Influence of glass frit on contact formation

Another main constituent of screen-printing pastes is the glass frit. Therefore, the in-fluence of the glass on the contact formation is another key issue for the understanding of the contact formation process of aluminium-containing pastes to boron emitters. Ac-cording to the model for contact formation of aluminium-free silver pastes of Schubert summarized in section 3.1 [90], the glass frit fulfills two main tasks in the contact forma-tion process. First, the lead glass etches the insulating passivaforma-tion/anti-reflecforma-tion layer.

3.4 Paste composition

This is necessary to enable the contact of the paste with the silicon surface. Second, the liquid lead that forms during the firing process reacts with the silver forming a liquid silver-lead phase. The dissolution of the silver is required to enable the growth of silver crystals into the silicon surface.

As has been shown in section 3.4.1, the contact structure of aluminium-containing screen-printing pastes differs from the one of pure silver pastes. This suggests that other mechanisms may be important in the contact formation of these pastes. There-fore, in this subsection, the role of the glass frit in the contact formation process will be investigated.

Variation of glass frit fraction

To analyze the influence of the glass frit fraction on the contact formation of aluminium-containing silver pastes, pastes with varying glass frit content were examined. In two experiments with different base material with a resistivity of ≈ 3 Ωcm or ≈ 7 Ωcm, slightly differing emitters (50 Ω/ or 60 Ω/) and the two different firing profiles pre-sented in figure 3.5 five pastes were analyzed. The aluminium and glass contents of the different pastes are summarized in table 3.2.

Table 3.2: Glass and aluminium content of the aluminium-containing pastes G1-G5.

G1 G2 G3 G4 G5

Al content medium medium low low low

Glass content low high low medium high

In figure 3.16 the specific contact resistance for the screen-printed contacts of paste G1-G5, obtained in the two experiments, are illustrated. For reasons of clarity, the glass content (low, medium, high) in the pastes is depicted by the height of the red bars in the lower part of the graph. The small squares in the boxplot represent mean values of ρc. For pastes G1 and G2 with medium aluminium fraction in both experiments the specific contact resistance decreases by increasing the glass content in the paste.

Additionally, the scattering of the measured values decreases. This result could be reproduced in further experiments with these two pastes not presented here. However, for the pastes G3-G5 featuring a low aluminium fraction, this trend is no longer clear.

In experiment 1 median and mean values of ρc show a decreasing trend with higher glass content but considering the scattering of the values indicated by the boxes, the measured values are comparable. In experiment 2 the median shows the same trend to higher glass contents, however, the mean values do not.

Since for the pastes G1 and G2 a reproducible reduction of the contact resistance with rising glass content was observed, samples of these pastes were selected for SEM

anal-Glasscontent

G1 G2 G3 G4 G5

0.5 5

1 10

Paste

ρ

C(m

Ω

cm2)

experiment 2 experiment 1

Figure 3.16:Specific contact resistance of screen-printed contacts with different glass content obtained in two experiments. The squares in the boxplot represent the mean values ofρc. For better comparison, the glass content is schematically depicted by the red bars in the lower part of the graph.

ysis. In figure 3.17 SEM images of the samples etched back in hydrofluoric acid are shown. In the images an influence of the glass frit content on the number of contact spots can be observed. With a higher glass content in the paste, more contact spots grow into the silicon surface. On SEM images of the samples etched back inaqua regia, beside the number of imprints, no difference between the two pastes can be observed.

In figure 3.18 cross-sections of contacts of the two pastes are shown. For paste G1 (low glass content) several intact aluminium particles can be seen distributed over the con-tact section (figure 3.18 (a)). These particles did not visibly mix with the surrounding paste components in the high temperature process. Therefore, the number of inhomo-geneous regions consisting of aluminium-containing glass and silver-aluminium phase

50 μm contact spots

G1-low

(a)

50 μm G2-high

contact spots

(b)

Figure 3.17: SEM micrographs of contact etched back in HF of pastes with different glass content (a) paste G1 - low glass content (b) paste G2 - high glass content. Some groups of contact spots are exemplarily marked by orange circles.

3.4 Paste composition

(encircled by green dashed lines) is small.

