**4.2 Texturing of multi-crystalline silicon wafers**

Multi-crystalline wafers offer a cost advantage compared to the mono-crystalline wafers and hence have been more widely adopted. However, the alkaline chemistry used for texturing mono-crystalline wafers does not work well for multi-crystalline wafers due to the presence of different grain orientations. An alternative acidic chemistry based on HF and HNO3 was developed to remove the saw damage and texture the multi-crystalline wafers simultaneously [17, 18]. The acidic solution-based texturing operates at temperatures below room temperature and hence leads to

reduced reaction gas emission, little heat generation, higher stability of the etching solution and better control of the etch rate [18]. A comparison of alkaline texturing and acidic texturing process for multi-crystalline wafers is shown in **Figure 7**.

The acidic texturing process of multi-crystalline wafer can be done in significantly reduced time compared to the alkaline texturing process and hence can be implemented in an 'inline' configuration where the wafers are passed through rollers immersed in the etching bath. A representative image of an inline process along with the typical acidic texturing process is shown in **Figure 8**. For a five lane configuration, the inline tool can have a throughput of up to 4,000 wafers/h. It is important to note that the wafer surface facing down in the etching solution is textured better than the top-side and is the 'sunny-side' for further processing. The acidic texturing process leads to formation of porous silicon on the textured surface which absorbs light and also increases the surface recombination [18]. Hence the porous silicon is removed using a dilute alkaline solution. Subsequently, an acidic clean (HF + HCl) is performed to remove oxides and metal contamination from the wafer surfaces.

It is important to note that the acidic texturing process discussed above is suitable for the slurry-wire sawn (SWS) multi-crystalline wafers. In the past few years, diamond-wire sawing (DWS) process has replaced the slurry-wire-based cutting due to process and economic advantages [19]. The saw damage of the SWS multicrystalline wafers is more than the DWS wafers, which have deep straight grooves and a much more smoother surface than the slurry-wire sawn wafers [19]. The saw damage for the SWS wafers plays an important role for initiating the texturing process, which does not occur for the DWS wafers.

Various methods have been proposed to texture DWS multi-crystalline wafers and are summarized in **Table 2** [20]. By tuning the various methods, reflectance of close to 0% can be obtained and hence the term 'black silicon' has been used for the texturing process of DWS multi-crystalline wafers. RIE was the first method for making black silicon and uses sulfur hexaflouride (SF6) to react with Si and gases like Cl2 and O2 for passivating and limiting the reaction [20]. Recently, commercial multi PERC solar cells with average efficiency of 21.3% have been demonstrated with RIE-based

#### **Figure 7.**

*Comparison of alkaline and acidic texturing for multi-crystalline wafers. Reflectance curves after deposition of SiNx:H are also showed for comparison [17].*

### *Industrial Silicon Solar Cells DOI: http://dx.doi.org/10.5772/intechopen.84817*

**Figure 8.**

*(a) Representative inline process with five lanes and (b) acidic texturing process flow for multi-crystalline wafers.*


#### **Table 2.**

*Various methods for texturing diamond-wire sawn multi-crystalline wafers [20].*

texturing process [21]. However, since RIE is a vacuum-based process the throughput is low as compared to a typical inline process and also additional pre-processing and post-processing is required to remove the saw damage and damage due to ion-bombardment, respectively. A variant of the RIE method which does not require vacuum or plasma has been implemented in a commercial tool [22].

One of the approaches for texturing DWS multi-crystalline wafers is to upgrade the existing acidic texturing-based chemistry with additives [23–25]. Such an approach can potentially have a lower CoO compared to the MACE-based approach [23]. Reflectance of such an additive-based approach has been demonstrated to be similar to the conventional isotexturing solution with solar cell efficiency of 18.7% for the Al-BSF-based structure [24].

MACE-based texturing is similar to the conventional acidic etching method with an additional step of catalytic metal deposition. The process flow consists of SDR, catalyst metal deposition, chemical etching and post-treatment. Efficiencies of 19.2% have been obtained for commercial multi Al-BSF cells using batch-type MACE texturing process [26]. Inline-type MACE-based commercial tool has been demonstrated with the possibility to tune the reflectance in the range of 12–23% and obtain average efficiency for Al-BSF and PERC structure of 18.8 and 20.2%, respectively [27]. Representative images of textured surface based on MACE process are shown in **Figure 9**. The cost of ownership (CoO) of the inline MACE process is potentially lower compared to the batch-based MACE process with scope to reduce it further by recycling Ag from the texturing bath [27].

**Figure 9.** *MACE textured DWS multi wafers, (a) surface with Ravg = 12% and (b) surface with Ravg = 22% [27].*
