Analysis of Passivation Layers and Stacks

Symmetric samples fabricated from 5^" 11 - 12 Ωcm n-type base material were used in order to evaluate the passivation quality of rear side passivation layers and stacks. The passivation quality is analysed by means of the effective minority charge carrier lifetime (further referred to as lifetime), which was measured before and after a high temperature step by photoconductance decay minority charge carrier lifetime measurements. For lifetimes exceeding 100µs, the gener-alized mode was used, while below the QSS mode was applied. The lifetime was evaluated for an injection level of ∆n = 10¹⁵ cm^-3, which is important to acknowledge, since the passivation quality depends strongly on the injection level.

The samples were exposed to different peak temperatures in order to investigate the sensitivity of the passivation layers/stacks on high temperature processing.

5.1.2.1. Sample Preparation

Figure 5.4.: Sample preparation processing chart and overview over used passivation laysers/stacks

Previously textured wafers were etched back in a 22 % NaOH solution in order to remove the texture and fabricate non-textured samples. This was done, since the rear of passivated solar cells is also not textured. Subsequently, a standard HCl/HF cleaning process was carried out. For the dissolution of thermal donors incorporated in the substrate, a high temperature phosphorus diffusion was done which resulted in a FSF, that was removed in a 22 % NaOH solution etching for 5 minutes. This was followed by a successive cleaning step of HCl/HF as well as a standard RCA cleaning step. Depending on the planned passivation as they are shown in figure 5.4, the samples were either coated with a 15 nm Al2O3-layer, a SiO2 layer or non of both. Furthermore, all samples were coated with a silicon nitride layer of 75 nm on the front side, either by means of direct PECVD (SiN_x) or remote PECVD (SiNA-SiN_x). Similarly, on the rear side, a 200 nm silicon nitride layer was deposited by means of remote PECVD or direct PECVD. A typical solar cell process was further emulated by a following firing step, which allows the incorporated hydrogen in the silicon nitride layer to be freed and saturate defects as well as dangling bonds [67]. The chain speed was held constant at 5400 mm/min, while the peak temperature was varied in intervals of 40‰ from 800‰ up to 920‰.

5.1. n-Type Base Passivation: Firing Stability and Passivation Quality

5.1.2.2. Passivation Quality Results

The passivation quality is evaluated by comparing measured lifetimes before and after a firing step, which is carried out at different peak temperatures. The results are discussed by directly comparing two similar passivation layers/stacks, which feature the same passivation but different capping layers. As capping layers remote PECVD and direct PECVD, referred to as SiNA-SiNx

and CT-SiN_x, silicon nitride are used.

Figure 5.5.: Lifetime comparison of samples with SiNA-SiN_x and CT-SiN_x passivation layers for different peak firing temperatures before and after firing at an injection level of

∆n = 10¹⁵ cm^-3(measured in generalized mode)

In the first analysis, the quality of silicon nitride surface passivation deposited by means of either remote PECVD or direct PECVD is compared (thickness of 200 nm). The lifetimes prior to a firing step are quite similar for both deposition techniques and are between 200 to 450µs (see figure 5.5).

This similarity is drastically changed after a firing step. While for SiNA-SiN_x, lifetimes of larger than 9 ms were obtained, CT-SiN_xled to only a small increase reaching up to 750 µs. Overall, considering the peak temperatures, a decrease of the passivation quality with increased firing peak temperature is visible for remote PECVD (SiNA-SiN_x) deposited silicon nitride, while for the direct PECVD deposited (CT-SiN_x) silicon nitride, the measured effective lifetime is varying strongly, but can be assumed not to be strongly influenced by the temperature. Altogether, having in mind the common firing set peak temperature of 860‰, the SiNA-SiN_x is better suited to fit processing needs compared to CT-SiN_x.

In figure 5.6, the effective minority charge carrier lifetimes of silicon oxide/silicon nitride stacks (15 min/200 nm) are compared, whereas the silicon nitride was either deposited by means of remote PECVD or direct PECVD. Processing of both samples featuring different silicon nitride capping layers was equal, except for the deposition method of the silicon nitride. While the lifetime for the SiNA-SiNx deposited silicon nitride before firing is reaching on average of 1 -2.5 ms, the CT-SiN_xcapping layer is resulting in lifetimes, that are much lower, about two orders of magnitude. Only lifetimes of roughly 20 - 40 µs were obtained, which were then measured using the QSS mode of the photoconductance decay assembly. This result is not changed after the firing step, while for low set firing peak temperatures an improvement for the SiNA-SiN_x capped samples in the case of silicon oxide/silicon nitride stacks can be found. Higher peak temperatures also led to a degradation of the lifetime compared to values obtained prior to the

5.1. n-Type Base Passivation: Firing Stability and Passivation Quality

Figure 5.6.: Comparison of SiO2/SiNA-SiNx and SiO2/CT-SiNx stacks for diverse peak firing temperatures before and after firing at an injection level of ∆n = 10¹⁵ cm^-3 (gen-eralized mode)

firing. This indicates, that the silicon oxide/silicon nitride stack is not stable for high firing temperatures, since only an improvement of minority charge carrier lifetime can be found for the lowest set peak firing temperature (800‰). Similar to this, the fired samples featuring a CT-SiN_x capping layer show also degraded lifetimes compared to the values measured before firing.

Figure 5.7.: Comparison of Al₂O₃/SiNA-SiN_x and Al₂O₃/CT-SiN_x stacks for different peak fir-ing temperatures before and after firfir-ing at an injection level of ∆n = 10¹⁵ cm^-3 (generalized mode)

Finally, the lifetimes of samples passivated by aluminium oxide/silicon nitride stacks (15 nm/

200 nm) are compared and are showing interesting results, as well. The lifetimes prior to the firing step are low for the stack featuring a SiNA-SiNx capping layer, whereas for samples with CT-SiN_xcapping layer, the lifetime is reaching up to 1.5 ms. Firing of the samples covered with a SiNA-SiNx capping layer results in a strong increase through all set peak firing temperatures to 2-8 ms. The CT-SiNx capping layer group has performed very well (5-9 ms) for set peak

5.1. n-Type Base Passivation: Firing Stability and Passivation Quality

firing temperatures of 840‰ and 880 ‰, but showed a decreased performance for a higher temperature of 920 ‰.