Influence of surface texture

To investigate the influence of different surface textures on the contact resistance and contact formation process of aluminium-containing silver screen-printed contacts to boron emitters, 3 Ωcm Cz silicon wafers were divided in three groups. The different groups received either an alkaline surface texture, were textured in a plasma process or textured in an acidic, isotropic texture (iso-texture) resulting in three different sil-icon surface morphologies. Then the samples were treated according to the standard procedure, with a 50 Ω/ BBr₃-based emitter passivated by 75 nm PECVD-SiN_x:H. In the following a silver-aluminium paste with a medium aluminium content was screen-printed on the wafers.

The heat transfer from an IR furnace on a silicon wafer is strongly dependent on the

3.6 Influences of wafer surface properties on contact formation

wafer surface morphology [150]. Therefore, set firing temperatures for the differently structured wafers were adjusted to obtain the standard firing profile on the wafers (com-pare figure 3.5) for all surface morphologies.

In figure 3.34 the specific contact resistance for the different samples measured by TLM is presented. The lowest value can be reached for plasma textured surfaces. On iso-textured wafers a rather high mean value of 21.3 mΩcm² is measured.

alkaline plasma iso

5

1 10

Surface texture

ρ

Ω

Figure 3.34: Specific contact resistance of screen-printed contacts on alkaline, plasma and iso-textured surfaces.

The differences in specific contact resistance can once more be attributed to the density of contact spots on the silicon surface: As can be seen in figure 3.35 (a) the highest number of contact spots can be found on the plasma textured wafer. Additionally, they appear in large groups. On the alkaline textured sample (figure 3.35 (b)) less contact spots can be found and the single groups consist of a smaller number of spikes. On the SEM image of the iso-textured surface only few contact spots can be found.

The large number of contact spots on the plasma etched surface can be due to plasma induced damages on the silicon surface forming during the etching process. Crystal defects could facilitate the growth of contact spots as it is the case for pure silver pastes [123]. Additionally, the silicon surface is enlarged by the porous structure of the plasma etched surface, which could lead to an increased number of contact points between paste and silicon surface. Another explanation could be that the silicon nitride, serving as passivation and anti-reflection layer, is not deposited homogeneously in the small pores of the plasma etched surface. At points where the passivation layer is thinner, contact spots could grow easier.

To investigate the contact formation process on differently textured silicon surfaces, contacts were etched back and analyzed by means of SEM and EDX. In figure 3.36 the left micrographs show the contacts etched in hydrofluoric acid, on the right the samples

50 μm

contact spots

(a)

50 μm

contact spots

(b)

50 μm

contact spots

(c)

Figure 3.35: SEM overview images of contacts etched in hydrofluoric acid: (a) alkaline textured, (b) plasma texture and (c) iso-textured surface. Some groups of contact spots are exemplarily marked by orange circles.

etched in aqua regia can be seen. On all samples etched in hydrofluoric acid, silver-aluminium contact spots (1) can be found. Close to the contact spots, the silicon surface is intact and the structure of the differently textured surfaces is visible (2). For better visibility, these areas are encircled by green dashed lines. In these regions, the silicon has not been etched by the glass frit contained in the paste. Additionally, it can be observed that the contact spots show the same surface morphology as the surrounding silicon. This can easily be seen at the edges of the pyramids and ditches on both the alkaline and isotextured samples, respectively (see red arrows). It is remarkable that even the small structures of the plasma-etched surface are adopted and can be seen on the surface of the contact spots in figure 3.36 (c). Further away from the contact spots the structure of the silicon surface looks blurred (3). Here the surface was etched by the glass frit, rounding the sharp structures of the texture.

On the samples etched back inaqua regia (right side images), the imprints of the contact spots can be seen as holes in the glass layer (1). For all surfaces the imprints have the shape of inverted silicon pyramids. Having a look at the glass, two different regions can be distinguished on all samples: Close to the imprints the glass shows a bright contrast (2). It appears thin, as the silicon surface structure can be seen through the

3.6 Influences of wafer surface properties on contact formation

Figure 3.36:SEM images of samples etched in hydrofluoric acid (left) andaqua regia (right).

(a),(b) alkaline textured, (c),(d) plasma textured, (e),(f) iso-textured silicon surface.

glass layer. This can be seen especially well for the alkaline and plasma textured surface in figures 3.36 (b) and (d). These glass regions correspond to the regions with intact silicon surface structure (2) on the samples etched in hydrofluoric acid. In distance to the contact spots, the glass features a darker contrast and appears thicker (3). Here the silicon surface is not apparent through the glass. The glass with the dark contrast is covering the silicon surface in regions showing a corroded silicon surface (compare contacts etched in hydrofluoric acid (3)).

To further analyze the difference between the two glass regions, EDX measurements were carried out on the samples etched back in aqua regia. In figure 3.37 EDX spectra taken at the different glass regions on an alkaline textured sample are shown. On plasma and iso-textured samples comparable spectra were obtained. The spectrum on the left (figure 3.37 (a)) was recorded at a region in distance to the contact spots (3). Silicon and oxygen peaks are detected originating from the silicon surface and the glass frit in the paste. On the right side (figure 3.37 (b)) a spectrum taken at a region where the glass features a bright contrast (2) is depicted. Here next to silicon and oxygen, nitrogen and aluminium can be found. During the firing process the aluminium in the paste melts and dissolves in the surrounding glass. The aluminium peak in the spectrum indicates that the glass close to the contact spots contains aluminium. The only source of nitrogen on the samples is the silicon nitride layer on the silicon wafers. Probably the dielectric layer below the aluminium-containing finger parts is not completely etched away.

Figure 3.37:EDX spectra of regions on contact etched inaqua regiataken in regions of dark (a) and bright (b) glass contrast in the right images of figure 3.36.

In figure 3.38 cross-sections of a screen-printed contact on an alkaline textured surface can be seen. Figure 3.38 (a) shows a magnified image of a contact spot. The position of the (former) silicon surface is marked by the red dashed line. It can again be seen that the surface of the contact spot continues the pyramidal shape of the silicon texture.

Between the contact spot and the bulk contact a thin layer is visible. This layer features a thickness of up to 75 nm. EDX point measurements taken at this layer show the presence of nitrogen. In figure 3.38 (b) a larger section of the contact close to the silicon surface is shown. Contact spots (1) can be seen, grown into the silicon surface below the inhomogeneous, aluminium-containing finger part (2). No contact spots can be found below aluminium-free (3) contact parts. At the silicon-contact interface EDX point measurements were carried out. The red marks show the positions of the measurements. No nitrogen could be detected between the aluminium-free contact

3.6 Influences of wafer surface properties on contact formation

1 μm

contact spot Al-containing region

residual SiNx

(a)

(1) no NN

(2) (3)