Influence of Contact Pattern on Contact Formation

SEM analysis was performed to measure the geometry of the formed eutectic, the depth of the local BSF and the presence of voids. The analysis is shown in Figure 6.11 and summarized in Table 6.1. The contact formation is strongly influenced by the contact spacing. As presented in the example, for a contact spacing of 0.1 mm (d_p<d_S), a thin eutectic layer is found, mainly due to the limited vertical Si diffusion in the Al matrix, and the BSF is deeply formed [Figure 6.11(a), (d), Figure 6.12(b)]. For small contact spacing a homogeneous Si distribution in hypereutectic composition is found in the Al matrix as normally presented in fully covered Al-BSF [Figure 6.11(d)]. For contact spacing larger than the spread of Si in Al (d_p>d_S), the dark-gray regions separate, and the presence of voids increases, enhancing their penetration depth in the Si. Above a contact spacing of 700 µm the presence of voids is strongly increased (no local BSF formed).

It has been suggested already in the previous study, that the explanation for void formations in locally contacted rear passivated solar cells, may be found in the Si diffusion in Al, which is increased by larger contact spacing and high peak firing temperatures. It has been also mentioned that the void formation may be related to the Kirkendall effect [143]. This effect occurs during the interaction of two materials with different diffusion rates within each other which are in contact

d

d

d

Figure 6.11: (a)-(c) SEM micrographs of samples with different contact spacings d_p, 100 µm, 250 µm, and 700 µm respectively. (d)-(f) Real pictures of the Al matrix after firing for (a)-(c), respectively, showing the dark-gray regions in Al,d_S, formed by the spread limit of Si in Al.

84 Chapter 6: Aluminum-Silicon Contact Formation

across an interface, such as solid Si and liquid Al during contact sintering.

Figure 6.12(b) shows a SEM micrograph of the Al-Si alloyed junction for a LCO of approximately 70 µm. The well-known three layers formed due to the interaction of Al and Si are presented (Al-matrix, eutectic layer, local BSF). Generally, the Al-matrix has a thickness of about 20 µm, the eutectic is found to be 15 µm deep in the Si bulk and approximately (70_±5) µm wide, depending on the dielectric ablation, and the thickness of the local BSF is up to 7 µm for standard to high temperature firing conditions. The depth of the voids for large contact spacings is approximately a factor of two larger than the depth of the eutectic layer [see Figure 6.12]. This leads us to another conclusion, that voids appear instead of eutectic layers due to the high overlap of melted Al during the sintering process.

By increased contact spacing there is no limiting factor for the diffusion of Si in the Al matrix, except the sintering temperature. This phenomenon will be discussed in section 6.7.

Another important conclusion is that the thickness of the local BSF decreases when increasing the contact spacing and that no local BSF was found below the

d

= 700 µm d

= 100 µm

15 µm

Figure 6.12: SEM analysis for the same Wafer,i.e. for the same firing temperature. (a) Void formed byd_p ≥700 µm; (b) local BSF up to 7 µm ford_p = 100 µm.

LCO

1 2 3

Figure 6.13: EDX line analysis of the Si content in the Al matrix for the samples presented in Fig. 6.12 (the dotted lines delimit the region presented by the SEM figures). (a) EDX scan ford_p ≥700 µm [Figure 4(a)] where a void is formed; (b) EDX scan ford_p= 100 µm [Figure 4(b)] extending over 3 LCO with eutectic layer homogeneously formed.

6.5. Study 3: Distribution of Silicon in Aluminum 85

Table 6.1: Analysis of the eutectic geometry, presence of voids and local BSF depth (all results_±0.5 µm).

dp [mm] Eutectic or void

local BSF [µm] Voids [%] Figure depth/length [µm]

-voids for increased contact spacing. Two types of -voids were found: -voids with and without local BSF (see section 6.7 for the analysis). This may be due to the reduced Si concentration in Al for large spacings. As illustrated in Figure 6.13, an EDS/EDX analysis was performed along the Al matrix (same procedure as shown in the last study), in order to follow the distribution of Si within the Al matrix. A line scan width of 10 µm was used. From the center of the LCO the analysis is performed to the left and right, counting for 300 s. The Si composition decreases exponentially with the length as described by the Fick’s law of diffusion [137, 141].

The presence of Si on each side of the LCOs in the Al matrix is demonstrated by the EDS/EDX analysis of Figure 6.13. For small contact spacing (d_p) the uncovered Si surfaces are close to each other, and therefore the low overlap of Al mass on each side of the LCOs saturates faster (low distances for the diffusion of Si in Al). This is shown by the almost constant Si concentration in Figure 6.13(b), which shows the result of an EDX line scan extending over 3 LCOs. Thus, the Si concentration in the Al matrix is constant and the eutectic layers are homogeneously formed. For a large contact spacing the growth of voids is increased. For this sample the Si concentration presents two maxima on each side of the opening [Figure 6.13(a)]. Due to the presence of the void during cooling, the contact area is located at the edges of the LCO, where a thin and narrow alloy is found with a respective small BSF formation.

The concentration of Si in the Al matrix is inhomogeneously distributed and much lower than for shorter contact spacings. If the distance of the LCOs is too large, no saturation of melted Al by Si occurs. For large contact spacing, the concentration of Si in the Al matrix may be too low to form an eutectic layer. A deeper explanation is found in the last section of this chapter.

86 Chapter 6: Aluminum-Silicon Contact Formation