Acidic Texturing Process
Acidic texturing (also known as iso-texturing) is a process used to roughen the surface of multicrystalline silicon wafers . Multicrystalline silicon wafers are comprised of many silicon grains which are oriented at varying angles and therefore unsuitable for anisotropic etch processes such as alkaline texturing. This is because different orientations would etch at different rates, leading to non-uniform thicknesses across the surface and polishing/planarization in others. Furthermore, grain-boundaries present in multicrystalline wafers would undergo defect etching as alkaline solutions may preferentially etch at pre-existing crystallographic defects, creating dislocations that can propagate through the wafer.
In contrast, the acidic texturing processes is isotropic, meaning that all crystal planes and orientations etch at the same rate. In order to create the surface features required for light trapping, the process relies on the presence of defects which will be preferentially etched. In most cases, the initial defects residual from the wafer sawing process. The resulting silicon surface is shown in Figure 1 and 2. These spherical caps reduce the reflection of the surface, however, it does not outperform the light trapping properties of the random pyramids formed using alkaline texturing on monocrystalline silicon wafers. A minimum reflection of 22-27% on uncoated silicon can be achieved prior to application of an antireflection coating.
Acidic texturing is typically achieved using a solution comprising of nitric acid (HNO3), hydrofluoric acid (HF) and acetic acid (CH3COOH) and is also called the “HNA” or “Trilogy” etch. In some cases, the acetic acid is replaced by de-ionized water. The etching process involves two primary reactions. At the silicon surface, nitrous oxide (HNO2) present in the nitric acid reacts with HNO3 forming nitrogen dioxide (NO2).
HNO3 + HNO2 → N2O4 ↔ NO2 (1)
The nitrogen dioxide migrates towards cathodic (negatively charged) sites at the silicon surface and accepts electrons from the silicon lattice to form the NO2– anion.
2NO2 → 2NO2– + 2h+ (2)
These resulting anions can react again with H+ present in the bath to reform HNO2 making the reaction autocatalytic. At the silicon surface, the resulting extraction of electrons injects holes, thus creating a region of positive charge. These regions form anodic sites which can be solubilised by hydrogen.
Si + 3h+ + H+ → Si4+ + ½ H2 (g) (3)
The resulting silicon cations combine with hydroxide anions present in the solution to form silicon dioxide.
Si4+ + 4OH– → Si(OH)4 → SiO2 + H2O (4)
In the 2nd step, oxidised silicon is then dissolved by HF to produce hexafluorosilicic acid and water and transported away from the silicon.
SiO2 + 6HF → H2SiF6 + 2 H2O (5)
During this process, acetic acid acts as a wetting agent, which increases the ability for HNO3 to uniformly distribute and remain in contact with the silicon surface. The resulting etch rate of silicon (2-4 µm/min) is faster than that of alkaline texturing, however, this is highly dependent on the chemical composition and the temperature of the reaction. The specific properties of the reactions are complex and can depend on other factors such as dopant type and concentration.
The composition of texturing bath solution can highly affect the texturing outcome and resulting surface morphology. By having a low HF:HNO3 ratio, the silicon surface can become increasingly polished resulting in a higher reflectance and thus lower JSC. Higher ratios will result in a rough texture with low reflectance, however, defect etching may result in lower VOC.
Porous Silicon Formation
Acidic texturing can result in the formation of porous silicon at the surface. This can occur when the texturing process is performed in the presence of illumination, or if a high concentration of HF is used in the solution ([HF] >>[HNO3]), where silicon is directly dissolved at the surface. Although porous silicon may help reduce the final surface reflection, the microstructure consists of a high surface to volume ratio. This makes the silicon harder to passivate and can result in high surface recombination velocities.
In order to prevent the formation of porous silicon, agitation of the texturing bath is essential. In some industrial processes, a short immersion of the wafers after texturing in a 1% potassium hydroxide (KOH) solution is used to remove any formed porous silicon layers.
Agitation of the acid bath
Agitation of the acid bath is essential to facilitating the movement of the solution around the bath. This can assist in reducing thermal gradients between the bottom and top of the solution—which can result in non-uniform etching across the wafer—and also facilitates ventilation of hydrogen gas pockets, and replenishing of the texturing solution.
One problem found commonly in textured wafers include the formation of holes or pits on the silicon surface. This can occur due to the build-up of the etching solution in a small region. Due to the exothermic nature of the process, heating of local sites can occur. If the solution is not agitated, these heated pockets can result in extremely high etch rates, resulting in the formation of localised sites where relatively large amounts of silicon have been removed.
Acidic texturing can be performed in an in-line process whereby wafers are transported between rollers through the etching solution so that both sides are textured. The solution bath is kept at temperatures between 6 to 10 °C to prevent over-etching and the solution is recirculated through a tank to be chilled. The typical etching process is maintained to be between 1-3 minutes long within a bath length of 6 meter.
Industrial baths are strictly monitored to maximise throughput and ensure consistency in etching. The addition of small amounts of H2SiF6 can assist in increasing the oxidizing potential of nitric acid, however, in excess, H2SiF6 can increase the viscosity of the solution resulting in lowered etch rates. In order to reach an optimal concentration of H2SiF6, dummy wafers are run in what is known as an “induction period” to allow for bath stabilisation. Monitoring tools continuously replenish solution (dosing) or bleed solution (overflow) to ensure constant bath volume and mixture ratios. A single bath may process more than 2 million wafers before it is necessary to be emptied entirely and refilled.