As the aluminium particles are covered by a thin native oxide layer, the reason is probably that the oxide layers of the intact particles were not penetrated completely, and therefore the exchange of the different paste components was limited. In the cross-sections of paste G2 fewer intact aluminium particles but more inhomogeneous aluminium-containing regions can be found. It is therefore concluded that the glass frit supports the etching of the oxide shell covering the aluminium particles and more silver-aluminium regions can form in the contact of paste G2. Therefore, the proba-bility that these regions get in contact with the silicon surface is enhanced and more silver-aluminium crystals can grow into the silicon resulting in a reduced specific con-tact resistance.

Considering ρc of the pastes with low aluminium content (G3-G5), it is supposed that for a given aluminium content there exists an optimal glass fraction above which satu-ration takes place. In section 3.8.2 it will be discussed that the glass frit accumulates around the aluminium particles during firing. In a paste containing a low amount of aluminium, a low glass content might be sufficient to wet the aluminium and support the etching of the oxide layer surrounding the aluminium particles. A further increase of the glass content could not result in an enhanced etching behaviour and no signif-icant improvement of the electrical contact characteristics would be observed. For a paste featuring a higher aluminium content, however, a low glass content might not be sufficient and a considerable influence of a higher glass content on the contact formation would be thinkable.

Figure 3.18:SEM micrographs of polished cross-sections of pastes with different glass content (a) paste G1 - low glass content (b) paste G2 - high glass content. The dark spheres represent aluminium particles that did not break up during the firing process. The inhomogeneous parts, encircled by green dashed lines, represent regions where aluminium particles have molten and mixed with the surrounding paste components. The dark stripes visible in (a) are residues of the polishing process and do not represent a difference in the microstructure of the contact.

Analysis of paste without glass frit

The results of the last subsection suggest that the glass frit supports the etching of the oxide shell of the aluminium particles and therefore the formation of a silver-aluminium phase and the growth of contact spots. However, with different mechanisms responsible for contact formation compared to aluminium-free silver pastes, the question arises if, beside for etching of the dielectric layer, the glass frit is mandatory to enable an elec-trical contact between aluminium-containing silver screen-printed contacts and a boron emitter. To check this, a glass-free aluminium silver paste with the same binder system as used in the other investigated pastes was produced and screen-printed on wafers with and without silicon nitride layer. For the investigation alkaline textured≈ 7 Ωcm n-type Cz silicon wafers with a 60 Ω/ boron emitter were used. Except for the depo-sition of SiN_x:H all samples were produced according to the standard procedure.

For the samples with silicon nitride, the TLM measurements did not yield reasonable values. This is not surprising as the dielectric layer is usually etched by the now missing glass frit and thus remains as an insulating layer between the metal of the contact and the silicon. In SEM micrographs of contacts etched back in hydrofluoric acid the silicon surface below the contacts is undamaged.

The samples without SiN_x:H show a specific contact resistance of 3.2 ±1.3 mΩcm². This seems high compared to the values for aluminium-containing pastes with glass frit (compare figure 3.14). There, depending on the aluminium fraction in the glass-containing paste, values between 0.7 and 7.8 mΩcm² were measured on wafers with silicon nitride layer. For a paste without glass frit and without insulating passivation layer between contact and emitter a direct metal-semiconductor contact exists at the points where the screen-printed contact gets in touch with the silicon. Therefore, lower contact resistances could be expected.

Figure 3.19 shows contacts of the glass-free paste on wafers without silicon nitride layer etched back in aqua regia. The black spots in the overview image (figure 3.19 (a)) represent local imprints in the silicon surface distributed over the contact area. Fig-ure 3.19 (b) gives a closer look of the interface. Most imprints have the shape of inverted pyramids like the imprints of the silver-aluminium contact spots observed for pastes containing glass frit. This observation shows that although no anti-reflection coating and no glass contained in the paste prevent a direct metal-silicon contact, the silicon below aluminium-containing parts of the contact is only dissolved locally and therefore contact spots only grow locally as well. This could be due to the porous structure of the aluminium-containing regions in the contact: the silver-aluminium phase gets only locally in contact with the silicon surface. Additionally, a thin native oxide with a thickness of a few nanometers might exist at the silicon surface [142]. Possibly, this

3.4 Paste composition

50 μm

imprints of contact spots

(a)

2 μm

imprints of contact spots

(b)

Figure 3.19: SEM micrographs of contact etched back in aqua regia for glass frit free paste on wafer without silicon nitride layer. (a) some contact spots are exemplarily marked with orange circles.

layer is not removed completely. Only at points where the silver-aluminium gets in direct contact with the silicon wafer contact spots can grow.

