Selective seed layer patterning of PVD metal stacks by electrochemical screen printing for solar cell applications

A proof of principle for electrochemical screen printing (ESP) as a patterning process for thin metal stacks that can be employed, eg, in interdigitated back contact (IBC) or silicon heterojunction (SHJ) solar cells, is demonstrated. By using the ESP process, a 125 × 125‐mm2 interdigitated back contact grid was successfully patterned into a 100‐nm physical vapor deposited (PVD) aluminum layer. Optimizations of the ESP process were performed to improve the patterning resolution. Rectangular trenches with a mean width of 36 ± 5 μm could be demonstrated on a 100‐nm–thick aluminum layer. Up to now, ESP can be applied to PVD aluminum, copper, or stacks of both materials. Finally, metal stacks of aluminum and copper were structured, which allow a more homogeneous current distribution for the ESP process and additionally for the subsequent copper electroplating because of the second metal layer underneath the layer to be structured. The successful transfer from wafer substrate to polymer foils increases the application options of ESP technology enormously, where the topography of the surface to be structured affects the printing results.


| INTRODUCTION
To increase the market share of highly efficient interdigitated back contact (IBC) solar cells, the production costs have to be reduced. A heterojunction back contact solar cell fabricated by Kaneka achieves a record efficiency of 26.7%. 1 Nevertheless, this cell type is still only produced for a niche market due to high processing costs. National Renewable Energy Laboratory (NREL) published several possibilities to reduce the main cost factors of such cells. 2 Until now, one main cost factor is the cell metallization. The present state of the art is screen printing of metallic pastes or plating into a temporarily applied resist mask. Here, either the material costs are high, or there are several process steps that are partly complicated.
In this work, an alternative metallization process route for IBC solar cells is presented, the so-called electrochemical screen printing (ESP).
This approach combines commercial screen printing and electrochemical etching in one single process step. The ESP is a maskless patterning technology, which allows a local removal of thin (<200 nm) physical vapor deposited (PVD) metal layer. It can be adjusted and controlled not only by printing parameters but also by electrical settings. It is a simple, precisely controllable, and fast structuring process. Nonhazardous water-based pastes are required for ESP. The printing speed is In the present work, the aluminum patterning is further developed to reduce the widths of removal lines to identify the current process possibilities in that respect. Also, process transfer to larger areas of up to 125 × 125 mm 2 is demonstrated. By using the ESP process, the proof of principle of the selective removal of a copper layer from Al/Cu stacks deposited on Si or polymer foils is presented for the first time. This opens many possibilities for follow-up processes, such as electroplating. Further, on a barrier layer, like TiN and TiW, it has to be added to the Al/Cu stack to prevent an interdiffusion of copper and aluminum to guarantee cell functionality. The barrier layer will be patterned by ESP or etching. 3 2 | APPROACH, THEORY, AND EXPERIMENTAL DETAILS

| Electrochemical screen printing
Electrochemical screen printing combines screen printing with electrochemical etching as known from electrochemical machining (ECM) technology to create a pattern in a full-faced thin PVD metal layer in one single process step. ECM technology enables high metal removal rates. 4 Thin metal layers (<200 nm) can be structured by utilizing the ESP process. Figure 1 depicts the principle of ESP, which is described in detail in Kamp et al. 5,6 The metal mesh of the screen acts as the cathode. The PVD metal layer on the substrate, eg, solar cell or foil, is set on anodic potential, and the contact is realized via the screen.
The needed electrically conductive, water-based nonhazardous pastes, which are under constant development by Fraunhofer ISE, are described in Gensowski et al. 7 The rheological properties of these water-based pastes can be adjusted by the particle content.
A high contour sharpness with a uniform width along the line length defines a successful printing result. Additionally, the patterned trenches have to be free of any metallic residuals to achieve electrical contact separation in the full-area PVD metal layer. The printing result is affected by the following variables: • Printing paste properties • Applied current and current distribution • Screen characteristics • Layout design-percentage of area to be patterned A significant advantage of the ESP is the precise process control by current/voltage profile and by printing speed during this process, which allows the etching depth to be precisely adjusted. This approach can be used for several applications in the photovoltaics sector, such as IBC solar cells and silicon heterojunction (SHJ) solar cells. 8

