Etching is a process which removes material from a solid (e.g., semiconductor or metal). The etching process can be physical and/or chemical, wet or dry, and isotropic or anisotropic. All these etch process variations can be used during solar cell processing.
Figure 1: Etching processes divided according to their physical, chemical, or combined (physical and chemical) nature.
Physical etching or sputtering is a dry process where the material is removed due to ion bombardment. The ion bombardment is delivered by a plasma. This process is known to [1]:
- be chemically unselective – depends only on the surface binding energy and the masses of the targets and projectiles,
- have a low etch rate – one atom per incident ion,
- be very sensitive to the angle of incidence of the ion and therefore anisotropic in nature, and
- the only etch process able to remove involatile products from the surface.
The chemical etch process can be either dry or wet. In both cases, an etchant reacts with the surface of the solid to form a byproduct that is volatile/soluble. This process is known to be:
- chemically selective,
- isotropic, anisotropic or defect orientated, and has a
- high etch rate.
There are two other types of etch technologies, ion-enhanced energy driven (also known as reactive ion-etching) and ion-enhanced inhibitor. The first combines the properties of physical and chemical etching by using a chemically reactive plasma. Common applications are edge isolation and surface texturing. The second is uncommon in solar cell manufacturing, and therefore is not relevant in this context.
Isotropic etching is where the etch rate is the same in all directions. In anisotropic etching, there is a preferential direction along the crystal planes where the etch rate is significantly higher, Fig. 2.
Figure 2: Schematic illustration of anisotropic and isotropic etching.
Dry etching is predominantly anisotropic. Reactive ion etching is used to edge isolate. During wet etching processes, the solid is immersed in a chemical solution (which can be either acidic or alkaline) and material is removed by dissolution. Wet etching can be used to remove residual saw damage, to texture, to polish, to clean material and/or to reveal defects in the wafers. There are three major wet etching types [3], see Fig. 3:
- Isotropic etching – As previously mentioned, the etch rate is the same in all directions. This type of etching is applied to remove saw damage from wafers or in chemical polishing.
- Anisotropic etching – the etch rate is different for different crystal plane orientations. This type of etching is used to texture monocrystalline wafers (e.g. alkaline etchants etch (100) silicon surfaces much quicker than (111) silicon surfaces creating random pyramids).
- Defect etching – Etching occurs primarily at the surface defects. This etching can be used to characterize wafers (e.g. Sopori etching to examine dislocations in silicon).
Figure 3: Schematic illustration of the three major wet etching processes.
Acidic and alkaline wet etching have different etch rates that define the anisotropy of the etching. The etch rate of alkaline etch solutions are generally lower than at he etch rates of acidic etching solutions. Consequently, alkaline etch processes are often performed at high temperatures (70-80 °C). Alkaline etching is typically anisotropic with an etch rate of 1-2 µm/min for low concentration (1-5% v/v) alkaline solutions. This type of etching is used to texture monocrystalline silicon. If the concentration is increased to 30-40% the etch rate increases to 2-4 µm/min and the etch mechanism becomes increasingly isotropic. This type of alkaline etch process is used to remove the saw damage from silicon wafer and for texturing.
Acidic etching is performed at low temperatures (6-20 °C) and is normally isotropic. The typical etch rates are 2-4 µm/min at 6-10 °C. Due to the different grain orientations within the same multicrystalline silicon wafer, acidic etching is used to texture this type of material. Acidic solutions are also used to perform defect etching (e.g. Sopori etching [2]).