A residual native SiO_xlayer could also explain the unexpected value of the specific con-tact resistance, as a direct concon-tact between metal and silicon only exists at the concon-tact spots and not over the complete contact.

In figure 3.20 polished cross-sections of contacts of the paste without glass (a) and a paste with glass (b) are compared. Except for the inhomogeneous regions contain-ing aluminium, the contact of the paste with glass shows a dense structure. For the paste without glass the contact structure is more porous. This is not unexpected given the importance of the glass for the densification behaviour of screen-printing

10 μm without glass

contact spots Si

(a)

10 μm

intact Al particle with glass

Al-containing Ag

contact spot Si

(b)

Figure 3.20: SEM micrographs of contact cross-sections of aluminium-containing pastes:

(a) glass-free paste, (b) paste containing glass frit. In (b) aluminium-containing regions are encircled by dashed green lines. These regions also exist in (a) but are difficult to detect and are therefore not marked.

pastes [90, 143, 144]. For both pastes local contact spots in the shape of inverted pyra-mids can be observed at the silicon surface (coloured red). It is important to notice that only very few intact aluminium particles can be found in cross-sections of the glass-free paste. This observation is in contradiction with the results of the previous subsection, where a higher density of intact aluminium particles was found for the paste with the lower glass content.

Figure 3.21 gives magnified views of cross-sections of contacts of the glass-free paste on wafers without silicon nitride layer. In figure 3.21 (a) a section of the contact in distance to the silicon surface can be seen. Beside the silver phase that shows an homo-geneous contrast, there are regions consisting of a silver-aluminium phase (encircled by green dashed circle). The aluminium-containing regions can be distinguished optically from the pure silver regions, as they contain thin, dark lines. These lines also define the border between silver-aluminium and pure silver phase. Possibly, they feature a higher aluminium content than the surrounding silver-aluminium phase. This cannot be verified by EDX as the lines are too thin. Another explanation could be that the lines represent grain boundaries between different grains of the silver-aluminium phase.

The aluminium-containing regions in the contact correspond to the inhomogeneous re-gions found in the contacts of glass-containing pastes. In the glass-containing contacts, however, the single regions are larger. This is partially due to the additional volume of the glass that enlarges the total volume containing aluminium. Additionally, the glass could accelerate the material exchange in the aluminium-containing finger parts and as a result increase their volume.

In figure 3.21 (b) the contact structure close to a contact spot can be seen. As for pastes with glass frit, contact spots can only be found where the silver-aluminium phase gets

2 μm

Ag

AgAl

(a)

2 μm

Si AgAl

Ag

Si

contact spot (b)

Figure 3.21: SEM micrograhps of polished cross-section of screen-printed contact of glass-free paste. Some aluminium-containing regions are encircled with green dashed lines. The boundaries between the silver-aluminium phase to the pure silver phase is difficult to see for glass-free contacts.

3.4 Paste composition

in touch with the silicon surface. The boundaries of the silver-aluminium phase to the pure silver phase are difficult to see because of the porous structure of the contact. In the silver-aluminium phase above the contact spot, next to the dark lines observed in figure 3.21 (a) dark spots are visible. The dark spots contain a high amount of silicon, as observed for glass-containing pastes.

The observations show that the formation of a silver-aluminium phase and the growth of contact spots in regions where this phase gets in contact with the silicon surface is also possible without glass contained in the paste, as long as the wafers have no passivation layer. This is in contrast to the contact formation of pure silver pastes on phosphorous emitters, where the lead-containing glass frit is necessary to enable the melting of the silver particles and the growth of silver crystals on the silicon surface. For aluminium-containing silver pastes, this role is supposed to be taken by the aluminium. Therefore, no glass is needed for this step in the contact formation process.