| IBC solar cells as application
Interdigitated back contact solar cells are currently industrially manufactured predominantly by SunPower, 9 while several more companies are in lab-or low-volume production. 10,11 Copper plating into a resist mask applied onto a PVD seed layer and screen printing of silver paste are the used metallization approaches. ESP as an alternative metallization processing route for IBC is illustrated in Figure 2. Such a process starts with a full-faced PVD metal stack of aluminum and copper on the rear side of the cell, similar to highly efficient IBC solar cell configurations shown in literature. 12 With the ESP process, the copper part of the stack can be patterned in the desired IBC design, so that the trenches are free of any residuals ( Figure 2B). The aluminum layer ensures a homogenous current distribution during the electrochemical printing process and the subsequent plating.
Immediately, after the ESP process, the printing paste is removed with water. In case that the paste has dried on the sample, the paste is removed with water in an ultrasonic cleaner. This ensures the complete removal of the printing paste.
In the next processing step, the patterned layers act as a seed layer for plating to increase the layer thickness ( Figure 2C). The uncovered aluminum areas in the trenches are passivated by its native aluminum oxide and will not be plated if a suitable plating process is applied. 8 A commercial copper-sulfate-based electrolyte from OMGroup is used.
The plating parameters also depend on the geometry of the patterned structures and the desired copper thickness.
The remaining aluminum in the grooves has to be removed by using sodium hydroxide solution as last step to achieve the required FIGURE 1 Schematic depiction of the electrochemical screen printing process during the ongoing structuring of a thin metal layer 5 [Colour figure can be viewed at wileyonlinelibrary.com] electrical contact separation between the n-type and p-type doped areas ( Figure 2D). Depending on the cell design, copper and aluminum will interdiffuse at temperatures reached in module operation, so it is certain that an additional diffusion barrier will be required. This layer would either be removed in the ESP process or would need to be insusceptible to plating similar to the aluminum.

| ESP on aluminum layers on Si substrate
The proof of principle of ESP on aluminum layers and a follow-up treatment of zincate activation and plating were published by Kamp et al. 5 In this work, the ESP process on 100-nm PVD layers was tested regarding its limits in resolution of line width. Quite narrow lines with an average width of 36 ± 5 μm were obtained by using 25-μm screen openings ( Figure 3). The total length of the rectangular trench is 42 mm ( Figure 3B). Figure  Our hypothesis is that the textured silicon surface can lead to an increased paste spreading and a more inhomogeneous printing result.
If the paste is on the pyramid tip, the paste spreading can be enhanced compared with if the printing paste is located between two pyramids. The paste spreading might also be increased by the process current, which causes the electrowetting. 13,14 Depending on the paste properties, eg, yield strength and viscosity, the electrowetting can be reflected in the electrical and optical aspect ratio. The aspect ratio of a printing paste without process current is higher compared with the aspect ratios of the printing paste with process current. Consequently, the contact angle between substrate and paste increases. So far, the effect of electrowetting was esti-  The confocal microscope image in Figure 4C shows aluminum residuals in the patterned trench. These residuals in the electrochemically etched structure can cause short circuiting between the two polarities.
Possible causes are inhomogeneous current distribution and blocked screen meshes. While patterning single metal layers with the ESP process, the inhomogeneous current distribution is one big challenge. The confocal microscope image in Figure 4C shows aluminum residuals, which are separated from the rest of the aluminum layer. It is not possible to remove this metal residual by using ESP process, as no current can reach this metal portion. The amount of metal residuals in the patterned trenches can be controlled to a certain degree by the used squeegee pressure and process parameters. A chemical posttreatment is beneficial to remove metal residuals in these trenches.

| ESP process on Al/Cu metal stack with IBC design
The ESP process was transferred to metal stacks with a similar process that was used under Section 3.1 to pattern single layers of the stack.
Patterning a stack offers several advantages compared with a single metal layer in terms of IBC solar cell application; ie, metal stacks lead to a more homogenous current distribution as more conductive material is available. This is especially true if one metal is etched preferentially as compared with the underlying one.
The thin Cu layer of the Al/Cu stack acts as a seed layer for the Cu plating. So plating is also possible without activation by using Al/Cu stacks. The process sequence of the metallization process for IBC solar cells on Al/Cu stack is depicted in Figure 2.

