OH)22- + 2H2.                    (1)

Diamond_Wire_Trans
Figure 1: Photograph of a monocrystalline silicon wafer which was sawn using diamond wire sawing. The sawing marks from the process are clearly visible on the wafer.

The etch rate depends on the [OH–] and [H2O] concentrations. The etch rate increases with increasing [OH-] concentration until some maximum point is reached. With further [OH-] increases, the etch rate decreases due to decreasing [H2O]. In Figure 2 we see that the etch rate after a point decreases with concentration, which is due to the lack of water in the process. The etch rate of KOH may vary from as low as 1 Å/s in dilute KOH at room temperature to as high as 2000 Å/s in 40 % KOH at high temperatures. The etching is anisotropic due to different energy levels of the back-bond states with the different crystal planes. However, depending on the etching condition, the level of anisotropy of the process can change. For KOH the anisotropy ratio is 30:1 (100):(111) at 100 °C (high etch rates) and >>100:1 at room temperature (low etch rates). In the saw damage etch process, the solution is 30-40 % KOH and the temperature is held at 70-80 °C. Such conditions result in high etch rates (2-4 µm/min) decreasing the anisotropy of the process, i.e. the saw damage etching is quasi-isotropic.

KOH.png
Figure 2: Etch rate of the [100] silicon crystal plane as a function of KOH concentration at 72 °C (graph adapted from [1]).

It is important to control the etch rate of the process. Over-etching leads to thinner wafers that could lower production yield due to breakage. Under-etching can lead to shunting and degradation of the minority carrier lifetimes because of residual surface micro-damag

[1] – H. Seidel et al, “Anisotropic Etching of Crystalline Silicon in Alkaline Solutions: I . Orientation Dependence and Behavior of Passivation Layers“, 1990 J. Electrochem. Soc. 137 3612