The formation of a silver-aluminium phase at temperature below the melting temper-ature of silver could happen via the formation of intermediate phases in the silver-aluminium-system [145]. Olia et al., e.g., investigated the diffusion behaviour of alu-minium and silver strips that were welded together [146]. They showed that at annealing temperatures of 400^◦C considerable amounts of aluminium diffuse into the solid silver, forming aδ-phase at the interface. The melting temperature of this phase is 726^◦C[146]

and therefore much lower than the melting temperature of silver (962°C, [145]). The formed δ-phase could therefore melt during firing. In the firing processes used to con-tact silicon wafers with screen-printing pastes, however, temperatures much higher than 400°C occur. Therefore, different phase transitions could take place. As the melting temperature of aluminium (660°C [145]) is reached during the firing process, the forma-tion of the silver-aluminium phase could also take place via a liquid aluminium phase.

To get more insights into these processes, in section 3.8 the temperature dependence of the contact formation process will be investigated.

For aluminium-free silver pastes on phosphorous emitters, silicon from the wafer sur-face is supposed to dissolve in the liquid silver-lead phase, formed in the firing process (see figure 3.1 (e)). The glass is therefore necessary for the formation of silver crystals.

The dominating process for aluminium-containing pastes is probably the dissolution of silicon in the silver-aluminium phase. The size of the metal spikes growing into the silicon is considerably increased for aluminium-containing pastes. Lago et al. [4] ex-plained this by the alloying of the aluminium in the paste with the silicon and the high diffusivity of silicon in aluminium [147, 148]. However, the experiments presented up to now confirm the former results that the spikes consist of silver with only a small fraction of aluminium. Additionally, an alloying of aluminium and silicon could not be

observed. A high diffusivity of silicon in the silver-aluminium phase could explain the occurrence of the deep silver-aluminium contact spots. This is further supported by the investigations of Lago et al. who showed that adding silicon to a silver paste with a high aluminium content reduces the density of deep metal spikes compared to pastes without silicon. This could be explained by a reduced diffusion pressure for silicon diffusion from the surface into the silicon containing paste. However, no investigations dealing with the diffusion of silicon in silver-aluminium phases are known.

For pastes containing glass frit, a higher glass content in the paste seems to result in an improved etching of the oxide shell covering the aluminium particles. Fewer intact aluminium particles can be found in the contact. Therefore, the density of inhomogeneous regions containing aluminium is increased and with this the number of contact spots. However, in the glass-free contacts almost no intact aluminium particles are present. A possible explanation for this apparent contradiction can be found by considering the volume expansion of the aluminium particles due to the phase transition during melting and the surface tension of the liquid aluminium [149]. The expansion of the aluminium particles results in a strong stress on the oxide shell covering them. In the presence of glass frit, this stress is partially reduced by the etching of the oxide shell and the formation of cracks in the shell. Additionally, the glass frit reduces the surface tension of the liquid aluminium particles. The diffusion processes are homogenized and the material exchange can occur over the small cracks in the oxide shell. In a paste with a low glass content, few openings in the oxide shell appear and the material exchange proceeds slowly. For increasing glass frit fraction, the oxide shells are etched more effectively and therefore get thinner. More openings in the shell appear, accelerating the material exchange. In the absence of glass frit in the paste no stress-release mechanism exists. When the internal pressure gets to high, the aluminium particles burst [149].

Therefore, no intact aluminium particles could be present in the fired contacts.

Although the glass is not needed for the formation of the silver-aluminium phase, its role in the contact formation process is not negligible. The densification behaviour of the screen-printing paste is influenced by the presence of glass in the paste as can be seen by the difference in the porosity for contacts with and without glass frit (compare

Although the glass is not needed for the formation of the silver-aluminium phase, its role in the contact formation process is not negligible. The densification behaviour of the screen-printing paste is influenced by the presence of glass in the paste as can be seen by the difference in the porosity for contacts with and without glass frit (compare

Im Dokument Contact Formation of Ag/Al Screen-Printing Pastes to Heavily B-Doped c-Si (Seite 68-77)