| Long patterning trenches of more than 28 m on Si substrate
In addition to working on an aluminum layer, the 125 × 125-mm 2 IBC grid shown in Figure 5 has been manufactured according to the metallization process route depicted in Figure 2. The layer thickness of PVD aluminum was 100 nm, and the layer thickness of PVD copper  The aluminum layer below the patterned copper seed layer allows an excellent current distribution for the plating. The layer thickness was increased homogenously to 7 μm ( Figure 5B). As in theory, the aluminum areas stay free of copper deposition. 8 To reach an electrical contact separation between n-type and p-type doped areas, the full- faced aluminum layer is etched selectively by using highly diluted NaOH. The trenches are free of any metal residuals, and the plated copper layer was not etched ( Figure 5C).
After the successful structuring of Al/Cu stack using ESP, a further metal layer in the Al/Cu stack will be necessary in the next step. This layer would act as a diffusion barrier to guarantee the long-term functionality of IBC solar cells. 15

| Results on polymer foil
As described in Section 3, the ESP process is a versatile process concerning its application. In addition to photovoltaics applications, this approach can also be used for flexible printed circuit boards. For this, the ESP process and the following process steps (Figure 2) were transferred to PET foil material.
A polymer foil was used as a substrate with a PVD aluminum layer of 100 nm and a PVD copper layer of 20 nm on top. The 125 × 125mm 2 IBC grid was fully patterned in 1.2 seconds. The ESP result is shown in Figure 6A, and the line width is 150 μm (screen opening of 100 μm). The confocal microscope image shows that the copper was selectively removed from the aluminum in the trenches. The electrochemically etched trenches are free of any Cu residuals. It is simpler to pattern planar surfaces than textures surfaces with the ESP process.
This approach was successfully demonstrated on PET foil substrate.
The 20-nm Cu-patterned layer acts as seed layer for Cu electroplating. This seed layer was thickened to a homogenous Cu layer with an average height of 6.7 ± 0.5 μm ( Figure 6B). The last step of the process sequence was to remove the aluminum from the patterned trenches. Figure 6C  The graph in Figure 6D shows resistance measurements between each line of the 125 × 125-mm 2 IBC grid on a 100-nm metal ESPstructured silver layer on PET foil that was patterned in another experiment. An electrical contact separation was achieved between most lines (infinite resistance). A few measurements at the beginning of the print show resistances in the ohm range. Contact problems at the beginning of the ESP process would need to be optimized to avoid this behavior. It is assumed that the electrical contact separation achieved on the 100-nm silver layer also applies to the ESP result of the metal stack (Al [100 nm]/Cu [20 nm]; see above).

| CONCLUSION
The ESP process was demonstrated on 125 × 125 mm 2 100-nm aluminum layers as well as on Al (100 nm)/Cu (20 nm) stacks on Si wafers and on foils, where single layers could be removed selectively. The printed pattern corresponds to a structure length of 28.48 m. The achieved electrochemical etched trenches on the metal stacks were in average 110 μm wide on Si substrate and 150 μm wide on foil substrate. These trenches were free of any residues in microscope inspection. Still, no full electrical contact separation was achieved on Si substrate, which has to be investigated in more detail. The patterned copper seed layer was thickened in average to a height of 7 μm by copper plating.
Furthermore, it was shown that currently, a resolution of 36 ± 5 μm on 100-nm aluminum layer could be achieved. The rectangular trench was 42 mm long, and electrical contact separation was reached. First steps comprising the key factors to successfully create an IBC cell metallization, such as patterning with the required resolution, selective plating, and selective etch back, have thus been demonstrated. These features will be used to create a pattern with all properties of a real IBC pattern in a first step (including a thin diffusion barrier layer between aluminum and copper, full contact separation, and greatly increased plating height) and actual IBC solar cells in a second step.
Additionally, this technology has the potential to be used not only in photovoltaics but also in other industrial branches for local structuring of thin metal layers.